I've been trying to calculate the molecular formula to some mass spectral exact mass features from a metabolomics experiment on a cultured organism. I found this resource:

http://www.chemcalc.org/mf_finder

I must say that it is very slow. Since I started running it for one feature (300m/z approximately), the program has not stopped searching for matches. I don't have prior experience working with this system, so I wonder if this is normal, or if might be because of the parameters I entered into the search?

This is what I entered into the search:

Accurate Mass- 300.123

MZ range- C0-100 H0-100 N0-10 O0-10 F0-3 Cl0-3 Br0-1 S0-3

Mass range- .05