a capillary absorption spectrometer and process are described that provide highly sensitive and accurate stable absorption measurements of analytes in a sample gas that may include isotopologues of carbon and oxygen obtained from gas and biological samples . it further provides isotopic images of microbial communities that allow tracking of nutrients at the single cell level . it further targets naturally occurring variations in carbon and oxygen isotopes that avoids need for expensive isotopically labeled mixtures which allows study of samples taken from the field without modification . the process also permits sampling in vivo permitting real - time ambient studies of microbial communities .