1054
Anal. Chem. 1980,
52, 1054-1059
Interference Effects from Aerosol Ionic Redistribution in Analytical Atomic Spectrometry James A. Borowiec, Andrew W. Boorn, John H. Dillard, Malcolm S. Cresser,' and Richard F. Browner* School of Chemistry, Georgia Institute
of Technology, Atlanta, Georgia 30332
Michael J. Matteson School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
The occurrence of lonlc redlstrlbutlon upon the generatlon of aqueous aerosols by pneumatic nebulization from solutions containing mixtures of ions is demonstrated. Fractionation of the aerosol accordlng to size shows that the extent of aerosol ionic redistribution (AIR) is critically dependent upon droplet size. It also varles with the ratio of the major to minor cation, as well as with the amount of the major ion. Examples of interference effects attributable to AIR in flame- and inductively coupled plasma-excited atomic emission spectrometry are discussed.
Soluble salts found in the atmosphere are believed to originate as ocean spray, carried into the atmosphere by the bursting of microscopic bubbles in the ocean surface ( I , 2 ) . However, ocean spray has been found in many instances to be unrepresentative of the ocean bulk from which it originates. Specifically, concentration ratios of ions present in the atmospheric aerosol may differ significantly from ratios of the same ions in the ocean bulk (3-13). This may lead to an enrichment of certain ions in the aerosol compared to the bulk. T h e bubble-burst mechanism has been studied by high speed photography (13-15) and appears to involve a two-stage mechanism of aerosol formation. On reaching the liquid surface, t h e bubble forms a thin film, which ruptures to produce drops in the micron and submicron size ranges. After film rupture t h e bubble collapses, leaving a small liquid jet which in turn breaks u p into drops. It is during the collapse of the bubble and the subsequent liquid jet that the process of ion enrichment is believed to occur. MacIntyre (6) and Wilkniss and Bressan ( 7 , 8 ) have suggested that the relative ionic potentials or charge-to-mass ratios of the ions are responsible for the enrichments observed. For example, sulfate was found t o be enriched with respect to chloride and potassium with respect t o sodium in the transition from bulk t o aerosol phase. However, the magnitude of the reported enrichment for fluoride over chloride, which was found to range between a factor of 10 and lo00 (10)suggests that a more complex mechanism than those indicated by either MacIntyre or Wilkniss and Bressan is active. Glass and Matteson (9) simulated natural bubble-bursting processes by experiments in which they used a controlled stream of bubbles to generate aerosols from binary salt solution mixtures. They found t h a t for a 3% (w/w) sodium chloride solution, the degree of enrichment of various minor ions (e.g. Li+, K+,Mg2+)in the aerosols was strongly dependent on t h e concentration ratio of ions in the bulk solution. In general, the greatest enrichments were found at large major-to-minor cation ratios. Furthermore, for the sodium/
'Present address:
Department of Soil Science, Aberdeen C n i v -
ersity, AB9 2UE, Scotland.
0003-2700/80/0352-1054$01 0010
magnesium ion pair, the degree of enrichment was found to vary periodically with droplet size in the range 0.7-8 l m . T h e relevance of such studies to aerosol production processes in analytical atomic spectrometry is clear. If it can be shown that similar ionic enrichments occur during pneumatic and ultrasonic nebulization steps as have been shown to take place during bubble bursting experiments, then an interference mechanism which has not been classified previously may be anticipated to operate under certain circumstances. If a n interference of this type occurs, its point of action will be during the nebulization step and so its magnitude is likely to be a function of the detailed parameters of nebulizer design and operation. Consequently, in addition to a dependence on variables such as ionic type and concentration ratio, t h e magnitude of the interference would be expected to be sensitive to variations both in the construction and operation of the specific nebulizer used. The function of the present preliminary investigation is therefore threefold: (1)to see if aerosol ionic redistribution occurs during aerosol generation by pneumatic nebulization; (2) to investigate the parameters influencing the incidence and extent of the phenomenon if it does occur; and (3) to study briefly the practical significance of the results thus obtained.
EXPERIMENTAL Aerosol Generation a n d Fractionation. Aerosols were generated with either a Perkin-Elmer atomic absorption (AA) nebulizer operated at 204 kPa differential pressure, or a Plasma-Therm inductively coupled plasma (ICP) crossflow nebulizer operated at 170 kF'a differential pressure (0.80 L min-' argon flow). The aerosols were directed t o a cascade impactor through a Perkin-Elmer spray chamber, inverted to avoid gravitational bias of the collected droplet fraction. This technique has been described in detail as a means of sizing aqueous aerosols (16),so only details relevant to the present experiments will be described here. The bulk solution was aspirated at a carefully controlled flow using a peristaltic pump (Buchler Multistaltic). For the AA nebulizer, the aspiration rate was set at 4.60 mL min-', and for the ICP nebulizer the rate was set at 1.20 mL min-'. Collection time was 1 2 min for runs with the AA nebulizer and 40 min for runs with the ICP nebulizer. After each run, the plates were carefully removed from the cascade impactor, and the deposited aerosol was washed from each plate into a separate 50-mL volumetric flask. The contents of each flask therefore reflected the composition of the aerosol deposited on the corresponding stage of the cascade impactor. Great care was necessary to avoid contamination of the plates, especially by sodium which was readily picked up during handling and drying steps. Analysis of Fractions. Fractions were analyzed by flame emission spectrometry. In every case where interference was considered even remotely possible, the absence of significant ionization, spectral, or matrix interferences was confirmed experimentally under the conditions used for analysis and over and beyond the sample concentration ranges encountered in the appropriate run. '? 1980 American
Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980 2.2
ICP Emission Spectroscopy. Results reported for the plasma emission studies were obtained with a Plasma-Therm ICP 2500 argon plasma, operated at 1.5 k W unless otherwise specified; measurements were made at a height of 15 mm above the coil, unless otherwise specified. Reagents. All cation solutions were prepared from analytical reagent grade metal chlorides.
RESULTS AND DISCUSSION In these experiments, concentration ratios of ions in aerosols generated from binary solution mixtures were compared to the ratios of the same ions in the bulk solution. One ion in the binary mixture (the major ion) was always present at high concentrations, generally in the range 2000- 18 000 pg mL-l. T h e other ion (the minor ion) was present a t much lower concentrations, generally between 50 and 200 times lower than t h e major ion. T h e aerosol spray generated by the nebulizer and collected on t h e plates of the cascade impactor is separated by the cascade impaction process into various discrete size fractions. Each stage of the impactor collects one specific size fraction, and with the impactor used in these studies the total range of fractions collected was from 0.44-11 pm. Median collection diameters for the stages were: 0.5, 0.9, 1.6, 2.7, 4.0, 5.2, 7.4, and 9.5 pm. In the operation of comparable nebulizers under conditions identical to those described here, this size range has been demonstrated to contain at least 80%, and typically 9070, of the sample mass leaving the spray chamber. I t can therefore be considered as representative of the aerosol which actually reaches the flame or plasma. By determining the concentration ratio of ions in the droplets collected on each stage of the impactor, and comparing this ratio with the ratio of ions in the bulk solution, possible ionic enrichments can be detected. An enrichment factor, Ed, was defined such that:
2.0
I
1.8
I
; 1.6
L
0
a
c
C
E 1.4 S
.-
L
w C
1.2
1.0
0.8
4
2
0
8
6
Droplet d i a m e t e r ,
10
pm
Figure 1. Variation in enrichment factor with droplet size for a range of cation pairs at 200:l w/w ratio. Major cation present at 10000 pg mL-'. (0)Magnesium/sodium, (A)lithium/calcium, (A) potassium/lithium, ( 0 )sodium/calcium, (0)magnesium/lithium
0 120 160
where CMi+is the concentration of minor ion and ChIa+ the concentration of major ion. The enrichment factor is a function of droplet diameter and so is specified for a collected median droplet diameter, d. A value of E d greater than unity represents an enhancement of the minor ion in the aerosol and a value less than unity represents a depletion of the minor ion. Preliminary Studies. Initially, solutions containing 10000 pg mL-' of the major cation and 50 pg mL-' of the minor cation (200:l ratio) were selected for examination. Such solutions have a number of properties which ensure both that meaningful comparative studies between different ion pairs can be carried out and that the data resulting are relevant to realistic analytical situations. First, in bubble-bursting aerosol ionic redistribution studies, significant enhancements have been observed only a t high concentrations of either major cation or anion. Secondly, Cresser and Browner (16) have demonstrated conclusively that with solute concentrations greater than approximately 0.5 70 by weight, evaporation effects exert a negligible influence upon the aerosol droplet size distribution for droplets down to 1 pm, and only a very small effect for droplets down to 0.5 pm. The smallest size of droplets collected by the cascade impactor used in the present investigation is 0.44 pm. This ensures that direct comparison of the enhancements observed for different cation pairs is possible. Thirdly, the use of solutions with relatively high concentrations facilitates reasonably precise analysis of the solutions obtained from the cascade impactor plates, without the need for collection of the aerosol for too extensive a period. The high concentrations also reduce possible contamination effects to a negligible level, as indicated by periodic blank runs.
1055
i
0.5
1
I
I
2
5
.
, , , ,
10
D r o p l e t d i a m e t e r , pm
Figure 2. Droplet size distribution for AA nebulizer. Ar flow: 7.60 L
min-'. Aspiration rate: 4.60 mL min-'. Ordinate axis is a mass distribution function. This normalizes the mass percent collected on each plate of the cascade impactor against the difference in logs of the maximum and minimum effective cutoff diameters for each plate. The effect of uneven size collection ranges is thus corrected
Finally, in the authors' view, a matrix cation level of 10000 pg mL-' represents a reasonable estimate of the highest matrix concentration likely to be nebulized in routine analysis other than in exceptional circumstances. Ionic E n r i c h m e n t s w i t h a n AA Nebulizer. The results obtained for five ion pairs, using a Perkin-Elmer AA nebulizer, are shown in Figure 1. These curves clearly den-tonstrate that aerosol ionic redistribution (AIR) does occur in pneumatically generated aerosols. Moreover, there are similarities between these curves and those observed for natural (11, 12) and bubble-burst induced (9) aerosols, in that periodic variations of enrichment factors with droplet size are observed in all cases. If Figure 1 is considered in conjunction with the particle size distribution curve obtained by the method of Cresser and Browner (16) and shown in Figure 2, it will be seen that the
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1056
I.€
1.5
-
1.4
L
1.4
/
/
0
m
C
1.3
1.2
c L
Y C
1.2
1 .o
1.1
0.8
z
2
0
6
4
Droplet
diameter
8
10
vrn
m
-
Figure 4. Effect of lithium concentration on calcium enrichment at a fixed 200: 1 lithium-to-calcium ratio. Lithium concentration. ( A ) 18 000 pg mL-', ( 0 )10000 pg mL-'. (0) 2000 pg mL-'
1.0
E u_ E
0.9
0.8 I
0.7
0.6
Effectof lithium concentration on sodium enrichment at a fixed 200: 1 lithium-to-sodium ratio. Lithium concentration: (m) 18 000 pg mL-', (A) 15000 pg mL-', (A)12 500 pg mL-', ( 0 )10000 pg mL-', ( 0 )5000 pg mL-' Figure 3.
interference attributable to AIR which might be anticipated in atomic absorption spectrometry, using this nebulizer under the specified conditions, would in most instances be relatively small. T h e greatest enrichments generally occur in droplets less than 1.5 pm in diameter, and the contribution made by these droplets to the overall droplet distributions is quite small. The magnesium/sodium curve is exceptional, and in this instance a substantial AIR enhancement effect might be anticipated. The considerable enrichment often encountered in relatively large droplets (ca. 9.5 pm) may be of less significance, because for a well designed nebulizer/spray chamber combination, relatively few droplets of this diameter reach the flame (17). However, it is quite possible that a spray chamber with an impact bead or mixer paddle will cause secondary fragmentation of droplets. In this situation, initially large droplets with a high enrichment factor, which would normally be lost to the system by settling or impaction processes, will shatter into smaller droplets which may reach the flame. This would be expected to lead to further enhancement effects for droplets less than 9.5 pm in diameter. In the present study, the mixer paddles were removed from the Perkin-Elmer spray chamber, eliminating secondary fragmentation at the paddle as a possible source of additional AIR. It should be pointed out here that although the PerkinElmer nebulizer/spray chamber combination is not fitted with a conventional impact bead, it has been demonstrated (17) that the mixer paddle support arm may actually function as an impact bead, causing a shift in the droplet size distribution to smaller droplets. If the paddles had been left in the chamber in the present work, the contribution to the distribution curve shown in Figure 2 from droplets greater than 5-pm diameter would have been drastically reduced ( I 7 ) .
E f f e c t of M a j o r C a t i o n C o n c e n t r a t i o n . The effect of the major cation concentration upon the incidence and extent of AIR will be of great importance analytically. From the enrichment factors shown in Figure 1, it can be seen that AIR interferences may be anticipated to occur for solutions containing high concentrations of matrix element. In order to determine a t what absolute concentration of major cation the AIR effect appears, the enrichment of sodium in a lithium matrix was investigated for five lithium solution concentrations in the range 5000 to 18000 pg mL-l. The lithium-to-sodium ratio was held constant a t 200:l in these solutions. The curves (Figure 3), while indicating rather complex behavior, do show definable trends. At moderate lithium concentrations (5000 and loo00 Kg mL-'), a substantial sodium enrichment is observed in small and large droplets, with a much lower enrichment in the droplets between approximately 2 and 8 pm. For the highest lithium concentrations (15 000 and 18000 pg mL-'), on the other hand, substantial depletion of sodium ions is seen for droplets less than 1-pm diameter. Droplets between 2- and 8-pm diameter show moderate (at 15 000 pg mL-' Li) or high (at 18000 pg mL-' Li) enrichment factors, but droplets above 8-pm diameter show little enrichment. The sample aspiration rate was maintained at 4.60 mL min-' by a peristaltic pump throughout the present investigation, so that an increase in solution viscosity due to increasing lithium chloride concentration would not influence the aspiration rate. However, an 18000 pg mL-l lithium solution will have viscosity and surface tension values sufficiently far from dilute solution values that a shift in the droplet size distribution from that shown in Figure 2 might be expected. This was found to be the case in practice, with the mean droplet diameter shifting slightly toward a greater value with increasing lithium chloride concentration. However, the change was small, and may not be significant in the present context. Throughout the series of experiments, a periodic variation in the degree of enrichment with droplet size fraction was observed. For the nebulizer used the curvature of the enrichment-factor vs. droplet-size graphs was reproducible, but not easily explicable. The lithium/calcium system was also studied for an ionic ratio of 200:1, in order to see if similarities existed with the lithium/sodium system. The curves (Figure 4) show that the detailed trends are very different for the two systems. Effect of Major-to-Minor Cation Ratio. Figure 5 illustrates the effect of decreasing the lithium-to-sodium ratio on the enrichment factor vs. droplet size graph, at a constant lithium concentration of 10 000 pg mL The value of Ed is clearly very dependent on the major-to-minor cation ratio. It can also
'.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1057
loor
1.5
1.4
-
1.3
-
0
9
.-VI0
L
C
: ._
E
..-VIn
1.2
0
L
E
W C
W
1 .I
1 ,( 2
4 Droplet
6 diameter,
8
10
prn
Figure 5. Effect of lithium-to-sodium ratio on sodium enrichment at a fixed lithium concentration of 10000 pg mL-': (A)200:1, (A)100:1,
Sodium, pg m l - '
( 0 )50:l
Figure 6. Effect of lithium at 10 000 pg mL-' on the determination of sodium by flame emission spectrometry ( 0 )In the presence of 10000 pg mL-' lithium, (0)no lithium
be seen from Figure 5 t h a t possible AIR interferences will become negligible a t concentration ratios below 50:l. T h e upper limit of t h e relationship was not investigated, as concentrations of matrix cation in excess of 10 OOO pg mL-' are not often encountered practically. ICP Nebulizer Studies. In addition to the extensive studies carried out with an AA nebulizer, more selective investigations were made with an ICP crossflow nebulizer. The operating conditions for ICP nebulizers are somewhat different from AA nebulizers, a n d so it might be anticipated that enhancement effects would also be different. In general, ICP nebulizers are run with approximately the same gas velocity as AA nebulizers, but with larger sample-to-gas volume flow ratios. As a consequence, droplet size distributions are often shifted toward larger values (18). For the systems studied, the same general trends were observed with the crossflow ICP nebulizer t h a t were found for the concentric AA nebulizer. However, the magnitudes of the effects were often different, a n d enhancements were generally higher. For example, a solution of 10000 pg mL-' Mg and 50 pg mL-' Na gave rise to sodium enhancements approximately three times greater than those observed with the AA nebulizer (see Figure 1). Examples of AIR Interferences. F l a m e A t o m i z a t i o n . From the preliminary studies, detectable AIR interferences would be expected for certain ion pairs a t appropriate concentration levels. The systems studied were lithium/sodium and magnesium/sodium. Analytical curves for the flame emission determination of sodium in a fuel-rich air/acetylene flame, with and without t h e addition of 10000 pg mL-' lithium, are shown in Figure 6. T h e blank value for the sodium contamination in the highly concentrated lithium solution is equivalent to 1 pg mL-' sodium. Any AIR effect can be seen to be slight, showing as a small negative curve to t h e analytical plot a t increasing sodium-to-lithium ratios. T h e effect cannot be due to ionization effects, as the sodium plot in the absence of lithium is linear, and also the addition of 3000 pg mL-' lithium to a 1 pg mL-' sodium solution produced no detectable enhancement. Incomplete matrix volatilization or solution viscosity effects could certainly account for the shallower slope of the sodium curve in the presence of lithium, but not for the concentration-dependent curvature. In practical analysis, an important consequence of this behavior is that the standard addition method for the determination of sodium in a lithium matrix will be invalid a t high lithium concentrations.
The second system chosen for study was the effect of 10 000 pg mL-' of magnesium on the determination of sodium, where a substantial enhancement might have been anticipated from the trends shown in Figure 1. A suppression of sodium emission from an air-acetylene flame in the presence of this high level of magnesium was actually observed. T h e occurrence of a net negative interference is attributable to incomplete volatilization processes, which clearly mask any positive AIR effect which may be active in the nebulization process. ICP A t o m i z a t i o n . In the ICP, volatilization interferences are generally found to be very slight, in contrast to the situation with the air/C2H2flame (19). Positive AIR effects should therefore be more noticeable as they are less likely to be masked by negative volatilization interferences. In addition, enhancement factor studies w i t h a n ICP crossflou nebulizer have shown considerably greater ionic enhancements with the magnesium/sodium system than similar studies w i t h an AA nebulizer. In order to test this predicted trend, the same ion pairs (Li/Na and Mg/Na) were examined as for the flame atomization experiments. The analytical curves for sodium at the 589.0 nm Na(1) line, over the range 0-200 pg mL-', were compared with a n d without the addition of 10000 pg mL-' lithium. At 50-, loo-, and 250-fold mass excesses of lithium over sodium, enhancements of 22, 34, and 49% respectively were observed. These enhancements appear to be the direct c:onsequence of a positive AIR effect, without any significant negative volatilization interferences to counterbalance them. There is the possibility of a slight ionization suppression interference here (19,20),but its predicted magnitude is small and, in any case, it should lead to a constant enhancement rather than to a concentration-dependent enhancement. T h e analytical curves for sodium in the presence and absence of 10 000 pg mL-' magnesium were also determined and are shown in Figure 7 . For this system, the magnitude of the enhancement was considerable, typically greater than a factor of two a t all sodium concentrations up to 50 pg mL-'. Furthermore, the positive trend was maintained a t a variety of measurement heights in the plasma (6-25 mm above the coil) and plasma incident powers (0.6-2.5 kW). T h e ionization potential of magnesium is sufficiently high that ionization suppression interference can be largely discounted as a major contributory factor to the observed effect. The curvature of the analytical line toward the concentration axis a t high sodium concentrations i n t h e presence o f m a g n e s i u m can be attributed to the influence of variations in the major-to-minor
1058
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
=
/
n
1
2
n
Figure 8. Three stages of secondary droplet fragmentation, showing regions in which ionic redistribution may occur (shaded regions)
0 0
10
20
30
40
50
Sodium, p g mi-’
Flgure 7. Effect of magnesium on the ICP emission spectrometric determination of sodium. ( 0 )I n the presence of 10000 p g mL-’ magnesium, (0)no magnesium
cation ratio on the magnitude of the AIR effect. Correlation of Observed AIR Interferences with Cascade Impactor Data. In order for there to be a satisfactory and unequivocal correlation between the magnitude of t h e interferences observed in these studies and the type of data obtained from the cascade impactor runs, and shown in Figures 1-5, a more quantitative link must be shown. Fortunately, this information is available in data from the cascade impactor plates. When a solution containing 10 000 pg mL-’ of magnesium a n d 50 pg mL-’ of sodium was nebulized for 40 min and collected on the cascade impactor, the total mass of sodium collected was determined to be 81.5 l g . The mass of sodium which would be anticipated to collect on the plates of the impactor in this time in the absence of an AIR effect can be calculated from the product of the nebulization efficiency (en), t h e sample aspiration rate ( F J ,the sodium concentration of the aspirated solution (50 pg mL-l) and the collection time. T h e value for t, is already available from the mass of magnesium collected on the plates during the run (as no AIR effect is seen on the ion of high concentration) and the sample aspiration rate. For the crossflow nebulizer run at 0.80 L min-’ argon flow and 1.20 mL min-’ sample aspiration rate, t, = 1.80%. This value is accurate because the sample collected on the plates of the cascade impactor corresponds very precisely with that which would reach the ICP in normal operation. T h e predicted mass of sodium to be collected in the absence of an AIR effect can now be calculated to be 42.0 pg. T h e theoretical value of the AIR enhancement is therefore 81.5/42.0 = 1.94. T h e experimental value for this ion pair determined in the ICP under stated conditions is 2.1, which can be considered as very satisfactory agreement between predicted and experimental data. AIR Mechanism. I t can be reasonably assumed, from the general agreement between trends found for bubble-bursting experiments and the data found in this study, that ionic redistribution effects occur by similar processes in aerosols whether pneumatically generated or produced by the collapse of surface films. In both instances, it may be imagined that the chemical properties of the droplets are determined primarily by surface effects. In the case of bubble-bursting, a thin film of liquid is formed, with two liquid/air interfaces, which ruptures to give rise to aerosol droplets. In the case of pneumatic nebulization. various processes which could give rise to this film can be envisioned. The primary breakup of
the liquid stream may proceed by “stripping” of the surface layer (21) from the bulk liquid. Small droplets may further rupture owing to surface instabilities, which could cause elongation, “necking” in the middle and breakup into a range of droplet sizes, including small satellite drops (Figure 8). It is likely that with the system studied, droplets formed in the range 0.5-10 pm are the result of a multistage process, where enrichment could occur sequentially a t each stage. Conditions in the surface layer of a dilute and static solution are described by the adsorption isotherm of Gibbs:
r, = - [ C , ( b d k ) / R T ]
*
[6y,/GC,(bulk)]
(2)
where rLis the surface depletion of species i at bulk concentration C,(bulk), R the gas constant, T the absolute temperature and [GyL/6C,(bulk)]the concentration dependence of the surface tension for species 1 . Generally by/bC is positive for cations, and so a surface depletion is found. Values for depletion ratios for binary mixtures of ions can be calculated from experimental values of Sy/SC ( 5 ) ,the value of Tha,-l/rKCl being 1.23, for example. The validity of the Gibbs equation has been confirmed experimentally under equilibrium conditions by a number of workers [e.g., MacBain and Swain (22)]. However, under nonequilibrium conditions, such as those experienced during bubble-bursting or pneumatic nebulization, ionic ratios are often far in excess of those predicted from the Gibbs equation. In our experiments, the values for small droplets particularly are much greater than would be expected. T h e redistribution of ions a t the water/air interface may also be considered in terms of an electrical double-layer effect. Loeb (23) considers that at a gas/liquid interface a significant number of water molecules are oriented with their oxygen atoms facing outward and their hydrogen atoms facing toward the water bulk. The polarization charge thus established a t the surface leads to the formation of a double layer. Initially, the positive charge at the surface attracts a layer of anions, which in turn attracts a looser, more diffuse layer of cations. The layer thickness and potentials depend upon pH and the ionic nature and concentration of solute species. Smaller, more mobile ions might be anticipated to be more readily drawn to such a double layer than less mobile ions with larger hydrated radii (9). This could be a major factor in determining the relative degree of enrichment of various ions. Possible AIR Effects in Other Systems. While the incidence of AIR interferences has been shown to be a factor of major importance in certain model binary ion systems, the extent of these interferences in the complex matrices often encountered in the analysis of practical samples is as yet unknown. It may be that matrix volatilization interferences or ionization interferences will be the dominant source of interference in many practical analyses. In addition, detailed studies of acid effects on aerosol ionic redistribution have not ;vet been carried out. Most trace metal determinations are
Anal. Chem. 1980, 52, 1059-1064
carried out in dilute acid, but some recent studies with the ICP have been with solutions prepared in 10% hydrochloric acid (19). This might mask any natural AIR interferences. Work by Boumans and DeBoer (20) with dual ultrasonic nebulizers has shown effects which seem to fit into the pattern of AIR interferences which we have observed with pneumatic nebulizers. By using two separate nebulizers, these workers were able to distinguish clearly between aerosol and plasma processes. Enhancements for zinc a t low concentrations, in binary mixtures containing potassium at high concentrations ( u p to 4000 Fg mL-') were found, and these were attributed to processes occurring in the desolvation system. However, t h e zinc enhancement might well be explained as an AIR effect, and this would also account for the similarity in the interferences noted for ammonium, potassium, and cesium ions, which cannot otherwise readily be explained. Although these effects fit the pattern of AIR interferences, direct measurement of ionic redistribution in ultrasonically generated aersols will be necessary in order to unequivocally establish its role in causing the observed enhancements.
CONCLUSIONS Aerosol ionic redistribution has been shown to have the potential to cause significant interferences in analytical atomic spectrometry when pneumatic nebulizers are used for sample introduction. Its predicted influence in flame AAS is relatively small, due to the nature of the operation of AAS nebulizers and the likely dominance of volatilization and ionization interferences in many practical analyses. In ICP spectrometry, on the other hand, AIR interferences may cause significant measurement errors. A particular sample matrix might not be anticipated on the basis of previous knowledge to give rise to interference problems due to volatilization or ionization effects in the plasma. However, an unexpected AIR interference may be active. Furthermore, changes in operating conditions for the nebulizer, or indeed a change in nebulizer, may cause the magnitude of the interference to alter drastically. consequently, in solutions containing high concentrations of matrix elements, matching of sample and standard matrices is essential if highest accuracy
1059
is to be achieved. Nevertheless, it should be emphasized that the magnitude of AIR interferences is very much a function of nebulizer design and operation. While significant interferences can be present when running a n ICP system within the normal range of operating conditions, it seems likely that the anticipation of a possible problem, combined with careful optimization of conditions t o minimize interference, should allow the full ICP analytical potential t o be realized. It is hoped that a fuller parametric investigation of AIR interferences currently underway will lead to guidelines for such optimization.
LITERATURE CITED (1) Aitken, J. Trans. Roy. Soc. Edinburgh 1881, 30, 337. (2) Boyce, S. G. Science 1951, 173, 620. (3) Judson, C. M.; Lerew, A. A.; Dixon, J. K.; Salley, D. ,J. J . Phys. Chem. 1953, 57,916. (4) Bloch, M. R.; Kaplan, D.; Kertes, V.; Schnerb, J. Nature(London) 1986, 209, 396. (5) Bloch, R. M.; Luecke, W. J . Geophys. Res. 1972, 77,5100. (6) MacIntyre, F. Tellus 1970, 22,451. (7) Wilkniss. P. E.; Bressan, D. J. J . Geophys. Res. 1971, 76,736. (8) Wilkniss, P. E.; Bressan, D. J. J . Geophys. Res. 1972, 77,5307. (9) Glass, S. J.; Matteson. M. J. Tellus 1973, 25, 272. (10) Sugawara, K. Oceanogr. Mar. B i d . Ann. Rev. 1984, 3 , 59. (11) Barker, D. R.; Zeitlin, H. J . Geophys. Res. 1972, 77,5076. (12) Woodcock, A. H.; Duce, R. A.; Meyers, J. L. J . Atmos. Sci. 1971, 28, 1252. (13) Blanchard, D. C. "Progress in Oceanography", M. Sears, Ed.; Pergamon Press: New York. 1963: DD 71-202. (14) Blanchard, D. C. "From Raindrops to Volcanoes", Doubleday and Co.: Garden City, N.J.. 1967; pp 46-127. (15) MacIntyre, F. J Geophys. Res. 1972, 77,5211. (16) Cresser. M. S.; Browner, R. F. Spectrochim. Acta. Part 6 1980, 35, 73. (17) Cresser. M. S.; Browner, R. F. Appl. Spectrosc. 1980, in press. (18) Novak, J W.; Browner, R. F. Anal. Chem. 1980, 52,576. (19) Larson, G. F.; Fassel. V. A.; Scott, R. H.; Kniselev. R. N. Anal Chem. 1975, 47,238. (20) Boumans, P. W. J. M.; De Boer, F. J Spectrochim. Acta, Par7 6 1976, 37. 365 (21) Lane, R. W. Ind. Eng. Chem. 1951, 43, 1312. (22) MacBain, J.; Swain, R. Proc. R . Soc. London, Part A 1938, 254,608. (23) Loeb, L. B. "Static Electrification"; Springer: Berlin. 1958; pp 61-80. ~~~
RECEIVED for review September 10,1979. Accepted February 29, 1980. This material is based upon work supported by the National Science Foundation under Grant No. CHE 77-07618.
Zeeman Effect in Flame Atomic Absorption Spectrometry K. G. Brodie" and
P. R. Liddell
Varian Techtron Ry. Limited, P.O. Box 222, Springvale, 3 777, Victoria, Australia
An electromagnet is used to apply a modulated transverse magnetic field to a 6 t m length burner. A fixed linear polarizer is used to eliminate the T component. Sixty-six elements are examined in the appropriate acetylene fuelled flame, using either nitrous oxide or air as the supporting gas. Zeeman sensitivity ratios range from 3 6 % to 94%. The accuracy of background correction for non-atomic absorption in the flame Is superior to conventional background correction systems. Detection limits for some elements are better due to the reduction in flame noise. However, the reflex nature of the analytical curve reduces the linear dynamic range and introduces the possibility of ambiguous results.
There has recently been considerable interest in the use of 0003-2700/80/0352-1059$01 .OO/O
the Zeeman effect for background correction in atomic absorption spectrometry. Since background absorption is a more serious problem with graphite furnace atomizers than with flames, most of the work has been with furnaces ( I ) . However, there have been several studies using flames and a variety of configurations have been utilized. The magnetic field has been applied to the flame (2-6) or the light source (5, 7-9) and has been applied parallel ( 4 ) or perpendicular ( 2 . 3, 5-9) to the light path. We have discussed the merits of the various approaches to Zeeman atomic absorption spectrometry (ZAAS) with graphite furnace atomizers ( I ) and selected the modulated longitudinal field on the furnace as the preferred approach. Application of the field to the furnace meant that background correction was carried out a t exactly the same wavelength as the atomic absorption, special lamps were not required, and self-abC 1980 American Chemical Society
