About some basic concepts for metrology in analyti

Authors: Paul De Bièvre

Publish Date: 2012/05/11

Volume: 17, Issue: 3, Pages: 355-356

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Abstract

There should be renewed attention for ‘counting’ as form of measurement 1 to serve for example in microbiology where counting of bacteria is important or in studying chemical reactions between protein molecules where counting ‘active sites’ on the surfaces of these molecules is revealed to be a simple way to measure the potential for interacting of these molecules in the human body In such cases expressing measurement results in terms of the quantity ‘mass’ and its units—be it kg or mg—or of the quantity ‘amountofsubstance’ and its units—be it mole or mmol is not very helpful Doing that in terms of ‘number of entities’ entry 14 Note 3 in 2 with unit ‘one’ symbol ‘1’ entry 110 Note 3 in 2 is more appropriate and … simpler Thus it is good to remind ourselves of the ‘particulate’ or ‘granular’ structure of matter an inherent property very different from the property of ‘mass’ It entails the usefulness and desirability of—indeed—counting of things atoms molecules ions bacteria active sites but also phenomena such as events all of them being covered by the generic term ‘entities’ Entities are nothing new we have been measuring radioactive decay in terms of number of events in becquerel or in terms of number of decay products such as number of daughter nuclides We are thinking in terms of a number of periods of a specified radiation in the 133Cs atom or express ‘indications’ entry 41 in 2 in counts per second in digital outputs all of them covered by the term ‘entities’On the macroscopic scale ‘counting’ must surely have been the earliest—and the easiest—form of measurement counting cattle soldiers horses apples or coconuts which determined degree of richness or power A more modern application of counting occurs in banks daily and on a massive scale counting currency unitsThat was ultimately recognized in analytical chemistry For centuries matter was looked upon as something being ‘continuous’ Although Demokritos in ancient Hellas implied already the ‘granular’ nature of matter it took Dalton Avogadro Cannizzaro Berzelius Lavoisier and other chemists about two thousand years later to definitively establish that matter had a ‘granular’ structure That was even before the physicists coined the very concept of the modern ‘atom’ Hence it was logical—and simple—to arrive at an appropriate unit for counting a number of particles generic name entities ‘one’ Granularity being recognized as a phenomenon in nature the concept ‘one entity’ to describe the molecular world appears difficult when we have to study and describe the interaction of very large numbers of molecules However that can be easily remedied by defining a fixed multiple of the natural unit ‘one’ and establish that as a convenient unit for handling and describing these interactions on the macroscopic level For convenience we can choose to give that number a name in the mind of an analytical chemist that is a ‘mole’ symbol ‘mol’ We can replace counting a very large number by measuring ratios of very large numbers It reduces the problem of measuring very large ratios to measuring numberratios An example of that is what we do in a titration The ‘equivalence point’ tells us when a large but unknown number of ions is equivalent in fact equal to a large but known number of other ions yielding the simplest numberratio of all 1/1 ie ‘1’ In isotope dilution mass spectrometry IDMS we do something similar we compare a large but unknown number of atoms of one isotope of an element in the “unknown sample to be measured” to a known number of atoms of another isotope of the same element called a ‘spike’ present in what is usually called a spike solution Also here we have an “equivalence point” or indicator of equality of large numbers a numberratio 1/1 ie ‘1’ Present isotope mass spectrometers are ‘measuring systems’ entry 23 in 2 in which the indications entry 41 in 2 of ion currents are linear over a large range and that enables to measure ratios of large numbers of the isotopic atoms involved differing from that numberratio 1 by several orders of magnitude for example 1/99 or 999/1 which can be measured easily and very precisely even very accurately if calibrated because the quantities being measured have now become electric currents and their ratiosThinking in ‘number of entities’ brings a very clarifying insight to the modern analytical chemist it does not need the concept of ‘amountofsubstance’ we did not need it in the above text In addition ‘amountofsubstance’ has sometimes been described as ‘the quantity of which the mole is the unit’ That is a reasoning which requires to choose the unit the mole and define it before the related concept ‘amountofsubstance’ can be described The logical here should be that we first choose and describe the quantity we have in mind before we assign a unit to itThe introduction of intercontinentally agreed definitions of concepts such as ‘quantity’ ‘base quantity’ ‘base unit’ in VIM 2 is of great help in clarifying our thinking and in our description of chemical phenomena as well as in expressing measurement results in analytical chemistry

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