Anal. Chem. 1002, 64, 2036-2041
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Inductively Coupled Plasma-Mass Spectrometry as an Element-Specific Detector for Field-Flow Fractionation Particle Separation Howard E. Taylor' and John R. Garbarino United States Department of the Interior, Geological Survey, Water Resources Division, Box 25046, M.S. 458, Denver Federal Center, Denver, Colorado 80255-0046
Deirdre M. Murphy and Ronald Beckett Water Studies Centre and Department of Chemistry, Monash University, Melbourne, Australia
An inductlvely coupled plarma-massspectrometer was used for the quantitativemeasurement of trace ekments In spocifk, submicrometer dzefraction particulates, separated by sedimentationfleld-flow fractlonatlon. Fractions were collected from the eiuont of the field-flow fractionation centrifuge and nebulized,with a Bablngton-type pneumatic nebulizer, into an argon inductivdy coupled piarmtimass spectrometer. Measured Ion currentswere usedto quantlfy the malor, minor, and trace element compodtion of the rlzo-mparated coiiddai (
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F t g w 5. Darllng river (Australia),eupended partlcuiate matter (A)
UV detector (dm'ld V,) and selected element (dmild V,) fractograms;
(B)massdeaed(dm'ldd)andelement-based(dm',ldd)siredlstrlbutlons;
and (C) elemental composition (dmildmq dlstrlbutions. Inductively Coupled Plasma-Mass Spectrometric Elemental Analysis. Selected fractions collected from the output of the SdFFF device were introduced directly into a Sciex Elan Model 260 inductively coupled plasma-mass spectrometer operating with argon gas and equipped with a Babington-type pneumatic nebulizer to permit suspended particulates to be incorporated into the sample aerosol without the risk of clogging. Operating characteristicsand conditions ofthis system have been described elsewhere.~~3' All ion currents were adjusted by their appropriate isotopic abundance and relative sensitivity factors, obtained by measuring standard calibration solutions. This provides a computed intensity directly proportional to the total mass of the respective element in units of ions per second. (30) Garbarino, J. R.;Taylor, H.E. In Applications of Inductively Coupled Plasma Mass Spectrometry; Date, A. R.,Gray, A. L.,Eda.; Blackie: London, 1989; pp 71-89. (31)Garbarino, J. R.;Taylor, H.E. Appl. Spectrosc. 1980, 34, 584592.
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Flgwo 6. Cadmlum ion adsorption on Yarra River (Australia)SPM; (A) UV detector (dm'ld V,) and Cd lon current (dm'cd,.ald V,) fractograms; (B) mass-based (dm'ldd) and adsorbed Cd-based (dm&,ldd) size dlstrlbutions; and (C) adsorption density (dm&,.aldA) dlstrlbutlon.
RESULTS AND DISCUSSION
Metal Colloids. Fractograms showing both the UV and specific element traces (Pb, Fe, and Al) for individual runs of the lead chromate (PbCrOd), goethite (FeOOH), and alumina (AlzO3) colloid samples are shown combined on Figure 2A. Chromium was also measured, and a specific element fractogram was essentially identical to Pb, but was not plotted to avoid visual confusion in Figure 2. The various traces demonstrate that there is good agreement between the UV detector response, which is used as a measure of the mass concentration of particles eluted, and the relevant normalized element ion currents. A SdFFF run of a mixture of the three inorganic colloids was also made, and the raw fracbgrams (UV trace, Al and Pb ion currents) are shown in Figure 2B. This mixture contained approximately equal amounts of PbCrOr and AlzO3 but only about one-fifth the mass concentration of FeOOH.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
The Pb peak in the mixture and the corresponding portion of the UV curve is in excellent agreement with the individual PbCrO, run shown in Figure 2A. The Al element and W traces in the mixture are also in quite good agreement with each other although the peak of the fractograms in the mixture are shifted significantlyto lower elution volume (i.e. 190 mL, cf. 240 mL for the individual AlzO3 run). In addition, there is no trace of Fe in the mixture at the position expected based on the individual FeOOH run (therefore the Fe ion current fractogram trace is not included in Figure 2B, for clarity). Thus, it would appear that the size distribution of FeOOH is altered in the mixture possibly by aggregation or heteroaggregation involving the A l 2 0 3 . Thus larger aggregates may not have eluted within the run time. We note that at the pH of the experiment (about pH 9) the surface charge of the goethite and alumina could well be of oppositepolarity as the isoelectric points for these oxides are at about pH 7 for goethite and pH 9 for alumina. However, the pyrophosphate present in the carrier would be expected to modify the Surface charge considerably. The UV curve for the mixture supports this hypothesis, showing only a minimal signal a t the position where the FeOOH was expected, and the shift in the A1203 peak maximum may indicate that this heteroaggregation has selectively removed more of larger sized A1203 material, thus altering ita size distribution. The fractograms for individual runs of FeOOH and A 1 2 0 3 given in Figure 2A were converted to both mass-based and element-basedsize distributions, and the results are plotted in Figure 3. The size distribution for the PbCrOr was not included because the mean particle size for this sample was less than 0.07 pm, and thus its peak was not significantly resolved from the void peak. In the case of FeOOH (Figure 3A),the UV-based and Fe-based size distributions are in good agreement. It would seem that errors due to the particle size dependenceof the light scattering process, which is responsible for the attenuation of the signal in the UV detector, is quite small, at least for this fairly narrow size distribution material. The UV-based and Al-based size distributions obtained for A1203 (Figure 3B) are also in general agreement, respectively, although there seems to be a small but significant shift of the UV curve to larger particle size. Light scattering intensity of smaller particles in this range is less on a per mass basis relative to larger particles, and this could possibly be responsible for such a shift in the UV curve. Further work is currently in progress to clarify this point. River Water Colloids. YarraRiver. The UV and selected element fractograms and size distributions for the Yarra River suspended particulate material (SPM) are given in Figure 4A with the corresponding frequency functionsplotted in Figure 4B. This sample contains only small particles (X0.25 pm), and much of the material is less than the minimum resolvable size of about 0.07 pm for the experimental conditions used to obtain the separation. The elemental composition plots in Figure 4C indicate that the Mg and Fe concentrations in the particles are similar, suggesting a fairly uniform mineralogy across the size distribution of the sample, although the Al concentration shows a general increase with particle size. Darling River. The fractograms, particle size distributions, and elemental composition distributions obtained for the Darling River suspended colloid sample are presented in Figure 6A-C, respectively. The Darling SPM appears to be broader than the Yarra SPM with significant amounts of sample being present up to 0.5 pm in diameter. The concentrations of Mg was constant across the size range studied (0.07-0.5 pm), indicating a fairly uniform mineralogical distribution of the aluminum silicates. In contrast the
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Fe composition decreases gradually with increasing size until about 0.3 pm after which it decreases rapidly with the larger particles in the sample containing negligible amounta of Fe. These trends illustrate the detailed information on the composition of complex particulate materials that can be obtained using the SdFFF-ICP-MS methodology. Cadmium Adsorption Characteristics. An adsorption experiment was conducted in which a Cd spike was added to the Yarra River water colloid concentrate. The fractogram and size distributions, frequency functions, and adsorption density distribution obtained are given in Figure 6 A 4 , respectively. It appears that the amount of Cd adsorbed per mass of solid absorbate increases as particle size decreases (see Figure 6B). This is the trend generally found for such pollutant uptake experiments and is consistent with the increase in specific surface area expected as the particle diameter decreases.1Ql25 The adsorptiondensity distribution represents the amount of Cd adsorbed per unit area of particle surface and thus may give an indication of how the adsorption a f f ~ t yof the adsorbate particles vary across the size range covered by the fractionationexperiment. For a homogeneous sample surface, we would thus expect this surface adsorption densityto remain constant. However, for the Yarra River suspended sediment samples, the adsorption density displayed an increase with particle size. This may be due to variations in the mineralogy or surface characteristics of the particles with more active components being associated with the larger size ranges. Alternatively it could be due to an anomalous increase in specificsurface area due to deviations from the spherical shape and smooth, nonporous particles which are assumed in the calculations used. In particular, aggregation processes may produce complex agglomerated particles that are likely to distort the simple particle size-surface area relationship (i.e. area or diameter expected for most fully dispersed mineral samples).
CONCLUSION The results of this study demonstrate that the elemental composition of
