Isotope scrambling and error magnification in multiple-spiking isotope dilution

Correction of creatine-creatinine conversion during serum creatinine quantification by two-dimensional liquid chromatography and double-spike isotope dilution tandem mass spectrometry

BACKGROUND AND AIMS

Development of a candidate reference method based on bidimensional liquid chromatography coupled to ESI-MS/MS and double spike isotope dilution for serum creatinine quantification capable of correcting for creatinine-creatine interconversion during sample pretreatment. Study of the impact of the creatine-creatinine interconversion during the analysis of human serum samples.

MATERIALS AND METHODS

13C1-creatinine and 13C2-creatine are added to the serum sample. Separation carried out by bidimensional liquid chromatography combining reversed phase and a strong cation exchange chromatography. The heart cut, containing creatine and creatinine, is automatically transferred to the second dimension. Quantification carried out by double spike isotope dilution tandem MS/MS.

RESULTS

Minimization of spectral interferences and ion suppression due to matrix effects while increasing sample throughput compared to the direct coupling of cation exchange chromatography to the ESI source. Trueness of the method studied with the satisfactory analysis of two certified reference materials. Satisfactory intra- and inter-day precisions obtained analysing a serum pool and control sera. Analysis of 93 serum samples revealed negligible interconversions with no correlation with creatine levels.

CONCLUSIONS

The method provides adequate analytical figures of merit for serum creatinine determination according to CSLI guidelines. Negligible creatine-creatinine interconversion is promoted with the applied sample preparation procedure.

Measurement of mercury species in human blood using triple spike isotope dilution with SPME-GC-ICP-DRC-MS

The measurement of different mercury compounds in human blood can provide valuable information about the type of mercury exposure. To this end, our laboratory developed a biomonitoring method for the quantification of inorganic (iHg), methyl (MeHg), and ethyl (EtHg) mercury in whole blood using a triple-spike isotope dilution (TSID) quantification method employing capillary gas chromatography (GC) and inductively coupled dynamic reaction cell mass spectrometry (ICP-DRC-MS). We used a robotic CombiPAL(®) sample handling station featuring twin fiber-based solid-phase microextraction (SPME) injector heads. The use of two SPME fibers significantly reduces sample analysis cycle times making this method very suitable for high sample throughput, which is a requirement for large public health biomonitoring studies. Our sample preparation procedure involved solubilization of blood samples with tetramethylammonium hydroxide (TMAH) followed by the derivatization with sodium tetra(n-propyl)borate (NaBPr(4)) to promote volatility of mercury species. We thoroughly investigated mercury species stability in the blood matrix during the course of sample treatment and analysis. The method accuracy for quantifying iHg, MeHg, and EtHg was validated using NIST standard reference materials (SRM 955c level 3) and the Centre de Toxicologie du Québec (CTQ) proficiency testing (PT) samples. The limit of detection (LOD) for iHg, MeHg, and EtHg in human blood was determined to be 0.27, 0.12, and 0.16 μg/L, respectively.