Question

A plot of detector signal versus ppm Ca²⁺ yielded a best fit straight line with a slope of 1.38 volts/ppm. Multiple blank measurements resulted in a standard deviation of 0.18 volts. Using a 2:1 signal-to-noise ratio criteria, calculate the detection limit for Ca²⁺ in ppm when this method is employed.

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Answer 1

LOD (limit of detection)
The detection limit of an individual analytical procedure is the lowest amount of analyte in a sample which can be detected but not necessarily quantitated as an exact value. Several approaches for determining the detection limit are possible.

Based on visual evaluation: The detection limit is determined by the analysis of samples with known concentrations of analyte and by establishing the minimum level at which the analyte can be reliably detected.
Based on Signal-to-Noise Approach: Determination of the signal-to-noise ratio is performed by comparing measured signals from samples with known low concentrations of analyte with those of blank samples and by establishing the minimum concentration at which the analyte can be reliably detected. A signal-to-noise ratio between 3 or 2:1 is generally considered acceptable for estimating the detection limit.
Based on the Standard Deviation of the Response and the Slope:

The detection limit (DL) may be expressed as:=3X STANDRD DEVIATION OF LOW CONCEN/ SLOPE OF THE CALIBRATION LINE

The detection limit = 3*0.18volts/1.38volts/ppm = 0.391ppm