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GC-TOF: A Fresh Lease of Life


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GC-TOF: A Fresh Lease of Life

Chromatography is one of the most widely used techniques in atmospheric science today – particularly in projects like my own, looking at volatile trace gases. Trace gas emissions are incredibly important in global geochemical cycling, even if the gas phase of the element is fleeting. Thankfully, due to recent advances in technology – notably in nano- & pico-second timing electronics – sensitivity of gas chromatographic instruments has come on leaps and bounds, allowing comprehensive analysis of these volatile trace gases.

Perhaps at the forefront of these advances, is the GC-TOF, standing for Time of Flight. Although, these advances could more accurately be referred to as a renaissance for ToF analysis…

The Theory

What struck me as I began to learn my way around the TOF here at UEA, was how long the technique had been around. To me, the TOF was an all-singing-all-dancing piece of magic, but it wasn’t always that way.

TOF Mass Spectrometry was introduced in the 1960s, however, it was restrained by the quality of electronic timing mechanisms. That problem has been successfully solved in recent times leaving TOF analysis as a very powerful tool.

The principles behind GC-TOF MS are relatively simple. It involves measuring the time required for an ion to travel from an ion source to a detector over a known distance. If all of these ions receive the same kinetic energy during instantaneous acceleration forth source, due to their different mass to charge ratios , they’ll have different velocities and therefore arrive at the detector at varying times. As TOF instruments have no accelerating potential between the source and the detector – a set up known as field free – as ions move through this region they will separate out into packets according to their velocity. As a result of this, ions of all mass to charge values can theoretically be detected the initial ion packet without the need to scan across a range of ions.

However, linear TOF instruments were not without limitations. Due to production and pulsing of ion packets, ion sampling efficiency led to the development of orthogonal sampling from a continuous ion beam. In these systems, ion packets are pushed out out a continuous collimated ion beam perpendicular to the axis of the flight path. Because the original ion beam is collimated, and some collision cooling can take place within the ion beam, there is reduced radial energy dispersion of ions which translates to an improved resolving power and sensitivity of the instrument.

A further alternative, and future development of the TOF system, is the reflecting TOF analysers. The reflectron present in these systems acts as an ion mirror with an electric field that opposes and is of greater magnitude than the electric field of the ion accelerator – replacing the field free region of linear TOFs. The refletron serves two purposes, firstly to reduce the dimensions of the instrument, and secondly to improve the resolving power of the TOF MS.

Here, ions of the same mass to charge ratio but differing kinetic energies enter the opposing field of the reflection and penetrate to different extents as a result of their kinetic energy. The ions with the highest kinetic energies penetrate the furthest. This causes the ions to decelerate until their kinetic energy dissipates at which point they are accelerated by the electrical field in the opposite direction. As ions with the highest kinetic energies penetrate the furthest, they will gain more kinetic energy upon reflection. This has the overall effect of altering the length of the ion flight paths in proportion to their kinetic energies, hence ions with a distribution of initial energies will be focused at the detector with respect to arrival time.

Implications

The implications of GC-TOF analysis are vast. Due to the high sensitivity and high resolution of the data collected, research in fields such as “volabolomics” can advance at a high rate in the coming years.

The TOF is almost unique in it’s ability to analyse a multitude of mass to charge ratios in a single run, saving both time and researcher effort, which are both invaluable resources. This technique potentially allows for immediate identification of known compounds, deconvolution of co-eluting compounds and minimises the steps required to identify unknown compounds.

To mention my own research, I will be using the GC-TOF to analyse a suite of volatile compounds produced by diatoms at different points of their life cycle. However, this forms the base layer for a far more interesting and ambitious project. Use of the GC-TOF potentially allows the formation an index of volatile compounds produced by certain species. If these volatiles are then observed in the field, the observer could refer to this index, and narrow down the number of species present that could be producing it, and therefore gain insight into the compounds origin.

The GC-TOF has come a long way in the past 60 years, and now stands on the brink of illuminating complex emission patterns in the natural world, and furthering our understanding of trace gas emissions."

Posted by on Mon, 30 Oct 2017



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