Gas Chromatography Lecture Notes PDF

Instrumental Analysis Chapter 23: Gas Chromatography  Gas Chromatography (GC) A technique for separating volatile/vaporized substances by percolating a stream of inert gas over the stationary phase GC is based on partitioning of solu...

Instrumental Analysis Chapter 23: Gas Chromatography  Gas Chromatography (GC) A technique for separating volatile/vaporized substances by percolating a stream of inert gas over the stationary phase GC is based on partitioning of solutes between the S.P and an inert carrier gas. Sample is vaporized before entering the column. The carrier phase has no rule in separation. GC is classified into: Gas-solid chromatography (GSC) (adsorption) Gas-liquid chromatography (GLC) (partition) Flow meter Flow controller Injector Septum Detector GC chart Pressure regulator  Recorder Oven Gas Column supply 72 Instrumentation of GC 73  Principle of GC A gaseous mobile phase flows under pressure through a heated tube either coated with a liquid stationary phase or packed with liquid stationary phase coated onto a solid support. The gas must be dried before use by passing it through tubes containing molecular sieves. The analyte is loaded onto the column via a heated injection port, where it evaporates. (operated at 50 oC higher than applied temp for GC column. ❖Any compound can be volatilized (compounds such as amino acids, sugars, and proteins decompose at high temperatures can not be analyzed). The sample may be organic or inorganic (GC can not separate ionic compounds). The oven temperature (from ambient to 400 oC) is then either held constant or programmed to rise gradually. The Separation of a mixture occurs on the column according to the relative lengths of time spent by its component in the stationary phase. ❖ Why separation using GC depends on the stationary phase only? Because the mobile phase acts as a carrier for the sample, it will not react with it. 74  Gas Chromatography Like ordinary chromatography, but sample and mobile phase are volatile (gas) Gas Mobile phase: Inert, Available, pure, low cost Low density gas- faster separation Compatible with the detector High density gas-better separation Typical inlet pressure 10-50 psi Common gases (Helium, Nitrogen, Hydrogen) There is two types:- GSC, and GLC ( according to the stationary phase ). 1. Stationary phase of GSC :- ▪ The stationary phase is:- solid (Adsorbent) , finely divided powder and has high adsorption power. ▪ Applied for the analysis of gas sample , most gas analysis can be conducted at RT. Used for low M.wt species that are not retained on GLC e.g. components of air H2S, CO, CS2, CO2, NO2 Examples :- Molecular sieve:- inorganic synthetic material similar to natural zeolite ( hydrated silicate of Ca and Al ), Activated silica gel., Activated charcoal. , 75  Stationary Liquid Phases 2- Stationary phase of GLC :- It is liquid held over inert support, it should be non-volatile at the temperature of application. Requirements: Low volatility / non-volatile (b.p. > 100°C higher than the maximum operating temperature for the column); thermal stability; chemical inertness; have a certain polarity and solubility of the components to be analyzed (how to select it? Polarity of S.P & sample should be matched The principle of like dissolves like" is applied Most Common Stationary Phases 1. Separation of mixture of polar compounds: Carbowax & polyethylene glycol 2. Separation of mixtures of non-polar compounds: (Diphenyl dimethyl Polysiloxane) 76 77  GC columns 1. Packed columns :- i. Made of glass , stainless steal or Teflon. ii. Its length 2 meter with 0.5 cm diameter. (P.S: 30-300 µm, < 0.5 cm id) iii. Uniformly packed with finely divided inert spherical solid support which uniformly coated with thin coat of liquid S. P. iv. Pressure (high) and flow rate of carrier gas (high, about 30-50 mL/min). v. Sample loading Capacity (larger). vi. Inefficient, poor resolution Packed Column 78  GC columns 2. Capillary/open tubular columns :- a) Wall coated open tubular ( WCOT ) i. It is capillary tube made of glass , stainless steal or Fused silica (FSOT, thinner, more flexible, low reactivity with good physical strength). ii. Its length 10-100 meter. (id: 0.1- 0.5 mm) iii. The inner wall is coated with thin liquid S.P =1 µm. iv. Pressure (low) and flow rate of carrier gas (low, about 1 mL/min). v. Sample loading Capacity (smaller). vi. Efficient, Better resolution vii. Short analysis time 79  Capillary/open tubular columns b) Support coated open tubular ( SCOT ) :- Thin liquid around solid support (30 µm) lining the inner tube, but it is larger in diameter & less efficient than WCOT. ( adv: larger sample loading capacity than WCOT). c) Porous layer open tubular (PLOT) :- consist of an open glass with thin layer of porous solid attached to it. 80  FSOT WCOT SCOT Packed Length, m 10-100 10-100 10-100 1-10 Inside dia., mm 0.1-0.5 0.25-0.75 0.5 2-4 Efficiency, plates/m 2000-4000 1000-4000 600-1200 500-1000 Total plates (20-400)x103 (10-400)x103 (6-120)x103 (1-10)x103 Sample size, ng 10-75 10-1000 10-1000 10-106 Rel. back pressure Low Low Low High Relative speed Fast Fast Fast Slow 81  Sample Preparation: Samples GC must be volatile. Samples which are non-volatile are converted into a volatile derivative. The most commonly method is the silylation (reaction of trimethylsilyl, - Si(CH3)3, with an active hydrogen atoms in the analyte (carboxylic acids, amines, imines, alcohols, phenols, and thiols). Inorganic metals (aluminum, beryllium, and chromium) can be analyzed by GC via formation of stable, volatile metal chelates with trifluoroacetylacetone (TFA) and hexafluoroacetylacetone (HFA). Derivation of Glucose with Trimethylchloro silane (TMCS) (Silylation) 82  Advantages and Limitations Advantages C: Limitations Samples must be volatile and thermally stable High resolution (many components in a given- sample can be identified Most detectors are destructive (samples are and quantitatively determined ). decomposed and can not be collected) Dirty samples such as blood or tissues High speed (short time of analysis). require clean up. High sensitivity (10-9 -10-12 gm). GC cannot identify the compound surely and High accuracy. another instrument such as IR or mass spectrometer must be used for. complete compound identity. Some training and experience are necessary. 83  Gas Chromatography GC Detectors 1- Electron capture detector (ECD) The sample is eluted from the column. It pass over a radio-active substance which lead to high reactivity and emission of equivalent amount of electrons. The electrons are collected by an anode , amplifier then detector. Temperature :- 225c Carrier gas :- nitrogen. Temp. Sensitivity :- some Uses :- for halocarbons. but never for hydrocarbons , why? Because all the bonds in hydrocarbons are covalent that prevent the emission of electrons. 84 2- Thermal conductivity detectorTCD) Conductivity bridge containing two samples & two references. The sample is eluted from the column and ended in a place with high temp., high conductance and high current which eventually lead to a signal. Temp. sensitivity :- high ( the oven must be controlled ). Response :- all substances. Max. temp. :- 450c Carrier gas :- helium Use :- water 85 3- Flame ionization detector (FID) Substances are mixed with helium ( or any radio-active substance ) which lead to sample ionization and detecting signals. Temp. sensitivity :- none Max. temp. :- 400c Carrier gas :- nitrogen Response :- to all substances Uses :- water soluble compounds Q :- ACCOUNT on GC detectors with SCETCHES. 86