Operation and Calibration of Analytical Balance

Analytical Balance (Calibration) Standard Operating Procedure (SOP) for operation and calibration of analytical balances. As per USP, balance calibration parameters are Internal adjustments, Linearity, Sensitivity, Repeatability (Uncertainty), and Accuracy, etc. 

SOP for Analytical Balance: Operation and Calibration

1.0      PURPOSE

    • The purpose of this SOP is to describe the procedure of Operation, Calibration, and Maintenance of Analytical balance in the Quality Control Department.

2.0     SCOPE

    • This SOP is applicable to the analytical balances for operation and calibration of balances in the Quality Control Department.

3.0     REFERENCES :

    • Operation Manual supplied by the manufacturer.
    • SOP for Handling of Out of Calibration
    • SOP for Maintenance of Laboratory Instruments
    • USP<1251> Weighing on an analytical balance.
    • SOP for Preparation of internal and external (Third Party) Calibration schedule and calibration practices.
    • 21 CFR 211.68
    • SOP for Instrument / Equipment usage log book.

4.0     RESPONSIBILITY :

    • QC Analyst shall be responsible for-
    • The operation, calibration, and maintenance of the instrument as per the SOP.
    • Maintaining the history card, Instrument usage logbook, and weight prints as per the SOP.
    • QC Head shall be responsible to ensure that the instrument is operated, calibrated and maintained and the related documentation is done as per the SOP.
    • QA shall be responsible to ensure the implementation of the system as per the SOP.
    • Plant Head and Quality Head shall be responsible for the approval of the SOP.

5.0     ABBREVIATIONS :

    • “Cal Int”      :  Internal Calibration
    • “Cal done”   :  Calibration Done
    • IPA               :  Isopropyl Alcohol
    • NA                :  Not Applicable
    • NMT             :  Not More Than
    • NIST             :  National Institute of Science and Technology

6.0     DEFINITION :

    • Hysteresis: Hysteresis of the mechanical parts occurs during weighing. (Note: Hysteresis in the balance is caused by excessive stretching of the springs, and it is primarily due to overloading or to the accidental dropping of an object onto the pan.)

Also read: SOP for Audit Trail Review and Privilege Policy

7.0     PROCEDURE FOR OPERATION AND BALANCE CALIBRATION:

    • General Notes and Precautions:

    • The balance door should not be open during weighing.
    • Temperatures of the balance and the material to be weighed should the same.
    • Do not expose Hygroscopic material more time in the air during weighing.
    • Air currents should not present near the balance.
    • Level properly the balance.
    • Hysteresis of the mechanical parts should not occur.
    • Do not cause vibration during Laboratory operations.
    • Pick up the weight with a forceps, place it carefully on the pan.

    • Place the weight in the center of the balance pan to eliminate the corner-weighing difference.
    • Do not overload the balance pan more than the capacity of the balance.
    • Many chemicals, such as salts are corrosive, and material of this nature should not spill on the balance pan or inside the balance housing. 
    • Before weighing or calibration, the balance should exactly in a horizontal position (check the level indicator, which should be in the center).
    • If any unevenness is observed in its location. Correct it by turning leveling feet until the level bubble is in the center of the indicator.
    • The weight class chosen for the calibration should be such that the tolerance of the weights used does not exceed 0.1% of the amount weighed.
    • The weights used for calibration shall be calibrated against standard weights traceable to NIST.

    • All the weights taken should be ± 10% of the target weight as specified in the individual monograph.
    • The analyst shall always use the calibrated weight box for calibration purposes.
    • Calibrate weight boxes by outside agency yearly ± 30 days.
    • After the receipt of the calibrated weight box along with the calibration certificate limit for allowable tolerance limit (for external calibration) should be calculated as minimum and maximum weight from the limit in terms of % mentioned in Attachment-1 for all the weight which require for the calibration.
    • e.g. Actual mass value found after calibration is 200.0050 gm, then 199.8050 gm to 200.2050 gm is the minimum and maximum weight limit as per ±0.1% of the actual mass value.
    • Every time after calibration of weight box format of Annexure -1 shall be reprinted and implemented including following.
      • Minimum and Maximum limit calculated as above for all weights
      • Weight box no
      • Valid up to date
    • Similarly, Reprint Annexure -2 and implemented including the following.
      • Actual mass value & theoretical weight
      • Weight box no.
      • Valid up to date.
    • After calibration submits the raw data for checking.

    • The analyst shall update the calibration status label and make an entry in the instrument usage log book.
    • If the weight print facility of balance is not working, then inform the service engineer for rectification of this problem.
    • Till rectification of this problem, Check the weight during weighing and signed by the section head.
    • If print facility available attach the print out with a template.
    • In case the weight prints taken are not considered in the calculation ( e.g due to  unclear  visibility, misprinting, not as per defined statistics mode or weighing mode, etc.),  
    • Attach such weight prints in the template and write justification against the weight print on the respective template.
    • Take fresh standard/ sample weights for the batch to continue the analysis.
    • If the calibration status label is not provided by the certified agency then affix the label on the weight box as per Annexure-3.

(Note: The  above mentioned general notes and precaution are applicable to all the analytical balance available in the Quality control department )

    • Cleaning Procedure:

    • Remove powder if any, from inside the balance using soft nylon brush & wipe with clean cotton balls.
    • Remove weighing pan & wipe it with a soft nylon brush, if required, wipe with clean cotton soaked in 70 % IPA solution & dry the pan with cotton balls completely before mounting.
    • Wipe the carefully inner side of the balance with clean moist cotton with 70% IPA solution in water, without disturbing the balance & finally mop with clean dry cotton.
    • In case of dirty spots or stains on the body, use cotton balls soaked in 70 % IPA solution in water.
    • Operation procedure:

    • Ensure that balance is on firm, vibration-free position and in a room without temperature fluctuations.
    • Check that balance is exactly in a horizontal position, compensate it if any minor unevenness in its location by turning two leveling feet at the rear of the balance housing until the air bubble is in the center of the leveling control (internal circle).
    • Connect the power supply of instrument with the mains and turn power switch “ON”.

    • Press “ON / OFF” key, the display will show normal weight(0.0000g).
    • Allow the balance to warm up for initialization 30 minutes.
    • Press the “O/T” key of the menu bar, the display shows “0.0000 g”.
    • If we are working with butter paper/container, place it on the weighing pan, close the glass draft shield, then press the “O/T” key to tare the scale.
    • Press the “Print” key from the menu bar, to take a printout of the printer is attached.
    • Before executing the tests, the analyst should place the weights in the vicinity of the balance for an appropriate time to reach sufficient thermal equilibrium.
    • If possible, Carry out all tests with a single test weight in order to minimize handling errors, but multiple test weights are permitted.
    • Weighing Procedure – Good Weighing Practices :

    • Take appropriate size butter paper/receiver of with respect to the quantity of sample.
    • Add slowly the desired quantity of the sample to the center of the butter paper /receiver.
    • The weight taken should be ± 10% of the specified weight or as specified in the individual monograph.
    • Addition weighing – Good Weighing Practices :

    • Addition weighing is used for solid samples or liquid samples for which volatility is not an issue.
    • Place the empty butter paper / or receiver on the balance in the center of the pan, and press the appropriate tare (Zero) key on the balance (On stabilization of indicator).
    • Add the desired amount of material to the butter paper / or receiver, and allow the balance to display stabilize reading (On stabilization of indicator).
    • Record the weight and transfer the weighed material to the final flask or beaker as per requirements,
    • Then reweigh the original weighing butter paper / or receiver by placing it in the same position on the pan
    •  Note- Do not change the set tare of the balance between these two weighings.
    • The difference between the two weights represents the transferred material weight.
    • Dispense weighing – Good Weighing Practices :

    • Dispense weighing is used for weighing of emulsions or viscous liquids such as ointments.
    • Tare the balance (On stabilization of indicator), place the sample container on the balance in the center of the pan in a suitable container (bottle, tube, transfer pipette or syringe) and record the weight (On stabilization of indicator).
    • Transfer the desired amount of material to the final flask or beaker as per requirements, then reweigh the sample container by placing back on the pan of balance [ Note- Do not change the set tare of the balance between these two weighings].
    • The difference between the two weights represents the transferred material weight.
    • Liquid and volatile material weighing – Good Weighing Practices:

    • Weighing of liquid and/ or volatile material (which has a low boiling point) shall be done as per procedure defined below.
      • Place the empty receiver (In case of material is highly volatile, add some amount of diluent) on the balance in the center of the pan and press the appropriate tare (Zero) key on the balance (On stabilization of indicator).
      • Add the desired amount of material to the receiver.
      • On stabilization of indicator displayed weight will be transferred material, which should be printed and keep with the raw data.
    • Hygroscopic samples – Good Weighing Practices:
    • Hygroscopic material readily absorbs the moisture from the atmosphere and steadily gain weight.
    • Therefore hygroscopic samples shall either weigh promptly or placed in a vessel with gas-tight closure.
    • For a gas-tight vessel, the analyst shall tare the vessel and enclosure, add the desired amount of sample in the vessel and replace the enclosure and record the weight.
    • The difference between the two weights represents the transferred material weight.
    • Viscous liquid material weighing – Good Weighing Practices:

    • Place the empty receiver on the balance in the center of the pan and press the appropriate tare (Zero) key on the balance (On stabilization of indicator).
    • Add the material to the receiver, slowly so maximum require the volume to the vessel.
    • On stabilization of indicator displayed weight will be transferred material, which should be printed and keep with the raw data.
    • Semi-solid material weighing – Good Weighing Practices:

    • Place the empty receiver on the balance in the center of the pan and press the appropriate tare (Zero) key on the balance (On stabilization of indicator).
    • Transferred the approximate required quantity of semi-solid material to the receiver (by touching the inner surface of the receiver ) with the help of a capillary tube or spatula.
    • On stabilization of indicator displayed weight will be transferred material, which should be printed and keep with the raw data.

OR

    • Place the clean and dry glass coverslip on the balance in the center of the pan and press the appropriate tare (Zero) key on the balance (On stabilization of indicator).
    • Transfer approximate required quantity of semi-solid material to the glass coverslip with the help of a capillary tube or spatula.
    • On stabilization of indicator record the displayed weight (Display weight shall be printed and keep with the raw data ) and transfer the glass coverslip directly into the receiver.
    • Warm or cool samples – Good Weighing Practices:

    • Equilibrate the sample that is warm or cool in the laboratory to room temperature.
    • As if warm samples weighed then apparent weight is smaller than the true weight because of heat convention.
    • Aseptic or bio-hazardous materials weighing – Good Weighing Practices:

    • Perform the weighing of the sterile or bio-hazardous samples within the confines of the clean bench, biosafety cabinet, isolator or similar containment device.
    • Airflow within the hood potentially can cause balance instability, so after a balance has been installed under the hood, perform a rigorous qualification study with suitable weight artifacts in order to determine the acceptability of the balanced performance in this environment.
    • Corrosive materials weighing – Good Weighing Practices:

    • Many chemicals, such as salts are corrosive, and material of this nature should not be spilled on the balance pan or inside the balance housing.
    • Take extra care to weigh such type of material.
    • In the event of a spill re-qualification of balance may require, depending on the nature of the spill.

Analytical Balance Calibration

    • Balance Calibration Parameter :

    • The calibration shall consist of the following parameters:
      • Internal adjustments
      • Linearity
      • Sensitivity
      • Repeatability(Uncertainty)
      • Accuracy
    • Parameter wise Balance Calibration Frequency :

    • Internal Adjustment: Calibration Frequency:  Perform on every working day, after a power failure, after any maintenance and relocation of balance.
    • Linearity: Calibration Frequency:  Perform on every working day, after a power failure, after any maintenance and relocation of balance.
    • Sensitivity: Calibration Frequency: Monthly and after any maintenance and relocation of balance.
    • Repeatability (Uncertainty): Calibration Frequency: Monthly and after any maintenance and relocation of balance.
    • Accuracy: Calibration Frequency: Monthly and after any maintenance and relocation of balance.
    • Eccentricity: Calibration Frequency: Monthly and after any maintenance and relocation of balance.
  • Internal Adjustment – Balance Calibration:

    • If necessary, turn on the power, and allow the balance to equilibrate for at least 30 minutes before proceeding with the calibration.
    • Note : Perform internal calibration, If facility available in balance.
    • Level the balance.
    • Press the key ‘ cal’ given in the front panel of the balance until the display of ‘Cal-int’ is displayed.
    • Release the key. The instrument performs calibration automatically.
    • Wait till the message ‘cal done’ is displayed and Take the print out . if the printer is attached.
    • Sign the printout with the date and paste in the calibration template and complete the entry in Annexure-1.
  • Linearity – Balance Calibration:

    • Carry out the Linearity using 200 mg, 100 g, and 200 g weights.
    • Put the external weight of 200 mg on the pan and note the measurement in the Template (Annexure-1).
    • Put the weight 100 g and 200 g on the pan and record the weight in Annexure-1.
    • Deviation NMT ± 0.05% of the actual mass value.
  • Sensitivity – Balance Calibration:

    • Carry out the Sensitivity by using 200 g weights.
    • Put the external weight of 200 g on the pan and note the measurement in the template (Annexure-2).
    • Put the weight 200 g on the pan and record the weight in Annexure-2.
    • Deviation NMT +05% of the actual mass value.
  • Repeatability ( uncertainty) – Balance Calibration:

    • Carry out the measurement uncertainty by using 200 mg and 200 g weight.
    • Put the weight of 200 mg on the pan and note the 10 measurements in the Template of measurement uncertainty test (Annexure -2)
    • Calculate the measurement of uncertainty as follows.              

                                     Standard deviation x 2

Repeatability =         ———————————————

                                         Actual weight of the standard weight

Note: In case the calculated standard deviation of 10 replicate measurements is smaller than0.41d value then replace the standard deviation value with 0.41d value. Where “ d” is the scale interval (readability ) of the balance.

    • Repeatability should not be more than 0.001 or 0.10% and the measurements should be within ± 10% or 0.0010. of the actual weight
    • Carry out the same procedure for 200 g weight.
  • Accuracy checkBalance Calibration:

    • Carry out full-scale calibration by using 10 g, 100g, and 200 g.
    • Deviation NMT 0.10% of the actual mass value.
  • Balance Calibration – Eccentricity:

    • Take the 100 g weight and weigh 5 times on different places of the pan as shown in figure-1 (when the balance pan is circular) and figure-2 (When the balance pan is square) and record the observation in the Template for Corner test (Annexure-2).

Figure-1

Figure 1 - Eccentricity - Balance Calibration

    • Every four readings (other than central reading) should lie in the range of ± 0.05% of central reading and actual mass value of weight.

Figure-2

Figure 1 - Eccentricity - Balance Calibration

8.0     ANNEXURES FOR BALANCE CALIBRATION:

EDITABLE FORMATS

 

 

 

Also Visit: Calibration of UV Spectrophotometer

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Janki Singh is experienced in Pharmaceuticals, author and founder of Pharma Beginners, an ultimate pharmaceutical blogging platform. Email: [email protected]

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