What Precautions should be taken for correct weighing of Laboratory Samples?

Adopt correct weighing practices

After deciding on the weighing balance that you require for your sample you have to start the actual weighing.You have to keep in mind that the accuracy and precision of your results is largely dependent on the reliability of your weighing. It is at this stage that you have to take adequate precautions and adhere to the standard operating procedures to ensure reliability of your reports.

Balance Calibration

• Weighing of samples is always against standard reference weights. Ensure that your laboratory has a set of certified standard weights and their calibration is validated as per the prescribed procedure through a recognized national calibration laboratory
• Keep your balance always calibrated using standard calibration procedures against daily, weekly and monthly schedules
• Standard weights should never be touched with hands

Environmental Control

• Horizontal positioning of balance should be daily checked with the help of inbuilt spirit level
• The balance should be kept on a vibration free support
• Balance should be kept in an area with controlled temperature and humidity
• Silica gel bag should be placed inside weighing chamber to keep the chamber free from humidity. The bags should be periodically recharged or replaced.
• Balance should not be exposed to direct sunlight as this can cause temperature variations inside the weighing chamber
• Balance should not be located next to doors or windows as opening and closing gives rise to air drafts.
• Always weigh samples after closing the weighing chamber doors
• Keep the inside of the weighing chamber and surrounding bench space scrupulously clean so as to avoid errors and cross contamination of samples.

Handling of Weights

• Weights should not be touched with bare hands as grease or oil from hands can lead to errors. Always use a pair of clean forceps
• Place weights gently onto the pan. The weights should be kept in the centre of the pan so as to avoid errors due to eccentricity
• After use weights should not be left outside on the work bench but kept inside the slots in the weight box and keep the box closed so as to avoid minimum environmental exposure

Weighing of Samples

• Every time use a clean spatula of appropriate size
• Butter paper weighing can introduce errors. Instead weigh the sample quantity required in the volumetric flask and after weighing of samples make up the volume with the diluent
•  Allow the reading to stabilize before recording
• Accurate weighing of samples of small amounts requires additional precautions such as use of disposable gloves, use of head caps to prevent fall of hair or dandruff and use of face mask to prevent breath from disturbing the reading
• Reading should be recorded directly into the laboratory notebook. Making note of readings on scrap paper, filter paper or hands should be strictly forbidden
• Add printer to the balance for direct print out on weight slips to avoid transcription errors. Preserve weight slips along with the laboratory records.

Weighing of samples is a critical component of any analytical determination. You can eliminate errors by adopting the recommended weighing practices as a regular habit.