The Linear Dynamic Range of a Detector

The linear dynamic range of a detector is not the same as its dynamic range, as already stated, because the linearity of most detectors deteriorates at high concentrations of solute and in some instances, also at very low concentrations of solute. The linear dynamic range of a detector is also quoted in orders of magnitude of concentration and is given the symbol Dl e.g. 

The first reported concentration is usually that which will provide a signal equivalent to twice the noise level and the second reported concentration is the limit at which the response factor was determined. At present, manufacturers do not usually differentiate between D and and do not quote a range for the response index r, however, it is hoped that in the future such data will be made available. Some manufacturers do mark the least sensitive setting of the detector as non-linear (N/L), which is a step towards a more rational approach to specifying linetu dynamic range. 

The response for a given detector will be different for different solutes in the case of the UV detector, the response will be a function of the extinction coefficient of the solute and for a refractive index detector, the refractive index of the solute. For this reason the response of two detectors of the same type and geometry can only be compared if the same solute and mobile phase are employed. When comparing the response of detectors of the same type but different geometry, then other factors have also to 

The linearity of most detectors deteriorates at high concentrations and thus the linear dynamic range of a detector will always be less than the dynamic range. The symbol for the linear dynamic range is usually taken as (D r) and might be specified in the following form for the FID as an example 

Detector response can be defined in two ways. It can be taken as the voltage output for unit change in solute concentration in which case in a similar way to detector sensitivity and dynamic range, the solute used for measurement must to be specified. Alternatively, it can be taken as the voltage output that would result from unit change in the physical property that the detector measures, e.g. refractive index or carbon content. In the latter case the dimensions of the response will vary with the nature of the property being measured. 

The detector response (RJ can be determined by injecting a known mass of the chosen solute (m) onto the column and measuring the response from the dimensions of the peak. Again assuming the 

---

Linear Dynamic Range - (D ) - 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 taken as the ratio of the concentration at which the response index falls outside its defined limits, to the minimum detectable concentration or sensitivity. The linear dynamic range is important when the components of a mixture being analyzed cover a wide concentration range. 

The linearity of most detectors deteriorates at high concentrations and, thus, the linear dynamic range of a detector will always be less than its dynamic range. 

The linear dynamic range (DJ of a detector is that range of solute concentration over which the numerical value of the response index falls within defined limits. For example, the linear dynamic range of a detector such as the FID might be specified as... 

Section 12.2.6.1), In reality, deviations from linearity usually occur at high concentrations (saturation effect). An easy way to determine the linear dynamic range of a detector is to plot the signal-to-concentration (or sample size) ratio versus the logarithm of concentration (sample size) (see Fig. 8),... 

The linear dynamic range of a detector is that range of solute concentration over which the response of the detector is linear. 

The linear dynamic range of a measuring system can be obtained from the linearized form of the detector response, i.e., from the logarithmic form of eqn [1] (Figure 9) or from eqn [6], as demonstrated in Figure 11. The latter procedure does not permit determination of the linearity coefficient and can only indicate its deviations from unity. 

Another important characteristic of a detector is the linear dynamic range a range of analyte concentrations that produce a calibration line best described with a linear equation. Non-linear calibrations with subtle curvatures may be also used in the linear dynamic range of the detector. 

Accuracy is a measure of the closeness of a measurement to the true value. Precision is a measure of how reproducible the measurements are. For many detectors, the accuracy of a measurement is maintained by user calibration. For some detectors, however, such as photodiode array detectors, accuracy relies on internal calibration. The linear dynamic range of the detector is the maximum linear response, divided by the detector noise. The detector response is said to be linear if the difference in response for two concentrations of a given compound is proportional to the difference in concentration between the two samples. Most detectors become nonlinear as the sample concentration increases. 

The linear dynamic range of the FID covers at east four to five orders of magnitude for 0.98commercially available detectors shows considerable... 

The inner cylindrical electrodes are 1.26 cm in diameter and 0.625 cm long with a separation distance to the inner cylinder of about 0.009 cm. The linear dynamic range of the detector was reported to be 3.5 x 10" but this range probably included concentrations that are too high for practical column operation. 

Bulk property detectors generally have neither the sensitivity nor the linear dynamic range of solute property detectors and, as a consequence, are less frequently used in modem LC analyses. Furthermore, none can be used with gradient elution, flow programming or temperature programming and so they place considerable restrictions on the choice of chromatographic system. They do, however, have certain unique areas of application, some of which have already been mentioned, but their use probably represents less than 5% of all LC analyses. 

One of the most important decisions that is left to the analyst when operating a liquid chromatograph is the choice of detector sensitivity. In some instruments the output from the sensor is monitored continuously over its entire dynamic range and so sensitivity is not an optional experimental parameter. Nevertheless, in this case, the sample size determines the concentration range over which the eluted solutes are monitored and thus an optimum sample size must be chosen. The detector should never be operated at its maximum sensitivity unless such conditions are enjoined by limited sample size or column geometry. Provided that there is adequate sample available, and the sample concentration when eluted is within the linear dynamic range of the detector, the maximum sample size that the column can tolerate should be used. This ensures that the detector noise is always minimal... 

Following the establishment of specificity, the method(s) should be validated to allow for use in release and stability testing. Such validation is typically less stringent than for final methods (sec Chapter 12), but should demonstrate specificity, linearity, range, accuracy, and analysis repeatability for the API. For related substances, specificity should be demonstrated and the limit of detection (LOD) and limit of quantitation (LOQ) should be established for the API to serve as surrogate values for the LOD and LOQ of impurities for which authentic substances are not available. To achieve a sufficient LOD and simultaneously keep the API in the linear dynamic range of the detector, it may be necessary to use different sample concentrations for the analyses of the API and related substances. It is additionally beneficial to repeat the separation on new columns from different batches to ascertain that the separation obtained can be maintained column to column. 

It is observed that the relationship between signal and concentration is linear over a finite range of concentrations. At higher concentrations, various phenomena (scattering, nonlinear optical processes, inner filter effect, etc.) lead to deviations from linearity. The range of concentrations where a linear relationship between signal and concentration holds is known as the linear dynamic range of the detector. 

The dynamic range of a detector (D ), is that range over which the detector continues to respond to changes in solute concentration and is not the same as its linear dynamic range. The dynamic range may extend from 1 x lO to 1 x lO g/ml. The use of a detector outside its linear... 

---