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PhytoChemia Acta

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Do It Faster, Makes Us Stronger – Fast GC

11 May 2017

Benoit Roger, Ph. D., & Alexis St-Gelais, M. Sc., chimiste

We have read many times on various oils-oriented Facebook groups indications that gas chromatographic columns should be as long as possible to achieve proper separation. Similarly, it is often stated that short analysis runs negatively impact the quality of data obtained. And under the most generally used conditions to perform gas chromatography, both of these statements are true.

But this does not mean that they are always true.

If you take a look at, for example, our blogpost about serrata-type frankincense, you will notice that the total run time for the chromatogram depicted was a bit over 20 minutes. This clashes with the almost 100 minutes used by many laboratories, and even our standard 60-minutes runs depicted in our reports dating from 2015 to this day. But comparing strictly on the basis of time is misleading in this case. This is because the frankincense serrata run was made under full fast GC conditions; our usual reports are done under semi-fast GC conditions; and most other labs work with conventional (non-fast) GC.

So wait, what it that fast GC thing? We apologize in advance, because the explanation is a little tedious. We try below to present it as simply as possible. It all is a matter of reducing the time needed for an analysis.

Let’s go back to the beginning. Since the introduction of gas chromatography and the development of capillary column in the 1950s [1], considerable research has been conducted to enhance the efficiency of GC systems. Among other points to improve: the separation speed… Short analytical time meant higher throughput and lower costs for analytical labs. It also allowed to have faster answers in continuous process control and gave the possibility to replicate analysis in an acceptable time [2]. Principles and theory of fast GC were established in 1960s but to this day, it still is relatively rarely encountered, except in some research papers, and it keeps an aura of novelty [3]. Columns suitable for this purpose are still less often sold, and even sometimes outright unavailable.

Here are the founding principles of fast GC as they are presented in a document from Supelco [4]– a very good read to go a bit more in depth. The retention time of a given compound, and thus analytical time, can be determined using several complex equations. Three key parameters can be modified to achiever shorter runs [5] :

  • Use a shorter column, which reduces the distance that has to be travelled (and thus the time required) before a compound is detected;
  • Ramp the temperature faster in the GC, knowing that higher temperatures gradually decrease retention of the compounds by the column;
  • Increase the velocity of the carrier gas, to carry the compounds faster through the system.

The problem is that by doing this without changing any other GC parameters, you will also lose some resolution – as it also depends in part on these parameters. Resolution describes the system’s ability to separate compounds from each other: this is the very purpose of the chromatographic process. The challenge is thus getting the fastest separation achievable while keeping a good resolution to separate as many compounds as possible.

The resolution potential of a given column is determined by some equations we will not report here because you could have to use peppermint essential oil to treat the headache you would get trying to understand each term… But you can remember the information that resolution can be improved by:

  • Using narrower columns – this maximises the interactions between the stationary and mobile phases;
  • Increasing the carrier gas diffusivity – in layman’s term, using hydrogen instead of helium as a carrier gas, as this property is specific to each gas;
  • Increasing stationary phase diffusivity – again in layman’s term, using a thinner film of stationary phase, so that compounds can more easily transfer from it to the mobile phase.

The good news is that if you apply all these modifications, you can recover more resolution than you lost by reducing the column length, ramping the temperature faster, and increasing the carrier gas velocity – which were required to achieve faster analysis. Another good news: if analytical speed is not highly critical, you can adjust the temperature ramp and gas velocity to optimize resolution.

Thus, fast GC combines all of this: short, narrow columns, with a thin stationary phase film, ran under hydrogen at high velocity, using quick temperature ramps. And this typically allows to achieve as good (if not even a little better) separations in less than 20 minutes instead of 60-90 minutes, in the case of essential oils.

The following chromatograms (Figure 1 and 2) from the Supelco application note [4] show the same mixture of compounds analysed with the same column type in conventional (upper profile) and fast GC (down profile). It shows that fast GC improves both analysis time and resolution (noted as R on the figure; higher R means better resolution).

Figure 1. A mixture of volatile compounds analysed using conventional GC conditions.

Figure 2. The same mixture ran in fast GC conditions.

Then why does conventional GC still exists? Because there are also some counterparts. First of all, separation is not strictly the same in conventional and fast GC. When an analytical method is established and validated, it cannot always be replaced easily – this is why we have been running in “semi-fast” GC since 2015, with the objective of migrating to full fast GC in the future. Furthermore, the use of hydrogen in GC-MS induces several problems including some modifications in fragmentation patterns. All commercial spectral databases are built using helium, so this is a challenge. We were forced to keep helium for our GC-MS instrument. The last main problem is that, as the internal diameter of fast GC column is narrower, the maximum amount of sample that can be injected per analysis is smaller. This can induce some limitations or even issues in some specific cases.

However, when developing new GC methods, fast GC should be considered because it still has huge advantages in speed and appreciable gains in resolution. As a bonus, hydrogen is a renewable resource that can easily be produced on-site from water. Helium, widely used in conventional GC, is one of the most abundant atom in universe but on Earth, it is a non-renewable resource that is also in high demand in medical applications… So, fast GC is a bit more responsible, resource-wise.

In short, fast GC enables efficient and quick analysis of essential oils, with results that are as good as conventional GC in much shorter runs. This shows that a relatively old idea, coupled with the recent developments in columns availability, makes for a major improvement in the field of GC analysis for the upcoming years.

References

[1] James, A. T., Martin, A. J. P. (1952) Gas-liquid partition chromatography. A technique for the analysis of volatile materials. Analyst, 77, 915-932, DOI: 10.1039/AN9527700915

[2] Korytár, P., Janssen, H.-G. (2002) Practical fast gas chromatography: methods, instrumentation and applications. Trends in Analytical Chemistry, 21 (9-10), 558-572. DOI: 10.1016/S0165-9936(02)00811-7

[3] Klee, M. GC Solutions #28: Why don’t more people do fast GC?, [On Line], (page consulted on May 9, 2017), URL: https://www.sepscience.com/Techniques/GC/Articles/1070-/GC-Solutions-28-Why-Dont-More-People-do-Fast-GC

[4] Supelco. Fast GC: Increase GC speed without sacrificing resolution, (page consulted on May 9, 2017), URL: http://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Supelco/General_Information/t407096.pdf

[5] Skoog, D. A., West, D. M., Holler, F. J., Crouch, S. R. (2013) Fundamentals of analytical chemistry, 9th edition, Cengage Learning, 1072 p.

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