Hi,

What is your analyte: small molecule, polymer, protein, otherwise?

do share [email protected]

It’s impossible to help without knowing precisely how the results are complex.

Eef Dirksen

Hi,

Thank you for your message. My samples are crude plant extracts, which are complex mixtures containing various small molecules such as polyphenols, flavonoids, and other secondary metabolites. I am interested in identifying and characterizing these compounds.

If you need more specific information about the extraction method or the plant species, please let me know.

Best regards,

For your LC-MS analysis, the first critical step is to examine your total ion chromatogram (TIC) and extracted ion chromatograms (EICs) to identify whether you're observing the expected molecular ions and assess potential background interference. It would be helpful to know your specific LC conditions including column type (C18, HILIC, etc.), mobile phase composition, and ionization mode (ESI+ or ESI-), as these factors significantly impact your results. Common issues to immediately check include column bleed (look for polysiloxane clusters in positive mode) and plasticizer contamination (e.g., phthalates around m/z 391). For compounds that aren't ionizing well, consider switching ionization polarity or trying different mobile phase modifiers. If you have any MS/MS fragmentation data, even from unit mass resolution instruments, these patterns can provide crucial structural information, especially when combined with characteristic isotope patterns for elements like chlorine or bromine.

For your NMR data interpretation, begin by verifying whether your observed chemical shifts and multiplicities match your expected compound structure, paying particular attention to the solvent used (CDCl3, DMSO-d6, or D2O) as this affects proton chemical shifts. Broad peaks may indicate aggregation or paramagnetic impurities, while missing signals could suggest exchangeable protons in protic solvents. If you've collected 2D NMR data like HSQC or HMBC, these correlations can quickly confirm or challenge your structural assignments. The most powerful approach combines evidence from both techniques - cross-validate the molecular weight from LC-MS with the structural information from NMR to ensure consistency. For rapid reference, public databases like mzCloud for mass spectra and NMRshiftDB for chemical shifts can provide valuable comparison data, while troubleshooting guides like "Practical LC-MS Troubleshooting" can help identify common instrumentation issues. To provide more targeted advice, it would be extremely helpful if you could share your most pressing specific questions along with any available spectral data (even screenshots of key regions), acquisition parameters, and your hypothesized structures. This will enable more precise guidance to help you meet your deadlines effectively.