Analytical balances are the key instrument of quantitative analysis. It is used to accurately weighing of samples and standards. it is one of the primary activities of the analysis. The balances are able to provide accurate measurements. Due to the extremely sensitive nature of these instruments, there are several factors that can cause them to give erroneous readings.
United States Pharmacopeia (USP) has set strict requirements for balances used for weighing analytes for quantitative measures. A recent update effective August 1, 2019 highlights the need for accuracy and stresses the concept of minimum weight as basis for accurate weighing processes.
For an analytical balance to give an accurate reading, the instrument must be calibrated. Calibration is important because it defines the accuracy and quality of the measurements that are recorded by the balance.
Working Range: of a balance is the weight range within which it will give accurate results.
The operating range minimum and maximum means: The minimum value is the lowest value at which the balance will give an accurate reading as per repeatability test. The maximum value is the maximum capacity of the balance.
USP General Chapter 41 (Balances) sets three distinct requirements to be applied to materials that must be accurately weighed.
Accuracy: As per Chapter 41, the accuracy of the balance must also be tested. For the accuracy test, the weight must have a mass that is between 5% and 100% of the balance’s capacity and the measurement uncertainty of the weight must be ≤ 1/3 of 0.10% (≤ 0.033%). The weight must be calibrated to prove the uncertainty.
Repeatability: Includes at least 10 measurements using one test weight. It’s recommended to use a half-load weight (one that is approximately one-half of the balance’s weighing capacity) for this test.
Based on the repeatability acceptance criterion, (2 x s) / (smallest net weight) ≤ 0.10 %, all masses equal or larger than (2 x s) / (0.10 %) conform with this requirement. The smallest mass that satisfies this criterion is called minimum weight:
mmin = (2 x s) / (0.10 %) = 2,000 x s
Note that the smallest standard deviation (s) that may be used for the assessment against the 0.10 % repeatability requirement and for the calculation of minimum weight is 0.41 d, where d is the digital increment (readability) of the balance. This lower limit is based on the rounding error of a digital indication.
Calibration: “The weighing shall be performed using a balance that is calibrated over the operating range and meets the requirements defined for repeatability and accuracy.” Both acceptance criteria are expressed as relative limit values, 0.10 %. Calibration, the third requirement.
USP General Chapter 1251 elaborates more fully on routine balance tests by applying a risk-based approach on performance verification activities. Furthermore, it elaborates on the variability of the balance’s performance over time, which leads to the concept of the safety factor.
USP General Chapter 41 and General Chapter 1251 ensures that the operator can trust on the accuracy of the balance over the time.