Anal. Chem. 1989, 6 1 , 2237-2240 (35) Agathonos, P.; Karaiskakis, G. J . Appl. Polym. Sci. 1989, 3 7 , 2237-2250. (36) Kolladima, A.; Agathonos, P.; Karaiskakis, G. Chromatcgraphle 1988, 26, 29-33. (37) Katsanos, N. A.1 Vassilakos, Ch. J . ChrOmatOSr. 1989, 471, 123-137. (38) Obberhettinger, F.; Badii, L. Tables of Laplace Transforms; SpringerVerlag: Berlin, 1973. (39) Danckwerts, p. v. &S-Li9Uu ReaCtiOnS; McGraw-HiII: New 'fork, 1970; p 15. (40) Huq, A.; Wood, T. J . Chem. Eng. Data 1968, 13, 256-259.
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(41) Blackadder, D. A.; Nedderman. R. M. A Handbook of Unit Operations; Academic: New York, 1971; p 145.
RECEIVEDfor review November 14, 1988, Resubmitted April 28, 1989. Accepted July 3, 1989- The authors thank Professor A. Jannussis for his help in certain mathematical manipulations and for stimulating discussions. They alsothank M ~ ~ . M. Barkoula for technical assistance.
Determination of Trace Impurities in High-Purity Oxygen by Gas Chromatography with Photoionization Detection Hiroshi Ogino,* Yoko Aomura, Masatsugu Komuro, and T e t s u Kobayashi Technical Research Laboratory, Toyo Sanso Co., Ltd., 3-3, Mizue-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa 210, J a p a n
A gas chromatographlc system used for the analysis of trace impurities In high-purity oxygen (99.99-99.9999 % (v/v)) is described. This system consists of a gas chromatograph equipped with a photoionization detector (PID), a gas sampier, a precoiumn fllled with a catalyst, and a computerized Integrator with an interface. The analytical reproducibilities (relative standard deviation, n = 15) for Ar (0.50 ppm), N, (0.10 ppm), Kr (4.0 ppm), CH, (6.0 ppm), and Xe (0.30 ppm) were l . f % , 4.8%, O S % , OB%, and 4.2%, respectively. The detection limits were as follows: Ai, 0.03 ppm; N,, 0.04 ppm; Kr, 0.01 ppm; CH,, 0.01 ppm; and Xe, 0.01 ppm. These limits are in a practical concentration range (sub parts per million to several tens of parts per miillon) in high-purity oxygen.
INTRODUCTION Recently, the purity of oxygen used in certain semiconductor manufacturing processes is becoming more important. Gas analysis in the sub-parts-per-million range plays an increasing role in these industries. Several papers have been published on the analytical method for the determination of impurities, such as Ar and N2, in oxygen by gas chromatography (1,2). In these methods, a gas chromatograph equipped with a thermal conductivity detector (TCD) was used to determine any inorganic gases. However, their sensitivity is not sufficient to determine accurately such impurities at less than parts per million levels. It is hard to achieve a good separation of Ne, Ar, NO,Kr, and Xe in a large volume of oxygen with a molecular sieve (5A or 13X) column, even if the column is operated a t low temperature (e.g., -72 "C) (3). Also, the filaments of the TCD could be irreversibly damaged or interfered with by the huge amount of oxygen. Therefore, when a TCD for trace determination of inorganic gases is used, a preconcentration technique and the elimination of oxygen should be required. There are many problems, such as recovery of trace components and so on, inherently associated with the preconcentration technique. The latter was applied, consequently, to determine accurately trace impurities in high-purity oxygen. Fortunately, a photoionization detector (PID) with a high sensitivity for inorganic gases, which is based on the emission
from a direct current discharge in helium gas, is commercially available (4, 5 ) . There have been no published reports on the determination method of a series of impurities of less than 1 ppm in highpurity oxygen by gas chromatography. T o determine the concentration of such impurities in high-purity oxygen would be important and interesting for semiconductor industries. This paper describes a simple and accurate gas chromatographic system for the determination of trace levels of impurities, such as argon, nitrogen, krypton, methane, and xenon, in high-purity oxygen. EXPERIMENTAL SECTION Apparatus and Materials. A diagram of the equipment setup for this experiment is shown in Figure 1. The experimental apparatus basically consists of a gas sampler, precolumn (oxygen adsorber), and gas chromatograph (Hitachi, GC-263-30,Tokyo, Japan) equipped with a PID. This system, including the gas sampler, can be programmed with a computerized integrator (Shimadzu, C-R4A, Kyoto, Japan) through an interface (Shimadzu, PRG-102A) and is able to analyze and automatically report its results every 20 min. Analytical results are computed by an absolute calibration method from the peak areas. Operating conditions of the GC and detectors used in this experiment are listed in Table I. To remove oxygen as the major component from a sample gas, a precolumn was installed between the gas sampler and analytical column. The precolumn was a 1.5 m X 3 mm i.d. stainless steel tube and was filled with a copper-based catalyst Nikki Chemical, N-211, 30/80 mesh, Tokyo, Japan). This catalyst was selected after several catalysts were evaluated as to whether they could absorb all the oxygen in the sample gas and whether the catalyst could be regenerated easily and repeatedly. The precolumn was heated and controlled with a temperature control unit (Yokogawa, UT-10, Tokyo, Japan). High-purity helium (Toyo Sanso, [A] grade, 99.9999% ) was employed without further purification as the carrier and discharge gas. In an attempt to evaluate and demonstrate the performance of this system, several oxygen gases of various purity levels, obtained from different manufacturers, were analyzed. Most of the experiments were conducted with reference gas mixtures having the following composition: 11 ppm each of Ar and N2in He and 9.7,140, and 1400 ppm each of Ne, Ar, Kr, and Xe in He. All reference gases were prepared by the gravimetric method. Mass flow controllers (STEC, MS-400, Kyoto, Japan) were used to prepare the low-concentration samples in oxygen gas from their reference gases and high-purity oxygen if necessary.
0003-2700/89/0361-2237$01.50/0 0 1989 American Chemical Society
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ANALYTICAL
CHEMISTRY, VOL. 61. NO. 20, OCTOBER 15. 1989
1 " " l " " i " " i " " I " '
M
0
5
10
15
25
20
TIME (MINI 0
N L -J&J Flgure 1. Schematic diagram of apparatus for determination 01 impurities in high-purity oxygen: A, GC; 8, interface: C. integrator; D. precolumn (catalyst);E. heating con+roilw F, manually operated six-pat rotary valve; G, air-actuated SIX-portrotary valve: H, sample loop; 1. prewre transducer: J. line finer; K,stop valve: L. back-pressure mntml valve; M. sample inlet: N, sample gas artlet: 0, vacuum pump for llne purge: P, hydrogen gas inlet; 0,helium gas inlet: R. H, (or He) outlet.
Activation of the Precolumn. The oxygen elimination of the precolumn is based on the following reaction: 02
- +
+ 2cu
2cuo
Also, the precolumn is regenerated with hydrogen via the following reaction: CuO + H,
Cu
H20
First, valve F was manually switched to the activation position (as shown in Figure 1)and then high-purity hydrogen Voyo Sanao, [A] grade, 99.9999% (v/v)) gas was passed through at ea. 150 mL/min to purge any air remaining in the precolumn. The precolumn temperature was then raised and held at 100 OC for 30 min. Finally, the temperature was raised to 140 "C and held with hydrogen flowing for 60 min. After the activation, the precolumn was purged with high-purity helium through inlet Q of Figure 1. Valve F was switched hack to the analfical position. The activated precolumn was then connected in series with the GC.
RESULTS AND DISCUSSION Characteristics of the Catalyst. The capacity of oxygen adsorption of the catalyst is strongly dependent on the heating temperature. Its capacity at room temperature was too small. Therefore, it was necessary to heat the precolumn a t a constant temperature. However, the high temperature of over ca. 200 "C was unsuitable for the determination of hydrocarbons such as methane because such compounds could be decomposed on the catalyst. Considering the capacity of the oxygen adsorption and the thermal stability of methane with the catalyst, the temperature of the precolumn w a maintained ~ a t 140 2 "C. At this temperature, there was no evidence supporting any decomposition of methane. T o confirm this point, the same oxygen gas was analyzed independently by the use of a Fourier transform infrared spectrophotometer (JEOL,JIR-3510, Tokyo, Japan) equipped with a 10 m optical path length gas cell. The analytical result for methane obtained by the present method was compared with that of the the FT-IR. There was only a negligible difference, within experimental error, between the present and FT-IR methods: 0.90 i 0.01 ppm for GC-PID and 0.85 i 0.05 ppm for FT-IR. In order to determine the capacity of oxygen adsorption, 3.7 mL of oxygen was repeatedly injected into the GC using the automatic gas sampler. The injected oxygen was carried to the precolumn by the carrier gas and was completely eliminated with the catalyst in the precolumn. The residual gases were then separated with an analytical column and
Flgure 2. GC-MS selected ion profiles for residual gases passed through me precolumn. OCMS operating conditions: column, MS-SA, 60/80 mesh, 3 m X 3 mm i.d. stainless steel tube; carrier gas (He), 30 mLlmin: sample size. 5.38 ml; oven temperature, 100 " C OCMS transfer line temperature, 150 OC: ion source temperature, 200 'C; filament current, 3.20 mA electron energy, 70 e% gain, 2.0.
Table I. Operating Conditions of the GC.PID, and TCD GC analytical column MS-SA, 60/80 mesh, 3 m stainless steel tube reference columnG MS-SA, 60/80 mesh, 3 m stainless steel tube column temp 50 "C carrier gas He, 48 mL/min PID discharge gas He, 37 mL/min PID temp loo "C applied potential 750 V discharge current 150 PA sample loop 1.70 mL TCD current
detector temp sample Imp
X
3 mm i.d.
X
3 mm i.d.
180 mA 100 "C 4.47 mL
Installed for TCD. detected with the PID. The precolumn (1.5 m X 3 mm i.d. stainless steel tube), which contained ca. 12 g of catalyst, could adsorb ca. 250-2.280 mL of oxygen at 140 "C even after several regenerations. This means that if a sample loop of 2 mL was used, analyses of more than 100 runs can be done without any additional regenerations of the catalyst. The GC-MS (Shimadzu, QP-300) analysis of the residual gases was undertaken to confirm the adsorption efficiency of oxygen with the precolumn. The result showed that the selected ion monitoring profile, 32 m / z for oxygen, did not show any profile for the presence of oxygen, as shown in Figure 2. The oxygen, that is, could be preferentially and completely eliminated with the catalyst in the precolumn, which was operated under the proposed conditions described below. The activation and regeneration were performed by a method identical with that previously described. Optimum Conditions of PID. The sensitivity of the PlD is known to be dependent on the flow rate of helium as a discharge gas, the applied potential of discharge, the temperature of the detector, the purity of helium, and so on (4, 5). In order to confirm these effects and determine the optimum conditions of PID, a series of experiments were conducted using a reference gas having 11ppm each of AI and N2 to He. The operating conditions of the GC are given in Table I. Figure 3 shows the relation between the PID response and the flow rate of helium as the discharge gas. The response decreased almost exponentially with increasing flow rate of helium gas in the range examined in this experiment. The following experiments were carried out at ca. 40 mL/min,
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20,OCTOBER 15, 1989
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Table 11. Analytical Results of Determination of Impurities in Oxygen Gases with Different Purity Levels impurities, ppm
(I
sample
purity
A B C Dd
99.9% 99.99% 99.995% 99.9995%
Ar 425b 1.88 f 0.03 0.52 0.01 0.53 f 0.01
*
Guaranteed purity. Obtained by GC-TCD.
NZ
Kr
CH,
9.58 f 0.89 0.24 d~ 0.02 20.61 f 0.20 0.24 f 0.02
10.32 f 0.04 16.17 f 0.04
