Anal. Chem. 1994,66, 3274-3280
Determination of Zirconium, Niobium, and Molybdenum in Steel by Glow Discharge Mass Spectrometry Takako Takahashi' Analytical Research Laboratory, Marubun Corporation, 3-3-4 Minamisuna, Kotoku, Tokyo 136, Japan Tadashl Shlmamura School of Hygienic Sciences, Kitasato University, 1- 15- 1 Kitasato, Sagamihara Kanagawa 228, Japan Determinations of low-level Zr,Nb, and Mo in steel samples by glow discharge mass spectrometry are described. The possibilitiesof metal argide correction on the isotopesof those elements and the use of doubly charged ions are discussed. Selectionof suitable isotope(s) depends on the major and minor constituents of the sample. Correction of metal argide interferences MAr+, where M is Fe, Cr, Mn,or Ni, can generally be made by monitoring the ion intensities of the M+ and production rate of FeAr+/Fe+. Use of doubly charged ions for Zr, Nb, and Mo determination is not feasible because of the existence of doubly charged metal argides such as FeAr2+or low intensities of the doubly charged analyte ions. Recently the requirements for trace analyses for high-purity iron as well as low-alloy steels, which are based on high-purity iron, have been increasing. Traditionally, a variety of analytical methods are applied for the determination of major and trace elements in steels, such as atomic absorption spectrometry (AAS),' inductively coupled plasma atomic emission spectrometry (ICP-AES),2 X-ray fluorescence spectrometry (XRF),3 or spark emission spectrometry (SES).4 Those methods are well established, and numerous papers were published discussing specific analytical problems. AAS and ICP-AES are good for quantitative analysis with good sensitivities. AAS especially has a superior sensitivity for specific elements, and ICP-AES has the capability of multielement analysis. Both of these methods, however, usually require time-consumingacid-decomposingprocedures because of liquid-state sample introduction. It is also difficult to analyze gas components with them. On the other hand, XRF and SES do not require wet chemistry. XRF is good for quantitative analysis, but its sensitivity is somewhat poor. SES is widely used for steel analysis because of its wide-range coverage of the elements and good sensitivity, but it is poor in quantitative analysis. Glow discharge mass spectrometry (GDMS) has been utilized recently for the determination of trace- to ultra-tracelevel impurities in high-purity metals or semiconductors. The reasons for this are as follows: (1) almost all the elements can (1) Japanese Industrial Standard, G1257. Methods for Atomic Absorption Spectrochemical Analysis of Iron and Steel. 1994. (2) Japanese Industrial Standard, 01258. Methods for Inductively Coupled Emission Spectrochemical Analysis of Steel. 1989, (3) Japanese Industrial Standard, G1256. Methods for X-ray Fluoresence Spectrometric Analysis of Iron and Steel. 1982. (4) Japanese Industrial Standard, G1253. Methods for Photoelectic Emission Spectrochemical Analysis of Iron and Steel. 1983.
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Ana!ytlcalChemlstry, Vd. 66,No. 19, October 1, 1994
be analyzed to mostly sub-ppm levels except for the noble gases, (2) major to ultratrace constituents can be analyzed at the same time, (3) there are uniform sensitivities among the elements, (4) there are opportunities for quantitative analysis. Consequently, for most of the cases, the purpose for using GDMS is total element monitoring from major- to sub-ppmlevel constituents. Thus, GDMS is oneof theideal instruments for multielements monitoring in steel samples. Determination of trace levels of Mo in steel by GDMS, however, is rather difficult. Generally, in a plasma generated by glow discharge, the production rate of polyatomicion species associatedwith thedischarge gas (Ar) isquite high. In GDMS, these polyatomic ions can sometimes cause serious interference problems. In the case of Mo determination in a steel sample, all of the Mo isotopes except "Mo are interfered with by FeAr+. Moreover, in many cases, the steel sample contains minor component elements M such as Cr, Mn, or Ni. Those elements cause complicated interfering metal argide ions, MAr+, and these ions interfere with not only Mo isotopes but also Zr and N b isotopes. Unfortunately, to resolve Nb+, Zr+, or Mo+ from MAr+, the required mass resolution would be more than 10 000. Therefore it is difficult to avoid those spectral interferences in conventional GDMS. Typical limits of quantitative analysis of Mo in steel with typical running conditions would be more than 100 ppm. On the other hand, if doubly charged ion species are utilized for the analysis, the interferences described above can be eliminated. Instead, Ti isotopes and doubly charged polyatomic species like MArZ+, would be a problem. Because Zr, Nb, and Mo are important elements for steel analysis, lack of data for those elements results in an inferior quality of total element monitoring. If correction of the interferences can be done, GDMS would be one of the most promising methods for steel analysis. We investigated the production rate of MAr+ and MAr2+ in the glow discharge plasma, the degree of interferences to the isotopes of Zr, Nb, and Mo in the steel, and the possibility of quantitative correction of those species for each isotope. EXPERIMENTAL SECTION Instrumentation. The instrument used for the analyses was the VG9000 (FI Elemental Analysis Ltd.) GDMS system, which had an inverse-geometry double-focused mass analyzer equipped with a discharge cell cooling system. High-purity Ar gas (6N grade) was introduced as the discharge gas via a getter pump gas purifier system. The discharge cell used was "Mega Cell", which produced an improved ion intensity. 0003-2700/94/0366-3274804.50/0
Q 1994 American Chemical Soclety
Table 1. Elomonlal Concentratlonr In NIST Stool Sampler and Iron Metrorlter
sample
Ti
SRM 661 SRM 662 SRM 663 SRM 664 SRM 665 SRM1765
0.020 0.084 0.050 0.23 0.0006 0.0055
Guin Tocopilla @
V
0.01 1 0.041 0.30 0.106 0.0006 0.0040 1.3 x 3.4 x
1 ~b 7 10-66
Cr
Mn
co
Ni
NIST Low-Alloy Steel (Certified Conc, 96 by Mass) 0.69 0.66 0.032 1.99 0.30 1 .os 0.30 0.60 0.31 1.50 0.048 0.32 0.066 0.258 0.15 0.142 0.0072 0.0057 0.007 0.041 0.051 0.144 0.0012 0.154 Iron Meteorite (8by Mass) 0.000186 0.63: lo.@ 0.00656 0.47b 5.36
Zr
Nb
Mo
0.009 0.20 0.050 0.069
