Physical Basis of Atomic Absorption Spectrometry II. Influence of Temperature Gradients on Spatial Distribution of Neutral Atoms in the Long Pathlength Cell Atsushi Ando,' Keiichiro Fuwa* and Bert L, Vallee Biophysics Research Laboratory, Department of Biological Chemistry, Harvard Medical School, and the Division of Medical Biology, Peter Bent Brigham Hospital, Boston, Mass. The "long pathlength cell" has markedly lowered the detection limit for some elements in atomic absorption spectrometry, as predicted by Beers law. The detection of other elements, however, does not respond in a kimilar manner. To account for marked variations in the behavior of different elements, the concentration distributions of 21 elements have been studied as a function of pathlength, using an air-hydrogen flame. Enhanced sensitivity of detection is shown to be a complex function of the pathlength, the nature of the flame, its shape and temperature, volatility of the elements, and their capacity for oxidation and reduction. The physicochemical basis of these observations and their pertinence to analysis by atomic absorption spectrometry are discussed.
developed recently (6). The data reveal the importance of oxidation-reduction conditions, of chemical reactions occurring at high temperature and of the resultant molecular species in determining the sensitivity of detection in long pathlength atomic absorption systems. EXPERIMENTAL
Present address, Geochemical Research Section, Geological Survey of Japan, 135 Hisamotocho, Kawasaki, Japan. a Present address, Department of Agricultural Chemistry, University of Tokyo, Tokyo, Japan.
Apparatus. The experimental arrangement for the measurement of absorption intensities along the cell path is shown in Figure 1. It differs from the conventional mount by placing the absorption cell perpendicular to the axis of the hollow cathode beam; in this manner atomic absorption can be measured at any distance from the burner. The absorption values are recorded after the light beam from the hollow cathode tube passes through the cell at right angles to its long axis. Burner. The platinum burner is the only component of the present system which differs from that used previously (I). It is placed at the entrance of the cell, leaving only a narrow gap (2 mm), and reducing substantially the amount of entrained air ( I , 6). Flame conditions are controlled easily, permitting studies of atomic absorption along the cell path under stable conditions. The air flow was maintained at 2.8 l./min and at 15 psi, providing constant aspiration of the sample solutions at a rate of 2 ml/min, while the hydrogen flow rate was varied from 2.5 to 14 l./min, Le., from 0.5 to 10 psi. Absorption Cell. Either Vycor (Corning) or quartz (General Electric) tubing was employed, either 1 m or 2.5 m long and with an inside diameter of 1 cm. The quartz tube served only for the measurement of resonance lines below 2200 A. Hollow Cathode Tubes. Westinghouse Electric Corp., single-element devices were used under operating conditions as recommended by the manufacturer. Monochromator and Detector. A Zeiss M4QIII monochromator and photomultiplier (EM1 9526 B) were employed. The circuits for the high voltage power supply and amplifier have been described (7). Measurement of Temperature. The temperature inside the absorption cell was measured both by electrical and optical means. A calibrated chromel-alumel twisted junction thermocouple (Pacific Transducer Corp., Model 326-CA, meter reading) served for temperature below 1000 "C. The exposed, twisted part was thinly coated with a zirconium oxide film to avoid the catalytic effects of the metal surface (8). The line reversal method employing the sodium D line was employed to measure temperatures above 1000 "C, using a Leeds and Northrup 8622-C optical pyrometer and an ocular spectroscope (9).
(1) K. Fuwa and B. L. Vallee, ANAL.CHEM., 35, 942 (1963). (2) I. Rubeska and B. Moldan, Appl. Opt., 7, 1341 (1968). (3) K. Fuwa, P. Pulido, R. McKay, and B. L. Vallee, ANAL.CHEM., 36, 2407 (1964). (4) P. Pulido, K. Fuwa, and B. L. Vallee, Anal. Biochem., 14, 393 (1966). (5) E. J. Agazzi, ANAL.CHEM., 37, 364 (1965).
(6) . , A. Ando. M. Suzuki, K. Fuwa, and B. L. Vallee, ANAL.CHEM., 41, 1974 (i969). (7) B. L. Vallee and M. Margoshes, ibid., 28, 175 (1956). (8) R. Smith, C. M. Staffor& and J. D. Winefordner; ibid., 41, 946 (1969). (9) R. Mavrodineanu and H. Boiteux, "Flame Spectroscopy," John Wiley & Sons, New York, N. Y., 1965, pp 29-30.
USEOF LONG ABSORPTION cells in atomic absorption spectrometry remarkably lowers the detection limit for certain elements (I). Thus, the long absorption path, achieved by passing a hydrogen-air flame through a 90 cm long Vycor or an alundum tube, yields, e.g., from 10 to 100 times greater sensitivity for zinc and cadmium than that obtained with conventional devices. Such increases in sensitivity, apparently in direct proportion to the length of the cell (I), suggested a relationship similar to that expressed by the Beer-Lambert law in molecular absorption spectrometry. However, the effect of pathlength and the limit of detection varied considerably for different elements, In addition to the cell dimensions, i.e., length and internal diameter, sensitivity depends on several other variables, e.g., the cell material, its characteristic reflectivity, flow rates of the oxidant and of the fuel gas which control both the temperature gradient and the oxidation reduction environment along the path. While the advantages of the long pathlength have been recognized and utilized empirically (2-5), their physicochemical basis has not been examined systematically and is as yet understood poorly. To gain insight into the marked variation of the behavior of different elements in long path cells, the concentration distribution of the neutral atoms of twenty-one elements has been studied as a function of the distance-up to 2.5 m-from the flame source. Both absorbance and temperature along the path were measured, or a solution of each element was atomized into the air-hydrogen flame using a platinum ring burner
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ANALYTICAL CHEMISTRY, VOL. 42, NO, 8, JULY 1970
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Monoc hromator
C"l.",
Ring lamp
Figure 1. Schematic diagram of experimental arrangement The absorption cell is placed at right angles to the monochromator and can be moved so that light from the hollow cathode can traverse it at any distance from the burner head Reagents. STANDARDSOLUTIONS.Of the 21 elements studied, 16 were chloride salts. Silver and lead were the nitrates; gold, platinum, and palladium were the ammonium chloro salts, i.e., ammonium chloraurate, chloroplatinate, and chloropalladite. Standard stock solutions of 1000 pg of metal/ml were prepared by dissolving spectroscopically pure metals or salts (Johnson Matthey Co.) in acid or metal-free water (vide infra). Each standard stock solution contained 1N hydrochloric or nitric acid. Most working solutions were prepared by diluting the standard stock solution to a suitable concentration range, 1-50 pg/ml, with metal-free water which contained concentrations of free acid varying from 0.001 to 0.05N. Mercuric chloride was dissolved in metal-free water to prepare a solution containing 100 pg/ml of the metal/ml. The working solution for tin contained 1N hydrochloric acid to counteract hydrolysis of this element. WATER. Metal-free water was obtained by passing tap water through a mixed cation-anion exchange resin, followed by distillation in an all-quartz still (10). Procedure. A solution of each element was atomized into the air-hydrogen flame when the cell was placed parallel to the entrance slit of the monochromator, rather than perpendicular to it, as is conventional. In this fashion absorption intensities across the cell can be measured at any distance from the burner (Figure 1). Throughout, only data with signal-to-noise ratio above 2.0 have been plotted as significant. When the 2.5-m cell was employed, water condensed at the distal end of the cell on aspirating solutions into the flame, but this could be prevented by a tape heater wound around the outer cell surface. The gas temperature inside the cell was measured by inserting the thermocouple junction into the cell from the open end. Temperature was found to vary as a function of distance from the burner (see Results) and, at any given distance, it was dependent on position of the thermocouple in relation to the cross section of the cell. Thus, the temperature was highest in the center of the cell, decreasing toward the periphery and being lowest near the cell wall but somewhat higher at the top than elsewhere. Therefore, the temperature was always measured with the thermocouple in the same position, designed to measure a representative quantity. The same thermocouple geometry was employed throughout: the twisted junction of the couple was bent to form a "V" about 5 mm high, which was then moved through the center of the cell. At 300 "C the thermocouple was calibrated with a standard mercury thermometer and at 1000 "C with an optical pyrometer. The results agreed well with the calibration values (10) R. E. Thiers, "Trace Analysis," J. H. Yoe and H. J. Koch, Jr., Eds., John Wiley & Sons, New York, N. Y.,1957, pp 637-666.
Element Ag
As Au Ca Cd co Cr cu Fe Hk? K
Table I. Resonance Lines Wavelength, A Element 3281 Mg 1937 Mn 2428 Na 4224 Ni 2288 Pb 2407 Pd 3579 Pt 3247 Sn 2483 Sr 2537 Zn 7665
Wavelength, A 2852 2795 5890 2320 2833 2476 2659 2246 4607 2138
supplied by the manufacturer. At 300 "C the temperature measurements with the thermocouple were reproducible to & l o " and at 1000" to *15". In the region of highest temperature, beyond the range of the thermocouple (0-1000 "C), Le., within 10 cm of the burner head, temperature values were approximated by means of the line reversal method (9). Suitable resonance lines were chosen for the absorption measurement of each element (Table I). RESULTS
Temperature. Over a range of 2 to 14 l./min of hydrogen the temperature along the cell path is a function of hydrogen flow rate. The temperature is maximal at a hydrogen flow rate of 2.5 to 3 l./min. Above this the temperature progressively decreases over the whole length of the cell, proportionate to the hydrogen flow rate (Figure 2). An increase in the rate of nebulization of water further lowers the temperature along the cell path. While nebulizing an aqueous solution of sodium chloride at a rate of 2.0 ml/min. and at a hydrogen flow rate of 3 I./min the temperature, measured 4 and 7 cm from the burner head, was almost constant at 1430 "C (1703 OK) and 1360 "C (1633 OK), respectively. The increase in temperature when nebulizing sodium chloride containing solutions is apparent only, owing to slight intrinsic differences and values obtained by the thermocouple procedure on one hand and the optical reversal method on the other. The former measurements were performed at 10,15, and 20 cm from the top of the burner (Figure 2) while those for the optical reversal procedure were performed at 4 and 7 cm, respectively, from the burner head (vide supra). Owing to the fact that close to the burner head the temperature changes rapidly, such differences in distance are critical. In addition, measurements by means of the ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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moo
I
HYDROGEN I/mm
l0OO0-
;
5}
4
8W'-
Figure 2. Temperatures along cell path and effect of hydrogen flow rate, during aspiration of water
sooe.
:I
8 IO
12.5 14
Notice that the temperature is highest at 2.5 liters of
hydrogen/minute and falls to lower values as the flow rate is lowered to 2 liters/rninute
400°.
ZOO0.
Flow rote
of
distilled w a t e r :
2 0 rnl/rnln
0"7
D
optical reversal method yielded temperatures inherently slightly higher than those by means of the thermocouple. Thus the differences in the temperature measurements in the presence and absence of sodium chloride are apparent only and reflect methodology. Atomic Absorption Profiles. PATHLENGTH. Figure 3 shows typical absorption profiles of calcium, 25 pg/ml, copper, 10 pg/rnl, and cadmium, 2.0 pg/ml, when absorption is measured as a function of the distance from the burner head. Concentrations of the elements were chosen to yield similar absorbances. The integrated areas under the curves are indices of total radiation absorbed by the respective atoms, and, hence, of detection sensitivity. At a distance of about 40 cm, cadmium absorption reaches a maximum and remains constant thereafter, up to 100 cm from the burner. This apparently accounts for the remarkable detection sensitivity for this element in the long pathlength system ( I ) . In contrast, calcium atoms absorb maximally about 3 cm from the burner head but absorption ceases beyond 20 cm. This is consistent with the failure of the long pathlength cell to decrease the analytical detection limit of calcium ( I I ) . The atomic absorption profile
(11) S. R. Koirtyohann and C . Feldmann, Develop. Appl. Spectros., 3, 180(1964).
of copper is intermediate between those of calcium and cadmium. Absorption rises to a maximum at about 20 cm and ceases at a distance of about 80 cm from the burner, corresponding again to the effect of pathlength on the analytical sensitivity of copper. In these instances enhancement of analytical sensitivity is a direct function of the length of the path in which absorption occurs. CONCENTRATION. An increase in the concentration of an element apparently does not alter the length of the path over which absorption will occur. While the area under the curve increases in proportion to the increase in concentration of the element, the length of the effective absorbing path remains virtually unaltered. These circumstances are documented by the atomic absorption profiles of copper at concentrations from 5 to 50 pg/ml (Figure 4). Increasing concentrations of copper increase the area under the curves, but not the length of the path over which absorption occurs. Comparable observations were made for the other elements studied. HYDROGEN FLOW RATE. The effect of hydrogen flow rates on the absorption profile of lead is shown in Figure 5. The area under the curve is maximal at a hydrogen flow rate of 3 to 4 I./min, where sensitivity of detection is highest. Greater or lesser flow rates depress absorption. The data suggest that at high hydrogen flow rates the absorbing path for this element is increased somewhat.
W
u Figure 3. Atomic absorption profiles of calcium (25 pg/ml), copper (10 pg/ml), and cadmium (2.0 pg/ml)
a p 4m 4.
DISTANCE FROM BURNER, cm
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ANALYTICAL CHEMISTRY, VOL. 42, NO, 8, JULY 1970
If
cu
50 DISTANCE FROM BURNER, cm
0
Figure 4. Atomic absorption profiles of copper as a function of indicated concentrations U: 50;
A: 25; 0 : 10; X : 5 p g / d
Table 11. Hydrogen Flow Rates for Maximal Sensitivity Hydrogen flow rate5 for maximal Element sensitivity, l./min 3 Ag, Au, Hg, Pd, Pt 4 Cd. Cu. Na. Pb, Zn Mg
7
8 Ca, Co, Cr, K, Mn, Ni, Sr 12 As, Fe, Sn a Air flow rate: 2.8 l./min. Cell: 1 cm id. X 91 cm long, Morganite alumina.
The hydrogen flow rates which result in optimal analytical sensitivities are summarized in Table I1 and vary somewhat for the 21 elements studied. Such differences result from the interaction of several variables, including the temperature and the oxidation-reduction environment along the cell path (uide infra), as well as the specific absorbing species. Generally, a reducing atmosphere increases the sensitivity of detection of easily oxidizable elements, exerting effects equal in magnitude to those of the temperature gradient. EFFECTS OF ACIDSAND OXIDIZING AGENTS. The effect of hydrochloric, nitric, and phosphoric acid (0.5 N ) and hydrogen peroxide (3 %) on the absorption profiles for lead (Figure 6a) and iron (Figure 66) were examined, In the latter instance, perchloric instead of phosphoric acid was studied. The effect of hydrochloric acid was minimal. Nitric acid and
hydrogen peroxide lower the atomic absorption of both elements, presumably owing to their promotion of oxide formation. Sulfuric, phosphoric, and perchloric acid render the absorption signal very unsteady, perhaps as a consequence of condensation on the cell wall. Hence, these involatile acids are quite ineffective adjuvants for atomic absorption analysis when performed in the long path absorption cell system. Since background due to molecular absorption can become an important factor in this system (12-14), the signaljnoise ratio can vary markedly in the presence of certain compounds. Thus depending upon the wavelength of the particular element being measured, sulfur dioxide and nitrous oxide resulting from sulfuric and nitric acids, respectively, can cause variable atomic absorption signal/noise ratios (2). The present studies suggest that among those examined hydrochloric is the best diluent, while nitric acid seems to be a poor second, Absorption Profiles under Conditions of Optimal Sensitivity of Detection. The absorption profiles for all elements measured are shown together with the temperature gradient in Figures 7 a and b. In each instance measurements were performed at the hydrogen flow rate shown to yield optimal sensitivity (Table 11). The absorption profiles of these 21 elements differ significantly and have been arranged in three groups, the characteristics of which are exemplified by the behavior of calcium, copper, and cadmium, respectively (Figure 3). The sharp absorption maxima of calcium, strontium, and chromium are found very close to the burner head (Figure 7a). Zinc, cadmium, and mercury atoms maximally absorb radiation, between 30 to 120 cm from the burner, falling off gradually thereafter for zinc and cadmium, respectively, but with a plateau being maintained up to at least 2.5 m for mercury (Figure 7b). The absorption profiles of the balance of the elements studied are intermediate between the calcium group and the group IIB metals. Those of iron, cobalt, sodium, and palladium are broader than those of the calcium group (Figure 7a, left) but those of the remainder (Figure 7a, right) begin to approach those characteristic of the very volatile metals (Figure 7 b ) . Apparently, in addition to temperature and hydrogen flow rate, certain physicochemical properties of the individual elements importantly affect atomic absorption as a function of cell length.
(12) S. R. Koirtyohann and E. E. Pickett, ANAL.CHEM., 38, 585
(1966). (13) Ibid.,p 1087.
(14) Zbid.,37, 601 (1965).
-4
t h i n Hydrogen flow rate
w u z a
Figure 5. Effect of indicated hydrogen flow rates on atomic absorption profile of lead, 25 pg/ml
P
Pb
40
60
0
CELL LENGTH, crn ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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of (a) lead 40 pg/rnl and (b)iron 25 pg/rnl
0.3-
Fe
y
z a m
/Acid
free, chloride
0.2
LL
$ m a
0.1
0
IO
0
40
20
80
CELL LENGTH, cm
b These absorption profiles have been characterized by two operational functions deduced from the graphs (Figure 8). L,,, is that distance from the burner where the absorbance of a given element is maximal, while LljZis that distance at which it again falls to half this maximal value. These parameters permit simple comparisons of the atomic absorption characteristics of different elements (Table 111). The three categories described above are discernible clearly. The Llp values for both calcium and strontium are 5 cm, closely similar to those of chromium and platinum. The L l p of the majority of elements ranges from 25 to 60 cm, but that of zinc is 100 cm, of cadmium 300 cm, and for mercury is beyond
Table 111. LIlzand LmaX (cm) for Various Metals Element
Ca, Sr Cr, Pt 25
Ni
Co, Fe, Na, Pd Cu, K, Mg, Mn, Sn Ag, As, A i , Pb
Zn Cd Hg
4co.
-
10
100 300
25
35 50 401 50 N 0 )
-
-
20 30
40
120
m
Sr
11400
;;I
%
-
m
Figure 7a. Temperature (- -) and optimal pathlength (-) for Ag, As, Ca, Co, Fe, Ni, Mn, Pb, Sn, and Sr (25 pg/ml); Na (2.5 pg/rnl); Cu, K, Mg (5 pg/ml); Au, Cr, Pd, Pt (50 pg/ml)
500
m
too 3
0
25
50
100
0
25
CELL LENGTH, cm 822
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
a 3 c a
50
too
0.5
1400
0.4
-4
m
W
y
-
Figure 76. Temperature (- -) and optimal pathlength for Cd (2.5 Fg/ml) (m); Zn (5 pg/ml) (A-A) and Hg (100 pg/ml) (0-0). Hydrogen flow rates as in Table II
a m
B
0.3 500
v)
m
z
5
[L
0
m C
0.2
a
U
100
0.1
m
a
r
0
25
0
100
50
150
200
2 50
CELL LENGTH, cm
I
Maximum
1000--
100-
E
I I
I L ma*
V
I I
!
‘3 50-
L half C E L L L E N G T H , cm
Figure 8. Symbolic representation of the maximal absorption, L,,,, and of the distance at which half ofLma,( L i s )is observed
the limits of measurement of the 2.5-m cell employed. As expected, L,,, reflects the same pattern. Correlation between Vapor Pressure of Metals and Lliz Values. Previous studies have estimated the vapor pressures of neutral atoms in the flame to be approximately 10-4 to 10-6 mm Hg. These values are the result of calculations for alkali elements incorporating the flow rates of gases and sample solutions into Saha’s equation (15, 16). The temperature at which these vapor pressures are achieved, T,, has been correlated with the absorption profile for each element, as expressed by Llp (Figure 9). The value of T p at a vapor pressure of mm Hg for each element was obtained by extrapolation of the vapor pressure us. temperature curve based on Dushman’s data (17). Since all studies were carried out at the hydrogen flow rates yielding the best sensitivity for a given element (Table 11), optimal concentration of the metal vapor of all the elements should be achieved. The vapor pressure of metals and Lli2 for most elements correlate well, with the exception of the readily oxidized elements, Le., Ca, Sr, Mg, and Cr. (15) A. Ando and R. Ishida, Birnko Kenkyu, 12, 128 (1964) (in Japanese); C.A., 62, 12 (1965). (16) R. Ishida, Sci. Light (Tokyo), 9, 134 (1960); Sci. Abstr., 6, 3002 (1962). (17) S. Dushman, “Scientific Foundations of Vacuum Technique,” John Wiley & Sons, New York, N. Y . 1949, pp 752-754. Reprinted in the “Handbook of Chemistry and Physics,” 49th edition, 1968-1969, The Chemical Rubber Co., Cleveland, Ohio, p D-111.
01 1000
0
TP =
10-6
2000
m m ti9
Figure 9. Temperature at which metals studied achieve a vapor pressure of 10-6 mm Hg, T,, and the Ll,z values of these elements. The two curves bracket the range of T, values for most of the elements studied
The elements fall into three groups: the behavior of the very volatile IIB elements is distinct from that of the less volatile ones, while the majority falls into the narrow range delineated by the two curves of Figure 9. The anomalous behavior of Ca, Sr, Cr, and perhaps Mg, exhibiting a much lower L1i2 than expected based on their vapor pressures, is in keeping with their ready formation of stable oxides. DISCUSSION The pertinence of the Beer-Lambert law to atomic absorption analysis has been discussed ( I ) . Assuming that the gases are transparent and homogeneous, its predictions, verified for solutions, would be expected to apply to the gas phase. Absorption would be expected to increase as a function of pathlength, providing the concentration of the absorbing species, i.e., the neutral atoms, is uniform and other factors do not alter this species, its potential absorbing path, or its ANALYTICAL CHEMISTRY, VOL. 42, NO. 8 , JULY 1970
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homogeneity. While studies of the atomic absorption of zinc and cadmium were in accord with such expectations (I), relatively little work has been reported that bears on these questions. The absorption profiles and estimated life time of the free atoms of eleven elements (Ag, Au, Co, Cu, Hg, In, Mg, Na, Ni, Pb, T1) along the flame path have been measured using a propane burner and a quartz cell, aligned vertically with respect to the monochromator (18). Neutral atoms of a number of metals, Le., Ag, Au, Cd, Hg, and to a lesser degree Cu, Ni, and Pb, exist outside of the flame zone (la),a finding confirmed by other investigators (19) for Ag, Co, Cu, Fe, and Pb, using the experimental arrangement employed in earlier (1)and the present studies. Preliminary attempts to correlate the dissociation energy of oxides and the vapor pressures of metals with the enhancement of atomic absorption in the long pathlength cell were inconclusive (20,21). These and earlier studies (1)have shown that the atomic absorption of most elements studied is not a simple function of the abundance of different metals, and the anion species in solution. Dependent on the prevalent temperature and on oxidation-reduction conditions, a salt particle vaporizes and dissociates, resulting in the formation of neutral atoms, the only species responsible for atomic absorption at the specific resonance lines of a given element. In addition, a number of other chemical reactions occur which are not related directly to these primary processes, e.g., the formation of hydrides, oxides, hydroxides, and other molecular species (22). All of these processes jointly govern the characteristics of the absorption profiles observed, and, hence, the ultimate limit of detection achieved with the long pathlength flame system. Mansfield et al. (23) and de Galan and Winefordner (24) have analyzed the dependence of the product of the aspiration efficiency, E, and the fraction of the free atoms, 0, to relate factors such as gas velocity, droplet size, and flame temperature to the type of atoms and the flame gas temperature and its composition, in, for instance, laminar C2H2/air flames. Similar analyses of the present somewhat different system are indicated. The present studies were designed to provide an understanding of the physical-chemical basis underlying the variations in sensitivity of detection for individual elements in the long pathlength system. The vapor pressure of a metal plays a dominant role in determining the effectiveness of the long absorption cell in providing increased sensitivity for most elements. Sensitivity of detection for Cd, Hg, and Zn is a function of pathlength over a range of about 1 m, perhaps owing to the high vapor pressures and stabilities of the neutral atoms of these elements along the flame path. Over a much shorter range such proportionality to pathlength can be observed for measurements of Ag, Au, As, Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Pd, and Sn, but fails to apply to Sr. In all instances the physicochemical properties of the element at the temperature of the flame apparently determine the resultant behavior in the analytical system. (18) Yu.B. Zeljukova and N. C. Poluektov, Zh. Anal. Khim., 18, 435 (1963). (19) J. Stupar, Microchim. Acta, 1966,722. (20) I. Rubeska and B. Moldan, Analyst, 93,148 (1968). (21) I . Rubeska, Anal. Chim. Acta, 40, 187 (1968). (22) A. G. Gaydon, “The Spectroscopy of Flames,” John Wiley & Sons,New York, N. Y . , 1957, p 74 and pp 223-225. (23) J. D. Winefordner, C. T. Mansfield, and T. J. Vickers, ANAL. CHEM., 35, 1607 (1963). (24) L. de Galan and J. D. Winefordner, J. Quant. Spectros. Radiat. Transfer, 7 , 251 (1967).
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While the concentration profile characteristics correlate highly with the vapor pressures of the various elements (Figure 9), the absolute concentrations of the neutral atoms can depend on their known chemistries (25). This might provide a basis for the observed absorption profiles as follows: mercuric chloride readily forms very stable monoatomic species which are stable at room temperature and which do not react directly with oxygen even at high temperatures, owing to the high vapor pressure of this element. The vapor pressures of Cd, Pb, and Zn are exceptionally high, accounting for their absorption over a long path. While the free atoms of the noble metals Au (Ag), Pd, and Pt are also resistant to oxidation, their vapor pressures are relatively low, probably accounting for the lowered absorption profiles observed as the temperature along the cell path decreases. Copper forms many compounds in the air-hydrogen flame, e.g., CuH, CuOH, and CuO (25). The low volatility and indissociable oxides of Co, Cr, Fe, and Ni likely account for their rather attenuated absorption profiles. The oxides and hydroxides of Ca and Sr form readily and are involatile, leading to the sharp, limited absorption of these atoms. All data obtained must be considered with reference to the particular flame conditions employed, of course, The airhydrogen system used in these studies is a low temperature flame having, theoretically, a maximum temperature of about 2100 “C (22). Owing to heat losses, the measured temperature values are usually somewhat lower. Thus a value of 1967 “C has been found (26). Others, varying from 1909 to 2014 “C as a function of hydrogen flow rates are found to be lowered to 186Ck1830 “C, when water is atomized (8). In the present study, the average temperature value was 1430 “C when water was atomized at a flow rate of 2 ml/min. Under these conditions and at comparable concentrations, the absorption signal of elements such as AI, Mo, Si, Ti was either not detectable or minimal, as with Ba. These elements are either nonvolatile or readily form undissociable oxides or hydroxides in the flame; higher temperature flames may be required for their detection (27). However, except in these cases, the air-hydrogen flame possesses major advantages . when used in conjunction with the long absorption cell system; background absorption is very low over a wide wavelength range; it is quiet and stable, and-in contrast with acetylene flames-is completely free of carbon residue. The distribution of neutral atoms in the flame is a complex function of temperature and composition. In turn, the temperature depends on the hydrogen flow rate which controls the oxidation-reduction environment in the flame path : a lean mixture raises the temperature along the path, thus, prolonging distance over which volatilization occurs. However, such a mixture also promotes the formation of oxides, thereby reducing the population of neutral atoms. A hydrogen-rich flame, while cooler, provides a reducing atmosphere which impedes the formation of oxides. Thus the optimal hydrogen flow rate and temperature are likely to differ for different elements or groups of elements. These results show that the neutral atoms of most elements are stable at relatively low temperatures (500 1000 “C). To maintain the population of neutral atoms within the flame, higher temperatures are required only for involatile or readily oxidizable elements. These observations form part of the basis for the variation
-
(25) N. V. Sidgwick, “The Chemical Elements and Their Compounds,” Clarendon Press, Oxford, 1950. (26) E. Pungor, “Flame Photometry Theory,” Van Nostrand, London, 1967, pp 158-159. (27) J. B. Willis, Appl. Opt., 7 , 1295 (1968).
in the absorption profiles of the various elements in the long pathlength system. While providing understanding of the physical bases which underlie the behavior of the various elements in the long pathlength cell system, these studies also indicate means by which to establish a more optimal analytical system for different elements. All data obtained for the elements under study have been categorized in terms of the Ll12, defined as that distance along the cell path where absorption will fall to one half of the maximal absorptivity. The Llp value of each element may be employed to select the optimal cell length for analysis. With the exception of cadmium, mercury, and zinc, the LllZ value for most of the elements is approximately 35-60 cm, and, hence, a length of 60 cm provides the optimal analytical sensitivity. Cells exceeding this length might well decrease sensitivity owing to the interference produced by the flame reactants in areas of the cell where specific absorption no longer takes place. At the temperature enployed here, long cells are not particularly advantageous analytically for the measurement of elements such as calcium and strontium which form involatile oxides and where absorption is limited to a small area of the flame as a consequence. The use of hotter flames for this purpose requires further study. The ring burner employed in these studies is very stable and, when used in conjunction with the absorption cell, virtually
isolates the flame path from the ambient air. As a consequence, outside air does not dilute the elemental vapor as in conventional open flames. Thus both greater efficiency and sensitivity are achieved while ready control of the oxidation reduction environment along the flame path is possible. Highly sensitive measurement of absorption in the far UV region, e.g. of As 1937 A (6), is also possible using a nitrogen or argon atmosphere with a minimal amount of air entrained to support combustion. These detailed studies have described the atomic absorption behavior of a number of elements as a function of the distance along the cell path. Several important parameters have been shown to determine absorption. These include the temperature, the vapor pressure of the various elements, and the chemical reactivity of the elements with other components of the combustion mixture. Additional studies of this type will be required to define completely the basic considerations pertinent to the atomic absorption of each element in the long pathlength system, and should lead to even greater applicability of the atomic absorption system for analytical purposes. RECEIVED for review January 22, 1970. Accepted April 20, 1970. Work supported by the International Lead Zinc Research organization, Inc., New York, N. Y .
ElectrochemicaI Behavior of Tr ipheny Itin Compounds and Their Determination at Submicrogram Levels by Anodic Stripping Voltammetry M. D. Booth and B. Fleet Department of Chemistry, Imperial College of Science and Technology, London, England
The electrochemical reduction of triphenyltin compounds in nonaqueous solution has been investigated by polarography, controlled potential electrolysis, and cyclic voltammetry and found to involve two 1-electron reductions. The first step involves the formation of a radical ion which is further reduced in the second step to the triphenyltin anion. The appearance of a prewave in the polarographic method indicated strong adsorption at the mercury surface of the radical ion. The phenomena has been utilized in the determination of trace amounts of triphenyltin compounds by anodic stripping voltammetry to a limit of detection of 10-8M. In addition, a procedure has been developed for determining triphenyltin compounds in potato crops.
IN RECENT YEARS, the interest in organotin compounds has been noticeably greater than in any other organometallic system ( I ) . They are finding increasing application as catalysts, stabilizers, and especially as biocides. The fungicidal properties of organotin compounds was discovered in 1954 (2) but the high phytotoxicity of the alkyl-tins prevented their use in agriculture. It was found, however, that the phytotoxicity of the corresponding aryl derivatives was much lower, and in (1) A. G. Davies, Chem. Brit., 4(9), 403 (1968). (2) G. J. M.Van der Kerk and J. G. A. Luijten, J . Appl. Chem., 4, 314 (1954).
the last ten years triphenyltin compounds have been widely used as fungicides (3). Methods of analysis for triphenyltin compounds have mostly been based on the determination of inorganic tin after breakdown of the complex (4), but a direct spectrophotometric procedure has been described by Hardon et al. (5) Electrochemical methods have been applied but again only for the determination of inorganic tin following breakdown of the complex (6-9). For the purposes of fungicide residue analysis this type of procedure is nonspecific. Triphenyltin compounds are reduced directly at the dropping mercury electrode, and this offered the possibility of a direct procedure. A general survey of the electrochemistry (3) K. R. S. Ascher and S. Nissim, World Rev. Pest Conrr., 3 (4),
188 (1964). (4) K. Burger, Report of the Farbewerke Hoescht A.-G., Gendor1958. ( 5 ) H. J. Hardon, H. Brunink, and E. W. Van Der Pol, Analyst, 85, 847 (1960). (6) S . Gorbach, and R. Bock, 2.Anal. Chem., 163,429 (1958). (7) J. Vogel and J. Deshusses, Helv. Chim. Acta, 47. 181 (19641. . , (8) Thompson-Hayward Chemical Company, Pest.. Anal. Manual Vol 11, 1967. (9)P. Nangniot and P. H. Martens, Anal. Chim. Acta, 24, 276 (1961). ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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