(UV) Tips and Tricks - How is the minimum limit of quantitation determined?

It is important to know the minimum limit of quantitation when measuring the concentration of a solution. The minimum limit of quantitation is the lowest limit of concentration that can be measured. It is determined from the calibration curve equation as the concentration corresponding to the absorbance equivalent to ten times the discrepancy. The discrepancy is determined as the standard deviation of repeated measured values on a blank solution (or a low-concentration sample solution).  Note that the "limit of quantification" is different from the "limit of detection".  The limit of detection is defined as three times the standard deviation of the blank.

An example of determining the minimum limit of quantitation in a caffeine solution is described below.

Fig. 1 shows the absorption spectra of aqueous solutions of caffeine corresponding to 2 mg/L, 4 mg/L, 10 mg/L, 20 mg/L, and 40 mg/L caffeine concentrations.

Fig. 1 Absorption Spectra of Aqueous Solutions of Caffeine (Black: 2 mg/L, Green: 4 mg/L, Purple: 10 mg/L, Red: 20 mg/L, Blue: 40 mg/L)

Fig. 2 shows the calibration curve created from the absorbance at a peak wavelength of 273 nm. The calibration curve equation was determined as:
Absorbance (Abs) = 0.05191 × concentration (mg/L) …. (1)

Fig. 2 Calibration Curve for Aqueous Solutions of Caffeine

Table 1 shows ten repeated measured values for a blank solution (pure water) at 273 nm and the standard deviation σ. 10 σ was determined to be 0.002624 Abs. The concentration corresponding to the 10 σ absorbance, that is, the minimum limit of quantitation, can be determined from Equation (1), as follows:
Minimum limit of quantitation = 0.002624/0.05191 = 0.051 mg/L

Table 1. Ten Repeated Measurements on a Blank Solution and Standard Deviation σ.

The minimum limit of quantitation for caffeine was determined in this example. However, note that the minimum limit of quantitation differs according to the substance, as the amount of absorbed light (absorption coefficient) differs from substance to substance.

References
1) Chapter 7, Genba de Yakudatsu Kagakubunseki no Kiso (Fundamentals of Chemical Analysis to Assist Onsite) (2006),
edited by The Japan Society for Analytical Chemistry, Supervising Editor Shoji Hirai (published by Ohmsha, Ltd.)