Chemical or Solute Vaporization Interferences in Flame Atomic Emission and Absorption Spectrometry The Alkaline Earth Anion Systems Velmer A. Fassel and Donald A. Becker Institute of Atomic Research and Department of Chemistry, Iow pa State University, Ames, Iowa 50010 The published literature on the flame emission spectrometric determination of trace elements in solution contains numerous accounts of chemical or solute vaporization interference effects. One of the most widely documented interferences of this type is the depression of alkaline earth emission signals by increasing concentrations of phosphate, sulfate, or borate in solution. The data presented in this paper show that these interferences can be eliminated at the interferent concentrations normally encountered by providing smaller aerosol particle size distributions, higher flame temperatures, and longer residence times in the flame. The data also refute the claim that flame atomic emission techniques are more susceptible to interferences of this type than atomic absorption spectrometry.
THEREare many accounts in the published literature on flame atomic emission and absorption spectrometry concerning the influence certain constituents in a sample may exert on the emission intensities or absorbances of spectral lines of the analytical element. These effects are commonly referred to as interferences and they have been classified in accordance with their mechanism of origin ( I , 2). Among these interferences are those whose degree of Occurrence can be specifically attributed to interelement effects arising from the chemical nature of the sample. In Gilbert’s classification, these interelement effects are identified as chemical interferences and this term has found general acceptance. Although claims have been made (3, 4) that atomic absorption measurements are far less susceptible to these interferences than emission measurements, experimental or theoretical justification for this conclusion appears to be lacking. One of the most widely observed chemical interferences arises from the influence other chemical constituents (concomitants) in the sample exert on the degree of conversion of aerosol droplets into free atoms of the element (analyte) to be determined. Because free atoms released in the flame are measured in both absorption and emission, the interference effect must be the same for both techniques, unless the excitation of the analyte element is also affected by the concomitants. There have been numerous inferences (4-9) that preferential excitation or collisional deactivation
of the analyte atoms by concomitant atoms or molecules occurs to a significant degree in the flames commonly employed for analytical purposes. However, theoretical considerations (10) suggest that these excitation interferences are much less likely to occur than has been often suggested. Moreover, experimental documentation of such interferences is lacking ( I ) . It is true that historically many journal pages have been devoted to description of various chemical interferences with the flame atomic emission determination of various metallic elements. When a knowledge of this history is combined with the rather persistent reiteration of undocumented claims that flame emission measurements are far more susceptible to these interferences than absorbance observations, the analyst may logically conclude that indeed this is so. Current published accounts of reinvestigations and resubstantiations (11) of one of the most widely documented interferences (the depression of the emission signal of calcium by phosphate) would lend credence to the analyst’s conclusion. It is the purpose of this paper to emphasize that particular caution must be exercised’in accepting these conclusions. The Alkaline Earth-Phosphate Interference. Historically, the emission intensity suppression of alkaline earth line or band radiation by the presence of phosphate ions in the analyte solution has been widely studied. Excellent reviews on the nature of this depression effect have been given by Alkemade and Voorhuis (12), Fukushima (13), and Herrmann and Alkemade (14). Phenomenologically, the interference mechanism may be outlined as follows: During the dehydration of the aerosol droplet, the calcium has every opportunity to react with phosphorus at the combining ratio to form a compound of high thermal stability-e.g., Ca2P20? or Ca3(P0&. The knees of the suppression curves (see Figure 1) may then logically be interpreted as the chemical combining ratio of the calcium and phosphorus containing species. Any excess phosphorus beyond this point should evaporate quickly, probably as P206and should cause no further interference. With reference to the interference effect, the question arises whether the alkaline earth-phosphorus containing compound will evaporate and dissociate as efficiently as the compounds formed in the absence of phosphorus. There is abundant experimental evidence in the literature that there
(1) P. T. Gilbert, in “Analysis Instrumentation,” Proceedings of
Xth National Analysis Instrumentation Symposium, Plenum Press, New York, N. Y., 1964. (2) C. Th. J. Alkemade, ANAL.CHEM., 38, 1252 (1966). (3) T. S. West, Analyst (London), 91, 69 (1966). (4) T. S. West, in “Trace Characterization: Chemical and Physical,’’ National Bureau of Standards Monograph 100, W. W. Meinke and B. F. Scribner, Eds., U. S. Government Printing Office, Washington, D. C., 1967, p 281. ( 5 ) A. Walsh, in “Advances in Spectroscopy,” Vol. 11, H. W. Thompson, Ed., Interscience, New York, N. Y.,1961, pp 1-22. (6) A. Walsh and J. B. Willis, in “Standard Methods of Instrumental Analysis,” Part A, F. J. Welcher, Ed., D. Van Nostrand, Princeton, N. J., pp 105-17, 1966. 1522
ANALYTICAL CHEMISTRY
(7) J. W. Robinson, “Atomic Absorption Spectroscopy,” Decker, New York, N. Y., pp 15-31,1966, (8) W. Slavin, At. Absorption Newsletter, (24) 15 (1964). (9) A. Walsh, Appl. Opt., 7 , 1259 (1968). (10) C. Th. J. Alkemade, ibid., p 1261. (11) R. Smith and J. D. Winefordner, Spectros. Lett., 1,157 (1968). (12) C. Th. J. Alkemade and M. H. Voorhuis, 2.Anal. Chim., 163, 91 (1958). (13) S. Fukushima, Mikrochim. Acta, 1959, 596. (14) R. Herrmann, C. Th. J. Alkemade, and P. T. Gilbert, “Chem-
ical Analysis by Flame Photometry,” Interscience, New York, N. Y., 1963.
>
c
VI
z W
z
5
W
w
a
Ll
IO
o‘.l
Ca 42274
-
I
111’11
10
100
MOLE imio (Poa/ca)
Figure 1. Comparison of phosphate interference effect as observed with a Beckman burner 1 : l ethanolReinvestigation of results using 25 &ml: water solvent, 20 mm above primary reaction zone; water solvent at 15 mm (0), 5 mm (A), and zero mm ( 0 ) above A from reference (la,V from the primary reaction zone. reference (17), 0 from reference (I8),and from reference (16)
is often not enough time for the aerosol particle to vaporize completely during its transit through the flame. For a fixed point of observation in the flame, the observed emission intensities or absorbances will therefore depend primarily on the degree of volatilization achieved at that point. For example, the observations by Alkemade and Voorhuis (12) and Fukushima (13) that the interference effect is greatly diminished by sampling the emission higher in the flame clearly supports this deduction. The physical factors governing the rate at which evaporation takes place have been discussed by Herrmann and Alkemade (14, pp 27-9), Baker and Garton (19), Hieftje and Malmstadt (20), and by Zeegers, Smith, and Winefordner (21). Because of space limitations these factors will not be discussed in detail here. It is sufficient to emphasize that a decrease in aerosol particle size distribution, and increases in flame temperature and aerosol residence time in the flame should contribute toward the reduction or complete elimination of many of these interferences. Recent developments in burner design have provided the analyst with opportunities to exercise a higher degree of optimization of these variables. It is somewhat surprising that little effort has been devoted to taking advantage of these developments in directly eliminating all vestiges of several important chemical interferences, without resorting to the addition of releasing agents (16, 22, 23) or dilution of the sample. As part of an extensive study on the nature of several widely reported chemical interference effects, we have observed that several of these interferences may be reduced to nondetectable (15) G. L. Baker and L. H. Johnson, ANAL.CHEM., 26,465 (1954). (16) J. Yofe and R. Finkelstein, Anal. Chim. Acta, 19, 166 (1958). (17) A. C. West, Ph.D. Thesis, Cornel1 University, New York, N. Y . , 1961. (18) W. A. Dippel, Ph.D. Thesis, Princeton University, Princeton, N. J., 1954. (19) C. A. Baker and F. W. J. Garton, Document AERE-R-3490, Atomic Energy Research Establishment Harwell, U. K., 1961. 40, 1860 (20) G. M. Hieftje and H. V. Malmstadt, ANAL.CHEM., (1 968). (21) P. J. T. Zeegers, R. Smith, and J. D. Winefordner, ibid., 40, (13) 26A (1968). (22) J. B. Willis, Specrrochim. Acta, 16, 259 (1960). (23) J. I. Dinnin, ANAL.CHEM., 32, 1475 (1960).
levels. Since the first report of our studies (24, 25), several brief accounts on the reduction or elimination of several important chemical interferences have appeared (26, 27). In this paper, we present the results of a study on the alkaline earth-phosphate, -sulfate, and -borate interference systems. The data will show that the depressant effect exerted by these anions on alkaline earth emission can be eliminated at interferent concentrations normally encountered by taking advantage of the optimal nebulization techniques and burner systems as described below. Nebulizer-Burners. In the present study, two different nebulizer-burners were employed. One of these was the premixed oxygen-nitrogen-acetylene burner previously described by D’Silva, Kniseley, and Fassel (28) and a nitrous oxide-acetylene slot burner of the type commonly employed in atomic absorption spectroscopy (29). The advantages of the latter as an emission source were recently discussed by Pickett and Koirtyohann (26). In operation both of these burners refine the nebulized sprays so that a smaller drop-size distribution is introduced into the flame. The refinement of the primary drop size distribution to a smaller value in spray chambers of the type used in the nitrous oxide-acetylene slot burner has been adequately discussed (14, pp 24, 107, 308; 30). In addition to the refinement of drop size distribution, a considerable preliminary evaporation of solvent occurs in spray chambers before the aerosol particles reach the flame (14, p 24). In the oxygen-nitrogen-acetylene burner, the premixing channel in effect serves both as an extended impact plate or barrier surface (14, pp 105; 31, 32, pp 103-5) and as a spray chamber in lowering the drop size distribution formed at the Beckman burner nozzle. With reference to the temperatures of the flames formed by the premixed burners described above, measurements by Willis et a f . (33) have shown that the nitrous oxide-acetylene flame produces a temperature ranging from 3000 OK down to 2820 OK depending on the stoichiometry and site of observation. For the oxygen-nitrogen-acetylene flame, iron electronic temperature measurements gave a value of 2700 O K in the region of the flame commonly observed for analytical purposes (34). Thus both flames provide temperatures considerably higher than those which prevailed in many of the flames employed in the past for the determinations of the alkaline earths. A Beckman nebulizer-burner assembly (14, pp 121-5), which provides a highly turbulent flame, was employed for the re-examination of several interference effects reported in the (24) V. A. Fassel, 18th Annual Mid-America Symposium on Spectroscopy, Chicago, Ill., May 1967, No. 85. (25) V. A. Fassel and D. A. Becker, Proceedings of “XI11 Col-
loquium Spectroscopium Internationale,” Ottawa, Adam Hilger, London, 1968, p 269. (26) E. E. Pickett and S. R. Koirtyohann, Specfrochim. Acta, 23B, 235 (1968). (27) V. Mossotti and M. Duggan, Appl. Opt., 7 , 1325 (1968). (28) A. P. D’Silva, R. N. Kniseley, and V. A. Fassel, ANAL. CHEM., 36, 1287 (1964). (29) J . Fiorino, R. N. Kniseley, and V. A. Fassel, Specrrochim. Acra, 22B, 413 (1968).
(30) J. B. Willis, ibid., 23A, 811 (1967). (31) E. Rauterberg and E. Knippenberg, Angew. Chem., 53, 477 (1940). (32) R. Mavrodineanu and H. Boiteux, “Flame Spectroscopy,” Wiley and Sons, Inc., New York, N. Y . , 1965. (33) J. B. Willis, J . 0.Rasmuson, R. N. Kniseley, and V. A. Fassel, Spectrochim. Acta, 23B, 725 (1968). (34) R. B. Myers, R. N. Kniseley, and V. A. Fassel, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1967, No. 115. VOL. 41, NO. 12, OCTOBER 1969
1523
Table I.
Operating Conditions for the Various Burners
Gauge pressures Burner and gas mixture Beckman turbulent oxyacetylene (14, pp 21-24) Premixed oxygen-nitrogenacetylene (28) Premixed nitrous-oxide acetylene (29) Premixed oxy-acetylene (29)
Flow rates (IJmin) oxygen 3.2 acetylene 2.1 oxygen 3.8 acetylene 4.8 nitrous oxide 11.4 acetylene 6.4 oxygen 13.8 acetylene 15.2
(psi.)
15 3 15 3 15 5 15 5
09
L
\
Ca 4227
A
Co 4227;
"lAc,;6;4 0.8
literature. Relatively high temperatures [in the region of 2800-3000 "K (3411 can be obtained with this burner for oxygen-acetylene mixtures, but chemical interferences which are particularly sensitive to drop size distribution tend to occur to a high degree in this flame. In part this stems from the inefficient evaporation of the larger droplets during their transit through the flame. In fact, Gibson, Grossmann, and Cooke (35) have reported that some aerosol droplets, when water solvent is used, pass through the flame without being completely evaporated. Another contributing factor to the rather severe chemical interference effects often observed in these burners is the comparatively high initial velocity of the unburnt oxygen and acetylene emerging from their orifices. In turn the correspondingly high rise velocity of the aerosol particle through the flame therefore imposes a limitation on the time available for complete vaporization of the droplet residue (14, pp 28, 300-307). Effect of Organic Solvents. The enhancements in absorption or emission signals afforded through the use of organic solvents for the sample solution, or the addition of organic components to the solution, have been adequately documented (14, pp 208-10; 36-41). Systematic studies on the mechanism of this enhancement effect have clearly shown that the gain in signals depends in a rather complicated and subtle way on the combined action of many factors. Clearly, several of the factors which contribute to spectral enhancements should at the same time lead to a minimization or elimination of condensed-phase chemical interferences. In particular, the presence of organic components with surface tensions lower than water, either as solvents or additions to the solution, should lead to the formation of a finer spray by the process of pneumatic nebulization (14, pp 22-4; 39, 40). If the vapor pressure of the organic component is at the same time greater than that of water, and is combustible as well, the increase in the evaporation constant (20) should contribute to a marked increase in the overall efficiency of vaporization of the aerosol droplet and its residue. EXPERIMENTAL FACILITIES
Two different experimental facilities were used in collecting the data reported in this paper. One of these facilities employed a Jarrell-Ash, Model 78-462, 1.0 meter Czerny(35) J. H. Gibson, W. E. L. Grossmann, and W. D. Cooke, ANAL. . CHEM., 35,266 (1963). (36) H. Bode and H. Fabian, 2.Anal. Chem., 163, 187 (1958). (37j R. Avni and C . Th. J: Akemade, Mikrochim. Acta, 1%0,
460. (18) J. W. Robinson, Anal. Chim. Acta, 23, 479 (1960). (39) J. E. Allan, Spectrochim. Acta, 17, 467 (1961). (40) I. Atsuya, Sci. Rept. Res. Inst. Tohoku Uniu., Ser. A , 18, 65 (1966). (41) G . R. Kingsley and R. R. Schaffert, J. Biol. Chem., 206, 807 (1954). 1524
ANALYTICAL CHEMISTRY
1
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0 1
,,
, ,,,,,, , (
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j
MOLE RATIO ( P O ~ / C P )
Figure 2. Comparison of phosphate interference on calcium Present investigation : 02-CsH~ slot burner (0),C2H2-N20 slot burner (U), C2H1-N2-02 premix burner (0);all measurements made at 20 mm above the burner with 200 pg Ca/ml in 1:l ethanol water nebulizing. From reference (12): air-C2H2 indirect nebulizer burner with 116 kg Caiml in water nebulizing at 112 mm (A) and 32 mm (V)above the inner cones
Turner mounting grating spectromet:r provided with a 1180 grooves/mm grating blazed for 5000 A. The reciprocal !inear dispersion of this instrument at the exit slit was 8.2 A/mm in the first order. The associated electronic recording system employed with this spectrometer has been previously described (42). The other combination of instruments was assembled around a Jarrell-Ash Model No. 82-000, 0.5 meter Ebert mounting grating spectrometer, provided with The reciprocal 1180 grooves/mm grating blazed at 5000 linear dispersion at the exit slit was 16 A/mm in the first order. The signal from an S-13 response multiplier phototube (EMI-6256B) was amplified by a high-speed picoammeter (Keithley Model 417) and recorded with a Texas Instruments Servowriter. The external optical system consisted of a fused quartz plano-convex lens which formed a 1 :1 image of the flame on the entrance slit. The ethanol-water solutions (50 ethanol by volume) of the metal chlorides or perchlorates were prepared from reagent grade chemicals. The phosphate, sulfate, and borate solutions added to the test solutions to provide a range of anion/cation mole ratios were prepared from their corresponding acids. A continuously variable infusion pump (Harvard Apparatus Co., Inc., Dover, Mass.) operating at a rate of approximately 1 ml/min was used for all experiments with the premixed air-oxygen-acetylene flame. Higher infusion rates, approximately 3 ml/min, were used for experiments with the nitrous oxide-acetylene slot burner flame. Pertinent data on the gas flow rates employed with the respective nebulizers and burners are summarized in Table I.
4.
RESULTS AND DISCUSSION Reinvestigation of Calcium-Phosphate Interference System in Total Consumption Turbulent Flames. In our discussion
above, we called attention to several severe limitations imposed by total consumption turbulent flames-toward the minimization of chemical interferences. In spite of these limitations, our reinvestigation of the depression of the (42) V. A. Fassel, R. H. Curry, and R. N. Kniseley, Spectrochim. Acra, 18, 1127 (1962).
Investigators Fassel and Becker Fassel and Becker Baker and Johnson
Table 11. Summary of Experimental Conditions Calcium concentration Solvent Burner Oxidant-Fuel (rglm EtOH-H20 Oz-CzHz 25 (as clod-) Beckman (4030) Hz0 0zXzHz 25 (as clod-) Beckman (4030) HzO OzCzHz 40 (as C1-) Beckman
(15)
West (17)
Beckman
0z-H~
Dippel (18)
Beckman (4030)
Oz-CzHz
Yofe and Finkelstein
Beckman
OrHz
25 (aNOS-)
HzO
100 (as C1-)
HzO
40 (as C1-)
H20
( 16)
calcium emission by increasing phosphate concentrations revealed an unexpectedly small depressant effect when water solvent was used, and no detectable depression for an ethanolwater mixed solvent. In Figure 1, our results are graphically compared with typical literature data. It is apparent from Figure 1, that up to molar ratios (P043-/Ca) of 100, we observed no interference when ethanol-water solutions were employed, and only a 14% depression was detected for water solvents. These observations are in contrast to the depressions of the calcium emission signal from 36 to 54% reported by previous investigators. The absence of significant interference when ethanol-water solutions are nebulized may simply reflect the beneficial actions of organic solvents in reducing chemical interference effects. Because the oxygen-hydrogen flame employed in two of the studies mentioned in Figure 1 (16, 17) is approximately 300 O K cooler than its oxygen-acetylene counterpart, the higher degree of interference in the former flame may also be reconciled. However, a comparison of pertinent experimental parameters indicated in the publications and summarized in Table I1 does not give any sound basis for expecting the much lower degree of interference under the environmental conditions employed in our measurements. There is, for example, no apparent logical basis for the disagreement in the magnitude of the depression for the Dippel (18) and Baker and Johnson (15) results for oxygen-acetylene flames. Thus, the alignment of the experimental results in Figure 2 strongly suggests that an observed chemical interference effect may be so unique that there is little expectation of precise duplication of the degree of depression by other investigators; and that subtle differences in the experimental conditions may play a very significant role in establishing the degree to which a chemical interference may occur. For example, it is well known that the performance of the Beckman total consumption burner as a nebulizer is sensitively dependent on the precision of capillary alignment in the burner nozzle. By indicating this factor as a possibility, we are by no means implying that it is the underlying cause for the variations in the degree of depression observed by the different investigators. The focus of the present paper is not primarily concerned with the question of whether quantitative agreement on interference effects can be achieved by different investigators. Of far greater importance is the question of whether an interference can be easily and confidently eliminated. As noted above, the recent development of burners capable of providing high temperature premixed flames has given the analyst a greater opportunity of directly eliminating many interfer-
Observation site 20 mm above burner Noted in figure captions As observed with flame
attachment for Beckman DU As observed with flame attachment for Beckman DU As observed with flame attachment for Beckman DU As observed with flame attachment for Beckman DU
E
0 1
50 100 MOLE RATIO (POq/Sr)
500 1000
Figure 3. Comparison of phosphate interference on Mg, Ba, and Sr 0
This investigation; observations in interconal zone of premixed O2-N2-CZH~ flame; 25 pg of metal/ml in 1 :1 ethanol water
From reference (18)
0 From reference (16) ences of this type. Because these same flames are destined to replace the turbulent versions as excitation sources, there is a compelling reason for evaluating their capability in completely eliminating one of the much publicized interferences. Observations with Premixed Flames. Figure 2 shows that, even at the 200 pg Ca/ml level, no significant depression of the calcium emission signal occurs up to P04+/Ca molar ratios of 100 for O Z - N K ~ H ZN, ~ O X Z Hand ~ , 02-CzHz flames under the experimental conditions specified in the figure caption. At molar ratios greater than 100, the solids content of the solution significantly affected the physical properties of the solution so that the nebulization efficiency was altered. Thus, the depression in emission beyond molar ratios of 100 should VOL. 41, NO. 12, OCTOBER 1969
1525
COOH 5540 i\
0.4
c
A MOLE RATIO (SO,
IC0 1
Figure 4. Comparison of sulfate interference on calcium 0 A 0
V
This investigation; observations in interconal zone of premixed O2-Nz-C2H2flame; 25 pg Ca/ml in 1:l ethanol water nebulizing From reference (15) From reference (17) From reference (16)
not be completely assigned to a vaporization or chemical interference effect. For comparison purposes, the interference effects observed in several previous investigations are also shown on the figure. Results similar to those shown in the top two curves of Figure 2 were obtained for aqueous solutions nebulized into the N20-C2H2 flame. The calcium phosphate interference doesn't appear to depend on flame stoichiometry or observation height in the N20-C2H2 flame. The interference curves obtained by atomic absorption are similar to those obtained by emission, thus refuting the claim that flame atomic emission is more susceptible to these interferences than atomic absorption. Figure 3 shows that no significant interference effect of phosphate ion on magnesium, strontium, and barium emission was observed below molar ratios of 500 in the premixed 02-NK2H2 flame when 1 :1 H20/EtOH solvent was employed. Analogous results were obtained in the N20-GH2 slot burner flame in emission. For comparison purposes, the typical depression effect observed in Beckman turbulent flames are shown in the figures. In a paper published after our initial
1526
ANALYTICAL CHEMISTRY
disclosure of the results summarized above, Mossotti and Duggan (27)'presented data obtained with a N20-CzH2 premixed total consumption burner which are in complete accord with our results. Thus the Mossotti and Duggan results and the experimental data summarized in Figures 1 to 3 clearly show that one of the most notorious chemical interferences effects, whose description has occupied so much space in the literature, may be either completely eliminated or reduced to negligible proportions through the proper selection of environmental conditions. It is of interest to note that certain reports in the atomic absorption literature (43-45) confirm some of the emission observations summarized above, whereas other absorption reports (46, 47) indicate that phosphate enhances calcium absorption rather than exerting the classical depressant effect. Sulfate and Borate Ion Interference with Calcium. The depression of calcium emission by increasing concentrations of sulfate and borate ion has also been recorded in the literature. In Figure 4, the degree of depression of calcium emission by sulfate as observed in the premixed 02-N2-C2H2 flame is compared with observations made by various investigators in Beckman burner turbulent flames. Again the present investigation shows that no interference is evident below a sulfate/calcium molar ratio of 500. Analogous results were obtained with the NzO-C~HZslot burner flame with 1:1 ethanol-water solutions nebulizing. The borate interference, which has been reported to be similar to the sulfate interference in its depressant effect (43, is also virtually nonexistent in the premixed 02-N2-C2H2 flame and N z O - C ~ Hslot ~ burner flame. RECEIVED for review December 23, 1968. Accepted August 5, 1969. Work' performed in the Ames Laboratory of the U. S. Atomic Energy Commission. (43) J. E. Allan, Analyst (London), 83,466 (1958). (44) W. Leithe and A. Hofer, Mikrochim. Acta, 1961,168. (45) W. Slavin, At Absorption Newsletter, 6 , 9 (1967). (46) M. D. Amos and J. B. Willis, Spectrochim. Acra, 22, 1325 (1966). (47) D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta, 36, 312 (1966). (48) M. E. Doty, Ph.D. Thesis, Kansas State University, Manhattan, Kansas, 1963.
