General Chemistry II

General Chemistry IIGas ChromatographyModule 8Instrumentation for GCProvide a regulated flow of carrier gas to the columnInlet system to vaporize and mix the sample with the carrier gasA thermostatted oven to optimize the temperature for the separationIn-line detector to continuously monitor the separationData handling system to record the chromatogramBlock Diagram of a GCGas Chromatograph

Gas ChromatographyCommon type of chromatography used in organic chemistry for separating and analyzing compounds that can be vaporized without decomposition.Typical usesTesting the purity of a particular substanceSeparating the different components of a mixture into separate componentsIdentify a compoundGC ColumnsTwo main types:PackedCapillary

Packed columns are 1.5 - 10 m in length and have an internal diameter of 2 - 4 mm. The tubing is usually made of stainless steel or glass and contains a packing of finely divided, inert, solid support material that is coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed. Thus numerous columns are available that are designed to separate specific types of compounds. GC ColumnsTwo main types:PackedCapillary

Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between 25-60 meters are common. The inner column walls are coated with the active materials (WCOT columns), some columns are quasi solid filled with many parallel micropores (PLOT columns). Most capillary columns are made of fused-silica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil.

CHAPTER 23: Figure 23.2

CHAPTER 23: Equation 23.7

Poly(siloxane) Stationary Phases

CHAPTER 23: Table 23.1

CHAPTER 23: Equation 23.1

Ionic Liquid Stationary Phase

Carrier GasThe choice of carrier gas (mobile phase) is important, with hydrogen being the most efficient and providing the best separation. Helium has a larger range of flow rates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used.

CHAPTER 23: Figure 23.11

Van Deemter Curves

CHAPTER 23: Figure 23.13

Microliter Syringe

Sample Inlet RequirementsTo vaporize and mix the sample with the carrier gas prior to the start of the separation withoutReducing the separation potential of the columnIn the absence of thermal degradation, adsorption or modification of the sampleWithout discrimination of sample components by volatility, molecular weight, or polarityWith quantitative recovery for both trace and major sample components

Packed Column InjectorCHAPTER 23: Figure 23.14

Split Injector for Open-Tubular Columns

GC TemperatureTemperature-dependence of molecular adsorption and of the rate of progression along the column necessitates a careful control of the column temperature to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times. For some cases temperature is ramped either continuously or in steps to provide the desired separation. This is referred to as a temperature program. Electronic pressure control can also be used to modify flow rate during the analysis, aiding in faster run times while keeping acceptable levels of separation.

Ionization DetectorsAt typical operating conditions the common carrier gases used in GC behave as perfect insulators allowing the conductivity due to very few charged species to be easily measuredFlame Ionization Detector (FID)Thermionic Ionization Detector (TID or NPD)Photoionization Detector (PID)Electron-Capture Detector (ECD)Helium Ionization Detector (ECD)GC DetectorsTwo most common detectorsFlame ionization detectorsThermal conductivity detectorBoth are sensitive to a wide range of components, and both work over a wide range of concentrations. Both detectors are also quite robust.Detection SystemsFlame ionization detectorThermal conductivity detectorElectron-capture detectorThermionic detectorElectrolytic conductivity detectorPhotoionization detectorAtomic Emission detectorFame photometric detectorMass spectrometry detectorCHAPTER 23: Figure 23.18

Flame Ionization DetectorNear universal responseCH* + O* CHO+ + eMDA = 10-12 g/sLinear range 106Fast responseLow dead volumeExceptional stability

Thermionic Ionization DetectorSelective for N and P compounds (other elements by modifying the thermionic source)MDA = 10-13 g/s N 10-14 g/s PLinear range 104Selectivity 104 g C/g N 105 g C/ g P 0.5 g P/ g N

CHAPTER 23: Figure 23.19

Electron-Capture Detector

Electron-Capture DetectorHigh-energy beta electrons generated by the decay of a radioactive isotope source used as the primary source of ionizing radiationThese particles produce a large number of secondary electrons through multiple collisions with the carrier gas molecules forming a plasma of thermal electrons (0.02 to 0.05 eV), radicals and positive ionsApplication of a pulsed potential to the ionization chamber allows collection of the thermal electrons and a standing (baseline) current establishedElectron-Capture DetectorWhen compounds with a high electron affinity enter the ionization chamber thermal electrons are removed by formation of negative ionsThe increased rate of neutralization of these ions with positive ions, or their reduced drift velocity during collection of the thermal electrons, is responsible for the detector signalThe decrease in the detector standing current is proportional to solute concentrationDetector response is temperature dependent

Electron-Capture DetectorCompound selective detector(electron affinity)AB + e ABAB + e A + B Selectivity range 107MDA = 10-14 g / mLLinear range = 104

Bulk Physical Property DetectorsRespond to some difference in a carrier gas property due to the presence of the analyteThermal Conductivity Detector (TCD)Near universal detectorMDA = 10-9 g / mLLinear range 104Non-destructive detectorDetectors with very small volumes availableCHAPTER 23: Figure 23.17

Thermal Conductivity Detector