Why one laboratory's 98.4% and another's 96.1% may not contradict — one year on

Asking directly, because I could not find a straight answer: Why one laboratory's 98.4% and another's 96.1% may not contradict — one year on

Working through the identity arithmetic and I would like it checked.

retatrutide has a monoisotopic mass close to 4731.3 Da. On an electrospray instrument I would expect to see the multiply charged series rather than the intact singly charged ion, so for the doubly charged species I calculate (4731.3 + 2 x 1.00728) / 2, and for the triply charged the analogous expression.

The observed values in the report sit within a few ppm of those. My question is what that actually establishes, because I have seen people treat a mass match as a purity result and I do not think it is one.

Transfer between laboratories: a method can be transferred from one lab to another, but the receiving lab needs to demonstrate that they can achieve the same performance. This requires comparative testing and sometimes small method refinements.

post #2 is right about the mechanism and I think understates the practical bit.

Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.

Worth separating two things that the opening post runs together.

Two things before anyone answers the substance.

First, the context in the first post is clear and specific. Second, the question is framed so that an answer can actually address it. Both are the norm here and both matter more than they sound.

Picking up post #2: that is the part I would want checked first.

Linearity: the detector response is proportional to compound concentration across the working range. Demonstrated by running standards at multiple concentrations and showing R-squared values typically ≥0.99.

Precision and repeatability: within-run and between-run variability of the method. Acceptance criterion is typically a relative standard deviation of ≤2% for area measurements.

For anyone arriving from a search: the marked solution above is the direct answer, and the replies underneath it add the caveats that make it safe to use.

On post #4 — agreed on the reasoning, with one qualification.

Robustness: the method gives consistent results when minor parameters vary. Tested by deliberately varying pH, temperature, flow rate, and mobile phase composition within reasonable ranges and demonstrating that results stay within acceptance.

Accuracy: the method measures what you intend to measure. For purity methods, this is tested by spike-and-recover experiments: add a known amount of impurity to a sample and measure whether you recover the added amount.

I read post #8 twice before replying, because I had assumed the opposite.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

Worth separating two things that post #7 runs together.

Two things before anyone answers the substance.

First, the context in the first post is clear and specific. Second, the question is framed so that an answer can actually address it. Both are the norm here and both matter more than they sound.

post #11 is right about the mechanism and I think understates the practical bit.

System suitability: injections run at the start of a batch to establish that the instrument and column are performing. Acceptance criteria typically include replicate precision (RSD ≤2%), peak tailing (0.8–1.5), theoretical plates (>2000), and resolution (>1.5).

Limits of detection and quantitation: LOD is the lowest concentration that produces a signal above background. LOQ is the lowest concentration at which the method meets precision and accuracy acceptance criteria. Both are determined empirically.

Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.