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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
Photometric Detection of Oxygen Zbigniew Mielniczuk,’ Christopher G. Flinn, and Walter A. Aue’ 5637 Life Sciences Building, Dalhousie University, Halifax, N. S., Canada
T h e determination of molecular oxygen is a frequent but often difficult task in analytical chemistry. A wide variety of methods can be employed depending on the type of sample and analytical requirements. Often gas chromatography (1-3) is used prior to detection by thermal conductivity (4). Further detection approaches include coulometry ( 5 ) ,ionization by metastable helium atoms (6), capacitance (7), quenching of flame emission (8), flame ionization (9, 10) and others. I n this study we are exploring a different approach: the selective and highly sensitive detection of oxygen via its chemiluminescent reaction with phosphorus vapor. T h e emission of light in the oxidation of phosphorus has aroused the interest of scientists for more than a century (11) and the literature on the subject, especially the early literature, is voluminous (12). T h e greenish glow (the “cool flame”) can be observed when phosphorus vapor reacts with oxygen of a pressure within sharply defined upper and lower limits. The glow is generated by a complex isothermal branched-chain type reaction (13) and formulas are available for the calculation of critical oxygen pressures in relation to phosphorus and inert gas pressures as well as to reactor dimensions (14). Gilbert comments: “The glow of white phosphorus has been much studied b u t is still poorly understood.” (15).
EXPERIMENTAL “Prepurified” grade nitrogen or helium was further purified (a necessary procedure) by passage through a heated scavenger cartridge (Supelco, Bellefonte, Pa.) and used as carrier for both reagent and sample. One fraction of the carrier (10 mL/min) was bubbled through molten white phosphorus at ca. 50 “C; the other (10 to 30 mL/min, usually ca. 20 mL/min) was used to transport gaseous samples (from a six-port valve, or a gas chromatographic injection port) to a quartz tube situated in the viewing area of a photomultiplier, where the two gas streams met. Residual phosphorus vapor and any volatile reaction products were disposed of by connecting the detector exhaust tube directly to the exhaust duct of a fume hood; nonvolatile products were removed about once a week. The detector was kept at 100 “C (for chromatographic expedience; the chemiluminescence can be equally well observed at room temperature). Figure 1shows a schematic of the simple arrangement of the phosphorus doping vessel and chemiluminescencedetector (drawn approximately to size; the tubes in view of the PM tube are in. and 1/16 in. 0.d. The luminescent area, visually, is a few mm2). Parts of a Bendix photometric detector system (SPED electrometer, photomultiplier unit with EM1 9524 B tube, and random, epoxy-potted glass fiber light guide; but without interference filter) were used for the optical channel. An aperture of fixed size (ca. 0.2-mm diameter) was used when attenuation (-2OO:l) of the light became desirable. The detector assembly was carefully shielded from room light. The sampling valve was a Valco 6-port model, with various lab-made loops. The detector, and the sampling arrangement with sample reservoir and appropriate valves, were mounted on opposite sides of a Varian 1200 GC column bath. A gas chromatographic column (Linde 5A molecular sieve) could thus be used, but was bypassed in most experiments. Sample mixtures were prepared from purified nitrogen (or helium) either in a 400-mL reservoir equipped with a septum for injection of small volumes of atmosphere with a gas-tight syringe, or by adding a constant, very small stream of “high purity” grade nitrogen-whose oxygen content was determined as 14 ppm-to a larger carrier stream. The combined stream, as well as the reservoir, were under slight overpressure ‘Present address, Institute of Food and Nutrition, Warsaw, Poland. 0003-2700/78/0350-0684$01 .OO/O
and could be sampled by the six-port valve. Sampling valve loops ranged from 0.05 to 5 mL. In separate experiments, the spectra emitted by the cold flame of phosphorus were scanned by a grating monochromator (part of the Varian AA-5 unit), using an appropriately modified detector version, and good exhaust facilities (considerable amounts of O3 are formed in this reaction in addition to various phosphorus compounds).
RESULTS AND DISCUSSION T h e spectra emitted by the reaction of phosphorus with excess oxygen in the gas phase are shown in Figure 2; using both dry and humid oxygen. The spectra contain nothing new ( 1 5 ) but are interesting in the present context. T h e band system in the UV is the y system of PO. T h e origin of the continuum has not been established. The bands superimposed on the continuum in the presence of water are those of HPO. The light as seen by the P M tube in the detector proper will, of course, have a somewhat different spectral distribution. For one, the type of glass fiber light guide used in this study is known to attenuate somewhat S2 emission a t 394 nm but pass H P O emission at 526 nm. It is, of course, opaque to the P O bands in the UV. However, the dominant continuum fits well into the spectral range of both light guide and photomultiplier and, is, without much doubt, responsible for t h e detector’s response. Figure 3 shows a calibration curve for peak heights resulting from valve injections, without a GC column, of O2 as measured on the analytical setup over a period of one month, using different loops, sample streams, etc., as indicated by different data point representation. T h e line through the points was deliberately drawn at exactly 45”. T h e deviations, in our opinion, are primarily due to difficulties associated with the preparation and manipulation of samples rather than to the reaction in the detector itself. (Typical GC peaks showed similar linearity, both in peak height and area, but were tested only over a much more restricted range.) T h e upper part of the linear range can be extended by increasing the phosphorus supply as was shown in separate experiments. The lower limit of response is given essentially by photomultiplier noise; it could most likely be lowered considerably by using a geometrically optimized reaction chamber immediately bordering the photocathode of a cooled P M tube; and by a further decrease in the oxygen content of the carrier gas, e.g., by a preliminary reaction with a separate phosphorus source. Furthermore, an even supply of P4 vapor (no fluctuations due to bubbling N2 through liquid phosphorus) may be helpful. Even with the rather simple arrangement used, however, the minimum detectable amount of O2 was ca. 2 X lo-” g (extrapolated from the response of 9 X g to S:N = 2:l). The minimum detectable concentration depends, obviously, on flow conditions and sample size. We had little trouble detecting 0.1 ppm O2 and, with proper instrumental change and optimization (and calibration standards available), analysis in the lower ppb ranges should be possible. I t must be noted in this context, however, that many studies in the literature show chemiluminescence to occur only within a certain range of O2pressure. Even though most of these reports concern reactions involving phosphorus in solid rather than in vapor form, it would need to be experimentally proved that the minimum detectable amounts or concentrations can be lowered by appropriate instrumental changes. Our own 0 1978 American
Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
8 8 x I d ' l g O2
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1
Figure 1. Schematic of phosphorusdopingtube and chemiluminescent reaction zone
COLD FLAME SPECTRUM P V O S P Y 3 ? U S - 3 X V G E h GAS - PYASE
A
HPo
C PEMI 2JMIU ESCEYCE
A-TN
',
200
" ~'hv.+++v
+dC
2 80
360
440
"h. 520 600 WAVELENGTH nm
Figure 2. Spectra emitted by the reaction of phosphorus vapor with excess dry and humid oxygen. Upper spectrum offset
";
DETECTION
OXYGEN
A - ~ h i.05
Figure 4. Repeated valve injections at two oxygen levels as indicated. 0 125-mL sampling loop, static samples
SRY C X Y S E Y
-I
IO2
./"
in experimental conditions (temperature, N2 or He as carrier, excess of phosphorus) appeared to have little, if any, effect on detector performance. Judging from what is known about phosphorus chemiluminescence, the reaction could be assumed to be specific for the elemental forms of oxygen (including, perhaps, some peroxides). In our study, a few gases and a wide variety of volatile organics (hydrogen chloride, ammonia, carbon dioxide, methanol, acetic acid, ethyl acetate, nitroethane, acetone, n-propylamine, chloroform, carbon tetrachloride, l-bromobutane, chlorobenzene, pyridine, 2-heptene, diallyl ether, hexane, acetylacetone, diacetyl, thiophene, p-quinone and piazselenole) were injected without eliciting any significant response. However, the question of selectivity was not further investigated.
ACKNOWLEDGMENT We are most grateful to Shubhender Kapila for putting together the first prototype.
LITERATURE CITED (1) J . J a n i k , "Chromatography of Nonhydrocarbon Gases", in Chromatography", 3rd ed., E. Heftmann, Ed., Van Nostrand Reinhoid, New York, N.Y., 1975,p 882-914. (2) H. Hachenberg: "Industriii Gas ChromatographicTrace Analysis", Heyden 8 Son Ltd., London, 1973. (3) P. G. Jeffery and P. J. Kipping, "Gas Analysis by Gas chromatography", Int. Ser. Monogr. Anal. Chem., Vol. 17,2nd ed.,Pergamon Press, Oxford,
I _
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Figure 3. Calibration curve for oxygen (in nitrogen) as supplied by valve sampling of static samples as well as flowing stream mixtures of (purified) carrier and small amounts of "high purity" nitrogen (14 ppm 02). No GC column. Carrier from sampling valve to detector: ca. 30 mL/min (purified) N,. Sampling loops ranged from 0.05 to 5.0 mL. Values above 3 X lo-' g 0,: Direct injection of atmosphere by gas-tight syringe
opinion is that this should be possible without major difficulties. Figure 4 shows six successive (valve) injections in two regions of the calibration curve three orders of magnitude apart. The deviations are not greater than those to be expected from parts of the instrument other than the detector, e.g., recorder overshoot, PM tube noise, inconsistencies in turning of the sampling valve, etc. I t is obvious from the way this device was tested that it could be used either as a gas chromatographic detector or as a gas stream monitor. Response was well reproducible and no significant interferences were noticed. Various changes
1972. (4) H. Kern and M. Elser, Inrernational Symposium on Microchemical Techniques 1977",Davos, Switzerland. (5) G. Burton, A. B. Littlewood, and W. A. Wiseman. in "Gas Chromatography 1966",A. B. Littlewood, Ed., Eisevier, Amsterdam, 1967. (6) C. H. Hartmann and K. P. Oimick, J . Gas Cbromfcgr., 4 (5),163 (1966). (7) J. D. Winefordner, H. P. Williams, and C. D. Miller, Anal. Chem., 37, 163 (1965). (8) C.V. Overfield and J. D. Winefordner, J . Cbromatogr., 30,339 (1967). (9) B. A. Schaefer, J . Cbromatogr. Sci., 10, 110 (1972). (IO) P. Russev, T. A. Gough. and C. J. Wooliam, J . Cbromatogr., 119,461 (1976). ( 1 1 ) E. Newton Harvey, "A History of Luminescence", The American Philosophical Society, Philadelphia, Pa., 1957. (12) Gmelin "Phosphorus" Tell B (16).1964;Teil C (16),1965. (13) N. Semenoff: "Chemical Kinetics and Chain Reactions", Clarendon Press, Oxford, 1935,p 163. (14) F. S.Dainton and H. M. Kimberley, Trans. Faraday Soc., 46,629 (1950). (15) P. T. Gilbert. in "Analytical Flame Spectroscopy". R. Mavrodineanu, Ed., Macrnillan, Toronto, 1970,p 253.
RECEIVED for review November 9, 1977. Accepted January 3, 1978. This study was supported by NRC grant 9604, AC grant 6099, and a grant by the Faculty of Graduate Studies, Dalhousie University.
