Measuring the streaming potential of HPCE capillaries is an excellent way to measure the condition of a capillary after a reconditioning process. The streaming potential is a voltage generated when a 2-bar pressure pulse is applied to the capillary to move the buffer down the capillary when the high voltage has been turned off. This low flow generates a charge separation called the streaming current. Since there is also electrical resistance, there is a voltage according to Ohm’s law (volts = amps × resistance in ohms). In HPCE, the streaming potential is usually in the mid-mV range, which is low compared to the kilovolts usually applied to the capillary during separation.
Dr. Vadim Okun of Lumex (Mission, BC, Canada) presented an interesting webinar that described how the streaming voltage could be measured and used to check the reconditioning of an analytical HPCE capillary. This is useful since the throughput of HPCE can be reduced by overly cautious run-to-run regeneration protocols, especially with uncoated capillaries. Or, incomplete regeneration can lead to migration time instability, which makes peak recognition and assignment of response factor and peak identity problematic. Okun shows that by using the streaming potential to guide capillary regeneration, the %CV of migration times can be improved dramatically. This can be especially useful when run times or sample queues are long.
The measurement requires a separate subsystem in the Capel-205 instrument, which is manufactured by Lumex. With the Capel-205, the high run voltage is turned off and the streaming potential is measured by a program initiated by a short pneumatic pressure pulse that moves the background electrolyte (BGE) down the capillary. This generates charge separation, which is compensated for by the opposing charging current. The output is a square wave voltage signal in the mV range. The voltage of the square wave can be compared with measurement made in prior runs. If the streaming potential is the same, then subsequent CE runs with the same sample should be highly similar in migration time and peak response. If the difference is large, one should anticipate quite different migration time and response factors for subsequent HPCE experiments.
Practically, between HPCE runs, the capillary is often reconditioned by flushing with a rinse buffer using a validated regeneration protocol. After the analytical run is the regeneration cycle. Once the capillary is filled with the BGE, the streaming potential can be measured prior to the next run. Comparing the results with prior runs will be a guide to capillary regeneration. Was regeneration sufficient? Reproducible?
The message to take back to the lab is that one should be aware that streaming potential has practical utility for monitoring and predicting the performance of HPCE separations. This is especially important when the workload requires many successive runs, with little or no operator attention.
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]
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