High pigment concentration measurement

When dealing with leaves such a condition is not usually met as the pigment concentrations encoutered in vivo are usually very high. The kinetic analysis is further complicated because the photosynthetic apparatus will adapt to the light conditions prevailing at different depths in the leave. However the fluorescence from the uppermost cell layers can be measured selectively using an appropriate choice of the excitation and emission wavelengths (3). Fig. 1 shows a typical fluorescence induction curve measured from a DCMU inhibited leave. The fluorescence reached its maximal value in essentially 100 msec. Contrary to what has been observed with thylakoids the fast rise was not followed by a secondary slow rise (6) so that Ffj]ax remained constant for at least 10 sec. 

In ocular applications, Raman spectroscopy can quickly and objectively assess composite lutein and zeaxanthin concentrations of macular pigment using spatially averaged, integral measurements or images that quantify and map the complete MP distribution with high spatial resolution. Importantly, both variants can be validated with HPLC methods in excised human eyecups and in animal models. 

As pigments are most often used in paints and varnishes a dispersion with a high energetic input should be used if not otherwise stated. This can conceal effects occurring at lower energy levels as, for example, more agglomerates can be broken up. A common feature of both instrument types mentioned above is the use of a very dilute solution. The exact concentration is dependent on the specific type of instrument used, as well as on the material and particle size to be measured, but is nearly always below 1% (weight). 

Although the effect of the concentration of suspension on the results of the measurement of pigments has never been proven, the development of techniques able to cope with concentrations closer to the applications if of interest These would make it possible, for example, to determine the particle size distribution in a dispersion paint or in a reaction vessel where a pigment is produced by the precipitation process. A measurement technique having no problems, in principle, with high concentration dispersions is the scattering of ultrasonic waves. Nevertheless the instruments on the market have up to now failed to realize the great expectations of this technique. 

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