Anal. Chem. 1088, 60, 394-403
304
(15) Haw, J. F. Ph.D.Thesis, Virginia Polytechnic Instltute and State Unlverslly, 1982. (16) Lyerla, J. R., Jr.; Levy, 0. C. TOPICS In Carbon-I3 Nuckar Magnetic Resonance; Levy, G. C.. ed.. Wiiey-Interscience: New York, 1974; Voi. 1, Chapter 3. (17) Boere, R. T.; Kdd, R. G. Annu. Rep. NMR Spectrosc. 1982, 73, 320-382. (18) Tiffon, B.; Ancian. B. J . Chem. Phys. 1982, 76(3), 1212-1216. (19) Stephen, K.; Lucas, K. Viscosity of Dense Flu&; Plenum: New York, 1979; p 117. (20) Haw, J. F.; Glass, T. E.; Dorn, H. C. Anal. Chem. 1983, 55, 22-29.
(21) Peter, S.;Brunner, G. Angew. Chem., Int. Ed. Engl. 1978, 17, 746.
RFCEIVJD for review June 29, 1987. Resubmitted September 25, 1987. Accepted October 26, 1987. The authors acknowledge financial support for portions of this work provided by Dow Chemical Co., Mobil Research Corporation, and the U S . Army (USA Belvoir Research, Development, and Engineering Center, Ft. Belvoir, VA).
Determination of Nitrogen Dioxide in Air Compressed Gas Mixtures by Quantitative Tunable Diode Laser Absorption Spectrometry and Chemiluminescence Detection Alan Fried,*’ Robert Sams, William Dorko, James W. Elkins? a n d Zhong-tao Cai3
National Bureau of Standards, Center for Analytical Chemistry, Gaithersburg, Maryland 20899
Tunable dlode laser absorption spectrometry (TDLAS) and chemllwnherrcence detection (CD) were both employed for the detennhatkn of seven NOJak compcessed gas mbdwes in the 2.3-2500 ppm (by mole) concentration range. I n the dlode laser determkratbns,we employed a vatiety d dyferent measurement and calbratlon approaches based upon direct ab9orp#onandseoondharmonicdetectkn. Suchredundancy enabled us to carry out a careful analysis thus mlnbnlzlng systematic BCTQI’E. Afler HNO, was accountad for, measured In all but one cylinder examined, the TDLAS and CD NO, determlnatlonswere generally found to agree to better than 3%. FoukrtransfonnWraredabmrptkn~rometrywas also empkyed h the detemrinatkn d one pMucular cylinder, the nomlnal2500 ppm cylhrder. The resuiting NO, determlnatbn was wllhln 0.2% of that obtained by TDLAS. However, a small remsbrlng discrepancy on the order d 5 4 % wasstHl observed between these two results and those detennkred by CD even after HNO, was accounted for. Further studies pointed to the presence of an additional nitrogencontalning compound In this cyllnder.
The reactive gas nitrogen dioxide (NO,)is one of many atmospheric molecules that significantly impact out environment. It is well-known (I, 2) that this gas plays an important role in the oocurrence of “acid rain” and the generation of photochemical smog. Concentrations of several hundred park per million (ppm) by mole of nitrogen oxide compounds, primarily nitric oxide (NO),NO,, and nitrous oxide (N,O), are produced by the combustJon of fossil fuels. Following the emission of these and other pallutants to the atmosphere, from motor vehicles and various stationary sources, a large fraction of the generated NO is photochemically oxidized to NOz. Commercial chemiluminescence detectors (CDs) have traditionally been employed in certifying NO and NO, compressed gas standards for field measurements of such staPresent affiliation, National Center for Atmospheric Research, Boulder, CO 80307,sponsored by the National Science Foundation. *Present affiliation, NOAA GMCC, Boulder, CO 80303. SGuest worker from the S anghai Institute of Ceramics, the People’s Republic of China.
I
This article not subjact to
tionary emission sources. Detection is accomplished via the chemiluminescent reaction between NO, contained in the sample stream, and a large excess of O3 added to the sample flow. The excited NO, thus generated is measured with a photomultiplier tube, the response of which is proportional to the NO concentration in the sample stream. For measuring NO2,the sample gas must first pass through a converter to effect the conversion of NO, to NO. With the possible exception of photolytic conversion, as described in detail by Bollinger et al. (3), CD converters will most certainly also convert other nitrogen-containing compounds to NO. Determinations of NO, employing commercial CDs, which typically use hot stainless steel or molybdenum to effect conversion, would be accurate as long as other nitrogen-containing compounds are not simultaneously present or, if possible, are first removed from the sample stream prior to conversion. However, in compressed gas cylinder mixtures other nitrogen oxide compounds may in fact be present causing systematic errors in NOz determinations. Because a growing number of laboratory and atmospheric studies require accurate compressed gas NO, standards, alternative measurement methodologies are essential in the certification process. Of particular importance in this regard, are measurement techniques that are very selective. In the present study, the quantitative technique of tunable diode laser absorption spectrometry (TDLAS) was employed in conjunction with CD, and in one case with Fourier transform infrared W-IR)absorption spectrometry,to achieve this end. The study described herein presents the first detailed NO2 intercomparison between very selective spectroscopic techniques and the more conventional technique of CD for compressed gas standards determinations. At the time this study was being conducted, however, a similar intercomparison between TDLAS and CD was reported by Walega et al. ( 4 ) for various reactive nitrogen compounds in ambient air. An intercomparison was also carried out on a 9 ppm NOz compressed gas mixture. A 15%higher CD response was observed which the authors suggested may have come from a cylinder impurity detected by CD and not by TDLAS. However, since this was not the main focus of the study, additional measurements to unequivocally identify the source of this discrepancy were not carried out. Similar results were observed through the present study, and as will be discussed, the cause
U.S.Copyrbht. Publlshed 1988 by the Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988 Cold Head Diode Laser
8 JL
Lock-In Amp
OAP
Dual Channel Signal Averager Input
i
c Stopcock
0paque.Limit Absorplion Cell
hpYt
l
Vertically Displaced / Beams
OAP: Ofl Axis Parabola BS: Beam Splitter VDFMP: Vertically Displaced Flat Mirror Pairs P: Baratron Pressure Sensor
Flgure 1. Schematic of experimental apparatus. In each vertlcally displaced flat-mirror pair (one at the entrance and one at the exit of the monochromator)one mlrror Is above the plane of the figure.
of this discrepancy was subsequently determined. Tunable diode laser absorption spectrometry, if employed properly, can be an extremely powerful technique since it can be applied to a large number of important atmospheric gases with very high selectivity and sensitivity. The selectivity is a direct result of the high resolution inherent to the technique (5,6). A great deal of care, however, must be exercised when carrying out quantitative measurements in order to avoid various sources of systematic errors. We present in this study, comprehensive details that should be considered when carrying out accurate quantitative analysis. Although specific details are given for the determination of a selected group of NOz/ air compressed gas mixtures a t nominal concentrations of 250,500,1000 (two different cylinders), and 2500 ppm, the same procedures have general applicability to other gas standards. Additional determinations were carried out by both TDLAS and CD on two cylinders in the 1-10 ppm range, a particularly important range for trace atmospheric studies of NOz. Such standards can be conveniently employed in dynamic dilution systems to generate sub-parts-per-billion NO2 concentrations. In the following sections, we describe ow tunable diode laser absorption system, the various calibration and calculational approaches employed in the NO2 determinations, and assessments of the total uncertainty including systematic errors for each technique.
EXPERIMENTAL SECTION Tunable Diode Laser Absorption System. The tunable diode laser absorption spectrometerand electronicsare illustrated schematically in Figure 1. The output of a tunable diode laser (Spectra Physics, Laser Analytics Division) was collimated into a 6 mm diameter beam employingan all-reflectiveoptical system (7-9) and directed through a 70 cm long cell. This cell was filled with a few hundred millitorr of pure NOz to establish the 100% absorption level (opaque limit) for the particular diode lasing output mode employed. Once established,the cell was evacuated to less than 7 X lo-' kPa (5 X lo4 Torr). Following the opaque limit cell, the diode laser beam was directed through two different arms of a double beam setup. The main beam passed through a pellicle beam splitter and through a 50-cm Pyrex sample absorption cell containing BaF2windows. Approximately 10% of the beam (the reference beam) was reflected off the beam splitter and through an interferometer for relative wavenumber calibration. The sample and reference beams were then both imaged on the entrance slit of a monochromator employing a pair of vertically displaced flat mirrors, as shown in Figure 1. After passage through the monochromator, which was employed for gross mode selection, each beam was focused onto separate mercury-cadmium-telluride photoconductive detectors.
395
As will be discussed,the reference arm interferometer was very important for accurately determiningabsorptionline widths, which in turn were essential for accurate quantitative analyses using the direct absorption approach. Initially, a 3-in. germanium (Ge) etalon was employed for this purpose. However, most of our measurementswere carried out employing a high-resolutionWcm air-spaced confocal interferometer from Burleigh Instruments, Inc. As discussed by Jennings (IO),this interferometer presents many advantages over Ge etalons for quantitative diode laser measurements. The free spectral range of the confocal interferometer was determined by using accurate OCS line positions in the 1700-cm-' range calculated by Maki from constants given in ref 11 as well as determined from accurate etalon length measurements. Both determinations, which agreed to within 0.1% , resulted in a free spectral range of 0.005 01 cm-l at 296 K. All TDL measurements were carried out at 296 h 2 K. Because of ita high resolution and narrow fringe widths, the transmission through the confocal interferometer was typically less than 1%. Since the reference arm only contained 10% of the original laser intensity, the resulting intensity at the output of the interferometer was often too low to be of use. During interferometer measurements,the beam splitter shown in Figure 1was replaced with a mirror mounted on a kinematic mount for precise repositioning. Interferometer scans were therefore carried out utilizing the full laser intensity. Scans were performed both before and after absorption scans. The remarkable stability of the confocal interferometer eliminated the need for simultaneous scans. Sample N02/air mixtures were continuously drawn through the sample cell at reduced pressures ranging between 0.6 and 11.8 Torr. A pair of stainless steel needle valves at both the cell inlet and outlet were used to control the cell flow rate and pressure. Calibrated 10 and lo00 Torr MKS Baratron pressure transducers were connected to the center of the cell for sample pressure measurements. Spectral Features of NOz. We employed in this study the strongest band of NOz, the v3 band, which absorbs in the infrared (IR) at 6.2 km. Each vibration-rotation line in this band, as well as in other bands of NOz in the IR, is characterized by a pair of spin-split lines designated by a plus or minus symbol following the line assignment. In some cases the individual components are well resolved, while in other cases the individual pain are only partidy resolved or totally unresolved. The NO2lines employed in this study comprise all three cases. Although the sensitivity can be considerablyhigher employing the latter, unresolved pain such as the 151,15Land the 142,1zalines in the 1606.6-cm-' region, for example, require a precise knowledge of the spin splittings for accurate quantitative analysis based upon the peak absorbance at line center. A typical sampling pressures around 11Torr, for example, a change of only 3.6 X lo-* cm-I in the spin splitting of the unresolved 142,lzcomponents would effect a 5.9% change in the combined l i e center absorption coefficient that one would calculate. Since our present knowledge of such splittings contains uncertainties of this order and larger, unresolved spin-split pairs were only employed in the present study in relative direct absorption and second harmonic measurements where standards were simultaneously employed. The absolute direct absorption results reported here were based upon measurements performed on a single isolated absorption line: the 94,6+line shown in Figure 2. This line, which absorbs at 1605.500cm-I, is well resolved from ita spin-split partner as well as from other lines of NO2. Lasing in this particular spectral region, moreover, was achieved with nearly 100% of the intensity in a single mode. Tunable Diode Laser Data Acquisition Modes. Both direct absorption and second harmonic (second derivative) detection were used to acquire absorption data. In both modes, the diode laser was scanned by utilizing two different methods: conventional slow scan integration and sweep integration, first described by Jennings (12). With the former method of scanning, the diode laser radiation was chopped, and we recorded the transmission spectrum as the laser wavenumber was slowly stepped by a computer-generated ramp voltage. Each detector output was directed into a lock-in ampl5er and ultimately into the computer. The computer averaged the signal from each lock-in for a specified period of time (typically0.4-1.2 s) before stepping the diode laser wavenumber. A complete scan of a spectral region (up to ap-
396 r
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988 i
!
I
I
I
I
!
I
I
1
/
Q4,6-
f 10
I
Slow Scan Integration
= 0.012
94,6+
I
!
I
1605.4542
1805.4766
1
1605.5000
Wavenumbers cm-’
Wavenumber (cm-’)
Figure 2. NOz direct absorption and confocal interferometer scans, employing sweep integration data acquisition, as a function of diode tuning current (xaxis). The 94#+ absorption line, 1605.500 cm-‘ (ref 20) was employed in all direct absorption determinations in this study. The peak absorbance of this line In this scan was 0.011.
proximately 0.1 cm-’), containing four to six individual vibration-rotation lines, took up to 8 min, depending upon the total number of frequency points. By use of sweep integration, the diode laser tuning current was sawtooth modulated at approximately100 Hz. An entire spectral region, including both spectra and Ge interferometerfringes, were thus recorded in approximately 10 ms. A Princeton Applied Research (PAR) Model 4203 dual-channelsignal averager digitized and stored the data in 1024 channels. Data in each channel were then coadded with subsequent data taken by repetitively scanning over the same spectral region. Typically, loo00 sweeps were coadded in 1.7 min, and the resulting averaged spectrum was then directed into a computer and stored on disk for subsequent analysis. When spectra were recorded by use of the high-resolution confocal interferometer, only one channel of the signal averager at any given time was employed. Under these conditions we were able to increase our scan rate to 200 Hz. Data acquisition employing sweep integration presents major advantagesover the more traditional slow scan integration method (12). The resulting spectra are more reproduciblefrom scan to scan and display higher signal-bnoise ratios (SNRs),as shown in Figure 3. The frequency instabilities evident in the slow scan trace make it extremely difficult to carry out meaningful line fitting analysis. Consequently, with one exception, all final direct absorption determinations were acquired utilizing sweep integration. Second harmonic detection presents several major advantages over direct absorption in terms of sensitivity and ease of implementation, as discussed in ref 5, 13, and 14. Second harmonic measurements were carried out by simultaneously superimposing a 5-8 kHz sine wave, from an external waveform generator, on the diode laser tuning current. A lock-in amplifier referenced to this frequency was used to synchronously detect the second harmonic signal (10-16 kHz) of the frequency-modulated absoqtion. In these measurementsthe diode beam was not chopped, and initially, as with preliminary direct absorption measurements, we employed the technique of slow scan integration. The output of each lock-in amplifier was directed into the computer for data averaging and storage. In later measurements, however, we coupled sweep integration with second harmonic detection to achieve advantages inherent to both techniques. Final second harmonic determinations presented in this study include both methods of diode laser scanning. The increased sensitivity of second harmonic detection, however, is gained at the expense of a relative response. As a result, accurate quantitative analysis requires, for all practical purposes, calibration standards. This was particularly true here where we coupled sweep integration with second harmonic detection. In this configuration, the diode output wavenumber was simulta-
Flgure 3. Comparison of slow scan and sweep Integration data acquisltbn modes for the same NOplines of Figure 2 employing identical time constants of 1 s.
neously swept, approximately through one full line width, and modulated at 10 kHz. The resultant signal was processed with a lock-in amplifier (PAR Model 126) employing a minimum time constant of approximately 100 ps and then directed into the PAR signal averager, as previously described. In this case, however, the time constant for the second harmonic lock-in amplifier could not be made less than the time to acquire data in each signal averager channel (5 ps). The recorded second harmonic spectrum as a result was distorted. Thus, calibration standards were a necessity. For this purpose, we employed a high emission rate NOz permeation tube calibration system, as will be discussed. Despite the many advantages inherent in second harmonic detection, both direct absorption and second harmonic detection were employed throughout this study. In some cases, both methods were used for a given determination,thus employing two different calibration approaches and ensuring against systematic errors that might unknowingly arise in any one determination. Finally, the high emission rate permeation tubes used in second harmonic detection have relatively short lifetimes of only a few months. Routine cylinder analysis thus requires frequent tube preparation, an inconvenience avoided with direct absorption. However, as will soon become apparent, direct absorption is susceptible to more systematic errors than second harmonic detection. Extensive intercomparisons are thus essential in identifying experimental conditions where such errors can be minimized or totally avoided. NOz Permeation Calibration System. A series of all-Teflon permeation tubes, approximately 10 cm long, were prepared in accordancewith recommended procedures (15) at various times throughout this study and maintained at constant temperature to within 10.01 *Cin a thermostated condenser. At temperatures between 20 and 30 OC, various combinations of tubes and flow rates produced NOz concentrations ranging from 10 ppm up to 832 ppm. The NOz concentration generated by this calibration system was determined in the usual manner by gravimetry in conjunction with accurate flow rate measurements. Each permeation tube was weighed periodically to obtain the NOz emission rate, and the air flow over the permeation tubes was controlled by a Tylan mass flow controller. Absolute flow calibrations, performed with every measurement, were determined with an estimated (la) uncertainty of *0.4% by using accurately calibrated wet test meters (WTMs) and bubble meters (16). The combined (2a) uncertainty in the NOz permeation concentration, estimated by adding in quadrature the various sources of uncertainty, ranged between 0.8% and 1.2% over the course of this study. The purity of the gas emanating from the permeation tubes was checked by TDLAS, CD, and FT-IRahrption spectrometry. The former was accomplished by scanning the output of one of our tunable diode lasers through the 1722-cm-’ spectral region which contains strong absorption features for both HNO, and HNOP Neither of these gases was found. Likewise, other than N204which is in equilibrium with NOpat atmospheric pressure, no obvious impurities can be detected by FT-IRspectrometry
. . 3000 2wo
. . 28w
I . L 28W 18W
woo
2oW
, 17w
.
, 1Km
.
. 15w
,
. 1400
.
.
.
1m
. 1200
Wavenumber
FbUm 4. (a) FT-IR scan of 2300 ppm NO, permeation standard flowing through a 20-m sample cell at atmospheric pressure In tlw search fw permeation tube impurities. The unapodlzed resolution was 4 cm-' and the temperature was 293 K. The negative CO, absorption is due to m r e C 0 2 in the background spectrum than In the sample spectrum. (b) FT-IR scan of nmlnal2500 ppm cylinder number 7 flowing through a 20-cm sample cell at atmospheric pressure and a temperature of 293 K. Bands in the 1722- and t300-cm-' spectral regions due to HNO, are clearly present in this cylinder spectrum but are absent in the permeation tube spectrum in (a). I n both cases. N,0, bands can be seen,
as discussed in the text.
in the high concentration permeation output of Figure 4a Characteristicabsorption features from HNO,, if present, would occur in both the 1722- and 1300-em-' spectral regions, as shown in Figure 4b (to be discussed shortly). Experimental details of this system as well as the analytical procedures employed are described by Elkins et al. (17). Additional supporting evidence verifying the accuracy of our permeation calibration procedures was obtained by FT-IR. The NOz concentration emanating from the permeation calibration system was determined by measuring the integrated hand absorbance at 2900 em-' (the v1 + v3 band) using an integrated hand strength from ref 18. The concentration thus determined was within 1%of the gravimetric determination. Similar results regarding the purity of these tubes were obtained by CD employing a nylon scrubber. Nylon quantitativelyremoves all acidic gases, and the resulting decrease in the CD response is thus a measure of the total acidic gas concentration from all nitrogen oxide speeies such as "0, and "0%. An invariant CD response, measured both with and without the nylon scrubber, further substantiated our findings that such species were not present. NO, Concentration Determinations by TDLAS. Determinations were performed hy direct absorption using one of three different approaches in the data reduction. In some cases, two, and sometimes all three, approaches were used in the final determination of a cylinder concentration. In each of the direct absorption approaches, the transmission spectrum for the sample gas was recorded as the diode laser output wavenumber was swept through absorption features of interest. A 100% transmission base-line scan was also recorded for each determinationhy scanning through the same spectral region with the absorptioncell evacuated or by replacing the sample gas with "zero air". As described previously, 100% absorption (opaque)
base-line scans were also recorded with each measurement set. The incident intensity (I,) at each point over an entire scan was calculated hy digitally subtracting correspondingpoints of both base-line scans. Various saturated features were linearly fit to obtain the 100%ahsorption base line over the entire scan. The n next digitally intensity (0for each sample gas ahsorption s ~ a was ratioed point hy point to the incident intensity (I,) thus calculated to give the sample transmission (I/&) spectrum. Absorbances ( A ) at each point were then calculated hy using the following: A = -In ( I / I o ) (1) In the frat approach, NO, cylinder concentrations ([Nod)were determined from peak absorbances (A(",))at line center (v,). In this procedure, the line center absorbance calculated from expression 1 was used together with the line center absorption coefficient (oI(Y,), cm-' atm-') and the total sample pressure (P, atm) and path length (L, cm), in the well-established B e e r Lambert ahsorption expression [Nod = A(~o)/&o)pL (2) The value for the ahsorption coefficient at line center was calculated in each and every case by using the integrated absorption coefficient with a Voigt line-shape function employing the Humlicek algorithm (19). The following molecular parameters were used in this calculation at 296 K for the 9,,.M line: (1)an integrated absorption coefficient of 1.138 cmP atm-', recently tabulated by Toth (20);(2) a calculated Doppler-broadenedhalf width (HWHM) of 0.00146 cm-'; and (3) an averaged airbroadening coefficient of 0.073 em-' atm-', as tabulated by Toth and by the 1980 AFGL Trace Gas Compilation (21). The ahsorption coefficient in the latter compilation is within 0.4% of that determined by Toth. On the basis of these measurements
308
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
The NO2concentration of the seven compressed gas mixtures examined in this study was determined in a straightforward manner by CD, [N021cD,employing NO/N2 standards (Standard Reference Material gas mixtures) of concentration [NOISMusing the following relation: (5)
NO1 Permeation Calibration System
Figure 5. Schematic of experimental setup showing the 50-cm TDL absorption cell incorporated between the CD converter and reaction chamber.
and subsequent measurements carried out in our laboratory, we placed a reasonable estimate for the 2a uncertainty in the 94$+ absorption coefficient at around f2.4%. The corresponding 2a uncertainty in the air-broadening coefficient for this line was conservatively estimated at around f20%. At the three distinct sampling pressures of 0.6, 1.0, and 11.8 Torr in this study, the Voigt line profile is predominately Gaussian in character. Thus the resulting contribution to the uncertainty in the calculated line center absorption coefficient from the air-broadening uncertainty amounts to 0.6%, 1.2%,and 8.7%,respectively. Most of the direct absorption studies were performed at the two lowest pressures where the &-broadening uncertainty has minimal effect on the final uncertainty of the results. The second calculation approach employed in a few NO2 cylinder determinations was very similar to the first. However, instead of measurements only at line center, the entire integrated line profile was used in the calculation in accordance with f A b ) dv (3) In this expression, a and J A ( v ) are the integrated absorption coefficient and integrated measured absorbance for the 94,6+line, respectively. The latter was determined by fitting the entire line profile to a Voigt function using a nonlinear least-squares fitting routine. The confocal interferometer fringes were first fit in this procedure to linearize the wavenumber axis. In addition to concentration,this line-fittinganalysis also calculatealine widths. Unlike the first two approaches, the final method (approach 3) employed was a relative determination. As in our second harmonic measurements, NO2 permeation tube standards were used in the calibration instead of absorption coefficients. In this case, peak absorbancesat line center for both the cylinder (A(v&.l) and the permeation (A(vo),,,) gases were employed in the calculation [NO21 =
A(V0)CYI
[N021,, A(v0),,
(4)
In second harmonic determinations, the corresponding ratios of second harmonic signals for both the cylinder and permeation gases were used in an expression similar to eq 4. Chemiluminescence Detection System. A commercial chemiluminescencedetector (Thermoelectron Corp., Model 10) using a hot stainless steel converter was employed throughout this study. Some measurements of NO2were conducted with this detector unmodified. However, we also took advantage of the nondestructive nature of TDLAS and incorporated the TDL absorption cell between the chemiluminescence detector (CD) converter and reaction chamber, as shown in Figure 5. The TDL absorption cell was placed far enough downstream of the hot stainless steel converter so that the gas equilibrated to room temperature by the time it reached the absorption cell, as verified by a thermistor placed in the gas stream. In this combined detector configuration, both the TDL and the CD thus sample the same flow stream under identical conditions of flow, pressure, and temperature. This configuration also allowed us to measure the residual unconverted NO2passing through the converter. The CD converter efficiency was directly obtained at all concentrations of interest by comparingthese measurementswith those obtained upstream of the converter. By use of an NO diode laser, this combined detector configuration,furthermore,enabled u8 to check whether or not the CD response solely originated from nitrogen oxide species or was caused by hydrocarbons and/or organosulfur compounds in the sample stream (22).
In this expression, S, and Ssarepresent, respectively, the CD signals for the NO2/= cylinder and NO/N2 SRM mixtures. The flow ratio f N i t / f & accounts for capillary flow differences between the sample gas (air matrix) and the NO reference gas (nitrogen matrix). This flow ratio was empirically determined (1.036) by measuring the CD NO2response upon alternately flowing N2 and air through our NO2 permeation tube calibration system. The CD converter efficiency, CE, in eq 5 was determined by TDLAS, as previously described, and by CD employing the above NO2permeation standards. Averaging the results of both techniques, we determined a converter efficiency of 99.3% f 1.4% (2a) at a converter temperature around 660 "C. The converter efficiency was studied as a function of concentration and temperature using our calibrated permeation source. At converter temperatures around 660 "C, no change in this value was observed over the entire concentration range of interest.
RESULTS AND DISCUSSION b this section, we discuss the various TDLAS measurement techniques and calculational approaches employed for each cylinder determination. Determinations by CD, and in one caee by FT-IR, are subsequently presented and intercompared with the TDLAS results. Various sources of systematic error can readily occur in TDLAS measurements, particularly in the direct absorption mode. Fortunately, the large number of intercomparisons performed in this study utilizing different measurement and calculational approaches made the presence of such occurrences obvious. Using this redundancy, we were able to cull out from our final analysis inaccurate data that on first appearances looked quite correct. In the direct absorption mode, comparisons between calculated and measured line widths were particularly important in this regard. Systematic errors in the direct absorption mode will, in most every case, be manifested by a distorted line shape with an increased line width relative to that calculated. Increased line width can result from a variety of different causes. The diode laser may scan erratically across absorption features of interest, affecting integrated and, to some extent, peak absorbance measurements. However, this is minimized by using sweep integration. A second cause of increased line width occurs whenever the laser line width is no longer infinitely narrow with respect to the absorption features under study. The increased width resulting from this "excess laser line width" causes a comparable decrease in the peak absorbance a t line center (12,23,24). The integrated absorbance, however, is unaffected. Increased line width can also occur when, as is frequently the case, closed-cycle refrigerator system are employed. The cycling compressor piston in such systems is typically mounted directly on the diode laser cold head, and the resulting mechanical shocks induce frequency-jitter on the recorded high-resolution spectrum. As shown by Sams and Fried (25),this perturbation can significantly affect the integrated absorbance while the peak absorbance at line center is minimally changed, if at all. In this situation, the increased line width is correlated with an increased integrated absorbance, directly opposite to that caused by excess laser line width. Concentration determinations based upon the integrated line profile, as a result, are systematically too high, sometimes by as much as 21% (25). As discussed by Sams and Fried (25),repetitive determinations using both approaches can be very helpful in ascertaining the cause of the distortion. Instrumental broadening
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
399
Table I. Summary of All NOz Cylinder Intercomparisone" ([N021CD
cylinder
nominal concn,
no.
PPm
1 2 3 4 5
2.3 8.9 250 500 lo00
time periodb
lo00 2500
mixing chamber Yes YES
mixing effect
[NOZIFT.~, PPm
none none
I
I I1 & 111 I1 & I11 IV
Yes
none
yes Yes
11.8 f 7.1% 8.2 f 1.0%
Y 88
none
yes
6.7 f 1.6%
no
-
100 1.48 f 0.04 (n = 13) 2.7 f 2.7% 5.58 f 0.14 (n = 11) 3.6 f 4.5% 2.1 f 14.3% 233 f 4 (n = 16) 483 f 10 (n = 11) 1.2 f 3.7% 880 f 16 ( n = 7) 13.4 f 5.5% 873 f 16 ( n = 5) 2.4 f 6.5% 852 f 17 (n = 6) 2.5 f 6.6%
none
no no
no
[N021CD [NOZITDL, PPm
1.44 f 0.02 (n = 2) 5.38 f 0.21 (n = 12) 228 f 33 (n = 15) 477 f 15 (n = 6) 762 f 46 (n = 6) 852 f 55 (n = 3) 831 f 54 (n = 13)
nQ
I1 & I11 IV 6 7
-
[NOZITDL)/
773 f 24 (n = 6) 2118 f 188 2123 f 97 (n = 6) 1968 f 84 (n = 10) 2100 f 90 (n = 11) 2122 f 87 (n = 4)
790 f 15 (n = 6) 2232 f 42 (n = 8)
2.2 f 3.6% 4.9 f 4.7%
2255 f 42 (n = 1) 2298 f 48 (n = 3)
6.9 f 4.4% 7.7 f 4.2%
The NOz values given are unweighed averages of many different measurement sets within a given time period. The uncertainties given were calculated from the root-mean-squareof all the 2a (within) uncertainties of the entire data set within this time period. Each (within) uncertainty was calculated by error propagation analysis. bThetime periods for cylinders 5 and 7 are different. CAll[NOZlcDvalues were obtained employing nylon scrubbers to first remove "0,. due to excess laser line width and mechanically induced frequency-jitter are both highly variable with changing tunable diode laser conditions of temperature and current. For a fured set of conditions, however, the latter distortion often results in greater run-to-run variability (25). Consequently, peak absorbance as well as integrated-fit determinations were intercompared throughout this study. When both determinations, or alternatively when the retrived and calculated linewidths, were in agreement, we obtained definitive proof that neither error source discussed above was operative. This situation, which is clearly the most desirable, was frequently but not exclusively encountered here. Other systematic error sources that may be encountered in the direct absorption mode occur when secondary lasing modes are present but are not rejected by a monochromator (26). Secondary modes can also be simultaneously absorbed by other lines of the sample gas not intentionally being studied creating difficulty in properly determining the 100% absorption level (9). The secondary mode absorption causes structure on the opaque limit scans in this case. Such structure, which can cause systematic errors as large as 10% (9),also makes this type of error particularly obvious. By careful examination of the opaque absorption features in each spectral region of interest, the presence of this error can be detected and easily avoided. In addition to the direct absorption approach, second harmonic determinations can also contain systematic errors which are caused by a nonlinear response between the sample and calibration gases (27). With the above systematic error sources in mind, we proceeded to carry out concentration determinations. Tunable diode laser as well as CD determinations, [NOPITDLand [NO&D, for a given cylinder mixture were usually carried out over several days. Results from each instrument carried out over a single day, which constituted a measurement set, were averaged. Final TDL and CD determinations were obtained in all cases except two by taking the unweighted average of all the individual measurement set results. These determinations are presented in the summary Table I along with the number of independent measurement seta in parentheses. For the two remaining cylinders, cylinder numbers 5 and 7 shown in Table I, extensive measurements were carried out at variow discrete times over a 1-year period. In addition to a slight apparent time dependence in some of the determinations,two different sampling procedures were also employed. Results from each time period were therefore tabulated and compared separately for these two cylinders.
The resulting 2a uncertainty presented with each final concentration was determined by first calculating the 2a (within) uncertainty for each measurement set. This was accomplished by adding in quadrature estimates for the various uncertainties from each term in the appropriate concentration expression. This procedure was repeated for each measurement set, and a final value for each determination was calculated from the root-mean-square average of all the within uncertainties. This procedure yields a more representative final uncertainty than that obtained by merely taking twice the standard deviation of the final average (the between component). The between component does not properly include the individual uncertainties in terms such as the absorption and pressure-broadening coefficients as well as in terms for the various calibrant gas concentrations. For the absolute direct absorption TDL measurements, the total 2a uncertainty was comprised of the following: (1)the precision in measuring the line center absorbance or fitting the integrated absorbance, which ranged between 0.5% and 5% for the former and approximately 8% for the latter; (2) the absolute accuracy of the pressure calibrations, which ranged between 0.9% and 1.7% for the 10-Torr head at pressures around 1.0 and 0.6 Torr, respectively, and 3.3% for the 1000-Torr head at pressures around 11.8Torr; (3) a 0.1% uncertainty in the absorption path length; (4) the 2.4% uncertainty in the absorption coefficient for the 94,6+line; and (5) the 0.6%-8.7% upper limit for the uncertainty contribution due to the pressure-broadening coefficient in peak absorbance determinations. Using these values, we calculate a combined 2a uncertainty for direct absorptiondeterminations of NOz in the 3-11 % range, with most values averaging around 5%. For the second harmonic measurementa, a corresponding analysis yields a total 2a uncertainty predominately in the 1-5% range. The 2a uncertainty in each CD measurement set ranged between 1.7% and 2.6%,and was comprised of the following: (1) the measurement precision for both the NO, cylinder and the NO standard, typically 0 . 2 4 6 % ;(2) a 1.4%uncertainty in the converter efficiency; (3) a 0.4% capillary flow correction uncertainty; and (4) the uncertainty in the NO SRM concentration, which ranged between 1% and 2%. For the nominal 1000 ppm cylinder number 5 shown in Table I, TDLAS determinations were carried out over four different time periods spanning one year. In time period I, final determinations were based upon second harmonic detection employing permeation standards. In subsequent time
400
ANALYTICAL CHEMISTRY, VOL. 80,
NO.5,
MARCH 1, 1988
Table 11. Intercomparison of the Various Tunable Diode Laser NO2 Determinations for Cylinder Number 5 O Time Periods I1 and I11 second
av
harmonicb
peak direct
detection
absorption
857 f 16 842 f 13
858 f 93
850 h 15
(0.9%)
858 f 93
Grand Average = 852 f 55 Time Period IV relative peak peak direct
absorption 819 i 40 842 53 832 f 28 825 k 58 842 h 25 831 f 35 825 f 25
*
av
831 f 40
normalized integrated fit
direct absorption*
805 f 65 828 f 135
829 f 36 819 17 852 51 859 i17
*
approachesin both time periods validates both the procedures and standards employed. The TDLAS determinations for the nominal 2.3,8.9, and 250 ppm NOz cylinders shown in Table I were all carried out using second harmonic detection calibrated by NOz permeation standards. For the 500 ppm and the second 1000 ppm cylinder (cylinder number 6), on the other hand, TDLAS determinations were exclusively based upon peak direct absorption measurements. The measured line half widths in both cases were within 1.9% of that calculated. This agreement, which was within our measurement precision, indicates negligible laser distortion in our peak determinations from any of the causes previously discussed. The nominal 2500 ppm mixture, cylinder number 7, was analyzed by TDLAS over four different time periods spanning 6 months. In time periods I and IV shown in Table I, NOz concentrations were determined by using the relative peak direct absorption approach employing eq 4. Second harmonic detection was employed in time periods I1 and 111,and thus all the determinationswere based upon permeation standards. Fourier transform infrared spectrometry was also used in analyzing this cylinder. Two different band systems were employed; the u3 and the u1 u3 bands shown in Figure 4. Band strengths from ref 28 and 18 were respectively used in these calculations. At such high concentrations, a fairly large fraction of the NOz exists in the form of N204,even after expansion to atmospheric pressure for FT-IR measurements (Figure 4b). However, at the maximum sampling pressure of 11.8 Torr in our TDL absorption cell, we calculate, using rate constants from Hampson (29),that 99.9% of this gas is converted to NOz in less than 50 ms. Since this time constant is manyfold smaller than the residence time of our TDL inlet plumbing, each Nz04molecule is detected by TDLAS as two molecules of NOz. Similarly with chemiluminescence detection, the low sampling pressure as well as the high converter temperature ensures complete decomposition of NzO4. For comparison purposes therefore, we corrected our atmospheric pressure FT-IR measurements by adding twice the Nz04 concentration to the measured NOz concentration. The former was obtained from FT-IR integrated absorptionmeasurements of the vll band shown in Figure 4b using the average band strength of 1377 cm-2 atm-' from ref 30 and 31. A N2O4 concentration of 45 ppm was thus determined, in close agreement with an equilibrium value of 42 ppm calculated. The resulting FT-IR determinationof 2118 ppm for cylinder number 7 was in very good agreement with that determined by TDLAS (2123 ppm) during the same time period, as shown in Table I. The total 2u uncertaintity given with the FT-IR result was approximately 9% and was estimated by adding in quadrature the measurement precision (4%), the band strength uncertainties (7-8%), and the uncertainty due to the Nz04contribution (3%). By contrast, initial TDLAS and CD intercomparisons resulted in significant differences that could not be accounted for by the combined uncertainties: in each and every case examined except one, the TDLAS determinations of NOz were considerably lower than the CD values by 6-26%. Subsequent studies were therefore carried out to identify the presence of other reactive nitrogen compounds detected by CD employing a hot stainless steel converter. Initial studies focused on the most likely candidate, "OB. In parts a and b of Figure 6, reference spectra of HNO8 were recorded by TDLAS in the 1722-cm-l region using, respectively, direct absorption and second harmonic detection. As can be seen, the second harmonic spectral features from the nominal 1000 ppm cylinder number 5 in Figure 6c positively confirms the presence of this gas. A few lines in the reference spectrum due to HN02 were not observed in the cylinder spectrum.
+
(1.7%)
817 f 106
(2.8%)
840 f 33
Grand Average = 831 f 54 O A l l concentrations are in ppm. The uncertainties given with each value are the estimated 20 values calculated by error propa-
gation analysis, as discussed in the text. The uncertainties with each average and grand average were calculated from the rootmean-square of all the individual values. * Determinations based upon NOz permeation standards. All other determinations were based upon the 91,B+absorption line parameters. The percentage difference between determinations is given in parentheses. periods, a new sampling procedure was employed. As will be further discussed, this resulted in a dramatic increase in the TDLAS NO2determinations. Both second harmonic and peak direct absorption results were employed in the final analysis for time periods I1 and 111. As further shown in Table 11, by the percentage discrepancy given in parentheses, these results were in excellent agreement to within 1%. The accuracy of the direct absorption results was independently substantiated by agreement between the measured and calculated line widths. The resulting grand average and associated uncertainty for these time periods, 852 f 55 ppm, are also presented in Table I. In the final time period IV (approximately 5 months following time period ID), all three direct absorption calculational methods were used in the analysis, thus further ensuring against the various sources of systematic error previously discussed. The fit-to-calculated line width ratios in this particular case did indicate the presence of compressor frequency-jitter distortion. The integrated-fit determinations resulted in NOz concentrations that were substantially larger than those obtained from the peak absorbance at line center. Relative peak determinations, which were based upon calibration standards and thus negligibly affected by frequency-jitter distortion, were in agreement with the absolute line center determinations, as shown in Table 11. The distorted integrated-fit values, when normalized by the increased width, resulted in agreement with the other two determinations. Averaging all the results shown in time period IV,we obtain a TDL NOz cylinder concentration of 831 k 54 ppm. This value, which is also tabulated in Table I, is slightly lower than, but within the total uncertainty of, the 852 f 55 ppm value determined in time periods I1 and 111. The resulting agreement among the various TDL techniques and calculational
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
401
Table 111. Summary of HN03 Intercomparieons' cylinder no. 1 2 3 4 5 6 7
nominal concn, ppm
time period
1.68 f 0.2 26 f 2 33 f 3 97 f 10 107 f 11 124 12 45 5 95 f 10
I I1 & I11 IV
["O~]FT.IR, ppm
% "03b
91 f 20
0 23 10 6.4 9.9 10.9 12.7 5.4 4.1
58 f 16
2.2
*
1000
2500
PPm
0
2.3 8.9 250 500 1000
["OBICD,
I I1
I11
51 f 5
IV
'The time periods here correspond with those in Table I. bPercentaaeof total nitrogen oxide concentration based on CD.
(al
(bl
(C)
Flgure 6. (a) Direct TDL absorptlon scan of 0.5 Torr of pure HNO, In a 20-cm absorptlon cell In the 1722.4-1722.8-cm-' region. (b) Second harmonic scan of this same reference mlxture. (c) Second harmonic scan of this same region for the 1000 ppm cylinder number 5.
The presence of HN03 was further verified in the nominal 2500 ppm cylinder number 7 by FT-IRspectrometry,as shown by the spectral features in Figure 4b. By use of both the v4 (1280-1365 cm-') and v5 (825-920 cm-l) bands, a HN03 concentration of 91 f 20 ppm was determined by FT-IR spectrometry. This is in close agreement with CD measurements of 95 f 10 ppm employing a nylon scrubber. At converter temperatures around 660 "C, no residual HN03 was detected in the converter effluent flow by TDLAS. This redundancy employing two different techniques with entirely different inlet systems is important since it negates arguments concerning the possibility that HN03may be outgassing from a contaminated cell or inlet lines. Likewise, arguments concerning a contaminated cylinder regulator can be dismissed because a high-purity, low-volume (7 cm3internal volume), single-stage regulator, which was carefully flushed or evacuated in all cases, was employed in these measurements. Moreover, for some of the measurements an evacuated bar stock valve was used in place of the regulator, and the same results were obtained. Additional HN03 measurements by CD were carried out on this same 2500 ppm cylinder a t three different times 1/2 to 4 months later. A continuous decrease in the HN03 concentration from an initial value of 95 ppm to a final value of
51 ppm was observed over this time span. As shown in Table 111, an FT-IR "OB concentration of 58 f 16 ppm corroborated this decrease. Additional cylinder measurements revealed that HN03 was ubiquitous. Measurements, primarily carried out by CD employing a nylon scrubber, revealed the presence of this gas in over 30 different NO2cylinders examined to date from four different gas vendors. In the six of the seven cylinders examined in this study, the HN03 concentration was found to range from 2.2% to 23% of the total reactive nitrogen concentration measured by our CD ([NO,]), as shown in Table 111. In all cases except one we observed a similar decrease in HN03concentration with cylinder use. This decrease continued until a final, dramatically lower, stable value was attained. In the case of cylinder number 5, "03 was found to increase with cylinder use. A slight increase of 17 ppm was observed by CD between time periods I11 and IV ( 5 months apart). The total NO, concentration in this case remained invariant while the NO2 concentration by TDLAS was found to decrease by 21 ppm, approximately the HN03 increase. Although this suggests a HN03/N02 temporal correlation, more definitive studies are required. At this point we can only speculate concerning the unexpectedly high "03 concentrations and the time dependence with cylinder use. In addition to gas phase chemistry, heterogeneous reactions involving NO2 and H 2 0 on the surfaces inside the cylinder may also be important. In particular, H 2 0 vapor emanating from Teflon paste, which is used on the outlet valve of most aluminum cylinders to prevent thread scoring, may play a significant role in explaining the observed HN03 time dependence. Under this scenerio, a localized high concentrationof HN03may be formed near the cylinder outlet valve, which after moderate cylinder use, becomes expended. Once expended, lower HN03 concentrations indicative of the rest of the cylinder are observed. Without use of mechanical or thermal means, poor mixing in compressed gas cylinders can prevent this localized reservoir from readily dispersing. Further research is currently being carried out at NBS in this area. All CD determinations of NO2shown in the summary Table I were obtained with HN03 removed by employing nylon scrubbers. Once accomplished, we achieved excellent agreement in most cases between CD and TDLAS determinations of NOz. This agreement, which was generally in the 1-3% range, is quite remarkable considering the fairly reactive nature of NO2 and the independence of both measurement techniques. The percentage difference and the associated uncertainty, calculated by propagating the individual uncertainties used in determining this difference, are tabulated in the ninth column of Table I. Agreement between TDLAS and CD was still somewhat poor for the lo00 ppm cylinder number 5 in time period I and the 2500 ppm cylinder number 7 in time periods 11-IV, even
402
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
after accounting for HN03 Various tests were performed on both these cylinders to ascertain the cause of the remaining discrepancy. In one, we took advantage of the coupled detector configuration and simultaneously measured the NO concentration eluting from the hot stainless steel converter by both TDLAS and CD. The former was accomplished by using an NO diode laser at 1824.25 cm-'. The NO concentrations thus determined were in excellent agreement to within 0.4%,verifying that both detectors were in fact responding to the same sample. This agreement, and the lack thereof in corresponding measurements of NOz, further suggested the CD was responding to the presence of yet another nitrogencontaining species reduced to NO by the hot stainless steel converter. Compounds like NH3, HCN, and various organic nitrates and amines are thus likely candidates while hydrocarbons and/or organosulfur compounds not containing nitrogen, which normally could interfere with CD measurements, can be ruled out in this case. The most obvious compound responsible for this remaining discrepancy, Nz04,was previously ruled out. Nevertheless, a further search for this compound was initiated. In addition to searching for Nz04 absorption lines with a TDL in the 1760-cm-'region, we also carried out static TDL measurements in an attempt to observe an increasing NOz response with time. Both experiments were negative, thus ruling out N204as well as higher polymers of NOz. In other experiments, we incorporated a glass mixing chamber (300 cm3 in volume) in the flow path upstream of our TDL inlet. This chamber contained many tubulations and protrusions to effect turbulent mixing and was used to ensure a uniform gas sample from our compressed gas cylinders. As shown in the fourth column of Table I, this chamber was employed in the analysis of all but one cylinder. Headto-head intercomparisons were also carried out with and without this mixing chamber in the flow path. As shown, only for cylinders 5 and 7, where we continued to observe a remaining discrepancy, did this mixing chamber have a significant effect on our results. In the case of the former during time periods I1 and I11 (4 months apart), the resulting NOz concentrationsby TDLAS d r a m a t i d y increased by 12%over values previously obtained (time period I) without this chamber. This dramatic increase, furthermore, was consistently observed in head-to-head intercomparisons throughout time periods I1 and 111, completely eliminating the remaining TDLAS-CD discrepancy previously found. By contrast, identical NOz concentrations were measured in both flow configurations employing an 832 ppm NOz permeation standard. Tests were also carried out to ensure that the increased NOz observed by TDLAS was not simply caused by HN03 decomposing to NOz on the chamber walls. This was achieved by sampling the mixing chamber effluent both with and without a nylon scrubber in the flow path upstream to first remove "OB. The TDLAS NOz response did not change, as would be the case if decompositionwere prevalent. These observations were further verified by direct measurements of HN03 in both flow configurations using our FT-IR. Again, no evidence for decomposition was observed. These tests, together with the fact that the mixing effect was observed in two out of the seven cylinders examined, offers further evidence of a cylinder-dependent effect and not a sampling artifact. In Figure 7, we show direct absorption measurements of NOz from cylinder number 5 with and without this mixing chamber in the flow path. For each flow configuration, we recorded two independent measurements sets. As can be seen, the increased absorbance due to the mixing chamber is clearly distinguishable from the measurement precision. From these as well as additional measurements,a mixing chamber increase
c
4 Wavenumber (cm.' )
Flgwe 7. Direct absorption scans of NO2 in the 1605-cm-' region for the 1000 ppm cylinder number 5 with and without the glass mixing chamber in the flow path during time period IV. Two scans (30 000 sweeps each) were recorded in each flow configuration. The absorbance of the strongest line in the first doublet, the 12,,,,+ line, averaged 0.02281 f 0.0004 ( 2 4 and 0.02099 f 0.00025 (2a)with and without the mixing chamber, respectively, in the flow path.
of 8.2% f 1.0% waa determined in time period IV, which again resulted in excellent agreement between TDLAS and CD determinations of NOz. Before additional experiments could be implemented to investigate possible thermal decomposition and search for heretofore unknown bands by FT-IR, the mixing effect completely disappeared in cylinder number 5 during time period IV. Subsequent intercomparisons between TDLAS and CD resulted in excellent agreement regardless of whether or not this mixing chamber was employed. We also observed a mixing chamber effect in the analysis of the 2500 ppm cylinder number 7. As shown in Table I, this mixing chamber was employed in time periods 11and 111,and the resulting effect averaged 6.7% f 1.6%. A slight remaining discrepancy in the 7 4 % range, however, between TDLAS and CD still existed even after this chamber was employed. Unfortunately, because of very low cylinder pressure and the attendant unpredictable behavior, no other conclusions could be drawn from this cylinder. Although many questions still remain regarding this mixing effect, evidence suggests that the effect is most likely associated with an additional cylinder nitrogen-containing compound. Our observationsfurther suggest that this compound decomposes on the walls of the mixing chamber, generating among other things, free NOz which is detected by TDLAS. Such behavior implies a weakly bound NO2 complex that, depending upon the method of sampling, can remain intact even while flowing through the TDL absorption cell. Under these circumstances, such an NOz complex if present, would go undetected by TDLAS but not CD due to the high converter temperature. Obviously, a great deal more research is needed to (1) further characterize the mixing effect, (2) the correlation, if any, with "0,; (3) the mixing effect and HN03 time dependence with cylinder use; and (4) the exact identity of the additional compound by FT-IR.
CONCLUSIONS We have presented in this study comprehensive details that should be considered when carrying out accurate quantitative analysis by TDLAS. The enhanced frequency and amplitude stability of sweep integration over the more conventional slow
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
scan method is important for high-quality data. The advantages and potential systematic errors associated with each data acquisition mode and calculational approach were further discussed. If accurate standards are available, the simplicity, higher sensitivity, and lower susceptibility to systematic errors make second harmonic detection the method of choice. In the particular application of the present study, high-concentration NOz determinations, frequent permeation tube preparation presents a slight inconvenience in this regard. If the necessary time and equipment are available, the independent calibration methods associated with each technique can be used to advantage to give an added degree of confidence to the measurement accuracy. Such was the case in the present study where direct absorption determinations of NO2 based upon the absorption coefficient were in excellent agreement with second harmonic values based upon NOz permeation standards. By use of the various TDLAS approaches, NOz concentrations in seven compressed gas mixtures were determined and compared with values obtained by CD and, in one case, FT-IR. Both infrared techniques revealed that the CD was responding to the presence of "OB. This gas was subsequently measured by CD, employing a nylon scrubber, and was found in 29 out of 30 NOz cylinders from four different vendors. The concentration of this gas in six of the seven cylinders examined in this study ranged between 2.2% and 23% of the total nitrogen oxide concentration, corresponding to values between 2 and 124 ppm. After HN03 was accounted for, the TDLAS and CD determinations of NO2 were in agreement to within l-3% in almost every case. This concurrence of two independent techniques based upon different calibration standards enabled us to make definitive NOz determinations on compressed gas mixtures in the 2.3-1000 ppm range. Based on these results, determinations by both TDLAS and CD employing a nylon scrubber are recommended for the highest accuracy in certifying NOz compressed gas mixtures. Fourier transform infrared measurements, as further shown by the excellent agreement with TDLAS for one particular cylinder, provide yet an additional degree of confidence in the results. However, in much less demanding situations, CD employing a nylon scrubber can provide the necessary results with much less effort and expense. In the two cases where discrepancies still remained, rather extensive studies pointed to the presence of an additional nitrogen-containing compound. Turbulent flow, effected by sampling through a glass mixing volume containing many tubulations and protrusions, apparently caused this additional compound to decompose to NOz. The resulting NOz concentrations by TDLAS dramatically increased, totally eliminating the remaining discrepancy with CD in one case. In addition to high concentration NOz standards, the measurement methodology presented in this study was also important for definitive determinations in the 1-10 ppm range. Gas standards, such as the nominal 2.3 and 8.9 ppm mixtures analyzed in this study are at convenient working concentrations to input to dynamic dilution systems for generation of sub-parts-per-billion calibration standards in trace atmospheric studies. The accuracy of the resulting atmospheric measurements will obviously be directly tied to the accuracy of the standards. However, as we have found in this study,
403
significant errors as large as 60% can arise if the NO2 determinations provided by the gas suppliers are not further verified. Our limited data base for cylinders in the 1-10 ppm range shows that an inaccuracy in the NOz concentration results from the presence of HN03. In addition, these data also suggest that there are further losses, most likely associated with surface effects. In this case, cylinder surface passivation may not be totally effective. Registry No. NOz, 10102-44-0.
LITERATURE CITED Kerr, J. A.; Calved, J. G. "Chemical Transformation Modules for Euia rian Acid Deposltlon Models"; interagency report to the US EPA; Atmospheric Sciences Research Laboratory: Research Trlangle Park, NC, Dec 1964. "Nitrogen Oxides"; National Academy of Sciences Report; National Academy of Sciences: Washington, DC 1977. Bollinger, M. J.; Hahn, C. J.; Parrish, D. D.; Murphy, P. C.; Aibrkton, D. L.; Fehsenfeid, F. C. J. Geophys. Res. 1984, 8 9 (No. D6), 9623-9631. Walega, J. G.; Stedman, D. H.; Shetter. R. E.; Mackay, G. I.; Iguchi, T.; Schiff, H. I. Environ. Sci. Technol. 1984, 18, 823-826. Fried, A.; Sams, R. ASTM Spec. Tech. P U N . 1987, No. 9 5 7 , 121-131. Slemr, F.; Harris, 0. W.; Hastie, D. R.; Mackay, G. I.; Schiff, H. I.J . Geophys. Res. 1988, 91 (No. D5),5371-5378. Sams, R.; Fried, A. J. Mol. Spectrosc. 1987, 126, 129-139. Fried, A.; Sams, R.; Berg, W. Appl. Opt. 1984, 23, 1867-1880. Sams, R.; Fried, A. Appl. Spectrosc. 1988, 4 0 , 24-29. Jennlngs, D. Appl. Opt. 1984, 2 3 , 1299-1301. Wells, J. S.; Petersen, F. R.; Makl, A. J. Mol. Spectrosc. 1983. 9 8 , 404-412. Jennlngs, D. Appl. Opt. 1980, 19, 2695-2700. Reid, J.; Shewchun, J.; Garside. 6. K.; Ballik, E. A. Opt. Eng. 1978, 17, 56-62. Hastie, D. R.; Mackay. G. I.; Iguchi, T.; Ridley, 6. A,; Schiff, H. I. Envlron. Sci. Technol. 1983, 17, 352A-364A. Hughes, E. E.; Rook, H. L.; Deardorff, E. R.; Margeson, J. H.; Fuerst, R. 0. Anal. Chem. 1977, 4 9 , 1823-1829. Fried, A.; Hodgeson, J. Anal. Chem. 1982, 5 4 , 278-282. Elkins, J. W.; Kagann, R. H.; Sams, R. J. Mol Spectrosc. 1984, 705, 480-490. Perrln, A.; Flaud, J.-M.; Camy-Peyret, C. Infrared Fhys. 1982, 2 2 , 343-348. Humlicek. J. J. Ouant. Spectrosc. Radiat. Transfer, 1979, 2 1 , 309-313. Toth R., personal communication from the Jet Propulsion Laboratory, 1985. Rothman. L. S.AFGL Trace Gas Compilation Edltlon of August 1980; AFGL Hanscom AFB: Bedford, MA, 1980. Grosjean. D.; Harrlson, J. Environ. Sci. Technol. 1985, 19, 862-865. Strow, L. L. J. Ouant. Spectrosc. Radlat. Transfer 1983, 2 9 . 395-406. Fridovich, B.; Devl, V. M.; Das, P. P. J. Mol. Spectrosc. 1980, 8 1 , 269-272. Sams, R.; Fried, A. Appl. Opt. 1987, 26, 3552-3558. Mucha, J. A. Appi. Spectrosc. 1982, 3 6 , 141-147. Mucha, J. A. Appl. Spectrosc. 1984, 3 8 , 68-73. Devi, V. M.; Fridovich, 6.; Jones, 0. D.; Snyder, D. G. S.; Das, P. P.; Fluad, J.-M.; Camy-Peyret, C.; Rao, K. N. J. Mol. Spectrosc. 1982, 9 3 , 179-195. Hampson, R. F., "Chemlcal Kinetic and Photochemical Data Sheets for Atmospheric Reactions"; US. Department of Transportation Report No. FAA-EE-80-17, April 1980. Guttman, A. J. Ouant. Spectrosc. Radiat. Transfer 1962, 2 , 1-15. Bibart, C. H.; Ewing, G. E. J. Chem. Phys. 1974, 6 1 , 1284-1292.
RECEIVED for review August 28,1986. Resubmitted and Accepted October 20,1987. We wish to acknowledge the Office of Standard Reference Materials at the National Bureau of Standards for partial support of this research. In order to describe adequately materials and experimental procedures, it was occasionally necessary to identify commercial products by manufacturer's name or label. In no instance does such identification imply endorsement by the National Bureau of Standards.
