Anal. Chem. 1983, 55,477-487
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Determination of Trace Impurities in Highly Purified Nitrogen Gas by Atmospheric Pressure Ionization Mass Spectrometry Yasuhiro Mitsui, * Hidekl Kambara, Masuo Kojlma,' Hiroshl Tomlta,' Kenji Katoh,' and Kunitaka Satoh' Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 785,Japan
Trace impurities in highly purified nitrogen gas are qualitatlvely and quantitatively analyzed by highly sensitive atmospheric pressure ionlratlon mass spectrometry (APIMS). Clusters are successfully identified by cluster dlssociatlon through multlpie collisions with neutral rnolecules in an electric fleld. Ions having the same moleciular weights, such as CO,, N20, and C,H, or H,O and NH, are identifiedby differences in ionization potential. Ions having c7 higher ionlratlon potential than that of Kr, or Xe, can be quenched by the presence of 100 ppm of Kr or Xe. For quantitative analysis, the influence of coexisting components is investigated and callbratlon curves are obtained. Concentrations of CO,, NO, N,O, and 0, In a sample gas are 47 ppb, 12 ppb, 1.7 ppb, and 13 ppb, respectively. Detection IiniRs of CO,, NO, and 0, In thls system are 200 pptr (parts per trilllon), 60 pptr, and 10 pptr, respectively.
Highly purified nitrogen gas is used as a standard for air pollution measurement and as a carrier gas in the semiconductor industry. One of the serious problems of using the gas in these fields, however, ia that trace impurities in the gas have not been detectable a t the parts-per-billion level or below, since no adequate analytical imethods have been reported. The conventional methods to detect inorganic trace impurities have been optical absorption and photoacoustic spectroscopy (1-4). However, the detection limits of these methods are not enough for the latest air pollution measurements, which require quantitative analysis a t less than 20 ppb. Atmospheric pressure ionization mass spectrometry (APIMS) (5-11), which has a high sensitivity, seems to be applicable for this special analytical purpose. Here, qualitative and quantitative analyses of trace impurities in a gas are investigated by APIMS. Some problems must be removed for analysis of trace impurities. For cluster identification, a collisional dissociation technique a t high pressure (6, 11) is investigated. For identification of ions having the same molecular weights, a selective ionization technique (7) is investigated. For quantitative analysis, the influence of coexisting components is also investigated. This paper reports that APIMS can be successfully applied to qualitative and quantitative analyses for trace impurities, such as COz, NO, NzO, and 02 in a highly purified nitrogen gas.
EXPERIMENTAL SECTION A schematic diagram of the apparatus is shown in Figure 1. It consists of a gas supply system, an ion source operated at atmospheric pressure, a collision chamber, and an analyzing region. Primarily ions were produced by a corona discharge with a needle electrode. Discharge current was 1fiA. Ion source temperature
Present address: Analytical Chemistry Division, National Chemical Laboratory for Industry, Yatabe, Tsukuba, Ibaragi 305, Japan.
was kept at room temperature (25 "C) throughout the experiment. The collision chamber was separated from the ion source and analyzing region with two aperture electrodes and evacuated with a rotary pump, the pumping speed of which was 800 L/min. The length of the chamber was 6 mm. The diameters of the first and the second apertures were 0.1 and 0.3 mm, respectively. Generally, many kinds of clusters are produced by ionization at high pressure (5, 6,8-14). These clusters are dissociated by multiple collisions with neutral molecules in the collision chamber at a pressure of about 50 Pa (collision induced dissociation method, CID method). An electric field is formed in the chamber by supplying a drift voltage between electrodes 11 and 12 in Figure 1. The ion intensities are changed with the drift voltage. The details of the CID method in an API system were reported previously (6, 11). The drift voltage supplied between these two electrodes was changed from 0 to 40 V for identification of clusters and was fixed at 10 V for the other experiments. A quadrupole mass spectrometer, covering 1-600 daltons, was used. The analyzing region was evacuated with an oil diffusion pump with an effective pumping speed of 800 L/s. Two highly purified nitrogen gases (99.9995% pure), which were named B1 and B2, were used as sample gases. Glass tubing and metal fittings used in the gas supply system were thermally purified with a flowing sample gas until the major ions in the API spectrum became N4+,N3+,and Nzf. After that, the heating was stopped and the sample gas was allowed to flow until equilibrium between adhesive impurities adsorption on and desorption from the inner surface of the gas inlet system was established. In this condition, API spectra were measured for identification of impurities. For identification of ions having the same molecular weight, krypton or xenon gas was mixed with the sample gas. The concentration of Kr or Xe in the mixture was kept at 100 ppm. Many adhesive impurities in mixture gases were removed by a molecular sieve trap (6 in Figure 1). Calibration curves for quantitative analysis were obtained with standard gases containing 1.1ppm of COz, 0.6 ppm of NO, 2.1 ppm of NzO, or 5.2 ppm of 0, from Nippon Sanso Co. These standard gases were mixed with sample gas B1 at various flow ratios giving the concentrations of the impurities. The mixing ratio was monitored with flow meters. Many adhesive impurities in the standard gases were also removed by the molecular sieve trap. For the standard gases COz, NO, and NzO, which have a strong adsorption, the equilibrium between adsorption on and desorption from the molecular sieve was established. The trap (4in Figure 1)installed in the sample gas stream inlet system was cooled with liquid nitrogen in order to remove any impurities in the sample gas for investigation of the detection limit in this API method.
RESULTS AND DISCUSSION Impurities in Highly Purified Nitrogen Gas. The API mass spectrum of highly purified nitrogen gas B1 is shown in Figure 2. Ion intensities of sample gas B2 are different from those of B1, but the observed ions of these sample gases have the same mass numbers. This suggests that impurities in B1 and B2 are the same. 1. Identification of Cluster Ions and Molecular Ions. Some peaks in Figure 2 come from clusters. Identification of these clusters was carried out by the CID method. The ion intensity changes of B1 with the drift voltage are shown in Figure 3. The ions a t mlz 19, 28, 29, 30, 32, 37, 42, 47, 48, 50, 56, and
0003-2700/83/0355-0477$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983
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Flgure 1. Schematic diagram of the experimental apparatus: (a) gas supplying region, (b) ionization region, (c) differential pumping region (collision chamber), (d) analyzing region, (1)hlghly purified nkrogen gas (Bl or B2),(2)highly purified nitrogen gas including 500 ppm of Kr or Xe, (3) highly purified nitrogen gas including a small amount of COP, NO, N,O, or 0, (standard gas for calibration), (4) trap, (5) flowmeter, (6) molecular sieve trap, (7) liquid nitrogen, (8)vessel to mix gases, (9)flowmeter, (10)needle electrode, (11)aperture slit (0.1mm i.d.), (12)aperture slit (0.3mm i.d.), (13)quadrupole mass analyzer.
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