Page 161 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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7.2 Other Discrete Sampling Devices

There have been two other discrete sampling devices that have been used in conjunction with flame
spectroscopy. The Kahn sampling boat is a system whereby nebulizer inefficiency was avoided by
using a tantalum sampling boat from which the sample was evaporated as it was pushed into the
flame. The technique was only applicable to the more easily atomized analytes, but it did yield a useful
improvement in sensitivity. Unfortunately, the technique is prone to poor reproducibility. The Delves
sampling cup was a modification of the above technique. It was applied mainly to the determination of
lead in microsamples of whole blood or other easily combustible material. The tantalum cup was
replaced with a smaller (10 mm outer diameter, 5 mm deep, 0.15 mm thick metal foil) and more easily
positioned nickel or stainless-steel micro-crucible. The crucible may also be used during preliminary
chemical treatment (e.g. addition of hydrogen peroxide to destroy organic matter). It is mounted into a
device which enables it to be pushed close to the flame to allow charring, and then into the flame, to
allow atomization. A nickel (or, more recently, silica) absorption tube is mounted in the flame and the
atoms enter the tube through a hole half way along its length. Light passes through the tube, thus
improving the reproducibility by lengthening and defining the residence of the atoms in the flame. Both
of the techniques described above have fallen into disuse, mainly because of the advent of flow
injection.

7.3 Slotted Tube Atomizer.

Although the slotted tube atomizer is not strictly a sample introduction technique, it should be
mentioned as it has found use in a number of different sample introduction techniques for flame
spectroscopy. The majority of techniques with which it has been used are those that produce a transient
signal or where a long residence time for the atoms in the light beam is required owing to the very low
concentration of the analyte in the sample. Normally, the atoms pass through the flame extremely
quickly and hence spend a very short period of time in the light beam (10 s), but the tube atomizer
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retains the atoms for a far longer period and hence the sensitivity increases.

7.4 Flow Injection

Flow injection has been coupled with flame-and plasma-based instruments. It has a number of
advantages over conventional nebulization, including the ease of coupling, the possibility of
preconcentration and matrix removal