Detection Methods

Before the compound of interest can be detected, an appropriate method of determining must be chosen. Several studies have shown that it is also important to consider the chemical species of the metal and not to simply estimate the total amount of the inorganic form beforehand. The measurement of very low concentrations of organometallic species is a challenging task, so it is necessary to hyphenate the separation method of choice with the most efficient detection instrument. However, not all instruments are useful for determining organometallic species because a few of them are simply not sensitive enough. Various detection methods are available that can by connected to the appropriate separation technique. This section describes some approved detection methods, such as atomic absorbance spectrometry (AAS), the atomic fluorescence spectrometry (AFS), and inductively coupled plasma mass spectrometry (ICP-MS).

14.2.4.1 Atomic Absorbance Spectrometry

Flame AAS is an inexpensive yet effective method of determining the total amount present of a specific element. Organometallic species can be analysed by coupling flame AAS to chromatographic separation methods. The main disadvantage of this method is its relatively low detection limit. One way to improve the detection limit is to preconcentrate the sample using, for example, column absorption (Matusiewicz 1997; Pasullean et al. 1995). Another possibility is to use hydride AAS, which can be coupled to a chromatographic device. The separation can be performed by

HPLC. The eluent is transferred to a hydride generator. The reaction mixtures can be analysed by AAS. This separation and detection method is often used to investigate arsenic species (Zhang and Combs 1996), but it does not permit the analysis of metal isotopes.

14.2.4.2 Atomic Fluorescence Spectrometry

AFS is a highly sensitive method for detecting special elements such as mercury, which is often used to detect this metal. It is usually coupled with a GC in order to separate metal specimens, but other separation techniques can also be applied (Bohari et al. 2001). For example, Limper et al. (2008) used AFS coupled to cry-otrapping gas chromatography for the detection and determination of mercury compounds. Gaseous analytes like dimethylmercury can also be measured with this method. Its main disadvantage is the need for thermal decomposition in order to analyse the elemental species.

14.2.4.3 Inductively Coupled Plasma Mass Spectrometry

Another detection method that is currently widely applied is ICP-MS. This method is extremely sensitive to most elements. It can also be used to differentiate between the various isotopes of an element. ICP-MS can be hyphenated with different separation techniques such as HPLC, GC or GE. The multielement capability of the ICP-MS is also a great advantage. It allows different elements to be scanned for in the same sample, which means that the interactions between different elements in a particular experimental setup can be researched. Another advantage is the use of isotopic standards for the isotope dilution method (Heumann et al. 1994). This capacity for determination has the advantage that effects of the matrix on the metal of choice can be neglected. However, spectrometric interferences can occur and are disadvantageous. This is also true of plasma instability in the presence of organic solvents. Therefore, it is necessary to decrease the sample input, which can be done using appropriate separation techniques such as GC or by avoiding the use of organic solvents.

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