PolarCept
It is recommended to use the external PolarCept filter when measuring a mixture of gas (such as when sampling fruit) or interfering gases may be reported by the instrument. The external filter can be used with any of the three measurement modes, and is recommended for Fixed Volume and Trigger Modes. PolarCept should only be used with 1.5 mL of distilled or deionized water.
The PolarCept filter consists of a plastic molded part, hydrophobic filter and O-ring. It is also used with the “fixed volume tubing” and sample probe needle. A small plastic syringe is used to fill and empty PolarCept. Once the hydrophobic filter and O-ring are in place, it should be very difficult to remove it, creating a leak-proof seal. Additional hydrophobic filters are included as replacements, when the filter is soaked with water or damaged during removal.
Eventually the water in the PolarCept filter will become saturated with trapped interfering gases and should be replaced with fresh distilled water. Saturation rates will depend on the measurement mode and amount of interfering gases present in the sample environment. The table below shows example saturation times when measuring headspace of bananas (with a maturity index of 5) in Continuous Mode. This sample contains various mixed hydrocarbons, ethylene and VOCs. The total VOC in ppm listed is the signal reported by the C2H4 sensor in ppm.
| VOC concentration | PolarCept saturation (min) |
|---|---|
| 3 ppm | 20 |
| 100 ppm | 1 |
To fill the PolarCept filter, attach an empty syringe to the hydrophobic filter. The plunger of the syringe should be completely depressed. Lower PolarCept over a cup of distilled water and draw in 1.5 mL with the syringe. Attach the sample needle and fixed volume tubing to stop leaks. To empty PolarCept, re-attach the syringe and push the water out the sample needle end.
To properly use PolarCept, keep the sample needle pointed downwards while measuring, as seen in the example below on the left. The water in the filter should “bubble” as the gas sample is pulled through it, causing some of the interfering gases to be trapped.
See the following graph of measured ethylene concentration by the F-950 with and without using PolarCept, measuring the same headspace of bananas. After about 20 minutes, the water becomes saturated, and the signal beings to rise with PolarCept.