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Quick and Dirty Calibration Curves

Sometimes adequate quantitation is achieved using with only one or two calibration standards.

In either case, a linear relationship between analyte concentration and signal must be assumed. When using a single calibration standard, it is strongly recommended that a blank is also measured; otherwise, it must be assumed that the intercept of the calibration curve is zero. When using two calibration standards, it is customary to use analyte concentrations that bracket (one higher, one lower) that of the sample.

Example

A simple colorimeter is used to compare a standard Fe o-phenanthroline complex solution (225 ppm) with a sample solution. The standard solution gave an absorbance of 0.833, while the sample absorbance read 0.681. What is the concentration of analyte in the sample?

Answer: 183.9 ppm

The assumptions inherent in the answer calculated for the above problem should be realized: (i) it was assumed that the response was linear, and (ii) it was assumed that the sample matrix did not absorb any light (ie, a zero intercept was assumed).