Miniature conductivity and pH sensors using ion selective field effect transistors applied in cuvette-based dialysis spectroscopy

Abstract A recently introduced device and method for spectroscopically monitoring the behavior of macromolecules during chemical dialysis in a cuvette were applied to several synthetic and biomacromolecules, including biologic drugs. Via a dialysis membrane, the device partitions the cuvette contents into Fluid 1, containing the macromolecules, from Fluid 2 containing the dialysate, which is circulated through an external reservoir. When dialysis is carried out against electrolyte solutions, including ionic excipients, it is difficult to precisely measure the time-changing concentration of the electrolyte in the macromolecular solution in the cuvette, because conventional conductivity probes are too large to fit into a cuvette, together with the inserted dialysis device. Because of this, measurements of electrolyte concentration in Fluid 1 until now have been made by measuring the conductivity in the external Fluid 2 reservoir and inferring the conductivity in Fluid 1 via the ratio of volumes of Fluid 2 to Fluid 1 (typically 40x to 400x). This method is imprecise, since conductivity variations in Fluid 2 are small. Now, a recently available miniature (∼1 mm) ion selective field effect transistor (ISFET) for both conductivity and pH has been adapted to the dialysis system and fits into the cuvette. This gives an immense improvement in data precision and opens the door for the method being able to scan the effect on macromolecules over a wide range of electrolyte concentrations, which can aid in optimizing biologic formulations. A single dialysis experiment of this type may be functionally equivalent to high throughput testing of multiple discrete samples. Results are presented here for an integrated cuvette device containing (i) dialysis membrane, (ii) non-contact stirring impeller,(iii) dialysate circulation channels, and (iv) the miniature ISFET probe. It can be used in any instrument accepting 1 cm square cuvettes, without any modifications to the instrument or its native data gathering software.