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Quantitative Chromatographic Analysis
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| where ( | is the concentration of solute in the mobile phase at thepeak height in (g/ml), |
| (m) | is the mass of solute injected, |
| (w) | is the peak width at 0.6067 of the peak height, |
| (s) | is the chart speed of the recorder or printer |
| (Q) | is the flow rate in ml/min. |
The log of the peak height (y) where y is the peak height in millivolts (or whatever units in which the output is defined) is then plotted against the log. of the solute concentration at the peak maximum (
). From equation (1),
log(y) = Log A + (r) log () (5)
The slope of the Log/Log curve will give the value of the response index (r). For a truly linear detector, r = 1. Alternatively, with suitable software, the experimental data can be curved fitted directly to a power function and the value of (r) extracted from the results. The Incremental Method for determining the response index is probably the simplest and has the advantage of including any characteristics of the chromatograph that may affect the results.
The Linear Dynamic Range of a Detector
The linear dynamic range of a detector will always be less than the dynamic range. The symbol for the linear dynamic range is (DLR) and is specified as follows
DLR = 2 x 10-3 for 0.98 < r < 1.02
The lowest concentration of the linear dynamic range is the minimum detectable concentration or the detector sensitivity. The largest concentration is that where the response factor (r) falls outside the range specified.
Detector Response
Detector response is the voltage output for unit change in solute concentration or the voltage output that would result from unit change in the physical property that the detector measures (e.g. refractive index for a refractive index detector or carbon content for an FID detector). The detector response, (Rc), can be measured by injecting a known mass of the chosen solute (m) onto the column and measuring the response from the dimensions of the peak. Assuming the concentration of the solute at the peak maximum is twice that of the average, then
(6)
where, (h) is the peak height and the other symbols have the meaning previously attributed to them.
The response of a detector will differ between different solutes and between different detectors. Ipso facto the response of two detectors of the same type and geometry must be compared using the same solute and mobile phase.
