someone explain LC-MS to me like I have not read a paper in years
someone explain LC-MS to me like I have not read a paper in years. Searched first, found three threads that contradict each other, hence the post.
Resolution between two peaks depends on retention, selectivity and efficiency. A shallower gradient buys retention and usually resolution, at the cost of peak width and run time.
The system suitability argument, since it comes up whenever somebody posts a number without one.
Before the sample result means anything, the instrument has to be shown to be fit that day: replicate injections with an acceptable RSD on area, a tailing factor inside limits, adequate plate count, and a resolution check between the pair you care about.
None of that is exotic and all of it is routine in a laboratory that reports for a living. Its absence does not mean a number is wrong; it means the number is unanchored, and unanchored numbers should not be quoted to two decimal places on this board.
Reading a trace posted here, in the order I actually look at things.
Axes first — if the wavelength and the time axis are not labelled, I stop. Then the baseline: where has it been drawn, and does the drawing absorb anything. Then peak shape: fronting, tailing, a shoulder that never resolves. Then the blank, if one was run, for carryover.
Only after all of that do I look at the percentage, and by then I usually know how much weight it deserves. The number is the last thing on the page and the first thing everybody argues about, which is exactly backwards.
Tell me where this is wrong. That is the useful part of posting it.
best — the order this archive was captured in
Carryover from a previous high-concentration injection looks exactly like a small impurity. The blank injection is four minutes and it removes the ambiguity entirely.
baseline choice is a decision, not a measurement
LC-MS for identity, UV for relative quantity
What wavelength, and what was the gradient?
axis labels or the trace is decoration
ghost peak, check the plumbing first, it is always the plumbing
That is area percent, not mass percent. The trace cannot give you the second one.
Mass spectrometry answers identity. UV purity answers relative quantity under the run conditions. A document with one and not the other is answering half the question, and the half it answers should be stated.
system suitability before you believe any number on the run
System suitability — repeat injections, tailing factor, plate count, RSD on area — is what tells you the instrument was fit for the measurement that day. Without it, the purity figure is unanchored.
How old is the column and roughly how many injections has it seen?
Why two honest labs report different numbers on the same vial.
Start with the gradient. A shallower slope holds compounds on the column longer and usually separates close-eluting species better. A steeper one gets you a faster run and a fatter peak. If a related substance elutes near the main peak, one method resolves it and reports it separately, the other absorbs part of it into the main peak.
Then integration. Where the baseline is drawn under a shoulder is a decision made by a person or by a piece of software configured by a person. It moves the number.
A point or two of spread between services on this assay is ordinary. Treating one lab as ground truth is how people end up in arguments with suppliers that neither side can win.
Left up. The integration disagreement in this thread is the most useful thing on the board this week.
area percent is relative to what the detector saw and nothing else
What did system suitability look like on that sequence?
Same method at both labs, or two different gradients?
Chased a ghost peak for three weeks. It was the plumbing. It is always the plumbing.
Do you have the mass, or only the UV trace?
Is that baseline drawn by the software or by hand?
214nm sees the peptide bond, 280nm sees the aromatics
Same method at both labs, or two different gradients?
This is the whole methodological point of the board in one line.
- 1Carryover from a previous high-concentration injection looks exactly like a…7 comments in this branch · started by u/georgi_tamm
- 2What did system suitability look like on that sequence?7 comments in this branch · started by u/baseline_drifter