Isopropyl alcohol

For any gas to be detectable by an electrochemical sensor, it must first be “electroactive.” As defined by the International Union of Pure and Applied Chemistry (IUPAC) Analytical Chemistry Division Commission on Electroanalytical Chemistry, an electroactive substance is…

In voltammetry and related techniques, a substance that undergoes a change of oxidation state, or the breaking or formation of chemical bonds, in a charge-transfer step.

Most alcohols are electroactive. Indeed, considerable work has been done in the development of alcohol fuel cells, and several alcohol breath meters used by police agencies are based on electrochemical sensors.

In the toxic gas detection field, sensitivity to alcohols has also been exploited, but usually for dubious purposes. For example, certain brands of so-called formaldehyde detectors are really little more than re-labeled alcohol detectors, that have much greater response to the alcohols, than to formaldehyde. In contrast, the Interscan formaldehyde sensor has good rejection to alcohol interferents, especially the ones that are typically encountered in formaldehyde monitoring applications.

In the application of monitoring ethylene oxide (EtO), certain manufacturers have foolishly and irresponsibly instructed their customers to bump-test (challenge a gas detector with something other than a legitimate calibration standard) their instruments with alcohol. This is most often done by simply taking a rag or swab that has been wetted with alcohol, and placing it close to the sensor inlet.

The theory is that since it is “too difficult” to obtain the proper EtO calibration standard, the alcohol bump-test will at least assure the user that the monitoring instrument is still working. Unfortunately, this practice is dangerous and is to be avoided because:

  • Current occupational exposure limits for EtO mandate monitoring for it at 1 ppm or less. How a sensor might respond to perhaps 1000 ppm or more of alcohol is in no way related to its low-level EtO performance. As such, a false sense of security will almost surely be instilled. Worse, proper calibration might be performed infrequently or not at all.
  • Even if the bump-test procedure were somehow refined into a sort of surrogate calibration method, based on reproducible sensor response data, it is still not appropriate. Analytical best practices demand that an instrument be calibrated with the target compound to be detected. Imagine defending a lawsuit on EtO exposure, and having to admit that the instrument was “calibrated” with an alcohol.

As to how alcohols affect the Interscan electrochemical sensor, it all comes down to the most basic rule of toxicology, as articulated in the sixteenth century by a Swiss chemist named Paracelsus: “The dose makes the poison.”

Parts-per-million level concentrations of alcohol would simply be detected, and would react away, generally without incident. However, in many cases where our sensors are exposed to alcohol, the concentrations are not low. Rather, the concentrations are at levels in excess of 500 ppm, and even higher. Bear in mind that the OSHA PEL for isopropyl alcohol is 400 ppm.

At such relatively high concentrations, an Interscan sensor—intended for operation at ppm and sub-ppm levels—will surely react to the alcohol, but this overexposure will cause the instrument to go off-scale, until all the alcohol reacts away. Moreover, certain reaction products could build up on the sensing electrode, creating a long-term “background” effect, that can markedly affect sensor performance.

With prolonged or repeated exposure to macro levels of alcohols, an Interscan sensor can be effectively destroyed, and be rendered essentially useless for detection of the target toxic compound. This is why we have always warned our customers about alcohol bump-testing, and provide means for instrument shut-off and automatic re-start, should alcohol be used to scrub down work areas, that are being monitoring for EtO.

Be concerned, then, that alcohol contamination can void the sensor warranty, but be more concerned that exposing the sensor to macro levels of alcohol will compromise your monitoring program.

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