In the pharmaceuticals industry color is used as:
• an indicator of quality where there is a color difference between acceptable and unacceptable product
• a conformance criterion for FDA or company manufacturing standards
• an indicator of degradation and loss of potency over time
• to differentiate between medications
• to verify efficacy of dip test strips where a defined color is associated with the presence of an active ingredient
• part of a brand's identity.
HunterLab instruments can be used for color evaluation of pharmaceutical intermediates, final products, byproducts, and medical devices such as those listed below.
• Tablets and capsules
• Powders and granules
• Liquid solutions, syrups, and gels
• Ointments, creams, and lotions
• Dip strips
• Plastic parts.
(Instrument recommendations for specific applications are shown at the end of this Applications Note.) Recognizing the importance of color for pharmaceuticals, the U.S. Pharmacopeia includes guidance on color and color measurement in its collection of standards. The relevant monographs (which may be purchased through www.usp.org) are discussed below.
U.S. Pharmacopeia Monograph 631
Monograph 631, titled 'Color and Achromicity,' defines color and colorlessness (achromicity), the visual observing situation, and the three attributes of color (hue/value/chroma and the corresponding X, Y, Z), principles that apply to HunterLab's instruments. The monograph also describes how color should be evaluated visually using 'diffuse, uniform illumination under conditions that reduce shadows and nonspectral reflectance to a minimum.' When powders are evaluated, the surface viewed 'should be smoothed with gentle pressure so that a planar surface free from irregularities is presented.' If the illumination affects evaluations, 'those obtained in natural or artificial daylight are to be considered correct.' It stipulates that 'a suitable instrumental measurement may be used' instead of visual evaluation and explains how color difference measurements can be made by comparison of samples to known standards of similar color, such as Munsell chips for reflectance measurements and the A-T USP fluid standards for transmittance measurements. Both of these types of standards are shown in the pictures below.
The Munsell and USP standards serve as guides for color selection and a standardized means for verbal color communication, but lack the precision of tristimulus instrumental measurements necessary to meet today's quality assurance requirements. A suitable compromise is to define the product color in nominal terms using the Munsell or USP standards, while using tristimulus color measurements to verify color consistency.
All of the requirements of Monograph 631 can be met using HunterLab instruments. The visual observing situation (including the diffuse, uniform illumination specified) is standardized using HunterLab spectrophotometers and colorimeters. A smooth, regular powder surface can be presented to a LabScan XE by pressing the powder into a plaque or pouring it into a glass sample cup and reading it through the glass bottom of the cup. Powders may also be measured in a glass sample cup using a ColorFlex, D25LT, or MiniScan XE Plus. Powders can be measured using a ColorQuest XE, UltraScan XE, UltraScan PRO, or UltraScan VIS by pouring them into a glass transmission cell and measuring them through one of the glass sides of the cell. As far as the illumination requirement, a number of standard illuminants, including several daylight illuminants, are available with all HunterLab spectrophotometers. In addition, the xenon flash lamps of the ColorQuest XE, LabScan XE, UltraScan XE, UltraScan PRO, and UltraScan VIS themselves provide a very good match to D65 daylight. All HunterLab instruments can provide color difference measurements by comparison of samples to standards.
U.S. Pharmacopeia Monograph 1061
Monograph 1061, titled 'Color-Instrumental Measurement,' first gives a synopsis of color measurement, explaining how a color measurement instrument simulates the three components of the visual observing situation: the spectral energy of the illumination (the light source or illuminant), the absorbing characteristics of the sample (the sample itself), and the visual sensitivity of the observer (the human observer or CIE standard observer). It also provides the equations for converting spectral reflectance or transmittance to CIE X, Y, Z tristimulus values, from which all other color values may be calculated. (The formulas for CIELAB values are provided at the end of the monograph.) Then it gets into the requirements, the crux of the monograph. These requirements are listed in the table below in order of appearance in the monograph, along with an evaluation of whether HunterLab's instruments comply with each. This monograph describes measurement of opaque solids and transparent liquids. It is not applicable to measurement of hazy liquids or translucent samples.
Pharmaceutical products and appropriate instruments for their measurement are given in the table below. A web address is shown where an illustrated method for the product's measurement is available. Methods are always being added, so visit http://www.hunterlab.com and search under the Solutions menu if an address is not provided for your application. A method could be added at any time.
(See attachedpdf file for the complete article with photos and data tables)