01Use a distinguishable reference that behaves comparably

Gerber and colleagues' original AQUA study used synthetic stable-isotope-labelled peptides as internal standards for tandem-MS measurements. The labelled reference mimicked a selected native peptide while remaining distinguishable by mass. This established a practical peptide-based route to targeted quantitative measurements in the systems studied.Gerber and colleagues — Absolute quantification by tandem MS (opens in a new tab)

The reference must be distinguishable in the measured channels, but chemical similarity is valuable because it can help the pair experience comparable separation, recovery and ionisation effects. A conveniently nearby peak from an unrelated compound is not automatically an equally suitable substitute.

Known amount is a substantive requirement. An internal standard with an incorrect concentration can introduce bias into every apparently well-normalised result. A good-looking chromatographic peak does not verify the concentration of the reference solution.

02See when a shared change cancels

In an original example, an analyte produces 800 response units and its internal standard produces 400. Their ratio is 2.00. If a shared effect halves both signals to 400 and 200, the ratio remains 2.00. This is the useful compensation principle.

The third row shows the limit: an effect that changes only the analyte does not cancel. The presence of an internal standard cannot guarantee equal behaviour. The relationship between signal ratio and amount ratio also needs an appropriate calibration or established response relationship.

A constant ratio is therefore helpful but not sufficient. Both channels could be weak, interfered with or outside their useful measurement range. Inspect the underlying signals as well as the divided result.

03Identify which steps happen before addition

Suppose an unlabelled peptide loses material during an early transfer and the internal standard is added afterwards. The standard did not experience that transfer, so the final ratio cannot reconstruct the lost fraction by itself. Moving the addition point changes the scope of what the reference can monitor.

Chemical form creates another boundary. In protein quantification, a free labelled peptide does not automatically follow the release of that peptide from an intact protein during digestion. In an assay of an already free peptide, that particular release step may not exist. The relevant workflow must be identified rather than borrowed from another application.

04Check suitability for the actual sample

Arnold and colleagues studied peptides used to measure retinoid-metabolising proteins in biological matrices. Calibration and internal-standard choices affected quantitative results, and matrix effects differed between peptides. Their primary findings support validating the chosen pair in its intended matrix, rather than assuming one reference strategy works for every peptide.Arnold, Stevison and Isoherranen — Sample matrix and peptide quantification (opens in a new tab)

An adequate record identifies the reference sequence and label, assigned concentration, addition stage, monitored signals and how the ratio is converted to amount. It also addresses interference or overlap between analyte and reference channels.

The conclusion should name which variation the standard controls and which uncertainties remain. That makes internal standardisation a transparent part of the assay instead of an unexplained assurance attached to its final number.