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The establishment of EVISA is funded by the EU through the Fifth Framework Programme (G7RT- CT- 2002- 05112).

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Plasma source detection techniques for gas chromatography

Plasma sources for atomic and molecular spectrometry are very versatile detection systems for gas chromatography providing high sensitivity and selectivity for speciation analysis.

Requirements for a hyphenated technique for speciation analysis

Speciation analysis requires the very selective detection of chemical species at trace and ultra-trace levels. These highly demanding requirements with respect to sensitivity and selectivity can only be achieved by a combination of very selective separation techniques and the most sensitive and highly selectice detection techniques. Atomic spectroscopy has the potential of being one of the most selective, consistent, and versatile techniques of this kind because it exploits differences in the elemental composition of the analytes. While atomic absorption and fluorescence represent particularly convenient approaches, they are confined to the determination of a limited number of compounds and are not readily adaptable to multi-element tracking. Atomic emission spectrometry ist best suited for the multi-channel element-selective chromatographic detection.

Plasma sources play a key role for the detection techniques that can provide element selectivity and high sensitivity. The plasma consists of a mass of predominantly ionized gas at a temperature of 4000-10000 K. This state can be maintained directly by an electrical discharge through the gas (DCP) or indirectly via inductive heating of the gas by means of an electromagnetic field established using power generated at radio frequencies (ICP) or microwave frequencies (MIP). Analyte excitation results from electron impact and from collisions with metastable atoms of the plasma support gas (usually argon or helium).

The main advantages of plasma sources are:

continuous mode operation allowing relatively easy hyphenation with separation techniques,
high sensitivity for the elements of interest due to a plasma temperature which provides an ideal environment for atomic spectrometric detection,
simplified calibration, which may rely on one single standard, due to complete dissociation of compounds and/or linear behaviour over many orders of magnitude,
simultaneous detection that permits the calculation of empirical molecular formulae and/or allows for multi-element speciation analysis.

Depending on the separation technique used, special requirements must be fulfilled for obtaining a useful hyphenated technique. When using a separation technique working with a liquid mobile phase such as liquid chromatography or capillary electrophoresis, the plasma source must provide sufficient energy for the vaporization and atomization of the column/capillary effluent. When using gaseous phase separation such as gas chromatography (GC), energy requirements are less demanding. Excessive energy (temperature) of the source may even degrade its performance by destroying molecular information and enhanced diffusional losses.

Plasma source detection techniques for gas chromatography

The power requirement for the dissociation, atomization, excitation and ionization of molecules in the gas-phase are in the range of 50-150 Watts for flow-rates of conventional GC systems when solvent front peaks are not directed through the source but by-passed.

Such power requirements can be provided by a number of sources, such as:

Inductively coupled plasma (ICP)
Microwave-induced plasma (MIP)
Glow-discharge sources (GDS)
Direct current plasmas (DCP)

While the ICP can easily provide plasma power in the order of 1.5 kW, the use of Argon as the plasma gas limits the ionization power of the plasma, leading to a low degree of ionization of non-metallic elements (e.g., carbon, phosphorus, sulfur and halides).
On the other hand, the high plasma temperature obtained in the ICP leads to the nearly complete atomization of melocules preventing one to obtain molecular information.

The less powerful MIP has the advantage of providing a small dead volume of the discharge tube and the compatibility of the plasma with the low carrier gas flows used in capillary GC columns, which together make it selective and often less subject to interference. For these reasons, limits of detection are at least two orders of magnitude better for GC–MIP-OES than GC–ICP-OES. There is no doubt that these properties were responsible for the worldwide commercial success of the GC–MIP-OES system at the end of the 1980s. However, since the ICP-MS has reached the market, the wider application field of this instrument has ousted the dedicated GC-MIP-AES systems, so that today there are only very few commercial GC-MIP-OES systems available.

The first description of an GC-MIP-OES system dates back to 1965. Most publications have appeared during the years 1990-2000.