Home > Informatics > Instrument Qualification

Instrument Qualification

Be sure your new instrument is fit to meet regulated industry requirements

Siri H. Segalstad

I have always claimed that, if you write a good user requirements specification (URS) and assess the answers from the suppliers and then choose the best one, you will get what you need. I realized that it is not always so when a former colleague of mine recently approached me with her problem:

They had written a good URS for a new pH instrument, and included a requirement that the supplier would be responsible for the installation qualification (IQ), operation qualification (OQ) and performance qualification (PQ). The chosen supplier had said yes to all requirements and the instrument was purchased.

Unfortunately, the only “qualification” supplied was some documentation of measurements for one pH electrode. The instrument was to be used with a large number of different electrodes but none of these had any documentation whatsoever. As to the instrument itself, there was no other qualification documentation, only the manufacturer’s specification.

This, of course, should be verified in a qualification if you don’t trust the supplier. In this case, I am not sure I would. However, if they had been more professional and known more about qualifications, I might have trusted them.

It turned out that the supplier, or maybe just the salesperson, did not know anything about qualifications. This does intrigue me, as I thought that qualifications and validations would be well-known issues to the instrument suppliers by now. After all, they all deliver to the pharmaceutical industry, to accredited laboratories, and to other industries with demands for validation or qualification.

My colleague problem is actually two different problems:

• The first one is that the supplier did not deliver what he had promised. This is a business problem and has to be dealt with by the purchasing department or the company’s lawyers.
• The second one is to make sure that the instrument indeed is “fit for fight” in the regulated industry. As the supplier obviously is not able to deal with that, this will have to be done by the purchasing company.

pH meter qualification
Detectors for electrical characteristics often use electrodes which measure the difference between two solutions, where one often is a part of the electrode. The measurements can be:

• Current — Ampere A
• Potential — Volt V
• Resistance — Ohm Ω
• Conductivity = 1/Resistance — Ω^-1 = Siemens

A pH meter is not really measuring acidity, but the potential between the electrode and the solution, and then calculated into pH. Its detector measures the concentration of hydrogen ions as a potential, and calculates this to pH as the definition of pH = - log [H⁺].

So, the pH meter is actually a voltmeter using a high-resistance electrode. It is calibrated by dipping the electrode into buffers and adjusting the output. The buffers are commercially available, and should be traceable to a national standard. pH is temperature-sensitive, so the measurements should be done at the same temperature. The instrument usually has the possibility to adjust its values according to temperature. If the temperature of the electrode, the sample or the buffers is different, the result will be inaccurate.

The qualification of the pH meter will include qualifying the temperature accuracy, voltage accuracy, pH accuracy, signal/noise ratio and, finally, a holistic accuracy test for the complete instrument. The electrodes need to be checked as well before taken into use, but also regularly during use, as they tend to deteriorate over time.

pH meter procedure 
Each instrument will need a procedure where requirements calibration, maintenance and use is stated — and documentation to prove that it has been followed. The procedure shall describe how often and how the pH meter shall be calibrated and maintained. Usually a pH meter is calibrated before use every day, using two or three buffer standards that will cover the whole area for the samples to be measured. One of them is always a pH 7 buffer. Furthermore, the electrodes need their regular checks to see if they are still working as they should.

Method qualification 
Even if the instrument works as intended, the method of using the instrument needs to be qualified. pH is easy to measure in water. However, with salts, oils or other chemicals added, it may not show the correct value. The qualification should prove that the measurement for this matrix is correct and, if not, it should attempt to show the error factors. Parameters for method validation include:

• precision (repeatability and reproducibility)
• accuracy
• selectivity
• linearity
• range
• limit of detection
• stability
• ruggedness

Documentation of use 
The documentation of the daily use of the instrument includes which batches of buffers have been used and which electrode was used for the measurements. The instrument log also will have the information on calibration and maintenance activities.

For further information 
You can find information on instrument qualification in several books, including my own International IT Regulations and Compliance. There are also specialized books available, like Chung Chow Chan, Y. C. Lee, Herman Lam, Xue-Ming Zhang (Editors): Analytical Method Validation and Instrument Performance Verification (Wiley 2004).

Siri Segalstad is Principal, Segalstad Consulting AS and author of International IT Regulations and Compliance (Wiley, 2008). She may be reached at editor@ScientificComputing.com.