standard operating procedures

SOPs – Standard Operating Procedures – A General Overview

Aim

This document is not meant to be all encompassing but is designed to give readers an overview as to what should be in a SOP and what a SOP might look like.

Background

It was common in the past for to be photocopied from a book and notes/scribbles to be added to a method to adapt it to a Laboratory’s specific needs. This resulted in analysts using different versions of the same method and obtaining different results. It also hindered the communication of improvements that an individual analyst had made to a method, as it was not known how many copies of a method existed and whether they had all been updated to include the improved method.

A way to improve this chaos is to ensure that all methods and procedures shall be documented to the extent necessary to ensure proper implementation and consistency of application. Hence the introduction of the concept of SOPs – Standard Operating Procedures.

It is sometimes not practical for laboratories to use an SOP which is fully documented specifying each type and individual test procedure. For example, analysis of soil samples for polyaromatic hydrocarbons. It is simpler to write an SOP on the preparation of soil extracts for analysis by gas chromatography and another SOP of a more general nature regarding the analysis of extracts by gas chromatography.

Information required for all SOPs

All SOPs should contain the following information

Authorised by (Someone in the organisation needs to approve the SOP before it can be used)

·Copy number (A record of the number of copies of an SOP needs to be kept – each individual copy has its own number – identifies if there are any illicit versions in use )

Page x of y (e.g. Page 1 of 6 – appears at the bottom of each page – identifies if there are pages missing from a method)

Other information may include when the method should be reviewed, individual reference numbers for when an organisation has many SOPs, Issue numbers – to identify new versions of a method from old versions of a method which have been withdrawn.

Title

Determination of A analyte(s)or measurand(s)}, in C {matrix} using D {principle}

Scope

Enables a user to see quickly whether the method is appropriate for a desired application (see also the Annex- method validation and measurement uncertainty).

• the analyte (measurand) which can be determined by the method,
• the sample matrix from which the analyte (measurand) may be determined,
• the technique used in the method,
• the minimum sample size,
• known interferences which prevent or limit the working of the method,
• the concentration range of the method.

*It may be more appropriate to have a separate section for interferences.

Warning & Safety Precautions

handling the samples

handling or preparing solvents, reagents, standards, or other materials

operation of equipment

requirements for special handling environments, e.g. fume cupboards

Definitions

Define any unusual terms, use ISO definitions wherever possible. Quote sources. Include chemical formulae/structures if appropriate.

Principle

Outline the principle by which the analytical technique operates. This section should be written so that a summary of how the method works can be seen quickly.

Reactions

Where appropriate to clarifying the working of the method or calculations, include details of any relevant chemical reactions. This may be relevant, e.g. where derivatisation is involved, titrimetry etc..

Reagents & Materials

List all of the reagents materials, blanks, QC samples and standards and certified reference materials required for the analytical process, identified by letter or number. List:

• details of any associated hazards including instructions for disposal,
• appropriate grade of reagent/chemical,
• specialist suppliers for rare chemicals,
• need for calibration and QC materials,
• details of preparation, including need to prepare in advance,
• containment and storage requirements,
• shelf life of raw material and prepared reagent,
• required concentration, noting whether w/v, w/w or v/v,
• labelling requirements.

Points to note from example:

Each reagent is uniquely identified so as to avoid confusion.

Weigh 20.00 g… Indicates that a balance capable of weighing to 2 decimal places is required.

20.00 g ± 0.25 g… Tells the analyst that he needs to be between 19.75 g and 20.25 g.

Using a 50 cm3 measuring cylinder… Indicates accuracy required for this procedure.

Quantitatively transfer… Indicates that you need to transfer all the solid with washings to the beaker.

Appropriately labelled… This might include: Name of reagent, Concentration of reagent, Date reagent made, Expiry date of reagent, Name of analyst who prepared the reagent – Most computer systems can be used to produce sheets of labels that suits an individual’s and/or organisation’s needs.

Apparatus & Equipment

Describe in sufficient detail the individual pieces of equipment and how they are connected to enable unambiguous set-up. For glassware include grade where applicable. Include environmental requirements (fume cupboards etc.).

It may be appropriate to reproduce the relevant parts of the instruction manual in the SOP – i.e. how to set up and use the spectrophotometer. This will depend on how easy it is to get access to the manual.

Sampling & Sample Preparation

Include sufficient detail to describe how the test portion is obtained from the sample as received by the laboratory. All information on the precautions to be taken to minimise the risk of contamination of the samples needs to be included. Include information regarding the storage, conditioning and disposal of samples. Many laboratories will write a separate document about the receipt, labelling, storage and disposal of samples to save having to duplicate this information on individual SOPs.

Environmental Control

List any special environmental conditions and cross reference to safety section, e.g. temperature control.

Calibration

Identify the critical parts of the analytical process. These will have to be controlled by careful operation and calibration. Cross reference to the relevant sections above. It may be easier to incorporate this section with the analytical procedure section.

Include calibration of equipment – what needs to be calibrated, how, with what, and how often?

It is normally easier to write separate documents to cover the calibration of balances, automatic pipettes, thermometers, refrigerators, freezers and spectrophotometers to save having to put all this information into each SOP.

Quality Control

Explain the quality control procedures, frequency of quality control checks during batch analysis, pass/fail criteria, action to take in the event of a failure. It may be appropriate to incorporate this section in the analytical procedure section.

Analytical Procedure

Describe the analytical procedure, cross referencing previous sections as appropriate including reagents (with the appropriate identifier), apparatus and instrumentation. Indicate at which point in the analytical procedure the quality control, and calibration procedures should be performed.

Points to note:

If any of the samples are highly coloured or turbid…. Guidelines on how to handle “difficult” samples

Using an automatic pipette… Defines the accuracy that the reagent has to be measured to

For effluent samples or samples which are likely to contain significant levels of nitrite… Guidelines on how to handle samples whose nitrite concentration will probably exceed the range for the method.

Leave the colour to develop for 30 to 60 minutes – This implies that the colour development time is important. You will have needed to carry out some checks to show that these times are satisfactory.

Any samples that give concentrations greater than 0.25 mg dm-3, must be discarded and the analysis repeated using a diluted sample… Only concentrations in the working range of the method should be reported.

Consult a senior officer… There will always be times when the analyst encounters problems or unknown situations – the analyst should be encouraged to seek help from an experienced member of staff.

Log Books

Information to be recorded in the log book associated with spectrophotometer might include – serial number, service dates, calibration checks (e.g. wavelength), repairs (e.g. new lamp).

On a daily basis information recorded might include date, name of analyst, analysis (e.g. nitrite), wavelength used, absorbance of blank, concentration and absorbance of standard.

At the front of the log book should be sufficient information regarding the nitrite analysis for the operator to determine if there is a problem, e.g. nitrite analysis, wavelength 540 nm, acceptable absorbances of (a) blank 0.000 ± 0.005, (b) 0.25 mg dm-3 nitrite standard 0.250 ± 0.010. Repeatability measurement on the same solution ± 0.003.

Log Books; points to note:

Use of a standard, quality control and blank and a replicate sample and recording the information in a log book means that you can demonstrate with reasonable certainty that (a) the reagents were made up correctly, (b) the standards were made up correctly, (c) there was no contamination present, (d) the spectrophotometer was set up correctly and (e) the analysis can be repeated and give a result to within the expected limit.

Calculation

Lay out the formulae for calculating the results ensuring all terms are clearly defined and derived. Specify requirements for checking, cross reference to QC requirements and indicate how corrections should be dealt with.

Reporting Procedures Including Expression Of Results

Indicate how results should be reported, including; rounding of numbers; final units. It may be appropriate to indicate the level of associated with the method.

Precision , accuracy and detection limits

Literature References

References may include operating manuals as well as published methods and standards.

Annex – Method Validation and Measurement Uncertainty

Methods should be supported by experimental data providing information on accuracy, precision and selectivity . The major sources of uncertainty, relating to the method should be identified and the assigned values listed. The overall uncertainty should be listed together with an explanation of how it was derived.

In the case of the nitrite method that was based on a published method, the validation work involved:

• Reproducibility – showing comparable results were obtained.
• Repeatability – showing in the results obtained for a known sample measured five times on the same day.
• Making up a set of 5 standards to show that the calibration was linear for the range 0.01 to 0.25 mg dm-3 NO2.
• Checking the detection limit using spiked samples .
• Checking that the time interval quoted for the colour development time was correct and non-critical within the stated range.

We also looked at the method to identify major causes of variation in results – e.g. measuring the sample volume with a measuring cylinder. We found that the biggest variation was in different analysts making up the ‘same’ standard solutions – hence the need to define an acceptable absorbance range for the standard.