United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-84-030 May 1984
Project Summary
Laboratory Evaluation of
Tunable Atomic Line Molecular
Spectrometers for Benzene
Analysis
D. R. Scott and R. L Hedgecoke
Tunable Atomic Line Molecular
Spectroscopy (TALMS) is a high
resolution, differential absorption
technique used in the ultraviolet region
which was recently developed at
Lawrence Berkeley Laboratory. Under
Interagency Agreements EPA-80-D-
X1014 and AD-89-F-2A008 with
Lawrence Berkeley Laboratory.
prototype TALMS instruments were
designed, constructed, and delivered to
Research Triangle Park, North Carolina.
TALMS I was a large laboratory
instrument designed to demonstrate
the feasibility of TALMS detection of
toxic organic compounds. TALMS II
was a more compact monitor for
benzene designed for possible field
applications. Both instruments were
evaluated in the laboratory for
performance characteristics in the
analysis of benzene using the 253.7 nm
mercury line. The tests included
detection limit, precision, effect of
temperature (TALMS I), effect of
pressure (TALMS II), and interferences
from organic compounds other than
benzene.
It was concluded that the prototype
instruments evaluated can detect
benzene precisely above the detection
limit. However, the relatively high
detection limit of 6 ug precludes the use
of the instrument for direct ambient air
analysis. The apparent interferences are
relatively small and may be artifacts
which can be eliminated with improve-
ments in electronic signal processing. It
was recommended that changes to the
sample cell and electronics be made to
improve the detection limit and that
extension of the capabilities of TALMS
to other organic compounds be made.
Sample preconcentration techniques
should also be used with the
instrument.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory. Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
An analytical technique called Tunable
Atomic Line Molecular Spectroscopy
(TALMS) has been recently developed at
Lawrence Berkeley Laboratory. TALMS is
essentially a high resolution, differential
absorption spectroscopic technique. It is
performed by splitting a source atomic
emission spectral line into components
with a magnetic field (Zeeman effect). A
differential measurement is made
between the absorption of a Zeeman
component (or components) magnetically
tuned to match a sharp analyte
absorption feature and an unmatched
Zeeman reference component(s). The
sharp features in the absorption
spectra of the organic compounds are due
to rotational-vibrational fine structure in
the electronic transitions. The difference
in polarization between the split Zeeman
components permits the matching and
nonmatching wavelengths to be
alternately selected and the absorption
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measured very rapidly with an electro-
optical device called a Current Controlled
Retardation Plate. The differential
absorption is proportional to the amount
of molecular species whose absorption
line is matched by Zeeman component(s)
of the source lamp. Therefore, calibration
curves can be established following
Beer's law.
TALMS is a high resolution (high
selectivity) absorption spectroscopic
technique. The high resolution is due to
the narrow width of the atomic probe line,
ca. 0.1 cm ', the location of which can be
shifted by changing the magnetic field
strength. TALMS is capable of the
measurement of a large number of
organic compounds with high selectivity.
The number of compounds that can be
detected is limited only by the presence of
narrow, psuedo line structure in the
absorption spectrum of the compound in
the gas phase and the matching of an
appropriate atomic emission line to this
feature.
TALMS has a number of advantages as
an analytical technique. It is a direct
analysis technique requiring very little
sample handling or preparation. Another
feature of TALMS is its essential freedom
from background interference. Since the
difference in wavelength between the
Zeeman components of the source
emission line is typically 0.04 nm, any
particle scattering or semi-continuous
absorption will effect both components
equally. Therefore, the differential
absorption measurement will remove
this interference from the total signal.
The high resolution attainable with
TALMS should result in high selectivity
and unambiguous qualitative identifica-
tion of compounds.
As a result of Interagency Agreements
EPA-80-D-X1014 and AD-89-F-2A008
with Lawrence Berkeley Laboratory,
TALMS instruments were designed,
constructed, and delivered to the Envi-
ronmental Monitoring Systems
Laboratory (EMSL) at Research Triangle
Park, North Carolina. Two prototype
instruments were delivered, a laboratory
instrument (TALMS I) and a benzene
monitor (TALMS II). Both of these
instruments were evaluated in the EMSL
laboratory for performance character-
istics. This report is concerned with these
laboratory evaluations.
Results
Evaluation tests were performed on
TALMS I for detection limit; precision;
calibration curve linearity and stability;
interferences by other organic
compounds; and effects of temperature.
The early tests were performed without
knowing that the squeezer was not
operating properly. It was later replaced,
and the instrument performance
improved. The tests on TALMS I gave a
detection limit of 35 /jg benzene with a
median precision of 20%, relative
standard deviation. Apparent negative
interferences on benzene were found for
o-xylene, pyridine, and toluene of 2.8, 23,
and 16%, relative to an equal volume of
benzene. For a change in temperature
from 27 to 160°C an increase in signal
was found for benzene, pyridine, and
toluene of 21, 50, and 57% respectively.
TALMS I had a moderate amount of
downtime during these tests.
Evaluation tests were performed on
TALMS II for detection limit; precision;
calibration curve linearity and stability;
interferences by other organic com-
pounds; and effects of pressure.
Preliminary tests with a National Bureau
of Standards Reference Material for
benzene and a cryogenic trap were also
conducted. The evaluation of TALMS II
gave a detection limit of 6 fjg benzene
with a median precision of 7.2%, relative
standard deviation. Apparent negative
interferences on benzene were found for
pyridine, toluene, chlorobenzene, and
styrene of 7.0, 1.7, 0.5, and 5.2%,
respectively. Since the interferences
decreased from those found with TALMS
I, there is a possibility that these are
artifacts of the signal processing
electronics. The study of pressure in the
sample cell on TALMS signal showed a
decrease in signal intensity of 8% at 66
torr and 46% at 636 torr, relative to the
signal at 3 torr. This effect must be
allowed for in calibration and sample
analysis. After addition of an automatic
gain control unit, very fewproblemswere
encountered with TALMS II.
Conclusions and
Recommendations
TALMS I, a single beam system, was
designed and constructed as a bulky
prototype laboratory instrument to
demonstrate the feasibility of analysis of
toxic organic compounds by Tunable
Atomic Line Molecular Spectroscopy. It
was supplied with a mercury lamp and
was evaluated for benzene detection and
analysis. After initial problems with the
squeezer, which was later replaced, and
experience acquired by the EPA operator;
the instrument operated satisfactorily.
The precision of analysis was fair with a
median of 20%, relative standard
deviation, but the main problem for
ambient benzene analysis was the
detection limit of 35 fjg. Also, somewhat
surprising were the apparent
interferences from pyridine, o-xylene,
and toluene. Since TALMS is a high reso-
lution technique, this is unexpected.
Evidence collected with TALMS II
indicates that these interferences may be
artifacts of the signal processing system.
Another surprising result was the
increase in TALMS signal with increase
in temperature for benzene, toluene, and
pyridine.
TALMS II was designed and construct-
ed as a compact monitor for benzene. It
was much smaller than TALMS I and
used a dual optical system to compensate
for lamp and electronics drift. Except for
an initial problem leading to erroneous
interferences, which was corrected by
the addition of an automatic gain control
circuit, the instrument performed very
well. The precision with a median of 7%,
relative standard deviation, was good,
probably due to the dual beam system.
The detection limit of 6 fjg was much
better than thatforTALMS I, although not
adequate for direct ambient benzene
monitoring. The interferences found for
toluene and pyridine, from 1.7 to 7%,
were much less than the corresponding
values for TALMS I. This is an indication
that the apparent interferences may be
artifacts introduced by inaccuracies in
the signal processing electronics. The
effect of pressure on the TALMS signal is
an important problem and must be
accounted for in calibration and analysis
of samples.
At the present stage of development
the TALMS II instrument cannot be used
for direct analysis of ambient benzene
due to its detection limit of 6 fjg. It is
recommended that further work be done
to evaluate the use of concentration
methods, e.g., solid sorbents or cryogenic
concentration, with theTALMS monitors.
Also further extramural studies should be
funded to locate more intense analysis
lines for benzene, e.g., the Co line at
252.8 nm, and other toxic organic
compounds. A monochromator and
electromagnet should be added to
TALMS II to extend its capabilities, and a
sample cell with smaller volume should
be added. A study should be conducted at
Lawrence Berkeley Laboratory to find the
cause of the temperature effect, which
might be used to enhance the detection
limit. Some of the recommendedchanges
to TALMS II have been made in a later
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version, TALMS III, which has been
delivered to EMSL, Research Triangle
Park, North Carolina in December, 1983.
The EPA authors D. R. Scott and R. L. Hedgecoke are with the Environmental
Monitoring Systems Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "Laboratory Evaluation of Tunable Atomic Line
Molecular Spectrometers for Benzene Analysis," (Order No. PB 84-177 765;
Cost: $8.50, 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 authors can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
U.S. GOVERMNENT PRINTING OFFICE; 1984 — 759-015/7687
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