09 February 2016
Powdered substances can come in many types, colours, appearances and for many different purposes. The one consistent factor typical to most powders is that appearance is important. Variations in appearance can indicate an error with the product itself or the manufacturing process. As a result, it seems very cost-effective to put in place a way of monitoring the appearance of powdered product to reduce the amount of sub-standard waste product.
Some key factors to think of when considering colour analysis of powders are below.
Due to the opaque nature of powder, a spectrophotometer that takes only reflectance measurements can be used.
A colorimetric spectrophotometer would be a good instrument for this type of application. In particular, the ColorFlex EZ as it is a compact benchtop spectrophotometer that can accommodate sample cups securely and allow for consistent placement and measurement.
For the majority of powders, a standard 64mm glass sample cup with an opaque sample cover would be the best setup. These sample cups come pre-marked measurement limits to ensure a consistent preparation process every time. The powder would be loaded into the sample cup to a specific height, taking care to ensure that the surface presented for analysis at the bottom has no obvious anomalies and is as flat as possible. The sample would then be covered with an opaque sample cover to ensure no external factors contribute to the measurements.
Amount of measurements
Generally speaking, the more measurements taken, the more accurate the representation of the visual characteristics of the sample. The amount of measurements necessary, however, would depend on the particle size of the powder.
For very small particles, the surface area they would create in a sample cell would appear very smooth and uniform allowing for a singular measurement to be relatively accurate.
With samples of a larger particle size, a shadowing effect is caused. This effect is noticeable even to the human eye and gives the surface for measurement both lighter and darker areas. To combat this, average measurements should be taken of the sample. This can be done either by turning the sample between measurements, shaking the sample gently to re-position the particles or by discarding the sample after measurements and re-filling the sample cup with more of the sample from the same batch. Whatever the method chosen, however, it must be ensured that all samples are prepared for analysis in the same way.
After all measurements are taken, the results can then be averaged to give a more accurate set of numerical data relevant to the batch rather than just one singular sample.
To gauge the correct colour of the samples, ideally the CIE L*a*b* colour scale would be used to give how light to dark, red to green and yellow to blue the sample is. The L*C*h* colour scale is also a possibility as it would give data also representing how light to dark the sample is but also its hue angle and chroma value that depicts how saturated the sample colour is.
For white powders there is the option of using whiteness indices or even yellowness indices if yellowness is a common indication of an error in the production process.
Application of results
Essentially, what is the purpose of measuring the colour of the powder? Using a self-contained spectrophotometer would negate the requirement of software as the results would be shown on the instrument itself. These could be saved, printed or just noted down for historical records. Alternatively software could be used with the ColorFlex EZ to allow for a more visual display of any standard to sample differences and even provide a customisable report to send to a customer.
Whatever the purpose of the powder be it medicinal, chemical, food or different, keeping the expected colour as vibrant or as pure as it should be is a key aspect of quality control and one that should not be left to chance. Colour analysis needn’t be a complicated, time consuming task but rather a complementary aid to an efficient manufacturing process.