Gas Chromatography

Chromatography

• Chromatography– TLC (analytical)– GC (analytical)– Column (preparative)

• Strengths of TLC• Weaknesses of TLC

GC Basics

Gas-Phase Separation

• Partitioning between mobile gas and stationary column

• Highly effective separations

GC

• Injection• Column• Detector• Chromatogram

GC

• Injection

GC

• Column

Column Material

• Nonpolar – For nonpolar

material– vDW– Follows bp

• Polar– For polar material– Follows polarity

GC

• Detector– FID– TCD

GC

• Chromatogram– Retention time– Area under

peak

GC Parameters and Retention Time

• Length of column• Type of column• Temperature profile• Gas flow rate

Important GC Questions

• What is each peak? (identity)– Characterization– Determination of Unknown

• How much of each compound? (Quantity)– Absolute quantity– Relative quantity (product distribution)

Identity

• Retention time corresponds to Rf

• Spike with authentic sample

Gas Chromatography/Mass Spec

• Identifying components in mixtures

• Separation then identification

• GC-MS

Quantitation

• Area under peak corresponds to amount of compound causing signal

• Use standard plot to determine absolute concentration

• Use percent of total area to determine product distribution– Assumes same response

factor

Our Experiment• How does the mechanism affect the product

distribution of an elimination reaction?• Conduct two elimination reactions• Collect the MIXTURE of products by distillation• Analyze product distribution by GC

OH+

Cl+

acid

base

Practical Aspectsthermometer

heat with stirring!

icebath!

thermometer

icebath!

heat with stirring!

REFLUX FIRST

DISTILL SECOND