Standard operating procedure SOP for the assay of paracetamol tablets

The terms defined above are perhaps illustrated by using the example of the simple assay which we have mentioned before. The assay in Box 1.3 is laid out in the style of a SOP. This particular section of the operating procedure describes the assay itself but there would also be other sections in the procedure dealing with safety issues, the preparation and storage of the solutions used for extraction and dilution, the glassware required and a specification of the instrumentation to be used.

Box 1.3 Extract from a standard operating procedure for the analysis of paracetamol tablets

8. Assay procedure

8.1 Use a calibrated balance

8.2 Weigh 20 tablets

8.3 Powder the 20 paracetamol tablets and weigh by difference a quantity of tablet powder equivalent to 125 ± 10 mg of paracetamol

8.4 Shake the tablet powder sample with ca 150 ml of acetic acid (0.05 M) for 10 min in a 500 ml volumetric flask and then adjust the volume to 500 ml with more acetic acid (0.05 M).

8.5 Filter ca 100 ml of the solution into a conical flask and then transfer five separate 5 ml aliquots of the filtrate to 100 ml volumetric flasks and adjust the volumes to 100 ml with acetic acid (0.05 M)

8.6 Take two readings of cach dilution using a UV spectrophotometer and using the procedure specified in Section 9

The assay described in Box 1.3 assesses the precision of some of the operations within the assay. If a single analyst was to assess the repeatability of the assay, instructions might be issued to the effect the assay as described was to be repeated five times in sequence, i.e. completing one assay before commencing another. If between-day repeatability were to be assessed the process used for determining the repeatability would be repeated on two separate days. If the within-laboratory reproducibility were to be assessed two or more analysts would be assigned to carry out the repeatability procedure. In arriving at a SOP such as the one described in Box 1.3 there should be some justification in leaving out certain steps in the complete assay. For instance, weighing is often the most precise step in the process and thus repeat weighings of samples of tablet powder would not be necessary to guarantee precision; the precision of the extraction might be more open to question.

Each of the sections within an assay would have other SOPs associated with them governing, for instance, the correct use and care of balances as listed in Box 1.4.

Box 1.4 Procedure for the use of a calibrated balance SOP/OOI A/01

This balance is a high-grade analytical balance. It carries out internal calibration but as a double check it is checked with certified check weights. Any deviation of (he check weight values from those expected indicates need for servicing of the balance. Check weight calibration should be carried out once a week according to the instructions in SOP/OOIC/OI.

Caution: The logbook (form SOP/OOI AR/01) must be filled in. Any spillages on the balance must be cleaned up immediately and recorded in the log. This balance is to be used only for analytical grade weighings.


1. When carrying out weighing of amounts < 50 mg use tweezers to handle the weighing vessel.

2. Make sure the door of the balance is shut. Switch on the balance and allow it to undergo its internal calibration procedure. When it is ready the digital read-out will be 0.0000. Wait 30 s to ensure that the reading has stabilised.

3. Introduce the weighing vessel onto the balance pan. Close the door. Wait 30 s to ensure that the reading has stabilised and then send the reading to the printer.

4. If the tare is used in the weighing procedure, press the tare button and wait until the balance reads 0.0000. Wait 30 s to ensure that the reading has stabilised. If it drifts, which under normal circumstances it should not, press the tare button again and wait for a stable reading.

5. Remove the weighing vessel from the balance, introduce the sample into the vessel and put it back onto the balance pan. Close the door and note the reading.

6. Remove the sample and adjust the sample size to bring it closer to the required amount. Re-introduce the sample onto the balance pan. Close the door and note the reading.

7. Repeat step 5 until the target weight is reached. When the required weight is reached wait 30 s to ensure that the reading has stabilised. Send the reading to the printer.

N.B. An unstable reading may indicate that moisture is being lost or gained and that the sample must be weighed in a capped vessel.

Date of issue: 6/10/95 Signature:

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