The reason 2 ppm match is hard to answer is that the obvious measurement and the relevant quantity are not the same thing, and substituting one for the other is silent.
16 likes 1mo
Analytics · Mass spectrometry · continued
Second pass at: What a 2 ppm match establishes, and what it does not posts 31–50
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
endpoint_line, post #26: Thank you for the correction. I would rather find out here than later. Go to post
Two sentences on 2 ppm match and then I will stop, because the rest is speculation and the thread is better without mine.
What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.
n.kaufmann, post #2: Building on the opening post rather than restating it. Calibration matters: a high-resolution instrument out of calibration can report mass with ppm error large enough to be uninformative. Check when the instrument was last calibrated before trusting the reported accuracy. I have written this out at length because the short version… Go to post
Following this. I have the same question and no better information than the first post.
Post #32 and I disagree about the size of the effect, not about the direction.
Resolution: "high resolution" commonly means <5 ppm across the mass range. Unit-resolution instruments achieve ±1 Da at best and cannot distinguish two species differing by less than 1 Da in total mass.
6 likes 1mo
My understanding of 2 ppm match is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it.
0 likes 28d
c.wijnberg, post #34: Post #32 and I disagree about the size of the effect, not about the direction. Resolution: "high resolution" commonly means Go to post
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.
Worth saying I have only my own numbers here, and n is small.
Post #36 put the caveat in the right place and I want to underline it.
Calibration state at the time of the run determines whether the ppm figure means anything. A report that states when the instrument was last calibrated is unusual and is worth more than one that does not.
10 likes 24d
Reframing 2 ppm match slightly, because I think the disagreement is about the question rather than the answer. If the question is "does it happen", yes. If it is "how often", nobody here knows.
7 likes 22d
Post #36 describes the usual case. This is about the unusual one.
The documentation on 2 ppm match is better than this thread and I say that as someone who has posted in the thread.
17 likes 20d
Quantitation by MS: most quantitation is done by LC-UV detection at 214 nm, not by MS, because extinction coefficients are better known. MS can quantify if an internal standard is used but that requires preparation.
I am confident about the direction and much less about the magnitude.
24 likes 18d
Calibration matters: a high-resolution instrument out of calibration can report mass with ppm error large enough to be uninformative. Check when the instrument was last calibrated before trusting the reported accuracy.
Caveat: everything above assumes the paperwork is what it says it is.
11 likes 16d
k.batista, post #6: Taking post #3 at face value and following it one step further. A definition problem is doing most of the work in this 2 ppm match discussion. Once the term is pinned down I suspect the disagreement mostly goes away and what is left is small. Go to post
What I would tell a new member reading about 2 ppm match for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
Post #41 put the caveat in the right place and I want to underline it.
Purity and identity are different questions: LC-MS establishes that the species at a retention time has the expected mass. It does not establish how much of the sample is that species (that is what LC-UV purity answers).
Flagging that the sources on this are thinner than the confidence in the thread suggests.
17 likes 11d
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.
The number is defensible. The precision I gave it is not.
7 likes 9d
n.kaufmann, post #2: Building on the opening post rather than restating it. Calibration matters: a high-resolution instrument out of calibration can report mass with ppm error large enough to be uninformative. Check when the instrument was last calibrated before trusting the reported accuracy. I have written this out at length because the short version… Go to post
Where the 2 ppm match reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.
b.kowalski, post #39: Reframing 2 ppm match slightly, because I think the disagreement is about the question rather than the answer. If the question is "does it happen", yes. If it is "how often", nobody here knows. Go to post
I think the 2 ppm match question is answerable and has not been answered, which is a more optimistic position than most of this thread.
Where I part company with post #45, and it is a narrow parting.
Resolution: "high resolution" commonly means <5 ppm across the mass range. Unit-resolution instruments achieve ±1 Da at best and cannot distinguish two species differing by less than 1 Da in total mass.
12 likes 3d
b.jankowiak, post #13: The practical version of 2 ppm match is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached. Go to post
Post #49 is the version of this I will quote in future. One addition.
Small methodological point on 2 ppm match: repeating a measurement is cheap and resolves most of what is being argued about here at no cost to anyone.
This topic was referenced in
- Isotope patterns and why the monoisotopic peak is not the tallestAnalytics › Mass spectrometry · 16 replies
Suggested topics
| Topic | Participants | Replies | Views | Activity |
|---|---|---|---|---|
|
Mass error in ppm: the arithmetic and the caveat
Mass error in ppm: the arithmetic and the caveat Writing it up because I had to work it out twice and would rather nobody else did. Working through the identity arithmetic and I would like it checked.…
|
+108 | 125 | 1.5k | 5mo |
|
Distinguishing a deletion sequence by mass alone — the long version
On the subject in the title: Distinguishing a deletion sequence by mass alone — the long version Working notes rather than a conclusion. A narrow question about Distinguishing a deletion sequence,…
|
+38 | 44 | 60k | 11mo |
|
About the Mass spectrometry category
LC-MS and high-resolution MS, charge states, mass accuracy in ppm, and what identity confirmation does and does not establish. This post is a community wiki: any member at trust level 3 or above can edit it,…
|
+5 | 9 | 19k | 6h |
|
Mass error in ppm: the arithmetic and the caveat — a second dataset
On the subject in the title: Mass error in ppm: the arithmetic and the caveat — a second dataset Working notes rather than a conclusion. Working through the identity arithmetic and I would like it checked.…
|
+20 | 24 | 1.7k | 1d |
|
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…
|
+12 | 16 | 7.5k | 17mo |
Related topics — sharing the tags MALDI-TOF, mass error (ppm), counter-ion content
| Topic | Participants | Replies | Views | Activity |
|---|---|---|---|---|
|
Particulates in a reconstituted solution: identifying the likely source — what changed since
Particulates in a reconstituted solution: identifying the likely source — what changed since — setting out what I have, and where I think it stops being reliable. What changes if the standard account of…
|
2 | 129 | 14mo | |
|
Buffers, pH and why a formulation is a formulation
Buffers, pH and why a formulation is a formulation Writing it up because I had to work it out twice and would rather nobody else did. A formulation question rather than a pharmacological one, though the two…
|
+11 | 15 | 51k | 21mo |
|
Racemisation and why it is invisible to a mass measurement — what changed since
Racemisation and why it is invisible to a mass measurement — what changed since Writing it up because I had to work it out twice and would rather nobody else did. What changes if the standard account of…
|
+11 | 15 | 42k | 16d |
|
Revisiting: Aggregates and why a purity assay may miss them entirely
Posting this under the heading it deserves: Revisiting: Aggregates and why a purity assay may miss them entirely Everything below is what sits behind that. I would like to disagree carefully with the settled…
|
+48 | 53 | 918 | 8mo |
|
Coming back to: Solubility limits and what a cloudy solution is telling you
On the subject in the title: Solubility limits and what a cloudy solution is telling you Working notes rather than a conclusion. Reading back through what has been written here about Solubility limits, three…
|
+38 | 43 | 29k | 13h |