I. COI~NIDES-"Mass Spectrometric Analysis of Gas Mixtures

431

References 1 ABD EL I~AHEn~, A. A., and F. A. OSMA~:Z. analyt. Chem. 169, 328 (1959). -2 A.-~Ix, A. M., A. A. ABD EL l~n]znn~ and F. A. Os~=~: Z. analyt. Chem. 167, 8 (1959). -- a B~LOHE~,I~., I~.A. CLOSE and T. S. WEsT: Chemist-Analyst 47, 2 (1958); ef. Z. analy~. Chem. 166, 45 (1959). -- a CL.~RKL. J. : Analyt. Chemistry 30, 1153 (1958); ef. Z. analyt. Chem. 167, 440 (1959). Dr. A. A. ABD EL t~AltEE~, 39, Falaki Street, Flat 24, Cairo, Egypt, U.A.I~.

Hungarian Ministry of Heavy Industry, Dept. of Heavy Chem. Industry

New Method of Mass Spectrometric Analysis of Gas Mixtures Containing Molecules of Identical Mass Number By I. CORNm~S

(Received August 6, 1959) The mass s p e c t r o m e t r i c analysis of m i x t u r e s c o n t a i n i n g gases of n e a r l y t h e s a m e m o l e c u l a r w e i g h t m a y p r e s e n t difficulties i f t h e i r c h a r a c t e r i s t i c m a s s s p e c t r a (cracking p a t t e r n s ) are v e r y similar, or i f t h e i r f r a g m e n t p e a k s dissimilar in h e i g h t are v e r y s m a l l as c o m p a r e d to t h e p a r e n t Table 1 peak. -~{ass A n e x a m p l e for t h e l a t t e r case is n u m b e r /g2peaks CO peaks presented by mixtures containing carbon m o n o x i d e a n d n i t r o g e n 2. As m a y be seen f r o m T a b l e 1 none of t h e f r a g m e a t p e a k s

12 14 16 28 29

3.41

1.03 0.46 0.21 100.00 1.15

100.00 in t h e c h a r a c t e r i s t i c s p e c t r u m of CO a n d 0.75 N 2 reaches 3.50/0 of t h e p a r e n t peak*. This m a k e s t h e m a s s s p e c t r o m e t r i c d e t e r m i n a t i o n of t h e CO a n d N 2 c o n t e n t of t h e m i x t u r e in t h e u s u a l w a y v e r y i n a c c u r a t e a n d p r a c t i c a l l y impossible when considerable a m o u n t s of o t h e r gases h a v i n g high p e a k s in t h e m a s s region 1 2 - - 1 6 (e.g. m e t h a n e , a m m o n i a ) a r e also present. To d e a l w i t h p r o b l e m s of t h i s k i n d we t r i e d a n e w m e t h o d of m a s s s p e c t r o m e t r i c a n a l y s i s b a s e d on t h e fact, t h a t t h e forms of t h e i o n i z a t i o n efficiency curves of s u b s t a n c e s h a v i n g a l m o s t t h e same m o l e c u l a r weight m a y e x h i b i t m o r e or less m a r k e d differences. A n e x a m p l e is given in T a b l e 2, which contains ion c u r r e n t d a t a for CO a n d N 2 m e a s u r e d a t different ionizing electron energies in p e r c e n t a g e s of t h e r e s p e c t i v e ion

* Due to differences in mass discrimination in the lower mass range, spectra of CO and N 2 taken by instruments of different construction may exhibit somewhat higher low mass peaks 1, but still not exceeding 5~ of the parent peak.

432

I. Columns:

currents at 70 eV. These two sets of values m a y be taken as a characteristic spectrum of some different kind, say, the "ionization spectrum" of CO and N~. I t is, namely, obvious, t h a t if we measure the mass 28 peak of a mixture of CO and N 2 at two different electron energies, say, at 70 and 30 eV, the peak heights obtained (designated b y H and H ' respectively) must satisfy the equations H h ; . p l + h~. p~ = H'

hi" Pl @ h2"/)2 =

in which Pl and P2 are the partial pressures of CO and N 2 in the sample, /h and h~ the peak heights of the pure components of unit pressure at 70 eV, and h 1' and h~' the same at 30 eV. Table 2 With the four h-values measured in a Electron Mass 28 Mass 28 calibration run these equations m a y be energy (eV) peak 3r2 peak CO used to calculate the partial pressures, 100.0 just as the exactly analogous equations 70 100.0 91.4 40 87.2 of the conventional mass spectrometric 48.8 55.7 30 analyses. 27.2 24 20.4 The analogy of the two types of 15.1 20 8.6 spectra m a y be stated as follows. The conventional mass spectrum is scanned at a fixed electron energy b y varying the mass number, while the "ionization spectrum" is scanned at a fix mass number (e.g. M ~ 28) by varying the electron energy. The continuous character of the ionization spectrum should be noted in contrast to the conventional mass spectrum, which is, of course, a typical line spectrum. This fact offers wide possibilities to choose two "peaks" (i.e. two electron energies) most suitable for the separation of the two components. For example, the peaks at 70 and 30 eV in the ease of CO and N 2 exhibit a difference sufficient for appropriate separation and, besides, the 30 eV peaks are b y more than one order of magnitude higher, than any fragment peak in the conventional mass spectrum of the gases in question.

Experimental Results The new method of mass spectrometric analysis outlined above has been tried by analysing synthetic mixtures of CO and N2, the CO content of which we also controlled b y the use of an Orsat apparatus. Some results arc summarized in Table 3. The mass spectrometric data are in both eases averages of three measurements. The accuracy of the method as revealed b y these measurements is practically on the same level as t h a t of the method proposed b y I-IIOKAM and F o x 3. Following, however, their way of measurement one must determine and compare the initial linear sections of the ionization

433

Mass Spektrometric Analysis of Gas Mixtures

efficiency curves of t h e p u r e c o m p o n e n t s a n d t h e m i x t u r e itself. This r e q u i r e s m u c h time, m e a s u r e m e n t s o f v e r y small ion c u r r e n t s a n d m o n o e n e r g e t i c ionizing electrons, a f e a t u r e n o t p r o v i d e d for in convent i o n a l a n a l y t i c a l i n s t r u m e n t s . Our m e t h o d , on t h e o t h e r h a n d , requires o n l y t h e u s u a l s t a b i l i z a t i o n of t h e Table 3 e l e c t r o n e n e r g y secured in m o s t Content of Massspecc o m m e r c i a l i n s t r u m e n t s , and, besides, the synthegc trometric t h e p r o c e d u r e itself fits v e r y well mixture analysis 0/0 0/0 i n t o t h e course of a c o n v e n t i o n a l m a s s s p e c t r o m e t r i c analysis, due to Sample 1. CO 48.5 48.1 N2 51.5 51.9 t h e a n a l o g y of t h e t w o m e t h o d s , which fac~ should p r o v e useful in t h e Sample2. CO 24.6 25.1 case of a n a l y s i n g m u l t i c o m p o n e n t i% 75.4 74.9

mixtures. I n t h e course of these e x p e r i m e n t s t h e t w o values of t h e electron e n e r g y were set b y h a n d b y t h e use of a p o t e n t i o m e t e r . G r e a t e r a c c u r a c y a n d speed of m e a s u r e m e n t can c e r t a i n l y be a t t a i n e d w i t h fixed series resistors a n d a simple switch. The r e l i a b i l i t y a n d a c c u r a c y of t h e new m e t h o d in cases of a n a l y s i n g m u l t i c o m p o n e n t m i x t u r e s a n d m i x t u r e s w i t h low CO (or low I ~ ) c o n t e n t will be e x a m i n e d b y a f u r t h e r series of e x p e r i m e n t s in t h e n e a r future.

Summary A new m e t h o d of m a s s s p e c t r o m e t r i c analysis of m i x t u r e s c o n t a i n i n g gases of n e a r l y t h e same m o l e c u l a r weight is described. The m e t h o d is b a s e d on differences e x h i b i t e d b y t h e i o n i z a t i o n efficiency curves of t h e gases in question a n d requires a p r o c e d u r e of m e a s u r e m e n t a n d calcul a t i o n analogous to that of the conventional mass spectrometric analysis.

To show the applicability and accuracy of the method ment on CO-I~ mixtures are presented.

data of measure-

References 1 BARnyArD, G. P. : Modern Mass Spectrometry, lo. 194. The Institute of Physics London 1953. -- 2 BLEARS, J.: J. sei. Instruments, Suppl. 1~o. 1, 36 (1951). -ttICKA~, W. M., and 1%.L. Fox: J. chem. Physics 20, 1055 (1952). I. Co~I~)ES, Budapest XIV, 127. u. 1, Hungary

Z. unalyt. Chem., Bd. 171

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