Gas Chromatography

Gas chromatography (GC) systems are similar to HPLC systems in that they are used to identify, separate and quantify compounds of interest. However, GC systems use an inert, gaseous mobile phase, as opposed to a liquid phase, to bring about the separation of molecules. The basic components of a GC system include an injection system, an oven, a column, a detector, and a computer system to analyze results.

There have been significant changes in GC in the past decade, and most of them have focused on maximizing throughput and decreasing run time. One of the factors limiting throughput has been the rate at which proper oven temperatures can be reached. Recent efforts have led to the development of ultrafast GC systems that incorporate advanced column heating devices and controls that can rapidly heat and cool columns. Improving speed of analysis also has been the driving force leading to changes in column technology. The use of small, nanobore capillary columns has improved throughput without sacrificing efficiency or precision. “There is a market out there for ultrafast GC columns, although they need specialized instrumentation to run it,” says Rob Bunn, product manager for GC columns & consumables at Thermo Fisher Scientific. The GC columns also have undergone improvements in sensitivity that offer lower detection limits and ultralow column bleeds. “The deactivation process has improved significantly in the past few years and has led to low activity on the column,” says Bunn.

Another development in the GC field has focused on the use of multidimensional systems that incorporate different columns and detection systems to improve sample resolution and throughput. The strategy involves using multiple columns to facilitate the separation of co-eluting peaks, such as enantiomers, or of samples that contain complex mixtures or a large number of components. A switching valve is used to route portions of effluent from one column to another column, and under certain conditions, the columns can be operated independently to increase throughput. The ability to incorporate a variety of different detectors within the system also is a huge benefit. “Mass spectrometry is fabulous as a universal detector, but we are seeing resurgence in the use of selective detectors for very specific types of applications,” says Laura Chambers, senior product specialist for chromatography products at OI Analytical Corp., who works with customers to help them reconfigure their GC systems. “In some systems you can now have an MS and three other detectors that can work in tandem.”

However, to take advantage of these improved technologies, customers must first understand what it is that they need. “Customers really need to know what they want to do with the system or they are going to waste a lot of money buying things they don’t need,” says Chambers. Since most methods for GC analysis are well standardized and documented, the application and protocol often determine the types of columns, detectors and other accessories to be used. Chambers therefore advises lab managers to think carefully about what the GC system is going to be used for, the skill level of the personnel using it and where it is going to be used. “That will help them make cost-effective decisions as they go through their configuration processes,” she says. Taking the type of sample, the sample load and the sample preparation into consideration also is important. Thinking through these issues will determine if any special equipment is needed and will ensure that the samples don’t overwhelm certain components of the GC system. There also are other accessories like syringes, filters and septa that play important roles in sample analysis. “When people are involved in new method development, they try a series of different columns and sometimes find that they are not getting the results they are looking for,” says Bunn. “Often, when things don’t work people blame it on the GC column, but the choice of liner and the septa are equally as important.”

Planning ahead and consulting with the vendor are important, as technologies and applications continue to evolve. “Talk to your vendor, because they have experts who know those instruments and applications inside and out, and they can be an extraordinarily valuable resource,” says Chambers. Bunn also advises GC users to regularly scan resources on vendor Web sites. “There is not just product information, but there is detailed information on specific applications [as well as] work flow solutions—from sample collection to analysis. A lot of companies have resources on their Web pages that help users make informed decisions,” he says.

Tanuja Koppal, PhD, is a freelance science writer and consultant based in Randolph, N.J.

Agilent
The Multimode gas chromatograph inlet with split, splitless and programmable temperature vaporization (PTV) capability is available at a lower cost than the previous version and features lower maintenance requirements. In addition to split/splitless operation, the inlet’s temperature programming capability facilitates a wide range of injection volumes, analysis of thermally unstable samples, and better productivity through fewer sample-preparation steps. The inlet design incorporates the proprietary Turn-Top feature, which allows liners to be changed in seconds without special tools or training. Sensitivity can be enhanced through large volume injections and inlet discrimination can be reduced for high molecular weight components. The Multimode inlet is compatible with the Agilent 7890A GC, 5975C GC/MS, 7683 and 7693 autosamplers, and the CTC Combi PAL autosampler.
www.agilent.com

Phenomenex
The Zebron ZB-XLB-HT Inferno high-temperature GC column reduces total run time to less than four minutes. Stable up to 400°C, it delivers more consistent performance and longer lifetime than other GC phases. The high temperature capability allows bake-off of matrix contamination, present in milk and other food products, which would otherwise decrease column lifetime. Standard fused-silica columns are not engineered to withstand temperatures above 380°C and their coating begins to degrade, eventually becoming brittle and inflexible. The Zebron Inferno non-metal columns incorporate proprietary coating and bonding technologies that provide stability at high temperatures, low bleed and low activity. The company’s Zebron ZB-5ms column is ideal for routine analysis of milk products using the FDArecommended GC/MS method.
www.phenomenex.com

Shimadzu
The Multi-Dimensional Gas Chromatography system (MDGC) expands the capabilities of chemists who perform target compound analyses in complex sample matrices. The multidimensional heartcutting system, incorporating a “multi-Deans switch,” greatly increases separation power by combining two capillary columns of different selectivity for a more efficient separation of the target analytes from a complex sample matrix. The improved separation allows more positive identification and improved quantitative analysis of the target analytes. The “multi-Deans switch” directs flow from the first column directly to a monitor detector or to the second column and detector. This device is based on the use of an advanced electronic flow controller to balance the pressure on both sides of the transfer line between columns. Multiple heart-cuts can be made from the first column without shifting the retention times of compounds eluting after the cuts, greatly simplifying method development.
www.ssi.shimadzu.com

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