Anal. Chem. 1986, 58,371-374
37 1
Determination of Oxygen-Containing Impurities in Nitrogen and Argon by Metastable Transfer Emission Spectrometry J. W. Mitchell,* P. K. Wittman, and A. M. Williams
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
The vlablllty of metastable transfer emission spectrometry (MTES) as a practical quantitative trace-analysls tool Is examined via a detailed investigation of its appllcatlon for the quantitatlve determlnatlon of oxygen-contalnlng lmpuritles In electronlc grade nitrogen and argon. Intensity responses for NO (B 2 r X 2 r ) derlved from chemllumlnescent reactions of active nitrogen wlth 02,H20, COP, CO, and SO2 are callbrated by using serial dllutlon techniques providing coverage over the concentratlon range 2-3000 ppm. Quantltatlon of total oxygen Impurity levels uslng O2 or NO as standards permits finite measurement limits of 160 ppb In nitrogen and 700 ppb in argon due to resldual oxygen Impurities In extraordlnarlly well-purlled nitrogen. At 2.0 ppm, the lowest concentratlon level reproducibly calibrated wlth flowlng standards, the slgnal-to-background nolse ratlo of 17 was measured experimentally for NO emlssion at 3206.9 A.
-
Suitable analytical techniques for on-line monitoring of electronic gases during production, after storage, and during consumption in processing is critically needed where maintenance of ultrahigh purity must be assured with regard to atmospheric contaminants containing oxygen. Differentiation between various purity grades of argon and nitrogen is critical in selecting the most suitable product for use as a controlled atmosphere purge gas in semiconductor processing. Fortunately, seldom are all of the potential atmospheric oxygencontaining impurities, 02,H20, COP, CO, SO2,NO, NO2,and N20, present simultaneously. Nevertheless, few analytical methods are sufficiently sensitive, economic, and easily applied for quantitatively measuring low parts per million levels of these impurities. The Hersch cell has been used advantageously to monitor free O2contamination in the 1-100 ppm range in nitrogen and inert gases (I). The selectivity of the device precludes responses of the electrochemicalcell to other oxygen-containing impurities. .Atmospheric pressure ionization mass spectrometry (API-MS) has been applied successfully for the identification and semiquantitative determination of 02,COP,NO, and N20 in purified nitrogen by Mitsui et al. (2). They report that a 99.9995% pure nitrogen sample and standard gases containing 1.1ppm COP,0.5 ppm NO, 2.1 ppm N20,and 5.2 ppm O2 were obtained from commercial sources and prepurified by subjection to a liquid nitrogen trap. The standard gases were purified, mixed with the pure sample at various flow ratios to provide calibration, and detection limits were estimated from background spectra of purified nitrogen gas. Low parts per billion levels were determined, and exceptionally low detection limits were calculated from the background ion intensities. The development of the method required stringent efforts to identify ions of the same mass, to eliminate cluster ions, and to prevent other interferences. No information on analysis time was provided, but due to the required heating of the gas handling system between samples, throughput is unlikely to be rapid enough for routine analysis of samples. On-line monitoring would also be difficult. Sensitive detection 0003-2700/86/0358-037 I S 0 1.50/0
of moisture and trace impurities, C02, CO, CHI, N2, and 0 2 , in helium via spectroscopic detection in the He afterglow has been reported (3). Although detection limits of 0.1-10 ppm were estimated for the impurities, no detailed applications of the method were made, and analytical validation of the measurements was not investigated. Selective quantitative methods for the reliable determination of trace impurities (H20,CO, C02),total hydrocarbons, and nitrogen oxides may be based on derivative infrared diode laser absorption spectrometry in long path length cells. Although great progress in the use of this approach for the determination of moisture has occurred (4),the method requires exceptional expertise and provides information on a single species at a time due to spectral limitations of the laser. However, process monitoring for a single impurity, e.g., C02,CO, HzO, NO, or NO2, would be feasible. Additional ultrasensitive, simple, on-line techniques for quantitatively monitoring trace oxygen impurities would be beneficial. In 1982, Ramsey and Nelson (5) reported the potential of metastable transfer emission spectrometry (MTES) for detecting carbon and oxygen impurities in argon. Quantitative calibrations were obtained for hydrocarbon impurities and for COP,and detection limits were estimated from the data obtained. In the current paper, the quantitative determination of oxygen impurities by metastable transfer emission spectrometry has been investigated in detail. A system for exploitation of metastable transfer emission spectrometry and for on-line monitoring of purified nitrogen and argon to determine total levels of oxygen is reported.
EXPERIMENTAL SECTION An overall schematic of the experimental system is shown in Figure 1. The equipment used in assembling the system is listed in Table I. The luminescencewas observed axially in the flow tube through a high-grade quartz window with a 0.3-m monochromator. A dispersion of 24 A/mm produced by a 1200 g/mm grating gave a resolution of 0.5 at 3131 A. The slits on the monochromator were set to 5 mm height and 500 Mm width, which produced a spectra band-pass of 1.2 nm. Molecular emissions extended throughout a significant portion of the flow tube and could not be approximated as a point source. Focusing optics were not used since sufficient light throughput at the slit was already observed. The photomultiplier tube was contained in a thermoelectric water-cooled housing maintained at -20 "C. The signal from this detector was fed into an electrometer and converted to a voltage. This voltage was then used to drive a strip chart recorder to obtain a permanent record of the signal intensity. Optionally this signal could also be recorded on a 13.3-cm diskette by use of an Apple IIe microcomputer. Gas Handling System. All rotameters were calibrated directly with a Brooks Instruments (Teaneck, NJ) mercury sealed volu-meter flow rate calibrator. The linear range of the flow scale of each rotameter was used in various experiments in which calibration graphs were generated by mixing gases at different, known flow rates. The flows of nitrogen and the various sample gases were controlled by a high-accuracymetering value in conjunction with a rotameter of the appropriate flow range. In Figure 2 a close-up diagram of the serial dilution arrangement of the rotameters is shown. When the flows in both the diluent and sample rotameters 0 1986 American Chemical Society
372
, ,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
:m
!
53/
7 FOOT LENGTH MIXING COIL
VENT TO ATMOSPHERE
TO MTES SYSTEM
OILUENT ~
2% r--
Figure 2.
Serial dilution apparatus for preparation of gas standards.
comi
Figure 1. MTES spectrometer.
Table I. Equipment Used equipment microwave power generator microwave cavity flowmeters
model MPG 4M
supplier Opthos, Rockville, MD
McCarroll type
Opthos, Rockville, MD 603 rotameter, 602 Matheson, E. rotameter, 610A rotameter Rutherford, NJ gas regulators gas dependent Matheson, E. Rutherford, NJ pressure 222B head PDR-D-1 supply MKS Inst., Burlington, MA gauge valves 4710 series Matheson, E. Rutherford, NJ detector 1P28 PMT Hamamatsu, Middlesex, NJ detector 3461 Pacific housing Photometrics, Marlboro, NJ detector 33 Pacific temperaPhotometrics, ture Marlboro, NJ controller high-voltage 227 Pacific Precision, Marlboro, NJ supply quartz Supersil I1 Hereaeus Amersil, window Sayreville, NJ monochroma- HR-320 ISA Instr., tor Metuchen, NJ ISA Instr., diffraction 2400 g/mm Metuchen, NJ grating Keithley, electrometer 616 Cleveland, OH VWR, S. Omniscribe strip chart recorder Plainfield, NJ vacuum pump 1397 Welch, Chicago, IL bellows-sealed SLlOOS Veeco, Long Island. NY valve were changed, various concentrations of gaseous impurities could be made. A 7 ft length of 0.63-cm copper tubing was formed into a spiral and used to ensure’a homogeneous mixing of the diluent and sample gases. The vent to the atmosphere was used to discharge the excess volume of the resultant diluted sample gas. The remaining gas sample was then routed through another rotameter, now shown in the diagrams, to enter the system directly to the observation/detection region, or prior to, and consequently through the microwave discharge. The pressure in the flow tube was monitored by a capacitance-type transducer. The pumping system had a capacity of 500 L/min and was throttled with a 2.54-cm bellows-sealed vacuum valve. All tubing between the gas regulators and the flow cell was 0.63-cm copper tubing except for
the final connection, which was made with a 0.63-cm flexible stainless-steel bellows tube. Additionally the connection to the vacuum pump was a combination of 1.27 cm i.d. vacuum hose and a 1.27-cm flexible stainless-steel bellows tube. The flexible bellows tube acted as a strain relief and vibration isolation for the flow cell from both the vacuum pumping system and the gas inlet rotameters. The gas flow cell was constructed of Pyrex glass with a quartz window sealed to one end by means of an O-ring gasket. This Pyrex flow cell was formed from three individual sections that were sealed together via three O-ring gaskets. Coupling between the flexible stainless-steel bellows tubes, the gas flow cell, and low-pressure copper tubing was achieved with Cajon UltraTorr fittings. Microwave Excitation. A ll4-wave McCarroll-type cavity was used to efficiently couple the output of the microwave power generator to the flowing source gas. The cavity operated at 2450 MHz and was tuned such that the forward power was typically 80 W and the reflected power less than 1W. The cavity was forced air cooled by the in-house compressed air supply. The active nitrogen produced in the microwave induced plasma was transported to the observation/detection region via a concentric ring injector assembly. Gases. The gases used in this work were obtained from several suppliers: Nz (99.999% Research grade), O2(99.993% Ultra-Pure), and SOz in nitrogen (99.98%) from Airco, Rivertown, NJ. N2 and Ar (industrial grades) were supplied by Air Products, Allentown, PA. O2in nitrogen (2.00 A 0.02 mol %), NO in nitrogen (235 A 3 ppm), and CO in nitrogen (9.67 f 0.09 ppm, 97.0 f 0.8 ppm, and 967 A 7 ppm) were obtained from the National Bureau of Standards (NBS). Moisture in nitrogen (3000 A 30 ppm) was prepared by bubbling through tandum saturated LiCl bubblers. The regulators used were in accordance with the suppliers recommendation for each particular gas. Immediately following the regulators were 7-pm filters to trap any particulates that may originate in either the gas tank or the regulator. Operating Procedure. The vacuum pump was run continuously, although when not engaged in an experiment it was sealed off from the system by the bellows-sealed vacuum valve. The microwave induced plasma source gas (nitrogen) was opened to allow a flow rate of 800 mL/min, and then the bellows-sealed vacuum valve was opened slowly but fully. The source gas was then turned off at the high-accuracy valve, and the system was allowed to pump down. This was down to ensure the vacuum integrity of the system from day to day. Once this was established, the source gas was turned on again and the system was maintained at 2.5 torr for a few minutes. The forced air cooling to the cavity was turned on, and the microwave power was turned on and allowed to warm up. Then the microwave power was increased and the discharge was ignited by ”tickling” with a Tesla coil. The system pressure was adjusted to the optimum operating pressure of 7.50 torr. The microwave induced plasma was maintained for at least 5 min to allow the flow cell to approach a dynamic thermal equilibrium due to the heating caused by the plasma. The sample gas was mixed by means of the serial dilution rotameters and subsequently metered into the system through an additional rotameter not shown in the figures. The pressure of the system was then adjusted to maintain the 7.50 torr operating pressure
ANALYTICAL
CHEMISTRY, VOL. 58,
NO. 2, FEBRUARY 1986
373
Table 11. NO Intensity vs. Concentration"
std 235 36 3.6 2d
concn., ppm measd
re1 intend
O.R.'
2300 1700 280 200 f 20
36.7 3.3 2.0 f 0.2
"Linear least squares on log-log plot, y = 0.7422X + 2.0616 [coefficient of correlation = 0.99581. bSystem pressure = 9.3 torr, X = 3206.9 A. Outside linear range. Duplicate measurements. by the throttling valve on the vacuum pump.
RESULTS AND DISCUSSION Among the oxygen-containing impurities of interest, 0 2 , HzO, C02,CO, NzO, and SO2, CO is the single exception that would not undergo substantial fragmentation upon collisional energy transfer from active nitrogen created within the microwave discharge. Reaction of the atomized oxygen atom from each of the indicated initial molecules leads to production of the common product, excited state NO* (B2a -,X2a). Monitoring NO emission at suitable characteristic emission wavelengths, selected from reference tables (6),provides a means of analyzing nitrogen and its mixtures with inert gases for total oxygen impurity levels. Experimental conditions were optimized for maximum formation of NO* (B2a X2a) by changing microwave power, flow rates, and system pressure until maximum intensity was observed for NO* emission at 3206.9 A. It was critical to optimize the nitrogen afterglow emissions for the pink afterglow Nz+ (B2&+ X22,+)transition for the determination of oxygen impurities. Direct passage of the analyte gas stream through the microwave discharge, where the microwave induced plasma and collisional transfer of energy from active nitrogen both influenced fracture of molecular bonds of oxygen species, proved to be optimal. Nitric oxide (B2n X2a) emission intensities were 5-fold higher when the analyte was passed through the microwave discharge in comparison to direct introduction of the analyte into the observation/detection region (see Figure 1). In this way the oxygen present in CO was also converted to the chemiluminescent NO reaction product. In contrast, direct reaction of active nitrogen with CO at concentrations over the range from 9.67 f 0.09 to 967 f 7 ppm produced no detection of emissions from NO. Also, within the observation range of our spectrometer, 200-600 nm, no characteristic emission for excited states of CO was detected, ruling out this possible selective determination. Following optimization of conditions for generating the chemiluminescenceformation of NO from oxygen-containing impurities, the lowest possible background was sought for impurities present in the nitrogen supplied to the microwave induced plasma. In a thorough study of the purification of nitrogen (7), we reported the superior efficiency of a reactive resin column (8). When this procedure was used for purifying the source gas stream, background emissions from NO were reduced more effectively than for any other purification method investigated. Quantitative determination of detection limits was based on constructing suitable calibration curves. The lowest concentration oxygen standard available from the NBS, 2.00 f 0.02 mol % in N2,was straightforwardly diluted by using flow controllers and provided samples over the 300-3000 ppm range. At these. high concentrations a smooth but nonlinear calibration was obtained. Practical measurements at the upper level of -400 ppm could be performed. With the NBS nitric oxide in nitrogen standard, 235 f 3 ppm, the oxygen range from 2.0 to 235 ppm was examined. A linear response with concentration was obtained, and the data are reported in Table -+
+
-
01
1 IO OXYGEN CONCENTRATION lppm]
100
Figure 3. Calibration of system for response to O2concentration in nitrogen.
11. The signal-to-noise ratio and the background signal to the NO emission wavelength were found to correspond to a lower limit of detection of 46 ppb. This detection limit is calculated in accordance with the IUPAC convention of using a signal that is 3X the noise signal in the system. Calibration was achieved by constructing a serial dilution apparatus shown schematically in Figure 2. By use of the 2.00 f 0.02 mol % Oz, standard flows of gases through different calibrated rotameters were varied, and dilution factors were calculated. Based on known initial concentrations of the standard gas mixtures and the dilution factors, the final sample concentrations within the 2.0-100 ppm range were prepared and used for calibration. The calibration graph in Figure 3 shows an extrapolated limit of detection of 0.11 ppm. The nonlinearity of the signal intensity above 100 ppm is suspected to be related to impurity-to-activenitrogen ratios being large enough to influence the overall steady-state level of the latter. Intensities for molecular oxygen calibration graphs were a factor of 2 lower than corresponding ones obtained for equivalent concentrations of NO as the source of oxygen. Evidently, the product of equilibrium and collisional constants and geometry factors governing the chemiluminescenceproduction of NO from O2 are approximately a factor of 4 smaller than those governing the direct excitation of NO by energetic components of active nitrogen. Similar examinations were conducted with trace levels of CO and SOz in nitrogen. Comparatively depressed sensitivity was observed for NO emissions from the former. Due to competitive formation of CN, intensities from NO resulting from active nitrogen chemiluminescent reaction with CO were a factor of 6 less than was observed previously for calibration with NO. In the case of SOz over the concentration range of 1-15%, formation of deposits within the analytical system created serious interferences and precludes calibration for oxygen present in this form. Fortunately, in most practical samples of inert gases the levels of CO and SO2 are usually very low. Moisture is an important source of oyxgen contamination in gases. Emission by OH was not observed in the active nitrogen system, and conversion to excited-state NO is not very effective. The system is consequently insensitive to moisture levels below 1000 ppm but is linear in its response to moisture levels up to 6000 ppm. At levels below loo0 ppm the moisture has no effect upon the signal intensity of the other oxygen-containingimpurities, e.g., NO and 02. The NO emission intensity of a 25 ppm NO sample remained constant while the moisture concentration was varied between 0 and
374
ANALYTICAL CHEMISTRY, VOL. 58,NO. 2, FEBRUARY 1986
T a b l e 111. C o m p a r i s o n
of Nitrogen Purity
N2t a n k
3206.9-A N O / i n t e n s i t p
O2concn., ppm
4 5 6 7 8 9 10 11
17.5 f 1 3.5 f 0.2 105 f 3 12.5 f 0.5 9 f 0.5 10 f 0.5 81 f 0.7 61 f 1
7.1 f 0.3 1.1 f 0.05 48 f 0.9 4.8 f 0.1 3.3 f 0.1 3.7 0.1 42.6 f 0.2 30.6 f 0.3
“Signal
from photomultiplier tube, A
*
X
los.
CONCENTRATION OF OXYGEN [pprn)
Figure 5. Intensity vs. concentration of NO in nitrogen-argon mixture.
Ar-to-Nzratio of 1:5. There is an apparent enhancement due to the presence of Ar, which could presumably reduce collisional quenching of N2* by the reaction NOON 23-APR-84
NOON 24.APR-84
25.APR-84 NOON
Flgure 4. On-line monltorlng of oxygen in Centralized nitrogen gas supply.
1000 ppm. No interference was observed when a 50 ppm oxygen sample was used. As demonstrated experimentally, considerably different responses are observed for NO emission formed by reaction or excitetion of active nitrogen with 02,NO, and H20. Determination of the total oxygen impurity content by calibration with a single oxygen-containing standard, either O2 or NO, must therefore be used cautiously. However, acceptably reliable, quantitative, relative comparisons of impurities at the low parts per million level in various samples is possible. Results of analysis of several nitrogen gas tanks are reported in Table 111. The method has also been applied to on-line monitoring of oxygen impurities in a centralized nitrogen supply system. As indicated in Figure 4,fluctuating oxygen impurity levels occur. Mixtures of nitrogen with argon have been used for active nitrogen spectrometry. Thus, use of purified nitrogen with argon as the analyte permits the determination of oxygen impurities in the argon. Previous investigators report N2-to-Ar ratios ranging from 1:l to 2:1 as being the optimum for observing atomic emission from various metals excited by active nitrogen (9). In principle, direct observation of emission from O2in an argon plasma should be possible. Although we were successful in sustaining an argon plasma excited by the microwave discharge, no excited states were sufficiently long-lived to be transported within the observation zone of instrumentation optimized for active nitrogen spectrometry. In a study of the effect of Ar-to-Nz ratio on the intensity of NO emission a maximum was observed at an
-
N2(A3Z.,+)+ N2(X1ZC)
+
N2(X1Bg+) N2(X1Zg+)v(1)
To achieve greater sensitivity for analysis of Ar, a 1:20 mixture was used for determining the O2 concentration-NO intensity relationship shown in Figure 5. This calibrated response below 150 ppm O2 closely approximates similar graphs obtained for O2 in nitrogen alone. The linearity above 200 ppm 0 2 extending to 600 ppm, however, was not observed in the absence of Ar. By use of this standard addition calibration plot, the level of oxygen in the Ar or the total oxygen impurity level of Nz plus Ar can be determined by extrapolation. The observed detection limit for oxygen contamination in Ar, 0.7 ppm, may possibly be enhanced considerably by using ozonizer-type discharges to produce greater active nitrogen concentrations (IO). Registry No. N2,7727-37-9; Ar, 7440-37-1; 02, 7782-44-7.
LITERATURE CITED (1) Hersch, P. Anal. Chem. 1980, 32, 1030. (2) Mitsul, Y.; Kambara, H.; Kojlma, M.; Tomita, H.; Kotoh, K.; Satoh, K. Anal. Chem. 1983, 55, 477. (3) Taylor, G. W.; Dowdy, E. J.; Bieri, J. Michael Anal. Chim. Acta 1982, 136, 277. (4) Mucha, J. A. Appl. Spectrosc. 1982, 36, 393. (5) Ramsey, T. H.; Nelson, M. D. Anal. Chem. 1982, 5 4 , 826. (6) Pearse, R. W. B., Gaydon, A. G., Eds. “The Identification of Molecular Spectra”, 4th ed.; Wiley: New York, 1976. (7) Wlttman, P. K.; Mitchell, J. W. Appl. Spectrosc., in press. (8) Wlttman, P. K.; Mitchell, J. W.; Williams, A. M., submitted for publication In Anal. Chlm. Acta. (9) Na, H. C. Ph.D. Dissertation, University of New Mexico, Albuquerque, NM, May 1982. (10) Dodge, W. B., 111; Allen, R. 0. Anal. Chem. 1981, 53, 1279.
RECEIVED for review July 1, 1985. Accepted September 27, 1985.
