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Comparative study on the kinetic energy release observed by high voltage and sector voltage scan techniques
lnternah'orud Journal o f Mass S p e c t r o m e t r y and I o n Physics, 28 (1978) 225--231 ~) Elsevier Scientific Pub!i~b!ng C o m p a n y , ~m~t.erdam ~ Printed in T h e Netherlands
-225
COMPARATIvESTUDY ON THEEINETIC ENERGY- RELEASE - -OBSERVED BY HIGH VOLTAGE AND SECTOR VOLTAGE ScAN TECHNIQUES .
M.D. M I ~ A H E D D e p a r t m e n t o f Physics, F a c u l t y o f Science, Mansoura University, Mansoura ( E g y p t )
F.H. ABD E L - K A D E R
-
D e p a r t m e n t o f Physfc~, F a c u l t y o f Science. Cairo University, C~'ro ( E g y p t )
(First received 23 August 1977; in finni f o r m 23 N o v e m b e r 1977) ABSTRACT A caxeful s t u d y has b e e n carried o u t to c o m p a r e the l~inetic energy released in s o m e metastable tr~n-~itions occurring in the first a n d s e c o n d field-free regions of the reversed
Nie_r--Jobn~on double f o c u ~ n g m ~
s p e c t r o m e t e r . No detectable difference in kinetic
energy ~ I e a s e d w a s observed in s o m e cases. In other cases, the kinetic energy released, calculated tirol the H V scan technique, is s o m e w h a t higher than that in the E S A scan m e t h o d . In s o m e cases the differences in kinetic energies released m ~ y b e attributed to shorter observation times, wh~.qt variation of T with ion d e c o m p o s i t i o n time is indicative of the existence o f ions having different internal energies.
INTRODUCTION The shape of the metastable peaks can be related to the conversion of the precursor ions' intern~! energy into translational kinetic energy of daughter i o n s [ 1 ] . T h e a v e r a g e l r i n e t i c e n e r g y re_leased T , c a l c u l a t e d f r o m t h e w i d t h o f t h e m e t a s t a b l e p e a k , c a n b e u s e d as a n i m p o r t a n t s o u r c e o f i n f o r m a t i o n regarding structure of ions, mechanisms and activation energies. However, it h a s b e e n r e p o r t e d [ 2 , 3 ] t h a t t h e s h a p e o n l y p a r t l y r e f l e c t s t h e k-~netic e n e r g y o f t h e p r o d u c t s ; i t ~1.~o d e p e n d s u p o n t h e g e o m e t r y o f t h e m a s s s p e c t o m e t e r . The kinetic energy released, calc~!oted from the width of a metastable peak arising from a decomposition which occurs in the field-free region b e t w e e n t h e e l e c t r o s t a t i c a n d m a g n e t i c a n a l y s e r s o f a n A.E~WIS9 m a s s s p e c trometer of Niex Jobn.~on geometry, has been shown to be in good agreement with values obtained by other methods [1,4]. On the_other hand, H o l m e s e t al. [ 5 ] h a v e c o m p u t e d m e t a s t a b l e , p e a k s h a p e s f o r a 9 A . E . I . G E C M S g C 2 S m a s s spe_~Lrometer a n d f o u n d t h a t di.~h-topp.ed m e t a s t a b l e ~ do n o t azise f r o m - ~ n g l e - v a l u e d energy_ r e l e a s e s , b u t f r o m t h e r e l e a s e o f a r a n g e of energies . . . .
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226 The present paper presents results which show the dependence of the kinetic energy release in metastable transitions on the position at which the decomposition occurs in a Varian MAT CH5 reversed Nier--Johnson double focusing mass spectrometer. The relative accuracies of high voltage HV and electrostatic analyser ESA scan techniques were also of interest. All comparisons are for the Kinetic energy release for a specific transition in a selected molecule, without correction for the energy spread of the main beam_ EXPERIMENTAL
Measurements o f t h e m a s s s p e c t r a w e r e m a d e ua~ag a V a r i a n M A T C H 5 reversed Nier--Johnson double focusing mass spectrometer. The standard normal operating conditions were as follows: accelerating voltage, 3 kV; sect o r ~-oltage, 5 3 9 V ; i o n i z i n g e l e c t r o n e n e r g y , 7 0 e V ; t o t a l e m i s s i o n c u r r e n t , 1 mA; resolving power, ca. 3000_ The ion source pressure was varied over the r a n g e 1 × 1 0 - 7 - - 1 X 1 0 -s t o r t a n d t h e s o u r c e t e m p e r a t u r e w a s k e p t a t 1 3 0 ° C . The recordings of the mutastable spectra were cazTied out using the usual h'~h voltage [6,7] and sector voltage [8--10] scan procedures. The voltage was determined with a digital voltmeter. The high and sector voltages can be varied in 0.001- and 0.1-V steps, respectively. Each peak was recorded on a potentiometer recorder at least three times then, on a separate occasion, the ssmple was reintroduced and the procedures were repeated. T h e k i n e ~ c e n e r g y r e l e a s e d i n t h e m e t a s t a b l e t r a n s i t i o n rn~ -* m ~ + m 3 h a s been calculated using the relationship [11,12] : m2
w h e r e A W is t h e w i d t h o f t h e m e t a s t a b l e p e a k i n t e ~ , , , s o f t h e a c c e l e r a t i n g v o l t a g e V o r s e c t o r v o l t a g e E , a n d e is t h e e l e c t r o n i c c h a r g e . T h e v a l u e o f W t h e n c o r r e s p o n d s t o s i n g l e v a l u e o f V , i . e . W = V s, o r o f E , i . e . W = E s , a t which fragment ions would have been detected in the case corresponding to T = 0. The metastab!e peak width at half-height was used to determine the characteristic values of T. Kinetic energy release values were reproducible to better than +-0.005 eV. R E S U L T S A N D DI SC U SSI O N Methanol Only one metast~ble transition of the molecular ion minus hydrogen atom has bcen studied in the mass spectrum of methanol. The shape of the metas t a b l e p e a k f o r t b ~ t r ~ n s ~ L i o n is c l e a r l y o f t h e c H q h = t o p p e d e n e r g y r e l e a s e type [3]. Calculation of T led to values of 1.800 and 1.687 eV in the first a n d s e c o n d f i e l d - f r e e r e g i o n s , r e s p e c t i v e l y . T h e v a l u e 1 . 6 8 7 e V is c o m p a r a b l e
227 with the values reported in the literature [10,13]. However, the relaLively small difference between T in the present work and T reported may arise f r o m t h e f a c t t h a t t h e d i f f e r e n c e s i n i n s t ~ l m e n t a l c o n d i t i o n s a f f e c t t h e inii~AI molec~l~r internal energy distribution.
Benzene T h e r e a c t i o n s t u d i e d i n d e t a i l w a s H" l o s s f r o m t h e b e n z e n e m o l e c u l a r i o n . T h e s h a p e o f t h e m e t a s t a b l e p e a k is g a u s s e s . T h e l c i n e t i c e n e r g y r e l e a s e d i n the met~stable tr~n.~tion in the first and the second t~eld-free regions are 0.066 and 0.062 eV, respectively. These ~esults are in good agreement with the values reported by Beynon and co-workers [14,15] H~ng the HV scan technique.
2-.,4 c e t y lpyridine In the mass spec~lm of this molecule, the strongest signal arising from t h e r e a c t i o n w h i c h l e d t o h i g h k ~ n e t i c e n e r g y is t h a t r e s u l t i n g f r o m t h e p l i m i nation of carbon monoxide from the moleclJlar ion. This reaction has been explained by ass~]ming that the pyridine nitrogen atom induces the skeletal r e a r r a n g e m e n t p r o c e s s [ 1 6 ] . T h e p e a k ~ri~ing is m u c h b r o a d e r t h a n t h e u s u a l "~netastable peak" and has a pronounced dip in its centre. However, this p e a k is p r o b a b l y o f t h e f i a t - t o p p e d e n e r g y r e l e a s e t y p e a s r e p o r t e d b e f o r e [ 1 7 ] . C a l c u l a t i o n o f T s h o w e d n e a r l y e q u a l v a l u e s o f 0 . 5 4 6 e V i n t h e f~-st and second tqeld-free regions.
t~'ualdehyde T h e l o s s o f C H O a s a n e u t r a l r a d i c a l l e a d s t o t h e n o .... al i n t e n s e i o n o f ~tlctu~ C4I-1~9. T h e m e t a s t a b l e t r a n s i t i o n o f t h l s r e a c t i o n h a s b e e n o b s e r v e d . T h e m e t a s t a b l e p e a k is c l e a r l y o f t h e d i s h - t o p p e d e n e r g y r e l e a s e t y p e a s s e e n in Fig. 1. Calcl]l~tion of the klnetic energy released has been found to be 1.066 and 0.942 eV in the first and second field-free regions, respectively.
n-Butyl m e t h y l k e t o n e and pinakoline Metastable transitions of the molecular and f~agment ions in the mass spe~Ltm of n-butyl methyl ketone and pinakoline occur~ug in the first and second field-free regions were investigated. The Lwo isomeric metastable m o l e c H l a r i o n s d e c o m p o s e t o y i e l d ( M - - C H 3 ) ÷. E a c h o f t h e s e i o n s d i s s o c i a t e f u r t h e r t o g i v e a n i o n o f M / e 5 7 (C4I-I~9) i n a c l i ~ n c t r a t i o f o r e a c h o f t h e t w o s p e c i e s . T h e s p e c t ~ , m o f n - b u t y l m e t h y l k e t o n e is c h a r a c t e r i z e d b y t h e p r e s ence of a metastable peak resulting from the separation of ethylene from ( M - - C 2 H s ) ÷, r e s u l t i n g i n t h e C H 3 C O + i o n . F u r ~ h e r , i t a p p e a r s f r o m F i g . 2 t h a t the metastable resulting from the dissociation of the (M--CH3) ÷ ion in n - b u t y l m e t h y l k e t o n e is o f t h e g a u s s i a n e n e r g y r e l e a s e t y p e , w h i l e i n t h e pinakoline counterpart the peak has a pronounced d i p i n i t s c e n t r e a n d is p r o b a b l y o f t h e f i a t - t o p p e d t y p e . I t s e e m s t h a t t h e r e is m o r e t h a n o n e m e c h a nizam f o r t h e f o r m a t i o n o f t h e f r a g m e n t i o n M / e 5 7 . T h e k i n e t i c e n e r g i e s released in the first and the second field-free regions for the decomposition
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o f t h e m o l e c u l a r a n d s o m e f r a g m e n t i o n s in b o t h m o l e c u l e s h a v e b e e n calc,,]~ted and are presented in Table 1. All transitions mentioned above were shown to be due to ,,nlmolecular fragmentation by taking measurements over a range of pressure (1 X 10 -5-1 X 1 0 -7 t o r t ) i n t h e ffLrst f i e l d - f r e e r e g i o n . I n n o c a s e w a s a c h a n g e i n p e a k shape detectable. In addition, the effect of varying the ion accelerating voltage on the shape of the metastable peak has been investigated; it has been f o u n d t h a t t h e s h a p e o f t h e m e t a s t a b l e p e a k is c h a n g e d i n s o m e c a s e s f r o m g a u s s i a n t o " r o u n d e d " as t h e a c c e l e r a t i n g v o l t a g e is d e c r e a s e d [ 1 8 ] . Comparisons between T calc,,l~ted from the HV and ESA scan methods show no detectable difference in trinetic energy release d,,ring the molecular ion decomposition in the first and second field-free regions if the resulting metastable fragment ions have gaussian or fiat-topped peaks. On the other hand, if the shape of the metastable fragment ions resulting from metastable tr~n~tion of the molec,]lnr ions are of the dish-topped type, then T calcul a t e d f r o m H V v a r i a t i o n s t u d i e s is s o m e w h a t h i g h e r t h a n f o r t h e E S A s c a n method. In addition, the kinetic energy released in further dissociation of f r a g m e n t i o n s r e v e a l e d t h a t T f o r m e t a s t a b l e t r n n . ~ t i o n o c c u r r i n g i n t h e fLrSt f i e l d - f r e e r e g i o n is e q u a l t o i n s o m e c a s e s , a n d g r e a t e r t h a n i n o t h e r c a s e s , t h e value of T for the s~me transition occurring in the second field-free region. The kinetic energy released in a metastable decomposition originates from Lwo s o u r c e s , t h e i n t e r n a l e n e r g y o f t h e a c t i v a t e d c o m p l e x a n d t h e r e v e r s e activation energy [19]. For a given reaction the former, but not the latter, should depend on the s~mpling time. Hence the reproducible differences in T obtained in the present study could be aLL~ibuted to different ion lifetimes and thus internal energies for dissociations in the first and second field-free r e g i o n . I t is h o w e v e r n o t u n d e r s t o o d w h y o n l y f o r d i s h - s h a p e d p e a k s , w h e r e the contribution from the reverse activation energy should prevail, a pron o u n c e d d e p e n d e n c e o n t h e s ~ m p H n g t i m e is o b s e r v e d . ACKNOWT .P.DGEW~.NTS T h e D e u t s c h e F S r d e r u n g s g e s e l i s c h ~ f ~ f i i r E n t w i c k l u n g s ] ~ u d e r ( G A W I ) is th~nlced for grant No. 73. 2507. 9/4001 towards the purchase of the mass spectrometer. We are also grateful to Prof. Dr. M. EI-Nadi, Vice President of M a n s o u r a U n i v e r s i t y , f o r bi~ u s e f u l i n t e r e s t . REFERENCES 1 J ~ I . B e y n o n , R . A . S a u n d e r s a n d A_E. W~U;~m~, Z . Naturfozr~_h. A , 2 0 ( 1 9 6 5 ) 1 8 0 . 2 J M . B e y n o n a n d A_E. F o n t a i n e , Z . N a t u r f o r s c h . A , 2 2 ( 1 9 6 7 ) 3 3 4 . 3 J M . B e y n o n , A . E . F o n t ~ ; n e a n d J_R. L e s t e r , I n t . J. Mass S p e c t r o m . I o n P h y s . , 8 (1972) 341. 4 W. H i g ~ n g a n d K R . J e n n ; n g s , Tr~nq. F a r a d a y S o c . , 6 2 ( 1 9 6 6 ) 9 7 . 5 J_L. H o l m e s , O_D. O s b o r n e a n d G . M . W e e s e , I n t . J. M - ~ S p e c t r o m . I o n P h y s . , 1 9 (1976) 207.
231 6 M. Barber and R.M. Elliot, paper presented at the 12th Annual Conference on i ~ Spectrometry and Allied Topics, ASTM Committee E-14, Montreal, 1964. 7 K.R. J~nn~-gs , J. Chem. Phys., 43 (1965) 4176. 8 R . W . K i s e r , R . E . Sn]Hvan a n d M . S . L u p i n , A n a l . C h e m . , 4 1 ( 1 9 6 9 ) 1 9 5 8 . 9 J.H. Beynon, J.W. Amy and W.E. Baitinger, Chem. Commun., (1969) 723. I 0 C. R e i c h e r t , R . E . F r a s s a n d R . W . K i s e r , I n t . J . i ~ Spectrom. I o n P h y s . , 5 ( 1 9 7 0 ) 457. I I R . G . C o o k s , J~-I. B e y n o n , R . M . C a p r i o l i a n d G . R . I ~ - t e r , M e t a s t a b l e I o n s , EIsevier~ Am~t~'~,m, 1973. 12 D.T. TerwiIliger, J.H. Beynon and R.G. Cooks, Proc. Roy. Soc., Set.A, 341 (1974) 135. 13 K . C . S y m t h a n d T . W . S h a n n o n , J . C h e m . P h y s . 51 ( 1 9 6 9 ) 4 6 3 3 . 1 4 M. B e r t r a n d , J~T-I. B e y n o n a n d R . G . C o o k s , I n t . J . i ~ - ~ S p e c t r o m . I o n P h y s . , 9 ( 1 9 7 2 ) 346, 15 R . G . C o o k s , K . J . l ~ i m , T . K e o u g h a n d J . H . B e y n o n , I n t . J . M ~ S p e c t r o m . I o n P h y s . , 15 (1974) 271. 16 M.D. M i g a h e d , A . I . Helal a n d S.B. E l - K h o l y , Org. Mass S p e c t r o m . , 7 ( 1 9 7 3 ) 1 4 2 3 . 17 M . D . M i g a h e d , A . S h e h a p a n d Th.IVI. E I - S h e r b i n i , P r o c . M a t h . P h y s . S o c . ( A R E ) , 4 4 (1977) 173. 18 T.W. ~nnon, F.W. Mcr~fer~ and C.R. McK;n-ey, Chem. Commun., (1966) 478. 19 E.G. Jones, J.H. Beynon and R.G. Cooks, J. Ch~m. Phys., 57 (1975) 2654.
-225
COMPARATIvESTUDY ON THEEINETIC ENERGY- RELEASE - -OBSERVED BY HIGH VOLTAGE AND SECTOR VOLTAGE ScAN TECHNIQUES .
M.D. M I ~ A H E D D e p a r t m e n t o f Physics, F a c u l t y o f Science, Mansoura University, Mansoura ( E g y p t )
F.H. ABD E L - K A D E R
-
D e p a r t m e n t o f Physfc~, F a c u l t y o f Science. Cairo University, C~'ro ( E g y p t )
(First received 23 August 1977; in finni f o r m 23 N o v e m b e r 1977) ABSTRACT A caxeful s t u d y has b e e n carried o u t to c o m p a r e the l~inetic energy released in s o m e metastable tr~n-~itions occurring in the first a n d s e c o n d field-free regions of the reversed
Nie_r--Jobn~on double f o c u ~ n g m ~
s p e c t r o m e t e r . No detectable difference in kinetic
energy ~ I e a s e d w a s observed in s o m e cases. In other cases, the kinetic energy released, calculated tirol the H V scan technique, is s o m e w h a t higher than that in the E S A scan m e t h o d . In s o m e cases the differences in kinetic energies released m ~ y b e attributed to shorter observation times, wh~.qt variation of T with ion d e c o m p o s i t i o n time is indicative of the existence o f ions having different internal energies.
INTRODUCTION The shape of the metastable peaks can be related to the conversion of the precursor ions' intern~! energy into translational kinetic energy of daughter i o n s [ 1 ] . T h e a v e r a g e l r i n e t i c e n e r g y re_leased T , c a l c u l a t e d f r o m t h e w i d t h o f t h e m e t a s t a b l e p e a k , c a n b e u s e d as a n i m p o r t a n t s o u r c e o f i n f o r m a t i o n regarding structure of ions, mechanisms and activation energies. However, it h a s b e e n r e p o r t e d [ 2 , 3 ] t h a t t h e s h a p e o n l y p a r t l y r e f l e c t s t h e k-~netic e n e r g y o f t h e p r o d u c t s ; i t ~1.~o d e p e n d s u p o n t h e g e o m e t r y o f t h e m a s s s p e c t o m e t e r . The kinetic energy released, calc~!oted from the width of a metastable peak arising from a decomposition which occurs in the field-free region b e t w e e n t h e e l e c t r o s t a t i c a n d m a g n e t i c a n a l y s e r s o f a n A.E~WIS9 m a s s s p e c trometer of Niex Jobn.~on geometry, has been shown to be in good agreement with values obtained by other methods [1,4]. On the_other hand, H o l m e s e t al. [ 5 ] h a v e c o m p u t e d m e t a s t a b l e , p e a k s h a p e s f o r a 9 A . E . I . G E C M S g C 2 S m a s s spe_~Lrometer a n d f o u n d t h a t di.~h-topp.ed m e t a s t a b l e ~ do n o t azise f r o m - ~ n g l e - v a l u e d energy_ r e l e a s e s , b u t f r o m t h e r e l e a s e o f a r a n g e of energies . . . .
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226 The present paper presents results which show the dependence of the kinetic energy release in metastable transitions on the position at which the decomposition occurs in a Varian MAT CH5 reversed Nier--Johnson double focusing mass spectrometer. The relative accuracies of high voltage HV and electrostatic analyser ESA scan techniques were also of interest. All comparisons are for the Kinetic energy release for a specific transition in a selected molecule, without correction for the energy spread of the main beam_ EXPERIMENTAL
Measurements o f t h e m a s s s p e c t r a w e r e m a d e ua~ag a V a r i a n M A T C H 5 reversed Nier--Johnson double focusing mass spectrometer. The standard normal operating conditions were as follows: accelerating voltage, 3 kV; sect o r ~-oltage, 5 3 9 V ; i o n i z i n g e l e c t r o n e n e r g y , 7 0 e V ; t o t a l e m i s s i o n c u r r e n t , 1 mA; resolving power, ca. 3000_ The ion source pressure was varied over the r a n g e 1 × 1 0 - 7 - - 1 X 1 0 -s t o r t a n d t h e s o u r c e t e m p e r a t u r e w a s k e p t a t 1 3 0 ° C . The recordings of the mutastable spectra were cazTied out using the usual h'~h voltage [6,7] and sector voltage [8--10] scan procedures. The voltage was determined with a digital voltmeter. The high and sector voltages can be varied in 0.001- and 0.1-V steps, respectively. Each peak was recorded on a potentiometer recorder at least three times then, on a separate occasion, the ssmple was reintroduced and the procedures were repeated. T h e k i n e ~ c e n e r g y r e l e a s e d i n t h e m e t a s t a b l e t r a n s i t i o n rn~ -* m ~ + m 3 h a s been calculated using the relationship [11,12] : m2
w h e r e A W is t h e w i d t h o f t h e m e t a s t a b l e p e a k i n t e ~ , , , s o f t h e a c c e l e r a t i n g v o l t a g e V o r s e c t o r v o l t a g e E , a n d e is t h e e l e c t r o n i c c h a r g e . T h e v a l u e o f W t h e n c o r r e s p o n d s t o s i n g l e v a l u e o f V , i . e . W = V s, o r o f E , i . e . W = E s , a t which fragment ions would have been detected in the case corresponding to T = 0. The metastab!e peak width at half-height was used to determine the characteristic values of T. Kinetic energy release values were reproducible to better than +-0.005 eV. R E S U L T S A N D DI SC U SSI O N Methanol Only one metast~ble transition of the molecular ion minus hydrogen atom has bcen studied in the mass spectrum of methanol. The shape of the metas t a b l e p e a k f o r t b ~ t r ~ n s ~ L i o n is c l e a r l y o f t h e c H q h = t o p p e d e n e r g y r e l e a s e type [3]. Calculation of T led to values of 1.800 and 1.687 eV in the first a n d s e c o n d f i e l d - f r e e r e g i o n s , r e s p e c t i v e l y . T h e v a l u e 1 . 6 8 7 e V is c o m p a r a b l e
227 with the values reported in the literature [10,13]. However, the relaLively small difference between T in the present work and T reported may arise f r o m t h e f a c t t h a t t h e d i f f e r e n c e s i n i n s t ~ l m e n t a l c o n d i t i o n s a f f e c t t h e inii~AI molec~l~r internal energy distribution.
Benzene T h e r e a c t i o n s t u d i e d i n d e t a i l w a s H" l o s s f r o m t h e b e n z e n e m o l e c u l a r i o n . T h e s h a p e o f t h e m e t a s t a b l e p e a k is g a u s s e s . T h e l c i n e t i c e n e r g y r e l e a s e d i n the met~stable tr~n.~tion in the first and the second t~eld-free regions are 0.066 and 0.062 eV, respectively. These ~esults are in good agreement with the values reported by Beynon and co-workers [14,15] H~ng the HV scan technique.
2-.,4 c e t y lpyridine In the mass spec~lm of this molecule, the strongest signal arising from t h e r e a c t i o n w h i c h l e d t o h i g h k ~ n e t i c e n e r g y is t h a t r e s u l t i n g f r o m t h e p l i m i nation of carbon monoxide from the moleclJlar ion. This reaction has been explained by ass~]ming that the pyridine nitrogen atom induces the skeletal r e a r r a n g e m e n t p r o c e s s [ 1 6 ] . T h e p e a k ~ri~ing is m u c h b r o a d e r t h a n t h e u s u a l "~netastable peak" and has a pronounced dip in its centre. However, this p e a k is p r o b a b l y o f t h e f i a t - t o p p e d e n e r g y r e l e a s e t y p e a s r e p o r t e d b e f o r e [ 1 7 ] . C a l c u l a t i o n o f T s h o w e d n e a r l y e q u a l v a l u e s o f 0 . 5 4 6 e V i n t h e f~-st and second tqeld-free regions.
t~'ualdehyde T h e l o s s o f C H O a s a n e u t r a l r a d i c a l l e a d s t o t h e n o .... al i n t e n s e i o n o f ~tlctu~ C4I-1~9. T h e m e t a s t a b l e t r a n s i t i o n o f t h l s r e a c t i o n h a s b e e n o b s e r v e d . T h e m e t a s t a b l e p e a k is c l e a r l y o f t h e d i s h - t o p p e d e n e r g y r e l e a s e t y p e a s s e e n in Fig. 1. Calcl]l~tion of the klnetic energy released has been found to be 1.066 and 0.942 eV in the first and second field-free regions, respectively.
n-Butyl m e t h y l k e t o n e and pinakoline Metastable transitions of the molecular and f~agment ions in the mass spe~Ltm of n-butyl methyl ketone and pinakoline occur~ug in the first and second field-free regions were investigated. The Lwo isomeric metastable m o l e c H l a r i o n s d e c o m p o s e t o y i e l d ( M - - C H 3 ) ÷. E a c h o f t h e s e i o n s d i s s o c i a t e f u r t h e r t o g i v e a n i o n o f M / e 5 7 (C4I-I~9) i n a c l i ~ n c t r a t i o f o r e a c h o f t h e t w o s p e c i e s . T h e s p e c t ~ , m o f n - b u t y l m e t h y l k e t o n e is c h a r a c t e r i z e d b y t h e p r e s ence of a metastable peak resulting from the separation of ethylene from ( M - - C 2 H s ) ÷, r e s u l t i n g i n t h e C H 3 C O + i o n . F u r ~ h e r , i t a p p e a r s f r o m F i g . 2 t h a t the metastable resulting from the dissociation of the (M--CH3) ÷ ion in n - b u t y l m e t h y l k e t o n e is o f t h e g a u s s i a n e n e r g y r e l e a s e t y p e , w h i l e i n t h e pinakoline counterpart the peak has a pronounced d i p i n i t s c e n t r e a n d is p r o b a b l y o f t h e f i a t - t o p p e d t y p e . I t s e e m s t h a t t h e r e is m o r e t h a n o n e m e c h a nizam f o r t h e f o r m a t i o n o f t h e f r a g m e n t i o n M / e 5 7 . T h e k i n e t i c e n e r g i e s released in the first and the second field-free regions for the decomposition
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o f t h e m o l e c u l a r a n d s o m e f r a g m e n t i o n s in b o t h m o l e c u l e s h a v e b e e n calc,,]~ted and are presented in Table 1. All transitions mentioned above were shown to be due to ,,nlmolecular fragmentation by taking measurements over a range of pressure (1 X 10 -5-1 X 1 0 -7 t o r t ) i n t h e ffLrst f i e l d - f r e e r e g i o n . I n n o c a s e w a s a c h a n g e i n p e a k shape detectable. In addition, the effect of varying the ion accelerating voltage on the shape of the metastable peak has been investigated; it has been f o u n d t h a t t h e s h a p e o f t h e m e t a s t a b l e p e a k is c h a n g e d i n s o m e c a s e s f r o m g a u s s i a n t o " r o u n d e d " as t h e a c c e l e r a t i n g v o l t a g e is d e c r e a s e d [ 1 8 ] . Comparisons between T calc,,l~ted from the HV and ESA scan methods show no detectable difference in trinetic energy release d,,ring the molecular ion decomposition in the first and second field-free regions if the resulting metastable fragment ions have gaussian or fiat-topped peaks. On the other hand, if the shape of the metastable fragment ions resulting from metastable tr~n~tion of the molec,]lnr ions are of the dish-topped type, then T calcul a t e d f r o m H V v a r i a t i o n s t u d i e s is s o m e w h a t h i g h e r t h a n f o r t h e E S A s c a n method. In addition, the kinetic energy released in further dissociation of f r a g m e n t i o n s r e v e a l e d t h a t T f o r m e t a s t a b l e t r n n . ~ t i o n o c c u r r i n g i n t h e fLrSt f i e l d - f r e e r e g i o n is e q u a l t o i n s o m e c a s e s , a n d g r e a t e r t h a n i n o t h e r c a s e s , t h e value of T for the s~me transition occurring in the second field-free region. The kinetic energy released in a metastable decomposition originates from Lwo s o u r c e s , t h e i n t e r n a l e n e r g y o f t h e a c t i v a t e d c o m p l e x a n d t h e r e v e r s e activation energy [19]. For a given reaction the former, but not the latter, should depend on the s~mpling time. Hence the reproducible differences in T obtained in the present study could be aLL~ibuted to different ion lifetimes and thus internal energies for dissociations in the first and second field-free r e g i o n . I t is h o w e v e r n o t u n d e r s t o o d w h y o n l y f o r d i s h - s h a p e d p e a k s , w h e r e the contribution from the reverse activation energy should prevail, a pron o u n c e d d e p e n d e n c e o n t h e s ~ m p H n g t i m e is o b s e r v e d . ACKNOWT .P.DGEW~.NTS T h e D e u t s c h e F S r d e r u n g s g e s e l i s c h ~ f ~ f i i r E n t w i c k l u n g s ] ~ u d e r ( G A W I ) is th~nlced for grant No. 73. 2507. 9/4001 towards the purchase of the mass spectrometer. We are also grateful to Prof. Dr. M. EI-Nadi, Vice President of M a n s o u r a U n i v e r s i t y , f o r bi~ u s e f u l i n t e r e s t . REFERENCES 1 J ~ I . B e y n o n , R . A . S a u n d e r s a n d A_E. W~U;~m~, Z . Naturfozr~_h. A , 2 0 ( 1 9 6 5 ) 1 8 0 . 2 J M . B e y n o n a n d A_E. F o n t a i n e , Z . N a t u r f o r s c h . A , 2 2 ( 1 9 6 7 ) 3 3 4 . 3 J M . B e y n o n , A . E . F o n t ~ ; n e a n d J_R. L e s t e r , I n t . J. Mass S p e c t r o m . I o n P h y s . , 8 (1972) 341. 4 W. H i g ~ n g a n d K R . J e n n ; n g s , Tr~nq. F a r a d a y S o c . , 6 2 ( 1 9 6 6 ) 9 7 . 5 J_L. H o l m e s , O_D. O s b o r n e a n d G . M . W e e s e , I n t . J. M - ~ S p e c t r o m . I o n P h y s . , 1 9 (1976) 207.
231 6 M. Barber and R.M. Elliot, paper presented at the 12th Annual Conference on i ~ Spectrometry and Allied Topics, ASTM Committee E-14, Montreal, 1964. 7 K.R. J~nn~-gs , J. Chem. Phys., 43 (1965) 4176. 8 R . W . K i s e r , R . E . Sn]Hvan a n d M . S . L u p i n , A n a l . C h e m . , 4 1 ( 1 9 6 9 ) 1 9 5 8 . 9 J.H. Beynon, J.W. Amy and W.E. Baitinger, Chem. Commun., (1969) 723. I 0 C. R e i c h e r t , R . E . F r a s s a n d R . W . K i s e r , I n t . J . i ~ Spectrom. I o n P h y s . , 5 ( 1 9 7 0 ) 457. I I R . G . C o o k s , J~-I. B e y n o n , R . M . C a p r i o l i a n d G . R . I ~ - t e r , M e t a s t a b l e I o n s , EIsevier~ Am~t~'~,m, 1973. 12 D.T. TerwiIliger, J.H. Beynon and R.G. Cooks, Proc. Roy. Soc., Set.A, 341 (1974) 135. 13 K . C . S y m t h a n d T . W . S h a n n o n , J . C h e m . P h y s . 51 ( 1 9 6 9 ) 4 6 3 3 . 1 4 M. B e r t r a n d , J~T-I. B e y n o n a n d R . G . C o o k s , I n t . J . i ~ - ~ S p e c t r o m . I o n P h y s . , 9 ( 1 9 7 2 ) 346, 15 R . G . C o o k s , K . J . l ~ i m , T . K e o u g h a n d J . H . B e y n o n , I n t . J . M ~ S p e c t r o m . I o n P h y s . , 15 (1974) 271. 16 M.D. M i g a h e d , A . I . Helal a n d S.B. E l - K h o l y , Org. Mass S p e c t r o m . , 7 ( 1 9 7 3 ) 1 4 2 3 . 17 M . D . M i g a h e d , A . S h e h a p a n d Th.IVI. E I - S h e r b i n i , P r o c . M a t h . P h y s . S o c . ( A R E ) , 4 4 (1977) 173. 18 T.W. ~nnon, F.W. Mcr~fer~ and C.R. McK;n-ey, Chem. Commun., (1966) 478. 19 E.G. Jones, J.H. Beynon and R.G. Cooks, J. Ch~m. Phys., 57 (1975) 2654.