acid, 1:l hydroch1oric:hydrofluoricacid mixture for digestion of Hi-Vol samples gave results which agreed within 3%of those obtained by concentrated hydrochloric acid. Unexposed glass fibre filters from different lots showed no detectable amounts of tin. The average tin concentration of urban air around Toronto was found to be 18 pg per thousand cubic meters, based on analysis of 32 Hi-Vol filters, with a maximum of 122 hg and a minimum of 3 pg per thousand cubic meters. The concentrations of tin were higher in air particulate matter in and around lead refineries and can manufacturing operations. The 1ead:tin ratio in these areas was about 300 as against 50 for traffic locations. As low as 0.5 pg tin per thousand cubic meters of air can be measured by this method. Up to 50 samples can be easily analyzed in one man-day. Application to Other Materials. The results of tin determination in a few other sample matrices are listed in Table V. The certified tin concentrations in the Canadian Certified Reference Materials Project (CCRMP) Samples KC-1 and MP-1 and USGS W-1 rock sample show close agreement with the results obtained by the present method. The CCRMP samples are minerals containing very high levels of nickel, copper, iron, sulfur etc., and yet did not require either
coprecipitation of tin or addition of sodium oxalate before analysis. Food samples were appropriately diluted with 1% v/v, hydrochloric acid and analyzed.
LITERATURE CITED (1) E. B. Sandell, “Colorimetric Determination of Traces of Metals”, 3d rev. ed., lnterscience Publishers, New York, 1959,p 854. (2)P. 0. Juliano and W. W. Harrison, Anal. Chem., 42, 84 (1970). (3)J. R. Levine, S. G. Moore, and S. L. Levine, Anal. Chem., 42, 412
(1970). (4) I: Rubaska and M. Miskowsky, At. Absorpt. News/., 11, 57 (1972). (5) F. J. Fernandez, At. Absorpt. News/., 12, 93 (1973). (6) P. N. Vijan and G. R. Wood, At. Absorpt. News/., 13, 33 (1974). (7)F. J. Schmldt, J. L. Royer, and S. M. Muir, Anal. Lett., 8, 123 (1975). (8)F.D. Pierce, T. C. Lamoreaux, H. R. Brown, and R. S.Fraser, Appl. Spectrosc., 30, 38 (1976). (9)P. D. Goulden and P. Brooksbank, Anal. Chem., 46, 1431 (1974). (IO) A. E. Smith, Analyst(London), 100, 300‘(1975). (11) B. E. Burke, Anal. Chem., 42, 1536 (1970). (12)K. C. Thompson and D. R. Thomerson, Analyst (London), 99, 595 (1974).
RECEIVEDfor review April 12, 1976. Accepted July 6, 1976. Presented a t the 1976 Pittsburgh Conference on Analytical Chemistry and Applied Spect,roscopy,Cleveland, Ohio.
Studies on the Mechanism of Atom Formation in Graphite Furnace Atomic Absorption Spectrometry R. E. Sturgeon, C. L. Chakrabarti,* and C. H. Langford Metal Ions Group, Depatfment of Chemistry, Carleton University, Colonel By Drive, Ottawa, Ontario, Canada K IS 5B6
The mechanism of atom formatlon for a number of elements in a Perkin-Elmer HGA 2100 has been studled using a combined thermodynamic and kinetic approach. Assuming that an analyte surface-gas phase equillbrium exists within the furnace and the production of observable atoms Is characterized by a unimolecular rate constant, a plot of the logarithm of the absorbance as a function of the inverse of the absolute temperature yields a straight line from which the activation energy, E,, of the limiting step in the atomization pathway can be obtained. These Ea values reveal that three major atomlzation mechanisms are operative: thermal dissociation of the analyte oxide or hallde, and carbon reduction of the oxide followed by atomizatlon of the free metal.
The formation of analyte atoms in various flames has been discussed by many workers. As convenient cells for the study of high-temperature reactions, flames have been used to obtain spectroscopic evidence of reaction intermediates and bond dissociation energies of stable species (1-10). However, little has been reported regarding the atomization processes in graphite furnace atomizers. Whereas it is generally conceded that local thermodynamic equilibrium exists at various points throughout a flame ( I ) , it may be questionable to assume that equilibrium is attained in electrothermal atomizers. The possibility of high thermal gradients, the rapid rates of rise of temperature and the transient signals generated by these atomizers (11, 12) may not allow adequate time for physical and chemical processes to reach equilibrium before the atomic species have been lost. Despite the possibility that equilibrium may not be attained in electrothermal atomizers, several researchers have attempted to gain insight into the
dominant processes of atom production based on the assumption that equilibrium has been attained. For example, Campbell and Ottaway (13)have assumed that reduction of metal oxides by carbon is rapid, and have correlated the appearance temperature (the temperature at which atom formation is first observed ( 1 1 ) with the temperature a t which reduction of analyte oxides with solid carbon (the surface of the graphite cell) becomes thermodynamically favorable ( A G L t l o n 5 0): M,O,,,)
+
YC,,,
-
XM,,,
f
YCO,,)
(1)
Their study suggests that a large number of analyte species produce gaseous atoms through such a reduction reaction. Although correlation between the temperature and AGO of the above reduction reaction was observed for 18 of the 27 elements investigated, their data for the appearance temperature reflects the temperature of the cell wall at the point in time at which the peak, rather than the beginning, of the analyte signal occurs. Many of their appearance temperature values therefore tend to be too high. The difference between their reported temperature and the true appearance temperature for each element is a function of the heating rate of the graphite cell, the geometry of the cell, and the analyte species. On the other hand, Aggett and Sprott (14) have compared the appearance temperatures of various analytes in both graphite and tantalum atomizers. Comparison of these two appearance temperatures from each atomizer for the same analyte species indicates whether or not oxide reduction plays a role in the formation of analyte atoms. Lower appearance temperatures from the graphite atomizer are indicative of a reduction process. The free energy change for Equation 1was evaluated at the appearance temperature to determine if re-
1792 * ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
duction was thermodynamically possible. Of the 16 elements studied, only 4 (Fe, Ni, Co, and Sn) offered evidence of a reductive mechanism. These authors (14) have implicitly assumed thie surface of the tantalum atomizer to be coated with an inert Taz05 layer. Thermodynamically, tantalum reduction of analyte oxides a t the temperatures employed is highly favorable:
The assumption that this reduction reaction can be ruled out is valid only if Taz05 forms a coherent stable film on the surface of the metal. It has been determined (15, p 339) that growth of'the surface oxide proceeds by a mechanism of diffusion of the metal through the film to the surface. Provided this step I S slow in comparison to the atomization time of the analyte, reduction by tantalum is not likely to occur. The fact that a change in the appearance temperature was noted can be interpreted as confirmation of the fact that the reduction of analyte oxides by Ta is hindered by the presence of a surface film of Taz05(assuming that accurate measurements of the temperature were obtained for both atomizers). Fuller (16,17) has criticnzed ihe thermodynamic approach to atom production because it does not explain the fact that several elements may form thermodynamically stable carbides at and below the temperatures a t which AG~educt,on for Equation 1is zero. In addition, the thermodynamic approach cannot give any indication of the rates of atomization and, consequently, predict absorbance peak shapes. As a result, he has attempted to explain the dynamics of these transient signals using a kinetic approach. With copper as a working model, he obtained rate const ants for the production and dissipation of atoms in a heated graphite atomizer and assigned an activation energy of 13.8 X 104 J mol-1 (33 kcal molm1)to the production of copper atoms. Whereas such treatment has the potential for predicting; peak shapes and activation energies governing the production and dissipation of atoms, it is a necessary condition that an isiothermal environment exist within the atomizer, preferably throughout the entire duration of the signal. In addition, accurate atomic vapor temperatures are required. Commercially available atomizers cannot provide an isothermal environment over the entire duration of the absorbance pulses ( I I , 1 2 ) .As a result, a purely kinetic treatment of the entire absorption pulse is very difficult. In addition, Fuller (16, 17) has implicitly assumed that the atomic vapor temperature lis equal to that of the graphite surface. Deviations from thermal equilibrium will cause large errors in the estimation of rate constants and activation energies, particularly when activation energies of the order of 100 kcal mol-' are involved.
