flow the ions are transported out of the flame and pass as second step a collector
electrode. The interaction of the moving ions (electric charge) with the voltage field
of the electrode induces an electronic signal. This signal is highly sensitive allowing
the detection of very low amounts of organic substances (down to pg levels) but
excludes inorganic species like the carrier gas or water. Further on, the detection is
not specific or selective. More or less all organic compounds can be detected, but
noteworthy with different sensitivity. The latter point has a high relevance for
quantitation (see Sect. 5.2). As second important aspect for quantitative measurements, the linear correlation of amount of compound and electronic signal, the
so-called linear range, should cover a wide extent. The good sensitivity, its wide
linear range and the broad spectrum of detectable substances are the main reasons for
the dominant application of the FID.
For some more specific applications, more selective detectors are used. The
principal approach is always the same, organic substances are converted to an
electronic signal by ionization. However, the type of ionization changes and,
simultaneously, also the selectivity due to different ionization potentials for individual substance groups at softer ionization processes as compared to flame ionization.
As an example, ionization by β
À -radiation is performed by the electron capture
detector (ECD) via the
63 Ni-isotope (see Fig. 4.8). This soft ionization allows the
selective detection of halogenated compounds as well as sulphur-containing compounds but exhibits a poor sensitive for nonhalogenated organic substances. Therefore, this detector is applied especially in environmental analysis measuring
e.g. dioxine or PCB pollution. Further specific detectors are e.g. the NPD (nitrogen
phosphor detector) used in particular for pesticide analyses or the photo ionization
detector (PID) applied for PAH detection.
A systematic comparison of some commercially available detectors regarding the
sensitivity, linearity and selectivity is given in Fig. 4.9.
As already mentioned, gas chromatography is restricted to volatile compounds.
Since volatility of organic substances depends mainly on the weight and the polarity
of a compound, high molecular weight and polar substances are excluded. However,
the discrimination between detectable and non-detectable it is not sharp, in particular
for moderate polar substances such as carboxylic acids or mono alcohols. Many of
these compounds are slightly volatile but have a poor separation performance
resulting in bad peak forms such as tailing or fronting. In order to get these
compounds better detectable, derivatization is used. Derivatization covers chemical
reactions leading to less polar compounds by conversion of polar functional groups
into less polar ones. This approach is dominantly applied for carboxylic acids and
alcohols that can be transferred to less polar esters or silyl ethers, respectively.
Derivatisation agents are added to the analytes prior to analysis and comprise substances such as diazomethane, methanol/borone trifluoride solution, TMSH for ester
formation or MSTFA, BSTFA for silyl ether generation. Examples are illustrated in
Fig. 4.10.
4.1 High Performance Chromatography: GC, HPLC
47
electrode. The interaction of the moving ions (electric charge) with the voltage field
of the electrode induces an electronic signal. This signal is highly sensitive allowing
the detection of very low amounts of organic substances (down to pg levels) but
excludes inorganic species like the carrier gas or water. Further on, the detection is
not specific or selective. More or less all organic compounds can be detected, but
noteworthy with different sensitivity. The latter point has a high relevance for
quantitation (see Sect. 5.2). As second important aspect for quantitative measurements, the linear correlation of amount of compound and electronic signal, the
so-called linear range, should cover a wide extent. The good sensitivity, its wide
linear range and the broad spectrum of detectable substances are the main reasons for
the dominant application of the FID.
For some more specific applications, more selective detectors are used. The
principal approach is always the same, organic substances are converted to an
electronic signal by ionization. However, the type of ionization changes and,
simultaneously, also the selectivity due to different ionization potentials for individual substance groups at softer ionization processes as compared to flame ionization.
As an example, ionization by β
À -radiation is performed by the electron capture
detector (ECD) via the
63 Ni-isotope (see Fig. 4.8). This soft ionization allows the
selective detection of halogenated compounds as well as sulphur-containing compounds but exhibits a poor sensitive for nonhalogenated organic substances. Therefore, this detector is applied especially in environmental analysis measuring
e.g. dioxine or PCB pollution. Further specific detectors are e.g. the NPD (nitrogen
phosphor detector) used in particular for pesticide analyses or the photo ionization
detector (PID) applied for PAH detection.
A systematic comparison of some commercially available detectors regarding the
sensitivity, linearity and selectivity is given in Fig. 4.9.
As already mentioned, gas chromatography is restricted to volatile compounds.
Since volatility of organic substances depends mainly on the weight and the polarity
of a compound, high molecular weight and polar substances are excluded. However,
the discrimination between detectable and non-detectable it is not sharp, in particular
for moderate polar substances such as carboxylic acids or mono alcohols. Many of
these compounds are slightly volatile but have a poor separation performance
resulting in bad peak forms such as tailing or fronting. In order to get these
compounds better detectable, derivatization is used. Derivatization covers chemical
reactions leading to less polar compounds by conversion of polar functional groups
into less polar ones. This approach is dominantly applied for carboxylic acids and
alcohols that can be transferred to less polar esters or silyl ethers, respectively.
Derivatisation agents are added to the analytes prior to analysis and comprise substances such as diazomethane, methanol/borone trifluoride solution, TMSH for ester
formation or MSTFA, BSTFA for silyl ether generation. Examples are illustrated in
Fig. 4.10.
4.1 High Performance Chromatography: GC, HPLC
47