compounds which may occur in samples and can describe how to recognize their spectra. The system is not intended to process arbitrary collections of organic/organometallic compounds such as may arise in general purpose mass spectrometry. The ease with which spectrum recognition rules can be built up and tested permits the analyst to experiment with many different empirical procedures for the classification of spectra of different types of compounds, For many simple applications, the classifications produced simultaneously with continued data acquistion are adequate for the purpose of analysis. In other cases, these classifications can serve to identify which spectra require further processing by manual interpretation or by sophisticated file search identification methods like STIRS (16).
(4)W. L. Felty and P. C. Jurs, Anal. Chem., 45, 885 (1973). ( 5 ) B. R. Kowalski and F. C. Bender, J. Amer. Chem. Soc., 94, 5632 (1972). (6) B. R. Kowalski and F. C. Bender. J. Amer. Chem. SOC., 95,686 (1973). (7) H. T. Kaili, R. C. T. Lee, G. W. A. Milne, M. Shaplro, and A. M. Guarino, Science, 180,417 (1973). (8)E. R. Kowaiski. F. C. Bender, and H. D. Shepherd, Anal. Chem., 45, 617 (1973). (9)J. T . Clerc, P. Naegli, and J. Seibl, Chimia, 27, 639 (1973). (IO)A. Buchs, A. B. Delfino. A. M. Duffield, C. Djerassi, B. G. Buchanan, E. A. Feigenbaq, and J. Lederberg. Helv. Chim. Acta, 53, 1394 (1970). (1 1) D. H. Smith, E. G. Buchanan, W. C. White, E. A. Feigenbaum, J. Lederberg, and C. Djerassi, Tetrahedron, 29, 31 17 (1973). (12) S. R. Heller, H. M. Fales, and G. W. A. Miine, Org. Mass Spectrom., 7, 107 (1973). (13)P.W. Brooks, Ph.D. thesis, Bristoi. 1974. (14)G.Eglinton, PureAppl. Chem, 34, 611 (1973). (15)D. H. Smith, Anal. Cbem., 44, 536 (1972). (16)K . 4 . Kwok, R. Venkataraghavan, and F. W. McLafferty, J. Amer. Cbem. SOC., 95, 4185 (1973).
ACKNOWLEDGMENT The authors thank Geoffrey Eglinton, University of Bristol, for provision of facilities, guidance, and suggestions during the course of these studies; Paul W. Brooks for supplying a fraction from a sediment sample, and Mike J. Humberston for carrying out the analysis of this fraction.
LITERATURE CITED (1)L. R . Crawford and J. D.Morrison, Anal. Chem., 43, 1970 (1971). (2)R . Venkataraghavan, F. W. McLafferty, and G. E. Van Lear, Org. Mass Spectrom., 2, l(1969). (3)R. J. Mathews. Aust. J. Chem., 26, 1955 (1973).
RECEIVEDfor review January 24, 1974. Accepted September 25, 1974. Financial support for these studies was provided by the Department of Education and Science, and the Royal Norwegian Council for Scientific and Industrial Research. GC-MS facilities were financed by the Natural Environment Research Council and the data acquistion system by the Nuffield Foundation. The programs described herein might be available from the National Research and Development Corporation, Kingsgate House, 66/74 Victoria Street, London S W l E 6SL, U.K.
Determination of the Volatility of Metal Chelates by Atomic Absorption David C. Hilderbrand’ and Edward E. Pickett Departments of Chemistry and Agricultural Chemistry, University of Missouri, Columbia, Mo. 6520 7
Metal chelates have been studied by atomic absorptlon in an attempt to determine their volatility at elevated temperatures. The chelates were introduced into the optical cell of the instrument using a tantalum boat inserted in the flame or a heated injection port attached to the burner chamber. Radioactive tracers were used to determine the efficiency of volatilization of selected chelates from the boat. Temperature profiles were constructed by plotting absorbance ws. temperature of the injection port. The sulfur-containing chelates (dithiocarbamates etc.) were the most stable with respect to this rapid or “flash” volatillzation. The chelate solutions gave about a 30-fold enhancement in sensitivity as compared to the usual nebulizer-flame method of sample introduction for the most volatile chelates of each metal. The possibility of extending the injection port method of sample introduction to the analysis of actual samples has been investigated with encouraging results.
In recent years, the volatility of P-diketone chelates has been extensively studied in attempts to develop a gas chromatographic method of analysis for metals (1, 2 ) . Several Present address, Department of Chemistry, South D a k o t a State University. Brookings, S.D. 57006.
424
methods have been developed but, in general, the quantitative formation and extraction of the more volatile fluorinated chelates is difficult to achieve. More recently, synergic chelating systems involving a P-diketone and a neutral ligand such as tributyl phosphate have been used to increase extractability while maintaining thermal stability (3, 4 ) . Thermogravimetric data on metal chelates are limited mainly to the 8-hydroxyquinoline chelates which have been found to be non-volatile ( 5 ) .Thermal data on other chelates are limited. This paper reports a new method for determing the relative volatility of a series of metal chelates with respect to flash volatilization, which have not previously been reported to be volatile, as well as several volatile /3-diketonates. Flash volatility represents a new measure of thermal stability of compounds which are not volatile in conventional terms, i.e., for those which cannot be distilled or gas chromatographed. The possibility of using these volatile metal chelates in a new method of sample introduction for atomic absorption has been studied.
EXPERIMENTAL T h e m e t a l chelates were formed and extracted in the laboratory using n o r m a l solvent extraction procedures (6). T h e extraction efficiencies observed in the laboratory were determined f o r each procedure by atomic absorption analysis o f t h e digested chelate solu-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
jection ports were filled with steel wool to act as an evaporating surface and flow spoiler. The port was surrounded by an electrically resistive heating coil to allow measurements to be made at various desired temperatures. The chelate-containing solvent was injected directly onto the steel wool and the port was flushed with an inert carrier gas. In one part of the study, the injection port was connected directly to the front of a Perkin-Elmer pre-mix burner chamber. The absorbance of the metal atoms from the chelate was measured in the flame to determine the sensitivity of the system for trace metal analysis. Measurements were made at a series of temperatures. The sensitivity at each temperature was determined and a temperature profile constructed. The area under the absorption peak was determined and compared to the area under the absorption peak for the introduction of a known amount of metal by traditional atomic absorption. These data were used to calculate the fraction of the injected atoms reaching the flame. The injection port was also connected to a piece of glass tubing submerged in a cold finger. The injected chelates and solvents were thus condensed and collected. This mixture was digested and analyzed, and the fraction of the injected sample that was recovered was determined. Ultraviolet-visible spectra of the condensates were compared to the spectra of the chelate solutions prior to injection to determine if the chelates were being volatilized and transported unchanged. A Perkin-Elmer Model 303 Atomic Absorption Spectrophorometer was used in the study. A Nuclear Chicago Scaling Unit with an end window Geiger tube detector (Model 161A) was used for the radioactive tracer studies. The ultraviolet-visible spectra were obtained using a Cary Model 11 spectrophotometer.
Table I. Extraction Conditions Chelate
Acronym or common name
Acetylacetone
ACAC
Pyrrolidine carbodithioatea N-Nitrosophenylhydroxylamine Diethyldithiocarbamate Diphenylthio carbazone Diphenylcarbazide Ethyl xanthate
APCD Cupferron DEDTC Dithizone Diphenyl carbazide Ethyl xanthate HFA #
Solvent
Acetylacetone o r benzene Methyl isobutyl ketone Chloroform Carbon tetrachloride Carbon tetrachloride Isoamyl alcohol Carbon tetrachloride Cyclohexane
Hexafluoroacetylacetone Chloroform 2-Methyl-8-hydroxy - Methyloxine quinoline N-Nitroso-2-naphthyl- Neocupferron Chloroform hydroxylamine 2 -Nitroso-1 -naphthol 2 -Nitroso-1 - Chloroform naphthol 8 -Hydroxyquinoline Oxine Chloroform 1-(2-Pyridylazo)-2PAN Chloroform naphthol Tri-n-butyl phosphate TBP Cyclohexane Thenoyltrifluoro TFA Benzene o r acetone isobutyl alcohol a The designation APDC and the name, pyrrolidinedithiocarbamate, both in common use, are erroneous because the lone nitrogen atom thus is indicated twice. This name should be suppressed.
RESULTS AND DISCUSSION The chelating agents investigated can be classified according t o the atoms t o which the metal is bonded. T h e metal atom is chelated through two sulfur atoms in DEDTC, APCD, and ethyl xanthate chelates. The p-diketones, TFA, ACAC, and HFA, are bound through two oxygen atoms, one of which is enolized. Oxine and methyloxine chelates are bound through one oxygen atom and one nitrogen atom. Cupferron and neocupferron chelates are bound through two oxygen atoms, and dithizone and diphenylcarbazide chelate through the two nitrogen atoms that are LY to the phenyl groups or through their sulfur or oxygen atoms, respectively. P A N is bonded to metal atoms through a n oxygen atom and a nitrogen atom. The combinations of chelating agents and metals which formed readily extractable metal chelates were studied. T h e data in Table I1 indicate that the chelates in which the metal atoms are chelated through two sulfur atoms are most stable toward flash volatilization when vaporized in the boat. All of the chelates formed with APCD, DEDTC, or ethyl xanthate gave a n absorption signal when heated in the tantalum boat. Most P-diketone chelates also produced absorption signals when heated in the boat. The only exceptions were CoACAC, Co-TFA, and Zn-ACAC. The b-diketone chelates
tion. The metals studied were the first row transition metals, chromium through zinc. The chelating agent and solvent systems which were used are given in Table I. The volatility of the chelates with respect to flash volatilization was studied by a series of approaches. First, the chelates were studied using a tantalum Sampling Boat (Perkin-Elmer Corp., Norwalk, Conn.) and atomic absorption spectrophotometric determination (7). A suitable volume of chelate-containing solution was placed in the boat. The solvent was evaporated at low temperature to avoid destruction of the chelate. The boat was then placed directly in the flame of the atomic absorption unit and the observed absorption signal measured. The radioactive 64Cuisotope was used as a tracer for a series of copper chelates to determine the fraction of copper atoms remaining in the boat. The activity in the boat after evaporation of the solvent, but prior to volatilization, was measured. Following volatilization, the activity remaining in the boat was determined and the per cent of the copper volatilized was calculated. The volatility of the chelates was also studied using a gas-liquid chromatography-type injection port. Glass and stainless steel in-
-
~~~~~~
Table 11. Volatility of Chelates in the Boat Sensitivity'
Chelating agent
ACAC
APCD
Cupfenon
DEDTC
Diphenylcarbazide Dithuone
EthylUeocupxanthate HFA-TBP hlethyloxine fenon
Oxine
PAN
TFA
Element
Chromium B B B D Cobalt D C B B Iron B B C B Manganese B A D B Nickel C D B Zinc D A D A a Sensitivity is defined as the amount of metal required to give a D = extracted but no signal.
A C
B
B
C C
B
B
B
B B B C
B
B D
1%absorption signal. A = 0.5-5.0 ng; B =
D D
B
B A 5.0-50 ng; C
D B
C = 50-500
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
ng;
425
”
t
L’ 100
Temperature,
‘c
Temperature.
roo
\
, IO0
OC
Figure 1. Temperature profiles for cobalt chelates
Figure 2. Temperature profiles for cobalt chelates
- ACAC chelate; - - - APCD chelate; and -
- Ethyl xanthate chelate; - - - HFA-TBP chelate; and -
- - DEDTC chelate
Table 111.Volatility of Chelates in the Injection P o r t Chelating agent
DEDTC APCD Ethyl Xanthate ACAC HFA-TBP T FA Methyloxine Cupferron Neocupf e r r on Diphenylcarbazide Dithizone PAN
Volatile
Nonvolatile
C r , Co, Cu, Mn, Ni, Zn Cr, Co, Cu, Mn, Ni, Z n Co C r , Co, Cu, Ni Zn Co, Cu, Mn, Z n Co, Ni cu Ni Co, Cu, Mn Co, Cu, Mn, Ni, Z n Ni
Cr
cu Ni, Z n
Co, Mn, Ni, Zn cu
have been known to be volatile for a long time. Chelates of acetylacetone and its fluorinated derivatives have been studied extensively for gas chromatographic analysis. It is pQssible that these three chelates which did not produce signals were being lost by volatilization during the evaporation of the solvent. Many of the highly fluorinated derivatives of acetylacetone form chelates that are certain to be too volatile for the boat, and thus they would not be detected and were not studied. Moreover, the extraction efficiencies of many of these P-diketone chelates are poor, making them of little practical use for analytical purposes. Of the various chelating agents used, the least volatility was observed for chelates of cupferron, neocupferron, and diphenylcarbazide. T o determine the per cent of metal being volatilized from the boat, a radioactive tracer, 64Cu,was used. The activity was measured after solvent evaporation but prior to placing the boat in the flame. The activity was measured again after volatilization of the chelate to determine the activity remaining in the boat. The unexpected volatility of the dithio chelates observed in the sensitivity studies was again observed. An average of 47% of the copper in the form of the DEDTC chelate was volatilized compared to 12% for the PAN chelate, 9% for the ACAC, and 8% for the oxine. The chelates were also studied in the injection port which permitted better control of the temperature of the surface from which vaporization occurs. From this more definitive study of the volatility of the chelates toward flash volatilization, temperature profiles were constructed for each chelate by measuring the absorbance resulting from the introduction of 1 fig of chelated metal into the injection port at selected temperatures. Figures 1 and 2 show the profiles for cobalt chelates. In general, the greatest sensitivity and, therefore, volatility was observed for the dithio 426
.
100
e
- a
- TFA chelate
compounds and the HFA-TBP chelates. The TFA chelate was also appreciably volatile. The ACAC chelate might have required a higher temperature for volatilization although it was not found to be volatile in the boat. The chelates of cobalt not shown in the graph did not produce an absorption signal when injected into the port over the range of temperatures studied. Similar profiles have been observed for other metals ( 7 ) . There seems to be little effect of the metal on the volatility of the chelate formed. Some chelates of each metal that was studied were volatilized, and some of the chelates of each of the metals failed to produce a signal. The effect of the chelating agent on the volatility of the metal chelate can be observed in the data in Table 111; the volatile and nonvolatile chelates are listed according to chelating agent. As in the case of the boat, the chelates demonstrating the highest volatility, both in terms of the number of chelates giving signals and the sensitivity of those signals, were those in which the metal atom is bonded to two sulfur atoms (APCD, DEDTC, and ethyl xanthate). Most of the P-diketonates were also volatile to some extent under the conditions encountered in the injection port. The best sensitivity of any of the chelating agents was observed for the hexafluoroacetylacetone-tri-n-butylphosphate system. Undoubtedly greater volatility and better sensitivity could be observed for some of the chelates of other fluorinated /3-diketones. However, these chelates are not readily formed and extracted quantitatively and were not included in this investigation. The presence of T B P with HFA forms a synergic system in which both species are bonded to the metal atom ( 3 ) giving a more efficient extraction while increasing the temperature at which the chelate is volatilized. The thermal stability is also increased. The chelates of dithizone, diphenylcarbazide, cupferron, neocupferron, oxine, and methyloxine were all nonvolatile or only slightly volatile. Several of these chelates gave a slight signal when investigated in the boat. T h a t these same chelates do not give signals with the injection port may be due to the thermal gradient effect. In the boat, the temperature goes from room temperature to around 1000 OC in a second. The point where the sample solution is placed is cooled to a considerably lower temperature by the vaporization of the solvent and then climbs back to the original temperature. As a result, the signal for the metal appears some 5 to 20 seconds after the introduction of the chelate into the injection port. Some chelates that are only slightly volatile might decompose under the conditions of time and temperature in the injection port but not have time to completely decompose in the boat. Two methods were used to determine the percentage of the chelated metal which is actually being volatilized and reaching the optical path of the instrument. First, some of
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
the chelate solutions were volatilized from the injection port and then condensed and collected by the apparatus described above. The results are shown in Table IV. The highest percentage of recovery observed for any chelate of cobalt, the metal most extensively studied by this procedure, was 33% for the DEDTC chelate. As a further check on the fraction of the metal reaching the optical path, the area under the absorption curve for the samples introduced by the injection port was compared to the area under the absorption curve for nebulized samples. The per cent absorption signals were converted to absorbance and plotted. The area under the time by absorbance plot was then determined. The sample introduced through the injection port was 0.1 ml of 1.0 ppm copper as the DEDTC chelate in carbon tetrachloride or 100 ng of chelated copper. The average area under the curve was 532 mm2. An equal amount of copper nebulized by traditional methods gave an area of 145 mm2. This represented approximately a 3.7-fold increase in area for injection port introduced samples over an equivalent amount of nebulized copper. It is estimated that about 10% of the nebulized sample reaches the flame in traditional atomic absorption (8). Assuming identical free atom formations for both methods of sample introduction and that all other pertinent parameters are the same for the two methods, these data indicate that 37% of the chelated metal is reaching the optical cell of the instrument. This compares with the recovery experiment data (Table IV) wherein 24% of the copper was recovered by condensation after volatilization. The lower recovery from the cold finger apparatus may have been due to incomplete condensation following volatilization. It has also been suggested that free atom formation fractions involving chelates may be greater than for aqueous samples (9). It is possible that both methods give low estimates of the volatility of metal chelates. There are cool surfaces between the hot injection port and the flame or the cold finger whereon a fraction of the volatilized metal may be deposited in chelate form or in some other chemical state. T o determine if the volatilized chelate was reaching the optical cell intact or if decomposition was occurring, the ultraviolet-visible spectra for the volatilized and trapped solutions were compared to spectra of the original solution. The solutions studied were cobalt, manganese, nickel, and copper chelates of diethyldithiocarbamate. The spectra of the cobalt chelate before and after volatilization were identical when appropriate dilutions were made. The spectra for nickel DEDTC showed small shifts and changes in relative intensities of the peaks, indicating that the nickel chelate underwent partial decomposition. The chelates of copper and manganese showed large changes in peak locations and intensities. Use of these volatile metal chelates for analytical purposes has been considered. The injection port design is preferable to the boat design as it provides more opportunity for optimization of parameters and greater tempera-
Table IV. Percentage of Metal Recovered by Condensation after Volatilization in the Injection Port Metal
Chelating agent
Recovered, X
co co co co cu cu Mn Mn Zn Zn
TFA APCD HFA-TBP DEDTC HFA-TBP DEDTC HFA-TBP DEDTC HFA-TBP DEDTC
23 24 19 33 11 24 15 16 32 36
ture control. The injection port gives sensitivities that are up to 30 times better than those observed with traditional nebulizer-flame spectrometry using the current design. This might be increased further by as much as three times if complete volatilization of the chelate can be approached. The current injection port has a capacity of 1.0 ml of solution per injection. Preliminary use of the injection port technique for the determination of cobalt in plant samples has demonstrated promising reproducibility. The relative standard deviation for replicate determinations of a given sample solution was found to be approximately f6.0%. The precision of the analysis of a given plant tissue when carried through the procedure in sets of five replicate samples gave an average relative standard deviation of f7.3%.
ACKNOWLEDGMENT The authors express their appreciation to S. R. Koirtyohann and W. A. Aue for many helpful discussions. LITERATURE CITED (1) R. W. Moshier and R. E. Sievers, "Gas Chromatography of Metal Chelates," Pergamon Press, New York, N.Y., 1965, p 8. (2) R. Belcher. R. J. Martin, W. I. Stephen, D. E. Henderson, A. Kamalizad, and P. C. Uden. Anal. Chem., 45, 1197 (1973). (3) B. B. Tomazic and J. W. O'Laughlin, Anal. Chem., 45, 1519 (1973). (4) W. C. Butts and C. V. Banks, Anal. Chem., 42, 133 (1970). (5) C. Duval, "Inorganic Thermogravimetric Analysis," Elsevier. New York, N.Y., 1963, pp 331, 351, 364, 392, 411. (6) D. C. Hilderbrand, "Sample Introduction for Atomic Absorption," Ph.D. Thesis, University of Missouri-Columbia, 1971, pp 28-34 and references cited therein. (7) Reference 6, pp 67-77. ( 8 ) S. R. Koirtyohann and E. E. Pickettn "XIII Colloquiun Spectroscopicum Internationale, Ottawa, Canada, June 1967," Adam Hilger, London, 1968, p 270. (9) C. L. Chakrabarti and S. P. Singhal, Spectrochim. Acta, Part 6, 24, 663 (1969).
RECEIVEDfor review July 24, 1974. Accepted November 6, 1974. This paper is a portion of a thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Missouri-Columbia by one of the authors (DCH).
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