<p><strong>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.</strong></p>
<h3><strong>1 INTRODUCTION</strong></h3>
<p>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.</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/bb06ae8ec7524862b78dcdd53341132d/obrazek.png" alt="Tab. 1 of important compounds determined by gas chromatography in brewing analytics."/></p>
<p>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. </p>
<p>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)</p>
<p>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.</p>
<h3><strong>2 RESOURCES OF GASES</strong></h3>
<h5><strong>2.1 Helium</strong></h5>
<p>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). </p>
<p>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.</p>
<p>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.</p>
<p>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).</p>
<h5><strong>2.2 Hydrogen</strong></h5>
<p>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.</p>
<p>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.</p>
<p>Hydrogen is industrially produced from the steam reforming of natural gas. It can also be produced by the electrolysis of water.</p>
<p><em>2H₂O → 2H₂ + O₂</em></p>
<p>Hydrogen generators for gas chromatography are based on this principle (Chromacademy, online).</p>
<h5><strong>2.3 Nitrogen</strong></h5>
<p>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. </p>
<p>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).</p>
<h3><strong>3 CYLINDERS OR GENERATORS?</strong></h3>
<p>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.</p>
<p>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). </p>
<p>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).</p>
<h3><strong>4 SAFETY CONCERNS</strong></h3>
<p>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.</p>
<p>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).</p>
<p>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.</p>
<p>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).</p>
<h3><strong>5 ECONOMICAL ASPECTS</strong></h3>
<p>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.</p>
<h3><strong>6 CONCULSIONS</strong></h3>
<p>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.</p>
<p>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.</p>
<p>Another benefit is economical. The cost of hydrogen is significantly lower than that of helium.</p>
<p>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.</p>


Advantages and Disadvantages of Substitution of Helium as Carrier Gas in Gas Chromatography by Hydrogen. Part I. – Technical and Safety Aspects

<p><strong>Gas chromatography often uses helium, hydrogen, and nitrogen as carrier gases. The most common carrier gas is helium since it provides good separations, is inert, and is safe to use. In recent years, the availability of helium has decreased and its cost has increased significantly. So, it is necessary to choose a suitable substitution. This article is focused on the chromatographic parameters such as retention time and selectivity after switching to the use of hydrogen or nitrogen.</strong></p>
<h3><strong>1 INTRODUCTION</strong></h3>
<p>The availability of helium has decreased and its cost has in-creased significantly, and chromatographers have therefore considered switching to the use of another suitable gas (Horák et al., 2013).</p>
<p>In this article the use of helium and hydrogen in gas chromatography is compared and practical aspects of switching to the use of hydrogen as carrier gas in gas chromatographic methods are dis-cussed especially in brewing analysis applications.</p>
<h3><strong>2 THEORY</strong></h3>
<p>The number of theoretical plates (N) or the equivalent of column plate height (H = L/N, L means the column length) can express the efficiency of a chromatographic column. The equivalent of column plate height is the function of the speed average of carrier gas (ū) which is described by the Golay-Giddings equation (Golay, 1958a; Giddings et al., 1960). This dependence is the function of the column diameter (dc), film thickness (df), capacity factor (k) and diffusion coefficients of a compound in mobile and stationary phases. Different H versus ū curves for different gases are obtained for a given column and a given compound (Fig. 1).</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/95caa91953cf48a9bc7ec1314ae4f7ab/obrazek.png" alt="Fig. 1 H versus ū curves for carrier gases hydrogen (ring), helium  (square), nitrogen (rhombus). (Korytár, Matisová, 2001) "/></p>
<p>Several important facts ensue from these dependences.</p>
<p>For all carrier gases the value H is almost independent of the type of carrier gas. So when an optimal average speed of carrier gas is adjusted, all three types of gases give similar efficiency and resolution.</p>
<p>The optimal carrier gas speed is the highest for hydrogen in com-parison with helium or nitrogen. For current capillary columns (length 10–50 m, internal diameter 0.25–0.32 mm and film thickness 0.1–0.5 µm) and using the optimal speed, hydrogen is 1.5 times faster than helium and 3.3 times faster than nitrogen. Importantly, the equivalent of column plate height is approximately the same.</p>
<p>The slope of the H versus ū curves drops in the order nitrogen – helium – hydrogen. This means that, when nitrogen is used in a speed higher than the optimal speed, fast degradation of efficiency is attained. On the other hand, in the case of hydrogen the gas speed can be increased without significant loss of efficiency. Helium is characterized­ by the middle slope (Fig. 1). With decreasing column diameter the slope of the curve drops, the curve becomes flatter and the optimal gas speed increases. Due to this a substantial speed of carrier gas can be used without considerable degradation of efficiency (David, Sandra, 1999; Horák et al., 2009).</p>
<p>For above reasons it is clear that hydrogen is more suitable as helium substitution than nitrogen.</p>
<h3><strong>3 RETENTION TIME</strong></h3>
<p>Generally, the chromatographic parameters such as retention time, efficiency, resolution, and selectivity are influenced by the choice of mobile phase. However, in gas chromatography this effect is much less important that in liquid chromatography. This is because the carrier gas in gas chromatography rarely interacts chemically with the stationary phase or with the analyte. The main role of carrier gas is to transport the analyte in the gas phase through the column and act as the second phase in the partitioning mechanism.</p>
<p>Two properties of the gas play a significant role in the gas chromatographic process – diffusivity and viscosity. The diffusivity of hydro-gen and helium is roughly the same but hydrogen is slightly less than half as viscous as helium at the same temperature. For this reason hydrogen requires a lower pressure to achieve the same average carrier gas velocity than helium.</p>
<p>Due to the compressibility of the carrier gas the relationship be-tween linear velocity and pressure is somewhat nonlinear, but in general, the switching from helium to hydrogen will reduce retention times roughly in half if the inlet pressure is unchanged.</p>
<p>Retention times in gas chromatography are influenced by several factors. The distribution coefficient of a solute in the column, which is not affected by the choice of carrier gas; the column parameters such as length, internal diameter and film thickness; and the average carrier gas linear velocity (Chromacademy, online).</p>
<h5><strong>3.1 Isothermal operation</strong></h5>
<p>The linear velocity is influenced by the column dimension, pressure drop, and viscosity of the carrier gas. Hydrogen is slightly less than half as viscous as helium or nitrogen at the same temperature. For this reason hydrogen requires a lower pressure drop to achieve the same average carrier gas velocity as for helium or nitrogen. If the velocity is unchanged with hydrogen versus helium carrier, then retention times remain also the same, while the hydrogen inlet pressure will be about half as much as with helium.</p>
<p>On the other hand, if the same pressure were applied with hydrogen as with helium, then the hydrogen carrier gas would cause peaks to be eluted in less time, because the linear velocity would be faster than with helium. Retention times at the same pressure would decrease by about half.</p>
<p>At constant flow rates, the situation is intermediate between the effects at constant velocity and those at constant pressure (Chromacademy, online).</p>
<h5><strong>3.2 Temperature program</strong></h5>
<p>The above tenets can be applied also to isothermal operations. If column temperature program is used the situation is a little changed.</p>
<p>When a constant linear velocity can be maintained during the temperature program by an electronic pneumatic unit, the retention times will not change. But the control of linear velocity is not available for all pneumatic systems of gas chromato-graphs.</p>
<p>In the constant inlet pressure mode, hydrogen will cause peaks to be eluted earlier. Simultaneously their elution temperatures will also be reduced and this can change the relative retention times of peaks with diver-gent chemical characteristic of determined compounds.</p>
<p>The same effects, but to a lesser degree, will occur when constant column flow-rate mode is used (Chromacademy, online).</p>
<h3><strong>4 PRACTICAL EXAMPLE IN BREWING ANALYSES</strong></h3>
<p>Following a very careful description of the theory of gas chromatography, a method translation software was developed for easy translating of operating conditions when carrier gas is changed (Snyder, et. al., 1992; Quimby et. al., 1995). Thanks to this, no complicated and time consuming search for new acceptable conditions for the separation with a new carrier gas is necessary. One of these forms of translating software is e.g. the GC Method Translation software free available on Agilent Technologies web sites (Agilent, online).</p>
<p>Using this software one can demonstrate the changing separation conditions for some beer flavors during switching from helium to hy-drogen or nitrogen carrier gas.</p>
<p>Tab. 1 shows the changes during analysis of volatile beer flavors on J&amp;W DBWAX, 60 m, 0,32 mm, 0,25 µm column.</p>
<p>If the goal is to keep the retention time constant, it is necessary to maintain the same linear velocity for all carrier gases. In this case the inlet pressure decreases for hydrogen carrier gas about half as much as with helium. But the nitrogen average velocity of 36.4 cm/s is above the minimum equivalent of column plate, as can be shown from van Deemter’s curve (Fig. 1). So a decrease of efficiency can occur when nitrogen carrier gas is used.</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/09634264fc5f49d8ad8dad8fe560fe56/obrazek.png" alt="Tab. 1 Chromatographic conditions for separation of selected volatile beer flavours on J&amp;W DBWAX, 60 m, 0.32 mm, 0.25 μm column after switching from helium to hydrogen or nitrogen carrier gas in two model modes – A) constant retention times; B) “translation” of original method by GC Method Translation Software in “translate only” mode"/></p>
<p>Tab. 1 also includes the results of calculated parameters of the original method “translated” by GC Method Translation Software. “Translated” hydrogen average velocity is about 1.5 times faster than with helium but nitrogen average velocity is about 2.3 times lower than with helium. So, by using the calculated temperature program a 1.5 times faster analysis could be obtained by using hydrogen carrier gas in comparison with helium carrier gas. On the other hand, using nitrogen carrier gas leads to a 2.3 times slower analysis.</p>
<p>Faster gas chromatography has recently became more popular. The changes in separation conditions of selected low volatile beer flavors on a faster gas chromatographic column J&amp;W DB-WAX, 10 m, 0,18 mm, 0,18 µm are shown in Tab. 2.</p>
<p>The same average velocities for all carrier gases are necessary for the unchanged retention times, specifically 84 cm/s. Due to using column with 0.18 mm internal diameter, van Deemter’s curves become more flat and this high linear velocity can be applied for helium or hydrogen carrier gas. But this high linear velocity cannot be recommend-ed for nitrogen carrier gas without a strong efficiency degradation.</p>
<p>Another part of Tab. 2 demonstrates the results of calculated parameters of “translated” original methods using GC Method Translation Software. Due to the shallow slope of van Deemter`s curve for hydrogen for this column internal diameter it is possible to use high linear velocity (129 cm/s) for hydrogen carrier gas. So, another acceleration of analysis is reached (1.53 times relative to helium). When nitrogen carrier gas is used, linear velocity of 39 cm/s is in the optimal area of separation column efficiency but the run time increases about 2.1 times in comparison with helium carrier gas.</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/583c4d796ba54fb4a8d068e14dc4ab4c/obrazek.png" alt="Chromatographic conditions for separation of selected low volatile beer flavours on J&amp;W DB-WAX, 10 m, 0.18 mm, 0.18 μm column after switching from helium to hydrogen or nitrogen carrier gas in two model modes – A) constant retention times; B) “translation” of original method by GC Method Translation Software in “translate only” mode"/></p>
<h3><strong>5 CONCULSIONS</strong></h3>
<p>Hydrogen is the best choice when switching from helium to an-other carrier gas in gas chromatography. Due to the advantageous course of van Deemter’s curve hydrogen provides the highest separation column efficiency at the highest linear velocity in comparison with helium or nitrogen. Hydrogen therefore provides faster analysis at lower inlet pressures than any other compared carrier gases.</p>


Advantages and disadvantages of substitution of helium as carrier gas in gas chromatography by hydrogen. Part II. - Retention time and selectivity.

<p><strong>As shown in the previous articles of this series, hydrogen is the most suitable choice as a substitution for helium as a carrier gas in gas chromatography. This paper addresses some specifics concerning the use of hydrogen as carrier gas in different techniques of sample introduction on chromatographic column. Attention is also focused on the performance and sensitivity of different types of gas chromatographic detectors, especially in brewing analysis.</strong></p>
<h3><strong>1 SAMPLE INTRODUCTION</strong></h3>
<p>Several different techniques are used for sample introduction on chromatographic column. Tab. 1 shows the overview of the most important methods used in brewing analysis according to sample introduction methods. Individual techniques of sample introduction with hydrogen as carrier gas are discussed in this paper.</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/25dcf84fd0f14b62aa22057d57523f2a/obrazek.png" alt="Tab. 1 Sample introduction according the most important groups of analytes determined in beer by gas chromatography"/></p>
<h5><strong>1.1 Split/Splitless injection</strong></h5>
<p>The most common inlet for capillary gas chromatography is known as the Split/Splitless inlet, which can be operated in two modes, split or splitless. This inlet is used mainly for injection of liquid samples. In the injector the liquid sample is vaporized into the gas phase prior to transfer onto the capillary column.</p>
<p>The injection principle of the split/splitless inlet can be described as follows: A syringe containing the sample is used to pierce a septum closing the injector. Then the sample is rapidly introduced to the heated inlet where it is rapidly volatilizes to the gaseous form and the gaseous sample is swept onto the column by the carrier gas. Depending upon the mode of operation, all of the sample may be introduced onto the column (splitless mode) or part of the sample may be directed away from the column (split mode).</p>
<p>In splitless injection hydrogen is preferred over helium as carrier gas. Due to its higher velocity the analytes could be introduced faster into the column. This results in sharper peaks which allows for lower detection limits.</p>
<p>In split mode the readjusting of split ratio could be required when moving from helium to hydrogen carrier gas, especially if shorter analysis is achieved by increasing hydrogen linear velocity. This adjustment of the split ratio is necessary to allow for only the correct amount of sample to enter the column. The rest of the sample goes to a vent. The ratio of sample entering the column versus that going to the vent is the split ratio.</p>
<p>The split ratio is usually adjusted empirically to obtain a good balance between analytical sensitivity and peak shape. If the split ratio is too low, peak shape will be broad and may show the fronting behavior associated with overloading. Of course if the split ratio is too high, too little sample will reach the column and the sensitivity of the analysis will decrease as peak areas decrease (Chromacademy, online).</p>
<h5><strong>1.2 Head-space</strong></h5>
<p>Head-space technique is a primary and very often used procedure for the determination of volatile compounds. Head-space analysis can be generally defined as a vapor-phase extraction, involving the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid.</p>
<p>Head-space analysis can be practiced in two ways. If the sample is in equilibrium with the gas phase in closed vessel, then the method is called a static head-space. If a carrier gas is passed over, or through, the sample and the extracted volatile compounds accumulated in a sorbent trap, then the procedure is generally referred to as dynamic head-space or purge-and-trap (Chromacademy, online).</p>
<h5><em>1.2.1 Static Head-space method</em></h5>
<p>In static head-space analysis, the volatiles in the sample are equilibrated with a gas phase above the sample in a gas-tight closed vial. After a predetermined equilibration time, part of the gas phase is withdrawn from the vessel, and injected into a gas chromatographic column. Static head-space systems are available in two different modes – syringe-based system or valve and loop-based system.</p>
<p>In syringe based system the gastight syringe is purged by gas for programmed period after injection of the sample. This is important when the samples with high concentration of compounds of interest are determined and so the carry-over can be eliminated. An inert gas such as nitrogen is recommended for syringe purge (Kolb, Ettre, 2006; Horák at al., 2012).</p>
<p>In valve and loop-based system the analysis includes several following steps. Initially the vial is heated for a length of time at a specified temperature. Then the vial is pierced with a needle and pressurized with a gas. In the next step the valve is turned so that the flow of gas changes direction and a portion of the head-space flows into the sample loop. After that the valve is turned again so that gas in the sample loop is flushed through the transfer line and into the gas chromatographic column. Pressurization of head-space vials with hydrogen is not recommended as it will create an enclosed pressurized volume of flammable gas (Kolb, Ettre, 2006; Horák at al., 2012).</p>
<p>Hydrogen carrier gas can be used with head-space samplers if separate nitrogen or another inert gas is used for purge or vial pressurization (Chromacademy, online).</p>
<h3><strong>2 DETECTORS</strong></h3>
<p>The role of hydrogen in gas chromatography is not limited only to the use as a carrier gas. Hydrogen is also a significant gas in some detectors as a fuel or make-up gas. Tab. 2 presents an overview of the most important methods used in brewing analysis according to detector type (Horák at al., 2011).</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/81d3521ba9f84f9a9974592377bfb5e0/obrazek.png" alt="Tab. 2 List of compounds determined by gas chromatography in brewing analytics according to detector type. (FID – flame ionization detector, ECD – electron capture detector,
FPD – flame photometric detector)"/></p>
<p>All the above detectors use make-up gas. The make-up gas is delivered between the column and the detector. The purpose of the makeup gas is to improve the transfer of sample between the column and the detector and helps to rapidly sweep the detector volume, thus reducing peak broadening and distortion. Hydrogen is not the ideal make-up gas for combustion detectors (FID, FPD) as the stoichiometry of combustion (hydrogen to oxygen) will determine signal response. Nitrogen as the make-up gas should be the best choice. Tab. 3 presents a list of gases used with different detectors (Chromacademy, online).</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/00cde382166341e7ae6a6770b5ce28b5/obrazek.png" alt="Tab. 3 List of gases used with different detectors"/></p>
<h5><strong>2.1 FID and FPD detectors</strong></h5>
<p>Tab. 4 demonstrates maximum hydrogen gas flow rates which are typical for most gas chromatographs equipped with FID or FPD. The adjustment of correct stoichiometry of combustion (hydrogen to oxygen ratio) is necessary in determining the maximum sensitivity of the detector. The use of hydrogen as a carrier gas and as a fuel will impose restrictions to the make-up gas. So, for as long as the stoichiometry of the combustion (hydrogen to oxygen ratio) is not affected, hydrogen is a valid make-up gas. Otherwise it is necessary to use another gas, usually nitrogen. Generally, hydrogen is not used as make-up gas with FID or FPD (Chromacademy, online).</p>
<p><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/2cba16263e654925a6394cc43d0b9133/obrazek.png" alt="Tab. 4 Maximum hydrogen gas flow rates which are typically with most gas chromatographs equipped with FID or FPD"/></p>
<h5><strong>2.2 ECD detector</strong></h5>
<p>Only carrier gas and make-up gas are used with ECD. The electron capture detector ECD utilizes the fundamental observation that the conductivity of gases in an ionization chamber can be drastically altered by the presence or absence of contamination in the gas. Thus the ECD consists of an ionization chamber containing a radioactive source, usually nickel–63, with a stream of inert gas, usually nitrogen, flowing through it. The β-ray emanation from the source causes ionization of the inert gas with a consequent liberation of free electrons (Horák et al., 2011). Neither hydrogen nor helium ionize under normal ECD operating conditions and should not be used as the make-up gas (see Tab. 3).</p>
<h5><strong>2.3 Mass detector</strong></h5>
<p>No brewing or malting methods require the use of mass detector, but this detector has become the most often linked to gas chromatographs due to its indisputable advantages, especially in quantitative analysis.<br/>Most gas chromatographs equipped with mass detector use helium as carrier gas. Moreover, in this system the column outlet is kept in vacuum rather than at ambient pressure, as occurs with conventional detectors. This vacuum reduces the column head pressure required to provide a certain column flow rate (linear velocity). Thus, helium that has a higher viscosity than hydrogen and, consequently, a higher head pressure will suppress the expansion of the injection solvent during splitless injection while hydrogen with its lower viscosity will allow a greater and perhaps uncontrolled solvent expansion volume. Peak tailing or broadening can result. This problem can be solved by increasing the hydrogen flow rate. But a large volumetric gas flows into the detector and this makes it slightly more difficult to pump away for high vacuum equipment. This may increase the number of background molecules which can collide with the ions formed, leading to a potential reduction in sensitivity and a change in the relative abundance if ions within the mass spectrum.</p>
<p>Thus, injection with a hydrogen carrier gas is best performed using a smaller internal diameter column such as 0.15, 0.18 or 0.20 mm. Higher linear velocities can be achieved at lower volumetric flow rates and sufficient vacuum level can be reached within the system and there should be no substantial changes in the appearance of the spectra. </p>
<p>Hydrogen can act as a scrubber in the flow system because it can displace contaminants that can be adsorbed on roughened or unswept areas when using helium carrier. On the mass spectrometer, this contamination typically looks like hydrocarbon contamination and it may take some time to have a completely noise-free system. However, once cleaned, hydrogen seems to keep contaminants from building up on filaments and other areas (Heseltine, 2010).</p>
<h3><strong>3 CONCLUSIONS</strong></h3>
<p>The papers of this series show that hydrogen can be used as an effective replacement for helium in many gas chromatographic applications. It is excellent in a wide range of applicability, provides good efficiency of chromatographic separation, and, for the most part, analyses are faster than with other popular carrier gases like helium and nitrogen. It is much cheaper to purchase than helium and can be generated easily and cheaply with a very high level of purity by using hydrogen generators. </p>
<p>Due to its explosive properties, precautions must be observed in its use.</p>


Advantages and disadvantages of substitution of helium as carrier gas in gas chromatography by hydrogen. Part III. - Sample introduction and detectors.

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Vědecký recenzovaný online časopis Kvasný průmysl (eISSN 2570-8619), vychází jako samostatné periodikum v angličtině. Od ročníku 2015 je obsah indexovaný databází Web of Science (edice ESCI), dále je indexován v databázích CAS, CAB Abstracts, DOAJ.
Časopis je publikovaný v režimu otevřeného přístupu Platinum Open Access s licencí Creative Commons. 

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