13e Colloque annuel du RÉDiST

Direct Powder Introduction into the Inductively Coupled Plasma: Potential Versatility as an Analytical Tool for Geoanalysis.

De Silva, Nimal, ndesilva@ccs.carleton.ca, Department of Chemistry, Carleton University, Ottawa, Ontario, Canada K1S 5B6.

Résumé de conférence

Direct Solid Analysis by ICP techniques:

The most common approach for sample introduction in Inductively Coupled Plasma Emission (ICP-ES) and Mass (ICP-MS) spectrometry is solution nebulization, where droplets generated by a nebulizer are introduced into the centre of the plasma as a flow of fine aerosol. In spite of its inherent simplicity, solution nebulization demands that solids samples such as most of the materials of geological interest, undergo tedious and time consuming dissolution procedures prior to the ICP measurement. Direct analysis of solids offers promise for the rapid determination of trace elements in difficult matrices, particularly geological materials, with added versatility. Elimination of complex sample preparation procedures saves time, effort and reagents. It minimizes possible loss of analyte and contamination. Increased efficiency of sample transport into the plasma has the potential for providing improved detection limits. Problems associated with the formation of molecular ions originating from solvent molecules (in ICPMS) can be eliminated. Although the potential application areas are numerous, with the availability of suitable techniques for direct analysis of solids, perhaps it would be the geoanalyst who would be benefitted the most.

Available Alternatives for Direct Solid Analysis:

Several sample introduction techniques for direct solid analysis have evolved to different levels of maturity, some of which are even commercially available. Due to the varied nature of sample types, available size, physical form, and the particular analytical problem in hand, most of these techniques are complimentary rather than being competitive.

For example, laser ablation (LA) ICP-MS has become a popular and powerful tool for geological applications, where micrometer grains on a rock surface can be analysed conveniently, providing information on localized concentrations and corresponding isotope ratios. However, finding a suitable calibration strategy can be difficult. If the original sample is available in powder form, the sample preparation and pelletization can still be an elaborate process. ElectroThermal Vaporization (ETV) and Direct Sample Insertion (DSI) can be used to vaporize microgram quantities of solid samples directly into the plasma without prior digestion. Although calibration can be somewhat simple, the sample size used for a given analysis generally has to be limited to microgram quantities. Spark Ablation (SA) has been used for direct analysis of solid samples. As the sample has to be electrically conducting, geological powder materials generally have to be mixed with conducting graphite or a metallic powder at a high proportion, which may pose problems related to contamination and poor detection limits.

Direct Powder Introduction:

Direct Powder Introduction (DPI) has the potential to overcome some of the above difficulties, particularly if the sample already is in powder form, or has to be converted to a powder during the analytical process. However, there are some technological challenges that have to be addressed for practical analytical application of DPI-ICP. These challenges can be classified into three basic categories: (a) delivery of a small mass of a dry powder into a gaseous plasma in a quantitative, controlled, uniform, and practical manner, (b) representative volatilization of analytes from the solid particles inside the plasma, and (c) processing of non-steady signals that originate as a result of the discrete nature of particles entering the plasma.

We have developed a device to introduce powders directly into the ICP which meet most of the features expected from an ideal powder delivery device. A sample of analytical data obtained by interfacing this device with a photodiode array based spectrometer is shown in Table 1, which shows a typical analytical precision of 1-2% with good accuracy. Further refinements in DPI technology is currently underway in our laboratory.

Non-conventional applications of DPI-ICP:

In conventional sample dissolution, even a minute volume of the resulting homogeneous solution becomes representative of the original bulk sample. On the other hand, sample dissolution also destroys information about homogeneity and mineralogy of the sample. With the state of the art spectrometers (available in both ES and MS), the DPI technique can be exploited to extract information at individual particle level in addition to the measurement of bulk concentration of analytes in the powder. Some potential non-conventional applications are:

- Testing of heterogeneity of samples and standard reference materials at micro- to milli-gram levels, using a large number of sub-samples.

- Mineralogical studies. Information on elemental association or isotopic distribution at individual particle level.

- Distinguishing between surface coated versus matrix laden trace elements in sediments, etc., and their possible differences in isotopic ratio distribution (with MS).

- Direct analysis of pre-concentration media in dry powder form without elution.

- Fundamental studies of the behaviour of particles in the plasma.

This presentation will focus on the current developments of the DPI technology, and future directions pursued in our laboratory to exploit its full potential.

Table 1 - Determination of Cu in Sediments and Fly Ash by DPI-ICP-ES

SRM   Recomm/Certified Measured %RSD %Difference
LKSD-1   44 41.5 1.5 -5.6
LKSD-2   37 39.6 1.2 6.9
LKSD-3   35 36.3 1.0 3.8
LKSD-4   31 35.1 2.3 13.3
STSD-1   36 35.0 2.5 -2.7
STSD-2   47 47.4 1.4 0.8
STSD-3   39 40.2 0.7 3.1
STSD-4   65 59.5 1.3 -8.5
BCSS-1   18.5 18.5 0.4 Low Std.
MESS-1   25.1 26.1 0.3 4.1
1645   109 74.4 0.8 -31.7
1633a   118 117.7 0.9 -0.2
1633   128 128 0.4 High Std.

Sample size: 10-12 mg, Average of 6 measurements