Anal. Chem. 1998, 70, 3315-3321
Technical Notes
Quantitative Analysis of Trace Moisture in N2 and NH3 Gases with Dual-Cell Near-Infrared Diode Laser Absorption Spectroscopy Shang-Qian Wu,* Jun-ichi Morishita, Hiroshi Masusaki, and Tetsuya Kimishima
Tsukuba Laboratories, Nippon Sanso Corporation, 10 Ohkubo,Tsukuba 300-2611 Japan
This paper demonstrates our optical measurement system based on near-infrared tunable diode laser absorption spectrometry and reports the results of trace moisture determination in nitrogen and ammonia gases. A nearinfrared InGaAsP distributed feedback diode laser operating at room temperature was employed as the optical source. We used a dual-cell detection strategy to cancel common mode noise from the diode laser and remove the effect of the residual moisture absorption in the beam path outside the sample cell. We also used this method to successfully eliminate the interfering absorption of matrix gas molecules such as NH3. The detection limit of H2O absorption of 4 ppb in nitrogen and 12 ppb in ammonia was obtained using a single-pass absorption cell of only 92 cm in length and the average results of 10 scan measurements. This system has characteristics of both the high sensitivity and capability of in situ and real-time measurement. The reduction of impurities in process specialty gases is very important in manufacturing electronics devices such as ultra-largescale integrated circuits (ULSI) and optical devices such as laser diodes. Moisture is one of the major impurities decreasing the production yield of ULSI. The presence of trace moisture in process specialty gases such as HCl, HBr, and NH3 can degrade the gas delivery equipment and reactor performance due to corrosive actions.1 Pure ammonia gas has recently been used for blue LEDs and laser diodes manufacturing. Some references suggested that removal of trace moisture in NH3 might improve the electrical and optical properties of the final products.2 Therefore, the importance of monitoring moisture in ammonia at ppb levels has been increasing. To maintain moisture-free gas delivery systems, it is important to have analysis technologies that can measure trace moisture in process specialty gases with high sensitivity under in situ and real-time conditions. Several methods have been developed for analysis of trace moisture in bulk gases such as nitrogen, hydrogen, and argon. (1) Ohmi, T.; Nakamura, M.; Ohki, A.; Kawada, K.; Hirao, K. J. Electrochem. Soc. 1992, 139 (9), 2654-2658. (2) Vergani, G.: Succi, M.: Thrush, E. J.: Crawley, J. A.: Van der Stricht, W.: Torres, P.: Kroll, U. Proceedings of 43rd Annual Technical Meeting of Institute of Environmental Sciences; 1997; pp 262-272. S0003-2700(98)00011-0 CCC: $15.00 Published on Web 06/20/1998
© 1998 American Chemical Society
The most sensitive one is atmospheric pressure ionization mass spectrometry (API-MS), which can measure trace moisture in nitrogen at the ppt level. However, API-MS cannot be directly applied to the process specialty gases, because most process specialty gases have high reactivity which rapidly degrades parts of API-MS detectors. In addition to that, API-MS cannot detect trace moisture in some gases such as NH3, whose ionization potential is lower than that of moisture, because the chargetransfer reaction necessary for high sensitivity does not occur from the gases to moisture. Thus, optical sensing methods whose detectors could be isolated from the gases are the good alternative. Some optical detection methods have been reported for analysis of trace moisture in halide gases. Fourier transform infrared spectrometer (FT-IR) with a multipass white cell was utilized for trace moisture measurements in HCl and HBr.3,4 In these measurements, however, the effect of residual moisture absorption in the beam path outside the sample cell cannot be neglected even in a purge box. This is because moisture is a component of air and has many strong absorption lines in most spectral regions employed for detection. Mid-infrared lead salt tunable diode laser spectrometry is another method that is suitable for trace moisture detection in reactive and corrosive gases.5,6 These lasers make possible direct access to convenient spectral regions for the absorption measurements of fundamental vibrational bands of many molecules. The disadvantage of mid-infrared lead salt spectrometry is that the detectors and laser diodes have to be operated under cryogenically cooled conditions; besides, the output power of laser is relatively small. In recent years, near-infrared diode lasers have been used for various light sources due to their compactness and easy operation. Thus, absorption spectrometry with the near-infrared diode laser was developed to measure trace moisture and other gas species. Carlisle utilized a 1305-nm distributed feedback (DFB) diode laser with optical fiber and two-tone frequency modulation spectroscopy to measure trace moisture; a minimum detectable absorption of (3) Miyazaki, K.; Ogawara, Y.; Kimura, T. Bull. Chem. Soc. Jpn. 1993, 66, 969971. (4) Stallard, B. R.; Espinoza, L. H.; Rowe, R. K.; Garcia, M. J.; Niemczyk, T. M. J. Electrochem. Soc. 1995, 142 (8), 2777-2782. (5) Inman, R. S.; McAndre, J. F. Anal. Chem. 1994, 66, 2471-2479. (6) Kastle, R.; Grisar, R.; Tagke, M.; Dornisch, D.; Schplz, C. Microcontamination 1991, 9 (10), 303-310.
Analytical Chemistry, Vol. 70, No. 15, August 1, 1998 3315
5 × 10-7 was achieved.7 Johnson et al. used the same method and a 100-m White cell for trace moisture detection; optical densities corresponding 1.7 × 10-6 was detected.8 Feher reported a photoacoustic detection system with a 1500-nm DFB laser and reported a result of 8 ppb NH3 measurement sensitivity.9 We have also reported an optical measurement system based on 1380-nm DFB diode laser absorption spectrometry operating at room temperature. We applied this system with a 50-cm absorption length sample cell and single-beam optical setup for trace moisture measurements in HCl and obtained a detection limit of ∼500 ppb.10-12 However, only a few measurements of trace moisture in NH3 with laser absorption spectroscopy have been reported, because ammonia has many absorption bands interfering with H2O absorption lines from the near-infrared to mid-infrared spectral region, as mentioned below. Kastle et al. first demonstrated the detection of moisture in ammonia using a tunable lead salt diode laser and a 10-m multipass White cell.6 To avoid the effect of ammonia absorption, the strongest H2O absorption lines in the mid-infrared spectral region were not selected, but two H2O absorption lines at 1923.162 and 1922.342 cm-1 were utilized, at which the absorption of NH3 molecules were relatively small. As a result, a detection limit of a few ppm was obtained. This result was due to the limitation of high background moisture level. In general, gas-phase impurity analysis using tunable diode laser absorption spectrometry utilizes the difference in optical absorption between the matrix gases and impurities at the same wavelength. In other words, the ideal condition for absorption spectrometry is that the matrix gas is transparent, and the laser radiation is only attenuated by the absorption of impurities. However, in the near-infrared spectral region, there exist many weak absorption lines of active gases that arise from overtone or combination tone transitions of vibrational-rotational bands. If the absorbance of matrix gas molecules is larger than that of the impurity, detection by the laser absorption spectrometer will be limited by the interfering absorption of the matrix gas molecules. As a matter of fact, our understanding of the overtone or combination tone spectra of ammonia in the near-infrared spectral region is far from being complete. There is not enough of an absorption line database of ammonia in the spectral region, though the newest HITRAN 96 database collected 11 152 NH3 absorption lines from 4 to 5294 cm-1.13-15 Ohtsu et al. utilized two InGaAsP/ InP diode lasers to detect NH3 and H2O absorption lines. They reported on 21 NH3 lines from 1496 to 1504 nm and 1 H2O line at (7) Carlisle, C. B.; Cooper, D. E. Appl. Phys. Lett. 1990, 56 (9), 805-807. (8) Johnson, T. J.; Wienhold, F. G.; Burrow, J. P.; Harris, G. W. Appl. Opt. 1991, 30 (4), 407-413. (9) Feher, M.; Jian, Y.; Maier, J. P.; Miklos, A. Appl. Opt. 1994, 33 (9), 16551658. (10) Ishihara, Y.; Masusaki, H.; Wu, S.-Q.; Matsumoto, K.; Kimishima, T. Proceedings of 4th International Symposium on Semiconductor Manufacturing, Austin, TX, 1995. (11) Ishihara, Y.; Masusaki, H.; Wu, S.-Q.; Matsumoto, K.; Kimishima, T. Electrochem. Soc. Proc. 1995, 95-20, 387-394. (12) Masusaki, H.; Wu, S.-Q.; Ishihara, Y.; Matsumoto, K.; Kimishima, T. Proceedings of 49th Symposium on Semiconductors and Integrated Circuits Technology, Tokyo, 1995. (13) Rothman, L. S.; Gamache, R. R.; Goldman, A.; et al. Appl. Opt. 1987, 26 (19), 4058-4097. (14) Urban, S.; Tu, N.; Narahari, R. J. Mol. Spectrosc. 1989, 133, 312-330. (15) Guelachvili, G. J. Mol. Spectrosc. 1989, 133, 345-364.
3316 Analytical Chemistry, Vol. 70, No. 15, August 1, 1998
1496.5 nm, and the positions of each line were given precisely.16 Yanagawa et al. also described a diode laser absorption measurement of NH3 gas from 1530 to 1550 nm.17 However, the information on the NH3 lines between 1300 and 1400 nm is not sufficient. To supplement the lack of NH3 absorption line data in the near-infrared wavelength region, we measured NH3 absorption spectrum near 1371 nm with a high-resolution FT-IR (Nicolet, Magna 750) and a DFB diode laser spectrometer. As a result, a number of weak absorption lines of NH3 were observed by the diode laser clearly in this wavelength region. These lines did not stand alone, and they formed an absorption continuum of NH3. Consequently no wavelength at which the H2O absorption lines are interference free was found.18 Therefore, to analyze trace moisture sensitively in NH3 with the near-infrared tunable diode laser absorption spectrometry (NIR-TDLAS) system, the method to remove the interfering absorption of NH3 must be developed as the key technology. Girard and Mauvais described an application of an InGaAsP/ InP DFB diode laser oscillated at 7306 cm-1 and a 10-m multipass cell to trace moisture measurement in NH3.19 They used an asynchronous subtraction method to remove the effect of the interfering absorption of NH3. The background signal data of the pure NH3 absorption spectrum was memorized by a computer and then subtracted from that of the sample data. A calibration curve from 0.4 to 3.3 ppm was made, and the detection limit calculated from the S/N ratio was ∼30 ppb. This approach is simple, but it assumes that the background signal including the interfering absorption of NH3 and the noise are quite constant. However, it is hard to maintain them constant; thus this system may lead to a loss of accuracy. In previous work, we developed a dual-cell system of NIRTDLAS which utilized a synchronous subtraction method to remove the effect of the interfering absorption of NH3 without loss of accuracy. The concept of our method utilized in this work is quite simple. For an interference matrix species such as NH3, the reference cell was filled with pure NH3 at the same pressure as the sample cell, leading to the removal of the common mode noise from the optical source as well as the common signal of the interfering absorption of NH3. The reference cell plays an important role in removing the absorption signal from NH3 molecules like a “filter”. In consequence, this system could also improve the signal-to-noise ratio (SNR) due to cancellation of common mode noise from the diode laser and demonstrated the potential to measure moisture in NH3 at the ppb level.20,21 In this paper, we report quantitative analysis results of trace moisture in N2 and NH3 at the ppb level using the optimized dual-cell NIRTDLAS system and a well-designed gas handling system. (16) Ohtsu, M.; Kotani, H.; Tagawa, H. Jpn. Appl. Phys. 1983, 22 (10), 15531557. (17) Yanagawa, T.; Saito, S.; Yamamoto, Y. Appl. Phys. 1984, 45 (8), 826-828. (18) Wu, S.-Q.; et al., to be submitted. (19) Girard, J.-M.; Mauvais, P. Proceedings of 5th International Symposium on Semiconductor Manufacturing, Tokyo, Japan, 1996; pp 325-328. (20) Wu, S.-Q.; Masusaki, H.; Ishihara, Y.; Matsumoto, K.; Kimishima, T.; Kuze, H.; Takeuchi, N. Proceedings of 5th International Symposium on Semiconductor Manufacturing, Tokyo, Japan, 1996; pp 321-324. (21) Morishita, J.; Wu, S.-Q.; Ishihara, Y.; Kimijima, T. Jpn. J. Appl. Phys. 1997, 36 (Pt. 2), L1706-L1708.
Figure 1. Schematic diagram of the dual-cell near-infrared diode laser absorption spectrometry system.
EXPERIMENTAL SECTION Optical Setup. The optical setup of the dual-cell system is illustrated schematically in Figure 1. A near-infrared InGaAsP DFB laser emitting at 1371 nm was employed as the optical source (NEL, NLK1351CCA). The diode laser was mounted on a copper block which stabilized temperature using a thermistor sensing element and a feedback circuit to drive a Peltier device (Asahi Data Systems, ATC-160). The temperature of the diode could be controlled with an accuracy of better than 0.005 K. Tuning rates of the diode laser were ∼0.1 nm/K and 0.0085 nm/mA. Coarse adjustment of the diode laser wavelength was carried out by changing the diode temperature. The emitted light was collimated by a diode laser collimator with a 0.6 NA (Asahi Glass, LDIFA-001) and through an optical isolator (Isowave, I-13-M-TB), which maintained isolation of ∼38 dB at 1371 nm for minimizing optical reflections back into laser cavity. Then the laser beam was divided with a 50:50, polarizationpreserving beam splitter and directed onto two Ge PIN photodiode detectors with preamplifiers (Hamamatsu, B4246). Transmission impedance of the preamplifiers was ∼1.2 × 103 with a 10-kHz bandwidth. The light of the signal channel passed through a 92 cm long absorption sample cell which was made of a 316-L stainless steel tube attached with Brewster angle windows. The reference channel had the same setup as the sample channel. For interference-free matrix species such as N2, the reference cell was filled only with pure N2 at atmospheric pressure for the purpose of noise cancellation. All of the optics from the laser diode to detectors are purged by pure N2 gas in a purge box. The effect of ∼1 ppm of residual moisture in a purge box outside the sample cell was also canceled by adjusting the beam path length outside the reference cell from the beam splitter to the detector of the reference channel. The current of the diode laser was directly modulated with a function generator (Yokogawa, FC120), while the dc part of the injection current was tuned across H2O absorption lines in gases. Two lock-in amplifiers (EG&G 5210) were used for the phasesensitive detection at the second harmonic (8 kHz) of the modulation frequency. Phase settings of the lock-in amplifier were chosen to maximize the 2f signal, and care was taken to avoid any distortion due to the time constant employed. To obtain the
Figure 2. Absorption spectra of H2O and NH3 gases measured by the dual-cell laser diode absorption spectrometry system.
second-derivative spectra of H2O absorption with high SNR, we systematically optimized detection parameters such as modulation frequency, modulation amplitude and gas pressure in our system. It turned out that a modulation frequency of 4 kHz and a sample gas pressure of 50-140 mbar gave the best conditions. At this low frequency, we could easily modulate the diode laser frequency by modulating the output current of the laser driver itself. To reduce Etalon fringe noise, all lenses and the beam splitter were AR-coated. Two channel outputs from the lock-in amplifiers were digitized by an A/D converter and then recorded on a laboratory computer. The advantage of the dual-cell scheme can be summarized below. First, this method can eliminate the common mode noise from the diode laser including excess amplitude noise (1/f noise), residual amplitude modulation noise (RAM),22,23 optical feedback from the downstream optics, and Etalon fringes which arise in the optical path between the laser and the beam splitter. Second, the scheme can compensate the effect of moisture outside the sample cell in the beam path. And third, the effect of interfering (22) Wang, L. G.; Haris, H.; Carlisle, C. B.; Gallagher, T. F. Appl. Opt. 1988, 27 (10), 2071-2077. (23) Cooper, D. E.; Warren, R. E. Appl. Opt. 1987, 26 (17), 3726-3732.
Analytical Chemistry, Vol. 70, No. 15, August 1, 1998
3317
Figure 3. An example of noise cancellation by dual-beam detection strategy.
absorption of matrix gas molecules in the spectral region, if any, can be eliminated. The latter feature is especially important for trace moisture detection in NH3. Figure 2 illustrates the second-derivative spectra of NH3 and H2O at 1371 nm which were measured precisely at the same time. The upper trace is the NH3 absorption spectrum measured from the sample cell, and the lower trace is the 2 ppm H2O absorption spectrum from the reference cell. The pressures of both cells were kept constant at 66.7 mbar, and flow rate through the cells were ∼300 standard cm3/min (sccm). The H2O absorption line at 1370.96 nm is one of the most strong lines in this spectral region. However, the line is located on the wing of a weak NH3 absorption line which corresponds to an absorption intensity of ∼4 ppm H2O. It means that if the normal method of single beam was employed for detection of trace moisture in NH3, the linearity and sensitivity of calibration curve could be lost in the lowconcentration region. Figure 3 shows an example of noise reduction by the dual-cell system. The laser wavelength was tuned from 1370.57 to 1370.63 nm at room temperature. In this spectral region, there are no strong H2O absorption lines. The pressure in both the sample and reference cells was constant at 66.7 mbar and the flow rate of pure N2 through the cells were ∼500 sccm. The lower trace is the signal from sample channel, and the middle trace is that from the reference channel. Both the sample and reference channel signals were noisy and had offset biases originating from the RAM noise of the modulated diode laser. It is apparent that both channels had characteristics of the same phase fluctuation. The upper trace in Figure 3 is a result of dual-beam subtraction with this method. The processed signal had the feature of low noise with baseline offset of nearly zero. An order of magnitude increase of the SNR was obtained. Figure 4 is a result of one sweep measurement where laser wavelength was scanned from 1370.10 to 1371.80 nm. Because a number of weak absorption lines of NH3 exist in the relevant spectral region, the reference cell was filled with pure NH3 at 66.7 mbar, the sample cell was filled with NH3 at the same pressure, and then dual-cell measurement was carried out. The flow rate through the sample cell was ∼200 sccm, which was identical with the reference cell. The time constant of lock-in amplifiers was 300 ms. As can be seen from Figure 4, the interference of NH3 3318 Analytical Chemistry, Vol. 70, No. 15, August 1, 1998
Figure 4. Interfering absorption spectra of NH3 gas measured from the sample and the reference channels. The lower trace is a subtracted result.
is removed owing to the dual-cell technique. This measurement provides a good example which interferes with the target absorption. Gas Handling. Figure 5 displays a schematic flow diagram of the present gas handling system designed for corrosive gases. Three key techniques for trace moisture analysis were taken into account in designing the gas handling system. First, in order both to reduce the background level of trace moisture and to limit the adsorbing and outgassing of water vapor from the inner surface, we minimized the dead space and surface area of the system. All tubings and components employed in this system were made of electropolished stainless steel. The leak rate of the gas handling system was below the detection limit of helium leak detector (∼10-11 L/s). The pure N2 gas was supplied from a large liquid nitrogen tank located at our laboratory and purified by an in-line purifier (Nippon Sanso Corp.) which can remove impurities including moisture. The residual trace moisture in pure N2 used in the experiments was no more than 1 ppb. All parts and components in the system could be purged by pure N2 if it was needed. Second, in previous work, we found that when the matrix species changed, or the mixing ratio of two kinds of matrix gas species was varied, the absorption line width of impurity and the absorption signal intensity changed. It led to loss of measurement accuracy. To resolve this problem, a two-stage dilution system was built for the present experiments. The gas handling system was composed of six mass flow controllers (MFC), two back pressure regulators (BPRs), two metering valves, and a vacuum pump. The first stage, composed of MFC-1, MFC-2, and BPR-1, was used for the dilution of standard moisture supplied from a gas cylinder (H2O/N2 standard gas, Nippon Sanso Corp.). The moisture concentration of the standard gas was determined to 7.3 ppm by a chilled mirror hygrometer (EG&G, model 300). The maximum dilution rate of the first stage was 1:1000. The second stage, composed of MFC-3, MFC-4, and BPR-2, was used mainly for maintaining a constant volume ratio of NH3 to N2 in the sample cell. The ratio was maintained constant at 10:1 when H2O/NH3 calibration was performed. And third, as mentioned above, adjustment of the pressure of both the sample and reference cells was needed to optimize SNR
Figure 5. Schematic diagram of gas handling system used for measurement of trace moisture in ammonia gas.
Figure 6. Effciency of Nanochem purifier for removing moisture in ammonia gas. The moisture was added to pure ammonia. The results were measured with the NIR-TDLAS system.
of the NIR-TDLAS system. MFC-5, MFC-6, metering valves, and a vacuum pump were employed for controlling the flow rate and pressure of the sample and reference cells. We could control the cell pressure from 1 to 1333 mbar precisely. Another problem that has to be resolved for trace moisture in NH3 is how to obtain pure NH3. This is due to the fact that about tens to hundreds ppb of trace moisture are contained even in commercial ammonia gas of high grade. In fact, the dual-cell detection is a measurement of concentration difference between sampled and purified NH3 gas. A Nanochem purifier, which is designed for NH3 (Semigas Systems Inc.), was selected for removing the residual moisture in this measurement. It was confirmed that the purifier could remove moisture in nitrogen below 1 ppb with API-MS. Figure 6 shows the results that identified the efficiency of the purifier. The results of 523 and 5213 ppb measured by the dual-cell system are shown. RESULTS AND DISCUSSION Determination of H2O in N2. For a dual-beam approach, a careful balancing of the signals from two channels is needed.
Despite use of identical detectors, amplifier electronics, and 50: 50 beam splitter, the balance could not be easily achieved. This is because of spatial variations in detector response and drifts in the amplifier or the other circuits. Several methods of dual-beam detection including optical24-26 or electrical27-29 balancing of the signals from two channels have been reported. The measurements reported here were obtained with a digital processing method. Signal from the reference channel was multiplied by a certain factor and subtracted from that of the sample channel. The factor was empirically determined for an optical alignment, and once established, the value was reproducible for at least one month. In this experiment, the factor of cancellation was 1.03. The calculation of cancellation was performed automatically by the computer, and only the subtracted data were output as a result from the sample. A typical method for a quantitative analysis at low concentration using this system is (1) adjusting the cancellation factor, (2) measuring an average spectrum of the background at zero ppb concentration, and (3) measuring an average spectrum from the sample and subtracting the average spectrum of the background from the average spectrum from the sample. The quantitative measurements of H2O in N2 facilitated by standard addition were performed by adding standard gas of water vapor (H2O/N2) to pure N2 with a certain dilution rate, and then the spectra were measured from 14 to 7300 ppb. Figure 7 illustrates the spectra from 14 to 515 ppb of H2O absorption in N2. Every spectrum is an average of 10 sweeps. The laser wavelength was scanned from 1370.93 to 1371.99 nm at room temperature. Sample cell pressure was kept constant at 66.7 mbar, and flow rate through the cell was ∼500 sccm. The reference cell was filled with pure N2 at 1013.3 mbar. The time constant of lock-in amplifiers was 300 ms. (24) Gehrtz, M.; Bjorklund, G. C. J. Opt. Soc. Am. B 1985, 2 (9), 1510-1525. (25) Houser, G. D.; Gramire, E. Appl. Opt. 1994, 33 (6), 1059-1062. (26) Bacon, A. M.; Zhao; H. Z.; Wang; L. J.; Thomas, J. E. Appl. Opt. 1995, 34 (24), 5326-5330. (27) Haller, K. L.; Hobbs, P. C. D. SPIE Proc. 1991, 1435, 298-309. (28) Allen, M. G.; Carleton, K. L.; Davis, S. T.; Kessler, W. J.; Otis, C. E.; Palombo, D. A.; Sonnenfroh, D. M. Appl. Opt. 1995, 34 (18), 3240-3249. (29) Zhu, X.; Cassidy, D. T. Appl. Opt. 1995, 34 (36), 8303-8308.
Analytical Chemistry, Vol. 70, No. 15, August 1, 1998
3319
Figure 7. Second-derivative absorption spectra of H2O in N2 measured with the NIR-TDLAS system. The absorption length of the sample cell is 92 cm.
Figure 8. Absorption spectra of H2O in NH3 measured by the dualcell NIR-TDLAS system. The absorption length of the sample cell is 92 cm.
Since the average of the left-hand and right-hand intensities was in principle free from the effect of the baseline fluctuations of signals, the signal intensity in the experiments was defined as the left-hand and right-hand peak-to-peak average of the secondderivative spectrum. As displayed in Figure 7, the signal intensity of H2O absorption in N2 is directly proportional to the H2O concentration. The correlation coefficient, which was evaluated by least-squares fitting between the signal intensity and H2O concentration, was over 0.999, indicating good linearity. Calibration of H2O in NH3. The measurements of trace moisture in ammonia were also performed with the dual-cell NIRTDLAS system. Figure 8 plots the spectra of 110, 245, and 523 ppb of H2O absorption in NH3. The standard gas of water vapor (H2O/N2) was added to pure NH3 which was passed through the Nanochem purifier. Each spectrum is a 10-sweep average. The laser wavelength was scanned from 1370.93 to 1370.99 nm at room temperature. The sample and reference cells were treated as before. As shown in Figure 8, the signal intensity of H2O absorption in NH3 is directly proportional to the H2O concentration. The correlation coefficient, which was evaluated by leastsquares fitting between the signal intensity and H2O concentration, was over 0.9999, indicating good linearity. The slope value of the calibration curve is smaller than that in HCl measured before,20,21 about one-third of the value in N2. This is owing to the width of the H2O absorption line in NH3 being larger than the width of H2O in N2 at the cell pressure. Detection Limits of the NIR-TDLAS System. According to the results of our experiments, the detection limit based on one
scan measurement of H2O in NH3 was estimated first, and then, the detection limit of the average result of 10 scan measurements was calculated; we finally estimated the detection limit of H2O in N2 according to the ratio of two calibration curve slopes. Because several different definitions and standard methods for calculating the detection limit have been proposed,30-35 in this paper, we utilized the International Union of Pure and Applied Chemistry (IUPAC) definition, which focuses on intercept of the calibration curve and the signal variability measured between results at zero ppb concentration. The value of the detection limit (DL) is determined as the concentration level of intercept (i) of calibration curve plus t times the standard deviation (σb) of the blank signal (DL ) i + tσb).30,31 Here, i is the intercept of the calibration curve which is equal to 4.7 ppb. The t is the statistical multiplier of one-tailed t-statistic which would be required in place of 4.094 to keep the risk fixed at the nominally assumed 0.13% for 10 data.30,35 The σb is the standard deviation of the sample signal at zero ppb concentration which corresponds to 7.2 ppb. The values of standard deviation of the absorption intensity over the calibration range corresponds to concentrations from 3.8 to 7.2 ppb, the average is ∼6.3 ppb. As the result of calculation, the
3320 Analytical Chemistry, Vol. 70, No. 15, August 1, 1998
(30) Bzik, T. J.; Smudde, G. H.; et al. Microcontamination Conf. Proc. 1994, 653-671. (31) Long, G. L.; Winefordner. J. D. Anal. Chem. 1983, 55 (7), 712. (32) Ridgeway, R. G.; Ketkar, S. N.; et al. Microcontamination Conf. Proc. 1992, 293. (33) Ketkar, S. N.; Bzik, T. J. Microcontamination Conf. Proc. 1993, 563. (34) Werle, P.; Slemr, M. F. Appl. Phys. 1993, B57, 131-139. (35) Bzik, T. J. Microcontamination Conf. Proc. 1992, 224-243.
detection limit based on one scan measurement was ∼35 ppb. The detection limit of the NIR-TDLAS system for H2O analysis in NH3 can be estimated to be 12 ppb because our analysis is often based on an average of 10 scan measurements which can improve the SNR of ∼3 times. The detection limit of H2O in N2 is ∼4 ppb according to the ratio of two calibration curve slopes. CONCLUSIONS A dual-cell near-infrared diode laser absorption spectrometry system was developed for analyzing the trace moisture in nitrogen and NH3. Single-longitudinal-mode, room-temperature operating InGaAsP distributed feedback diode lasers were employed as optical sources. A water vapor absorption line at 1370.96 nm, which is combination tone of fundamental vibrational bands, was utilized for trace moisture measurements. The dual-cell technique succeeded in canceling optical source noise and interfering absorption of matrix molecules and achieved high sensitivity. Using this method, we measured trace moisture from 14 to 7300 ppb in N2 with 92-cm absorption length at 66.7 mbar pressure.
The detection limit of our NIR-TDLAS for H2O in N2 was ∼4 ppb. This result corresponds to an absorbance of ∼1.8 × 10-7. As application measurements of this system, we demonstrated the quantitative methods of trace moisture in NH3 despite the interfering absorption of the matrix gases. The trace moisture in NH3 from 110 to 1006 ppb was measured with the same system and a detection limit of ∼12 ppb was obtained. ACKNOWLEDGMENT The authors sincerely thank Dr. Nobuo Takeuchi and Dr. Hiroaki Kuze (Center for Environmental Remote Sensing, Chiba University) for their support during the research. This work was also supported, in part by the Japan Key Technology Center (JKTC).
Received for review January 5, 1998. Accepted May 8, 1998. AC980011Z
Analytical Chemistry, Vol. 70, No. 15, August 1, 1998
3321
