Metrological Traceability: Why Two Labs' Numbers Are Not Comparable Without It

Two laboratories measure the same vial and report two numbers. Both are internally consistent, both were produced carefully, and there is no way to tell which is closer to the truth — or whether the difference between them is real. The property that would settle it has a name, and most research-grade measurement does not have it.

Quick answer

What is metrological traceability? An unbroken chain of comparisons linking a measurement result back to a stated reference, usually an SI unit, with the uncertainty of every link documented. A 2023 study developing a certified reference material put the consequence plainly: quantitative results without traceability make comparisons between different studies difficult. The same work notes that establishing traceability to the SI unit of mass gives a material the potential to anchor a traceability chain, which makes quantification results standardised and comparable regardless of which quantitative method was used. That last clause is the whole point — traceability is what lets different methods arrive at numbers that mean the same thing.

What the chain is made of

Each link is a comparison with a documented uncertainty. A simplified version for a peptide quantity:

The chain is only as good as its weakest link, and a link with no documented uncertainty does not merely weaken it — it breaks it. Once one comparison in the sequence has no stated uncertainty, the uncertainty of everything downstream cannot be calculated.

Why it is harder for peptides than for simple chemicals

For a small molecule of known structure and high purity, the route is relatively short. For a peptide it is not, for three reasons that recur throughout this literature.

• Assignment is indirect. Characterising peptide standards in the cited work required oxidation and hydrolysis of the peptide, separation of the sulfur-containing amino acids, and isotope dilution mass spectrometry against an isotopically labelled hydrolysate — several steps, each contributing uncertainty.
• Conversion factors enter the chain. A reference method for quantifying total protein content in a food matrix identified the use of conversion factors to translate measured amounts into the reported quantity as one of its explicit challenges, alongside the estimation of uncertainty contributions and the combined uncertainty of the final result. Any step that multiplies by an assumed factor carries that factor's own uncertainty forward.
• Standards are scarce and variable. Only a few peptide standards are commercially available, and published work describes them as suffering from varying purity — covered in reference standards.

The practical distinction that matters most

There are two different things people mean by a reliable number, and conflating them causes most of the confusion.

For most research use, internal consistency is what the work actually requires, and it is achievable without any traceability chain at all. That is an important and liberating point: you do not need SI traceability to run a valid comparison between two lots. You need one defined reference, used consistently, with records.

What you cannot do without traceability is claim your number is the same quantity as a number from somewhere else — a supplier's certificate, a published paper, a collaborator's dataset. Those comparisons are approximate at best, which is the reasoning behind the advice in lot-to-lot variability and bridging to compare within a supplier rather than across suppliers.

What a research buyer can actually build

• A defined working reference. One well-characterised lot, measured carefully, used as the comparator for everything after it. Not traceable, but consistent.
• A written definition of it. Which lot, measured when, by what method, stored how. A reference nobody can describe is not a reference.
• A review interval. References degrade like everything else, and a certified material in the cited work had stability supported for months rather than years.
• The method recorded alongside every number. This is the single cheapest thing that preserves meaning. A value without its method is not reproducible even by you.
• Honest labelling of cross-source comparisons. When comparing your measurement to a certificate, record that the two were produced by different methods against different references. That is not a weakness in the record; it is the record being accurate.

Key takeaways

• Traceability is an unbroken chain of comparisons to a stated reference, with the uncertainty of every link documented.
• Published work states that results without traceability make comparisons between different studies difficult, and that traceability makes results comparable regardless of the quantitative method used.
• A link with no documented uncertainty breaks the chain, because downstream uncertainty becomes incalculable.
• Peptide chains are long: indirect assignment, conversion factors and scarce standards each add uncertainty.
• Internal consistency and traceability are different properties. Most research work needs the first, which is achievable without the second.
• Without traceability, cross-source comparisons — against a certificate, a paper, a collaborator — are approximate.
• A defined working reference, a written definition, a review interval and the method recorded with every number get a buyer most of the practical benefit.

Frequently asked questions

Do I need traceable measurements for research work?

Usually not. If your conclusions rest on comparisons within your own work, a consistently used internal reference is sufficient and far more achievable. Traceability becomes necessary when a number has to be meaningful to someone outside your laboratory.

Is a supplier's certificate traceable?

For research-grade material, generally no, and it will not claim to be. The number is valid within the supplier's own system. That is why comparing lots from one supplier is more informative than comparing suppliers.

What is a primary method?

One that produces an absolute quantity without needing a reference material of the same substance — quantitative NMR against a certified internal standard is the example discussed in how much quantification methods agree. Primary methods are what anchor the top of a chain.

Why do conversion factors matter?

Because they are assumptions that enter the arithmetic silently. If a measured quantity is multiplied by a factor to reach the reported quantity, the factor's own uncertainty travels with it, and the published reference method cited here names that as an explicit challenge.

Does any of this change how I buy?

Mainly it should change what you expect a certificate to settle. It establishes what the supplier measured, by their method, against their reference. Whether that matches your bench measurement is a separate question, answered by measuring — see measuring peptide concentration.

Sources

• Zhu M, Li J, Chu Z, Li L, Meng B, Zhao Y, et al. Development of cancer biomarker heat shock protein 90α certified reference material using two different isotope dilution mass spectrometry techniques. Anal Bioanal Chem. 2024;416(4):913–923. DOI
• Martinez-Esteso MJ, O'Connor G, Nørgaard J, Breidbach A, Brohée M, Cubero-Leon E, et al. A reference method for determining the total allergenic protein content in a processed food: the case of milk in cookies as proof of concept. Anal Bioanal Chem. 2020;412(30):8249–8267. DOI
• Schaier M, Hermann G, Koellensperger G, Theiner S. Accurate characterization of β-amyloid (Aβ40, Aβ42) standards using species-specific isotope dilution by means of HPLC-ICP-MS/MS. Anal Bioanal Chem. 2022;414(1):639–648. DOI

Citations retrieved via PubMed. These studies concern proteins, amyloid peptides and food matrices; the metrology transfers directly, the specific materials do not.

This article describes analytical measurement only. It contains no usage instructions, no dosing information and no protocols of any kind.

For laboratory and in-vitro research use only. Not for human or veterinary use.