Gas analysis – the hydrogen way

Hydrogen is considered by many to be the best carrier gas for gas chromatography and, in some of the more critical applications, it is already recognised as the carrier gas of choice. But what is driving this demand for ultra-high purity hydrogen and why is it proving so popular? Helium is still the most commonly used carrier gas, but this is steadily changing, with a growing preference for hydrogen in certain applications. One of the main drivers for change is global concern about depleting helium reserves. Even though industry calculations have shown that there are sufficient reserves of helium to last for several centuries, recently-imposed temporary helium supply restrictions have prompted chromatographers to look elsewhere and consider alternative options. Turning their attentions to hydrogen, some have found that using hydrogen as a carrier gas brings some significant benefits and GC manufacturers are introducing new equipment optimised for hydrogen use too. In fact, hydrogen is not new to gas chromatography. Among volume gas users in laboratories and the pharmaceutical processing industries in particular, hydrogen has been used as a carrier gas in GC and as a detector fuel gas for FID for decades. These methods of gas chromatography detection are highly sensitive and are capable of giving accurate readings of complex samples containing a large number of components at low concentrations. In the pharmaceutical sector, for example, this kind of gas analysis is a critical part of the manufacture of any modern drug and is carried out at every stage of the production process. When the purity of the hydrogen or air used in the GC-FID application falls short of the quality needed for such gas analysis, it results in ‘baseline noise’ – the appearance of unwanted peaks on the chromatograph. These imperfections can make it virtually impossible to measure the area beneath the peaks accurately. In some instances, it may not be possible to detect some components in the sample at all, simply because they are present at very low concentrations that are below the limit of detection. Clearly, one of the main factors contributing to the increased use of hydrogen in gas chromatography has been the increased availability of a true ultra-high purity hydrogen product, with critical impurities such as oxygen, water and hydrocarbons dramatically reduced compared to conventional hydrogen grades. To help meet demand in this area, Air Products has recently extended its BIP technology to hydrogen. Each Hydrogen BIP cylinder contains less than 20 ppb of water; 100 ppb of oxygen and 10 ppb of total hydrocarbons. This makes Hydrogen BIP gas many times purer than all convention hydrogen grades. Such advanced gas products are making hydrogen even more attractive as a carrier gas because of its potential to deliver highly-accurate analyses and improved limits of detection, time after time. Hydrogen’s high diffusivity means faster linear velocities can be achieved, which means shorter analyses, while still providing the same separation efficiency as helium. Responding to growing interest in using hydrogen as a carrier gas, Air Products has recently compiled research to compare the performance of ultra-high purity hydrogen, with critical impurities 100 times lower than standard grade hydrogen, against helium and nitrogen. The tests carried out with various samples reveal that ultra-high purity hydrogen performs well – delivering reliable results, with minimal baseline noise and reduced analysis time. In general, shorter analyses mean increased throughput and lower costs per sample. However it is worth noting that hydrogen may not be suitable for all applications and it is important to take expert advice particularly when using hydrogen as the mobile phase in GC-MS applications. When considering making the switch to using hydrogen as a carrier gas of choice, a number of important steps should be followed. Due to the flammable nature of hydrogen, the gas chromatograph will need to be modified and safety standards followed. In particular, it is necessary to ensure there is no build-up of gas by avoiding and detecting leaks and safely venting any hydrogen outlet stream. Recent advances in the equipment used for gas chromatography are also helping to improve safety when using hydrogen as a carrier gas. The modern gas chromatograph can detect leaks upstream in the column by monitoring gas pressure, which may be especially useful when working with hydrogen. If hydrogen leaks before reaching the column, the pressure drop inside the gas chromatograph is such that the hydrogen pressure cannot build up and reach the defined set point. The gas chromatograph interprets the permanent difference between the operative and the set point pressure as a leak and protects the equipment by shutting off the hydrogen valve. However, when the leak is downstream in the column or near the detector, this safeguard is not efficient and a hydrogen sensor needs to be installed to detect any hydrogen build-up in the oven. A further modification that needs to be made when switching from helium to hydrogen is to safely vent the outlet stream from septum purges and split ports. This is necessary because when helium is used it can be vented directly into the laboratory and poses no hazard at all. However, when hydrogen is used, the ports need to carry it to the laboratory’s flammable vent line instead. In addition, the laboratory should ideally install a proprietary gas detector system and a flow restrictor to minimise the risk in the event of a gas leakage. Once these modifications have been made, the laboratory can run a standard FID performance evaluation test to see for themselves how much improvement they can expect to achieve with ultra-high purity hydrogen compared to helium, with the base line being more stable and accurate. MATGAS, a leading R&D organisation based at the Universitat Autonoma de Barcelona in Spain, has recently made the change and is now using hydrogen as a carrier in gas chromatography. According to Dr Lourdes Vega, the switch has been positive. Provided it’s used at the right purity and the correct safety measures are taken, she said: “Hydrogen can be a highly effective carrier gas that can improve quality, speed up your process and help you save money.” For many laboratories a reliable source of supply is a key aspect. Depending on your hydrogen specification needs you can choose between cylinders and generators. Of course, before making a decision relative advantages and disadvantages should be considered. Both modes of supply offer the highest purity levels. But hydrogen generators may be a solution in remote locations or laboratories with storage limits. When considering switching from helium to hydrogen, those responsible for performing gas analysis may be looking for more detailed information about the benefits that might apply to their work. To help them with this, Air Products has recently produced a short film entitled Hydrogen as a Carrier and Fuel Gas in Gas Chromatography (www.airproducts.co.uk/H2BIP). While helium remains the most commonly used carrier gas, there is no doubt that hydrogen is growing in favour and the reasons for this are clear. Using ultra-high purity hydrogen for certain demanding analytical applications will significantly lower detection levels and provide an assurance of improved accuracy too. For these reasons, we are expecting demand for ultra-high purity hydrogen to continue to grow in the future. Author Gary Yates, specialty gas consultant at Air Products. Gary has a BSc in physics and a PHD in theoretical physics. He has extensive experience in advising companies in the use of analytical gases for a wide range of modern industrial applications. Contact www.airproducts.co.uk/BIP