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# Relative Response Factor | Pharma Qualification

Detector relative response factor (f)

The relative response factor of the detector expresses the sensitivity of a detector relative to a standard substance. It can be expressed in terms of an equality of masses, volumes or number of moles.

fi = (Ai / Ast) fst

where A is the area of ​​the peak of interest (subscript i) and pattern (subscript st), respectively, and fst is the response factor of the pattern compound. An arbitrary value is usually assigned to fst (for example, 1 or 100). When the relative molar response is expressed and n-alkanes are used as standards, the value of fst is the number of carbon atoms of the n-alkanes multiplied by 100 (eg 600 for n-hexane)

Detector sensitivity (S)

It is the output signal per unit of concentration or unit of mass of a substance in the mobile phase that enters the detector   calculate the sensitivity of a detector, the output signal is given as the area of ​​the peak in mV.min, As or AU.min (AU = absorbance unit). These values ​​are derived from the integrated value of the peak area converted into the specified units.

Alternatively, the peak area can be obtained by multiplying the peak height at maximum (in mV, A, or AU) by its width at half the height (in units of time). The area of ​​the peak calculated in this way will be 6% less than the true area, assuming that the peak is Gaussian.at enters the detector.

In the case of concentration-sensitive detectors, the sensitivity is calculated per unit of concentration in the mobile phase:

S = Ai Fc / Wi = E / Ci

where Ai is the area of ​​the integrated peak (in mV.min or AU.min), E is the height of the peak (in mV or AU), Ci is the concentration of a compound in the mobile phase when it reaches the detector (in g .cm-3), Fc is the mobile phase flow in the column, corrected to the column temperature (in cm3.min-1), and Wi is the mass (quantity) of the compound (in mg). The dimensions of the detector sensitivity are mV.cm3.mg-1 or AU.cm3.mg-1. In the case of thermal conductivity detectors these values ​​are sometimes called Dimbat-Porter-Stross sensitivity.

In the case of detectors sensitive to mass flow, the sensitivity is calculated per unit mass of the substance in the mobile phase that enters the detector:

S = Ai / Wi = Ei / Mi

where Ai is the area of ​​the integrated peak (As), Ei is the height of the peak (in A), Mi is the mass flow of the substance entering the detector per unit time (in gs-1), and Wi is the mass (quantity) of the substance (in g). The dimensions of the detector sensitivity are Asg-1 or Cg-1.

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