TOM480 Gas Chromatography Lab Report

TOM480 Gas Chromatography Lab Report - Department of...

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Department of Chemistry, The University of Michigan, 03 March 2008 Gas Chromatography-Optimized Separation Author: Thomas Bratton GSI: Amy Payeur Abstract:
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Department of Chemistry, The University of Michigan, 03 March 2008 Introduction Chromatography was first developed by the Russian botanist Mikhail Tswett in 1903 as he produced a colorful separation of plant pigments through a column of calcium carbonate. 2 It has since become a popular tool in laboratories in the separation and identification of compounds. The separated compounds are compared to known compounds as a reference. The basic idea of chromatography is that different molecules interact differently within a sample mixture and a medium. The sample mixture is known as the mobile phase and the medium is known as the stationary phase. The stationary phase is found in what is known as a column made of glass, plastic or stainless steel. An eluent (elution buffer) is used to recover the elute (the separated compounds) from the medium. 3 The amount of eluent(s) is/are recorded on a chromatogram over time. Qualitatively speaking, the number of peaks shown on the chromatogram determines the amount of compounds in the mixture. The area of the peaks can then determine the quantitative aspect of the mixture. Optimization of the peaks is highly desirable. Variables that control the shape of the peaks are the amount of volume as well as the concentration of the species injected into the GC, the flow rate (ml/min), and the temperature of the column. A symmetric gaussian type peak is desired. 4 Anti-symmetric peaks include characteristics known as fronting and tailing. Speed is another variable associated with GC. Optimization of the peaks is important and is usually found with low temperatures, however, the lower the temperature, the longer it takes for a particular run to complete. A balance between speed
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Department of Chemistry, The University of Michigan, 03 March 2008 and optimization, also known as efficiency is most desirable. In chromatography, efficiency is measured in the number of theoretical plates. The more plates the better the efficiency. An equation that describes efficiency is known as the Van Deetmer equation. Equation 1: Plate Height = H = A +B/u + Cu The Van Deemter Equation In this equation, the coefficient A is for multiple flow paths, B/u represents longitudinal diffusion, and Cu accounts for mass transfer to and from the stationary phase. The Van Deemter equation plots parabolic shaped lines where plate height (H) is minimized at a certain flow rate. Differences in efficiency are encountered due to differing parameters among instruments. Different types of chromatography include but are not limited to: Thin Liquid Chromatography (TLC), High-Performance Liquid Chraography (HPLC), Liquid Chromatography-Mass Spectrometer (LC-MS), and Gas Chromatography (GC). For this experiment the GC works the best in determining an unknown mixture. The GC uses a gas phase mobile phase rather liquid and employs a stationary phase that is non-polar. The non-polar stationary phase interacts the best with organic and volatile substances
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TOM480 Gas Chromatography Lab Report - Department of...

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