I think you can see the calculated mass by using software of your equipment. Please click help. Good luck

Mass of Hydrogen is 1.00794 not 1.0,mass of Carbon is 12.0107 not 12.0 like wise you calculate the exact mass of you molecule based on the atom present & compare with that of Q-TOF mass. you will have 4 decimal mass from calculation.

The calculated mass is the sum of atomic masses of the constituents. The monoisotopic ion (containing only one kind  isotope of each atom) is usually calculated, evidently, with the most abundant isotopes; for example the molecular ion of C6H6 contains 6 C-12 and 6 H-1: m/z = 6*12.0000 + 6*1.007825 (the mass of the electron is not usually deduced or included if the ion is positive or negative respectively). The C-12 mass is exactly 12 by definition, and all other isotope masses are calculated on this basis. Sorry Ashish Atodariya but you are wrong: you give the atomic masses of the natural isotopic mixtures. As the C-13 isotope is also present in carbon, about 1%, C6H6 contains about 6 % of molecules with one C-13 and the first “isotope peak” will be at m/z = 5*12 +1*13.003354 + 6*1.007825, with an abundance of 6% as regard to the monoisotopic peak. There is also a very small contribution of the H-2 (deuterium, very small abundance), then there is also a second isotope peak, etc… You should consult a basic book on MS to understand the full complexity of the exact mass and isotopes. Note also that for high masses (with more than hundred carbons…) the first C-13 peak may be larger (100*1%) than the monoistopic C-12 peak. Mass calculators are very useful to calculate exact masses but also for simulating the isotope “cluster”, i.e. the ensemble of isotopic peaks.

Hi,

You have two very interesting tools when you can have access to HRMS :

*Exact mass calculator which will calculate the theorical mass when you have the elemental structure 

http://www.sisweb.com/referenc/tools/exactmass.htm

*A tool that allows to generate a list of molecular formulas from an accurate mass :

http://www.chemcalc.org/mf_finder/mfFinder_em_new

The second tools can help you generate hypothesis of fragment ions when you have MS 2 spectrum.

Regards

Nagaru is right. According to IUPAC (Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013; Pure Appl. Chem., Vol. 85, No. 7, pp. 1515–1609, 2013.):

"Accurate mass and exact mass are not synonymous. Accurate mass refers to a measured mass, and exact mass refers to a calculated mass."

Thank you one and all name by name for ur valuable inputs.

You should calculate the theoretical mass by yourself. And many software can do this.

Good point Christoph !!!  I forgot to add in my answers that m/z calibration is fundamental ! If you calibrate the instrument with a few well defined peaks close to your unknown, ppm precision may be obtained. Using standard calibration, or worst an old calibration table, may lead to an order of magnitude less precise masses. 

It's depends on the software you use, usually with the molecular formula the program itself calculates. In general an error +/- 5ppm is accepted.

The easiest way is to use a software like chemsketch. You will just have to draw your component.

As someone said, CHEMDRAW is very useful to calculate the exact mass corresponding to one selected chemical formula. It is an excelent software.

The precise mass vs the calculated mass is the important information for formula identification. Another useful info is to draw a simulated spectrum for the expected ion, with the isotope peaks, and compare the shape of the "isotope cluster" with the experimental spectrum (in the absence of overlap, interferences...).

Hellow, anyone can help me to find answer I took LC-MS  [M+Na]  771.17 for the molecular weght alrady i found = 748 ok but beside base peak its appear other peak  413.25 and 381.25 wat dase mean this two value?

There are many possibilities. Is this spectrum obtained by MALDI or ESI as ionization source?

Like [M+Na] you can find additional adducts ions ([M+NH4], [M+K]). You can also find matrix clusters if you use MALDI (HCCA or DHB) or even combinations [matrix+K+Na]. Dimerization of your analyte may also occur [2M+H] or [2M+Na]. You must also consider formation of polyprotonated [M+2H] or [M+3H], mainly in ESI (multiple protonation is usual).

However, it is very likely that the peaks you mention are fragment ions, depending on your molecule. You should inspect all the possibilities.