Diagnostic method for binary pumpsLooking at the graph, we can see several charachteristics of the solvent flow.
- Absolute flow accurracy for any given programmed flow rate.
- Variance of flow at a given flow rate. (sometimes called 'pulsation')
- 'Rise time', or time to reach the increased programmed flow.
- 'Decay time', or time to reach the decreased programmed flow.
- The differences in the above between pump A, and pump B
This method was developed in the lab of Daniel Sykes, Professor of Analytical Chemistry, at The Pennsylvania State University. The background is that John Best, of Best Instrument 'interned' in the summer of 2006 for Dr. Sykes, with the intention of both learning HPLC, testing products, and developing HPLC methods for use in analytical chemistry lab courses. It was noted that one of the two pumps on the HPLC system was not performing well. The 'method' was simply to digitize pump flow rate relative to a programmed range of flow rates, first for 'pump A', then 'pump B'. This method turned pump A on at a low flowrate for a period of time, then increased the flow rate in steps. Then, pump A qould be turned off, and pump B would be taken through the same flowrates.
Pump A, (the malfunctioning pump) runs from T=0 to about T=20 minutes. Pump B runs from 20 minutes until just over 40 minutes. The method file is apparent from the graph, first, step one of the pumps up (or down) through a range of flowrates, then do the same for the other pump. The graph is enlightening. Note the flow rates for pump 1 for which serious anomalies occur. In the end, these anomalies were attributed to a bad piston seal. After a new seal was obtained through Restek the method was re-run. Unfortunatley, the 'post correction' data has been misplaced, but in essence, the flow profiles for pump A and pump B were nearly identical.
In light of flow variance, we should discuss using precisely mixed solvents (isocratic), vs. mixing solvents 'on the fly', and that may become part of a different atricle.