ORGANIC MASS SPECTROMETRY, VOL. 22, 186-187 (1987)

OMS Letters

Dear S i r :

EFFECT OF METlIOD OF I O N PREPARATION ON CHARGE STRIPPING MASS SPECTRA

It has been c l e a r l y demonstrated t h a t t h e charge

s t r i p p i n g mass s p e c t r a of s i n g l y charged p o s i t i v e

ions can be used t o c h a r a c t e r i z e gaseous ion s t r u c -

t u r e s ; i n many c a s e s such s p e c t r a p rov ide isomer

d i f f e r e n t a t ion where co l l i s ion - induced d i s s o c i a t i o n

mass s p e c t r a do n o t . ’ Although i t i s c l e a r l y e s t ab -

l i s h e d t h a t charge s t r i p p i n g mass s p e c t r a a r e depen-

dent on t h e i d e n t i t y of t h e n e u t r a l c o l l i s i o n gas,’”

i t i s l e s s c l e a r what r o l e t h e i n t e r n a l energy of

t h e r e a c t i n g i.ons p l ays i n de te rmining t h e mass

spectrum observed. Bowen et a13 and McLafferty

e t a14 have r epor t ed t h a t t h e charge s t r i p p i n g mass

s p e c t r a of i somer ic [C H 1 i ons prepared by 70 e V

and by 1 7 e V e l e c t r o n impact a r e t h e same, w i t h i n

exper imenta l e r r o r , sugges t ing no e f f e c t of r e a c t a n t

ion i n t e r n a l energy on t h e observed s p e c t r a . Dass

e t a15 have observed s i g n i f i c a n t d i f f e r e n c e s i n t h e

charge s t i i p p i n p mass s p e c t r a 0: [ C ~ : : ~ I + . i o z s p r e -

pared by charge exchange wi th d i f f e r e n t i n t e r n a l

e n e r g i e s but have a t t r i b u t e d t h e s e d i f f e r e n c e s t o

i somer i za t ion r e a c t i o n s of t h e p recu r so r i o n s p r i o r

t o c o l l i s i o n r a t h e r t han t o i n t e r n a l energy e f f e c t s

on t h e mass spectrum of a d i s t i n c t i o n i c species.

On t h e o t h e r hand, Bowers and co l l eagues6 have r e -

por ted t h a t t h e charge s t r i p p i n g mass s p e c t r a of

[CH3CH2NH3]+, [ CH3NH2CH3]+ and [ CH3CH20H2]+, pre-

pared by pro ton t r a n s f e r chemical i o n i z a t i o n , changed

wi th t h e t o t a l i on source p r e s s u r e . The pro tona ted

s p e c i e s were formed by a s t r o n g l y exothermic pro ton

t r a n s f e r r e a c t i o n ; w i th i n c r e a s i n g source p r e s s u r e

t h e exc i t ed s p e c i e s should b e more e f f e c t i v e l y de-

+. 3 6

a c t i v a t e d by c o l l i s i o n and they a t t r i b u t e d t h e ob-

se rved s p e c t r a l changes t o t h e e f f e c t of i n i t i a l

i n t e r n a l energy and/or angu la r momentum on t h e

cha rge s t r i p p i n g m a s s s p e c t r a . The p resen t l e t t e r

r e p o r t s obse rva t ions s i m i l a r t o those r epor t ed by

Bowers e t a16, bu t f o r more complex s p e c i e s , which

show t h a t cha rge s t r i p p i n g m a s s s p e c t r a of i o n s pre-

pared by p ro ton t r a n s f e r chemical i o n i z a t i o n depend

on t h e method of i on p r e p a r a t i o n ; w e a t t r i b u t e t h e s e

changes i n s p e c t r a t o t h e e f f e c t of i n i t i a l i n t e r n a l

energy.

+ + r 2 3

r +OH

CH CH=CH-CH CH =C(CH )C€l 3 II

CH =CH-CCH 2 1 2 3

Table 1 p r e s e n t s t h e charge s t r i p p i n g mass s p e c t r a of

t h e [C4H70]

f e r chemical i o n i z a t i o n . These s p e c t r a were ob-

t a i n e d u s i n g a ZAB-2FQ m a s s spec t rometer7 f o r i ons of

8 keV k i n e t i c energy us ing H e as c o l l i s i o n gas a t an

ind ica tcc! p r e s x r c of 2 S 12 xbar OF. t h e ion iza -

t i o n gauge l o c a t e d on t h e pumping l i n e below t h e

c o l l i s i o n c e l l . A l l mass-and energy-reso lv ing s l i t s

were f u l l open and s i g n a l s were measured i n te rms of

r e l a t i v e peak h e i g h t s . R e s u l t s are presented f o r

i o n s prepared by CH4 chemical. i o n i z a t i o n and by

i - C 4 H I 0 chemical i o n i z a t i o n .

s t u d i e d , t h e charge s t r i p p i n g mass s p e c t r a ob ta ined

f o r i o n s prepared by i-C4H10 C I show a g r e a t e r abun-

dance of [C4H70]’+ and a lower abundance of i ons

a r i s i n g by m u l t i p l e l o s s of H / H from t h i s i on

( i e . [C4H501 and [C4H40] ) t han do t h e s p e c t r a

ob ta ined f o r t h e same i o n s prepared by CH C I ; t h e

d i f f e r e n c e s a r e w e l l o u t s i d e t h e r e p r o d u c i b i l i t y of

+ ions 1 t o 3 , p repared by pro ton t r a n s -

-7

For t h e t h r e e s p e c i e s

2 2+ 2+

4

Table 1 . H e Charge S t r i p p i n g Mass Spec t r a of (C H 0)’ Ions 4 7

% of To ta l Ion S igna l

1 2 3

Product C H 4 C I i -C4H10CI C H 4 C I i - C 4 H 1 0 C I C H 4 C I i -C4H10CI

[ C4H701 ’+ 70.922.6 85.220.2 45.6f4.7 58.820.6 38.021.6 51.2+0.7

14.720.7 18 .3 t0 .1 27.321.3 25.7f1.1 [CGH6012+ 0 .8 f0 .3

[ C4H401 ’+ 26.9?2.4 12 .7 t0 .3 22.523.4 12.120.2

- 17.2 t2 .0 10.820.5 13.7t0.7 9.3t0.2 [ C4H501 2f 2 .2 t0 .3 1 .2 t0 .1

21.121.2 13.820.4

@ 1987 by John Wiley & Sons, Ltd. Received December 1986

OMS LETTERS

t h e measuzements which a r e g i v e n i n the Table . The

s p e c t r a o b t a i n e d f o l l o w i n g CH C I are i n r e a s o n a b l e

agreement w i t h t h o s e r e p o r t e d e a r l i e r . 8 4

T h e p r o t o n a f f i n i t i e s o f the C H 0 c a r b o n y l com-

pounds have b e e n r e p o r t e d as PA(CH =CHCOCH )=826 k J

mol-', PA(CH CH=CHCHO)=824 k J mol-l and PA(CH =C(CH3)

CHO)=808 k J m o l - . compounds by [ t -C4H9] (PA(i-C4H8)=824 k J mol-' )

i n i-C4H10CI s h o u l d b e a p p r o x i m a t e l y t h e r m o n e u t r a l ,

a l t h o u g h it i s p r o b a b l e tha t n o t a l l [C4Hg]+ i o n s

are i n t h e i r ground s ta te u n d e r o u r e x p e r i m e n t a l

c o n d i t i o n s ; i n any event the e x t e n t o f e x c i t a t i o n of

t h e [C4H70] i o n s formed s h o u l d b e s m a l l . By c9n-

t ras t , p r o t o n a t i o n of t h e c a r b o n y l compounds i n

CH C I is s u b s t a n t i a l l y e x o t h e r m i c even f o r ground

s ta te r e a c t a n t i o n s (f'g(CH4)=546 k J mol-I ) and

PA(C2H4)=684 k J mo1-l

n i f i c a n t f r a c t i o n of t h i s e x o t h e r m i c i t y t o b e re-

t a i n e d by the [C4H70] i o n s formed. The s i m p l e s t

i n t e r p r e t a t i o n of the r e s u l t s is tha t a t least p a r t

of t h i s excess i n t e r n a l e n e r g y is c a r r i e d o v e r t o

the [C4H70]

p r o c e s s and r e s u l t s i n an i n c r e a s e d e x t e n t o f f r a g -

m e n t a t i o n of t h i s i o n . In s u p p o r t of t h i s i n t e r p r e -

t a t i o n w e n o t e t h a t v a r i a t i o n of t h e s o u r c e p r e s s u r e

i n the CH C I s y s t e m l e a d s t o a v a r i a t i o n i n t h e

c h a r g e s t r i p p i n g m a s s s p e c t r a a s shown i n T a b l e 2

f o r [CH CE=TUCHOH]+; w i t h i n c r e s s i n g p r e s s u r e t h e

[C4H70]

c o l l i s i o n a l l y , r e s u l t i n g i n less f r a g m e n t a t i o n f o l -

lowing c o l l i s i o n a l i o n i z a t i o n . The s p e c t r a o f T a b l e

1 were r e c o r d e d a t the s o u r c e p r e s s u r e which gave

t h e optimum y i e l d of t h e p r o t o n a t e d s p e c i e s ; t h i s

o c c u r r e d at a s o u r c e i o n i z a t i o n gauge r e a d i n g o f

- 5 X mbar, a l t h o u g h , f rom p r e v i o u s e x p e r i e n c e

w e b e l i e v e that when these e x p e r i m e n t s were per -

formed t h e i o n i z a t i o n gauge w a s r e a d i n g low by ap-

p r o x i m a t e l y a f a c t o r of t e n .

formed by i-C4H10CI showed no dependence of t he i r

c h a r g e s t r i p p i n g m a s s s p e c t r a on s o u r c e p r e s s u r e

s i n c e t h e [C4H70]+ ions are formed i n i t i a l l y w i t h

v e r y l i t t l e e x c e s s internal energy .

the s p e c t r u m o b t a i n e d a t t h e h i g h e s t CH4 s o u r c e

p r e s s u r e i s v e r y c l o s e t o t h a t o b s e r v e d f o l l o w i n g

i -C4HIOCI .

4 6

2 3

3 1 2 Thus p r o t o n a t i o n of t h e c a r P 8 n y l +

+

4

) and o n e would e x p e c t a s i g -

+

2+ i o n formed i n the c h a r g e s t r i p p i n g

4

3 i o n s are more e f f e c t i v e l y d e a c t i v a t e d

The [C4H70]+ i o n s

I n t e r e s t i n g l y

T h e p r e s e n t r e s u l t s show c l e a r l y tha t the c h a r g e

s t r i p p i n g mass s p e c t r a o f i o n s formed b y p r o t o n

t r a n s f e r c h e m i c a l i o n i z a t i o n depend on the method

of i o n f o r m a t i o n i n a manner which i s c o n s i s t e n t

w i t h an i n f l u e n c e of i n i t i a l i n t e r n a l e n e r g y on the

c h a r g e s t r i p p i n g spec t rum. Obvious ly i n s u c h

e x p e r i m e n t s one s h o u l d p r e p a r e the reactant i o n s by

the least exothermic r e a c t i o n p o s s i b l e . It is w e l l -

187

e s t a b l i s h e d ' tha t c h a r g e s t r i p p i n g u s i n g N2 or 0 2 as c o l l i s i o n g a s i m p a r t s less e n e r g y t o t h e

c h a r g e - s t r i p p e d i o n and o n e might a n t i c i p a t e t h a t

t h e e f f e c t of i n i t i a l i n t e r n a l e n e r g y would b e more

pronounced when t h e s e c o l l i s i o n g a s e s a r e u s e d . I n

t h e p r e s e n t s y s t e m s the u s e of O2 o r N2 c o l l i s i o n

g a s e s r e s u l t e d a l m o s t e n t i r e l y i n f o r m a t i o n of

[ C H 012+.

s e p a r a t e l y , o f the c h a r g e s t r i p p i n g mass s p e c t r a of

[C H ]+ i o n s we have found t h a t t h e N2 and 0

c h a r g e s t r i p p i n g mass s p e c t r a a r e more s e n s i t i v e t o

the method of i o n p r e p a r a t i o n t h a n a r e t h e He c h a r g e

s t r i p p i n g mass s p e c t r a .

However, i n s t u d i e s , t o b e r e p o r t e d 4 7

2 9 11

T a b l e 2 . E f f e c t of CH4 P r e s s u r e on Charge S t r i p p i n g

Mass S p e c t r a o f 2

% of T o t a l Ion S i g n a l

3X10-6mbar 8X10-6mbar 2 . 5X10-5mbar

[C4H701 2+ 43 .3 55.2 60.6

[ C4H601 2+ 12 .6 14 .0 15.2

[ C4H50 1 2+ 1 9 . 3 13 .8 10 .8

[ C 4 H 4 0 1 '+ 24.9 17 .O 13.4

Yours

Alex G . H a r r i s o n

Department of Chemis t ry

Un iver 5 4. t y o f Tor o n t o

T o r o n t o , O n t a r i o

Canada M5S 1 A l

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1.

2 .

3 .

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6 .

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