Page 168 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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7.6 Chromatography

Several different types of chromatography have been coupled with atomic spectrometric detectors.
Most applications involving chromatography coupled with atomic spectrometry yield speciation data,
i.e. they separate different chemical forms of an analyte.

Liquid chromatography is the most common type because it is so easily coupled with the majority of
atomic spectrometric detectors. The flow rates associated with ion chromatography or HPLC (typically
1-2 ml min ) are compatible with the sample uptake rate of inductively coupled plasma-based
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instruments. However, the flow rate is too low to be coupled directly to a flame spectrometer, whose
uptake is typically 6-10 ml min . For this coupling to be made, it is frequently necessary to have a
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connection that contains a small opening to the atmosphere so that air may be entrained, thus preventing
the formation of a vacuum. Other components of the coupling may be required depending on the nature
of the chromatography. If organic solvents are used in the mobile phase (e.g. methanol, acetonitrile)
then these have the effect of quenching plasmas and, under extreme conditions, may lead to plasma
extinction. A desolvation device, e.g. a membrane drier tube or Peltier cooler, is therefore necessary.
Coupling of liquid chromatography with microwave-induced plasmas is still in the research stages,
because the plasma is so easily extinguished.

Gas chromatography has been coupled to atomic spectrometric detectors, but the coupling is less
straightforward than for liquid chromatography. This is because the analytes must be transferred from
the GC oven to the atom cell at an elevated temperature to prevent analyte condensation (loss of
analyte and therefore loss of sensitivity). A number of research papers have been published that
describe the construction of such transfer lines. For flame detection, a slotted tube atomizer or some
other quartz or ceramic tube is usually placed in the flame to improve sensitivity. For inductively
coupled plasma work, the transfer line usually leads directly to the back of the torch. Care must be
taken to ensure that the transfer line does not act as an aerial for the RF power. This could result in
damage to instrumentation and to operators. The positioning of the transfer line within the torch and the
sheathing gas flow should be optimized very carefully, since these parameters have a dramatic effect on
the sensitivity. A commercial piece of instrumentation is available that has coupled GC with MIP-AES
detection (see Chapter 4). As described previously, the MIP is very easily extinguished, and even
microlitre quantities of organic solvent frequently have to be vented from the system before the analyte
reaches the plasma. Other types of chromatography, e.g. capillary electrophoresis, are yet to make a
full impact on coupled techniques, but research is on-going.