crass section ratios. Lowering the gun voltage to increase oxygen sensitivity has been used in the tungsten oxide system (6). The minerals pyrite FeSz and molybdenite MoSz were analyzed to test the applicability of the oxide derived ur values to other materials. Assuming values for the relative scattering cross sections from the plot, the compositions were determined a t 1500 eV for *He. The pyrite composition was calculated to be FeS1.9, while that for molybdenite fell short of expectation a t MoSl,la. It would appear that a phenomenon other than those accounted for in the relative differential scattering cross section is occurring in molybdenite. The differential sputtering contribution in sulfides may differ from that in oxides. Further investigation is necessary in this area.
CONCLUSION This simple treatment presents an empirical approach to quantitative analysis by ion scattering spectrometry. With a simple correction factor, the relative scattering cross section, quantitative results can be drawn from the ISS data. However, as data have been taken only for oxides, caution must be exercised in extending this approach to other systems. Systems other than oxides, as well as glasses and powders, should be investigated so that reliable quantitative analysis can be performed on them in the future. Additional work should be done with various probe gases, such
as 20Ne and *OAr, and over a wider range of ion gun potentials so that a general theory can be determined. The ISS sensitivity for various elements is a simple function of atomic number with greatest sensitivity existing for the highest atomic number elements. The relative sensitivity of high atomic number elements as compared to low atomic number elements varies with the primary ion energy.
ACKNOWLEDGMENT Thanks are due to the members of the Ford Scientific Research Staff and especially to Jack Tabock for microprobe analysis of these samples, Joseph Sprys for electron diffraction, and John Bomback for helpful discussion.
LITERATURE CITED ( 1 ) D. P. Smith, Surface Scb, 25, 171, (1971). (2) F. W. Bingham, Sandia Corporation Research Report, SC-RR-66-506, TiD-4000 Physics (1966). (3) J. T. McKinney and J. A. Leys, Proceedings of 8th Conference on Electron Probe Analysis, New Orleans, La., 1973. (4) 0. Kubaschewski and B. E . Hopkins, "Oxidation of Metals and Alloys," 2nd ed., Academic Press, London, 1962. (5) "Columbium and Tantalum." F. T Sisco and E Epremian, Ed., John Wiley and Sons, New York, N.Y., 1963. (6) G. Nelson, Sandia Laboratories, Albuquerque, N.M., personal communication, 1973.
RECEIVEDfor review January 31, 1974. Accepted September 9, 1974.
Flame Emission Photometer for Determining Phosphorus in Air and Natural Waters Manfred J. Prager' Nucor Corporation, 2 Richwood Place, Denville, N.J. 07834
W. Rudolf Seitz2 NERC Corvallis, Environmental Protection Agency, Southeast Environmental Research Laboratory, Athens, Ga.30602
Phosphorus in natural waters can stimulate algal growth and lead to eutrophication ( I ). The standard analytical method for phosphorus is based on the reaction of orthophosphate with ammonium molybdate to form molybdophosphoric acid, which is determined colorimetrically ( 2 1. Other phosphorus fractions in natural waters are defined in terms of the operations used to treat the sample prior to analyzing for orthophosphate. Flame emission photometry is a possible alternative method that would not require converting phosphorus to orthophosphate. In a cool, fuel-rich, hydrogen-air flame, phosphorus forms excited POH, which emits a broad band spectrum peaking a t 526 nm. Drager and Drager ( 3 ) first proposed this method for the determination of phosphorus in air and invented a burner surrounded by a water-cooled jacket for exciting and observing the green emission. Brody and Chaney ( 4 ) constructed a detector based on this principle and used it to selectively measure phosphorus comP r e s e n t address, U n i t e d States Customs L a b o r a t o r y , 6 World T r a d e Center, New York, N.Y. 10048. Present address, D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of Georgia, Athens, Ga. 30602.
148
pounds separated by gas chromatography. Dagnall, Thompson, and West ( 5 ) and Syty and Dean (6) measured phosphorus in water using commercial spectrophotometers with sprayer-burners. The best sensitivity, 0.2 mg/liter phosphorus, was achieved with a nitrogen-hydrogen diffusion flame ( 5 ) . T o improve upon this performance by an order of magnitude, these investigators built a filter photometer and modified the burner. Further improvement in sensitivity to a detection limit of 0.014 mg/liter phosphorus required that the nebulization chamber be maintained a t about 900 "C in an electric furnace ( 7 ) .Flame photometry has been applied to the measurement of phosphorus in detergents (8, 9 ) , lubricating oils ( I O ) , and rocks (11 ). This paper describes the development of a flame emission photometer with improved sensitivity that may be used for the measurement of phosphorus in air or water. (By merely changing the interference filter, the instrument may be used for the determination of sulfur.) Increased sensitivity, required for natural water analysis, and applicability to both air and water samples have been achieved by appropriate design of the burner and the use of ultrasonic rather than pneumatic nebulization of water samples.
A N A L Y T I C A L CHEMISTRY, VOL. 47, N O . 1, J A N U A R Y 1975
L
ti YDROGEN
f
-
/
/
/
HIGH VOLTAGE POWER SUPPLY
0 i / O V AC
b d1 1
ASPIRATOR A I R , Iiter/mtn !-PHOTO
MULTIPLiER TUBE
Effect of varying air flow through aspirator on phosphorus and background emission for fixed hydrogen flows Figure 2.
0 - 0 - 0 Hydrogen flow 540 cm3/min. A - A - A Hydrogen flow 790 cm3/min. - W - Hydrogen flow 1070 cm3/min - - _ _ _Phosphorus emission.
Background emission COMPRESSOR VALVE'
li' VENl
Figure 1.
Flame emission photometer schematic
With ion exchange pretreatment to remove interfering cations. the photometer successfully measures dissolved phosphorus a t concentrations likely to be found in natural. waters. It, does not respond to particulate phosphate.
EXPERIMENTAL Photometer. Development, construction, and initial data obtained with the flame photometer have been described in detail elsewhere (22 ). The photometer is illust,rated schematically in Figure 1. The burner chamber is a hollow brass cylinder into which is inserted a piece of Teflon rod with a hole drilled into it to accommodate hypodermic needle tubing. Hydrogen passes through this tube and burns a t the end of it. The combustion air and sample enter the brass cylinder through a short length of stainless steel tubing screwed into the brass cylinder a t 90'. The flame is confined to the burner chamber by a restriction so that it is not directly viewed by t,he photomultiplier. The flame is pointed downward to facilitate water removal. Attached to the downstream end of the brass cylinder is a piece of Pyrex tubing through which the phosphorus emission is observed. The other end of the Pyrex tube is connected to an air aspirator. Compressed air is passed through the aspirator to reduce the pressure at the sample inlet and draw sample and ambient air into the burner. Part of the air is passed over the outside of the burner near the flame to cool it. This increases sensitivity and also reduces flame background. Flow rates are controlled by needle valves, and hydrogen flow is indicated on a Dwyer Model VFB flow meter. A Baird-Atomic interference filter with maximum transmittance at 525 iim and a 1.5-nm bandwidth a t half-peak transmittance allows the phosphorus emission to he selectively measured by an RCA 1P21 photomultiplier tube. The photomultiplier tube is operated a t 750 volts supplied by a Venus Scientific Model F-12 power supp!y. The photomultiplier current is measured by a Nucor Corp. electromrter and recorded on a IO-mV recorder. The flame is ignited by sparking between the flame tube and the brass cylinder burning housing. Nebulizer. Sample aerosol was generated ultrasonically by a Mistogen EN142 electrnnic nebulizer. The aerosol is blown out through a flexible plastic tube reduced to 0.62-cm i.d. a t the outlet. The outlet is aimed a t the sample inlet port of the photometer and positioned 4 cm away. When the aspirator air is turned on in the photometer, aerosol is drawn into the burner. The rate of aerosol generation varies from sample to sample (1.3). However, this does not affect ohserved emission intensity as long as the rate a t which aerosol comes from the nebulizer exceeds the rate a t which aerosol is drawn into the burner.
Sample can he added directly to the nebulizer (requiring 100150 ml for aerosol generation), or it can be added to cups that fit into the nebulizer vessel, thus reducing the minimum sample requirement,to 10 ml. Procedures. Interference studies and calibrations of emission us. phosphorus concentration in water were done by making standard additions t o 250 ml of solution in the nebulizer vessel. The cups were used t o measure response to phosphorus in different compounds, as well as for measuring the emission from river water spiked with known concentrations of soluble phosphorus. Because it takes 10-20 seconds for the aerosol to stabilize, the first 30 seconds of aerosol were blocked from the photometer. This led to sharper emission peaks and reduced the amount of water collecting in the sample inlet through aerosol condensation. When drops form in the sample inlet, they must he removed with a paper towel so that the quantity of aerosol reaching the flame is not reduced. Sample cups can he readily exchanged, and total time for one emission reading is approximately one minute. Reagents. Phosphorus standards were prepared by weighing phosphoric acid. Dilute standards (< 25 mg/l.) were prepared fresh daily. Twenty-five mesh Amberlite IR-120 ion exchange resin was used for removing metal ions. River water was spiked with soluble phosphorus using Nutrient Reference Samples provided by the EPA, Method and Performance Evaluation, Analytical Quality Control Laboratory, Cincinnati, Ohio. Phosphorus-containing vapors were generated by passing metered cylinder air through a short length of 6-mm Pyrex tube into which had been inserted a piece of filter paper wetted with triethyl phosphate. The saturated vapor of the sample emerged through a small orifice (hypodermic needle) and was drawn into the burner (14, 1 5 ) . The concentration of sample entering the instrument was estimated from the air flow rate over the triethyl phosphate and the vapor pressure and temperature of triethyl phosphate. The fact that the photometer responded linearly over a wide range to phosphorus-containing vapor generated in this way was taken to indicate that this method of vapor generation was satisfactory.
RESULTS AND DISCUSSION Flame Conditions. Figure 2 shows phosphorus emission intensity from H3P04 solutions and background intensity as a function of air flow through the aspirator for fixed hydrogen flows. At low aspirator air flow rates, increasing the flow increases the observed phosphorus emission, probably by drawing more aerosol into the flame. Increasing the aspirator air beyond a certain point, however, leads to a decrease in phosphorus emission and a sharp increase in background. The decrease in phosphorus emission occurs because the flame is no longer hydrogen rich (5, 6 ) . Because air flow to the burner varies inversely with hydrogen
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1 , J A N U A R Y 1975
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Table I. Emission Intensity per Unit Phosphorus for Different Compounds (Relative to Phosphoric Acid) Emission Compound
1.0 1.0 1.0 1.0 1.0 1.0 1.3 1.7
Triethyl thiophosphate
3.2
P
flow, the 'optimum hydrogen/air ratio occurs a t higher aspirator flow ratesfor higher hydrogen flow rates. Normal operating conditions for water samples were the following: hydrogen flow 800-1000 cm3/min; aspirator air, 8-9 l./min; and cooling air, 1 l./min. For air samples, the photometer is operated a t about 250 cm3 Hz/minute and 3 I./min aspirator air. The higher flow rates are required for water samples because extra energy is needed to vaporize the aerosol droplets entering the flame. Response t o Phosphorus. Emission intensity us phosphorus as phosphoric acid is linear from 0.003 mg/l., the detection limit, to 120 mg/l., the highest concentration tested. Figure 3 shows raw data over the range 0-0.15 mg/l. The emission for the deionized water blank is usually less than th. background emission in the absence of aerosol. Increasing the hydrogedair ratio increases the water blank relative to background. The response to phosphorus compound vapors in air samples is linear over the entire concentration range that was studied, 0.9 pg/l. to 2 mg/l. of phosphorus. The smallest concentration used is above the detection limit, but represents the lowest concentration that could be conveniently generated with adequate accuracy. Response to phosphorus is most sensitive immediately after the flame is first lighted, z.e., when the photometer is coolest. As the instrument warms up, the sensitivity for phosphorus decreases. A t least 15-minutes warmup is necessary to achieve sufficient stability to perform an analysis. A t the high hydrogen and air flow rates required for maximum sensitivity with water samples, prolonged continuous operation of the photometer (over one hour) leads not only to a gradual loss of sensitivity, but also to an increase in background emission. The background remains high when the flame is turned off, indicating' that heating of the photomultiplier tube is responsible. This effect is not observed a t the lower flow rates used for air samples. I t can be eliminated with water samples by working a t reduced flow rates. Nucor Corp. is presently modifying the detection system to eliminate this effect completely. Tahle I lists relative emission per unit phosphorus for several phosphorus-containing compounds. The more volatile phosphorus compounds give a larger signal than relatively involatile compounds. As aerosol reaches the burner, the volatile phosphorus compounds presumably vaporize faster and more completely in the flame than the nonvola150
Volatility
b p 215" 95"/10 mm bp 100"/16 mm
AHCH
V
Figure 3. Raw data showing phosphorus emission as a function of concentration from 0 to 0.15 mg/I. in 0.03 mg/l. increments
Intensity
Orthophosphoric acid Dibasic ammonium phosphate Polyphosphoric acjd Sodium pyrophosphate Metaphosphoric acid Sodium glucose phosphate Ru el ene Triethyl phosphate
tile compounds, leading to more efficient excitation of POH emission. A comparison of data obtained with air and water samples shows that the response to vapor phase triethyl phosphate is over ten times greater than the response to aqueous triethyl phosphate for a given amount of phosphorus reaching the flame per unit time. This indicates that differences in response per unit phosphorus for different compounds are due primarily t o volatility. Phosphorus in water does not normally occur in volatile compounds ( 1 6 ) ; this effect is not a problem for phosphorus analysis in natural waters. Some organic phosphorus compounds such as ruelene and triethyI thiophosphate are adsorbed fairly rapidly on the plastic medication cups. The cups used with adsorbing organic phosphorus compounds desorbed phosphorus slowly during subsequent use and had to be thrown out. Emission readings of these substances were taken immediately after the dilute solutions were prepared. Since significant concentrations of adsorbing phosphorus compounds were not expected in natural waters, other container materials were not investigated. Cation Interferences. Metal ions depress POH emission because the hydrogen-air flame does not have sufficient energy to excite the metal-phosphate salts that form as aerosol evaporates in the flame (5, 7-11). Calcium and magnesium depressed the emission from 1 mg/l. phosphorus as H3P04, by 85 to 95% a t a mole ratio of 1.5. Sodium reduced the phosphoric acid response less than 10%. When metal ions were added to water solutions of organophosphorus compounds, the phosphorus response was depressed much less. For example, calcium depressed the response from methyl parathion only about 15% and sodium had a smaller effect. Cation interference can he eliminated by pre-treatment of the solution to be analyzed with an ion exchange resin. Since particulate phosphorus in natural waters occurs as insoluble metal phosphates, usually with calcium or ion ( 1 6 ) ,it will not be excited by the hydrogen-air flame. S u l f u r Interference. The hydrogen-air flame efficiently excites sulfur S2 molecular emission peaking a t 394 nm ( 4 - 6 ) . Some sulfur emission occurs a t 525 nm, the wavelength of maximum phosphorus emission. As observed in the previous studies, emission intensity varies with the square of the sulfur concentration. The intensity of sulfur emission for a given sulfur concentration varies for different compounds ( I 7). In natural waters, the most common form of sulfur is sulfate. Sulfate is converted to sulfuric acid by the ion exchange resin that
A N A L Y T I C A L CHEMISTRY, VOL, 47. NO. 1 , J A N U A R Y 1975
~~~~~~~~~~
~
~
Table 11. Water Analysis Results
,Measured concn deion. water, m g i l .
XO.
Form of phosphorus
1 2O
Inorganic Inorganic
. . .b
0 . 05b
0.300
0.28 0.27
3O
Organic
0.170
4a
Organic
0.85
0.17 0.19 0.79
Sample
Yominal concn, m g / l .
~~
Measured
concn river water, m g l l .
0.038, 0 . 0 4 1 0.22 0.21 0.15 0.17 0.69 0.69
0.83
Recover?.,
I;
Source of river water
82 78
Filtered Oconee River Filtered Oconee River
88
Filtered Oconee River
85
Filtered Oconee River
For samples 2, and 3, and 4, t h e r i v e r w a t e r was run t w i c e u s i n g separate samples. T h e same s p i k e d deionized w a t e r was used f o r b o t h measurements. T h e r i v e r w a t e r b l a n k was e q u i v a l e n t t o a b o u t 0.04 m g / l . phosphorus.b W e r a n o u t o f q u a l i t y c o n t r o l s t a n d a r d before a r r i v i n g a t a s u i t a b l e a n a l y t i c a l procedure. D a t a are f o r a s y n t h e t i c s t a n d a r d .
LO: SULFUR
CONCFNTRATION. r n g l l i l e r
Figure 4. Selectivity of flame emission for phosphorus as a function
of sulfur concentration Selectivity is defined as (emission/gram phosphorus)/(emission/gram sulfur). The data shown are for phosphorus as H3P04 and sulfur as H2S04
removes metal ions. Figure 4 plots the relative emission intensities for phosphorus and sulfur as a function of sulfuric acid concentration. Above 5 mg/l., sulfur rapidly becomes a significant interference. If sulfur emission is suspected, its presence can be detected by diluting the sample to determine whether response is first or second order.
Emission from Soluble Phosphorus in River Water. The hydrogen-air flame will probably only excite POH emission from the dissolved phosphorus in natural waters. There will be little, if any, emission from particulate phosphorus. A spiking experiment was undertaken t o determine whether soluble phosphorus in river water would produce the same emission signal as the equivalent concentration of phosphorus in deionized water. Filtered Oconee River water was spiked with known concentrations of soluble phosphorus. Samples 1 and 2 contained inorganic phosphorus, and samples 3 and 4 contained organically bound phosphorus. A liter of river water was shaken with approximately two grams of ion exchange resin for about 30 seconds. The spike was added less than 10-15 minutes before running the sample to reduce the extent of reactions between the added phosphorus and particulate matter in the sample that got through the filter ( 1 8 ) .Sample was added to a nebulizer cup and introduced into the flame. A calibration curve was prepared from a deionized water blank and two H3P04 standards. Unspiked river water shaken with ion exchange resin served as a blank for the spiked river water.
Table I1 shows the results. The nominal concentration is taken from the literature accompanying the quality control samples used for spiking. The percentage recovery is the measured concentration of the spike in river water compared to the measured concentration of the same spike in deionized water. The emission from soluble phosphorus in river water is less than from soluble phosphorus in deionized water. This may be because some of the soluble phosphorus added to river water reacts to a particulate form that is not excited by the flame. The measured spike concentrations in deionized water agree with the nominal concentration within 10%. Sources of variation in measured phosphorus concentration include: variations in sensitivity with time, differences in the quantity and droplet size of the aerosol with different solutions, and uncertainties in peak intensity measurements.
ACKNOWLEDGMENT The authors thank George Yager of the Southeast Environmental Research Laboratory for help in setting up and running the photometer.
LITERATURE CITED (1) A. F. Bartsch, EPA Publication R3-72-001, Corvallis, Ore., 1972. (2) "Standard Methods for the Examination of Water and Wastewater," 13th ed.. American Public Health Association, Chicago, Ill., 1971, pp 518-534. (3) Draegerwerk. W. German Patent 1,133,918 (1962). (4) S. S. Brody and J. E. Chaney, J. Gas Chromatogr., 4, 42 (1966). (5) R. M. Dapnall, K. C. Thompson, and T. S. West, Analyst (London). 93, 72 (1968). (6) A. Syty and J. A. Dean, Appl. Opt., 7 , 1331 (1968). (7) K. M. Aldous, R. M. Dagnall, and T. S. West, Analyst (London),95, 1130 119701 - -, (8) A. Syty, Anal. Lett., 4, 531 (1971). (9) W. N. Elliot and R. A. Moslyn, Analyst (London),96, 452 (1971). (10) W. N. Elliot, C. Heathcote, and R. A. Moslyn. Talanta, 19, 359 (1972) (11) A. Syty, At. AbsorptlonNewsletf., 12, 1 (1973). (12) M. J. Prager, EPA Report R4-73-026 (1973). (13) J. Stupar and J. B. Dawson, Appl. Opt., 7 , 1351 (1968). (14) P. T. Gilbert, Anal. Chem., 38, 1920 (1966). (15) P. T. Gilbert, Final Report, Vol. II, Contract No. N-00140-67-C-1093, March, 1967. (16) J. K. Syers. R. F. Harris, and P. E. Armstrong, J. fnviron. Quality, 2 , 1 (1973). (17) R. M. Dagnall, K. C. Thompson. and T. S. West. Airalyst (London), 92, 506 (1967). (18) Y. S. R. Chen, J. N. Butler, and W. Stumn, Enwron. S o . Technol., 7 , 327 (1973). \
RECEIVEDfor review December 12, 1973. Accepted September 19, 1974. Part of the work reported here was performed under Environmental Protection Agency contract number 68-01-0111 with the NUCOR Corporation. Mention of commercial products does not imply endorsement by the Environmental Protection Agency.
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