or a mixture of inert gases containing precursor ions in a preferred embodiment the precursor ions are h 3 o no or o 2 but the inert gas or mixture of inert gases may contain other precursor ions or negative precursor ions the reactive metabolites in the breath are ionised by the process of chemical ionization to form product ions of the reactive species the ratio of the product ions to precursor ions gives the absolute concentration of the sample the analysis of each trace gas constituent is completed and displayed within seconds as stated above while the trace metabolites are preferably measured by selective ion flow tube mass spectrometry sift ms it is to be understood the measurements can also be made using other flow tube or drift tube methodologies or by atmospheric chemical ionization methods as illustrated in the fig1 which is a measurement showing bromamine in individual breath samples the subject a and b are asthmatics and the subjects c and d are healthy people fig2 is a measurement of breath samples showing chloramines the subject a is a healthy person and the subjects b and c suffer from copd fig3 is a measurement of a real time bromamine standard sample using sift ms and fig4 is a measurement of a real time chloramine standard sample using sift ms bromamine concentrations in individual expired breaths were.