Before reliable near infrared spectroscopic analysis - the critical sampling proviso. Part 2: Particular requirements for near infrared spectroscopy

Abstract

Non-representative sampling of materials, lots and processes intended for NIR analysis is often fraught with hidden contributions to the full Measurement Uncertainty MUtotal = TSE + TAENIR. The Total Sampling Error (TSE) can dominate over the Total Analytical Error TAENIR by factors 5-10-25, depending on the degree of material heterogeneity and the specific sampling procedures employed to produce the minuscule aliquot, which is the only material actually analysed. Part 1 presented a brief of all sampling uncertainty elements in the “lot-to-aliquot” pathway, which must be identified and correctly managed (eliminated or reduced maximally), especially the sampling bias, as a prerequisite to achieve fully representative sampling. The key for this is the Theory of Sampling (TOS), which is presented in two parts in a novel compact fashion. Part 2 introduces (i) application of TOS to process sampling, specifically addressing and illustrating how this manifests itself in the realm of PAT, Process Analytical Technology, and (ii) an empirical safeguard facility, termed the Replication Experiment (RE), with which to estimate the effective sampling-plus-analysis uncertainty level (MUtotal) associated with NIR analysis. The RE is a defence against compromising the analytical responsibilities. Ignorance, either caused by lack of awareness or training, or by wilful neglect, of the demand for TSE minimisation, is a breach of due diligence concerning analysis QC/QA. Part 2 ends with a special focus on: “What does all this TOS mean specifically for NIR analysis?”. The answer to this question will perhaps surprise many. There is nothing special that need worrying NIR analysts relative to professionals from all other analytical modalities; all that is needed is embedded in the general TOS framework. Still, this review concludes by answering a set of typical concerns from NIR practitioners.

Before reliable near infrared spectroscopic analysis - the critical sampling proviso. Part 1: Generalised theory of sampling

“…the TSE + TAE representing the entire from-lot-to-aliquot pathway, see section Replication Experiment (in Part 2 of this paper). 15 The focus in this review is on the interaction of lot (material) heterogeneity with the optional sampling processes available of which some are acceptable, many are notall to be revealed below. In fact, a specific nonrepresentative sampling procedure is bound to impact adversely on the quality of any analytical determination, very often to degrees significantly more influential than what is manifested by TAE NIR alone.…”

“…With TOS, the objective of conducting a Replication Experiments (see section Replication Experiment (in Part 2 of this paper)), 15 is specifically to include all sampling error-induced uncertainty complements, from the critical primary sampling stage as well as all sub-sampling and other operations taking place on the way to securing the final analytical aliquot. The proper focus below is universally "from-lot-to-aliquot".…”

“…This primary sampling task can only be performed based on the principles of TOS. Also: what is the spread of analytical results stemming from, say, 10 replicate primary samples (see section Replication Experiment (in Part 2 of this paper) 15 ) Photo credit: Jens Bo Holm-Nielsen (2005); reproduced here with permission.…”

“…If the provider of primary samples is not the analyst, as is often the case, after having read this review the analyst will be professionally TOS-informed and can therefore forcefully demand adherence to TOS in all stages before analysis from all agents involved! Section DS3077 (2013) "Horizontal sampling standard" (Part 2 of this tutorial) 15 presents the world's only international standard dedicated to applying the principles of representative sampling, in which further overviews of the interrelationships between TOS' Governing Principles (GP), Sampling Errors (SE) and Sampling Unit Operations (SUO) can also be found, with an exclusive view of practical sampling. Since 2013 this has been the de facto international sampling standard, 6 which is now (2022-2023) in the process of being inducted as an ISO standard.…”

“…In technological, industrial and societal application fields, crushing of the entire lot is almost never an economically viable option, for which reason considerable care and creativity must be invested at the primary sampling stage to be able to secure representative samples; here composite sampling is the only avanue. 2,5,15 Illustration copyright by KHE Consulting, brought here permission. Here again, manual grab sampling is a charicature, Figure 19 (right).…”