I have found a molecular ion of m/z 140 in LC-ESI(-)-MS. Does it mean that the molecular weight of my target compound is 141 or the exact mass of my target compound is 141? I have asked this, since for example a compound with 4 chlorine atoms has an exact mass approximately 2 amu lower than the molecular weight of the same compound, and this changes many things.
You mean average molecular weight?
What you measure in MS is the exact mass and you should see isotope effect (i.e. one main band and than few smaller in correspondence to occurrence of less abundant isotopes)
not with the lowest, but with the most abundant!
E.g. boron or selenium have lighter isotopes, but they are less abundant.
Thank you for your responses. Many people around me think that they directly measure the molecular weight of the compound, but this should not be the case. Thank you again.
Hi Akin,
remember that ESI source usually transfer protonated (positive mode) and desprotonated (negative mode) ions from solution to gas phase, this kind of ions are not considered molecular ions, they are protonated or desprotonated molecules [M+H]+ or [M-H]- that we cannot call as molecular ion, molecular ion have the same mass of the neutral molecule such as M.+ from electron ionization sources.
When we observe an ion using ESI technique we might observe in positive mode, protonated, sodiated, potassiated, amoniated species (there are more than that). We must have consider that the average molecular mass is not used in mass spectrometry, the values that we must consider is the exact mass of the ions with the isotopic distribution, the fun thing in MS is that we observe the isotopic distribution of the atoms that we have in the ion. Usually we use to work with the ion that contain most abundant isotopes often called most abundant monoisotopic ion.
regards
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