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Anal. Chem. 1983, 55, 1660-1665
are understandable since they are dependent on the system used and operating conditions.
CONCLUSIONS We have examined the use of EGA-ICP system for the speciation of metal salts. Encouraging preliminary results were obtained for the speciation of solid samples, as evidenced by the resolution of binary and ternary mixtures of salts of a common element. Quantitation was also found to be possible within accuracies of 1 to 9%. The EGA-ICP system offers several advantages over other proposed solid sample speciation techniques. The system is very simple, with both the vaporization and excitation energy supplied by the plasma discharge. This arrangement prevents loss of vaporized material during transport to the detector and allows for minimal dispersion of the sample between vaporization and excitation, which minimizes peak broadening. The EGA-ICP system also has a higher maximum operating temperature, which increases the range of samples than can be examined. Several limitations were encountered with the EGA-ICP system. If different components in a solid mixture decompose to a common product then speciation cannot be obtained by observing the emission of a single element. The possibility of solving this problem by simultaneously observing the emission of several elements must still be investigated. Several experimental factors play a role in determining actual resolution, the most important of these are sample size and temperature ramp rate. As the sample size decreases, peak resolution increases. This was noted in the case of the V203 + V204mixture, where the peaks were separable only with samples containing less than 0.5 pg of material. Decreasing the temperature ramp rate was observed to increase peak resolution. Optimization of these parameters is essential for successful speciation of samples with chemical forms that evolve at similar temperatures. The current experimental system allows control of sample temperature with a practical resolution of only 50-100 OC. A finer control of sample temperature in future designs may enhance speciation capability. In addition, the current ICP torch design does not restrict sample introduction to the central plasma region. Correction of this feature should result in better sensitivity and detection limits although the moderate present values of these parameters did not present a problem in this work. Chemical reactions between sample components and the graphite probe walls of sample matrix may also limit the separation of sample. In many cases, the probes employed
had to be aged by heating with sample several times before reproducible results could be obtained. The use of pyrolytic or other specially treated graphite or metal cups may alleviate this problem. As in all high temperature applications, the graphite cups also tend to deteriorate with repeated use. Attempts to model the rate limiting process which controls the temperature and rate of sample evolution resulted in the conclusion that volatilization follows an Arrhenius type rate expression. Although the energy of activation of the volatilization process was greater than the heat of vaporization for the modeled system, it was not possible to identify specific chemical reactions which may be involved in volatilization. Since many reasonable solutions seem to exist for the problems and limitations discussed above, further studies will be undertaken in order to expand the speciation abilities of EGA/ICP toward mixtures of solids. The high sensitivity of ICP emission spectrometry makes this approach especially attractive for trace level analyses.
LITERATURE CITED Muller-Vonmoos, M.; Kahr, G.; Rub, A. Thermochim. Acta 1977, 2 0 , 307. Morgan, D. J. J . Therm. Anal. 1977, 72,245. Gibson, E. K. Thermochim. Acta 1973, 5 , 243. Friedman, H. K. Thermochim. Acta 1970, 7 , 199. Gallagher, P. K. Thermochim.Acta 1980, 47, 323. Schuetzie, D.: Cronn, D.: Crlttenden, A. L: Charlson, R. J. Environ. Sci. Techno/. 1975, 9 , 838. Malissa, H.; Puxbaum, H.; Pell, E. 2.Anal. Chem. 1976, 282, 109. Gall, S . ; Paullk, F.; Pell, E.; Puxbaum, H. Z . Anal. Chem. 1976, 282, 291. iauer, C. F.; Natusch, D. F. S. Anal. Chem. 1981, 53, 2020. Wlndsor, D. L.; Heine, D. R.; Denton, M. B. Appl. Spectrosc. 1979, 3 3 , 56. Salln, E. D.; Horlick, G. Anal. Chem. 1979, 57, 2284. Fernandez, M. A.; Bastiaans, G. J. Anal. Chem. 1979, 51, 1402. Buzas, I . "Thermal Analysis"; Heyden: New York, 1974; Vol. 1. Rodgers, R. N.; Yasuda, S. K.; Zlnn, J. Anal. Chem. 1960, 32, 672. Van Den Broek, W. M. G. T.;de Galan, L. Anal. Chem. 1977, 49, 2176. "CRC Handbook of Chemistry and Physics", 52nd ed.; Chemical Rubber Co.: Cleveland, OH, 1972. Cotton, F. A.; Wiiklnson, G. W. "Advanced Inorganic Chemistry": Interscience: New York, 1972; pp 821-827. Duval, R.; Wadler, C. Anal. Chim. Acta 1960, 23, 257. "Merck Index", 9th ed.: Merck: Rahway, NJ, 1980; pp 709-711. Northway, S. J.: Fry, R. C. Appi. Spectrosc. 1980, 3 4 , 332. Northway, S. J.; Brown, R. M.; Fry, R. C. Appl. Spectrosc. 1980, 34, 338. Windsor, D. L.; Denton, M. B. Appl. Spectrosc. 1978, 32, 366. Hughes, S. K.; Fry, R. C. Anal. Chem. 1981, 53, 1111.
RECEIVED for review April 28,1983. Accepted June 13,1983. This work was supported by the Welch Foundation, Grant A-866, and the National Science Foundation, Grant CHE 79-21584.
Optimization of Response of Chemiluminescence Analyzers A, A. Mehrabzadeh, R. J. O'Brien,* and T. M. Hard Chemistry Department and Environmental Sciences Doctoral Program, Portland State University, Portland, Oregon 97207 The behavior of chemllumlnescence analyzers Is discussed in terms of the response equatlons for exponentlaldllution and plug-flow reactors. Three operational modes are dlstingulshed In each case, and their characterlstlcs and advantages are treated. The response equations are tested experlrnentally with the ozone/ethyiene chemiluminescent reactlon. Appllcation to the popular NO/O, analyzer is dlscussed.
Chemiluminescence (CL) as an analytical technique in both
the gas and liquid phases has been the subject of increasing study. A recent review (1) lists 71 references for a 2-year period. CL analyzers are widely used for atmospheric measurements of trace gases because of their sensitivity and relative simplicity. In particular, ozone and oxides of nitrogen (NO,) are routinely measured via CL, and commercial instruments are available for this purpose. The detection limits of these instruments range to the parts-per-billion level a t 1 atm total pressure, and modified or laboratory-constructed instruments have been made several orders of magnitude more
0003-2700/83/0355-1660$01.50/00 1983 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55,
sensitive. In its simplest sense, a CL analyzer is a transducer which converts a flow of molecules into a flow of detectable photons. However, any ad,ual gas-phase CL analyzer has a formidable number of design parameters. These are tlhe chamber volume, the pump or blower capacity, the reagent gas flow rate and mole fraction, the sample flow rate, and the chamber pressure. In addition to these controllable parameters, the sample total pressure will in general be variable, as will its temperature. Finally, the rate constants for the relevant chemical processes will control the behavior and determine the ultimate sensitivity of the analyzer. Since there are a t leatit five independent instrument-design parameters, it is not surprising that many reported CL studies have dealt with lesai than the complete range of variables. For instance, at a fixled chamber volume and pumping rat e one may simultaneouslly vary both reagent flow and sample flow until an empirical maximum response is obtained. This variation may result in changing chamber pressure, which need not be addressed in an empirical treatment. However, if the chamber pressure is altered by changing sample pressure, then either or bioth pressure changes may produce an unwanted change in response. Equations describing the maximum achievable response of a CL analyzer have been given (2-4). But, the conditions for reaching this maximum have not always been clearly stated, especially in regard to optimization of the reagent flow. Under some conditions a deceptive signal maximum is observed through variation of the chamber pressure, and this maximum has been misinterpreted. Steffenson and Stedmm (5)have given the general response equation for a hiO/03 plug-flow CL analyzer, observed a response “plateau” with varying samples flow rate at high chamber pressure, and interpreted this in terms of their equation. They optimized the reagent flow empirically for one set of conditions rather than for each case. They recognized that variation of pumping speed at constant sample flow would have produced a different type of response behavior, but did not treat this situation. Plug flow analysis is suitable for a small-volume reactor, but often large chamber volumes are desirable, to maximize response. The opposite extreme from plug flow is exponential dilution, where the incoming reactants are well-mixed throughout the chamber volume and no concentration gradients occur. This case has not been treated previously. Since we are interested in extending CL analysis to new atmospheric species, we have obtained generalized response equations for both the plug-flow and exponential-dilution analyzers and have mathematically optimized the response with respect to all relevant parameters (6).Here we discuss the operational modes of CL analyzers and experimentally test the behavior of the ozone/ethylene CL process for conformance with the equations. Finally, the underlying principles discovered in this analysis are applied to a discussion of the popular NO/03 CL analyzer.
THEORY The simplest mechanism for a chemiluminescent process involves the reaiction of a reagent (R) gaci with a detected (D) gas to produce an excited intermediate which fluoresces or is quenched. Additionally, R and D may combine to produce nonemitting products. The reaction of R with D may be pressure-dependent (three-body reaction). The analyzer is controlled by three uolrrme flows: reagent, sample, FD,and total, FM(crn3/s). (By contrast, the sample molecular flow is FD[pD], where [PD] is the total sample gas concentration or number denaiity in molecules/cm3.) The combination of these three flows and tlhe sample pressures controls the gas concentration [MI in the chamber through a continuity or mass-conservation equation. This equation places funda-
NO. 11, SEPTEMBER 1983
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mental constraints on the operation of a CL analyzer. If we define the response of a CL analyzer in terms of emitted photons/s per ambient concentration of detected gas, [DIo (molecules/cm3), a steady-state solution of the relevant differential equations results in the following response equation for an exponential-dilution (ed) instrument (6). \
\
/
-)(,+z
1 2+Z+l/Z)
In obtaining this equation, we have assumed that [R] >> [D] (required for instrument linearity) and that the removal of excited intermediate is controlled by quenching and fluorescence rather than by its flow out of the chamber. This equation is written in terms of the total emission rate within the chamber and must be corrected for the photon collection efficiency and the photomultiplier spectral response in any application. Here XRis the mole fraction of R in the reagent stream; Y is the partial rate factor (yield) for formation of excited intermediate in the reaction of R with D; kRD, kf, and k , are the rate constants for the overall reaction of R with D including all product channels and for the fluorescence and quenching (by M) of the excited intermediate; [PD] is the total concentration (number density) of the sample; and V the chamber volume. We define the inlet molecular flow ratio 2 as the ratio of the reagent to sample flows (molecules/s). Equation 1 is based upon a situation where all reactions between R and D are second order. If all the reaction channels for R + D are three-body, the term kRD should be replaced by km [MI in eq 1. If only one channel is pressure dependent, Y also will be pressure dependent. The two forms of eq 1 are based upon mass conservation, FM[M] = (1 + z)FD[PD], and each is appropriate to a particular experimental situation. We may distinguish three modes of operation for a CL analyzer, corresponding to constancy, or response limitation, of one of the three independent flow parameters: reagent, sample, or total flow. These are discussed separately below. The response of a plug-flow (pf) CL analyzer may be obtained by integrating the equations for the chemical and flow processes over the chamber volume. The result is Tpf
=
(2) This equation is equivalent to eq 9 of Steffenson and Stedman (5)but here it is expressed in terms of FMand 2, rather than in terms of the sample flow, F D . Mode 1. Total Outflow ( F M Limited. ) The first operational mode is the one which gives the maximum achievable response, and this response is ultimately limited by the pumping speed FMof the vacuum pump or blower. Inspection of eq l a reveals that maximum response occurs at high chamber concentration [MI (high chamber pressure) and is given by
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
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2
0.004
it
04'
Is\
$. 1I
',-,I
V 0.003 n
-4
-3
-2
-I
LOG CMI
0
t
2
CATM)
Figure 1. Mode 1 and mode 2 response for a CL analyzer based upon eq 1 and 2. The response Is plotted vs. chamber concentration [MI, which is controlled by FD variation in mode 1 and by f variation in mode 2. Curves 1 and 2 are for mode 1 (ed and pf, respectively) with constant Z = 1. Curve 3 is mode 1 with Z adlusted to ZoPtat each [MI. Curves 4 and 5 are for mode 2 (ed and pf) with f D / f= , 0.1.
,,
The first form of this equation has been given by several authors but may be subject to misinterpretation, for instance if it seems to imply that more signal may always be obtained by increasing F D or by decreasing [MI. The basic limitation of response by total volume flow F M is more clearly indicated by the second form of this equation. Note that the inlet-flow ratio 2 enters directly into the maximum response equation. At any given chamber density [MI, maximum response occurs a t a single value of 2. Any less reagent flow will cause insufficient reaction of D with R and any more will dilute the detected gas. The optimal value of 2 for a mode 1analyzer may be found by setting the 2 partial derivative of eq l a or 2 equal to zero. The result for exponential dilution is ZedoPt
= (1 4- ~ R D V X & ~ ] / F M ) - ~ / ~ (4)
and for plug flow the optimal 2 is given implicitly by
Unfortunately the last equation must be solved numerically. At sufficiently high chamber density [MI both values of Zopt approach zero; but it is unwise to drop 2 from any of the equations presented here. For mode 1operation at constant F M , as chamber pressure increases due to increasing inlet flows, the response will approach the r,, plateau asymptotically. This behavior is shown for exponential-dilution and plug-flow in Figure 1. Two mode 1 situations are illustrated: constant 2 = 1 (curve l),and adjustment to Zoptat each pressure (curve 3), the latter producing higher response. The plug-flow response for mode 1 is also shown for 2 = 1 (curve 2). These curves are drawn by using the rate constants and operating parameters of Kley and co-workers ( 3 , 4 )for the NO/03 CL analyzer treated further in the discussion section. The condition for reaching the plateau may be found by inspection of eq l a and is given by
[MI >> (1 + Z)FM/Z~RDVXR
(64
as well as
[MI >> kf/k,
((33)
Condition 6b implies strongly quenched emission, whose disadvantage is outweighed by the advantages of high sample throughput and adequate chamber residence time. Mathematical analysis of the plug-flow equation (2)gives the same
condition for reaching the plateau, although Figure 1illustrates that the inequality is not as rigorous for plug flow. This analysis also indicates that at low chamber density the plug-flow and exponential-dilution equations reduce to the same mathematical form. Thus curves 1 and 2 in Figure 1 differ only in the intermediate region near the transition to plateau response. Mode 2, Sample Flow ( F D ) Limited. The second operational mode applies to a situation of constant sample flow rate with varying out-flow or chamber pressure. This mode is useful for cases where the total sample volume is fixed, limiting the allowable maximum sample withdrawal rate. Under this situation, maximum response will not occur asymptotically at high pressure but rather as a true maximum which may be either higher or lower than ambient. The optimal reagent flow ratio Zoptmay be determined by partial differentiation of eq l b with respect to 2, resulting in ,Topt = 1, or equal inlet molecular flows irrespective of chamber pressure. The optimal chamber pressure may be found by partial differentiation of eq l b with respect to [MI, resulting in a cubic equation which can be solved (6). This maximum can also be found empirically by varying chamber pressure at the desired sample flow rate and Zopt = 1. In this mode the response is less than that given by eq 3, and greater response could be obtained by increasing F D as well as Fhl. This mode is illustrated in curves 4 and 5 (ed and pf, respectively) of Figure 1. Here F D was 10% Of F M and the dashed portions of the curves are for values of [MI attainable only by increasing FMbeyond the assumed pump capacity. Mode 3, Reagent Flow Limited. The final mode considered here applies to a situation where the reagent supply or cost limits the achievable response. Here, as in mode 2, maximum response is not achieved. This mode has potential advantages for eliminating the sensitivity of CL analyzers to changing sample pressure, but it apparently has never been recognized. Its application to the N O / 0 3 CL analyzer is presented in the discussion section. Pressure Dependence of CL Analyzers. CL analyzers are potentially sensitive to changes in sample pressure. This can be an annoyance for slight changes in barometric pressure for ground-level ambient air sampling. The problem is more severe for vertical atmospheric sampling since pressure decreases with altitude. Equations 1, 2, and 3 define the Concentration response of CL analyzers in photons/s per unit concentration, ambient. Thus the signal achieved by a CL analyzer is given by r[DIo. The mole-fraction response (photons/s per unit mole fraction) is given simply by r[PD]. In eq la, 2, and 3, multiplying by the sample pressure [PD] removes the dependence of response on sample pressure, In terms of r,, this means that an analyzer operating on the mode 1plateau will give a constant response even if sample pressure decreases, as long as the sample mole fraction or mixing ratio remains constant. That is, the analyzer in this mode is linear in mixing ratio, independent of the concentration or number density [Do] of the detected gas and of its environment, [PD]. However, since chamber concentration [MI will naturally drop with decreasing sample pressure, and since condition 6 shows that plateau response will only occur at high [MI, it is necessary to adjust the CL analyzer for the lowest pressures to be encountered in order to maintain insensitivity to sample pressure. Constant mole-fraction response is advantageous for vertical atmospheric sampling because it means the absolute instrumental sensitivity increases with altitude. In the general equation, eq la, the mole-fraction response does not vary with sample pressure, even during operation off the response plateau, provided the chamber pressure is held constant. Since choked flow into a CL chamber will cause
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
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-2
-I
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0 LOG 2
LOG H CATM)
Flgure 2. Ozone/uthylene chemiluminescent response r vs. chamber pressure [MI, with F, varying from 0.33to 208 standard cm3/s, and Z = 1. Here, F, was about constant, simulating mode 1. Points are experimental data Curve is based upon eq la, as described in text. The high flow limiting response could not be ireached with a suction
Ozone/ethylene chemilurnlnescent response r vs. ratio of reagent and sample flow rates, Z ,for mode 2 operation. Here, [MI = 1.6 X molecules/cm3;F , = 3.3cm3/s; reagent flow varied from 0.33to 83 standard cm3/s, and F, was adjusted to keep [MI constant. Flgure 3.
pump. -7.2
a decrease in [MIwith decreasing [PD], fl~owregulation would be required to achieve constant [MI in this situation. Thus mode 2 (fixed F D ,r < r-) may be inherently sensitive to [MI and hence to [PI,] (curves 4 and 5 in Figure 1). The situation is more complicated than this simple treatment indicates, and in fact constancy of either mole-fractton or concentration response with changing sample pressure can be achieved (6, 7). Mode 3 operation also can combat the dependence on sample pressure and is discussed below. Liquid-Phase CL Analyzer. Only the chemical mechanism chosen for analysis limits the applicability of the above equations and principles. Thus they apply equally to a liquid-phase CL analyzer operating with this simple pseudofirst-order mechanism. For a liquid analyzer, [P,] = [MI so the equations simplify somewhat.
n 0 \
y
-7.3
3 W
0
" \ n
;
-7.4
v) v)
0 0
b
-7.5
L (I)
-1
EIXPERIMENTAL SECTION Procedure. The ozone/ethylene CL reaction (8-11) was used to test the equations and principles developed here. The experimental data are shown as points in Figures 2-4. In all experiments, pure or air-diluted ethylene (the reagent) and several parts per million of ozone in oxygen or air were admitted into ani integrating sphere through separate restricting valves. The two upstreani pressures were maintained at 1.00 atm, and the two entering flows were measured at that pressure with tapered flowmeters (Gilmont F-2100, 2200, 2300). The ozone concentration was kept constant by maintaining constant flow through a mercury-lamp ozonizer and diverting most of the output to a fume hood. The sphere's volume was V = 1900 cm3,and its contents exited through a 5 cm diameter port and a large valve controlling F M to a vacuum pump (Welch 1402,2500cm3/s). The sphere's inner wall was coated with Eastman 6080 White Reflectance Coating for high diffuse reflectivity. Through a second 5-cm port with a fused-silica window, chemiluminescence was detected by an EM1 9807B photomultiplier whose KCsSb photocathode has a quantum efficiency of abo'ut 0.25 in the 350-450 nm region. The ratio of the area of this port to the total nonreflecting area of the sphere was 0.40, and other absorptive and geometric losses were estimated at 0.5. Photon pulses were amplified, discriminated from dynode-initiated pulses with an efficiency of about 0.8, using a Precision Instruments Model AD6 amplifier/discriminator, and counted with a Heathkit Model 1M-4110 counter to obtain the observed signal in photons/s. Background chemiluminescence at each experimental point was measured by replacing the reagent flow with air and was sub-
-7.6 -2
-I
0
L O G M CATM)
Ozone/ethylene chemiluminescent response r vs. chamber pressure [MI at constant F, = 33.3cm3/s,and Z = 1. F, was varied with the exit valve (mode 2). Note peak response at a particular value of [MI, in contrast with Figure 2. Flgure 4.
tracted t o yield the net signal. This procedure does not compensate for different quenching abilities of ethylene and air, but the correction was small. The signal was divided by the ozone concentration [Ill, (which was calibrated by ita 254-nm absorption, in a separate 75-cm cell) and by the instrumental photon detection efficiency (0.25 X 0.4 X 0.5 X 0.8) to obtain r in (photons s-I)/ (molecule c m 3 .
RESULTS Three types of experiments were performed. In the first, reagent and sample flows were held equal (2 = 1)and the response was measured as they were increased. The observed response at several values of sample flow from 0.33 to 208 cm3/s is plotted in Figure 2. Although this experiment simulated mode 1, the observed values of FD and [MI indicated that k M varied somewhat, producing some distortion of the curve shown in Figure 2. Here the reagent was pure ethylene, and ozone was generated in pure oxygen,
1664
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
with [Os] = [D], = 3.8 x 1013molecules/cm3. The results indicate that the ozone/ethylene CL does not reach the high-pressure plateau within the range of this experiment. That is, the plateau could be reached (condition 6b) only by pressurizing [MI above [PD] with a pressure rather than a suction pump. Failure to reach the plateau is a consequence of the very high value of kf/k, for the ozone/ethylene reaction. In the second experiment the chamber pressure [MI and the detected gas flow rate F D were held constant (mode 2) while the reagent and total (FM)flow rates varied. The results, shown in Figure 3, give the expected maximum at 2 = 1 as predicted for a mode 2 analyzer. (In the limit of low [MI, Zopt 1 for a mode 1 analyzer also.) Here [MI = 50 torr, F D = 3.33 cm3/s, XR= 0.1, and reagent flow = 0.33-83 cm3/s. The principal gas in both flows was air, to avoid dependence of the response on quenching-gas composition. The third experiment (Figure 4) was a search for the optimum pressure for chamber operation under conditions of constant FD(mode 2). In this case, all parameters were held constant except [MI and FM, which were adjusted with the exit valve. The reagent and sample composition were the same as in the second experiment, and FD and reagent flow were each constant at the relatively low value of 33.3 cm3/s. The results show the predicted maximum in response a t a single chamber pressure. The solid curves in Figures 2-4 are drawn by a leasbsquares analysis of eq 1. This equation was fitted to each data set by using a simplex minimization search for the set of kinetic rate constants which gives the best fit to all the data. These curves are shown to indicate that eq 1 does account for all the observed experimental behavior. The derived rate constants, several of which have not been previously reported, will be presented elsewhere with a discussion of the fitting procedure -+
(12).
DISCUSSION The ozone/ethylene CL process, with its low value of kRD and high quenching half-pressure kf/ k,, requires both large chamber volume (eq 6a) and high chamber pressure [MI (eq 6a, 6b) in order to achieve plateau response with moderate pump capacities. In this situation, if plateau-based insensitivity to ambient pressure is desired, condition 6a may be satisfied by decreasing FM (at a loss in response). However, Condition 6b can only be achieved by operating the chamber above atmospheric pressure (Figure 3). We have discussed (7)alternative methods to combat pressure dependence of the ozone/ethylene CL analyzer. The N O / 0 3 CL reaction is significantly different from ozone/ethylene because of its much faster reaction rate kRD and lower kf/k,. Consequently, condition 6b is satisfied a t pressures well below ambient. Analyzers based on this reaction have been operated either at reduced chamber pressures (typically 0.01 atm) or a t ambient pressure by the use of a blower rather than a vacuum pump. These analyzers are used routinely in air monitoring at ground levels and have been used to measure very low NO, concentrations in remote regions of the troposphere and stratosphere. Steffenson and Stedman (5), Kley and coworkers ( 3 , 4 ) ,Ridley and Howlett (2),and others have discussed aspects of the optimization and performance of this analyzer. However, the variation of response with reagent flow (2)and in particular with ambient pressure [PD] has not been adequately treated. The mole-fraction response r[PD] of this analyzer is illustrated in Figure 5 using eq 1 and 2 with the rate constants and analyzer parameters reported by Kley and co-workers ( 3 , 4 )(V = 1000 cm3, XR = 0.03, [MI = 0.02 atm, FM= (sample + reagent molecular flows)/[M] = 3800 cm3/s, reagent flow = 8.3 standard cm3/s, 2 = 0.12). Four pairs of
-1
0 1 LOG REAGENT FLOW CSTD. CC/SEC)
2
Figure 5. Mole-fraction response of the Nolo:, CL analyzer using the rate constants and instrumental parameters of Kley and co-workers (3, 4 ) . The response, ratioed to maximum response, is shown vs. reagent molecular flow (standard cm3/s). Four pairs of curves are shown; solid ones use exponential dilutlon and dashed ones plug flow, with chamber pressures ranging from 0.0002 to 0.2 atm. The vertical line is the reagent flow chosen by Kley et al. The numbered points correspond to operating conditions described in the text.
curves are shown, on a semilog scale, for the exponentialdilution and plug-flow cases for chamber pressures increasing in decades from 0.0002 to 0.2 atm. The responses, relative to the maximum achievable response ,r given by eq 3 using 2 = ZoPt for each [MI, are plotted against reagent flow from 0.1 to 100 standard cm3/s, The response of the analyzer operated at [MI = 0.02 atm is within 10% (ed) and 0.1% (pf) of the maximum response obtainable at that chamber pressure, and within 22% (ed) and 12% (pf) of the high-pressure asymptote. These differences in response are insignificant if the analyzer is used only at a single altitude or single value of [PD], However, if the analyzer is to maintain its performance over a range of altitudes, the dependence of chamber pressure on ambient pressure must be considered. If the sample inlet were a choked-flow restriction with fixed FD, then [MI (at low 2) would be proportional to [I?,], and the response of the analyzer would deteriorate with increasing altitude, as is evident in Figure 5. Operating point 1,corresponding to the conditions of Kley and co-workers ( 3 , 4 ) ,does fall at the response maximum of our calculated curve. Starting at this point, several moves are of interest. If F D is adjusted with altitude to maintain constant [MI, the analyzer stays at the same point, and the molefraction response remains constant as long as FM does not vary. However, if F D is not regulated and thus sample volume flow as well as reagent molecular flow remain constant (choked flow), with [MI proportional to [PD], the response drops precipitously with increasing altitude toward point 2. This drop in response can be combated in two simple ways without requiring automatic flow control for F D . In the first, the product VXR[M] is adjusted at ground pressure to move further onto the plateau, by increasing chamber volume, reagent mole fraction, and/or the chamber pressure. In this analyzer’s operating range, eq 6b is always satisfied, so changes in any of the latter three variables are equivalent (eq 6a). For instance, if the operating parameters move from point 1 to point 3, the analyzer will have the same response but will be insensitive to changes in chamber pressure from 0.2 to 0.02 atm. This move also has the benefit of a significant reduction in required reagent flow. Even greater insensitivity to ambient
Anal. Chem. 1983, 55, 1665-1668
pressure can be obtained by moving to point 4 where the curves from 0.2 down to 0.002 atm are very close to overlapping. Although this move halves the response, it allows a decrease in reagent flow of an order of magnitude. Such a decrease would be significant for balloon-borne instruments since eith,er the payload weight could be decreased or the mission prolonged. The move to point 4 fully exploits mode 3 operation for the analyzer. Further major increases in ground-level response can only be obtained by increasing F M or by improving the spectral overlap bletween the chemiluminescence and the photodetedor. Starting from operating point 1of Kley and cO-workers, a 10-fold increase in F M and FD,at constant XRand V, will move theiir analyzer to a point which is insensitive to changes in chamber preiesure from 2 to 0.02 atm. This move (out of the plane of Figure 5) would place the analyzer in a mode 3 position similar to point 4 and would result in a $fold increase in mole-fraction response. However, the move would require about a 10-fold increase in reagent flow, to 100 standard cm3/s, A smaller increase in reagent flow would suffice if V were increased. The sensitivity of NO/03 analyzers is limited by the ozone self-lumiinescence. For small values of 2 = Zopt,the chamber ozone COtnCentratiOn = [R] (XR[M]FM/~RDV)~/'. Thus if the dependence of this background on [O,] were known, the analyzer's sensitivity could be optimized with respect to this variable concurrently with the others. For pseudo-first-order homogeneous ozone emission, the background signal is proportional to the square root of XR[M]/V. If the reaction is heterogeneous, the inverse dependence upon V would probably be stronger. +
CONCLUSION The large number of variable parameters operative in a CL analyzer are difficult to optimize with a purely empirical approach. The equations presented here illustrate how these parameters interact, and which ones most influence the response of a CL analyzer under various operating conditions. With unlimited sample volume, eq 3 indicates the response
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is limited only by the pump or blower capacity. Condition 6a indicates that reagent mole fraction and chamber volume can compensate for a low value of kRD. However, only chamber pressure [MI can compensate for an unfavorable quenching half-pressure (eq 6b). Thus any CL process with known rate constants can he evaluated in a straightforward fashion. If the relevant rate constants are unknown, they may be measured routinely in experiments similar to those shown in Figures 2-4. This process will be described more fully (12). If only an empirical optimization is desired, the above prinicples will guide the investigatpr to an efficient approach.
LITERATURE CITED Wehry, E, L. Anal. Chem. 1982, 5 4 , 131R-150R. Ridley, B. A.; Howlett, L. C. Rev. Sci. Instrum. 1974, 45, 742-746. Kley, D.; McFarland, M. Atmos. Techno/. 1980, 12, 63-69. Kley, D.; Drummond, J. W.; McFarland, M.; Llu, S. C. J. Geophys. Res.1081, 86, 3153-3161. (5) Steffenson, D. M.; .Stedrnan, D. H. Anal. Chem. 1974, 4 6 , 1704-1709. (6) Mehrabzadeh, A. A.; O'Brlen, R. J.; Hard, T. M., manuscript In preparation. (7) O'Brlen, R. J.; Hard, T. M.; Mehrabzadeh, A. A. Envlron. Scl. Techno/., In press. (8) Nederbragt, (3. W.; van der House, A,; van Duljn, J. Nature (London) 1965, 206, 07. (9) Finlayson, B. J.; Pitts, J. N., Jr.; Atklnson, R. J. Am. Chem. SOC. 1974, 96, 5356-5367. (10) Pitts, J. N., Jr.; Finlayson, B. J.; Akimoto, H.; Kummer, W. A,; Steer, R. P. Adv. Chem. Ser. 1972, No. 113. 246-263. (11) Kelly, T. J.; Gaffney, J. S.; Phllllps. M. F.; Tanner, R. L. Anal. Chem. 1983, 55, 135-138. (12) Mehrabzadeh, A. A.; O'Brlen, R . J.; Hard, T. M., manuscript In preparation. (1) (2) (3) (4)
RECEIVED for review February 23, 1983. Accepted June 13, 1983. This work was supported, in part, by N.S.F. Atmospheric Chemistry Program Grant ATM 8003312, U.S. E.P.A. Office of Research and Development Grant R807733, and the Portland State University Research and Publications Committee. Although the research described in this article was funded in part by the U S . E.P.A., it has not been subjected to the Agency's required peer and administrative review and therefore does not necessarily reflect the view of the Agency and no official endorsement should be inferred.
Determination of Phosphorus by Gas-Phase Chemiluminescence after Hydride Generation Kazuko MatRumoto,* Kitao Fujiwara, and Keiichiro Fuwa Department of Chemistry, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
A continuous phosphlne (PH,) generatlon technlque was developed and applled to the senstlve detectlon of phosphorus by gas-phase chemllumlnescence wlth ozone oxldatlon (detection Ilmlt, 8 ng of PimL). Phosphate Ion In aqueous sohtlon was converted to phosphlne by passlng the sample mlst (produced by a ultrasonic nebullzer) through an incandescent carbon tube. No speclflc reducing reagent Is necessary for the reactlon. The method Is a rapld and slmple procedure wlth low contamlnatlon and high sensltlvity.
The hydride generation technique has contributed much to the iimprovement of the sensitivity in atomic absorption,
emission, and fluorescence spectrometries (1) of As, Sb, Bi, Se, Te, Ge, Sn, and Pb. We have successfully applied this technique to the molecular absorption spectrometry for ammonium-nitrogen (2). In an attempt to extend this hydride generation technique to other elements, we have corisidered its applicability to phosphorus: solubility of phosphine, PH,, in water is low (0.26 mL/mL of water at 17 "C) and bp is -87.7 O C , which means if phosphine is once produced in a sample solution, it can be easily evolved from the solution and is trapped in a liquid N2 bath like other hydride-forming elements. However, the conversion of phosphate (P043-)into phosphine requires a stronger reducing reagent than those used in usual hydride generation. The oxidation-reduction potential for a P043--PH032- couple is -1.12 V (vs. NHE),
0003-2700/83/0355-1665$01.50/00 1983 American Chemical Society
