Gas Chromatograph Analyzer Configuration
The configuration of a typical gas chromatograph is shown in Figure 13. The electronics section controls the automatic timing of the analyzers batch process, and it controls the sample system.
With a smart sample system, the electronics section can monitor its condition. The chromatograph oven, which is controlled by the electronics section, can either be isothermal or have a
programmed temperature ramp. The oven is controlled to maintain the temperature of the sample valve, the columns, and the detector. The sample conditioning section is one of the most critical parts of the analyzer system. An improperly designed sample system is one of the primary reasons for analyzer failures and high maintenance costs. With modern electronics, the sample system is being monitored so that if a failure occurs, the sample system is shut down to protect the analyzer.
Carrier gas, which is usually stored in pressurized cylinders, is supplied to the analyzer through a two-stage regulator.
Figure 13: Configuration of Gas Chromatograph Analyzer
Oven/Temperature Control
The operation of the oven is critical to the reproducible and reliable operation of the analyzer because it has a dramatic effect on the retention times and column performance. The temperature of the oven is also critical to the components of the oven. The sample valve’s temperature must be hot enough to vaporize a liquid sample quickly, but not hot enough to thermally decompose the sample. The column temperature should be hot enough to achieve the desired component separation within the desired cycle time; however, the temperature should be low enough to extend the life of the columns. The detector’s temperature must be hot enough to avoid condensation of the sample.
The oven temperature can be maintained at a constant value (isothermal), or the oven temperature may be increased over time (programmed temperature). The operations of isothermal ovens and programmed temperature ovens are described below.
Isothermal
The isothermal oven must maintain a constant temperature (Figure 15) over the entire column, and it should be able to maintain the temperature to ±0.1°C. The temperature gradient (the difference between the highest and lowest mean temperatures) should be kept to a minimum. The operating range of the oven should be from 5-10° above ambient up to 400°C.
Programmed Temperature
Temperature programming is used to separate gas components with a wide range of boiling points. The oven temperature is gradually increased after the sample is injected to speed up the elution of the components with high boiling points. The programmed temperature technique is used mostly in the laboratory; however, some manufactures offer the option for process GCs.
The column is heated at rates that vary from 0.25°C/min to 20°C/min (Figure 14). The oven should be able to heat the column from ambient temperature to 400°C within 40 minutes. The programmed temperature oven should be able to maintain the actual temperature to ±2°C of the desired temperature.
Figure 14: Isothermal and Programmed Temperature Profiles
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Carrier Gas Supply System
The function of a carrier gas system is to supply a clean, dry, and constant pressure of carrier gas to the analyzer. The choice of carrier gas primarily depends on the type of detector that is used (although for best column efficiency nitrogen would be preferred). For example, thermal
conductivity detectors require the use of helium of hydrogen because they provide the maximum thermal conductivity difference (sensitivity) between the carrier and the sample components.
Nitrogen is used when hydrogen in the sample needs to be measured, which provides maximum sensitivity for the detector. When hydrogen needs to be measured along with other components, two carrier gases (nitrogen and helium or hydrogen) and two detectors are recommended.
Nitrogen is for the hydrogen measurement, and helium or hydrogen is for measurement of the other components.
Carrier Gases
Carrier gases carry the sample components from the sample valve through both the column and the detector. The goal of the carrier gas is to provide a stable transport and detection medium for the sample components. The carrier gas should be:
• inert to avoid reaction with the sample or solvent
• pure and dry
• appropriate for the detector
The properties of common carrier gases are shown in Figure 15. Helium, hydrogen, and nitrogen are the most commonly used carrier gases.
Molecular
Figure 15: Properties of Common Carrier Gases(3)
Flow/Pressure Regulation
Reliable operation of the gas chromatograph is dependent on a carrier gas flow rate that is accurately controlled. Varying carrier flow rates will effect retention times of the components as well as affect the stability of the detector. Pressure regulators that are not affected by ambient temperature changes should be used. Carrier gas pressure should be supplied to the analyzer at about 15 psi above column head pressure to provide optimum conditions for the pressure
regulator at the analyzer. Two cylinders of carrier gas should be used for each analyzer, and they should be installed as shown in Figure 16. The cylinders are installed so that one cylinder can be replaced without affecting the carrier gas flow through the analyzer.
Figure 16: Recommended Carrier Gas Cylinder Installation
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Detectors
The detector responds to the gases that emerge from the column. In practice, detectors operate on a deferential principal; that is, they show a constant base line with carrier gas flowing, generate a signal as the components pass, and then return to baseline. The signal that is generated is used to measure the amount of component that passed through the detector. Due to their stability in a harsh environment and their sensitivity to a wide range of components, the following detectors have become the limited choices in process gas chromatographs:
• thermal conductivity detectors (TCD)
• flame ionization detectors (FID)
• photo ionization detectors (PD)
• flame photometric detectors (FPD)
• electron capture detectors (ECD)
Thermal conductivity and flame ionization detectors are most commonly used. The type of
detector that is selected is based on the type and concentration of the components to be measured.
The detector is usually selected by the analyzer vendor. Figure 17 shows a guide that is used for selecting a detector based on the type and concentration of the component. All five detectors will be explained on the following pages.
Figure 17: Guide to the Selection of Gas Chromatograph Detectors
Thermal Conductivity Detectors
Thermal conductivity is the ability of a substance to transmit heat by conduction. Thermal conductivity detectors use this thermodynamic property of gas to detect the components as they exit the column. As a general rule, the lower the molecular weight (MW) of a gas molecule, the higher is its thermal conductivity. To illustrate the differences in the thermal conductivities of gases, assume that two identical ingots of steel are heated to the same temperature. One ingot is placed in an atmosphere of pure helium (MW=4), and the other ingot is placed in an atmosphere of pure nitrogen (MW=28). The ingot of steel in the helium atmosphere will cool faster due to the higher thermal conductivity of helium.
The thermal conductivity technique is appropriate for GC analysis because the thermal conductivity of the carrier gas (hydrogen or helium) is significantly different from the thermal conductivities of sample gas components (see Figure 18). The thermal conductivity of a gas changes slightly with the operating temperature.
Argon 44.22 Hydrogen 458.72
n-Butane 40.91 Methane 85.54
Ethane 54.50 Nitrogen 64.06
Ethylene 52.07 Oxygen 65.91
Helium 368.63 Propane 45.46
Figure 18: Thermal Conductivities of Typical Gases - k in [cal/(sec)(cm2)(°C/cm) x 10-6] at 38° C
Two thermal conductivity detector elements are used for stability. The two detectors are sometimes split into two pairs (Figure 19). One detector is used to measure the separated
components and the reference detector is subjected to pure carrier. These two detectors should be identical and exposed to the same conditions. The detectors are then electrically connected in a Wheatstone bridge circuit, or in a constant temperature circuit.
Figure 19: Two Thermal Conductivity Detectors in a Gas Chromatograph Analyzer
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The Wheatstone bridge (Figure 20) is used to measure any change in the resistance of the elements. The bridge provides the best measurement results when it is balanced. The bridge is balanced when the measuring and reference gases have the same thermal conductivity
characteristics. With carrier gas flowing across both detectors, the circuit is then balanced, or zeroed, electronically. Zeroing causes the circuit to force the reference detector to operate at the same temperature as the measuring detector.
With a constant voltage applied and the circuit balanced, components cross the measuring element causing a change in its resistance. A corresponding increase in current, which is the basis for the output, will occur. Due to the increase in current, detectors using this circuit are susceptible to burning out.
With a constant voltage detector circuit, as the resistance of the element changes, the voltage is adjusted to maintain the same current. The change in voltage becomes the basis for the output.
This circuit dramatically improves the life of the elements.
Thermal conductivity detectors are selected, typically, when the sample gas component
concentrations are greater then 0.1 mole %. The detector element can either be a filament or a thermistor. The element that is chosen must respond accurately to variations in carrier gas
impurities and sample component concentrations. The operation of the filament and the thermistor are described below.
Figure 20: Wheatstone Bridge
Thermistor - Thermistors are sintered mixtures of manganese, cobalt, and nickel oxides, plus trace elements, all of which give them the desired electrical properties. The thermistor, which is in the form of a small bead, is mounted on a platinum wire. The thermistor is then coated with glass to make it inert. For this reason, a thermal conductivity detector that uses a thermistor is often referred to as a thermistor bead detector.
Figure 21 shows two thermistors in the measuring cell block. The two thermistors are a matched set. The two elements are heated by the electrical current, which flows through them. The measuring cell, in which the element resides, is connected to the effluent of the chromatograph column. The element is cooled, to some extent, by the carrier gas flow. As the gas components elute off the column, less of the heat generated by the thermistor is removed, and the resistance of the element changes. Because the gas component absorbs less of the heat than the carrier gas, an imbalance in the circuit will occur proportional to the change in thermal conductivity. This change is directly related to the concentration of the component. The output of the detector is plotted over time in the form of a chromatogram.
Figure 21: Thermistor Bead Detector
Thermistors are very sensitive; however, they have a limited temperature range and poor stability.
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Filament - Figure 22 shows a duel pair of filaments in a measuring cell. The four filaments are heated by an electrical current that flows through them. The measuring element cell, where the filaments reside, is connected to the effluent of the chromatograph column. The filaments are cooled, to some extent, by the carrier gas flow. As the gas components elute off the column, less of the heat generated by the filament is removed, and the resistance of the element changes.
Because the gas component absorbs less of the heat than the carrier gas, an imbalance in the circuit will occur proportional to the change in thermal conductivity. This change is directly related to the concentration of the component. The output of the detector is plotted over time in the form of a chromatogram.
Figure 22: Filament Elements in a Measuring Cell Block(5)
Ionization Detectors
An ionization detector is used when trace amounts of gas molecules need to be measured. In general, the operation of ionization detectors is based on the fact that the electrical conductivity of a gas is proportional to the concentration of charged particles (ions) within the gas. Figure 23, shows an ionization detector with an unspecified ionizing source. The ionization source ionizes the molecules in the effluent gas from the column. The presence of ions within the electrode gap causes a current, I, to flow through a measuring resistor, R2. The resulting voltage drop E0 is amplified by an electrometer, and the signal is sent to a recorder. The "bucking voltage" is used to zero the circuit when only carrier is flowing.
Figure 23: Ionization Detector(4)
Two types of ionization detectors are used in gas chromatography, flame ionization and photo ionization. Both detectors are used to measure gas component levels from 0.5% to levels as low as a few ppb. Although both detectors operate on the same principals, there are several
differences, which are described below.
Flame Ionization
A flame ionization detector is component-specific, it measures low level organic compounds.
Figure 24 shows a list of compounds to which an FID will not respond.
He Xe COS N2O CO2
Ar O2 H2S NO2 H2O
Kr N2 SO2 NH3 SiCl4
Ne CS2 NO CO SiHCl3
SiF4 Figure 24: Compounds That Give Little or No Response to the
Flame Ionization Detector(4)
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The hydrogen air flame in Figure 25 is the ionizing source. The sample gas components are mixed with the hydrogen fuel, and then they are burned. At the high temperature of the flame,
compounds that contain carbon bonds break down into positive and negative ions and electrons.
A metal grid (the Collector) surrounds the flame to collect the current that passes through the flame. The number of ions that are formed, and thus the current that is conducted, is roughly proportional to the number of carbon atoms in the flame.
Figure 25: Flame Ionization Detector(4)
Flame ionization detectors can measure low level inorganic molecules such as carbon monoxide, carbon dioxide, carbonyl sulfide and formaldehyde by converting them to methane with the use of a methanator.
Methanator - A methanator consist of 2 feet of 1/8” tubing, packed with a nickel-coated catalyst (Figure 26). The methanator is placed in front of the detector. As the components leave the end of the column, they are readily converted to methane by the catalyst in the presence of hydrogen.
The methane that is produced can then be measured using the flame ionization detector.
Figure 26: Methanator
Air Treater - Carrier gas impurities (e.g., hydrocarbons) that are present between the electrodes will cause a constant current to flow. This constant current is called the “background current.” It is desirable to minimize the background current so that small changes in current flow can be more easily detected. The best method of removing trace hydrocarbons from the instrument air is to use an air treater to heat the air in the presence of a catalyst, which converts the hydrocarbons to inert carbon dioxide (Figure 27).
Figure 27: Air Treater
Photo ionization
The photo ionization detector (Figure 28) contains a sealed ultraviolet lamp with a specific energy (in eV). Lamps with energies of 9.5, 10.0, 10.2, 10.9, and 11.7 eV are available. As the gas components elute from the column, their UV-absorbing molecules are ionized by the ultraviolet light. Compounds whose ionization potentials are lower then the lamp’s ionization energy are affected. In the presence of the radiation the components become ionized. A pair of electrodes are located in a chamber, which is adjacent to the ultraviolet source. An electric potential is applied across the electrodes to create an electrical current. When the ions that were formed pass through the electrodes, a current output is generated proportional to the concentration of ions. The current from the detector is amplified and passed to the recorder.
Figure 28: Photo Ionization Detector
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Flame Photometric
Flame photometric detectors are used to measure trace quantities of compounds that contain sulfur. Figure 29 shows a schematic of a flame photometric detector. The sample gas stream from the column is mixed with a constant flow of hydrogen near the burner tip. Sulfur-containing compounds are burned in the flame to produce a blue light. The light is transmitted through an optical cable to a 394 nm band-pass filter. The band-pass filter isolates the desired wavelength range before the light reaches the photomultiplier tube. The photomultiplier tube produces an output current that is proportional to the square of the sulfur concentration. The output current is amplified, and it is converted to a voltage by an electrometer circuit.
a
Figure 29: Schematic of a Flame Photometric Detector
Electron Capture
Figure 30 shows a schematic of an electron capture detector (ECD). The detector contains two electrodes through which the nitrogen carrier gas passes. One of the electrodes is treated with a radioisotope (tritium or nickel-63), which emits high-energy electrons as it decays. The electrons ionize the nitrogen molecules to produce an abundance of low-energy electrons in the carrier gas stream. These low-energy electrons, which are collected by the positively-charged electrode, produce a steady current. If the gas sample contains molecules that absorb the low-energy electrons, the amount of current is reduced. The loss of current is a measure of the electron affinity of the sample gas component.
The ECD is very sensitive to certain molecules such as alkyl halides, conjugated carbonyls, nitriles, nitrates, and organometals; however, it is insensitive to hydrocarbons, alcohols, ketones, etc.
Figure 30: Electron Capture Detector
Columns
The separation of sample components occurs in the chromatograph column. There are two basic types of chromatograph columns, adsorption and partition columns. Variations of the types of columns are packed and capillary. Within these variations, hundreds of column materials are used, and others are constantly being developed. Although there are many columns, their basic operation is the same. The operation of these columns are described on the following pages.
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Adsorption Columns
Adsorption columns are used in gas-solid chromatography for the separation of gases, such as nitrogen, oxygen, carbon dioxide, and hydrogen sulfide. These columns can also be used to separate hydrocarbons in the C1 to C3 range. Components are separated by their differences in adsorption, which can be defined as their tendency to adhere to the adsorbent. Packing materials are surface-active solids, such as activated alumina, charcoal, molecular sieves, silica gel, and synthetic zeolites. The mole sieve column separates the sample gas components by molecular size. The particles in the mole sieve column contain molecular-sized holes that trap small molecules that fit into them. A length of tubing acts as a series of sieves. The larger molecules pass through the column faster than the smaller molecules, which enter the pores. For petroleum service, adsorption columns have a limited range of application except when used in combination with other columns.
Partition Columns
Partition columns are used in gas-liquid chromatography for separating complex hydrocarbon samples. The column packing is a granular solid (support), which is coated with a nonvolatile liquid stationary phase (Figure 31).
a
Figure 31: Enlargement of Support and Liquid Stationary Phase
The packing exposes a large liquid surface to the vaporized sample components as they migrate through the column. The sample components are partitioned between the gas and liquid phases.
The components that are least soluble in the liquid pass rapidly through the columns and emerge first. The components that are most soluble in the liquid are retained longer and emerge later.
The components that are least soluble in the liquid pass rapidly through the columns and emerge first. The components that are most soluble in the liquid are retained longer and emerge later.