Hydrogen is gaining popularity in use as a carrier gas because it offers the benefits of speed of analysis, resolution, and sensitivity over other gases. and a fuel gas for GC-FID applications. The selection of an appropriate hydrogen supply source for GC-FID contributes to the usefulness and reliability of the system. Options for hydrogen supply include point-of-use hydrogen cylinders, multi-cylinder configurations, and hydrogen generators. Today cylinder hydrogen is the most commonly used supply source in the laboratory, but hydrogen generators are gaining in popularity. Understanding the benefits and drawbacks of each option helps in choosing the best hydrogen supply source for a system.
Cylinder hydrogen is the most commonly used supply source in laboratories. There are two different approaches in using hydrogen cylinders for GC supply:
Point-of-use cylinders provide the user with good control of the gas delivery system. For the best GC performance, the hydrogen delivery pressure must be constant. Locating the cylinder near the GC minimizes the pressure drop in the supply line between the gas source and the instrument, providing consistent delivery pressure. Because of the proximity of the gas source to the GC, delivery pressure monitoring and adjustment are easily accomplished in using this arrangement.
Multi-cylinder configurations used with switchover systems provide a more continuous supply of gas, extending the length of time before cylinder changeout is required. GC operation is not interrupted during cylinder changes, as changeout of the primary cylinder is performed when while the secondary side of the switchover system is delivering gas. This more continuous hydrogen supply also enhances reliability, as it offers lessbecause it reduces the potential for running out of carrier or fuel gas and compromising the analysis.
The flammable nature of hydrogen and the high pressure of cylinders (2400 psig) present concerns for cylinder storage and handling in labs. In addition, the size and weight of hydrogen cylinders present hazards to personnel performing cylinder changeouts. Care must be taken in while handling cylinders during changeouts, and cylinders should be secured to the wall or bench top using appropriate cylinder holders and restraints. Because of these concerns, many labs are seeking ways to reduce or eliminate the use of hydrogen cylinders.
High purity hydrogen (zero grade or better) is required for use as a carrier gas to ensure a clean baseline. Contaminant levels for a given grade of hydrogen may vary from cylinder to cylinder, and still meet specification, as long as the total level in each cylinder is. This cylinder-to-cylinder variation may be a consideration for sensitive analyses, as it introduces another variable into the operation.
Oxygen and moisture cannot be prevented from entering the system during cylinder changes. To minimize the impact of these contaminants on the GC system, high purity gas handling equipment should be used. Regulators should be of brass or stainless steel barstock construction (depending on the sensitivity of the analysis being performed), with stainless steel diaphragms and metal-to-metal seals. Barstock construction ensures that the regulators have minimal internal dead space that may trap ambient contamination. Stainless steel diaphragms do not adsorb and release oxygen or moisture into the system, and a metal-to-metal seal will minimize ambient air leakage into the regulator. To minimize diffusion of ambient contaminants into the system, stainless steel tubing should be used from the cylinder to the GC. To further protect the system from oxygen and moisture, point-of-use purifiers should be installed in the hydrogen lines just prior to the GC to remove any residual contaminants. These components add cost to the hydrogen supply system, but are essential for system performance.
High purity hydrogen generators are becoming more popular as hydrogen supply sources in laboratories. Generators are available in different hydrogen production capacities typically ranging from 150 ccm to 600 ccm, and can be used to supply multiple GCs. Hydrogen generators offer a number of safety, reliability, and convenience benefits to the user.
Gas generators offer significant safety benefits over gas cylinders. The use of a hydrogen generator eliminates the need for personnel to handle high-pressure cylinders, and the storage of large quantities of flammable hydrogen on the premises. A generator typically contains about 100 ccm of hydrogen at any given time, significantly less than the contents of an average cylinder. Some hydrogen generators are designed with various alarm functions to alert the user to operating problems, and automatic shut down capabilities that are triggered if the unit malfunctions. All of these factors contribute greatly to the overall safety of in the lab.
Once installed and operating properly, hydrogen generators provide a continuous supply of high purity gas. The need to change and store cylinders is eliminated, saving time and cost. Eliminating cylinder change outs changeouts also reduces downtime due to interruptions in carrier or fuel gas supply, and minimizes the potential for ambient air contamination.
Hydrogen generators will typically supply hydrogen at purity levels of 99.9999%. This purity level remains constant over time, eliminating gas purity as an operating variable in the analysis. This consistent purity provides reliability for the GC system.
The relatively small size of hydrogen generators allows them to be conveniently located on the lab bench, without consuming a lot of valuable bench space. Eliminating the hydrogen cylinders can also free up a lot of valuable storage space in the lab.
It is important to be aware that some hydrogen generator models are capable of supplying hydrogen at only 60-100 psig. This pressure limitation may be a problem for labs where the generator is located remotely from the GC that it supplies, or where multiple instruments are fed from one generator. The generator should be located as close as possible to the instrument that it supplies to minimize the effects of supply line pressure drop. Depending on the complexity of the lab and the number of GCs in use, selecting a generator with a broad delivery pressure range may provide more optimal performance.
Changes in operating mode may also present delivery pressure challenges. Many labs are considering the use of fast GC techniques, which require a hydrogen carrier gas pressure of 120 psig. Selecting a generator with a higher pressure capability will provide more operating flexibility, and allow for expansion or changes in technique.
Hydrogen generators typically cost from $5000-$8000 depending on the capacity. This relatively high front-end capital investment may be a consideration for some laboratories. However, it is important to compare this one-time capital cost expenditure to the ongoing costs of cylinder product, cylinder rental, and cylinder handling to fully understand the impact of the investment.
Some generators require the addition of a caustic electrolyte solution for the production of hydrogen. Other models require the monitoring and changing of desiccant cartridges to remove residual moisture from the hydrogen. If this maintenance is not performed, the generator will not operate properly, and reliability will suffer. Understanding the required operating and maintenance procedures will help ensure optimal generator performance.
While cylinders are the most common hydrogen supply source for labs, they present a number of safety and handling concerns. The flammability of hydrogen and the high pressure of the cylinders pose storage and handling problems. The need to inventory and change cylinders regularly may also be inconvenient and costly. Many users are seeking ways to eliminate the hydrogen cylinders from their labs to improve safety and productivity.
To combat these concerns, many users are converting to hydrogen generators for their GC hydrogen supply. Generators offer the benefits of continuous supply and consistent purity, while eliminating many of the safety concerns associated with hydrogen cylinders. The safety, reliability, and convenience of hydrogen generators make them an attractive alternative for GC hydrogen supply.
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