Two-Dimensional Gas Chromatographic Analysis of Trace Benzene in Styrene Application

A two-dimensional gas chromatographic method for the analysis of trace benzene in styrene is described. This method employs the Agilent 6890N GC system equipped with a Deans switch device. The method can be used in a prefractionator mode to analyze trace benzene in styrene according to the proposed ASTM method [1]. It can also be used in a heart-cutting mode to transfer more precisely just the benzene peak from the primary column to the secondary column. The Agilent 6890N electronic pneumatics control and flow calculation software are used for setting up the precise flows and pressures required. Excellent calibration and sensitivity of this system is demonstrated in both prefractionator mode and heart-cutting mode, and the interference can be greatly reduced by the heart-cutting. Two-Dimensional Gas Chromatographic Analysis of Trace Benzene in Styrene Application Introduction The American Society of Testing and Materials (ASTM) method D5135 uses a single column GC to measure the overall purity of styrene, including benzene down to 1000 ppm [2]. However, industry now desires to lower the benzene content in styrene to 10 ppm. Styrene also contains trace amounts of C8 and C9 hydrocarbons that cannot be separated from trace benzene with a single GC column. A fast analytical method for trace benzene in styrene is needed for final product inspections, process control, establishing specifications, and research work. ASTM has proposed a two-dimensional gas chromatography (2-D GC) method to completely separate trace levels of benzene from other contaminants in styrene. This method operates in a prefractionator mode where the primary column retains the high boiling styrene components while the low boiling contaminants (including benzene) are transferred to a second column. After the transfer is complete, the benzene is easily separated from the interfering hydrocarbons on the second column. This method uses two columns of different polarity coupled together with a device that allows one or more discrete effluent segments to be transferred from the first to the second column. There are two different switching devices that can be used with this method. The first device uses a rotary valve to transfer the benzene from column one to column two. The second uses a fluidic switch to transfer the benzene. The fluidic switch is similar in design HPI