Chemical ionization by a needle electron source

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/ _ n t e r n a t / o n a / J o u r n a l of Mo~zSpectrometry and Ion Physw.s, 2 4 (1977).453+~W~3 ~~.: : ~ : ~ ~.) Elsevier S c ~ ' e n t i ~ P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e f l m a d s ~ : . ~ . . . ~. . . . . : . . . . ~ • -_: ~ . - _ . . ~ . .-~-:..--'~-~ . • -

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CHEMICAL

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IONIZATION BY A NEEDLE ~

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.HIDEKIKAMBARA

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Central Research Laboratory. HitaoAi Ltd. Kokubunj~ Tokyo (Japan)-

(Received 19 October 1976)

C h e m i c a l I o n i z a t i o n ( C I ) t ~ d i z e d b y e m p l o y i n g a s i m p l e ~,eedle e l e c t r o n s o u r c e m d e s c r i b e d , in w h i c h t h e r e a g e n t gas is i o n i z e d w i t h e l e c t r o n s e m i t t e d f r o m t h e n e e d l e elec/ z o d e . T h i s ion s o u r c e c a n b e u s e d f o r o x i d i z i n g o r r e d u c i n g r e a g e n t g a s e ~ T h e i o n s o u r c e c h a r a c t e r i s t i c s s u c h as pressure a n d d i s c h a r g e v o l t a g e d e p e n d e n c e s o f r e a c t a n t i o n s are investigated. The GI m a s s ~ of some aleohois ~ inve~J.il~ltc~d. This ~ [ pt0~e~i is m o r e e f f i c i e n t t h a n t h a t o f c o n v e n t i o n a l C I s o u r c e s b u t y i e l d s similar s p e c t r a .

INTRODUCTION

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Chpmical ionization is a phenomenon resulting from ion-molecule reactions tmder pressures in therange 0.1---2tort [1--3]. In comparison to elec. tron impact (El) ionization, chemical ionization (CI) provides ~mple mass patterns and often very intense quasi.molemflsr ion peaks. For this reason, CI is becoming accepted as a t e c h n i q u e c o m p l e m e n t a r y t o EI in mass spectrometry, " " ~. . T h e CI practices require gas mixtures at pressures.of c~. 1~tozr i n t h e ion source. S a m p l e g a s is m i x e d with t h e reagent gas, t h e major• componen~ of t h e gas mixbtre: Methane, i s ~ b u t a n e , a m m o n i a or water are generally used "i.'" as t h e reagent gas. In conventional CI ion sources primary ions are formed b y t h e impact of electrons o n t h e reagent gas, These ions t h e n react further w i t h the reagent gas t o p ~ u c e t h e stable ions characteristic o f each gas, called r e ~ r t a n t ions. R e a c t a n t ions ionize t h e sample o ~ t h r o u g h i o n - m o l e c t d e xeacti0ns- " Tl~e E1 i o n source generally used-in CI operations h ~ s e ~ m ' a l d e f i c i e n c i e s f o r analytical application. T h e electron beam emitted f r o m a n inc~_descent •~_~ment requires very high electron ~ and. a . . . L ~ . . ~ o n current for

stable

beam operatioa

rough ia . m

electron entrance apem

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:The u s e of, a n i n c ~ d e s e e n t _m_ament.in t h e p r e s e n c e . o f .an o _x~_n i t gas a t high : iz~l~s, s h e r t e n s the lifetime-of:the !on SoUSe..dueto contamination f r o m

the pyrolysis:products formed. When water vapour as wen.as oxysen: gas : i e o e x i m c ~ i s used.for: a ! n ~ e n t p s , ,..,.

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• +~:!!i~+shortened+ A"~mcther dff+ficulty in using this ion source/s revealed when + attempting t o mea.m~ the pressure in the ionization chamber. To avoid these drawbacks, several new ion sources have been proposed. Hoegger et.aL-~ [4]!:~ep0rte~ a new C I i o n s o u r c e ~ u b ~ l i ~ . g a - h f g h - ~ d n c y flow discharge. However, this method requires a large piece of equipment for ionization and does n o t appear practical Hunt et aL [ 5 ] reported an ion source utilizing the Townsend discharge for. positive and negative GI processes by employing two. meshed electmdes~wRh a. 1/8. in separation.He

eliminated the inhezent drawbacks by using electrons created by the discharge. This is very useful for active reagent gases suchas oxygen or water. The drawbacks m f e m ~ t o above are also eliminated in a simple CI ion source which produces ions by electron impact and this is described in this paper. The electrons come from the corona ~ e o f a needle electrode at the same pressure as in the ionization chamber. This new ion source is very simple in its~constmCtion and has a l o n g operational lifetime.

Basic figure o f the n e w Cri m n source

The drawbacks inherent in conventional CI ion sources result fl~m the incandescent filament and differential p , m p i n g between the ion and the electron sources. These drawbacks can be eliminated if electrons are emitted from some other cold. source operating at the same pressure as the CI ion source. Negative corona discharge was employed as an elec~con source in this expe~riment. A schematic ~ of the new CI ion souxce is shown in F i g . 1. It con.qi~ts o f a needle elecixode, a.repeller electrode w i t h ~ mesh and an aperture electzode. Most.of the electrons are generated by the needle electrode and chemical i c m ; ~ o n occ~r~ between the repeller and the aperture electrodes. ~These two regions ~ce hereaftex called the d i s c ~ and ionization regions, respectively. Supplied voltages for these electrode~ are --500,

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o f t h e needle el..ec~,ode tip is s0.small. ~mit_me electric field a!~m~d:me": tip is s t m ~ enough to accelerate electrons and ionize molecuhs. " • ' The electrons g e n e ~ d near the tip dri~ over m e ionize moleeu e , me repeUer e, and en . le ion on l on, electrons eotUde with gas molecules and lose their energy by exciting or i0n~ing molecules in the ionization re, on. T h e mean free path of the electrons, denoted by I,is represent~d b y the effective cross section o and the gas density p in eqn. (1). ! = :1lop

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The value of I is estimated to be about 0.5 m m at I tort'with p = 3.6- 10~I cc and o = 5 - 10 - ~ cm ~. ionization of the molecules requires that the electrons have an energy level greater than 15 eV. If the electric field strength before the repeller electrode is over 300 V cm -~, the electrons axe accelerated to the ionizing energy tevel. The distance R between t h e n e e d l e electrode and the repeller electrode is 4 _ram and the. supplied voltage between them is 500 V. So the electric field E before the repeller electrode is estimated to be 300 V cm -~ using eqn. (2) [6]

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where Vo, r and re(= 0.03 ram) axe the potently! difference, distance farom the tip and radius of curvature at the tip apex, respectively. T h e actual electric field is lower during discharge; however it is sufficient for realizing ionization. The electrons ionize molecules a f t ~ passing through the repeller electrode. I n the ionization region, the direction of the electzic field i s the reverse o f that in t h e discharge region. Ionized molecules drift to the aperture electrode while ion--molecuh reactions are taking p h c e s o t h a t chemical ionization is accompli~.hed. Almost all the emitted electrons are used to generate primary ions so that a relatively ~mall number of electrons, such as 20/IA,

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considerably

reduces the contamination trouble inherent in CI ion sources. • Apparatus

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A Schematic view of the apparatus, originally used fob.an Ah'nospheric Ionization (API) study [7] in our ~boratory, is shown in Fig. 2. The API had a twod~age differential pumping system and a needle electrode for corona discharge. The intermediate region of the API a p l ~ . tus was used fo~_this study~..:_ -,.:"i .-:. ,~.:: :-.- -...-.- .:'.. -,.. ::..! :! ,'-i:: . ~: --. ,..:-.-:.., .... :.--: .....

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Fig. 2. S c h e m a t i c v i e w o f t h e e ~ p e r i m e n t a l apparatus: 1) discharge r e g i o n ; 2 ) ion;~'-~tion region; 3 ) n e e d l e e l e c t r ~ , e; 4 ) s a m p l i n g , aperture; 5 ) repelle~; 6 ) f o c u s i n g e l e c t r o d e ; 7 ) ap,:,~L,~re e l e c h ' o & ; 8 ) v a ~ b l ~ 1,~1~ valve; 9 ) valve: 1 0 ) M e L e o d gauge; 1 1 ) lens; 1 2 ) E I i o n s o u r c e ; ~ 3 ) f i l a m e n t ; 1 4 ) l e n s ; 1 5 ) q u n d r u p o l e m a s s analyzer;, 1 6 ) e l e c t r o n multiplier.

ionization region. A rotational M ~ gauge is connected to the ionization chamber which is ummlly operated in the range 0.5--1 tort. EIectzons ale p.rni~ed fl'om the needle electrode, placed 4 mm from the repeller electrode. A sewing needle was employed as the needle electrode. T h e repeller ele~z~le is. made o f a 1 0 ~ m e s h i ~ n ~ n s c ~ e n . The needle electrode is supplied, with - - 5 0 0 V through a 4 M ~ resistor. The repeller electrode voltage.was 2 0 V, The distance between the repeller elecl~ode and t h e electrode with:the-s~m~pling aperlm~: (of 0,3-ram d h , ) . i s 2.5 ram. A. focusing electrode, with a 2-tamalia, aperture is be twee n. the repeller and aperture electrodes..The analyzing region is evacuated to 5 X 10 "s t o r t b y a 1000-1/a oil diffusion pump with a cold t~ap,which has a 20(M/s real pumping speed. A q-~dmpole mass. spectrometer, which covers the.x~age 1--150 ~mu. was used as a mass analyzer. -The electron current emitted from the needle electrode was 22 I~A at I tour and 500 V discharge voltage. An ion" current of about 1 - lO-1°A w a s d e t e c t e d in the analyzing region. The repeller and the foc-.,dng electrodes were adjusted to obtain the m~ximum ion cun~.nt at the collector.-J:...- ' . . '. . . . - . . - - ~ . . • ....

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Reactant ions were investigate d at room temperature for v a d o u s m e t h a n e pressure~..The results, w e r e compared :with- previous w o r k [8] -which utilizedi a conventional C I i o n source_~_-- :.:--:: '-i_ :-.:::. ::-~.:.i:- !~-_:~ : -::,:~::-i .;:-:;.: ~, -~:~- A discharge voltage :of500-V:.was e n o u g h " t o - o b t s i n a s t s h l e ~ ' e m : - : rent at •

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needed at low pressures. For example, --1.5 kV wasnecessary for a sthblei:~.

at 0.I ~orr.:Thedischaege cu~ent changed .withvaziations ~ "~;~.":':~r

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sure i n the ionization chamber. The rplnfionship between them is sh0wn in:i:::i

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then gr~u~11y d ~ . If the discharge voltage and the ~ c e between the needle-and the repeller electrodes is fixed ( a n d t h e ~ - b r a t i 0 n curve for various reagent gases-hasbeen measured elsewhere), the pressure in t~e ionnization chamber can be determined from the discharge cun~nL The pressure dependence o f the reactant ion intensities is shown in Fig. 4 for methane as reagent gas combined with a small amount•of benzonitri]e sample gas. The ~ e voltage was --550 V and e~cb electrode .was adjusted to yield the maximum ion cun~nt; The main reactant ions were and C~H~ and :their ion intensities remained un~hanged for pressu~ variations in t h e ionization chamber. Ion i ntensiti~ of C~I~;I C~H~ and ~- CNH+, which are f o r m e d after higher order ion--molecule reaction~ increased with: pressure because the ion--molecule collision t~me inczeased. Ions suc~ as CIT3 and CH~, which are primary ions, were obseJ~ed at 0.35 tort. However, these were not observed at higher pressures. These ~sults agree with those o f previous workers with conventional CI ion sotmce¢ Ion intensities at 0.5 tocr i n ~ with the discharge voltage; however, the mass spectzal pattern did not change greatly as is shown in Fig. 5. Thi~ s~ggests that the CI mass pattern is not extremely sensitive to electron energy, which agrees with previous results [8]..The ion i n a n i t i e s strongly depend: on the total d L ~ - h ~ curRnt a t l o w e r pr~-ures. Actually, t h e i o n intensities were in proportion to the total discJ3srge cucmnt at 0.1 •ton-. However, the dependence of ion inten.~ity on discharge current weakened at Pressure above 0.5 tour. Ion intensity did not depend on discharge ~rrents over 20/LA, although it depended on it in the range of several #A. In normal operation, the discharge current was 20/JA. This was enoughto provide the c I spectra.

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On the other b-nd, /~ has been reported that ion inten.~ty increases linearly with .electron c~,rrent,at least up to 2 0 0 ~ A dectzon current, in a conventional CI ion source. [8]. H u n t et al. [5] proposed their ion source on the condition that the electron current was 100--150 p A in normal opera-

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the ionization chamber. Thus, the electrons emitted from t~he needle electrode are distributed around the aperture and ionize molecules more effio cientlyin the n e w apparatus. The CI spectra of butyl alcohols and amyl alcohol were investi~tecL •Parent ions of butyl alcohols and iso-amyl alcohol were not observed in the EI mode. However, qUasi-parentions were observed in-the CI m o d e at 0.5 tozz, 500-V ~ voltage and 20#zA electroncuzzent~The CI mass spectrum of n-butyl alcohol togethex with the EI mass spect~]m is shown in Fig. 6. The EI mass spectzum was obtained from API* data~.The n-butyl alcohol parent ion is bazely observable in the EI mode; h o w e v ~ many fragment ions ~re o ~ , with st~ng intensities.These fragment ions reflectthe stzength of the.binding energy. The C - ~ bonds next to the O H have the smallestdiss~ation enezgiesso that i~=gmentiom of .CH~OH'.(31:amu)mblmez~. m tee m~n~io~peaL However, p ~ t o n a f f i n ~ i s impo_rtantin CI. Proton trRn~-

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yieIds the qu~i-pazent ion:75 ~trn~Waft.mole~Lde~~p~r~tl~~l'om ~he nlOle--. .cute to. leav~ ions o f . ~ . 5 7 ~'r.u When the proton tzan~er reaL~tion oc(mrs. These reaction p r o c e ~ . a ~ e . ~ can~ed out.so gently that fragmentationldoes noL ~ a s Often as i ~ t h e ];:t" process.- - ". . . . . . : . T~e CI m ~ ~ ' O ~ . ~butY] a~coholltog~th~.~th~e. F~I"~spe~ t~]m from API data am shown in Fig..7. The m~in ion.p~_~I~-~in the EI mass. spectrom are CH3CHOI~ (45ainu) aud CH3CH2CHOH+ (59 ~mu), which also reflect the chemical bond strength~ The parent ion is only 0.26% of the m ~ i n fragment peak at 45 atom The CI spectrum Of sec-butyl alcoho ! resem, bles that of n-butyl alcohol in spite Of a rather different-EI mass pattern. The water~hninated ion was the main ion in the CI mode and the quasi-parent ion intensity was abou~ 1 0 ~ of that of the main i o n s .

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~ d t~e.pa_,~t ion .~m~t~is less~.O.O~%'ot tbe.:mami~agmentpeak m t e ~ t y : The C~-spee~:iof re.butyl a]eobo] ~e~b]es-~ose Of-n: and l~e-bU~l alcohol From these C~ ~ ~em~-me bo,ld . s ~ ~ ~tte~ce~ do'not g~atly affect ~e:CI massp a ~ wit~ the ~ c ~ i t ions C ~ , O:~ and HsO*. : ~ : T h e m ~ spectra of iso-~rnyl alcohol for both the EI and CI modes are shown in Fig. 9. Many fragment ions are observed in the EI mode and the parent ion ofiso-amyl alcohol is 0.01% of the ma/n fragment peak ion intensity. On the other hand, the main peak was at 71 ainu in the CI mode and

the quasi-parent ion was as large as l%"of the m ~ i n ~ . '~ILLS".C]~p 8 ~ closely approximates the CI mass pattern of n~rnyl alcohoi ~ported by

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Hirose [8]. The ion inte_n~ty ratio of the quasi-parent and main fragment ions of ~ y l alcohol is about ten times less t/ran those of the butyl alcohols. A c o m p r e h ~ explanation for this diffel~nce has not been posed to date. Therefore, furtherinvesti~tions should be'carried out r e ~ .. ing the ionization mechanism as well as the stenc structure o f these m o l e •

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p zessu~ and ~ e CU~m..~ of .the needle apex. . . . . . . . . . .. - ~-:..~..-. Electrons c a m e f r o m a_~."dir~."0n- ~ to the:aPertu~ ~eetr0de; how- " ever, tliere ~ no dif~eulties i n t h e CI operati0n. ~ ion source can be easily mortared ;jus~ ~ fr0nt o f : a n E I ion :source. As a result, electrical switching between the EI and CI mode can be realized. . . . .

1 9. 3 4 5 6

M.S.B. Munson and F-H. Field, J. Amer. Chem. Soc., 88 (1966) 2621. D,F. Hunt, Prog. Anal. C h ~ : , 6 (19"/3) 359. G.W.A. Milne and MJ. Lacey, Crit, Rev. Anal. Cb~,-, 45 (1974). B. Hoegger and P. Bommer, I n t J. Mass Spectrom. Ion Phys., 13 (1974) 35. D.F. Hunt, C/~I. MeEwen and T.M. Harvey, Anal. Chem., 4~ (19#5) 1 ~ 3 0 . H.D. Beckey, H. Krone and F.W. Roellgen, J. Sci. ~ (J. Phys. E) Set. 2, 1 (1968,) 118. H. l~mbara and L Eanomata, Anal. Chetm, 49 ( 1 9 ~ ) 2~0. ' 8 H. ttirose, H. T ~ S. Okudaira and M. I~oh, m ~ Spectrose.; 21 (1973) 1221 ~

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