QUALITY CONTROL: Refractive Index IV: Implementation
Loyd V. Allen, Jr., PhD, RPh
Throughout this four-part series, we have provided several applications of a relatively simple and inexpensive analytical method for both quality assurance and quality control (QA/QC). In this issue, we will generally look at the implementation of a QA/QC program in a pharmacy.
There are a number of different instruments available for purchase in different price ranges. A digital refractometer is recommended with a 5-decimal readout. A digital refractometer operates on the same general critical angle principle as a traditional handheld refractometer; the difference is that light may be from an LED light source that is focused on the underside of a prism element. A sample is placed on the measuring surface, and some of the light is transmitted through the sample and lost, while the remaining light is reflected onto a linear array of photodiodes creating the shadow line. The refractive index (RI) is directly related to the position of this line on the photodiodes. The instrument then correlates the position to the RI. The refractometer should be suitable for a recommended wide range of indices (1.3 to 1.7).
When used for quantitative analysis, various concentrations of the substance in a liquid, such as water, can be prepared in grams of solute per 100 mL of solution. When working with mixtures of organic liquids, greater linearity is obtained when solutions are prepared on a volume percent basis. Once the standard curve is developed, an unknown can be easily compared. This is a relatively simple method of checking actual concentrations of simple solutions.
Qualitative analysis can be used for at least two different situations. Pure liquids will have a specific RI so confirmation of the liquid can be obtained by reading its RI and comparing with standards or literature values. Second, the same substance at the same concentration should have the same RI. If a different substance has been substituted, then the RI may be different and can then be confirmed with further analysis.
Calibration and Standards
Calibration is one of the most important concepts related to the use of a refractometer. All measuring systems are prone to change over time and a refractometer is no exception. Calibration is the act of establishing, under specified conditions, the relationship between values read on a refractometer and the corresponding values represented by a reference standard and the subsequent adjustment of the refractometer to match those values. The RI of water can be used as a standard; the nD20 of distilled water is 1.3330: the nD25 of distilled water is 1.3325.
As mentioned previously, temperature is critical in obtaining an accurate refractometer reading. It is important that the refractometer and the sample be at the same temperature. This may require waiting a short time period for equilibration to occur. Due to the difference in the mass of the refractometer and small sample, about 10 to 20 seconds for every 5°C difference in temperature should suffice; this can be determined experimentally for a specific laboratory, instrument, and sample.
For a measurement, place only a few drops of the substance, sufficient to form a film, on the sample area. Allow for temperature equilibration and operate the refractometer according to the manufacturer's instructions to obtain the RI value. Clean the sample area as required and prepare for the next sample or for storage.
Documentation can be achieved using an external printer or by recording the values in a documentation log, compounding record, or other suitable record.
With the ease of operation and low cost involved, there is little reason for a pharmacy to not incorporate RI in their daily QA/QC activities.