When gas chromatography (GC) is used to separate a complex mixture, selection of the appropriate carrier gas and the optimum source for the carrier gas in GC are critical decisions for the laboratory manager. The manager should select the carrier gas that provides the desired separation in the minimum period of time to optimize the throughput of the laboratory. In addition, once the appropriate gas has been selected, the manager must then evaluate the various potential sources of that gas to determine how it should be supplied to ensure laboratory safety, convenience, and minimize the cost of the gas.
Historically, nitrogen or helium has been employed as the carrier gas in GC. When hydrogen is used, it is typically provided to the chromatograph via a high-pressure tank with appropriate pressure reduction valves and tubing. While this approach is fairly straightforward, it suffers from a number of disadvantages, including the dangers inherent in working with pressurized gas, the cost of the tanks, and the inconvenience of having to replace tanks on a periodic basis.
Hydrogen— an appropriate carrier gas for GC
Hydrogen is an extremely useful carrier gas for GC and provides a number of significant benefits compared to the use of helium or nitrogen. The major benefit of hydrogen is the fact that it can lead to a dramatic reduction of the time required for a given separation. In addition, hydrogen frequently allows for the use of a lower temperature for separation, thereby increasing column longevity. Besides its use as a carrier gas, hydrogen is used in GC as a fuel gas for flame-ionization detectors (FIDs) and as a reaction gas for Hall detectors.
Three gases are commonly used as carrier gases in GC: nitrogen, hydrogen, and helium. While nitrogen provides somewhat higher chromatographic efficiency than hydrogen, the overall consideration is to obtain the required separation in the minimum period of time. The van Deemter plot (Figure 1) shows that the use of nitrogen provides a shorter theoretical plate (0.22 mm) than hydrogen (0.28 mm), which leads to a greater number of plates for a column and provides better resolution than either hydrogen or helium. It should be noted that the maximum efficiency for nitrogen is obtained at a linear velocity of 8–10 cm/sec while the optimum linear velocity for hydrogen is approximately 40 cm/sec, which leads to a four-fold decrease in the average analysis time. While the efficiency of nitrogen is somewhat better than hydrogen, the decrease in analysis time is significant and suggests that the throughput of the laboratory can be dramatically improved by using hydrogen as the carrier gas.
A chromatogram showing the separation of a standard reference mix using hydrogen as the carrier gas is presented in Figure 2. This separation was performed in less than nine minutes using an Rtx® -1 column (0.53 mm id, 5 μm); the same separation took more than four times as long with nitrogen. A very satisfactory separation was obtained and the retention times are extremely repeatable.
Since the use of hydrogen provides a significant reduction in the time for separation, the analyst could reduce the column temperature for separation. Although this will lead to a slightly longer separation time, the lifetime of the column will be greater, leading to further economic benefit.
While both hydrogen and helium are more satisfactory gases than nitrogen, there are several drawbacks to the use of helium. It is quite expensive, is a non-renewable resource, and has limited availability in many parts of the world. In contrast, hydrogen is readily available via the electrolysis of water or as a high-pressure bottled gas.
Generation of hydrogen via the electrolysis of water
The generation of hydrogen in the laboratory via the electrolytic dissociation of water provides a convenient, safe, reliable andeconomical method to provide the gas for GC. This two-step process is described in equations 1 and 2.
- H2O —> 2H+ + O2-
- 2H+ + 2e-—> H2
The protons that are formed via the dissociation of water are allowed to cross a membrane and form molecular hydrogen.
A schematic diagram of a typical hydrogen generator is presented in Figure 3. The hydrogen generator includes an electrochemical cell that contains a solid polymer membrane to support electrolysis. The system operates at a potential of approximately 7 V (depending on the desired flow rate).
A specially designed palladium membrane is included in the design to optimize the purity of hydrogen (99.99999+ %). The palladium membrane is heated to greater than 600 °C so that only hydrogen and its isotopes can pass through the pores; this provides gas with an oxygen content less than 0.01 ppm and a moisture content less than 1.0 ppm, at flow rates up to 800 mL/min at a pressure of 100 psi.
This type of hydrogen generator produces a steady, dependable, and precise flow of gas. As an example, the chromatogram shown in Figure 4 demonstrates FID baseline stability over a two-hour period. In addition, a series of ten runs was performed for the simulated distillation mix described in Figure 2, with extremely reproducible retention times (Table 1).
Benefits of a hydrogen generator
A hydrogen generator provides a continuous stream of gas at a flow rate that is required to maintain a number of gas chromatographs and provides three major benefits:
- Minimizes the safety hazards of hydrogen tanks
- Eliminates the inconveniences of hydrogen tanks
- Is considerably less expensive than hydrogen tanks
Minimizing safety hazards
When a hydrogen generator is employed, only a small amount of gas at low pressure is produced in a given period of time and the gas is ported directly to the chromatograph. Typically, the generator has a storage compartment that holds only 50 mL of stored gas at a maximum pressure of 4 atm. In contrast, if the contents of a full tank of hydrogen were suddenly vented into the laboratory, up to 9000 L of the gas would be released, creating the possibility of an explosion and/or reducing the breathable oxygen content of the atmosphere, thereby creating an asphyxiation hazard to the laboratory occupants.
When a new gas tank is required, the analyst must transport a tank from a secure storage area to the laboratory. A standard tank is quite heavy and can become a guided missile if the valve is compromised during transportation. With the hydrogen generator, there are no transportation issues and the output from the generator is permanently plumbed into the chromatograph. If a leak were to occur, there is little danger of explosion or asphyxiation as the quantity of gas is small.
When a hydrogen generator is employed, the gas is supplied on a continuous basis and can be provided on a 24 /7 basis if desired. In contrast, when tank gas is employed, tanks must be replaced on a periodic basis. If the need for replacement occurs during a series of analyses, the analyst must interrupt the analytical work to restart the system, wait for a stable baseline, and may have to recalibrate the system.
The hydrogen generator is a self contained unit that requires the user to simply add water on a periodic basis. The tank can be refilled during operation, so there is no down time. When a hydrogen tank is employed, it is necessary to replace the tank. In contrast, “multiple gas chromatographs can be operated with essentially no interaction with a hydrogen generator,” according to Dr. Lionel Nesbitt of Mastertaste Foods, a manufacturer of natural fruit flavors in Clark, NJ.
In many facilities, spare gas tanks are stored outside in a remote area (for safety reasons) and it may be time consuming to get a replacement cylinder. When it is necessary to get a tank, the chromatographer may need to get an individual who is qualified to handle the tanks. Many users, including Reza Bibiano of Genzyme, have indicated that replacing used tanks can be a significant inconvenience, especially in inclement weather if the tanks are stored outside.
In addition to the significant safety and convenience benefits, a hydrogen generator can provide a significant economic benefit compared to the use of gas tanks. The running cost of operation of a hydrogen generator is exceedingly low as the raw materials to prepare hydrogen are deionized water and electricity. On a periodic basis, the deionizer bag (which is used to filter the water that is recycled during the operation of the instrument) should be replaced; in most instances, the bag is replaced twice a year at an expenditure of approximately $100. It has been estimated that the running costs and maintenance for the hydrogen generator is approximately $225/year.
A recent cost estimate for a laboratory that uses two— three cylinders of hydrogen per week is in the range of $15,000–25,000/year. While the calculation of the precise cost of each approach for a given user is dependent on a broad range of local parameters and the amount of helium or hydrogen that is used, it is quite clear that the use of the hydrogen generator leads to a considerably lower cost than the use of tank gas.
When tank gas is employed, there are many hidden costs, including transportation costs, demurrage costs, and paperwork (e.g., a purchase order, inventory control, and invoice payment). In addition, the value of the time required to get the tank from the storage area, install the tank, replace the used tank in storage, and wait for the system to re-equilibrate after the tank has been replaced has an economic cost.
The cost benefit of a hydrogen generator increases dramatically as the number of gas chromatographs in the laboratory increases. As an example, the laboratory of Reza Bibliano of Genzyme operates six GC systems with hydrogen carrier gas using a single hydrogen generator. In the past, it was necessary to replace the hydrogen tank approximately every three weeks — now the single generator meets all the needs. The gas generator has been in operation for three years with no difficulties.
In a typical example, Henkel Loctite (Rocky Hill, CT), a manufacturer of high-technology sealants, adhesives, and coatings, required one tank of helium per week to supply carrier gas for each of two GC/FID systems in the analytical services laboratory. The out-of-pocket cost of the gas was over $8,500/year. When the lab moved to a new facility, it replaced the tank helium with hydrogen and obtained better quality separations for high-sensitivity methods. This approach saved nearly $20,000/year.
The use of hydrogen as a carrier gas for GC provides more rapid separations than nitrogen, with a minimum loss in chromatographic efficiency. The mode of supplying hydrogen is via the electrolysis of water. A hydrogen generator creates a steady stream of gas at a low pressure and stores a very small quantity of the actual gas, so that safety issues due to the potential of an explosion are dramatically minimized. In addition, the hydrogen generator is more convenient than tank gas, requires essentially no maintenance, and reduces the cost of hydrogen relative to the use of tank gas. A single hydrogen generator can provide the carrier gas for several GC systems as well as the gas needed for detectors.
Dr. Peter Froehlich has over 30 years of experience in the analytical instrumentation industry. He was awarded a Ph.D. from Purdue University and has a background in chromatography and spectroscopy. He is the President of Peak Media, 10 Danforth Way, Franklin, MA, 02038 and can be reached at 508 528-6145; firstname.lastname@example.org.