A method of identifying molecules of biological origin is disclosed. The molecules are identified and the basis of the accurately determined mass to charge ratio of the molecules and at least a further physico-chemical property such as elution time or charge state. Further physico-chemical properties may be used. The experimentally determined accurate mass and physico-chemical properties can then be compared with a look-up table of information. The look-up table may generated or physico-chemical properties of data in a conventional database may be calculated. The ability to recognize and preferably identify the same molecules in two different samples may be used to determine whether a particular biological molecules has been expressed differently in an experimental sample relative to a control sample.