Editorial: Quality Assurance in Compounding, Part VI
United States Pharmacopeia (USP) Chapter <741> Melting Range or Temperature
Over the past couple of years, several compounding pharmacists have inquired about the use of a melting point apparatus to confirm the melting points of incoming compounding materials; basically, a double-check for comparison against the Certificate of Analysis.
USP Chapter <741> Melting Range or Temperature is the chapter covering this topic. It defines the melting range or temperature of a solid as those points of temperature where the solid coalesces and is completely melted, unless defined otherwise for Class II and III drugs. Any apparatus or method capable of equal accuracy can be used, and the accuracy of the apparatus should be checked at suitable intervals using one or more of the six USP Melting Point Reference standards, preferably those that melt nearest the melting points of the substances being tested. It should be noted that most drugs have melting points less than 300�C with most of them less than about 250�C.
A very general procedure is as follows (more detail is provided in the USP):
- Comminute the test material to a very fine powder.
- If a hydrate, render it anhydrous (this can be done by placing a dehydrating material (Indicating Drierite) in a sealable container and setting the container of the test material (open) into the sealable container, sealing the larger container and letting it set overnight.
- Depending upon the device used, place the required quantity of dried, fine powder on the sample holder.
- Follow the instructions of the manufacturer of the melting point apparatus being used.
- Document the results obtained.
The temperature at which the substance is observed to collapse definitely at any point is defined as the beginning of melting. The temperature at which the substance becomes liquid throughout is defined as the end of melting or the "melting point." Both temperatures should fall within the limits of the melting range required for that substance.
There are many types of apparatus that can be used, and this is a relatively simple and straightforward method after one becomes experienced in its use.
Loyd V. Allen, Jr., Ph.D., R.Ph
Editor-in-Chief
|