User:Einf20/Flow Field Thermal Gradient Gas Chromatography

Flow Field Thermal Gradient Gas Chromatography (FF-TG-GC), also known as Hyper-Fast GC, is a separation method based on classical gas chromatography that uses very short separation columns, a resistively heated enveloping capillary and a negative spatial temperature gradient to perform gas chromatographic separation of substances within seconds to few minutes. For various applications, FF-TG-GC drastically improves speed, sensitivity and selectivity over conventional gas chromatography. The first FF-TG-GC prototype was developed in 2013 by Peter Boeker and Jan Leppert at the University of Bonn. In 2017, Boeker and Paul Chambers founded the company HyperChrom SA, which has evolved, marketed and supported FF-TG-GC since then, together with HyperChrom Deutschland GmbH.

Principle of measurement

General setup

The basic design of the FF-TG-GC corresponds to the design of a conventional gas chromatograph, consisting of an injector with gas supply, a heatable fused silica separation column and a detector, e.g. a flame ionisation detector (FID) or a mass spectrometer (MS). Explanatory concept diagrams can be found on the manufacturer's website.

In FF-TG-GC, the column oven used in conventional GC is replaced by a resistively heatable stainless steel envelope capillary which is inserted helically into a support structure, the so-called helix tower. A 2 to 4 metre long classic, manufacturer-independent fused silica separation column is manually inserted into this enveloping capillary. The temperature of the enveloping capillary is measured and controlled without direct contact using IR sensors. Due to the low thermal mass of the heating capillary and the efficient transfer of the heat of the heating capillary to the separation column situated within, the temperature of the separation column can be controlled very quickly and precisely. Heating rates of up to 3000 °C per minute and cooldowns from 350 °C to 50 °C within 10 seconds are possible. The temperature of the separation column can be controlled over its entire course from injector to detector. However, the programming of fast heating rates (analogous to the oven programme in conventional gas chromatography) is limited to the main part of the column, the helix; the remaining heating zones are heated statically.

Setup of the helix tower and principle of the negative thermal gradient

The helix tower, which is the support structure for the heating capillary, fixes both the heating capillary and the inserted separation column in a helical channel. Through this channel, an air flow can be directed from inside the helix tower in a controlled manner that eliminates random convection in the vicinity of the heating capillary and thus significantly increases reproducibility. A so-called flow field is created. By using flow dampers inside the helix tower, this flow field can be adjusted in a way that a stronger air flow is directed over the heating capillary in the lower area of the helix (which is closer to the detector). The result is a continuous, stronger cooling of the column section closer to the detector; a negative spatial temperature gradient is created. Both the flow field and the thermal gradient give the technology its name.

With temporal temperature programming only, as is common in classical gas chromatography, an analyte will begin to migrate along the column when it reaches a certain temperature. During the migration to the detector, the temperature of the column increases following the temporal temperature programme, causing the analyte peak to broaden. Due to the negative temperature gradient, the analyte migrates across the column at a constant temperature, and the peak broadening does not occur. Physically, this effect can also be explained by a temperature-induced slow peak front at comparatively high velocities of the analytes in the peaktail. The combination of these effects leads to a spatial peak focussing. The negative thermal gradient thus increases the chromatographic resolution and reduces the elution temperature at the same time, which is particularly advantageous for temperature-sensitive analytes such as explosives.

In addition, the helix tower offers the possibility of water cooling via an external recirculating chiller. The chiller keeps the environment of the separation column thermally very stable and minimises retention time fluctuations. In addition, this enables starting temperatures from 25 °C.

Suitable detectors

Theoretically, the FF-TG-GC can be used with all detectors commonly used in gas chromatography. However, a particularly high recording frequency of the detectors is needed. As analyte peaks have very small peak widths of usually well below one second, peaks are otherwise characterised with an insufficient number of data points or are not detected at all. Particular practical experience exists in using especially fast flame ionisation detectors as well as quadrupole mass spectrometers (especially in SIM mode) and time-of-flight mass spectrometers (also suitable in scan mode). The selection of a suitable detector highly depends on the respective application.

Column connectors

Conventional column connectors are unsuitable for use in FF-TG-GC because they have too high an unpurged volume. This volume causes peaks to tail for up to several seconds, which is not relevant in conventional gas chromatography; however, in FF-TG-GC, with run times of e.g. 50 seconds, such peaktailing would be very disturbing and lead to frequent coelutions. The heated transitions between injector, helix tower and detector, the so-called transfer ovens, therefore include specially designed purged connectors through which different separation columns can be interconnected. By purging the resulting additional volume with pre-conditioned carrier gas, the negative effect on the peak shape is prevented and the use of transfer lines or guard columns is made possible without impairing the analytical results. This also allows the injector to be backflushed during the measurement in order to minimise the introduction of solvent-contaminated carrier gas from the injector and to avoid coelutions of early analytes with the solvent.

Applications

The applications for which FF-TG-GC is suitable correspond roughly to the applications for which classical gas chromatography is used as well, i.e. qualitative and quantitative analyses of highly volatile to medium volatile substances. However, due to the limited column length and the slightly reduced resolution, there are also unsuitable applications, especially if they require a longer separation column due to particularly challenging separation problems.

The particular advantage of FF-TG-GC is a considerably increased measuring speed (up to a factor of 50) with a relatively high separation performance, which is roughly comparable to that of a 20 m column (0.25 mm internal diameter) in conventional gas chromatographs. It is therefore particularly suitable for comparatively easy separation problems where a significant increase in measuring capacity is advantageous. This is the case, for example, in laboratories with high sample throughput. This strength of the FF-TG-GC is also of enormous advantage when a short time from injection of the sample to the measurement result is required, for example in incoming goods inspections, process controls or public safety issues.

References

[[Category:Chromatography]]