Gas Chromatography

GAS LIQUID CHROMATOGRAPHY Principles Partition of molecules between gas (mobile phase) and liquid (stationary phase).

Gas Chromatography Filters/Traps Data system Regulators RESET Regulators Syringe/Sampler Air Hydrogen Gas Carrier Inlets Detectors Column

Schematic Diagram of Gas Chromatography

Separation process in GC Gaseous analytes is transported thru the column by gaseous mobile phase, called the carrier-gas Mobile phases: gas Stationary phases: non volatile liquid on the inside of column or on a fine solid support

Walt Jennings, GC Pioneer

Open tubular columns Made of fused silica Open tubular columns offer Higher resolution Shorter analysis time Greater sensitivity Lower sample capacity ID of open tubular column: 0.1-0.53 mm, length: 15-100m The thickness of stat. phase: 0.1-5 um Liquid stat. phases Polar column is the best for the polar analyte

Columns for GC Columns for GC/MS DB-1ms  •  HP-1ms  •  DB-5ms  •  HP-5ms SemiVol  •  HP-5ms  •  DB-35ms  •  DB-17ms  •  DB-225ms  •  DB-XLB Polysiloxane Polymers HP-1  •  DB-1  •  DB-5  •  HP-5  •  DB-17  •  DB-23  •  DB-35  •  HP-35  •  HP-50+  •  DB-200  •  DB-210  •  DB-225  •  DB-624  •  DB-1301  •  DB-1701  •  DB-1701P  •  Ultra 1 and Ultra 2 Polyethylene Glycol (PEG) DB-WAX  •  DB-WAXetr  •  HP-INNOWax  •  DB-FFAP  •  HP-FFAP  •  DuraGuard Chiral CycloSil-B  •  Cyclodex-B  •  HP-Chiral ?/a> High Temperature DB-1ht  •  DB-5ht  •  DB-17ht Life Science DB-ALC1 and DB-ALC2  •  HP-Blood Alcohol  •  DB-EVDX  •  HP-Fast Residual Solvent

Packed columns Contain a fine solid support Useful for preparative separations Solid support: Teflon Uniform and small particle size: improve column efficiency

Retention Index (KOVAT Index, RI) Nonpolar stationary phase Volatility of the solutes: principal determination Strongly polar stationary phase H-bond and dipole-dipole interaction RI describes the retention behavior of a compound log tr’ (unknown) – log tr’ (n) RI= 100*(n + (N-n)--------------------------) log tr’ (N) – log tr’ (n)

Temperature programming Raising column temperature solute Vp  decrease retention time and sharpens peaks

Sample injection (Split injection) Split is preferred if the analyte > 0.1% of the sample For high resolution work Split delivers only 0.2 – 2% of the sample to column Split ratio Septum purge

Splitless injection For trace analysis Glass liner Injector temperature is lower than that of split Needs cold trapping (solvent trapping) On-column injection: Used for samples that decompose above their boiling point

GC Detectors Because the physical and chemical properties of carrier gas differ widely from those of a vapor, a wide range of detection methods can be employed.   We now monitor some physical, rather than chemical properties of the effluent gas stream. This is because most physical changes such as thermal conductivity, light adsorption to ionization potentials, and heats of combustions can be converted into an electrical signal which can then be amplified and recorded in some other way.

? FID ECD NPD FPD TCD

Thermal Conductivity Detector (TCD) Non destructive, simple, robust, and cheap detector. Its sensitivity and linearity are only moderate -good enough for routine analysis. The rate of heat loss can be used as a measure of the gas composition. A heated filament is cooled by the flow of pure carrier gas. Heat is transferred by conduction when gas molecules strike the heated filament. When the carrier gas is contaminated by sample components, the filament looses less heat and becomes hotter. This change is used to generate the signal.

Gas C.G.S. units at 0C   105 Molecular weight Hydrogen Helium Methane Nitrogen 41.6 34.8 7.2 5.8 2 4 16 28

Flame ionization Detector (FID) The most useful and commonly used detector in GC analysis. High sensitivity, selectivity for carbon containing compounds and wide range of linearity. Destructive, mass sensing detector.

Cations generated in the flame are counted and increase electric current which produce the detector signal. The current is approximately proportional to the amount if carbon in the form of volatile organic compounds which enter the flame in the column effluent. It normally requires three separate gas supplies (Hydrogen, helium, and air) with their precision flow regulators.

Typical gas chromatogram of the aroma extract isolated from Eucalyptus polyanthemos Schauer

Nitrogen Phosphorus Detector (NPD) Sensitive and a specific detector for a compound containing nitrogen or phosphorus. The specific response of this detector makes it especially useful for the analysis of pharmaceuticals and in particular in environmental analysis including herbicides.

The heated alkai bead (Rubidium or cesium bead) emits electrons which are collected at the anode and provides background current. When a solute that contains nitrogen or phosphorus is eluted, the partially combusted N and P materials are adsorbed on the surface of the bead. This adsorbed material reduced the work function of the surface and, as a consequence, the emission of electrons is increased. The sensitivity of phosphorus of NPD is 3 times higher than that of nitrogen.

2 1 Typical gas chromatogram of a mixture of N-methylhydrazine derivatives obtained from the products formed by cod liver oil oxidation with Fe2+/H2O2. Peaks: 1 = 1-methylpyrazole (malonaldehyde); IS = internal standard (2-methylpyrazine)

Flame Photometric Detector (FPD) FPD is an emissivity detector and has unique properties that response quite specific the compounds containing phosphorus and sulfur.

These chemi-luminescent species are monitored at selected wavelength. Phosphorus and sulfur containing hydrocarbons produce chemi-luminescence at specific wavelengths when burnt in the hydrogen flame. These chemi-luminescent species are monitored at selected wavelength. An optical filter permits light of the specific wavelength to enter the photomultiplier to produce a signal.  R-P HPO* HPO + h (526 nm)   R-S S2* S2 + h (393 nm)

Electron Capture Detector (ECD) ECD has very high sensitivity and good selectivity for halogenated compounds. It’s probably one of the most sensitive GC detector available (minimum detectable concentration ca. 10-13 g/mL) Widely used in analysis of pesticides. A low energy of -ray source is used in the sensor to produce electrons and ions. The first source to be used was tritium absorbed into a silver foil but this was replaced by the far more thermally stable 63Ni source.

The carrier gas (nitrogen or argon) is ionized by the -radiation from the source:    N2 N2+ + e- The small, mobile electrons are attracted to the anode before they can recombine with the nitrogen cation. When a component (AM that contains halogenated atoms) that is capable of capturing electrons enters the detector, it will combine with the electrons produced by the ionization. AM + e- AM- AM- + N2+ AM + N2 + energy The decrease in the number of electrons and ions will lead to a fall in current and this is related to the concentration of the component entering the detector.