The New USP Chapter 41
On December 1, 2013, the new regulations of USP Chapter 41 published in June 2013 went into effect. In the U.S., this chapter is mandatory and its implementation is overseen by the U.S. Food and Drug Administration (FDA).
However, it all basically continues to revolve around one important procedure: Weighing accurately...
Area of Application
So far, the USP Chapter 41 was entitled “Weights and Balances." When it was revised and released in December 2013, the title was shortened to “Balances,” which clearly indicates the chapter's major area of application. Accordingly, Chapter 41 does not refer to minimum sample weights any longer. Instead, it defines the terms "repeatability" and "accuracy." Both concepts are of crucial importance as they together define the operating range of a balance.
The new USP regulations require that balances used for weighing must be calibrated.
Repeatability
According to the new USP Chapter 41, “Repeatability” defines the starting point of a balance’s operating range.
How do I calculate it?
- Perform 10 measurements with the exact same weight
- Calculate 2 x the standard deviation (std. dev.) / nominal value ≤ 0.10%
- Calculate the starting point of the operating range: 2 x SD x 1000
- If the std. dev. < 0.41 digits, replace it by 0.41 digits
What does this mean?
The higher you go in a balance’s operating range, the more accurate it is, relatively speaking. Your results will be in a similar range as with the parameters formerly used in this calculation, so you should not expect any significant changes.
Accuracy
In order to test a balance’s accuracy, Chapter 41 requires you to use a test weight that has a mass between 5% to 100% of the balance’s maximum capacity.
How do I calculate it?
- The measurement uncertainty of a weight must be ≤ 1/3 of 0.10%
- Perform one measurement using a test weight with a mass between 5% to 100% of the balance’s capacity
- The difference of the value measured should be ≤ 0.10% of the weight value
What does this mean?
The USP describes the quality of the weights to be used for this test.
This means you must use a calibrated weight to prove the uncertainty of that weight. However, you do not need to use higher class weights. OIML Class F1 or F2 weights are usually sufficient.
Chapter 1251
The key information about USP Chapter 1251 on “Weighing on an Analytical Balance” is that it is only a recommendation – you are not required to follow the procedures described in this chapter.
Chapter 1251 describes the following, for example:
- Balance qualification
- Balance calibration
- Risk analysis (risk-based approach)
We believe there are even better approaches than the ones suggested in Chapter 1251. Therefore, we recommend that you contact a Sartorius product specialist who can advise you.