Increasing the Life-Span of a Probe to Cover a Warranty Period

Q. I work in a quality control laboratory and use a pH meter with a single-junction reference probe to test samples taken from various pharmaceutical (drug) production lines. The probe I use has a six-month warranty. It slowly stops working after just a couple weeks. I am replacing it once a month. We make sure to store it correctly according to the manufacturer’s specifications and even use probe cleaner at the end of each shift to ensure that it stays clean. What can I do to increase the lifespan to at least cover the warranty period?

A. The most common problem seen with pH readings in pharmaceutical-drug testing is a progressively sluggish response. If a probe calibrates quickly but does not respond well in a sample or if it slowly quits working within a few weeks, it is most likely that the probe is not compatible with the sample. Single-junction probes are good for measuring weak ionic water-based solutions and drinking water. Because you are working with drugs, which are much more complex molecules, single-junction references are not suitable. There is nothing that can be done to prolong the life of a single-junction probe in this situation; they are simply not compatible.

Double and flushable junction probes are more appropriate for pharmaceutical applications because samples tend to be more complex. Large molecules will clog the single-junction probes over time. Thus, they may work well initially then become slow to respond over time. If a double junction probe is responding sluggishly, it simply may need to be cleaned. You can continue to clean the probe as you have (i.e., daily). Cleaning prevents proteins, fatty acids, and oils from building up on the glass and causing a delayed response. Proper cleaning and storage of probes is imperative for obtaining the longest life and their optimal accuracy.

If you encounter a situation in which the probe does not change the measurement values at all, the probe has probably shorted out. This happens most often during shipping or with rough handling. The wires used for the cable and reference are thin and somewhat fragile, and will give a steady reading if broken. We have also seen this happen often in production and manufacturing environments.

—Jennifer Minor-Newton, technical application specialist for Cole-Parmer