(continued from page 5, Chromatography)
Through spectroscopy, analyte gases are identified by their interaction with light. One branch of spectroscopy, infrared (IR) spectroscopy, deals with the ability of matter to absorb light from the midinfrared region of the light spectrum, or wavelengths between 2.5 and 25.0 µm.12
By identifying the wavelength at which a substance demonstrates absorbance maxima and comparing this wavelength value to known libraries of chemical compound spectral data, it is possible to identify the sample. After the absorbance characteristics of the sample have been identified, the Beer-Lambert law, which relates absorbance to concentration, can be used to determine sample concentration. The Beer-Lambert law is:
where A = absorbance (a unitless quantity), E = absorptivity (m-1 cm-1), l = length of the light path through the sample (cm), and c = concentration of the sample (mole/L).
Absorptivity (E) is the proportionality constant between the amount of light absorbed and the gas concentration. An IR instrument can be calibrated to yield this constant by measuring substances of known concentration. Knowing E and the path length (l), absorbance (A) can be measured for an unknown sample, and the Beer-Lambert equation can be used to compute concentration:
When IR radiation is sent through a sample it interacts with the molecules, causing the chemical bonds to vibrate as the molecules absorb the radiation. Each functional group of gases is characterized by the tendency to absorb IR radiation of a particular wavelength, regardless of the structure of the rest of the molecule. Should two different substances exhibit absorbance maxima at the same wavelength, they can still be distinguished from one another because they will differ in another aspect, namely, in their molar extinction coefficients, or absorbance of IR radiation per mole of each substance.13
The only substances that cannot be analyzed with IR spectroscopy are those that exist as single atoms with no chemical bonds, such as noble gases, and homonuclear diatomic molecules, because they have no dipole movement. These two types of materials will not absorb IR radiation.12
An IR spectrometer can be interfaced with a sterilizer to continuously monitor the water vapor per unit volume and EtO concentration during the process through direct sampling of sterilizer headspace. In the case of gas blends, such an analyzer can also monitor and record the concentration profiles of the components. Differentiating the gases is useful in determining whether there is selective gas absorption by the load. To be used with a sterilizer, the spectrometer is programmed with the spectral information of each analyte gas. This information can be considered the optical fingerprint of each gas (see Figure 1 below).
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Figure 1. Infrared spectral analysis of EtO, water vapor, and CO2. (Data courtesy of Spectros Instruments, Inc.) |
The gas sample is carried from the vessel to the detector site, where it flows through a tube only to return, unchanged, to the sterilizer. Gas can be conducted through the beam path, or the beam can be reflected through a window in the sterilizer so that it crosses the headspace gas and returns to the detector. The IR source is placed at one end of the tube and the IR detector at the other. The IR beam passes through the gas sample, and the detector collects the spectral reading and transmits the data to a microprocessor (see Figure 2).
The computed gas identification and concentration data can either be printed or used by the system to actually control the process. One advantage of this technology is that it permits simultaneous monitoring of more than one analyte gas. The relationships that develop between gases and between gases and product or packaging types, as well as the process itself, can be easily controlled.