This Solanine example is an example that the Seven Golden Rules can be used for elemental composition determination obtained from unit mass resolution mass spectrometers (inaccurate mass spectrometers). The linear ion trap is a common “work-horse” in mass spectrometry labs and although no mass accuracy and resolving power is advertised it can reach between 50 and 100 ppm mass accuracy and 1000-2000 resolving power. More importantly it shows that accurate isotopic abundances are more important than accurate mass measurements if sufficient resolution is provided by chromatography.
Seven Golden Rules
Using data from the LTQ (selected resolving power = 1700) m/z of 868.54548 [M+H] was measured (ESI positive, Nanomate) and a mass of [M+H] 868.50582 was calculated with MWTWIN. The mass accuracy was 46 ppm isotopic abundances [A+1] = 50.72%; [A+2] = 15.68%; [A+3] = 3.61% and the isotopic abundance errors were: [A+1] error = 1.43%; [A+2] error = 0.45%; [A+3] error = 0.15%; [A+4] error = 0.06%.
The Seven Golden Rules (see supplement) calculated 5856 formulae within 100 ppm mass accuracy (elements CHNSOP) and found 1380 formulae within 5% isotopic pattern range. The formula with the highest rank (99.97) was C45H73NO15 which refers to the correct formula and could lead to the correct annotation of the isomer structure if additional constraints are used.
More than 44 formula candidates were found in PubChem within the 5% isotopic pattern error range. Hence these formulae must be considered as potential candidates. Assuming that we would only research natural compounds – a match against the natural compound formula list would refer in only two hits. One formula C45H73NO15 refers to 5 different alkaloid isomers, among them the correct structure solanine. The other formula refers to a small peptide. Using a target formula database removes 99.9% (5853) wrong candidate formulas. Download the example in the software section.