## Referenced Kendrick mass-defect calculation and screening tools for lipid classes

**Introduction:**Kendrick mass-defect analysis of high-resolution mass data may conveniently be used for identification of members of a homologous series of various classes of lipids. The Kendrick mass is defined by setting the mass of a chosen molecular fragment, typically CH2, to an integer value in atomic mass units:

*Kendrick mass = IUPAC mass* (14/14.01565)*The Kendrick mass defect is defined as the difference between the exact Kendrick mass and the nominal Kendrick mass (integer):

*Kendrick mass defect(KMD) = Kendrick mass - nominal Kendrick mass*The Kendrick scale effectively converts the mass of CH2 from 14.01565 to exactly 14. Thus, homologous series (namely, compounds with the same constitution of heteroatoms and number of rings plus double bonds, but different numbers of CH2 groups) will have an identical Kendrick mass defect. The Referenced Kendrick mass-defect (RKMD) employs additional steps to remove the mass defect of a lipid core/backbone of interest and normalizes these values such that homologous species with saturated chains have a RKMD of 0 and species with varying degrees of unsaturation have integer RKMD values of -1, -2, -3 and so on (see example of a RKMD plot below).

*Referenced Kendrick mass defect(RKMD) = (experimental KMD - reference KMD)/0.013399*(where 0.013399 is the mass defect contribution of 2H)

**Calculate the Referenced Kendrick mass-defect (RKMD) for a lipid class of interest from the menu options below.**

The RKMD value may then be used to screen a list of precursor ions for that lipid class.