Exact mass measurements with accuracy in the low partsper-milllon range, suitable for elemental composition assignments, are obtained by Fourier transform mass spectrometry (FTMS) In the absence of calibration compound. It is de-termined that good results are obtained when measurements are made under the same conditions as was a calibrant and somewhat poorer results are obtained when other conditions are chosen and measurements are “corrected”. The effects of both magnetic and electric field Instabilities on mass measurement accuracy are considered and examples of ac-curate mass measurements yielding elemental composition assignments for unknown samples are presented.

In many cases, structure elucidation of unknown materials is facilitated by making accurate mass measurements of mass spectral fragment ions. If mass measurement accuracy of 5-10 ppm or better is obtained, elemental compositions of fragment ions can usually be determined or, atthe very least, the number of compositions which need to be considered can be drastically reduced. Traditionally, such measurements are made relative to a calibration compound which is also present in the ion source and for which elemental compositions of mass spectral peaks have previously been determined. Obviously, this method introduces “chemical noise” and can interfere with