United States
                    Environmental Protection
                    Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
                    Research and Development
EPA/600/S3-89/028 Aug. 1989
&EPA          Project Summary
                    Construction  and Testing of
                    Electrochemical  NO2  PSDs

                    M. W. Findlay, Jr., J. R. Stetter, and C. Yue
                     The objective of this project was to
                   develop, test and deliver a prototype
                   electrochemical instrument  capable
                   of real-time measurement of low-ppb
                   levels of NO2. The unit must  be small
                   enough  for  use  as  a personal
                   monitor.
                     The work was conducted im two
                   distinct tasks that were performed
                   concurrently:  (1) improvement of the
                   currently available NO2 sensing tech-
                   nology to provide a sensor with suf-
                   ficiently high signal/noise ratio and
                   stable background signal for continu-
                   ous low-ppb level measurements,
                   and (2) the design, assembly, testing,
                   and delivery of a portable instrument
                   incorporating this sensor technology
                   to  provide capability  for monitoring
                   personal exposure to ambient NO2
                   concentrations.
                     During this project, the following
                   work was completed:
                     A very low  background, low noise,
                   attitude-insensitive sensor was devel-
                   oped and tested to provide detection
                   of  NO2 at levels as low  as 5 ppb. A
                   portable, modular prototype  NO2
                   monitor was  developed  and tested.
                   The instrument was  interfaced  to a
                   commercial data logger to allow col-
                   lection and storage of real-time con-
                   centration data,  as well as a  con-
                   tinuous  display  of  ambient  NO2
                   levels. Three of these  units were con-
                   structed  for evaluation.  In addition,
                   preliminary design  of a second gen-
                   eration data logger/controller that can
                   provide optimum ease  of  use and
                   maximum accuracy was completed.
                     Future work will address further im-
                   provement in the NO2 sensor stabil-
                   ity, reduction in NO2 instrument size,
                   development of the  optimum  data
                   logger/controller, expansion of the
                   monitor to measure  other com-
 pounds, and the investigation of mul-
 tiple-function, multiple-compound,
 sensor-array based real-time  mon-
 itors.
  This  Project Summary was devel-
 oped by EPA's Atmospheric Research
 and Exposure Assessment Laboratory,
 Research  Triangle Park, NC, to an-
 nounce key  findings of the research
 project that  is fully documented in  a
 separate report o.f< th& same title (see
 Prefect Repoxt aedering; Information at
 back.)
 introduction
  The Atmospheric Research and Expo-
 sure Assessment Laboratory, Research
 Triangle Park, North Carolina, has re-
 sponsibility for:  assessment of environ-
 mental monitoring technology and sys-
 tems for air,  implementation of agency-
 wide quality assurance  programs for air
 pollution  measurement systems,  and
 supplying  technical  support to other
 groups in the Agency including the Office
 of Air and  Radiation, the Office of Toxic
 Substances,  and  the  Office of Solid
 Waste.
  The  monitoring of pollutant gases at
 sub-ppm and low-ppb levels is of primary
 concern in indoor air and non-industrial
 locations such as the home. The trend
 toward more airtight homes, which began
 during  the energy crisis of the early
 I970's, has caused concern among health
 experts about increased levels of indoor
 pollutants such as the sulfur and nitrogen
 oxides. It is the objective of this work to
 develop a personal sampling   device
 (PSD)  which will allow real-time moni-
 toring of low concentrations of NO2 in the
 presence of  ambient levels of potential
 interferents such as NH3, CO, H2S, com-
 mon hydrocarbons, and  SO2.

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  Amperometric electrochemical sensors
 provide portable real-time analysis capa-
 bility and have been used for ppm level
 measurement of NO2. These  sensor
 systems do not suffer interferences from
 NO, N20, or  nitrous acid  but often
 possess too much noise  and drift to
 provide  accurate data at ppb  levels.
 Major sources of  drift have been tenta-
 tively  identified  as temperature  and
 relative humidity fluctuations.
  The  work discussed  in the full  report
 has  focused upon improvement  of the
 existing  electrochemical  technique so
 that  it  can be used at ppb levels. This
 requires gaining an understanding of the
 reactions of NO2  in the sensor system.
 This knowledge is required to design and
 build an  improved real-time  portable
 instrument for the measurement of NO2
 in the field.

 Procedure
  All gases  and gas mixtures were
 supplied by  Scott  Specialty gases in
 pressurized cylinders at a concentration
 of 49-400  ppmv  in  zero  air. Working
 standard mixtures of NO2/air  were pre-
 pared  fresh several  times daily by
 dilution  of  the  stock gas  mixture. Low
 levels of NOg in air were prepared using
 a calibrated permeation device (KinTek
 Laboratories, Houston, TX).
  Gas flow through the sensors was con-
 trolled using Spectrex AS-400 pumps and
 Matheson Model 602 (0-1L) and 603 (0-
 5L)  flow  meters  fitted with  Clippard
 needle  valves. Elevated  temperature
 experiments were  conducted in a GCA/
 Precision Scientific Model  18 oven. Low
 temperature experiments were performed
 in a  modified GE TA242TH refrigerator.
Temperature  was  monitored with a 0-
 110°C mercury thermometer.
  Commercial sensors were obtained
 from the  Energetics Sciences Division of
 National Drager (ND or ESI). Solid elec-
trolyte  sensors using Nafion (El DuPont
TM)  cation exchange material and aque-
ous electrolyte sensors with low surface
area (LSA) and high surface area  (HSA)
sensing electrodes were designed  and
built  by TRI  under  carefully  controlled
conditions.
  Sensors were tested  in  the drawn-air
("pump-after-sensor") flow configuration
recommended  in  earlier  work.  A  low
volume exposure chamber was used to
test the ND sensors. The volume of the
chamber used with the  TRI sensors was
decreased by 50% from the prior work in
order to increase the linear flow  rate of
analyte past the working electrode. This
 design change should improve response
 time and signal magnitude.
  All  of the bench-top experiments were
 performed  at  room temperature  (ap-
 proximately 22°C) and the potential of the
 NO2  gold  sensing electrode  was main-
 tained constant at  +800 mV  vs. SHE
 (-200 mV vsi  Pt/air reference electrode)
 throughout'the  tests. The  sample
 (analyte) flow; was 100 cc/min through the
 sensor and, since the gas mixtures were
 prepared from  laboratory air,  most tests
 were  performed at  30.-60%  RH.  Only
 tests  of the 49 ppm  NO2/air commercial
 mixture were1  dry  ("approximately"  0%
 RH).        i


 Results and Discussion

 The Proposed Instrument
  After discussion of the specifications of
 the "ideal" instrument, the hardware de-
 signs for the NO2  instrument were  pre-
 pared. A  block diagram of the N02 unit is
 as shown in Figure 1. The components of
 the instrument were identified as:
 1.  Pump/sensor  -  module  1: This
    component is the  center of the unit. It
    contains Jthe sensor, pump,  poten-
    tiostat cirpuit,  pump motor  control
    circuit,  circuit  to  provide  a stable
    power supply,  temperature  circuit,
    and all signal measurement circuits.
    There are' also provisions for an alarm
    circuit  and LED display for con-
    tinuous readout, but these were not
    be included in  the  prototype instru-
    ments.   !
 2.  Data  logger/controller  -  module 2:
    This component of the unit will con-
    tain the A/D channels for signal input
    and digital input/output channels for
    instrument control. The input chan-
    nels will monitor  the sensor  output,
    temperature, battery  voltages, sensor
    bias,  and [other pertinent parameters.
    The output channels  will allow control
    of the pump motor circuit and analog
    switch for auto-gain set and auto-zero
    measurements.  An evaluation  of
    existing  and possible data  loggers
    relative to these  specifications was
    completed as a part of this work.
3.  Battery pack - module 3: This module
    contains jthe  rechargeable  Ni-Cad
    batteries used  for pump power. To
    provide the voltage  to operate  the
    pump motor circuit as well as power
    the pump, a 7.2 V pack was originally
    specified.1 A 9V battery will  be in-
    cluded  in  module   1 to  keep  the
    sensor  on standby and ready for use
    even  when the instrument  was  not
    deployed.

 Sensor Development
  TRI  has obtained excellent response
 from catalyst films vapor deposited onto
 porous  membranes. By  controlling  the
 nature of this  film, the  signal can  be
 optimized. Commercially available high
 surface area  (HSA)  sensors and the TRI
 Nafion sensor  were used as  "perform-
 ance  standards"  for  evaluation of  the
 "improved" responses from the TRI LSA
 sensors tested in this work.
  Figure 2 compares  the  response of a
 commercial NO2 sensor to that of the TRI
 Nafion sensor  and  the  acid  electrolyte
 low surface area working electrode sen-
 sor. The work outlined briefly below was
 focused  on  optimizing  the sensor
 performance  further, and  characterizing
 its response and lifetime behavior.


 Membrane Selection:
  Choice of a membrane  for the sensing
 electrode  is critical and  the  membrane
 can have a profound effect upon analyti-
 cal  performance. For use in a personal
 monitor,  it is necessary  that the  mem-
 brane be rugged and possess a very high
 resistance to water flow through  the
 pores. A series of tests  were performed
 to compare the properties of several por-
 ous membranes to determine the one
 most suited for the NO2 monitor.
  The  membrane properties evaluated
 included  tensile,   or  tear,  strength,
 porosity,  weeping pressure (the pressure
 required to force electrolyte through the
 membrane),  and response  character-
 istics. Several  of the membranes tested
 provide  suitable substrates. The Gortex
 (0.45 iim and 1.0 pm pore size), Celgard
 (0.02 and  0.04  pm)  as well as the Zitex
 material (E606122  and E606-223) were
 tested in TRI sensor designs.

 Catalyst Loading:
  The magnitude of the  signal from a
 given concentration of analyte gas is pro-
 portional  to the catalyst  loading on the
 membrane until the reaction in the sensor
 (at the working electrode) is no longer
 catalyst limited. The optimum loading  for
 a sensor is the one that  produces con-
 stant response and for which slight varia-
tions in catalyst activity  (due to  small
changes in temperature  or specific sur-
face area) do not substantially affect the
sensor signal.
  A series of tests were performed  to es-
tablish  the relationship between catalyst

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                                                      NO2 Analyzer
                                              NO2 Analyzer
                                                                              + 888.88
                                                                             a      n
                            Battery
                             Pack
                           1.2 V Ni-Cd
       Instrument
     (Sensor, Pump,
       Electronics)
Data Logger
(4 Channel)
                     Figure 1. NO2 Instrument at the end of this phase of the project, showing the three
                              modules and connectors.
loading and sensor response to NO2  for
each of the potential membrane mater-
ials.  These included the three different
porous membrane materials and several
catalyst loadings for each membrane:
600, 900, 1200, 2100,  and 3000  Ang-
stroms. The array of sensors constructed
from the different electrode and  mem-
branes were evaluated for signal magni-
tude  (pA/ppm),  background  current
(ppm), noise  level (ppb), and response
time.
  Based upon  signal  magnitude and
physical properties, the G-1.0 membrane,
was chosen as the  most suitable mem-
brane for the  sensor. Initial tests of  the
precision and  stability of sensors  using
this material were encouraging. Precision
of the sensor response was  typically
within  ± 5%, and baseline drift less than
20  nA,(> 100 ppb)  over 4 hrs of testing
at 1-5 ppm N02 levels. The data indicate
that the performance of the sensor could
be  improved even further with  a higher
loading of electro-catalyst.


Attitude Sensitivity:
  In order to use the sensor for a  porta-
ble monitor,  it is necessary  to  make  it
position and  attitude  insensitive.  An
hydrophilic polypropylene wick  material
was packed behind the sensing  electrode
membrane so that it extended down into
the electrolyte. This wick insures continu-
ous contact between  the sensing elec-
trode and the electrolyte; even if the sen-
sor were turned upside down.

Characterization of TRI NO2
Sensor
  Based upon the data from the optimi-
zation  study described above,  several
sensors were  constructed for  testing in
the personal modular  prototype monitors
that were built. A series of tests were per-
formed to  characterize the  response of
the new TRI  sensor  described  above.
These tests included evaluation  of  the
following characteristics:

Signal Magnitude:
   The  signal  for  NO2  from  the  low
surface area sensor  is typically  0.2-0.5
liA/ppm. Signal noise is  less than 5 nA,
allowing the detection of about  5  ppb
NO2. The  improvements realized  in the
signal to background  ratio with the  LSA
sensor should  allow the  optimum moni-
toring for N02  at the  lowest levels. How-
ever,  the versatility of  the  monitor  will
allow use of many different sensors in the
unit.  The  HSA sensors  may  provide  a
better response at extremely high levels
of pollutant while the LSA sensors are
    optimized for performance  at low  NO2
    concentrations.


    Response of Potential
    Interferents:
       Several compounds may be present in
    significant concentrations  in ambient  air.
    Table 1 lists a number  of compounds
    likely to be encountered during the use of
    the instrument. Of the compounds tested,
    only H2S yielded a signal large enough to
    be of concern.
       The use of a selective filter to  remove
    the  H2S from the sample  was studied.
     Initial tests of the filter at NO2 levels of 1
    ppm indicate little scrubbing NO2 by the
     H2S filter. This filter needs to be studied
     in detail at low concentrations of  NO2, to
     determine  the effect on  response time
     and  calibration  of the  NO2  personal
     monitor.


     Effects of  Flow Rate:
       For a sensor operated  in a flowing air
     stream, it is necessary to determine the
     effect of the rate  of air flow upon the re-
     sponse of the sensor. The LSA sensor is
     independent of flow rate in the range 45-
     300  cc/min. Since the  air flow in the in-
     strument is set at 100 cc/min, any fluctu-
     ations  in  pump  speed will not greatly

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M. W. Findlay, Jr., J. R. Stetter, and C.  Yue are  with Transducer Research, Inc.,
  Naperville. IL 60540.
James D. Mulik is the EPA Project Officer (see below).
The complete report,  entitled "Construction and  Testing, of Electrochemical NO2
  PSDs," (Order No. PB 89-169 874/AS; Cost: $15.95, subject to change) will be
  available only from:
        National Technical Information Service
        5285 Port Royal Road
        Springfield, VA 22161
        Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
        Atmospheric Research and Exposure Assessment Laboratory
        U.S. Environmental Protection Agency
        Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penally for Private Use S300

EPA/600/S3-89,'028

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