European Pharmacopoeia Chapter 2.1.7 

Nothing makes a particular balance model compliant or non-compliant with Chapter 2.1.7 of the Ph.Eur. However, there are some general requirements listed in Chapter 2.1.7, such as it must be ensured “the installation and operating conditions do not have a negative impact on the performance of the balance.”

The only specific requirement is the balance must be grounded, which is usually the case (via electrical supply) for state-of-the-art models.
 

Chapter 2.1.7 specifies that “Any weighings performed as part of tests prescribed to establish compliance with a monograph of the Ph.Eur. must be carried out according to the principles outlined in this chapter.” When determining whether a balance is affected, users should consider this statement. If in doubt, they should consider whether the result of the weighing is relevant for the assessment of compliance with the corresponding monograph. It should also be noted that the preparation of a buffer solution can be affected by the new chapter of the European Pharmacopoeia.
 

During calibration, a relationship is established between the known value of test weights and the associated indicated value of the balance. Chapter 2.1.7 states “… instruments must be periodically calibrated …by the user or by a suitable competent body.”

There are three requirements stipulated in Chapter 2.1.7 of the Ph.Eur.:

• Calibration must establish traceability of measurement results to the SI units.
• Calibration results must include an associated measurement uncertainty.
• It is recommended to perform an “as found” calibration before any maintenance operation that significantly alters measurements and to perform a second calibration (“as left”) afterwards. 

Significant operations according to Chapter 2.1.7 include repairs, transfer of the balance to another location or mechanical adjustments.

We recommend having calibrations conducted by an accredited calibration laboratory, as in this case, competence is ensured through the accreditation and the above requirements are fulfilled.
 

The Pharma Software package available for Cubis^® II lab balances helps customers successfully meet requirements set by both the new Ph.Eur. chapter 2.1.7 and the USP, chapter 41.

Our USP Advanced QApp is used to check the performance of balances and offers a guided workflow to determine minimum sample weight acc. to USP Chapter 41 or the new Ph.Eur. Chapter 2.1.7. Our Minimum Weight application monitors weight determined by the USP Advanced QApp and checks it against permitted ranges as per different guidelines. These applications can be paired with add-on advanced software programs for determining and monitoring the measurement uncertainty. 

• In the applicability of the respective chapter, the Ph.Eur. (“analytical purposes”) is somewhat broader than the USP (“substances that must be weighed precisely”).
• The Ph.Eur. explicitly requires the traceability of measurement results to the SI unit.
• The Ph.Eur. explicitly states calibration of the balance must include a measurement uncertainty calculation.
• For the repeatability check, the Ph.Eur. requires a test load no bigger than 5% of the maximum capacity of the balance but at least 100 mg. The USP does not give any comparable restrictions.
• The “sensitivity” check of the Ph.Eur. is similar to the “accuracy” check of the USP, but the acceptable deviation of 0.05% in the Ph.Eur. is smaller than 0.10% in the USP.
• For both performance checks, the Ph.Eur. explicitly requires single-piece test loads, while this is not the case in the USP.

For the repeatability check, a single-piece test load - no bigger than 5% of the maximum capacity of the balance but at least 100 mg – is used. This weight is placed at least 10 times on the weighing pan. Before each measurement, the balance is set to zero. Repeatability is satisfactory if two times the standard deviation of measured values divided by smallest net weight defined by the user is no greater than 0.10%. In cases where standard deviation (s) is smaller than 0.41 × d (where d is the actual scale interval of the balance), s is replaced by 0.41 × d.

For the sensitivity check, a single-piece test load with a weight between 5% and 100% of the capacity of the balance is placed on the balance. Sensitivity is satisfactory if the difference between the indicated value of the balance and the nominal weight of the test load (or its conventional mass, see Question #15 in the FAQs) is no more than 0.05%.

The smallest possible sample weight on a balance, or minimum sample weight (mmin), is 2000 times the standard deviation of the repeatability measurement. Thus, if the standard deviation s is, for example, s = 0.00015 g, the smallest possible sample weight is mmin = 0.3000 g = 300 mg.

The additional requirement of replacing the determined standard deviation with 0.41 × d in cases where the standard deviation is less than 0.41 × d (where d is the actual scale interval of the balance), sets the smallest possible sample weight that can be weighed on a balance at 820 × d. For a four-digit analytical balance with d = 0.0001 g, for example, this means that the smallest possible minimum sample weight can never be less than 0.0820 g or 82 mg.