Preparation of creatinine standard stock solution

The liquid chromatography isotope dilution mass spectrometry (LC/ID-MS) has recently been used for the certification of organic reference materials. We are developing a new definitive method of LC-ID/MS as to determination creatinine in serum. We prepared a stock standard solution of creatinine in 10 mmol/l of acetic acid at a concentration of 8.84 mmol/l. With this acetic acid concentration, creatinine dissolved completely in a few minutes. This stock standard solution was stable at least for 1 year and has widely applications (such as GC-MS, LC-MS, LC, colorimetric method, etc).