Measuring Powder in Very Small Quantities
Using the LabScan XE

Batch-to-batch color consistency (whiteness, for some) is an important indicator of quality for many powders. In the pharmaceutical industry, for instance, the whiteness of powder may be an indication of its freshness or purity. Frequently only a small amount of powder may be available for measurement.

A HunterLab LabScan XE 0/45 spectrophotometer in the port-up configuration with the variable sample illumination (VSI) option can be used to measure the reflectance of powders held in the semi-micro powder holder. This is the method advocated by HunterLab for the measurement of finely-ground material available only in very small quantities.

The LabScan XE

The Application

Powders have several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.

• Powders come in the form of fine particulates—not a solid sample—and must be measured through a clear glass window in order to be effectively made into a solid.

• Powders exhibit light trapping between the particles and will be sensitive to ambient light and to small differences in the optical configuration of the instrument. Using a sufficient sample thickness will minimize these effects.

• Powders may be slightly fluorescent, which means that they will be sensitive to the UV content of the light source. If fluorescence is an issue, use of the UV control option is recommended.

Powders in the pharmaceutical industry have the additional characteristic of being very expensive, and so normal powder sampling techniques are not economically suitable. Sampling techniques that use a very small sample size (approximately 0.4 cc) are more cost-effective in this situation.

Note: If inter-instrument agreement is a concern when measuring powders, all the instruments used for those measurements MUST be the same model to minimize measurement differences.

Recommended Color Scale	CIE L*a*b* as a full color descriptor
Recommended Single-Number Index	YI D1925 (2/C), YI E313, WI E313 for indication of yellowness and/or whiteness, Y Brightness
Recommended Illuminant/Observer	D65/10°. C/2° may also be used.

Measurement Method

1. Configure your software to read using the desired color scale, illuminant, and observer.
2. Install the standard 1/4-inch port plate, and standardize the instrument in this configuration.
3. Replace the standard port plate with the modified port plate for the semi-micro powder holder (HunterLab Part Number CMR-2439).
4. Place the powder to be measured into the powder holder.
5. Pack the powder down using the plunger. Do not overtighten the powder holder. The powder should be at least 1/2-cm deep in the holder.
6. Place the powder holder into the port plate.
7. Take a single color reading of the powder. Dump, refill, and read the powder at least three times from the same batch. Average the three color readings for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
8. Record the average color values for the sample batch.