Influence of the Air-space Volumes in Preparation Vessels on the Accuracy of Concentration for Volatile Organic Compound Standard Solution

The factors influencing the accuracy of the concentration for a volatile organic compound (VOC) standard solution in its preparation procedure were evaluated by designing experiments using an orthogonal arrays table (L-18). The factors tested were A: kinds of solvent, B: airspace volume in vessel, C : period after picking up bottles of raw material from freezer and D : storage temperature of stock solution. Eighteen kinds of trans-1,2-dichloroethylene solutions that had different sets of the four factors were prepared and measured by gas chromatography. The results showed that the air-space volume in the preparation vessels had the largest influence on the accuracy for a concentration of the VOC standard solution. Three 1000 mg L-1 1,1-dichloroethylene solutions were prepared from stock solutions (100000 mg L-1), which had different air-space volumes (9%, 39%, 70%) in the vessels, respectively. The measured concentrations of the three 1000 mg L-1 solutions decreased along with an increase in the air-space volume for the stock solution. These results were in good agreement with the calculated values based on Raoult's law using the vapor pressures of 1,1-dichloroethylene and methanol. At a 9% air-space volume of the vessel, 0.070% of 1,1-dichloroethylene and 0.013% of methanol were calculated to be vaporized. When the vapor pressure of solute was higher than that of the solvent, a larger influence on the concentration of solute was expected. When solutions were prepared with a 5% or less air-space volume of the vessel, however, the influence on the accuracy of the concentration became a minimum (less than 0.01%). Moreover, the air-space volume of the sample vials for GC measurements was also found to influence the accuracy of the concentrations of VOC compounds of higher vapor pressure.