This gas blending system is a major enhancement to the automated VOC analyzer. It includes hardware to generate humidified air utilizing air from the existing Whatman TOC air purifier and compressor. This humidified air is used to generate daily system blanks and the calibration standard blend for introduction into the ATD-400 system. Calibration samples are generated by dilution of static span gases of 30ppb to 10ppm concentrations with humidified air using a micro gas blender. This digital electronic dilution system (Trace Analytical, Inc., Menlo Park, CA) is capable of blend ratios of as much as 1:10,000. It is a low volume blend apparatus with a clearing time of less than 5 minutes and all wetted parts are constructed of stainless steel.

Fig. 1: Automatic Calibration and Humidified Blank Dilution system

The system has been modified in several ways to incorporate it into the VOC analyzer. By adding pressure regulators and electrically actuated solenoid valves we can control the delivery of blanks and standard via the Turbochrom data system (using timed events). A humidifier has been installed for the dry air and a humidity probe block is installed downstream of the wetted flowpath to allow the monitoring of humidity for quality audits and initial set-up. A humidity probe and meter are available options if they are not currently available at the site.

Fig. 2: Calibration and Blank Humidifier

This blending system reduces errors in long term evaluation of the system performance by allowing the use of a single 30L canister of a standard at 1 ppmv concentration, which can provide daily calibrations for periods of up to several months. Using higher pressure cylinders could allow the use of the same standard over the entire season, thus reducing the variability of canister cleaning and refill errors. Additionally, standards used contain only 10-12 components used as reference peaks for retention time stability as well as Propane and Benzene used for calculation of system recoveries. At the 1-10 ppm level these standards are less expensive, more stable and often more accurate then those provided at ppb levels.