Disinfecting MET ONE Sensors with Vaporous Hydrogen Peroxide

An aseptic production environment is one of the requirements of pharmaceutical manufacturers, in order to reduce the existence of microbes and hence the contamination of the final product. As a strong oxidizer and effective disinfectant, vaporous hydrogen peroxide (VHP) is commonly used for disinfection. A 3% VHP with an exposure time of 45min is used as part of a typical clean room disinfection cycle.

VHP exposure can damage the optical particle counters by bleaching the sensor’s internal surfaces and corroding the mirror surface. This, in turn, increases reflections within the sensor, thus providing erroneous particle counts. This article discuses the test conducted by Hach Ultra to gain insights into the impact of VHP on the sensor in MET ONE remote air particle counters. As part of the test, the sensor and the mirror were examined.

Experimental Procedure for the Sensor

A gaseous vapor of 5-6% concentration of hydrogen peroxide was produced by partly filling a clean vessel with an aqueous solution of 30% hydrogen peroxide in deionized water. A Gastec Dosimeter tube was used to measure the VHP concentration.

The sensor to be tested was positioned outside the vessel and attached to an isokinetic probe placed at a distance of 6mm from the aqueous solution surface. An external vacuum source was used to draw in the vaporous hydrogen peroxide onto the sensor.

After continuously exposing the sensor for 72 hours, it was examined to get a like comparison with the testing carried out by Particle Measuring Systems (PMS). After 744 hours of continuous VHP exposure, the sensor was ready for final evaluation. This exposure was equal to 1,090 disinfection cycles with an exposure time of 45min.

Experimental Procedure for the Mirror

Besides the aforementioned vaporous hydrogen peroxide test, the mirror immersed in an aqueous solution of 30% hydrogen peroxide for 600 hours. After this period, the mirror was taken out from the solution and visually inspected. The visual inspection was followed by the reinstallation of the mirror in the sensor to determine whether any changes are occurred in calibration.

Experimental Results

The results summarized in Table 1 showed the stability in calibration voltage and noise over time, suggesting that the ability of the sensor to count 0.3-0.5µm-sized particles had not changed significantly.

Table 1. Standard optical black coating

DATE CAL VOLTAGE (VOLTS) NOISE 0.3 MICRON PEAK VOLTAGE 0.5 MICRON PEAK VOLTAGE
21-APR-2006 1 17 34.65 MV 335.26 MV
25-APR-2006 0.998 17 34.65 MV 334.52 MV
1-MAY-2006 0.95 17 34.40 MV 337.45 MV
31-MAY-2006 0.952 17 34.65 MV 335.01 MV

The results for the mirror after being subjected to direct VHP exposure for 600 hours are shown in Figures 1, showing negligible deterioration on the reflective surface of the mirror but some degradation on the edges. However, these edges are not functional optical surfaces.

Photographs of the mirror after 600 hours direct exposure to the 30% aqueous solution of hydrogen peroxide.

Figure 1. Photographs of the mirror after 600 hours direct exposure to the 30% aqueous solution of hydrogen peroxide.

The exposure time for the testing carried out by Hach Ultra was more stringent when compared to the testing performed by PMS (Table 2).

Table 2. Comparison of Hach Ultra and PMS testing

Hach Ultra Testing PMS Testing
Sensor Simulated VHP Disinfection Cycles 1,090 96
Mirror Submergence in 30% Aqueous H2O2 (Days) 25 2.67

Conclusion

The results corroborated the VHP resistance of the critical sensor components present in the Hach Ultra products. The key sensor components of the MET ONE 4500, 4800, 4900, 5800, 5900 and 2300 products can be designated as VHP-resistant.

Since sampling during disinfection is required neither by the US FDA nor EU GMP, it is recommended to cap the product intake during disinfection practices. However, other components of the MET ONE products, including filter and pump, may be prone to damage caused by oxidizers such as VHP.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Characterization.

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