System Suitability: What a Laboratory Checks Before Your Sample Is Run
A purity figure is the output of an instrument, and an instrument can be wrong. System suitability is the set of checks a laboratory runs before your sample is injected to establish that the instrument was capable of producing a meaningful answer that day. It is routine, it is unglamorous, and its absence from a report is more informative than most people realise.
What it is
Before analysing a sample, the laboratory injects a known standard several times and measures whether the system behaves within pre-set limits. If it does not, the run is not valid and the sample is not reported. The point is to separate instrument failure from sample failure, because on a chromatogram the two can look identical.
The parameters that are checked
- Retention time repeatability. Successive injections of the same standard should elute at the same time. Drift indicates a pump, temperature or mobile-phase problem, and a shifting retention time makes identity assignment unreliable.
- Peak area precision. Typically expressed as relative standard deviation across replicate injections. High variance means the quantitation underneath the purity figure is noisy.
- Theoretical plates. A measure of column efficiency. A column that has lost plates broadens peaks, and broad peaks merge impurities into the main peak, which inflates purity.
- Tailing factor. Asymmetry in the peak shape. A tailing peak overlaps whatever elutes behind it, and integration then has to guess where one ends and the next begins.
- Resolution between critical pairs. The two species hardest to separate on that method. If they are not resolved on the standard, they will not be resolved in your sample either.
Why it decides what a purity figure means
Purity is the main peak area divided by total area. Every parameter above changes that ratio. A degraded column with low plate count and a tailing main peak will report a higher purity than a healthy one analysing the identical vial, because impurities that should have resolved are instead buried under the main peak.
This is one of the mechanisms behind two laboratories reporting different figures for the same material without either being dishonest. Why certificates of analysis disagree covers the others, and reading an HPLC chromatogram covers what a shoulder on a main peak indicates.
What to look for on a report
Most certificates supplied with research peptides do not report system suitability at all. That is normal rather than damning, but it means the purity figure arrives without evidence that the measurement was in control. Where a report does include it, the useful checks are whether the acceptance criteria are stated alongside the results, and whether the results sit comfortably inside them rather than at the boundary.
A laboratory operating under an accredited scope will have system suitability built into the method, whether or not it appears on the customer-facing report. What ISO/IEC 17025 accreditation actually covers sets out what that does and does not establish, and what a third-party peptide test actually measures covers the rest of the document.
What it does not establish
System suitability establishes that the instrument was working. It says nothing about whether the right method was chosen for the compound, whether the detection wavelength suits the sequence, or whether the sample submitted represents the lot. Those are separate questions, and a perfectly controlled measurement of the wrong thing is still the wrong thing.
All material is supplied for laboratory research use only. It is not a drug, not a supplement, and not for use in humans or animals.
