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USP SECTION 41 (MEASUREMENT UNCERTAINTY): U.S. PHARMACOPEIA Section 41 Weights and Balances, details a formula to calculate the measurement uncertainty of a balance when the balance is used for Assay weighing. This same formula is referenced when determining the minimum sample weight of a balance.

USP Section 41 states that “when substances are to be ‘accurately weighed’ for Assay the weighing is to be performed with a weighing device whose measurement uncertainty (random plus systematic error) does not exceed 0.1% of the reading.” The formula for arriving at this measurement uncertainty value is three times the standard deviation, of at least ten replicate measurements, divided by the amount weighed. The resulting value shall not exceed 0.001.

We always want to remember that as we increase in weight, even though there may be a greater variance between the replicates with each increase, the measurement uncertainty value decreases. The heavier the weight applied, the greater the chances of that particular weight applied passing the test.

MINIMUM SAMPLE WEIGHT: The minimum sample weight, for Assay weighing, is calculated using the above mentioned test method. The minimum sample weight is determined by identifying the smallest mass value that passes the USP Uncertainty Test.

If a 10mg test load was desired on a 5-place analytical balance and the results of the test came to 0.16% of the applied weight. This particular balance, at that particular location, could not be used because its measurement uncertainty exceeds 0.1%. The unit would then need to be tested at 20mg or the balance would be needed to be upgraded to a 6-place micro balance. If the resulting value had been 0.14% instead of 0.16%, then the value could be rounded down to the same significant figures as the stated criteria (0.1%) and therefore would pass becaue it would be equal to the tolerance.

One of the more controversial aspects of this section is what in fact constitutes the “sample weight”. Is the sample to be measured alone considered the “sample weight”? Or is it the “sample weight” with a tare vessel? The “sample weight” in this case would be just the sample of product alone without the tare vessel. So a tare vessel can not be used to increase the weight and bring the sample into a desired range that will pass the USP Uncertainty Test.

As a general rule of thumb, the recommended minimum sample weight will is approximately 3000 to 5000 divisions (i.e. 30.00 mg or 50.00 mg on a 5-place analytical balance). In laboratory environments that are close to ideal (i.e. little to no vibration, air currents, people traffic etc.), then 2000 divisions (20mg) may also pass the Uncertainty Minimum Sample Weight Test. As stated earlier, when applying the USP Uncertainty Test, the heavier the weight applied, the greater the chance of the value coming within the 0.1% tolerance. In referencing the above example again, at a test load of 1000 divisions (10.00 mg), this balance would be required to produce 10 almost perfect replicates, leaving very little room for any deviations. At this mass, only 10 replicates with 1 deviation of 1 count would allow this 10mg to pass, which in real-world conditions, is not probable.

DEFINITIONS

Assay: A quantitative or qualitative analysis of the composition of a material.

Random Error: Errors due to the play of chance. Variability between successive measurements due fluctuating environmental conditions, different observers, etc. An error, which is in general, different each time a measurement, is made.

Systematic Error: Factors that consistently affect the variable being measured. An error that affects all measurements similarly, (mechanical problem with balance) and or can be eliminated by upgrading the system.

With quality awareness at an all-time high, traceability, accuracy, and the respective documentation are a must for all weighing and measuring equipment, balance calibration weights included. NIST traceable weight calibrations have become more than just sufficing regulatory and customer audits. Whether it is for internal balance weight verifications or balance calibration, having the needed traceability and accuracy are essential in any weighing operation.

Balance calibration weights, also known as precision weights, mass standards, mass sets, calibration masses, and certified weights, give the process a safety net in ensuring the balances and weighing process are accurate and traceable. We all want to be weighing to the same standard, whether it’s at 1kg, 100g, 1g, or 1mg. With that in mind it is important that our weight calibration provider meets the needed criteria for ensuring accuracy and NIST traceability.

To start, balance weight calibration providers should be ISO/IEC 17025 and or ANSI/NCSL Z540-1 laboratory accredited, preferably through A2LA or NVLAP as the accreditation body. This calibration accreditation for precision weight calibration will ensure that a recognized third party regulatory agency is continually assessing and holding that respective precision weight calibration provider accountable for ensuring NIST traceability and accuracy and overall technical competence.

In addition to having the calibration accreditation, the weight calibration provider’s quality program should also be implementing the following: having their Primary Mass Sets calibrated directly by NIST with a rigorous measurement assurance program implemented and maintained for the integrity of the NIST traceable values on their Primary Mass Sets; technical personnel are qualified and experienced (NIST trained and certified preferably); precision weight calibration procedures and weighing designs are that of the metrological industry standard and or strongly based on NIST calibration procedures and recommendations; and finally the weight calibration provider is an active participant in a proficiency testing program (periodic round robins) to measure the weight calibration provider’s ability to maintain the NIST traceability and accuracy on their mass sets. All of the above will help ensure that the resulting weight calibration value provided on the issued NIST Traceable Calibration Certificate will be accurate, traceable, and valid throughout the world.

With the above being said, it is also important that the weight calibration and certification provider has the capability to calibrate your precision calibration weights ranging from 1mg up to 30kg. And of course that the weight calibration measurement values are precise and accurate enough to meet the extremely tight tolerances for Ultra Class Weights, ASTM Class 0 Weights and ASTM Class 1 Weights, along with the comparative OIML Class E2 Weights and OIML Class F1 Weights. You do not want to be sending your balance precision calibration weights to separate weight calibration vendors because the weight calibration provider can only calibrate up to 10kg due to capability or accuracy limits.

And finally, the weight calibration provider must have the flexibility and turnaround time for weight calibrations that will meet your schedule and requirements. A standard NIST Traceable Weight Calibration Certificate usually has a next calibration due date of 12 months, if the balance calibration weights are sent in to the weight calibration provider and are at the facility of the weight calibration provider for a month, not only is it one month less that you can be using your precision calibration weights but in many companies, especially in the pharmaceutical industry, certified weight calibrations are date sensitive and must have the calibrations completed within that specified calendar month or it may cause paperwork headaches.

In summary, like anything else that is critical, there is a lot to be considered in assessing who should be calibrating your balance calibration weights. Not only do you need to ensure the technical competence of the weight calibration provider to ensure traceability and accuracy, but you also need to take into account the weight calibration provider’s capabilities for measurement range and finally their timeliness in getting your balance calibration weights back to you and your process.