7 Introduction to Gas Chromatography/High Resolution Mass Spectrometry B. J. KIMBLE Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
University of California-Berkeley, Berkeley, CA 94720
The combination of a gas chromatograph (GC), a mass spectrometer (MS) and an on-line computer is now well-established as an exceptionally powerful and versatile tool for identifying and quantitating the components present in organic mixtures. As with any successful technique, many improvements have been incorporated in recent years in order to provide the maximum amount of information that can be derived from a GC/MS analysis. Significant developments include the use of: a) glass capillary columns (1-3), for minimized surface effects and increased chromatographic resolution; b) direct interfacing of capillary columns to the mass spectrometer ion source (4-7), to reduce sample losses in a molecular separator and to preserve chromatographic resolution by elimi nating large-volume connectors; c) high pressure ionization methods (e.g., chemical ionization, 8-14), to provide complementary in formation to assist in the interpretation of electron impact spectra; d) improved computer techniques, for automated data acquisition, data handling (15-17), spectrum recognition (18-23) and interpretation (24-26). An a d d i t i o n a l approach f o r t h e improved u t i l i t y o f t h e GC/MS t e c h n i q u e would be t o d e t e r m i n e t h e r e c o r d ed mass v a l u e s i n each spectrum w i t h s u f f i c i e n t a c c u r a c y t o be a b l e t o compute t h e e x a c t e l e m e n t a l comp o s i t i o n o f each o f t h e i o n s p e c i e s c o n t r i b u t i n g t o t h e spectrum (27) . Such an achievement would p r o v i d e t h e maximum amount o f s t r u c t u r a l i n f o r m a t i o n t h a t c a n be d i r e c t l y d e r i v e d from a mass spectrum — i . e . , a d a t a ©0-8412-0422-5/78/47-070-120$10.00/0
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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s e t c o m p r i s e d o f e x a c t e l e m e n t a l c o m p o s i t i o n s and t h e i r r e l a t i v e abundances. The r o u t i n e a v a i l a b i l i t y o f e l e m e n t a l c o m p o s i t i o n s p e c t r a from a GC/MS a n a l y s i s would s u b s t a n t i a l l y a s s i s t t h e i n t e r p r e t e r / a n a l y s t i n s e v e r a l ways, a l l o f w h i c h d e r i v e from t h e i n c r e a s e d s t r u c t u r a l s p e c i f i c i t y i n h e r e n t i n such d a t a . I n t h e case o f mass s p e c t r a which a r e , i n f a c t , interprétable by a t r a i n e d spect r o m e t r i s t , exact elemental composition i n f o r m a t i o n would e l i m i n a t e t h e guesswork i n v o l v e d i n r e l a t i n g mass v a l u e s and mass d i f f e r e n c e s t o s t r u c t u r a l u n i t s , and t h u s , would r e s u l t i n more r a p i d and r e l i a b l e i n t e r pretations. T h i s i s p a r t i c u l a r l y t r u e when t h e r e c o r d e d spectrum r e s u l t s from t h e c o - e l u t i o n o f two o r more components, and t h e i n t e r p r e t i v e p r o c e s s has t o i n c l u d e t h e c o r r e c t assignment o f masses i n t o t h e i r appropriate "sub-spectra". Elemental composition i n f o r m a t i o n would a l s o f a c i l i t a t e t h e l o c a t i o n and q u a n t i t a t i o n o f s p e c i f i c components i n m i x t u r e s , s i n c e the i n t e n s i t i e s f o r a p a r t i c u l a r e l e m e n t a l c o m p o s i t i o n c o u l d be p l o t t e d as a f u n c t i o n o f scan number; such an e l e m e n t a l c o m p o s i t i o n chromatogram would c o n s t i t u t e a more s p e c i f i c s t r u c t u r a l i n d i c a t o r t h a n do n o m i n a l mass chromatograms. Of p a r t i c u l a r u t i l i t y , e l e m e n t a l c o m p o s i t i o n s p e c t r a would p r o v i d e d i r e c t s t r u c t u r a l i n f o r m a t i o n i n t h e case o f " h a r d - t o - i n t e r p r e t " s p e c t r a o f t r u l y unknown components — t h a t i s , when l i t t l e o r no c h e m i c a l information i s a v a i l a b l e to a s s i s t i n the i n t e r p r e t a t i o n . A l s o , unexpected compounds c a n a r i s e i n a v a r i e t y o f s i t u a t i o n s as a r t i f a c t s o r c o n t a m i n a n t s , o r due t o i n a d e q u a t e f r a c t i o n a t i o n and/or d e r i v a t i z a t i o n p r o c e d u r e s , e t c . ; and o f c o u r s e , " d i f f i c u l t " s p e c t r a occur p a r t i c u l a r l y i n analyses o f mixtures c o n t a i n i n g s e v e r a l hundred components. W i t h t o d a y s i n c r e a s i n g need t o s t u d y more and more c o m p l i c a t e d samples ( p a r t i c u l a r l y e n v i r o n m e n t a l m i x t u r e s such as sewage e f f l u e n t s , and samples f o r w h i c h m i n i m a l s e p a r a t i o n and f r a c t i o n a t i o n steps are d e s i r e d ) , i t i s t h i s aspect o f complex m i x t u r e a n a l y s i s t h a t would p a r t i c u l a r l y benef i t from t h e a v a i l a b i l i t y o f e l e m e n t a l c o m p o s i t i o n spectra. To d a t e , t h e concept o f e l e m e n t a l c o m p o s i t i o n assignments i n GC/MS a n a l y s e s has been e x p l o r e d i n a v a r i e t y o f ways, and a p p l i c a t i o n s o f t h e t e c h n i q u e seem l i k e l y t o i n c r e a s e as s u i t a b l e commercial i n s t r u m e n t a t i o n becomes more w i d e l y a v a i l a b l e . U n f o r t u n a t e l y , a somewhat ambiguous t e r m i n o l o g y has a l r e a d y e v o l v e d conc e r n i n g use o f t h e p h r a s e s " a c c u r a t e mass measurement" and, i n p a r t i c u l a r , " h i g h r e s o l u t i o n " . In general, 1
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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t h e s e terms a r e used t o c o n t r a s t w i t h t h e measurement o f n o m i n a l masses ( e . g . , m/e 100 o r 101) and i n s t r u m e n t o p e r a t i o n a t n o m i n a l mass r e s o l u t i o n ( e . g . , Μ/ΔΜ < 1,000); however, i t s h o u l d be r e c o g n i z e d t h a t i n p r a c t i c e t h e s o - c a l l e d a c c u r a t e mass measurements may n o t be p a r t i c u l a r l y " a c c u r a t e " , n o r t h e mass r e s o l u t i o n p a r t i c u l a r l y " h i g h " . F o r s i m p l i c i t y , t h e acronyms GC/LRMS and GC/HRMS w i l l be used here t o denote GC/MS a n a l y s e s i n v o l v i n g t h e c o n t i n u o u s r e c o r d i n g o f complete mass s p e c t r a a t n o m i n a l (low) r e s o l u t i o n and h i g h e r mass r e s o l u t i o n s r e s p e c t i v e l y . I t s h o u l d a l s o be n o t e d t h a t t h e terms " a c c u r a t e mass measurement" and " h i g h r e s o l u t i o n " a r e n o t synonymous; t h i s may be i l l u s t r a t e d by summarizing t h e approaches t h a t have been r e p o r t e d i n the l i t e r a t u r e concerning the use o f elemental c o m p o s i t i o n i n f o r m a t i o n i n GC/MS a n a l y s e s . E l e m e n t a l c o m p o s i t i o n m o n i t o r i n g o f GC e f f l u e n t s . I n t h i s approach, t h e o b j e c t i v e i s t o u t i l i z e t h e mass s p e c t r o m e t e r as a h i g h l y s t r u c t u r a l l y s p e c i f i c GC d e t e c t o r f o r q u a n t i t a t i v e s t u d i e s , p a r t i c u l a r l y when the compound o f i n t e r e s t i s p r e s e n t i n a h i g h l y complex m i x t u r e (28-35). The mass s p e c t r o m e t e r i s o p e r a t e d a t some i n c r e a s e d mass r e s o l u t i o n ( e . g . , 10,000) w i t h a s i n g l e a c c u r a t e mass ( c h a r a c t e r i s t i c o f t h e components o f i n t e r e s t ) f o c u s s e d c o n t i n u o u s l y on t h e d e t e c t o r . The i n c r e a s e d mass r e s o l u t i o n aims t o reduce t h e i n t e r f e r e n c e s from o t h e r i o n s h a v i n g t h e same n o m i n a l mass but o f a d i f f e r e n t e l e m e n t a l c o m p o s i t i o n , thus en h a n c i n g compound s p e c i f i c i t y w i t h r e s p e c t t o c o - e l u t i n g components from t h e gas chromatograph, i n s t r u m e n t back ground, column b l e e d , e t c . E l e m e n t a l c o m p o s i t i o n assignments from s p e c t r a r e c o r d e d a t n o m i n a l mass r e s o l u t i o n . Here, t h e masses o f the r e c o r d e d peaks i n each spectrum a r e d e t e r m i n e d w i t h g r e a t e r t h a n n o m i n a l mass a c c u r a c y , and e l e m e n t a l c o m p o s i t i o n s a r e computed from t h e mass measurements (36; see a l s o 37, 3 8 ) . However, as t h e mass r e s o l u t i o n i s n o t i n c r e a s e ? , problems a r i s e i f t h e nominal mass r e s u l t s from i o n s o f more t h a n one e l e m e n t a l compo sition i . e . , i f t h e peak i s a c t u a l l y an u n r e s o l v e d doublet or m u l t i p l e t . This disadvantage represents a p a r t i c u l a r problem i n t h e a n a l y s i s o f complex m i x t u r e s when t h e components o f t h e m i x t u r e may n o t be chromat o g r a p h i c a l l y r e s o l v e d , and more c o m p l i c a t e d s p e c t r a a r e o f t e n produced. The u s e o f a double-beam mass s p e c t r o m e t e r f o r GC/MS a n a l y s e s (39^-41) i n which a mass c a l i b r a n t and t h e sample a r e a n a l y s e d s e p a r a t e l y but s i m u l t a n e o u s l y u s i n g two s e p a r a t e i o n s o u r c e s , a
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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common mass a n a l y z e r and two s e p a r a t e d e t e c t o r s s h o u l d improve mass measurement a c c u r a c y b u t does n o t s o l v e the problem o f inadequate r e s o l u t i o n o f doublet o r m u l t i p l e t masses i n t h e sample s p e c t r a . A n o t h e r t e c h n i q u e , w h i c h u t i l i z e s a q u a d r u p o l e mass s p e c t r o meter t o " s i m u l t a n e o u s l y p r o d u c e p o s i t i v e and n e g a t i v e i o n s p e c t r a , may a l s o be d e v e l o p e d f o r a c c u r a t e mass measurement i n GC/MS s t u d i e s (£2).
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1 1
E l e m e n t a l c o m p o s i t i o n assignment from s p e c t r a r e c o r d e d a t i n c r e a s e d mass r e s o l u t i o n . I n t h i s c a s e , t h e aim i s t o c o n t i n u o u s l y r e c o r d complete mass s p e c t r a t h r o u g h o u t a c h r o m a t o g r a p h i c a n a l y s i s a t a s u f f i c i e n t l y h i g h mass r e s o l u t i o n t o e l i m i n a t e u n r e s o l v e d peaks and w i t h s u f f i c i e n t mass measurement a c c u r a c y t o e x a c t l y d e t e r m i n e a l l o f t h e e l e m e n t a l c o m p o s i t i o n s p r e s e n t i n each spec trum. I n p r i n c i p l e , t h i s GC/HRMS method c a n be under t a k e n u s i n g mass s p e c t r o m e t e r s equipped f o r e i t h e r p h o t o p l a t e d e t e c t i o n (43-45) o r e l e c t r i c a l d e t e c t i o n (44-51, see a l s o 5 2 ) . I n p r a c t i c e , however, i t i s t h e l a t t e r method (which p a r a l l e l s n o m i n a l mass GC/MS o p e r a t i o n ) w h i c h has t h e p o t e n t i a l f o r p r o v i d i n g t h e maximum f l e x i b i l i t y and speed i n terms o f c o m p u t e r i z e d o p e r a t i o n , r e p e t i t i v e spectrum a c q u i s i t i o n , l a r g e spectrum c a p a c i t y , and t h e g e n e r a t i o n o f t o t a l i o n i z a t i o n and mass chromatograms. I t i s t h i s GC/HRMS approach u t i l i z i n g magnetic s c a n n i n g and e l e c t r i c a l d e t e c t i o n w h i c h w i l l be d i s c u s s e d more f u l l y h e r e . I t i s not intended t o describe d e t a i l s o f i n s t r u m e n t d e s i g n and s e t - u p p r o c e d u r e s , b u t r a t h e r t o present f o r the chemist/user the p r i n c i p a l p a r a m e t e r s i n v o l v e d , and t h e i r r e q u i r e m e n t s , l i m i t a t i o n s and i n t e r - r e l a t i o n s h i p s ; i t i s t h i s knowledge t h a t becomes i m p o r t a n t i n p r a c t i c a l terms f o r t h e e f f e c t i v e and e f f i c i e n t i n t e r p r e t a t i o n o f GC/HRMS d a t a . Background The p r i m a r y parameters o f i n t e r e s t i n o b t a i n i n g e l e m e n t a l c o m p o s i t i o n s p e c t r a a r e mass measurement a c c u r a c y (a measure o f a b i l i t y t o a c c u r a t e l y d e t e r m i n e the t r u e mass) and dynamic mass r e s o l u t i o n ( t h e degree t o w h i c h masses c a n be r e s o l v e d under magnetic s c a n n i n g conditions). I n v e s t i g a t i o n o f t h e s e parameters de pends, o f c o u r s e , on t h e s p e c i f i c mode o f s p e c t r o m e t e r and computer o p e r a t i o n ; t h e d i s c u s s i o n h e r e i s based on d e v e l o p m e n t a l GC/HRMS s t u d i e s u n d e r t a k e n u s i n g a modi f i e d ΑΕΙ MS-902 d o u b l e - f o c u s s i n g mass s p e c t r o m e t e r t o g e t h e r w i t h o n - l i n e , r e a l - t i m e d a t a a c q u i s i t i o n and r e d u c t i o n (47-49, 5 3 ) .
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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I n o u t l i n e , a spectrum i s produced by exponen t i a l l y s c a n n i n g over the mass range from h i g h mass t o low mass. The r e s u l t i n g a n a l o g s i g n a l from the de t e c t o r a m p l i f i e r i s d i g i t i z e d a t e q u a l time i n t e r v a l s and the d i g i t a l v a l u e s f o r each peak d e t e c t e d above a c o n s t a n t p r e s e t t h r e s h o l d v a l u e a r e saved f o r a n a l y s i s , t o g e t h e r w i t h the a b s o l u t e time o f o c c u r r e n c e o f t h e peak w i t h r e s p e c t t o the s t a r t o f the scan (54). The d i g i t a l v a l u e s f o r each peak d e f i n e the peak p r o f i l e , from w h i c h the peak i n t e n s i t y i s d e t e r m i n e d from i t s a r e a (5J5, the sum o f t h e d i g i t a l v a l u e s f o r t h a t peak) and the peak p o s i t i o n i n time i s d e t e r m i n e d from t h e t i m e computed f o r the peak c e n t e r - o f - g r a v i t y o r cent r o i d (54). Mass/time f u n c t i o n . In the i d e a l c a s e , t h e r e f o r e , t h e mass (M) o f any peak i s an e x p o n e n t i a l f u n c t i o n o f t h e e l a p s e d time ( t ) from the s t a r t o f the s c a n ( F i g u r e 1 ) . That i s , M « "
t / T
(1)
e
,f
where τ i s t h e t i m e c o n s t a n t " o f the e x p o n e n t i a l f u n c t i o n , i . e . , t h e time t a k e n t o scan between any mass, M , and the mass M /e (where e=2.718). At the s t a r t o f the s c a n , t = 0 and M i s e q u a l t o the maximum mass value, M , so t h a t e q u a t i o n (1) becomes x
x
m a x
M = M
max
e"
t / x v
(2) J
S c a n r a t e . The s c a n r a t e i n t h e commonly quoted u n i t s o f seconds/mass decade (a mass decade c o v e r s the range Μ t o M /10) can be d e r i v e d from t h i s e q u a t i o n by con s i d e r i n g t h e t i m e d i f f e r e n c e ( t - t i ) f o r two masses Mi and Mi/10: χ
x
2
M
>
M
e
• max "
t l / T
a n d
M
'
-
M
max
β
'*
ΐ Λ
These e q u a t i o n s combine t o g i v e t -ti
= τ l o g 10 = τ (3) G 10 where τ i s the time t o scan one mass decade, i . e . , the s c a n r a t e i n seconds/decade. 2
1 0
R e s o l u t i o n . As n o r m a l l y d e f i n e d , the mass r e s o l u t i o n , R, a t 10% v a l l e y s e p a r a t i o n o f two e q u a l mass peaks i s g i v e n by
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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Resolution Mass Spectrometry
= Μ/ΔΜ
(4)
where M i s t h e n o m i n a l mass and ΔΜ i s t h e a c c u r a t e mass d i f f e r e n c e f o r t h e two mass peaks ( F i g u r e 2 ) . The time d i f f e r e n c e , A t , between t h e maxima o f t h e s e two peaks can be d e t e r m i n e d from t h e r a t e o f change o f mass w i t h time (from e q u a t i o n 2) g i v i n g -M
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d
M
= —5**
. '
t / T
e
·dt
which approximates t o τ f o r s m a l l v a l u e s o f ΔΜ and A t . (2) g i v e s AM -At M ~T~
D i v i d i n g by e q u a t i o n
and u s i n g e q u a t i o n ( 4 ) , t h i s reduces t o At = " I
(5)
From symmetry c o n s i d e r a t i o n s , t h e time d i f f e r e n c e , A t , between t h e peak maxima i s e q u a l t o t h e time t a k e n t o scan a c r o s s one o f t h e s e peaks between t h e p o i n t s w h i c h a r e a t 5% o f t h e maximum peak h e i g h t ( F i g u r e 2 ) . Thus, i f τ and At a r e known, then t h e r e s o l u t i o n c a n be c a l c u l a t e d f o r each s i n g l e peak t h r o u g h o u t t h e mass range. A l s o , e q u a t i o n ( 5 ) shows t h a t f o r c o n s t a n t mass r e s o l u t i o n , R , and s c a n r a t e τ (=τ l o g 1 0 ) , t h e w i d t h s o f a l l s i n g l e t peaks measured a t 5 1 o f r h e i r maximum h e i g h t s h o u l d be c o n s t a n t . E q u a t i o n s 3 and 5 , combined t o g i v e 1 0
t =
- T
1
0
/ R
log
e
10 ,
(6)
p e r m i t v a r i o u s i n s t r u m e n t parameters t o be c a l c u l a t e d and compared w i t h e x p e r i m e n t a l r e s u l t s . F o r example, f o r a c o n s t a n t r e s o l u t i o n (10% v a l l e y ) o f 10,000 and a s c a n r a t e o f 6 seconds/decade, t h e p e a k w i d t h f o r a l l masses a t t h e i r 5% l e v e l s i s g i v e n by At =
s e c = 260.6 ysec ΙΟ* χ 2.3026
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
PERFORMANCE
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HIGH
ρ _ M . M _ " M -M, ΔΜ ~ 2
Τ
At
Figure 2. Mass resolution (10% valley defi nition)
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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Resolution Mass Spectrometry
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For a d i g i t i z a t i o n r a t e o f 50 kHz, t h e d a t a p o i n t s a r e s e p a r a t e d i n time u n i t s o f 1/50,000 s e c o r 20 y s e c , so t h a t i n t h i s example, t h e w i d t h s o f a l l s i n g l e t peaks s h o u l d be d e f i n e d by 260/20 = 13 d a t a p o i n t s between t h e i r 5% l e v e l s . Mass measurement a c c u r a c y . I t i s t h e a b i l i t y t o mea sure masses i n a spectrum w i t h h i g h a c c u r a c y t h a t i s the b a s i s f o r d e t e r m i n i n g t h e c o r r e c t e l e m e n t a l com p o s i t i o n s . As p e r f e c t l y e x a c t mass measurements cannot be a c h i e v e d i n p r a c t i c e , t h e approach used t o g e n e r a t e a h i g h r e s o l u t i o n spectrum i s t o compute, f o r each measured mass, a l i s t o f p o s s i b l e e l e m e n t a l compo s i t i o n s (56) whose a c c u r a t e masses f a l l w i t h i n a s m a l l mass "wincfow" around each measured mass. I n o r d e r t o r e s t r i c t t h e e l e m e n t a l c o m p o s i t i o n l i s t i n g t o somewhat manageable p r o p o r t i o n s , t h e u s e r i s r e q u i r e d t o s e l e c t the t y p e s and maximum numbers o f each element t h a t he c o n s i d e r s r e l e v a n t on c h e m i c a l grounds. He i s a l s o r e q u i r e d t o i n p u t t h e " w i d t h " o f t h e mass window t o be used f o r t h e c a l c u l a t i o n s i . e . , t h e measured mass ± 6ppm, o r a mass window o f 26. The v a l u e o f 6 must be chosen c a r e f u l l y so t h a t the c o r r e c t e l e m e n t a l c o m p o s i t i o n f o r a p a r t i c u l a r measured mass i s i n c l u d e d i n t h e l i s t o f p o s s i b i l i t i e s . I f t h e chosen v a l u e o f 6 i s t o o s m a l l , t h e n t h e c o r r e c t c o m p o s i t i o n may n o t be l i s t e d ; however, i f t h e chosen v a l u e o f δ i s s i g n i f i c a n t l y l a r g e r than necessary, then the r e s u l t i n g l i s t o f c o m p o s i t i o n s r a p i d l y becomes t o o u n w i e l d y t o e v a l u a t e . The s e l e c t i o n o f t h e a p p r o p r i a t e v a l u e o f 6 f o r any p a r t i c u l a r s e t o f i n s t r u m e n t oper a t i n g parameters depends d i r e c t l y on a knowledge o f t h e mass measurement a c c u r a c y d e t e r m i n e d e x p e r i m e n t a l l y under t h o s e c o n d i t i o n s . The a t t a i n m e n t o f h i g h mass measurement a c c u r a c y i s dependent upon many f a c t o r s (57-60) such as adequate d e f i n i t i o n o f a c t u a l peak shapes, tTïë absence o f abnormal peakshapes, d e f i n i t i o n o f t h e a c t u a l mass/time f u n c t i o n , s o f t w a r e a l g o r i t h m s f o r mass c a l c u l a t i o n , and mass r e s o l u t i o n . The dependency on mass r e s o l u t i o n r e l a t e s t o t h e p r e s e n c e o f u n r e s o l v e d peak d o u b l e t s o r m u l t i p l e t s ( F i g u r e 3 ) ; f o r these peakshapes, t h e p o s i t i o n o f t h e c e n t e r - o f - g r a v i t y o f t h e o v e r a l l peak p r o f i l e i s i n a c c u r a t e , r e s u l t i n g i n i n a c c u r a t e mass measurements f o r t h e i o n s p e c i e s a c t u a l l y p r e s e n t . I n g e n e r a l , improved mass measurements c a n be o b t a i n e d by a v e r a g i n g t h e measurements from s e v e r a l s p e c t r a ( 6 1 , 6 2 ) . T h i s p r o c e s s r e d u c e s t h e e f f e c t o f t h e random e r r o r s a s s o c i a t e d w i t h a c c u r a t e mass d e t e r m i n a t i o n s , and, t h u s , a s m a l l e r mass window c a n be used i n t h e
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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generation o f p o s s i b l e elemental compositions f o r averaged mass measurements. S t a t i s t i c a l l y , the mass measurement a c c u r a c y would be e x p e c t e d t o be improved by a f a c t o r o f /n where η i s t h e number o f measurements averaged f o r a p a r t i c u l a r a c c u r a t e mass.
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P r a c t i c a l C o n s i d e r a t i o n s and t h e E v a l u a t i o n o f Instrument Performance? GC and f a s t - s c a n n i n g HRMS. I n p r a c t i c a l t e r m s , t h e a b i l i t y t o d e t e r m i n e a c c u r a t e mass measurements f o r s p e c t r a r e c o r d e d from c h r o m a t o g r a p h i c e f f l u e n t s c e n t e r s on the performance o f t h e f a s t - s c a n n i n g h i g h r e s o l u t i o n mass s p e c t r o m e t e r and i t s d a t a system (57, 63^-67). As f o r GC/LRMS, t h e gas chromatograph and GC/MS i n t e r f a c e may be o f v a r i o u s d e s i g n s p r o v i d e d t h a t : a) the i n c r e a s e d p r e s s u r e i n t h e i o n s o u r c e due t o the c a r r i e r gas p e r m i t s o p e r a t i o n o f the mass spectrometer ; b) each c h r o m a t o g r a p h i c peak i s o f s u f f i c i e n t d u r a t i o n so t h a t s p e c t r a w i l l be r e c o r d e d f o r each component; c) a s u f f i c i e n t amount o f each component i s i n t r o duced v i a t h e GC i n l e t system i n t o t h e i o n s o u r c e so t h a t r e c o g n i z a b l e s p e c t r a c a n be recorded. As t h e s e d e s i g n c o n s i d e r a t i o n s have been t h e s u b j e c t o f many p u b l i c a t i o n s ( e . g . , 6 8 ) , o n l y the most i m p o r t a n t a s p e c t s f o r GC/HRMS w i l lïïe"d i s c u s s e d f u r t h e r h e r e . The main parameters f o r GC/HRMS o p e r a t i o n a r e summarized i n T a b l e 1; many o f t h e s e p a r a m e t e r s , o f c o u r s e , a l s o p e r t a i n t o GC/LRMS a n a l y s e s , b u t they become somewhat more c r i t i c a l f o r t h e c o n c o m i t a n t measurement o f a c c u r a t e masses under f a s t - s c a n n i n g h i g h r e s o l u t i o n c o n d i t i o n s . The t a b l e a l s o i n d i c a t e s t h e p r i m a r y optimum r e q u i r e m e n t s f o r each parameter. F o r s e v e r a l o f these, there are c o n f l i c t i n g requirements due t o t h e i n t e r - r e l a t i o n s h i p s t h a t e x i s t between t h e d i f f e r e n t p a r a m e t e r s , and t h e a c t u a l v a l u e s u t i l i z e d f o r a n a l y s e s w i l l n e c e s s a r i l y be a s e t o f v a l u e s t h a t a c h i e v e t h e optimum compromise. Scan c y c l e t i m e . The o v e r a l l c y c l e time f o r r e c o r d i n g a s i n g l e spectrum i s a f u n c t i o n o f the s c a n r a t e , t h e mass r a n g e , and t h e time r e q u i r e d t o r e s e t the magnetic f i e l d between t h e minimum and maximum mass v a l u e s . As d e s c r i b e d above, t h e t h e o r e t i c a l s c a n r a t e , τ , i s a c o n s t a n t and i s d i r e c t l y p r o p o r t i o n a l t o t h e time c o n s t a n t o f t h e e x p o n e n t i a l f u n c t i o n , τ. I n p r a c t i c e , 1 0
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
7.
KIMBLE
Gas Chromatography/High Resolution Mass Spectrometry
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T a b l e 1,
129
P r i m a r y C o n s i d e r a t i o n s f o r GC/HRMS
Parameter
Requirement
mass measurement accuracy
maximize t o l i m i t number o f p o s s i b l e e l e m e n t a l comp o s i t i o n s generated
dynamic mass resolution
maximize t o reduce number o f u n r e s o l v e d p e a k s , hence i n c r e a s e mass measurement accuracy minimize t o increase sensitivity
sensitivity
maximize t o reduce sample quantity requirements
scan t i m e ( f u n c t i o n of mass range and s c a n r a t e )
m i n i m i z e scan time t o p r e s e r v e chromatographic r e s o l u t i o n i n t o t a l i o n i z a t i o n and mass chromatograms maximize mass range f o r u s e f u l information maximize s c a n r a t e (sec/decade) for increased s e n s i t i v i t y and mass measurement accuracy
magnet r e s e t t i m e
m i n i m i z e t o reduce "wasted" t i m e and p r e s e r v e chromatographic resolution i n t o t a l i o n i z a t i o n and mass chromatograms
GC peak e l u t i o n time
m i n i m i z e f o r h i g h e r chromatographic r e s o l u t i o n maximize w i t h r e s p e c t t o s c a n c y c l e time ( i . e . , s c a n time + magnet r e s e t time) to ensure s u f f i c i e n t time f o r r e c o r d i n g mass s p e c t r a
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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the mass/time f u n c t i o n i s n o t p e r f e c t o v e r t h e whole mass range; t h a t i s , τ i s n o t c o n s t a n t b u t v a r i e s w i t h mass as i l l u s t r a t e d i n F i g u r e 4. The use o f an i n t e r n a l mass s t a n d a r d such as p e r f l u o r o k e r o s i n e ( P F K ) , w h i c h c o n t r i b u t e s peaks o f known a c c u r a t e mass t h r o u g h out t h e mass r a n g e , p r o v i d e s a means f o r d e t e r m i n i n g the a c t u a l mass/time f u n c t i o n over s m a l l s e c t i o n s o f the mass range. Thus, a v a l u e o f τ can be a s s i g n e d t o each o f t h e c a l i b r a t i o n masses, and from each o f t h e s e v a l u e s t h e measured s c a n r a t e , τ , i n seconds/decade can be c a l c u l a t e d from e q u a t i o n 3. The v a r i a t i o n o b s e r v e d f o r a n o m i n a l s c a n r a t e o f 6 seconds/decade f o l l o w s t h e p r o f i l e f o r τ shown i n F i g u r e 4, and ranges between about 6.5 seconds/decade a t m/e 580 and 5.5 seconds/decade a t m/e 80; t h e a c t u a l o v e r a l l s c a n t i m e between m/e 800 and m/e 65 i s about 7.2 seconds (48, 49). I t s h o u l d be n o t e d t h a t t h e upper s e c t i o n oF~~the mass range (~ m/e 800-650) i s n o t e a s i l y u s a b l e f o r ac c u r a t e mass a s s i g n m e n t s due t o t h e low i n t e n s i t y o f t h e PFK c a l i b r a t i o n masses i n t h i s r e g i o n , and t h e r a p i d r a t e o f change o f τ i n t h i s r e g i o n p r e c l u d e s a c c u r a t e e x t r a p o l a t i o n o f τ above t h e h i g h e s t r e l i a b l e c a l i b r a t i o n mass. The t i m e t a k e n t o r e t u r n t o t h e i n i t i a l maximum mass v a l u e f o r t h e n e x t scan ( t h e magnet r e s e t t i m e ) s h o u l d , o f c o u r s e , be m i n i m i z e d as f a r as p o s s i b l e t o r e d u c e "wasted" t i m e and m i n i m i z e t h e o v e r a l l scan c y c l e t i m e . As r e p o r t e d p r e v i o u s l y (£8), magnet c i r c u i t m o d i f i c a t i o n s u n d e r t a k e n on a MS-902 mass spec t r o m e t e r have p e r m i t t e d t h i s t i m e t o be r e d u c e d t o 2.4 seconds w h i l s t m a i n t a i n i n g mass c a l i b r a t i o n up t o - m/e 650 f o r t h e subsequent s c a n . W i t h t h e s e i n s t r u m e n t p a r a m e t e r s , t h e r e f o r e , t h e o v e r a l l scan c y c l e t i m e ( i . e . , s c a n t i m e + magnet r e s e t time) i s j u s t under 10 seconds.
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1 0
GC peak e i u t i o n t i m e . The v a l u e f o r t h e o v e r a l l scan c y c l e t i m e i s i m p o r t a n t f o r t h e a d j u s t m e n t o f t h e gas c h r o m a t o g r a p h i c o p e r a t i n g parameters ( e . g . , f l o w r a t e , column t e m p e r a t u r e and programming r a t e ) . In order to e n s u r e t h a t mass s p e c t r a can be r e c o r d e d f o r any GC peak, t h e t i m e f o r e i u t i o n o f a peak has t o be a d j u s t e d so t h a t i t i s a t l e a s t t w i c e t h e scan c y c l e t i m e . As i l l u s t r a t e d i n F i g u r e 5, t h e minimum f a c t o r o f two d e r i v e s from t h e unknown time r e l a t i o n s h i p between t h e s t a r t o f a s c a n and t h e commencement o f e i u t i o n o f a GC peak. I t i s n e c e s s a r y , t h e r e f o r e , i n t h i s c a s e , t o ensure t h a t a l l c h r o m a t o g r a p h i c peaks e l u t e i n t o t h e mass s p e c t r o m e t e r i o n s o u r c e over a time p e r i o d o f a t
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
KIMBLE
Gas Chromatography/High
Resolution Mass Spectrometry
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Mass A
Centroid of Mass A
Figure 3. Effect of inadequate mass resolution on mass measurement ac curacy
Figure 4. The variation of τ as a function of mass
Figure 5. The extreme relationships between scan cycle time and GC peak eiution time; scan cycle time equals one-half of the GC peak eiution time
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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l e a s t 20 seconds so t h a t a t l e a s t one u s a b l e spectrum can be r e c o r d e d . The u s e o f a GC peak e i u t i o n time o f j u s t t w i c e the scan c y c l e time r e s u l t s i n mass chromatograms which show a v a r i e t y o f p r o f i l e s from t h e same GC peak ( s e e F i g u r e 6 ) . S i m i l a r f e a t u r e s may a l s o be seen w i t h t o t a l i o n i z a t i o n chromatograms, which c a n c o m p l i c a t e the matching o f such o u t p u t w i t h gas chromatograms r e c o r d e d by a flame i o n i z a t i o n d e t e c t o r ; q u a n t i t a t i v e comparisons a r e a l s o more d i f f i c u l t t o u n d e r t a k e w i t h such v a r i a t i o n s i n peak h e i g h t s . The scan c y c l e t i m e , t h e r e f o r e , d i r e c t l y l i m i t s t h e o v e r a l l gas chromato g r a p h i c performance t h a t c a n be u t i l i z e d e f f e c t i v e l y . Mass measurement a c c u r a c y . The a b i l i t y t o a c c u r a t e l y measure masses under GC/HRMS c o n d i t i o n s i s , o f c o u r s e , the o b j e c t o f t h e e x e r c i s e . However, i t i s t h e i n t r o d u c t i o n o f t h e sample v i a a gas chromatograph t h a t p r e s e n t s major problems f o r d e t e r m i n i n g a c c u r a t e mas s e s . Because o f t h e s h o r t r e s i d e n c e time o f each component i n t h e i o n s o u r c e , a w e l l - d e f i n e d peak p r o f i l e f o r each mass has t o be d e t e r m i n e d i n a v e r y s h o r t time (~ 260 psec a t a s c a n r a t e o f 6 seconds/decade and a r e s o l u t i o n o f 10,000). A l s o , o n l y one o r two r e a s o n a b l y i n t e n s e s p e c t r a c a n be e x p e c t e d t o be r e c o r d e d from each component. T h i s v i r t u a l l y e l i m i n a t e s t h e a b i l i t y t o average the measured masses t o improve a c c u r a c y , and, t h e r e f o r e , i t i s n e c e s s a r y t h a t h i g h a c c u r a c y measurements s h o u l d be r e l i a b l y produced f o r each i n d i v i d u a l spectrum. The l a r g e volume o f d a t a g e n e r a t e d from a GC/HRMS a n a l y s i s a l s o demands a h i g h degree o f r e l i a b i l i t y as d e t a i l e d i n v e s t i g a t i o n s i n t o anomalous f e a t u r e s a r e e x t r e m e l y time-consuming. I n a d d i t i o n t o t h e g e n e r a t i o n o f e l e m e n t a l com p o s i t i o n l i s t i n g s f o r measured a c c u r a t e masses, a d e t a i l e d knowledge o f a t t a i n a b l e mass measurement a c c u r a c y i s a l s o i m p o r t a n t f o r GC/HRMS s t u d i e s i n o r d e r t o g e n e r a t e a c c u r a t e mass chromatograms ( o r e l e m e n t a l c o m p o s i t i o n chromatograms) t o l o c a t e t h e e i u t i o n times o f s p e c i f i c components o r compound c l a s s e s f o r f u r t h e r d e t a i l e d s t u d y (£7). I n t h i s c a s e , the u s e r s e l e c t s the e l e m e n t a l c o m p o s i t i o n o f i n t e r e s t and t h e c o r r e s p o n d i n g a c c u r a t e mass i s c a l c u l a t e d . An a p p r o p r i a t e mass window (δ) i s s p e c i f i e d and, f o r each spectrum, the i n t e n s i t y o f t h e a c c u r a t e mass which i s l o c a t e d w i t h i n t h i s s p e c i f i e d mass window around t h e c a l c u l a t e d mass, i s saved. The i n t e n s i t i e s p l o t t e d as a f u n c t i o n o f s c a n number c o n s t i t u t e t h e e l e m e n t a l c o m p o s i t i o n o r a c c u r a t e mass chromatogram. Here a g a i n , s e l e c t i o n o f t h e s i z e o f the mass window depends on knowledge o f t h e
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
7.
KIMBLE
Gas Chromatography/High
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mass measurement a c c u r a c y ; t h e c h o i c e i s c r i t i c a l t o ensure t h a t t h e c o r r e c t masses a r e l o c a t e d b u t t h a t " e x t r a n e o u s " masses a r e e x c l u d e d as f a r as p o s s i b l e . M e n t i o n s h o u l d a l s o be made o f t h e g e n e r a t i o n o f nomin a l mass s p e c t r a and chromatograms from a c c u r a t e mass data. Here t h e i n t e n s i t i e s o f a l l a c c u r a t e masses t h a t f a l l w i t h i n t h e mass range between (X.0000-0.4000) amu and (X.0000+0.6000) amu a r e summed t o g i v e t h e i n t e n s i t y o f n o m i n a l mass X. The mass measurement a c c u r a c y becomes i m p o r t a n t f o r t h e a c c u r a t e masses t h a t may l i e c l o s e t o t h e l i m i t s o f t h i s r a n g e , and cons i d e r a t i o n s h o u l d be g i v e n t o t h e c o n v e n t i o n s used f o r d e f i n i n g t h e n o m i n a l mass o f an i o n ( 6 9 ) . In v i e w o f a l l t h e s e b a s i c c o n s i d e r a t i o n s , i t i s e s s e n t i a l t h a t a d e t a i l e d e v a l u a t i o n o f a c c u r a t e mass measurement c a p a b i l i t y s h o u l d be u n d e r t a k e n f o r a g i v e n s e t o f o p e r a t i n g parameters t o ensure r e l i a b l e s i n g l e scan s p e c t r a and a c c u r a t e mass chromatograms. The p r a c t i c a l d e t e r m i n a t i o n o f mass measurement accuracy i s undertaken f o r a p a r t i c u l a r s e t o f i n s t r u ment o p e r a t i n g p a r a m e t e r s by r e c o r d i n g a number o f s p e c t r a o f a s e l e c t e d compound whose p r i n c i p a l masses are u n i q u e l y known. Because o f t h e c o n t i n u o u s v a r i a t i o n i n sample f l o w r a t e w h i c h r e s u l t s from samples i n t r o d u c e d v i a a gas chromatograph, t h e b a s i c e v a l u a t i o n i s made under GC/HRMS o p e r a t i n g c o n d i t i o n s ( i . e . , w i t h normal c a r r i e r gas f l o w r a t e , column temp e r a t u r e , e t c . ) , but w i t h i n t r o d u c t i o n o f the e v a l u a t i o n compound v i a a b a t c h i n l e t system a t c o n s t a n t sample f l o w r a t e . A c t u a l GC/HRMS e v a l u a t i o n c a n t h e n be checked by i n t r o d u c i n g a known compound v i a t h e gas c h r o m a t o g r a p h i c i n l e t and v e r i f y i n g t h a t t h e r e s u l t i n g mass measurements a r e c o n s i s t e n t w i t h t h e b a s i c e v a l u a t i o n f o r s i m i l a r masses o f s i m i l a r i n t e n s i t i e s . The compound s e l e c t e d f o r e v a l u a t i o n o f mass measurement a c c u r a c y s h o u l d produce i o n masses t h a t a r e s i n g l e t peaks w e l l - s e p a r a t e d from o t h e r masses a t t h e o p e r a t i n g mass r e s o l u t i o n . I d e a l l y , t h e y s h o u l d a l s o a d e q u a t e l y c o v e r t h e mass range o f i n t e r e s t , and s h o u l d be o f a p p r o x i m a t e l y t h e same r e l a t i v e i n t e n s i t i e s t o p e r m i t e v a l u a t i o n o f mass measurement a c c u r a c y as a f u n c t i o n o f b o t h mass and i n t e n s i t y . In p r a c t i c e , a compromise has t o be made w i t h r e s p e c t t o t h e s e l a t t e r r e q u i r e m e n t s , and a v a r i e t y o f compounds f o r t h i s purpose have been r e p o r t e d i n t h e l i t e r a t u r e ( e . g . , p e r c h l o r o b u t a d i e n e , £1, and h e p t a c o s a f l u o r o t r i b u t y l amine, 48,49). E v a l u a t i o n o f t h e measurements o b t a i n e d f o r t h e s e l e c t e d masses from s e v e r a l scans can be u n d e r t a k e n by a v a r i e t y o f r e l a t e d methods. One approach (61)
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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described the production o f a histogram which p l o t s the p e r c e n t a g e o f t h e t o t a l number o f mass measurements as a f u n c t i o n o f t h e i r mass measurement e r r o r s (the d i f f e r e n c e between t h e measured mass and t h e t r u e mass). T h i s method, however, s u f f e r s from t h e drawback t h a t o n l y a l i m i t e d number o f scans were e v a l u a t e d , and t h e r e s u l t i n g h i s t o g r a m was g e n e r a t e d from c o m p o s i t e e r r o r measurements from i o n s o f d i f f e r i n g masses and a l s o differing relative intensities. As GC/HRMS a n a l y s e s n e c e s s a r i l y i n v o l v e t h e p r o d u c t i o n o f l a r g e numbers o f s p e c t r a and r e q u i r e a c o r r e s p o n d i n g computer c a p a b i l i t y , a more d e t a i l e d s t a t i s t i c a l e v a l u a t i o n c a n be u n d e r t a k e n i f a l a r g e number ( e . g . , > 100) o f s p e c t r a a r e r e c o r d e d and e v a l uated automatically. I n t h i s approach (7£), f o r each s e l e c t e d a c c u r a t e mass, M , t o be e v a l u a t e d , t h e mea s u r e d mass i s l o c a t e d i n each spectrum w i t h i n an e s t i m a t e d mass window around t h e t r u e mass, Mt. F o r each v a l u e o f M , s e v e r a l p a r a m e t e r s a r e c a l c u l a t e d from a l l of the recorded s p e c t r a : a) The number o f s p e c t r a (N) f o r w h i c h t h e measured mass was found w i t h i n t h e mass window. (The e s t i m a t e d mass window s h o u l d be chosen so t h a t i t r e p r e s e n t s t h e a b s o l u t e maximum mass measure ment e r r o r t h a n c a n be t o l e r a t e d f o r s p e c t r a l interpretation e.g., ±20 ppm. Thus, f o r a l l v a l u e s o f Μ^, Ν s h o u l d c o r r e s p o n d t o t h e number o f s p e c t r a r e c o r d e d ; i . e . , a l l o f t h e measure ments w i l l be w i t h i n 20 ppm o f t h e i r t r u e v a l ue) . b) The mean mass e r r o r , E. ( T h i s may be p o s i t i v e or n e g a t i v e w i t h r e s p e c t t o M t ) . c) The s t a n d a r d d e v i a t i o n o f t h e mass measurement e r r o r s (σ ppm). d) The s t a n d a r d d e v i a t i o n o f t h e mean mass e r r o r (o = σ//Ν). e) Mean peak i n t e n s i t y . f ) S t a n d a r d d e v i a t i o n o f t h e i n t e n s i t y measure ments. (For a d d i t i o n a l e v a l u a t i o n o f i n s t r u m e n t p e r f o r m a n c e , mean and s t a n d a r d d e v i a t i o n v a l u e s a r e a l s o c a l c u l a t e d f o r a v a r i e t y o f o t h e r p a r a m e t e r s a s s o c i a t e d w i t h each s e l e c t e d mass; t h e s e i n c l u d e τ, W. , W., , R, as ex p l a i n e d below). * These v a l u e s p e r m i t a more d e t a i l e d e v a l u a t i o n o f the r e l a t i v e e f f e c t s o f t h e s y s t e m a t i c and random e r r o r s a s s o c i a t e d w i t h mass measurement a c c u r a c y . I n the absence o f s y s t e m a t i c e r r o r s , t h e mean e r r o r Ε s h o u l d approach zero ( i . e . , t h e mean measured mass s h o u l d e q u a l t h e t r u e mass), and t h e r e i s a 99.78% t
t
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0
b
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In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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chance t h a t t h e mean mass w i l l be w i t h i n 3 o o f t h e t r u e v a l u e . A l s o , f o r a normal d i s t r i b u t i o n , 99.78% o f t h e measurements f o r each mass s h o u l d f a l l w i t h i n 3σ o f the mean measured v a l u e , i . e . , v i r t u a l l y a l l o f t h e measurements s h o u l d f a l l w i t h i n a s p r e a d o f 6σ ppm. A s u i t a b l e c h o i c e f o r d e t e r m i n i n g t h e mass window (2δ) f o r e l e m e n t a l c o m p o s i t i o n l i s t i n g s and chromatograms would r e s u l t , t h e r e f o r e , from u s i n g a δ v a l u e e q u a l t o the sum o f t h e maximum v a l u e o f 3σ f o r t h e masses c o n s i d e r e d , and t h e maximum a b s o l u t e v a l u e o f Ε f o r t h e same masses (see F i g u r e 7 ) . The use o f t h e 3σ v a l u e s ( c o r r e s p o n d i n g t o 99.78% of t h e measurements f o r a normal d i s t r i b u t i o n ) r a t h e r t h a n 2σ v a l u e s (95% o f t h e measurements) becomes im p o r t a n t f o r GC/HRMS a n a l y s e s because t h e l a r g e number of s p e c t r a ( s e v e r a l hundred, each p o s s i b l y c o n t a i n i n g s e v e r a l hundred a c c u r a t e m a s s e s ) , t h a t a r e g e n e r a l l y r e c o r d e d n e c e s s i t a t e s a h i g h degree o f r e l i a b i l i t y as w e l l as a c c u r a c y . I n g e n e r a l , t h e number o f e l e m e n t a l c o m p o s i t i o n s computed f o r a t y p i c a l measured mass (M) becomes unmanageable f o r a mass window l a r g e r t h a n about M ± 15 ppm, p a r t i c u l a r l y f o r more complex mole c u l e s t h a t may c o n t a i n many t y p e s o f atoms ( e . g . , C, C , H, N, 0, S i , F ) . Large mass measurement e r r o r s are a l s o p r o b l e m a t i c f o r the g e n e r a t i o n o f elemental c o m p o s i t i o n chromatograms s i n c e t h e mass measurement d i s t r i b u t i o n (spread) may o v e r l a p w i t h t h e d i s t r i b u t i o n of measured masses f o r a n o t h e r e l e m e n t a l c o m p o s i t i o n p r e s e n t i n some o f t h e s p e c t r a . Thus, i n t e n s i t i e s o f e x t r a n e o u s masses may be i n c l u d e d i n t h e f i n a l o u t p u t even though t h e " c o r r e c t " mass window has been s e l e c ted, and t h i s would r e s u l t i n one o r more a r t i f a c t peaks i n t h e e l e m e n t a l c o m p o s i t i o n chromatogram. I n p r a c t i c a l terms f o r s p e c t r a l i n t e r p r e t a t i o n , i t i s a l s o i m p o r t a n t t o know t h e minimum a b s o l u t e mass i n t e n s i t y v a l u e f o r w h i c h t h e d e s i r e d mass measurement a c c u r a c y can be a s s u r e d ; d a t a o u t p u t c o n t a i n i n g l o w e r i n t e n s i t y v a l u e s c a n t h e n be e v a l u a t e d w i t h a p p r o p r i a t e c a u t i o n . F i n a l l y , i t i s i m p o r t a n t t o remember t h a t t h e c o n s i d e r a t i o n o f mass measurement a c c u r a c y r e l a t e s o n l y to w e l l - d e f i n e d normal peak shapes. Any s i g n i f i c a n t d i s t o r t i o n o f such peak shapes, such as u n r e s o l v e d d o u b l e t s , s h o u l d be r e c o g n i z e d and t h e e f f e c t on mass measurement a c c u r a c y t a k e n i n t o a c c o u n t .
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
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Sensitivity. I n GC/HRMS, an e v a l u a t i o n o f o v e r a l l s y s tem s e n s i t i v i t y must c o n s i d e r n o t o n l y t h e d e t e c t i o n o f s p e c i f i e d masses from a g i v e n q u a n t i t y o f a component i n j e c t e d i n t o t h e gas chromatograph (as f o r GC/LRMS), but a l s o t h e mass measurement a c c u r a c y t h a t c a n be
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
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Mass Chromatoçram
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Figure 6. Representation of an accurate mass chromatogram for equal GC peak shapes; scan cycle time equals one-half of the GC peak eiution time
Figure 7. The evaluation of mass measurement accuracy; M = true mass, M = mean measured mass, Ε = mean error, σ = standard deviation of the measured masses. t
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In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
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o b t a i n e d f o r t h e s e masses. T h i s more s t r i n g e n t c r i t e r i o n r e s u l t s i n somewhat l e s s s e n s i t i v i t y compared t o d e t e c t i n g t h e p r e s e n c e o f a p a r t i c u l a r mass, s i n c e t h e peak p r o f i l e s must be s u f f i c i e n t l y w e l l - d e f i n e d t h a t the peak c e n t e r - o f - g r a v i t y o r c e n t r o i d can be a c c u r a t e l y and r e l i a b l y d e t e r m i n e d . S e v e r a l d i s c u s s i o n s have appeared i n t h e l i t e r a t u r e c o n c e r n i n g the t h e o r e t i c a l a s p e c t s o f s e n s i t i v i t y and mass measurement a c c u r a c y (57_, 7 1 ) . B a s i c a l l y , t h e l i m i t i n g f a c t o r concerns the s t a t i s t i c s a s s o c i a t e d w i t h the i o n a r r i v a l times a t the d e t e c t o r , and t h i s d e t e r mines how w e l l the r e s u l t i n g d i g i t i z e d peakshape i s d e f i n e d ; t h e f r e q u e n c y o f d i g i t i z a t i o n o r the number o f d a t a p o i n t s / m a s s peak i s a l s o a c o n s i d e r a t i o n here ( 7 2 ) . I t has been p r e d i c t e d (71) t h a t f o r a mass r e s o l u t i o n o f 10,000, mass measurement a c c u r a c i e s w i t h i n 10 ppm ( = 3σ, o r 99.78% c o n f i d e n c e l e v e l ) s h o u l d be a c h i e v e d w i t h 30 i o n s / p e a k , and w i t h i n 17 ppm (=3σ) f o r 10 i o n s / p e a k . E s t i m a t e s made from a c t u a l mass measure ments (73) i n d i c a t e t h a t 10-15 i o n s / p e a k a r e r e q u i r e d t o g i v e a mass measurement a c c u r a c y o f 30 ppm (=3σ). The r e l a t i o n s h i p between t h e a b s o l u t e peak i n t e n s i t y , I (the sum o f t h e d i g i t a l v a l u e s f o r a peak p r o f i l e ) , and the number o f i o n s , N, c o m p r i s i n g t h a t peak may be d e t e r m i n e d (61) from the e q u a t i o n I . Ν "
r
ÎRef
where G i s t h e g a i n o f t h e m u l t i p l i e r the primary v a r i a b l e under normal o p e r a t i n g c o n d i t i o n s . (the other parameters a r e : R = input r e s i s t a n c e of the m u l t i p l i e r a m p l i f i e r i n ohms, e = charge on an e l e c t r o n i n cou lombs, f = f r e q u e n c y o f d i g i t i z a t i o n i n s e c " , ν = v o l t a g e i n c r e m e n t r e p r e s e n t e d by one b i n a r y " b i t " on the A/D c o n v e r t e r i n v o l t s ) . As w i t h GC/LRMS, i t i s i m p o r t a n t t h a t t h e o v e r a l l s e n s i t i v i t y i s d e t e r m i n e d f o r a s e l e c t e d component i n t r o d u c e d t h r o u g h t h e gas chromatograph, p a r t i c u l a r l y t o t a k e i n t o a c c o u n t any l o s s e s t h a t may o c c u r when using a molecular separator i n t e r f a c e . I t i s also n e c e s s a r y f o r GC/HRMS t h a t one o r more i o n masses ( e . g . , basepeak and/or m o l e c u l a r i o n , e t c . ) a r e s e l e c t e d i n o r d e r t o s p e c i f y the mass measurement ac c u r a c y t h a t can be o b t a i n e d from an i n j e c t i o n o f a g i v e n amount o f the compound. In a d d i t i o n , the l i m i t e d number o f h i g h r e s o l u t i o n s p e c t r a t h a t can be r e c o r d e d f o r any p a r t i c u l a r GC peak under GC/HRMS c o n d i t i o n s a l s o has t o be c o n s i d e r e d . In t h e l i m i t i n g c a s e , when 1
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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the scan c y c l e time ( e . g . , -10 seconds) i s about oneh a l f o f t h e GC peak e i u t i o n t i m e , t h e maximum r e c o r d e d i n t e n s i t y f o r any mass may be e x p e c t e d t o v a r y by a f a c t o r o f about two (see F i g u r e 6 ) , depending on t h e time o f s t a r t i n g a scan i n r e l a t i o n t o t h e time when the GC peak b e g i n s t o e l u t e a r e l a t i o n s h i p which i s unknown i n p r a c t i c e . F o r an i n j e c t i o n o f Ν ng o f t h e s e l e c t e d component, t h e maximum sample f l o w r a t e a t t h e top o f t h e GC peak c o r r e s p o n d s t o about N/10 ng/sec f o r a GC peak e i u t i o n time o f 20 seconds. Measured mass i n t e n s i t i e s , t h e r e f o r e , w i l l be p r o p o r t i o n a l t o sample f l o w r a t e s between about N/20 and N/10 ng/sec. I f the v a r i a t i o n o f mass measurement a c c u r a c y w i t h r e s p e c t t o a range o f measured i n t e n s i t i e s has been d e t e r m i n e d under t h e same c o n d i t i o n s , t h e n an assessment can be made f o r t h e mass measurement a c c u r a c y e x p e c t e d f o r s e l e c t e d masses r e c o r d e d from an i n j e c t i o n o f Ν ng under a g i v e n s e t o f e x p e r i m e n t a l c o n d i t i o n s ( i . e . , s c a n r a t e , mass r e s o l u t i o n , m u l t i p l i e r g a i n , e t c . ) . Dynamic mass r e s o l u t i o n . A knowledge o f t h e a c t u a l mass r e s o l u t i o n o f each r e c o r d e d peak i s an i m p o r t a n t measure o f t h e o v e r a l l p e r f o r m a n c e o f a f a s t - s c a n n i n g h i g h r e s o l u t i o n mass s p e c t r o m e t e r , and d i r e c t l y r e l a t e s t o t h e a b i l i t y t o a c c u r a t e l y measure masses. A l s o , i t i s f r e q u e n t l y n e c e s s a r y t o be a b l e t o d e t e r m i n e whether o r n o t c e r t a i n mass d o u b l e t s would be e x p e c t e d t o be r e s o l v e d under t h e i n s t r u m e n t c o n d i t i o n s used. As d i s c u s s e d i n t h e s e c t i o n above on mass r e s o l u t i o n , t h e mass r e s o l u t i o n (10% v a l l e y d e f i n i t i o n ) o f any peak c a n be d e t e r m i n e d from i t s w i d t h a t 5% maximum peak h e i g h t (W51) and τ ( e q u a t i o n 6 ) , where b o t h W51 and τ a r e measured i n t h e same u n i t s ( e . g . , seconds o r number o f d a t a p o i n t s ) . The a c t u a l v a r i a t i o n o f τ t h r o u g h o u t t h e mass range (see F i g u r e 4) means t h a t t h e r e s o l u t i o n c a l c u l a t i o n f o r a p a r t i c u l a r peak s h o u l d i n c o r p o r a t e t h e a p p r o p r i a t e v a l u e o f τ f o r t h e mass peak c o n s i d e r e d ; t h i s v a l u e i s i n t e r p o l a t e d f o r each peak d u r i n g t h e mass assignment p r o c e s s based on t h e v a l u e s c a l c u l a t e d f o r t h e s u r r o u n d i n g c a l i b r a t i o n masses. The d e t e r m i n a t i o n o f t h e a c t u a l peak w i d t h ¥5% f o r any peak i s somewhat more i n v o l v e d as i t cannot be d i r e c t l y measured from t h e s e t o f d i g i t a l v a l u e s t h a t c o n s t i t u t e a peak p r o f i l e . F o r i d e a l (Gaussian) peakshapes, however, i t i s p o s s i b l e t o c a l c u l a t e in terms o f t h e peak a r e a , A ( t h e sum o f t h e d i g i t a l v a l u e s f o r t h a t peak p r o f i l e ) , and t h e maximum peak h e i g h t , Η ( t h e maximum d i g i t a l v a l u e f o r t h e peak profile). From t h e g e n e r a l e q u a t i o n f o r a G a u s s i a n
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
7.
Gas Chromatography/High
KIMBLE
Resolution Mass Spectrometry
f u n c t i o n , t h e w i d t h a t 5% o f t h e maximum peak ( i . e . , a t H/20) c a n be shown t o be g i v e n by W
height
= 2σ y/1 log 2Ô = 4.8955 σ
5%
139
(7)
e
where σ i s t h e s t a n d a r d d e v i a t i o n o f t h e f u n c t i o n . a r e a A under t h i s c u r v e i s g i v e n by
The
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A = Ησ/27 where Η i s t h e maximum peak h e i g h t , so t h a t , e l i m i n a t i n g σ gives W
5I
ïïj
=
ï
l o
Se
2 0
1
- '
9 5 3
ff
( c f . W51 = 1.90 A/H from c o n s i d e r a t i o n o f a t r i a n g u l a r peakshape). Therefore, the r e s o l u t i o n ( i . e . , 10% v a l l e y d e f i n i t i o n ) i s g i v e n by R
5%
=
i t
=
=
? *
( 9 )
(where τ i s measured i n number o f d a t a p o i n t s ) . Preliminary evaluation of this c a l c u l a t i o n using a c t u a l d a t a (70) i n d i c a t e s t h a t f o r w e l l - d e f i n e d normal peak shapes, t F e v a l u e s c a l c u l a t e d f o r W51 a r e a p p r o x i m a t e l y c o n s t a n t , as would be e x p e c t e d , and a l s o c o r r e s pond t o v a l u e s measured d i r e c t l y from p l o t s o f i n d i v i d u a l peak p r o f i l e s . In p r a c t i c e , t h e u t i l i t y o f t h e c a l c u l a t i o n o f R5^ i s l i m i t e d t o t h e e v a l u a t i o n o f peak p r o f i l e s . T h i s i s because each peak i s r e c o r d e d as a s e r i e s o f d i g i t a l v a l u e s t h a t exceed a c o n s t a n t p r e s e t t h r e s h o l d w h i c h i s chosen t o e l i m i n a t e " n o i s e " b u t t o maximize the o v e r a l l dynamic range. I f t h e d i g i t a l v a l u e s f o r two c l o s e mass peaks do n o t c u t t h e t h r e s h o l d v a l u e a t the v a l l e y ( F i g u r e 8 a ) , t h e n t h e p r o f i l e w i l l be r e c o g n i z e d as t h e p r o f i l e o f a s i n g l e peak and t h e r e s u l t i n g c a l c u l a t i o n s w i l l be i n e r r o r . I n o r d e r t h a t t h e p r o f i l e be r e c o g n i s e d as two d i s t i n c t p e a k s , t h e maxima f o r e q u a l - s i z e d peaks must be s e p a r a t e d by W^h "the w i d t h o f each peak measured a t t h e t h r e s h o l d ( F i g u r e 8b). This t h r e s h o l d peakwidth i s e a s i l y determined from t h e number o f d a t a p o i n t s r e c o r d e d f o r each peak, and t h i s number c a n be used t o c a l c u l a t e a " t h r e s h o l d r e s o l u t i o n " , R , f o r each mass peak from t h e e q u a t i o n tV|
R = th
τ
R
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
(10)
PERFORMANCE
MASS
SPECTROMETRY
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
HIGH
Figure 8. (a) Peak profile for doublet separated with resolution R = t/W (10% valley definition); (b) Peak profiles for peaks resolved at threshold, R< = T / W . 5 %
5%
A
Î A
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
7.
KIMBLE
Gas Chromatography/High
141
Resolution Mass Spectrometry
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
In terms o f t h e i n t e r p r e t a t i o n o f a c t u a l d a t a , t h e t h r e s h o l d o r " e f f e c t i v e " r e s o l u t i o n w i l l more a c c u r a t e l y r e f l e c t the o v e r a l l instrumental a b i l i t y to r e s o l v e c l o s e l y p o s i t i o n e d mass peaks and thus t o s u b s e q u e n t l y d e t e r m i n e t h e i r a c c u r a t e masses and e l e mental c o m p o s i t i o n s . The d e c r e a s e i n e f f e c t i v e r e s o l u t i o n due t o t h e use o f t h e t h r e s h o l d p e a k w i d t h f o r t h e c a l c u l a t i o n r a t h e r than Ws%, can be i l l u s t r a t e d by assuming t h a t the t h r e s h o l d p e a k w i d t h f o r a G a u s s i a n peakshape i s ap p r o x i m a t e d by W
« 6σ
th
(11)
where 6σ i s t h e base w i d t h w h i c h d e f i n e s 99.781 o f t h e peak a r e a . T h e r e f o r e , from e q u a t i o n s (7) and ( 1 1 ) , W , i s g i v e n by W
th
• ;nw5T 5* - · * w
1
2
W
w
5%
C a l c u l a t e d v a l u e s f o r Rs%, W^-h and R^h a r e shown i n T a b l e 2 f o r t h e case o f a n o m i n a l s c a n r a t e o f 6 seconds/decade and a n o m i n a l r e s o l u t i o n (Rs%) o f 10,000; c o r r e s p o n d i n g v a l u e s a r e a l s o g i v e n f o r t h e range o f τ e n c o u n t e r e d f o r t h i s n o m i n a l scan r a t e . These c a l c u l a t i o n s demonstrate a s i g n i f i c a n t d e c r e a s e i n e f f e c t i v e r e s o l u t i o n f o r W^h 6σ, a d e c r e a s e o f 18.41 compared t o t h e r e s o l u t i o n a t 5% maximum peak h e i g h t , and a d e c r e a s e o f up t o 25% compared t o t h e supposed r e s o l u t i o n o f 10,000. F u r t h e r d e c r e a s e s i n e f f e c t i v e r e s o l u t i o n a r e t o be e x p e c t e d f o r l a r g e peaks s i n c e a p e a k w i d t h o f 6σ f o r a G a u s s i a n peakshape c o r responds t o t h e w i d t h a t 1.11% o f t h e maximum peak h e i g h t ; f o r l a r g e peaks, t h e a c t u a l t h r e s h o l d w i l l be s i g n i f i c a n t l y below 1.11% o f maximum peak h e i g h t , and the t h r e s h o l d p e a k w i d t h w i l l thus be l a r g e r t h a n 6σ. =
T a b l e 2.
Scanrate, τ
1 0 >
Comparison o f r e s o l u t i o n v a l u e s c a l c u l a t e d from p e a k w i d t h a t 5% o f maximum p e a k h e i g h t (10% v a l l e y d e f i n i t i o n ) and p e a k w i d t h a t 1.11% o f maximum p e a k h e i g h t . τ, sec.
sec/decade
W
W
th 5% =4.8955σ, =6σ, psec ysec
R
5%
R
th
5.5
2. 389
260.6
319.4
9,167
7,479
6.0
2. 606 2. 823
260.6 260.6
319.4 319.4
10,000 10,833
8,159 8,838
6. 5
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
142
HIGH
PERFORMANCE
MASS
SPECTROMETRY
In a d d i t i o n t o t h e a c t u a l d e t e r m i n a t i o n o f t h e e f f e c t i v e r e s o l u t i o n f o r each r e c o r d e d peak, t h e s e c a l c u l a t i o n s c a n be u t i l i z e d t o i d e n t i f y "abnormal" peakshapes t h a t c a n o c c u r when o p e r a t i n g a h i g h r e s o l u t i o n mass s p e c t r o m e t e r under GC/HRMS c o n d i t i o n s (60, 7 4 ) . A v a r i e t y o f such peak shapes c a n be r a p i d l y i d e n t i f i e d by comparison o f t h e c a l c u l a t e d p e a k w i d t h a t 5% h e i g h t (W51) and t h e measured t h r e s h o l d p e a k w i d t h (W h); abnormal peakshapes i n c l u d e : s m a l l " n o i s y " peaks, f l a t - t o p p e d ( " s a t u r a t e d " ) p e a k s , t a i l i n g p e a k s , and u n r e s o l v e d d o u b l e t s o r m u l t i p l e t s . A l l o f t h e s e peakshapes w i l l a d v e r s e l y a f f e c t t h e a c c u r a t e d e t e r m i n a t i o n o f t h e peak c e n t e r - o f - g r a v i t y and hence the measurement o f a c c u r a t e mass. The e f f e c t o f abnormal peak shapes on t h e q u a l i t y o f a c t u a l d a t a o u t p u t i s i l l u s t r a t e d i n F i g u r e 9. The p l o t shows t h e a c c u r a t e mass chromatogram f o r m/e 119.0861 ( = C H ) from a GC/HRMS a n a l y s i s o f t h e neut r a l f r a c t i o n o f an e x t r a c t o f p e t r o l e u m r e f i n e r y w a s t e w a t e r ( 7 5 ) . The c o m p l e x i t y o f t h i s f r a c t i o n i s e v i d e n c e d by t h e u n r e s o l v e d "hump" w h i c h i s a l s o p r e s e n t i n the f l a m e i o n i z a t i o n gas chromatogram and many o f t h e o t h e r mass chromatograms g e n e r a t e d from t h e GC/HRMS d a t a . N o t a b l e i n t h e chromatogram shown a r e the two a b r u p t zero i n t e n s i t y v a l u e s l o c a t e d a t scan numbers 96 and 143, i n d i c a t i n g t h a t no a c c u r a t e masses were l o c a t e d i n t h e s e scans w i t h i n the mass range m/e 119.0861 ± 20ppm, i . e . , c o r r e s p o n d i n g t o t h e e l e m e n t a l composition C 9 H 1 1 . D e t a i l e d i n v e s t i g a t i o n o f t h e s e two scans r e v e a l e d t h a t C 9 H 1 1 i o n s were i n d e e d p r e s e n t i n the s p e c t r a and t h a t t h e a b s o l u t e i n t e n s i t i e s were c o n s i s t e n t w i t h the o v e r a l l p r o f i l e observed i n t h e a c c u r a t e mass chromatogram. P l o t s o f t h e a p p r o p r i a t e peak p r o f i l e s , a l s o shown i n F i g u r e 9, i l l u s t r a t e t h e abnormal peak p r o f i l e s t h a t were a c t u a l l y r e c o r d e d ; t h e v e r t i c a l l i n e s denote the p o s i t i o n s o f t h e c a l c u l a t e d c e n t e r s - o f - g r a v i t y f o r t h e two p r o f i l e s . I n b o t h cases the c a l c u l a t e d c e n t e r - o f - g r a v i t y was s h i f t e d s u f f i c i e n t l y so t h a t t h e c o r r e s p o n d i n g a c c u r a t e mass was o u t s i d e the mass window used t o g e n e r a t e t h e chromatogram. Both o f t h e s e peak, p r o f i l e s would be e a s i l y r e c o g n i z e d as b e i n g abnormal by comparison o f t h e i r area/maximum h e i g h t r a t i o s ( g i v i n g t h e c a l c u l a t e d W51 v a l u e s ) and the measured t h r e s h o l d p e a k w i d t h s , Wth. In b o t h c a s e s , t h e a v a i l a b i l i t y o f s o f t w a r e a l g o r i t h m s f o r d e t e c t i n g p r o f i l e minima and r e t h r e s h o l d i n g would e f f e c t i v e l y overcome t h e e r r o r s i n t r o d u c e d by t h e s e u n r e s o l v e d peak p r o f i l e s .
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t
9
n
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF PITTSBURGH on August 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch007
7.
Gas Chromatography/High
KIMBLE
τι·
12993.
0
10
sun η/ε»
20
30
143
Resolution Mass Spectrometry
us.i
40
90
t0
70
80
90
100
110
120
130
140
190
190
C,H 0 T
C.H„â
2754B9.5 24591 113.06 817 60
10
275934.9 10231 119. OB 520 60
10
SCAN 143
SCAN 96
Identification and Analysis of Organic Pollutants in Water
Figure 9. Accurate mass chromatogram for m/e 119.0861 (=C H ). ted from scans 96 and 143. 9
tl
Peak profiles plot-
In High Performance Mass Spectrometry: Chemical Applications; Gross, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
144
HIGH
PERFORMANCE
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SPECTROMETRY
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Conclusions I t has been t h e i n t e n t i o n o f t h i s c h a p t e r t o summarize f o r the chemist/user the primary aspects o f GC/HRMS which need t o be c o n s i d e r e d when u n d e r t a k i n g e v a l u a t i o n and i n t e r p r e t a t i o n o f such d a t a . The degree o f d a t a e v a l u a t i o n n e c e s s a r y i n h i g h r e s o l u t i o n mass s p e c t r o m e t r y i s s i g n i f i c a n t l y g r e a t e r than f o r l o w r e s o l u t i o n s t u d i e s . Any e v a l u a t i o n must i n c l u d e mass measurement a c c u r a c y and i t s dependency on mass and peak i n t e n s i t y , dynamic r e s o l u t i o n f o r each mass peak, and t h e many p o s s i b l e e l e m e n t a l c o m p o s i t i o n s g e n e r a t e d f o r each mass peak. The d i f f i c u l t y i s compounded i n GC/HRMS because o f t h e l a r g e amounts o f d a t a t h a t a r e g e n e r a t e d d u r i n g each a n a l y s i s ( e . g . , 15-25 d a t a p o i n t s / mass peak, 500-1500 mass peaks/spectrum, 300-600 spectra/analysis). F o r t h e s e r e a s o n s , the p r i m a r y u t i l i t y o f GC/HRMS i s f o r complex samples whose components cannot be d e t e r m i n e d by i n t e r p r e t a t i o n o f t h e i r nominal mass s p e c t r a . Even w i t h t h e p r o d u c t i o n o f guaranteed h i g h q u a l i t y d a t a , a f l e x i b l e i n t e r a c t i v e and u s e r - o r i e n t e d s o f t w a r e system i s e s s e n t i a l i n o r d e r t o f u l l y u t i l i z e the a v a i l a b l e i n f o r m a t i o n w i t h i n a r e a s o n a b l e t i m e frame. Such s o f t w a r e c o n s i d e r a t i o n s i n c l u d e t h e a b i l i ty to r a p i d l y review elemental composition l i s t i n g s t h a t a r e l i m i t e d by u s e r - s p e c i f i e d c r i t e r i a , such as l i s t i n g s f o r s i n g l e masses, mass d i f f e r e n c e s , l i m i t e d mass r a n g e s , and f o r masses h a v i n g i n t e n s i t i e s above a s p e c i f i e d a b s o l u t e o r r e l a t i v e v a l u e . The a b i l i t y t o i n c l u d e f u n c t i o n a l groups o f e l e m e n t a l c o m p o s i t i o n s ( e . g . , C H , C O 2 C H 3 , O S 1 C 3 H 9 , e t c . ) as i n p u t f o r generating elemental composition l i s t i n g s i s also a s i g n i f i c a n t a s s i s t a n c e i n data i n t e r p r e t a t i o n (76). In p a r t i c u l a r , s o f t w a r e a l g o r i t h m s which u t i l i z e c h e m i c a l c o n c e p t s t o l i m i t t h e number o f c o m p o s i t i o n a l poss i b i l i t i e s g e n e r a t e d f o r each mass (26, 77-79) a r e n e c e s s a r y t o speed-up t h e i n t e r p r e t a t i o n p r o c e s s and a s s i s t i n the generation of a s e l f - c o n s i s t e n t set o f e l e m e n t a l c o m p o s i t i o n s f o r each h i g h r e s o l u t i o n spectrum o f i n t e r e s t . The u s e o f e l e m e n t a l c o m p o s i t i o n s g e n e r a t e d f o r a c c u r a t e mass d i f f e r e n c e s i s a l s o u s e f u l i n t h i s context, s i n c e the elemental compositional p o s s i b i l i t i e s f o r a r e l a t i v e l y s m a l l mass d i f f e r e n c e a r e s u b s t a n t i a l l y fewer than f o r t h e a c c u r a t e masses themselves. As f o r GC/LRMS, t h e a u t o m a t i c i d e n t i f i c a t i o n o f t h e s p e c t r a o f i n t e r e s t would a l s o s i g n i f i c a n t l y speed-up t h e o v e r a l l d a t a i n t e r p r e t a t i o n p r o c e s s . A l g o r i t h m s such as t h a t d e s c r i b e d by B i l l e r and Biemann (15) s h o u l d be implementable f o r r e a s o n a b l y 6
5
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i n t e n s e , a c c u r a t e l y measured masses; however, more complex a l g o r i t h m s (17) would r e q u i r e t h e a b i l i t y t o r e c o r d a s u b s t a n t i a l number o f s p e c t r a t h r o u g h o u t t h e t i m e o f e i u t i o n o f a gas c h r o m a t o g r a p h i c peak. In e s s e n c e , t h e r e f o r e , t h e r o u t i n e u t i l i t y o f t h e GC/HRMS t e c h n i q u e demands an e x t e n s i v e s o f t w a r e capa b i l i t y f o r the r a p i d e v a l u a t i o n of instrument p e r f o r mance, f o r t h e p r o d u c t i o n o f r e l i a b l e h i g h - q u a l i t y d a t a , and f o r t h e e f f i c i e n t h a n d l i n g o f t h e d e t a i l e d information contained i n the elemental composition s p e c t r a r e c o r d e d o f gas c h r o m a t o g r a p h i c e f f l u e n t s . I n p a r t i c u l a r , t h e r o u t i n e use o f such s o f t w a r e f o r i n strument e v a l u a t i o n and performance m o n i t o r i n g i s v i t a l i f t h e p o t e n t i a l o f t h e GC/HRMS t e c h n i q u e i s t o be e x p l o i t e d i n a m e a n i n g f u l way.
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