North American Sterilization & Packaging Company
17 Park Drive, Franklin, New Jersey 07416 USA
Phone:1.973.209.4388    1.800.392.6310       Fax:1.973.209.6374        Email: info@naspco.com

(Continued from Page 7, Gas Diffusion)

Speeding EtO-Sterilized Products to Market with Parametric Release (continued)

Gas Analysis: MW Spectroscopy

Microwave (MW) spectroscopy functions like IR analysis--through the absorption of electromagnetic radiation. The MW spectrum contains the wavelengths between 0.1 and 30 cm.

The object of the MW technique is to determine the MW frequencies that are selectively absorbed by certain materials.¹⁰

The data allow insight into the atomic, molecular, and crystalline structure of samples. At low pressures, there is a linear relationship between absorbance maxima and molecular concentration.

Like IR, MW radiation is not energetic enough to induce an electronic transition, but it can cause changes in the vibrational and rotational dipole motion of the molecule, which is the method used by this technology. The MW spectra of analyte gases at low pressures are characterized by very narrow bandwidth absorption lines.¹⁴

In the 1980s, with a grant from the Medical Devices Directorate of the Department of Health in the United Kingdom and from private industry, a group of researchers at the University of Wales College of Medicine (Cardiff) began investigating MW spectroscopy for direct analysis of EtO concentration and water vapor during sterilization. In the past decade, they have successfully designed, prototyped, demonstrated, and patented an MW molecular rotational spectrometer that not only measures sterilant and water vapor inside a vessel using 100% EtO or EtO/diluent mixtures, but, like the IR spectrometer, also measures multiple analyte gases at the same time during the entire cycle.^15,16

The researchers have documented and confirmed that the system provides accurate, reproducible, and unambiguous determination of EtO sterilization process parameters, making it suitable for parametric release. Interfaced with an automated sterilizer control system, it represents another viable solution to gain parametric release. The group has obtained a U.S. patent for their spectrometer.¹⁷

The work of the researchers was an extension of a method for MW gas analysis that had been proposed in 1967.¹⁸ In this original procedure, a gamma coefficient was monitored while a sample of analyte gas was exposed to an increasing incident power until the maximum signal (S_max) was determined. S_max was found to be directly proportional to the concentration (N) of the analyte gas (the number of molecules). In the new system, a constant flow of gas from the sterilization vessel is circulated through a continuous loop. First, the gas flows from the vessel to a gas cell (a cavity with a preset resonant frequency) and then, once the analysis is complete, it is returned to the vessel.¹⁸ The pressure of the sample inside the spectrometer cavity is linearly proportional to the headspace pressure of the sterilizer. Therefore, the concentrations of EtO and water vapor inside the spectrometer cavity are also proportional to the concentrations of EtO and water vapor inside the sterilizer.

The prototype unit was programmed to emit an MW source frequency range, which the researchers called a scanning window.¹⁸ This window included the full linewidth of the absorption lines of the analyte gases: EtO (scanning window, 4 MHz; absorption line, 23.123 GHz) and water vapor (scanning window, 1 MHz; absorption line, 22.235 GHz).¹⁴ Studies of the system revealed that the linear correlation between the output signal and the concentration of EtO in the spectrometer cavity yielded results of ±4% accuracy. Considering the high concentrations inside a vessel, which can range from 350 to 1200 mg/L, such a level of accuracy is more than acceptable for the control and monitoring of a parametric release sterilization system.¹⁸

Several qualities of this system make it suitable for monitoring and controlling a sterilization cycle. First, because the partial pressures of EtO and water vapor in the analyte sample and the partial pressures of these gases inside the vessel are proportional, and the conductance of the inlet and outlet valves of the gas cell are constant, this system can relate the concentration inside the analyzer to that of the vessel without having to monitor the pressure inside the vessel.¹⁵ Second, the research team reports that in their instrument, the frequency of the resonant cavity does not require an identical source frequency, as other types of spectrometers do. The ability to vary the source frequency means that the concentration of several gases in a mixture could be measured effectively.¹⁸

This website is designed for monitors with a resolution of 1024 x 768
and
to be viewed with MS Internet Explorer or Netscape Navigator Web Browsers

Send mail to fdpp@fdpp.com with questions or comments about this web site.
Copyright © 2001 North American Sterilization & Packaging Company, Inc.