1 Observation of Metastable Transitions in a High Performance Mass Spectrometer
High Performance Mass Spectrometry: Chemical Applications Downloaded from pubs.acs.org by 181.215.39.219 on 11/30/18. For personal use only.
K. R. JENNINGS Department of Molecular Sciences, University of Warwick, Coventry, United Kingdom
A metastable transition is the unimolecular or collision induced decomposition of an ion after leaving the source exit s l i t and before reaching the collector of a mass spectrometer. The daughter ions produced in these decompositions give rise to metastable peaks in mass spectra produced in a magnetic deflection instrument. During the past ten years, various methods of observing metastable transitions in high performance double focussing instruments of various geometries have been introduced (1,2,3,4). This article aims to give a comparative account of these methods together with illustrations of the use of some of them in the author's laboratory. No single method has established i t s e l f as ideal for a l l applications, and before considering the methods in detail, one must be clear what information is required or may be obtained from a study of metastable transitions. If one is concerned with obtaining detailed information about the potential energy surface over which a particular decomposition occurs, the method must be capable of yielding information on peak shapes, and if possible, the energy resolution should be high enough to reveal any structure in the peak. On the other hand, if one's prime aim is to establish whether or not there is a metastable peak arising from the process m ->m or alternatively to assign as precisely as possible the mass of either m or m when one of them is known, a method which produces a very narrow peak, the position of which on the mass or voltage scale can be measured with high precision, is to be preferred. The choice of method may also be influenced by a consideration of whether one is particularly interested in collision-induced decompositions or whether one particularly wishes to avoid them. Other factors such as sensitivity, rapidity of scan, +
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d i s c r i m i n a t i o n e f f e c t s and the i n t e r p r e t a t i o n o f t i v e peak i n t e n s i t i e s must a l s o be c o n s i d e r e d . Methods o f O b s e r v i n g Applications.
Metastable
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I n a d o u b l e f o c u s s i n g i n s t r u m e n t , the t h r e e pos s i b l e v a r i a b l e s a r e V, the a c c e l e r a t i n g v o l t a g e , E, the e l e c t r i c s e c t o r v o l t a g e and B, the m a g n e t i c f i e l d s t r e n g t h . These may be scanned i n v a r i o u s ways t o a l l o w the c o l l e c t i o n o f d a u g h t e r i o n s formed i n metas t a b l e t r a n s i t i o n s w h i c h o c c u r i n the f i e l d f r e e r e g i o n s between the s o u r c e and the f i r s t s e c t o r and between the two s e c t o r s . The major c h a r a c t e r i s t i c s o f the f i v e methods w h i c h have so f a r been used a r e summarized i n T a b l e I . In d i s c u s s i n g these methods, i t w i l l be as sumed, u n l e s s o t h e r w i s e s t a t e d , t h a t the mass s p e c t r o meter has a geometry i n w h i c h the e l e c t r i c s e c t o r p r e cedes the m a g n e t i c s e c t o r . The f i r s t method s i m p l y makes use o f the o f t e n d i f f u s e peaks o f low i n t e n s i t y w h i c h a r e o b s e r v e d i n a normal mass spectrum i n w h i c h Β i s scanned w i t h V and Ε f i x e d . The m e t a s t a b l e peaks may be masked by i n t e n s e normal p e a k s , and assignment i s u s u a l l y not d i f f i c u l t f o r peaks g i v e n by a low m o l e c u l a r w e i g h t compound. However, i t becomes i n c r e a s i n g l y d i f f i c u l t to a s s i g n low i n t e n s i t y m e t a s t a b l e peaks i n the mass spectrum o f a compound o f h i g h m o l e c u l a r w e i g h t . I f o n l y the more i n t e n s e m e t a s t a b l e peaks are o f i n t e r e s t , few problems a r e e n c o u n t e r e d , and the method has the advantage t h a t the normal mass spectrum and m e t a s t a b l e peaks a r e ob t a i n e d i n a s i n g l e scan. One o f the most w i d e l y used methods i s t h a t i n w h i c h the a c c e l e r a t i n g v o l t a g e V i s scanned w i t h Ε and Β fixed. In t h i s method the d a u g h t e r i o n m i s selec t e d under normal o p e r a t i n g c o n d i t i o n s but w i t h V and Ε s e t a t t y p i c a l l y h a l f the maximum v a l u e . A s c a n o f V i s t h e n i n i t i a t e d and a peak i s o b t a i n e d whenever d i f f e r e n t p r e c u r s o r i o n s m i f u l f i l l the r e q u i r e m e n t that V /V„ = m /m (1) +
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where V i s the a c c e l e r a t i n g v o l t a g e r e q u i r e d f o r the c o l l e c t i o n of m i o n s formed i n the s o u r c e and V i s the v a l u e r e q u i r e d t o t r a n s m i t m i o n s formed from m i i o n s i n the f i e l d - f r e e r e g i o n between the source and e l e c t r i c s e c t o r . T h e r e f o r e , the method i s w e l l s u i t e d to g i v e i n f o r m a t i o n on p r e c u r s o r i o n s o f a g i v e n daugh t e r i o n . The s i g n a l i s p r o d u c e d by the c o l l e c t i o n o f d a u g h t e r i o n s o f a f i x e d mass and energy, d e f i n e d by 0
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Metastable Transitions
JENNINGS
Table I.
Scan Β
C h a r a c t e r i s t i c s o f D i f f e r e n t Methods f o r Observation of Metastable Transitions.
Fixed
Ease o f KE Assignment R e l e a s e
V, Ε
Often D i f ficult
Ε, Β
Usually to nearest nominal mass
Yes
i) D i f f i c u l t i i ) Usually to nearest nominal mass
Yes
V, Β
Yes
Feature 2
G i v e s m /mi r a t i o . Low i n t e n s i t y r e l a t i v e t o normal peaks. O v e r l a p pro blems. 2
G i v e s a l l mi o f s e l e c t e d m . Range l i m i t e d by p r a c t i c a l Vi/Vo r a t i o . Source c o n d i t i o n s vary during scan. +
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i ) Ε p r e c e d e s B: g i v e s a l l mi/m r a t i o s w i t h o u t mass a n a l y s i s . Requires EM between s e c t o r s , i i ) Β p r e c e d e s E: gives a l l m from s e l e c t e d m i . Wide range; c o n s t a n t source c o n d i t i o n s . 2
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V V E
Β
Good; peaks narrow
No
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Gives a l l m from s e l e c t e d m i . Range l i m i t e d t o mi/m ~3. Source c o n d i t i o n s vary during scan. 2 +
2
Β/Ε
V
Good; narrow peaks
No
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Gives a l l m from s e l e c t e d m i . Wide range; c o n s t a n t source c o n d i t i o n s . 2 +
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the s e l e c t e d v a l u e s o f Β and E. The spectrum i s an energy spectrum o f p r e c u r s o r i o n s w h i c h fragment t o y i e l d t h e s e daughter i o n s , and t h i s i s r e a d i l y used t o g i v e t h e masses o f p r e c u r s o r i o n s by means o f e q u a t i o n (1). I f t h e f r a g m e n t a t i o n i s accompanied by t h e r e l e a s e o f t r a n s l a t i o n a l energy, t h e m e t a s t a b l e peak i s b r o a d ened. I f t h e 3 - s l i t w i d t h i s r e d u c e d i n an i n s t r u m e n t o f N i e r - J o h n s o n geometry, t h e i n c r e a s e d d i s c r i m i n a t i o n a t t h e β-slit causes t h e peak-shape t o change and f a c i l i t a t e s t h e e v a l u a t i o n o f t h e energy r e l e a s e . Un der such c o n d i t i o n s , t h e i n t e n s i t y i s r e d u c e d b u t s i g n a l a v e r a g i n g may be used t o improve t h e s i g n a l - t o n o i s e r a t i o , and, i n c e r t a i n c a s e s , t h i s has r e v e a l e d f i n e s t r u c t u r e i n m e t a s t a b l e peaks. S e v e r a l fragmen t a t i o n s which l e a d t o the p r o d u c t i o n o f 0 Η ions y i e l d m e t a s t a b l e peaks w h i c h p o s s e s s f i n e s t r u c t u r e (5J). The s i g n a l g i v e n by t h e Ci»rU i o n s formed i n t h e process +
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i s r e p r o d u c e d i n F i g u r e 1. T h i s peak s t r u c t u r e i s r a t i o n a l i z e d by assuming t h a t t h e two p r o c e s s e s y i e l d l i n e a r and c y c l i c forms o f t h e C H ions, the enthal p i e s o f f o r m a t i o n o f w h i c h d i f f e r by 1.09 eV. The d i f f e r e n c e i n e n e r g i e s r e l e a s e d i n t h e two p r o c e s s e s w h i c h g i v e r i s e t o t h e two components o f t h e peak i s 0.52 eV. T h i s i n d i c a t e s t h a t a p p r o x i m a t e l y h a l f o f t h e d i f f e r e n c e i n e n t h a l p i e s o f f o r m a t i o n appears as t h e difference i n energies released. A s i m i l a r conclusion i s r e a c h e d from t h e s t r u c t u r e i n t h e m e t a s t a b l e peak a r i s i n g from t h e p r o c e s s +
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A f u r t h e r use o f t h i s type o f s c a n has been i n t h e i n v e s t i g a t i o n o f t h e energy r e l e a s e w h i c h accompanies the f r a g m e n t a t i o n o f an i o n formed i n an i o n - m o l e c u l e r e a c t i o n i n a h i g h p r e s s u r e source. For t h i s type o f e x p e r i m e n t , t h e c o n d i t i o n s r e q u i r e c a r e f u l adjustment s i n c e i f the pressure i s too low, the y i e l d o f f r a g menting i o n i s low and i f t h e p r e s s u r e i s t o o h i g h , c o l l i s i o n - i n d u c e d decompositions occur o u t s i d e the s o u r c e and mask t h e peak g i v e n by t h e u n i m o l e c u l a r decomposition. I n a s t u d y (6) o f t h e f r a g m e n t a t i o n o f the C H 3 O H 2 " i o n , i t was founcT t h a t b e s t r e s u l t s were o b t a i n e d w i t h a s o u r c e p r e s s u r e o f hydrogen o f 0.05 Torr w i t h a t r a c e o f methanol. M e t a s t a b l e peaks a r i s i n g from two f r a g m e n t a t i o n s c o u l d be o b s e r v e d 4
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Metastable Transitions
JENNINGS
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