Direct Determihation of Phosphorus by Atomic Absorption Flame Spectrometry G. F. Kirkbright a n d M a u r i c e Marshall Chemistry Department, lmperial College, London, U.K
t h e direct determination of phosphorus by atomic absorption spdctromet!y at its 177.5-, 178.3-. and 178.8-nm resonance lines using a nitrogen-separated nitrous oxideacetylene flame and a microwave-excited phosphorus electrodeless discharge lamp source is described. Phosphorus may be determined in aqueous solutibn as ophosphate with a sbnsitivity of 4.8 and 5.4 wg/ml (for 1% absorption) at the 177.5- and 178.3-nm lines; the corresponding detection limits obtained at these wavelengths were 29 and 21 y g / m l , respectively. The high temperature and relatively transparent nature of the fuel-rich flame make it a convenient atom cell for the atomization of phosphorus: no significant chemical or physical interferences have been observed at the levels investigated. The results of a preliminary investigation of the application of the method to the direct determination, of phosphorus in milk powder and yeast and beef extracts are described.
The determination of phosphorus by methods which involve measurement of the emission intensity from HPO molecules or, the phosphorus continuum in flames has been reported by several workers (1-4). Emission spectrometric methods for determination of phosphorus in solution based on excitation of phosphorus atoms produced in a high-frequency plasma source have also been described (5, 6). Little information is available, however, on the direct determination of phosphorus by atomic absorption spectrometry @AS). The characteristics of the groundstate lines of the phosphorus atom are shown in Table I. Sullivan and Walsh (7) reported the determination of phosphorus by AAS a t the 177.499-nm resonance line using a hollow cathode sputtering cell, while Khartsyzov and L’Vov (8) successfully determined phosphorus by AAS using an electrically heated graphite cuvette. The latter workers stated that the highest absorption sensitivity (for 1% absorption) was obtained a t the 177.499-nm line, although it is not clear whether all six ground-state lines of phosphorus were examined in this work. The energy available i n . a flame of moderate or high temperature should be sufficient to enable the formation of phosphorus atoms from phosphorus-containing species introduced into it. As the ground-state lines of phosphorus lie a t wavelengths less than 200 nm, where the atmosphere and most flames absorb very strongly, until recently i t has been assumed that only poor sensitivity could be obtained for phosphorus by AAS in flame cells owing to the assumed (1) S. S. Brody and J. E. Chaney, d. Gas. Chromatogr.. 4,42 (1966) (2) R. M . Dagnall. K . C. Thompson, and T. S. West, Analyst (London). 93, 72 (1968). (3) D. W. Brite, Anal. Chem., 26, 553 (1954). (4) A . Davis, F. J. Dinan. E. J. Lobbett, J D. Chazin, and L. E. Tufts, Anal. Chem.. 36, 1066 (1964). ( 5 ) R. H . Wendt and V . A . Fassel, Anal. Chem., 37,920 (1965). (6) G. W . Dickinson and V . A . Fassel, Anal. Chem.. 41, 1021 (1969). (7) A. Walsh. Proc. Collop. Spectrosc. l n t . . 70th. 13 (1963). (8) B. V . L’Vov and A . D. Khartsyzov, Zh. Prikl. Specfrosk.. 11, 413 (1969).
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necessity of employing the nonresonance lines above 200 nm for its determination. Thus, Manning and Slavin (9) reported the determination of phosphorus in the nitrous oxide-acetylene flame using the 213.547/213.620-nm linepair and the 214.911-nm line. These lines originate above the ground state in levels whose population is less than 1% of the total atomic phosphorus population; relatively low-sensitivity values of 290 and 540 pg/ml for 1% absorption a t 213.547/213.62 and 214.911 nm have been reported under these conditions (9). In order to determine phosphorus directly in a flame a t its ground-state lines below 200 nm, the flame employed must not only be hot enough to atomize phosphorus efficiently, but it must transmit radiation from the primary source a t these wavelengths. Further requirements for the direct determination a t these wavelengths are a suitable intense phosphorus atomic line source and a vacuum or inert gas-purged monochromator. Earlier papers from this laboratory have described the high transparency below 200 nm of the fuel-rich nitrogenshielded nitrous oxide-acetylene flame and its use for the direct .AAS determination of arsenic, selenium, sulfur, and iodine (10-12). This paper reports the use of this flame for the atomization of phosphorus and the transmission of radiation a t the ground-state lines emitted by a phosphorus microwave electrodeless discharge lamp (EDL) source to effect the direct determination of this element by AAS.
EXPERIMENTAL Apparatus. The instrumental assembly employed for AAS using a microwave-powered EDL source, nitrous oxide-acetylene slot burner with facilities for nitrogen shielding and Hilger and Watts E796 vacuum polychromator, in which provision is made for nitrogen purging of the optical path between these components, has been described previously (11). The detection system was also similar to that employed in earlier work (13) in which the output from the photomultiplier was led through a short coaxial cable (ca. 8 in. in length) via a 470-ohm load resistor to the input of a digital frequency meter (Model TSSA 7737A, Venner Electronics Ltd., New Malden, U.K.). AC mains power to the digital frequency,meter and E H T supply unit for the photomultiplier was filtered uia an RF mains filter. (Model L1829, Belling and Lee Ltd., Enfield, U.K.) to exclude the possibility of mains-borne interference. The frequency meter was provided with an adjustable pulse height trigger level, which in this study was set a t 60 mV to further discriminate against spurious pulses due to external ’interference. Manual adjustment of the “profile” control of the polychromator permitted wavelength scanning over the narrow spectral range encompassing the phosphorus 177.499-, 178.287-, and 178.768-nm lines. Two phosphorus atomic line sources were investigated. A phosphorus hollow-cathode lamp (S. J. Juniper and Co., Halstead, Essex) prepared from indium phosphide contained in a nickel-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
D . C. Manning and S . Slavin, At. Absorption Newslett.. 8, 132 (1969). G. F . Kirktiright and L. Ranson,Anal. Chem.. 43, 1238 (1971). G. F . Kirkbright and M . Marshall.Ana/.Chem.. 44, 1288 (1972). G . F. Kirkbright. T. S. West, and P. J. Wilson, At. Absorption Newslett., 11, 53 (1972). G. F Kirkbright, M . Marshall and T. S. West, Anal. Chem., 44, 2379 (1972).
Table I . Characteristics of Principal Resonance Lines of Phosphorus Atom Wavelength, nm
167.168 Transition
Lower energy level, cm-1 Upper energy level, cm-
167.461
167.971
177.499
3sZ,3p3-35,3p4
178.287
178.768
3p3-3p2 (3P) 4s
0
0
0
0
0
0
gia,b gkb"
59820 4 2
56090 4
6
4
0.23
59535 4 6 0.68
56340 4
fiebd
59716 4 4 0.45
0.154
0.102
55939 4 2 0.051
>50%
>50%
>50%
125%
125%
125%
Uncertainty in f i k valueb
a g i = statistical weight of lower state. bValues taken from NBS Monograph 22, Atomic Transition Probabilities, Vol. II. 'gk weight of upper state. a f i k = oscillator strength for the transition.
cathode shell and filled with neon a t a pressure of CQ. 5 Torr was operated by a conventional power supply (Techtron Pty, Melbourne, Australia). Electrodeless discharge lamp sources were prepared from either Vitreosil or Spectrosil tubing (8-mm bore, Thermal Syndicate Ltd. U.K.) blanks which were carefully outgassed before use by heating under vacuum. EDL sources were prepared containing red phosphorus and phosphorus pentoxide a t a pressure of between 2 and 4 Torr of argon. The three-quarter wave resonant cavity employed for microwave excitation of the EDL sources (Model 210L, Electromedical Supplies Ltd, Wantage, U.K.) was modified by the provision of external water cooling to remove excess heat. Reagents. A stock solution containing 5000 pg/ml of phosphorus was prepared by dissolution of analytical reagent grade diammonium hydrogen o-phosphate (5.33 g) in 250 ml of distilled water. This solution was then diluted as required for use. Stock solutions of the diverse ions used in the interference studies were also prepared from analytical reagent grade salts of the elements.
RESULTS Spectral Source Characteristics. Although emission from the excited-state lines of phosphorus was detectable, the phosphorus resonance lines below 200 nm from the hollow-cathode lamp source could not be observed a t sufficient intensity to permit their use for AAS with the existing optical arrangement. EDL sources were successfully prepared from both red phosphorus and phosphorus pentoxide. Experiments made to systematically vary the conditions within these sources led to the conclusions that sources prepared using red phosphorus gave higher line emission intensities than those prepared from phosphorus pentoxide, a very small charge of red phosphorus (ca. 100 pg) produced the most intense and stable sources, an argon filler gas pressure of 3 Torr gave best results, both Vitreosil and Spectrosil silica tubing was suitable, and the sources require careful tuning in the three-quarter wave cavity using a reflected power meter. In operation for AAS, the EDL sources prepared from red phosphorus were initiated a t high power (60 W ) , the applied power was increased to maximum (200 W), and the cavity tuning stub was adjusted to minimize the reflected power (ca. 20 W). The applied power was then reduced stepwise, retuning the cavity a t each reduction, to the optimum operating power (ca. 60 W). The preset channels of the spectrometer employed did not permit examination of the source emission at the 167.168-, 167.461-, and 167.971-nm phosphorus lines; it is also improbable that the optical arrangement used would allow transmission of radiation a t these wavelengths. The profile control of the spectrometer was used to scan over a narrow wavelength range (approximately i1 nm) centered
= statistical
Table II. AAS Data for Phosphorus Wavelength, nm
Relative emission intensity of source, arbitrary units Flame transmission factor, % Sensitivity for phosphorus, ppm P/1% absorption Detection limit for phosphorus, ppm for 50% rel. std. dev.
177.499
178.287
17a.768
1
5.6
8.1
28
56.5
4.8
29
5.4
21
64
8.8
37
on 178.3 nm. Only three atomic lines were observed from the phosphorus EDL sources in this range, a t wavelengths appropriate for their identification as the P 177.5-, 178.3-, and 178.8-nm lines. Insignificant background emission was detected a t wavelengths between these lines; this may, however, result partly from the narrow fixed spectral band-width (0.03 nm) of the spectrometer. The relative emission intensities of the three phosphorus lines from the EDL source were measured with no flame burning but with nitrogen flowing through the burner shielding device to provide a transparent light path to the monochromator. The values of the relative intensities obtained are shown in Table 11. The most intense line observed is that at 178.8 nm, whereas it might be expected that the 177.5 nm line would show greatest intensity. Although the relative source emission intensities in Table I1 are uncorrected for variation in the photomultiplier response with wavelength, this correction is very small over the narrow wavelength range between 177.5 and 178.8 nm. Any theoretical prediction of the intensity ratios assumes thermal equilibrium within the source; this is unlikely to be the case. In addition, if any residual oxygen is present in the optical path, the 177.5-nm line would be absorbed more strongly than the other two lines. Flame Transmission Characteristics. No significant radiation intensity from the EDL source was observable a t the phosphorus resonance lines unless the spectrometer was evacuated or purged with nitrogen. Similarly, the background absorption of the nitrous oxide-acetylene flame used without nitrogen was very great a t these wavelengths; the transmission of the unseparated flame was negligible a t 177.5 nm and only 1-2% a t the 178.3- and 178.8-nm lines. As expected from our earlier work with
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W
a
3.6
3.7
3.8
3.9
4.0
4.1
ACETYLENE (tltar~/mln)
Figure 1. Effect of acetylene flow rate on background transmission of nitrogen-separated nitrous oxide-acetylene flame (1) At the phosphorus 177.5-nm line, (2) at the phosphorus 178.3-nm line, (3) at the phosphorus 178.8-nm line. The nitrous oxide flow rate equals 7.2 I./min
sulfur ( I I ) , however, the fuel-rich nitrous oxide-acetylene flame is relatively transparent a t the phosphorus resonance lines when nitrogen shielding is employed. Figure 1 shows the percentage of transmission obtained a t different fuel flow rates, and Table I1 lists the maximum percentage of transmission observed in the slightly fuel-rich flame a t each line. The 100% transmission in each case refers to the intensity recorded without the flame but with the optical path purged with nitrogen and nitrogen flowing through the burner shield a t the optimum flow rate for flame separation. No background emission from the flame was detected a t the phosphorus resonance lines examined. Atomic Absorption Characteristics of Phosphorus. Absorption signals were observed a t each of the three phosphorus resonance lines studied when o-phosphate solutions were introduced into the nitrogen-separated nitrous oxide-acetylene flame via the indirect nebulizer and spray chamber assembly. That these signals were attributed to atomic absorption by phosphorus atoms was indicated by the absence of such a signal when species other than those containing phosphorus were introduced into the flame and by the absence of any absorption a t the 180.7-nm sulfur resonance line when phosphorus-containing species were nebulized. The optimum acetylene flow rates for highest sensitivity (for 1% absorption) did not differ significantly from those required to produce maximum flame transmission a t each wavelength, and these flow rates were therefore used in all further work. The absorbance signals were proportional to the concentration of phosphorus, and linear calibration graphs were obtained over the ranges 80-600 pg/ml a t 177.5 nm, 40 to 400 pg/ml. a t 178.3 nm, and 80 to 600 pg/ml a t 178.8 nm. The corresponding sensitivities (for 1% absorption) were calculated a t each wavelength from the mean absorption produced on repetitive nebulization of 200 pg/ml phosphorus solutions. Similarly, the detection limit attainable a t each line with our experimental assembly, defined as that concentration of phosphorus in solution which produced a relative standard deviation of 5070, was calculated from repeated absorbance measurements on a solution containing 40 pg/ml of phosphorus ( i e . , near the limit of detection). The values of the sensitivity and detection limit obtained are shown in Table 11. No flame emission from phosphorus was observed a t the wavelengths studied. 1612
It is evident from Table I1 that although somewhat greater sensitivity is attainable at the 177.5-nm line than a t the 178.3-nm line, the detection limit is significantly better at the 178.3-nm line. This effect most probably results from the lower standard deviation in the signal intensities received a t the detector with the more intense 178.3-nm line; allowing for the difference in flame background absorption a t the two lines, the intensity ratio I( 178.3):Z(177.5) received a t the photomultiplier from the EDL source is ea. 15:l. Random air entrainment and flame turbulence would also give rise to greater noise levels a t 177.5 nm than a t 178.3 nm. From a practical analytical viewpoint, the line which gives the lowest detection limit, that is 178.3 nm, is likely to be the most useful. Justification of this choice of the phosphorus 178.3-nm line may also be made by examination of the values of the relative standard deviation a t the 1 7 7 5 , 178.3-, and 178.8-nm lines a t a concentration within the working range for each. Thus, when a 200-pg/ml phosphorus solution was nebulized, the following percentage of relative standard deviations in absorbance were obtained: 177.5 nm (9.5%); 178.3 nm (1.8%),and 178.8nm (20.5%). Interference Studies. The effect of the presence of foreign ions on the determination of phosphorus by AAS a t 178.3 nm was investigated. An ion was considered to interfere when it produced a change in absorbance of greater than three times the standard deviation obtained in the absorbance produced for a pure phosphorus solution of the same concentration. No significant chemical or physical interference was observed in the absorbance signal produced a t 178.3 nm by 150 pg/ml phosphorus (as orthophosphate) in aqueous solution in the presence of 40-fold weight excesses of the following ions: Al, Ba, Ca, Co, Cr, CU, K, Li, Mg, Mo, Na, Ni, Zn, chloride, bromide, fluoride, nitrate, sulfate, borate, carbonate, and EDTA. Iodide was observed to cause considerable enhancement in the phosphorous absorbance at 178.3 nm; nebulization of a potassium iodide solution alone gave rise to a similar high absorbance at 178.3 nm. No interference was observed from iodide, however, on the determination of phosphorus a t 177.5 or 178.8 nm. The interference thus appears to be spectral in origin rather than a chemical or physical effect. on the phosphorus atomic concentration in the flame. Examination of the atomic spectrum of iodine, however, shows the existence of a resonance line a t 178.276 nm. The separation between this line and the phosphorus 178.287nm line is rather greater than that expected t o give rise to significant overlap when a sharp line source is employed. I t is possible that there is some error in the wavelength assigned for this line, as its measurement is difficult in the vacuum ultra-violet region. Additionally, it is possible that the emission profile of the phosphorus line from the EDL source a t 178.287 nm is broadened considerably by self-absorption; this would result in greater overlap with the iodine 178.276-nm line, particularly if the absorption profile of this line is also broadened significantly at high optical density in the hot nitrous oxide-acetylene flame. Accuracy. The accuracy of the direct AAS method for the determination of phosphorus was tested by its determination in samples of milk powder, beef extract, and yeast extract whose phosphorus content had previously been determined by absorptiometric measurement of reduced phosphomolybdic acid (molybdenum blue) after wet ashing of the samples. Weighed samples of each (between 3 and 5 g) were dissolved in hot distilled water and the solutions were diluted to 100 ml. The phosphorus content of these solutions was then determined on 40-ml aliquots by making standard additions of o-phosphate solution (equivalent to 100 pg/ml of phosphorus), diluting to
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9. AUGUST 1973
Table I l l . Determination of Phosphorus In Some Foodstuffs
Molybdenum blue method after wet ashing
Direct AAS method Sample
Yeast extract Beef extract Milk
powder
Results, %
1.43,1.32, 1.27 0.73a,0.704. 0.749 1.00,1.12, 1.13
Mean f std. dev., %
Results. %
Mean f std. dev.. YO
1.34f 0.08
1.32,1.34, 1.31,1.19 0.714,0.726, 0.740,0.696 1.02,0.975, 0.991,0.955
1.29 f 0.06
0.73f 0.02 1.08 f 0.07
100 ml with distilled water, and recording the absorbance a t 178.3 nm when the solutions were nebulized into the flame. The phosphorus content in the samples was then calculated from the absorbance produced by the sample solution to which no standard addition of phosphorus had been made. The results obtained are shown in Table I11 and are in good agreement with those obtained for the same samples by wet ashing and solution absorptiometry. Although the precision obtained for these analyses by the AAS method is somewhat lower than by the absorptiometric procedure, this reflects only the detection limits attainable with our particular experimental assembly. The AAS procedure is considerably more rapid than the solution absorptiometry method for these water-soluble samples, as it is not necessary to effect preliminary wet ashing.
DISCUSSION The preliminary study reported here indicates that the direct determination of phosphorus (as orthophosphate) by AAS may be made a t 178.3 nm in the nitrogen-separated nitrous oxide-acetylene flame with freedom from chemical and physical interferences from other ions present in the solution. Useful analytical sensitivity (for 1% absorption) is obtained a t each of the three phosphorus resonance lines examined. These sensitivity values will probably not be significantly improved as long as an EDL source and nitrous oxide-acetylene flame are employed for the determination. The detection limits, however, depend critically on the instrumentation employed and may be
0.72f 0.02 0.99 h 0.03
subject to considerable improvement with further refinement of the optical and electronic assembly. The spectral overlap interference observed for iodine on the phosphorus absorption signal gives rise to the possibility of the determination of phosphorus using a n iodine EDL source. This technique may prove advantageous, as EDL sources for iodine which exhibit greater intensity and stability than those for phosphorus may be prepared. Phosphorus is an essential nutrient and must frequently be determined in foodstuffs and other samples of agricultural and clinical origin. The convenience of the direct atomic absorption procedure for these samples lies in its rapidity. The application of this technique should also prove advantageous for determination of phosphorus in samples where the need for selectivity is greater than that for sensitivity, e.g., fertilizers, ores, slags, rocks, alloys, and detergents. The application of the method reported here to these determinations is currently being studied and will be reported in a later publication
ACKNOWLEDGMENT We wish to thank Rank, Hovis, McDougall (Research) Ltd. for the provision of analyzed samples and the British Steel Corp. for the loan of some of the equipment used in this work. Received for review October 18, 1972. Accepted January 15, 1973. We are grateful to the Science Research Council for the award of a CAPS studentship to one of us (M. M.).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, A U G U S T 1973
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