As Analysis Requirements Become Ever More Sophisticated So Does the Need for Specialty Gas Mixtures
As analytical requirements seek quantification at ever lower concentration levels the gas mixtures required for verification and calibration become more difficult to obtain or prepare, and often impossible to store. Concentrations may be extremely low (parts per billion, or even parts per trillion.) Analytes are often highly polar, or highly reactive, and may react with each other or the matrix. Perhaps a truly zero matrix is not possible for the concentration range desired. Or perhaps a series of concentrations is needed to build a calibration curve.
The obvious answer is to prepare the mixture dynamically and use it immediately. Typically a small flow of the analyte is diluted with a much larger flow of the matrix gas, and both flows are controlled by electromechanical flow controllers. Conventional dynamic blending, however, also presents problems. For example, to create a 1.0 ppm(v/v) mixture from a pure gas requires mixing 1.0 cc/min of analyte into 1,000 l/min of matrix gas! Not a practical solution. Using 0.1 cc/min of analyte flow would only reduce the matrix flow to 100 l/min, and controlling flow mechanically below 0.1 cc/min by mechanical devices is a practical impossibility. Multiple dilution stages can be used, but a flow of analyte vapor is still needed. Suppose the analyte is liquid?
Permeation Tubes offer a solution. They provide a means of controlling extremely low flows of a wide range of analytes.
Permeation Tubes
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