United State*
Environmental Protection
Aflency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
EPA-600/2-79-113
June 1979
Research and Development
Development of a
Monitor for HCN in
Mobile  Source
Emissions

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                RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology  Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:

      1.   Environmental  Health Effects Research
      2.   Environmental  Protection Technology
      3.   Ecological Research
      4.   Environmental  Monitoring
      5.   Socioeconomic Environmental Studies
      6.   Scientific and Technical Assessment Reports (STAR)
      7.   Interagency  Energy-Environment Research and Development
      8.   "Special" Reports
      9.   Miscellaneous Reports

This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
 NOLOGY series. This series describes research performed to develop and dem-
 onstrate instrumentation, equipment, and methodology to repair or prevent en-
 vironmental degradation from point and non-point sources of pollution. This work
 provides the new or improved technology required for the control and treatment
 of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.

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                            EPA-600/2-79-113
                            June 1979
        DEVELOPMENT OF A MONITOR FOR HCN
            IN MOBILE SOURCE EMISSION
                        by
    Darrell E. Burch, Pamela S.  Marrs Davila,
      Francis J,  Gates and John D.  Pembrook
  Ford Aerospace and Communications Corporation
             Aeronutronic Division
          Newport Beach, California 92663
              Contract No.  68-02-2716
                  Project Officer

                    Fred Stump
Emissions Measurement and Characterization Division
   Environmental Sciences Research Laboratory
 Research Triangle Park, North Carolina  27711
   ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
       OFFICE OF RESEARCH AND DEVELOPMENT
      U.S. ENVIRONMENTAL PROTECTION AGENCY
 RESEARCH TRIANGLE PARK, NORTH CAROLINA  27711

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                             DISCLAIMER
This report has been reviewed by the Environmental Sciences Research Laboratory,
TJ. S. Environmental Protection Agency, and approved for publication.  Approval
does not signify that the contents necessarily reflect the view and policies
of the U. S. Environmental Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
                                   ii

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                                  ABSTRACT

     Three real-time monitors for measurement of HCN concentrations in mobile
source emissions have been designed, built, tested and delivered to the
Environmental Protection Agency (EPA).  The important design parameters for
these nearly identical instruments were determined during the first phase of
the program by performing tests with a versatile gas-filter correlation spec-
trometer built previously in our laboratory for the EPA.  The instruments
employ a gas-filter cell to provide sensitivity to HCN while discriminating
against other infrared active gases such as H20, CC-2, NH3 and many hydro-
carbons that occur in mobile source emissions.  These gases absorb near 3
micrometers, the approximate center of the narrow spectral band employed by
the instrument.

     Samples are contained in a temperature-controlled cell that uses a
20-pass optical system with an optical path length of 15.5 m.  An H20 monitor
built as an integral part of the instrument measures the H20 concentration,
making it possible to account for a small amount of interference by this gas
in the sample.  The rms noise-equivalent-concentration of HCN is less than
0.02 ppm.  The combined error after accounting for ^0 interference for most
dilute samples is less than 0.1 ppm of HCN.
                                    iii

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iv

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                            CONTENTS


Abstract

Figures                                                             v

Abbreviations and Symbols                                          vi

    1.   Introduction                                               1

    2.   Conclusions                                                3

    3.   Recommendations                                            5

    4.   Optical Diagram and Layout                                 6

    5.   Gas-Filter Cell and Principles of Detection               11

    6.   Electronics and Processing Detector Signal                17

    7.   H20 Monitor                                               21

    8.   Instrument Performance                                    25

    9-   Conversion of the Versatile Gas-Filter                    31
          Correlation Spectrometer

   10.   References                                               34

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                                FIGURES

Number                                                               Page

  1.   Optical Layout  of the HCN Instruments                           7

  2'   Optical Diagram of the Retroreflector-Grid Assembly              8

  3.   Side View of the Detector Optics                                 8

  4.   Images Formed by the Multiple-Pass Optics                       10
         on Mirror C2

  5.   Diagram of Gas-Filter Cell Assembly                             12

  6.   Transmission Spectra of HCN, HO, NH  and C H                   15

  7.   Block Diagram of Signal Processing Electronics                  18

  8.   Optical Diagram of the H_0 Monitor                              22

  9.   Log-log Plots of the Output Signal vs HCN Concentration         26
         for the 5 Different Sensitivity Ranges
                                     vi

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                   ABBREVIATIONS AND SYMBOLS
AGC        -- automatic gain control

           -- acetylene
D.R.       — discrimination ratio, ratio of the concentration of an
              interfering gas species to the concentration of HCN that
              produces the same output signal.  This ratio may be posi-
              tive or negative.

GFC        — gas-filter cell

HCN        -- hydrogen cyanide

1pm        -- liters per minute

m          -- meter

mm         -- millimeters

NHg        -- ammonia

ppm        -- parts per million (by volume)

RTF        — relative transmission function of the grating assembly
V           — voltage

Vc         — component of amplified voltage at the carrier frequency, 360 Hz.

Va         — component of amplified voltage at the modulation frequency, 30 Hz.

V         — Va/Vc(related directly to HCN concentration)

Hm         -- micrometer (unit of wavelength)

sec        -- second

dc         « direct current

Hz         -- cycles per second

rms        — root mean square
                                   vii

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                                  SECTION 1

                                INTRODUCTION

     Certain types of catalytic mufflers used in automobile exhaust systems
have been found to produce HCN when the vehicle is tuned fuel-rich.  Under
some extreme conditions, the HCN concentration in the exhaust may reach un-
safe levels as high as 20-50 ppm.  Because of the very high toxicity of HCN,
it is important that the concentration of this gas be controlled below a
harmful level.  The chemical process by which the HCN is generated is not
fully understood, nor is it possible to predict the HCN concentration result-
ing from different vehicle operating conditions.  It is, therefore, essential
that vehicles that are potential producers of high concentrations of HCN be
tested so that proper controls and safety precautions can be established.

     At the time of the discovery that significant amounts of HCN could be
produced, there were no available instruments capable of real-time measure-
ment of the concentration of this gas in vehicle exhaust.  This report deals
with the development of an infrared instrument for this purpose.

     Gas-filter cell (GFC) techniques have been proven by several groups of
workers to be quite useful for instruments to measure the concentrations of
many different gases that contain much structure in their absorption spectrum.
Gas-filter cell instruments depend for their sensitivity and discrimination
on a filter that consists of a small cell that contains the gas species to be
measured.  The infrared spectrum of HCN contains much structure, making this
gas a good candidate for GFC techniques.

     The first phase of the present project involved a series of tests to  in-
vestigate the feasibility of an HCN instrument based on GFC techniques and to
determine the important parameters and expected performance of such an instru-
ment.  The tests were performed on a versatile GFC Spectrometer built pre-
viously for the EPA.l  It is convenient to interchange many of the components
in this instrument, such as detectors, gas-filter cells, gratings, filters,
etc.  It is possible to employ virtually any desired spectral bandpass from
0.3 |im to 14 M/m.  After the required tests were performed with the versatile
GFC spectrometer, it was converted for use as an HCN monitor and returned  to
the EPA where it has been used for several months.  This converted instrument
is adequate for most of the measurements to be made with it, although the
performance is not as good as can be expected from an instrument designed
specifically as an HCN monitor.
1.   Burch, D.E,, F.J. Gates, D.A. Gryvnak and J.D. Pembrook. "Versatile Gas
     Filter Correlation Spectrometer", Prepared by Aeronutronic Ford Corpo-
     ration under Contract No. 68-02-1227.  EPA Report No. 600/2-75-02^,
     June 1975.

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     The second phase of the project called for designing, building, testing,
and delivering three HCN monitors to be used in analyzing exhaust from mobile
sources. Results of the tests performed in the first phase served as the
basis for the design of these instruments.  These three nearly identical in-
struments employ a narrow infrared spectral interval near 3 (Jta that contains
several sharp absortion features of the HCN absorption spectrum.  The same
basic GFC technique employed in previous instruments provides good sensitiv-
ity, stability and discrimination.  With the exception of Section 9, most of
the remainder of this report deals with these three instruments.

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                                  SECTION 2

                                 CONCLUSIONS
     The capability of a gas-filter correlation instrument to measure the con-
centration of HCN in automotive exhaust has been demonstrated.  Although
several exhaust gases absorb significantly in the vicinity of the 3 pm HCN
band, it is possible to use this absorption band to measure the HCN concentra-
tion and to reduce the interference by the other gases to an acceptable level.
The most serious interference results from H20; some interference also results
from C02, C2H2 and NH^.  Interference by other exhaust gases in their normal
concentrations is negligible.  Much of the good discrimination against other
gases that absorb in the same spectral interval is due to the gas-filter cell;
however, the spectral bandpass of the instrument must be selected carefully
to obtain adequate discrimination.  A cell that contains C2H2 is placed in
the monitoring beam to reduce further the interference by this gas.

     Even with the instruments adjusted for optimum discrimination, there is
still excessive interference by H20.  This interference is accounted for by
measuring the H20 concentration and correcting the output signal by an amount
that is  related to the H20  concentration.  The relationship between the con-
centration of H20 and the interference it produces is determined ahead of
time from samples of H20 +  N2.  This relationship is not linear, making it
impractical  to account automatically for the  interference with simple electro-
nic circuits as we have done on other instruments built previously.  The H20
concentration is measured by automatically comparing the transmittances over
2  adjacent spectral intervals; one  interval near 1.9 Mm includes strong H20
absorption lines, and the other contains only weak lines.

     A  constant amount of HCN vapor can be kept for several months, and possi-
bly  longer,  in the small gas-filter-cell with quartz windows fused to a quartz
cell body.  Less than 0.5 mm  of  surface of epoxy cement is in contact with
the HCN vapor in the small  opening used to fill the cell.  Epoxy cement or
silicone rubber cement cannot be  used to seal windows to the cell body because
the HCN causes the cement to deteriorate.

     Two important  features of  the  GFC  assembly contribute  to the  good stabil-
ity  of  the  zero  setting  (the  output signal when there  is no absorbing  gas in
the  sample  cell).  A  temperature-controlled  enclosure maintains  the HCN in the
GFC  at  a nearly  constant  temperature,  thus maintaining  nearly constant  trans-
mittance.  The monitoring beam  of radiant  energy  is  transmitted  through the
rotating GFC assembly with  essentially no  deviation  in  position  or direction
of the  beam  as the assembly rotates.  Therefore,  the  stability of  the  output
signal  is almost  independent  of  any unavoidable wobble  or  shift  in the  assem-
bly  as  it rotates.  This  feature  represents  an  improvement  over  the mo. 3  com-

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mon type of GFC Instrument, which employs a rotating mirror chopper with a
stationary GFC.  Wobble in the rotating mirror chopper causes deviations in
the position and direction of the reflected beam.  This instability in the
optical beam leads to instability in the zero setting.

     The electronic system processes the detector signal  in such  a way that
the output signal varies almost linearly with HCN concentration and is insen-
sitive to small changes in source brightness or detector  sensitivity.  Good
signal-to-noise ratios are made possible by the convenient tungsten-iodide
energy source  and the liquid-nitrogen-cooled InSb detector.  Adequate sensi-
tivity for a 0-1 ppm of HCN full-scale range is achieved  by the 15.5 m optical
path  in  the multiple-pass cell.  Five separate sensitivity ranges (1,3,10,30,
and 100  ppm) make it possible to measure concentrations from approximately
0.05  to  100 ppm.

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                                   SECTION 3

                                RECOMMENDATIONS

     The performance of the instruments meets the requirements for the type
of measurements for which it is designed.  However, results of some of the
tests performed on the completed instruments indicate a few design changes
that could improve the performance,  A few of the changes are discussed below.

     Some unexpected instability in the zero-setting has been attributed in part
to the non-uniform brightness of the energy source.  The source is a tungsten-
iodide bulb with a coiled filament; therefore, the image formed on the entrance
slit of the grating assembly does not illuminate the slit uniformly.  Slight
shifts in the optics of the sample cell move the image on the slit and cause
slight shifts in the zero setting.  If this shift proves troublesome during
day-to-day operation, the bulb could be replaced with a Nernst glower, or
some other acceptable source that would illuminate the entrance slit more
uniformly. ,_A Nernst glower is much less convenient to operate than the
tungsten-iodide bulb, but in the spectral interval of interest, the radiance
of a Nernst glower is 507o-100% greater than that of the bulb.  Therefore, a
slightly higher signal-to-noise ratio could also be achieved with a Nernst
glower.

     The liquid-nitrogen-cooled detector should be mounted so that it is
better insulated thermally from the other components.  This would probably
improve the stability of the zero-setting.  Two changes in the sample cell
are recommended to reduce errors due to adsorption of HCN on the cell walls.
The cell body could be glass lined or made of solid glass, and the volume
could be reduced by reducing the diameter.  The latter change would reduce
the rate that the sample must be flushed to avoid significant reduction in
the HCN vapor concentration due to adsorption.

     Several of the design features of the instruments are based on the
problems related to the real-time measurement of low concentrations of HCN
in automotive exhaust.  Certain of these design features could be changed
for an instrument to monitor the HCN concentration in other kinds of samples.

     Sensitivity could be increased by increasing the length of the sample
cell.  This change would be quite practical if instrument size was not an im-
portant factor and if large volumes of sample gas could be used. If C2H2 and
NHg were not present in the sample, the sensitivity to HCN could be improved
by employing a wider spectral bandpass shifted to a nearby region where the
HCN adsorption is stronger.  Sensitivity could also be increased by changing
to a narrow spectral interval near 14 M-m that includes some very strong rtCN
adsorption lines.  This spectral interval is not practical, however, if the
sample contains a high concentration of C02, as does automotive exhaust. Dis-
crimination could also be improved by operating the sample cell at reduced
pressure; however, this would complicate the gas-handling system.
                                     5

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                                  SECTION 4

                         OPTICAL DIAGRAM AND LAYOUT
OPTICAL DIAGRAM

     Figure 1  is an optical diagram of an HCN analyzer.  Radiant  energy from
the  source, a  tungsten-iodide bulb, enters the heated enclosure that  surrounds
the  gas-filter cell (GFC) assembly.  Lens LI, which  serves  as  an  entrance  win-
dow, forms an  image of the source on the 1.4 mm wide by  3.5 mm high slit
(slit  1) mounted on the  inside wall of the heated enclosure.   Flat mirror  Nl
rotates the converging beam from lens LI through an  angle of 90°  so that it
passes through the rotating GFC assembly approximately 2 cm below the axis
of rotation.   The beam alternately passes either through the GFC  or the at-
tenuator at a  frequency  of 30 Hz.  A reticle attached to the rotating GFC
assembly chops the beam  at 360 Hz.  Field lens L2 serves as an exit window for
the  heated enclosure and forms an image of LI near mirror N3 .   Mirror N3
 (f.l.  =7.5  cm)   forms  a magnified image of slit 1  next to the front surface
of mirror C2 in the sample cell.  Lens L3 functions  both as an entrance win-
dow  for the sample cell  and as a field lens, imaging mirror N3 near mirror Cl.

     The path  of the beam through the multiple-pass  optics  is  complex; a brief
description is given in  this section under "Description  of  the Sample Cell."
After  passing  through the sample cell 20 times, the  beam is focused by spheri-
cal  mirror C6  onto the 0.4 mm wide by 5.1 mm high entrance  slit (slit 2) of
the  grating assembly.  The image overfills the slit  in the  horizontal direc-
tion but underfills it in the vertical direction.

     The beam  of radiant energy entering the grating assembly  is  rotated 90°
by flat mirror Gl and collimated by spherical mirror G2  (f.l.  = 38.1  cm).  The
collimated beam is dispersed by the diffraction grating.  Spherical mirror G3
picks  off the  dispersed  energy in the wavelength interval of interest and
forms  a dispersed image  of slit 2 in the grid plane  of the  retroreflector-
grid assembly.  A retroreflector with a grid, illustrated in Figure 2, passes
the  desired spectral interval.  The grid has two 7.62 mm high  openings cut in
it. One of the openings  is 0.56 mm wide and the other is 0.94  mm  wide.  The
openings are separated by a 0.20 mm wide metal strip.  The  beam exiting from
the retroreflector assembly follows the reverse path of  the beam  entering  the
retroreflector assembly,  except the returning beam is displaced vertically.
As a result of the vertical displacement of the beam, the returning radiant
energy passed by the grid strikes mirror G2 below the incoming beam from mir-
ror Gl.  Radiant energy  in the spectral intervals passed by the grid  is fo-
cused by spherical mirror G2,  forming an undispersed image  near the germanium
lens  LD of the detector optics shown in Figure 3.

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GAS
IN
                                                                                               GAS
                                                                                               OUT
                    f)
                                         SAMPLE  CELL
                          GRATINC

                         ASSEMBLY
 POWER

SUPPLY

SECTION
                                                                                                  VIEWING
                                                                                                  WINDOW
Figure  1.   Optical  layout  of the HCN  instruments.

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oo
                                                                   InSb DETECTOR
                                                                      FILTER FD
                                                                          LENS LD
                                                                                                 —GLASS DEWAR
                              COLD SHIELD
                              SAPPHIRE WINDOW
                                                                           MIRROR G6
        Figure  2.   Optical diagram of the  retroreflector-
                     grid assembly.
Figure 3.    Side view  of detector optics,

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     The converging beam  from mirror  G2  passes  underneath flat  mirror Gl and
is incident on flat mirror G6, which  is  mounted 45°  from the  vertical.  After
striking mirror G6, the beam travels  vertically to  the  germanium lens LD.  This
7 mm focal length lens forms an  image of the  grating on the 1 mm x 1  mm ele-
ment of the InSb detector.  Filter FD has a spectral bandpass approximately
0.6 urn wide centered near 3 Mm.  This cold filter reduces the detector noise
by reducing the amount of background  energy incident on the detector;  it also
blocks overlapping orders of shorter  wavelengths that are passed by the grat-
ing assembly.

     The phovoltaic InSb  detector is  mounted  in an  evacuated  glass dewar and
is cooled with liquid-nitrogen.  The  glass dewar has a  liquid nitrogen hold
time of approximately 3 hours and is  potted with RTV in a stainless steel  cy-
lindrical housing.  (The  housing is not  shown in Figure 3.)   The detector  is
surrounded by a cold shield, and the  bandpass interference filter is  mounted
in the cold shield.  Both of these components are contained in  the glass de-
war.

     The stainless steel  grating assembly is  spaced  off from  the baseplate of
the instrument by three stainless steel  spacers in  such a way that stresses
in the baseplate will not cause  stresses in the grating assembly.

DESCRIPTION OF SAMPLE CELL

     The sample cell body consists of an 80-cm  long  piece of  7.6 cm ID (3.0")
stainless steel tubing with flanges welded to each end.   The  flanges  provide
support for the various mounts and adjustment plates for the  multiple-pass
optics.  End covers are screwed  to each  flange  and 0-rings provide leak-proof
s.eals.  Optical path lengths of  337 cm to 2465  cm may be obtained by  adjusting
the multiple-pass optics; it is  intended that the optics remain adjusted to
20 passes, which give a 1553 cm  path  length.

     The cell is maintained at 55eC by a temperature controller that  controls
the average power dissipated by  the heaters.  One heater is coiled around  the
cell body, and a power resistor  mounted  on each end  plate provides additional
heat at the ends. The endplates  are within ±2°C of the  cell body when  the  cell
temperature is stabilized.  The  temperature near the center of  the cell  body
may be determined by monitoring  the output of an iron-constantan thermocouple
embedded in the cell body.  Foam rubber  insulation is wrapped around  the cell
body and secured with aluminum tape;  black foam rubber  insulates the  ends.
The cell is welded to two stainless steel mounting plates which are spaced off
the baseplate with 1.27 cm  (0.5") thick  phenolic spacers to minimize  conduc-
tive heat losses through  the baseplate.

     Gas flows into and out of the cell  through tubing  attached to the 1/4"
Swagelok fittings.  Gas enters the end of the cell to which mirror C2 is
attached and exits the cell at the opposite end, as  indicated in Figure 1.

     Figure 1 illustrates the multiple-pass optical  components  mounted "'nside
the sample cell.  Spherical mirrors Cl and C3 are cut from the  same mirror
blank with a 76.2 cm (30") radius of  curvature.  The distance between the
front surfaces of mirrors Cl and C3 and  the front surface of  spherical mirror

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C2 is approximately 76 cm (30").  The magnified image of slit 1  formed by
mirror N3 is located adjacent to the front surface of C2.  Mirrors Cl and C3
form multiple images of the source on mirror C2.  The number of  images,  and
thus the number of cell passes made by the beam, can be set by adjusting the
aximuth adjustment screw for mirror Cl.  Figure 4 illustrates the  image pattern
on mirror C2 as viewed through the viewing window when the cell  is adjusted
for 20 passes.

     Part of the beam from mirror N3 spills over mirror Cl onto  mirror C3,
which forms an image  of the source adjacent to mirror C2 near lens L5.  This
energy exits the sample cell after only 2 passes and is use in the I^O moni-
tor discussed in Section 7.

     After passing through the sample cell 20  times, the beam is incident on
mirror C4 (focal length = 15.0 cm).  This mirror is adjusted to  make the beam
strike flat mirror C5, which directs the beam  to spherical mirror C6.  Mirror
C6, with a focal length of 6.0 cm, forms an image of slit 1 on slit 2, the
entrance slit of the grating assembly.  An anti-reflection coated sapphire
window is bonded to the exit port of the sample cell.  Lens L4 is on a mount
attached to the grating assembly and functions as a field lens.
  EXIT WINDOW
  (TO H20 OPTICS)
2   6  10  14  18
                                                       SAMPLE CELL WALL
                                                             ENTRANCE WINDOW
    Figure  4.   Images formed by multiple-pass cell optics on mirror C2.
                Numbers by the images indicate the number of passes of the
                beam through the sample volume when the image is formed.

                                      10

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                                   SECTION 5

                 GAS-FILTER  CELL AND PRINCIPLES  OF DETECTION
GAS-FILTER CELL
     It was found that  gas-filter  cells made  by us  in the  past  are  not  suit-
able for HCN because HCN  slowly  attacks the epoxy seal,  causing it  to leak.
Thus, the HCN gas-filter  cell  should not  incorporate  glass-metal interfaces
with epoxy used as a bond.  This restriction  on the design made fabrication
of a leak-proof cell difficult because quartz windows had  to be fuzed to  the
quartz body of the gas-filter  cell.

      Figure 5 is an assembly  diagram of the  assembly, which is  composed of
 four parts:  the holder, the gas-filter cell, the attenuator windows, and the
 reticle.   The entire assembly is mounted on  the shaft of the 1800-rpm
 hysteresis synchronous motor.  As the GFC assembly spins, the beam of radiant
 energy, located approximately 2 cm below the axis,  passes alternately through
 the attenuator and then through the gas-filter cell  at a frequency of 30 Hz.
 A heated enclosure surrounds  the spinning assembly.  (Figure 8  in Section 7 is
 a diagram of the enclosure showing some of the components mounted on or with-
 in it.)  The temperature of the enclosure, and thus  the temperature of the
 GFC, is maintained at  approximately 50°C by  a temperature controller that re-
 gulates the power dissipated  by two power resistors  mounted on  the enclosure.
 Insulation is attached to the body of the enclosure,  and a phenolic mounting
 spacer minimizes conductive heat losses to the base  plate of the instrument.


     The GFC consists of  a quartz cell body with quartz windows  fused to each
side of the cell body.  The cell body is made from  a  disk 5.7 cm in diameter
and 6.4 mm thick.  The  disk is cut approximately in half to form the cell
body, and 6 equally spaced radial slots are drilled out of the  cell body. Ribs
are left between these  slots for added strength.  Shallow grooves not shown in
Figure 5 cut in the cell body  interconnect the 6 slots.  Another groove con-
nects these grooves with  a small hole drilled in the  straight edge  of the cell
body.  After the 1.5mm  thick  quartz windows are  fused to the cell body, the
6 radial slots and interconnecting grooves in the cell body form a cavity that
contains the HCN.   The cell is filled with HCN by attaching a special fitting
to the hole drilled in the straight edge of the cell  body.   The  gas flows into
the slot volumes via the interconnecting grooves between the slots.   After the
cell is filled,  the hole in the straight edge of the  cell body  is sealed with
epoxy cement.

     The attenuator consists of one 1.5-mm thick quartz window and a glass
window approximately 0.5-mm thick.   The glass window  absorbs enough eneigy to

                                     11

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                                         QUARTZ WINDOWS-
HOLDER
                                       ATTENUATORS•
                      Figure 5.   Diagram of the  GFC assembly.

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make the average  transmittance ot  the attenuator over the spectral region of
interest approximately  the same as that of the GFC.

    The GFC and the  attenuator windows are mounted in the black-anodized
holder.  This 6.9 cm diameter holder has 10 radial slots, 6 slots on the side
containing the GFC and  4 slots on  the side containing the attenuator.  The 6
slots  in  the GFC are aligned with 6 slots in the holder.   Only 4 slots are on
the attenuator side  of  the holder  because the GFC, when mounted in the holder,
extends into the  attenuator side of the holder.   The edge of the GFC therefore
covers part of the area where the  other two slots would normally be.   Employ-
ing only 4, instead  of  6,  slots in the attenuator side does not reduce the
average value of  the chopped energy reaching the detector during each cycle
of the GFC assembly  because the average transmittance of the attenuator side
must be reduced to match the average transmittance of the GFC side.   The glass
attenuators selected to match the  average transmittances  of the two  halves of
the GFC assembly  are more  transmissive than they would be if 6 slots were used
instead of 4.

    The disk-shaped  reticle is 1.3 mm thick and 6.9 cm in diameter.   It has
12 symmetrically  placed radial slots that are aligned with the slots in the
gas-filter cell and  the slots in the holder.   As explained in the previous
paragraph, no energy passes through 2 of the reticle slots since there are
only  10 slots in  the holder.   Each of the reticle slots is narrower  than its
corresponding slot in the  GFC or the holder,  and thus the reticle slots limit
the beam as it passes through the  gas-filter cell assembly.   As the  reticle
spins  at 30 revolutions per second, it chops the beam and produces  a strong
360 Hz component  on  the detector output signal.

PRINCIPLES OF DETECTION

    Detailed explanations  of the spectroscopic principles of detection and
gas-filter correlation  principles  are given in several reports published by
us.  '  '   The instrument produces  a dc output signal proportional to the
1.    Burch,  D.  E., F. J.  Gates, D.A. Gryvnak and J.D. Pembrook. "Versatile Gas
      Filter  Correlation  Spectrometer", Prepared by Aeronutronic Ford Corpo-
      ration  under Contract No. 68-02-1227. EPA Report No. 600/2-75-024,
      June  1975.

2.    Burch,  D.E., and D.A. Gryvnak.  "Infrared Gas-Filter Correlation Instru-
      ment  for  In-Situ Measurement of Gaseous Pollutants." Prepared by Philco-
      Ford  Corporation for EPA under  Contract No. 68-02-0575.  EPA Report No.
      EPA-650/2-74-094.   Also, Burch, D.E., and D.A. Gryvnak. "Cross-Stack
      Measurement of  Pollutant Concentrations Using Gas-Cell Correlation
      Spectroscopy."  Chapter 10 of Analytical Methods Applied to Air Pollution
      Measurements, Stevens, R.K. and W.  F. Herget, (eds) Ann Arbor, Ann Arbor
      Science Publishers  Inc., 1974.

3.    Burch,  D.  E., Gates, F. J., Gryvnak, D.A., and J. D. Pembrook. "Versatile
      Gas-Filter Correlation Spectrometer." Prepared by Aeronutronics Ford
      Corporation for EPA under Contract  No. 68-02-1227.  EPA Report No. EPA-
      600/2-75-024, June  1975.

                                      13

-------
concentration of HCN in the sample cell.  When no HCN is in the sample cell,
the two halves of the gas-filter cell assembly cycle are electronically  and
optically balanced to produce zero output.  When HCN is added to the  sample
cell, the gas absorbs a fraction of the energy that passes through  the at-
tenuator when it is in the beam.  However, when the gas-filter cell is in the
beam, the HCN in the GFC absorbs most of the energy at wavelengths  where the
HCN in the sample cell absorbs.  Therefore, the addition of HCN in  the sample
cell effectively decreases the energy incident on the detector when the  at-
tenuator is  in the beam more than it reduced the energy when the GFC  is  in  the
beam.  Thus, the detector signal is chopped at 360 Hz with a modulation  at
30 Hz, the rotational frequency of the  gas-filter cell.  The detector^signal
is processed by the electronics to produce a dc output signal proportional  to
the fractional modulation of the energy incident on the detector, and thus  to
the concentration of HCN in the sample  cell.

     Any absorbing gas species other than HCN in the sample cell will not
produce a 30-Hz modulation if there is  no correlation between the spectral
structure of the gas and the spectral structure of the HCN in the gas-filter
cell.  When  there is no correlation between the spectral structures,  the gas
absorbs exactly the same fraction of the energy passing through the gas-fil-
ter cell as  that passing through the attenuator.  Thus, under this  idealized
condition, no 30-Hz modulation is produced and there is no interference  in
the output signal by the gas.  Of course, the instrument can not operate pro-
perly if the absorbing gas absorbs a large fraction of the radiant  energy in-
cident on it.  Unfortunately, there is  usually a small amount of correlation
between the  spectral structures of the  gas species to be detected and another
gas that absorbs in the same spectral region.

      Figure  6  shows  superimposed  spectra  of HCN and  three  other automotive
exhaust gases  that absorb  in  the  spectral region of  interest.  Reducing  in-
terference by  these  three  gases, H20, C2H2 and  NH3, while maintaining good
sensitivity  to HCN has been the most difficult  problem in  the HCN analyzers
The unpublished curves in Figure 6, obtained with a Fourier Transform Inter-
fermometer by Dr. Herget4,  show spectral features for each gas.  Careful
consideration of these features is required in  order to optimize the  dis-
crimination.  The dashed curve represents the absorption spectrum for 4  75  m
of atmospheric path at 20°C and is labelled "H20^since H20 is the  only'
significant  atmospheric absorber in this spectral region.  The other  three
curves are spectra of the three gases indicated in a 10-cm absorption cell
at 20 C.  The HCN sample consisted to 6650 ppm of HCN in N2 at 1-atm  total
pressure.   The NH3 and C2H2 samples were pure at pressures of 40 torr and
20 torr, respectively.
4.    Herget, Dr. William F., Environmental Protection Agency, Research
     Triangle Park, K.C., 27711  (Private communication)
                                     14

-------
(jt
          3325
                          3330
3335             3340

          WAVENUMBER (cm'1)
                                                                         3343
                                                                                         335O
                                                                                                         3355
                   3.005
                                    3.000
            2.995

          WAVELENGTH (gm)
                                                                       2990
                                                                                         2.895
                                                                                                           2390
        Figure  6.  Transmission  spectra of HCN,  H20, NH3  and C2H2.   The relative transmission function of  the
                    grating assembly is indicated by the curve labelled RTF.

-------
     The "relative transmission function," RTF, of the retroreflector-grid
assembly is illustrated graphically in Figure 6.  This function  is determined
by the width of the entrance slit, the widths and spacings of the slots  in
the grid (see Figure 2), and the dispersion in the plane of the  grid.  An
opaque strip between the two grid slots causes the minimum in the relative
transmission function near 3348 cm"-'-.   Reducing the relative transmission
near 3348 cm   greatly reduces the interference by C2&2- ^ the  complete
spectral interval were employed with approximately the same relative transmis-
sion over the entire interval, the G^2 would interfere positively in the
measurement of HCN concentration and would thus cause the instrument to  in-
dicate too high of a concentration of HCN.  Blocking out some of the energy
near the almost coincident HCN and £<2&2 lines close to 3348 cm~^ reduces
the positive correlation between the spectral structures of these two gases.

     The 1/3 - atm of C2H2 in the series cell further reduces the inter-
ference by C2H2 in the sample.  The gas in the series cell absorbs much
of the radiant energy from thi monitoring beam at wavelengths where this
gas absorbs strongly.  Thus, the absorption by the C2H2 in the sample cell
is greatly reduced.  The series cell is so-named because it is placed op-
tically "in series" with the sample cell and the monitoring beam passes
through it at all times.  By contrast, the gas-filter cell is only in the
monitoring beam during half of each revolution of the gas-filter cell
assembly.

     The spectral bandpass was selected experimentally to minimize inter-
ference by ^0, the automotive exhaust gas that interferes most.   Ammonia
(NH3) also absorbs within the spectral interval passed by the instrument,
but interference by this gas is not serious because of the small amount of
correlation between the spectral structures of NH^ and HCN.
                                    16

-------
                                  SECTION 6

                ELECTRONICS AND PROCESSING OF DETECTOR SIGNAL
    The electronics process the signal  from the detector and produce a dc
voltage that is proportional to the concentration of HCN in the  sample gas.
The relationship between the output voltage and the HCN concentration is de-
termined by calibrating the instrument  with samples of known concentration.
A block diagram of the electronics used to process the detector  signal is
shown in Figure 7.  The 360-Hz component of the signal from the  detector
serves as the carrier for the 30-Hz component  that is to be measured.  The
30-Hz component of the detector signal  resulting from any  30-Hz  variation of
radiant energy on the detector that is  not also chopped at the carrier fre-
quency is rejected by the electronics.   The electronics consist  basically of
two demodulators in series.  The  first  demodulator operates at the carrier
frequency, 360 Hz; the average output of this  demodulator  is proportional to
the amount of radiant energy chopped at 360 Hz.  If the chopped  energy inci-
dent on the detector is also modulated  at 30 Hz, the 30-Hz component of the
output signal from the 360-Hz demodulator is proportional  to the modulation.
This 30-Hz component then passes  through a series of gain  controls and am-
plifiers to the 30-Hz sychronous  demodulator.  The output  of the 30-Hz syn-
chronous demodulator is proportional to the 30-Hz modulation of  the 360 Hz
signal, and thus to the concentration of HCN in the sample gas.  The reference
signal for the 30-Hz synchronous  demodulator is supplied by the  30-Hz reference
pick-up mounted on the heated enclosure where  it senses the rotation of the
GFC assembly.

    When  H20 is in the sample gas, the 30-Hz  signal also  contains a component
due to the H^O.  This interference is accounted for analytically from the
output of an H^O monitor described in Section  7 that produces a  dc signal
proportional to the ^0 concentration.  Several instruments built previously
by us have employed "^0 Correction" circuitry that used the output of the
HjO monitor to produce an electrical signal in the electronics that process
the signal from the detector in the main channel.  This electrical signal was
proportional to the ^0 concentration and was  adjusted to  correct automati-
cally  for the interference  by ^0 in the main  channel.  However, this correc-
tion circuitry can not be used in the same manner in the HCN analyzers be-
cause  the interference by ^0 is  not proportional to the ^0 concentration.
Therefore, it is necessary  to read the  ^0 concentration from the ^0 monitor
and determine the correction from a curve determined experimentally for each
unit.

    A  time constant switch  mounted on the main electronics card  makes it
possible to select an electronic  time constant of 0.3, 1,  3 or 10 sec.  The


                                      17

-------
00
             He Terence
              Pickup
              (30 Hz)
           Tuned  Reference
             Amplifier
              (30 Hz)
             Adjustable
               Phase
              Shifter
                                  Adjustable
                                    Phase
                                   Shi fter
i
From
Detector
                                                   Preamplifier
1
1
Tuned Amplifier
(360 Hz)
                                                   Automatic Gain
                                                  Control Amplifier
                                                    (Constant Vc)
                                                                                     Individual
                                                                                  Span Adjustments
I  I
                                                                                    Range Switch
                                                                                   Tuned Amplifier

                           Span Adjustment
                               30 Hz
                             S ynchronous
                             Demodulator
                                                                                   Amplifier with
                                                                                   Adjustable Time
                                                                                      Constant
                                                  Output Jack
                                                     10 V
                                                  Full Scale
                                                      H20
                                                    System
                                                  Electronics
                                                                                                               Adjustable
                                                                                                               Attenuator
                                          Output Jack
                                          (V )  10 V     Output Jack
                                          Full  Scale (va) 1 mV - 10 V
                                                       Full Scale
                                  Figure  7.   Block  diagram of  the  signal-processing  electronics.

-------
main output signal  can  be  determined from a panel meter or from either of two
output jacks.  A  signal that corresponds to a full-scale reading of the panel
meter produces a  10 volt dc signal on one of the output jacks.   The corres-
ponding full  scale  reading for the other output jack can be adjusted from less
than 1 mv to  10 v.  Thus, the instrument can be used with a wide range of re-
corders or meters,  provided the input impedance is sufficiently high.

    During the normal mode of operation it is desirable that the output signal
be proportional to  the  ratio, Va/Vc = V", where Va is the average demodulated
30-Hz component of  the  detector signal and Vc is the 360-Hz component of the
detector signal.  The instrument calibration relates the HCN concentration of
the sample gas to V, and  it is desired that this relationship be independent
of source brightness, detector sensitivity, etc.  During the normal mode of
operation, Vc is  maintained at a constant value by the automatic gain control
(AGC) circuits.   If, for example, the signal from the photodetector decreases
because of dirt on  a window or a decrease in source brightness, the AGC in-
creases the  amplification  of both the 360-Hz signal and the 30-Hz signal by
the same  factor.  Therefore, V is kept directly proportional to the output,
Va, and maintains a constant relationship with the HCN concentration.

    The 30-Hz reference pickup consists of a light-emitting diode (LED) and a
small phototransistor.   Aluminum tape is wrapped around half the circumference
of  the reticle,  and the other half is black.  As the GFC assembly spins at
1800  rpm,  light  from the LED is reflected by the aluminum tape but not by the
black area.   The  reflected light is sensed by the detector and produces a
30-Hz signal. The  amplifier for the 30-Hz reference pickup is tuned and con-
tains an  adjustable phase  shifter to produce a "clean" signal with the proper
phase for the 30-Hz synchronous demodulator.

    The  zero-balance assembly electronically accounts for any misbalance bet-
ween  the  two halves of  each cycle of the GFC assembly when there is no HCN in
 the  sample cell.  This  electronic assembly would not be required if it were
practical to maintain a perfect optical balance between the two halves. Ap-
proximately ± 10 percent variation in the transmittance through either of the
 two halves of the cycle can be accounted for by this electronic zero-balance
 assembly.   The assembly changes the amplification ahead of the 360 Hz de-
modulator during one of the two halves of the GFC assembly cycle.  A signal
 from the  30-Hz reference pickup switches the zero-balance assembly between
 the two different gains with the proper phase.  The difference in the gains
 between the two halves  is  adjusted with the "zero" potentiometer mounted on
 the top of the instrument.

     Five  ranges  of  sensitivity are provided: 1,3,10,30, and 100 ppm full
 scale.   The gain in each range can be adjusted independently over a factor of
approximately 5  if  different sensitivities are desired.  A five-position
 switch mounted on the top  of the instrument makes it convenient to change from
one range to another.  An  additional potentiometer mounted on the main elect-
ronics card  is also provided to change the gain of all 5 ranges simultaneously
by  the  same  factor.  This  single adjustment is usually adequate for making
small corrections in the span calibration.
                                      19

-------
    A four-position switch mounted on the main electronics card makes it
possible to use the panel meter to monitor quantities other than the output
signal.  The 30-Hz reference signal and the average carrier voltage, Vc, can
be checked with this meter to see if they are in the correct operating range.
When the selector switch is in the "signal-in" position, the meter reads -he
level of the signal going into the 360 Hz synchronous demodulator.  With the
selector switch in the "signal out" position, the meter reads the level of
the output signal.  The signals on the output jacks are not affected by the
position of the selector switch.
                                     20

-------
                                  SECTION  7

                                     MONITOR
OPTICAL

     The H20 monitor produces a  dc voltage  that  is  proportional  to  the  con-
centration of H20 in the multiple-pass  sample  cell.  The ^0  concentration is
determined by comparing the  transmission  of the  sample  at  two wavelengths,
one in the region of strong  ^0  absorption  near  1.9 Mm  and one in a nearby
region of weak H20  absorption.

     The optical components  employed  by the H20  system  are shown in Figure 8.
The bottom portion  of  the  figure is a side  view  of  the  H20 detection  system
optics, and the top portion  of the figure is a top  view.   The heated  enclosure
for the GFC assembly is shown.   Components  mounted  inside  the enclosure are
shown as they would be viewed with the  top  off,  for the top view, and the
back wall off, for  the side  view.  Detectors DA  and DB  and their associated
filters, FA and FB, are mounted  on an aluminum block, which is mounted  in the
top section of the  heated  enclosure.  The spinning  GFC  assembly, which  is
mounted on the motor shaft inside the heated enclosure, is also  shown in the
figure.

     When the sample cell  is set at 20  passes, mirror C3 forms an image of
the source adjacent to mirror C2 and  next to lens L5. (See Figure 4)  This
energy exits from the  sample cell and is  used  in the H20 detection  system.
Lens L5, with a focal  length of  10.1  cm,  forms an image of C3 near  lens L6.
The combination of  flat mirrors  HI and  H2 raises the beam  of  energy exiting
from the sample cell by 5.7  cm and directs  the beam into the  top section of
the heated enclosure.  Lens  L6,  with  a  focal length of  3.8 cm, forms  images
of slit 1 on detectors DA  and DB.

     Detectors DA and  DB have photoconductive  PbS elements, 4 mm x  4mm,  that
are mounted on a small aluminum  block as  shown in Figure 8.   Filter FA  acts
as a dichroic beam-splitter, transmitting a narrow  spectral interval  in the
region of strong H20 absorption  near  1.9  (Jm.   The wavelengths not transmitted
by filter FA are reflected to filter  FB,  which transmits a narrow spectral
interval of weak H20 absorption.  The gains of the  preamplifiers are  adjusted
so that the 360 Hz  signals from  the two detectors are equal when no absorb-
ing gas is in the sample cell.   It is apparent that the addition of 1^0 to
the sample cell would  reduce the amount of  energy incident on detector  DA
more than it would  reduce  the amount  of energy incident on detector DB.  The
resulting difference in the  detector  signals is  proportional  to  the H^O con-
centration.

                                      21

-------
                                      HEATED
                                    ENCLOSURE
      SAMPLE
      CELL
         \
                 L5
                 HI
o
                       t
                      O
    Ley
                                                  GFC
                                                ASSEMBLY
Figure 8.  Optical diagram of the H^O monitor.
                               22

-------
     Since small variations  in  the  temperatures  of the  detectors  and  filters
can produce drift in the zero-reading  of  the  H20 monitor,  these components
are maintained,at approximately 50° C by the heated enclosure.  We have  found
from experience with similar instruments  that this 2-detector  type  of monitor
is quite adequate for the present purposes.   By  employing  two  fixed filters
and two detectors,  it is possible to keep the optics  of the  H20 monitor re-
latively simple and not employ  any  moving optical components that are not
required by the HCN channel.

SIGNAL PROCESSING

      The  bias voltage  for detectors DA and DB is provided by a ± 15 Vdc
power supply used with the main electronics.   A separate preamplifier ampli-
fies  the  output of each detector,  and the outputs of both preamplifiers are
 fed into  a difference  amplifier.  When the sample cell is free of any 1^0,
the monitor is zeroed  by adjusting the outputs of the preamplifiers so that
they are  equal, thus producing a zero input to the difference amplifier.  This
 is done with a fine zero control that is mounted on the front panel of the in-
 strument  and a coarse  zero control that is mounted on the E^O electronics card.
The addition of 1^0 to the sample cell creates a misbalance between the signals
 from the  two preamplifiers and results in a signal from the difference ampli-
 fier.  This signal passes through a tuned amplifier and is then demodulated to
 produce a dc voltage that is proportional to the t^O concentration.  A refer-
 ence signal for the synchronous demodulator is obtained from the output of the
 preamplifier for detector DB.

      A variable-gain amplifier makes it possible to adjust the "span" so that
 full-scale readings of the panel meter can be made to correspond to 1^0 con-
 centrations between approximately 1 percent and 10 percent.   An output jack
 mounted on the instrument can be used to monitor the t^O output signal.  The
 HoO output signal  is displayed on the panel meter mounted on the top of the
 instrument when the 1^0 button is pressed.

      An automatic  gain control (AGC) circuit is employed so that the amplified
 difference signal  is nearly independent of slight changes in such things as
 source brightness  or dirt on windows that would change the amount of energy
 incident on both detectors by about the same factor.  The AGC circuit main-
 tains  the signal from the preamplifier for detector B at a constant level.
 For example, if a  change in the optics occurs that would ordinarily decrease
 the detector signal, the bias voltage is increased automatically to com-
 pensate for the decrease in radiant energy.   The bias voltages for both de-
 tectors are increased by the same factor; therefore, to a good approximation,
 the change in bias voltage  increases the outputs of both detectors by the
 same  factor.  It follows that a given output of the difference amplifier re-
 presents a certain fractional difference in the outputs of the two preampli-
 fiers.  This difference can then be related to the E^O concentration and is
 nearly independent of small changes in source brightness,  dirt on windows,etc.

      The amount of E^O interference is different for each of the 3 units;
 therefore a separate curve relating the interference error to the output of
 the HoO monitor is provided for each unit.  As a typical example, the max-
 imum HoO interference on Unit 2 corresponds to 0.7 ppm of HCN and is produced

                                      23

-------
by approximately 1.5% H^O.  At either higher or lower concentrations, the
interference is less.  Because of the non-linear relationship between the I
interference and the concentration of this gas, it is not possible to use the
ttjO "Correction" circuitry originally designed and built to account automati-
cally for the H20 interference. Instead, the 1^0 concentration is determined
from the output signal of the t^O monitor, and the correction determined from
the interference curve is applied analytically to the output signal of the HCN
channel.
                                    24

-------
                                  SECTION 8
                            INSTRUMENT PERFORMANCE
SENITIVITY AND CALIBRATION

     Hydrogen cyanide vapor  adsorbs  readily  to  the  surfaces  of many  types  of
containers,  including metal  tubing and the stainless  steel sample  cell.  The
adsorption,  and  subsequent desorption,  can cause  serious  errors  in sampling
dilute mixtures  of HCN + N2-  A larger fraction of  the  HCN can adsorb  on the
container walls  when the concentration is low than  when it is high.  This  makes
it difficult to  make accurate dilute mixtures that  can  be used reliably as
standards to calibrate instruments.   This difficulty  was  partially overcome  in
calibrating  the  HCN instruments by using sample path  lengths much  shorter  than
the  15.5 m path  length at which the  instrument  is to  be used.  The output  sig-
nal  of the instrument is proportional to the product  of path length  and HCN  in
concentration.   Therefore, we were able to use  higher concentrations of HCN  in
the  standard HCN + N£ mixtures.  The lowest  concentration of the standard  sam-
ples was approximately 500 ppm.

     Two  short  calibration  cells (1.5 cm and 6.0  cm)  were placed in  the monitor-
 ing  beam near  the series cell  (see Figure 1), and the multiple-pass  cell  was
 adjusted  to  4  passes.  The multiple-pass cell and the short  cells  were maintained
 near 50°C, normal operating  temperature of  the  multiple-pass cell.  All
 standard  calibration  samples were maintained at essentially  1.00 atm and were
 flushed  through the cells long  enough that  adsorption on the walls did not
 reduce the vapor concentration  significantly when the output signal  was
 measured.

     Figure  9  shows  typical  log-log  plots of the  output signals  for  the 5
 different  sensitivity ranges vs the  equivalent  concentration at  the  normal
 path length  of 15,5 m.   A 10 volt output signal corresponds  to full-scale  de-
 flection of  the panel meter. When  the concentration  is lower than approximately
 10 ppm,  the  output signal is directly proportional  to the concentration.   An
 accurate  set of curves  is provided  for each  instrument  to relate HCN concentration
 to the output  signal  for the 30 ppm  and 100  ppm ranges.

 NOISE  AND DRIFT


     Most  of the noise  on the  output signal  originates  in the detector and
 preamplifier when the optical  components are clean  and  aligned properly.   If
 some of  the  optical components, particularly the  multiple-pass mirrors, are
 not  aligned  properly,  additional noise may result from  vibrations  in the optics.
The  noise  level corresponds  to  the  same concentration of HCN regardless of the
 sensitivity  range employed.
                                      25

-------
ro
                0.7
                0.5
0.1
0.1
                     0.5   0.7  1
                                                         2           57

                                                     CONCENTRATION OF HCN (ppm)
                                                                              10
                                                                               50   70  100
                    Figure 9.  Log-log plots of the output signal vs HCN concentration for the  5  different
                               sensitivity ranges.  Concentrations below 10  ppm can be determined Directly
                               from the output signal  or  the panel meter.

-------
     The noise level of each  instrument was measured  by recording  the  output
signal with a strip chart recorder.  The  sample  cell  was filled with No,  and
the recording was made for  10 minutes  at  each  of two  different time constants:
10 sec. and 3 sec.  Table 1 summarizes the rms noise  levels  (expressed in
ppm of HCN) estimated from  these  recorder tracings.

     Flushing gas through the multiple-pass sample  cell at rates up to approxi-
mately 10 1pm does not significantly increase  the noise level.  Even higher
flow rates may be usable without  increasing the  noise level, but they  have not
checked.
                TABLE  1    RMS NOISE MEASUREMENTS EXPRESSED IN

                                        PPM OF HCN
  Instrument No.                3  Sec Time  Constant            10 Sec Time Constant

        1                             0.015                         0.012

        2           .                  0.010                         0.007

        3                             0.007                         0.005
     A small drift  in  the  zero-setting (output  signal with no absorbing gas in
the sample  cell)  appears to  be  correlated  to  filling the detector with liquid
nitrogen.   Slight shifts in  the grating assembly due to temperature differences
apparently  lead to  very slight  changes in  the spectral bandpass, which in turn,
cause shifts in the zero-setting.   The drift  can be minimized by keeping the
detector cooled and not filling it  with liquid  NZ to within  1" of the top.  If
these precautionary procedures  are  followed,  the drift in zero-setting can be
reduced to  less than 0.2 ppm of HCN per hour.


INTERFERENCE

     Table  2 summarizes typical interferences by several gas species that occur
in automotive exhaust.  The  second  column  gives the concentration of the inter-
fering gas  species  in  a typical sample studied.   All interference test samples
were at 1 atm total pressure.   The  average absorptance A, given in the third
                                      27

-------
                       TABLE 2,, TYPICAL INTERFERENCE DATA
Gas
Concentration
    (ppm)
Interference
   Error
 (ppm of HCN)
Discrimination
    Ratio
H20
H20
H20
C02
TSIH 3 (ammonia)
CH^Cmethane)
C2H2( acetylene
C2H2
C2H/^(ethylene
C2H6( ethane)
CjH.g( propane)
C^H^gC butane)
6 , 000
13,000
25,500
60,000
386
3860
83
340
682
3860
3860
3860
3860
0.03
0.06
0.11
0.009
0.052
-,0. 0015
0,032
0,122
0.227
0.003
0.017
0.217
0.115
+0.38
+0.60
+0.39
-0.12
-0.54
0.01
-0.13
+0.00
+0.94
-0,04
+0. 015
-0.036
-0.37
+15,800:1
+21,700:1
+65,400:1
-500,000:1
-715:1
> 386,000:1
-640:1
CO
+730:1
- 96,500:1
+260,000:1
-107,000:1
- 10,400:1
                                       28

-------
column, was determined  from (IO-I8)  /I   where T  and I  are the 360 Hz input
signals observed with the  sample in  and the sample out, respectively.   The inter-
ference, or false  signal,  produced by each sample appears in the fourth column.
The discrimination ratio,  D.R.,  given in the fifth column,  represents the ratio
between the concentration  of an interfering gas and the concentration of HCN
required to produce the same output  signal.  A negative value of D.R.  indicates
that the interfering gas produces a  negative output signal.   Note that propane
absorbs significantly,  but it does not produce a significant interference signal.
This indicates  that there  is little  correlation between the spectral structures
of propane and  of  the HCN  in the GFC.

     It is unlikely that any of the  gases listed in Table 2 would occur in con-
centrations high enough to produce significant interference, except for H20, CO2,
NH3 and C^.   The interference by C02 or NH3 is nearly proportional to the con-
centration of the  gas;  therefore, the discrimination ratio for each of these two
gases  is essentially independent of  the concentration.   In most cases the
interference by NH^ is  probably negligible, but a small correction may be
needed if the concentration of this  gas is unusually high.   The correction for
C02 interference usually corresponds to less than 0.1 ppm of HCN.   The
interference by C2H2 or H20 is not directly proportional to the gas concentration.
of each o-f these gases  is  provided for each instrument.  Interference by ^0 is
positive for all H20 concentrations  below approximately 3% (30,000 ppm).   The
interference increases  with increasing H«0 concentration up to approximately
13,000 ppm; as  the concentration increases further, the interference decreases.
Interference by C2H2 is negative for small concentrations then becomes
positive for higher concentrations.

     The most serious  interference results from H20.   The concentration of this
gas is measured by the  H20 monitor,  making it possible to account for the inter-
ference by using previously obtained interference data on H20 + N2 mixtures.
The very small  amount  of interference by C02 can be accounted for from the
known  discrimination ratio and the concentration, which is usually known
accurately enough  for  this purpose.   In situations when the NHo or C2H2 concen-
tration is usually high, it may be necessary to measure the concentration of the
gases  so their  interferences can be  accounted for accurately.


GAS HANDLING

     Measurements  of the HCN concentration can be made either while a sample
is stationary in the sample cell or  while it is flowing.   If measurements are
to be  made on a stationary sample, checks should be made to see if a signifi-
cant amount of  the HCN  adsorbs on the walls of the incoming-gas lines or the
sample cell.  Adsorption depletes some of the HCN vapor from the mixture,
causing the instrument  output signal to be lower than it would be in the
absence of adsorption.   Adsorption may take place for several minutes after
a sample has been  introduced into the cell.  The continuing adsorption re-
sults  in a decrease in  the output signal.   Errors due to adsorption can be
reduced by pre-conditioning the gas  lines and cell by filling them for several
minutes with a  sample that contains  approximately the same HCN concentration
as the sample to be measured.   This  mixture can then be flushed out by the
sample to be measured.


                                       29

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     If a sample of high HCN concentration has been in the sample  cell,  care
must be exercised to avoid errors in the next sample if its HCN concentration
is much lower than the original sample.  In this case, HCN adsorbed on  the
walls from the original sample may come off the walls and cause an erroneously
high reading.

     It is recommended that samples be flushed through the cell at  5-10  1pm  if
enough sample is available.  If only a limited amount of sample gas is  available,
the cell can be preconditioned to reduce errors due to adsorption.  No  sig-
nificant errors are being caused by adsorption if the output signal is
essentially constant while the flushing rate varies by 50% to 100%.

     False output signals corresponding to a few tenths of a ppm of HCN may
result while a zero gas is being flushed from the cell with a sample of exhaust
gas.  A false signal may also occur during the opposite flushing process.  These
false signals may occur even if neither of the two gases would produce  a signal
if  sampled separately.  The signals are apparently due to the difference in
the indices of refraction of the two gases.  When the gases are not mixed uni-
formly in the sample cell, the difference in indices causes a slight shift of
the monitoring beam where it enters the grating assembly.   A slight shift in
the zero-setting results.  When one of the gases has completely displaced
the other one, the false signal disappears and the output signal represents the
HCN concentration in the sample cell,

     A sample of gas in the cell is approximately 85% flushed out  by another
gas when the volume of the incoming gas flushed through is 3.7 liters, the
volume of the sample cell.  When twice the cell volume has been flushed through,
between 98 and 99% of the original gas has been removed.
                                      30

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                                  SECTION 9

                   CONVERSION OF THE VERSATILE GAS-FILTER

                          CORRELATION SPECTROMETER

     This section describes the tests performed  as  the  first phase of the
program on the versatile gas-filter correlation  spectrometer and  the conver-
sion of the spectrometer for use as an HCN analyzer.  The  spectrometer has
been described in detail in a previous report .  Locations  of the  components
discussed below are shown in the optical diagram in Figure 1 of the same re-
port, along with definitions and discussions of  the operating principles and
the important instrument parameters.

     The work performed during this first phase  of  the  program served two
purposes.  The first was to perform the tests required  to  determined an op-
timum set of instrument parameters and to determine the performance that could
be expected from gas-filter cell instruments built  specifically for use as HCN
analyzers.  Conversion of the EPA - owned versatile gas-filter correlation
spectrometer for use as an HCN analyzer constituted the second task of the
first phase.  After the spectrometer was converted, it  x^as returned to the EPA
to use in tests on automotive exhaust until three new instruments designed
specifically as HCN monitors were built and tested.

DETERMINATION OF INSTRUMENT PARAMETERS

     The relatively strong HCN band centered near 3312  cm   (3(J*n) was selected
instead of the stronger band near 712 cm"1 (14|Jm).  Serious interference by
COo could be expected if the lower wavenumber band  were used because of the
strong absorption lines of this gas in the region of the HCN band.  In addi-
tion, convenient window materials are not available for the low wavenumber
region.
1.  Burch, D.E., F. J. Gates,  D.A.  Gryvnak  and J. D. Pembrook.  "Versatile Gas
    Filter Correlation Spectrometer,"  Prepared by Aeronutronic  Ford Corpora-
    tion under Contract No.  68-02-1227.   EPA Report No. 600/2-75-024, June  1975.
                                      31

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      The 300 line/mm grating, the Nernst glower  source,  and  the  liquid-
 nitrogen-cooled PbS detector were installed  in the versatile spectrometer.
 Various sample cell lengths were employed during the  tests,  but  the  multiple-
 pass sample cell was adjusted to 28 passes (L =  28m)  before  the  instrument
 was calibrated and returned to the EPA.  By  using a variety  of grids in  the
 retroreflector-grid assembly and by adjusting the grating position,  we were
 able to use many different spectral b -ndpasses for the  tests.  The most  im-
 portant factor in the choice of the spectral bandpass is the need to minimize
 the interference by H20, 02% and NH.$.  As explained  in Section  5, use of the
 strongest portion of the 3|jun HCN band is not practical  because of excessive
 interference by C2H2 and ^3-  Bandpasses of many different  widths and center
 positions were tried.  The bandpass selected for the versatile spectrometer
 extends from approximately 3338 cm"1 to 3356 cm"1.  The minimum  width of the
 bandpass is determined by the amount of radiant  energy  required  to produce an
 acceptable ratio of the detector signal to noise.

      While assembling the 3 new instruments  built under Phase II, we deter-
 mined  that somewhat better discrimination could  be obtained  by using a grid
 with two openings that produces the spectral transmittance function  illustra-
 ted in Figure 6.  Because of the improved detectors used in  the  new  instru-
 ments, we were able to achieve a satisfactory signal-to-noise ratio  while
 employing this bandpass, which is narrower than  the one employed in  the  con-
 verted versatile spectrometer.

        Epoxy cement used to seal windows to the body of  the gas-filter cell
was found to decompose because of reaction with HCN in the cell.  Leaks develop-
ed in the seal within a few weeks after a cell was filled.  This  problem was
overcome by fusing qu?rtz windows to the quartz 'cell body.  A  one-half atmos-
phere mixture of 15.8% HCN, 6.7% C2H2 and 77.5% N3 was introduced into the
1.1 cm  long cell through a small-bore tubing,  and the cell was sealed by filling
the tubing with epoxy cement.   Only approximately 0.1 mm2 .of epoxy is in con-
tact with the HCN vapor.   Infrared spectrum of the HCN in the  cell have been
scanned periodically and compared with a spectrum scanned a few days  after the
cell was filled.   The comparisons indicate that the amount of HCN in  the cell
has not changed by more than 10% in nearly a year.  The change, if any, may be
much less than this amount.

      Discrimination against C2H2 and NH3 is  improved  further by  a "series"
 cell in the optical path near the entrance window of  the multiple-pass sample
 cell.  The 500 torr of C2H2 and 250 torr of  NH3  in this 5-cm long series cell
 absorb much of the energy near the centers of the strong absorption  line of
 these  gases.  Thus, adding either of these two gases  to the  sample cell has
 less effect on the detector signal than it would have without the series cell
 in place.

      After inserting the series cell, C2H2 and NH3 still interfered  slightly.
 The NHg interfered positively, i.e., it caused the output signal to  be too
 large.   This interference was reduced further by placing an  "attenuator  cell"
 in the alternator section of the instrument  in the position  normally occupied
 by a neutral-density attenuator.  The 1-cm long  attenuator contains  450  torr
 of NH3.  Negative interference by C2H2 was greatly reduced by the 25 torr of
 C2H2 added to the HCN in the GFC.  With this combination of  gas-filter cell,

                                      32

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series cell and attenuator cell, the interference signal caused by C2H2 and
NH^ at the highest concentrations normally found in automotive exhaust cor-
responds to less than the minimum detectable concentration of HCN.

     A slight interference by I^O in the sample is accounted for in the same
manner as in the three monitors described in the first 8 sections of this
report.  An H^O monitor  similar to the one described in Section 7 measures
the H^O concentrations.
                                      33

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                             SECTION 10

                             REFERENCES

1,    Burch, D.  E.,  F.J. Gates,  D.A. Gryvnak and J. D. Pembrook.
     "Versatile Gas Filter Correlation Spectrometer", Prepared by
     Aeronutronic Ford Corporation under Contract No. 68-02-1227.
     EPA Report No. 600/2-75-024, June 1975.

2,    Burch, D.E. and D.A.  Gryvnak. "Infrared Gas-Filter Correlation
     Instrument for In-Situ Measurement of Gaseous Pollutants." Pre-
     pared by Philco-Ford Corporation for EPA under Contract No. 68-
     02-0575.  EPA Report No. EPA-650/2-74-094.  Also, Burch, D.E. and
     D.A. Gryvnak,  "Cross-Stack Measurement of Pollutant Concentrations
     Using Gas-Cell Correlation Spectroscopy."  Chapter 10 of Analytical
     Methods Applied to Air PollutionMeasurements, Stevens, R.K. and
     W.F. Herget, (eds) Ann Arbor, Ann Arbor Science Publishers Inc.,1974.

3.    Burch, D.E,, Gates, F,J.,  Gryvnak, D.A.,  and J. D. Pembrook.
     "Versatile Gas-Filter Correlation Spectrometer."  Prepared by
     Aeronutronics Ford Corporation for EPA under Contract No. 68-02-1227.
     EPA Report No. EPA-600/2-75-024, June 1975.

4.    Herget, Dr. William F., Environmental Protection Agency, Research
     Triangle Park, N.C.,  27711,  (Private Communication)
                                 34

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                                   TECHNICAL REPORT DATA
                            (Please read Inaaitcrions on the rc\ erse before completing,'
 REPORT NO.
     EPA-600/2-79-113
2.
 TITLE AND SUBTITLE
     DEVELOPMENT OF A MONITOR FOR HCN IN MOBILE
     SOURCE EMISSIONS
                              6. PERFORMING ORGANIZATION CODE
                              3. RECIPIENT'S ACCESSIOONO.
                              5. REPORT DATE
                                June 1979
 AUTHOR(S)
     D.E. Burch, P.S.  Davila, F.J. Gates, and
     j.D. Pembrook
                                                           8. PERFORMING ORGANIZATION REPORT NO.
                                  U-6470
9. PERFORMING ORGANIZATION NAME AND ADDRESS
     Ford Aerospace  and Communications Corporation
     Aeronutronic Division
     Ford Road
     Newport Beach,  California  29663
                              10. PROGRAM ELEMENT NO.

                                1AD712  BA-54  (FY-78)
                              11. CONTRACT/GRANT NO.
                                Contract No.  68-02-2716
12. SPONSORING AGENCY NAME AND ADDRESS
     Environmental  Sciences Research Laboratory—RTP,
     Office of Research and Development
     U.S. Environmental Protection Agency
     Research Triangle  Park, N.C.  27711
                          NC
13. TYPE OF REPORT AND PERIOD COVERED
  Final 4/77 - 10/78	
                              14. SPONSORING AGENCY CODE
                                EPA/600/09
15. SUPPLEMENTARY NOTES
 16. ABSTRACT
          Three real-time monitors for measurement  of HCN concentrations in mobile
     source emissions  have been designed, built,  tested and delivered to the En-
     vironmental Protection Agency (EPA).  The  important design parameters for these
     identical instruments were determined during the first phase of the program of
     performing tests  with a versatile gas-filter correlation spectrometer built
     previously in our  laboratory for EPA.  The  instruments employ a gas-filter cell
     to provide sensitivity to HCN while discriminating against other infrared active
     gases such as H20,  CO», NH  and many hydrocarbons that occur in mobile source
     emissions.  These gases absorb near 3 micrometers,  the approximate center of
     the narrow spectral band employed by the instrument.

          Samples are  contained in a temperature-controlled cell that uses a 20-
     pass optical system with an optical path length  of 15.5 m.   An HO monitor
     built as an integral part of the instrument  measures  the HO concentration,
     making it possible  to account for a small  amount of interference by this gas
     in the sample.  The rms noise-equivalent-concentration of HCN is less than
     0.02 ppm.  The combined error after accounting for HO interference for most
     dilute samples is less than 0.1 ppm HCN.
17.
                                KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
                                              b.lDEIMTIFIERS/OPEN ENDED TERMS
                                           c. COSATI Field/Group
     * Air pollution
     * Motor vehicles
     * Emission
     * Hydrogen cyanide
     * Monitors
     * Development
                                                13B
                                                13F
                                                07B
13. DISTRIBUTION STATEMENT
    RELEASE  TO PUBLIC
                 19. SECURITY CLASS (ThisReport)
                   UNCLASSIFIED
                 20. SECURITY CLASS (This page)
                   UNCLASSIFIED
              21. NO. OF PAGES
                  A3	
                                                                         22. PRICE
EPA Form 2220-1 (9-73)
                                             35

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