Interfering Gases
No analytical method is completely specific. Gases present in the environment, other than the “target” gas of a measurement, may affect instrument response. Interferences are not necessarily linear, and may also exhibit time dependent characteristics. The following table details the approximate cross-sensitivity of the ethylene sensor to several common interfering gases. A 20% cross sensitivity means that 100 ppm of the interfering gas would read the same as 20 ppm of the target gas (ethylene). Therefore, 20 ppm H2S should read <40 ppm on the F-950: 10 ppm NO2 should read <5 ppm: 10 ppm Cl2 should read -0.1 ppm.
| Gas | Interfering gas (ppm) | Cross-Sensitivity (%) |
|---|---|---|
| H2S | 20 ppm | < 200 |
| NO2 | 10 ppm | < 50 |
| Cl2 | 10 ppm | < -1 |
| NO | 50 ppm | < 80 |
| SO2 | 20 ppm | < 50 |
| CO | 400 ppm | < 30 |
| H2 | 400 ppm | < 0.5 |
| C2H4 | 80 ppm | < 100 |
| NH3 | 25 ppm | < 0.1 |
| HCHO | 4 ppm | 90 |
| CO2 | 50000 ppm | < 0.1 |
Ripening fruit emit a complex mixture of hydrocarbons, including ethylene. Oxidation of these other gases in the electrochemical sensor cannot be readily distinguished from ethylene. This causes the ethylene value to be falsely high in the presence of interfering gas.
Felix Instruments has tested a method to absorb some of the competing gases and provide better ethylene measurements. This method, PolarCept, uses distilled water in an external chamber and has been shown to filter out some polar hydrocarbons and alcohols to produce less interference.