Instrumentation Richard F. Browner School of Chemistry Georgia institute of Technology Atlanta. Ga.30332
Andrew W. Boorn Sciex 55 Glen Cameron Road, X202 Thornhill, Ontario L3T 1P2. Canada
Sample lntmduction Tihniques fior
Selection of the best sample introduction procedure for an analysis requires consideration of a number of points. These include: the type of sample (e.g., solid, liquid, gas), the levels, and also the range of levels for the elements to be determined, the accuracy required, the precision required, the amount of material available, the number of determinations required per hour, and special requirements, such as whether speciation information is needed. The measurement techniques available, whether flame atomic absorption spectracopy (FAAS), inductively coupled plasma (ICP) atomic emission spectroscopy, or dc plasma (DCP) atomic emission spectroscopy,will also have a major effect on the choice of the procedure selected. Although this paper will concentrate specifically on sample introduction techniques, in any real analysis sample introduction is an extension of sample preparation. As a consequence, the selection of a suitable sample introduction technique can depend heavily on available and effective sample preparation procedures. Generally, though, sample preparation will not be discussed here, except where sample introduction and sample preparation are intimately linked.
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0003-2700/84/0351-875A$01.50/0 0 I984 American Chemical Society
. . . . . T o understand the limitations of practical sample introduction systems it is necessary to reverse the normal train of thought, which tends to flow in the direction of sample solutionnebulizer-spray chamber-atomizer. and consider the sequence from the opposite direction. Looking a t sample introduction from the viewpoint of the atomizer, the choice of procedure will hinge on what the atomizer can usefully accept. Bearing in mind that every atomizer has certain reasonably well defined, but different, properties of temperature, chemical composition etc., an introduction procedure must he selected that will result in rapid breakdown of species in the atomizer, irrespective of the sample matrix. To ensure efficient free atom production, the following parameters must be known for each analyte-matrix-atomizer combination: maximum acceptable drop size, optimum solvent loading, both aerosol and vapor, maximum acceptable analyte mass loading, appropriate gas flow patterns for effective plasma penetration (for the ICP), and suitable observation height. This last parameter should be selected in conjunction with the gas flow pattern of sample introduction such
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that adequate residence time is provided for the introduced material to desolvate, vaporize, and atomize. In certain cases, for instance, when organic solvents are introduced to an ICP. it also may be necessary to adjust the atomizer operating characteristics to account for the change in plasma properties induced by the solvent. Here, an increase in forward power to the plasma (e&, from 1.25 to 1.75 kW) is necessary to aid the decomposition of organic species. Overall, then, it is the properties of the atomizer that dictate the design and operation of the sample introduction system. This is particularly true for liquid sample introduction with pneumatic nebulization.
Present Underbtanding of Sample introduction Processes There is a great deal of intuitive, but relatively little experimentally based, knowledge in this field. Clearly, there is some upper limit to the size of drop that can be vaporized in the typically 1-2 ms available in the atomizer. Yet there are no tables available that specify the upper limit of drop size suitable for each matrix and atomizer. Such tables would be of great help to
ANALYTICAL CEMISTRY, VOL. 56, NO. 7, JUNE 1984
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
practicing analytical chemists. Of course, the production of these tables is not a trivial matter, and would require rather involved experimental procedures at the state of the art in particle generation and characterization. Nevertheless, when these data do become available, which undoubtedly they will in due time, this should allow researchers to steer around the majority of interference problems other than those of spectral overlaps, for which tabular data are already available (I,2 ) . ICP Systems. The data available on solvent-loading limitations for organic solvents with the ICP have been characterized recently ( 3 ) .Some typical limiting aspiration rates are shown in Table I, together with evaporation factors, E, of the solvents. These data are useful as a guide for organic solvent introduction to the ICP. The evaporation factor is a measure of the rate of mass loss from an evaporating drop, and is given by:
E = 48 D , u P , M ~ ( ~ R T ) - ~ (1) where D , is the diffusion coefficient of the solvent vapor, u is the surface tension, P, is the saturated vapor pressure, M is the molecular weight of the solvent, 6 is the density, R is the gas constant, and T i s the absolute temperature. In general, the ICP has decreasing tolerance to solvents as their evaporation factors increase, and there is an inverse correlation between evaporation factor and limiting aspiration rate. However, the alcohols have a much greater quenching effect on the plasma than their evaporation factors would indicate, and they may readily extinguish the plasma under normal operating conditions. It is always possible to remove at least part of the solvent vapor by condensation. Two groups of workers have attempted this and shown that the tolerance of the ICP to organic solvents is greatly improved when a large fraction of the solvent vapor is removed from the gas stream passing to the plasma ( 3 , 4 ) .This is an indication of how the sample introduction process can be modified to produce analyte closer to the optimum for the atomizer. No published data are available on limitations of aqueous sample introduction to the ICP, although clearly water loading in the plasma has a direct influence on plasma properties. In fact, it has been shown for certain ionic lines that doubling the water loading entering the plasma can cause a 100-fold reduction in analytical signal (5). From a practical standpoint, three important conclusions can be reached. First, it is necessary to introduce sample to the atomizer with drops no larger than a certain maximum size
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(dmx).Second, the solvent introduction rate must fall within a certain permissible hand of values. Third, to maintain good system reproducibility, it is essential that all these parameters be controlled carefully over the long term. Any significant change in drop size or solvent loading reaching the atomizer could have an adverse effect, both on system accuracy and system reproducibility. From this standpoint, the need to maintain a constant temperature in the plasma box in an ICP system becomes clear, as a means to reduce the baseline drift caused by variable solvent vapor loading (5). Atomic Absorption Systems. Flames are generally far less susceptible to variations in solvent loading than ICPs are, although the introduction of organic solvents to an air-acetylene flame can lead to a significant temperature drop. This in turn could cause the onset of interferences due to sample matrix problems. For flame AAS systems, the design of nebulizers and spray chambers appears to have been empirically optimized to provide the best aerosol drop size in the flame for interference-free analyte vaporization. Solvent loading appears to be a secondary factor. In the past 20 years AA nebulizers and spray chambers have undergone a steady progression. They have changed from devices producing very coarse aerosols, with a corresponding high incidence of vaporization interferences, to devices producing much finer aerosols, which are largely free from this type of interference. In fact, recent data indicate that it is possible to virtually eliminate all matrix-induced vaporization interferences in AAS (e.g., calcium-phosphate, siliconaluminum, silicon-manganese, etc.) (67).This is accomplished by shift-
Table 1. Limiting Organlc Aspiration Rates for ICP a
Methanol Ethanol
Xylenes Acetone MlBK Diethyl ether
Chlomfwm
ing the aerosol distribution reaching the flame to smaller values, through modification of the spray chamber design. In many respects it is surprising that this process has taken so long, as early work, particularly that of Stupar and Dawson, gave a clear indication of the importance of aerosol drop size in minimizing interference (8).Since publication of this paper, there appears to have been very little work carried out on the systematic study of aerosol properties and interference effects. Fortunately, it appears now that such improvements can he accom. plished relatively simply. As a counterbalance to any complacence that this statement might imply, it should also be noted that nebulizers that produce improved detection limits for many volatile elements have been marketed recently. These devices operate by letting a higher proportion of large-diameter droplets reach the flame. While this can, in certain in.
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Figure 1. Drop size distribution for AA nebulize1 1984
77.3 771 321
Defined as maxlmum whnt uptake POBslble fa stable opemtlon lw 1 h.
LC 83-22323
NO. 7, JUNE
47.2 45.6 18.5 264
* Ar ICP operated at 1.75XW rl power.
640 pages(lS84)Clothbound
878A * ANALYTICAL CHEMISTRY, VOL. 56,
0.1 2.5 1.0 0.1 3.0
