Turbulence as a source of excessive baseline noise during high-speed isocratic and gradient separations using...

The occurrence of turbulent flow conditions in the connection tubing or at the entrance of the detection flow cell has been identified as a potential source of excessive detector noise in the signal of the diode array UV-absorption detector flow cell of a commercial HPLC system used to perform high-speed separations on wide-bore columns (4.6 mm i.d.). This excessive noise was found to occur abruptly if the flow rate is increased beyond a certain critical value or during gradient runs, if the mobile phase viscosity falls below a certain critical value. Several detector cells of different dimensions were studied to investigate the dependency of the effect on the detector cell design and the system tubing. Evidence for the turbulent flow origin of this noise is that its onset always occurred at a given fixed value of the Reynolds number. This critical Reynolds number could also be used to predict the onset of detector noise during gradient elution runs. Second, evidence for the turbulent flow origin of the noise was that it could be reliably eliminated using a flow splitter after the column to reduce the flow rate below its critical value before entering the detector. The loss in separation efficiency and detection sensitivity accompanying this flow splitting solution was found to be so small that it does not weigh up against the huge advantage of the possibility to eliminate the excessive detector noise.