Identification of Trace Contaminants in Environmental Samples by Selected Ion Summation Analysis of Gas Chromatographic-Mass Spectral Data D. W. Kuehl US.Environmental Protection Agency, Environmental Research Laboratory-Duluth, 620 1 Congdon Boulevard, Dulufh, Minn. 55804 The gas chromatograph-mass spectrometer (GC/MS) s3;stem has become a powerful tool for analysis of trace organic contaminants in environmental samples, largely because of the development of the computerized measurement of mass spectra ( 2 ) . Since then software techniques such as reconstructing mass chromatograms ( 2 ) ,matching unknown spectra to a library of known spectra ( 3 ) ,and correlating GC retention indices with MS data ( 4 ) have greatly aided the chemist in identifying components of residues. Biller and Biemann ( 5 ) have reported dramatically improved GC resolution by plotting the summed abundance of ions that maximize a t a given spectrum number. The spectra generated are referred to as “reconstructed mass spectra” and the summed abundance plot as a “mass resolved gas chromatogram.” The result is spectra free of interferences from unresolved GC peaks, column bleed, etc. A rapid method to specifically characterize polychlorinated biphenyls (PCB’s) in environmental residues by the level of chlorination has been described (6). It was observed that careful selection of the ionic masses, “subset-masses”, used in producing mass chromatograms resulted in a reconstructed gas chromatogram showing only PCB peaks. The masses chosen were 224,260,294,330, and 362 for dichloro- through hexachlorobiphenyls. In each case the ion chosen was not the molecular ion but the weaker M 2, M 4, or M 6 ions. To compensate for the loss in sensitivity, a digital computercontrolled quadrapole mass spectrometer was used in a selected ion-monitoring mode. However, with present instrumentation the technique is applicable only to the quadrapole MS and is not readily applied to the magnetic sector instrument. We wish to describe a data-processing technique for the analysis of GC/MS data that combines the selectivity of “subset masses” with the sensitivity of ion-abundance summations. The technique has been termed “Selected Ion Summation Analysis” (SIS Analysis) and is defined as an ion-intensity profile generated at the termination of repetitive scanning GC/MS analysis by summing the intensities of selected masses and reporting the summation vs. spectrum number if, and only if, all specified masses are present. The SIS Analysis has been shown to be successful for a wide
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variety of environmental pollutants including pesticides, chlorinated industrial products, and polynuclear aromatic hydrocarbons.
EXPERIMENTAL Walleyes (Stizostedion vitreum uitreum) from Saginaw Bay of Lake Huron and lake trout (Saluelinus namaycush) from the Apostle Island area of Lake Superior were prepared according to Veith et al. (7) and were analyzed on a Varian MAT CH-5 GC/MS system previously described (8).
RESULTS AND DISCUSSION BASIC language software has been developed to interact with Varian MAT Spectrosystem 100 MS DISKOS software to provide Selected Ion Summation Analysis of GC/MS data. The new software allows the operator to enter a set of key masses from the mass spectrum of a compound of interest and a range of mass spectra to be searched. The computer then searches through each spectrum for all specified masses and a report is printed if, and only if, a spectrum contains all of the specified masses. The report consists of the spectrum number, intensity of each mass, and the summation of the intensities of the masses. The two major advantages of this technique, an increase in compound-detection sensitivity and an increase in specificity of compound identification, was demonstrated by examining Lake Superior lake trout data for dieldrin, a chlorinated pesticide. In the mass spectrum of dieldrin (Figure 1)all ions greater than m/e 150 are less than 20% relative intensity, and their fragmentation pattern is so complex that no particular chlorine ion cluster can readily be used to identify dieldrin.
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Figure 2a. GC/MS data mass range intensity summation plots used to search for dieldrin in Lake Superior lake trout
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Mass spectrum of dieldrin as found in Lake Superior lake ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
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UPPER TRACE SELECTED ION SUMMATION PROFILE LOWER TRACE MASS CHROMATOGRAM
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mle 358 360 362 364
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NUMBER
Figure 3. Comparison of GC/MS data single ion intensity and SIS plots for hexachlorobiphenyl isomers in Lake Superior lake trout
However, the spectrum does have four characteristic chlorine ion clusters at mle 263,277,345, and 380 that are generally used to identify dieldrin. Figure 2a is the conventional "mass-range chromatogram" where a sum of ion intensities for mass ranges 261-265, 275-279, 343-347, and 378-384 are plotted vs. spectrum number. The plot is complex, and dieldrin cannot readily be determined to be present in the sample. A less complex plot may be generated if single ion intensity profiles are used (Figure 2b); however, the sensitivity gained by a mass-range summation is sacrificed by using this technique. A search for dieldrin by using SIS Analysis is presented in the top trace of Figure 2b. The plot shows only one strong peak for which GC retention time may be readily calculated and/or a library search of the spectra at the peak maxima may be made. The confirmation of dieldrin has been accomplished rapidly and efficiently.
Table I. Average Increase in Sensitivity of PCB by Chlorination Number for SIS Peaks Compared to M 6 Peaks
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Chlorination No. of PCB
Average increase in sensitivity
No. of peaks
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4 5 6
3.4
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7.5
6
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6
The utility of SIS Analysis was further demonstrated when SIS Analysis was compared to single ion mass chromatograms for PCB's in Lake Huron walleye. Mass chromatograms for the M 6 ions of tri-, tetra-, penta-, and hexachlorobiphenyls were plotted on separate graphs along with SIS traces of the corresponding PCB. Figure 3 is a plot for the hexachlorobiphenyl isomers. No loss of peak resolution was observed from the single ion trace to the SIS trace. In addition, an increase in compound sensitivity was observed for PCB's that ranged from a factor of 3.4 to 7.5 (Table I).
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(1) R.
LITERATURE C I T E D A. Hites and K. Biernann, Anal. Chem., 40, 1217 (1968).
(2) R. A. Hites and K. Biernann, Anal. Chem., 42, 855 (1970). (3) R. A. Hites and K. Biemann, in "Advances in Mass Spectrometry", Vol. 4, E. Kendrick, Ed., The Institute of Petroleum, London, 1968,p 37. (4) H. Nau and K. Biernann, Anal. Chem., 46,426 (1974). (5) J. E. Biller and K. Biemann, Anal. Left., 7 , 515 (1974). (6)J. W. Eichelberger. L. E. Harris, and W. L. Budde, Anal. Chem., 46, 227 (1974). (7) G. D. Veith, D. W. Kuehl, and J. Rosenthai, J. Assoc. Off. Anal. Chem., 58,
l(1975).
( 8 ) D. W. Kuehl, G. E. Glass, and F. A. Puglisi, Anal. Chem., 46, 588 (1974).
RECEIVEDfor review November 4,1976. Accepted December 17, 1976.
Sealed Stainless Cell for Differential Scanning Calorimetry on Volatile Systems C. J. H. Schouteten, S. Bakker, B. Klarema, and A. J. Pennings" Department of Polymer Chemistry, State University of Groningen, The Netherlands
Differential scanning calorimetry ( 1) (DSC) is nowadays widely employed in studying thermal properties and transitions of many systems. Its range of applications has been extended to volatile materials by introducing sealed cells. Aluminum volatile sample pans, sealed by cold-welding ( 2 ) ,and stainless steel large volume capsules ( 3 ) ,sealed by rubber O-rings, are commercially available (Perkin-Elmer). Freeberg and Alleman ( 4 ) as well as Knappe, Nachtrab, and Weber ( 5 ) designed cells that can withstand considerably higher pressures in the order of 20 atm. These designs apply a Teflon disk or ring-shaped washer and a screw-thread to provide a tight seal between the cover and the pan of the cell. The purpose of this paper is to describe a simple cell of stainless steel that was sealed by resistance-welding by means of transformed capacitor discharge (6).This type of sealing appears to be quite suitable for this application, since the actual welding time is of the order of s so that the parts are heated only slightly. The welded capsule could withstand pressures over 80 atm and turned out to be particularly useful when applied to differential scanning calorimetry on volatile systems. EXPERIMENTAL The dimensions of the stainless steel (type 316) cover and pan are 522
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
indicated in Figure 1. Both parts, and in particular the rims, were ultrasonically cleaned in dichloromethane prior to sealing by resistance-welding.The welding was performed with a PECO Impuls schweiss Maschine Typ FP3K equipped with a ElektrodenkraftSteuergerat Typ LA40LE4 (PECO,Munchen-Passing,W. Germany). In this study cover and pan were welded by means of copper elec075
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Figure 1. Dimensions'(in mm) of the stainless steel sealed cell
