Normalization of mass spectra acquired by mass spectrometric imaging

Abstract

Mass spectra acquired by imaging mass spectrometry (IMS), in particular MALDI imaging of tissue sections, are each normalized by one of: the p-norm of the mass spectrum transformed by applying an exclusion list, the p-norm of the mass spectrum transformed by square rooting the intensity values, the median of the mass spectrum, and the median absolute deviation of the noise level of the mass spectrum.

1 . A method for normalizing mass spectra of a mass spectrometric imaging data set, comprising normalizing each mass spectrum by one of:
 (i) applying an intensity value exclusion list to that mass spectrum and then computing the p-norm of the resulting mass spectrum,   (ii) square rooting the intensity values of that mass spectrum and then computing the p-norm to the resulting mass spectrum,   (iii) the median of the mass spectrum, and   (iv) the median absolute deviation of the noise level of the mass spectrum.   
     
     
         2 . The method of  claim 1 , further comprising:
 (a) deriving a first mass image from the mass spectrometric imaging data set wherein each mass spectrum has been normalized by one of (i)-(iv);   (b) normalizing each mass spectrum by computing a p-norm of that mass spectrum without applying an exclusion list;   (c) deriving a second mass image from the mass spectra normalized in step (b); and   (d) comparing the first and second mass images and normalizing the mass spectra using the method in step (b) when the first and second mass images are substantially similar otherwise normalizing the mass spectra using the method in step (a).   
     
     
         3 . A method for normalizing mass spectra of a mass spectrometric imaging data set comprising the steps:
 (a) calculating first normalization factors for each mass spectrum by a p-norm;   (b) calculating second normalization factors for each mass spectrum by one of: the p-norm of the mass spectrum transformed by applying an exclusion list, the p-norm of the mass spectrum transformed by square rooting the intensity values, the median of the mass spectrum, and the median absolute deviation of the noise level of the mass spectrum;   (c) normalizing each mass spectrum by the corresponding first normalization factor, when the first and second normalization factors match in a statistical test, otherwise normalizing each mass spectrum by the corresponding second normalization factor.   
     
     
         4 . The method of  claim 3 , wherein the statistical test is a correlation. 
     
     
         5 . The method of  claim 4 , wherein a correlation coefficient greater than 0.8 indicates a match of the first and second normalization factors. 
     
     
         6 . The method of  claim 3 , wherein the statistical test is a chi-square analysis of the distributions of the normalization factors. 
     
     
         7 . The method of one of  claims 1  to  3 , wherein the mass spectra of the mass spectrometric imaging data set are acquired by MALDI imaging. 
     
     
         8 . The method according  claim 7 , wherein a tissue section is analyzed by MALDI imaging to produce the mass spectrometric imaging data set. 
     
     
         9 . The method of one of  claims 1  to  3 , wherein the p-norm is the total ion count. 
     
     
         10 . The method of one of  claims 1  to  3 , wherein the mass spectrometric imaging data set is acquired from a tissue section and the method further comprises the steps of deriving a mass image from the normalized mass spectra; and displaying the mass image in order to determine and visualize the spatial distribution of compounds in the tissue section.