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Quantitative Chromatographic Analysis
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| Solute | r = 0.94 | r = 0.97 | r = 1.00 | r = 1.03 | r = 1.05 |
| 1 | 11.25% | 10.60% | 10.00% | 9.42% | 9.05% |
| 2 | 88.75% | 89.40% | 90.00% | 90.58% | 90.95% |
The advantages to defining linearity in this way is that a claim of linearity can be experimentally substantiated and if the detector is not linear, then a correction can be applied if (r) is known. The response index of all detectors (GC and LC) should be experimentally determined, even if a value for the response index is provided by the detector manufacturer.
The Determination of the Response Index of a Detector
There are two methods used to measure the response index of a detector, the incremental method of measurement and the logarithmic dilution method of measurement. The former will be described here
The Incremental Method of Linearity Measurement
The measurement requires the detector itself and a chromatograph with its associated electronics and recorder or computer system. The solute is chosen as typical of the type of substances to be analyzed and a phase system that will elute the solute in a reasonable time. Initial sample concentrations are chosen that will be appropriate for the sensitivity of the detector under examination. Duplicate samples are used and the eluted peaks either recorded or the data stored by the computer. The sample solution is then diluted by a factor of three and the procedure repeated. This procedure is continued until the height of the eluted peak is the same order of magnitude as the noise level.
If the detector has no data acquisition and processing facilities, and peak areas or peak heights cannot processed by a computer, the peaks from the chart recorder must be used to provide the calibration data. The width of the peak at 0.607 of the peak height is measured and from the chart speed and the mobile phase flow rate the peak volume can be calculated. The concentration of solute at the peak maximum will be twice the average peak concentration and can be calculated from the following equation
