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is mass sensitive, the response depends on the mass per unit time passing through it and not the mass per unit volume that passes through it. This is clear when the operation of the flame ionization detector (FID) is considered. In practice, for example, the eluent from a capillary column is mixed with a hydrogen, or hydrogen/nitrogen stream, which is then burnt at a small jet and the ions produced measured by an appropriate pair of electrodes. The response of the detector will depend on the mass of solute eluted per unit time from the capillary column and, thus, will be independent of the hydrogen or hydrogen/nitrogen flow (as this will not effect the rate of solute elution, in terms of mass per unit time). In contrast, if the FID was a concentration device (like the katherometer), as the hydrogen or hydrogen/nitrogen flow dilutes the sample, the response will be directly related to the flow rate of the diluting gas. It follows, that for a mass sensitive device, the sensing volume

(or both) the ions can be separated from one another on the basis of their individual masses. By means of a suitable scanning procedure, each individual ion mass is then sensed and its mass identified. The advantages of this type of analytical approach and its value combined with a GC instrument are very obvious.   There are three basic types of mass spectrometer, the sector mass spectrometer, the quadrapole mass spectrometer (which includes the mass analyzer) and the time-of-flight mass spectrometer. All three types of mass spectrometer have been used (and, indeed, are still used) in combined configurations with gas chromatographs. It follows, that the basic principles of all three types of mass spectroscopic systems will need to be described. It should be noted, however, that the quadrapole mass spectrometer in one of its various forms is by far the most popular mass spectrometer to be used in a combined system, but the function of the sector instrument is the simplest to

is connected to the sensor at the base and make�up gas can be introduced into the base of the detector if open tubular columns are employed as these columns are usually operated with hydrogen or helium as the carrier gas. The electron capture detector is extremely sensitive, probably one of the most sensitive GC detectors available (minimum detectable concentration ca. 10-13 g/ml) and is widely used in analysis of pesticides. Unfortunately, its sensitivity is often given in terms of the minimum mass of solute eluted, which can be misleading. The detector is concentration sensitive and thus the concentration of the solute for a given mass will vary with the position it is eluted in the chromatogram (for a given mass of solute, an early peak would be narrow and have a small volume and a high concentration at the peak maximum: however, if eluted as a late peak it would be broad, have a relatively large volume and a lower concentration at the peak maximum). Consequently, a mass of solute

nbsp; Figure 22. The Flame Ionization Detector The ionization process is not very efficient, only 0.0018% of the solute molecules produce ions, (about two ions or electrons per 105 molecules). Nevertheless, because the noise level is very small, the minimum detectable mass of n-heptane is only 2 x 10-12 g/sec. At a column flow rate of 20 ml/min. this is equivalent to a minimum detectable concentration of about 3 x 10-12 g/ml. The detector responds to mass per unit time entering the detector, not mass per unit volume consequently the response is almost independent of flow rate. This is particularly advantageous and allows it to be used very effectively with capillary columns. Although the column eluent is mixed with the hydrogen prior to entering the detector, as it is mass sensitive and not concentration sensitive, the diluting effect has no impact on the sensitivity. The FID detects virtually all carbon containing solutes, with the

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

Detectors for Use with Capillary Columns   Detectors for use with capillary columns must have high sensitivities due to the limited sample size that can be used whith such columns. In addition, due to the very small peak volumes produced by the column, the sensing volume must also be extremely small. As the flame ionization detector (FID) is mass sensitive as opposed to concentration sensitive (see book 4 of this series for the meaning of mass and concentration sensitivity) the dilution by hydrogen does not effect the detector response. Thus, the FID has both the high sensitivity and the small sensor volume that is necessary and is, consequently, ideal for use with capillary columns. The nitrogen phosphorous detector (NPD) is also appropriate for capillary columns (the function of which is very similar to that of the FID) for the