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IP 570/13a recommends verification procedure

Stanhope-Seta’s Proficiency Testing Scheme is now a recommended practice in IP 570

A new verification procedure has been recommended in IP 570/13a Determination of hydrogen sulphide (H2S) in marine fuels – Rapid liquid phase extraction method. Participation within the PT Scheme is stated as the preferred means to achieve this recommendation.

Verification of the performance of method IP 570 is paramount, however use of verification materials is difficult. Apart from being a toxic gas, H2S is highly reactive and, with a high vapour pressure and very low boiling point, can easily be lost during sampling, storage or measurement. The medium to long term storage of verification samples containing H2S is therefore problematic due to potential sample degradation.

The Seta R & D Team developed a novel technique to stabilise a solution containing hydrogen sulphide and produce a stable verification fluid that enables users to quickly ascertain the performance of their H2S analyser. Following positive feedback from the Energy Institute SC G-5 H2S Task Group, experiments continued to try to extend the shelf life and in December 2012 the first production batch was supplied to willing participants in a formal ruggedness study.

In the first successful study of its kind 23 laboratories participated from locations in the European Union, UK, USA, Asia and Africa. Participants were asked to test samples within a time period and enter data into a specific website to allow analysis and review. Two tests on each sample were requested in order that repeatability could be ascertained. A z-score was then calculated for each lab to indicate how close they were to the population mean. When a z-score is outside -2 to +2 it is recommended that the lab should carry out an investigation to understand why. There were four outlier laboratories overall, and four labs which failed the z-score test. The remainder of the labs had good precision and accuracy.

The results were presented at the January 2013 EI H2S TG meeting at the Energy Institute. In a parallel study, using the same batch of material, it was demonstrated that the shelf life could be extended to 3 months from the date of manufacture. However, the logistical challenge of getting the verification fluid samples through the strict QC tests, delivering it to users and testing it in time still existed. To address this issue a proficiency testing scheme (or PT Scheme) was proposed.

IP 570 Proficiency Testing Scheme

A PT Scheme offers regular samples with data gathering and statistics to show precision, which has many advantages. For example additional information can be gleaned such as a laboratory ‘participant’ mean which would be a good indicator for sample degradation. At the end of the data gathering exercise a published report provides participants with all the results so labs can see what the norm is and evaluate their own performance in carrying out IP 570.

It is worth noting that ISO 17025 (General Requirements for the competence of testing and calibration laboratories) states that the monitoring of tests and calibrations can be undertaken by regular use of certified reference materials, secondary reference materials or participation in an inter-laboratory comparison or proficiency testing programme.

Seta launched the H2S IP 570 Proficiency Testing Scheme at the start of 2013 with monthly reports provided for participating laboratories. In the 2013 June & October EI H2S Task Group meetings the results were displayed and discussed. In summary 90% of results pass the z-score test.

A questionnaire is sent to participants whose results fail the z-score test to enable them to take a logical approach to problem solving. In general, follow-up investigations are successful in identifying a problem with labs who participate monthly subsequently achieving a good z-score the month after an issue is identified. Looking at the cross–monthly data there are no signs of sample degradation due to shipping or handling with the calculated H2S value, quality control mean and laboratory participant mean all within good agreement from month to month.

For further information

Visit the IP 570 PTS web page

Visit the EI for test method IP 570/13a

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