TuO-07

TuO-07

FTMS APPROACHES TO STRUCTURE ELUCIDATION IN NATURAL PRODUCTS CHEMISTRY

David J Bourne

Australian Institute of Marine Science, PMB 3, Townsville MC, Qld, 4810

Electrospray ionisation has become the favoured method for the study of large biomolecules as this technique leads to the formation of multiply charged ions which fall into the mass range of most mass spectrometers. The combination of ESI and the modern FTICR mass spectrometer brings another dimension to the study of these compounds. The remarkable high resolution of these instruments means it is much easier for the biochemist to unequivocally assign charge state and hence molecular mass of large proteins with multiple charge states generated by electrospray ionisation. Hence instruments like the Bruker are marketed predominantly as tools of trade for the study of macromolecules.

We have found that the FTMS offers the investigator of smaller molecules some very important benefits - principally resolution and mass accuracy. The instrument is routinely operated in low resolution (broadband) mode but with careful tuning the resolution usually lies somewhere between 8,000 and 15,000. Calibration is normally external and if ESI source conditions or tuning parameters are not drastically altered between calibration and analysis we expect (and get) mass assignment errors of less than 5 ppm on ions of interest. Magnetic sector mass spectrometers are capable of this resolution, and have been used for decades for mass measurements. The difference lies in the speed and utility of the process with the FT. More than fifty samples can be analysed in a day and for each sample there is accurate mass information on every component in the chosen mass range.

High resolution, mass accuracy and linearity of the mass scale allows us to utilise another aspect of the FT. In natural products chemistry we are often faced with families of like compounds (homologues etc). To ascertain whether the compound in a extract are related it is simple exercise to calculate mass differences between the ions of interest. If the nominal mass difference between two compound is 14 daltons the question arises whether or not this is a CH₂ homologue or an additional OH with one more degree of unsaturation. This question can be easily answered by subtraction of the two ion masses and this facility is not unique to the FT; it can be done with a sector instrument but it will require more time, more effort and the complication of internal calibration to do this. Within minutes with the FT we can have an accurate mass picture of a mixture and quickly know what chemical relationships exist between components.

Where accurate mass difference is unique on the FT is in MSMS studies. Resolution usually degrades to less than 5000 but the facility of accurate mass difference remains. With MSMS studies formulas can be assigned to neutral losses from parent to daughter ion.

Some examples will be presented of the utility of ESI FTICR MS in structure elucidation.