تسجيل الدخول

تسجيل

Single component systems

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p404-405

2026-11-08

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Single component systems

Where only one component in the sample absorbs significantly, the wavelength is chosen to coincide with the centre of a broad maximum in the spectrum to minimise wavelength-setting errors. For example, for 1,2,4,5-tetrazine in aqueous solution, a wavelength of 535 nm would be appropriate. If the spectrum has no suitable maximum, a flat absorption minimum can be used, provided that the consequent loss of sensitivity is acceptable. Wavelengths near the extremities of the UV and visible ranges must be avoided, because of the effects of stray-light errors. Accurate measurements of a drug in solution may be difficult because of nonspecific absorption, i.e. absorption by other components in the sample. In these circumstances, the geometric correction devised by Morton and Stubbs (1946) is sometimes applied. This assumes that the nonspecific absorption varies linearly with wave length over the range measured. Taking a solution of pure drug, two equi-absorptive points are selected, one at a lower wavelength (λ1) and the other at a higher wavelength (λ3) than that of the peak maximum (λ2). Any irrelevant absorption in the sample increases the observed absorbance of one equi-absorptive point (usually λ1) more than the other (λ3). A simple geometrical calculation involving absorbances at λ1 and λ3 enables the absorbance at λ2 to be corrected for the nonspecific absorption (Donbrow 1967). The assumption of linearity of the irrelevant absorption can be tested by subtracting the theoretical curve for the calculated quantity of pure material and inspecting the residual difference spectrum.

The classic example of a pharmacopoeial assay based on the Morton–Stubbs correction is that for vitamin A alcohol and the ester (British Pharmacopoeia 2002, p. 1783). Other techniques proposed for the correction of nonspecific absorption include difference spectrophotometry, second-derivative spectrophotometry, the use of orthogonal polynomials, and chemical or physical transformation of the drug to give absorption at a longer wavelength.