19 Negative Ion M a s s Spectrometry E. C. HORNING, D. I. CARROLL, I. DZIDIC, and R. N. STILLWELL
Pesticide Analytical Methodology Downloaded from pubs.acs.org by YORK UNIV on 12/04/18. For personal use only.
Institute for Lipid Research, Baylor College of Medicine, Houston, TX 77030
Negative ion mass spectrometry will come into use in the future in many applications, and it is likely to prove particularly valuable in pesticide work and in toxicological studies. Most manufacturers of mass spectrometers are now prepared to supply instruments that can be used for either positive or negative ion mass analysis, or for concurrent operation by rapid switching, and numerous older instruments are being modified for negative ion studies. Current investigations are usually directed to gaining additional information about basic processes involved in negative ion formation and to exploring applications. Source conditions for both positive and negative ion mass spectrometry are usually discussed in terms of source pressure and style of ionization. The source conditions that have been used for negative ion formation are in Table I. The pressure in the source, while an important technological variable, is not a determinant of the mass spectrum. It is possible, for example, to duplicate electron impact ionization (EI) mass spectra of steroids under chemical ionization (CI) conditions, with nitrogen as the charge and energy transfer reagent, by using a short residence time for the ions in the source. In general, the internal energy of product ions is inversely related to the degree of equilibration attained in the source, and this can be varied by varying the residence time of ions in the source. The usual mode of operation f o r atmospheric pressure i o n i z a t i o n (API) (1,2) leads to h i g h l y s t a b l e ions which have been thermally and chemically e q u i l i brated with the c a r r i e r gas and other ions i n the source, and very few fragment ions are u s u a l l y observed. The highest degree o f fragment i o n formation, short o f e l e c t r o n impact i o n i z a t i o n r e a c t i o n s , i s observed under the low pressure chemic a l i o n i z a t i o n (LPCI) c o n d i t i o n s employed by Brandenberger ( 3 ) . Negative ions are formed under a v a r i e t y o f c o n d i t i o n s . The best-known r e a c t i o n s are those of e l e c t r o n attachment and d i s s o c i a t i v e e l e c t r o n attachment, since these are the r e a c t i o n s 0-8412-0581-7/80/47-136-353$05.00/0 © 1980 American Chemical Society
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which occur i n e l e c t r o n capture d e t e c t o r s . Low energy e l e c trons are the reagent, and these are present when gases such as n i t r o g e n , argon and argon/methane are used under API or CI c o n d i t i o n s . Proton removal or proton t r a n s f e r to a b a s i c i o n w i l l a l s o lead to anion formation. T h i s r e a c t i o n can be used to c l a s s i f y organic compounds i n terms of gas phase a c i d i t y . For example, p i c r i c a c i d i s a strong gas phase a c i d ; a l i p h a t i c a l c o h o l s are weak gas phase a c i d s . Oxygen s u b s t i t u t i o n r e a c t i o n s l e a d i n g to phenolate ions occur f o r some substances. Many c h l o r i n e - s u b s t i t u t e d compounds and some aromatic hydrocarbons r e a c t with 02^ i o n s ; t h i s type of r e a c t i o n i s discussed l a t e r . Adduct formation, i n v o l v i n g a h a l i d e i o n , leads to negative ions under some circumstances. There are a l s o a few a d d i t i o n r e a c t i o n s of s p e c i a l i z e d reagents that w i l l lead to negative i o n s . Much of our experience i n negative i o n s t u d i e s i s based upon atmospheric pressure i o n i z a t i o n mass spectrometry ( 1 ) . The c o n d i t i o n s used by Brandenberger (3) f o r i d e n t i f i c a t i o n s t u d i e s i n v o l v e low pressure chemical i o n i z a t i o n with n i t r o u s oxide or methane. Ordinary CI (0.3-1 Torr) c o n d i t i o n s have been used i n a number of l a b o r a t o r i e s . Current evidence suggests that there may not be a s i n g l e c o n d i t i o n that can be recommended as being best i n a l l a p p l i c a t i o n s of negative i o n mass spectrometry, and that at l e a s t two or perhaps more c o n d i t i o n s w i l l u l t i m a t e l y f i n d wide use. Our approach to the question of "best" c o n d i t i o n s i s to use low energy c o n d i t i o n s i n q u a n t i t i v e a n a l y t i c a l work, and to use higher energy c o n d i t i o n s i n i d e n t i f i c a t i o n and s t r u c t u r a l s t u d i e s . There are a v a r i e t y of reasons f o r t h i s , but i n general the e f f e c t of higher energy c o n d i t i o n s i s that fragmentation i s i n c r e a s e d , and t h i s i n c r e a s e s the amount of data a v a i l a b l e f o r i n t e r p r e tation. In the f u t u r e , i t i s p o s s i b l e that two i o n i z a t i o n steps w i l l be used i n the same a n a l y t i c a l system f o r both q u a l i t a t i v e and q u a n t i t a t i v e a n a l y s i s . At present, with p o s i t i v e i o n s , i t i s p o s s i b l e to convert an i n i t i a l l y formed ion to fragment ions by c o l l i s i o n with n e u t r a l gas molecules. When two mass a n a l y z e r s are used, the general process has been c a l l e d MS/MS by M c L a f f e r t y . Much l e s s i s known about the s t a b i l i t y of negative ions under varying c o n d i t i o n s . Our recent API negative i o n mass spectrometry s t u d i e s (4^) have been c a r r i e d out with a source ( F i g u r e 1) having the p h y s i c a l dimensions of a commercially a v a i l a b l e e l e c t r o n capture d e t e c t o r . With an e l e c t r o d e i n p l a c e , and with a n i c k e l - 6 3 f o i l i n p l a c e , the source chamber f u n c t i o n s as an e l e c t r o n capture d e t e c t o r . A very small n i c k e l - 6 3 f o i l i s used i n the aperture r e g i o n . The gas stream from a gas chromatograph i s s p l i t before e n t e r i n g the source, so that d e t e c t i o n i s achieved i n three ways. The s p l i t stream i s d i r e c t e d i n part to a standard e l e c t r o n capture d e t e c t o r and i n part to the API source, which allows both a source e l e c t r o n
19.
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E T A L .
Negative
Ion Mass
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capture response and mass a n a l y s i s of ions from the source be recorded simultaneously. E l e c t r o n attachment s t u d i e s have, with some exceptions, given the expected r e s u l t s . Table I I contains a l i s t of com pounds or compound types known to g i v e molecular negative r a d i c a l ions by e l e c t r o n attachment, i n order of decreasing electron a f f i n i t y . F i g u r e s 2 and 3 show the experimental r e s u l t of i o n i z i n g b e n z i l with thermalized e l e c t r o n s ; the o n l y product i s an M i o n , and a l l three expected responses are observed. Azulene, one of the compounds i n Table I I , shows the expected response under API c o n d i t i o n s , but benzophenone does not. When these experiments were repeated under CI c o n d i t i o n s , a l l compounds i n Table I I gave molecular negative ions. E l e c t r o n attachment r e a c t i o n s may end with M*" formation or with the formation of a fragment i o n ( s ) through cleavage to a s t a b l e anion and a n e u t r a l r a d i c a l (which i s not observed). These r e a c t i o n s , when halogenated ( C l , B r , I ) compounds are i n v o l v e d , g i v e the corresponding h a l i d e i o n . Many examples of T
M + e
•
M
T
[VF]
(M-R)- + R-
7
[Μ ] —-fr(M-X) · + X~ d i s s o c i a t i v e e l e c t r o n attachment r e a c t i o n s are known. Figures 4 and 5 show the mass a n a l y s i s and d e t e c t o r responses f o r methyl parathion. An M"" i o n i s not observed. The fragment ions at m/z 138, 141, 154 and 248 correspond to s t a b l e anions which are formed by the e l i m i n a t i o n of a n e u t r a l r a d i c a l with or without rearrangement. Q u a n t i f i c a t i o n can be c a r r i e d out by s e l e c t e d i o n d e t e c t i o n o f (M-15)~ i o n s , o r by use of one of the other i o n s . The s t r u c t u r e i s evident from the fragment ions. Proton t r a n s f e r r e a c t i o n s are not a t present u s e f u l i n p e s t i c i d e s t u d i e s . These r e a c t i o n s are gas phase analogs of r e a c t i o n s o c c u r r i n g i n s o l u t i o n which r e s u l t i n the i o n i z a t i o n of a c i d s . Basic i o n s are used as reagents. The ions which are employed under API c o n d i t i o n s (5^) a r e C l ~ and 0 ? , f o r the i o n i z a t i o n o f r e l a t i v e l y strong a c i d s , while under CI c o n d i t i o n s i t i s p o s s i b l e to use the s t r o n g l y b a s i c ions F~, 0 or HO" f o r the i o n i z a t i o n o f very weak a c i d s . Oxygen s u b s t i t u t i o n r e a c t i o n s occur f o r many halogenated compounds and f o r aromatic hydrocarbons. The best known r e a c t i o n s are those of aromatic p o l y c h l o r o and n i t r o c h l o r o compounds ( 6 ) . 1
T
T
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TABLE I.
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SOURCE CONDITIONS FOR NEGATIVE ION FORMATION
Source C o n d i t i o n
API
760
Torr
equilibrated
CI
0.3-1
Torr
not e q u i l i b r a t e d
CI
