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
1665
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 (1) (2) (3) (4)
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 prepa-
ration.
(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 prepa-
ration.
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
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
which is more negative than that for a H20-H2 couple (-0.828 V vs. "E). This means that phosphate cannot be reduced in aqueous solution by the usual reagents such as NaBH4 or Zn in acid solution and the reduction can take place only in a dry environment; however a strong reducing reagent may be used. In fact, all the synthetic methods of phosphine in literature are dry reactions, starting from compounds of P(II1) (3). Therefore, in order to reduce phosphate (PO:-) in aqueous solution, the reaction must start from desolvation of the sample solution. In our previous work, we studied phosphine generation coupled with ICP atomic emission spectrometry, where phosphate in aqueous solution was first precipitated by adding CaClz as calcium phosphate, which was then reduced to Ca3Pz by heating (ca. 1100 "C) together with aluminum powder (4). The detection limit of the method was 20 ng of P or 2 pg of P/mL. Although the detection limit expressed as absolute amount is considerably low, it is relatively high when expressed as concentration, since a commercial graphite furnace atomizer of AAS was utilized for the reaction and therefore the sample amount applied a t a time was only 10 pL. In addition, the method requires tedious reaction procedure. Considering the requirements for phosphine generation and for overcoming the drawbacks of the previous method, we have contrived a completely innovative hydride generation system, where phosphate solution is nebulized with a ultrasonic nebulizer and is continuously introduced into a flow-through carbon furnace. Phosphate is reduced by heated carbon to phosphine while passing through the furnace. The point of the system is that, although the reaction apparently seems to occur between phosphate solution and heated carbon, the real reactant is desolvated phosphate particle; i.e., the reaction proceeds under dry condition. The use of a flow-through furnace has made it possible to treat a milliliter amount of sample solution by continuously nebulizing it into the furnace. Gas-phase chemiluminescence with ozone oxidation is adopted for the detection of phosphine, which has already been proved to be highly sensitive to the hydrides of As, Sb, Sn, and Se (5, 6). In the present experiment, the chemiluminescence, yielded from the reaction of phosphine with ozone, is detected nondipersively by a photon counter. Although ICP would also be a sensitive and phosphorus-specific detector, we select the chemiluminescence detector; since the amount of phosphine produced in the present system is small, the latter detector is preferred because of its higher sensitivity. The detection limit of phosphorus is 8 ng of P / m L and the method was successfully applied to the determination of phosphorus in organic phosphate esters. EXPERIMENTAL SECTION Apparatus. A schematic illustration of the system is depicted in Figure 1. The flow-through carbon furnace was constructed based on the design reported in ref 7 : the carbon tube is 6 mm ad., 3.5 mm i.d., and 50 mm long. The voltage-regulating direct current power supply for the furnace is 600 A, 30 V, at maximum and was custom-made. The glass ultrasonic nebulizer chamber was laboratory-constructed. A ultrasonic generator from a commercial humidifier (KA-406, Toshiba Co.) is utilized for the nebulizer. The ozone generator is Model 0-1-2 from Japan Ozone Co., and the photon counter, Model C-1230from Hamamatsu TV Co., is used for the chemiluminescence detection with a photomultiplier tube, Model R649 from Hamamatsu TV Co., which is cooled at -20 "C with an electric cooling unit, Model C659-A, Hamamatsu TV Co. The output of the photon counter is recorded with a strip chart recorder, a Seiko Desktop Recorder. The ultrasonic nebulizer is designed so that sample solution is introduced into the nebulizer chamber with a peristaltic pump. After each measurement, the solution is discarded through an outlet with the aid of carrier gas flow. The nebulized solution is carried by He carrier gas into the furnace, where phosphate is reduced to phosphine by the incandescent carbon. The upper
ilr---
I
rn 141
1
U
E X H'AUST
Flgure 1. A schematic diagram of phosphine generation with the gas-phase chemiluminescence detection system: (1) He cylinder, (2) O2 cylinder, (3) ultrasonic nebulizer, (4) high-frequency generator, (5) flowmeter, (6) flow-through furnace, (7) water trap (immersed in dry ice-ethanol), (8) hydride trap (immersed in liquid N2), (9) reaction chamber, (10) PM housing (PM: Hamamatsu TV, Model R649), (11) peristaltic pump, (12) power supply, (13) ozone generator, (14) photon counter and PM power supply, ( 1 5 ) PM coollng unit, (16) strip chart recorder, (17) ozone trap (ascorbic acid), (18) cooling water. H e + PH,
4
exhaust
1
/I
Flgure 2. A schematic Illustration of the reaction chamber. The dimension units are millimeters.
end of the carbon tube is capped with a quartz funnel, which is placed upside down on the top surface of the furnace and is tightly bound to the furnace with rubber bands so that all the phosphine gas is introduced without leak into a water trap, which is immersed in a dry ice-ethanol mixture in order to remove water vapor from the sample gas. The phosphine is collected in a liquid N2 cooled U-shaped tube, which is 4 mm i.d., half packed with silica wool. All the components in the system are connected with PVC tubes as shown in Figure 1. After the phosphine is trapped, it is vaporized upon removal of liquid N2 and is introduced into the chemiluminescence chamber, where the reaction with ozone takes place. An excess of ozone is absorbed in a trap packed with ascorbic acid, as shown in Figure 1. The chemiluminescence reaction chamber is shown in Figure 2. The window facing the detector is made of quartz, but other parts are Pyrex glass. The chamber is covered with an aluminum cylinder in order to remove room light. The optimum flow rates of He and O2 depend on the dimensions of the chamber. If He flow rate is too small compared to that of 02,ozone gradually backflows into the He gas inlet and chemiluminescence takes place in the peripheral of the chamber. This situation largely decreases sensitivity. The chamber volume is closely related with optimum gas flow rates and is an important factor for obtaining high sensitivity. Chemicals. The phosphate solution was prepared from reagent grade potassium hydrogen phosphate dissolved in water. Reagents used for the test of other elements possibly producing hydrides in the present system were as follows: NaBH,, water glass TeCl.,, NaN02,NaN03, Na2S03,Na2S04,and (Na20/SiOz),As203, NH4Cl. Solutions for Ge, Sn, Pb, Sb, Bi, and Se were prepared by dissolving the metal with appropriate acid.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
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10 0 time(sec)
A temporal emission profile of gas-phase phosphine chemiluminescence.
Flgure 4. CURRENT ( A )
Flgure 3. The effect of the furnace current on the chemiluminescence
intensity. Procedure. Twenty milliliters of sample solution is placed in the ne'bulizer chamber. The phosphine trap is immersed in the liquid N2 Dewar and the furnace is turned on to red heat (-1500 "C). The sample mist produced in the nebulizer is introduced into the furnace. After phosphine is trapped for 1min, the ozonizer is started and the furnace is turned off in order to protect the carbon tube from deterioration. The phosphine trap is removed from liquid N2and is immediately immersed in water, which vaporizes phosphine more rapidly than just allowing the trap stand in air. The peak height of the chemiluminescence signal recorded on a st,rip chart recorder is used for calibration. Care must be taken throughout the experiment that ozone does not enter the furnace, which immediatelydeterioratesthe carbon tube: the situation that only ozone flows in the system without He flow easily leads to such carbon damage. It takes 2 min to measure a sample according to the procedure mentioned above. R E S U L T S AND DISCUSSION Optimization of Experimental Conditions. The dependence of the sensitivity on oxygen flow rate was examined from 600 to 1500 mL/min with He flow rate held constant at 200 mL/min. Since no significant change of the sensitivity was observed, oxygen flow rate was fixed a t 900 mL/min in the following experiments. Also the effect of the helium flow rate was studied while varying it from 25 to 240 mL/min. The sensitivity slightly increases with the increase of the flow rate; however, above 200 mL/min the inner pressure of the system becomes so high that the gas sometimes leaks from the interface of the furnace with the funnel. Accordingly, the helium flow rate was fixed at 200 mL/min. Although the effect of the furnace temperature must be studied, it was impossible to measure it, since the carbon tube is totally covered with a brass jacket, and consequently a pyrometer is useless for such case. Therefore, we express the sensitivity as a function of the furnace current and voltage. The result is ishown in Figure 3. Since the power supply regulates voltage, the current gradually decreases on repeated heatings, as the carbon tube gradually deriorates and its resistance increases. According to the current decrease, the sensitivity also varies. The overall change of the sensitivity during the life span of a tube is as follows. When the tube is new, sensitivity of the measurement is poor. But after several heatings, it starts increasing on every heating and finally rleaches a constant state. This constant state continues for about 20 heatings, after which sensitivity rapidly increases for a few heatings and the tube becomes so brittle that it finally breaks. The sensitivity dependence on the current is shown in Figure 3 and can be described as follows. The sensitivity is low when the current is less than 150 A (less than 1.6 V), but a t current higher than that, it increases rapidly
Table I. Relative Chemiluminescence Intensities of Inorganic Ions in Gas-Phase Chemiluminescence with Ozone Oxidation inorganic ion ~
0
~
P,Q,4B Si Ge Sn Pb AS
Sb Bi
Se Te NH,' NO,NO,-
so,,s0,z-
3
-
concn, d m L
re1 intens
0.2 0.2 20 20 20 20
100 100 5
20
0.2 20 20 20
20 20 20 20 20 20
0
0 6 0
95 0 0 0
280 25 20 15 50 50
with increasing current. Although the sensitivity still continues increasing, the tube is severely damaged when the current is higher than 250 A (3.5 V) and cannot be used in analytical work. Accordingly, the following experiments were carried out a t 250 A. Chemiluminescence of Phosphine with Ozone Oxidation. The reaction of phosphine with ozone produces a faint white emission. It is so weak that the spectrum could not be measured with a conventional monochromator system. Therefore, nondispersive measurement was attempted by using interference filters changed every 10 nm from 400 to 600 nm. The emission was detected throughout this range with a broad maximum a t around 480 nm. This emission spectrum seems different from that of HPO, which appears when phosphate solution is introduced into a flame (8). We carefully examined the spectrum region where HPO emission is expected (528 nm), but no significant peak was observed. Detection Limit a n d Analytical Calibration Curve. The calibration curve is linear from 100 ppb to 100 ppm. In the lower concentration region, the curve is still linear but the slope is slightly decreased. A temporal emission profile is shown in Figure 4. The detection limit expressed as a concentration corresponding to twice the standard deviation in five repetitive measurements of blank solution is 8 ppb. The reproducibility obtained from five measurements of 50 ppb solution is 9%. Chemiluminescence of O t h e r Inorganic Elements. Since the present system is based on nondispersive detection of the emission, the observed signal is not specific to phos-
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
P 0.5%
+
P
AS
8
6
AS
0.2%
1
0.5%
d
0.2 a
4
//
2
0
Aetent ion t i me I rnin . I
Figure 5. Separation of phosphine and arsine with Tenax OC column. The column was 50 crn long and 3 mrn i.d.
phorus. Accordingly, other hydride-forming elements may interfere with the phosphorus determination. The emission intensities of these elements and several anions, which may also produce hydrides in the present system, were examined. The results are summarized in Table I. Contrary to our expectations, most hydride-forming elements, except Te and As, do not give significant chemiluminescence signal. Tellurium gives slight emission, but the sensitivity is less than 5% that of phosphorus. Arsenic is the only element that shows sensitivity comparable with that of phosphorus and would probably cause severe positive interference in phosphorus determinations. Table I also demonstrates that the sensitivity does not depend on the chemical form, as illustrated by the pair-of Po43-and P2OT4-.The elimination of arsenic signal would be possible if a gas chromatographic system has been developed and evaluated. The details of the system are described in the following section. Separation of Analyte Emissions. In order to separate phosphorus emission from that of arsenic by differential volatilization, the hydride trap was left standing in the air to
warm up to room temperature. Although this procedure considerably decreased the warming up rate compared with that of immersing the trap in water, separation of the two analfie5 was still impossible. Therefore, in the next attempt, chromatographic separation was carried out with a 50 cm column (3 mm 4 ) packed with Tenax GC, 60/80 mesh. As Figure 5 shows, the two analyte emissions can be separated with this column. The chromatogram obtained when only phosphorus is introduced also shows that the phosphorus peak, previously considered as a single peak, actually consists of two peaks. Both peak heights are confirmed to be linearly dependent on phosphorus concentration, therefore representing two phosphorus species. The ratio of the two peaks is not affected by the existence of HC1, HzS04,or "OB. From a purely synthetic viewpoint, it is reported that small amounts of PzH4 and other phosphorus hydrides are always present as impurity in PH3, whatever synthetic method is employed (3). The two peaks may be due to these other phosphorus hydrides. We also attempted the separation with several other columns but Tenax GC gives the best result at present. The use of a commercial gas chromatograph would more completely separate the three peaks. Since arsenic peak overlaps on the second phosphorus peak a t present, the first peak is used for analysis. Application. The present method was applied to the determination of phosphorus in commercial reagents, ATP, and sodium p-nitrophenylphosphate. Appropriate amounts of both reagents were weighed and dissolved in water to a constant volume (final concentrations were ca. 100 ppb). The results are as follows: ATP, calcd 10.3%, found 10.3 f 1.0%; sodium p-nitrophenylphosphate, calcd 8.35%, found 8.30 f 0.86%. ACKNOWLEDGMENT The authors are indebted to K. Kawasaki of Shimadzu Go. for his helpful suggestion in constructing the flow-through furnace. Registry No. PH3, 7803-51-2; As, 7440-38-2; Te, 13494-80-9; phosphorus, 7723-14-0; carbon, 7440-44-0. LITERATURE CITED (1) Robbins, W. B.; Caruso, J. A. Anal. Chem. 1979, 51,889A-899A. (2) Takahashi, M.; Tanabe, K.; Saito, A,; Matsumoto, K.; Haraguchi, H.; Fuwa, K. Can. J. Spectrosc. 1980, 25, 25-28. (3) Brauer, G. "Handbook of Preparative Inorganic Chemistry"; Academic Press: New York, London, 1963; p 525. (4) Matsumoto, K.; Fuwa, K. BunsekiKagaku 1981, 30, 188-190. (5) Fujiwara, K.; Watanabe, Y.; Fuwa, K.; Winefordner, J. D. Anal. Chern. 1982, 54, 125-128. (6) Fraser, M. E.; Stedman, D. H.; Henderson, M. J. Anal. Chern. 1982,
54. . , 1200-1202.
(7) Molnar, C. J.; Winefordner, J. D. Anal. Chem. 1974, 46, 1419-1422. (8) Haraguchi, H.; Fuwa, K. Anal. Chem. 1076, 48, 784-786.
RECEIVED for review February 22, 1983. Accepted May 12, 1983. This work was financially supported by the Japanese Ministry of Education under Grant No. 56740228.
