Analytical Balances and USP interpertation of <41> and <1251>.
Hi all new to this company. Just handed our approved SOP for "Use, Verification, and Maintenance of balances in QC" Also given USP docs <41> Weights and Balances and <1251> Weighing on an Analytical Balance.
I am scratching my head over <41> "Unless otherwise specified, when substances are to be "accurately weighted" for Assay, the Weighing is to be performed with a weighing device whose measurement uncertainity (random + systomatic error) does not exceed 0.1% of the reading Measurement uncertainty is satisfactory if three times the standard deviation of not less than ten replicate weighings devided by the ampount weight, does not exceed 0.001. The question is "weighing device" is that the analytical balance or the actual weights (class 1.1 or 2 or 3)? Does this paragraph only describe how to choose the class of weights and not to the analytical balance with regards to tolerance of the weighted number? Am I totally off the mark? Our company's SOP has us measure a weight 10 times, get a Standard Dev mul by 3 and devide by weight and then compare that number to accept <0.001. They call that the "Balance measurement of uncertainty" The balance acceptance criteria is the mean +/- 0.001% (yes %) of the mean. Our balances are failing at the 10gram weight (passes Std Dev formula <0.001) but fails the mean +/- value. Their answer is to get a bigger weight (100g) and then the mean +/- value passes. The justification is we choose a weight that is in the middle of the balance range as oppoised to the weight we typically measure around (10g).
I know I am wordy; but, appriciate any help.
The other question comes from <1251> page 2 check for drift. "The check for drift at the most sensitive position will show whether a problem exists; the variation in the observed weight foes not exceed +/- 0.2mg. They give an example of a 20g weight. The question is is the +/- 0.2mg tolerance apply to the entire range of the analytical balance, (i.e, 0 - 220g)? Our company'
Analytical Balances and USP interpertation of <41> and <1251>
The interpretation of USP<41> has had a lot of people confused.
The way that this has been explained to me is as 'minimum weight'. The calculation allows you to determine the balance measurement uncertainty of the balance and by using a correlation curve this can then help you to determine to minimum weight which can be accurately measured on a balance.
Therefore if your balance only meets the criteria when using 100g (very poor!!) then the balance should be appropriately labelled to ensure that nothing less than 100g is weighed out using the balance.
The 'weighing device' does, I believe refer to the balance, suitable weights should be used and these should have traceable calibration.
I've found that analytical balances, routinely maintained tend to have a minimum weight of 10-25mg (sartorius), I have performed this qualification test on approximately 50 balances. I always perform the measurement uncertainty at the minimum weight (calculated) to unsure the 'actual' meets the 'theoretical'.
The drift question USP<1251> will apply throughout the range as the drift shouldn't be affected by the weight on the balance. You're checking the internal operations of the balance and as long as your balance is situated suitably, once settled the reading should be reproducible and shouldn't drift without influence from external factors.
Hope this helps.