Page 169 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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7.7 Solid Sampling Techniques

There are a variety of solid sampling techniques that have been used to introduce samples.
Electrothermal vaporization of a solid or liquid sample from a rod or a tube into plasma instruments
yields detection limits far superior to those obtained by conventional nebulization. Sample transport
approaches 100% and, if a solid sample is vaporized, the dilution factors associated with the dissolution
of the sample are not present. In addition, interferences arising from constituents of the matrix, e.g.
chlorides, oxides, may be removed during the charring stage (as in ETAAS), thereby facilitating
interference-free determination of the analyte when it is vaporized into the plasma. The process is very
similar to that for ETAAS. Sample is dispensed on to the vaporisation device and it then undergoes a
temperature programme so that it is dried, ashed and then vaporized. A sheath of gas (typically argon)
then transports the analyte to the plasma, where it is detected as a transient peak. For some analytes
(notably those that form very refractory oxides), the addition of a matrix modifier assists in the
vaporization of the analyte into the plasma. Fluoride-based modifiers such as PTFE (in the form of a
suspension) or Freon gases have proved very popular, because they vaporise these analytes as their
fluorides at a much lower temperature than in the modifier's absence. This helps to prolong the lifetime
of the electrothermal device in addition to improving the transport of the analyte to the plasma. The
plasma is energetic enough to dissociate the analyte fluorides, so that analyte atoms (or ions) may be
detected.

Another popular method of solid sample introduction is laser ablation. Here, a laser (often an Nd:YAG
laser) is focused on to the surface of a sample (although occasionally focusing either just above or
below the surface is used). Pulsing of the laser leads to localised vaporization of the sample, which can
then be transported to the atom cell by a flow of inert gas. This technique avoids the need for sample
dissolution and so yields good detection limits. Laser ablation has several advantages over other
introduction techniques. It is particularly useful when only a very limited amount of sample is available.
The laser can be focused on to a very small area so that single crystals/particulates may be analysed.
By repetitive ablation of the same spot, depth profiling of a sample may also be made. The main
drawback with the technique is that it can be very difficult to calibrate accurately. This problem arises
from the fact that very closely matrix-matched standards must be used to calibrate, and it can be hard
to find samples of known composition that are matched closely enough. Despite this problem, many
workers have succeeded in analysing a variety of matrices. Another problem arises from the small
sampling area (ca 50 µm diameter spot), so that a representative sample is not obtained unless the
sample is very