Gas Chromatography Detectors - Classification of Detectors > Page 1

Introduction

A chromatography detector is a device that locates in the dimensions of space and time, the positions of the components of a mixture that has been subjected to a chromatographic process and thus permits the senses to appreciate the nature of the separation.

 

The definition, by necessity, must be broad, as it needs to encompass all types of detecting systems ranging from elaborate electronic devices to the human eye or even the sense of smell. Tswett in his pioneering chromatographic separation of some plant pigments used the human eye to determine the nature of the separation and, even today, as one of the more common separation techniques is thin layer chromatography, the human eye is still one of the more frequently used detectors. Similarly, essential oil chemists smell the eluent from a gas chromatography (GC) column in organoleptic assessment.

The detector, as well as being an essential supporting device for the gas chromatograph has also played a critical role in the development of the technique as a whole. There has been a synergistic interaction between column development and detector development. The need to develop higher column efficiencies has demanded higher detector sensitivities which has provoked the development of more sensitive detectors. In turn, the more sensitive detectors has encouraged the improvement of column performance. In fact, the rapid development of GC in the 1950s was possible because or the swift introduction of high sensitivity linear detectors.

Classification of Detectors

Detectors can be classified into two types, bulk property detectors and solute property detectors. The bulk property detector measures some bulk physical property of the eluent (such as dielectric constant or refractive index) and the solvent property detector, measures some physical or chemical property that is unique to the solute (such as heat of combustion or fluorescence). Detectors can also be classified as concentration sensitive devices such as the katharometer or mass sensitive devices such as the flame ionization detector (FID). Another method of classification is to define detectors as specific or non-specific. An example of a specific detector would be the nitrogen phosphorous detector (NPD), which as its name implies detects only those substances that contain nitrogen or phosphorous. A non-specific detector would be the katharometer detector which senses all vapors that have specific heats or thermal conductivities different from those of the carrier gas. In general (though not always), non specific detectors have lower sensitivities than the specific detectors, the reasons for which will be discussed in due course. In this treatment of GC detectors the classification of bulk property detector and solute property detectors will be used.