discharge in terms of two independent systems, the expanding cloud of electrode material vapor and the current modulated supporting plasma. Further, reactions should be considered in two spatially separated regions of the discharge, the channel region where the electrode material strongly interacts with the supporting plasma and the fringe region where the electrode material interacts only weakly, and in an indirect way with the plasma. The channel region is dominated by charge transfer excitation reactions involving material sampled directly into singly and doubly ionized levels. Neutral atom sampling may be active early in time and channel recombination is probably important late in time. Direct sampling into useful states of the neutral atom seems unlikely in regions of high current and high particle density. Attempts at controlling channel intensities must be made in the space charge region since this is where channel radiation patterns are determined. The fringe regions are governed primarily by step-wise relaxation from doubly ionized levels of aluminum into singly ionized levels, and finally into levels of the neutral atom. Relaxation may be by direct recombination into low lying levels or by stepwise cascading. Further experiments are suggested on a system having more intermediate states which radiate in the visible and ultraviolet regions. An analytically important consequence of step-wise fringe relaxation is the lack of direct current control over the radiation patterns of neutral atom species. The neutral atoms are separated from the current sensitive doubly charged ions by two or more kinetically controlled relaxation steps. Thus
neutral atom radiation will respond only in an indirect way to instrumental changes in the discharge current. The nature of the interaction between the discharge channel and the expanding cloud of electrode material presents several possibilities for obtaining chemical and instrumental control over relative line intensities. Control may be exerted on the time and volume of interaction by changing the rate of channel growth relative to the radial movement of electrode material. The radial and axial velocities of electrode material can be changed by changing the chemical nature of the host material, thus changing the residence time of electrode material in both the channel and the space charge regions. The nature of the electrode material may also control the relative line intensities of the plasma species through a resonance condition in charge transfer reactions. Finally, the nature of the interaction can be changed by controlling the composition of the plasma by varying the pressure and chemical composition of the discharge atmosphere. RECEIVED for review July 24, 1969. Accepted October 30, 1969. The financial support of the National Science Foundation through Grant GP-7796 is gratefully acknowledged. The donation of the apparatus used in spark source construction from General Motors Research Laboratories, Warren, Michigan is appreciated. One of us (R. D. S.) gratefully acknowledges the Ethyl Corporation, the American Oil Foundation, the University of Wisconsin Graduate School, and the Wisconsin Alumni Research Foundation for financial aid in the form of graduate fellowships.
Atomic Absorption Interferences of Tin P. 0.Juliano and W. W. Harrison Department of Chemistry, Unicersity of Virginia, Charlottesville, Va. 22901 Interferences of various metals and inorganic acids on tin atomic absorption in a premix air-hydrogen flame were observed over a wide range of flame conditions. In general, the inorganic acids depressed tin absorption, while the alkali and alkaline earth metals produced enhancements. The study of the effect of refractory oxide formers showed that aluminum had a negative effect, while titanium exhibited a positive effect. Among the transition metals investigated, cobalt and copper produced enhancements while zinc had no effect. Comparative studies done in the airacetylene and nitrogen-hydrogen-entrained air flames showed that although tin has a greater sensitivity in the hydrogen flames, it is also much more subject to interferences than in the air-acetylene flame.
N o DETAILED STUDY of interferences in the atomic absorption determination of tin has appeared in the literature. Agazzi (1) reported that 100 ppm (as P) of phosphate and pyrophosphate depressed tin absorbance by 50 and 25 %, respectively, in a long path absorption cell. Using the common premix burner, Capacho-Delgado and Manning (2) determined the extent of interference with phosphoric, sulfuric, and nitric acids in both the air-acetylene and air-hydrogen
x
( I ) E. J. Agazzi, ANAL.CHEM., 37,364 (1965). (2) L. Capacho-Delgado and D. C. Manning, Spectrochinz. Acta, 22, 1505 (1966). 84
flames at various concentrations. They also observed that lead, copper, zinc, and nickel, at concentrations of 1000 ppm do not interfere by more than 3 in the air-acetylene flame, whether individually or in combination. Amos and Willis (3) have noted that 5000 ppm of sodium (as the hydroxide) depressed tin absorption by 15 in the air-hydrogen flame, whereas no interference was found in the air-acetylene flame. The present study is primarily an investigation of the various interferences on tin absorption in the air-hydrogen flame. Although high sensitivity is obtained, previous study ( 4 ) has shown that tin is subject to certain severeinterferences in such a flame. Before the higher sensitivity of the air-hydrogen flame can be used to best advantage, it should be determined what general ionic interferences occur and to what extent. Interferences were observed in this study over a wide range of concentrations and flame conditions in order to determine the effect of these parameters. Many of the experiments were repeated in an air-acetylene flame to allow meaningful comparisons between the two common flame systems. Since inert gas-hydrogen-entrained air flames are rapidly proving to be very sensitive for tin, a comparative investigation of certain interferences in the nitrogen-hydrogen-entrained air flame was included. (3) M. D. Amos and J. B. Willis, ibid., p 1325. (4) W. W. Harrison and P. 0. Juliano, ANAL.CHEM., 41, 1016 (1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
burner head. At these conditions the acetylene flame is a n intense yellow while the hydrogen flames are almost invisible. These standard conditions were selected because they produced a maximum tin absorption, but the effect of varying flame composition and monitored height in the flame was also determined for each interferent-tin system. Unless otherwise stated, the flame parameters will be as summarized above.
I
2.0
RESULTS AND DISCUSSION
I
‘/o
I
4 PHOSPHORIC
2
I
6 ACID
I
8 (V/V)
10
Figure 1. Effect of phosphoric acid on tin atomic absorption in air-hydrogen flame a t various fuel to oxidant ratios Sn, 50 ppm; height above burner head, 5 mm; air, 8.8 liters/min.; curve A , fuel to oxidant ratio of 2.0; B , 2.7; C , 3.3
EXPERIMENTAL
Apparatus. A Jarrell-Ash Model 82-360 atomic absorption/flame emission spectrometer was used to obtain all data. A Perkin-Elmer premix burner with a three-slot Boling head was used with a single pass through the flame. The reference point for the measurement of height in the flame was so set that zero height is read when the light beam from the hollow cathodz tube just touches the top of the burner head. The analytical wavelength was 2246 A, and the light source was a tin hollow cathode tube (Perkin-Elmer No. 621G), operated at 20 mA. Gas flow rates were carefully regulated and were monitored with calibrated flow meters. Reagents. All solutions were made in 10% hydrochloric acid (concentrated acid v/v) in order t o stabilize tin in solution. Hydrochloric acid has no effect on tin absorption as previously reported ( 2 ) . A standard tin stock solution was prepared from reagent grade metal (Baker and Adamson). All the acids and the inorganic salts used were either reagent grade or the highest quality available. Metal solutions were made from their chloride salts with the exception of aluminum, calcium, and copper which were prepared from the nitrate, carbonate, and sulfate, respectively. Zinc and titanium solutions were prepared from the metal. Appropriate blanks were prepared for each test solution at each concentration employed for zeroing of the spectrometer. Laboratory distilled water passed through a mixed bed, ion exchange column (Type R-2 Cartridge, Illinois Water Treatment Co., Rockford, Ill.) was used for all dilutions. Flame Parameters. For maximum sensitivity, “reducing” flames are usually used in the atomic absorption analysis of tin. The standard flame parameters used in this study were set to produce flames of high fuel to oxidant ratios. The settings are summarized below. Air-hydrogen flame: fuel to oxidant ratio, 2.7; air flow rate, 8.8 liters/min.; height in the flame, 5 mm above burner head. Air-acetylene flame: fuel to oxidant ratio, 0.21 ; air flow rate, 11.O liters/min. ; height in the flame, tangent to burner head. Nitrogen-hydrogenentrained air flame: fuel to inert gas ratio, 1.4; nitrogen flow rate, 10.4 liters/min.; height in the flame, 5 mm above
Effect of Acids. Capacho-Delgado and Manning ( 2 ) have studied the effects of phosphoric, sulfuric, and nitric acids on tin absorption in a n air-hydrogen and a n air-acetylene flame, Our results were somewhat similar to theirs in that nitric acid produced almost no effect in either flame while phosphoric and sulfuric acids caused serious absorption depressions in the air-hydrogen flame of up to 50% and 57< by volume of the acids. The magnitude of these effects could not be explained by the ditkences in measured sample flow rate with and without the acids present. Furthermore, in the air-acetylene flame, phosphoric acid caused a n absorption enhancement. Because flame composition has been shown to be a n important parameter in interference studies ( 5 , 6), the fuel-to-oxidant ratio of the air-hydrogen fiame was varied to see what effect, if any, would be produced 011 the acid-tin interference. Nitric and sulfuric acid effects showed little change as the flame composition was varied, but the phosphoric acid depression indicated a dependency on this parameter. As shown in Figure 1, this interference could be made to appear negative or positive by variation of the fuel-to-oxidant ratio. Sulfuric and nitric acids did not exhibit a n absorption enhancement in this flame under any conditions studied. To determine if this was a n effect unique to the a h h y d r o g e n flame, the experiments were repeated in a n air-acetylene flame where the fuel-to-oxidant ratio was similarly varied. Again, the phosphoric acid interference was strongly dependent upon the flame composition with the relative absorbance decreasing as the flame becomes more reducing. The similarity of response in both the hydrogen and acetylene flames, which are quite unlike in terms of available reactive species, suggests that the effect is probably not a perturbation of basic flame chemistry, but rather is related to the specific chemistry of phosphorus and tin in the flame. Effect of Alkali Metals. The experimental results in Figure 2 show that in a n air-hydrogen flame, cesium, rubidium, and potassium enhanced tin atomic absorption while lithium and sodium produced slight depressions. The results for sodium agree reasonably well with the value of 15% reduction of tin absorption reported by Amos and Willis (3) for 5000 ppm of sodium. Experiments done in the air-acetylene flame produced similar results except that the depressive effects of lithium and sodium were essentially eliminated. Alkali metals are known to produce ionization interference because of their low ionization potentials (5, 6). Cesium, rubidium, and potassium have ionization potentials of 3.87, 4.16, and 4.32 eV, respectively, while those of lithium and sodium are significantly higher (5.36 and 5.12 eV, respectively). This large difference in ionization potential from
(5) W. Slavin, “Atomic Absorption Spectroscopy,” Interscience, New York, N. Y . , 1968. (6) R. Herrmann and C. Th. J. Alkemade, “Chemical Analysis by
Flame Photometry,” 2nd revised ed., Interscience; New York, N. Y . , 1963.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
e
85
1.41
-j
al
a-
$
’”
m 0.9I
0
Na ’
0
. 500
0
1000
PPM ALKALI
8
1500
I
I
500
1000 PPM
8 2000
I
1500 METAL
2000
Figure 3. Effect of alkaline earth metals on tin atomic absorption in air-hydrogen flame
METAL
Figure 2. Effect of aikali metals on tin atomic absorption in air-hydrogen flame
Sn, 50 ppm; see text for standard flame conditions
Sn, 50 ppm; see text for standard flame conditions
state directly would be very useful but is hampered by the very high concentration of tin solution required to obtain a reasonable Sn+ absorption and also by the lack of agreement in assigning lines t o the SnI’ state (9-11). An alternative proposal would be t o assume that the ionized species is not the tin atom but a more easily ionized tin molecular species. The ionization of alkaline earths in a nitrogen-oxygen-hydrogen flame represents a lower energy process than that of direct ionization of the atom (12). Hydroxide ionization was proposed as one possible explanation and was demonstrated by mass spectrometric methods for strontium (13). In the case of tin, the chloride can probably be eliminated as the ionized species because it dissociates in the flame, as evidenced by the lack of interference from hydrochloric acid, thereby leaving the oxide and hydroxide as possibilities. Thus, the suppression of a significant tin oxide or tin hydroxide ionization by alkali metals could also lead t o the tin atomic absorption enhancement shown in Figure 2, since the resulting increase in the oxide or hydroxide population would in turn increase tin atomic population through a shift of the equilibrium among these species. Sensitive methods for monitoring the ionic hydroxide and oxide concentrations are required t o study the problem further. I n addition to ionization effects, solute vaporization interferences should also be considered as possible causes for tin atomic absorption perturbations. Alkemade (14) has pointed
potassium to sodium could contribute to the disparate effects within the family shown in Figure 2. If it is assumed that the increase in tin atomic absorbance results from complete suppression of tin ionization, then about 20% of the tin atoms were in ionic form in both the airhydrogen and air-acetylene flames. This value is very high, particularly for tin which happens to have a rather high ionization potential (7.30 eV). These observations suggest the possibility of chemi-ionization of tin. In the air-acetylene flame the presence of ions of high ionization potentials, namely, HCO+ and HaO+, are presumably formed in the following steps (7).
+ 0 + H C O + + eHCO+ + HzO + CO + H30+ CH
(1)
Ionization of the metal atoms has been suggested in the following manner (8).
R+
+M
-
M+
+ R (disintegrates)
(3)
where M is the metal atom and R+ is either HCO+ or H30+. The high ionization potentials of HCO+ and H 3 0 + make this reaction energetically favoiable. The formation of these ion species requires a large amount of energy and, as seen in Reaction 1, this is provided by the formation of the C-0 bond. In the air-hydrogen flame, no such highly energetic ion species are known to be present in significant quantities; the O-H and H-H bonds are much weaker than the C-0 bond. However, from the similarity of the atomic absorption interference curves of the alkali metals in both hydrogen and acetylene flames, it appears that tin is undergoing the same type of ionization in both flames. A study of the tin ionic (7) C. P. Fenimore, “Chemistry in Premixed Flames,” Pergamon Press, New York, N. Y., 1964. (8) E. M. Bulewicz and P. J. Padley, Combust. Flame, 5 , 331 (1961). 86
0
(9) W. R. Brode, “Chemical Spectroscopy,” 2nd ed., John Wiley and Sons, New York, N. Y., 1943, p 570. (10) G. R. Harrison, “M. I. T. Wavelength Tables,” John Wiley and Sons, New York, N. Y., 1939, p 570. (11) W. F. Meggers, “Tables of Spectral-Line Intensities, Part I,” National Bureau of Standards Monograph 32, U. S. Government Printing Office, Washington, D. C., 1961, p 390. (12) T. M. Sugden and R. C. Wheeler, Discussions Faraday Soc., 19, 76 (1955). (13) K. Schofield and T. M. Sugden, Tenth Intern. Symp. Cornbustion, Combustion Institute, Pittsburgh, Pa., 1965, p 589. (14) C. Th. J. Alkemade, ANAL.CHEM., 38,1252 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
3
.
0
r
W W
O_ 1.3
0
v,
m Q
\
9
3w
E
l
I
- -
0.9
0
100
PPM
200
300 ( M g )
TIN
n a
Figure 4. Effect of tin on alkaline earth absorption in airhydrogen flame
I 500
1000 1500 2000 METAL Figure 5. Effect of cobalt, copper, and zinc on tin atomic absorption in air-hydrogen flame Sn, 50 ppm; see text for standard flame conditions
"'"0
PPM
Analyte concentrations: Mg, ppm; Ca, 4O0ppm; Sr, 40 epm. Analytical wavelengths: 2852 A for Mg, 4227 A for Ca, 4607 A for Sr
out that such effects can create positive or negative deviations. The addition of potential interfering agents could produce enhancements or depressions of tin absorption depending upon whether a more or less volatile compound o r matrix formation occurs during the desolvation process. Other possible explanations for tin atomic absorption interferences include oxygen transfer reactions between tin-oxygen species and concomitants as well as shifts in free tin atom population with variation of experimental parameters. Quite likely a combination of these effects may be responsible for the net results of tin as shown with the various groups of elements tested in the present study. Effect of Alkaline Earths. The investigations of the effect of alkaline earth metals of tin atomic absorption in the airhydrogen flame showed that enhancement is obtained from all members of the family with the exception of barium which has no effect (See Figure 3). The higher ionization potentials of the alkaline earth metals (7.61, 6.09, 5.67, and 5.19 eV for magnesium, calcium, strontium, and barium, respectively) as compared with the alkali metals would seem to preclude any significant thermal ionization, although chemi-ionization is a possibility. The more easily ionized oxides and hydroxides of the alkaline earths could possibly contribute to the electron concentration in the flame. However, this is contradicted by the fact that barium and calcium, having lower ionization potentials than magnesium, exhibited smaller interference effects. The enhancement of tin absorption by the alkaline earth metals led to the investigation of whether a similar enhancement of alkaline earth atomic absorption by tin would occur. Such a result would be obtained if simple ionization suppression occurs. In the cases of magnesium and calcium, as shown in Figure 4, negative interferences were observed instead, after an initial rise in absorbance at low tin concentration. Strontium, again reacting in a different manner than the other alkaline earths, was strongly enhanced, the enhance-
ment factor reaching as high a value as 9.0 at a fuel-to-oxidant ratio of 3.3 and a tin concentration of 2000 ppm. The effect of tin on barium absorption was not done because the large barium concentration needed to obtain reasonable absorption readings would have necessitated the introduction of very large amounts of tin if the interferent-to-analyte ratio was to be maintained. The tin interference on magnesium and calcium in Figure 4 suggests the possibility of at least two conflicting processes. The initial enhancement, perhaps a case of ionization suppression, quickly reaches a maximum after which a second process, possibly some form of solute vaporization interference, becomes significant, producing an adsorption depression. For the extremely rich fuel-to-oxidant ratio of 3.3, the latter effect appears to predominate even at low tin concentrations and only a n absorption depression is obtained. The magnesium interference on tin increased as the flame became less fuel-rich, while the depressive effect of tin on magnesium decreased under the same variation of conditions. Effect of Transition Elements. Cobalt, copper, and zinc were selected as representative transition elements to study their effect upon tin atomic absorbance. Copper is known to be almost completely atomized in the flame (15). Thus it would not form the oxide or hydroxide to any significant extent, eliminating ionization interference due to molecular species, and because of its high atomic ionization potential (7.68 eV), should not readily undergo such ionization. However, copper, like the alkaline earths, enhanced tin absorption in the air-hydrogen flame, as shown in Figure 5. By decreasing the fuel-to-oxidant ratio, the interference further increased. When the reverse case, the effect of tin on copper atomic absorption was studied, tin had only a negligible effect
(15) L. de Galan and J. D. Winefordner, J . Qunnt. Specrrosc. Radiat. Tramfir, 7,251 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
87
I
A
0.0' 0
I
500
I
1000 PPM
I
1500
I
2000
TIN I
Figure 6. Effect of tin on cobalt atomic absorption in airhydrogen flame at various fuel to oxidant ratios
500 PPM
Co, 40 ppm; wavelength, 2521 A; height above burner head, 5 nun; air, 8.8 liters/min.; curve A , fuel to oxidant ratio of 2.0; B, 2.7; C,3.3; D,4.0
under all flame conditions tested. A repeat of these experiments in a n air-acetylene flame showed that copper again enhanced tin absorption, but by only about 10% at the normal analytical conditions and tin once more had no effect on copper absorption. Cobalt exhibited in the air-hydrogen flame a n interference on tin similar to that produced by magnesium. The positive interference persists at all measured heights in the flame as well as at the several fuel-to-air ratios studied. f h e case of tin interference on cobalt absorption at 2521 A was also studied to check again for possible mutual interference between the two metals. As shown in Figure 6, a severe depression of cobalt absorption by tin was observed. Thus, the effect of cobalt on tin is to increase the tin atomic ground state, while the effect of tin on cobalt is to greatly depopulate the cobalt atomic state. The mechanism of cobalt interference would appear to be ditTerent from that exhibited by copper but similar to that of magnesium. Zinc, like copper a weak oxide former, might be expected to exhibit a similar interference effect on tin, if oxide formation is assumed to play a role in the absorbance shifts. In Figure 5 , it is seen instead that zinc has very little effect on tin absorption and that what little interference is shown is in a negative direction. Likewise, tin has no measurable effect o n zinc absorption. Effect of Refractory Compound Formers. Aluminum interferences on calcium and magnesium are by now classic examples of interferences in flame spectroscopy. The effect of aluminum of tin atomic absorption at different heights in the air-hydrogen flame is seen in Figure 7. Shown is a striking similarity to the effect of aluminum on magnesium in a n air-acetylene flame (16). In both cases the depressive effect of aluminum decreased as higher portions of the flame (16) W. W. Harrison and W. H. Waldin, ANAL.CHEM.,41, 374 (1969). 88
I
1000
I
1500
2000
ALUMINUM
Figure 7. Effect of aluminum on tin atomic absorption in air-hydrogen flame at various heights in flame Sn, 50 ppm; curve A , beam tangent to burner head; B, 5 mm above burner head; C, 10 mm; D, 15 mm. See text for standard flame conditions
were monitored, and as the fuel-to-oxidant ratio was decreased. At higher portions of the flame, an initial rise in tin absorption was observed, suggesting the presence of two conflicting interference processes. The experiment concerning aluminum interference on tin was then repeated in an airacetylene flame. In this case, a 10 to 2 0 x enhancement of tin absorption at all flame heights and flame conditions was observed. If aluminum and tin form a refractory mixed oxide, as was suggested in the case of aluminum and magnesium ( I 7 ) , then it appears that the aluminum-tin mixed oxide is so weakly bonded as to be completely dissociated in the airacetylene flame while still creating negative interference in the cooler fuel-rich air-hydrogen flame. The process accounting for the initial rise in tin absorption at high portions of the air-hydrogen flame appeared to become the predominant process in the air-acetylene flame. Since titanium has also been reported to interfere with magnesium absorption in a manner similar to aluminum, a determination of titanium interference on tin absorption was investigated. The results showed that titanium strongly enhances tin absorption in both the air-hydrogen (see Figure 8) and air-acetylene flames with the enhancement becoming smaller as higher portions of the flame were monitored. This is in contrast to the previously described effect of aluminum on tin in the air-hydrogen flame wherein a negative absorbance effect was produced which progressively decreased in the upper region of the flame. This would indicate that no stable mixed refractory oxide is formed by titanium with tin. Interferences in Nitrogen-Hydrogen-Entrained Air Flame. The recent introduction of the inert gas-hydrogen-entrained air flame to atomic absorption greatly increased the sensitivity (17) I. Rubeska and B. Moldan, Atzal. Chim. Acta, 37, 421 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
i;
------- : , I
1.61
1.o
B V
m
m
I
1.
Q
--F---4
II \
w 1. >
I
/I
n -.-nl
0
0
I
I
I
500
1000
1500
PPM
I
20 PPM
I
I
I
40
60
80
100
PHOSPHORIC ACID
Figure 9. Effect of phosphoric acid on tin atomic absorption in the nitrogen-hydrogen-entrained air flame
2000
TITANIUM
Sn, 15 ppm; see text for standard flame conditions.
Figure 8. Effect of titanium on tin atomic absorption in airhydrogen flame at various heights in flame Conditions and labels, same as Figure 7.
of tin analysis. Both argon (18, 19) and nitrogen (20) have been used as the inert gas. Nitrogen, which was used in this study, has the advantage of lower cost over argon although its use would result in a lower flame temperature because of its higher heat capacity. With a premix burner Dagnall e t al. (21) reported a maximum temperature of only 850 OK. At such a low temperature, tin species in the flame should be prone to compound formation. The results in Figure 9, a study of phosphoric acid interference, show a rapid absorbance decrease down to a phosphorus-tin atom ratio of 0.67 after which very little change was noted, indicating the formation of a compound with a stoichiometry corresponding t o that of stannous phosphate. The effect was almost completely independent of flame composition and height in the flame. To further evaluate chemical interference in this flame, the effect of cesium on tin absorption was determined. Cesium should have very little affinity for tin. However, a large depressive effect, though not as severe as that of phosphoric (18) W. W. McGee and J. D. Winefordner, Anal. Chim. Acta, 37, 429 (1967). (19) H . L. Kahn and J. E. Schallis, Atomic Absorption Newsletter, 7, 5 (1968). (20) R. M. Dagnall, K. C. Thompson, and T. S . West, Analyst, 93,518 (1968). (21) Ibid., 92,506 (1967).
acid, was observed contrasting drastically with the positive ionization interference obtained in the air-hydrogen flame. Compound formation is probably not the process involved here. The main effect may be due to solute vaporization interference (14). As the sample droplets are vaporized in the flame, tin atoms become occluded in a cluster of cesium species which, at the low temperature present in the flame, i s difficult to disintegrate. At the temperature of the nitrogenhydrogen-entrained air flame it appears that very little salt is needed to hinder atomization. Unlike the case of phosphoric acid, cesium interference was virtually eliminated at a height of 15 mm above the burner head. This could be attributed to the effect of air entrainment which becomes significant by that height above the burner head. The resulting combustion of the fuel possibly increases the temperature sufficiently to cause the disintegration of the tin-cesium particles formed after the evaporation of water from the sample droplets. The air-hydrogen and nitrogen-hydrogen-entrained air flames are very prone to interferences, particularly the latter. The air-acetylene flame provides a medium for tin atomic absorption which is considerably less subject to interferences, particularly that due to organic solvents (4), and usually produces a smaller effect for those tin interferences common to the hydrogen and acetylene flames. The lower sensitivity of the air-acetylene flame for tin may be offset by preconcentration techniques. RECEIVED for review August 28, 1969. Accepted November 6, 1969.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
89
