Model 491M/B/SD dynamically blends precisely known VOC/TOC gas standards with parts-per-billion and parts-per-trillion analyte concentrations. First stage (491M/B) of dilution combines analyte flow from Trace Source(TM) permeation tubes with precisely controlled flow of pure dilution gas to form ppb concentration mixture. In second stage (491M/SD), aliquot of ppb mixture is further diluted to form ppt mixture. Standard additions techniques may also be used to insure accuracy.
The 491M/B/SD MODULAR GAS STANDARDS GENERATOR dynamically blends precisely known VOC/TOC gas standards with parts-per-billion (ppb) and parts-per-trillion (ppt) analyte concentrations. Two modules provide two sequential stages of dilution. The first stage 491M/B combines the analyte flow from Trace Source(TM) permeation tubes with a precisely controlled flow of pure dilution gas to form a parts-per-billion concentration mixture. In the second stage 491M/SD, an aliquot of the ppb mixture is further diluted to form a ppt mixture. Trace Source(TM) permeation tubes are now available for 500 analyte compounds.
The 491M/B/SD uses dynamic blending to circumvent the common problems with ppb and ppt standards. Typically, ppb and ppt VOC/TOC gas standards are not stable in storage due to reactions and adsorption in the container. Dynamic blending avoids this problem. With dynamic blending the key challenges are controlling the analyte vapor flow and assuring true "zero" dilution gas. The modular, two-stage dilution approach of the 491M/B/SD uses permeation tubes with measurable rates to control analyte flow. Combined dilution flow required is 10 l/min or less. It also allows the "standard additions technique" to be used to insure accuracy in spite of significant, unknown dilution gas contamination.