2362
Anal. Chem. 1907, 59, 2362-2369
Evaluation of an Improved Single-Beam Gas Chromatography/Fourier Transform Infrared Interface for Environmental Analysis Donald F. Gurka*
Quality Assurance and Methods Development Division, U S . Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Las Vegas, Nevada 89114 R i c h a r d Titus
Department of Chemistry, University of Nevada, Las Vegas, Nevada 89114 Peter R. Griffiths, David Henry, a n d Aldo Giorgetti Department of Chemistry, University of California, Riverside, California 92521
A prototype, gas chromatography/Fourier transform Infrared (GCIFT-IF?) light-pipe interface has been constructed and evaluated for environmental analysis. Thls Interface reduces the signal loss, which normally occurs at hlgh Hghtplpe temperatures, to 20% of that obtalned at amblent. By use of thls Interface, the minimum identlfiable quantfty (MIQ) was determined for 52 typical envlronmental contamlnants and ranged from 20 to 120 ng. This corresponds to IdentHIcatlon llmits of 10-60 ppb for a 2-pL gas chromatographic (GC) Injection of a 1-L water sample, which has been extracted and the extract concentrated to 1 mL, and 200 ppb to 1 ppm for a 5 0 3 sample treated in a sbnliar fashlon. The MIQ's for this Interface average about 13 times more sensltive than those measured on the same FT-IR spectrometer equipped with a 1982 vintage commercial interface. I t is estlmated that caplliary gas chromatography/quadrupole mass spectrometry (GCIMS), used in the full scan mode, Is now 3 to 34 times more sensitive than the light-plpe GC/FT-It7 technique. Gas-phase FT-IR group frequencles of typical envlronmental contaminants have been provided to allow the use of thls new interface as a rapid and sensitive, class-speclfic screenlng technique for envlronmental samples. Weakly absorblng, or overlapplng, group frequencies necessitate the use of several confirmatory class-speclflc frequencles for complex envlronmental samples.
The GC/FT-IR technique has been reported as a viable technique for environmental analysis (1-4). The structural information provided by this technique is often not obtainable from other techniques and, used in conjunction with GC/MS, is particularly powerful (1,5,6). The most frequent criticisms leveled against this technique are the small size of the vapor-phase infrared database and its insensitivity relative to the GC/MS method (7). However, it has recently been noted that for environmental analysis the MS database is really not much larger than the existing FT-IR database ( 5 ) . The sensitivity of packed column GC/FT-IR for routine environmental analysis has been reported as 0.5-10 wg (8) and that of capillary GC/FT-IR as 350-1000 ng (9). Cooper and Taylor have reported capillary GC/FT-IR identification limits as low as 57 ng for some low boiling organic compounds at a light-pipe temperature of 230 "C using a narrow-band MCT detector (10). Earlier literature claimed low nanogram sensitivities for this technique (11) but usually for low boiling, very strong infrared absorbers, under nonroutine operating
conditions (e.g. low light-pipe temperatures, nominal injector split ratios). The frequent citation of detectian limits based on strong infrared absorbers like isobutyl methacrylate overlooked the fact that GC/FT-IR response factors exceed an order of magnitude (9) for environmentally important compounds. Griffiths et al. have discussed various modifications to commercial GC/FT-IR interfaces which, if implemented, should result in a substantial sensitivity enhancement. These modifications include eliminating the loss in signal intensity which occurs a t high light-pipe temperatures, the use of smaller focal area detectors (12),and better light-pipes (13). These light-pipes should exhibit better transmission properties than those prepared by the earlier procedure of Azarraga (14). Wilkins et al. have reported an interface which eliminates most of the light-pipe temperature effect (15) and Griffiths et al. have recently reported an interface which eliminates 80% of the temperature effect and employs a small focal area, MCT detector (16). However, no rigorous sensitivity evaluation of this interface for environmental analysis was reported. Earlier it was reported that full scan capillary GC/MS was 40-440 times more sensitive than GC/FT-IR (9). This estimate was based on a comparison of GC/FT-IR and GC/MS instrumental reponse factors which were cross-correlated by assigning an identification limit of 5 ng of phenanthrene for the capillary GC/MS technique (when used in the full scan mode). Full scan GC/MS phenanthrene environmental analytical identification limits of 20 (17) and 10.8 ng (18) have been reported in other laboratories. Full scan GC/MS identification limits of g/wL of solvent have been reported elsewhere (19) but routine achievement of this capability for environmental analysis may necessitate very expensive sample cleanup procedures. But as with GC/FT-IR, GC/MS response factors also span a range which is greater than an order of magnitude (9)*
One useful environmental application of more sensitive GC/FT-IR systems would be for compound class screening using group frequency regions. Although the appropriate software capability has existed for years, the relative insensitivity of GC/FT-IR, coupled with the additional loss in sensitivity resulting from using only a portion of the Fourier domain information, precluded its use for group frequency screening in trace analysis. Unlike the selected ion monitoring mode of GC/MS which leads to a sensitivity gain, because only the masses of interest are scanned, a GC/FT-IR selected frequency (group frequency) detection approach results in a sensitivity loss because the Fourier approach collects all available frequencies. A sensitive compound class screen could
0003-2700/87/0359-2362$01.50/08 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
be used to locate compound types of interest or as a library search prefilter for FT-IR and FT-IR/MS analysis (20). EXPERIMENTAL SECTION GC/FT-IR Instrumentation and Data System. The FT-IR system consisted of a Digilab (Cambridge, MA) FTS-2OB spectrometer (purchased in 1976) equipped with a medium-band (cutoff 700 cm-') 0.25 mm2 focal chip MCT detector. This MCT has a D* of 4.6 X 1O'O cm Hz112/Wat 1 kHz. The light-pipe was 1.0 mm X 15 cm and was made by the method of Griffiths and Yang (13). The GC/FT-IR interface reported in ref 16 was used in this study. The beam exiting the light-pipe of this interface was focused onto a torroidal mirror and then through a 1.06-mm aperture onto the MCT 0.25-mm2 detector element. In this fashion a demagnification of about 4 was obtained. The data system included a Data General (Southboro, MA) Nova/4 computer, a 16-bit analog-to-digital (A/D) converter, Digilab (Cambridge, MA) HI-COMP 32 high-speed array processor, and a Control Data Lark Model 50-Mbyte disk system. With the Digilab GC/S software, four interferograms per second were collected to magnetic disk. The US.EPA 2300 vapor-phase FT-IR search library was used. Chromatography was performed with a Hewlett-Packard (Palo Alto, CA) Model 5880A with a J and W Scientific, Inc. (Rancho Cordova, CA), 1.0-pm DB-5 film, fused silica capillary column (FSCC) GC column (30 m x 0.32 mm) at a flow rate of 1mL/min of helium using no makeup gas. All GC injections were performed with a J and W Model I1 on-column injector. The end of the FSCC column was interfaced to the FT-IR light-pipe. the FT-IR transfer lines and light-pipe were maintained at 280 OC. After an initial hold time of 1 min, the GC oven was ramped from 40 to 280 OC at 10 OC/min and was held at 280 "C for 20 min. All FT-IR spectra were referenced against a 40-scan file of the empty light-pipe. GC/FT-IR Software. Data acquisition and group frequency chromatogram reconstructions were performed with the Digilab GC/S software. FT-IR group frequencies were determined from the US EPA vapor-phase library spectra with software supplied by the Battelle-Columbus Laboratories. Tentative spectral assignments were made by spectral comparison with available EPA vapor-phase or, if necessary, condensed-phaseFT-IR spectra (21). RESULTS AND DISCUSSION Evaluation of I n t e r f a c e Sensitivity. The prototype interface was evaluated with standard solutions representative of typical environmental contaminants. The MIQ's of 52 compounds were determined and compared with those previously reported for a 1982 vintage commercial capillary GC/FT-IR interface (22). The MIQ was defined as the minimum quantity of compound injected on-column which results in a library search in which the analyte of interest appears in the first five search hits. Under these conditions the Gram-Schmidt (GS) peak signal noise (S/N) is about 5:l. The MIQ's are listed in Table I and range from 8- to IO-fold better for the prototype interface with an average sensitivity enhancement of l3-fold. This enhancement is somewhat larger than can be expected from reducing the blackbody effect and the noise-equivalent-power (NEP) advantage realized from using the smaller focal area detector (15). The additional sensitivity is probably derived from a more sensitive MCT detector and/or a better light-pipe constructed by the technique of Griffiths and Yang (13). The light-pipe used in this study had a volume of 132 ILLas opposed to the 94-kL volume of the light-pipe used earlier. There was no apparent loss in chromatographic resolution. Figure 1 shows the infrared spectrum of 400 ng of nitrobenzene measured on the commercial interface and 50.ng measured on the prototype system. The spectrum obtained with the new interface exhibits a S/N for the asymmetric nitro stretch which is 1.6 times greater than that obtained with the old interface, even though only one-eighth as much material was injected. Figure 2 shows the GS infrared reconstructed chromatogram of a standard solution containing 50 ng each of 10 common environmental contaminants. The chromato-
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Flgure 1. 400 ng of nitrobenzene measured on commercial Rterface (a) and 50 ng measured on the prototype interface (b).
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Flgure 2. Gram-Schmidt reconstructed chromatogram of 50 ng/ component standard mixture: (1) 1, 4-dichlorobenzene; (2) 1,2-dichlorobenzene; (3) N-nitrosodipropylamine; (4) nitrobenzene; (5) isophorone; (6) acenaphthene; (7) dibenzofuran; (8) fluorene; (9) hexachlorobenzene; (10) bis(2-ethylhexyl) phthalate.
gram S / N indicates that most of the solution components are detectable a t the low nanogram level. Figure 3 shows the FT-IR spectra of 25 ng of dioctyl phthalate and 50 ng of fluorene. Both compounds are high boiling environmental contaminants, and represent a strong FT-IR absorber and a weak absorber, respectively. The use of low-boiling, strong FT-IR absorbers as sensitivity standards by FT-IR manufacturers has led to much confusion about the capability of this technique and prompted the late Tomas Hirschfield to state in 1985, "Currently quoted limits of detection (FT-IR) are but sheep in wolves' clothing, substantially more impressive in appearance than in reality" (23). We believe that
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987 36 4
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Figure 5. GC/FT-IR spectrum of compounds tentatively identified as benzaldehyde (a) and rn-xylene (b).
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Figure 4. GCIFT-IR reconstructed chromatogram of sediment extract.
the compounds listed in Table I adequately represent the compound types to be found in the environment and thus represent a true test of this interface for routine environmental monitoring. However, it is understood that these are instrumental identification limits and that true sample identification limits will usually be higher. Interface Performance with Sediment Extract. The infrared recohstructed chromatogram (IRC) of a 0.4-pL injection of a sediment extract is shown in Figure 4. Over 60 compounds are detected with nearly base-line resolution. The infrared spectra of the components producing the most intense and the least intense FT-IR responses are shown in Figure 5. The compounds corresponding to these responses were identified as benzaldehyde and m-xylene, respectively. Thus the interface demonstrated the capability to identify compounds spanning a linear dynamic range of over 10-fold and to achieve this with base-line resolution. This is undoubtedly a result of increased interface sensitivity which allowed a superior result at a low column loading. The lower sensitivity of a commercial GC/FT-IR interface used in the past required significant extract concentration, leading to poorer chromatographic separation through column overloading. The infrared spectra of 50 compounds detected in the extract were examined and compared with those obtained earlier with a commercial interface (see Table 11). Even though only one-quarter as much extract was injected, 17 new detections with high S / N were obtained. These detections led to three new tentative identifications (M-xylene, p-chlorotoluene, and 2-chlorobenzaldehyde). Characteristic strong bands a t 2048
and 2118 cm-l identified the compounds a t 13.92 and 16.97 min as isothiocyanates (24). These are probably degradation products from thiocarbamates or thioureas. In addition the new functionalities provided by this interface included seven esters, two alcohols, and one dichlorobenzyl chIoride. Thus the new interface substantially increased the amount of useful information, relative to that obtained by the old.
Prospects for Routine Environmental Monitoring with the New Interface Using Multigram Scanning. The most likely environmental use of GC/FT-IR is for rapid screening. Because the time interval required for the doubling of known chemicals is known to be continually decreasing (25), and economical identification is based on the availability of reference spectra, rapid screening by infrared (or GC/MS) can, a t best, lead to compound class assignment. Since class assignment is dependent on the availability of gas-phase group frequencies, these were determined for several hundred spectra in the US EPA library and are listed in Table 111. As reported earlier, these group frequencies are slightly shifted to higher frequencies (0-30 cm-l) relative to condensed-phase IR group frequencies (26). The availability of these group frequencies permits the rapid screening of extracts for compound classes by the utilization of selected frequency regions (sometimes called chemigrams or multigrams). Ten infrared group frequency regions were scanned (postrun) for the sediment extract and these are listed in Table 11. Thirty-nine of the 50 GS peaks appeared in at least one frequency window with 29 of these in the saturated C-H region (2932-2970 cm-'). Four of these regions were examined in an attempt to distinguish between the three aromatic aldehydes and the nine esters tentatively identified
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
2365
Table I. Fused Silica Capillary Column Gas Chromatographic/Fourier Transform Infrared On-Line Identification Limits compound isophorone nitrobenzene dimethyl phthalate dibenzofuran 2,4-dinitrotoluene N-nitrosodimethylamine 1,3-dichlorobenzene diethyl phthalate 4-chlorophenyl phenyl ether di-n-butyl phthalate di-n-propyl phthalate butyl benzyl phthalate 2-methylnaphthalene l,4-dichlorobenzene bis(2-chloroethyl) ether hexachloroethane 4-chloroaniline 2-nitroaniline 3-nitroaniline 4-nitroaniline 1,2,4-trichlorobenzene naphthalene 2-chloronaphthalene 2,6-dinitrotoluene bis(2-chloroisopropyl) ether bis(2-ch1oroethoxy)methane 4-bromophenyl phenyl ether N-nitrosodipropylamine N-nitrosodiphenylaminec 1,2-dichlorobenzene acenaphthene acephthylene 1,3-hexachlorobutadiene fluorene anthracene hexachlorobenzene hexachlorocyclopentadiene phenanthrene fluoranthene pyrene phenol 2-chlorophenol 2-cresol 4-cresol 2-nitrophenol benzoic acid 2,4-dichlorophenol 4-chlorophenol 2,4,6-trichlorophenol 2,4,5-trichlorophenol 2,4-dinitrophenol 4,6-dinitro-2-cresol
identification limita ng injected rglL 40 25 20 40 20 20 50 20 20 20 25 25 110 50 70
50 40 40 40 40 50 40 110
20 50 50 40 50 40 50 40 50 120 40 40 40 120 50 100
20 12.5 10 20 10 10 25 10
10 10 12.5 12.5 55 25 35 25 20 20 20 20 25 20 55 10
25 25 20 25 20 25 20 25 60
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sensitivity improvement* 7.5 16 20 15 15 8 20 30 40 16 16 6 14 8 10 10
10 14
8 10 7 15 20 6 10
16
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70 50 50 50 40 70 50 100 120 120 60 60
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8 11 11 11 10 11 11
8 8 8 8 8
a Based on a 2-pL injection of a 1-L sample that has been extracted and concentrated to a volume of 1.0 mL (using the MIQ definition). *Sensitivity improvement relative to values (using the MIQ definition) measured with the same FT-IRspectrometer using a commercial interface and reported in Appl. Spectrosc. 1985, 39, 827. Determined as diphenylamine.
by handchecking. This was considered a severe test because the group frequencies of these compound types are quite close. The group frequency regions for the 1701-1740 (aldehyde), 1748-1759 (ester), 2801-2851 (asymmetric aldehydic C-H), and 2619-2766 cm-' (symmetric aldehydic C-H) regions are shown in Figure 6. Whereas handchecking identified only three aryl aldehydes in the extract, the 1701-1740 cm-' chromatogram revealed at least four peaks. Each of these four peaks also appears in the ester 1748-1759 cm-' and the aldehydic proton asymmetric stretch 2801-2851 cm-l regions. Examination of the 2619-2766 cm-l symmetric aldehydic proton stretch regions finally eliminated aldehyde false positives. Thus overlapping group frequency regions necessitate the use of several confirmatory spectral regions for reliable compound class assignment.
Spectral handchecking resulted in 30 aromatic class assignments but the 3013-3090 cm-l multigram aromatic C-H stretch region located only 10 with a S/Ngreater than 5. The aromatic C-H multigram out-of-plane bend region a t 748-891 cm-I detected 24 compounds including all of the 10 peaks in the aromatic C-H multigram regions. Use of these two regions in conjunction leads to no false positives but 20 false negatives. If the 1018-1091 cm-I aromatic substituent-sensitive region is used in conjunction with the 748-891 cm-' region, the number of aromatic class confirmations, with S/Ngreater than 5, increases to 17. An attempt was made to distinguish between benzylic and aromatic chlorine chlorine by use of the chlorobenzylic region C-H stretch (2970-2978 cm-'), the chlorine substituent sensitive region (1018-1096 cm-'), and the chlorine bound to
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
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