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Rapid ultra-trace determination of beryllium by gas chromatography
Talaata 1968. Vol. 15. Pp. 87 to 94. Pcrgamon Press. Printed in Northern Ireland
RAPID ULTRA-TRACE DETERMINATION OF BERYLLIUM BY GAS CHROMATOGRAPHY WILLIAM
D. Ross
Monsanto Research Corporation and Dayton, Ohio 45407, U.S.A. and ROBERT E. SIEVER~ Aerospace Research Laboratories, ARC, Wright-Patterson Air Force Base, Dayton, Ohio 45433, U.S.A. (Received 23 May 1967. Accepted 10 JuIy 1967)
Summary-The electron capture detector has been used to measure ultra-trace quantities of beryllium separated as berylli~(II) trifhroroacetylacetonate by gas chromatographic techniques. The lower limit of detectability is co. 4 x 10-r*g of beryllium. Calibration plots extend from 8 x 10-l’ to 4 x lo-l1 g of beryllium. Samples of beryllium in aqueous solution at four concentrations (l-18 x 10-t, l-18 x HYB, 295 x lO+, and 8.84 x 10-log/ml) were analysed quantitatively by combining solvent extraction and gas chromatography. The distribution of beryllium during the extraction procedures was determined independently by use of radioactive beryllium-7, but the use of tracers is not required in the recommended procedure. Interference studies were made on cations and anions found in biological samples. At the concentrations used in the extraction procedure and the gas chromatographic process, none of the fifteen ions studied interferes appreciably.
THE analysis of metals by gas chromatography has been the subject of recent intense research activity as a result of the remarkable sensitivity, simplicity, and speed of the technique. l-ls In addition, interferences by co-occurring elements are much less frequently encountered in the gas chromatographic technique than in other methods for metal analysis, because the components of the sample are simultaneously separated and measured. The utilization of fluorinated ligands offers significant advantages over the use of the protonated analogues for the formation of volatile metal chelates. The volatility and the excellent response of the electron capture detector for metal complexes derived from trifluoroacetylacetone, the anion of which may be depicted as cFJ_c~=~~ *
I
TFA
make this compound one of the most promising chelating agents in quantitative analyses.l*s-u The ease of quantitative extraction with this complexing agent is another attractive quality.6-7~10*20M orie and SweeP and Moshier and Schwa&erg’ have demonstrated that samples of alloys, and aqueous solutions of mixtures of aluminium, gallium and indium, can be analysed by gas chromatography of metal trifluoroacetylacetonates formed by solvent extraction. In these studies, the sensitivity of the methods developed was limited by the type of detector (thermal conductivity) employed. If electron capture detectors are used, the method becomes much more sensitive. Indeed the technique is more sensitive for some elements than are neutron-activation, atomic-absorption, or other classical methods now in use. 87
88
WILLIAMD. Ross and ROBERT E. SIEVERS
In this study it has been established that beryllium is detectable in quantities as low as 4 x 10-ls g when the electron capture detector is used in the measurement of the trifluoroacetylacetonate chelate. This observation stimulated research to establish a rapid quantitative analytical method for beryllium at extremely low concentrations, based on gas chromatography. EXPERIMENTAL Apparatus Gas chromatography. An ionization detector (diode type) cell, Model A-4150, was used in a Barber Colman Model 20 gas chromatograph. This detector was used in a non-pulsed electron capture mode by reducing the cell potential from a modified power supply. The maximum accessible cell potential was 200 V. The original electrometer was modi6ed to improve the sensitivity. Rearrangement of the circuit provided for stepped multiplication of 1,2, 5, and 10 times the original signal available to the recorder. Gas chromatographic columns were fabricated from Teflon (DuPont) tubing because of its inert characteristics. Borosilicate glass was chosen for fabrication of injection port liners. These inert materials served to reduce the likelihood of sample decomposition. As in the chromatography of all potentially toxic compounds, it is strongly recommended that the effluent carrier gas stream be vented to a fume hood, or at least passed through a carefully tended cold trap. Mixer. The agitation of the reagents during equilibration in the solvent extraction step was achieved by using a Spex Industries, Inc. Mixer/Mill. Samples (ranging m size from 1 to 10 ,ul) of the organic solutions were introduced into the glasslined injection port of the gas chromatography apparatus with a Hamilton microsyringe. y-Cowtter. Model VSlB Well-Type Scmtdlation Counter and a Model DSIB Scaler purchased from Nuclear Measurements Corporation. Reagents
Beryilium trij?uoroacetyIacetonate. The synthesis of beryllium trifluoroacetylacetonate was conducted in a fume hood, with care being exercised to ensure that the samples did not wntaminate the laboratory. Metallic beryllium was dissolved in perchloric acid and the solution was used to prepare Be(TFA), by solvent extraction. I1 The crystalline product was isolated by evaporation of the benzene layer. The identity and purity of Be(TFA), were determined by measuring melting points (authentic samples melt at 112 f 1‘l****) and obtaining emission spectrographic and infrared spectrophotometric data. We have recently established that powdered samples of elemental beryllium and numerous other metals react directly with tritluoroacetylacetone or other fluorocarbon B-diketones to form the corresponding metal chelates.” Standard solutions were prepared by dissolving weighed amounts of Be(TFA), in Mallinkrodt Nanogradequality or Matheson Coleman and Bell Pesticidequality benzene. The solutions were never stored longer than 24 hr (in polyethylene bottles) before the calibration curve measurements were made. The calibration points were obtained by injecting varying amounts of solutions of three concentrations: 7.04 x 10-t glml, 1.41 x lo-’ g/ml, and 2.82 x lo-* g/ml (weight of chelate per ml; all other concentrations are expressed in weight of beryllium per ml). Beryflium. Standard aqueous solutions of beryllium(I1) were prepared by dissolving 0.0515 g of high purity metallic beryllium in concentrated perchloric acid, then heating, filtering, and diluting with triply distilled water. Parent solutions were stored in polyethylene bottles, and solutions containing less than 5OOppm were prepared just before the experiments were performed. Four wncentrations were used: 1.18 x 10-t g/ml, 1.18 x lo-* g/ml, 2.95 x 10-e g/ml, and 8.84 x BY0 glmL Solutions containing tracers were made by adding radioactive Be (Nuclear Science and Engineering Corporation) to naturally occurring ‘Be solutions. An amount of ‘Be sufficient to yield a concentration of 2 x 10-*s g/ml was added after it was determined that this concentration provided a convenient radioactivity level yet did not affect the response of the electron capture detector (1 ml of the radioactive solution produced approximately 80000 wunts/min at 950 V). 7Wj?uoroacetyIacetone.For the solvent extraction studies a O+lOSMsolution of trifluoroacetylacetone (Pierce Chemical Co.) in benzene was employed.
Rapid ultra-trace determination of beryllium by gas chromatography
89
Analytical procedure A l-00-ml aliquot of the aqueous solution of beryllium to be analysed was measured into a borosilicate &s-s culture tube (75 mm x 16 mm. with a volume of 9 ml). To this was added 1.00 ml of a solutiocof the chelating a‘gent [O-005M sol&ion of H(TFA) in benzene] and l+lO ml of sodium acetate buffer solution (l&f). The vessel was sealed with a screw cap [fitted with Teflon tape (DuPont) under the cap to prevent leakage]. The tube was then shaken for 1 hr in the Spex Mixer/Mill agitator. The sample was centrifuged until the organic and aqueous layers were completely separated (1 min is adequate). An aliquot of the organic layer (O-50 ml) was transferred to a fresh vial, and an equal volume of 0.OlM aqueous sodium hydroxide was added to remove the excess of uncomplexed @and. The mixture was immediately shaken (manually) for 15 sec. the washed organic layer was quickly separated, and the gas chromatographic measurements were made. The washed organic layer was introduced as l-O-~1 (or larger at very low Be concentrations) samples into the gas chromatography apparatus by means of a microsyringe. Five replicate analyses were made. Standard solutions of Be(TFA), in benzene were analysed periodically to obtain and check the calibration curve. Znstrument operating conditions In this kvestigition the following instrumental operating conditions were utilized. Column dimensions. 4 ft x 0.06 in. bore. DuPont Teflon Column packing, 5 k SE52 (methyl-ph&yl) silicone gum rubber on 60-80 mesh Gas Chrom Z (Applied Science Laboratories) Column temperature, 80” Column effluent flow-rate, 50 ml/min Column inlet pressure, 28 psig Eluent, prepurified nitrogen Detector voltage, 10 V Scavenger flow-rate, 214 ml/min, prepurtied nitrogen Injection port temperature, 168” Detector temperature, 200” RESULTS
AND
DISCUSSION
Beryllium trifluoroacetylacetonate can be quantitatively determined by gas chromatography in even smaller amounts than those reported for other metal trifluoroacetylacetonates. The beryllium chelate is easily eluted and an excellent peak is obtained at a column temperature of 80’ (Fig. 1). A calibration curve was made by analysing standard solutions of Be(TFA), in benzene. The calibration curve (Fig. 2) consists of a plot of the chromatographic peak heights and peak areas us. the logarithm of the weight of beryllium. Periodic analyses of freshly prepared standards should be made to ensure that the detector response is stable and unchanged relative to that indicated by previously determined calibration curves. Standard solutions prepared from weighed amounts of Be(TFA), can be analysed alternately with the unknowns to recheck the calibration curves. Solvent extraction methods were used to convert the beryllium in aqueous samples into Be(TFA), with concomitant transfer to the organic phase. A substantial excess of the chelating agent must be present to ensure near-quantitative extraction of the beryllium from the aqueous phase and to prevent excessive absorption losses on the walls of the vessels. Trifluoroacetylacetone is eluted just prior to the beryllium chelate and exhibits such pronounced tailing that the presence of large amounts of H(TFA) presented a serious problem because of the overlap of the H(TFA) and Be(TFA), chromatographic peaks. This difficulty is illustrated in the first chromatogram in Fig. 1. When the organic phase is washed with sodium hydroxide solution, however, the excess of unreacted ligand is removed from the organic layer, and the interference is eliminated. The second chromatogram in Fig. 1 shows the effect of treatment with the sodium hydroxide solution.
90
WILLLM D. Ross and ROBERTE. SIEVERS
HO-FA)
lo (TFA)z
i
\
i,
-+-++-A min
-+-+4-d min
FIO. I.-Chromatogram obtained from injection of a l-,ul sample of the extracted organic layer before (left) and after (right) washing with aqueous sodium hydroxide. The sample contained 2.7 x lo-l1 g of beryllium.
Fro. 2.-Calibration
curve prepared from standard solutions of Be(TFA), in benzene. -ePeakheights;---O---Peakareas.
Several precautions should be observed in the handling of the extremely dilute samples. At higher concentrations difficulties are not ordinarily encountered, but losses of various ionic and molecular species by adsorption on the walls of storage and reaction vessels becomes much more significant at lower concentrations. One may minimize losses by the use of vessels with low adsorptive properties and by keeping to a minimum the storage time of dilute solutions containing beryllium.
Rapid ultra-tracedetermination of berylliumby gas chromatography
91
The analytical results obtained on unknowns are shown in Table I. The values obtained are consistently a little low due to losses incurred in the extraction and wash steps. Reasonably good accuracy was achieved considering the extremely small quantities being determined. If greater accuracy is desired, this can be achieved at some loss in convenience and time by the use of tracers to measure the losses and TABLEI.-BERYLUUM DETERMMATfON Sample no. 1A :E 1D Mean 2A 2B 2c 2D Mean 3A 3B 3c 3D 3E Mean :; 4C 4D 4E Meall z: 5c 5D 5B Meall 6A z 6D 6B MW
Sample size.PI
Be detected by G.C., g
’ ;c;z
2.12 x lo-” 212 x 10-l’ 214 x 10-l’ 1.97x IO-” 2.09 x 10-l’ 2.16 x lo-” 2.16 x lo-” 2.02 x 10-1’ 2.18 x IO-‘l 2.13 x lo-” 1.14x lo-” 1.13x lo-‘1 1.15x lo-” I.10 x lo-” 1.10x lo-‘1 1.12 x 10-l’ 1.14 x IO-” 1.13x lo-” 1.16x lo-” 1.08 x lo-” 1.10x lo-11 1.12x lo-” 2.88 x 10-l’ 2.69 x lo-” 279 x 10-l’ 2.63 x 10-l’ 274 x lo-” 2.75 x 10-l’ 6.11 x IO-” 5.84 x 10-l’ 5.84 x 10-l’ 6.15 x 10-l’ 6.15 x 10-l’ 6.02 x 10-l’
BY GAS CHROMATOGIUPHY WlTHOUT TlUfXRS
Be measured
Be
Mean error,g/ml
Relative error, %
1.18x lo-’
-1.3 x lo-*
11.0
1.18 x
-1.1 x IO-’
9.3
-6.0 x lo-10
5.1
-6.0 x UF”
5.1
-29
x lo-‘@
6.8
-1.32 x lo-lo
14.9
1.18 x
1.18 x
2.95 x
8.84 x
G.C:‘g,ml 1.06x 10-1 1.06 x lo-’ 1.07x lo-’ 0.99 x 10-T 1.05x lo-’ lo-’ 1.08x lo-’ I.08 x lo-’ 1.01 x lo-’ 1.09x IO-’ 1.07x lo-’ lo-’ I.14 x 10-a 1.13x IO-’ 1.15x 10-a I.10 x 10-8 1.10x lo-’ 1.12x IO-@ lo-* 1.14x 10-a 1.13x 10-a 1.16x lO-s 1.08x IO-’ 1.10x 10-e 1.12x 1oJ lo-’ 2.88 x lo-’ 2.69 x lo-’ 2.79 x 10-a 2.63 x lo-’ 2.74 x lo-’ 2.75 x lo-, lo-~@ 7.64 x 10-l” 7.30 x lo-10 7.30 x lo-‘0 7.69 x lo-10 7.69 x 10-l” 7.52 x KF”
permit recalculation of the beryllium concentration with the losses being taken into account. In four analyses, the losses were determined by independent radiotracer measurements to be 2.5, 2-5, 2-O and 3.0% respectively. When the losses are taken into account, the relative errors fall to the values shown in the last column of Table II. Accordingly, when tracers are used, the relative errors are reduced from 5-l to 2.5 % for the two 1.18 x lo-8 g/ml samples, from 6-8 to 4.7% for the 2.95 x lO_, g/ml sample, and from 14.9 to 12-l % for the 8.84 x 10-l” g/ml sample. Experience with particular types of samples in a given concentration range may permit one to estimate loss correction factors that will effectively improve the accuracy without the use of tracers.
92
Ww
D. Ross and ROBERTE. Sravaas
TABLE IL-IMPROVEMBNTOF ACCURACY BY sIMuLTANEousUSE OF TRACERSWITSl
QAS CHaohwrooaAPHY MaAsuaEMa~
Sample I10.
:
Be taken in aqueous p-9
1.18 1.18 2.95 8.84
gM
x x x x
% Be lost, determined by tracer measurements
Be measured by G.C., g/ml (av. of 5 runs)
1O-L 10-O IO-@ 10-l”
1.12 1.12 2.75 752
x x x x
10-s 10-5 lo-’ lo-‘0
Corrected result based on tracer measurements, glml 1.15 1.15 2.81 7.77
;:; 2.0 3.0
x x x x
Relative error %, with tracer corrections
10-e 10-e 10-O lo-‘0
2.5 2.5 142.:
It is expected that this analytical technique should be useful in biomedical systems for determination of trace amounts of beryllium. Previous workerss27 have discussed in detail the digestion and dissolution of a variety of samples to prepare aqueous solutions for analysis. It was necessary to design experiments to determine whether several naturally occurring cations and anions interfere in either the extraction or the chromatographic steps of the analysis. Table III shows the results of the interference TABLBIf.T.-MCa
Compound A. &ions CaCI,*2H,O C~(HJ’U*HIG KNO, Na&% NaF Mixture of anions CaCI,.2H,O Ca@U’C&H,O KNO, N%SG, NaF B. Mixture of cations Al co cu Fe Mg Mn Zn
SrUDrIS ON THE EXllUcIIoN OF BeRYLLnJM(n)
Concentration, glml
AND
G.C.
ANALYSIS
Be(TFA), peak heights, mm With anion Without anion or cation or cation
Be, glml
Ratio
x x x x x
200/l 4011 200/l 400/l 100/l
120 120 120 120 121
120 120 120 120 120
lo-* 10-e IO-8 10-a IO-8
9.70 1.99 9.94 2.41 6.01
x x x x x
10-e 10-e 10-a lo-’ IO-@
537 5.37 5.37 5.37 5.37
5.82 1.19 5.96 1.45 6.01
x x x x x
10d 10-S lo-* I 10-6 10-O
5.37 x 10-0
564/l
119
120
140 1.40 I.40 1.40 140 1.40 140
x x x x x x x
1.18 x 1O-8
118/l
10
10
studies. None of the species evaluated caused any appreciable interference at the concentrations used. Scribner” noted that strong interference occurred in the extraction of beryllium when fluoride was present at a concentration of 0.25iU. He has recently independently confirmed that fluoride concentrations below IWM do not interfere with the extraction of beryllium. 21 At intermediate fluoride concentrations partial extraction occurs. Consequently, if tluoride concentrations greater than 10-4M are expected, tracers should be used to establish the percentage of beryllium extracted.
Rapid ultra-trace determination of beryllium by gas chromatography
93
It is significant that quantitative extractions can be effected in the presence of EDTAF Therefore, should it be necessary, gross quantities of numerous co-occurring metal ions could be retained in the aqueous layer by use of this masking agent. If desired, the effective sensitivity of this technique could probably be improved by various modifications. Since only a very small portion (5 ~1) of the organic phase is used in the chromatographic analyses, perhaps the organic solution could be concentrated to improve the effective sensitivity. An alternative approach is to use smaller aliquots of benzene with higher concentrations of the chelating agent, thereby producing a greater concentration of the complex in the benzene layer after extraction. These options could be exercised in addition to the obvious expedient of preconcentration of the aqueous solution to be analysed. AcknowZe&menr-Theauthors wish to thank Mrs. Geneva Harris for her excellent technical assistance. This paper was presented in part at The Sixth International Symposium on Gas Chromatography, Rome, Italy, September 1966, and preliminary results were reported in the Proceedings. Zusammenfassung-Mit dem Elektroneneinfangdetektor wurden Ultraspurenmengen Beryllium gemessen. die gaschromatographisch als Beryllium(U)-trifluoracetylacetonat abgetrennt wurden. Die untere Nachweisgrenze ist etwa 4 - 1O-18g Beryllium. Eichkurven gehen von 8 - 10-l” bis 4 * lo-” g Beryllium. Berylliumproben in wiiDriger Liisung wurden bei vier Konzentrationen (1,18 - lO_‘, 1,18 - lO_*, 2,95 - 10-O und 8,84 * IO-10 g/ml) quantitativ durch Kombination von fltissigfliissig-Extraktion und Gaschromatographie analysiert. Die Verteilung von Beryllium wlhrend der Extraktionsvorgslnge wurde unabhiingig mit radioaktivem Beryllium bestimmt, der Gebrauch von Tracem ist jedoch bei dem empfohlenen Verfahren nicht notwendig. Die Stbnmg durch in biologischen Proben vorkommende Kat- und Anionen wurde untersucht. Bei den bei der Extraktion und bei der Gaschromatographie verwendeten Konzentrationen start keines der untersuchten fiinfxehn Ionen merklich.
1. 2. 3. 4.
5.
R&&-On a utilM le detecteur il capture d%lectrons pour mesurer des ultra-traces de beryllium s6pare a l’ttat de trifluoroa&ylac&onate de b&yllium(II) par des techniques de chromatographie en phase vapeur. La limite inferieure de detection est d’environ 4 x 10-l* g de beryllium. Les courbes d’etalonnage vont de 8 x 10-l* a 4 x 10-11g de beryllium. On a analyd quantitativement des &chantillons de beryllium en solution aqueuse ii quatre concentrations (1,18 x l(r’, 1,18 x lO_*, 2,95 x lO+ et 8,84 x 10-log/ml) en combmant l’extraction par solvant et la chromatographie en phase vapeur. Le partage du beryllium pendant les techniques d’extraction a ette determine independamment par l’emploi de b&yllium radioactif mais l’utilisation de traceurs n’est pas necessitee par la technique recommand& On a Ctudie l’interference de cations et d’anions que l’on trouve dam des echantillons biologiques. Aux concentrations utilisees dans la technique d’extraction et la methode de chromatographie en phase vapeur, aucun des quinze ions ttudies ne g6ne de facon appreciable. REFERENCES D. K. Albert, Anal. Gem., 1964,36,2034. T. Fujinaga, T. Kumamoto and Y. Ono, Nippon Kagaku Zasski, 1965,&I, 1294. R S. Juvet, Jr. and R. P. Durbin, Anal. Chem., 1966,38,565. R. S. Juvet, Jr. and R. L. Fisher, ibid., 1966, 38, 1860. G. P. Morie and T. R. Sweet, ibid., 1965,37, 1552.
6. Idem, Anal. Chim. Acta, 1966,34,314. 7. R W. Moshier and J. E. Schwarberg, Talanta, 1966, 13,445. 8. R. W. Moshier and R. E. Sievers, Gas Chromatography of Metal Chelates. PergamonPress,Oxford,
1965.
94
Wm
D. Ross and &BERT
E. Sravnks
9. W. D. Ross. AnaI. Ckem., 1963,35,1596. 10. W. D; Ross; R. E. Sievers and G. wheeler, Jr., ibid. 1965,37,598. 11. W. D. Ross and G Wheeler. Jr.. ibid.. 1965.37.598. 12. J. E. Schwa&erg, R. W. M&e; and;. H. k&h, Tukmra, 1964,11,1213. 13. R E. Sievers, J. C. Connolly and W. D. Ross, J. Car CZtromutog., 1967,5, 241. 14. R E. Sievers, G. Wheeler, Jr. and W. D. Ross, Advances in Gas Ckromato upky, Ed. by A Zlatkis and L. S. Ettre, p. 112. Pteston Technical Abstracts Co., Evanston, fr* lhnois, 1966. 15. R E. Sievers. G. Wheeler, Jr. and W. D. Ross, Anal. Ckem., 1966,38,306. 16. D. Tsurumatsu, Y. Ishihara, K. Sato and K. Nalcasawa, Bunseki Kapku, 1966,15,181. 17. H. Veening, W. E. Bachman and D. M. Wiin, Z. Car CZiromutog. 1%7,5,248. 18. F. M. Ado and R. S. Juvet, Jr., AnaZ. Ckem., 1966,38,569. 19. W. G. Scribner, M. J. Botchers and W. J. Treat, ibid., 1966,38,1779. 20. W. G. Scribner, W. J. Treat, J. D. Weis and R. W. Moshier, ibid. 1965,37,1136. 21. W. G. Scribner, private communication. Work performed under U.S. Air Force Contract AF 33(615)-1093. 22. R A. Staniforth, Doctoral Dissertation, Ohio State University, 1943. 23. J. Cholak and D. M. Hubbard, Anal. Ckem., 1948,20,73. 24. H. A. Laitenen and P. Kivalo, [bid., 1952,24,1467. 25. C. W. Sill and C. P. Willis, ibid., 1959, 31, 598. 26. T. Y. Toribara and R. E. Sherman, ibid., 1953,25,1594. 27. J. Walkley, Am. Znd. Hyg. Assoc. J., 1959,20,241.
RAPID ULTRA-TRACE DETERMINATION OF BERYLLIUM BY GAS CHROMATOGRAPHY WILLIAM
D. Ross
Monsanto Research Corporation and Dayton, Ohio 45407, U.S.A. and ROBERT E. SIEVER~ Aerospace Research Laboratories, ARC, Wright-Patterson Air Force Base, Dayton, Ohio 45433, U.S.A. (Received 23 May 1967. Accepted 10 JuIy 1967)
Summary-The electron capture detector has been used to measure ultra-trace quantities of beryllium separated as berylli~(II) trifhroroacetylacetonate by gas chromatographic techniques. The lower limit of detectability is co. 4 x 10-r*g of beryllium. Calibration plots extend from 8 x 10-l’ to 4 x lo-l1 g of beryllium. Samples of beryllium in aqueous solution at four concentrations (l-18 x 10-t, l-18 x HYB, 295 x lO+, and 8.84 x 10-log/ml) were analysed quantitatively by combining solvent extraction and gas chromatography. The distribution of beryllium during the extraction procedures was determined independently by use of radioactive beryllium-7, but the use of tracers is not required in the recommended procedure. Interference studies were made on cations and anions found in biological samples. At the concentrations used in the extraction procedure and the gas chromatographic process, none of the fifteen ions studied interferes appreciably.
THE analysis of metals by gas chromatography has been the subject of recent intense research activity as a result of the remarkable sensitivity, simplicity, and speed of the technique. l-ls In addition, interferences by co-occurring elements are much less frequently encountered in the gas chromatographic technique than in other methods for metal analysis, because the components of the sample are simultaneously separated and measured. The utilization of fluorinated ligands offers significant advantages over the use of the protonated analogues for the formation of volatile metal chelates. The volatility and the excellent response of the electron capture detector for metal complexes derived from trifluoroacetylacetone, the anion of which may be depicted as cFJ_c~=~~ *
I
TFA
make this compound one of the most promising chelating agents in quantitative analyses.l*s-u The ease of quantitative extraction with this complexing agent is another attractive quality.6-7~10*20M orie and SweeP and Moshier and Schwa&erg’ have demonstrated that samples of alloys, and aqueous solutions of mixtures of aluminium, gallium and indium, can be analysed by gas chromatography of metal trifluoroacetylacetonates formed by solvent extraction. In these studies, the sensitivity of the methods developed was limited by the type of detector (thermal conductivity) employed. If electron capture detectors are used, the method becomes much more sensitive. Indeed the technique is more sensitive for some elements than are neutron-activation, atomic-absorption, or other classical methods now in use. 87
88
WILLIAMD. Ross and ROBERT E. SIEVERS
In this study it has been established that beryllium is detectable in quantities as low as 4 x 10-ls g when the electron capture detector is used in the measurement of the trifluoroacetylacetonate chelate. This observation stimulated research to establish a rapid quantitative analytical method for beryllium at extremely low concentrations, based on gas chromatography. EXPERIMENTAL Apparatus Gas chromatography. An ionization detector (diode type) cell, Model A-4150, was used in a Barber Colman Model 20 gas chromatograph. This detector was used in a non-pulsed electron capture mode by reducing the cell potential from a modified power supply. The maximum accessible cell potential was 200 V. The original electrometer was modi6ed to improve the sensitivity. Rearrangement of the circuit provided for stepped multiplication of 1,2, 5, and 10 times the original signal available to the recorder. Gas chromatographic columns were fabricated from Teflon (DuPont) tubing because of its inert characteristics. Borosilicate glass was chosen for fabrication of injection port liners. These inert materials served to reduce the likelihood of sample decomposition. As in the chromatography of all potentially toxic compounds, it is strongly recommended that the effluent carrier gas stream be vented to a fume hood, or at least passed through a carefully tended cold trap. Mixer. The agitation of the reagents during equilibration in the solvent extraction step was achieved by using a Spex Industries, Inc. Mixer/Mill. Samples (ranging m size from 1 to 10 ,ul) of the organic solutions were introduced into the glasslined injection port of the gas chromatography apparatus with a Hamilton microsyringe. y-Cowtter. Model VSlB Well-Type Scmtdlation Counter and a Model DSIB Scaler purchased from Nuclear Measurements Corporation. Reagents
Beryilium trij?uoroacetyIacetonate. The synthesis of beryllium trifluoroacetylacetonate was conducted in a fume hood, with care being exercised to ensure that the samples did not wntaminate the laboratory. Metallic beryllium was dissolved in perchloric acid and the solution was used to prepare Be(TFA), by solvent extraction. I1 The crystalline product was isolated by evaporation of the benzene layer. The identity and purity of Be(TFA), were determined by measuring melting points (authentic samples melt at 112 f 1‘l****) and obtaining emission spectrographic and infrared spectrophotometric data. We have recently established that powdered samples of elemental beryllium and numerous other metals react directly with tritluoroacetylacetone or other fluorocarbon B-diketones to form the corresponding metal chelates.” Standard solutions were prepared by dissolving weighed amounts of Be(TFA), in Mallinkrodt Nanogradequality or Matheson Coleman and Bell Pesticidequality benzene. The solutions were never stored longer than 24 hr (in polyethylene bottles) before the calibration curve measurements were made. The calibration points were obtained by injecting varying amounts of solutions of three concentrations: 7.04 x 10-t glml, 1.41 x lo-’ g/ml, and 2.82 x lo-* g/ml (weight of chelate per ml; all other concentrations are expressed in weight of beryllium per ml). Beryflium. Standard aqueous solutions of beryllium(I1) were prepared by dissolving 0.0515 g of high purity metallic beryllium in concentrated perchloric acid, then heating, filtering, and diluting with triply distilled water. Parent solutions were stored in polyethylene bottles, and solutions containing less than 5OOppm were prepared just before the experiments were performed. Four wncentrations were used: 1.18 x 10-t g/ml, 1.18 x lo-* g/ml, 2.95 x 10-e g/ml, and 8.84 x BY0 glmL Solutions containing tracers were made by adding radioactive Be (Nuclear Science and Engineering Corporation) to naturally occurring ‘Be solutions. An amount of ‘Be sufficient to yield a concentration of 2 x 10-*s g/ml was added after it was determined that this concentration provided a convenient radioactivity level yet did not affect the response of the electron capture detector (1 ml of the radioactive solution produced approximately 80000 wunts/min at 950 V). 7Wj?uoroacetyIacetone.For the solvent extraction studies a O+lOSMsolution of trifluoroacetylacetone (Pierce Chemical Co.) in benzene was employed.
Rapid ultra-trace determination of beryllium by gas chromatography
89
Analytical procedure A l-00-ml aliquot of the aqueous solution of beryllium to be analysed was measured into a borosilicate &s-s culture tube (75 mm x 16 mm. with a volume of 9 ml). To this was added 1.00 ml of a solutiocof the chelating a‘gent [O-005M sol&ion of H(TFA) in benzene] and l+lO ml of sodium acetate buffer solution (l&f). The vessel was sealed with a screw cap [fitted with Teflon tape (DuPont) under the cap to prevent leakage]. The tube was then shaken for 1 hr in the Spex Mixer/Mill agitator. The sample was centrifuged until the organic and aqueous layers were completely separated (1 min is adequate). An aliquot of the organic layer (O-50 ml) was transferred to a fresh vial, and an equal volume of 0.OlM aqueous sodium hydroxide was added to remove the excess of uncomplexed @and. The mixture was immediately shaken (manually) for 15 sec. the washed organic layer was quickly separated, and the gas chromatographic measurements were made. The washed organic layer was introduced as l-O-~1 (or larger at very low Be concentrations) samples into the gas chromatography apparatus by means of a microsyringe. Five replicate analyses were made. Standard solutions of Be(TFA), in benzene were analysed periodically to obtain and check the calibration curve. Znstrument operating conditions In this kvestigition the following instrumental operating conditions were utilized. Column dimensions. 4 ft x 0.06 in. bore. DuPont Teflon Column packing, 5 k SE52 (methyl-ph&yl) silicone gum rubber on 60-80 mesh Gas Chrom Z (Applied Science Laboratories) Column temperature, 80” Column effluent flow-rate, 50 ml/min Column inlet pressure, 28 psig Eluent, prepurified nitrogen Detector voltage, 10 V Scavenger flow-rate, 214 ml/min, prepurtied nitrogen Injection port temperature, 168” Detector temperature, 200” RESULTS
AND
DISCUSSION
Beryllium trifluoroacetylacetonate can be quantitatively determined by gas chromatography in even smaller amounts than those reported for other metal trifluoroacetylacetonates. The beryllium chelate is easily eluted and an excellent peak is obtained at a column temperature of 80’ (Fig. 1). A calibration curve was made by analysing standard solutions of Be(TFA), in benzene. The calibration curve (Fig. 2) consists of a plot of the chromatographic peak heights and peak areas us. the logarithm of the weight of beryllium. Periodic analyses of freshly prepared standards should be made to ensure that the detector response is stable and unchanged relative to that indicated by previously determined calibration curves. Standard solutions prepared from weighed amounts of Be(TFA), can be analysed alternately with the unknowns to recheck the calibration curves. Solvent extraction methods were used to convert the beryllium in aqueous samples into Be(TFA), with concomitant transfer to the organic phase. A substantial excess of the chelating agent must be present to ensure near-quantitative extraction of the beryllium from the aqueous phase and to prevent excessive absorption losses on the walls of the vessels. Trifluoroacetylacetone is eluted just prior to the beryllium chelate and exhibits such pronounced tailing that the presence of large amounts of H(TFA) presented a serious problem because of the overlap of the H(TFA) and Be(TFA), chromatographic peaks. This difficulty is illustrated in the first chromatogram in Fig. 1. When the organic phase is washed with sodium hydroxide solution, however, the excess of unreacted ligand is removed from the organic layer, and the interference is eliminated. The second chromatogram in Fig. 1 shows the effect of treatment with the sodium hydroxide solution.
90
WILLLM D. Ross and ROBERTE. SIEVERS
HO-FA)
lo (TFA)z
i
\
i,
-+-++-A min
-+-+4-d min
FIO. I.-Chromatogram obtained from injection of a l-,ul sample of the extracted organic layer before (left) and after (right) washing with aqueous sodium hydroxide. The sample contained 2.7 x lo-l1 g of beryllium.
Fro. 2.-Calibration
curve prepared from standard solutions of Be(TFA), in benzene. -ePeakheights;---O---Peakareas.
Several precautions should be observed in the handling of the extremely dilute samples. At higher concentrations difficulties are not ordinarily encountered, but losses of various ionic and molecular species by adsorption on the walls of storage and reaction vessels becomes much more significant at lower concentrations. One may minimize losses by the use of vessels with low adsorptive properties and by keeping to a minimum the storage time of dilute solutions containing beryllium.
Rapid ultra-tracedetermination of berylliumby gas chromatography
91
The analytical results obtained on unknowns are shown in Table I. The values obtained are consistently a little low due to losses incurred in the extraction and wash steps. Reasonably good accuracy was achieved considering the extremely small quantities being determined. If greater accuracy is desired, this can be achieved at some loss in convenience and time by the use of tracers to measure the losses and TABLEI.-BERYLUUM DETERMMATfON Sample no. 1A :E 1D Mean 2A 2B 2c 2D Mean 3A 3B 3c 3D 3E Mean :; 4C 4D 4E Meall z: 5c 5D 5B Meall 6A z 6D 6B MW
Sample size.PI
Be detected by G.C., g
’ ;c;z
2.12 x lo-” 212 x 10-l’ 214 x 10-l’ 1.97x IO-” 2.09 x 10-l’ 2.16 x lo-” 2.16 x lo-” 2.02 x 10-1’ 2.18 x IO-‘l 2.13 x lo-” 1.14x lo-” 1.13x lo-‘1 1.15x lo-” I.10 x lo-” 1.10x lo-‘1 1.12 x 10-l’ 1.14 x IO-” 1.13x lo-” 1.16x lo-” 1.08 x lo-” 1.10x lo-11 1.12x lo-” 2.88 x 10-l’ 2.69 x lo-” 279 x 10-l’ 2.63 x 10-l’ 274 x lo-” 2.75 x 10-l’ 6.11 x IO-” 5.84 x 10-l’ 5.84 x 10-l’ 6.15 x 10-l’ 6.15 x 10-l’ 6.02 x 10-l’
BY GAS CHROMATOGIUPHY WlTHOUT TlUfXRS
Be measured
Be
Mean error,g/ml
Relative error, %
1.18x lo-’
-1.3 x lo-*
11.0
1.18 x
-1.1 x IO-’
9.3
-6.0 x lo-10
5.1
-6.0 x UF”
5.1
-29
x lo-‘@
6.8
-1.32 x lo-lo
14.9
1.18 x
1.18 x
2.95 x
8.84 x
G.C:‘g,ml 1.06x 10-1 1.06 x lo-’ 1.07x lo-’ 0.99 x 10-T 1.05x lo-’ lo-’ 1.08x lo-’ I.08 x lo-’ 1.01 x lo-’ 1.09x IO-’ 1.07x lo-’ lo-’ I.14 x 10-a 1.13x IO-’ 1.15x 10-a I.10 x 10-8 1.10x lo-’ 1.12x IO-@ lo-* 1.14x 10-a 1.13x 10-a 1.16x lO-s 1.08x IO-’ 1.10x 10-e 1.12x 1oJ lo-’ 2.88 x lo-’ 2.69 x lo-’ 2.79 x 10-a 2.63 x lo-’ 2.74 x lo-’ 2.75 x lo-, lo-~@ 7.64 x 10-l” 7.30 x lo-10 7.30 x lo-‘0 7.69 x lo-10 7.69 x 10-l” 7.52 x KF”
permit recalculation of the beryllium concentration with the losses being taken into account. In four analyses, the losses were determined by independent radiotracer measurements to be 2.5, 2-5, 2-O and 3.0% respectively. When the losses are taken into account, the relative errors fall to the values shown in the last column of Table II. Accordingly, when tracers are used, the relative errors are reduced from 5-l to 2.5 % for the two 1.18 x lo-8 g/ml samples, from 6-8 to 4.7% for the 2.95 x lO_, g/ml sample, and from 14.9 to 12-l % for the 8.84 x 10-l” g/ml sample. Experience with particular types of samples in a given concentration range may permit one to estimate loss correction factors that will effectively improve the accuracy without the use of tracers.
92
Ww
D. Ross and ROBERTE. Sravaas
TABLE IL-IMPROVEMBNTOF ACCURACY BY sIMuLTANEousUSE OF TRACERSWITSl
QAS CHaohwrooaAPHY MaAsuaEMa~
Sample I10.
:
Be taken in aqueous p-9
1.18 1.18 2.95 8.84
gM
x x x x
% Be lost, determined by tracer measurements
Be measured by G.C., g/ml (av. of 5 runs)
1O-L 10-O IO-@ 10-l”
1.12 1.12 2.75 752
x x x x
10-s 10-5 lo-’ lo-‘0
Corrected result based on tracer measurements, glml 1.15 1.15 2.81 7.77
;:; 2.0 3.0
x x x x
Relative error %, with tracer corrections
10-e 10-e 10-O lo-‘0
2.5 2.5 142.:
It is expected that this analytical technique should be useful in biomedical systems for determination of trace amounts of beryllium. Previous workerss27 have discussed in detail the digestion and dissolution of a variety of samples to prepare aqueous solutions for analysis. It was necessary to design experiments to determine whether several naturally occurring cations and anions interfere in either the extraction or the chromatographic steps of the analysis. Table III shows the results of the interference TABLBIf.T.-MCa
Compound A. &ions CaCI,*2H,O C~(HJ’U*HIG KNO, Na&% NaF Mixture of anions CaCI,.2H,O Ca@U’C&H,O KNO, N%SG, NaF B. Mixture of cations Al co cu Fe Mg Mn Zn
SrUDrIS ON THE EXllUcIIoN OF BeRYLLnJM(n)
Concentration, glml
AND
G.C.
ANALYSIS
Be(TFA), peak heights, mm With anion Without anion or cation or cation
Be, glml
Ratio
x x x x x
200/l 4011 200/l 400/l 100/l
120 120 120 120 121
120 120 120 120 120
lo-* 10-e IO-8 10-a IO-8
9.70 1.99 9.94 2.41 6.01
x x x x x
10-e 10-e 10-a lo-’ IO-@
537 5.37 5.37 5.37 5.37
5.82 1.19 5.96 1.45 6.01
x x x x x
10d 10-S lo-* I 10-6 10-O
5.37 x 10-0
564/l
119
120
140 1.40 I.40 1.40 140 1.40 140
x x x x x x x
1.18 x 1O-8
118/l
10
10
studies. None of the species evaluated caused any appreciable interference at the concentrations used. Scribner” noted that strong interference occurred in the extraction of beryllium when fluoride was present at a concentration of 0.25iU. He has recently independently confirmed that fluoride concentrations below IWM do not interfere with the extraction of beryllium. 21 At intermediate fluoride concentrations partial extraction occurs. Consequently, if tluoride concentrations greater than 10-4M are expected, tracers should be used to establish the percentage of beryllium extracted.
Rapid ultra-trace determination of beryllium by gas chromatography
93
It is significant that quantitative extractions can be effected in the presence of EDTAF Therefore, should it be necessary, gross quantities of numerous co-occurring metal ions could be retained in the aqueous layer by use of this masking agent. If desired, the effective sensitivity of this technique could probably be improved by various modifications. Since only a very small portion (5 ~1) of the organic phase is used in the chromatographic analyses, perhaps the organic solution could be concentrated to improve the effective sensitivity. An alternative approach is to use smaller aliquots of benzene with higher concentrations of the chelating agent, thereby producing a greater concentration of the complex in the benzene layer after extraction. These options could be exercised in addition to the obvious expedient of preconcentration of the aqueous solution to be analysed. AcknowZe&menr-Theauthors wish to thank Mrs. Geneva Harris for her excellent technical assistance. This paper was presented in part at The Sixth International Symposium on Gas Chromatography, Rome, Italy, September 1966, and preliminary results were reported in the Proceedings. Zusammenfassung-Mit dem Elektroneneinfangdetektor wurden Ultraspurenmengen Beryllium gemessen. die gaschromatographisch als Beryllium(U)-trifluoracetylacetonat abgetrennt wurden. Die untere Nachweisgrenze ist etwa 4 - 1O-18g Beryllium. Eichkurven gehen von 8 - 10-l” bis 4 * lo-” g Beryllium. Berylliumproben in wiiDriger Liisung wurden bei vier Konzentrationen (1,18 - lO_‘, 1,18 - lO_*, 2,95 - 10-O und 8,84 * IO-10 g/ml) quantitativ durch Kombination von fltissigfliissig-Extraktion und Gaschromatographie analysiert. Die Verteilung von Beryllium wlhrend der Extraktionsvorgslnge wurde unabhiingig mit radioaktivem Beryllium bestimmt, der Gebrauch von Tracem ist jedoch bei dem empfohlenen Verfahren nicht notwendig. Die Stbnmg durch in biologischen Proben vorkommende Kat- und Anionen wurde untersucht. Bei den bei der Extraktion und bei der Gaschromatographie verwendeten Konzentrationen start keines der untersuchten fiinfxehn Ionen merklich.
1. 2. 3. 4.
5.
R&&-On a utilM le detecteur il capture d%lectrons pour mesurer des ultra-traces de beryllium s6pare a l’ttat de trifluoroa&ylac&onate de b&yllium(II) par des techniques de chromatographie en phase vapeur. La limite inferieure de detection est d’environ 4 x 10-l* g de beryllium. Les courbes d’etalonnage vont de 8 x 10-l* a 4 x 10-11g de beryllium. On a analyd quantitativement des &chantillons de beryllium en solution aqueuse ii quatre concentrations (1,18 x l(r’, 1,18 x lO_*, 2,95 x lO+ et 8,84 x 10-log/ml) en combmant l’extraction par solvant et la chromatographie en phase vapeur. Le partage du beryllium pendant les techniques d’extraction a ette determine independamment par l’emploi de b&yllium radioactif mais l’utilisation de traceurs n’est pas necessitee par la technique recommand& On a Ctudie l’interference de cations et d’anions que l’on trouve dam des echantillons biologiques. Aux concentrations utilisees dans la technique d’extraction et la methode de chromatographie en phase vapeur, aucun des quinze ions ttudies ne g6ne de facon appreciable. REFERENCES D. K. Albert, Anal. Gem., 1964,36,2034. T. Fujinaga, T. Kumamoto and Y. Ono, Nippon Kagaku Zasski, 1965,&I, 1294. R S. Juvet, Jr. and R. P. Durbin, Anal. Chem., 1966,38,565. R. S. Juvet, Jr. and R. L. Fisher, ibid., 1966, 38, 1860. G. P. Morie and T. R. Sweet, ibid., 1965,37, 1552.
6. Idem, Anal. Chim. Acta, 1966,34,314. 7. R W. Moshier and J. E. Schwarberg, Talanta, 1966, 13,445. 8. R. W. Moshier and R. E. Sievers, Gas Chromatography of Metal Chelates. PergamonPress,Oxford,
1965.
94
Wm
D. Ross and &BERT
E. Sravnks
9. W. D. Ross. AnaI. Ckem., 1963,35,1596. 10. W. D; Ross; R. E. Sievers and G. wheeler, Jr., ibid. 1965,37,598. 11. W. D. Ross and G Wheeler. Jr.. ibid.. 1965.37.598. 12. J. E. Schwa&erg, R. W. M&e; and;. H. k&h, Tukmra, 1964,11,1213. 13. R E. Sievers, J. C. Connolly and W. D. Ross, J. Car CZtromutog., 1967,5, 241. 14. R E. Sievers, G. Wheeler, Jr. and W. D. Ross, Advances in Gas Ckromato upky, Ed. by A Zlatkis and L. S. Ettre, p. 112. Pteston Technical Abstracts Co., Evanston, fr* lhnois, 1966. 15. R E. Sievers. G. Wheeler, Jr. and W. D. Ross, Anal. Ckem., 1966,38,306. 16. D. Tsurumatsu, Y. Ishihara, K. Sato and K. Nalcasawa, Bunseki Kapku, 1966,15,181. 17. H. Veening, W. E. Bachman and D. M. Wiin, Z. Car CZiromutog. 1%7,5,248. 18. F. M. Ado and R. S. Juvet, Jr., AnaZ. Ckem., 1966,38,569. 19. W. G. Scribner, M. J. Botchers and W. J. Treat, ibid., 1966,38,1779. 20. W. G. Scribner, W. J. Treat, J. D. Weis and R. W. Moshier, ibid. 1965,37,1136. 21. W. G. Scribner, private communication. Work performed under U.S. Air Force Contract AF 33(615)-1093. 22. R A. Staniforth, Doctoral Dissertation, Ohio State University, 1943. 23. J. Cholak and D. M. Hubbard, Anal. Ckem., 1948,20,73. 24. H. A. Laitenen and P. Kivalo, [bid., 1952,24,1467. 25. C. W. Sill and C. P. Willis, ibid., 1959, 31, 598. 26. T. Y. Toribara and R. E. Sherman, ibid., 1953,25,1594. 27. J. Walkley, Am. Znd. Hyg. Assoc. J., 1959,20,241.