Advantages and Disadvantages of Substitution of Helium as Carrier Gas in Gas Chromatography by Hydrogen. Part I. – Technical and Safety Aspects
Pixabay/Michael Schwarzenberger: Advantages and Disadvantages of Substitution of Helium as Carrier Gas in Gas Chromatography by Hydrogen. Part I. – Technical and Safety Aspects
Gas chromatography plays a significant role in the determination of flavors, which is important not only for monitoring the quality of final product but also when testing new technological procedures. Correct setting of all conditions having an influence on the chromatographic separations, including carrier gas, is necessary for acquiring accurate and reproducible results in gas chromatographic methods. Some difficulties with substitution of helium as carrier gas by another gas are discussed in this series of papers. Laboratories may be forced to make this change due to “helium crisis”. This paper is focused on technical and risk aspects of this substitution.
Official methods of brewery institutions such as European Brewery Convention (EBC), Mitteleuropäische Brautechnische Analysenkommission (MEBAK), The Institute of Brewing (IOB) or The American Society of Brewing Chemists (ASBC) include gas chromatographic procedures used for the determination of beer flavors. Beer flavors or some contaminants can be determined by this separation procedure in intermediate products as well as in final beer. The review of the main of these compounds is shown in Tab. 1.
The use of a very good optimized analytical procedure is required in order to obtain accurate and reproducible results. In gas chromatographic determination the attention must be also focused on the correct setting of all conditions having an influence on the chromatographic separations including carrier gas.
The choice of carrier gas has a substantial influence on the speed of analysis (Blumberg, 1993; Blumberg, 1997a; Blumberg, 1997b). Helium, nitrogen, hydrogen or argon is very often used in gas capillary chromatography. The great advantage of helium consists in its universal use in all types of detectors, mass detectors included. For this reason helium is the most commonly used carrier gas. Helium and nitrogen are non toxic, non-flammable and also very safe. On the other hand, helium is much more expensive in comparison with other carrier gases. Hydrogen is not very popular because of the risk of explosions. Compared to helium or nitrogen the use of hydrogen provides significant advantage in speed of analysis, sensitivity and resolution within a time unit. (Korytár and Matisová, 2001; Matisová and Dömötörová, 2003)
Helium has gradually been in short supply and its price has been increasing. The series of our papers is focused on grasping this problem and its solution when hydrogen is used instead of helium as carrier gas in gas chromatography. This part is focused on technical and risk aspects.
2 RESOURCES OF GASES
In spite of being the second most abundant element in the known universe, helium is rare on Earth. Helium is present in the Earth‘s atmosphere, especially in its outer part called the heterosphere and, due to its extra low weight, escapes into space. Most terrestrial helium is present as the isotope ⁴He (its nucleus consists of 2 protons and 2 neutrons) and only in trace amounts as the isotope ³He (2 protons and 1 neutron).
Helium is a non-renewable source and most terrestrial helium has been created by the natural process of radioactive decay of heavy elements such as thorium and uranium.
Helium is refined from natural gas deposits using a relatively complex process called cryogenic fractional distillation. The United States is world’s leading supplier of helium. Helium has been gained here from 1917. Its stock is expected to be depleted by 2018. Other territories with large concentration of helium include Algeria and Qatar. However, some experts believe that total depletion could come within a generation.
Low natural gas demand has led to sparse supplies of helium in some regions and an increase in price. So, many users worry about their ability to obtain helium when they need it more than about the increased price. For this reason it is necessary to find helium alternatives also in gas chromatography (Chromacademy, online).
Hydrogen is not only the lightest and the simplest of all elements but it is also the most common substance in the universe. At standard temperature hydrogen is stable but its reactivity sharply increases during warming. Hydrogen can be found combined with other elements with the exception of precious gases. Its compounds with carbon, oxygen, sulfur and nitrogen represent the fundamental units of life on Earth.
Hydrogen is a flammable, colorless, odorless, tasteless, non-toxic gas. It poses fire and explosive hazard when its concentration in air exceeds 4%. Proper safety precautions should be used in order to prevent an explosion. It should be noted that the concentration of hydrogen is very unlikely to exceed 4 % even in the smallest laboratory due to the large volume of air and to the relatively low rate at which hydrogen is produced or consumed during a typical gas chromatography analysis.
Hydrogen is industrially produced from the steam reforming of natural gas. It can also be produced by the electrolysis of water.
2H₂O → 2H₂ + O₂
Hydrogen generators for gas chromatography are based on this principle (Chromacademy, online).
Nitrogen is a colorless, odorless, tasteless gas. It is also inert, nontoxic and very safe. Nitrogen is the largest constituent of the Earth’s atmosphere. Nitrogen occurs in all organisms, primarily in amino acids.
As an industrial gas, nitrogen is produced by the low temperature fractional distillation of liquid air. Nitrogen generators are used in laboratories. These generators produce ultra high purity nitrogen (99.999+) suitable for gas chromatography. The gas generators are designed to take compressed air from the existing laboratory supply or via an integrated oil-free compressor. This flow of air then passes through the carbon molecular sieve column. These columns eliminate under pressure all compounds present in air (oxygen, carbon dioxide and humidity). This technology is called PSA (Pressure Swing Adsorption) (Parker, online).
3 CYLINDERS OR GENERATORS?
As seen above, helium is available only in cylinders. Nitrogen and hydrogen are available also in gas cylinders or can be produced by gas generators.
Gas cylinders represent uncomplicated and cheaper solution especially for the beginning. Most laboratories use 20 or usually 50 l cylinders for gas storage at 20 MPa. Gas cylinders are equipped with a two-stage regulator. The first stage reduces the pressure of the gas from the cylinder, usually from 20 to 3 MPa. The second stage reduces the pressure of the gas usually from 30 bar to the desired line pressure, usually of the order of 0,5 MPa. When using gas cylinders it is necessary to monitor the decreasing amount of gas in the cylinder and guarantee a timely exchange of empty cylinder for a full one. Manipulation with gas cylinders must be carried out carefully and all safety instructions must be kept (Linde-gas, online).
Gas generators eliminate the use of gas cylinders in the laboratory. Their big advantage is the minimum requirements for their servicing. Nitrogen generators require compressed air, so an air compressor must be included and this entails an increase in noise level (Chromservis, online). Hydrogen generator needs only deionized water for continuous hydrogen production with high purity (99.9995%) (Labicom, online).
4 SAFETY CONCERNS
The use of gas cylinders is safe but their presence in the laboratory represents a certain safety risk. In case of emergency, e.g. fire, an explosion of the cylinders can occur due to high temperature even with such inert gases as helium or nitrogen. Cylinders often involve long lines leading to gas chromatographic systems and the hookups are often at the end of benches or in other rooms. With the long tubing, large volumes of gas are present under pressure in the lines, and the possibility of venting of these lines through a line break cannot be excluded. This could allow for the entire venting of the volume of one or more cylinders into the laboratory. The consequences can be not only economical and could include loss of expensive gas, interruption of running analysis, column breakage in the gas chromatographic hot oven and, in case of hydrogen, an explosive level of the gas in the laboratory.
Gas generators with their safety shutoffs and monitoring safety features such as gas flow, gas pressure or leak detection only allow for small volumes of gas in their lines and units. If a sudden release of pressure or flows is detected, the gas generators will turn off. Generators only store about 60 cm3 of gas. (Peak Scientific, online).
Modern gas chromatographs equipped with electronic pressure control module also incorporate this feature. So the destruction of chromatographic column can be prevented. The only instance in which this check might fail would be a column break at or near the detector, hence maintaining back pressure at the inlet. To guard against this, column installation should be carefully and properly carried out and the column should never rest against the internal oven walls as this may reduce its mechanical strength, leading to possible breakage. Hydrogen leak from damaged column to gas chromatographic oven can be eliminated by in-oven hydrogen detector with a cut-off relay.
When switching the carrier gas from helium to hydrogen, it might be necessary to change the external tubing connections to the gas chromatograph. Copper tubing used for delivery of the carrier gas should be replaced with stainless steel tubing as copper tubing will oxidize and harden with time. Hardened copper tubing is quite brittle and may break if bumped while stainless steel tubing is much more robust. To avoid contamination to the gas chromatographic system, clean, preferably GC-quality tubing is recommended to use (Bartram and Froehlich, 2010).
5 ECONOMICAL ASPECTS
In terms of balance sheet it must be considered that the cost of hydrogen is significantly lower than that of helium. Afterwards it is necessary to decide to invest in gas generator or to use gas in gas cylinders. The return on investment time will depend on the cost of the generator relative to the number of cylinders it replaces over time. A type of gas chromatographic detectors is another fact influencing the return on investment time. The most used detector in brewing laboratory is the flame ionization detector. This detector needs hydrogen whose consumption is much larger than the volume exhausted by carrier gas. Generally, the shortest return on investment time would be attained if the hydrogen generator is considered in relation to replacement of helium cylinders instead of the less expensive hydrogen cylinders.
We are showing several reasons why, in spite of being explosive, hydrogen in appropriate installations and under appropriate safety conditions seems as an ideal replacement for helium as a gas chromatographic carrier gas.
The benefits rest in environmental concerns. Hydrogen is a “green” gas. Hydrogen is available via the elecrolysis of water and is not a critical resource. In contrast, helium is a by-product of natural gas or petroleum production and there are environmental concerns with the production and purification of the gas.
Another benefit is economical. The cost of hydrogen is significantly lower than that of helium.
Availability concerns represents one more advantage – because the generation of hydrogen is from water, it is always accessible on the market, so the chromatographer need not be concerned about availability issues.