Anal. Chem. 1980, 52, 1821-1824
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Base Peak Profiles of Gas Chromatography-Mass Spectrometric Data Obtained from Thermal Desorption of Activated Carbons Katherine Alben Environmental Health Center, Division of Laboratories and Research, New York State Department of Health, Albany, New York 1220 1
format is suited to t h e accumulation of historical sample profiles which can be conveniently compared. The methods described provide a basis for monitoring water treatment processes, in particular, removal of organics by adsorption onto activated carbon. The numerical format can be interpreted by a water supply engineer looking for changes in water quality. If desired, the data can be communicated accurately to a chemist for further interpretation. T h e use of base peak profiles emphasizes the presence of the major volatile compounds adsorbed on carbon. This information provides a n interpretation of t h e substances most able to compete for available sites on the activated carbon, either because of a predominant concentration in the effluent system or because of a preferential carbon affinity (10). This approach does not preclude a search of the sample profile for substances of known toxicological concern, which may or may not be the most abundant compounds adsorbed. T h e presence of select compounds of interest in repetitive scan mass spectral data, such as the priority pollutants ( I I ) , is typically determined from profiles of the masses of characteristic ions and known values for their retention indices. This paper proposes t h e use of base peak profiles in addition to current methods of data reduction, so that a treatment process can be evaluated not only with respect to the toxic substances it is intended to remove but in recognition of the most abundant substances present, which can control its effectiveness.
Thermal desorption gas chromatography-mass spectrometry is a sensitive, specific method for analyzing volatile compounds adsorbed on activated carbons used in water treatment. Computer-calculated base peak profiles reveal the major sample contaminants in a numerical format. Complementary base peak profiles generated by electron impact- and chemical ionization-mass spectrometry analysis further improve the uniqueness of sample profiles, suggesting their potential for monitoring of water treatment processes.
T h e chemical analyses traditionally used to monitor water treatment processes are gradually giving way to instrumental analyses. Thus, measurements of biological or chemical oxygen demand are being supplanted by determinations of the total organic carbon content of water during various stages of treatment ( I ) . However, these methods do not reveal variations in t h e chemical identity of organic compounds entering a treatment plant. T h e need remains for a rapid. sensitive, and specific measurement which can be used to document changes in composition of influent and effluent streams. More specific monitoring methods for volatile organics were developed by Suffet et al. using profiles of intensity vs. time generated by a gas chromatograph with a flame ionization detector ( 2 , 3 ) . Since this detector responds to most organic compounds, the specificity of the method is determined by the chromatographic column. T h e quadrupole mass spectrometer is a universal detector, a t least as sensitive as a flame ionization detector but more specific in the information it provides. Its disadvantage is the complexity of the data, which requires an on-line computer for acquisition, storage, and interpretation. Numerous schemes have been devised to eliminate redundant information and retain only t h e key features of mass spectra (4-6). T h e advantage of chromatographic information. particularly relative retention indices, to catalog mass spectral data, has also been demonstrated (7. 81. These methods have all facilitated t h e search of mass spectral data for known compounds and the interpretation of mass spectra to identify unknown compounds. However, simplified methods of mass spectral analysis for monitoring water treatment processes remain to be developed. For this project. methods of sample preparation and data reduction were devised to fingerprint the major volatile compounds adsorbed on activated carbon used in water treatment. Profiles of activated carbons from several water treatment facilities were obtained by thermal desorption combined with gas chromatography-mass spectrometry (GC-MS). The method of sample introduction was chosen to simplify sample preparation steps which are part of more traditional carbonchloroform extraction procedures (9). The data were formated as plots of base peak (and intensity) vs. mass spectrometer scan number, which is equivalent to time. Both electron impact (EI) and chemical ionization (CI) data were acquired. In either case, storage of the data in a numerical base peak 0003-2700/80/0352-1821S01.00/0
EXPERIMENTAL SECTION This method was developed with a Ferkin-Elmer Sigma 1 gas chromatograph with a flame ionization detector. A Bendix flasher oven was mounted directly in front of the gas chromatograph injection port for thermal desorption. Helium carrier gas (25 mL/min) passed from the flasher through a 1.9-cm 23-gauge syringe needle inserted in a fixed position through the septum of the gas chromatograph's injector. A glass column (1.8 m, 2-mm id.) packed with Carbopack C-O.l% SPI OOO was used for analysis. Cartridges of activated carbon (approximately 0.1 g) were desorbed at 250 "C for 5 min in the flasher, while the GC column was subambiently cooled with liquid nitrogen t o -10 "C. After the carbon was desorbed, the helium carrier gas was switched to bypass the sample cartridge, and the GC column was programmed from 50 "C, held for 1 min, t.o 200 "C at 8"/min, and held at 200 "C for 30 min. GC-MS profiles were obtained under identical chromatographic conditions by using a Finnigan 4000 Electric quadrupole mass spectrometer interfaced to a Nova 3 data system. Sample cartridges were desorbed at 250 "C in a modified capillary injection port. Care was taken to avoid contamination of the column as it cooled. The jet separator interface from the gas chromatograph to the mass spectrometer was maintained at 250 "C. The mass spectrometer was operated in a 70-eV 131 mode with scans from 50 to 600 amu every 2 s. For comparison, some data were acquired in a CI mode, using methane as reagent gas and scanning from 100 to 600 amu. RESULTS AND DISCUSSION Thermal desorption-GC-MS data for three activated carbon samples are plotted in Figure 1. The intensity of each base peak, vertically displaced by its mass in amu, is plotted vs. C
1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 12, OCTOBER 1980
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Table I. Summary of Base Peak Profiles for Major Constituents Thermally Desorbed from Activated Carbon Samples sample I C lb la intens, intens, intens, base peak, amu scan ions x scan scan ions x ions x 10-3 10-3 no. no. no. mol wt 10-3 CI compd E1 50 56 57 57 57 57 57 57 58
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the scan number. In this way, data which is essentially three dimensional is displayed in two dimensions. Major components are listed in Table I. T h e data in Figure 1 are entirely numerical: scan number or time, base peak, and intensity. Current mass spectral data systems typically display data as profiles of total or reconstructed ion current vs. time or as specific ion mass chromatograms for selected ions. However, the number of discrete masses that can be displayed on a computer terminal is limited. A display of all masses scanned saturates the terminal and is excessively detailed for monitoring water treatment processes.
The information contained in the base peaks of each scan is essential for interpreting a sample's composition. For production of base peak profiles, the computer is programmed to search for and plot (or list) only the base peak of each scan and its corresponding intensity. The base peak profiles in Figure I clearly fingerprint the levels and chemical identity of the major sample constituents. Analyses of duplicate carbon samples indicated that the base peak profile is highly reproducible. In Figure 2, base peak profiles are compared for duplicate samples collected a t the same time and place from a particular carbon filter in a treatment plant. The position of corresponding peak maxima
1824
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
for the 29 major components differs by an average of one scan (2 s). The intensities of the corresponding peak maxima differ by a n average of 27%. This value was obtained by renormalizing intensities to the same sample weight, based on their wet weights before thermal desorption. However, a better correction factor would be obtained by using dry sample weights, determined for the activated carbon samples after thermal desorption. Repetition of a particular base peak in a sample profile suggests the presence of possible isomers or compounds with similar composition. Examples are the multiple occurrences of base peak 105 amu in sample l a and base peak 57 amu in sample IC. However additional analytical information is needed to confirm the extent t o which compounds with the same E1 base peak are actually related. For a particular sample, complementary E1 and CI base peak profiles can be acquired to further reduce ambiguities. This is illustrated in Figure 3 for a granular sample (same as Figure la). Depending on the choice of reagent gas, CI causes very little fragmentation compared to EI: protonated molecular ions are the predominant species formed, except for compounds such as hydrocarbons and alcohols, which can undergo hydride abstraction or functional group elimination (12). Therefore the base peak typically occurs a t the parent mass plus 1 amu. These data, as summarized in Table I, illustrate the advantages of CI measurements to construct unambiguous base peak profiles. Only isomers with the same elemental composition are labeled by the same CI base peak and must be differentiated by their mass spectral scan number. For example, the six compounds with base peak 105 amu in the E1 data are found to be isomers of one compound with parent mass 120 amu. In contrast, the six compounds with base peak 91 m u in the E1 data are found to be compounds with parent masses 92 amu (below the mass range scanned), 106 amu (three isomers), and 126 amu (two isomers). The application of CI is somewhat restricted, however, with methane as the reagent gas. Methane's high hydrocarbon background favors scanning in a higher mass range (e.g., 100-600 amu) than for E1 analysis (50-600 amu). Samples such as l b and IC contain compounds with low molecular weights, which necessitate scanning a t low masses. These compounds were not sufficiently abundant to be easily distinguished from background reagent gas ions. Moreover, with methane as reagent gas, fragmentation may occur for certain compounds. For example, trichloroethane was not seen in the CI data of sample l a , which suggests loss of HC1 and base peak 97, below the mass range scanned. I t would be interesting to experiment with other reagent gases in order to minimize background interference and fragmentation. T h e range of application of base peak profile analysis is determined by the method used to introduce samples into the mass spectrometer. Thermal desorption-GC restricts the application to volatile organic compounds, which can be identified by interpreting the fragmentation patterns in their complete E1 spectra. Most of the compounds found by thermal desorption in this work were also identified in carbon-chloroform extracts of carbon from the Torresdale
treatment plant in Philadelphia (2,3). As shown in this paper, thermal desorption can be applied to compounds more volatile than chloroform, which would be lost in the process of solvent extraction and concentration or obscured in the solvent peak of the gas chromatogram. Thermal desorption is also simpler and more sensitive. For this project sample sizes on the order of 0.1 g of carbon were adequate to observe the major contaminants with instrumental detection limits of 10 ng. This sensitivity is characteristic of thermal desorption-GC-MS analyses, since the sample is not diluted in a volume of solvent (mL) much greater than can be injected into the gas chromatograph (pL). Achievement of this sensitivity from carbon chloroform extracts requires larger samples of carbon ( - 50 g) . For monitoring other aspects of water treatment, base peak profiles could be constructed by using alternative methods of sample preparation and/or introduction. Volatile organics transported in the process stream can be extracted for analysis by the gas-phase purge and trap procedure and then thermally desorbed into the GC-MS system (11). For nonvolatile substances, the range of analysis could be extended by using pyrolysis and/or thermal degradationrGC-MS (13),which has been successfully applied to the determination of biological materials (14,15) and natural and synthetic polymers (16-18). As in the analyses for volatile organics, discussed in this paper, chemical ionization should be a valuable complementary technique to generate unambiguous base peak profiles of the degradation products of nonvolatile materials.
ACKNOWLEDGMENT I am grateful to the Division's Toxicology Center for making time available to use the Finnigan 4000.
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1979). J. Watson, "Introduction to Mass Spectrometry", Raven Press, New York, 1976,pp 112-126. T. Risby and A. Yergey, Anal. Cbem., 5 0 , 326A (1978). R . Symuleski and D. Wetzel, Environ. Sci. Techno/., 13, 1124 (1979). A. Yergey, T. Risby, and H. Golomb. Biomed. Mass Spectro., 5 , 47
(1978). H. Schulten and W. Gortz, Anal. Cbem., 5 0 , 428 (1978). E. Gallegos in "Analytical Chemistry of Liquid Fuel Sources", P. Uden, S.Siggia, and H. Jensen, Eds., American Chemical Society, Washington, DC, 1978,pp 13-36. N. Igiauer and F. Bentley, J . Cbromatogr. Sci., 12, 2 3 (1974).
RECEIVED for review December 3,1979. Accepted July 3,1980.
