Gas Chromatography

Gas chromatography (GC) continues to be a workhorse technique, but users now face choices of upgrading their systems to newer time-saving technologies or possibly switching to hydrogen carrier gas. Participants in this forum are Eric Phillips, Thermo Fisher Scientific; Mark Taylor, Shimadzu Scientific Instruments; and Hans van den Heuvel, Bruker Chemical & Applied Markets.

Gas tanks for GC are rapidly becoming a thing of the past. How can GC users be encouraged to switch to hydrogen carrier gas?

Phillips: GC users will not need any encouragement to switch from helium to hydrogen as a carrier gas. The drivers for this change are a combination of scientific and economic factors, with some politics thrown in. Hydrogen can provide faster analysis times and more efficient separations on GC columns. If we use the van Deemter equations and graph those, hydrogen provides a wider and faster optimal flow rate range for chromatographic separations. If we add the expense of purchasing helium tanks, not to mention the production through distillation of natural gas and shipping, it is no wonder that laboratories are looking for other carrier gases. Of course, nitrogen could be used. It is plentiful. However, there is a problem because it is just not as efficient in GC. Separations are longer and peaks are wider, not a good combination. Hydrogen is also plentiful. The easiest way to get to it is with a hydrogen generator, which uses water. These will just use a relatively small amount of electricity to provide the hydrogen gas. So, the movement to hydrogen as a carrier gas for GC has been happening for some time. It is increasing more rapidly because of economic (and partially political) concerns.

There are some things to keep in mind when moving to hydrogen generators though. The gas must be dry, high purity, and of sufficient pressure to perform the analyses. Moisture is still not good for GC columns. The generator used must also produce enough pressure to cover the flows that are needed by the GC even at high temperatures or high injector split flows. The chromatographic results may be a little different than what you are used to with helium as a carrier gas. Some adjustments to the GC or GC–mass spectrometry (MS) method may be needed and should be expected. The GC–MS system should be built with hydrogen carrier gas in mind. This will prevent a lot of problems from occurring when tuning the system.

Taylor: We see two issues with switching to hydrogen as a GC carrier gas, the first being safety and the second being method redevelopment. As helium becomes more expensive and less available, the demand for alternatives will increase. Hydrogen will be the natural replacement as long as the issues mentioned above are addressed.

There are already some safety measures in place. Most hydrogen generators have a pressure sensor that will force the shutdown of hydrogen production in the case of a leak. Most modern GC systems have a similar feature in their electronic carrier gas delivery hardware. New GC models will have hydrogen sensors in the oven that will have the ability to sense the presence of small leaks and to stop carrier flow, thus adding an additional level of safety assurance.

In terms of method development, hydrogen has a higher optimum linear velocity than helium, and the minimum in the van Deemter curve is much broader. This means that it will provide optimal separations at a broader range of linear velocities. It will also mean that users will be able to potentially speed up their analyses without compromising peak resolution.

Modern GC software will include helium-to-hydrogen method translation, allowing users to quickly make the switch. They can either maintain their current peak retention times by not adjusting the helium liner velocity from their previous method or further optimize the chromatography by speeding up the linear velocity and shorten their analyses.

van den Heuvel: Helium is considered one of the best carrier gases for GC, is only available from gas tanks, and is running short. On the one hand, helium is nonrenewable once vented in the atmosphere and is a by-product from natural gas, which has a declining availability. On the other hand, the applicability of helium is increasing with MRI scanners being the most helium demanding instrument today. Both trends will lead to an increased market price for helium and, thus, an increased operation cost of the GC. Alternatively, nitrogen can be used, however its use as carrier gas requires much higher operating pressures compared to helium or hydrogen. The benefit of hydrogen is that it can be produced rather easily, ensuring high purity at a relative low price. The only disadvantage of hydrogen is its flammability. Risk minimization is achieved by local production of small amounts and relative low pressures, and the ability of GC systems to shut of the gas supply as soon as gas supply settings are not met.

Will the aging and replacement of older instruments cause labs to adopt newer, more costly (but time-saving) methodologies and technologies, such as open-tubular columns as packed column replacements, portable GC systems, fast GC, or automated robotic sample handling, or will labs simply replace their systems with similar less costly instruments? What standardizations and regulations would help drive the upgrade process?

Phillips: Changes to any business will only be done for some kind of advantage. An advantage can take many forms: social consciousness, cost savings, business growth, or increasing revenues. These are the same for laboratories seeking to add scientific and analytical ability. When moving to new instrumentation, a laboratory will look at a combination of all of these. It is not a given that new instrumentation will cost more. This is seen in the computer industry too. Prices can remain constant or even decrease while performance and capability increase.

As equipment ages, it will eventually be replaced by equipment that can save time and money for the lab. The new equipment should also be able to grow with the laboratory. There are no regulations that become easier to meet, and the new instrumentation will need to able to do the work that is asked of the lab now and in the future. In some cases the new instrumentation may actually cost less. Over the years, for example, single-quadrupole GC–MS systems have decreased in price.

Current technology provides flexibility to the laboratory that would not have been possible three years ago. Ion-trap GC–MS technology has been available for decades. These instruments can provide significant functionality to the laboratory. Triple-quadrupole GC–MS-MS systems are typically more expensive than either the single quadrupole and ion trap. However, when compared to systems they will be replacing, the capital expense may be much less. Triple-quadrupole GC–MS-MS systems can provide unprecedented levels of selectivity and precision that until recently were unheard of for the same price. This translates into the ability to meet regulatory limits for targeted analytes now and into the future.

There are trends in GC–MS where a lower-cost technology has caught up to higher-cost technology in terms of performance, or at least close enough to start a change. This can be seen in the analysis of dioxins and other persistent organic pollutants (POPs). In the past, the only way to do this analysis was using high-resolution magnetic sector (HRMS) instruments. This is set in the regulations, but more importantly these analyses just couldn’t be done any other way. Now there are triple-quadrupole GC–MS-MS systems that can reach levels of sensitivity, selectivity, and precision that come close to rivaling HRMS at a much lower cost. Used together, a triple-quadrupole GC–MS-MS system and an HRMS instrument can dramatically increase the throughput of these complicated analyses. There is also a push to have instruments be allowed to confirm dioxins and other POPs. If this is allowed by regulators, then new instrumentation could be dramatically less expensive than older instruments.

Taylor: As your question suggests, many manufacturing industries and general testing labs that use GC follow methods as written by governing agencies such as the EPA or ASTM. The data generated through the verbatim use of these methods tend to be accepted without question.

When making capital equipment purchases, the majority of managers typically face one of two choices. One, they can purchase the most economical instrument to satisfy current methodologies. Or, they can potentially spend more on newer technologies that may ultimately be better and faster but initially require an investment in method development and validation. Most will make the safe decision and choose the former. A few will take the challenge of the latter and adopt newer technologies with the goal of ultimately becoming more efficient in the lab.

van den Heuvel: GC has been a commodity product for a long time already, and a number of changes have developed in GC. For a long time, capillary columns were being used with the benefit of increased resolution and analysis speed, although some methods for bulk analysis still require the capacity of packed columns. One big advantage of capillary columns over packed columns is the validation. Capillary columns are produced under more strict methodologies including minimal requirements on plate number, inertness, and polarity than compared to packed columns. Also, the traceability of the components used to manufacture a capillary column like the glass of the column is much more detailed than with packed columns. One of the goals is to ensure a constant quality and performance. This is essential in case a column needs to be replaced and the new column is performing identically.

Besides the faster GC analysis, today’s GC systems have increasingly better diagnostics and preventive capabilities. The instruments have become more computer-based and are able to validate if the GC system is working according to expectations (for example, using injection counters to alert when the septum, syringe, or column needs to be replaced). Also with the introduction of electronic controlled devices like the electronic flow control, the instrument is not only able to identify but also respond on undesired situations. For example, the GC system does not allow temperature programs above the maximum column temperature and when the set value of an electronic flows control is reached, the instrument shuts down all potential damaging devices like oven temperature and hydrogen flow. Both of these developments help to improve the performance, the uptime of the GC system and reduces potential costs due to damaged columns or detectors after a leak error.

GC developments have also reacted to the trend to manage the Lab instruments from a central point removing the computer near the GC system. As a result, today’s GC systems have an extensive high-resolution graphical interface to control the instrument also locally. This is useful for maintenance when operation is at the instrument and devices like the oven temperature can be set low instantly without walking to the central computer.