Colour analysis analytical techniques

According to the predominant component, the binders are usually divided into protein, oil, polysaccharide, and resin binders. In this section we shall focus on protein binders but it is worth mentioning that in the majority of natural non-protein binders a minority protein component is usually present as well. Thus many of the analytical techniques described here can be (with certain limitations) applied to them as well. Although in colour layers of artworks and particularly in paintings protein binders are relatively abundant (up to 10%), their identification is often limited by a small amount of sample that is usually available for analysis (tens or hundreds of micrograms at most [6]). 

Identification and quantification of natural dyes need high performance analytical techniques, appropriate for the analysis of materials of complicated matrices containing a small amount of coloured substances. This requirement perfectly fits coupling of modern separation modules (usually high performance liquid chromatography in reversed phase mode, RPLC, but also capillary electrophoresis, CE) with selective detection units (mainly mass spectrometer). 

Chemical analysis. Chloride ions in the extraction solution described above can be determined by different analytical techniques such as colour-based titration, poten-tiometric tritration, chloride-ion selective electrodes, etc. All of the analytical techniques have been shown to give comparable results with good accuracy, provided that frequent calibration with standard solutions is carried out. For very small amounts of liquid (as for example obtained by pore-water expression) the ICP technique (inductively coupled plasma spectroscopy) has been used successfully. However, this instrumentation is available only in specialised analytical laboratories. 

UVA IS spectrophotometry is a well developed routine technique, which is used extensively in QA/QC laboratories, but not so frequently as a process analytical technique because of lack of selectivity of the spectra (exceptions are monoaromatic hydrocarbons). Visible and near-ultraviolet spectra often do not contain so much industrially useful information. However, the method is suited for the determination of components that can be readily distinguished from the sample matrix (e.g. UV absorbers in polyolefins, but not in EPs colour measurements of ABS). Provided that the analyte has a UV chromophore, UV absorbance measurements provide much greater sensitivity than NIR measurements. This enables antioxidants and other additives to be determined at the low ppm level. Polyolefins are transparent in the UV, which enables calibrations for the analysis of antioxidants to be matrix independent. 

There is a wealth of published work on the analysis of colouring agents in cosmetics, especially, lipsticks. Marmion (1991) reviewed the literature that was published from the mid-1960s through the 1980s. This part will highlight selected early studies that are now considered classics and will also review the methods published since Marmion s (1991) work, grouped by the analytical technique employed. 

A more recent development is a technique known as flow injection analysis, in which a discrete volume of a liquid sample is injected into a carrier stream. Reagents required for the development of the analytical property of the analyte, e g. colour developing reagents for spectrophotometry, are already present in the stream. The stream then flows straight to the detector and the technique depends upon the controlled and reproducible dispersion of the sample as it passes through the reaction zone. Thus the reaction does not necessarily need to develop to completion,... 

Genuine chemical methods were preceded by techniques such as the touchstone,26 and also by the direct investigation of the material or object by the use of the senses. A material could be felt, it could be tasted, its colour could be observed, it could be smelt, and the noise it could be induced to make could be listened to. The use of such methods of organoleptic analysis in the work of the Georgian King, Vakhtang VI (1675-1737) have been described.27 Probably the first demand for a quantitative analytical method arose from a desire to estimate the purity of samples of gold. While... 

The reflectometer performs the assessment and in some systems it also monitors the reaction. The analyte present in the water of the sample in partnership with the reagents in the test strip, produces the required reaction and the dye or colour is formed that produces a certain reflectance on exposure to radiation. This enables quantitative analysis comparable in precision and accuracy with classical photometry. A special feature of such techniques is that in most cases, undiluted sample material may be used. Also, the range of... 

In addition to transfer techniques in quantitative analyses, such as polarography, titrimetry, spectroscopy and other analytical methods used after separation by TLC, the in situ optical measurement is the most widely employed technique for quantitative determinations. In most cases UV-absorption is used, while coloured substances can be determined by absorption measurement in the visible range of the spectrum. Fluorescent substances are preferably determined by fluorescence measurement. Infrared absorption techniques are not used in routine analysis up to this date. 

In particular in the industrial environment, where most in-poIymer analyses are being carried out, analyst time needs to be minimised. To this extent, autosamplers, robots, fast analysis techniques (e.g. ASE , fast GC), hyphenation and standardised data output formats for further manipulation and transmission are wanted. Automation is advantageous (Table 8.4). Ideally, the whole process may be automated analysis, data reduction and output. Unfortunately, standardisation of data handling procedures is still far off. This determines continuous, multiple efforts for training of analysts. Analytical methods should also be easy to maintain caUbra-tion should be required at minimal levels. Sample preparation should minimise time, effort, materials and volume of sample consumed. Sample pretreatment is ideally superfluous. There should also be little inherent doubt on the representativity of the analysis (of special concern for those techniques employing minimal sample amounts 0.1-1 fj,g). The method should be able to qualitatively identify the specific analyte(s) of interest, on the basis of expected behaviour (e.g. retention time, colour... 

---