Gas chromatography (GC) was at one time commonly called “GLC,” where the “L” stands for liquid. Inside GC columns are particles of a ceramic or inert material coated with an extremely viscous liquid stationary phase that interacts with the analyte. By contrast, HPLC stationary phases are bonded to the base material. GC is a common technique used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. GC is typically used for separating the different components of a mixture, improving the purity of a particular substance, or identifying a particular compound. GC makes it possible to separate the volatile components of a very small sample and to determine the amount of each component present. The essentials required for the method are an injection port through which samples are loaded, a “column” on which the components are separated, a regulated flow of a carrier gas which carries the sample through the instrument, a detector, and a data processor. In gas chromatography, the temperature of the injection port, column, and detector are controlled by thermostatted heaters.
In gas chromatography, the mobile phase is a carrier gas, usually an inert gas such as helium or an unreactive gas such as nitrogen. The gaseous compounds being analyzed interact with the walls of the column, which is coated with different stationary phases. This causes each compound to elute at a different time, known as the retention time of the compound. The comparison of retention times is what gives GC its analytical usefulness. The most widely used gas selected by the respondents is helium and cylinders (standard – 46 percent and portable – 19 percent) are the number source for delivery.
Different types of gases respondents use:
|Zero air gas (<1 ppm hydrocarbons)||9%|
Many GC systems are available as basic units that can accommodate different detectors and different sample injectors. This allows users to customize the GC system according to their own particular needs, providing full flexibility. If you are in the market for a new GC system, vendors recommend that you should first choose a base system, then a detector type and, finally, a method of sample introduction. The base system is the fundamental part of the modular GC unit, accommodating the GC column and basic hardware. GC detectors have been evolving rapidly to provide greater sensitivity. Flame ionization detectors (FIDs) have been the most widely used, as they detect any molecule containing carbon. Numerous other detector types have been introduced over the years, but the most interesting is the mass detector, which is essentially a miniaturized mass spectrometer. Mass detectors provide unequivocal identification of peaks emerging from the chromatograph based on the molecules’ molecular weights and fragmentation patterns. Other detectors are sensitive only to specific types of substances, or work well only in narrower ranges of concentrations
|Flame ionization (FID)||31%|
|Thermal conductivity (TCD)||15%|
|Electron capture (ECD)||11%|
|Flame photometric (FPD)||5%|
|Hall electrolytic conductivity||2%|
Although many of the chromatographic instrumental techniques have matured and automation is commonplace, sample preparation is still considered to be slow, labor-intensive, and even a bottleneck in laboratory processes. Some high-throughput laboratories take advantage of the latest autosampling technology to process hundreds and sometimes thousands of samples a day.
Related gas chromatograph components respondents are using in their lab:
|Regulators, valves, fittings||22%|
|Purge & trap||10%|
For the service and repair of their GC instrument, 45 percent of the respondents use instrument manufacturers either as a service contract or time and material.
|Instrument manufacturer service contract||23%|
|Instrument manufacturer time/material||22%|
|In-house service department||12%|
|Multi-vendor service provider||4%|
Around the mid-1980s, column chromatography began to give way to capillary column GC, in which the liquid phase is coated onto the inner walls of a thin capillary tube that can be anywhere from 15 to 100 meters long, and wound into a coil. Capillary column chromatography represented a significant advance in the field and greatly improved the ability of columns to separate the multiple components found in complex drug and arson samples. However, a few applications still require packed columns.
The most common problems respondents encounter with their GC columns are “short column life” and “interactions with sample or solvent.” Problems respondents encounter with GC columns:
|Short column lifetime||23%|
|Interactions with sample or solvent||22%|
|Low reproducibility after changing columns/batches||18%|
|Lack of support from vendor||11%|
Top ten features / factors that are important to respondents in the decision-making process for a new GC system:
|Availability of supplies and accessories||83%|
|Ease of maintenance||85%|
|Easy transfer of methods||84%|
|Quality of data||92%|
|Ruggedness and reliability||87%|
|Service and support||90%|
Completed Surveys: 223
For more information on gas chromatography systems, visit www.labmanager.com/GC