nbsp; The Absolute Mass Detector The absolute mass detector adsorbs the material as it is eluted from the column onto a suitable adsorbent and continually weighs the mass adsorbed.       Figure 61 The Absolute Mass Detector

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

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

peak is proportional to the total mass eluted whereas with the integral response the step height of the integral curve is proportional to the total mass eluted. The differential curve is often used to identify the retention time which is the point where the signal crosses from positive through zero to negative. The Dynamic Range of the Detector A detector has two response ranges, the dynamic range and the linear dynamic range and the two range are not synonymous. The dynamic range of a detector is that concentration range over which a concentration dependent output is produced. The minimum of the range will be the concentration at which the output is equivalent to twice the noise level and the maximum that concentration where the detector no longer responds to a concentration increase. The dynamic range is usually given as a concentration ratio and is thus, dimensionless. Detector Linearity The linear dynamic range of a detector is that concentration range over which the

. 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

peak height (or the peak area) using the numerical value of the response index. Thus. in effect, the useful linear dynamic range of a detector for quantitative purposes can be significantly extended by employing correction procedures when using the response index. It should be pointed out that the logarithmic dilution method should not be used if the linearity is to be measured by the method recommended by the E19 committee of the ASTM. Detector Response There are two ways of defining detector response, either as detector output (usually in mv) per unit change in solute concentration or as the detector output per unit change in the units of detector measurement (e.g. the sensitivity of a conductivity detector would be defined in terms of detector output per unit change in electrical conductivity). The detector response (RD) is determined by injecting a known mass (m ) onto the column and measuring the peak height (h)  in (mv), then