Made from low expansion borosilicate glass (Pyrex® or equivalent ASTM Type 1 Class A glass).

A coaxial insert consists of an upper glass alignment tube with a precise outer diameter that fits very closely within the matching 5 or 10 mm thin wall outer glass NMR tube, ensuring accurate coaxiality and centering of the insert within the outer NMR tube. The lower glass stem tube, of a smaller diameter, typically holds an external lock or reference material. The smaller diameter stem tube is precision sealed to the upper alignment tube using a highly accurate, state-of-the-art glass working lathe, maintaining the coaxial alignment necessary for the acquisition of high quality spectra. The accompanying table presents the most commonly used sizes for Bruker and Varian probes.