Chromatography
Avoiding Safety Issues and Reaping the Benefi ts of Hydrogen as a Carrier Gas in GC
Matt James, Kirsty Ford, Tony Edge, Avantor Introduction to explosive mixtures
One of the challenges of an analytical laboratory is making changes in an ever-changing world where external pressures often drive practical changes within the laboratory environment. Often these are in the form of new regulations, stipulating better more robust data, more sample analysis, or pressures from productivity, where fi nancial pressures encourage the analytical scientist to reduce the analysis times. However, more recently the world of analytical chemistry has been targeted with another driver. The world is becoming more and more aware of the impact that the human race is having on valuable natural resources, and so terms like sustainability are being introduced into the laboratory environment.
One area that is being impacted is gas chromatography (GC). GC has been a front- line analytical technique for several decades and is routinely used for environmental, petrochemical, pharmaceutical, and food sample analysis [1, 2, 3, 4]. Although there has been a gradual move to perform the analysis of more polar compounds using HPLC, there is still a substantial amount of analysis being performed using this technique. The predominant carrier gas used in GC is helium, which is proving diffi cult to source. Helium is actually one of the most abundant elements in the universe, however on earth it is only generated through radioactive decay. It is very light and seeps through the earth only getting trapped by pockets of natural gas, which effectively means that it is a non-renewable resource. There is, thus, a drive in the industry to look at other possible carrier gases, with the benefi ts of reducing costs. The primary replacement carrier gas is hydrogen, and this article will look at the benefi ts and pitfalls associated with introducing hydrogen into a working laboratory, along with some other changes that will reduce analysis times without compromising the chromatographic performance of the GC system.
One of the major perceived challenges associated with the introduction of hydrogen is associated with safety. The reaction between air (oxygen) and hydrogen is stoichiometrically a trivial one, however the actual chemistry is very complex, and one that can exhibit a thermal runaway, or explosion under the correct conditions. Hydrogen gas forms combustible or explosive mixtures with atmospheric oxygen over a wide range of concentrations in the range 4.0% – 75% and 18% - 59% [5]. In terms of understanding why this reaction is so dangerous it is necessary to understand the chemistry.
The reduced hydrogen oxygen model [6], Figure 1, is often used to model the reaction, although this does not necessarily present a complete picture of all of the reaction mechanisms that are occurring, it does give an understanding of the underlying chemistry behind the observed phenomena.
When solved, the reduced model generates two reaction states, a low energy one, and a high energy one which is associated with the thermal runaway or explosion. There is not a gradual transition between the two states and the difference can be quite dramatic when varying a parameter by only a small amount. The reaction scheme generates three explosion limits, in the form of a characteristic ‘z’ shape when looking at a pressure - temperature plot. Understanding of this reaction scheme is of particular importance to the petroleum and the automobile industries, since it is the basis of all combustion reactions [7, 8, 9]. Looking at the reaction scheme it is specifi cally R6 and R7 that form the basis of this explosive reaction when at atmospheric pressures. These reaction steps generate free radicals and water, the free radical propagates the reaction, whereas the formation of water generates heat, which in turn increases the rate of reaction. The reaction step that slows the reaction down is the wall termination or R5. The M here represents any gas phase collision partner. The idea that a reaction can have two very different reaction states is not unique to combustion reactions, and many other examples exist [10, 11] and in the world of mathematical modelling are referred to as part of catastrophe theory.
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Figure 2. The three explosion limits that exist for the hydrogen oxygen reaction. Figure 2: The three explosion limits that exist for the hydrogen oxygen reaction.
The non-linear nature of the reaction kinetics is itself fascinating as it can result in oscillatory behaviour when associated with continuous fl ow reactors, or in the presence of carbon monoxide have been shown to exhibit chaotic behaviour. Indeed, when the experimental data is plotted as a 2 dimensional plot where x(t) is plotted against x(t+n), where n is a regular time interval [12], higher levels of structure can be seen which is an indicative sign that the chemical kinetics are chaotic in nature. This experimental system has been investigated in some depth and other artifacts of a chaotic system have also been shown to exist including, bistability [13], period doubling bifurcations [14] and next maxima return plots [12].
Figure 1. The reduced hydrogen – oxygen reaction scheme [6].
Hydrogen is lighter, less viscous, and has a lower density than other fuels. As a consequence of these properties’ hydrogen will disperse readily which means in a non-contained environment there is a reduced risk of building up high concentrations, however it does also mean that it is more likely to leak from any pipe work etc. This would suggest that in a large well ventilated room small leaks would not present a problem, however the major safety issue is the source of the hydrogen. The two most common sources of hydrogen that are used by modern chromatographers are gas cylinders, which will contain up to 50 L of gas pressurised to 200 bar, or a hydrogen generator which typically stores around 60 mL of gas pressurised to less than 10 bar. It is very evident that for the safety aware chromatographer that the hydrogen generator provides a substantially safer environment and one that could be readily employed within a laboratory, whereas a gas cylinder would require extra safety precautions, and would require the storage of the hydrogen cylinder outside of the laboratory facility.
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INTERNATIONAL LABMATE - JULY 2023
Pressure
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