Morton and Stubbs

Morton and Stubbs method (and the method using derivative curves, cf. 11.18) allows an efficient baseline correction if it is assumed that absorption varies linearly in the measurement range. Thus, in the situation represented in Fig. 11,24a, it is essential for quantification to correct absorbance A2 at the maximum X2 by subtracting the values indicated as x and y ... 

Figure 11.24—Illustration of calculations obtained using the Morton and Stubbs method.

Figure 9.27 Illustration of the concepts behind Morton and Stubbs calculation.

The method of Morton and Stubbs (as well as the method using derivative curves (cf. Section 9.15), leads to an efficient correction of the baseline on condition that the underlying absorption varies linearly in the zone of measurement. In a situation such as that represented by Figure 9.27a, in order to quantify the... 

The spectral curve of vitamin A alcohol and acetate in various solvents has a maximum at 325-328 m//, with the steeper slope at wavelengths greater than 325 m// and a pronounced shoulder at about 312 m/u. If extraneous material in vitamin A solution absorbs linearly within the wavelength region of this curve a simple geometrical correction can be applied. This correction holds if the irrelevant absorption is constant or increases and decreases uniformly. However, gross errors are introduced if the formula is used in cases of non-linear irrelevant absorption. The formula usually ascribed to Morton and Stubbs takes the form—... 

In order to take advantage of the shoulder at 312 mjLC and also to limit the mathematical uncertainty of an equation of this type Morton and Stubbs chose points on the curve such that... 

If the Morton and Stubbs correction is in excess of +3 per cent or — IS per cent or if the absorbance ratio requirements are not met proceed as below. 

The topic of irrelevant absorption (often called background) has attracted many workers. Morton and Stubbs devised a geo-... 

Morton, R. A., and A. L. Stubbs Absorption Spectra of Hydroxyaldehydes, Hydroxy-ketones and their Methyl Ethers. J. chem. Soc. 1940, 1347. 

The classic example of a pharmacopoeial assay based on the Morton-Stubbs correction is that for vitamin A alcohol and the ester. Other techniques proposed for the correction of non-specific 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. 

Morton, R. A., Stubbs A.L., (1946). Photoelectric Spectrophotometry Applied to the Analysis of Mixtures and Vitamin A Oils, Vol.Tl, pp. 348-350 Morton, R.A., Stubbs, A.L. (1947). A Re-examination of Halibut-liver Oil. Relation Between Biological Potency and Ultraviolet Absorption Due to Vitamin A, Biochem. /., Vol.41, pp. 525-529... 

In order to eliminate irrelevant absorption, Lambertsen and Braekkan (1959) used a mathematical procedure such as is customary in analysis of vitamin A (Morton-Stubbs correction). In this procedure, extinction is measured at three different wavelengths and it is assumed that irrelevant absorption is linear in this region. The authors used 280, 292 (absorption maximum), and. 301 mix as fixation points and obtained as corrected extinction-. Ecorr. = 2.778 Em (l- 352 Em -t- 1.626 Em)-... 

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