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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 the response index is employed, any slight non linearity can be taken into account by correcting the 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

In practice, no detector has a truly linear response (despite manufacturers claims) but most detectors will have a response approaching that of linear. It is difficult to apply a standard to detector linearity, but the Response Index (1) does help comparisons to be made between one detector with that of another. Providing the response of the detector approaches linearity then its response can be described by the following simple equation,                                        y = Acr     where (r) is the response index and the other symbols have the For a truly linear detector, r=1, and the extent to which (r) deviates from unity would be a measure of its non linearity.

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

The Logarithmic Dilution Method of Linearity Measurement This method of linearity measurement was introduced by Lovelock (2). The procedure requires some special apparatus that is diagramatically represented in figure 3. Figure 3 The Logarithmic  Dilution Apparatus. A known mass of solute is introduced into a well–stirred vessel through which passes a flow of gas. The exit gas is arranged to pass directly into the detector. As a consequence, the mixture is continuously diluted and the concentration of the solute in

Summary of Detector Criteria 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.

variable wavelength) the electrical conductivity detector, the fluorescence detector and the refractive index detector. These detectors are employed in over 95% of all LC analytical applications. These four detectors will be described and for those readers requiring more information on detectors are referred to Liquid Chromatography Detectors. The subject of detector specifications will not be discussed here but will also be dealt with in detail there. Detector sensitivities and detector linearity will, however, be given for each of the four detectors. The UV Detector The UV detector is by far the most popular and useful LC detector that is available to the analyst at this time. This is particularly true if multi-wavelength technology is included in this class of detectors. Although the UV detector has some definite limitations (particularly for the detection of non polar solutes that do not possess a UV chromaphores) it has the best combination of sensitivity, linearity