Atomization of Suspensions in Atomic Absorption Spectrometry W. W. Harrison and P. 0. Juliano Department of Chemistry, University of Virginia, Charlottesuille, Va. 22901 Atomization of suspensions in atomic absorption spectrometry offers certain advantages. Suspension atomization allows the introduction of elements in specific chemical form, such as oxides, into flames. The preparation of suspensions and the evaluation of polymeric materials as suspending agents are described. The atomization of stannic oxide, stannic sulfide, and stannous oxide suspensions in the turbulent air-hydrogen and oxygen-acetylene flames is compared with that of stannic tin in solution. The interferences of organic and inorganic materials on the tin absorption of suspensions are reported. The direct atomization of suspensions was found to be subject to few interferences from other suspensions or ionic species. Application of the technique to chemical analysis showed that results were dependent on the physical nature of the sample. CONVENTIONAL NEBULIZER-BURNER systems commonly used in atomic absorption spectrometry require that samples be put into solution prior to absorption measurements. I n the study of flame atomization processes, uncertainties may be created by the solution samples in that it is often not clear what compounds are produced in the flame during the desolvation-atomization steps. Proposed flame reactions leading to atomization may depend o n certain species, the presence of which in the flame could only be determined by auxiliary probe techniques, such as mass spectrometry. Advantages could result from the ability to introduce into a flame specific selected compounds in a finely divided molecular whole form rather than as solvated ions. For example, certain metal atomization processes have been suggested to result from the initial formation of oxides with subsequent reduction to a n atomic population. The response characteristics of these oxides directly introduced into a flame could then perhaps provide informative data. We have, therefore, investigated the experimental feasibility of aspirating suspensions into suitable flames for atomic absorption wherein the aspirated samples would maintain their molecular integrity prior to flame reaction. There are also certain practical analytical advantages to the projected use of suspensions in atomic absorption spectrometry. If samples which involve laborious dissolution steps, such as ores or cements, could be analyzed directly from suspensions, great savings in time could be expected. We have attempted to show in this study that by proper selection of standards, this could be a n advantageous approach for certain materials. Gilbert ( I ) suggested the utility of aspirating suspensions for flame emission studies. Using a soil suspension in 1 :1 glycerol-isopropanol, he reported a qualitative analysis from flame emission spectra in turbulent oxygen-hydrogen and oxygen-acetylene flames. H e concluded that quantitative analyses could also be carried out, but did not report any such work. Recently, Levedev ( 2 ) analyzed the alkali metals in minerals also by flame emission through atomization of prepared suspensions in water. He found the sensitivity of solutions to be consistently higher than that of suspensions and (1) P. T. Gilbert, ANAL.CHEM.,34, 1025 (1962). (2) V. I. Levedev, Zh. Anal. Khim., 24, 337 (1969).
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the intensity of emission of the analysis element to be dependent upon the particular mineral. He also observed that when suspension particle size is sufficiently reduced, emission intensity becomes independent of this parameter. Muzgin and Lisienko (3) determined the feasibility of doing analysis by spraying suspensions into a spark discharge and measuring the emission of the analyte. To keep their sample in suspension, they bubbled air through the mixture during analysis. Ramirez-Munoz et al. (4) have reported the use of suspension techniques in the atomic absorption analysis of insoluble materials. This study was undertaken to investigate the suspension aspiration of selected tin compounds with the aim of comparison to results obtained from similar studies on tin solutions and also t o explore the application of suspension atomization to the analysis of tin by atomic absorption spectrometry, extending our previous tin studies (5,6). EXPERIMEXTAL
Apparatus. The instrument used was a Jarrell-Ash atomic absorption flame emission spectrometer, Model 82-360, modified for single pass measurements. A tin hollow cathode tube (Westinghouse WL 22941 AX), operated at 10 m h was used as light source. The tin resonance line at 2246 A , reported to be the most sensititive for atomic absorption (7), was used for all the experiments. Large bore Beckman total consumption burners were used for the direct aspiration of suspensions in the hydrogen and acetylene flames. Aspiration rate measurements revealed that no significant differences in rates could be observed between the aqueous solutions and suspensions using the large bore burners. F o r absorption and emission profile measurements, zero height was set at the point of tangency of the light beam with the burner tip. Reagents. Powdered reagent grade stannic oxide (Baker and Adamson) and stannous oxide (J. T. Baker) were used for the preparation of suspensions while the stannic sulfide (Research Organic/Inorganic Chemical Corporation) used was 99% pure. Gelatin (Difco Laboratories) and starch (J. T. Baker) solutions were prepared by dissolving the weighed amount of material in hot water. Triton X-100 was obtained from Rohm and Haas Company, the activated charcoal from Fisher Scientific Company. In order to prevent the hydrolysis of tin, stock solutions prepared from the metal (Baker and Adamson) were made in 10% hydrochloric acid (concentrated acid v/v). Dilutions were made using laboratory distilled water passed through a mixed bed ion exchange column (11linois Water Treatment Co.). To compensate for any tin impurity that might be present in reagents, appropriate blanks were prepared for each test solution and suspension. Preparation of Suspensions. The water-insoluble tin compounds used in this investigation were prepared as aqueous suspensions. Stock suspensions were prepared by weighing
(3) V. N. Muzgin and D. G. Lisienko, Zh. Anal. Khim., 24, 666 (1969). (4) J. Ramirez-Munoz, M. E. Roth, and W. F. Ulrich, 20th Pitts-
burgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 2-7, 1969. ( 5 ) W. W. Harrison and P. 0. Juliano, ANAL.CHEM.,41, 1016 (1 969). (6) P. 0. Juliano and W. W. Harrison, ibid., 42, 84 (1970). (7) L. Capacho-Delgado and D. C. Manning, Spectrochim. Acta, 22, 1505 (1966).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
the desired amount of material into a volumetric flask which was then partially filled with water, shaken manually until complete dispersion had been achieved, and then diluted to the mark. I n the case of the ore samples, proper dispersion could not be achieved without prior grinding in a mortar and pestle. With proper dispersion, pipetting of aliquots from the stock suspensions was quite feasible. After the stock suspension was thoroughly shaken, the required volume was transferred by ordinary solution pipets. In the normal time required for the pipetting step, n o settling problem was noted. The stock suspension aliquots were then diluted with distilled water to provide working suspension standards. When absorption readings were taken for four replicate aliquot suspensions from a common stock stannic oxide suspension, reproducibility was within the reading error of the instrument. A properly formed suspension could easily be volumetrically manipulated and aspirated. Measurement of Absorption. To minimize the effect of settling, samples were shaken prior to aspiration and readings taken as soon as a stable reading was attained, a process which normally requires no more than 5 seconds of suspension aspiration. Duplicate readings were always taken. When 12 successive absorption measurements were taken for a single suspension, shaking the sample thoroughly before each individual reading, the deviation was again within instrumental limitations of 1 %. Measurement of Polymeric Stabilization of Suspensions. The stability of colloids in water can be increased if the particles are coated with a high molecular weight lyophilic species such as gelatin (8). While we are not concerned with colloids in this study, the possibility of suspension stabilization by gelatin o r other polymers had been reported before (4) and, in this study, the effect of gelatin, starch, and Triton X-100 (alkyl phenoxy polyethoxy ethanol, a surface active polymer produced by Rohm and Haas Company) o n the stability of suspensions was determined. All three materials improved suspension stability (gelatin > starch > Triton X-100) but were not used with the tin compounds in this study because they did not require the addition of these materials. No significant absorbance change was noted within the period required for sample measurement. However, with other suspension materials, such stabilizing agents might be of value o r even necessary. Particles passing through a 200-mesh screen have been suitable for suspension studies without stabilizing agents. RESULTS AND DISCUSSION Atomization of Tin Suspensions. When suspensions are introduced into a flame, the atomization process may be expected to proceed through desolvation of the spray droplet, which may contain several suspension particles, followed by vaporization of the residue and its subsequent decomposition. The available tin compounds studied have relatively low vaporization o r decomposition points: stannous oxide decomposes at 1080 "C, stannic oxide sublimes at 1800 to 1900 "C, and stannic sulfide decomposes at 600 "C. There is, therefore, reason to expect reasonably efficient vaporization and atomization of these suspensions. Figure 1 shows the absorption profiles of stannic oxide suspension in the turbulent air-hydrogen flame a t varying fuel to oxidant ratios. These results for stannic oxide suspensions were then compared t o data from tin solutions using the same concentrations, burner, and experimental parameters. Almost identical absorption profiles were obtained, and the effect of fuel to oxidant ratio was likewise very similar to Figure 1. I n short, there seemed to be little difference in (8) F. Daniels and K. A . Alberty, "Physical Chemistry," 3rd ed., John Wiley and Sons, Inc., New York, N.Y., 1967, pp. 293-5.
"I
0001
io
'
'
,
20 30 40 HEIGHT ABOVE
I
1
1
50 60 70 BURNER (rnrn)
ao
Figure 1. Atomic absorption profiles of stannic oxide suspension in the turbulent air-hydrogen flame Sn, 250 ppm; air, 2.5 liters/min; curve A , fuel to oxidant ratio of 4.7; B, 7.0; C , 9.4;
D,12.0
aspirating the stannic oxide suspension as compared to the same concentration of tin in solution. The absorption profiles of stannic sulfide and stannous oxide are not shown because of their similarity, also, to Figure l . The sensitivities of both stannic oxide and stannic sulfide suspensions, which were within 2 % of each other, were within 8 % of that of tin solution. However, the comparison of tin suspension and tin solution relative sensitivities must also consider concentration. At the tin working concentrations for a turbulent air-hydrogen flame, usually 200 ppm and under, working curves prepared for suspension and solution are almost congruent. At higher concentrations, the two curves bend off considerably toward the concentration axis with the suspension showing the greater deviation. Thus, as the concentration increases, the sensitivity of the solution is increasingly greater than the suspension. Comparing still another tin compound, stannous oxide, a sensitivity of only about 10% that of tin solution was found a t the same concentrations and conditions where the stannic oxide and stannic sulfide yielded sensitivities almost equal to the tin solution. The absorption profiles of stannous oxide were, however, similar in shape t o the other suspensions. SnO emission at 3585 A was measured for the tin samples. The SnO emission intensities of tin solution, stannic oxide suspension, and stannous oxide suspension were found to be in the relative order of 1.00:0.93 :0.23 and all yielded maxima appearing a t a position in the flame higher than the tin atomic absorption maximum. The most probable reason for the lowered sensitivity of stannous oxide is that the dissociation of the stannous oxide suspension particle is not as efficient a s the others. The fact that the atomization of stannic oxide and stannic sulfide closely approximates that of tin solution suggests that the atomization of these species may proceed in a similar manner. No significant difference in the position of tin absorption maximum was observed for the compounds studied. The behavior of stannic oxide in a turbulent nitrogen-hydrogen-entrained air flame using the same burner was also studied briefly. The sensitivity and absorption profiles were similar to that of tin solution, indicating that the
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
249
3'51-----T
I
040/
3.0
W
0
f
am
020
Q:
010
00Q1
0
30
20
10
HEIGHT
ABOVE
40
60
50
70
05
BURNER (mm)
0
Figure 2. Atomic absorption profiles of stannic oxide suspension and stannic tin solution in the turbulent oxygen-acetylene flame
I
'/o
I
20 40 METHANOL
I
I
I
60 80 100 BY VOLUME
Figure 3. Effect of methanol on atomic absorption of stannic oxide suspension in the turbulent air-hydrogen flame
Sn, 200 ppm; air, 2.5 liters/min; fuel to oxidant ratio of A , 4.7; B, 7.0; C, 9.4
suspension atomization is relatively efficient even a t low flame temperatures. The absolute sensitivity for both solution and suspension was increased by 80z over the air-hydrogen flame under comparable conditions, in keeping with previous reports for laminar burners (7). If the low vaporization rate of stannous oxide in the airhydrogen flame is caused by poor thermal dissociation, then the use of a hotter flame should reduce this factor. To investigate this possibility, the behavior of stannous oxide suspension and tin solution in a turbulent oxygen-acetylene flame was observed. Tin absorption of stannous oxide suspension was lower than that of the solution by only a factor of four, a n improvement over the factor of ten lowering in the 250
40
60 BY
80
100
VOLUME
Figure 4. Effect of methanol on the atomic absorption of stannic oxide suspension in the turbulent oxygen-acetylene flame
Sn, 1000 ppm for both solution and suspension; oxygen, 2.5 liters/min; curves A and B, fuel to oxidant ratio of 1.2; C and D, 1.7; curves A and C, stannic oxide suspension; Band D,tin solution
0.01 0
20
70 METHANOL
Sn, 500 ppm in SnOz; oxygen, 2.5 liters/min; curve A , fuel to oxidant ratio of 1.2; B, 1.7
air-hydrogen flame. When the experiment was repeated for stannic oxide and stannic sulfide suspensions, tin absorption was lowered by about 30 and 50 %, respectively, as compared with tin in solution. Much of this difference compared t o the air-hydrogen flame is due t o the reduced absolute sensitivity for tin in the oxygen-acetylene flame. Higher concentrations of tin are required and, as previously noted, the disparity between the solution and suspension increases under these conditions. This would not, however, account for all the sensitivity effect nor the difference in the relative behavior of stannic oxide and stannic oxide to each other in the two flames. The suspension absorption profiles, as in the case of the airhydrogen flame, resemble that of tin solution as shown in Figure 2 where the results for both solution and stannic oxide suspension are plotted. Effect of Organic Materials. Previous results (5) have indicated that organic compounds depress tin atomic population in hydrogen flames. If the flame chemistry is itself perturbed by the organics, the tin atomic population produced from the atomization of tin suspensions should be depressed in a similar manner. Suspensions of stannic oxide in watermethanol solutions were prepared and the absorption readings in a turbulent air-hydrogen flame were measured. The large depressive effects, shown in Figure 3, are practically the same type of interference curves obtained for tin solutions in the same flame under the same conditions. When the experiments were repeated in the turbulent oxygen-acetylene flame, enhancements were observed for both stannic oxide suspension and tin solution as shown in Figure 4. Thus, the organic interference on stannic oxide suspension, as typified by methanol, is similar to the interference o n tin solution when turbulent flames are used and also similar to that previously observed with laminar burners for tin-organic solutions (5). As a further check o n the organic interference problem, it seemed worthwhile to add elemental carbon and to compare the effect to that obtained from organic solutions. Such a n experiment is made possible by the use of suspensions.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
Graphite was preferred because of the availability of samples of high purity but was not suitable because of its poor suspension-forming properties. Carbon in the form of powdered charcoal dispersed more easily and was used for the experiment in spite of its somewhat lower purity. A depressing effect of carbon o n tin absorption from tin solution was observed which reached a n apparent maximum value of approximately 21 depression at about 4000 ppm of carbon suspension. The maximum depression of 81 % produced by methanol is significantly higher than that produced by carbon, but the carbon content of even 1% methanol by volume is equivalent to approximately 3000 ppm. Hence, the concentration range in the study of the effect of carbon was not equivalent to that used in the study of the effect of methanol. The difficulty of preparing stable suspensions more concentrated than 5000 ppm of carbon prevented the extension of this experiment to higher concentration ranges. The experiment was repeated for stannic oxide suspension where carbon produced a somewhat smaller relative depression of absorption than for tin solution. With a turbulent oxygen-acetylene flame, the powdered carbon suspension does produce a tin atomic absorption enhancement although much lower than that produced by methanol, for both solution and suspension, with practically negligible enhancement under fuel rich conditions. This would suggest that the tin enhancement effect of organic solvents in acetylene flames is primarily due to faster desolvation, a physical process, rather than any significant contribution of the organic solvent to the fuel content of the already organic acetylene flame. The enhancement for solution samples was again greater than that for suspension. Ionic Interferences. The effect of phosphoric acid, which was observed to produce a large depressive effect on tin in solution, was monitored for stannic oxide suspension in the turbulent air-hydrogen flame. The depressive effect was significantly less when suspensions were used, as seen in Figure 5 showing the effects of phosphoric acid on solution and suspension. The experiment was repeated in the turbulent oxygen-acetylene flame with, again, a much smaller interference for the suspensions. The effects of cesium, as a typical ionization interference, and aluminum, as a typical refractory oxide former, were also tested and determined to be very small for both solutions and suspensions. Effect of Other Suspended Materials. The effect of other suspended materials on the analyte in suspension was also investigated. This situation could be significant when the sample consists of a mixture of insoluble materials, such as soil or other geological samples. Interference is less likely than in solution because the analyte and potential interferent do not mix intimately with each other ionically prior to atomization. The effects of silica and alumina o n stannic oxide suspension were determined in the turbulent air-hydrogen flame. No significant effect was produced by either silica or alumina up to 1600 ppm a t a tin concentration of 200 ppm. Application to Analysis of Tin Ore Concentrate. Tin is found naturally as stannic oxide, the mineral form called cassiterite. No major tin deposit is found in the United States and most of the tin used in this country comes from reclaimed tin or is imported from other countries. Imported tin usually comes as tin ore concentrate instead of the ordinary ore in order to minimize shipping costs. The analysis of tin ore concentrate involves fusion of the sample with alkali followed by the dissolution of the fused material into water. This rather tedious and laborious sample preparation suggested the great advantages which could result if analysis
z
D l 0.4
0 PPM
2000 4000 6000 PHOSPHORIC ACID
Figure 5. Effect of on atomic absorption suspension and stannic turbulent air-hydrogen
phosphoric acid of stannic oxide tin solution in the flame
Sn, 200 ppm for both suspension and solution; curves A , B , C , interference on stannic oxide suspension; curves D,E, F, interference on tin solution; air; 2.5 liters/min; curves A and D fuel to oxidant ratio of 4.7 and 30 mm above burner; curves B and E , ratio of 7.0 and height of 37 mm; curves C and F, ratio of 9,.4 and height of 40 mm; wavelength, 2246 A could be performed aia the direct aspiration of tin ore concentrate suspensions, eliminating the fusion dissolution. Efforts were made then to obtain standard samples of tin ore concentrate for evaluation of the proposed analysis method. Upon contacting several tin manufacturing companies in this country, NBS No. 138 Tin Ore Concentrate was the only standard located. The standard analysis supplied by NBS reports a 7 4 . 8 x tin content. If it is assumed that all the tin present is in the stannic oxide form, the standard would be 95,Oz pure in Sn02. However, suspension atomization and analysis of the standard against a working curve prepared from pure stannic oxide suspensions showed that very low tin values were obtained. In a turbulent air-hydrogen flame, a value of approximately one fourth the true tin value was indicated, while with the hotter oxygen-acetylene flame the results increased to about one third of the true value. Thus, the atomization of tin from the ore concentrate proceeded much less efficiently than that from the stannic oxide suspension. Cassiterite is a mineral composed of tetragonal crystals which is evidently much more diffcult t o break up than the pure stannic oxide. There were no obvious differences in sample particle size or suspension properties. In the case of any analytical method, the standards used should ideally be as similar to the samples as possible. It would therefore be desirable to run a series of standard tin ores, using certain ones to define a working curve and using others as “unknowns” for evaluation of the analytical method. Unfortunately, only one NBS tin ore is available, so a simulated analysis was performed using selected portions of this material as both standard and unknown. A series of standard suspensions was prepared from a stock suspension of the tin ore concentrate. Three suspensions to serve as analysis samples were also prepared from this stock suspension and designated A samples. T o avoid the preparation of all the
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
251
Table I. Analysis of Tin Ore Concentrate in the Turbulent Air-Hydrogen Flame Tin concentration, ppm Samples Actual Analysis Deviation
,
03 0 ~ W
A
355 24 1 964
350 24 1 951
-1.4 0.0 -1.3
B
519 1038 1298
525 1045 1334
+1.2 +0.7 +2.8
v
P 020-
R
L A
0 000
500 PPM
1000 TIN
1500
Figure 6. Calibration curve prepared from tin ore concentrate absorption readings against actual tin concentration computed from NBS standard analysis Flame, turbulent air-hydrogen; air, 2.5 liters/min; fuel to oxidant ratio, 9.4; heigbt above burner, 40 mm; wavelength, 2246 A; 0 , standard samples; m, A samples; A, B samples
samples from a common stock suspension, a second stock suspension was used to prepare a series of B samples. The working curve is shown in Figure 6 for the tin standard suspensions. The curve was drawn for a best fit through the absorbance values of the standards with absorbance figures also shown for the designated samples a t their normal concentration to indicate the extent of their deviation. Using the
working curve in its conventional sense and obtaining apparent concentrations for each sample, Table I was prepared t o indicate that quite reasonable results can be obtained when the standards and samples are closely matched. It is recognized that this represents the ideal case wherein the two are indeed from the same material. However, it does not seem unreasonable to expect that acceptable results could be obtained for ores of similar content. Currently under investigation for other elements are additional types of materials, such as cements, limestones, and geological samples for which more than one standard are available. We feel that the use of suspensions in flame spectrometry has some utility which should be considered. Not only may selected compounds be introduced directly into the flame in the solid state as a possible aid to the study of flame chemistry, but a very practical technique may result for those sample types which involve lengthy sample dissolution procedures. RECEIVED for review July 7, 1970. Accepted October 29, 1970.
High Sensitivity Internal Reflection Spectroelectrochemistry for Direct Monitoring of Diffusing Species Using Signal Averaging Nicholas Winograd' and Theodore Kuwana Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 Internal reflection spectrometry (IRS) at optically transparent electrodes (OTE) appears to be an excellent technique for quantitative studies of species produced or perturbed electrochemically within a distance 6 of the electrode surface. Quantitative applications employing IRS at OTE, however, require an understanding of the sensitivity variation of absorbance. This variation was formulated as a sensitivity factor, Neff,in the quantitative relationships describing absorbance and could be correlated with the optical constants of the system and the Fresnel equations describing the reflectivity. Further, the absorbance as a function of time during an electrochemical potential perturbation could be quantitatively analyzed, and good agreement was found with experimental results. Signal to noise ratios were greatly improved by signal averaging techniques, and small changes (
