A method of mass spectrometry comprising: providing a first sample comprising a first mixture of components, molecules or analytes; providing a second different sample comprising a second mixture of components, molecules or analytes; and probabilistically determining or quantifying the relative intensity, concentration or expression level of a component, molecule or analyte in said first sample relative to the intensity, concentration or expression level of a component, molecule or analyte in said second sample; wherein said first sample and/or said second sample comprise a plurality of different biopolymers, proteins, protein digest products, peptides, fragments of peptides, polypeptides, oligonucleotides, oligonucleosides, amino acids, carbohydrates, sugars, lipids, fatty acids, vitamins, hormones, portions or fragments of DNA, portions or fragments of cDNA, portions or fragments of RNA, portions or fragments of mRNA, portions or fragments of tRNA, polyclonal antibodies, monoclonal antibodies, ribonucleases, enzymes, metabolites, polysaccharides, phosphorolated peptides, phosphorolated proteins, glycopeptides, glycoproteins or steroids, characterised in that the method comprises assigning a prior probability distribution function Pr for the relative amount or concentration L of components, molecules or analytes present in each sample; assigning a prior probability distribution function Pr(k) for the overall response factor k of components, molecules or analytes in the samples; predicting what would be observed for each mass peak intensity given probability distribution functions Pr and Pr(k); adjusting the value of L or the probability distribution function Pr and/or adjusting the value of k or the probability distribution function Pr(k); predicting what would be observed for each mass peak intensity given these adjustments; comparing mass peak intensities that are predicted with those that are observed; accepting or rejecting the adjustments; and determining the ratios of relative concentrations L of each component, molecule or analyte in each of said samples. providing a first sample comprising a first mixture of components, molecules or analytes; providing a second different sample comprising a second mixture of components, molecules or analytes; and probabilistically determining or quantifying the relative intensity, concentration or expression level of a component, molecule or analyte in said first sample relative to the intensity, concentration or expression level of a component, molecule or analyte in said second sample; wherein said first sample and/or said second sample comprise a plurality of different biopolymers, proteins, protein digest products, peptides, fragments of peptides, polypeptides, oligonucleotides, oligonucleosides, amino acids, carbohydrates, sugars, lipids, fatty acids, vitamins, hormones, portions or fragments of DNA, portions or fragments of cDNA, portions or fragments of RNA, portions or fragments of mRNA, portions or fragments of tRNA, polyclonal antibodies, monoclonal antibodies, ribonucleases, enzymes, metabolites, polysaccharides, phosphorolated peptides, phosphorolated proteins, glycopeptides, glycoproteins or steroids, characterised in that the method comprises assigning a prior probability distribution function Pr for the relative amount or concentration L of components, molecules or analytes present in each sample; assigning a prior probability distribution function Pr(k) for the overall response factor k of components, molecules or analytes in the samples; predicting what would be observed for each mass peak intensity given probability distribution functions Pr and Pr(k); adjusting the value of L or the probability distribution function Pr and/or adjusting the value of k or the probability distribution function Pr(k); predicting what would be observed for each mass peak intensity given these adjustments; comparing mass peak intensities that are predicted with those that are observed; accepting or rejecting the adjustments; and determining the ratios of relative concentrations L of each component, molecule or analyte in each of said samples.