Description_x000D_ General description_x000D_ For more information on other Sigmatrix products visit the Sigmatrix Selection Guide._x000D_ Suitability_x000D_ This solution is provided ready-to-use as a 2% (w/v) recombinant HSA (Human Serum Albumin) in PBS (Phosphate Buffered Saline), pH 7.4. This formulation has been shown to be appropriate for use as a sample diluent or matrix for preparing calibrants/controls for clinical diagnostic LC-MS/MS assays. Beneficially, as the HSA source is recombinant, the matrix should not contain any measurable contaminants typically found in even doubly stripped serum. SigMatrix Ultra has been successfully used in LC-MS/MS assays for clinically relevant concentrations of total testosterone (20–2,000 pg/mL), estradiol, and estrone (2–1,000 pg/mL). Calibrators prepared in SigMatrix Ultra showed good linearity (r2 >0.97) and no interference at the lowest calibrator concentrations for all analytes tested. Additionally, extracted blank SigMatrix Ultra spiked with analyte specific isotopicallylabelled internal standards showed no interferences above the lower limits of quantitation at the retention time of the respective analytes._x000D_ Physical form_x000D_ SigMatrix Serum Diluent is an aqueous solution containing 2% highly purified recombinant human albumin (expressed in rice) in phosphate buffered saline (pH 7.4) with no preservatives