Particle Size Analysis: a decade in quality assurance
By Lydia McIntyre-Brown, Senior Marine Taxonomist and NMBAQC PSA Administrator
In a process that is affected by numerous variables, how do we produce data that are reliable and comparable?
Particle Size (PS) analysis is an essential tool for classifying sedimentary environments but with numerous laboratories and individuals undertaking the analysis how do we assure that data is reliable and comparable?
When I finished my degree in Oceanography and Marine Biology and entered the “real world” I never imagined that I would find myself involved in the Quality Assurance of PS analysis for accompanying biological data – a field that I have now been involved in for ten years!
PS analysis plays a fundamental role in classifying sedimentary environments and has a wide range of applications in many fields.
The size of the particles present has a profound influence on many other processes such as transport and deposition, chemical reactivity, porosity, permeability and entrainment which in relation to biological data all influence the species assemblages which live there.
The NMBAQC Scheme
Although quality assurance may not be the most inspiring of topics to write (or read!) about, it is vital for producing good quality, reliable and comparable data.
In the early 1990s work undertaken for the National Marine Monitoring Programme (NMMP) was completed by a variety of different laboratories, producing data for a centralised database held by the UK Environment Agency, it was agreed that a UK national quality assurance scheme was needed to QA this data and The North-East Atlantic Marine Biological Analytical Quality Control (NMBAQC) Scheme was launched in 1994. Although, at this time the N stood for National, in 2016 this was changed to NE Atlantic to reflect the wider scope of the scheme and be more inclusive to our European neighbours.
All UK Competent Monitoring Authorities (CMAs) and their contractors undertaking sample analysis for statutory marine monitoring programmes are required to participate within an externally run analytical quality control scheme – the NMBAQC fulfils this role.
The Scheme provides a source of external quality assurance for laboratories involved in the production of marine biological data contributing to UK national and EU monitoring programmes and also aims to develop and promote best practice in relation to sampling and analytical procedures through training exercises, workshops and literature guidance.
The Scheme is currently comprised of seven modules relating to different areas of analysis (Fish, Invertebrates, Particle Size, Zooplankton, Phytoplankton, Macroalgae and Epibiota); I have been involved in the running and development of the PS module since 2011. The PS module has two components: the PS ring test and the PS-OS (own-sample).
The PS Ring Test has been operating since the Scheme’s inception in 1994/95 and involves the distribution of pre-prepared artificial samples comprised of natural sediments and/ or commercial aggregates. Essentially each lab receives “identical” samples to analyse; results are collated, compared and participants are given feedback on their performance.
Particle size ring test samples ready for dispatch to participants
The PS-OS module, introduced by APEM in 2015, involves the re-analysis of participants in-house samples by the Scheme’s Benchmark Laboratory and offers a way to quality assure data on a sample or project-based level.
PS methodologies have evolved over the years and as technology has improved the use of sieves and high-tech laser diffraction analysers mean we can now measure particles from 90000 micrometres right down to 0.01 micrometres (that’s one hundred thousandth of a millimetre)!
However, this technology adds variables such as model used, particle refractive index and ultrasonics to name a few, that can have an impact on the final data distribution of a sample before you even add in human disparities in sample preparation.
The introduction of the NMBAQC Best Practice Guidance (Mason 2016) first introduced in 2011 has been instrumental in assuring that laboratories are following a standard procedure and along with a series of workshops, individual sample reports and annual reports help ensure that all participants are able to produce quality data and are able to get advice and feedback from experts when required.
The Best Practice Guidance is updated to reflect advancements in technology and methodology. In the time that I have been involved with the PS module of the NMBAQC Scheme there has been a vast improvement in the consistency and quality of data produced, with the majority of participants following the Best Practice Guidance and any recommendations provided in workshops and annual reports.
Cumulative particle size distributions showing the improvement in sample analysis from Scheme Year 17 (2011; sediment distributed as PS39) to Scheme Year 27 (2021; sediment distributed as PS78). The closer together the lines are the more similar the data produced are.
Although we may not always be happy with the initial result of QA analysis, getting data quality assured by external auditors is the best way to maintain standards and identify any potential errors or mistakes in data and correct them.
How APEM can help
APEM has unrivalled knowledge of the NMBAQC Scheme giving us the ability to offer extra project level audits for clients for both biological and PS data or samples NMBAQC Scheme Year 27 (2020/21) is almost complete however Scheme Year 28 (2021/22) is nearly upon us and there is still time for laboratories to sign up to participate.
To find out more about the NMABQC Scheme visit the Scheme website or to find out more about the services APEM can offer in terms of project level PS audits contact the team.
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The scientific community is full of acronyms; however, when it comes to quality control few are as important as NMBAQC