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Gas Chromatography: Alive, Kicking and Value-Driven
by Angelo DePalma
Despite reports of its demise, gas chromatography is still alive, kicking, and thriving. Instruments pack more value than ever. Prices have remained stable while products improve in function, capabilities, and ease of use. Anyone who has packed their own half-inch diameter aluminum column in a stairway, or integrated peaks by cutting paper traces and weighing them on a balance, can appreciate the sophistication of today’s high-throughput, computer-driven instruments.
The GC business is thriving as well. Mark Taylor, GC product manager at Shimadzu (Columbia, MD), says his company’s sales of GC instruments grew “significantly” last year, in part thanks to a brisk replacement market driven by big industries like chemicals and energy. Shimadzu’s flagship GC, the GC-2010, debuted about four years ago.
GC system prices have remained stable over the past few years, but instruments are gaining greater capability. About one in five new GC instruments comes with a mass detector. Years ago one could spend an entire career operating a mass spectrometer. Today anyone can walk up to a table-top GC-MS instrument, push the “on” button, wait three minutes and begin a run.
Mass detectors cost ten to fifteen times as much as ultraviolet or flame ionization, but MS provides unequivocal molecular weight information for every peak, plus compound identification using a library.
Analyte polarity and thermal stability are still issues with GC, as is column clogging, so chromatographers must be diligent as ever with sample preparation. High-throughput laboratories will routinely snip off the first four inches of a capillary column, change the septum and liner, and start over “fresh” at the beginning of every work day.
Offering an array of detector and injection unit options, Shimadzu’s GC-2014 covers a wide range of applications from capillary to packed column analysis. |
Mr. Taylor sees some “exciting stuff coming down the pike” for next-generation instruments: Columns becoming narrower, carrier gas pressures and flows increasing, retention times shortening, resolution rising, and the chromatographic process as a whole coming under much tighter control. Other significant trends include wider acceptance of 2D GC-MS, where peaks are “heart cut” and automatically injected into a second column of different polarity.
End-users are no longer willing to delve into the inner workings of a GC (or NMR, mass spectrometer, or liquid chromatograph for that matter). Operators want instruments that are easy to use, like appliances, says Mike McMullen, VP for chemical analysis at Agilent. “They’re looking for rugged, reliable answer machines, and are interested in technology only up to the point it helps them work more effectively. Our job as vendors is to hide the complexity.” Agilent’s GC business focuses on food safety and analysis, forensics (including homeland security), environmental testing, and workplace drug testing.
Shimadzu’s Taylor agrees. Customers want a black box, a solution, that will do the job, he says. “They’re not buying a GC, they’re buying analyzers.” Approximately 70% of Shimadzu’s GC business is directed at specific analytes.
Productivity, Throughput
Regardless of the lab, the industry, or the application, throughput and productivity remain top priorities for GC users. “Anything vendors can do to drive productivity or enhance current capabilities is viewed positively,” says Tom Gluodenis, industry marketing manager for forensics at Agilent (Wilmington, DE).
GC vendors strive to ensure that every second of analysis time is used efficiently through the use of high-speed ovens, by minimizing automated liquid sampling overlap, providing back-flushing injectors during a run, and through simultaneous peak monitoring and scanning.
Peak monitoring, also known as single-ion monitoring (SIM), relates to situations where the analyst knows and focuses on one particular peak of interest. Scanning examines at all peaks. During SIM-scan both operations occur without sacrificing time or sensitivity.
Expanded ion/compound libraries can save tremendous amounts of time for confirming the presence of a compound in toxicology, forensics, or chemical/environmental analysis. Many instruments come with their own standard libraries, or connect with national databases for toxic chemicals or drugs of abuse. Augmenting these services with metabolites or breakdown products adds to the reliability and veracity of data.
Agilent has recently introduced several innovations for GC and GC-MS. The first, Capillary Flow technology, manipulates gas flows within small volumes, eliminating concerns for leakage. Through another, QuickSWAP, users can service the oven without shutting down the vacuum for the mass detector, an operation that takes up to an hour. Agilent capillary GC instrumentation also incorporates automated back-flushing, which purges the injector area during the burnout period. The last innovation is an implementation of the Deans switch, which takes a heart cut from a peak and shoots it into a second column. According to Mr. Gluodenis these four improvements provide up to three times the information compared with conventional GC, in roughly half the cycle time.
Don’t Try This at Home
One of the most recent gas chromatograph introductions in the industry is the Agilent 7890A GC, featuring Capillary Flow Technology for shorter cycle times and turn-top design for convenient changing of liners, among many other innovations. |
A group of professors at the University of Illinois Urbana-Champlain have founded a startup company, Cbana (the name is about to change), to commercialize university GC-related inventions covering sample preparation, ultra-miniaturized instrumentation, and detector technology.
The three researchersKeith Cadwallader, Ph.D. (Food Science), Mark Shannon, Ph.D. (Mechanical Engineering) Richard Masel, Ph.D. (Chemical Engineering)are working on high-capacity “pre-concentrators” that remove specific analytes from air (compared with activated carbon which removes everything). “Just as you use a specific column for various molecule types, some day we may use a specific pre-concentrator,” says Dr. Shannon.
On the miniaturization front they have a microfabricated GC measuring 1x1 inch and a few millimeters high, which the scientists claim provides the same resolution as a full-sized GC. A huge benefit of microchanneled GCs is the ability to heat evenly and quickly. Their third main research area involves using enzymes or other biomolecules as detectors for microfabricated GC instruments. Biochemical detecting promises hitherto unthinkable sensitivity and specificity.
The group’s other main interest is in what they term an olefactory detector, which consists of Prof. Cadwallader sitting in a chair and sniffing aroma-active components of food or environmental samples as they emerge from the GC.
No, Cadwallader is not crazy. While the best manufactured GC detectors operate in the ppm to ppb range, the human nose can detect compounds in the ppt range. “It’s very safe because the quantity of material coming through is at pictogram levels,” he says. Olefactory detection compliments other detection modalities nicely. Eluent streams may be split between human and FID or mass detection to add an extra dimension to the data.
Target compounds include esters, pyrrolenes, sulfur compounds, and chlorinated anisoles, the chemical found in corks that is responsible for ruining the taste of wine. “You can smell these compounds at part per trillion levels but they are otherwise very difficult to analyze,” Cadwallader says.
For those who prefer the non-olefactory route, it is possible to achieve super-high sensitivity through more traditional means.
Christos Stamoudis, Ph.D., who directs the Industrial Hygiene and Chemistry Laboratory at Argonne National Laboratory (Argonne, IL), monitors 120 creeks and wells at the Argonne site for seventy compounds. Stamoudis uses the “purge-and-trap” method for concentrating analytes from water samples, followed by standard EPA-registered methods to analyze VOCs. For a busy environmental analytical group, compound identification and computerization have been the most significant trends, followed by very small, high-throughput columns and the ability to make large on-column injections. “Larger injections provide greater sensitivity, and reduces the need to concentrate samples,” notes Dr. Stamoudis.
Where conventional GC detects down into the ppb range, Stamoudis’s group has recently modified standard sampling and injection methods to allow ppt determinations of common environmental contaminants. Among his improvements are switching to single-ion monitoring, using larger injection volumes, and playing with the instrument’s split ratios. He is experimenting further, for example with a pesticide-detection method that uses direct hexane extraction with little or no evaporation, and injection of about ten times the usual amount onto the column.
Field-Worthy Instrumentation
Environmental field monitoring still mostly depends on collecting samples and analyzing at a central laboratory. Now sampling and testing can take place on site, in real time, through portable GCs from Photovac (Waltham, MA) or SRI Instruments (Torrance, CA). Portables vary in functionality. SRI’s compact designs are hefty but transportable. Photovac develops fully-functional GCsinjector/sampler, column, oven, detector, and computerin a 10- to 12-lb package about the size of a briefcase. The instruments use ultrazero air for halogenated hydrocarbon detection, or nitrogen for the electron capture detector.
In addition to the general purpose Voyager and petrochemical-specific P10/PetroPRO GC systems, Photovac also sells stand-alone flame ionization and photoionization detectors for measuring total volatile organic compounds. Think of these units, used mainly for leak detection, as ultra-portable GCs without the chromatography column.
Photovac VP of product development Jim Norgaard has been looking into ultra-miniaturized GC devices after hearing a talk by Chia-jung Lu of the University of Michigan on a microfabricated GC-on-a-chip. Liu’s device, in which components are etched into silicon using standard photolithography, provides a glimpse into the future of GC. Agilent and other companies already sell microchanneled, chip-sized liquid chromatography and electrophoresis systems.
The deal-breaker for environmental GC markets, Photovac’s specialty, is not the chip’s reliability or analytical capabilities, but its power supply. Instruments that detect chemical and petrochemical leaks must be “intrinsically safe” by U.S. and E.U. safety regulations, that is, designed so they cannot possibly spark a conflagration of analyte gases. Ultra-miniaturized instrumentation can take up to 7W of power, which according to Norgaard is too high. “Ours consume between one and two Watts, and even that presents serious challenges to meet intrinsic safety requirements,” he admits.
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Angelo DePalma, a chemist-turned-freelance writer, lives in Newton, NJ. He may be contacted at adp@tellurian.net.
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Choosing a Portable Gas Chromatograph
Environmental field work has been driving development of ultra-portable GC systems. Jim Norgaard and colleague George Hoag of Photovac provide a shopping list of features for anyone considering the purchase of a portable GC:
• Compactness: less than 15 lbs, portable, with on-board consumables for eight hours of field operation and built-in display and controls (no laptop needed)
• Ruggedness: operation in a variety of physical conditions (dampness/wetness, dirt, cold, darkness); operable while wearing gloves
• Ease of use: graphical user interface if possible, automated methods for field use, familiar operations/terminology in embedded software
• Low-maintenance, multi-detector options: photoionization detection (for halogenated and non-halogenated VOCs), electron capture (halogenated electrophilic compounds), thermal conductivity detector (fixed gas detection and hydrocarbons)
• Breadth of application: capable of handling unanticipated requirements in addition to standard methods for narrow compound subsets; ability to resolve co-eluants and/or interferences
• Rapid operation: high sample throughput and quick turnaround time; straightforward sample introduction, short analytic runs, in-run
maintenance
• Detection range: low ppb ranges for environmental contaminant chemicals; high linearity and precision from 0.13 to 103 of the critical action level
• Variety of sampling techniques, including: loop and syringe injection, plus sampling by probe and bagged samples
• Associated analytical tools for interpreting results and adjusting protocols in the field |
Laboratory Equipment
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