PBS6-128584
GC/FT-IR and GC/FT-IR/MS Techniques for
Routine Environmental Analysis
California Univ., Riverside
Prepared for
Environmental Protection Agency
Las Vegas, NV
Nov 85
^ ^
-------
£B86-120584
EPA/600/4-85/078
November 1935
GC/FT-IR AND GC/FT-IR/KS TECHNIQUES FCR ROUTINE ENVIRONMENTAL ANALYSIS
by
Peter R. Griffiths and Charles L. Wilkins
Department of Chemistry
University of California, Riverside
Riverside, California 92521
Cooperative Agreement Number CR-811730-01
Project Officer
Donald F. Gurka
Qualiity Assurance Division
Environmental Monitoring Systems Laboratory
Las Vegas, Nevada 89114
ENVIRONMENTAL MONITORING SYSTEMS LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
-------
TECHNICAL SEPOBT DATA
(titan teed in unctions on !h; riverje before compitl'mgj
1. REPORT NO. )Z.
EPA/600/4-85/038 j
3. FtECIPttZHTS ACCtiSSiOH
PR&fc 12358A/SS
•t. TITLE AND i.UOTlTLE-
GC/FT-IR AMD GC/FT-IR/MS TECHNIQUES FOR ROUTINE
ENVIRONMENTAL ANALYSIS
5. REPORT DATE
November 1935
S. PERFORMING OrtCANIZATIQN CODE j
7. Aurncnisi
Peter R. Griffiths and Charles L. Vlilkins
1 PEFlFOFtMjNC: ORGANIZATION REPORT MO,
9. PGflr-OPMIMC ORGANISATION NAME AND ACTRESS
Department of Chemistry
University uf California, Riverside
Riverside, California 92521
10. PROGRAM CLEMENT N0r
11. CONTRACT/GfiANT NO
Cooperative Agreement
No. CR-8L1730-01
12. SPONSORING A3ENCY/JAME.AND AD5HESS , . , ,,
Environmental Mom ton rg Systems Laboratory - LV, MV
Office of Research and Development
U.S. Environmental Protection Agency
13. TYPE OF REPORT AN3 PERIOD COVERED
14. SPONSORING AGENCV CODE
EPA/600/07
IS. SUPPLEMENT ARV MOTES „ . . _
Donald F. Gurka
Environmental Monitoring Systems Laboratory
Lr Vri" HV 3PP1
l€. ABSTRACT LJ-"
-This report documents progress on three taslcs related to the design and
testing of procedures and techniques for analyzing volatile and semivolatile
components of environmental samples. The tasks include; (1) develop and test
a procedure to use infrared molar absurptivities and internal standards for t'.ie
routine quantification of environmental contaminants, (Z) prepare and test
computer software to use for the on-the-fly analysis (both qualitative and
quantitative) of mixtures of volatile components by direct-linked gas chrcmato-
graphy/Fourier transform infrared/mass spectrometry, and {3} develop, test, and
construct a high-sensitivity gas chromatography/Fourier transform infrared
system, and retrofit the Fourier transform infrarec spectrometer at the Environ-
mental Protection Agency's Environmental fbnitoring Systems Laboratory in Las
Vegas, Nevada.
17. KEV WORDS AND DOCUMENT ANALYSIS
a. * DESCRIPTORS
t>.IDENTlFlE&SjOPEN ENDED TERMS
c. C05ATJ Flfid/C/OUp
;ie. distribution statement
*9 seCOP'TV ClASS iTfai Jifp&rtj
UNCLASSIFIED
2i. no. of p^ces
40
(RELEASE TO PUBLIC
20. SECURITY CLAliS tTtiis wgf/
UNCLASSIFIED
2'J. PHICE
EPA. Form 2220-1 (Re*. 4-77) PREVIOUS ECfTiow n oeKucrc
-------
NOTICE
The information in this document has been funded wholly or in part by
the United States Environmental Protection Agency under Cooperative Agreement
Number CR-811730-01 to the University of California at Riverside, Riverside,
California. It has been subject to the Agency's peer and administrative review,
and it has been approved for publication as an EPA document. Mention of trade
names or commercial products does not constitute endorsement or recommendation
for use.
i i
-------
ABSTRACT
This report documents progress on three tasks related to the design and
testing o; procedures and techniques for analy£ing volatile and semi volatile
components of environmental samples. The tasks include (11 develop and test
a procedure to use infrared mo'ar absorptivities and internal standards for the
routine quantification of environmental contaminants, <2) prepare and test
computer software to use for the on-the-fly analysis (both qualitative and
quantitative) of mixtures of volatile components by direct-linked gas chroroato-
graphy/Fcurier transform infrared/mass spectrometry, and (3) develop, test, and
construct a High-sensitivity gas chromatography/Fourier transform infrared
system, ind retrofit the Fourier transform infrared spectrometer at the Environ-
mental Protection Agency's Environmental Monitoring Systems Laboratory in Las
Vegas, Nevada.
[n response to Task i, a method to estimate the quantity of each component
separated by gas chromatography based on the results of spectral search program
is described, and a
-------
CONTENTS
Page
Abstract iii
Figures vi
Tables vii
Abbreviations and Symbols viii
1. Introduction 1
2. Conclusions and Recommendations 4
Task 1 4
Task 2 4
Task 3 4
3. Results and Discussion 6
Quantitation Without Identification (Task No. 1) 6
The GC/FT-IR/MS Interface (Task No. 2) 9
Optimizing the GC/FT-IR Interface (Task Mo. 3) 22
Reference? 31
v
-------
1
2
3
4
5
6
7
8
9
10
11
12
FIGURES
Page
GC/FT-IR/MS parallel split 10
Pulse sequence used for (above) EI-FT-MS and (below) CI-FT-MS. . 11
FT-IR integrated absorbance reconstruction (2995-2870 cm-1) for
the SOCAL fuel additive chromatogram 12
Chromatogram reconstructed from electron impact data for the
GC/FT-IR/EI-CI FT-MS analysis of the SOCAL fuel additive ... 13
Mass spectra from EI (above) and CI (below) date for aliphatic
component of SOCAL fuel additive 14
Gram-Schmidt infrared reconstruction of-chromatogram of
peppermint oil sample 17
FT-MS electron impact data reconstruction of chromatogram of
peppermint oil sample 18
Infrared spectrum of first peak in Figure 6 19
Interferogram intensity at the centerburst versus cone angle . . 24
Shield-cone attachment for GC/FT-1R lightpipe 25
GC/FT-IR system with adjustable oven and collection optics ... 26
Collection optics using a KBr lens whose distance from the end
of the lightpipe, x, can be varied 28
Possible images of the lighcpipe (the glass end of which is
shown shaded) onto a detector (shown as a bold circle) .... 30
vi
-------
TABLES
Nupib?r Page
1 FT-13 search restl ts far aliphatic component of SOCAL fuel
additive , i6
2 FT-lk Horary search resul u: for first peak cf peppermint oil
samp's. . - 2D
3 FT-IR library search results For peak identified as menthane
(peppernint oil). ........ 21
4 Comparison of various AMM algorithms For the 45 "unKnown"
compounds Z2
v i i
-------
ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
AMM -- accurate mass measurement
CAS -- Chemical Abstracts Service
CI — chemical ionization
EMSL-LV -- Environmental Monitoring Systems Laboratory, Las Vegas,
Nevada
EI -- electron impact
EI-CI -- alternate electron impact, chemical ionization
EPALIB -- EPA Library of Vapor-Phase Infrared Spectra
FID -- flame ionization detector
FT — Fourier transform
FT-IR -- Fourier transform infrared
FT-MS -- Fourier transform mass spectrometry
GC -- gas chromatography
GC/FT-IR -- coupled GC and FT-IR
GC/FT-MS — coupled GC-and FT-MS
GC/FT-IR/FT-MS — coupled GC, FT-IR and FT-MS
GIFTS -- gas chromatography infrared Fourier transform software
HQI -- hit quality index
IR -- infrared
KBR -- potassium bromide
MCT -- mercury cadmium telluride
MS — mass spectrometry
NEP -- noise equivalent power
SNR -- signal-to-iioise ratio
UCR -- University of California, Riverside
WCOT -- wall-coated open-tubular
SYMBOLS
d^ -- diameter of an image
d0 -- diameter of an object
f -- focal length of a lens
m-j -- calculated quantity of component, i
Qi -- HQI for component, i
x -- distance between object and lens
y -- distance between image and lens
vi i i
-------
SECTION 1
INTRODUCTION
Currently the U.S. Environmental Protection Agency (EPA) screens the gas
chromatographicable portion of sample extracts for a few hundred target organic
compounds (1,2). Si..ce over sixty thousand manufactured chemicals are currently
regulated under the Toxic Substance Control Act, it is apparent that many
regulated compounds are not identified (let alone determined) in environmental
samples. The present protocol for the analysis of volatile and semivolatile
compounds by the EPA involves a separation by gas chromatography (GC) and
measurement of the mass spectrum of each separated component. Mass spectrom-
etry (MS) is a fast, sensitive instrumental analytical technique, and the
linkage of gas chromatography and mass spectrometry (GC/MS) represents a good
first step towards the total characterization of environmental samples. Never-
theless! GC/MS does have several drawbacks.
First, the mass spectra of many isomeric compounds are very similar, yet
the toxicity and/or carcinogenicity of the individual isomers may be very
different. For certain compounds, it may be difficult to produce a discernable
molecular ion when ionization is initiated by electron impact (EI), and, even
when chemical ionization (CI) methods are used the molecular ion (M+) of cer-
tain compounds may be present at ? level considerably below the intensity of
other ions in the mass spectr^:. Under these circumstances, a priori interpre-
tation of the mass spectrum may be difficult if not impossible. Finally, even
a semiquantitative determination of all peaks in the gas chromatograms of •
complex environmental samples is usually impossible without an unambiguous
identification of each peak component and the availability of calibration
standards for each component.
The application of Fourier transform infrared spectrometry (FT-IR) both in
place of mass spectrometry, as well as the application of FT-IR in addition to,
mass spectrometry for the identification of components separated by gas chro-
matography (GC/FT-IR and GC/FT-IR/MS, respectively) have been proposed as
alternative techniques to GC/MS. Although GC/FT-IR measurements may never have
superior sensitivity to the corresponding GC/MS measurements, the capability of
infrared spectrometry to distinguish between isomers and the reproducibility of
GC/FT-IR spectra from one instrument to another should enhance the potential
for unambiguous structural assignments of components of complex mixtures separated
by GC. Because of the greater reproducibility of infrared spectra, relative to
GC/MS spectra (reproducibi1ity requires precise quaarupole tuning), GC/FT-IR
also lias the potential to improve the capability of obtaining estimates of the
quantity of each component eluting from the chromatographic column, whether or
not this molecule has been unequivocally identified or has merely been assigned
to a certaii chemical class.
i
-------
Although GC/FT-IR was first shown to be feasible as early as 1967 (3), it
did no* become a truly useful technique until the early 1930's when the
sensitivity of GC/FT-IR measurements was increased to the point that nanogram
quantities (typically 10-100 ng) cf many components of complex mixtures separ-
ated by capillary GC now yield identifiable infrared spectra. The state of the
art of capillary GC/FT-IR was summarized in a review by Griffi ths ct al. (4).
Since that paper appeared, further investigations (5,6) have indicatecTthat
detection limits can be decreased further, perhaps even to the point that the
practical detection limits of GC/FT-IR can be made comparable to those of GC/MS
for complex environmental samples. In this event, a GC/FT-IR/MS system would
become a truly powerful tool for environmental analysis.
A directly linked GC/FT-IR/MS system hai indeed been reported by several
laboratories (7-9) and although one of these groups (7) has reported several
multi-component sample analyses, no environmental analyses have been reported
to date for such a direct linked system. Nevertheless, both Gurka et a].,
(10,11) and Shafer et al., (12) have reported the usefulness of indeperTaent
GC/MS and GC/FT-IR analyses on hazardous waste extracts.
The availability of a viable GC/FT-IR/MS system should greatly increase
the chemical information from environmental samples, thereby allowing better
risk assessment which in turn should lead to more efficient cleanup and dis-
posal measures at chemical waste sites. Several aspects of the development of
such a system are being addressed in this project.
This cooperative agreement covers three separate but related tasks:
Task Mo. 1: (a) to develop a procedure to use infrared molar abscrptivi-
ties and internal standards for the routine quantification of on-the-fly
environmental contaminants;
-------
on those techniques which are the most amenable to currently available GC/FT-IR
technology and, therefore, the most likely to provide meaningful results within
the time frame of this project; and (b) to construct a high sensitivity GC/FT-
IR system and to retrofit the existing EMSL-LV FT-1R rpectrometer, from parts
supplied by the EMSL-LV.
3
-------
SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
TASK 1
a. Measurement of the absorbance of the most intense peak in the
spectrum or that of certain functional groups does not permit the
quantity of a given component present in the lightpipe to be deter-
mined accurately.
b. A method based on a weighted average of the quantities of the samples
given as the top few matches by spectral searching routines appears
to have great potential for allowing the quantity of an unidentified
peak component to be estimated.
c. A major limitation to this method (see a ,b. above) appears to be the
quantitative accuracy of the EPA library of vapor-phase infrared spectra
(EPALIB).
d. We recommend that efforts to improve the quality and size of GC/FT-IR
reference data bases be renewed, possibly through the initiation of a
collection of reference spectra submitted to a central computer by
Current GC/FT-IR users. This is an economic approach to increasing
the data base because reference spectra would be donated.
TASK 2
a. A parallel interface between a gas chromatograph, an FT-IR spectrom-
eter, and a Fourier transform mass spectrometer was constructed and was
applied to the identification of the components of several complex
mixtures.
b. Accurate mass measurements have been used to establish the molecular
formula of GC eluates and when combined with infrared spectral search
data, accurate mass measurements appear to present a powerful method
for compound identification.
c. A low-cost quadrupole mass spectrometer has been installed and will
be interfaced first to a commercial GC/FT-IR interface and then to an
optimized GC/FT-IR interface being constructed under Task 3.
TASK 3
a. Methods of reducing the effect of detector nonlinearity because of
unmodulated radiation emitted by the lightpipe are being developed.
4
-------
The use of a cooled lightpipe in conjunction with a cone to collect
scattered radiation has been shown to improve the sensitivity of a
commercial GC/FT-IR system.
An optical configuration has been designed to reduce the size of the
infrared-detector below the size of detectors now being used in
commercial GC/FT-IR systems. This will result in an increased
system sensitivity.
-------
SECTION 3
RESULTS AND DISCUSSION
QUANTITATION WITHOUT IDENTIFICATION (Task Ho. 1)
This task involves the development of a technique for obtaining an esti-
mate of the quantity of each GC eluate from its GC/FT-IR spectrum. The first
approach was to determine whether it was possible merely to use the absorbance
of the r,iost intense band in the spectrum to provide an estimate of sample
quantity. Using the EPA Vapor-Phase Infrared Spectral Library (EPALIB), it was
determined that the absorptivities of the strongest band (measured in cm^ g~l)
varied by well over an order of magnitude. Changing to a molar absorptivity
(cm2 mole"1) did not alter tin's conclusion.
Since many functional groups have greatly different polarities, we felt
that by limiting the search to compounds containing a particular functional
group, the standard error of estimate could be reduced. For carbonyl-contain-
ing compounds, the standard deviation of the absorptivities of the carbonyl
stretching mode was approximately 50 percent of the mean value of the molar
absorptivity, meaning that approximately 65 percent of all carbonyl compounds
could be determined within a factor of two. The variations were presumably due
to the contribution of such canonical forms as:
X . c = 0 <—> +X = C-0"
to the structure, so that displacement, of the partially charged oxygen atom
during the C = 0 stretching mode would lead to a larger change in dipole moment
for a given displacement of the oxygen atom.
When a similar investigation was performed for compounds containing the OH
group, the standard deviation was found to be larger than the mean value of the
molar absorptivity. In part this surprising result was due to several errors
in the data base, but even after some of the erroneous entries had been deleted,
the standard deviation was still very large. An approach based on the assump-
tion of a constant molar absorptivity was, therefore, abandoned in favor of a
technique which appears to have the potential of giving a far superior result.'
Through ar. internal search of EPALIB, it v/as recognized that the best
matches to the spectrum of a selected probe molecule often had similar struc-
tures. The molar absorptivities of these compounds were also often found to be
quite similar. For a given peak in a chromatogram, a good match will be
obtained if the reference spectrum of that compound is in the data base. In
this case, the amount of sample passing through the lightpipe can be estimated
6
-------
from the quantitative information given in the FPALIB header and the lightpipe
dimensions and carrier-gas flow rat.?.
If the "unknown" is not represented in the data base, a spectral search
will lead to the closest analogs to the correct molecule appearing in the
search output. In fact, even if a reference spectrum of this sample is in the
data base, it can often be difficult to unequivocally assign its chemical
structure because of the effect of spectral baseline noise, coeluticn peaks, or
the presence in the data base of reference spectra of compounds with a similar
chemical structure.
We have, therefore, developed a method for estimating sample quantity
based on the hit quality index (HQI) of a spectral search. The quantitative
information listed in the header of each reference spectrin in EPALIB is weighted
by the reciprocal of the liQI in order to derive a quantitative estimate of the
amount of sample present in the lightpipe.
The HQI is a measure of the difference between the spectrum of the unknown
and each reference spectrum in the data base. The smaller the HQI, the greater
the probability of matching the spectrum of the unknown and the library reference
spectrum. Typically, if the HQI of the best match is less than half the value
of the HQI of the next best and subsequent matches, there is a good probability
that the "unknown" has been identified, and the sample quantity can be calculated
using the data in the EPALIB header.
In practice this is rarely found to be the case and unequivocal assign-
ments are only occasionally made using the EPALIB infrared data bas.e alone.
The procedure we are developing takes the quantitative value afforded by each
of the top N hits and weights them by the reciprocal of the HQI. As an example,
consider the output of a (hypothetical) spectral search:
Order of Hit HQI Compound Calculated Quantity (from EPALIB)
1 Qi A mj
2 Q2 B rr>2
On
mi
mN
7
-------
In this case, the quantitative estimate of the analyte without direct knowledge
of the identity of the analyte is calculated as:
1
"est
N
I
i=l
N
I Qi
i = l
-1
(3)
Qi'
The success of this approach was tested by an internal search in EPALIB.
The spectrum of a given compound, e.g., phenol, was input into the Digilab
GIFTS GC/FT-IR search software. The first hit, of course, was always the
selected probe molecule (with an HQI of 0.000, since the "unknown" and the
"reference" spectra were identical). Neglecting this entry, the next N entries
were treated according to equation (3) and the result compared with the sample
quantity listed for the probe molecule.
For some compounds the results of this approach were excellent. For
example, the value of sample quantity calculated for phenol from the five
closest matches was only in error by 3 percent from the value listed in the
EPALIB header for phenol. Other compounds, especially those with exceptionally
high peak absorptivities, gave poorer results. Nevertheless, for the great
majority of compounds tested the error was less than a factor of two. In the
second year of this project, further investigations of this technique will be
made. It is believed that the principal source of error in this approach is
the poor quality of the EPA vapor-phase library, which contains many errors.
Since many people are using vapor-phase infrared reference spectra, it has
been suggested that the collection of a library, in addition to the
EPALIB, be initiated. This could take the form of a computer "bulletin board"
to which users could subscribe. Any user with high quality vapor-phase
("GC/FT-IR") reference spectra could submit these spectra to the bulletin boa-d
for evaluation and, later, inclusion. Many users have between 10 and 200
reference spectra suitable for inclusion in this data base. In the meanwhile,
experiments to determine the level of inaccuracy of the quantitative estimates
of sample quantity in the headers of several entries in EPALIB will be made in
the next * months. Eventually the user contributed spectra would be merged
with EPALIB.
THE GC/FT-IR/MS INTERFACE (Task No. 2)
A linked analytical technique comprising capillary GC/FT-IR/MS ccn provide
chemical information about mixtures of volatile and semivolatile compounds to
aid in component elucidation. The infrared and r.iass spectral data generated are
complementary and can be combined to give a less ambiguous chemical identifica-
tion. In the initial phase of this work a Fourier transform mass spectrometer
(FT-HS) was used to measure the mass spectra. At the end of the first year a
quadrupole mass spectrometer (Hewlett-Packard Mass Selective Detector) was
installed.
8
-------
For GC/FT-IR/MS, two configurations are possible, a series or a parallel
interface. The series interface was not considered appropriate for the FT-MS
measurements for two reasons. Since the GC effluent has to traverse the FT-IR
gas cell first, some "dead volume" is introduced. The interface, therefore,
somewhat degrades chromatographic resolution before the mass spectral analysis.
More importantly, a very low pressure is required for FT-MS. Since the differ-
ence in relative sensitivities between FT-IR and FT-MS is currently at least
two orders of magnitude, a parallel split of the GC effluent was favored
(Figure I). Retention times measured by FT-IR and FT-MS correspond very
closely using this parallel configuration.
The parallel splitter consists of a low dead volume heated tee in which
the effluent path to the mass spectrometer is placed 90° to fiat of the original
GC effluent path. This junction has a 2-crn piece of 10-pm (i.d.) fused silica
capillary tubing to act as a ca. 200:1 splitter and as a restrictor to maintain
low (10"° torr) pressure in tfie mass spectrometer vacuum system. The flow path
to the FT-IR lightpipe is virtually unobstructed.
There are several advantages to using an FT-IR/MS in this system. Besides
the high-split ratio, which allows almost all of the GC effluent to pass to the
less sensitive FT-IR, several types of mass spectral data can be obtained
during one chromatographic run. Due in part to the rapid data acquisition of
the FT-MS, electron impact (EI) and chemical ionization (CI) mass spectra can
be acquired alternately. To prevent CI reagent gas (methane) from interfering
in EI acquisition, the gas is pulsed into the FT-MS only during the CI experiment.
The respective pulse sequences are shown in Figure 2.
To demonstrate that the interface is amenable to the study of complex
mixtures, a SOCAL fuel additive was separated via capillary GC and analyzed
with the GC/FT-IR/CI/FT-MS system. The flame ionization detector has a very
low-dead volume and the chromatogram measured using this detector may be'con-
sidered to give the best attainable resolution. The GC/FT-IR chromatogram
reconstruction (Figure 3) can be seen to closely approach the chromatographic
resolution of the FID trace. Each FT-IR data file was acquired with a time
resolution of 1 second (32 scans coadded). The alternate EI-CI data acquisi-
tion for the FT-MS results in slightly degraded chromatographic resolution.
Fifty EI scans coadded plus a CI acquisition result in a 2-second time
resolution. Despite the longer time acquisition, many peaks are still resolved
in the FT-MS EI reconstruction (Figure 4).
Most of the components of the SOCAL mix are aliphatic hydrocarbons. One
example of an identification of an eluate using FT-IR, EI-FT-MS and CI-FT-MS
will be given. A hydrocarbon eluting after 12 minutes in the MS reconstruction
and file 564 in the FT-IR reconstruction is identified as a branched hexane.
No molecular ion is seen in the mass spectrum (Figure 5) corresponding to the
mass spectral and infrared spectral search results. The number one hit for the
FT-MS and FT-IR, however, are identical (2,2,5-trimethylhexane) and give a
molecular weight of 123. Although alkanes are known to give (M-H)+ ions via
chemical ionization (hydride ion abstraction), no peak at mass 127 is observed.
The lack of the (M-H)+ ion could be due to the significant branching of the
alfcane compound. As the extent of branching increases, the (M-H)+ intensity
can drop markedly and has been observed to account for as little as 0.5 percent
9
-------
Figure 1. GC/FT-IR/fo'S parallel split.
10
-------
E!
5ms
n
1ms
, ri
5ms
_n
3ms
n
30ms
_J""1
CI
5ms
n
2ms
n _
100m$
— s—l
5ms
n
70G;tls
f—
3Cms
—r~ i
Quench
Delay
Electron Beam
Delay
Eon Excete
& Detect
Quench
Trigger Valve
Delay
Beam
RX&! Delay
Excite
& Detect
Figure 2. Pulse sequence used for (above) EI-FT-MS and (below) CI-FT-MS.
11
-------
SOCAL
2995-2870 cm"1
1 I r " ' I I I i ' ¦ 1 T *
40 200 360 520 630 840 1000 1160 1320 1480
Data Points
Figure 3. FT-1R integrated absorbance reconstruction 12995-2370 ofT^t for tfte
SGCAL fuel additive cftrcsistogram.
-------
G. C. Run Tsme in (Minutes
Figure 4. Chromatogram reconstructed from electron impact data for the GC/FT-IR/E1-CI
FT-MS analysis of the SOCAL fuel additive.
-------
57
El
i7"i
113
t
SO 70 SO 90 100 110 120 130
Mass In A.M.U.
57
SO i[
^ ..—'I
71
a
85
113
-ijM.
50 @0 70 SO 30 100 110 120 130
ass sra A.&8.U.
Figure 5. Hess spectra fr crs El I above 1 and CI tbelov? data for aliphatic
CMpcment of SOCAl fuel additive.
1*
-------
of total additive ionization [13). The peak at 113 amu corresponds to loss of
a methyl group [15 amu). The infrared search results '.Table 5.) indicate similar
t»r;rc'i&i toi-trocar-issns i ri ttes list.
Ancthar s ar.pl e n/hicfi was analysed by K/FT~I?.jTI-."5 v-:a> p?3$»rar:rt oi1.
This -sjng alsc ccrtairs potentially difficult compounds to identify. This
ait, ftawever, has j&en character! zee! by a cnr-eca to graphic svppl isr ard c eti be
considered as a "known" sarvle. Peppermint oil contains isoners of cyclic
hydrocar'aors as well as isc..rers of cyclic ketones. Tl"e concentration dynamic
range was also a concern because 95 percent of the sasp3e is ^presentei> one reconstruction and net in tie other, dependiig on
quantities present, molar abivrptivities, etc.
Figure 8 shews tne infrared spectrjpn of the First Vi figure S.
Despite the low SNR, the FT-IR results do give the correct identification of
alpha pinene (Tablr1 ?). The mass spectral El results give a molecular ion of
1.36 amu and a strong CI (M+flt ion at 137- The successful infrared search
re-siiHs nay tse partially attributed tc the rathsr unique structure of the
cyclic -alpha pinene hydrocarbon.
The differentiation of menthone (12.3 minutes! from isomer thane (12.7
minutes) was hampered by the partial coelution of isomenthore with other later
eluting components. The infrared spectra of these isomers are nearly identical.
The mass spectra are also not much help in differentiation. The msntftone peak
is identified, hoover, by the infrared spectral search {Table 3) arid supported
by mass spectral molecular weight and CI {M+H) data. The isomenthorre peak
could not have bacn identified by IR data alone. With molecular weight
information obtained froni tlw mass spectrum, the isomer v>as readily differen-
tiated J since menthone had already been identified}.
The spectral library search rasuits frara tne SS/F1-5R/FT-H5 axpsrinent art-
very irfamative in t^iemsaJves. kith SDotS1 sets of dstd, a vary powerful -"deanti-
fic^tion tool is possible. Ore may a&k, however, whether one set of resu'.ts
should be weighted a^a'nst tfie atfer. An algoritnra to accept or reject search-
result combinations has recently ieen published (14V. Tte al^Drit!^ utilises
accurate mass measurement fAWI] FT-MS results to establish the most accurate
molecular farsuia passible. The mess error for such a determination was typi-
cally less than 10 ppra at mass 250. An error of'less than 10 jvpm has been
found to lead to unambiguous determinations of molecular formula.
Forty-five model compounds of various group-types and polarities were used
at concent: atious that might typically be found in a thick-film capillary SC
separation. Infrared and mass spectral search results were incorporated so
that if ths calculated molecular formula is not represented any of the first
five MS search results, the FT-IR/MS search results are not use6. If nar.e of
the first five matches of the IR search results coincide with the determined
molecular foraula, then the compound is considered urjidentifieri. ;io compounds
were incorrectly identified, such t^at the MS sampling conditions appear to be
adequate. Table 4 stirtraarizes the performance af the various algorithm combina-
tions possible with the data collected. The accurate mass FT-tR/MS variation
15
-------
TABLE 1. FT-1R SEARCH RESJLTS FOR ALIPHATIC CCMPC'JEX" OF SOCtL FijEL ADOIT1WE
EPALIP3
! Jibber
HQ I
Identification
43
2491
331
452
802
2494
599
38
994
993
51
10?
116
121
125
127
129
141
161
164
Hexarie, 2,2 ,5-Trimetfcyl -,
Hexane, 2,5-OimethyI-,
(C3Hjg>
Isooctane/so-called/
Heptane, 2,2,4,6 ,6-Pentamethyl -,
(Ci2H2s)
Hexane, 2,2,4-Trin:ethyl
fc9h303
Heptane, 2,2-0i[tiethy"! -,
fCgH2o)
1 soptra/. MsuWte
-------
\k
'\S.~v
4w
•yK'AHh^, -&$
Jw
190
T
X
910
Data Points
1150
Ts90
Figure 6. Gram-Schmidt infrared reconstruction of chromatogram of
peppermint oil sample.
17
-------
G. C. Run Time in Minutes
Figure 7. FT-MS electron impact data reconstruction of chroma togram of
peppermint oil sample.
18
-------
Alpha Pinene
(a)
Jl
.0095n
oj .GOSS-
IP
c
ra
•2 .0037
o
tn
jQ
<
.0008-
.0021
T
y, li
i|1N
i!
'Ay
f
(b)
.4
I.
4)1
f|lfpFr']inj
i ! 1 r~—i 1
4000 3530 3060 2590 2120 1650 1180 710
Wavenumbers
Figure 8. Infrared spectrum of first peak in Figure 6. (a) EPALIB reference
spectrum of a-pinene, (b) infrared spectrum of first peak in Figure 6.
19
-------
TABLE 2. FT-IR LIBRARY SEARCH RESULTS FOR FIRST PEAK OF PEPPERMINT OIL SAMPLE
EPALIBa
Number HQI Identification CAS" Number
2244 1318 Bicyclo/3.1.l/Hept-2-ene, 2,6,6- 80-56-8
(c10H16)C
3303 1319 Alpha Pinene 80-56-8
3013 1339 Bicyc1o/3.1.1/Hept-2-ene-2-ethino 128-50-7
(c11h1801>
29^25 1352 Ethane 7.1-84-0
(c2h6)
3304a 1353 Beta-Pinene 127-91-3
327 1355 Cyclohexane, 1,4-Dimethyl-, 589-90-2
121 1360 Cyclohexane, Cis-1,3-Dimethyl-, 638-04-0
(CsHi6)
457 1363 2-Butene, 2,3-Dimethyl-, 563-79-1
(c6h 12)
a Entries with an EPALIB number greater than 3300 have been added to the data
base at the University of California, Riverside. The similarity of the HQI
values given by the first two hits (both a-pinene but measured at different
locations) is noteworthy,
b Chemical Abstracts Service.
c Elemental composition.
libraries contain erroneous entries, the redundancies in the mass spectral
library minimize the effect of these errors. Such, unfortunately, may not be
the case for the vapor-phase infrared data base. Nevertheless, for environ-
mental samples the limitations of the GC/FT-IR data base may be less in prac-
tice than might be construed from the above discussion. For example, only
6,000 of the ma?s spectra in this data base are those of manufactured chemicals.
In addition, none of the 38,000 mass spectra were measured on-the-fly but were
bled in via the probe. This implies that many of thesu compounds -.'/ill not pass
20
-------
TABLE 3. FT-IR LIBRARY SEARCH RESULTS FOR PEAK IDENTIFIED AS MENTHOHE
(PEPPERMINT OIL)
EPALIb"
Number
HQI
Identification
CASb
Number
3306
84
Menthone
(C10H18°1)°
89-80-5
3307
155
Isomenthone
(CioHi8°i)
491-07-6
732
429
4-0ecanone
(CiQ^OOl)
624-16-8
985
452
4-0ctanone
(C8H16°1>
589-63-9
285
460
6-HendecanoTie
(CliH220i)
927-49-1
622
544
3-Nonanone, 2-Methyl-,
(c10H20°l)
5445-31-8
734
607
3-Undecanone
(CuH220i)
14476-37-0
621
653
3-Decanone, 2-Methyl-,
(c11h22°1>
d
984
654
2-Heptanone, 3-Butyl-,
(°1 lH22°l)
d
2133
706
4-Heptanone, 2-Methyl-,
(CgHieCi)
626-33-5
a Entries with an EPALIB number greater than 3300 have been added to the data
base at the University of California, Riverside. The similarity of the HOI
values given by the first two hits (both a-pinene but measured at different
locations) is noteworthy.
b Chemical Abstracts Service.
c Elemental composition.
d CAS Number not given in EPALIB header.
21
-------
TABLE 4. COMPARISON OF VARIOUS AMM ALGORITHMS FOR THE 45 "UNKNOWN" COMPOUNDS
Number Identified
Algorithm
Correct Tncorrect
Eliminated
FT-IR
FT-IR/AMfo
FT-MS
AMM, FT-IR/MS
FT-HS/AMM
FT-IR/MS
32
33
26
30
32
35
13
8
19
12
0
0
0
4
0
3
13
10
OPTIMIZING THE GC/FT-IR INTERFACE (Task No. 3)
Our work on the design and construction of an optimized interface for
GC/FT-IR can be divided into two parts: (1) the design of an optimized single-
beam GC/FT-IR interface, and (2) the design of a dual-beam system, much of
which is directly dependent upon the results of the research of the single-beam
system. Areas under study for the single-beam design include (1) determining
the optimum lightpipe dimensions for a wide bore 0.32 mm i.d., 1-um thick,
stationary phase) wall-coated open-rtubular (WCOT) fused-silica column, (2)
using slower collection optices (i.e., a mirror with a long focal length) at
the end of the lightpipe. The use of an optical configuration of this design
will allow the use of smaller detectors that have a lower noise equivalent
power (NEP), and (3) using cold shield apertures and smaller detectors to avoid
nonlinear conditions at elevated lightpipe temperatures.
Part I: Single-beam Studies
We have taken two approaches towards optimizing single-beam GC/FT-IR
measurements. In the first, ways of modifying a standard commercial GC/FT-IR
system have been investigated, while in the second, a completely new GC/FT-IR
system is being designed and h':ilt.
Standard System Modifications
Methods are needed to reduce the signal loss that takes place at the
detector when the lightpipe is heated. This decrease in signal leads to an
increase in the baseline noise level in ratio-recorded spectra and a consequent
reduction in signal-to-noise ratio (SNR). Gurka, Laska, and Titus (15) have
shown the magnitude of this effect for temperatures ranging from ambient to
250°C. A similar profile was observed 1n our laboratory for measurements using
a Nicolet 60-SX GC-FT-IR spectrometer. The best explanation of this effect
is that the detector (and/or preamplifier) become saturated and are driven into
a non-linear response by the unmodulated infrared radiation emitted by the hot
lightpipe (6). This heat is focused on the detector along with the modulated
mid-IR radiation, causing the response of the detector to the modulated signal
(the interferogram) to decrease.
22
-------
It may be noted that the effect of saturation by unmodulated radiation is
usually to reduce the intensity of the interferogram without leading to gross
photometric errors in the computer spectrin (16). The effect of an excessively
large photon flux of modulated radiation, on the other hand, can lead to severe
photometric inaccuracy (17). Fortunately, the latter effect does not appear to
be observed in the case of most GC/fT-IR data.
The magnitude of the signal loss is dependent on the optical configuration
used in the FT-IR spectrometer and the GC/FT-IR interface. For example, the
Hicolet 6000 and 60-SX GC/FT-IR system lose from 70-75 percent of original
signal upon heating the lightpipe to about 250°C, An older (1976) Nicolet
7199 system, however, does not lose much more than 20 percent of the IR signal
upon heating the lightpipe. This difference appears to be a result of a
difference in design of the collection optics of the two GC/FT-IR systems,
since the optics of the 60-SX collect a higher percentage of the radiant energy
emitted by the lightpipe than the older optics. By preventing this unmodulated
radiation from reaching the detector while focusing the modulated source
radiation onto the detector, a more linear detector/preamp response and better
retention of IR signal should result.
We found that if a short length of lightpipe, the internal diameter
(1.d.) of which was slightly bigger than the i.d. of the lightpipe, was located
after the exit aperture of the lightpipe (detector end), an immediate increase
in interferogram signal resulted for the Uicolet 60-SX GC/FT-IR system. The
use of such an aperture, even cooled by water, made no difference at all for
the older 7199 system.
He also noted that when a water-cooled cone was placed over the end of the
short lightpipe, a signal increase resulted even at ambient temperature. The
increase in signal appears to result frora a combination of the lightpipe acting
as a cold shield, thus preventing unmodulated radiation from the end of the
GC/FT-IR lightpipe from reaching the detector, and the cone collecting modulated
radiation transmitted by the GC/FT-IR lightpipe but scattered by the heat-
shield lightpipe,. then passing it to the detector.
Polished alumina?! cones were then fabricated to determine an optimum cone
angle. The reflective cones were collecting the component of the modulated
infrared signal that had been scattered by the lightpipes. A 20 percent
increase in ambient signal was achieved for the 60-SX GC/FT-IR interface and as
much as 38 percent increase was observed for the 7199 system with a lightpipe
of same i.d. (i mm) and similar length. Figure 9 illustrates the angle dependent
nature of these cor.es on the infrared signal. We noted that positioning the
cone near the lightpipe still resulted in a signal loss upon heating. A short
lightpipe extension was used to move the cone further from the end of the hot
lightpipe. The final sliield-cone attachment is illustrated in Figure 10.
Design of a Mew Single-beam GC/FT-IR System
To determine the optimum dimensions for GC/FT-IR lightpipes, we found it
necessary to design and build an interface capable of supporting lightpipes of
various dimensions while keeping set-up time to a minimum. Figure 11 illu-
strates the interface designed for this purpose. As can be seen, an adjustable
23
-------
0.T8-
o
o
! ] i 1 1 j j
0.00 S.00 12.00 18.00 24.00 3000 36.00 42.00
Cone Ang?e (Degrees)
Figure 9. Interferogram intensity at the centerburst versus cone antjle.
24
-------
t e
( t
1 1
I t
1 I
! 1
v. «r
X /
rih
Figure 10. Shield-cone attachment for GC/FT-IR light-pipe.
(A) Polished cone reflector; (C) mounting nut; (C) short light-pipe piece;
(D) water cooled beam block; (E) light-pipe end fitting; (F) light-pipe heater
black; (G) gold coated light-pipe.
25
-------
Transfer Lines
M: Mirror
D: Detector
Figure 11. GC/FT-IR system with adjustable oven and collection optics.
-------
oven is employed such that while the front-end is fixed in position, the rear-
end can be moved to accomodate lightpipes of varying lengtn. Because the
frOnt-end ij fixed relative.to the focusing mirror, the lightpipe's fror.t-end
will alw'-.j be in correct alignment with the b^m converging on it. However,
as the .'jhtpipe is shortened or lengthened, the collectior. optics and detector
need to be moved as a single unit either forward or backward- Ftounting the
components of the interface on plates which ride on a connecting track as shown
makes alignment of tfie system an easy and quick task.
An investigation cf the optimum optical configuration for collecting the
beam emerging from the lightpipe is being mace, using lightpipes held in this
variable-length oven. The goal of this study is to determine the combination
of optics and detector that will result in the highest possible interferogram
signal-to-noise ratio being measured for a given lightpipe. In the experiment
we are performing, a KBr lei>s and a detector are mountad upon a track. The
optical configuration is illustrated in Figure 12. A lens follows the
equation:
1/f - 1/x + 1/y (1)
where f is the focal length of the lens and x ard y are distances of the object
di = d0y/x (2)
where d-j is the diameter of the image at the detector and is the diameter of
the object (in this case, th? internal diameter of the lightpipe). By varying
the position of the lens and detector along the track, the tradeoffs betv.een
the solid angle of radiation collected versus detector sue [and hence its noise-
equivalent-power (HEP)] rsay be determined. The results of this experiment are
very much dependent upon the lightpipe employed but should be completed in
about 2 months time (September 1985). Shortly thereafter the EKSL-L.V GC/FT-IP,
system will be retrofitted to realize the increased sensitivity -esulting from
this optimized single-beam system.
Finally, as noted above, we have seen tfcat at elevated temperaturest
no/i-linear responses resulting from saturation of the pre-amplifier or mercury
cadmium-telluride (HOT) detector artd thus lower spectral signal-to-noise levels
are common. For this reason we are studying fundamental methods to circumvent
these problems.
One such method utilizes a cold shield either placed directly after the
lightpipe (as described above) or, preferably, at a focus between the lightpipe
and detector. A second method is simply to match the size of the detector to
that of the image of the liahtpipe such as to exclude any image of the hot
glass at the end of the lig;,tpipe. He have found that if this method is to be
used, extreme care must be taken to eliminate aberrations of the image by the
optics. Three possible cases Illustrating poor and good operating conditions
are illustrated in Figure 13. Only the lower case, when the image of the
cavity cf the lightpipe exactly coincides with the detector, represents the
circumstance for which unmodula ted energy, non-linear conditions would be
minimized. Since in practice it is very difficult to reproduce this condition
because of optical aberrations introduced by the post-1 ightpipe optics, we
27
-------
j-
-------
Figure 13. Possible images of the lightpipe (the glass end of which is
shown shaded) onto a detector (shown as a bold circle). (Top) Image of the
lightpipe is too small; (Center) Image is aberrated, (Below) Image is the
correct size. For the first two cases, radiant energy emitted by the
lightpipe is focussed on the detector; the lower case is the
optical condition.
29
-------
believe that the best solution to the problem involves the use of detectors
matched to the lightpipe image in conjunction with a cold shield placed at a
focus between the lightpipe and detector.
Dual-beam GC/FT-1R
The complete design of a dual-beam system is 'iccessaril idem upon
the results of the investigation into single-beair. jC/FT-JR s For this
reason, much of the design is yet to be completed, One area e have been
involved during the first year of this project is the constr_ . id evalua-
tion of a dual element detector. By utilizing a detector with a . lements
housed within the same dewar, we believe that nonlinear conditio . ^served at
high levels of radiation flux incident upon the large single detecvor used in a
dual-beam FT-IR system) may be avoided by dividing the energy between two
elements rather than concentrating on one. These elements must be closely
matched to optimize the signal addition step, *hich is carried out electronically
before the resultant interferogram is digitized.
30
-------
REFERENCES
1. Gurka, O.F., Meier, E.P., Beckert, W.F., and Haeberer, A.F. Paper pre-
sented at the 3rd National Conference on Itenagenerit of Uncontrolled
Hazardous Waste Sites. Washington, D.C., November 1982.
2. Federal Register. 44, 69464, Monday, December 3, 1979.
3 Low, M.J.D. and Freeman, S.K. Anal. Chem. 1967, 39, 194.
4. Griffiths, P.R., de Uaseth, J.A., and Azarraga, L.V. Anal. Chem. 1983,
55, 1361A.
5. Vang, P.W.J., Etfiridge, E.L., Lane, J.L., and Griffiths, P.R. Appl.
Spectrosc. 1984, 38, 813.
6. Yang, P.W.J, and Griffiths, P.R, Appl. Spectrosc. 1984, 38, 816.
7. Wilkins, C.L., Giss, G.N., White, R.L., Brissey, G.M., and Onyiriuka,
E.C. Anal. Chem. 198?., 54, 226(3.
8. Shafer, K.H., Hayes, T.L., and Tabor, J.E. Proc. Soc. Photo-Opt. Instrum.
Eng. 1981, 189, 160.
9. Crawford, R.U., Hi rsciifel d3 T., Sarfrcrn, 3-K., and Wong, C,H. Anal.
Chem. 1982, 54, 817.
10. Gurka, D.F. and Betowski, L. Anal. Chem. 1984, 54, 1819.
11. Gurka, D.F., Hiatt, M., and Titus, R. Anal. Chem. 1984, 56, 110?.
12. Shafer, K.H., Hayes, T.L., Brasch, J.W., and Jakobsen, R.J. AnaT. Chem.
1984, 56, 237.
13. Harrison, A.G. "Chemical Ionization Mass Spectrometry", CSC Press, Inc.,
Cleveland, Ohio, 1983, p. 83.
14. Laude, D..\., Johlnian, C.L., Cooper, J.R., and Wilkins, C.L. Anal. Chem,
1985, 57, 1044.
15. Gurka, D.F., Laska, P.R., and Titus, R. J. Chronatogr. Sci. 10^2, 20, 145.
16. Kuehi, D. and Griffiths, P.R. Anal. Chem. 1978, 50, 418.
17. Chase, D.B. Appl. Spectrosc. 1985, 39, 491-
31
-------
------- |