Estimation of the minimum detectable concentration and optimum instrumental conditions in precision absorption spectrophotometry of substances in the condensed phase

An expression for the signal-to-noise ratio in precision absorption spectrophotometry of molecules in the condensed phase is derived and used to calculate the theoretical minimum detectable sample concentration, C(mt), and to predict the best instrumental condition for measurement. Single-beam and double-beam instruments are considered. The theoretically calculated values of C(mt) for several organic molecules are compared with the practical minimum detectable sample concentrations, C(mp), which are those concentrations giving an absorbance of 0-002. As long as the source of radiation is intense, and the root-mean-square noise-to-signal ratio for incident radiation passing through the blank is known, then the expression for C(mt) is extremely simple and easy to evaluate for any given substance whose molar absorptivity is known. The influence of instrumental parameters on the value of C(mt) is evaluated, and only the monochr omator slitwidth, the number of combined blank and sample signal measurements, and the frequency response bandwidth of the electrometer-readout system have any significant effect on the value of C(mt) (for any given wavelength setting of the monochromator).