A computer implemented process for producing a representation of a reference spectrum for a hypothetical solution having a first pH condition, for use in determining the composition of a test sample, is disclosed, The process involves producing a position value for at least one peak of the reference spectrum in response to a measured pH condition of the test sample and a property of at least one peak in a base reference spectrum for the hypothetical solution, the base reference spectrum being associated with a pH condition of the hypothetical solution that is different from the measured pH condition.