United States
Environmental Protection
Agency
Environmental Monitoring Systems >
Laboratory **
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/066 Jan. 1986
Project Summary
Gas Chromatographic/Fourier
Transform Infrared Analysis of
Trace Organics: Feasibility of
Analysis After Collection of
Organics on Tenax/GC
Sorbent Cartridges
R. A. Palmer, J. W. Childers, and M. J. Smith
The combination of sorbent cartridge
thermal desorption with capillary col-
umn GC/on-the-fly FTIR has been
shown effective for the detection and
identification of volatile organics in
laboratory-generated mixtures, includ-
ing the distinction between isomeric
species, at the level of a few hundred
nanograms per compound per cartridge.
Traces of water desorbed from the
cartridges must be reduced by the
addition of a dryer unit between the
desorption chamber and the GC col-
umn. Methods of lowering the detection
and identification limits to less than
100 ng per~ compound per cartridge are
proposed.
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
The purpose of the project described in
the full report was to evaluate the feasibil-
ity of using on-the-fly GC/FTIR to detect
and identify volatile organics adsorbed on
TENAX-GC cartridges. While the two
analytical procedures of: (1) thermal
desorption of TENAX-GC cartridges with
subsequent GC separation and (2) GC
separation followed by FTIR detection
have been separately utilized for the
analysis of organic pollutants, it appar-
ently has not been shown that TEN AX-GC
cartridge desorption can be successfully
combined with GC/FTIR analysis. The
primary goal of this project was to deter-
mine the feasibility of such a combination
by analyzing well characterized labora-
tory-generated sample cartridges. Sample
cartridges were loaded with realistic
amounts (with respect to concentrations
found in ambient samples) of various
mixtures of organic compounds. Special
emphasis was given to the identification
of geometrical isomers. Instrumental
problems addressed include removal of
interferences due to water co-eluting
with target compounds, spectral cleanup
to facilitate library based identifications,
and development of an analytical method
to coordinate the various instrument
subsystems necessary for precise anal-
yses.
In principle, GC/FTIR should be com-
plementary to GC/MS for the analysis of
complex mixtures of volatile organic
pollutants. Whereas GC/MS can often
give molecular weights, provide informa-
tion leading to an empirical formula, and
provide some information about molecu-
lar structure, this information is in some
cases insufficient to enable precise struc-
tural identification; this is particularly
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true for geometrical isomers. GC/FTIR,
on the other hand, is capable of identifying
structural patterns of molecules and
providing information about the functional
groups present. By nondestructively prob-
ing the vibrational frequencies of mole-
cules it often affords a means of precise
determination of structural formula.
Experimental
Samples for evaluation were prepared
by exposing 1.35 g TENAX-GC cartridges
to measured volumes of helium spiked
with known concentrations of various
target compounds (primarily volatile
chloro-organics). These cartridges, con-
taining typically 200-500 ng per com-
pound, were thermally desorbed at 200°C
and the target compounds subsequently
trapped at -150°C using a Nutech* model
320 cartridge desorption, cryotrap and
sample injection system. The cryotrapped
compounds were then flashed through a
Perma-Pure Nafion dryer and injected
onto the temperature programmed capil-
lary column (SE-54 fused silica WCOT) of
a Varian 3700 gas chromatograph. Figure
1 shows a schematic diagram of the
analytical system. The infrared spectral
analysis of the GC eluent was carried out
using the "on-the-fly" GC interface asso-
ciated with the IBM-Bruker model 9195,
Na-purged FTIR spectrometer. A spectral
resolution of 8 cm"1 and range of 800-
4000 cm"1 were used. The typical 1
second residence time of a GC peak in the
light pipe allows for the registration of 5
interferograms per peak. After standard
transformation and background correc-
tion, IR spectra of eluted peaks were
compared for identification to the Sadtler
IRVAP-8 x 16 spectral library. For aid in
locating peaks, the effluent from the light
pipe of the GC interface was returned to
the GC for FID analysis.
Vent Carrier
ColdNi
A — Desorption Chamber
B — Dryer
C -~ 6-Port GC Valve
D - Release Valve
E -» Interferometer
F - Light Pipe
Plotter
IBM 9195
FTIR
Console
> and %
Monitor
V
Aspect
^ 2(}uu %:
Computer
Disk
Drive
Figure 1.
Bloc' diagram of thermal desorption-GC/FTIR system.
Results and Discussion
The relatively small amounts of water
present in cartridge desorbed samples
such as were used in this study pose no
serious problem for a FID-monitored GC.
However, even with the addition of the
Nafion dryer unit, sufficient water re-
mains in the GC eluent to be a significant
problem for IR detection. The GC column
used, although efficient for the separation
of the relatively non-polar organic com-
pounds of interest, tends to bleed polar
compounds such as water over a wide
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
region of the chromatogram. The result is
a residual broad band observed in the
FTIR generated real-time chromatogram
(the Gram-Schmidt trace), as well as a
background level of water which shows
up in all spectra obtained. Fortunately,
this background is easily corrected by
subtracting from the spectrum the trans-
form of summed interferograms obtained
just before or after the peak interval. An
example of this is shown in Figure 2.
Because of the exact registration of the
interferograms and their transforms (the
spectra), this background correction is
very efficient. Subsequent computer
searches of the Sadtler IRVAP-8 x 16
library (8000 spectra), which is stored on
disk, consistently produced hits in agree-
ment with the independently determined
identity of the compounds in the test
mixtures.
Of particular interest in the evaluation
of the desorption-GC-FTIR system, are
the results obtained for the mixture of
several isomeric compounds as given in
Table 1. The results of computer searches
of the Sadtler IRVAP-8 x 16 library are
alsoincluded inTable 1 .The library search
program returns the 10 best spectral
matches and an associated "Hit Quality"
number for each in the range 0 to 1000
(1000 indicates a perfect match). The
column of Table 1 labelled "Hit Quality"
gives a ratio where the first number
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4000
Figure 2.
3000 2000
Energy (cm''')
WOO
m-dichlorobemene
a) Absorption spectrum gen-
erated from adjacent base-
line (background) correction).
b) Uncorrected spectrum from
peak interval.
c) Corrected spectrum with
peaks used for search iden-
tified.
indicates "goodness of fit" of the subject
spectrum to the correct library spectrum,
and the second number indicates "good-
ness of fit" of the subject spectrum to the
#1 ranked (but possibly incorrect) match.
In the results summarized in Table 1 it
should be noted that each of the last four
compounds eluted was identified from its
background corrected spectrum by routine
search of the IRVAP library as either the
first or second best "hit." The correct
identities of the third and fourth com-
pounds were also included in the best 10
hits, though these two results would
certainly not have independently sup-
ported correct identification. Because of
the co-elution of m- and p-xylene(peak 1),
these two compounds could not be identi-
fied by routine search of the Sadtler
library. (The combined spectrum is not
recognizable as any individual compound.)
In order to identify successfully these two
compounds a bona fide reference spec-
trum of one or the other isomer should be
substracted from the combined experi-
mental spectrum. The residual spectrum
could then be submitted to the search
routine with expectation of successful
identification of the other isomer. A user
generated library containing the refer-
ence spectra of m- and p-xylene must be
created for this purpose.
It should also be noted that the search
results in Table 1 were obtained without
using any restrictions on the peak picking
program of the search routine, nor was
any smoothing applied to the spectra
before submitting them to the search
routine. In addition it should be empha-
sized that the library contains 8000
spectra of a wide variety of compounds
and that these were obtained using
different equipment (although the resolu-
tion is the same). In several of the
searches it is clear that relatively strong
Table 1. Geometrical Isomer Discrimination Mixture
Compound
m-xylene
p-xylene
o-xylene
o-chlorotoluene
m-chlorotoluene
p-chlorotoluene
m -dichlorobenzene
o-dichlorobenzene
Cartridge
Loading (ngj
381
}
380
389
477
473
471
567
575
Peak No.
1
2
3
4
5
6
7
Search
Hit No.
*
10
7
1
2
1
1
Hit Quality
262/434
380/500
444/444
658/676
294/294
310/310
*m-and p-xylene co-elute See text.
spurious (noise) peaks have been identi-
fied in the peak picking routine which
would be eliminated by reasonable in-
crease in the peak picking threshold. The
search of a more selective library of
spectra generated using the same GC/
FTIR instrument could be expected to
yield more consistently low hit numbers
and higher hit qualities than the search of
the standard Sadtler library. Information
on retention time, compound volatility
range and column material would elimi-
nate many compounds in the spectral
library from consideration.
Another serious problem inherent in
the method as executed in this study is
that it is difficult to identify peaks in the
Gram-Schmidt trace because of the per-
vasive water background. Useful spectra
can be obtained from the transform of
co-added interferograms recorded during
the passage of a compound through the
light pipe even when the corresponding
peak is effectively obscured by the back-
ground in the Gram-Schmidt trace. Since
such peaks are usually clearly identified
in the (subsequent) FID detected chro-
matogram, it appears that modification of
the equipment so as to allow the use of
the FID to trigger the saving of inter-
ferograms (rather than using the Gram-
Schmidt algorithm for this purpose) would
yield a significant improvement in, not
only the speed of the method, but also its
reliability and detection limits.
Conclusions
The combination of sorbent cartridge
(TENAX-GC) thermal desorption with
capillary column GC/on-the-fly FTIR has
been shown effective for the detection
and identification of volatile organics in
laboratory-generated mixtures, including
the distinction between isomeric species,
at the level of a few hundred nanograms
per compound per cartridge. The use of a
dryer element immediately following the
desorption unit has been shown essential
for selective removal of traces of water
also desorbed from the TENAX-GC, which
otherwise seriously degrade the detection
limits. The necessity of the use of the
dryer element restricts the use of the
method to non-polar compounds. Detec-
tion limits for the system as currently
configured are determined by the ability
to observe peaks in the Gram-Schmidt
(real-time) FTIR chromatogram. Potential
detection and identification limits are less
than 100 ng per compound per cartridge,
or sub-ppb levels of detection for 20L air
samples drawn through the sorbent car-
tridges. Hardware and software improve-
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ments designed to achieve these limits
and also retain semi-automatic data
collection are described below.
Recommendations
From the results obtained to this point,
several additional improvements to the
method can be recommended which will
reduce the degree of operator intervention
required, decrease analysis time, and
significantly lower detection and identi-
fication limits.
1. A user-generated library of spectra of
target compounds should be estab-
lished to supplement the Sadtler
library. These spectra should be
obtained using the complete desorp-
tion-GC/FTIR system, from authen-
ticated mixtures loaded at the level of
ca. 1000 ng per compound per
cartridge.
2. Changes in the hardware and/or
software should be developed which
will use the FID to trigger the saving
of interferograms, rather than using
the Gram-Schmidt algorithm.
3. An objective protocol for raising the
sensitivity of the peak-picking pro-
gram of the library search should be
developed. Factors such as retention
time, column material and range of
compound volatilities need to be used
to exclude subgroups of compounds
in the spectral library.
R. A. Palmer, J. W. Childers, and M. J. Smith are with Duke University. Durham.
NC 27706.
W. A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "Gas Chromatographic/Fourier Transform Infrared
Analysis of Trace Organics: Feasibility of Analysis After Collection ofOrganics
on Tenax/GC Sorbent Cartridges," (Order No. PB 86-118 932/AS; Cost:
$11.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems 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
Penalty for Private Use $300
EPA/600/S4-85/066
0000329 PS
60604
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