Laboratory balances are equipment for measuring the weight of samples (liquid, solid, and other forms). They are specially designed for laboratory requirements such as increased response time, reliability, compliance, and safety.
Laboratory balances are the core of every measurement performed in the pharmaceutical industry. They are commonly used for weighing analytical reference solutions, samples for performing laboratory tests, and samples for buffer preparation for various purposes such as pH calibration and spectrophotometer application.
Laboratory balances are not dedicated for use in laboratories. Instead, they can also be used in production areas, for measuring samples in areas such as In - Process labs and manufacturing sections.
Types of Laboratory Balances in the pharmaceutical industry
Let's discuss some types of balances used in the pharmaceutical industry, which are categorized as follows
Analytical balances
Analytical balances are designed to measure small mass samples, typically in the range of milligrams. Their ability to sense a small weight makes them so precise that they can also be affected by environmental factors such as airflow or tiny particles. To prevent the balance from these factors, balances are enclosed in a transparent enclosure.
Their ability to make precise measurements make them an ideal choice for the Pharma industry, where the measurement of small quantities of the sample can have a great impact on the final product or lab test. Additionally, analytical balances can perform measurement in more than one unit of weighing.
Precision balances
Precision balances are like analytical balances, but with lesser accuracy. They are designed to measure masses, higher than the capacity of analytical balance – in the range of grams or kilograms. These balances are cheaper than analytical balances, due to their reduced accuracy and preciseness. Since they are designed to weigh large masses, they have bigger pan sizes.
Precision balances are rigorous in nature, and are not easily affected by environmental conditions such as airflow and humidity levels.
Micro balance
Microbalances can measure masses of extremely small weight, typically in the order of million parts of a gram. They can be used as independent instruments or can be integrated with other instruments, such as moisture analyzer.
Since microbalance is capable of detecting very smaller quantities, they require a specialized and controlled environment for their placement and usage. They must be installed away from doors, windows or airflow passages to prevent affecting air during measurements.
Moisture balances/analyzers
Moisture balances or analyzers are specialized balances that help in detecting the moisture of the sample, rather than its weight. These balances work on the principle of thermogravimetric or Loss on Drying - LOD.
In moisture analyzer, the sample for which the moisture to be determined is placed on the pan. The analyzer first determines its weight with the moisture in it. Then, sample is died until the entire moisture evaporates from the sample. The heater are so effective that it dries the sample within seconds, without taking too much long.
After drying, the analyzer again weights the sample. The difference between the weight of the moist and dried sample is interpreted as moisture content in the sample.
United States and European pharmacopoeial requirements for laboratory balances
Pharmacopoeia is the collection of instructions, standards and quality rules for safe and effective manufacturing of medicines in a country or a region. It consists of tests to identify and confirm the safe composition of pharmaceutical materials (excipients, active and finished products), analytical methods, micro logical testing and QC testing.
The relevant regulatory bodies such as the United States FDA consult the relevant pharmacopeia, when reviewing Pharma manufacturers operations during their inspection.
The United States and European pharmacopeia contains specific chapters for laboratory balances. The aim of both pharmacopeia is to emphasize on accurate weighing for laboratory balances.
United States Pharmacopeia
United States Pharmacopeia has two dedicated sections for laboratory balance. Chapter no 41 "balances" and chapter no 1251 "weighing on an analytical balance".
Chapter no 41 "balances"
In this chapter, the USP States three critical requirements for laboratory balances
Calibration
Calibration is the verification of output of a laboratory balance against standard reference value.
The USP requires that every laboratory balance must be calibrated during its entire life period. However, USP does not suggest any frequency for performing calibration.
For determination of calibration frequency, most Pharma product manufacturers rely on industry best practices such as manufacturer's recommendation, balance usage, calibrating after every new installation, and after any major fault.
Calibration documentation is also necessary and its reviewed during the inspection. Calibration certificate is the most common documentation.
Calibration certificate is a formal document that provides details of the calibration performed. Some common information that a certificate provides, includes the following
Repeatability and minimum sample weight
Repeatability is defined as the sameness of output, every time, a same load is put to weight. The repeatability must be independent of the sample weight because USP states that its must hold true for every weight within its operating range. Additionally repeatability must not be performed it's by minimum sample weight. The only requirement of USP is that sample weight must be within the range of the operating condition.
Repeatability according to USP is considered satisfactory if two times the standard aviation of measured value divided by the smallest net weight is not greater than 0.1 0%.
Repeatability test must be performed at its intended place of installation, because this test is significantly affected by external conditions at its intended place of installations.
Accuracy
Accuracy represents degree of closeness of average weight displayed with respect to object’s actual weight. If a laboratory balance not meets the criteria of accuracy, it is not suitable for use in Pharma manufacturing organization.
According to USP the accuracy of a balance is satisfactory if the dispute value lies within 0.1 0% of the true test weight value.
The test weight is defined as a weight having a mass between 5% and 100% of the maximum load of the balance. Its maximum permissible error loss does not exceeds 1/3 of the applied test.
Chapter number 1251 "weighing on an analytical balance"
Chapter number 1251 focuses on qualification and operation of electronic balance. This chapter can also be applied to all balances used for analytical purposes in pharmaceutical organization. However, a section named Performance qualification is specifically related to laboratory balances.
Performance qualification
This section presents some meteorological tests and its tolerance, and four types of tests are performed – sensitivity, linearity, eccentricity error and repeatability.
Sensitivity
It is the smallest weight, that can deviate the reading from 0 to 1 unit, and is defined as change in weighing value divided by the change in load usually measured between zero and the capacity of the balance.
According to USP, the acceptance criteria of main scale must not exceed 0.05%.
Eccentricity
Eccentricity is deviation between the displayed load, when the test weight is placed asymmetrically off the center of the gravity of the load. This test is usually performed in the center of gravity and the four quadrants. Test load is usually 30% of the balance capacity.
The deviation should be not more than 0.05% for accurate weighing.
Linearity
It is the ability of a weighing scale to follow a linear relationship between a load and the indicated weighing value. It is perform usually from 3 to 5 points over the working range of the laboratory balance.
The linearity should not exceed the acceptance criteria of 0.05%, where accurate being is preferred.
Repeatability
As mentioned above, repeatability is ability of a balance to display a weighing value when a particular weigh is put on the weighing scale. It is expressed as a standard aviation of more than one weighing value.
For readability test, under chapter no. 1251, the test weight should be less than 10% of maximum working range.
European pharmacopoeia
European pharmacopeia is a single document for the quality of Pharma and medicinal products in the European countries. It is meant to be followed by all pharma product manufacturers during product development, production and marketing.
For the laboratory balance, chapter number no 2.1.7 “Balances for analytical purposes,” is the relevant one. Some highlights of chapter 2.1.7 include the following.
Calibration must be performed regularly, and must be followed by random performance checks in between them.
Calibration
The users should define proper calibration frequency and calibration procedures. During calibration traceability to a national standard should be demonstrated.
Calibration must be performed after maintenance, repairs, change of location of the balance. All the critical information related to calibration should be mentioned in a calibration certificate.
Performance test
Performance tests must be performed randomly in between calibrations, to check and verify the accurate working of a laboratory balance. It also enables detecting any error in the balance reading.
Chapter 2.1.7 in indicates two tests for performance testing - repeatability and sensitivity.
Repeatability
Like Chapter 41 of USP, repeatability is found out by standard deviation, which can be found out by weighing the test weight 10 times consecutively, while the balance should be zero between each reading, every time.
If two times a standard aviation divided by the smallest net weight is not greater than 0.1 0%, the repeatability test is passed.
According to European pharmacopeia, the test load should be less than or equal to 5% of the maximum but at least 100 mg.
Sensitivity
Sensitivity is used to check the accuracy of a balance. Balance cannot be used for pharma manufacturing, if the sensitivity test fails.
During this test, test weight, greater than or equal to 5% of maximum capacity is placed on the balance. If the deviation remains in the range of 0.05% test is passed. Otherwise, the test is failed, and balance cannot be used in this condition.
About the Author
Muhammad Asim Niazi has a vast experience of about 11 years in a Pharmaceutical company. During his tenure he worked in their different departments and had been part of many initiatives within the company. He now uses his experience and skill to write interested content for audiences at PharmaSources.com.
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Pharma Sources Insight JANUARY 2024
