Advantages and disadvantages of substitution of helium as carrier gas in gas chromatography by hydrogen. Part III. - Sample introduction and detectors.
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.
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.
Tab. 1 Sample introduction according the most important groups of analytes determined in beer by gas chromatography
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.
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).
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.
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.
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).
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.
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).
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.
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).
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).
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).
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).
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)
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).
Tab. 3 List of gases used with different detectors
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).
Tab. 4 Maximum hydrogen gas flow rates which are typically with most gas chromatographs equipped with FID or FPD
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).
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. 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.
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.
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).
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.
Due to its explosive properties, precautions must be observed in its use.