TRACE
Applications of Mass Determinations FRED P. AB RAM SON1
E. I. du Pont de Nemours & Co. Instrument Products Division Monrovia, Calif. 91016
r p H E 1971 Analytical Chemistry -*· Division's Summer Symposium, "Analytical Chemistry: A Key to Progress in National Problem Areas," discussed many analyses of significance to the welfare of the population or the economy of the country. The purpose of this paper is to present methods for a number of these analyses based on a single instrumental technique—mass spec trometry—and the rationalization of these methods over traditional approaches. Several of these anal yses have been reported by use of mass spectrometry b u t only for a particular type of problem. For ex ample, ΛΓ-nitrosodimethy lamine (1, 2), chlorinated pesticides (8-8), organophosphorus pesticides (9), poly chlorinated biphenyls (10-14), and organomcrcury (15) have been subjected to a variety of mass spectrometric analyses, although di-
1 Present address, Departments of Pharmacology and Pathology, George Washington University School of Medi cine, Washington, U.C. 20005.
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ethylstilbesterol has not. By use of t h e most current capabilities of mass spectrometry, coupled with appro priate ancillary techniques such as gas chromatography, data acquisi tion and processing, and specific ion detection, the limits for each of these adulterants lie in the low nanogram or even in the picogram range. T h e advantages of mass spectrometry are : Relatively uniform high sensitivity for all materials which are volatilizable, as opposed to the electron capture gc technique where differ ences in sensitivity of 10° or greater are observed (16) Excellent selectivity from inter fering materials by use of specific ion detection (14) or high resolution (6) T h e ability to identity unexpected materials easily T h e freedom from false positive results Nonreproduction of gc times ex cept experiments for which the mass spectrometer is not being scanned
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
Nominal Mass Spectrometry Experiments
A D u Pont Instruments Model 21-490 single-focusing mass spec trometer was used to detect the output from an Aerograph Model 1400 gas chromatograph. The mass spectrometer was equipped with the special differential pumping system and an all-glass g c / m s interface based on the molecular beam design of W a t e r m a n and Stern (17) which permits a major fraction of the mate rials of interest eluting from the chromatograph to be transmitted to the mass spectrometer. In all scanning experiments the mass spec trometer was controlled by a D u Pont Instruments Model 21-094 data acquisition and processing system with a scan rate of 2 sec/ decade. T h e library search ac cessory was also implemented. T h e gc/ms interface temperature was 250°C, and the ion source tempera ture was 230°C. Tor single-ion detection experiments, the amplifier output was registered on a potentio-
SPECIAL REPORT Mass spectrometry combined with gas chromatography and/or data acquisition and processing and specific ion detection can be used to advantage to determine various adulterants of interest, such as DES, chlorinated pesticides, PCB's, and organomercury in the low nanogram or even in the picogram range
Spectrometry to Trace >f Environmental Toxic Materials
metric recorder through a n appro priate filter network. Chlorinated Pesticides. Of prin ciple importance t o t h e analysis of chlorinated pesticides is t h e main tenance of t h e chemical integrity of these highly labile materials. Metallic surfaces a n d reactive sites in t h e chromatographic a p p a r a t u s and g c / m s interface must be elim inated. Unlike gas chromatographic methods which are n o t particularly sensitive t o decomposition following t h e column, t h e identification b y mass spectrometry requires intact molecules t o provide recognizable mass spectra. T h e experiments which follow show t h a t t h e criterion is fully met. Reference spectra a t t h e 100 ng/μΐ level were t a k e n for lindane, heptachlor, aldrin, chlordane, dieldrin, ρ , ρ ' - D D E , o,p'D D D , endrin, ο,ρ'-DDT, p,p'D D D , and ρ , ρ ' - D D T . These reference spectra were added t o t h e already in use (18) in t h e library routines. T w o isomers of search chlordane were separated a n d h a d similar spectra.
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Figure 4. Specific ion d e t e c t i o n by use of mass 296 at 70 eV with 1.7-ng s a m p l e of d i e t h y l s t i l b e s t e r o l - d i m e t h y l ether. Col u m n t e m p e r a t u r e , 220°C; Dexsil 300 GC
Figure 5. Specific ion d e t e c t i o n by use of m a s s 74 at 22 eV with 0.25-ng s a m p l e of DMNA. C o l u m n : 10% Carbowax 400 a n d 5% KOH on 100/120 Gas C h r o m P; 6-ft X Vs-in. Pyrex; 90°C; 30 m l / m i n He flow; interface t e m p e r a t u r e , 150°C
ANALYTICAL CHEMISTRY, VOL. 4 4 , NO. 14, DECEMBER 1972
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Special Report
a more highly chlorinated biphenyl species and the molecular ion from a biphenyl of lower substitution. For example, the fragment ions Ci2H 2 Cl6 + resulting from HC1 loss from heptachlorobiphenyl will over lap all of the molecular peaks from hexachlorobiphenyl except the species containing six 37C1 atoms. This species is of low relative abun dance. To circumvent this diffi culty, the identification of chlorine content was made starting from the highest number of chlorines (seven) to the lowest (three) and eliminating chromatographic peaks from any future consideration once any as signment was made. T h e spectra of these polychlorinated biphenyls are rather similar with molecular ion species as base peaks and loss of Cl2 generating the major fragment ion cluster. This molecular ion screening proced ure shown in Figure 6 is the super position of the five mass chromatograms, each approximately atten uated to reflect the absolute inten sities of each species, with the total ion chromatogram, all generated from the data acquisition and pro cessing system. T h e only possible missed assignments occur where a less substituted biphenyl coelutes with a more heavily substituted species, and only the latter is iden tified. Four such areas may exist, centered at scans 57, 93, 103, and 123, where there are larger peaks for the Cl 6 species t h a n for the CI7. This sort of fragmentation is not expected and probably represents several points of coelution. None theless, this procedure has identified 31 components in this mixture with out using any special chromato graphic care. In this procedure, merged components with their centroids only 1 scan apart (ca. 6 sec) have been separated (Figure 6). This technique will be of general significance in the screening of complex mixtures where individual components or groups of components can be categorized by their mass spectra.
mass spectrometry to exclude from observation masses of elemental composition other t h a n t h a t de sired. In this way, materials which would cause interference even by the use of nominal mass singlepeak monitoring may be avoided. One traditional method for per forming high-resolution experiments is peak matching (22) by use of an oscilloscope to monitor the output from the mass spectrom eter which is being slowly alternated (ca. 1 sec) between two masses, one a reference mass and one the unknown mass. Exact mass is determined by superimposing the position of the unknown peak with
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the position of the reference mass on the oscilloscope and reading the difference in mass from a calibrated dial. Translating this approach to a recorder, one observes a continuing alternation of peaks: reference, unknown, reference, unknown. The equivalent of superimposing two peaks by use of the oscilloscope is placing the unknown peak ex actly between the alternating ref erence masses when using a recorder. Alternatively, by knowing the mass of a species whose observation is desired, the mass measurement cir cuit may be set in advance to cause this mass to fall exactly between the reference masses. I n this way,
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Figure 6. Mass chromatogram deconvolution of Arochlor 1260 with chlorine n u m b e r a s structural probe. Top chromatogram represents total ionization, and lower five mass chromatograms identify chlorine n u m b e r s which are indicated on total ionization trace
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