Liquid Chromatography Detectors - Detector Specifications > Page 2

Unfortunately, even today, there is no LC detector that has an equivalent performance to the flame ionization detector (FID) used in GC. In general, LC detectors have sensitivities of two to three orders of magnitude less than their GC counterparts and linear dynamic ranges one to two orders of magnitude lower. 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 effect on the resolution in GC separations, detector dispersion can actually destroy a separation achieved in an LC column if the system is not designed correctly.

Dealing with the other specifications, the dynamic range and linear dynamic range are the same as those defined in book 4. The response index, the measure of detector linearity, can also be determined in exactly the same way, either by the incremental method of calibration, or the logarithmic dilution method. In the logarithmic method of calibration, mobile phase, now a liquid, is passed continuously through an enclosed stirred vessel containing a known mass of solute, the eluent passing directly into the detector. The logarithm of the detector output is plotted against the logarithm of the calculated solute concentration and the magnitude of the response index determined from the slope of the curve in the manner described in book 4. The response, noise and sensitivity are measured in exactly the same way as for GC detectors. Pressure sensitivity and pressure tolerance have a more important significance in LC as in multidimensional LC, the detector may be situated between two or more columns and thus must tolerate pressures up to the input pressure (e.g., several thousand p.s.i). Pressure sensitivity and flow sensitivity are also more important in LC due to the relatively high pressures involved and the sensitivity of many sensors to pressure changes (e.g., the refractive index detector and the UV detector). However, LC columns have a high impedance to flow and so pressure pulses are often smoothed out in the column and do not reach the detector. Dispersion that takes place in a column is very important and will be dealt with in some detail.