Formulas were assigned allowing 4 -50 C, 4 -100 H, 2 -40 O, 0 -2 N, 0 -1 S and 0 -1 C. Combinations of N, S and 13 C were not allowed, nor were double bond equivalence minus O greater than 10. A 5 th order polynomial calibration was performed using a series of expected ions, as in a previous paper , and formulas were assigned if the mass offset was less than 1.5 ppm.