Isotope patterns and why the monoisotopic peak is not the tallest

Isotope patterns and why the monoisotopic peak is not the tallest — setting out what I have, and where I think it stops being reliable.

Isotope patterns: what I expected, what I found, and the gap between them.

I am posting the gap rather than a theory about it. My theories about gaps like this one have not held up well.

Monoisotopic and average mass are different numbers and both appear on documents. At this molecular weight the difference is a few daltons, which is more than the tolerance being claimed.

Reading it back, the second half matters more than the first.

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

The bit of isotope patterns that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.

Picking up post #3: that is the part I would want checked first.

Second-hand on isotope patterns, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.

Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak.

It cost nothing to check and would have cost something not to.

Post #3 is the version of this I will quote in future. One addition.

If you are new and reading this thread for the answer to isotope patterns: the answer is conditional, the conditions are in the third reply, and the rest of the thread is worth skipping.

Post #3 describes the usual case. This is about the unusual one.

Where the isotope patterns discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.

The honest summary of what a mass result buys you: it narrows the field of what the material could be, considerably. It never closes it, and no certificate should be read as though it had.

I read post #7 twice before replying, because I had assumed the opposite.

Isotope patterns: I would want to see the raw numbers rather than the summary before agreeing. Summaries lose exactly the information that would settle this.

An observation about isotope patterns that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.

Resolution and mass accuracy are different specifications. An instrument can resolve two species and still assign their masses imprecisely, and the reverse is also possible.

I would rather be precise about what I do not know than vague about what I do.

This follows post #11 rather than contradicting it.

Agreed on isotope patterns, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part nobody states.

Worth separating two things that post #10 runs together.

A methods point on isotope patterns rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.