Page:NIOSH Manual of Analytical Methods - 1024.pdf/7

 1,3-BUTADIENE: METHOD 1024, Issue 2, dated 15 August 1994 - Page 7 of 8

the “B” solenoid valve. Originally, it was connected to the capped port of the tee in the “B” septum purge line. (If desired, switching between normal operation of the “B” system and backflushable pre-column operation could be easily achieved by adding a manually operated three-way valve.) Replumb the components of the “C” system as shown, and extend lines from the normally open (NO) port of the “C” solenoid and the “C” backpressure regulator into the oven. Connect the lines and columns with a zerodead-volume cross (e.g., Part # ZX1, Valco, Houston, TX) and graphite ferrules. Set the initial oven temperature to 50 °C and the “C” backpressure regulator to 185 kPa. With the solenoid valves activated (inject mode), set the “C” flow control to 20 mL/min and the “B” controls so that the effluent from the analytical column and the “C” split vent total 10 mL/min. Then, with the solenoid valves deactivated (backflush or normal mode), adjust the “B” backpressure regulator until the flow from the “C” split vent returns to the value previously measured. This establishes a reverse flow of 10 mL/min through the pre-column. Program the oven to hold the initial temperature (50 °C) for 2 min, then rise to 120 °C at 20 °C/min, and hold for 8 min. Adjust the time from injection to backflush by injecting standards and progressively decreasing the time from 2 min until the methylene chloride peak is removed without attenuating the butadiene peak. It may be necessary to clear higher hydrocarbons from the analytical column by programming the oven to 200 °C at 30 °C/min and holding 4 min. Program the solenoid valves to be activated after each run to prepare for the next injection. Using the backflushable pre-column, there remains a slight problem with retention drift. While in inject mode, the pre-column strips residual water from the carrier gas. This activates the aluminum oxide surface of the analytical column and causes retention to increase. The effect is most noticeable when starting up after the system has been idle. When beginning a sequence of samples, it is advisable to analyze solvent blanks until the retention drift (e.g., of vinylidene chloride) becomes tolerable. APPENDIX B. CONVERSION OF 1,3-BUTADIENE VOLUME TO MASS MacCallum and McKetta [5] determined the compressibility factor,, which corrects for non-ideal behavior, for 1,3-butadiene at temperatures,, ranging from 10 to 75 °C, and pressures, , from approximately 420 to 1050 mm Hg. Multiple regression of the observed values against, , and , yields the following equation (standard error of the estimated is 0.000635 for 13 degrees of freedom): , where:	a = 1.00095,

,

,

.

The mass,, of 1,3-butadiene, corrected for compressibility and the presence of water vapor (when the gas is stored above water), may be calculated by the following equation: , µg, where: = vapor pressure of water @  °C (mm Hg), = volume of 1,3-butadiene (µL), 54.09	= molecular weight of 1,3-butadiene (g∙mol–¹), 62.36	= gas constant (mm Hg∙L∙mol–¹∙K–¹), 273.2	= absolute temperature of 0 °C (K).

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition