Reflectance colour measurement spectrophotometry

In this work hybrid method is suggested to determine anionic surfactants in waters. It is based on preconcentration of anionic surfactants as their ion associates with cationic dyes on the membrane filter and measurement of colour intensity by solid-phase spectrophotometry method. Effect of different basic dyes, nature and hydrophobicity of anionic surfactants, size of membrane filter pores, filtration rate on sensitivity of their determination was studied. Various cationic dyes, such as Methylene Blue, Crystal Violet, Malachite Green, Rhodamine 6G, Safranin T, Acridine Yellow were used as counter ions. The difference in reflection between the blank and the sample was significant when Crystal Violet or Rhodamine 6G or Acridine Yellow were used. 

Approximately 10 pi of sample is required and after development, the density of the colour is measured via the transparent bottom layer by reflectance spectrophotometry. There is a relatively complex relationship between the signal and concentration, but with suitable transformations a perfectly acceptable calibration curve is obtained. 

There are a bewildering variety of methods and instruments available to the food technologist in the field of colorrr measurement. When one is approaching the subject for the first time or when attempting to devise a method for a material outside the normal experience, the wealth of possibilities available sometimes makes the choice difficrrlt. It is the purpose of this chapter to attempt to identify a systematic approach in order to ease the task. The approach is concerned primarily with the use of tristimulus colorimetry and reflectance spectrophotometry. These two techniques will be discussed in detail along with colour scales and formulae commonly in use. The key points which may influence measured results will also be described followed by advice on the selection, preparation and presentation of samples. Finally, detailed references to work carried out in the field of food colour measirrement and where to access further information will be provided. 

In a technique called solid-phase spectrophotometry absorption of a colour complex of the analyte sorbed on a solid support is measured without subsequent stripping of the chromogenic species. Solid-phase spectrophotometry offers the advantage of in situ preconcentration of the analyte. Therefore, it is (several) orders of magnitude more sensitive than the corresponding conventional spec-trophotometric methods [11]. UV microscopy (cfr. Chp. 5.3.2) may find application in studies aiming at the study of the physical distribution of additives UV microspectroscopy is discussed in Chp. 5.6.1. For UVA IS reflectance, cfr. ref. [12]. 

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