Gas Chromatography (GC)

Overview

Gas Chromatography (GC) is a common separatory technique used to analyze (qualitatively and/or quantitatively) volatile compound(s) within a mixture.  Samples can be gases, liquids or solids although for the latter two cases they must be first dissolved in a volatile solvent.  For substances to be considered volatile, and thereby suitable for GC analysis, their vapor pressures should typically be <300C and their molecular weight <1000 Da.  The compounds should also NOT undergo decomposition when vaporized.    

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The process of GC is relatively simple.  The sample, typically containing a mixture of compounds, is injected into a flow of an inert carrier gas (i.e. the mobile phase) which transports it down a column containing an appropriate chemical (i.e. the stationary phase).  The column is located in an oven allowing the temperature to be increased during the analysis, to values as high as 200-350C (or higher for some applications).  During a sample ‘run’ the oven is typically heated to a set temperature and held for a period of time before being ramped to a higher temperature (ramp rates are typically ~10-80C/min) and held again.  Multiple hold and ramp events can occur within a run.  The sample components interact with the stationary phase within the column.  Those compounds that have a strong interaction take longer to travel down the column (i.e. elute) compared to those which interact weakly.  Heating the column facilitates the desorption of species that are tightly held to the stationary phase.  This process results in the separation of the sample components.  As the individual components exit the column, they are detected by a suitable detector.  The time it takes for a compound to traverse the GC system is termed the 'Retention Time'.  Each component will have a suitably different retention time if analysis conditions have been chosen to allow for their appropriate separation within the column.  Ultimately, separation is based on differential partitioning of the components between the mobile and stationary phases. 

 

While the general GC process is fairly easy to understand, many variations exist in actual instrument setup due to the wide variety of carrier gases, columns, stationary phases and detectors in use today.  Further, numerous analytical conditions affect the quality of component separation of an analyzed mixture including column temperature, carrier gas flow rate, column length and amount of material injected.  Both experience and method development skills are necessary to choose the appropriate instrument set-up and analysis conditions for a given sample that results in sufficient resolution (i.e. peak separation), detection limits and suitable analysis times.

 

Sample Introduction

Samples can be injected into the instrument manually using a syringe or via an autosampler, which also uses a syringe, but is more controlled and reproducible.  Injectors can be operated in a Split or Splitless mode.  Other less common methods of introducing samples exist as well and are used for special applications.    

 

The Split method of injection is most common and involves introducing only a percentage (typically 0.2-20%) of the sample onto the column.  The remainder is vented out of the injector to waste.  This is a fast method resulting in narrow analyte bands on the column and subsequently narrow peaks in the chromatogram.  It also is easy to use and automate and helps protect the column from contaminants and involatile sample components that may be present.  However, it is not suitable for trace analysis since a portion of the sample is discarded.   

 

The Splitless method of injection transfers the entire sample volume onto the column.  This is primarily used for trace analysis applications.  However, this is a much slower process than the Split method resulting in broader analyte bands on the column.  Further, this method is more susceptible to column contamination since the entire sample enters the column. 

 

Gases

The carrier gas most commonly used in GC is He although H2 has grown in popularity of late due to the significant increase in cost for the former.   Other gases sometimes used include N2 and Ar.  The decision on which gas to use often depends on the detector being utilized as well as cost although retention times are also affected by the nature of the carrier gas. 

 

Some detectors such as those involving combustion require additional gases to be added into the detector.   They need both a fuel gas (commonly H2) and an oxidizing gas (commonly air).  Many detectors also utilize a ‘Make-Up’ gas to help sweep the analyte through the device to reduce peak broadening which ultimately affects resolution and peak separation.  Common make-up gases include N2, He and an Argon/Methane mixture.    

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Columns

Two types of columns exist, Capillary and Packed columns. 

 

1.  Capillary columns are typically 0.1-0.75mm in diameter, 15-60m in length and wound in a circle.  The stationary phase can be simply coated onto the wall (referred to as 'non-bonded') or it can be immobilized and/or incorporated (via cross-linking) into the column wall (referred to as 'bonded').  The latter is preferable as less 'bleeding' of the stationary phase occurs during the analysis.    

 

2.  Packed columns consist of a tube filled with inert particles (e.g. Chromosorb 101 which is a porous styrene--divinylbenzene cross-linked polymer) with typical particle sizes of 125-250um.  These particles are coated with an appropriate stationary phase.  Packed Columns are less commonly used in GC as they have lower resolution than capillary columns.   

 

Columns are easily switched out as needed as specific types are often required for different types of analyses.  Further, columns do degrade with use and sometimes are ‘poisoned’ by components within a sample.  As such they must be periodically replaced. 

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Stationary Phases

A wide variety of stationary phases are used in GC columns with the actual one chosen dependent on the compounds being analyzed with the general chemical principle of 'like dissolves like' being considered.  Understanding the interaction of the analytes of interest with the stationary phase is important because this is key to the separatory aspect of the technique. 

 

The analysis of Polar compounds such as alcohols, acids, ethers, esters, thiols and amines utilize Polar stationary phases (e.g. Polyethylene Glycol) while Non-Polar compounds such as alkanes require a Non-Polar Stationary phase (e.g. siloxane, hydrocarbon/siloxane mixture).  Polarizable compounds such as alkenes, alkynes and aromatics often require a very polar stationary phase (e.g. poly(biscyanopropyl siloxane)). 

 

A large variety of stationary phases exist within each of these 3 categories of compounds.  The type of stationary phase utilized is one important factor in determining the maximum temperature of a particular GC analysis.  For example, the maximum temperature of the non-polar stationary phase polydimethylsiloxane is 350C while that of the polar polyethylene glycol is only 250C.   

 

Detectors

A wide variety (more than 15 total) of detectors can be utilized in GC and are based on the analysis of various physicochemical properties.  Each detector has its own set of advantages and disadvantages regarding what type of compounds can be analyzed, what carrier gas can be used and sensitivity.  Many GC instruments contain multiple detectors located in series to one another to improve system versatility.  However, in such a scenario any destructive detector, such as those involving a flame, must be placed at the end of the series.  Those more commonly used in GC are FID, TCD, ECD, FPD, PID and NPD detectors.

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Flame Ionization Detector (FID)

The FID Detector is an extremely common detector with extremely good sensitivities typically in the low pg range.  It is used to detect hydrocarbons and other carbon-containing compounds although the presence of heteroatoms (e.g. O and N) can reduce the sensitivity.  FID is based on the detection of ions formed during combustion of organic compounds in a flame after exiting the column.  Two electrodes are used to provide a potential difference to detect the ions.  As an organic compound enters the flame, primarily C ions are generated with the current produced between the electrodes being proportional to the amount of organic compound present.  The current is measured with an electrometer and amplified.  Both H2 and O2 (or air) are typically added into the FID detector if the carrier gas doesn’t already contain them in suitable concentrations.  FID is a destructive detector. 

 

Thermal Conductivity Detector (TCD)

The TCD detector is a good general-purpose detector.  Its sensitivities (0.4-0.8ng) and resolution are worse than the FID detector but it can detect all compounds, is non-destructive and does not require additional gases such as O2 and H2.  TCD works by comparing the thermal conductivity of the gas flow containing the analyte to the pure carrier gas containing NO analyte.  Thermal conductivity is measured by passing the gas over an electrically heated filament in a temperature-controlled cell.  The presence of an analyte passing over the filament changes the thermal conductivity of the column effluent (most often reducing it), causing the filament to heat up which changes its resistance. This resistance change results in a measurable voltage change.  

 

Electron Capture Detector (ECD)

The ECD Detector is a selective detector with excellent sensitivities (~5-50 femtograms) to analytes containing electronegative functional groups such as halogens, peroxides and nitro groups making it a common choice for use in environmental, forensic and pharmaceutical labs.  This detector is not sensitive to compounds such as hydrocarbons, amines or alcohols meaning it is not a good ‘general purpose’ choice.  However, it is non-destructive allowing it to be placed in series with other detectors. 

 

The ECD detector contains a foil of 63Ni, which is a radioactive Beta emitter, and two electrodes with a potential difference between them.  The Beta particles (which are high energy electrons) cause ionization of the carrier gas flowing through the detector allowing a current to be measured between the electrodes.  As molecules containing electronegative functional groups pass through the detector, they capture some of the electrons (thus the name ‘electron capture’) which reduces the current measured between the electrodes (because there is less ionization of the carrier gas) which is then recorded.  Typically, a 'make-up' gas such as argon/methane or N2 is added to the gas flow exiting the column to help slow down the Beta particles.  Without this process the Beta particles move too rapidly to be captured by the analytes.  The make-up gas improves the detector ionization yield and thus, its sensitivity.        

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Flame Photometric Detector (FPD)

The FPD Detector is another selective detector primarily used to measure S and P-containing compounds.  Detection limits are very good being in the low pg range for S and sub-pg range for P.  Like other flame-based detectors, it is destructive.  The FPD detector is similar to the FID in that the sample exists the column into a hydrogen rich flame.  However, FID measures ions that are produced following combustion of organic compounds while FPD measures emitted light from the compounds as they luminesce in the flame.  Band pass filters are used to selectively detect compounds containing S or P which each emit distinct wavelengths.  The filters allow only a specific range of light to pass through to a photomultiplier tube (PMT) for detection.  Different filters can be used for the analysis of other heteroatoms such as halogens, N, B, Se and metallic elements. 

 

Pulsed Flame Photometric Detector (PFPD)

The PFPD Detector is a variation of the FPD which keeps the fuel/air ratio within the detector below that needed to maintain a constant flame.  Instead, the flammable gases build up within which are then ignited in a pulse.  The emission of heteroatoms such as S and P is delayed relative to that of C species which are highly exothermic, rapid and irreversible.  This time delay results in increased sensitivity and selectivity for compounds containing these heteroatoms.  FPD and PFPD detectors are used for a variety of applications such as the analysis of petrochemicals, pesticides, chemical warfare agents and flavor and fragrances in the food and beverage industries.        

 

Photoionization Detector (PID)

The PID detector is a selective detector used for the analysis of aromatics and C-containing compounds with double bonds as well as amines with very good detection limits (~1-10 pg).  It is a non-destructive detector.  In PID, an ultraviolet light is used to ionize the compounds eluting from the column.  Any compound with ionization potentials below the photon energy are ionized.  The resulting ions are attracted to an electrode and the current generated measured.  The current produced is proportional to the concentration of the ionized components.  PID detectors are commonly used in a variety of environmental applications including some outlined in EPA protocols.

 

Nitrogen Phosphorus Detector (NPD)

The NPD Detector, also known as a TSD (Thermionic Specific Detector), is another selective detector used for the analysis of N and P, as the name implies.  It has excellent sensitivities in the sub pg range.  The NPD and FID detectors have some similarities.  While the sample passes through a hydrogen/air flame in the FID, it passes over a heated Rb or CsCl alkali bead.  The hot bead emits electrons by thermionic emission.  The electrons are collected at an anode to measure current.  When components containing N or P elute from the column the partially combusted materials are adsorbed onto the bead surface.  The alkali ions on the surface facilitate ionization of N- and P-containing compounds which are then attracted to the anode.  This, in turn, increases the current measured at the anode.   

 

Uses 

GC is best employed when looking for known compounds within a sample since it doesn't positively identify most analytes.  In other words, multiple compounds may have similar or identical retention times if they have similar molecular weights and affinity to the stationary phase.  Typically, GC analysis involves comparing retention times of signals from the sample with that from standards containing the compounds of interest analyzed under the exact same conditions.  If a sample contains unknowns then analysis by an additional technique such as GCMS is necessary to aid in their identification. 

 

GC is used in a wide variety of industries.  It’s useful for looking for the presence of contaminants in environmental samples, industrial processes and food and beverages.  It’s also commonly used in quality control applications to ensure the purity of products.  Many products such as drugs or chemicals used in semiconductor processing must be extremely pure.  The superb detection limits of GC help insure their purity.