High Performance Mass Spectrometry: Chemical Applications

8 Analytical Applications of Postive and Negative Ion

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 28, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch008

Chemical Ionization Mass Spectrometry DONALD F. HUNT and SATINDER K. SETHI Department of Chemistry, University of Virginia, Charlottesville, VA 22901

As early as 1916 Dempster (1) observed an ion at m/e=3, which was correctly identified as H3 . By 1925 it was well established that this ion was produced by a secondary process resulting from c o l l i s i o n between ion (H+2) and neutral species ( H ) in the mass spectrometer ion source. Studies of such ion molecule c o l l i s i o n s were largely neglected u n t i l 1952, when interest was revived by the observation of the ion, CH+5 formed by the reaction, (2) +

2

.

CH+4 + C H —-> CH+5 + CH.3. 4

The b i r t h of Chemical Ionization Mass Spectrometry (CIMS) took place when Field (3) and Munson (3a) realized that an ion such asCH+5could ionize sample molecules by transferring a proton to them in the gas-phase. Such an ionization process is t o t a l l y different from ionization of a molecule by removal of an electron, as is done in most other mass spectrometric methods. Here it is a chemical reaction between the primary ion (reagent ion) and the sample molecule which is responsible for ionization of the sample. It is possible to control both the energetics of sample ion formation as well as the type of structural information obtained in the resulting mass spectrum. Different CI reagents undergo different ion-molecule reactions with the same sample molecule, and each ion-molecule reaction affords different structural information about the sample in question. Ion molecule reactions have been developed to identify different organic functional groups and to differentiate, primary, secondary and t e r t i a r y alcohols (4), 1 ° , 2 ° . and 3° amines (5), c y c l i c alkanes from ©0-8412-0422-5/78/47-070-150$10.00/0 Gross; High Performance Mass Spectrometry: Chemical Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


8.

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Positive and Negative Ion Chemical Ionization

151

o l e f i n s ( 4 ) , s u l p h u r c o n t a i n i n g a r o m a t i c s from nons u l p h u r c o n t a i n i n g a r o m a t i c s , (6) and even i n some cases the o x i d a t i o n s t a t e o f the heteroatom i n polya r o m a t i c h y d r o c a r b o n s (7) . R e c e n t l y t h e r e have been a t t e m p t s t o d i s t i n g u i s h s t e r e o i s o m e r s i n t h e gas-phase u s i n g o p t i c a l l y a c t i v e r e a g e n t gases ( 8 ) . In a d d i t i o n t o an e f f i c i e n t t e c h n i q u e f o r t h e p r o d u c t i o n o f a wide v a r i e t y o f p o s i t i v e i o n s , CI i s a l s o an e x c e l l e n t method f o r g e n e r a t i n g n e g a t i v e l y charged sample i o n s . Due t o t h e l a r g e c o n c e n t r a t i o n o f t h e r m a l o r near t h e r m a l energy e l e c t r o n s , produced d u r i n g i o n i z a t i o n o f t h e CI r e a g e n t gas, t h e r e s o n a n c e e l e c t r o n c a p t u r e mechanism o p e r a t e s e f f i c i e n t l y t o produce l a r g e c o n c e n t r a t i o n s o f n e g a t i v e sample i o n s under CI c o n d i t i o n s . A d e s c r i p t i o n o f o u r i n i t i a l r e s e a r c h e f f o r t i n n e g a t i v e i o n CI w i l l be p r e s e n t e d l a t e r i n the chapter. P a r t i c u l a r l y noteworthy i s the development o f methodology w h i c h f a c i l i t a t e s s i m u l taneous d e t e c t i o n o f b o t h p o s i t i v e and n e g a t i v e i o n s on a q u a d r u p o l e mass s p e c t r o m e t e r (6). In a d d i t i o n t o t h e above work we have a l s o r e c e n t l y d e v e l o p e d methodology f o r o b t a i n i n g CI mass s p e c t r a o f n o n v o l a t i l e s a l t s and t h e r m a l l y l a b i l e m o l e c u l e s , under CI c o n d i t i o n s u s i n g q u a d r u p o l e i n s t r u m e n t s w i t h f i e l d d e s o r p t i o n e m i t t e r s as s o l i d probes b u t i n t h e absence o f an e x t e r n a l l y a p p l i e d f i e l d (9). We have a l s o demonstrated t h a t a c c u r a t e mass measurements ( NHÎ

ΔΗ = -127 K c a l / m o l e ; P.A.(Crit) - 127 ΔΗ = -207 K c a l / m o l e ;

show t h a t r e a c t i o n w i t h Ht w i l l

P.A. (NHt) = 207

produce p r o t o n a t e d

Gross; High Performance Mass Spectrometry: Chemical Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

154

HIGH

PERFORMANCE

MASS

SPECTROMETRY

sample i o n , [ M + H ] , h a v i n g 26 Kcal/mole more energy than those g e n e r a t e d by p r o t o n t r a n s f e r from CH+. Due t o v e r y h i g h p r o t o n a f f i n i t y o f ammonia, the NHi* w i l l o n l y t r a n s f e r a p r o t o n t o m o l e c u l e s w h i c h a r e more b a s i c t h a n ammonia. A c c o r d i n g l y , the NH^ i o n f i n d s u t i l i t y as a r e a g e n t f o r s e l e c t i v e l y i o n i z i n g b a s i c components i n a m i x t u r e o f o r g a n i c compounds. In many c a s e s e x t e n s i v e f r a g m e n t a t i o n o f the sample i s d e s i r a b l e i n o r d e r t o o b t a i n as much s t r u c t u r a l i n f o r m a t i o n as p o s s i b l e . F r a g m e n t a t i o n under EI i s due t o h i g h i n t e r n a l energy o f the m o l e c u l a r i o n a l t h o u g h the f r e e r a d i c a l c h a r a c t e r o f Mt lowers a c t i v a t i o n energy f o r many o t h e r w i s e i n a c c e s s i b l e decomp o s i t i o n pathways. F o r t u n a t e l y E l - t y p e s p e c t r a can be o b t a i n e d under CI c o n d i t i o n s by u s i n g p o w e r f u l one e l e c t r o n o x i d i z i n g agents l i k e N j * . The n i t r o g e n r a d i c a l c a t i o n formed by e l e c t r o n impact on N2 gas a t 1 torr


+

N* + e(80eV)

-> N *

+ N* + e

AB + NÎ'

•+ A B '

AB + N*

+ AB

2

+

+ e

+ N

2

+ N

2

(1) ΔΗ - -(2-8)eV

+ e

(2)

ΔΗ - -(0-3)eV

(3)

can t r a n s f e r 2-8eV o f energy t o the sample m o l e c u l e (AB) d u r i n g the i o n i z a t i o n s t e p ( E q - 2 ) . Extensive f r a g m e n t a t i o n o f the r e s u l t i n g Mt i o n r e s u l t s and a spectrum i d e n t i c a l t o t h a t produced by EI methodology i s o b t a i n e d . M e t a s t a b l e (N*) can a l s o i o n i z e the sample as shown i n ( E q - 3 ) . S e l e c t i v e Reagent Gases f o r P o s i t i v e Ion CIMS Argon-Water : When an argon-water m i x t u r e i s employed as the CI r e a g e n t gas, the s p e c t r a o b t a i n e d e x h i b i t f e a t u r e s c h a r a c t e r i s t i c o f b o t h c o n v e n t i o n a l EI and B r o n s t e d a c i d CI s p e c t r a (T4) . Use o f t h i s reagent gas m i x t u r e i s p a r t i c u l a r l y v a l u a b l e when an i o n c h a r a c t e r i s t i c o f the sample m o l e c u l a r weight and abundant fragment i o n s c h a r a c t e r i s t i c o f m o l e c u l a r s t r u c t u r e a r e b o t h r e q u i r e d t o s o l v e the a n a l y t i c a l problem a t hand. E l e c t r o n bombardment o f A r / H 0 (20/1) a t 1 t o r r produces i o n s a t m/e 4 0 ( A r ) , 8 0 ( A r J ) , and 1 9 ( H 0 ) as w e l l as a p o p u l a t i o n o f m e t a s t a b l e argon n e u t r a l s (Ar*). P r o t o n t r a n s f e r from H 0 t o a sample m o l e c u l e i s u s u a l l y o n l y s l i g h t l y e x o t h e r m i c and seldom r e s u l t s i n e x t e n s i v e f r a g m e n t a t i o n o f the r e s u l t i n g M+l i o n . In c o n t r a s t e l e c t r o n t r a n s f e r from sample t o A r i s h i g h l y e x o t h e r m i c (4-6eV) and p r o d u c e s from the sample 2

+

+

3

+

3

+



8.

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155

an energy r i c h r a d i c a l c a t i o n w h i c h s u f f e r s fragment a t i o n t o produce a E l - t y p e s p e c t r u m . The a b i l i t y t o r e c o r d b o t h E I - and C l - t y p e s p e c t r a i n a s i n g l e s c a n i s p a r t i c u l a r l y u s e f u l when t h e maximum s t r u c t u r a l i n f o r m a t i o n p o s s i b l e i s d e s i r e d and t h e q u a n t i t y o f sample a v a i l a b l e f o r analysis i s only s u f f i c i e n t f o r a s i n g l e e x p e r i m e n t . A m i x t u r e o f n i t r o g e n and w a t e r a f f o r d s s p e c t r a i d e n t i c a l t o t h o s e o b t a i n e d w i t h argon and w a t e r as t h e CI r e a g e n t . F o r t h e purpose o f c o m p a r i son, c o n v e n t i o n a l EI and C I ( A r - H 0 ) s p e c t r a o f d i - n p e n t y l a m i n e a r e shown i n F i g u r e 1. R e a c t i o n o f t h e amine w i t h H 0 a f f o r d s a s i n g l e i o n [ M + l ] . I n cont r a s t t h e EI spectrum d i s p l a y s a r e l a t i v e l y weak molecular ion. 2

+

+

3

Deuterium o x i d e : When D 0 i s employed as t h e CI r e a g e n t , a l l a c t i v e hydrogens a t t a c h e d t o N,S, o r 0 atoms i n an o r g a n i c sample undergo exchange d u r i n g t h e l i f e time o f t h e sample i n t h e i o n s o u r c e o f t h e mas s spectrometer. A r o m a t i c hydrogens have a l s o been shown t o undergo exchange {IS). I f t h e mol. wt. o f t h e sample i s a l r e a d y known from p r e v i o u s C l f C H O s p e c t r a , t h e number o f a c t i v e hydrogens i n t h e m o l e c u l e can be c o u n t e d by i n s p e c t i o n o f t h e mol. wt. r e g i o n o f t h e CI (D 0) spectrum. D i f f e r e n t i a t i o n o f 1°, 2°, and 3° amines i s e a s i l y a c c o m p l i s h e d i n t h i s manner (5>) . I n the CI ( D 0 ) spectrum o f 6 - k e t o e s t r a d i o l ( F i g u r e 2 ) , the M+l peak o b s e r v e d i n t h e w a t e r CI s p e c t r a a t m/e=287 i s s h i f t e d t o m/e=290. T h i s l a t t e r i o n c o r responds t o d - k e t o e s t r a c T i o l + D and r e s u l t s from exchange o f t h e two a c t i v e hydrogen atoms i n t h e d i o l f o l l o w e d by d e u t e r a t i o n . 2

#

2

2

+

2

Ammonia E l e c t r o n bombardment o f ammonia g e n e r a t e s NHÎ a l o n g w i t h ( N H ) H and (NH ) H+. These i o n s f u n c t i o n as weak B r o n s t e d a c i d s and w i l l o n l y p r o t o n a t e s t r o n g l y b a s i c s u b s t a n c e s l i k e amides ( 1 6 ) , amines (17) , and some a , 3 - u n s a t u r a t e d k e t o n e s (Γ8Γ) . The r e s u l t i n g sample i o n s seldom undergo f r a g m e n t a t i o n because o f t h e low e x o t h e r m i c i t y a s s o c i a t e d w i t h p r o t o n t r a n s f e r r e a c t i o n s ( F i g u r e 3 a ) . A l d e h y d e s , k e t o n e s , e s t e r s , and a c i d s , w h i c h a r e n o t s u f f i c i e n t l y b a s i c tô a c c e p t a p r o t o n from NH+, show i o n s i n C I ( N H ) s p e c t r a r e s u l t i n g from t h e e l e c t r o p h i l i c attachment o f NH^ t o t h e molec u l e CFigure 3b) ( 1 9 ) . A n o t h e r i n t e r e s t i n g a s p e c t o f CI (NH ) r e s e a r c h i s the f i n d i n g t h a t ammonia can be employed as a r e a g e n t gas f o r t h e d i r e c t a n a l y s i s o f o r g a n i c s i n w a t e r . The +

3

2

3

3

3

3


HIGH

100*

1 El

sort 30,

44


SPECTROMETRY

43. 58

70.

IOO-f

it!

MASS

1UDT

MWI57

ο CI

PERFORMANCE

,157 *f 158

43

50i

44 301

1561

158, 20

60

"V·

100

—ι— 140

170

Figure 1. EI and CI (N +H 0) mass spectra of di-n-pentylamine. The intensity of reagent ions is 50 to 100 times greater than as shown. 2

2

IOC

Analytical Chemistry

Figure 2. CI (H 0) and CI (D 0) mass spectra of 6-ketoestradiol. The inten sity of reagent ions is 50 to 100 times greater than as shown. 2

2


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157


ammonium i o n i s n o t s u f f i c i e n t l y a c i d i c t o p r o t o n a t e w a t e r . A c c o r d i n g l y , o r g a n i c s i n water c a n be s e l e c t i v e l y i o n i z e d when ammonia i s employed as t h e CI reagent gas. The NH+ i o n c a n a l s o be used as s t e r e o c h e m i c a l probe o f o r g a n i c s t r u c t u r e s . S i m p l e a l c o h o l s a r e n o t i o n i z e d under CI (NH ) c o n d i t i o n s . I n c o n t r a s t , d i o l s i n w h i c h t h e two h y d r o x y l groups c a n s i m u l t a n e o u s l y form i n t r a m o l e c u l a r hydrogen bonds t o t h e NH+ i o n a r e i o n i z e d (20). D i f f e r e n t i a t i o n o f t r a n s d i a x i a l d i o l s from t h e 3 T e q u a t o r i a l o r a x i a l - e q u a t o r i a l isomers i s e a s i l y a c c o m p l i s h e d by CI (NH ) mass s p e c t r o m e t r y (7) (Figure 3c). L i k e ammonia, methylamine, i s a l s o a u s e f u l r e agent gas. Aldehydes and k e t o n e s r e a c t w i t h CH NH i n t h e CI s o u r c e t o form p r o t o n a t e d S c h i f f bases ( 2 1 ) . The r e a c t i o n i s q u i t e s e n s i t i v e t o t h e s t e r i c e n v i r o n ment o f t h e c a r b o n y l group ( 7 ) .


3

3

3

3

N i t r i c O x i d e : N i t r i c o x i d e i s one o f t h e most v e r s a t i l e r e a g e n t gases f o r p o s i t i v e i o n CIMS. E l e c t r o n bombardment o f n i t r i c o x i d e a f f o r d s NO w h i c h f u n c t i o n s as an e l e c t r o p h i l e , h y d r i d e a b s t r a c t o r , and one e l e c t r o n o x i d i z i n g agent toward o r g a n i c samples. Depending on t h e type o f o r g a n i c f u n c t i o n a l groups p r e s e n t , any o r a l l o f t h e above r e a c t i o n s may be o b s e r v e d . We f i n d t h a t n i t r i c o x i d e CI s p e c t r a a r e p a r t i c u l a r l y u s e f u l f o r i d e n t i f y i n g organic f u n c t i o n a l groups i n sample m o l e c u l e s , f o r d i f f e r e n t i a t i n g o l e f i n s from c y c l o a l k a n e s , and f o r f i n g e r p r i n t i n g h y d r o c a r b o n mixtures. Of p a r t i c u l a r i n t e r e s t i s t h e f i n d i n g t h a t CI (NO) s p e c t r a c a n be employed t o d i f f e r e n t i a t e p r i mary, s e c o n d a r y , and t e r t i a r y a l c o h o l s . N i t r i c o x i d e CI s p e c t r a o f t e r t i a r y a l c o h o l s c o n t a i n o n l y ( M - 1 7 ) i o n s formed by a b s t r a c t i o n o f t h e h y d r o x y l group t o form n i t r o u s a c i d . S p e c t r a o f s e c o n d a r y a l c o h o l s e x h i b i t three i o n s ; ( M - l ) , which corresponds t o a p r o t o n a t e d k e t o n e ; ( M - 1 7 ) ; and (M-2+30) . The l a t t e r i o n i s g e n e r a t e d by t h e o x i d a t i o n o f t h e a l c o h o l f o l lowed by a d d i t i o n o f N 0 t o t h e r e s u l t i n g k e t o n e . Spectra o f primary alcohols also e x h i b i t ions corr e s p o n d i n g t o ( M - l ) and (M-2+30) . I n a d d i t i o n , however, an i o n , ( M - l ) , u n i q u e f o r p r i m a r y a l c o h o l s i s o b s e r v e d . T h i s i o n i s p r o d u c e d by h y d r i d e a b s t r a c t i o n from C i o f t h e a l d e h y d e formed on o x i d a t i o n o f t h e p r i m a r y a l c o h o l . F i g u r e s 4 a , b, c show CI(NO) s p e c t r a of three isomers o f pentanol. +

+

+

+

+

+

+

+

+


158

HIGH

Scheme:

PERFORMANCE

MASS

SPECTROMETRY

NO* as a S e l e c t i v e CI Reagent.

TERTIARY ALCOHOLS: +

(R) C-OH

R C (M-17)

3

3

SECONDARY ALCOHOLS: Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 28, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch008

+

(R) CH-0H - ^ - * ( R ) C - 0 H — K R ) aC=0 i i ^ ( R ) C 0 * N 0 (M-l) (M-2+30) +

2

2

2

+

-*(R) CH

+

2

(M-17)

+

PRIMARY ALCOHOLS: +

RCH -0H —

^RC H-OH

2

(M-l)

^RCH=0 —

^ΗΟ···ΝΟ

+

(M-2+30)

+

+

+

->RC =0 (M-3)

+

D i f f e r e n t i a t i o n o f o l e f i n s and c y c l o a l k a n e s h a v i n g the same M. W. i s a l s o e a s i l y a c c o m p l i s h e d u s i n g n i t r i c o x i d e as r e a g e n t . Spectra of cycloalkanes e x h i b i t only an [ M - l ] i o n s whereas those o f o l e f i n s c o n t a i n b o t h [M-l]+ and [M+30] i o n s ( 1 9 ) . The l a t t e r s p e c i e s r e s u l t s from e l e c t r o p h i l i c a d d i t i o n o f N 0 t o t h e double bond ( F i g u r e 4d, e ) . CI(NO) s p e c t r a o f h y d r o carbons c l o s e l y resemble those o b t a i n e d under f i e l d i o n i z a t i o n c o n d i t i o n s . Over 80% o f t h e i o n c u r r e n t i n CI(NO) s p e c t r a o f most h y d r o c a r b o n s i s c a r r i e d by t h e [ M - l ] i o n . T h i s s i t u a t i o n stands i n sharp c o n t r a s t t o t h a t o b t a i n e d under e i t h e r EI o r C I ( C H O c o n d i t i o n s , where e x t e n s i v e f r a g m e n t a t i o n o f h y d r o c a r b o n m o l e c u l e i s observed. In a d d i t i o n t o t h e above r e s u l t s , i t i s p o s s i b l e to use CI(NO) s p e c t r a t o i d e n t i f y many f u n c t i o n a l groups i n o r g a n i c m o l e c u l e s . Spectra of a c i d s , alde hydes and k e t o n e s show M+30, M-17; M+30, M - l ; and M+30 i o n s r e s p e c t i v e l y . One drawback t o t h e use o f n i t r i c o x i d e as a CI r e a g e n t i s t h a t i t i s a s t r o n g o x i d i z i n g agent and t h e r e f o r e r a p i d l y d e s t r o y s h o t m e t a l f i l a ments used t o produce t h e beam o f i o n i z i n g e l e c t r o n s . To overcome t h i s p r o b l e m we have d e v e l o p e d a Townsend e l e c t r i c discharge ( f i l a m e n t l e s s ) source f o r producing a beam o f i o n i z i n g e l e c t r o n s o r i o n s ( 6 ) . +

+

+

+


+

8.

HUNT

159


A N D SETHI

160 180 Figure 3. CI (NH ) mass spectra of: (a) triethylamine, (b) cyclohexanone, (c) Ό-(-) ribose. The intensity of reagent ions is 50 to 100 times greater than as shown. S

locr 8
10eV)

Resonance e l e c t r o n capture Dissociative electron capture

+

•A~+B +e

Ion p a i r p r o d u c t i o n

+

A +B"+e W i t h the e x c e p t i o n o f a s m a l l p o p u l a t i o n o f low energy secondary e l e c t r o n s produced under EI c o n d i t i o n s d u r i n g p o s i t i v e sample i o n f o r m a t i o n , most of the e l e c t r o n s a v a i l a b l e under EI c o n d i t i o n s p o s s e s s energy i n e x c e s s o f lOeV. A c c o r d i n g l y , most n e g a t i v e sample i o n s are produced by e i t h e r i o n - p a i r f o r m a t i o n o r by d i s s o c i a t i v e e l e c t r o n c a p t u r e mechanisms, and most o f the sample i o n c u r r e n t i s c a r r i e d by low mass f r a g m e n t s , s p e c i e s l i k e O , HO", C l " and CN~, e t c . Ions of t h i s type p r o v i d e l i t t l e s t r u c t u r a l i n f o r m a t i o n about the sample m o l e c u l e i n q u e s t i o n . In c o n t r a s t t o the above s i t u a t i o n , Wurman and Sauer (23») showed t h a t the t h e r m a l i z a t i o n o f e l e c t r o n s can o c c u r i n a f r a c t i o n o f m i c r o - s e c o n d i n the p r e s e n c e o f gases l i k e methane and iso-butane. The r e s u l t i n g l a r g e p o p u l a t i o n o f t h e r m a l e l e c t r o n s makes r e s o n a n c e e l e c t r o n c a p t u r e the dominant mechanism f o r f o r m a t i o n o f n e g a t i v e i o n s under CI c o n d i t i o n s . Once formed the n e g a t i v e l y charged sample i o n s s u f f e r up t o s e v e r a l hundred s t a b i l i z i n g c o l l i s i o n s w i t h n e u t r a l r e a g e n t gas m o l e c u l e s b e f o r e they e x i t the i o n i z a t i o n chamber. U n l i k e the r e s u l t s o b t a i n e d by h i g h energy e l e c t r o n i m p a c t , s p e c t r a r e c o r d e d under CI c o n d i t i o n s e x h i b i t abundant m o l e c u l a r anions (M-) f o r many t y p e s of molecules. F u r t h e r , t h o s e m o l e c u l e s t h a t fragment under n e g a t i v e i o n CI c o n d i t i o n s g e n e r a l l y do so by e l i m i n a t i o n o f s m a l l m o i e t i e s from the p a r e n t a n i o n . S i n c e the s t r u c t u r a l f e a t u r e s w h i c h s t a b i l i z e a negat i v e charge on an o r g a n i c m o l e c u l e are not u s u a l l y the same as t h o s e t h a t s t a b i l i z e a p o s i t i v e c h a r g e , e l e c t r o n c a p t u r e n e g a t i v e i o n CI s p e c t r a t e n d t o p r o v i d e s t r u c t u r a l i n f o r m a t i o n complementary t o t h a t a v a i l a b l e i n the p o s i t i v e i o n mode. 1



8.

HUNT


A N D SETHI

Perhaps t h e most e x c i t i n g f e a t u r e o f n e g a t i v e i o n CIMS i s t h e f i n d i n g t h a t t h e s e n s i t i v i t y a s s o c i a t e d w i t h i o n f o r m a t i o n by e l e c t r o n c a p t u r e i n t h e CI s o u r c e can be 100-1.000 t i m e s g r e a t e r t h a n t h a t a v a i l a b l e by any p o s i t i v e i o n methodology. T h i s r e s u l t s u g g e s t s t h a t n e g a t i v e i o n CIMS w i l l soon become t h e method o f c h o i c e f o r t h e q u a n t i t a t i o n o f many o r g a n i c s i n complex m i x t u r e s by GCMS. Key t o t h e s u c c e s s o f t h e n e g a t i v e i o n t e c h n i q u e i s t h e development o f c h e m i c a l d e r i v a t i z a t i o n procedures which f a c i l i t a t e i n t r o d u c t i o n o f groups i n t o t h e sample under a n a l y s i s t h a t enhance b o t h f o r m a t i o n o f m o l e c u l a r a n i o n s , M , by e l e c t r o n c a p t u r e and s t a b i l i z a t i o n o f t h e r e s u l t i n g M toward unde s i r a b l e fragmentation. Preliminary studies indicate t h a t p e n t a f l u o r o b e n z a l d e h y d e and p e n t a f l u o r o b e n z o y l c h l o r i d e a r e e x c e l l e n t reagents f o r t h i s purpose. R e a c t i o n o f t h e s e two r e a g e n t s w i t h p r i m a r y amines and phenols f a c i l i t a t e s d e t e c t i o n o f these c l a s s e s o f compounds a t t h e femtogram ( 1 0 ~ g) l e v e l by n e g a t i v e i o n GC-CIMS methodology ( T a b l e I ) . 1

1

1 5

TABLE J_.

Detection L i m i t s f o r D e r i v a t i v e s of Primary Amines and Phenols

Compound

GCMS D e t e c t i o n L i m i t 10 χ 1 0 "

Signal/Noise

1 5

g

4/1

5

g

4/1

P e n t a f l u o r o b e n z o y l amphetamine

>)TMS

Κ*-

Pentafluorobenzylidene dopamine-bis-trimethyl s i l y l CO-C F 6

1

5

ether

20 χ 10

1 5

g

6

Δ ' -Tetrahydrocannabinol pentafluorobenzoate


4/1

161

HIGH

162


P u l s e d P o s i t i v e and N e g a t i v e

PERFORMANCE

Ion CI

MASS

SPECTROMETRY

(PPINICI):

S i m u l t a n e o u s r e c o r d i n g o f p o s i t i v e and n e g a t i v e i o n CI mass s p e c t r a on F i n n i g a n Model 3200 and Model 3300 q u a d r u p o l e mass s p e c t r o m e t e r s i s a c c o m p l i s h e d by p u l s i n g the p o l a r i t y o f the i o n source p o t e n t i a l (±110V) and f o c u s i n g l e n s p o t e n t i a l (±10-20V) a t a r a t e o f 10 kHz as i l l u s t r a t e d i n F i g u r e 5. Under t h e s e con d i t i o n s , p a c k e t s o f p o s i t i v e and n e g a t i v e i o n s a r e e j e c t e d from the i o n - s o u r c e i n r a p i d s u c c e s s i o n and e n t e r the q u a d r u p l e mass f i l t e r . U n l i k e the magnetic i n s t r u m e n t s , i o n s o f i d e n t i c a l m/e, but d i f f e r e n t p o l a r i t y , t r a v e r s e the quadrupoTe f i e l d w i t h e q u a l f a c i l i t y and e x i t the r o d s a t the same p o i n t . Detec t i o n o f i o n s i s a c c o m p l i s h e d s i m u l t a n e o u s l y by two continuous diode m u l t i p l i e r s operating w i t h f i r s t dynode p o t e n t i a l s o f o p p o s i t e p o l a r i t y . The r e s u l t i s t h a t p o s i t i v e and n e g a t i v e i o n s a r e r e c o r d e d s i m u l t a n e o u s l y as d e f l e c t i o n s i n o p p o s i t e d i r e c t i o n on a c o n v e n t i o n a l l i g h t beam o s c i l l o g r a p h . E l e c t r o n C a p t u r e - EI Type S p e c t r a : As n o t e d e a r l i e r when N or argon i s used as r e a g e n t gas, the p o s i t i v e i o n CI s p e c t r a are essen t i a l l y i d e n t i c a l to t h a t o b t a i n e d under EI c o n d i t i o n s . In the n e g a t i v e i o n mode, sample i o n s a r e formed by e l e c t r o n capture. N e g a t i v e i o n s are not produced from n i t r o g e n or a r g o n under CI c o n d i t i o n s . U s i n g PPINICI t e c h n i q u e w i t h N as a r e a g e n t gas we can s i m u l t a n e o u s l y d e t e c t and r e c o r d EI type s p e c t r a on the p o s i t i v e i o n t r a c e and i o n s produced by resonance e l e c t r o n c a p t u r e on the n e g a t i v e i o n t r a c e . S i n c e the s t r u c t u r a l f e a t u r e s t h a t s t a b i l i z e p o s i t i v e and n e g a t i v e fragment i o n s a r e not u s u a l l y the same, the above methodology p e r m i t s one t o s i m u l t a n e o u s l y r e c o r d spec t r a w h i c h c o n t a i n complementary s t r u c t u r a l i n f o r m a t i o n . An example o f t h i s case ( F i g u r e 6a) i s the ΕΙ-EC spec trum o f a m y t a l . 2

2

E l e c t r o n Capture - Bronsted

A c i d Type S p e c t r a :

I f methane o r i s o b u t a n e i s used as r e a g e n t gas f o r PPINICIMS, B r o n s t e d a c i d type CI s p e c t r a a r e produced on the p o s i t i v e i o n t r a c e and e l e c t r o n c a p t u r e s p e c t r a a r e g e n e r a t e d on the n e g a t i v e i o n t r a c e . N e g a t i v e i o n s d e r i v e d from methane o r i s o b u t a n e are not o b s e r v e d i n t h i s mode o f o p e r a t i o n .


8.

HUNT

5

PPN1-C1

POS EM

RODS RODS

U

ooooo*


163


A N D SETHI

-•1600V I600V

H ~I

- - -++ + - -

^

^ „ „ u ,i LB 0

NE G EM •V 1 6 0 0 V

2 ION "^LENS SOURCE +I0V + 4-10 V P U L S E RATE

|0 KHz Analytical Chemistry

Figure 5. Pulsed positive negative ion chemical ionization (PPNICI) mass spectrometer: FIL-filament, EM-electron mul tiplier, and LBO-Hght beam oscillograph

1001

28.

χ 50

-

50j

6J> 100

Φ Ώ

d ζ 3

ιοω —

ο ζ 3

ι

ω
10,000) i n p a r t t o r e s o l v e sample i o n s and r e f e r e n c e i o n s produced s i m u l t a n e o u s l y . The a c c u r a t e mass o f t h e sample i o n i s t h e n c a l c u l a t e d by e x t r a p o l a t i o n from the measured mass o f a nearby r e f erence i o n . Drawbacks o f t h e above methodology i n c l u d e the h i g h c o s t o f the n e c e s s a r y i n s t r u m e n t a t i o n , l o w s e n s i t i v i t y w h i c h always accompanies o p e r a t i o n a t h i g h r e s o l u t i o n , sample i o n s u p p r e s s i o n due t o t h e l a r g e q u a n t i t i e s o f r e f e r e n c e compound r e q u i r e d t o produce abundant r e f e r e n c e i o n s a t h i g h mass, and t h e d i f f i c u l t y i n u s i n g the method f o r GC-MS a n a l y s i s due t o t h e s l o w s c a n speeds u s u a l l y r e q u i r e d t o m a i n t a i n adequate ion current a t high r e s o l u t i o n . To overcome t h i s p r o b l e m , A s p i n a l eit a l . ( 3 2 ) , i n 1975, d e v e l o p e d a t e c h n i q u e f o r o b t a i n i n g a c c u r a t e mass measurements a t low r e s o l u t i o n u s i n g a d o u b l e beam m a g n e t i c s e c t o r mass s p e c t r o m e t e r . I n t h i s method two p o s i t i v e i o n beams, one c o n t a i n i n g i o n s from t h e i n t e r n a l s t a n d a r d and one beam c o n t a i n i n g i o n s from t h e sample a r e p r o d u c e d i n two s e p a r a t e i o n s o u r c e s , p a s s e d t h r o u g h the same m a g n e t i c f i e l d s i m u l t a n e o u s l y , and c o l l e c t e d s e p a r a t e l y a t two e l e c t r o n m i l t i p l i e r s . A c c u r a t e mass measurements (

High Performance Mass Spectrometry: Chemical Applications

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