The case in front of me: a quantified content assay · liraglutide.
I would like the axes of comparison first and the recommendation second.
I have tried the first option and it works; the question is whether the second is better rather than merely different.
What is the actual trade-off, and does it matter at the scale I am working at?
The part that matters: most research-grade certificates report purity without content, which is exactly backwards from what users actually need.
A content assay is always paired with a purity assay because purity tells you what fraction of the measured mass is the target and content tells you the total measured mass.
| Δ mass (Da) | Most likely cause | Distinguishing feature |
|---|---|---|
| +1 | Deamidation (Asn or Gln) | New peak, slightly earlier retention |
| −17 | Loss of ammonia | Often with deamidation |
| −18 | Dehydration / succinimide | pH-dependent, reversible |
| +16 | Oxidation (Met, Trp) | Earlier retention, light-related |
| −128 | Missing Gln or Lys | Deletion sequence from synthesis |
| 0 | Isomer: racemisation or scrambling | Same mass, shifted retention |
If a sample shows high purity but low content, the explanation is usually that the standard used for quantitation had a different purity than claimed.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
Worth noting that the standard certificate carries its own uncertainty, usually on the order of two to three per cent, which the sample result inherits.
Ask for both the purity and the content, and do not accept purity alone.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsSpecifically, content assay and purity are orthogonal measurements answering orthogonal questions, and the confusion between them is one of the most expensive misreadings in this space.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
Stated carefully, quantitation against a certified reference material assumes the sample and standard are treated identically through the analytical method, which is why the method for calibration matters as much as the method for measurement.
Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.
The limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.
If a supplier gives you content without the standard's purity, ask them to provide it.
The relevant detail is that reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
Specifically, running multiple independent aliquots of the same sample should give results that agree to within the method precision, which is usually one to three per cent.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
Content assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
Where content data have been published from testing services on common peptides, the spread between services on identical material is typically a few per cent.
The caveat is that content assay costs more than purity, so most people do not do it, which is exactly why it is valuable on the first lot from a new supplier.
Ask for both the purity and the content, and do not accept purity alone.
edited 25 May 2024 by Dr_Hanne_Solberg — clarified the distinction between purity and content
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.