The effect of trace gases on neodymium ion formation

International Journal of Mass Spectrometry and Ion Physics, Elsevier Scientific Publishing Company, Amsterdam -Printed

THE EFFECT

OF TRACE

LR. FLETCHER

and K.J.R.

GASES

ON NEODYMIUM

36 (1980) 253-257 in The Netherlands

253

ION FORMATION

ROSMAN

Department of Physics, Western Australkn 6202, Western Austmlio (Australia)

Instit+e

of Technology,

South Bentley,

LW.T_ TWADDLE Department of Chemistry, Western Australian 6102. Western Austmlia (Australia) (First received 29 October

1979;

Institute

of Technology,

South Bentley,

in final form 28 April 1980)

ABSTRACT During measurements of Nd isotope abundances, significant ionization suppression and enhancement effects have been observed and correlated with the presence of oxygen and water vapour in the ion source region of a solid source mass spectrometer_ These effects are of particular interest at present because many laboratories are developing the Sm-Nd technique for use in geochronologicai studies.

INTRODUC’ITON In

recent

years

there

hs

been

a rapid

expansion

of

interest

in Sm-Nd

geochronology Cl] and numerous laboratories, including our own, are establishing facilities for this purpose. It was decided to adopt one of the established mass spectrometric procedures for neodymium analysis 123 but in implementing this technique unexpected difficulties were encountered. Although neodymium has been isotopically analyzed by mass spectrometry by a variety of procedures [2-5], high-precision analyses for geochronological applications have generally been made by loading samples in the chloride form on single or triple filament assemblies [2,3]. We initially intended to adopt the former approach but experienced unacceptably low ior&zation ef&iencies. We now believe this to be due to a difference between the residual gas composition in the ionization region of our instrument and those which successfully employ single filament procedures. In this paper measurements which illustrate the form and magnitude of the effect will be described, and also a brief review of some earlier work related to this prob lem will be presented. OOZO-7381/80/0000-

0000/$02.25

0

1980

Ekevier Scientific

Publishing Company

EXPRRIk4RNTA.L A leak inlet system and gas-analyzing mass spectrometer fAE1 MSXO) were fitted to the ion source chamber of a 30.5cm radius, 90” magnetic actor solid source mass spectrometer. This permitted samples of nitrogen, oxygen and water vapour to be introduced and their pressures monitored_ The gas analyser also served to examine the purity of the leaked gases. A system&c fiiamentand that difference between .tbe pressur e adjacent to the ionizing r&stered by the MS10 was expected, the difference possibly being as huge as an order of magnitude. However, since the intention of the study is to demonstrate the nature of performance changes resulting from pressure changes, this does not significantly affect the conclusions. Neodymium oxide (Specpure, Johnson Matthey) was dissolved in HCl and 0.5-g samples of neodymium were evaporated onto 0.027 X 0.75~mm rhenium filaments. Drying was done in air, using a IL-A current to heat the filaments. The single filament data were recorded at a filament temperature of 1300°C whilst an ionizing filament temperature of 1900°C was used for the triple filament assembly. Ion current measur ements were made by direct collection of ions in a deep Faraday cage using a Gary 401 electrometer with a 10114Z input resistor. At each stage of the tests, one of the test gases was leaked into the system at a steady rate, and its effect on the formation of Nd+ and NdO* observed. At the same time the background levels of the other test gases were monitored to guard against extraneous effects. The test gas was pumped’away after each measurement and re-introduced at the next desired partial pressure. This permitted drift corrections to be made and cumuiative OP residual effects to be assessed. The “background” partial pressures of various other gases were also monitored, but no variations were observed which correlated with Nd’ or NdO’ fIuctuations, RESULTS For the single filament ion source the effects of introducing oxygen, nitrogen and water vapour are shown in Fig. 1. The~efficiency of NdO’ formation is obviously increased by the introduction of_ oxygen and,. to a lesser extent, by water vapour, However, the introduction of uitrogen had no significant effect on the fomnation of NdO”. Nd”. ions were rsot observed in these tests and therefore must be CO.%% of the total ion production. With the triple filament ion .source, measurable beams of both Nd* and_ NdO’ were- obtained (Fig. 2). .Agai.u, nitrogen pressure variations did not affect either ionic species, but botbNdCi? a&d Nd+ beams were influenced by oxygen and :water v&pour; Although NdO’ production was generally at the ‘expense bf ..Nd+- there -was ‘eventual. Guppression of both ion beams at higher -oxygen pressur es. Over then-‘.PPhoIe-pre&.zre kmge iz~vestig&ed (&a. lo-’ to >10w5 torr) the relationship between NdO’/Nd’ .and the pa&i&

255

SINGLE

FILAMENT

PARTIAL

PRESSURE

x

16’

(TORR)

Fig. 1. The response of NdO+ ion production to the single filament ion source. A relative beam intensity Na effects were measured with ~(02) = p(Ha0) = 3 x >< lo-’ torr; and Hz0 effects with ~(0~) = 3 x lo-’

presence of 02, Ha0 and Nz for a of 1 represents 62 mV for each gas. 10e7 torr; 02 effects with p(Hz0) = torr.

2 TRIPLE

I

FILAMENT

b--O- +

2

0

P(H,Ol

0

4 X lo-’

4 lo-’

FILAMENT

6

(TORR)

2 P
TRIPLE

3

0

CTORR)

6

0

*

I

4

2 PCOIIX

4 lo4

6



Fig. 2. (left). The response of NdO‘ and Nd+ ion production to tire presence of 02 and Hz0 for a triple fi+nent.ion source. For the “02” data a relative beam intensity of 1 represents 97 mV of Nd+ and 20 mV of NdO*, with p(Ha0) = lo* torr_ For the “HaO” data a .reIative beam ii&en&y of 1 represents 36 mV of Nd’ and I1 mV of NdO+, with n(H20) = 3 x -10L7 t&r. Fig.

3 (night)_ The NdO?Nd” ratio as a function of Hz0 ion source. From the data used for Fig. 2.

: filament

and O2 pressure for the triple

pressures of oxygen and water vapour (Fig. 3) is approximately linear. In all cases, when oxygen and water vapour pressures were reduced to background levels, ion emissions also returned to original levels, indicating that there were no changes in the samples or permanent changes in filsment character&tics resulting from exposure to the gases. DIsCUSSION

Many of the effects of oxygen on thermal ionization processes have been known and have received considerable attention under differing experimental condition; in the past [ 6lO]. However, because of the empirical manner in which practical solid source mass spectrometric procedures frequently develop, the dependence of a technique on existing vacuum conditions may be overlooked. In the case of neodymium, because the ionization process is so dependent upon the residual gas in the ion source, considerable time savings can be made during development of a procedure if this correlation is recognized. Also, any gain in beam intensity will have a significant influence on the precision of the measurements. The observed increase in NdO’ production with increasing oxygen pressure can be accounted for by an increase in the work function of the ionizing filament. The effect of adsorbed oxygen on surface work function has been studied extensively for tungsten. As far back as 1924, Kingdon [S] measured the effect. More recently Johnson and Vick [73, Greaves and Stickney [8], and Chao and White [S] reported similar studies. Chao and White [S] also extended these studies to rhenium surfaces and reported a change in the work function similar to that observed for tungsten. Their reslilts indicate that at 1900°C a change in oxygen pressure from IO-’ to 10m6 tx\rr will increase the work function by ,?pproximately 0.1 eV. This will produce a significant increase in the ionization efficiency of such a surface for neodymium, and the magnitude of the change is in agreement with our observations. Of particular relevance to the analyses of neodymium by triple filament is the mass spectrometric study of Weiershausen IlO] who measured the effect of oxygen on the ionization of neodymium, samarium and praseodymium on tungsten and rhenium surfaces. Where comparison is possible our data are consistent with his. The enhanced production of NdO’ and suppression of Nd+ is an obvious response to the availability of oxygen at the filament surface. His data also imply that considerable gains in Nd” ion production may be achieved by using higher ionizing filament temperatures (>lSOO” C) since the depression of the Nd’ ion current due to oxygen is less significant under these conditions. In all cases, water vapour acts similarly to oxygen but its effects are.much less pronounced. Presumably it decomposes and acts as an oxygen donor with an effect similar to that observed when molecular oxygen~ is introduced.

257

CONCLUSIONS

At the filament temperatures appropriate for the isotopic analysis of neodymium, the par-Sal pressures of oxygen and water vapour within the mass spectrometer ion source exert a significant infhrence on the ion currents which can be generated, and hence on the quality of the isotopic data. For single filament rhenium ion sources NdC9’. production increases as the partial pressure of oxygen or water vapour increases, as expected from the response of the rhenium work function to oxygen. In triple filament sources, oxygen and water vapour pressures up to ca. 10S6 torr cause both the enhancement of NdO’ beams and the suppression of Nd’ beams emitted from neodymium chloride samples. However, because contemporary mass spectrometers achieve source pressures of 19OO”C) and cryogenic pumping in the ion source region should provide even more favourable conditions for Nd’ production. ACKNOWLEDGEMENTS

The authors thank Dr. J.R. de Laeter for reading the manuscript and providing helpful criticism. G. Brown -and A. Planken carried out the modifications to the mass spectrometer and C_ Sackson assisted with the preparation of the manuscript. This work was supported by the Australian Research Grants Committee. REFERENCE8 1 M.E. Bickford and W.R. Van Schmus, Rev. Geophys. 8pace Phys., 17 (1979) 824. 2 D.A. Papsnastassiou, D.J. De Paolo and G-J. Wasser burg, Proc. 8th Lunar Sci Conf., (1977) 1639. 3 RX. O’Nions, P.J. Hamilton and N.M. Evenson, Earth Planet- Sci. Lett., 34 (1977) 13. 4 L-D. Nguyen and M. de Saint-Simon, Int. J. Mass Spectrom. Ion Phys., 9 (1972) 299. 5 I-L Bass&e, J. Cesario, D. Poupard and R. Naudet, Rot. Symp., Librde, 23-27 June.1975, I.A.E.A., p. 385. 6 K_H. Kingdon, Phys. Rev., 24 (1924) 510. 7 MC. Johnson and F.A. Vick, Proc. R. Sot. London, Ser. A, 158 (1937) 55. 8 W. Greaves and R.E. Stickney, Surf. Sci., ll(l968) 395. 9 B.Y. Chao and F.A. White, Int. J. Mass Spectrom. Ion Phys., 12 (1973) 423. 10 W. Weiershausen, Ann Phys., 15 (1965) 252.