Mass Spectrometry in Inorganic Chemistry

16 A Modified Mass Spectrometer Ion Source for the Study of High Temperature Vaporization

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Vaporization of Z i n c , Cadmium, Arsenic, Selenium, Cadmium Arsenide, and Cadmium Selenide. J. B. W E S T M O R E University of Manitoba, W i n n i p e g , Canada H.

FUJISAKI

Research Institute for Scientific Measurements, T o h o k u University, Sendai, Japan A. W. TICKNER Division of A p p l i e d Chemistry, National Research C o u n c i l , Ottawa, Canada The

construction

range 100° which

the vaporizations

presented. izations

of an ion source

to 400°C.

Zη(solid)

Zη(gas)

Cd

Cd

The

the

under results vapor

(gas)

As (gas) 4

Cd As

)

2(solid

CdSe

(solid)

The accommodation from solid arsenic

where n = 5, 6, 7, 8

Se (gas)

Se(solid)

4

and some

data are presented.

As(solid)

As

in

conditions

were

(solid)

3

The

occur are analyzed

Thermodynamic

studied

for vaporizations

is described.

n

3Cd

+

(gas)

Cd

As (gas) 4

+

(gas)

Se (gas) 2

coefficient or from Cd As

be small for the vaporization

3

of Se

2

2

for the vaporization

of

is small and also

may

from

CdSe.

231 In Mass Spectrometry in Inorganic Chemistry; Margrave, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

232

MASS S P E C T R O M E T R Y IN INORGANIC C H E M I S T R Y

h e most

commonly

used method

of

studying relatively involatile

m a t e r i a l s i n a mass spectrometer is b y means of a K n u d s e n or effus i o n c e l l . T h e s e are n o w a v a i l a b l e as accessories to c o m m e r c i a l i n s t r u ments.

A m o l e c u l a r b e a m f r o m the c e l l intersects a n e l e c t r o n b e a m i n

the i o n i z i n g r e g i o n .

T h e recorded

i o n c u r r e n t is p r o p o r t i o n a l to the

c o n c e n t r a t i o n of a p a r t i c u l a r m o l e c u l a r species i n the b e a m a n d c a n b e r e l a t e d to its p a r t i a l pressure i n s i d e the effusion c e l l .

B y v a r y i n g the

t e m p e r a t u r e of the effusion c e l l a n d r e c o r d i n g the i o n c u r r e n t o w i n g to the v a r i o u s species e m e r g i n g f r o m the c e l l at e a c h t e m p e r a t u r e , t h e r m o d y n a m i c d a t a c a n b e o b t a i n e d . I f the effusion c e l l c a n b e c a l i b r a t e d so Downloaded by MICHIGAN STATE UNIV on May 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch016

that absolute p a r t i a l pressures c a n b e d e t e r m i n e d , t h e n free energies a n d entropies as w e l l as enthalpies c a n b e o b t a i n e d f o r the v a p o r i z a t i o n reactions. A m o d i f i e d i o n source has b e e n c o n s t r u c t e d i n w h i c h the i o n i z i n g e l e c t r o n b e a m a c t u a l l y passes t h r o u g h a c h a m b e r c o n t a i n i n g v a p o r a b o v e the c o n d e n s e d phase as s h o w n i n F i g u r e 1. T h e details of its c o n s t r u c t i o n w i l l b e g i v e n i n the e x p e r i m e n t a l section.

T h e p a r t i a l pressure of the

v a p o r i n the i o n i z i n g r e g i o n is d i r e c t l y c o n t r o l l e d b y the t e m p e r a t u r e of the sample. O n e a d v a n t a g e of this i o n source over the c o n v e n t i o n a l K n u d s e n c e l l source

arises because the s a m p l e v a p o r is a c t u a l l y g e n e r a t e d

i n the

i o n i z i n g r e g i o n . T h i s results i n greater s e n s i t i v i t y a n d the a b i l i t y to w o r k at l o w e r v a p o r pressures.

U s i n g a F a r a d a y c u p collector a n d v i b r a t i n g

r e e d a m p l i f i e r , i o n currents w e r e m e a s u r e d at pressures e s t i m a t e d to b e i n the range 10" to 10" torr. G r e a t e r s e n s i t i v i t y s h o u l d b e a c h i e v e d w i t h 7

4

an e l e c t r o n m u l t i p l i e r detector.

It becomes possible to s t u d y systems at

l o w e r temperatures a n d different conditions t h a n p r e v i o u s l y , a n d i n some cases different b e h a v i o r has b e e n o b s e r v e d .

S i n c e at these pressures the

m e a n free p a t h is m u c h l a r g e r t h a n the c e l l d i m e n s i o n s i t is possible to work

at c o n d i t i o n s

where

collisions i n the

vapor

are of

negligible

importance. A d i s a d v a n t a g e of this i o n source is that because of the r e l a t i v e l y l a r g e area of the slits some v a p o r c a n diffuse b a c k i n t o the i o n i z a t i o n r e g i o n f r o m o u t s i d e the c e l l . A l t h o u g h the a m o u n t of b a c k diffusion is r e l a t i v e l y s m a l l , i n some cases i t p r e v e n t e d a n u n a m b i g u o u s

conclusion

as to w h e t h e r a s m a l l e r m o l e c u l e arose b y v a p o r i z a t i o n f r o m the c o n d e n s e d phase or b y t h e r m a l d e c o m p o s i t i o n of a l a r g e r m o l e c u l e o n t h e mass s p e c t r o m e t e r

filament.

A f u r t h e r c o n s e q u e n c e of the r e l a t i v e l y l a r g e

slit area is the effect o n the steady state pressure i n the s a m p l e c h a m b e r . T h i s effect w i l l b e a n a l y z e d i n the t h e o r e t i c a l section. T h e p o t e n t i a l of the s a m p l e c h a m b e r c o u l d b e v a r i e d w i t h respect to the

filament.

I n this w a y the c u r r e n t c a u s e d b y a p a r t i c u l a r i o n c o u l d b e

In Mass Spectrometry in Inorganic Chemistry; Margrave, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

16.

WESTMORE ET AL.

High

Temperature

Vaporization

233

d e t e r m i n e d as a f u n c t i o n of the electron energy, a n d a n i o n i z a t i o n effic i e n c y c u r v e c o u l d b e o b t a i n e d . A l t h o u g h the curves d i d n o t h a v e the i d e a l i z e d shape, a p p e a r a n c e potentials o b t a i n e d f r o m t h e m a g r e e d w e l l w i t h l i t e r a t u r e values w h e n a d i r e c t c o m p a r i s o n was possible. T h e elect r o n e n e r g y scale w a s c a l i b r a t e d w i t h a n i n e r t gas, u s u a l l y k r y p t o n or xenon.


Experimental D e s c r i p t i o n o f t h e I o n S o u r c e . T h e i o n source w a s o r i g i n a l l y d e s c r i b e d b y M a n n a n d T i c k n e r ( I I ) a n d later m o d i f i e d ( 1 7 ) . T h e details of the m o d i f i e d a p p a r a t u s n o w f o l l o w . T h e i o n source o f a c o n v e n t i o n a l 9 0 ° m a g n e t i c sector mass s p e c t r o m e ter was m o d i f i e d as s h o w n i n F i g u r e 1. O r i g i n a l l y the n o r m a l p u m p i n g o n the mass spectrometer t u b e w a s s u p p l e m e n t e d b y a 70 l i t e r s / s e c . m e r c u r y diffusion p u m p e q u i p p e d w i t h a l i q u i d n i t r o g e n t r a p a t t a c h e d d i r e c t l y to the i o n source c h a m b e r to r e d u c e the b a c k g r o u n d of r e s i d u als i n the i o n source. H o w e v e r , there w e r e a p p r e c i a b l e v a r i a t i o n s i n p u m p i n g speed, a n d h e n c e this extra d i f f u s i o n p u m p w a s o n l y u s e d f o r i n i t i a l h e a t i n g of the s a m p l e c h a m b e r . C o p p e r c o o l i n g coils w e r e a d d e d to the ends of the source c h a m b e r to p r e v e n t the r u b b e r O - r i n g s f r o m being overheated. T h e entire furnace assembly was m o u n t e d on plate P i w h i c h was a t t a c h e d to the s a m e m o u n t i n g posts as the f o c u s i n g a n d c o l l i m a t i n g plates of the mass spectrometer. P l a t e P was s u p p o r t e d w i t h respect to P i b y f o u r rods. A l l of the plates a n d s u p p o r t i n g rods w e r e m a d e of C h r o m e l A . 2

T h e e l e c t r o n b e a m was s u p p l i e d b y a tungsten filament F . I t e n t e r e d the i o n source f u r n a c e t h r o u g h a s m a l l slit a n d e m e r g e d t h r o u g h a s l i g h t l y l a r g e r slit to b e m e a s u r e d b y means of t r a p T . P e r m a n e n t magnets m o u n t e d e x t e r n a l l y s u p p l i e d a c o l l i m a t i n g field of a b o u t 400 gauss. A n exit slit w a s p r o v i d e d i n the e n d of the f u r n a c e , a n d a c i r c u l a r h o l e 1 c m . i n d i a m e t e r i n p l a t e P i opposite the slit a l l o w e d sufficient p e n e t r a t i o n of t h e field f r o m the f o c u s i n g plates to w i t h d r a w the ions. T h e t w o slits f o r t h e e l e c t r o n b e a m a n d the i o n exit slit w e r e d e f i n e d b y pieces of C h r o m e l f o i l s p o t - w e l d e d onto the i o n source f u r n a c e . T h e sizes of the slits c o u l d c o n s e q u e n t l y b e v a r i e d , a n d t h e i r t o t a l area a v e r a g e d b e t w e e n 4 to 6 s q . m m . T h r e e t o o l steel p i n s 0.5 m m . i n d i a m e t e r m o u n t e d o n P i a n d c o r r e s p o n d i n g holes i n t h e e n d of the i o n source f u r n a c e l o c a t e d the exit slit w i t h respect to the c o l l i m a t i n g slits of the mass spectrometer. E l e c t r o n s f r o m the filament w e r e a c c e l e r a t e d b y a p o t e n t i a l a p p l i e d b e t w e e n the filament a n d t h e f u r n a c e . T h i s p o t e n t i a l c o u l d b e v a r i e d b e t w e e n a b o u t 5 to 100 volts a n d was m e a s u r e d w i t h a d i g i t a l v o l t m e t e r w h e n i o n i z a t i o n efficiency curves w e r e b e i n g d e t e r m i n e d . T h e furnaces w e r e m a c h i n e d f r o m m o l y b d e n u m w h i c h , i n a d d i t i o n to its l o w v a p o r pressure a n d h i g h m e l t i n g p o i n t , possessed h i g h t h e r m a l c o n d u c t i v i t y a n d w a s n o t a t t a c k e d b y the v a p o r s of the metals to b e e x a m i n e d . E a c h f u r n a c e h a d its o w n h e a t i n g e l e m e n t of a n o n i n d u c t i v e t y p e c u t f r o m C h r o m e l A sheet a n d w o u n d o n w i t h m i c a i n s u l a t i o n . S i n c e it w a s n o t necessary to k n o w the t e m p e r a t u r e of the i o n source f u r n a c e


234



w i t h great a c c u r a c y , i t was c o n v e n i e n t to p l a c e a t h e r m o c o u p l e o n its w a l l i n s i d e t h e h e a t i n g element. T h e r a d i a t i o n shields w e r e m a d e of t a n t a l u m a n d w e r e s u p p o r t e d b y stainless steel screws a n d b o r o s i l i c a t e glass spacers. A l l e l e c t r i c a l leads w e r e b r o u g h t out at the e n d of the i o n source c h a m b e r b y means of f e e d - t h r o u g h insulators.

Figure 1.

Mass spectrometer ion source

A. Ion source furnace; B. Sample furnace; C. Radiation shields; D. Molybdenum crucible; E. Thermocouple; F. Filament; G. Sample support; H. Cooling coils; J. Feed through insulator of Teflon; Pj and Pi. Mounting plates; and T. Electron beam trap

A r e g u l a t e d a.c. p o w e r s u p p l y w i t h a n o u t p u t v o l t a g e constant to 0 . 1 % w a s u s e d to s u p p l y p o w e r for h e a t i n g the furnaces. E a c h f u r n a c e r e c e i v e d its p o w e r f r o m the s e c o n d a r y w i n d i n g of a n i s o l a t i n g t r a n s f o r m e r w h i c h a l l o w e d the f u r n a c e h e a t i n g elements to operate at the p o t e n t i a l of the i o n source, a p p r o x i m a t e l y 2000 volts a b o v e g r o u n d . T h e t e m p e r a t u r e of e a c h f u r n a c e w a s c o n t r o l l e d m a n u a l l y b y a d j u s t i n g a v a r i a b l e a u t o transformer w h i c h s u p p l i e d the voltage to the p r i m a r y w i n d i n g . T h i s


16.

WESTMORE

ET AL.

High

Temperature

235

Vaporization


a r r a n g e m e n t a l l o w e d different t e m p e r a t u r e s to b e m a i n t a i n e d i n the t w o furnaces over a sufficiently w i d e range. P o w d e r e d samples w e r e p l a c e d i n c r u c i b l e s m a c h i n e d f r o m m o l y b d e n u m . O n e c r u c i b l e h a d a s i n g l e l a r g e c o m p a r t m e n t w h i l e another h a d seven c o m p a r t m e n t s of e q u a l size d r i l l e d i n i t . S a m p l e s c o u l d b e p l a c e d i n different n u m b e r s of holes to v a r y the effective s a m p l e area, as w i l l b e d i s c u s s e d i n the t h e o r e t i c a l section. T h e c r u c i b l e w a s fitted i n t o a s p e c i a l l y m a d e q u a r t z c u p w h i c h h a d a c i r c u l a r h o l e i n the b o t t o m so t h a t i t c o u l d b e m o u n t e d o n a s u p p o r t m a c h i n e d f r o m b o r o n n i t r i d e or q u a r t z . A h o l e d r i l l e d i n the axis of the s a m p l e s u p p o r t a l l o w e d the i n s e r t i o n of a C h r o m e l P - A l u m e l t h e r m o c o u p l e w h i c h m e a s u r e d the s a m p l e t e m p e r a t u r e . T h e t h e r m o c o u p l e w i r e s w e r e b r o u g h t out of the v a c u u m w i t h o u t joins b y p a s s i n g t h e m t h r o u g h s m a l l holes i n a T e f l o n p l u g w h i c h was c o m p r e s s e d b y f o r c i n g i t i n t o a t a p e r e d h o l e b y s c r e w i n g d o w n a t h r e a d e d n u t onto i t . T h e t h e r m o c o u p l e was c a l i b r a t e d at the m e l t i n g p o i n t s of z i n c , l e a d , a n d t i n a n d at the b o i l i n g p o i n t of w a t e r . R e c a l i b r a t i o n after use s n o w e d t h a t the c a l i b r a t i o n s h a d c h a n g e d b y less t h a n 0.6° i n a l l cases. T h e e.m.f ,'s w e r e m e a s u r e d o n a p o t e n t i o m e t e r w i t h a n a c c u r a c y of ± 0 . 0 0 2 m v . T e m p e r a t u r e e q u i l i b r i u m c o u l d b e a t t a i n e d i n a f e w m i n u t e s , a n d the t e m p e r a t u r e w a s h e l d constant to w i t h i n 0.5° w h i l e the i o n currents w e r e b e i n g m e a s u r e d . It w a s f o u n d t h a t the a m o u n t b y w h i c h t h e t e m p e r a t u r e of the i o n source f u r n a c e e x c e e d e d that of the s a m p l e f u r n a c e h a d l i t t l e effect o n the results. S i n c e the temperatures of the t w o furnaces w e r e i n t e r d e p e n d e n t to some extent, a r e l a t i v e l y s m a l l t e m p e r a t u r e difference of a b o u t 5 ° w a s u s e d i n most of the e x p e r i m e n t s to e x t e n d the r a n g e of the s a m p l e f u r n a c e t e m p e r a t u r e to as l o w a v a l u e as possible. T o correct f o r a n y v a r i a t i o n i n the o v e r - a l l efficiency of the i o n source d u r i n g a series of measurements a s m a l l reference pressure of k r y p t o n or x e n o n was m a i n t a i n e d i n t h e i o n source c h a m b e r b y a l l o w i n g the gas to l e a k i n f r o m a reservoir i n the s a m p l e l i n e . K r y p t o n a n d x e n o n w e r e chosen b e c a u s e t h e y w e r e c h e m i c a l l y i n e r t , t h e i r i o n i z a t i o n potentials are a c c u r a t e l y k n o w n a n d are the l o w e s t of the i n e r t gases, a n d t h e i r a t o m i c w e i g h t s w e r e closer to those of the samples t h a n w e r e the o t h e r i n e r t gases. A t e a c h t e m p e r a t u r e the i o n c u r r e n t c o r r e s p o n d i n g to the i n e r t gas w a s m e a s u r e d as w e l l as the i o n c u r r e n t c a u s e d b y t h e s a m p l e . Theoretical Evaluation of Steady State Conditions in the Sample Cell.

A t the

s t e a d y state t h e rate of e v a p o r a t i o n of m o l e c u l e s f r o m t h e surface of the s a m p l e is e q u a l to the s u m of the rate of loss of m o l e c u l e s f r o m the c e l l a n d the rate o f c o n d e n s a t i o n of v a p o r m o l e c u l e s onto the s a m p l e surface. T h e t h e o r e t i c a l m a x i m u m rate of e v a p o r a t i o n of m o l e c u l e s of m o l e c u l a r w e i g h t M f r o m a surface is g i v e n b y the w e l l - k n o w n L a n g m u i r - K n u d s e n e q u a t i o n (4,7,9)

as

rate of evaporation = Ρ ( 2 Τ Γ Μ Κ Γ ) "

1/2

moles c m . " sec." 2

1


236


w h e r e F is the s a t u r a t e d v a p o r pressure. S i m i l a r expressions are o b t a i n e d f o r c o n d e n s a t i o n a n d effusion: rate of condensation = P(2TTMRT)~ ^

moles c m . " sec." 2

1

rate of effusion = Ρ ( 2 Τ Γ Μ Κ Γ ) " ^ moles c m . " sec.' 2

1

1

w h e r e ρ is the pressure of v a p o r a b o v e the surface. I n m a n y cases rates of e v a p o r a t i o n a n d c o n d e n s a t i o n

are

much

s m a l l e r t h a n these t h e o r e t i c a l m a x i m u m rates. A n e v a p o r a t i o n coefficient, a , or a c o n d e n s a t i o n coefficient,

Mass Spectrometry in Inorganic Chemistry

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