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Gas chromatography (GC), much like liquid chromatography (LC), is used to separate compounds by sending a mixture through a column with which the constituent compounds differentially interact, detecting them as they emerge. But while “LC compounds tend to be polar and usually in the liquid phase, GC compounds tend to be nonpolar or of low polarity, and tend to have cooler boiling points,” points out Mark Taylor, GC Product Manager for Shimadzu Scientific Instruments (www.ssi.shimadzu.com).
Temperature (like solvent in LC)—sometimes as a gradient—is used to ensure that eventually our analyte will dissolve into the gas phase, says Meredith Conoley, Senior Director of Global GC and Petrochemical Operations for Bruker Corp. (www.bruker.com).
A GC system, then, essentially consists of an inlet where the sample is introduced, along with a carrier gas like helium or nitrogen, a separation column inside an oven, and a detector. (Systems specialized for simply measuring hydrogen or methane, for example, may not even require an oven.)
The choice of system components is typically dictated by the type of sample matrix, the nature of the compounds being tested for, the concentration range expected, and the sensitivity desired. There is no golden arrow, but there are systems that can handle most applications.
Modern GC is a well-developed, mature technology that has been in use since the 1950s to overcome matrix interferences in separating mixtures and identifying or quantifying their components (or groups of components). It is “used extensively in petrochemical and refining industries, QA, manufacturing labs, and environmental analysis,” points out Taylor. It can also be used in clinical and forensics settings, to look for things like fatty acid methyl esters, blood alcohol content, and drugs of abuse.
Generally the types of compounds examined by GC include petrochemicals, permanent gases, light hydrocarbons, and light and small molecule polymers, he adds—“pesticides, PCBs, herbicides, and a lot of nonlife-science-related compounds.”
Range of instrumentation
Although GC systems are typically modular, the components are not wholly independent of each other. For example, “the type of injector port is usually tailored to the type of column you’re going to use,” says Daniel Snow, Laboratory Services Director of the University of Nebraska-Lincoln’s Water Sciences Laboratory. And the type of column may be constrained by the type of detector.
GC injectors
According to Snow, until 10–15 years ago, the most common injectors were simple, nonpurged, nonvented ports that fed into a packed column. “If you’re injecting gases, many use an injection valve which has a loop for holding the gas sample in. I personally have used a gas-tight syringe to inject a gas sample.”
Things got a little more complicated as GCs were connected to mass spectrometers (MS) and capillary columns began to replace packed columns. There are now a host of different injector options, some for fairly specialized applications. SRI Instruments (www.srigc.com), for example, offers a dozen injector types for liquid, solid, gas, and even SPME fiber samples, notes President, Hugh Goldsmith. Many of these are heated to allow a sample to vaporize before it enters the column, offer room for the gas to expand, or (like the common split/splitless) can be vented for the bulk of the carrier gas to escape.
In the 1960s it was realized that GC columns need not be straight tubes; most are now sold wound into 9” spirals. “Virtually any chromatography column … will fit in virtually any oven” (with the exception of some very small units which take only 5” spirals), says Conoley.
There are two principal types of columns used on a GC. A packed column is typically a 5-mm-i.d. glass or metal coil that is 1–5 m in length and filled with a packed stationary phase. A capillary column is typically a 10–100-m-long, ≤0.25-mm-i.d. glass coil in which the stationary phase is bonded to the wall of the tube, allowing the sample to interact with it as it flows through the center. A variety of stationary phases are available.
Most of the field has migrated to capillary columns. They generally yield sharper peaks with quicker analysis times, and MS requires their lower leakage capacity (≤1 mL/min of carrier gas reaching the detector), notes Goldsmith. Yet, he points out, their smaller capacity means “they can only digest a relatively small amount of sample.” In addition, they are much more expensive than packed columns.
Also, there are still some things that you can only really still separate using a packed column (without an expensive, elaborate setup), adds Snow, such as the fixed or permanent gases.
There is also a larger format of capillary column, with its 10–30 mL/min flow rate closer to that of a packed column, called a .53-mm capillary column—“perfect for typical GC detectors like FID and TCD,” Goldsmith says.
GC detectors
Conoley estimates that about 80% of GCs sold (not counting those hooked up to MS) are fitted with either a flame ionization detector (FID) or thermal conductivity detector (TCD). The former is considered a “selective” detector that will respond to a certain class of compounds (like hydrocarbons) but not others (noncombustible gases, for example). A TCD, on the other hand, is a “universal” detector that responds to nearly any gas. There are also “specific” detectors designed to recognize something like a particular atom or functional group.
The large GC manufacturers all make mass spectrometers—frequently costing multiples of the GC itself—that couple to their GC (for what is called GC/MS) and allow the resulting mass spectrum to be compared to a library of compounds for positive identification. Like other detectors, each MS has applications it is best suited to; a discussion of the preferred matrices, compounds, sensitivities, selectivities, dynamic range, cost, robustness, ease of use, and other characteristics of each is far beyond the scope of this article.
Purchasing considerations
There are entry-level systems that are inexpensive—on the order of $5000–8000, depending on the detector—often dedicated to a single application, notes Taylor. A more versatile instrument will generally have the flexibility to do different analytical techniques, and add or change injectors, columns, and detectors. Some manufacturers, like Shimadzu, will pair only their highest-end GCs with MS.
More versatile GCs have the ability to simultaneously accommodate several different injector ports and several different detectors (how many varies among vendors and models). Yet as a rule they are not compatible with other manufacturers’ components (columns being the exception). Thus, one of the first decision points will be which manufacturer’s overall system to purchase.
Because of modularity it is generally safe to select only those components needed to meet your short-term needs, knowing that such instruments are largely future-proofed. But Goldsmith cautions that, because most instruments are too big to easily ship (SRI’s being an exception), upgrades require a service call that itself can add thousands of dollars to the cost of a field upgrade.
Almost all GC ovens can heat up to 400º, some to 450º, and at least one to 500º (from DANI Instruments S.p.A., www.danispa.it). Yet “in reality most people never go above 350º,” points out Conoley. “There are more applications that will want cryocooling [by liquid nitrogen or CO2] than there really are those that want oven temperatures over 400º.”
Total cost of ownership can vary greatly among instruments—from initial price of the instrumentation and length of warranty, to service contract and service calls, and perhaps shipping, says Goldsmith. ”SRI even offers a built-in air compressor that can obviate the need to purchase bottled air, countering the “urban myth that you need ultra clean air.”
Other purchasing considerations include the availability of computer control, autosamplers, and robotics. In addition, it is important to know whether a system comes with a data processing system or requires it as an add-on.
Perhaps the only real breakthrough in recent years is the advent of GC×GC orthogonal chromatography, says Taylor. GC×GC requires higher pressure flow regulators, faster data collection capability, and heating and cooling capacities found out-of-the-box only on newer, high-end, instrumentation.
The best bet may be to assess what applications you plan to run now and in the future, and search the literature for what others have done to accomplish similar things, advises Snow. And then consult specialists at two or three different vendors to help design the best GC system for those applications.
Josh P. Roberts has been a full-time biomedical science writer for more than a decade. After earning an M.A. in the history and philosophy of science, he went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology; e-mail: [email protected].
Please see our Gas Chromatograph / GC System (GC Instruments) section to find manufacturers that sell these products