Weighing is a critical job. In quantitative analysis, role of weighing is very important which may direct impact on accuracy of analytical results. In most of analytical errors, weighing used to be cause however, it is difficult to identify the error during weighing but by following some precautions, analyst can eliminate the analytical errors due to weighing.
Generally Electronic balance are used in laboratory for weighing operation, where we place the material which needs to weight, automatically weight displayed on balance and we can take print copy or transfer the weight print in electronic form.
Here we are going to discuss how many methods are validated or approved by regulatory authorities. Before that it required to discuss on some basic steps and precautions which should be follow must.
Weighing activity should be distributed into three part i.e. planning, verification and weighing.
Planning:- Before weighing, following activities must do.
1- Collect all the materials which require at time of weighing i.e. spatulas, forceps, weighing paper or weighing boat, beaker, dropper, diluent, dried volumetric flask, material which need to weight and respective testing procedures.
2- Some of material are stored in refrigerator or in hot condition. Keep the material in desicatar atleast half an hour before to attend room temperature.
3- Read the safety data for respective material before handling.
Verification:- After arrangements of all required contents following things need to verify.
1- Verify the balance area environment condition i.e. temperature and humidity.
2- Verify the appropriate usage of analytical or microbalance. Ensure the load limit of respective balance which should not be exceeded. Also ensure balance usage range.
3- Ensure balance is placed on anti-vibration platform which eliminates the fluctuation in weight.
4- Ensure the sprit levels are in centre.
5- Check the calibration and daily verification status. Calibration frequency on monthly basis is suitable or as per manufacturers recommendations. However, the first user should verify the weight on daily basis against standard weight of E1 class. The verification reading should be documented. If daily verification is within acceptance limit, balance can be use further otherwise service required for respective balance.
6- Balance, pan, and sarrounding area to be neat and clean otherwise clean with camel's hair brush or equivalent brush.
7- Ensure balance pan are properly positioned and door of balance are properly working.
8- Ensure there is no any error or warning message on balance display. If any, resolve the same before use. Some of balance model shows calibrate me, in that scenario internal calibration required. If power fluctuations was occurred before you reach to balance then message displayed on balance Pannell. Internal calibration must in this condition.
9- Some time, if we restart the balance, leave the balance for atleast 1 hrs for equilibration. Some microbalance required 24 hrs for equilibration.
10- Drift problem is common issue in balance and we should aware about it. Balance drift can occurred without operation also. It is very easy to eliminate the drift issue but before it we should find the source of drift in respective balance. The possibility of drift is may be as:- balance doors are open, laboratory temperature is not consistent, levels are disturbed, sample getting moisture, balance pan is not cleaned etc.
11- Select the atleast 5 decimal places for better accuracy of weight.
12- Selection of receiver is important. Butter paper can be use but most precautions to be taken during handling of weight on butter paper. Weighing boat is best receiver as it can be use to collect material on balance pan also it is helpful during transferring the material in flask. There is different sizes of weighing boat are available we should select based on our requirements and capacity of balance load.
Weighing method:- As per USP chapter 1251, three methods are validated for weighing operation. These are as:-
Method-1
Place the receiver on centre of pan and tare the same with appropriate tare key. Add the material on receiver carefully and note the reading. Transfer the material in flask carefully and again place the receiver on the centre of balance pan. "Don't change the set tare weighing".
Note down the second reading which contains untransferred material. The difference between first weight and second weight is actual weight of transferring material.
Methods-2
Tare the balance. Place the receiver on centre of balance pan and add the material. Note the reading of receiver+material. Transfer the material in flask carefully and again place the receiver on centre of balance pan. "Don't change the set tare weighing". Note the reading of receiver+ untransferred material. The difference between first weight and second weight is actual weight of transferred weight.
Methods-3
This method is mostly used for quantitative analysis. Place the receiver on centre of balance pan and tare the same. Add the material on receiver carefully and note the reading. Transfer the material in flask carefully and rinse the receiver with diluent as all the materials completely transferred in flask. The first noted weight is actual weight of transferred material.
Weighing of liquid material:- As liquid materials are mostly volatile in nature hence more precautions to be taken. Keep the flask with some amount of diluent on centre of balance and close the balance doors properly. Tare the balance. Add the liquid sample with help of capillary or pipette as suitable by open the required balance doors. After material added, close the door of balance and wait for balance reading stable and note the reading.
For some liquid sample, method-1 can be use also.
Based on above described practices user can eliminate weighing and analytical errors. It is also important as every balance should be timely serviced and daily verification along with schedule calibration to be done.
Abhishek Singh is the author of Pharma technical support. He is pharmaceutical professionals, having experience of more than 12 years in Quality.