Elemental Composition

Elemental composition analysis is one of the most informative techniques in mass spectrometry. With appropriate mass spectral data, elemental composition can quite often determine the unambiguous elemental composition of a molecular ion.

This technique, however, only produces sensible results from high accuracy mass spectra. For ions less than or equal to m/z 200, a measured accurate mass within 5 ppm is often required to determine elemental composition. At higher m/z values, measured accurate mass values of significantly less than 5 ppm may be required.

The algorithm that is used exhaustively computes all possible combinations of elemental formulae subject to the constraints provided by the user (see below). In some cases, millions of possible formulae are evaluated, but because of the speed of the algorithm, this happens very quickly.

This feature will be very useful to calculate the elemental composition of one or more mass peaks of a spectrum, if the m/z values are of high accuracy, typically 5 ppm mass accuracy or better.

First of all, you should establish the constraints of the calculation, just by clicking on the applicable option of the menu, which will display the dialog box below:

You can also select the 'Double Bond Equivalence', 'Tolerance', 'Maximum Result Count', 'Electron Mode', 'Charge State', 'Adduct' and Fitness Threshold'.

Tolerance: Tolerance for matching computed composition to the target m/z. If spectral mass accuracy is expressed in ppm, then the tolerance for matching should also be in ppm. The typical values in Da should be 0.01 or less, or in ppm 5 or less. Tolerances much larger than this, generally give too many hits to be useful.

m/z: symbol used to denote the quantity, in units of Daltons (Da), formed by dividing the mass of an ion in unified atomic mass units by its charge number (regardless of sign).

Electron Mode: Electron mode can be even, odd, or both. Typically, EI (electron impact) produces positively charged, odd electron ions, whereas ESI (electrospray ionization) produces even-electron ions with either a positive or negative charge.

EI: Ionization of an atom or molecule by electrons that are typically accelerated to energies between 10 and 150 eV in order to remove one or more electrons from the molecule. The term electron impact is deprecated.

ESI: A process in which ionized species in the gas phase are produced from a solution via highly charged fine droplets, by means of spraying the solution from a narrow-bore needle tip at atmospheric pressure in the presence of a high electric field (1,000 to 10,000 V potential).

Charge State: is a signed value. The magnitude (1, 2, 3, ...) is a function of the experimental ionization technique, while the sign (+/-) is determined by the mode in which the experimental spectrum was produced and detected. For electron impact (EI) spectra, charge state is usually +1, while for ESI charge state is usually +1 or -1 for most organic compounds. Peptides may have higher charge states.

Adduct: Adducts are typically found in soft ionization techniques such as ESI. Proton (H) is most common for organic compounds, while H and/or Na (or other Group 1A alkali metal cation) may be found in protein, peptide or polymer spectra. NOTE: If an adduct is specified, the mass (* charge state) is subtracted from the target mass before compositions are computed. The composition calculation assumes a neutral (uncharged, non-adducted) mass.

Fitness Threshold: Results that match the other criteria but have fitness lower than this threshold will be discarded. Fitness ranges from 0.0 (no match to experimental spectrum) and 1.0 (perfect match). Fitness is computed by matching the experimental isotope cluster to the computed cluster for the formula, taking into account both / m/z and intensity.

Once you have finished with the constraints, click on the 'Elemental Composition' icon and select with the crosshair the desired peak of the Mass spectrum. A table with the results will be displayed (it is also possible to find this table under the 'View/Tables' menu). All the results obtained will satisfy the composition, DBE, and tolerance constraints, and will be ranked in decreasing order of fitness. Please bear in mind that a fitness value of 1 will be the perfect match.

The Update button will allow you to modify the constraints and apply these changes to the current Elemental Composition result.

Click on the applicable button of the Elemental Composition table to remove the highlighted area.

Please bear in mind that mass spectrometers measure monoisotopic m/z values which is different with the Molecular weight value (calculated using the average mass of each element weighted for its natural isotopic abundance). For example, for C20H23N, the chemical mw is 277.402, and monoisotopic mw is 277.182. Here you can see a list of the exact mass of some elements:

Element	Symbol	Exact Mass

Hydrogen	H	1.00783
	2H (D)	2.0141
Carbon	12C	12
	13C	13.0034
Nitrogen	14N	14.0031
	15N	15.0001
Oxygen	16O	15.9949
	17O	16.9991
	18O	17.9992
Fluorine	F	18.9984
Silicon	28Si	27.9769
	29Si	28.9765
	30Si	29.9738
Phosphorus	P	30.9738
Sulphur	32S	31.9721
	33S	32.9715
	34S	33.9679
Chlorine	35Cl	34.9689
	37Cl	36.9659
Bromine	79Br	78.9183
	81Br	80.9163
Iodine	I	126.9045