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X-ray fluorescence determination of cobalt on polyurethane foam
7alanra. Vol. 28. pp. 437 to 440. 1981 Printed I” Great Britain. All rights reserved
0039-9 14018l/070437-04$02.00/0 Copyright 8 1981 Pergamon Press Ltd
X-RAY FLUORESCENCE DETERMINATION COBALT ON POLYURETHANE FOAM
OF
A. CHOW,G. T. YAMASHITAand R. F. HAMON Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada (Received 31 December 1980. Accepted 21 January 1981)
Summary-Although the extraction and preconcentiation of several species by oolyurethane foam have been reported, very few methods have included a direct determination of the component on the polymer. Cobalt thiocyanate is highly extractable by polyether polyurethane from solutions of 3M NH&I and 1M NH,SCN. After extraction of the cobalt the foam discs can be analysed by X-ray fluorescence, which gives a linear response for samples treated identically. Cobalt at levels as low as 0.05 ppm can be quantitatively extracted and cobalt determined in the presence of several other metal ions.
extraction and preconcentration of various metals from aqueous solution by polyurethane foam has received considerable investigation’*’ over the past few years because of the high extraction, large capacity and low flow-resistance. This method requires careful control of the solution composition to ensure efficient extraction and some degree of component separation. Several authors have used polyurethane foam as a preconcentration or separation method before recovery for analysis by atomicabsorption spectrometry or radiochemical analysis. Others have determined only the extraction capability of the foam, by determination of the metal concentration in the solution before and after extraction. The extraction of cobalt thiocyanate by polyurethane foam has been reported by Braun et al.,3-5 Hamon6*’ and Maloney et a/.’ This method has a high distribution coefficient and is a fairly rapid process which may be applied for a variety of metals. The direct determination of the extracted component in the polyurethane foam has seldom been reported. Radioiodine has been extracted from aqueous solutiong-’ ’ and from milk” into polyurethane foam holding a hydrophobic immobilizing phase containing iodine. These foams were analysed directly for iodine-131 by gamma spectrometry. Schiller and Cook13 investigated the extraction of ionized, colloidal and very finely precipitated gold with polyurethane foam. Although most of the results were based on measurements of gold-198 tracer left in solution, neutron-activation analysis of the gold-containing foam was used for a few samples. This required either a 4- or 12-day cooling period and although a very sensitive trace technique, did not give fully consistent results. Tanaka et ~1.‘~ extracted an alkylbenzenesulphonateCrysta1 Violet complex into polyurethane foam and estimated the concentration by direct visual comparison with a series of standards, or by spectrophotometry after recovery with methanol. Open-cell polyurethane foam loaded with various organic reagents has been used to detect metal ions The
437
directly by the colour formed.15 With this technique, a direct comparison could be made with extracted standards, or, with a foam column system, the length of the coloured region could be related to the original solution concentration. The determinations of zinc and lead with dithizone, copper with rubeanic acid, and cobalt thiocyanate with Amberlite LA-l were reported. Although neutron-activation analysis can generally be used for the direct determination of many elements isolated on polyurethane foam, the difficulties involved in standard preparation and in handling radioactive materials, plus the restricted availability of irradiation facilities, limit the general application of such a method. X-Ray fluorescence (XRF) equipment also has a wide range of application and is more commonly available to the analyst. The sensitivity of the particular analysis depends on the X-ray equipment available and the element sought. This paper describes the direct XRF determination of cobalt on polyurethane foam, after its extraction from thiocyanate solution. EXPERIMENTAL Apparatus
A Perkin-Elmer model 306 atomic-absorption spectrometer with a cobalt hollow-cathode lamp (Cathodeon Ltd.) was used to determine the cobalt extraction for the majority of the samples. The wavelength was set at 240 nm with a slit-width setting of 4 (band-pass 0.7 nm). Nickel, iron, copper, lead and zinc lamps were used as required. A Baird-Atomic model 530 gamma-spectrometer was used to measure the activity of 6oCo solutions. Direct X-ray analysis of the foam samples was performed with a New England Nuclear NER-496 X-ray fluorescence exciter system coupled to a Tennelec TC909 power supply. A Nuclear Semiconductor 512 amplifier, usually set at coarse gain 1, provided an increased signal for display on a Tracer Northern TN-1705 pulse hetght analvser. The NER-496 XRF exciter svstem used a 0.5-Ci 24’Am source distributed in a shielded annulus placed around the Si(Li) detector. Normally, each foam disc was placed on a 6.1 x 10-4-cm thick Mylar film 2 cm above
438
A. CHOW. G. T. YAMASHITAand R. F. HAMON
the beryllium window of the detector. One series of foams was analysed on an Applied Research Laboratories multichannel X-ray quantometer, with a tungsten X-ray tube. Polyurethane foams of the polyether type, 0.48 and 0.64 cm thick, were obtained from local commercial sources. The foams were squeezed in cylindrical flasks with Pyrex plungers closely fitted through the flask covers. Chromatography columns (25 mm outer diameter) with sinteredglass discs and plastic stopcocks were used for flowthrough extractions. Reagents A standard lOOO-ppm cobalt solution was prepared by dissolving 1.002 g of the metal (99.8% pure) in a little 6M hydrochloric acid, made up to 1 litre of solution containing 17, v/v of the acid, and diluted for use in standards and extraction solutions. Solutions of 5M ammonium chloride and 5M ammonium thiocyanate were prepared in water. All chemicals were of analytical reagent grade, except as noted, and the water used was distilled and demineralized. Procedure
The polyurethane foams were soaked in 1% hydrochloric acid for 15 hr with occasional squeezing, dried between clean paper towels, and then extracted in a Soxhlet with acetone for 6 hr. The foams were then squeezed and airdried and cut into circular discs by hand, with sharpened tubular bits. Initially, clean foam discs (approx. 3.5 cm in diameter) were placed in the cylindrical flasks with solutions containing ammonium thiocyanate and final cobalt concentrations of l-5 pg/ml. The preliminary studies showed that although the cobalt was not effectively extracted and thus did not provide a significantly sensitive method by XRF determination, the method was at least feasible. Previous work by Hamon6.’ has shown that high ionic strength improves the extraction of the cobalt-thiocyanate complex. Solutions containing 30 ml of 5M ammonium chloride, 10 ml of 5M ammonium thiocyanate, and appropriate volumes of lOO-ppm cobalt solution, were diluted with water to 50 ml to give final cobalt concentrations of 65 pg/ml in 3M ammonium chloride/lM ammonium thiocyanate. Smaller foam discs (about 1.8 and 2.3 cm in diameter) were squeezed continuously in these solutions for 30 min. After squeezing, the extracted samples and similar standards were analysed for cobalt concentration by atomic-absorption spectrometry. After the foams had been dried in air. each disc was centred on Mylar film held about 2 cm over the detector for X-ray analysis. The spectra were accumulated for 100 set and the peak height was integrated over the region from 6.6 x IO3 to 7.4 x IO3 eV. Each sample was integrated eight times (four times on each side), the values were averaged, and the average blank was subtracted to give the final count. The integrated counts obtained from each side of the foams showed a difference of less than 3%. A foam disc containing 250 pg of cobalt was placed at various heights. 1.8-2.5 cm, above the detector but there was no significant variation in the results. The effect of volume was studied with solutions of 50, 75. 100 and 150 ml containing 0.5 ml of IOO-ppm cobalt solution with 20 ml of a radioactive 6oCo solution in 3M ammonium chloride/l,%4 ammonium thiocyanate. Aliquots of 15 ml of each sample were counted to determine the initial 6”Co levels and then returned to their respective solutions, with a foam disc weighing 44 + 3 mg. After 10 min of continual squeezing the foams were removed to prevent continued extraction and l5-ml aliquots of solution were measured again for their activity. The aliquots and foams were then returned to their solutions for another 10 min of extraction and the procedure was repeated for a total extraction time of 30 min in the case of the SO- and 75ml solutions and of 60 min for the IOO- and
Table
1. Effect of time on extraction Extraction
Time, min 10 20 30 40 50 60
%
50 ml*
15 ml*
100 ml*
150 ml*
97.8 99.7 99.7
91.7 98.7 99.0
74.3 83.8 93.3 96.0 96.9 97.1
61.2 72.1 86.0 91.3 93.6 94.3
* Volume of l.O-ppm cobalt solution in 3M NH,CI/IM NH,SCN. extracted with 44 + 3 mg of polyurethane foam.
150-ml volumes. All activities were corrected for background. It is evident from Table 1 that cobalt is more readily extracted from smaller volumes, since after 20 min of squeezing. 99% was extracted from the 50- and 75-ml solutions while only 97% or less was extracted after 60 min from the larger volumes. To obtain quantitative extraction with these or greater volumes of solution would require more time or larger weights of foam. To determine the effective concentration range of this technique a series of solutions consisting of 50 ml of 3M ammonium chloride/lM ammonium thiocyanate containing 2.5-100 pg of cobalt and some 6oCo tracer were extracted for 30 min with foam discs approximately 2.3 cm in diameter (weighing 44 * 3 mg). The data showed that for this entire concentration range, 30 min of squeezing was sufficient time to yield 99.5 f. 0.2% extraction. Solutions with cobalt concentrations of 0.25-2.5 pg/ml but without radioactive tracer were similarly extracted. The atomic-absorption measurements showed a high degree of cobalt extraction for all the samples studied, and although the sensitivity of this determination at low concentrations was poor. the previous study with Wo had indicated a very high extraction. The calibration graph of the XRF data cs. cobalt concentration was linear, the equation being cps = 5.69 x initial Co concentration (pg/ml) + 0.75. Six foam discs 2.8 cm in diameter and 0.48 cm thick were squeezed in 50-ml solutions containing O-5 pg/ml cobalt and 3h4 in ammonium chloride and IM in ammonium thiocyanate. A second experiment used foams of 0.64 cm thickness. while all other conditions remained constant. Both series showed high extraction of cobalt, as evaluated by atomic-absorption spectrometry, and produced linear calibration curves for X-ray activity cs. cobalt concentration, as shown in Fig. 1. A set of five foam discs was prepared from 1-5 pg/ml cobalt solutions. Each of the discs from these solutions was counted separately by XRF for 100 sec. To observe the effect of foam thickness. the disc from the 5+g/ml solution was placed on top of each of the other foams in turn and the pair analysed by XRF. The X-ray data in Table 2(a) from both of these trials showed a linear variation with initial concentration. with a slight enhancement relative to the sum of the counts expected for the paired foams. This suggests that the geometry was not ideal and that thicker foams could be used somewhat more effectively for preconcentration and determination of the cobalt. Foam discs were used to extract cobalt from each of four similar solutions containing 5 pg mL. As well as bemg counted separately on both sides, the foams were stacked in combinations of two, three and four and counted. The data in Table 2(b) show an increasing deviation from the expected values as the number of stacked foam discs was increased, because of the decrease in efficiency of the counting geometry.
Determination
12 B A 6 -
L
0
I
2
3
4
439
of cobalt on polyurethane foam
5
was placed in a specially designed holder made from a 5 x 5 cm square of 1.5-mm thick plastic with a 2.3 cm (diameter) hole drilled in its centre. The hole was covered with Mylar film. which prevented any direct contact between the samples and the X-ray tube window. The air was evacuated from the sample chamber before readings were taken. The maximum scale reading (100 mV) was set for the foam from a 2.5+g/ml solution and all measurements were made in units of mV/50 sec. The results showed a linear relationship between cobalt concentration and X-ray measurements. The sensitivity was increased about 10 fold, as indicated by a signal-to-noise ratio of 2.2 for this system as compared to 0.2 for the XRF exciter system, both at the 0.25-ppm cobalt level. Passive extractions with a flow-through system could also be done by using chromatography columns with sintered-glass discs and foams of the same diameter as the column. With 0.64-cm thick foams having an average weight of 44 k 3 mg, 50-ml aliquots of 3M ammonium chloride/lM ammonium thiocyanate solutions with cobalt concentrations from 0.25 to 2.0 pg/ml were run through the foam twice at a Row-rate of approximately 1 ml/min. The extraction of cobalt was quantitative, as determined by atomic-absorption measurements, and the foams were quite suitable for XRF analysis.
C Cobalt 3 , ppm
Fig. 1. (0) 0.48~cm thick foams weighing 33.0 f 0.8 mg; (0) OH-cm thick foams weighing 44.4 f 0.9 mg, 50 ml of cobalt solution in 3M NH,CI/lM NH&N, extracted for 30 min. Copper, iron, lead, zinc and nickel at the I-pg/ml level were tested for interference in the analysis of SO-ml of I-&ml cobalt solutions which were 3M in ammonium chloride and IM in ammonium thiocyanate. Blanks containing 1 pg/ml of the potential interferents but no cobalt were also tested. The ammonium chloride used here was a technical grade and therefore its solution was premixed with ammonium thiocyanate solution (to give 3:l molar ratio) and passed through a column of polyurethane foam plugs in order to remove any excess of iron or zinc, as thiocyanate complexes. All extractions were done for 30 min with continuous squeezing. The X-ray fluorescence analysis was run with a coarse gain setting of 2 to spread the spectra for easier study, with integration between 6.7 x-10’ and 7.4 x 103eVm for cobalt,- 6.2 x IO3 and 7.4 x lo3 eV for iron. 7.4 x lo3 and 7.8 x 10s eV for nickel. 8.0 x IO3 and 8.4 x 10’ eV for copper, and 8.4 x lo3 and 9.0 x lo3 eV for zinc. No lead peaks were observed at these settings. The extraction of cobalt and each interferent was determined by atomic-absorption spectrometry. All samples examined showed 100% extraction of cobalt, with the extraction of the interferent elements ranging from 0% for nickel to 90% for iron. The determination of cobalt was unaffected by the presence of any of these metals. X-Ray fluorescence was also used in the secondary emission mode. The ““‘Am y-source was inverted to cause monochromatic X-rays to be emitted by the pure target metal and these X-rays were used to cause X-ray fluorescence of the sample. The targets used were molybdenum, tin, and dysprosium. Each target produced significantly lower X-ray counts for the cobalt samples of foam and a lower signal-to-background ratio than did direct irradiation of samples of foam. In addition to analysis with the energy-dispersive NER-496 exciter system, the 0.2525 pg/ml samples were run on an Applied Research Laboratories X-ray quantometer unit with a wavelength-dispersive function, to obtain higher sensitivity. The tube current was set at 27.3 mA and the tube voltage was 37.5 kV. Each foam sample
DISCUSSION
The direct determination of cobalt thiocyanate on polyurethane foam by X-ray fluorescence has been found to be both quantitative and qualitative. Results show linear calibration over the cobalt concentration range from 0.25 to 2.5 ppm. The lowest concentration studied, 0.05 ppm, gave 99% extraction over a 30-min period, which suggests that cobalt at even lower concentrations could be extracted provided that larger volumes of solution, thicker foams and longer extraction times were used. Use of an X-ray quantometer gave higher sensitivity, and direct analysis at the ng/ml level should be possible. Table 2. Effect of various foam combinations Original cobalt concentration, PPm
X-Ray fluorescence. cps Expected
I .o
(4
(b)
2.0 3.0 4.0 5.0 5.0 + 1.0’ 5.0 + 2.0 5.0 + 3.0 5.0 + 4.0 5.0 5.0 5.0 5.0 5.0 + 5.0 5.0 + 5.0 + 5.0 5.0 + 5.0 + 5.0 + 5.0
38.8 45.7 51.8 59.0 -
51.5 78.4 105.0
Found 6.8 13.7 19.8 27.0 32.0 44.0 52.3 58.3 65.3 25.4 26. I 26.9 26.6 46.5 56. I 62.4
Samples: 50 ml in 3M NH&I/lM NH.,SCN. Extraction time: 30 min. Foam size: 44 f 3 mg. * Lower concentration foam placed closest to detector.
440
A.
CHOW,G. T.
YAMASHITAand R. F. HAMON
Nickel, lead, iron, zinc, and copper do not interfere with the cobalt determination since their complexes are either unstable or not extracted onto the foam, as in the case of nickel and lead, or they can be electronically separated according to their XRF energies, as seen with iron, zinc and copper. The electronic separation shows that this technique can be extended to other metal thiocyanates. The capacity of the foam is a limiting factor, however, since deviations in completeness of extraction and linearity of response occur when the total amount of thiocyanate complexes extracted (including interfering thiocyanates as well as cobalt thiocyanates) exceeds the capacity of the foam. Several other metal complexes are known to be extractable by polyurethane foam and should also prove useful for direct determinations on the foam. Overall, the XRF method may be applied to a wide variety of elements preconcentrated by polyurethane foam, without need for a recovery procedure, and with few (if any) interferences. Acknowledgement-This work was financially supported by the Natural Sciences and Engineering Research Council of Canada.
REFERENCES
T. Braun and A. B. Farag, Anal. Chim. Acta, 1978, 99, 1. Analyst, 1979, 2. G. J. Moody and J. D. R. Thomas, 104,l. 3. T. Braun, A. B. Farag and M. P. Maloney, Anal. Chim. Acta, 1971, 93, 191. 4. T. Braun and A. B. Farag, ibid., 1978, 98, 133. 5. T. Braun and M. N. Abbas, ibid., 1980, 119, 113. of Manitoba, 6. R. F. Hamon, Ph.D. Thesis, University 1981. I. R. F. Hamon and A. Chow, Abstracts 60th CIC Conference, Montreal, Canada, 1977. 8. M. P. Maloney, G. J. Moody and J. D. R. Thomas, Proc. Chem. Sot. Anal. Div., 1977, 14, 244. 9. S. Palagyi and T. Braun, .I. Radioanal. Chem., 1979, 51, 267. 10. S. Palagyi and E. Bila, Radiochem. Radioanal. Left., 1978, 32, 87. 11. T. Braun and S. Palagyi, Anal. Chem., 1979, 51, 1697. Radiochem.~ Radioanal. 12. S. Palagvi and R. Markusova. Left., lG8, 32, 103. 13. P. Schiller and G. B. Cook, Anal. Chim. Acta, 1971, 54, 364. K. Hiiro and A. Kawahara, Bunseki 14. T. Tanaka, Kagaku, 1973, 22, 523; Chem. Abstr. 1973, 19, 87234. 15. T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 73, 301.
0039-9 14018l/070437-04$02.00/0 Copyright 8 1981 Pergamon Press Ltd
X-RAY FLUORESCENCE DETERMINATION COBALT ON POLYURETHANE FOAM
OF
A. CHOW,G. T. YAMASHITAand R. F. HAMON Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada (Received 31 December 1980. Accepted 21 January 1981)
Summary-Although the extraction and preconcentiation of several species by oolyurethane foam have been reported, very few methods have included a direct determination of the component on the polymer. Cobalt thiocyanate is highly extractable by polyether polyurethane from solutions of 3M NH&I and 1M NH,SCN. After extraction of the cobalt the foam discs can be analysed by X-ray fluorescence, which gives a linear response for samples treated identically. Cobalt at levels as low as 0.05 ppm can be quantitatively extracted and cobalt determined in the presence of several other metal ions.
extraction and preconcentration of various metals from aqueous solution by polyurethane foam has received considerable investigation’*’ over the past few years because of the high extraction, large capacity and low flow-resistance. This method requires careful control of the solution composition to ensure efficient extraction and some degree of component separation. Several authors have used polyurethane foam as a preconcentration or separation method before recovery for analysis by atomicabsorption spectrometry or radiochemical analysis. Others have determined only the extraction capability of the foam, by determination of the metal concentration in the solution before and after extraction. The extraction of cobalt thiocyanate by polyurethane foam has been reported by Braun et al.,3-5 Hamon6*’ and Maloney et a/.’ This method has a high distribution coefficient and is a fairly rapid process which may be applied for a variety of metals. The direct determination of the extracted component in the polyurethane foam has seldom been reported. Radioiodine has been extracted from aqueous solutiong-’ ’ and from milk” into polyurethane foam holding a hydrophobic immobilizing phase containing iodine. These foams were analysed directly for iodine-131 by gamma spectrometry. Schiller and Cook13 investigated the extraction of ionized, colloidal and very finely precipitated gold with polyurethane foam. Although most of the results were based on measurements of gold-198 tracer left in solution, neutron-activation analysis of the gold-containing foam was used for a few samples. This required either a 4- or 12-day cooling period and although a very sensitive trace technique, did not give fully consistent results. Tanaka et ~1.‘~ extracted an alkylbenzenesulphonateCrysta1 Violet complex into polyurethane foam and estimated the concentration by direct visual comparison with a series of standards, or by spectrophotometry after recovery with methanol. Open-cell polyurethane foam loaded with various organic reagents has been used to detect metal ions The
437
directly by the colour formed.15 With this technique, a direct comparison could be made with extracted standards, or, with a foam column system, the length of the coloured region could be related to the original solution concentration. The determinations of zinc and lead with dithizone, copper with rubeanic acid, and cobalt thiocyanate with Amberlite LA-l were reported. Although neutron-activation analysis can generally be used for the direct determination of many elements isolated on polyurethane foam, the difficulties involved in standard preparation and in handling radioactive materials, plus the restricted availability of irradiation facilities, limit the general application of such a method. X-Ray fluorescence (XRF) equipment also has a wide range of application and is more commonly available to the analyst. The sensitivity of the particular analysis depends on the X-ray equipment available and the element sought. This paper describes the direct XRF determination of cobalt on polyurethane foam, after its extraction from thiocyanate solution. EXPERIMENTAL Apparatus
A Perkin-Elmer model 306 atomic-absorption spectrometer with a cobalt hollow-cathode lamp (Cathodeon Ltd.) was used to determine the cobalt extraction for the majority of the samples. The wavelength was set at 240 nm with a slit-width setting of 4 (band-pass 0.7 nm). Nickel, iron, copper, lead and zinc lamps were used as required. A Baird-Atomic model 530 gamma-spectrometer was used to measure the activity of 6oCo solutions. Direct X-ray analysis of the foam samples was performed with a New England Nuclear NER-496 X-ray fluorescence exciter system coupled to a Tennelec TC909 power supply. A Nuclear Semiconductor 512 amplifier, usually set at coarse gain 1, provided an increased signal for display on a Tracer Northern TN-1705 pulse hetght analvser. The NER-496 XRF exciter svstem used a 0.5-Ci 24’Am source distributed in a shielded annulus placed around the Si(Li) detector. Normally, each foam disc was placed on a 6.1 x 10-4-cm thick Mylar film 2 cm above
438
A. CHOW. G. T. YAMASHITAand R. F. HAMON
the beryllium window of the detector. One series of foams was analysed on an Applied Research Laboratories multichannel X-ray quantometer, with a tungsten X-ray tube. Polyurethane foams of the polyether type, 0.48 and 0.64 cm thick, were obtained from local commercial sources. The foams were squeezed in cylindrical flasks with Pyrex plungers closely fitted through the flask covers. Chromatography columns (25 mm outer diameter) with sinteredglass discs and plastic stopcocks were used for flowthrough extractions. Reagents A standard lOOO-ppm cobalt solution was prepared by dissolving 1.002 g of the metal (99.8% pure) in a little 6M hydrochloric acid, made up to 1 litre of solution containing 17, v/v of the acid, and diluted for use in standards and extraction solutions. Solutions of 5M ammonium chloride and 5M ammonium thiocyanate were prepared in water. All chemicals were of analytical reagent grade, except as noted, and the water used was distilled and demineralized. Procedure
The polyurethane foams were soaked in 1% hydrochloric acid for 15 hr with occasional squeezing, dried between clean paper towels, and then extracted in a Soxhlet with acetone for 6 hr. The foams were then squeezed and airdried and cut into circular discs by hand, with sharpened tubular bits. Initially, clean foam discs (approx. 3.5 cm in diameter) were placed in the cylindrical flasks with solutions containing ammonium thiocyanate and final cobalt concentrations of l-5 pg/ml. The preliminary studies showed that although the cobalt was not effectively extracted and thus did not provide a significantly sensitive method by XRF determination, the method was at least feasible. Previous work by Hamon6.’ has shown that high ionic strength improves the extraction of the cobalt-thiocyanate complex. Solutions containing 30 ml of 5M ammonium chloride, 10 ml of 5M ammonium thiocyanate, and appropriate volumes of lOO-ppm cobalt solution, were diluted with water to 50 ml to give final cobalt concentrations of 65 pg/ml in 3M ammonium chloride/lM ammonium thiocyanate. Smaller foam discs (about 1.8 and 2.3 cm in diameter) were squeezed continuously in these solutions for 30 min. After squeezing, the extracted samples and similar standards were analysed for cobalt concentration by atomic-absorption spectrometry. After the foams had been dried in air. each disc was centred on Mylar film held about 2 cm over the detector for X-ray analysis. The spectra were accumulated for 100 set and the peak height was integrated over the region from 6.6 x IO3 to 7.4 x IO3 eV. Each sample was integrated eight times (four times on each side), the values were averaged, and the average blank was subtracted to give the final count. The integrated counts obtained from each side of the foams showed a difference of less than 3%. A foam disc containing 250 pg of cobalt was placed at various heights. 1.8-2.5 cm, above the detector but there was no significant variation in the results. The effect of volume was studied with solutions of 50, 75. 100 and 150 ml containing 0.5 ml of IOO-ppm cobalt solution with 20 ml of a radioactive 6oCo solution in 3M ammonium chloride/l,%4 ammonium thiocyanate. Aliquots of 15 ml of each sample were counted to determine the initial 6”Co levels and then returned to their respective solutions, with a foam disc weighing 44 + 3 mg. After 10 min of continual squeezing the foams were removed to prevent continued extraction and l5-ml aliquots of solution were measured again for their activity. The aliquots and foams were then returned to their solutions for another 10 min of extraction and the procedure was repeated for a total extraction time of 30 min in the case of the SO- and 75ml solutions and of 60 min for the IOO- and
Table
1. Effect of time on extraction Extraction
Time, min 10 20 30 40 50 60
%
50 ml*
15 ml*
100 ml*
150 ml*
97.8 99.7 99.7
91.7 98.7 99.0
74.3 83.8 93.3 96.0 96.9 97.1
61.2 72.1 86.0 91.3 93.6 94.3
* Volume of l.O-ppm cobalt solution in 3M NH,CI/IM NH,SCN. extracted with 44 + 3 mg of polyurethane foam.
150-ml volumes. All activities were corrected for background. It is evident from Table 1 that cobalt is more readily extracted from smaller volumes, since after 20 min of squeezing. 99% was extracted from the 50- and 75-ml solutions while only 97% or less was extracted after 60 min from the larger volumes. To obtain quantitative extraction with these or greater volumes of solution would require more time or larger weights of foam. To determine the effective concentration range of this technique a series of solutions consisting of 50 ml of 3M ammonium chloride/lM ammonium thiocyanate containing 2.5-100 pg of cobalt and some 6oCo tracer were extracted for 30 min with foam discs approximately 2.3 cm in diameter (weighing 44 * 3 mg). The data showed that for this entire concentration range, 30 min of squeezing was sufficient time to yield 99.5 f. 0.2% extraction. Solutions with cobalt concentrations of 0.25-2.5 pg/ml but without radioactive tracer were similarly extracted. The atomic-absorption measurements showed a high degree of cobalt extraction for all the samples studied, and although the sensitivity of this determination at low concentrations was poor. the previous study with Wo had indicated a very high extraction. The calibration graph of the XRF data cs. cobalt concentration was linear, the equation being cps = 5.69 x initial Co concentration (pg/ml) + 0.75. Six foam discs 2.8 cm in diameter and 0.48 cm thick were squeezed in 50-ml solutions containing O-5 pg/ml cobalt and 3h4 in ammonium chloride and IM in ammonium thiocyanate. A second experiment used foams of 0.64 cm thickness. while all other conditions remained constant. Both series showed high extraction of cobalt, as evaluated by atomic-absorption spectrometry, and produced linear calibration curves for X-ray activity cs. cobalt concentration, as shown in Fig. 1. A set of five foam discs was prepared from 1-5 pg/ml cobalt solutions. Each of the discs from these solutions was counted separately by XRF for 100 sec. To observe the effect of foam thickness. the disc from the 5+g/ml solution was placed on top of each of the other foams in turn and the pair analysed by XRF. The X-ray data in Table 2(a) from both of these trials showed a linear variation with initial concentration. with a slight enhancement relative to the sum of the counts expected for the paired foams. This suggests that the geometry was not ideal and that thicker foams could be used somewhat more effectively for preconcentration and determination of the cobalt. Foam discs were used to extract cobalt from each of four similar solutions containing 5 pg mL. As well as bemg counted separately on both sides, the foams were stacked in combinations of two, three and four and counted. The data in Table 2(b) show an increasing deviation from the expected values as the number of stacked foam discs was increased, because of the decrease in efficiency of the counting geometry.
Determination
12 B A 6 -
L
0
I
2
3
4
439
of cobalt on polyurethane foam
5
was placed in a specially designed holder made from a 5 x 5 cm square of 1.5-mm thick plastic with a 2.3 cm (diameter) hole drilled in its centre. The hole was covered with Mylar film. which prevented any direct contact between the samples and the X-ray tube window. The air was evacuated from the sample chamber before readings were taken. The maximum scale reading (100 mV) was set for the foam from a 2.5+g/ml solution and all measurements were made in units of mV/50 sec. The results showed a linear relationship between cobalt concentration and X-ray measurements. The sensitivity was increased about 10 fold, as indicated by a signal-to-noise ratio of 2.2 for this system as compared to 0.2 for the XRF exciter system, both at the 0.25-ppm cobalt level. Passive extractions with a flow-through system could also be done by using chromatography columns with sintered-glass discs and foams of the same diameter as the column. With 0.64-cm thick foams having an average weight of 44 k 3 mg, 50-ml aliquots of 3M ammonium chloride/lM ammonium thiocyanate solutions with cobalt concentrations from 0.25 to 2.0 pg/ml were run through the foam twice at a Row-rate of approximately 1 ml/min. The extraction of cobalt was quantitative, as determined by atomic-absorption measurements, and the foams were quite suitable for XRF analysis.
C Cobalt 3 , ppm
Fig. 1. (0) 0.48~cm thick foams weighing 33.0 f 0.8 mg; (0) OH-cm thick foams weighing 44.4 f 0.9 mg, 50 ml of cobalt solution in 3M NH,CI/lM NH&N, extracted for 30 min. Copper, iron, lead, zinc and nickel at the I-pg/ml level were tested for interference in the analysis of SO-ml of I-&ml cobalt solutions which were 3M in ammonium chloride and IM in ammonium thiocyanate. Blanks containing 1 pg/ml of the potential interferents but no cobalt were also tested. The ammonium chloride used here was a technical grade and therefore its solution was premixed with ammonium thiocyanate solution (to give 3:l molar ratio) and passed through a column of polyurethane foam plugs in order to remove any excess of iron or zinc, as thiocyanate complexes. All extractions were done for 30 min with continuous squeezing. The X-ray fluorescence analysis was run with a coarse gain setting of 2 to spread the spectra for easier study, with integration between 6.7 x-10’ and 7.4 x 103eVm for cobalt,- 6.2 x IO3 and 7.4 x lo3 eV for iron. 7.4 x lo3 and 7.8 x 10s eV for nickel. 8.0 x IO3 and 8.4 x 10’ eV for copper, and 8.4 x lo3 and 9.0 x lo3 eV for zinc. No lead peaks were observed at these settings. The extraction of cobalt and each interferent was determined by atomic-absorption spectrometry. All samples examined showed 100% extraction of cobalt, with the extraction of the interferent elements ranging from 0% for nickel to 90% for iron. The determination of cobalt was unaffected by the presence of any of these metals. X-Ray fluorescence was also used in the secondary emission mode. The ““‘Am y-source was inverted to cause monochromatic X-rays to be emitted by the pure target metal and these X-rays were used to cause X-ray fluorescence of the sample. The targets used were molybdenum, tin, and dysprosium. Each target produced significantly lower X-ray counts for the cobalt samples of foam and a lower signal-to-background ratio than did direct irradiation of samples of foam. In addition to analysis with the energy-dispersive NER-496 exciter system, the 0.2525 pg/ml samples were run on an Applied Research Laboratories X-ray quantometer unit with a wavelength-dispersive function, to obtain higher sensitivity. The tube current was set at 27.3 mA and the tube voltage was 37.5 kV. Each foam sample
DISCUSSION
The direct determination of cobalt thiocyanate on polyurethane foam by X-ray fluorescence has been found to be both quantitative and qualitative. Results show linear calibration over the cobalt concentration range from 0.25 to 2.5 ppm. The lowest concentration studied, 0.05 ppm, gave 99% extraction over a 30-min period, which suggests that cobalt at even lower concentrations could be extracted provided that larger volumes of solution, thicker foams and longer extraction times were used. Use of an X-ray quantometer gave higher sensitivity, and direct analysis at the ng/ml level should be possible. Table 2. Effect of various foam combinations Original cobalt concentration, PPm
X-Ray fluorescence. cps Expected
I .o
(4
(b)
2.0 3.0 4.0 5.0 5.0 + 1.0’ 5.0 + 2.0 5.0 + 3.0 5.0 + 4.0 5.0 5.0 5.0 5.0 5.0 + 5.0 5.0 + 5.0 + 5.0 5.0 + 5.0 + 5.0 + 5.0
38.8 45.7 51.8 59.0 -
51.5 78.4 105.0
Found 6.8 13.7 19.8 27.0 32.0 44.0 52.3 58.3 65.3 25.4 26. I 26.9 26.6 46.5 56. I 62.4
Samples: 50 ml in 3M NH&I/lM NH.,SCN. Extraction time: 30 min. Foam size: 44 f 3 mg. * Lower concentration foam placed closest to detector.
440
A.
CHOW,G. T.
YAMASHITAand R. F. HAMON
Nickel, lead, iron, zinc, and copper do not interfere with the cobalt determination since their complexes are either unstable or not extracted onto the foam, as in the case of nickel and lead, or they can be electronically separated according to their XRF energies, as seen with iron, zinc and copper. The electronic separation shows that this technique can be extended to other metal thiocyanates. The capacity of the foam is a limiting factor, however, since deviations in completeness of extraction and linearity of response occur when the total amount of thiocyanate complexes extracted (including interfering thiocyanates as well as cobalt thiocyanates) exceeds the capacity of the foam. Several other metal complexes are known to be extractable by polyurethane foam and should also prove useful for direct determinations on the foam. Overall, the XRF method may be applied to a wide variety of elements preconcentrated by polyurethane foam, without need for a recovery procedure, and with few (if any) interferences. Acknowledgement-This work was financially supported by the Natural Sciences and Engineering Research Council of Canada.
REFERENCES
T. Braun and A. B. Farag, Anal. Chim. Acta, 1978, 99, 1. Analyst, 1979, 2. G. J. Moody and J. D. R. Thomas, 104,l. 3. T. Braun, A. B. Farag and M. P. Maloney, Anal. Chim. Acta, 1971, 93, 191. 4. T. Braun and A. B. Farag, ibid., 1978, 98, 133. 5. T. Braun and M. N. Abbas, ibid., 1980, 119, 113. of Manitoba, 6. R. F. Hamon, Ph.D. Thesis, University 1981. I. R. F. Hamon and A. Chow, Abstracts 60th CIC Conference, Montreal, Canada, 1977. 8. M. P. Maloney, G. J. Moody and J. D. R. Thomas, Proc. Chem. Sot. Anal. Div., 1977, 14, 244. 9. S. Palagyi and T. Braun, .I. Radioanal. Chem., 1979, 51, 267. 10. S. Palagyi and E. Bila, Radiochem. Radioanal. Left., 1978, 32, 87. 11. T. Braun and S. Palagyi, Anal. Chem., 1979, 51, 1697. Radiochem.~ Radioanal. 12. S. Palagvi and R. Markusova. Left., lG8, 32, 103. 13. P. Schiller and G. B. Cook, Anal. Chim. Acta, 1971, 54, 364. K. Hiiro and A. Kawahara, Bunseki 14. T. Tanaka, Kagaku, 1973, 22, 523; Chem. Abstr. 1973, 19, 87234. 15. T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 73, 301.