Hello Jubyeong Chae ,

When analysing molecules by mass spectrometry, there will nearly always be a discrepancy between the theoretical or calculated exact mass, and the value obtained experimentally.

There are a number of reasons why these vary such as the accuracy and resolution of the detector (depending on the type of mass spectrometer you are using to perform the analysis) as well as the sample itself as ESI-MS can produce signals which contains additional molecules (called adducts) which are "attached" to the molecule you are analysing. Due to this, you will observe at least the addition of a proton (so as to charge the molecule and allow it to be analysed in the electric fields of the mass spectrometer) if not sometimes the addition of salt adducts (sodium, potassium etc) which will add on further mass.

Including all of the above, mass spectrometrists will use a "mass tolerance" either stated as "+/- Daltons" or "+/- ppm" (parts per million) which indicates the difference between calculated and experimental mass.

The Barrow Group at Warwick University have a good site which may be of interest to you (https://warwick.ac.uk/fac/sci/chemistry/research/barrow/barrowgroup/calculators/mass_errors/).

Good luck.

Best,

Peter

Dear, Peter

Thank you for your kind answer Peter.

As follows your advice, I visited that site and calculated mass tolerance then, I found this mass error about 100 ppm. The gap of mass value is about 0.5

Can this data be used as evidence of structure? Isotope pattern was matched with that of calculation. I don't know the available 'tolerance range'.

Although I agree with Peter, I'd like to add that Dalton is a mass unit, and MS deals with m/z (mass to charge). I do not exactly understand what is "mass tolerance" (in fact m/z tolerance). Roughly: If your instrument is a quadrupole 0.5 (better 0.3) uncertainty on m/z unit is necessary to two m/z values which differ by 1 (for a singly charged ion). This will give no info on the atomic composition. If you have a high resolution instrument (FTICR, Orbitrap, TOF) with a precision in the ppm range, you may induce the elementary composition.