Commutability is the hallmark of all measurement science and is particularly important in analytical chemistry at large. Commutability is the feature of being comparable across borders and universal over all time.
Using appropriate and traceable standards/reference materials during calibration combined with using validated and standardized methods provides for commutability.
Since the mid 1980s, the Environmental Protection Agency (EPA) has pushed the forefront of standardizing assays and methods in testing then delivering validated and standardized methods for all laboratories to us throughout the United States and in the world. Shortly thereafter, the Food and Drug Administration (FDA) developed Good Laboratory Practices (GLP) whereby a method has to be proven as valid, standardized and then used throughout industry for a particular purpose. The United States Pharmacopeia (USP) has published a stringent set of guidelines that govern the validation and standardization of methods.
Contrast this with the world of forensics.
For example, blood ethanol analysis by way of Headspace Gas Chromatography using a Flame Ionization Detector (HS-GC-FID) is a routine method of analysis and perhaps the most frequently run forensic assay in the US today. Yet, unlike EPA, GLP or USP methods, there are no standardized methods, instrumentation or assays developed and implemented in the testing world. Some laboratories conduct ethanol determinations by enzymatic assay, others use direct liquid injection on packed columns by way of GC-FID. Still others use single column GC-FID. One laboratory runs GC-FID with MS. Not all laboratories use dual column HS-GC-FID. Those that do run dual column HS-GC-FID some consciously decide not to quantitate on two columns. What constitutes a batch as well as what constitutes proper Quality Control is never uniform among laboratories. Acceptance criteria vary in a seemly arbitrary way. Should there be blanks between unknowns? Should inorganic salt be used? How much dilution should be preformed by way of ISTD?
And the list goes on and on showing a great amount of variance between and among laboratories to the point that the core question remains: where is the commutability of the number generated USP, VIM 3 and TAM sense of the word?