AUric acid should be contained from National Institute of Standards and Technology (NIST) for comparison and measurement of reference materials obtained from other sources. Standard operating procedure should include weekly crosschecks of these standard material Use NIST Standard Reference Material (SRM) to calibrate instrument and to analyze for nitrogen content of other reference material using instrument parameter as in E. Other primary standard material for instrument calibration may be used if nitrogen content is verified by NIST SRM. Except for uric acid (which liberates HCN when heated), dry reference materials 2 h at 105 C before use. Store reference materials in desiccator. Suggested reference materials