A Capillary Approach to ASTM D3606: Test Method for Determination of Benzene and Toluene in Finished Motor and Aviation Gasoline Fast : Cycles in <10 Minutes Excellent Repeatability & Linearity Robust Solution using AC Deans switching technology No Matrix Interference Flexible carrier gas (He, N 2, H 2 ) Keywords: Aromatics, ASTM D3606, Dean s switching, Benzene, Toluene INTRODUCTION Several governmental regulations now require ethanol to be blended into motor gasoline. The original D3606 method was developed for finished motor and aviation gasoline that did not include ethanol. In this method, any Ethanol present in samples would present major challenges for the (micro)packed columns resolving benzene and ethanol, interfering with the benzene quantification. Figure 1. Plumbing diagram for Benzene / Toluene analyzer according ASTM D3606 WCOT capillary columns are more efficient than packed or micropacked column technologies, and resolve Ethanol from Benzene, thus providing more accurate values when ethanol is present. In addition, the proposed method provides full separation for Butanol, a blend component that is gaining interest in the market. INSTRUMENTATION The test method covers the determination of benzene and toluene in finished motor and aviation gasoline by capillary column gas chromatography. Benzene can be determined between the levels of 0.06 and 5.0 volume % and toluene can be determined between the levels 0.5 and 20.0 volume %. The precision for this test method was determined using conventional gasoline as well as gasolines containing oxygenates (ethanol and/or ethers such as methyl tert-butyl ether, ethyl tertbutyl ether, tert-amyl methyl ether and butanols). The method may be used for finished motor and aviation gasolines that contain ethanol up to 85 volume % and butanol up to 20 volume %. An internal standard, Methyl Isobutyl Ketone (MIBK), is added to the sample which is then introduced through the Automatic Liquid Sampler in the Split / Splitless Inlet (S/SL). A pre separation is made on the nonpolar pre column. Benzene, Toluene and Internal Standard are cut from the matrix by applying a pressure switch, directing the effluent from the pre column either to the monitor column or to the analysis column. After toluene has eluted, the flow through the nonpolar column is reversed, flushing out the components heavier than toluene. Benzene, Toluene and Internal Standard are separated from the remaining matrix on the analysis column in a temperature programmed oven run and detected by the Flame Ionization Detector (FID). The detector response is recorded, the peak areas are measured, and the concentration of each component is calculated with reference to the internal standard. The developed method is flexible towards carrier gas. This note reports data obtained with Hydrogen, but Helium and Nitrogen data are comparable.
Backflush Cut 1 Cut 2 Cut 3 APPLICATION NOTE OPTIMISING D3606 The standard method ASTM D3606 is further optimized by using an AC Deans switching device and isothermal oven settings. AC deans switching provides the possibility to cut only the peaks of interest to the analytical column, thus eliminating matrix interference virtually completely. The deans switching device is used as backflush system to keep the column clean from heavy sample matrix and cut down cycle time. The method settings of the oven have been changed to isothermal which results in faster effective cycle times since no oven cool down is required. It also provides best retention time and area repeatability. PRECOLUMN CHROMATOGRAM A pre-column chromatogram is recorded by injecting a Resolution Mixture containing all peaks of interest. Benzene, Toluene and Internal Standard are cut from the matrix by applying a pressure switch, directing the effluent from the pre column to the analysis column. After toluene has eluted, the flow through the nonpolar column is reversed, flushing out the components heavier than toluene. Figure 2. Precolumn Chromatogram Figure 3. Analysis column Chromatogram
VALIDATION The system and methodology of the Benzene and Toluene analyzer are thoroughly tested for separation efficiency, repeatability, response linearity, detection limit and recovery. REPEATABILITY Area and retention time are the two primary measurements in gas chromatography. The precision in which they are measured ultimately determines the validity of the generated quantitative data. Retention time and area precision require that all parameters (temperatures, pressure, flow, injection) are controlled to exacting tolerances. Area and retention time repeatability for the AC Benzene and Toluene analyzer according ASTM D3606 are measured for 6 consecutive runs for a standard blend (Cal 4) containing approximately 0.7 % Benzene and 5.0 % Toluene (figure 4). Retention time repeatability of some key components is calculated in table 2. Figure 4: Repeatability overlay of 6 consecutive runs of Calibration level 4. Benzene Toluene Benzene Toluene IS Injection Conc % Conc % Ret time Ret time Ret time 1 0.73 5.06 6.466 9.074 7.774 2 0.72 5.05 6.465 9.073 7.774 3 0.73 5.06 6.467 9.076 7.775 4 0.72 5.05 6.466 9.075 7.775 5 0.72 5.05 6.467 9.076 7.776 6 0.72 5.03 6.466 9.075 7.775 Average 0.72 5.05 6.466 9.075 7.775 stdev 0.0043 0.0115 0.0008 0.0012 0.0008 RSD % 0.60% 0.23% 0.01% 0.01% 0.01% Table 2. Area and Retention time repeatability of a standard blend (calibration level 4)
LINEARITY The linearity of response for the analyzer is verified by analyzing 7 different calibration mixtures in a range of concentration covering the scope of ASTM D3606. Seven standards covering the range 0.06 to 5.0 % volume for benzene and 0.5 to 20.0 volume % for Toluene are prepared. For both component the linearity plots are created (see figures below). Both calibration lines have a linearity correlation >0.9999. VERIFICATION OF DETECTABILITY To verify the detectability of the system, standard calibration mixture 7 is injected to ensure that a signal to noise of 50 to 1 or greater is attained for benzene (figure 7). The signal of Benzene is approximately 8 pa, while the noise level is approx. 0.04 pa (6 x SD). The resulting signal to noise level is approx. 200 to 1. Figure 5. Linearity plot Benzene Figure 6. Linearity plot Toluene Figure 7. Calibration level 7 for verification detection limit benzene
Cut 3 Cut 2 Backflush Cut 1 APPLICATION NOTE SAMPLES The performance of the system is validated by analyzing a wide scope of samples, without added oxygenates or containing methanol, ethanol, MTBE, ETBE, TAME, Butanol or combination of these. An example of a low level Benzene (0,20 volume %) and Toluene (0,38 volume %) containing sample is pictured underneath. Remaining sample matrix Figure 8: Typical Gasoline Reference Sample, analyzed with optimized ASTM D3606 method CONCLUSION The AC ASTM D3606 analyzer is a specialized solution for Determination of Benzene and Toluene in Finished Motor and Aviation Gasoline by Capillary Column Gas Chromatography. The standard method ASTM D3606 is further optimized by using AC deans switching device and isothermal oven settings. AC deans switching provides the possibility to cut only the peaks of interest to the analytical column, providing unambiguous identification. The deans switching device is also used for backflushing heavier sample matrix to keep the system clean, and shorten runtimes. The use of isothermal oven eliminates the need for oven cool down resulting in even faster cycle times, and provides for excellent result repeatability. AC Analytical Controls has been the recognized leader in chromatography analyzers for gas, naphtha and gasoline streams in crude oil refining since 1981. AC also provides technology for residuals analysis for the hydrocarbon processing industry. Applications cover the entire spectrum of petroleum, petrochemical and refinery, gas and natural gas analysis; ACs Turn-Key Application solutions include the AC Reformulyzer, DHA, SimDis, NGA, Hi-Speed RGA and Customized instruments. 00.00.261 - Copyright 2014 PAC L.P. All rights reserved