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 4, Sampling)

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

Gas Analysis: Chromatography

After determining how to draw the gas samples, sterilization engineers must also decide on a gas analysis method. The two most common ways to analyze in-chamber gases are chromatography and spectroscopy.

Gas chromatography (GC) is already well known in the EtO industry for its applications in monitoring work areas and analyzing chemical residuals on products after processing. It can also be used to directly analyze headspace gases in EtO sterilization. In this application, a gas sample is injected into a heated port. This can be done either manually by collecting the sample with a syringe and injecting it through a rubber septum, or automatically with a computer-controlled vacuum pump that extracts the sample from the vessel and channels it through a heat-traced line leading to the analyzer.

After injection, the sample is vaporized with a heating element and then mixed with a carrier gas, which is usually ultrapure nitrogen or helium. This gaseous mixture then passes through a silica column. The column, which is inside a temperature-controlled oven, is packed with ground firebrick or diatomaceous earth that has been coated with a layer of a liquid that has a very high boiling point, such as a silicone or squalene.

When the gases enter the column, they adsorb onto the liquid layer. The higher the solubility of the analyte gases, the slower they will move through the column and exit into the GC detector. The gases desorb from the column and enter the detector separately, allowing identification and quantification.¹⁰

Each substance generates a concentration pulse as it enters the detector. This pulse is recorded as a chromatogram, a graph showing the detector response as a function of elution time. The number of peaks on a chromatogram reflects the number of components present in the sample. The position of the peaks, which reveal retention times when they are compared with known standards, allow identification of the sample gas components. Retention time is the time it takes for the peak to reach the detector after injection. The height and areas of the peaks show the quantity of each component. Several types of GC detectors are available: thermal conductivity detectors, flame ionization detectors, and element-specific detectors.

Unlike many other gas analysis methods, GC operates at atmospheric pressure, creating special requirements for the sampling system. Some sterilization cycles condition at very low pressures. The dynamic environmental conditioning cycle, for example, conditions at less than 1.0 psia. GC operates at a pressure of ~14.7 psia. Therefore, sampling for EtO during the sterilant dwell cycle, which usually occurs close to or above atmospheric pressure, is easier because the pressure difference between the GC and the sterilizer offers less resistance than in other cycles.

Automated sampling can be accomplished with specially designed pumps that can overcome the vacuum inside the vessel and bring the gas to the column, where it is then allowed to rise to atmospheric pressure before analysis. Naturally, the most important factors affecting this part of the system are the sample lines, which must be perfectly sealed and heat traced, and the pump, which must perform at a consistent level.

Calculating the Gas Concentrations. The initial pressure (P₁) and temperature (T₁) of the gas sample will change dramatically as it is removed from the vessel. And as the gas enters the GC injection port, these changes (P₂ and T₂) will need to be factored into the final concentration calculation.

Calculation of the analyte gas concentrations inside the GC column to determine the process concentrations inside the sterilizer is accomplished with a gas law that combines Boyle's law (on pressure and volume) and Charles's law (on volume and temperature).¹¹

Boyle's law:

Charles's law:

Combined gas law:

The combined gas law, together with Avogadro's law, which describes the relationship between volume (V) and number of moles of a gas (n),

can be used to determine density (d) or the mass per unit volume of a gas in SI units (kg/m³ or mg/L). Because gas density is inversely proportional to volume, the following can be derived:

or

where d₁ = gas density inside the sterilizer and d₂ = gas density inside the GC column.

The GC control system receives as inputs the temperature and pressure inside the sterilizer (P₁ and T₁) and the temperature and pressure inside the column (P₂ and T₂), and will detect the density of the analyte gas components injected into the port (d₂). This leaves only one unknown: the gas density inside the sterilizer (d₁). The proportional relationship between d₂ and the density of the gas inside the sterilizer (d₁) can be expressed in terms of the combined gas law:

This equation can be resolved to yield the density of the analyte gas inside the sterilizer (d₁) as:

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.