The 1991 Jekyll Island meeting i ~ t e t h u i l s .With this a p proach his group can identify a greater range of compounds more accurately.
Uy Alnn 5'enninn Environmental analysis is compared to searching for a needle in a haystack. hut oft e n a better analogy i s searching for an unknown "something" in a haystack. When the analyte is not preselected or is poorly identified. then versatile analytical techniques are required. This problem was addressed by several speakers at the recent International Syinl~osium on Eni+onmental hnalytical Chemistry. held i n ekvll Island, GI\. "Most analyses of water for trace semivolatile organics are still performed by removal of the organic analytes from the water. then GC/>IS [gas chromatographyiinass spectrometry] and computer matching," says Juhn McCuire from EPA's Eni,irunmental Research Laboratory i n Athens. G A "That works fur targets, but fails to identify those pollutants not extracted, xvhich are generally highly polar. and t h u s e compounds not determined by G U M S because of high molecular \\,eights." Worse yet, for thuse nontarget compounds not matched by t h e c om p n t er , Mi: G u ire finds that testing laboratories spend on average five to ten seconds identifying each analyte from the spectral data.
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"There are literally millions of potential pollutants. Identifying those additiunal pollutants that can be chromatographed doesn't require Prdcis articles ore reports of I I I C C mi\: techniques. hut the application ings of unosuo1 significance. inter- r i f existing ones." 'I'n impruve the notional or nationnl dei~elopmrnts at:i:urai:y uf analyses. McGuire a n d of environmrntul irriporl(mcc sighis colleagues advocate basing nificnnt puhlii; policy dt.i.i:lupidaiililication on spectral data from twu u r mnre standard analytical mcnts. and reloted items.
1536 Enwon SCI Techno1 VoI 25 NO 9 1991
Multispectral analysis Multisliei:tral anal!. w's . as practiced hy McGuire's g r o u p , employs Fonrier transforni infrared spectrnsi:(ipy [FT-IR) and several variations (if mass spectrometry [US) t o identify compounds eluting off the GC column. Infrared spectra identify substructures such as 1::-anide or 1:artioriyl functionalities: chemical ionization MS-a "gentle" method of ionizing a niolei:ule-provides the niolecular weight: and high-resolution MS characterizes molecular fragments. As a test. McGuire and his c:olleagues rechecked the results uf a contract laboratory's water anal the Superfund program. They found that the three r e p t e d target compounds were reliably identified by GCIMS. However. the analysis included three tentative identifications and 1 6 peaks listed as unknowns that represented some of the largest peaks in the GC trace. Keanalysis of these 19 peaks using the multispec:t r a1 a p pr oach fo 11 nd t i v o tent at i r e iden ti f i cations that arc prohahly ini:orrei:t, an unresol\,ed peak due to fast GC run times. a missed analysis of tctrahydrofurarl [THF). a series of propylene glycol uligiii~rs,and three subs t i t n t c d rcdiicorl ilitir:n7.iifurari compounds. Even inure iriipressivvo. i i i llir ixiiirsii rif investigating the m u l t i spectral mcthod MiGuire's gruup
0013 936X 91 0925 1536502 50 0
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American Chemical Society
uncovered an error propagated in a routine spectral matching. A compound that produced a GC/MS spectrum attributed to methyl benzofuran revealed an IR band characteristic of an alcohol group. With this clue, the EPA scientists correctly identified the analyte as indenol, which has an MS fragmentation spectrum essentially identical to that of methyl benzofuran. The multispectral approach does require much more than 10 seconds of analysis and an investment in new instrumentation. However, once a compound is correctly identified by this approach, it joins other spectra in computer matching programs.
New hyphenated techniques For most analysts, GCIFT-IR is a relatively new iechnique. "IR has traditionally not been sensitive enough for environmental analysis," explained Charles Wurrey bf the Denartment of Chemistrv at the University of Missouri-Kansas City. Yet FT-IR offers some attractive features: spectral differentiation of isomers, routines for spectral subtraction to eliminate impurities or background matrix, peak areas that can be quantified, established theoretical underpinnings, and, as previously described, significant structural information. To make this technique practical, interfaces for coupling GC with FTIR have been developed that require samples of only -100 pg. Three designs were described by Wurrey: lightpipes that afford IR spectra of gas phase samples; cryotrapping, which freezes samples on a slide that moves back and forth beneath an IR microscope; and matrix isolation. It is this last method that Wurrey's group has explored. In matrix isolation, GC effluent is sprayed across a gap and onto an Au-coated Cu disk refrigerated to 1 2 K. The He carrier gas is vacuum pumped away, whereas trace amounts of Ar along with the eluted analytes freeze onto the disk. As material collects, the disk revolves so as to deposit samples in a helical pattern. With this a p p r o a c h , Wurrey claims to detect tetrachlorodibenzodioxins (TCDDsI at < 1 ppb levels. However, spectral matching of samples in Ar matrixes requires special libraries [as does gas phase IR via lightpipes), and does not provide real-time data. "It will not replace GUMS," says Wurrey, "but can be a confirmatory technique."
Damib Barcel6 from CID-CSIC in Barcelona, Spain, described another multispectral approach based solely on GCIMS techniques. His group is developing these analytical methods to identify a wide range of pesticides and their metabolites in soil and water. Following separation by GC, unknowns are analyzed by a combination of positive- or negative-ion chemical ionization MS, highresolution MS, or even MSIMS. duce bine
If vou nave a d
problem where your sample is limited, these techniques may
be useful. on in For i-dal either m e ovide ion sa tural infhrmation, whereas negative-ion chemical ionization yields less structural information but is highly selective. GC/MS/MS can differentiate between closely related pesticides such as simazine and atrazine. However, admits Barcel6, MS/MS is far from a routine operation. In these experiments, product ions were produced by Ar+ collisioninduced dissociation, which required the analyst to select the proper ion collision energy. Barcel6 has also explored LC/MS with chemical ionization for the detection of more polar molecules such as organophosphates and oxometabolites. Although the detection limits are not as good as GCI MS, the Spanish scientists are able
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to detect organophosphates with LC/MS at levels of 1.0 nglg. Another promising technique, plasma source-MS (PS-MS), was discussed by Gary Heiftje of Indiana University's Department of Chemistry. According to Heiftje, this technique provides semiquantitative data for elemental and isotope concentrations. Furthermore, w i t h proper laboratory protocols-such as carefully covered samples, and laminar flow hoods and benchesdetection limits in the parts-pertrillion range can be obtained. However, PS-MS problems can include isobar interferences (overlapping MS peaks because of polycations with the same weight as the target analyte), and high analyte concentration and sample matrix interferences. To reduce these problems, Heiftje's research group
