Gas chromatographyLecturer: Somsak Sirichai

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Advantages of GC

• Fast analysis, typically minutes• Effi cient, providing high resolution• Sensitive, easily detecting ppm and often ppb• Nondestructive, making possible on - line coupling; e.g., to mass spectrometer• Highly accurate quantitative analysis, typical RSDs of 1 – 5%• Requires small samples, typically μ L• Reliable and relatively simple• Inexpensive

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Disadvantages of GC• Limited to volatile samples

• Not suitable for thermally labile samples

• Requires spectroscopy, usually mass spectroscopy, for confirmation of

peak identity

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Gas chromatography:mobile phase: gasstationary phase: usually a nonvolatile liquid,

but sometimes a solidanalyte: gas or volatile liquid

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GC: Block Diagram

Figure 1: Block diagram of a typical gas chromatograph.

He, N2 or H2

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Flow MeasurementThe two most commonly used devices are a soap - bubble flowmeter and a digital electronic flow measuring device.

Fig. 2.2 Flow meters: ( a ) Soap film type. ( b ) Digital electronic type

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CARRIER GAS

The main purpose of the carrier gas is to carry the sample through the column. It is the mobile phase and it is inert and does not interact chemically with the sample.

A secondary purpose is to provide a suitable matrix for the detector tomeasure the sample components. Below are the carrier gases preferred for various detectors:

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Gas SamplingGas sampling methods require that the entire sample be in the gas phase under the conditions in use. Gas - tight syringes

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Liquid Sampling

Solid SamplingSolids are best handled by dissolving them in an appropriate solvent and by using a syringe to inject the solution.

Since liquids expand considerably when they vaporize, only small sample sizes are desirable, typically microliters. Syringes are almost the universal method for injection of liquids. The most commonly used sizes for liquids are 1, 5, and 10 μ L.

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Column: Open Tubular ColumnColumns: (1) Packed column and (2) Open Tubular Column (capillary column)

The vast majority of analyses use long, narrow open tubular columns made of fused silica (SiO2) and coated with polyimide (a plastic capable of withstanding 350oC) for support and protection from atmospheric moisture

Compared with packed columns, open tubular columns offer:• higher resolution• shorter analysis time• greater sensitivity• lower sample capacity

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FIGURE 23-2 (a) Typical dimensions of open tubular gas chromatography column. (b) Fused-silica column with a cage diameter of 0.2 m and column length of 15–100 m. (c) Cross-sectional view ofwall-coated, support-coated, and porous-layer columns.

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Column: Packed Columns• Packed columns contain fine particles of solid support coated with

nonvolatile liquid stationary phase, or the solid itself may be the stationary phase.

• Compared with open tubular columns, packed columns provide greater sample capacity but give broader peaks, longer retention times, and less resolution. (Compare Figure 23-8 with Figure 23-3.)

• Despite their inferior resolution, packed columns are used for preparative separations, which require a great deal of stationary phase, or to separate gases that are poorly retained.

• Packed columns are usually made of stainless steel or glass and are typically 3–6 mm in diameter and 1–5 m in length.

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FIGURE 23-3 (b) Chromatogram of vapors from the headspace of a beer can, obtained with 0.25-mm-diameter 30-m-long porous carbon column operated at 30C for 2 min and then ramped up to 160C at 20/min. [Courtesy Alltech Associates, State College, PA.]

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Temperature and Pressure Programming

A large fraction of all gas chromatography is run with temperature programming, in which the temperature of the column is raised during the separation to increase analyte vapor pressure and decrease retention times of late-eluting components.

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Carrier Gas

• Helium is the most common carrier gas and is compatible with most detectors. For a flame ionization detector, N2 gives a lower detection limit than He.

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• Figure 23-11 shows that H2, He, and N2 give essentially the same optimal plate height (0.3 mm) at significantly different flow rates. Optimal flow rate increases in the order N2 < He < H2. Fastest separations can be achieved with H2 as carrier gas, and H2 can be run much faster than its optimal velocity with little penalty in resolution.

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Figure 23-12 shows the effect of carrier gas on the separation oftwo compounds on the same column with the same temperature program.

FIGURE 23-12 Separation of two polyaromatic hydrocarbons on a wall-coated open tubular column with different carrier gases. Resolution, R, increases and analysis time decreases as we change from N2 to He to H2 carrier gas. [Courtesy J&W Scientific, Folsom, CA.]

The main reason H2 was not used more often in the past is that concentrations 4 vol% in air are explosive.

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Most analyses are run at carrier gas velocities that are 1.5 to 2 times greater than the optimum velocity at the minimum of the van Deemter curve. The higher velocity is chosen to give maximum efficiency (most theoretical plates) per unit time. A decrease in resolution is tolerated in return for faster analyses.

Gas flow through a narrow column may be too low for best detector performance, so extra makeup gas is sometimes added between the column and the detector. Makeup gas that is optimum for detection can be a different gas from that used in the column.

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Sample Injection

Injection into open tubular columns:

split injection: routine means of introducing small sample volume into open tubular column • interest constitute > 0.1% of the sample

splitless injection: best for trace levels of high boiling solutes in low boiling solvents • for trace analysis of analyse that are less than 0.01% of the sample

on-column: best for thermally unstable solutes and high solvents; best for quantitative analysis • use for samples that decompose above their boiling point

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The ideal detector for gas chromatography has the following characteristics:

1. Adequate sensitivity. In general, the sensitivities of present-day detectors lie in the range of 10–8 to 10–15 g solute/s.2. Good stability and reproducibility.3. A linear response to solutes that extends over several orders of magnitude.4. A temperature range from room temperature to at least 400oC.5. A short response time that is independent of flow rate.6. High reliability and ease of use. To the greatest extent possible, the detector should be foolproof in the hands of inexperienced operators.7. Similarity in response toward all solutes or, alternatively, a highly predictable and selective response toward one or more classes of solutes.8. Nondestructive of sample.

Detectors

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Thermal Conductivity Detectors

• The thermal conductivity detector (TCD): simple and universal— responding to all analytes.

• consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas.

• The heated element may be a fine platinum, gold, or tungsten wire or, alternatively, a small thermistor. The electrical resistance of this element depends on the thermal conductivity of the gas.

• Figure 32-10a shows a cross-sectional view of one of the temperature-sensitive elements in a TCD

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Figure 32-10 Schematic of(a) a thermal conductivity detector celland (b) an arrangement of two sampledetector cells (R2 and R3) and tworeference detector cells (R1 and R4).(Reprinted from F. Rastrelloa,P. Placidi, A. Scorzonia, E. Cozzanib,M. Messinab, I. Elmib, S. Zampollib,and G. C. Cardinali, Sensors andActuators A, 2012, 178, 49,DOI:10.1016/j.sna.2012.02.008.Copyright (2012), with permissionfrom Elsevier.)

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Thermal Conductivity Detector Characteristics • Minimum Detectability (LOD): ~ 10-9g (~10ppm) • Response: all compounds • Stability: moderate • Temperature Limit: ~ 400oC • Gases – Carrier: (usually) Helium / Makeup: Same as carrier

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Flame Ionization Detectors (FID)

In FID, eluate is burned in a mixture of H2 and air. Carbon atoms (except carbonyl and carboxyl carbons) produce CH radicals, which are through to produce CHO+ ions in the flame

CH i +Oi → CHO+ + e−

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FIGURE 6.12 Schematic diagram of an FID

CH i +Oi → CHO+ + e−

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Flame Photometric Detector (FPD)

• Compounds are burned in a hydrogen-air flame very similar to an FID detector

• Sulphur and phosphorous containing compounds produce light emitting species (sulphur – 394nm / phosphorous – 526nm). A monochromatic filter allows only one of the wavelengths to pass and a photomultiplier tube is used to measure the amount of incident light and a signal is generated. A different filter is required for each detection mode.

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Flame Photometric Detector Characteristics

• Minimum Detectability (LOD): ~ 10-100pg (sulphur); 1-10pg (phosphorous)

• Response: Sulphur or phosphorous containing compounds (one at a time)

• Stability: moderate

• Temperature Limit: ~ 300oC

• Combustion gases: Hydrogen and Air. Makeup: Nitrogen

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Electron Capture Detectors (ECD)

• ECD has become one of the most widely used detectors for environmental sample because it selectively responds to halogen containing organic compounds, such as pesticides.

• Compounds such as selective halogens, peroxides, quinones, and nitrogen groups are detected with high sensitivity.

• insensitive to functional groups such as amines, alcohols, and hydrocarbons

• an important application of the ECD is for the detection and quantitative determination of chlorinated insecticides

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Figure 27-10 Schematic diagram of an ECD

usually nickel-63

The sample equate from a column is passed over a radiative beta emitter, usually Ni-63. An electron from the emitter causes ionization of the carrier gas (ofter nitrogen) and the production of the burst of electrons. In the absence of organic species, a constant standing current between a pair of electrons results from this ionisation process. The current decreases significantly, however, in the presence of organic molecules containing electronegative functional groups that tend to capture electrons.

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Nitrogen-phosphorus Detector (NPD)

• selective for the determination of nitrogen- or phosphorus-containing compounds

• similar to FID in that it is based on the measurement of ions that are produced from eluting compound, but NPD does not use a flame for ion production

• NPD generates ions by using thermal heating at or above a surface that can supply electrons to any electronegative species that surround it, forming negatively charged ions. This mechanism of ion formation is particularly efficient for nitrogen or phosphorous-containing compounds, which makes the NPD selective for such chemicals

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FIGURE 6.20 Schematic drawing of the NPD

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Photoionisation Detector (PID)

• Compounds eluting into a cell are bombarded with high-energy photons emitted from a lamp. Compounds with ionisation potentials below the photon energy are ionised. The resulting ions are attracted to an electrode, measured and a signal is generated.

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Photoionisation Detector Characteristics

• Minimum Detectability (LOD) ~ 25-50pg (aromatics); 50-200pg (olefins)

• Response: Depends on lamp characteristics, conventionally used for aromatics and olefins (10 eV lamp)

• Stability: good

• Temperature Limit: ~ 200oC

• Makeup gas: Same as carrier

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Figure 21.18 A transmission quadrupole MS with an EI ionisation source for use in GC/MS.

Gas Chromatography/Mass Spectrometry

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Mass Spectrometer Characteristics

• Minimum Detectability (LOD): ~ 1-10ng (SCAN); 1-10pg (SIM)

• Response: Any compound that produces fragments (or ions) within the selected mass range of the analysing device.

• Stability: good

• Temperature Limit: ~ 250oC

• Carrier gas: Helium

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