Made of top quality 3.3 borosilicate glass. With plastic stopper. Designed according to ISO 1042, DIN 12664, USP standards and “Eichordnung” (the German Federal Weights and Measures Regulations). The conformity with “Eichordnung” is certified through the official “DE-M (year)” sign on each flask. Calibrated to contain (TC, In). USP standards are more accurate than ISO 1042. Each flask has a batch number printed on the flask. Batch certificate included (to be downloaded free of charge here)