Gas chromatography. Advantages of GC. Disadvantages of GC

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Advantages of GC Gas chromatography Fast analysis, typically minutes Effi cient, providing high resolution Sensitive, easily detecting ppm and often ppb Nondestructive, making possible on - line

1 Advantages of GC Gas chromatography 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 Lecturer: Somsak Sirichai 1 2 Disadvantages of GC Limited to volatile samples Not suitable for thermally labile samples Requires spectroscopy, usually mass spectroscopy, for confirmation of Gas chromatography: mobile phase: gas stationary phase: usually a nonvolatile liquid, but sometimes a solid analyte: gas or volatile liquid peak identity 3 4

GC: Block Diagram Flow Measurement The two most commonly used devices are a soap - bubble flowmeter and a digital electronic flow measuring device.

( b ) Digital electronic type 5 6 CARRIER GAS The main purpose of the carrier gas is to carry the sample through the column.

2 GC: Block Diagram Flow Measurement The two most commonly used devices are a soap - bubble flowmeter and a digital electronic flow measuring device. He, N2 or H2 Figure 1: Block diagram of a typical gas chromatograph. Fig. 2.2 Flow meters: ( a ) Soap film type. ( b ) Digital electronic type 5 6 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. Gas Sampling Gas sampling methods require that the entire sample be in the gas phase under the conditions in use. Gas - tight syringes A secondary purpose is to provide a suitable matrix for the detector to measure the sample components. Below are the carrier gases preferred for various detectors: 7 8

Liquid Sampling 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.

3 Liquid Sampling 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. Column: Open Tubular Column Columns: (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 350 o C) for support and protection from atmospheric moisture Solid Sampling Solids are best handled by dissolving them in an appropriate solvent and by using a syringe to inject the solution. Compared with packed columns, open tubular columns offer: higher resolution shorter analysis time greater sensitivity lower sample capacity 9 10 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 m. (c) Cross-sectional view of wall-coated, support-coated, and porous-layer columns

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.

) 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.

4 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 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. 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.] 15 16

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.

Optimal flow rate increases in the order N2 < He < H2.

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

5 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 Figure 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. Figure shows the effect of carrier gas on the separation of two compounds on the same column with the same temperature program. The main reason H2 was not used more often in the past is that concentrations 4 vol% in air are explosive. FIGURE 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.] 19 20

6 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. 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 Detectors 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 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 400 o C. 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

Thermal Conductivity Detectors The thermal conductivity detector (TCD): simple and universal responding to all analytes.

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.

sample detector cells (R2 and R3) and two reference detector cells (R1 and R4). (Reprinted from F. Rastrelloa, P. Placidi, A. Scorzonia, E. Co

7 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 temperaturesensitive elements in a TCD Figure Schematic of (a) a thermal conductivity detector cell and (b) an arrangement of two sample detector cells (R2 and R3) and two reference 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 and Actuators A, 2012, 178, 49, DOI: /j.sna Copyright (2012), with permission from Elsevier.) Thermal Conductivity Detector Characteristics Minimum Detectability (LOD): ~ 10-9 g (~10ppm) Response: all compounds Stability: moderate Temperature Limit: ~ 400oC Gases Carrier: (usually) Helium / Makeup: Same as carrier 27 28

8 Flame Ionization Detectors (FID) In FID, eluate is burned in a mixture of H 2 and air. Carbon atoms (except carbonyl and carboxyl carbons) produce CH radicals, which are through to produce CHO + ions in the flame CH i +O i CHO + + e CH i +O i CHO + + e FIGURE 6.12 Schematic diagram of an FID 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

Flame Photometric Detector Characteristics Minimum Detectability (LOD): ~ 10-100pg (sulphur); 1-10pg (phosphorous) Response: Sulphur or phosphorous containing compounds (one at a time) Stability:

Makeup: Nitrogen 33 34 Electron Capture Detectors (ECD) Figure 27-10 Schematic diagram of an ECD ECD has become one of the most widely used detectors for environmental sample because it selectively

9 Flame Photometric Detector Characteristics Minimum Detectability (LOD): ~ pg (sulphur); 1-10pg (phosphorous) Response: Sulphur or phosphorous containing compounds (one at a time) Stability: moderate Temperature Limit: ~ 300 o C Combustion gases: Hydrogen and Air. Makeup: Nitrogen Electron Capture Detectors (ECD) Figure Schematic diagram of an 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. usually nickel-63 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 35 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. 36

Nitrogen-phosphorus Detector (NPD) selective for the determination of nitrogen- or phosphoruscontaining compounds similar to FID in that it is based on the measurement of ions that are produced from

10 Nitrogen-phosphorus Detector (NPD) selective for the determination of nitrogen- or phosphoruscontaining 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 FIGURE 6.20 Schematic drawing of the NPD 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. Photoionisation Detector Characteristics Minimum Detectability (LOD) ~ 25-50pg (aromatics); pg (olefins) Response: Depends on lamp characteristics, conventionally used for aromatics and olefins (10 ev lamp) Stability: good Temperature Limit: ~ 200 o C Makeup gas: Same as carrier 39 40

Gas Chromatography/Mass Spectrometry Mass Spectrometer Characteristics Minimum Detectability (LOD): ~ 1-10ng (SCAN); 1-10pg (SIM) Response: Any compound that produces fragments (or ions) within the

11 Gas Chromatography/Mass Spectrometry 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: ~ 250 o C Carrier gas: Helium Figure A transmission quadrupole MS with an EI ionisation source for use in GC/MS

Harris: Quantitative Chemical Analysis, Eight Edition CHAPTER 23: GAS CHROMATOGRAPHY

Harris: Quantitative Chemical Analysis, Eight Edition CHAPTER 23: GAS CHROMATOGRAPHY Chapter 23. Gas Chromatography What did they eat in the year 1,000? GC of Cholesterol and other lipids extracted from