OptLab-SPX   Color Examination according to European
                       Pharmacopoeia and DAB

The European Pharmacopoeia (EP) and related national pharmacopoeiae describe the color examination of pharmaceutical raw materials. The principle uses five series of stable colored solutions, each of which is diluted to 7 or 9 shades. The color of a sample is examined by visual comparison to one or more of these references.

Visual comparisons nowadays are generally looked upon skeptically, because they are subject to individual conditions. Illumination, surrounding colors as well as personal differences and feelings will affect color examinations. Furthermore, individual visual inspections may not be reproducible and cannot be quantified.

On the other hand the color of a (non-fluorescent) sample is strictly defined by the sample transmission spectrum in the visible spectral range (400–700 nm). Such spectra can be scanned easily and reliably with a standard UV/Vis spectrometer and stored on a PC in digital form. However, a spectrum typically comprises 30–50 or more data points and such large data sets are not easy to process.

It has therefore been suggested to perform the described color inspection objectively by processing the recorded sample and reference spectra mathematically. This requires that the necessary reference spectra be scanned once and used as the basis for the color inspection. The scheme calculates a characteristic value which is almost linear to the dye concentration. Thus it is simple to use this single value for comparing the color depth of the solutions.

OptLab-SPX is designed to perform this calculation and comparison automatically. The result of this calculation is a decision as required by DAB/EP (sample is less/more strongly colored than …) together with the generated characteristic values. The result can be printed together with the spectrum diagram or can be stored. OptLab contains all characteristic values of all reference solutions described in DAB/EP in 1 cm cells together with the data of the weakly colored solutions in 5 cm cells.

    Results of a DAB/EP color calculation for a weakly yellow sample

The advantages of this procedure are that an objective value which can be statistically treated is applied and that the reference solutions must not be prepared for each measurement. But it must also be taken into consideration that the described color inspection is only valid if the hue of the sample and reference solutions is similar. Substantial differences in hue will generate meaningless results.

If required, OptLab-SPX can generate the required characteristic values for other cell path lengths or other reference solutions. The user can add any number of additional calculations.

There currently are efforts in pharmaceutical working groups to replace the above procedure by standard procedures for color description. Starting in the 1930s, algorithms were defined by CIE which are used in many industrial areas for color description. Typically, the results are defined as a triple of color values, e. g. XYZ or Lab values. These values are the basis for further calculations, such as the color difference data. Various national and international standards, such as DIN, ISO or ASTM, have adopted these principles. OptLab also features the determination of these more general color values; thus the software is prepared for future developments in pharmaceutical color investigation.

Conclusion: The OptLab-SPX software packages feature a new objective procedure to replace visual color inspection in the pharmaceutical industry. This results in a substantial reduction of effort and high objectivity of the procedure. A numerical result replaces an individual, subjective assessment.