Anal. Chem. 1994,66, 241 1-2419
Application of Tunable Diode Laser Absorption Spectroscopy To Trace Moisture Measurements in Gases Ronald S. Inman and James J. F. McAndrew' Air LiquHe, 5230 South East A venue, Countryside, Illinois 60525
We have applied tunable diode laser absorption spectroscopy (TDLAS) to trace moisturedeterminationin hydrogen chloride gas and in nitrogen. To the best of our knowledge, these are the first TDLAS measurements to combine operation in a reactive gas matrix with low ppb moisture measurements.Cas handling methods, which were previously applied to trace moisture measurement using other equipment, are combined with a TDLAS system that uses second derivative detection and signal averaging techniques. The system is calibrated by direct introduction of a moisture standard into the absorption cell, and its response time has been evaluated using a gas handling system capable of rapid switching betweenhumidified and dry gas streams. The reproducibility (2 X standard deviation of the signal) of the system is currently 0.4 ppb moisture for analyses in nitrogen, when 10 runs of 5 s each are averaged. The sensitivity (slope of the calibration curve) is considerably reduced in HCl, and the reproducibility in HCl is 4 ppb. For the absorption line used, the pressure broadening coefficient of moisture in HCl has been shown to be approximately three times its value in nitrogen, which partially accounts for the reduction in sensitivity in HCI. The current limitation on measurable moisture levels is the background of the system. The lowest background levels measured to date are 10 ppb in nitrogen and 63 ppb in HCI. The modern semiconductor manufacturing industry requires ppb and lower levels of all impurities in process gases.' Moisture is considered a key impurity because it is extremely difficult to eliminate, and it adversely effects many manufacturing processes. In nitrogen, argon, and hydrogen, several analytical techniques, most notably atmospheric pressure ionization mass spectroscopy (APIMS),2 deliver the requisite sensitivity for moisture and other impurities. APIMS is not compatible with reactive gases, however, and a need is perceived for extremely sensitive techniques compatible with a wider range of gases. FT-IR is currently the technique of choice for reactive gases in many gas analysis laboratories, and sensitivities to 100 ppb moisture in HC1 (for an 8-m path and a 5-min measurement time) have been r e p ~ r t e d . ~ [Wherever the abbreviations ppm, ppb, or ppt or used, they refer to molar concentrations, which are the same as concentrations by volume because we assume the ideal gas law is obeyed by all constituents; thus, ppm means part per million by volume, etc.] Unfortunately, FT-IR seems unable to deliver the 1 ppb sensitivity that is expected to be needed in the long term. (1) Ohmi, T.; et al. Microcontamination 1990, 8 (3). 27. (2) Ronge, C.; Murphy, D. T.; Shadman, F. Microcontamination 91 Proceedings; Canon Communications: 1991; p 153. (3) Miyazaki, K.; Kimura, T. Bull. Chem. Soc. Jpn. 1993, 66, 3508. 0003-2700/94/0366-247 1$04.50/0 0 1994 American Chemical Society
Tunable diode laser absorption spectroscopy (TDLAS) is a technique of considerable flexibility and sensitivity, which has become widely used in environmental monitoring, spectroscopy, chemical kinetics, etc.4 A great deal of work has been directed toward improving the sensitivity that can be achieved, and several groups have reported results at or close to the quantum noise limit using a variety of modulation scheme^.^-^ In addition, a large number of techniques have been developed aimed at curing specific problems, such as interference fringes, which are of key importance in practical applications.sl0 If we assume that TDLAS can detect an absorbance (base 10) as small as 10-6 (smaller absorptions have been observed in the several instances of quantum-noise limited operation), then it should have a sensitivity on the order of 0.2 ppb moisture when a 10-m path length is used and the laser is tuned to the relatively strong infrared absorption line at 1456.888 cm-'. In addition to sensitivity and compatibility with a wide variety of matrices, TDLAS offers the important advantage of fast response to changes in the humidity of a gaseous sample, whereas most moisture sensors are sluggish at best. Mucha applied the method of standard additions to moisture measurement in nitrogen and oxygen using TDLAS more than 11 years ago." We have based our determination of moisture in the sample gas on his "standard addition" approach, but we do not follow his method of "optical dilution" whereby a high concentration standard in a short path length cell is used to calibrate measurements of low concentration samples in a long path length cell. The latter method is elegant and useful, but we prefer at this stage of our investigations to be sure that the sample is exposed to the same surfaces as thecalibration gas. In his 1982 paper, Mucha made moisture measurements in the ppm range and quoted a minimum detectable concentration of 200 ppb. In a later review,'* he gave a brief description of some measurements in which he could estimate a detection limit of 5 ppb moisture based on signal to noise ratio, but mentioned that practical moisture measurements were limited to an uncertainty of 20-50 ppb by persistent background moisture in his multipass cell (it is unclear whether the background fluctuated by 20-50 ppb or this was the absolute value of the background). (4) Optical Sensing for Environmental Monitoring, Oct. 11-14,1993, A d W M A and SPIE (Proceedingsto be published in 1994), includes a session on TDLAS with many examples of operation in rugged, field environments. (5) Carlisle, C. B.; Cooper, D. E.; Prier, H. Appl. Opt. 1989, 28, 2567. (6) Werle, P.;Slemr, F.; Gehrtz, M.; Brauchle, C. Appl. Phys. B 1989, 49, 99. (7) Pavone, F. S.; Inguscio, M. Appl. Phys. B 1993, 56, 118. (8) Webster, C. R. J. Opt. Soc. Am. B 1985, 2, 1464. (9) Silver, J. A.; Stanton, A. C. Appl. Opt. 1988, 27, 1914. (10) Sun, H. C.; Whitaker, E. A. Appl. Opt. 1992,31 (24), 4998. (1 1) Mucha, J. A. Appl. Spectrosc. 1982.36 (4), 393. (12) Mucha, J. A.; Barbalas, L. C. ISA Trans. 1986, 25 (3), 25.
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Silver and Stanton applied TDLAS to atmospheric humidity measurements and developed an instrument suited to airborne 0perati0n.l~ In their experiments, accuracy at relatively high moisture levels was of more concern than the measurement of trace concentrations. Carlisle and Cooper14 described high sensitivity measurements of water vapor using a TDLAS system which included an optical fiber. They used a 1.31-wm InGaAsP diode as high-quality optical fibers are available in that spectral region. They could measure a very small absorbance (5 X at 7665 cm-I with pure water vapor in a short path (50 cm) cell, but the absorption cross section at 7665 cm-I is more than 3 orders of magnitude smaller than that available for the line at 1456.888 cm-I, and they did not address the introduction of actual, low-moisture gas samples to their system. Kastle et al.15 described the measurements of moisture in ammonia down to a few ppm that were probably the first measurements of moisture in a reactive gas by TDLAS. Their multipass cell was not optimized for ppb moisture, and they encountered an extremely large background which interfered with their measurements, even at the ppm level. They attempted to remove this effect by observing the variation in signal with the flow rate of gas through their multipass cell and extrapolating to infinite mass flow to find the contribution due to the sample gas. This approach is interesting, but it assumes that the background moisture due to the cell is constant and leads to a loss of precision. Schaeffer et a1.16described a multipass absorption cell for high-temperature UHV operation and showed a noiseequivalent concentration of 10 ppb moisture in their cell using a 100-mpathlengthandan absorptionat 1684.836 cm-l (about twice as intense as that at 1456.888 cm-I mentioned above). Because of the lack of a suitable low concentration standard, they calibrated their system by using high concentration samples at low pressure in the cell and determining moisture in those samples with a frost point hygrometer. Although this procedure is often useful for gases which do not interact with surfaces, here it is risky because the background contribution from the cell needs to be carefully assessed and can be different at different cell pressures, especially if the mass flow rate through the cell varies. In the present work, it was considered important to be able to determine whether samples can be introduced to theTDLAS hygrometer in a representative manner, so the system was calibrated by direct introduction of a sample of verified moisture concentration to the analysis cell. Previous workers have not generally had access to moisture standards at the ppb level and so have been obliged to use a variety of other approaches. The moisture generator used here is based on a permeation device: its design and verification have been described previo~s1y.l~The delivered moisture level can be calculated from the weight loss of the permeation tube over time, although we prefer to verify this by comparison with a hygrometer of established performance.
Diode Laser Optics and Modulation Scheme. Figure 1 is a schematic of the diode laser optical system. The diode was supplied by Laser Analytics Division of Laser Photonics Corp. and is mounted in a Laser Analytics liquid nitrogen-cooled coldhead. A 1-in.-diameter, aspheric, A.R.-coated, E/ 1 ZnSe lens is used to collimate the laser beam. The detector is a Graseby HgCdTe Model 1710112 with a 10-MHz bandwidth amplifier. Laser current is controlled by an ILX Lightwave LDX-3620, and temperature is controlled by a Lake Shore DRC-9 1OA. Figure 2 is a schematic of the laser modulation and signal processing electronics. A current ramp is applied to the diode to vary the output light frequency. This ramp is supplied by a Kinetic Systems Model 3 115 D/A converter controlled by an IBM PC-compatible computer. This ramp is applied with a frequency of 1.95 kHz, and the resulting signal is fed to a DSP Model 2012 transient recorder via a DSP 1412-2 amplifier. Successive ramps are signal averaged using a DSP Model 4101 averaging memory and output to the PC via an RS-232 link. DSP-supplied software is used to control data acquisition and display. An additional sinusoidal modulation at 1 MHz is generated by a Tektronix FG501A function generator and delivered to the diode via a bias tee. The modulation signal is frequency doubled and mixed with the detector output, and the mixer output is the signal input to the 1412-2. The detected signal is therefore the second derivative of the absorption signal. This setup is very similar
(13) Silver, J. A.; Stanton, A. C. Appl. Opt. 1987, 26 (13), 2558. (14) Carlisle, C. B.; Cooper, D. E. Appl. Phys. Left. 1990, 56 (9), 805. (15) KPstle, R.;et al. Microconramination 1991, 9 (lo), 27. (16) Schaeffer, R. D.; Sproul, J. C.; O'Connell, J.; van Vloten, C.; Mantz, A. W. Appl. Opr. 1989, 28 (9), 1710. (17) Mermoud, F.; Brandt, M. D.; McAndrew, J. J. F. Anal. Chem. 1991,63, 198.
(18) McAndrew, J. J. F.; Brandt, M. D.; Li, D.; Kasper, G. Microconraminntion 1991, 9 ( l ) , 33. (19) (a) McAndrew, J. J. F.; Brandt, M.D.; Kasper, G.;Kimura, T. Microcontaminntion 91 Proceedings: Canon Communications: 1991; p 352. (b) McAndrew, J. J. F.; Brandt, M. D.; Kasper,G.;Kimura,T. Microconramination 92 Proceedings; Canon Communications: 1992; p 386.
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We have attempted to minimize the background moisture level and response time of our system. For any humidity measurement technique of high sensitivity, the background moisture level is a key issue in determining detection limit. For example, APIMS systems are available with sensitivities better than 1 ppt (part per trillion) for moisture in nitrogen,2 but background moisture levels restrict most measurements to levels about 100 ppt. TheTDLAS measurements of Kastle et al.ls were primarily limited by background moisture, as mentioned above. Minimization of background moisture in the sample cell and optimization of response time can be achieved by (i) choosing materials or surface finishes known to interact less with moisture (e.g., electropolishing stainless steel), (ii) optimizing the flow path in the analysis cell and delivery systems so as to avoid dead volumes, and (iii) using sufficient sample flow to deliver the necessary dilution level. It is possible to rank components such as particle filters, valves, tubing, etc. in terms of their moisture interactions by injecting a "pulse" of several ppm moisture and comparing the delay and distortion of the input pulse due to different components. This is referred to as a "transfer function" technique,18 and we have used it, for example, to select electropolished stainless steel tubing with relatively low levels of moisture interacti~n.'~
EXPERIMENTAL SECTION
to pumps t
Detector Laser
U
Figure 1. Optical schematic of TDLAS hygrometry bench.
Current Source r
out
DIA Converter
-
(1.95 WRampSource) Mk
Signal Generator (1 MHz sine wave)
1
Out
I
Signal Averager
I
1
I
I
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Transient Recorder .. .
l
i
1
I
I
Doubler
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n Detector 1 -
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Figure 2. Diode modulation and signal processing electronlcs.
to that described previously by Bomse et a1.20 The amplitude of the 1-MHz modulation applied to the diode was determined by comparison with the width of a moisture absorption line at a cell pressure of 250 mTorr and a sufficently high moisture concentration to see the signal easily in “direct detection” mode, Le., without the 1-MHz modulation. At this low pressure, the absorption linewidth was assumed equal to the Doppler width. The 1-MHz modulation amplitude (peak to peak) for measurements in (20) Bomse, D. A.; Stanton, A. C.; Silver, J. A. Appl. Opt. 1992, 31 (6), 718.
nitrogen ranged from 1.3 to 1.8 times the linewidth at the cell pressures of 20-38 Torr which were used. This ratio of modulation amplitude to linewidth is less than the optimal value of 2.2 and leads to a loss in sensitivity to moisture in the cell of up to a factor of 2.21 However, using a relatively small modulation amplitude enhances the distinction between the broader signal due to moisture at atmospheric pressure outside the sample cell and the signal due to the sample at reduced (21) Webster, C. R.; Menzies, R. T.; Hinkley, E. D. Infrared Laser Absorption: Theory and Applications. In Laser Remore Chemicol Analysis; Measure, R. M., Ed.;Wiley: New York, 1988.
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Moisture Generator
.........................................................................
................................
N
1-
G a s Cabinet
To Exho
Flgure 3. Handling system for reactive gases and moisture introduction.
pressure inside the cell and gives the best overall sensitivity for our system. For measurements in HCl, we used a larger modulation amplitude of 0.01 5-0.024 cm-l, which is again less than the optimal value, as the pressure-broadening factor for moisture in HCl is appreciably greater than in nitrogen (see below). Moisture Introduction and Gas Handling. Figure 3 is a schematic of the gas handling system. It was designed to be able to handle reactive gases and so a number of safety features are included, such as housing the reactive gas cylinder in a gas cabinet equipped with a purge panel (cabinet and panel supplied by Air Liquide Electronics, Countryside, IL), which enables moisture and other impurities to be effectively purged from the system before opening the reactive gas cylinder. The moisture generator is located on the nitrogen line indicated at the top of Figure 3 and is similar to that described p r e v i ~ u s l y . ~Nitrogen ~ is dried by passing it through a molecular sieve activated at 300 "C. Similar molecular sieve dryers have been verified by APIMS in our laboratory to generate no more than 5 ppb moisture (with some variation depending on activation procedure). This results in a background moisture addition less than 0.5 ppb when this stream is added to and diluted by the HC1 or nitrogen stream to be analyzed (see below). Standard moisture levels are generated by varying the flow of nitrogen over a permeation device (provided by GC Industries, Chatsworth, CA) maintained at 50 OC by a heater and temperature controller. The nitrogen flow is controlled by a 5 standard L/min mass flow controller (MFC, supplied by Unit Instruments, Torba Linda, CA; Model UFC-1100). In order to verify the performance of the moisture generator, moisture levels were determined 2474
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using a frost-point hygrometer supplied by General Eastern (Watertown, MA) which had been previously certified by NIST. The permeation rate of the tube was 3500 ng/min, determined by measurements in the range from 1 to 5 ppm. When it is desired not to add any moisture to the N2 or HCl stream, it is necessary to remove the permeation device. The hydrogen chloride stream was passed through a purifier designed for use in HC1 and supplied by Millipore Corp. The purifier is designed to remove multiple impurities from the HC1 stream, but here we are concerned only with its performance with respect to moisture. The specification of this purifier is less than 10 ppb moisture in the output stream. The output level has been verified to be less than 100 ppb by FT-IRa3 One of the objectives of our work is to improve the determination of the output moisture level of this and similar purifiers. The HCl was provided by Air Liquide Electronics (Morrisville, PA), and its moisture level before purification was roughly 2 ppm. Because it is not known how the permeation rate of a given device varies when the carrier gas is changed from nitrogen to a reactive gas such as HCl, the mosture standard is always generated in nitrogen. This nitrogen stream is combined with a larger stream of hydrogen chloride gas. The flows of the moisture-doped nitrogen and hydrogen chloride were controlled by two orifices operating under critical flow conditions. The flow of moisture-doped nitrogen through the first orifice was determined at a series of upstream (stagnation) pressures by placing an MFC in-line downstream and using it as a mass flow meter (Le,, with the control valve open continuously). The MFC had been previously calibrated using an NISTtraceable Sierra Cal-Bench, which measures flow by deter-
I*o
,
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i 10
I 50
100
150
200 250 300 Frequency Scan
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400
450
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Figure 4. TDLAS signal for 104 ppb moisture in nitrogen.
mining the velocity of a piston in a tube into which the gas streams feed. The measured orifice diameter was 35 pm, in reasonable agreement with the diameter calculated from the observed experimental stagnation pressure to flow ratio of 0.85 (standard cm3/min)/psi by assuming critical flow. The flow of hydrogen chloride was determined by a second orifice, approximately 200 pm in diameter. The sample cell was used to measure the flow of HCl through this orifice as follows: First, thevolumeof thecell was determined by flowing nitrogen into the cell using an MFC. The pressure in the cell was measured using a Leybold Diavac 1000 diaphragm pressure gauge (1 atm full scale). The rate of pressure increase (dP/ dt) was determined at various mass flows (dnldt). From the slope of this plot, the volume of the sample cell was estimated as 2.5 L, assuming ideal gas behavior. dP/dt in the sample cell was determined for various HC1 pressures upstream of the orifice, enabling the HC1 mass flow to be deduced from the nitrogen data. HCl mass flows were measured in the range of 400-750 standard cm3/min. The above procedure assumes HCl behaves as an ideal gas in the sample cell. The cell is never filled above 1 bar. For nitrogen, the compressibility at 300K and 1 bar is 0.99982, so the ideal gas assumption contributes avery small error. For HC1, the compressibility at 298K and 1 bar is 0.9945,so the error introduced by this assumption is less than 0.5%. The pressures upstream of the orifices on the nitrogen and HC1 lines vary in the range of 10-40 psig. The proportion of nitrogen in the HC1 stream is always less than 5% and its effect on linewidth is neglected. When the system is not in use, unpurified nitrogen from our house nitrogen system (moisture content less than 100 ppb) is delivered to the HC1 line upstream of the regulator and allowed to purge the entire system. For analyses in nitrogen (cf. Figures 4 and 5), purified nitrogen can be introduced to the HC1 line downstream of the hydrogen chloride purifier and delivered in place of hydrogen chloride by the same path as the HCl. This nitrogen is purified using a Nanochem resin purifier, which according to its manufacturer has been shown by APIMS to remove moisture to less than 1 ppb.22 (22) Nakahara, F.;Ohmi,T.;et al. Eualuationof NanoehemResinGas Purification System f o r Semiconducror Fabrication by APIMS; Report prepared for Hercules Corp.
Cell Design and Operating Parameters. The cell is of the Herriot designz3with a mirror separation of approximately 30cm. The total number of passes is usually 30 to give a total path length of 9 m. A larger mirror separation would have yielded greater sensitivity in terms of noise-equivalent concentration, but only at the cost of higher background due to greater surface area. The sample cell is operated with a sample flow of 600-750 standard cm3/min and a t a pressure of 20-40 Torr, so that the signal due to moisture inside the cell is readily distinguished from the much broader absorption due to moisture in the optical path outside the cell. The cell was operated at room temperature, except for the response time measurements (see below). The input beam to the multipass cell is reduced in diameter using a pair of focusing mirrors (250(M3)and 50 (M4) mm focal lengths]. The mirror separation and the distance of the second mirror from the Herriot cell input was optimized using a Gaussian beamZ4propagation program written by one of us (R.I.) so as to achieve the maximum number of passes in the multipass cell without any overlap of the light spots on the mirrors (which would lead to interference fringes). We attempted to optimize the cell design in terms of its flow characteristicsZ5and to characterize its performance by means of its response to a moisture input. We have used this technique previously,19 to characterize components of gas distribution systems. Our original cell included some polymeric materials incompatible with HCl, which wereeliminated from the final "working cell" design used for collecting the data presented in Figures 4,5, 8, and 9. The response time measurements were made on the original cell, heated to 60 OC, whereas the working cell was always operated at room temperature in order to avoid accelerating any corrosion. The effect, if any, of eliminating the polymers would probably be to reduce the response time. The fact that the working cell has been exposed to HCl and is operated at a lower temperature however probably means that its actual response time is longer than the data of Figure 6 indicate. The purposeof theresponse time data is therefore to provide a reference against which other cell designs can be compared under similar conditions to determine their suitability for moisture measurements. (23) Herriott, D.; Kogelnik, K.; Kompfner, R. Appl. Opt. 1964, 3 (4). 523. (24) OShea, D. C. Elements ofModern OptlcalDesign; Wiley-Interscience: New York, 1985. (25) Jurcik, B.; et al. US. Patent Application, submitted.
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25
Initial Concentration = 29.4 ppb added (38 ppb total)
1
1
Signal of 5 Corresponds to 9 ppb
1
~
The configuration of Figure 3 requires that the permeation device be removed to generate a “zeron gas. A considerable equilibration time is needed for this approach, which would obscure the response of the cell. Therefore, for the response time measurements, we used an arrangement which includes one nitrogen line with a moisture generator (as in Figure 3) and a second line with only a purifier and no permeation device, configured so that a continuous flow was maintained through each line and either could deliver flow to the sample cell while the other flowed to vent. This has been described in detail elsewhere.26 Because of the requirement of continuous flow to vent, it is difficult to set up a similar configuration for HC1.
RESULTS AND DISCUSSION Detection and Background Levels in Nitrogen. Figure 4 shows a typical signal for 104 ppb moisture in approximately 30 Torr of nitrogen. The frequency scan is in units of time determined by the ramping function used to vary the laser current and the laser tuning curve. The signal has the characteristic second derivative shape, i.e., a minimum corresponding to the minimum transmittance, with maxima on either side. We define the signal as the average of the difference between the minimum and each of the maxima (Le., the average of SLand SRin the figure). The scan in Figure 4 was obtained using the strongest moisture absorption line accessible on our diode, which is most likely at 1456.888 cm-l, based upon the diode’s output characteristics. We have not attempted to assign this line moredefinitively. Although the true assignment is of interest, its main usefulness would be to determine whether we are in fact using the strongest absorption line available. As the objective of our work is to make low-level moisture measurements, and the sensitivity of our system is already substantially higher than the smallest moisture levels we have observed, it seems more important to focus on reducing moisture levels in the system than on improving sensitivity. The spectrum took approximately 5 s to collect (SO00 scans at 1 kHz). These data were collected using a final low pass filter setting of 8 kHz, and 5000 scans were averaged for an effective bandwidth of 1.3 H z . ~There is some indication that
the absorption signal is superimposed on a broader peak due to moisture remaining in the purged region outside the cell. The primary source of noise is interference fringes, probably due to optical etalon effects, as is usually the case for diode laser measurements, here manifested as a Ywavynbackground signal. The fringe amplitude in the figure is equivalent to a concentration of approximately 5 ppb. Fringes can typically be reduced to an amplitude equivalent to 2-5 ppb. However, as discussed below, the fringes are reproducible, and thus it is possible to detect concentration changes somewhat smaller than the fringe amplitude. This has been pointed out previously by Mucha.I2 At 30 Torr, the expression of Olivero and L ~ n g b o t h u m ~ ~ gives 0.0088 cm-l for the Voigt linewidth (fwhm) of the 1456.888 cm-l transition, whereas the Doppler width is 0.0044 cm-l. Assuming our line assignment is correct and using a width of 0.0088 cm-I, the 5 ppb fringe amplitude corresponds to an absorbance of 1.7-2.8 X depending on whether we assume a Lorentzian or a Doppler line shape (the Voigt result being between these two). Figure 5 is a calibration curve collected by varying the flow over the permeation device to generate a series of moisture concentrations. The cell pressure was constant at 30 Torr, and the cell flow was 600 standard cm3/min. The moisture background is determined by extrapolation to zero moisture added (standard addition method) and is approximately 11 ppb in this case. Each data point corresponds to the average of several scans such as that in Figure 4. Because the features seen in Figure 4 cannot always be readily distinguished at lower moisture concentrations, we define the signal as follows: Signal is collected for a high concentration peak with low noise. The distance along the frequency axis from peak center (the minimum) to the maximum on each side is measured (these should be equal). Modulation and all other conditions are the same for all lower concentration scans. The value of the low concentration 2f signal is determined at its minimum (peak center) and at those points which are the same distance on either side of the minimum as the maxima were in the high concentration trace. This gives two readings for each spectrum (SLand SRin Figure 4), which are averaged to give a signal. Each point in Figure 5 is an average of up to 10 of these signals collected at each concentration. Our procedure could be improved upon by using a least squares fit to the entire peak as has been done elsewhere.2s This would utilize the information contained in all points rather than just three as our method does. The standard deviation of the signals collected at each concentration is a measure of the sensitivity of the analysis, to be compared with signal to noise measurements on other systems. The largest standard deviation of the data sets included in Figure 5 is 0.5% of signal and corresponds to a concentration of approximately 0.2 ppb at a cell moisture concentration of 35 ppb (24 ppb moisture added + 11 ppb moisture background). This is better than would be expected by equating the above fringe amplitude to random noise and dividing by the square root of the number of scans averaged. The reason is that the fringes are to some extent reproducible ~~
(27) Olivero, J. J.; Longbothum. J . Quant. Specfrosc.Radiclt. Transfer 1977,17, 233.
(26) Brandt, M. D. US. Patent 5,259,233, Counterflow Valve, 1993
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( 2 8 ) Fried,A.:Drummond,J.R.;Henry,B.;Fox,J.App/. Opt.1991,30(15), 1916.
from scan to scan, as noted above. If we again assume that we are in fact using the absorption at 1456.888 cm-l, 0.2 ppb corresponds to an absorbance of about 1 X 106. This is comparable to the sensitivity achieved by similar TDL systems and supports the conclusion that we are monitoring the line at 1456.888 or one of similar strength. The standard deviationof the signal at a given concentration is a measure of the sensitivity of the analytical system, Le., the smallest change in delivered concentration which could be detected when analyzing a continuous gas flow, occurring in a time short enough that calibration does not drift. The precision of the determination of the background level is determined by that of the intercept of the calibration curve. Here, a least squares analysis gives 10.6 f 1.3 ppb. This is an upper limit on the moisture output of the dryer used. As we know that similar dryers are capable of output levels no higher than 1 ppb moisture, it seems most of this background is contributed by our system. The moisture background mentioned above is the primary limitation of the system at present. The background level varies depending upon the exposure of the system to moisture. Once it has reached a level below 20-30 ppb (in nitrogen), the decay becomes sufficiently slow so that the background appears constant when measurements are made over a period of several hours using the usual cell flow of 600 standard cm3/min. If the background moisture is due to cell “outgassing”, we would expect it to decrease as the flow through the cell is increased. Preliminary measurements indicate that this is the case, and the background extrapolates to zero at large sample flows (>3 standard L/min). The signal departs from linearity at higher concentrations. This is probably an artifact, as the absorbance even at several hundred ppb is well within the linear range of the second derivative signal.29 It may be attributable to saturation of some of the signal processing electronic but, although undesirable, is not of concern in the present experiments where low concentrations are the primary interest. Time Response. The response time of the sample cell was measured using a specially constructed gas manifold to be sure that the results were characteristic of the cell itself and not of the gas handling system. As described in the Experimental Section, we maintained a continuous gas flow through a moisture generator similar to that included in Figure 3 and through a similar line which incorporated only a molecular sieve dryer and no permeation device. Risetimes of the order of 10s have been measured by APIMS for moisture inputs delivered by a similar system. The cell was heated to 60 OC with heating tape in order to reduce the interactions of the walls with moisture. Figure 6 shows the response of the system after switching from an input of 29 ppb to “zero” gas. The zero level here is less than 9 ppb, which includes contributions from cell outgassing and residual moisture in the nitrogen stream after purification. The contribution due to residual moisture in the purified nitrogen stream is not known. Purification was effected using a carefully activated 5-A molecular sieve. Figure 6 is part of a larger data set (6 h total), and each point corresponds to a measurement similar to that in Figure 4. (29) Mucha, J. A. Appl. Specrrosc. 1984, 38 ( l ) , 68.
There are some anomalously large fluctuations in signal (of amplitude corresponding to -4 ppb) just before the change in moisture. The reason for these is not understood, but they are not typical: for most of the session, fluctuations were of the order of 1 ppb. A 90%response (reduction in measured concentration from 29 ppb added to 3 ppb above background) is achieved in about 5 min whereas 99% response takes about 1 h. At higher concentrations, a faster response.is obtained: this is expected for any low-level moisture analyzer. For a step from 40 to 750 ppb and down to 40 ppb again, 90%response was achieved within 1 min and 99% within 5 min. Based on the cell volume of 2.5 L, the operating pressure of 30 Torr, and the flow of 600standard cm3/min, the residence time in the cell is of the order of 10 s. The decay in Figure 6 is clearly much slower than this. The initial decay is approximately exponential and corresponds to a residence time of about 2.5 min. Thus, it is clear that the response time is essentially completely governed by adsorption/desorption processes. It should be pointed out that although the performance illustrated in Figure 6 is much slower than simple gas purging, it is very fast compared to most other moisture analyzers. Applicationto HCI. We generated standard moisture levels in HCl by combining a HC1 stream with a smaller humidified nitrogen stream, as described above. No shift in the absorption frequency was observed between nitrogen and HC1 matrices, but the pressure broadening coefficient is a factor of about 3 larger in HC1. The pressure broadening of the absorption in nitrogen and in HCl was measured by comparing the width of a “direct” (i.e., not second derivative) absorption signal with the period of interference fringes generated by adding a window to the optical path. The fringes were due to an optical etalon between the diode laser source (either the diode itself or the window on the dewar) and the additional window. The interfering optics were identified by placing a second additional window in the optical path and jittering it back and forth by hand. When the second window was placed at any point between the laser source and the fixed window, the jittering affected the phase of the interference fringes, indicating that the second window was between the interfering optical elements. When the second window was placed anywhere in the beam path after the fixed window, jittering had no effect on the phase of the interference fringes, ruling out other optical elements as components of the etalon. The distance from the laser source to the window was 111 cm, for a free spectral range of 0.0045 cm-I. The width of a fringe was constant to within 6% in the vicinity of the absorption peak, so that measurement of partial fringes could be made without difficulty. Figure 7 is a plot of fwhm of the observed peak in nitrogen and in HC1. The zero-pressure intercept is approximately 0.005 cm-1 in both cases, compared to an expected Doppler fwhm of 0.0044 cm-I. The error bars in Figure 7 reflect 1 standard deviation in the measurements at each pressure. The errors in slope and intercept quoted in Figure 7 and below are statistics derived from the data and do not consider possible systematic errors, such as errors in ement of the separation of optical elements responsible for the interference fringes. The observed nitrogen broadening coefficient is 1.2 f 0.1 X AnaiyticalChemistry, Vol. 66,No. 15, August 1, 1994
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-
i
u
I
l
25
o.OIOt
Slow = 0 MXllppb
a
I
3
I
2a
d
/
0.007
9
a
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*
1) I 104 m-'/Ion 0053 t 0003 e"'
slope = ( I 2
10
L
-
5
0.006
0
0.005
r
I IO
0
20
30
250
300 350 Frequency Scan
400
450
500
Flgure 8. TDLAS signals for several moisture levels in HCI. The trace labeled 63 ppb corresponds to the background moisture level in the cell. The other two traces correspond to addltions of 35 and 119 ppb.
lo4 cm-l compared to a value of 1.09 X lo4 cm-* for the air-broadening coefficient in the HITRAN database.30 The HC1-broadening coefficient is 3.9 f 0.3 X lo4 cm-l, which is intermediate between typical values for the self-broadening coefficient of water31 (-5 X lo4 cm-l) and the nitrogenbroadening coefficient, which seems reasonable in view of the possibility of strong hydrogen-bonding interactions between HC1 and HzO. The achievable sensitivity in HCl relative to nitrogen will be reduced by a factor close to the ratio of the linewidths. For HCl, as for nitrogen, we used a modulation amplitude less than the optimum value in order to avoid losing the ability to distinguish the signal due to moisture in the cell from the broader absorption due to moisture in the light path outside the cell. The modulation amplitude is about half the linewidth, leading to a factor of about 3 reduction in signal relative to optimum conditions. * Figure 8 shows spectra for moisture in HCl for the zero gas, 35 ppb added and 119 ppb added. Figure 9 is the corresponding calibration curve. In order to measure the absorption signal following the procedure discussed in con(30) Rothman, L. s.;et al. J . Quanf.Specfrosc.Radiaf. Transfer 1992,48 ( 5 / 6 ) , 469. (31) Mucha, J. A. Appl. Specfrosc. 1982, 36 (2). 141.
2478
-
-lW 0 Background * 63 +I- 4ppD
~
~ 1W
2w
3w
400
Cone. .ddrd (ppb)
Flgure 9, Calibration curve for moisture in HCi by TDLAS.
Flgure 7. Width (fwhm) of H20absorption signal as a function of added gas pressure.
200
_
40
Pressure (torr)
150
Standard Oeviation d Background = 2 2 ppb
1
Analytical Chemistry, Vol. 66,No. 15, August 1, 1994
nection with Figure 4, we needed to obtain a spectrum with a well-defined second derivative shape. This was collected at a concentration of 1 ppm moisture in HC1. The distances from line center at which to measure the values corresponding to the maxima in the second derivative curve were defined using this trace. There is again a deviation from linearity at about 0.4 ppm, but the data are linear in the concentration range of interest. The standard deviations of the various data series are about equal in absolute terms and correspond to approximately 2 ppb. This is larger than would be expected from the nitrogen data, even allowing for the reduction in sensitivity. It may be attributable to changes in the adsorptiondesorption behavior of moisture in the presence of HCl. The moisture level in the dried HCl for this series of experiments was 63 f 4 ppb (from Figure 9). This may represent the performance limit of the HC1 purifier used, but more likely still includes some considerable contribution from cell outgassing, as in the case of nitrogen. The background moisture level remains essentially constant over a typical data collecting period of several hours, but decays slowly from day to day if the cell is not exposed to moisture.
CONCLUSIONS A system for analysis of moisture in gases using tunable diode laser absorption spectroscopy has been designed and built. The reproducibility (2 standard deviations) of the system, when 10 scans each collected over 5 s are averaged, is 1% of signal or 0.4 ppb at the lowest concentration sample studied (35 ppb). For a single scan, the background is due mainly to interference fringes and corresponds to 2-5 ppb. The system has also been shown to be suitable for use in hydrogen chloride. The reproducibility in HC1 (twice the standard deviation of a series of 10 5-s measurements) is 4 ppb and appears independent of concentration up to about 400 ppb. A continuous monitor based on this system should, therefore, be able to detect a change of 0.4 ppb in nitrogen or 4 ppb in HCl occurring for a period longer than 2 min. If the system is exposed to air or dry nitrogen between samples, the reproducibility of analyses will not be as good, as it will be influenced by the response of the gas introduction system. As with most analytical systems, the accuracy of the concentration measurement is not the same as the sensitivity of the system to a change in moisture level. The accuracy is very sensitive to the time elapsed since the last calibration and
the number of calibration points used. As an example, the accuracy of the residual moisture determination was 2.6 ppb in nitrogen (at a confidence level corresponding to approximately 2 a) and 8 ppb in HCl. The error estimate in the determination of the output concentration of purifiers (for example) is domimated at present by background moisture levels, which are the main limitationof the present system. For analyses in nitrogen, the lowest moisture background achieved at a cell flow of 600 standard cm3/min is approximately 10 ppb. In HC1, the moisture background is about 63 ppb.
ACKNOWLEDGMENT We would like to acknowledge Alan Stanton and others at Southwest Sciences, Inc., and Clint Carlisle of SRI International for helpful conversations regarding modulation techniques and other aspects of diode laser spectroscopy. Received for review February 22, 1994. 1994."
Accepted May 13,
Abstract published in Advance ACS Abstracts, June 15, 1994.
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