2401 GasLiquid Chromatography - San Diego …faculty.sdmiramar.edu/.../2401_GasLiquid_Chromatography.pdfvapor -slow injection of large samples causes band broadening and loss of resolution

August 20171 Gas Liquid Chromatography

2401 Gas (liquid) Chromatography

Chromatography Scheme

Gas chromatography - specifically gas-liquid chromatography - involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.

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Gas ChromatographyIn gas-liquid chromatography, a gaseous mobile phase transports gaseous solutes through a long, thin column containing stationary phase. In the figure below, a volatile sample is injected through a rubber septum into a heated port, which vaporizes the sample. The sample is swept through the column by He, N2 or H2

carrier gas, and the separated solutes flow through a detector, whose response is displayed on a computer.


Carrier Gas & Injection PortThe carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependent upon the type of detector which is used. The carrier gas system runs through a molecular sieve to remove water and other impurities.

For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapor - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro-syringe is used to inject sample through a rubber septum into a flash vaporizer port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a micro-liter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 µL. For capillary GC, split/splitless injection is used, see diagram. The injector can be used in one of two modes; split or splitless.

The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). The sample vaporizes to form a mixture of carrier gas, vaporized solvent and vaporized solutes. A proportion of this mixture passes onto the column, but most exits through the split outlet. The septum purge outlet prevents septum bleed components from entering the column


ColumnsThere are two general types of column, packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.

Capillary columns have an internal diameter of a few tenths of a millimeter. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns. Both types of capillary column are more efficient than packed columns.

In 1979, a new type of WCOT column was devised - the Fused Silica Open Tubular (FSOT) column. These have much thinner walls than the glass capillary columns, and are given strength by the polyimide coating. These columns are flexible and can be wound into coils. They have the advantages of physical strength, flexibility and low reactivity.


How Columns Work


Column TemperatureFor precise work, column temperature must be controlled to within tenths of a degree. The optimum column temperature is dependent upon the boiling point of the sample. As a rule of thumb, a temperature slightly above the average boiling point of the sample results in an elution time of 2 - 30 minutes. Minimal temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful. The column temperature is increased (either continuously or in steps) as separation proceeds.

The effect of temperature on gas chromatograms.(a) Isothermal at 45°C; (b) Isothermal at 125°c; (c) programmed at 30°C to 180°C


DetectorThere are many detectors which can be used in gas chromatography. Different detectors will give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. Detectors can also be grouped into concentration dependent detectors and mass flow dependent detectors. The signal from a concentration dependent detector is related to the concentration of solute in the detector, and does not usually destroy the sample. Dilution with make-up gas will lower the detectors response. Mass flow dependent detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector. The response of a mass flow dependent detector is unaffected by make-up gas. Have a look at this tabular summary of common GC detectors:

Detector Type Support Gases

Selectivity Limits Dynamic Range

Flame Ionization (FID)

Mass flow H2 and air Most organic compounds 100pg 107

Thermal Conductivity (TCD)

Concentration Reference Universal 1 ng 107

Electron Capture (ECD)

Concentration Make-up Halides, nitrates, nitriles, peroxides, anhydrides, organometallics

50 fg 105

Nitrogen-phosphorus Mass-flow H2and air

Nitrogen phosphorus 10 pg 106

Flame photometric (FPD)

Mass-flow H2 and air possibly O2

Sulfur, phosphorus, tin, boron, arsenic, germanium, selenium chromium

100 pg 103

Photo-ionization (PID) Concentration Make-up Aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulfurs, some oragnometallics

2 pg 107

Hall electrolytic conductivity

Mass-flow H2and O2

Halide, nitrogen, nitrosamine, sulfur


Thermal Conductivity DetectorA TCD detector consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. The TCD is not as sensitive as other detectors but it is non-specific and non-destructive.

Two pairs of TCDs are used in gas chromatographs. One pair is placed in the column effluent to detect the separated components as they leave the column, and another pair is placed before the injector or in a separate reference column. The resistances of the two sets of pairs are then arranged in a bridge circuit. The bridge circuit allows amplification of resistance changes due to analytes passing over the sample thermoconductors and does not amplify changes in resistance that both sets of detectors produce due to flow rate fluctuations, etc.


Thermal Conductivity DetectorTCD Movie


Flame Ionization DetectorThe effluent from the column is mixed with hydrogen and air, and ignited. Organic compounds burning in the flame produce ions and electrons which can conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. The current resulting from the pyrolysis of any organic compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this gives the advantage that changes in mobile phase flow rate do not affect the detector's response. The FID is a useful general detector for the analysis of organic compounds; it has high sensitivity, a large linear response

range, and low noise. It is also robust and easy to use, but unfortunately, it destroys the sample.


Flame Ionization DetectorFID movie


Electron Capture DetectorThe electron capture detector (ECD) is a highly sensitive detector capable of detecting picogramamounts of specific types of compounds. The high selectivity of this detector can be a great advantage in certain applications. Compared with the FID, it has much more limited linear response range, generally less than 2 orders of magnitude. The response can also vary significantly with temperature, pressure and flow-rate. The detector contains a sealed radioactive source, Ni-63, and thus, requires that certain radiological safety requirement be met. This figure shows a schematic diagram of an ECD. A flux of beta particles generated by the Ni-63 collide with the carrier gas molecules causing them to ionize by ejecting thermal (i.e., low energy electrons). The thermal electrons migrate to an anode (positive electrode) which generates a current signal. When a sample contains compounds that capture and remove thermal electrons, some of the electrons are prevented from reaching the anode and consequently, results in a reduction in the baseline current. This change in current provide the signal response for the electron-capturing compounds.


Electron Capture DetectorECD Movie


Mass Spectroscopy for ChromatographyThe physics behind mass spectrometry is that a charged particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass to charge ratio, m/e.

In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion. If the molecular ion is too unstable then it can fragment to give other smaller ions.

The collection of ions is then focused into a beam and accelerated into the magnetic field and deflected along circular paths according to the masses of the ions. By adjusting the magnetic field, the ions can be focused on the detector and recorded.


Mass Spectroscopy, Ionization SourceAn Most widely used electron impact (EI) source produces positive ions, negative ions and neutral species. the positive ions are directed toward the analyzers by electrostatic repulsion. The electron beam is so energetic that many fragments are produced that are useful in identifying the molecular species.


Mass Spectroscopy, AnalyzerMass analyzer separates the ions according to their m/z values.

The most common analyzer are listed in Table shown. The most common analyzers for GC/MS are the quadrupole mass filter and the ion trap. High resolution mass spectrometers use the double-focusing analyzer, the ion-cyclotron used with GC/MS


GCMS, InstrumentThe schematics of a complete GC/MS system is shown. In the diagram, the sample is injected into the capillary GC and the effluent enters the inlet of a quadrupole mass spectrometer. The molecules are then fragmented and ionized by the sources, are mass analyzed and are detected by the electron multiplier.


GC Mass Spectroscopy, InstrumentThe schematics of a complete GC/MS system is shown. In the diagram, the sample is injected into the capillary GC and the effluent enters the inlet of a quadrupole mass spectrometer. The molecules are then fragmented and ionized by the sources, are mass analyzed and are detected by the electron multiplier.


GC Method DevelopmentWhat method should be used when undertaking analysis by GC:

Consider the following when making a decision-

1. Goal of analysis – What is require? Is it qualitative or quantitative; maybe both? Are you in need of high precision and will your analyte be available in high concentrations or only trace amount? These factors create trade-offs in selecting techniques to be used.

2. Sample preparation – What type of sample is going to be analyze? The key to success is clean samples. Remember that your sample also needs to be volatile. "Garbage in means garbage out".

3. Detector- What type of information is needed? What type of compound are you analyzing? Some detectors are only good for certain samples (FID for hydrocarbons). Other detectors have limit in how much samples reaches it. Although MS is your best detector, calibration or availability may be difficult.

4. Column – Choices include what type of stationary phase, column diameter, length.. Table below provide some useful information when selecting a column.

5. Injection method – Split or splitless. The concentration of the analyte you are to analyze will help you decide.

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2401 GasLiquid Chromatography - San Diego …faculty.sdmiramar.edu/.../2401_GasLiquid_Chromatography.pdfvapor -slow injection of large samples causes band broadening and loss of resolution