Free Books and Brochures

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 detector output is linearly

detector, the detector sensitivity (XD) is given by The two important ranges that are specified for a detector are the dynamic range and the linear dynamic range. The dynamic range (DR) extends from the minimum detectable concentration (i.e. the sensitivity) to that concentration at which the detector no longer responds to any increase. The dynamic range is not usually pertinent to general analytical work but is important in preparative chromatography. The linear dynamic range or detector linearity is as important as sensitivity for any detector that is to be used for quantitative analysis. It is defined as the concentration range over which the detector response is linearly related to the concentration of solute passing through it

1. Dynamic Range � (RD) � The dynamic range of a detector is that concentration range over which it will give a concentration dependent output. The units are dimensionless.   2. The Response Index � (r) � The response index of detector is a measure of detector linearity and would be unity for a truly linear detector. In practice the value of (r) should lie between 0.98 and 1.02. If (r) is known, quantitative results can be corrected for any non linearity. 3. Linear Dynamic Range � (DL) � The linear dynamic range of a detector is that concentration range over which the detector response is linear within defined response index limits. It is also dimensionless and is important when the components of a mixture cover a wide concentration range. 4. Detector Response � (Rc) � The detector response can be defined as the detector output per unit change in concentration (e.g.� volts/g/ml) or, as the detector output per unit change of physicalpropertybeingmeasured (e.g.

noise = 2) when eluted as an early peak may not be detected or discerned when eluted as a late peak   The linear dynamic range of the electron capture detector is again ill-defined by many manufacturers. In the DC mode the linear dynamic range is usually relatively small, perhaps two orders of magnitude, with the response index lying between 0.97 and 1.03. The pulsed mode has a much wider linear dynamic range and values up to 5 orders of magnitude have been reported. The linear dynamic range will depend on the strength of the radioactive source and the detector geometry. If a response index lying between 0.98 and 1.02 is assumed, then a linear dynamic range of at least three orders of magnitude should be obtainable from most electron capture detectors. An example of a pesticide analysis employing an electron capture detector to monitor the separation is shown in figure 42

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

Alternative Method for Specifying Detector Linearity The E19 committee suggested an alternative procedure for defining linearity (3). They defined the linear dynamic range as follows,   �"the linear dynamic range of a detector is that range of concentration of a test substance over which the response of the detector is linear to within 5%, determined form a linearity curve".   The range should be expressed as a ratio of the highest concentration to the minimum detectable concentration. Although defining linearity by this method ensures an minimum linear performance and, consequently, a reasonable quantitative accuracy, the definition is not sufficiently explicit. Conversely, if