Element Subtraction Using the > Symbol

See also Entering Formulas.

Element Subtraction was added to allow one to quickly find the mass or empirical formula of a compound that has lost one or more elements due to a reaction or a neutral loss in mass spectrometry. For example:

C₆H₅Cl>HCl means C₆H₅Cl minus HCl, having a mass of 76.0959 Da (average mass), equivalent to C₆H₄

The elements to the right of the > symbol must all be present in the elements to the left of the > symbol, otherwise an error will occur. Furthermore, the number of atoms of a given element listed to the right of the > symbol cannot be greater than the number of atoms of the element to the left of the > symbol. Thus, the following formulas will result in an error:

If you include multiple > symbols in a formula, then the order of operation is right-to-left. For example:

C₆H₅Cl₃>H₃Cl₂>HCl
will first be converted to
C₆H₅Cl₃>H₂Cl
and then to
C₆H₃Cl₂

The mass of C₆H₃Cl₂ is 145.99342 Da (average mass), and consequently this is the mass returned for C₆H₅Cl₃>H₃Cl₂>HCl
If you choose Convert To Empirical Formula, then the empirical formula returned is C₆H₃Cl₂

Abbreviations may contain formulas having > symbols in them. Thus, if the abbreviation J is defined to be C₆H₅Cl>HCl (mass 76.0959 Da), then the formula H₂J will have a mass of 78.11184 Da. If you choose Expand Abbreviations for the formula H₂J, then the formula will be replaced with H₂C₆H₅Cl>HCl. However, if the original formula is J₂ and you choose Expand Abbreviations, then we cannot simply replace J₂ with (C₆H₅Cl>HCl)₂ since that is an invalid formula. Consequently, J is first converted to its empirical formula, giving (C₆H₄)₂ This same situation applies if J is in the middle of a formula. For example, H₂JO₂ will be converted to H₂C₆H₄O₂ when choosing Expand Abbreviations, or to C₆H₆O₂ when choosing Convert to Empirical Formula.

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