Accuracy

[rgamage]

I expect a peak at 515 and observed a peak at 516m/z. Is this close enough? What possible reasons can cause this difference?

[Ron]

Two possiblilities. 1. The sample has elements with large mass defect so monoisotopic mass is larger than nominal mass.
2. The mass spectrometer needs to be retuned to adjust the mass axis.
2 is most likely the case

I just thought of a third possibility. You didn't specify the type of instrument and ionization mode. If you are doing CI usually don't get molecular ion, you get molecular ion plus a proton.

[Don_Hilton]

You may be looking at something else. Give a bit more detail about what you are doign - instrument, compund, ionization thechnique.

[thohry]

If the instrument has passed a calibration (tuning), it can not be that big difference. You may try to re-tune the MS.