Balances are one of the most ubiquitous instruments in laboratories and serve an important role in a wide range of processes, from preparing carefully measured samples or reagents, to determining density, to calibrating pipettes. Accurate and precise mass measurements are critical to the success and repeatability of experiments, and there are many influences that can impact the performance of a laboratory balance when best practices are not followed. Here are a few do’s and don’ts to keep in mind during laboratory weighing procedures.
DON’T: Install a balance just anywhere
Setting up a balance without taking location into consideration is setting yourself up for failure – that is, drift, inaccurate measurements and poor repeatability. Vibrations, temperature, humidity and draft are some of the many factors that can distort weighing measurements, and certain locations are more prone to disturbances than others. For example, in highly-trafficked areas and areas near air conditioning systems or larger laboratory equipment, balances are more likely to experience vibrations or draft.1 Placing a balance near a window where it is exposed to direct sunlight can lead to drift due to temperature changes.
DO: Select a stable place to install your balance
As mentioned, you’ll want to keep in mind vibrations, temperature, humidity and draft. Installing a balance in a low traffic area away from other running equipment will minimize exposure to vibrations and draft from fans or people passing by. Rooms with sliding doors are ideal, as door movement can be a significant source of air flow.1 Corners are a good place to install a balance, as they tend to experience the least vibration within a building. Balances should also be placed away from air conditioning systems, heating systems and windows, and in rooms with a stable temperature and relative humidity between 40 and 60%. Lastly, balances should always sit on a level surface, and ideally on a weighing table that is resistant to vibrations and static.
DON’T: Weigh a sample in a container of inappropriate size or material
Excessively large weighing containers can increase the impact of flow forces and containers with large, uncovered openings pose a greater risk for evaporation of liquid samples. Additionally, certain materials are more susceptible to interfering forces; plastic surfaces, for example, are more likely to experience electrostatic charges than glass or metal, especially at lower humidities.1 However, metal containers that are magnetic can also throw off measurements due to interference with magnet components found in most laboratory balances. All of these factors should be taken into consideration when choosing what type of container to weigh your samples in.
DO: Weigh samples in a suitable container of minimal size
It is best practice to weigh samples in the smallest possible container while still ensuring that sample material will not overflow or spill out of the vessel. Tubes or flasks containing liquid samples should be sealed whenever possible to prevent sample loss through evaporation. The material of the weighing vessel should not react with the sample material and should ideally be static-resistant and non-magnetic. Keep in mind that glass materials can also be susceptible to electrostatic charges when relative humidity is low. Avoid using materials such as household parchment paper for weighing, as some material can still stick to this type of paper. Laboratory-grade weighing paper is preferable, as it is specifically designed to prevent sample loss in precision measurements.
DON’T: Handle weighing containers or test weights with your bare hands
(Not-so) fun fact: latent fingerprints can weigh up to 200 μg, which may not sound like a lot, but can become a big problem when several fingerprints are left on a test weight required for calibration of highly sensitive microbalances.2 Additionally, fingerprints are hygroscopic, meaning they absorb moisture that can add additional weight to test weights or sample containers. Components of fingerprint residue, such as salt and lactic acid, can even cause the surface of metal test weights to corrode over time. Furthermore, holding items in one’s hand for an excessive time before placing them on a balance can cause heat to transfer from the body to the item, which further impacts measurements.
DO: Handle containers and test weights with clean gloves or forceps
The use of clean, lint-free gloves eliminates the transfer of sweat and oils from the skin to items placed on the balance, preventing errors from added weight or corrosion of calibration materials.2 However, as hand warming can still be a concern, forceps or tweezers may be used to transfer items to the balance without impacting their temperature. Care should be taken to handle test weights with implements that will not scratch or damage them, such as forceps with plastic-coated tips to prevent contact between metal and metal.
DON’T: Use a balance immediately after turning it on
When power is introduced to a balance after a period of being shut down, the instrument will gradually warm up before its temperature stabilizes. Therefore, if sufficient time is not allowed for the temperature to stabilize before measurements are taken, measurements taken at different time points will be inconsistent. Frequent shutting down and powering up of the balance – every day, for example – increases the risk that measurements will differ due to temperature changes.
DO: Allow sufficient time for the balance to warm up before using
One should follow manufacturer’s instructions to determine the time needed for a balance to warm up, as the precise time frame varies from model to model. However, generally, more sensitive balances will require a longer warm-up time – up to 6 hours for semi-microbalances and up to 12 hours for microbalances.1 Due to the significant amount of time required for this stabilization process, it is advisable to keep balances continuously connected and in standby mode when not in use as opposed to disconnecting or shutting them down after use. When a balance is powered on again after being disconnected, tasks should be properly scheduled to allow sufficient time before weighing.
DON’T: Overload the balance
All laboratory balances have a specified weight limit, and balances should never be loaded beyond this limit at any time. Beyond the obvious fact that accurate measurements can not be made above this limit, overloading the balance can cause damage to the load cell or other delicate instrument components, introducing additional error or even rendering the balance nonfunctional. Similar damage can also be caused by dumping items onto the scale in a careless manner.
DO: Take care to avoid damage to the balance
To prevent damage and error, ensure the combined weight on the balance is below its maximum capacity – this includes both the container and the sample. This is another good reason to use weighing vessels of minimal size, rather than containers much larger than needed for the measured sample. Items should not be left on the weighing pan when the balance is not in use and items should be placed gently on the pan to minimize impact. If the balance is accidentally overloaded or impacted – for example, if something falls onto it – the instrument should be retested and recalibrated to ensure it is still in working order.
DON’T: Ignore sample properties when performing weighing measurements
While consistency is generally a good thing in the lab, treating every sample exactly the same neglects the risk of error posed by certain sample properties. Samples that are hygroscopic, such as silica gel, may absorb moisture from the air, causing an increase in weight measurement. Liquid samples, especially volatile liquids, can evaporate, losing weight and potentially resulting in condensation that contaminates the weighing pan. Samples coming out of a refrigerator or freezer, in addition to their temperature difference, may contain ice crystals that alter the sample weight. Similarly, samples coming from a heated environment can result in skewed measurements, as temperature gradients between the balance and sample result in air currents that exert force on the weighing pan and/or sample.1 Magnetic materials, as previously mentioned, can interfere with the magnetic field from key balance components that enables their precise measurements.
DO: Optimize weighing procedures to account for sample properties
Appropriate measures should be taken to accommodate samples that may pose a greater risk of error due to their properties. Hygroscopic and volatile samples should be measured quickly and covered or sealed to prevent the absorption or loss of liquids. Samples stored in a temperature environment that differs from that of the balance should be given sufficient time to adjust to room temperature before they are weighed. Magnetic materials should be demagnetized, if at all possible; if this is not an option, magnetic interference can be reduced by placing the sample on a mu-metal film, or distancing the sample from balance pan by placing it on a platform such as an inverted beaker.1 Finally, certain samples may require the use of balances with specific features to be accurately weighed - for example, live animals should be weighed on a balance with dynamic weighing capabilities.
References
1. "Reliable Weighing Results - Proper Handling of Laboratory Balances and Correct Handling of Samples," White Paper, Sartorius (2020). https://www.sartorius.com/en/products/weighing/lab-weighing-resources/reliable-weighing-results-proper-handling-of-laboratory-balances-and-correct-handling-of-samples
2. "Maintaining Your Lab Balance – Taking Care of Test Weights," White Paper, Sartorius (2021). https://www.sartorius.com/en/products/weighing/lab-weighing-resources/handling-test-weights-whitepaper