Weighing is one of the most fundamental of laboratory tasks, and careful weighing is a central pillar supporting the generation of high-quality laboratory results. New proposals from the US Pharmacopeia (USP) are aiming to help clarify, explain and harmonize weighing best practices. If enacted, these proposals will become legal requirements for some laboratories in the pharmaceutical industry. These proposals are also very useful as guidance for any laboratory where accurate weighing is important.
The specific proposed changes are to General Chapter 41-Weights and Balances, and General Chapter 1251-Weighing On an Analytical Balance. The text has been published in the Pharmacopeial Forum (PF) with a deadline for comments of 30 Nov 2012. The complete text of the proposed changes can be found at the PF Online. New users will have to register for the PF, and log-in before accessing the documents which are found in Volume 38(5). Registration and access to the PF is free of charge.
General Chapter 1251 offers guidance for best practices in weighing, and the proposed changes are extensive. Following an introductory paragraph is a new section titled Qualification with guidance subsections covering installation qualification, operational qualification and performance qualification for analytical balances. Four suggested performance qualification tests are tabulated and discussed along with guidance for setting appropriate acceptance criteria. Balance checks and minimum weight determinations are also covered within the qualification section.
The next major section of chapter 1251 discusses operation of the analytical balance. Receivers (or weighing vessels) are discussed, including particular advice for weighing solids, powders and liquids. Different types of weighing are described along with hints for dealing with problem samples such as statically charged, volatile, hygroscopic, or corrosive samples. Anyone who uses an analytical balance is sure to find helpful information in this chapter.
General Chapter 41 sets definite legal requirements for what it means to “accurately weigh” a substance. The tolerance for accurate weighing is changed from 0.1 to 0.10 which might seem like no change, but under USP rounding rules, a value of 0.149% which formerly would round to 0.1% and pass, would now round to 0.15% and fail to meet the new 0.10 tolerance. In effect, the addition of the trailing zero reduces the specification by approximately one-third. However, the coverage factor is changed from 3x to 2x, which increases tolerance by one-third. Similar to offsetting penalties in a game of American football, the net result is little change in minimum weighing for most users.
Other changes in General Chapter 41 include removal of specific class recommendations for weights, instead providing rules so that users can determine appropriate weight classes of weight calibration uncertainty. The ASTM E617 standard for weights, which formerly was listed as “incorporated by reference” is now listed as one of two “applicable standards” (the other applicable weight standard now being OIML R111).