Sensitivity or the Minimum Detectable Concentration

To unambiguously identify the presence of a peak and, in addition, be able to give some proximate estimation of its size for quantitative purposes, the peak height needs to be at least 5 times the noise level. The detector sensitivity, or the minimum detectable concentration, (Xd), is defined as that concentration of solute that will give a signal equivalent to twice the noise level and, consequently, the concentration of solute at the limiting (k ) value must be 2.5Xd. 

Detector Sensitivity or the Minimum Detectable Concentration has been defined as the minimum concentration of an eluted solute that can be differentiated unambiguously from the noise. The ratio of the signal to the noise for a peak that is considered decisively identifiable has been arbitrarily chosen to be two. This ratio originated from electronic theory and has been transposed to LC. Nevertheless, the ratio is realistic and any peak having a signal to noise ratio of less than two is seriously obscured by the noise. Thus, the minimum detectable concentration is that concentration that provides a signal equivalent to twice the noise level. Unfortunately, the concentration that will provide a signal equivalent to twice the noise level will usually depend on the physical properties of the solute used for measurement. Consequently, the detector sensitivity, or minimum detectable concentration, must be quoted in conjunction with the solute that is used for measurement. 

It is seen that the maximum value of the capacity factor Is inversely proportional to the detector sensitivity or, the minimum detectable concentration. It follows, that the detector sensitivity also sets an ultimate limit on the peak capacity that can be realized from a column. This limit however is fairly high as can be seen from the data given in Table (1) The capacity ratios and peak capacities were calculated for a column having an efficiency of 10,000 theoretical plates, a dead volume of 6.7 ml and a sample concentration of % v/v. 

Detector sensitivity or the minimum detectable concentration is the minimum concentration of an eluted solute that can be discerned... 

Where Ac is the detector concentration sensitivity or the "Minimum Detectable Concentration" which is the parameter of importance to chromatographers. 

Where Q, is the minimum detectable amount, R the detector noise level and s the detector sensitivity [135,146,151,152]. For a concentration sensitive detector the minimum detectable concentration is the product of Q, and the volumetric gas flow rate through the detector. The minimum detectable amount or concentration is proportional to the retention time, and therefore, directly proportional to the column radius for large values of n. it follows, then, that very small quantities can be detected on narrow-bore columns. 

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. 

Detector noise is the term given to any perturbation on the detector output that is not related to the presence of an eluted solute. As the minimum detectable concentration, or detector sensitivity, is defined as that concentration of solute that provides a signal equivalent to twice the noise, the detector noise determines the ultimate performance of the detector. Detector noise has been arbitrarily divided into three types, short-term noise, long-term noise, and drift, all three of which are depicted in Fig. 1. 

The first of these is the sensitivity. This expresses the minimum detectable concentration (often as a function of range) and is usually quoted in the absence of interference. It is defined mathematically by equating the product aC with the noise level of the system. It is usually stated in units of path-integrated concentration, most correctly in units of pgm , or alternatively in ppm m (where ppm is used to express the partial pressure of the target species as millionths of the total atmospheric pressure). 

Hence, all compounds having a conductivity less than the carrier gas will be detected by the TCD, which is a universal concentration-sensitive detector. The TCD is nondestructive, and may be used for preparative separations. The detector has however low sensitivity, and the minimum detectable (M D) mass is about 10 ng even using He or H2 as carrier gas. Other detector characteristics can be found in Table 2.4. The TCD is commonly used for determination of light and permanent gases in packed or PLOT columns. The TCD is well suited for portable gas chromatographs because it is easily miniaturized and does not require extra gases. 

A form of the viscosity detector has been used for exclusion chromatography, and prior experiments employing a single capillary have demonstrated the feasibility and usefulness of a continuous on-line viscosity detector in conjunction with a concentration detector (RI or UV). The sensitivity of the device, however, is rather poor, the minimum detectable concentration being about lO" g/ml. To date, the instrument has only been used in a recent application in polymer chromatograjdiy. 

The flow sensitivity of a detector will also be one of the factors that determines the long term noise and thus will influence the sensitivity or minimum detectable concentration of the detector.lt is usually measured as the change in detector output for unit change in flow rate through the sensor cell. Again, the refractive index detector is the most sensitive to flow rate changes. 

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