1. A method of spectroscopically determining the concentration c of a sample of a nonlinear absorbent that does not conform to Lambert-Beer's law and so exhibits a non-linear relationship between concentration and absorption, the method comprising: (a) forming a mathematical model of said nonlinear absorbent, said model including a plurality n, n.gtoreq.2, of model components each of which conforms to Lambert-Beer's law, the step of forming the mathematical model comprising: (i) taking calibration spectroscopic data of absorption a(.lambda.) at the plurality n of wavelengths .lambda. for each of a plurality of calibration samples of said non-linear absorbent, said calibration samples having known concentrations; and (ii) from the calibration spectroscopic data taken in step (a)(i), determining parameters of the plurality of model components that would allow the plurality of model components to model the non-linear absorbency behavior of said absorbent by satisfying the following equation: EQU a(.lambda.)=(.epsilon..sub.1 (.lambda.).multidot.c.sub.1 +.epsilon..sub.2 (.lambda.).multidot.c.sub.2 + .multidot. .multidot. .multidot. .epsilon..sub.n (.lambda.).multidot.c.sub.n).multidot.d where each model component .epsilon..sub.i (.lambda.).multidot.c.sub.i, 1.ltoreq.i.ltoreq.n, has a spectral absorption value given by .epsilon..sub.i (.lambda.) and a concentration c.sub.i ; and (b) applying the mathematical model to the sample of the absorbent, the step of applying the mathematical model comprising: (i) taking measurement spectroscopic data for a sample of said absorbent of unknown concentration at the plurality n of wavelengths; (ii) applying the equation set forth in (a)(ii) to said measurement spectroscopic data to determine the concentration values c.sub.i of the plurality of model components corresponding to the measurement spectroscopic data; and (iii) from the concentration values c.sub.i of the plurality of model components, calculating C.