Dear Rachal,

the molecular weight usually includes isotopic mass abundances and is not useful for MS (Carbon would not be calculated as c=12u but as a mixture of ~99% 12u and ~1% 12.0034). Hence chemdraw correctly sums up only the major isotopes to give the m/z of the most prominent peak. in ESI- you loose a proton (H=1.007) during ionization. Therefore what you measure is atomic mass minus ionization loss.

Mrs/Miss/mr. Cmp,

Your data as they are given in your posting are obtained with low resolution. In this context there is unable to talk abot mass accuracy below the second decimal sign. Or the H mass should be 1.01, not H=1.007. In this context in your case there is unable to talk about 'exact' mass weight. Moreover there is not provided the standard mass weight (experimental). How is the accuracy of your measurements and the standard deviations (3 replicates towards the standard value)?  

Is your question about the relative isotopic abundances you see, or the that the peaks are 1 da lower than the calculations?

About being 1 da lower- that is loss of hydrogen as your are running ESI-, this generates the charge.

The isotopic abundance- Chemdraw calculates the monoisotopic masses for you using the most common isotopes. Small molecules such as yours come close to these isotopic abundances, but there are molecules with a mixture of isotopes. Large molecules, such as peptides, show a mass spectrum where the molecule mass is closer to the average molecular weight. It may be possible that the source of the bromine in your molecule doesn't quite match the usual isotopic abundances.

Rachal,

In general, mass spectrometers measure the m/z of a peak with much higher accuracy than peak abundance. Your mass error is 0.01 m/z units (30 ppm), but your abundance error is in the range of 5-10%. This is typical of many mass spectrometers. Even high performance mass spectrometers that can measure m/z with sub parts-per-million accuracy will still have a difficult time improving on the peak abundance accuracy that you have obtained in your mass spectrum.

Chemdraw is not using the latest NIST values for isotopic abundance calculation, and Jonathan Amster is correct about the isotopic errors in MS.

What you see is indeed your [M-1]- species. Small deviations from the theoretical (calculated) isotope distribution are normal, and if you acquired your spectra a bit longer, those would probably appear much better. I must point out that MS doesn't deal in molecular weights in any way, ever.