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TEMPERATURE EFFECTS IN MATRIX ASSISTED FIELD DESORPTION MASS SPECTROMETRY
E. Bramer-Weger, S. Wong, M. Subhan, F. Röllgen
To cite this version:
E. Bramer-Weger, S. Wong, M. Subhan, F. Röllgen. TEMPERATURE EFFECTS IN MATRIX
ASSISTED FIELD DESORPTION MASS SPECTROMETRY. Journal de Physique Colloques, 1986,
47 (C7), pp.C7-441-C7-445. �10.1051/jphyscol:1986774�. �jpa-00225969�
TEMPERATURE EFFECTS IN MATRIX ASSISTED FIELD DESORPTION MASS SPECTROMETRY
E . BRAMER-WEGER, S. S. WONG, M. SUBHAN and F. W. ROLLGEN I n s t i t u t e of P h y s i c a l Chemistry, U n i v e r s i t y of Bonn, W e g e l e r s t r a s s e 1 2 , 0-5300 Bonn, F.R.G.
A b s t r a c t
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Matrix t e c h n i q u e s a r e f r e q u e n t l y a p p l i e d i n f i e l d de- s o r p t i o n mass spectrometry (FD MS) t o f a c i l i t a t e t h e d e s o l v a t i o n of molecular i o n s and t o reduce t h e thermal s t r e s s on molecules.For some o l i g o s a c c h a r i d e s mixed with NaI and t a r t a r i c a c i d and f o r v a r i o u s mixtures of sucrose t h e o n s e t temperature of molecular i o n emission, which i s c l o s e t o t h e b e s t e m i t t e r temperature f o r t a k i n g mass s p e c t r a , was determined. The o n s e t temperatures a r e t y p i c a l l y s i g n i f i c a n t l y below t h e melting p o i n t of t h e s o l v e n t - and matrix- f r e e sample. Using t h e same mixtures t h e o n s e t temperatures f o r p o s i t i v e i o n s were found t o be s l i g h t l y lower t h a n f o r n e g a t i v e i o n s . The i o n emission temperature i s reduced by t h e a p p l i c a t i o n of a polymer m a t r i x and by t h e admixture of an a p p r o p r i a t e s u r f a c e - a c t i v e compound b u t i s r a i s e d by t h e u s e of high s a l t concentra- t i o n s . The e f f e c t i s g r e a t e r with LiCl than with NaI.
I
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INTRODUCTIONI n f i e l d d e s o r p t i o n mass spectrometry (FD MS) / I / s o c t i o n i z a t i o n of t h e r m a l l y l a b i l e and n o n - v o l a t i l e compounds i s achieved by a f i e l d i n - duced d e s o l v a t i o n of i o n s from v i s c o u s l a y e r s / 2
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4/. For t h i s purpose sample l a y e r s a r e d e p o s i t e d on f i e l d anodes o r cathodes and h e a t e d t o induce t h e electrohydrodynamic d i s i n t e g r a t i o n p r o c e s s e s r e s u l t i n g i n t h e d e s o l v a t i o n of i o n s .For most samples e m i t t e r h e a t i n g i s a p r e r e q u i s i t e f o r t h i s i o n i z a t i o n mechanism. However, h e a t i n g can a l s o r e s u l t i n d e s t r u c t i v e e f f e c t s such a s thermal degradation of t h e sample and t h e removal of t h e sample l a y e r p r i o r t o formation of ion e m i t t i n g protuberances. The l a t t e r may a r i s e from an i n s u f f i c i e n t adhesion of t h e l a y e r t o t h e e m i t t e r s u r f a c e o r from temperature dependent p r o p e r t i e s of t h e l a y e r , r e l a t e d t o v i s c o s i t y and s u r f a c e t e n s i o n , which do n o t allow t h e formation of p r o t r u s i o n s of s u f f i c i e n t f i e l d enhancement.
Various m a t r i x t e c h n i q u e s have been i n t r o d u c e d t o f a c i l i t a t e t h e d e s o l - v a t i o n of i o n s and g e n e r a t e long l a s t i n g and abundant i o n s i g n a l s /5-7/.
I n t h e n e g a t i v e ion mode of FD MS (NFD MS) matrix e f f e c t s a r e used t o a v o i d e l e c t r o n detachment from i o n s by keeping t h e d e s o l v a t i o n f i e l d s t r e n g t h low /8
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l o / . For a n a l y t i c a l a p p l i c a t i o n s m a t r i x e f f e c t s , which cause a r e d u c t i o n of t h e thermal s t r e s s on molecules i n t h e FD p r o c e s s by lowering t h e emission temperature, a r e of p a r t i c u l a r i n t e r e s t . We have s t u d i e d t h e i n f l u e n c e of matrix mixtures and mixtures with ad- d i t i v e s on t h e o n s e t temperature of molecular ion emission f o r s e l e c t e d Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986774C7-442 JOURNAL DE PHYSIQUE
t e s t compounds. The b e s t e m i t t e r t e m p e r a t u r e s i n FD MS a r e c l o s e t o t h e o n s e t t e m p e r a t u r e s , t y p i c a l l y n o t more t h a n 1 0 t o 20 O C above. T h i s p a p e r r e p o r t s r e s u l t s o b t a i n e d f o r m o l e c u l a r i o n s of o l i g o s a c c h a r i d e s , mainly s u c r o s e , a p p l y i n g t h e p o s i t i v e and n e g a t i v e i o n mode of FD MS.
I1
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EXPERIMENTALThe e x p e r i m e n t s were performed w i t h a s i n g l e f o c u s i n g mass s p e c t r o m e t e r equipped w i t h a n FD i o n s o u r c e . Only b a r e 1 0 pm W-wires w e r e u s e d a s f i e l d anodes o r c a t h o d e s . I n t h e p o s i t i v e and n e g a t i v e i o n modes of FD, t h e e m i t t e r c o u n t e r e l e c t r o d e d i s t a n c e s were 3 and 4 mrn, and t h e a p p l i e d p o t e n t i a l d i f f e r e n c e s , 9 and 4 kV, r e s p e c t i v e l y .
The w i r e emitters were p l a c e d p a r a l l e l t o t h e c o u n t e r e l e c t r o d e s l i t . A c c o r d i n g l y t h e o n s e t t e m p e r a t u r e i s r e l a t e d t o t h e Semperature of t h e middle p a r t of t h e w i r e (T,). The mean t e m p e r a t u r e ( T ) of t h e w i r e e m i t t e r s was determined by r e s i s t a n c e measurements / 4 / . The t e m p e r a t u r e T m 1s g i v e n i n a good approximation by
Tm = 1.5
T -
0.5 TRwhere TR i s t h e t e m p e r a t u r e a t t h e end of t h e h e a t e d w i r e which i s u s u a l l y a t room t e m p e r a t u r e .
The o n s e t t e m p e r a t u r e s g i v e n i n t h e t a b l e s a r e t h e r e s u l t of r e p e a t e d measurements. They were o b t a i n e d under c o n d i t i o n s of a slow h e a t i n g r a t e of a few degrees/min and f o r an i o n s i g n a l l a s t i n g a t l e a s t 3 min. The r e p r o d u c i b i l i t y of t h e t e m p e r a t u r e d e t e r m i n a t i o n was w i t h i n +/- 5 OC.
The a c c u r a c y oE t h e t e m p e r a t u r e d e t e r m i n a t i o n was e s t i m a t e d t o be +/- 10 OC.
111
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RESULTS AND DISCUSSIONThe growth of f i e l d e n h a n c i n g p r o t u b e r a n c e s from a sample l a y e r and a c c o r d i n g l y t h e o n s e t t e m p e r a t u r e f o r m o l e c u l a r i o n e m i s s i o n i s d e t e r - mined i n a r a t h e r c o m p l i c a t e d way t h r o u g h a number of p a r a m e t e r s . For a homogeneous l a y e r i m p o r t a n t p a r a m e t e r s a r e v i s c o s i t y , s u r f a c e t e n s i o n and e l e c t r i c a l c o n d u c t i v i t y . S i n c e t h e s e q u a n t i t i e s change w i t h t i m e and s o l v e n t e v a p o r a t i o n , t h e e m i t t e r h e a t i n g r a t e a l s o a f f e c t s t h e on- s e t t e m p e r a t u r e . T h e r e f o r e we a p p l i e d slow h e a t i n g r a t e s t o measure low o n s e t t e m p e r a t u r e s o f m o l e c u l a r i o n e m i s s i o n .
T a b l e 1 g i v e s t h e measured o n s e t t e m p e r a t u r e s f o r FD of some o l i g o s a c - c h a r i d e s . NaI and t a r t a r i c a c i d were added t o t h e aqueous s o l u t i o n s of t h e s a c c h a r i d e s t o form e l e c t r o l y t e s o l u t i o n s and t o a c c o u n t f o r t h e f o r m a t i o n of c a t i o n i z e d m o l e c u l e s . The t e m p e r a t u r e s of T a b l e 1 a r e s i g - n i f i c a n t l y below t h e m e l t i n g p o i n t of t h e s o l v e n t - f r e e samples. For example t h e m e l t i n g p o i n t of w a t e r - f r e e s u c r o s e i s 185 "C / T I / . The i n - c r e a s e of o n s e t t e m p e r a t u r e w i t h t h e s i z e of t h e m o l e c u l e s r e f l e c t s an i n c r e a s e of t h e v i s c o s i t y o f t h e sample s o l u t i o n r a t h e r t h a n an i n - c r e a s e of t h e d e s o l v a t i o ~ e n e r g i e s . T h i s i s s u p p o r t e d by t h e f a l t t h a t t h e l e s s v i s c o u s s t a c h y o s e / t a r t a r i c a c i d s o l u t i o n gives+[M
+
HI s t a c h - yose i o n s a t a lower t e m p e r a t u r e t h a n a t which [M+
Na] i o n s a r e ob- t a i n e d from a s t a c h y o s e / N a I s o l u t i o n .A s shown i n T a b l e 2 f o r s u c r o s e t h e u s e of a m a t r i x such a s p o l y e t h y - l e n e o x i d e ( o r p o l y e t h y l e n e g l y c o l ) l o w e r s t h e o n s e t t e m p e r a t u r e s . T h i s e f f e c t a r i s e s from a l o w e r i n g of t h e v i s c o s i t y and s u r f a c e t e n s i o n by t h e polymer. R e p l a c i n g t h e s o l v e n t w a t e r w i t h g l y c e r o l , m o l e c u l a r i o n s c o u l d be d e t e c t e d a t room t e m p e r a t u r e (RT) and a t e l e v a t e d tempera-
Sample (weight r a t i o ) Molecular i o n Onset temperature ( O C )
s u c r o s e + NaI + water [M
+
Nal+( 2 : I : 10)
r a f f i n o s e + NaI + water [M
+
Na] + ( 2 : I : 10)stachyose + NaI + water [M
+
~ a ] +( 2 : I : 10)
stachyose
+
t a r t a r i c a c i d [M+
H I +( 2 : 1 ) + w a t e r
Table 2
-
Temperature range f o r t h e emission of molecular i o n s of sucroseSample (weight r a t i o ) Molecular i o n Temperature range ( O C ) sucrose + NaI
( 2 : I ) + water
sucrose + Nal' + PEO 4000
(2 : 1 : 1 ) + water
s u c r o s e + NaI + PEO 4000 + g l y c e r o l ( 2 : 1 : 1 : 5 ) + water
sucrose + g l y c e r o l (1 : 40) sucrose + g l y c e r o l
+ PEO 4000 (1 : 40 : 0 , 5 )
[ M
+
Na] + 85-
130[M
+
~ a ] + RT ( 4 min) 45-
85RT = room temperature; PEO 4000 = polyethylene oxide of mean molecular weight 4000 u.
t u r e s . Two temperature ranges of i o n emission a r e t y p i c a l f o r g l y c e r o l s o l u t i o n s . A t RT most of t h e sample i s l o s t i n a s h o r t time. The RT de- s o r p t i o n c o n d i t i o n s resemble t h o s e of an "electrohydrodynamic i o n i z a - t i o n " source /12, 13/ i n t h a t t h e i o n emission t a k e s p l a c e from a gly- c e r o l s o l u t i o n of low v i s c o s i t y . I n c o n t r a s t , t h e i o n emission a t higher temperatures i s determined from a h i g h l y v i s c o u s s t a t e of t h e r e s i d u a l sample l a y e r d e p l e t e d from g l y c e r o l .
Onset temperatures f o r f i e l d emission of s u c r o s e molecular a n i o n s from v a r i o u s mixtures a r e l i s t e d i n Table 3 . I n t h e n e g a t i v e ion mode of F D , polymers such a s PEO o r PEG 4000 a r e t y p i c a l l y a p p l i e d t o f a c i l i t a t e t h e d e s o l v a t i o n of i o n s . LiCl i s added a s an e l e c t r o l y t e t o achieve a s u f f i c i e n t c o n d u c t i v i t y of t h e sample l a y e r and Eor molecular anion
C7-444 JOURNAL DE PHYSIQUE
f o r m a t i o n . LiCl r a i s e s t h e v i s c o s i t y and t h e surEace t e n s i o n of t h e m i x t u r e . Accordingly t h e o n s e t t e m p e r a t u r e d e c r e a s e s w i t h t h e concen- t r a t i o n o f t h e s a l t . ~ u r t h ' e r m o r e a r e d u c t i o n o i t h e e m i s s i o n tempera- t u r e was a c h i e v e d by t h e admixture of t h e c a t i o n i c s u r f a c t a n t dodecyl t r i m e t h y l ammonium c h l o r i d e ( D O A ) , l o w e r i n g t h e s u r f a c e t e n s i o n . Tar- t a r i c a c i d used i n s t e a d o f L i C l a l s o r e s u l t s i n a lower o n s e t tempe- r a t u r e . However, t h e m o l e c u l a r a n i o n s i g n a l o f s u c r o s e was r a t h e r weak i n t h e p r e s e n c e of t a r t a r i c a c i d .
T a b l e 3
-
Onset t e m p e r a t u r e s f o r t h e e m i s s i o n of m o l e c u l a r i o n s of s u c r o s e from m i x t u r e s i n t h e n e g a t i v e i o n FD mode ( w a t e r was u s e d a s s o l v e n t a t a w e i g h t r a t i o PEG 4000 : w a t e r of 1 : 2)Sample ( w e i g h t r a t i o ) Molecular i o n Onset t e m p e r a t u r e ( O C ) s u c r o s e + LiCl + PEG 4000 [M
+
C ~ I -( 2 : 1 : 2) (1000 : 1 : 1000)
s u c r o s e + DOA + PEG 4000 [M
-
H I-
(20 : 1 : 20)
s u c r o s e + LiCl + DOA [M
+
~ 1 1 -+ PEG 4000 (20 : 10 : 1 : 20)
s u c r o s e + t a r t a r i c a c i d [ M
-
H I -+ PEG 4000 ( 2 : 1 : 2 )
s u c r o s e + t a r t a r i c a c i d [M
-
H I -+ DOA + PEG 4000 (20 : 10 : 1 : 20)
PEG 4000 = p o l y e t h y l e n e g l y c o l of mean m o l e c u l a r w e i g h t 4000 u DOA = dodecyl t r i m e t h y l ammonium c h l o r i d e
I n T a b l e 4 t h e c o r r e s p o n d i n g o n s e t t e m p e r a t u r e s f o r m o l e c u l a r i o n s of s u c r o s e o b t a i n e d i n t h e p o s i t i v e i o n mode of FD from t h e same and s i m i - l a r m i x t u r e s a s t h o s e of T a b l e 3 a r e g i v e n . I n t h e m i x t u r e s i n which DOA i s r e p l a c e d by t h e a n i o n i c s u r f a c t a n t sodium d o d e c y l s u l f a t e (DOS) t h e s u r f a c t a n t h a s a s i m i l a r e f f e c t on t h e e m i s s i o n t e m p e r a t u r e i n t h e p o s i t v e i o n mode a s DOA i n t h e n e g a t i v e i o n mode of FD. The o n s e t t e m p e r a t u r e s f o r p o s i t i v e i o n s a r e always lower t h a n t o r n e g a t i v e i o n s u s i n g t h e same m i x t u r e s o r m i x t u r e s of s i m i l a r p r o p e r t i e s . T h i s e f f e c t may a r i s e from d i f f e r e n c e s i n t h e a p p l i e d f i e l d t o t h e s u r f a c e of t h e w i r e e m i t t e r , and from t h e f i e l d s t r e n g t h l i m i t a t i o n i n t h e d e s o l v a - t i o n of a n i o n s from p r o t u b e r a n c e s c a u s e d by f i e l d induced e l e c t r o n de- tachment. T h i s i n t e r e s t i n g e f f e c t s h a s n o t been i n v e s t i g a t e d i n d e t a i l y e t . T a b l e 4 a l s o shows t h a t t h e o n s e t t e m p e r a t u r e s f o r m o l e c u l a r i o n e m i s s i o n from a m i x t u r e w i t h L i C l i s s i g n i f i c a n t l y h i g h e r t h a n from t h e c o r r e s p o n d i n g , m i x t u r e w i t h NaI ( s e e T a b l e 1 ) . Such a s a l t e f f e c t h a s a l r e a d y been r e p o r t e d f o r L i I , Nai and K I / 1 4 / . The d e c r e a s e i n o n s e t t e m p e r a t u r e p a r a l l e l s t h a t of t h e v i s c o s i t y , s u r f a c e t e n s i o n and of t h e d e s o l v a t i o n e n e r g i e s of t h e m o l e c u l a r i o n s .
Sample ( w e i g h t r a t i o ) M o l e c u l a r i o n O n s e t t e m p e r a t u r e ( OC) s u c r o s e + PEG 4000 + L i C l [M
+
~ i ] +( 2 : 2 : 1 ) (1000 : 1000 : 1 )
s u c r o s e + PEG 4000 + L i C l [ M
+
~ i ] ++ DOS (1000 : 1000 : 1 : 50)
s u c r o s e + PEG 4000 + t a r - [M
+
H I + t a r i c a c i d ( 2 : 2 : 1 )s u c r o s e + PEG 4000 + t a r - [M
+
H I +t a r i c a c i d + DOS (20 : 20 : 10 : 1 )
DOS = sodium d o d e c y l s u l f a t e
ACKNOWLEDGEMENT
-
The a u t h o r s a r e g r a t e f u l t o t h e W i s s e n s c h a f t s - m i n i s t e r i u m d e s Landes N o r d r h e i n - W e s t f a l e n f o r f i n a n c i a l s u p p o r t o f t h i s work.REFERENCES
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