Without reliable standards it is impossible to obtain reliable analytical data. One of the great challenges in analytical chemistry is coming up with calibration standards for uncommon chemicals. This can be especially challenging when making gaseous standards for monitoring gasification processes. Stites & Associates, LLC, has made great strides forward in solving this persistent problem using a permeation/diffusion calibrator.
The basic principle is illustrated below:
Purified gas (often air or nitrogen) is fed to a temperature controlled oven where a constant emission device releases the compound(s) of interest at a constant rate. The gas is then diluted with additional purified gas. The total amount of gas flowing is measured by a precision meter. By knowing the amount of compound being emitted from the constant emission device and the exact gas flow, the precise gas concentration of the compound can be calculated. By varying the amount of dilution gas, a range of concentrations can be generated. This range of standards can be used to create a calibration curve for many types of instruments. This technique can be applied to almost any compound that is a volatile liquid at the temperature of the oven (could be above normal room temperature).
The constant emission device can be a permeation tube or a diffusion device. A typical permeation tube is a short piece of permeable tubing (often made of Teflon), partially filled with a pure liquid compound and sealed at the ends. A typical device is shown below:
Figure 2
Permeation tubes usually have fairly low permeation rates. These are often in the range of just a few mg/min or less. Hence, permeation tubes are best applied when the calibration range is in the ppb or low ppm range. When working with gasification projects, the concentrations of compounds of interest are often in the range of several hundred and even several thousand ppm.
Higher emission rates can be created with diffusion devices. These is be shown schematically below:
The diffusion rate is fixed by the compound molecular weight and vapor pressure (depends on temperature), the internal diameter and length of the hollow tube and the operating pressure of the calibration system. With these devices emission rates of hundreds of mg/min are possible. With volatile compounds, concentrations of several thousand ppm are possible. These devices are particularly useful for accurately measuring gaseous compounds found in gasification projects. It is often very important to know the concentration of key compounds used to monitor the gasification process. Using pure compounds and a gravimetric procedure, the devices can be used to calibrate for a wide variety of compounds.
Stites & Associates has recently procured and commissioned a VICI Metronics 450 Dynacalibrator system. This system has two, selectable, precision dilution systems. This gives the calibrator a very wide dynamic range. The system has been commissioned for use with benzene up to 50 ppm. Higher levels for benzene and other compounds are under development using VICI Dynacal Diffusion Vials.
This system can be applied to a very wide range of compounds including:
- BETX
- PAH’s
- Alcohols
- Aldehydes
- Ketones
- Phenols and Cresoles
- Furans
- Plasticizers
- Halogenated Compounds
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