# Gas Chromatography Detectors - Detector Linearity > Determination of Response Index > The Incremental Method of Linearity Measurement > Page 7

### The Incremental Method of Linearity Measurement

This procedure provides a curve relating detector output to solute concentration over the concentration range of interest employing the associated chromatograph. Replicate sample are placed on the column and the eluted peaks monitored. The sample solution is made up in an appropriate volatile solvent at the maximum concentration of interest and duplicate samples placed sequentially on the column and the eluted peaks monitored. The sample is then diluted by three and the analyses repeated. The sample is again diluted and the process repeated until the peak heights are about five times the noise level (see later for a definition of noise).

The concentration at the peak maximum for each ample injected is calculated as follows,

_{}

where, (c) is the concentration of solute in the mobile phase at the

(m) is the mass of solute injected,

(w) is the peak width at 0.6065 of the peak height.

(s) is the chart speed in cm/min.

and (Q) is the mobile phase flow rate in ml/min.

The logarithm of the peak height (y) is
plotted against the logarithm of the solute concentration at the peak maximum
(c), (*cf. *equation 1)

The slope of the curve will give the response index (r) which will be p/4 or unity for a perfectly linear detector.