Analysis of the Polychlorinated Biphenyl Problem Application of Gas Chromatography-Mass Spectrometry with Computer Controlled Repetitive Data Acquisition from Selected Specific Ions James W. Eichelberger, Lawrence E. Harris, and W. L. Buddel Environmental Protection Agency, National Environmental Research Center, Analytical Quality Control Laboratory, Cincinnati, Ohio 45268
The polychlorinated biphenyls are a difficult-to-analyze class of environmental contaminants because of the large number of possible individual isomers and their physical and chemical similarity to the chlorinated hydrocarbon pesticides. Individual mass monitoring techniques were examined in search of a procedure which is sufficiently sensitive, but which eliminates the need for elaborate separations, and does not sacrifice too much of the qualitative information content inherent in a complete mass spectrum. Several sets of candidate masses were evaluated using PCB’s mixed with certain pesticides which are likely to be in a solvent extract of an environmental sample. The method was also tested with a contaminated sediment sample from an Ohio lake and appears useful for both qualitative and quantitative analyses.
The polychlorinated biphenyls (PCB’s) are widely discussed environmental contaminants which have been observed in air, water, sediment, and food chain samples (1-3). Qualitative and quantitative analyses of these mixtures of mono to perchloro position isomers are complicated by the large number of possible individual isomers and by the physical and chemical similarity of this set of compounds to the chlorinated hydrocarbon pesticides, e . g . , DDT, DDE, lindane, dieldrin. In an excellent review of the chromatographic and biological aspects of PCB’s, Fishbein ( 4 ) points out the difficulty of analytical detection and separation of these materials. A suitable analytical method should combine high sensitivity with high qualitative information content in the response in order to obviate the need for elaborate separations and cleanup. All reported analytical procedures for PCB’s employ some type of separation of isomers and closely related compounds. Since most of these compounds are mildly polar non-ionic organic compounds, they are amenable to gas chromatography which has been employed widely. Several standard detectors respond to these compounds a t environmentally significant levels, and the selective electron capture detector is capable of detecting these compounds well into the subnanogram level. However, it is now widely recognized that the qualitative information content in these detector responses is small and inadequate for analyses of mixtures of PCB’s, chlorinated hyTo w h o m correspondence s h o u l d be (1) (2) (3) (4)
addressed.
Chern. Eng. News, Dec. 13, 197.1, p 3 2 . Science, 175, 155 (1972), C.G. Gustafson, Environ. Sci. Techno/., 4, 814 (1970) L. Fishbein, J. Chromafogr., 68, 345 (1972).
drocarbon pesticides, and other organic environmental pollutants. The application of a mass spectrometer as a chromatographic detector generates the necessary qualitative information but with a substantial loss in sensitivity-perhaps a factor 103-105.A conventional mass spectrometer, which focuses on the detector each sequential ion in the required mass range of about 50-500 amu, spends only a very small fraction of the total scan time focusing ions of significant information content. Perhaps for as much as 95-9970 of the total scan time, the spectrometer is not focusing ions on the collector or is focusing ions which do not contain relevant information. During these periods, significant information-containing ions are continuously generated in the ion source and literally discarded with the resultant wasted sensitivity. This occurs whether the spectrometer is operated in a continuously rescanning mode ( 5 )or not. Substantial gains in sensitivity have been reported ( 6 ) where it was possible to rapidly switch focusing between several ions of known significance to the problem. This was accomplished with a magnetic deflection instrument by a device which quickly modified the accelerating potential; however, the technique was limited to a maximum of three ions separated by a factor of 10% in mass. The development of the digital computer controlled quadrupole mass spectrometer (7) opened the possibility of extensive use of program controlled data acquisition with sampling of specific ions, sequences of ions, and varying dwell times on each. Data acquisition from subsets of the ions used in conventional operations should enhance sensitivity without significant loss of information content. This technique is not the same as the “mass chromatogram” concept (8) which used conventional data acquisition, with the wasted ions, but displayed the change in abundance of selected ions under program control to facilitate interpretation. The application of subset data acquisition techniques to the problem of analyses of PCB’s is attractive because of the possibility of gaining needed sensitivity without sacrificing too much of the qualitative information content inherent in a complete mass spectrum. A major problem was to define which of the many ions from the large number of isomers it was most advantageous to utilize in the method. This paper reports the results of our study of the use of subset data acquisition techniques with a minicomputer (5) R. A . Hites and K . Biemann, Anal. Chem., 39,965 (1967). (6) C:G. Hammar. B. Holmstedt, and R. Ryhage, A m i . Biochem., 25, 532 (1968). (7) W. E. Reynolds, V. A. Bacon, J. C. Bridges, T. C . Coburn. E. Halpern, J. Lederberg, E. C. Levinthal, €. Steed, and R. B. Tucker, Anal. Chern.. 42, 1122 (1970). (8) R. A. Hites an3 K. Biemann, Anal. Chern., 42, 855 (1970).
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controlled quadrupole mass spectrometer. The system was equipped with a gas chromatograph inlet system and is the basis for a highly sensitive qualitative and quantitative analytical procedure for the analyses of PCB’s in the environment. The technique is clearly adaptable to a variety of other analyses. EXPERIMENTAL M a t e r i a l s . T h e commercial polychlorinated biphenyl mixtures were obtained from t h e Monsanto Company, S t . Louis, Mo. T h e mixture of seven characterized chlorinated biphenyl isomers was obtained from the Contaminants Characterization Program, E n vironmental Protection Agency, Southeast Environmental Research Laboratory, Athens, Ga. 30601. T h e characterization d a t a will be published in a future report. T h e pesticides DDD, DDE, DDT, and dieldrin were provided by the Pesticide Repository of the Perrine Primate Laboratory, Environmental Protection Agency, P.O. Box 490, Perrine, Fla. 33157. Solvents were t h e best available from the Burdick & Jackson Laboratories, Muskegon, Mich. 49442. Solutions were stored in glass containers with Teflon (Du Pont) lined lids. I n s t r u m e n t a t i o n . T h e gas chromatograph was a Varian Model 1400 modified by removal of its conventional detector. All separations were conducted with a 6-ft coiled glass column (i.d. 0.078 in.) packed with 1.5% OV-17 plus 1.95% QF-1 on Gas-Chrom Q 100/120 mesh. Carrier gas was helium a t 30 m l / m i n . T h e injector temperature was 220°C and the column 180°C. T h e effluent was directed through an all glass jet type enrichment device and an all glass transfer line’ into the ion source of a Finnigan Model 1015 quadrupole mass spectrometer. Enrichment device and transfer line temperatures were maintained a t 200-220
c..
0 7
T h e spectrometer was scanned by applying a mass set voltage to t h e quadrupole rods from a 15-bit digital-to-analog converter. At a given mass set voltage, only ions of a specific mass-to-charge ratio pass through the quadrupole field to the electron multiplier detector. The system operator controls t h e mass set voltages through a PDP-B/E computer (4096 12-bit words) a n d a control program. T h e program uses calibration d a t a a n d operator entered mass numbers, mass ranges, or subsets of mass ranges t o generate appropriate 15-bit numbers to drive the digital-to-analog converter. ‘rhus i t is possible for an operator to specify a nearly inexhaustible variety of scan sequences. On a signal from t h e operatur, the control program begins to generate repetitively t h e scan sequence until a given time has elapsed or t h e operator intervenes. As part of the setup procedure, t h e operator specifies a n integration time. This sets the time, between 1 and 4095 msec, during which the signal from t h e electron multiplier is integrated before it is converted by a 12-bit analog-to-digital converter into a n ion abundance measurement. The abundance d a t a are stored in a d a t a buffer of the computer’s core memory and subsequently on magnetic tape (Dectape) or a disk. All integration times used in subset scans were selected to make the cycle time for a subset scan approximately the same as a conventional 40-400 a m u scan, Z.C.. ahout 5 seconds. Thus, individual total ion abundance peaks appear at the same place on the spectrum index number (time) axis in both types of ion abundance chromatograms. T h e gain in sensitivity observed in a subset scan is the result of a time-averaging signal-to-noise ratio enhancement during the longer integration times. In addition to the integration time, there is a 2.3-msec settling time for each mass set voltage. This provides more t h a n adequate time for the rod voltageb t o stabilize before integration of the sigrial begins. All of the hardware and software required to accomplish these tasks are part of a standard Finnigan d a t a system developed by System Industries, Inc., Sunnyvale, Calif. 94086. Software used was System Industries version D which is based on previously described concepts ( 7 ) . Scan parameters and quantities of materials used to produce d a t a in Figures 2-6 are as follows: Figure 2. Five nanograms of each of seven characterized polychlorinated biphenyl isomers was used in each experiment; 40400 amu scan with an 11-msec integration time. P C B subset masses with a 540-msec integration time. Figure 3. Two micrograms of Aroclor 1254 was used in each experiment: 150 ng of DDE, 150 ng of dieldrin, 400 ng of DDD, and 400 ng of D D T w’ere added for t h e lower chromatogram; 40-400 a m u scan with a 10-msec integration time. Figure 4.
