I have an independent lab COA with HPLC-UV at 214 nm showing 98.7% main peak, but the same report's LC-MS total ion chromatogram has extra peaks at 0.6% and 0.4%. The UV doesn't show them. Sample was lyophilized powder stored 6 weeks at -20 C.
Mobile phase was 0.1% TFA in an ACN gradient. Is this a UV absorbance issue, a degradation artifact, or just MS over-sensitivity?
Anyone seen UV and MS purity diverge this much, and what did the extracted ion data show?
UV at 214 nm can miss things with no chromophore. What m/z were the extra MS peaks?
If TFA adducts are in plya, MS can make one peak look like three. Check deconvolution before calling it degradation.
I'd ask for extracted ion chromatograms, not just the TIC. TIC can exaggerate low-abundance ions.
If the 0.6% peak has the same m/z as the main peak but a different adduct, it's likely gas-phase, not sample. If it's a different m/z, then it's a real impurity.
Also 214 nm is weak for some compounds; 220 or 254 can help. Six weeks at -20 shouldn't cause ~1% unless moisture got in.
I've seen a 98.7% UV sample come back 99.1% by EIC because the TIC counted TFA clusters. But that depends on the lab's integration settings.
Agree on EICs. I've seen UV 98% vs MS 97.4%, but 1% extra peaks usually means method mismatch.
Did the lab report relative area or absolute? And was the MS normalized? That changes the math a lot.
The wavelength issue is real. If the main peak has low absorptivity at 214, the UV area% can look cleaner than it is.
I'd request DAD peak purity and a second wavelength before assuming degradation. Also check the blank: TFA clusters and plasticizers can show up in TIC.
A 0.6% and 0.4% pair is small enough that integration settings and baseline choice could erase or double them. The only way to settle it is same sample, same prep, EIC quantification against a standard. Ask the lab if they ran the standard in the same sequence.
I'd pay for EIC over TIC any day. Guessing costs more.
Same. Last time I asked for EICs, the TFA cluster vanished and the main peak area matched UV within 0.2%. TIC was just noisy. I’d request raw data and reintegrate before paying for a rerun.
214 nm makes me twitchy. If your compound has no strong chromophore there, area% is basically a guess. I’d ask for DAD spectra at peak apex, upslope, and downslope, plus 220 and 254. Peak purity first, degradation story second.
Relative area only, and MS was normalized to the main peak. Lab told me that after I asked twice. Once I converted to absolute counts, the 0.6% and 0.4% became one fuzzy bump. Math matters, folks.
Six weeks at -20 with a bad cap? I’ve seen vials pull moisture just from freeze-thaw cycles. But that usually shows earlier. I’d check the blank and a fresh prep before blaming storage.
Integration settings can absolutely invent peaks. My old lab had a baseline skim that doubled TFA clusters into “impurities.” Same raw file, different software, 0.0% extra. Get the method parameters.
EIC against a standard is the only clean comparison. Same prep, same column, same gradient. If the extra m/z doesn’t match standard retention and exact mass, then call it. Until then, it’s gas-phase chatter.
I’d ask the lab if they can re-process the existing data before charging for a rerun. Most will send EICs and DAD purity if you’re polite. We settled a 98.7 vs 97.4 mess that way last month.
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