Free Books and Brochures

all the ions in the source are accelerated virtually simultaneously. The ions then pass through the third electrode into the drift zone and are eventually collected by the sensor electrode. The time of flight mass spectrometer is not employed extensively in gas chromatography/mass spectroscopy combination systems as it is more commonly used to examine high molecular weight materials Many analysts that use GC/Mass Spectrometer combined systems are neither specialists in gas chromatography or mass spectrometry and may need the support of experienced gas chromatographers or mass spectroscopists for particularly challenging samples. For those who wish to study mass spectrometry further, an excellent discussion on general organic mass spectrometry is given in Practical Organic Mass Spectrometry edited by Chapman (9).   Gas Chromatography IR Spectroscopy (GC/IR) Systems   IR spectra were initially obtained off-line, by condensing the eluted solute in a cooled trap, making into

column dispersion in any chromatographic system.   Dispersion in the Detector Sensor Volume Irrespective of the type of detector or whether it is for LC or GC, if it is concentration sensitive, the device that actually senses the solute contained in the mobile phase must address a finite volume of the column eluent in order to measure the concentration of the solute and sense its presence. The caveat, that the sensor is concentration sensitive is important as, if the detector 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

up to 450oC. The detector can be used for the analysis of aqueous samples as steam has no effect on the source in the sensor. The sensor is thermostatted in a separate oven which can be operated at temperatures ranging from 100oC to 450oC. The column 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

. Only highly specific LC detectors have sensitivities that can approach those of GC detectors. See also the section on detectors in the HPLC supplement. Detector Specifications Detector specifications are like those for GC detectors and are listed as follows, 1. Dynamic Range 2. Response Index or Linearity 3. Linear Dynamic range 4. Detector Response 5. Detector Noise Level 6. Detector Sensitivity or Minimum Detectable Concentration 7. Total System Dispersion 8. Sensor Dimensions 9. Detector Time Constant 10. Pressure Sensitivity 11. Flow Sensitivity 12. Operating Temperature Range In general the specifications are the same for both GC and LC detectors with the exception of detector dispersion. Although, detector dispersion has a minimal

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

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 same reason. The micro-argon detector employs a scavenger flow which also, in effect, reduces the sensor volume, so this detector can also be used with capillary columns. The FID and the NPD detector will be described here. For a more detailed treatment of GC detectors see book 4 of this series.   The Flame Ionization Detector   The FID, invented by Harley and