Generating Nitrogen for Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is an extremely important tool in the elucidation of molecular structure and is commonly used to obtain information about the interaction between molecules in complex samples such as biological systems. Since an NMR signal is temperature dependent, many users pass a gas over the sample to maintain it at a constant temperature. The use of nitrogen is typically recommended for high-field NMR systems and NMR systems with cryogenically cooled probes.

Requirements for flow rates depend on the equipment used, e.g., the rates are higher for magic angle spinning probes (typically >200 L/min) than for regular high-resolution probes (normally 5–20 L/min). For a sample temperature close to or slightly below room temperature, a precooling accessory in line is required to achieve sufficient offset in the thermostating gas for stable sample temperature regulation. The required dewpoint of the temperature regulation gas will depend on the use and the type of unit. Without precooling, a dewpoint of <4 °C is usually sufficient; for precooling units the required dewpoint is typically at least 10 °C lower than the coldest spot of the precooling unit.

In some laboratories, nitrogen gas is provided by purchased cylinders or Dewars; however, use of an in-house generator to provide the gas is safer, more reliable, more convenient, and more economical. In addition, an in-house nitrogen generator is completely automatic and requires minimal maintenance. This paper describes how nitrogen can be generated in-house from a standard compressed air supply, and considers the various benefits of this approach for the NMR laboratory.

Design of an in-house nitrogen generator

The design of a typical in-house nitrogen generator is shown in Figure 1 (Parker Balston model N2-45, Parker Hannifin Corp., Haverhill, MA). The generator includes the following:

Figure 1 - Schematic of in-house nitrogen generator.

  • A prefiltration system has a coalescing filter that consists of borosilicate glass microfibers with a fluorocarbon resin binder, an activated carbon filter, and a particulates filter. The system removes liquids and particulate matter larger than 0.01 μm from the incoming air supply particulates to protect the membrane module from contamination, and is designed to withstand the dynamic changes in pressure, temperature, and airflow that occur on a regular basis in a typical compressed air system. Automated drains are provided to deliver the collected liquids to waste.
  • A hollow-fiber membrane permits oxygen and water vapor to permeate the membrane and escape through the sweep port while the nitrogen flows through the tube (Figure 2). While each individual fiber membrane has a small internal diameter, a large number of fibers are bundled together (Figure 3) to provide an extremely large surface area for the permeation of oxygen and water.

Figure 2 - Separation of nitrogen from compressed air via a membrane.

Figure 3 - Fiber bundle of nitrogen separation membranes.

  • A high-efficiency filter after the hollow-fiber membrane is provided to ensure that any particulate matter in the gas is removed. The final membrane filter removes particulate contamination to 0.01 μm (absolute) and ensures that a clean supply of high-purity nitrogen is provided to the sample compartment of the spectrometer.

Figure 4 - Parker Balston model N2-45 High-Flow Nitrogen Generator with Bruker 600 MHZ NMR system (Bruker GmbH, Bremen, Germany) (courtesy of Agriculture and Agri-Food Canada [Charlottetown, Prince Edward Island, Canada]).

The Parker Balston model N2-45 High Flow Nitrogen Generator (Figure 4) can provide up to 133 L/min of 99.5% pure gas at a maximum inlet pressure of 145 psig. The flow rate of gas is dependent on the inlet pressure; at a typical inlet pressure of 80 psig, the flow rate is 60 SCFH (23 L/min). If higher flow rates are desired, larger-capacity units are available. Nitrogen gas is hydrocarbon-free and phthalate-free and is suitable for high-sensitivity measurements. A series of alarms is provided in the event of problems, and relays can be employed to alert the operator. In addition, an optional oxygen monitor that employs a galvanic cell can be fitted to the system, which also includes relays to send a warning signal to an external device (i.e., stop data collection if the oxygen content is too high).

Providing nitrogen to an NMR facility using an in-house nitrogen generator

An in-house nitrogen generator offers a number of important benefits to the NMR spectroscopist since it minimizes safety hazards, increases convenience, reduces the overall cost of operation, and decreases the environmental impact of supplying the requisite nitrogen compared to the use of a purchased gas tank or a Dewar flask. Many NMR manufacturers recommend the use of nitrogen. Typical users include:

  • Dr. John Cavanaugh, Professor of Biochemistry at North Carolina State University (NCSU) (Raleigh), uses time domain NMR to study how anthrax samples react to environmental stress with the goal of eventually developing new therapeutic targets (anthrax bacteria are extremely stable and maintain their properties in boiling water). In these studies, it is necessary to maintain the physiological temperature of anthrax samples.
  • The University of Guelph NMR Centre (Guelph, Ontario, Canada) is equipped with six modern, high-resolution NMR spectrometers that are capable of a diverse array of solution and solid applications. The Centre provides service to all academic communities on campus as well as to commercial organizations, and uses Parker Balston nitrogen membrane-based generators to provide superdry, high-purity nitrogen to 600-MHz and 800-MHz NMR systems with cryoprobes.
  • The Crops and Livestock Research Center of Agriculture and Agri-Food Canada uses an in-house nitrogen generator in conjunction with an Atlas Copco GA-11 compressor (Stockholm, Sweden) to provide 99.5% nitrogen for its Bruker 600 MHZ NMR system (Figure 4) to study a variety of samples in metabolomic research.

Benefits of in-house nitrogen generation

Minimizing safety hazards

When an in-house nitrogen generator is employed, only a small amount of the gas is present at a low pressure at a given time and the gas is ported directly to the NMR spectrometer. For example, the Parker Balston model N2-45 generates a maximum of 67 L/min of gas at a maximum pressure of 125 psig. In contrast, a number of serious hazards can exist when purge gas is supplied to the instrument via a high-pressure tank. A full tank has a pressure of >2000 psi, and a leak could displace some of the air in the laboratory, potentially leading to asphyxiation of laboratory personnel. In addition, significant hazards can exist during the transport and installation of a gas tank. A standard tank is quite heavy, and can become a guided missile if the valve on a full tank is compromised during transport (in many facilities, specially trained technicians replace the gas tanks). Additionally, use of a pressurized tank can be extremely hazardous in earthquake-prone areas. These hazards are eliminated with an in-house generator. In some facilities, a Dewar-based system is used to supply the gas, which can result in freezer burns since the temperature of liquid nitrogen is –195.8 °C.

Convenience

An in-house nitrogen generator can provide gas 24 hr/7 days a week without any user interaction other than routine replacement of the coalescing prefilter. Dr. Chris Kirby, a physical chemist who manages the NMR facility at Agriculture and Agri-Food Canada, reports that his facility has had the nitrogen generator for over two years without needing any service, other than changing the filters twice a year (which takes approximately 2 hr). In contrast, when tank gas is employed, the user must pay close attention to the level of gas in the tank and replace it periodically, which can be time-consuming. If the gas in the tank is exhausted, the timely collection of useful spectra may be compromised; if the NMR is collecting data on an unattended basis, important data may be lost. An additional concern is that when a tank is replaced, air could enter the system. If this happens it may be necessary to bleed the lines before continuing operation. Thomas Buser, Operations Manager at Bruker Ltd. (Milton, Ontario, Canada), reports that many users employ Parker Balston nitrogen generators with the facility’s high-frequency NMR systems since the in-house systems provide a reliable and convenient supply of nitrogen.

In many facilities, spare purge gas tanks are stored outside in a remote area for safety reasons, and it is time-consuming to get a replacement tank. When this is necessary, the analyst may have to hire an individual who is qualified to handle the tanks. Many spectroscopists have indicated that replacing used tanks can be a significant inconvenience, especially in inclement weather if the tanks are stored outside.

A high-resolution NMR employs a powerful magnet, and special care must be taken when nitrogen tanks are used that are made from steel. The tank must be kept far enough away from the magnet so that it is not attracted by the magnet, and changing tanks requires caution to ensure that the magnet does not affect the movement of the tank (for example, if a hand tool is dropped in the general vicinity of the magnet, it will not hit the floor).

If a tank needs to be replaced during a series of analyses, the analyst must interrupt work to replace the tank and wait for a stable baseline. In addition, if a series of automated analyses are desired (i.e., overnight or on a weekend), the analyst must ensure that a sufficient volume of the gas is on hand before starting the sequence. According to Valerie Robertson, NMR facility manager at the University of Guelph, the in-house generating system provides a consistent flow of nitrogen with minimal maintenance. A maximum flow of 160 L/min of nitrogen (98% pure) is readily obtained with 120 psi inlet air, which meets the stringent requirements of the facility.

The frequency of tank replacement depends on usage of the system. Changing a nitrogen tank is clearly an inconvenience, and leads to a reduction in the useful operating efficiency of the facility. In addition to the actual time required to change the tank, the laboratory staff must verify that there are sufficient replacement tanks in storage and order replacement tanks as appropriate. The use of an in-house nitrogen generator eliminates the need to keep track of and change gas cylinders.

Cost

The cost of obtaining the necessary gas using an in-house nitrogen generator is considerably lower than that of obtaining the gas from external sources. The total cost of operation of an in-house nitrogen generator is extremely low, since the raw materials needed to prepare the required gas are laboratory air and electricity. Running costs and maintenance for the generator add up to a few hundred dollars per year; many users find that the payback for an in-house system is a year or less.

Dr. Cavanaugh at NCSU originally used tank gas from a local gas supplier, which cost approximately $100/tank to support the facility’s NMR systems (Bruker 700 NMR and Varian 600 NMR [Palo Alto, CA]). NCSU’s usage of nitrogen necessitated that the tank be replaced weekly, which was a major expense and inconvenience; in addition, laboratory personnel had to monitor the supply of the gas continually. Dr. Cavanaugh reported that the instrument paid for itself very quickly by eliminating the need to purchase gas Dewars and cylinders.

In addition to the actual price of a nitrogen tank, the overall cost for using tank gas includes the time and cost of obtaining the gas tank as well as the value of the time involved in changing tanks, the required paperwork (e.g., generating a purchase order and payment of the invoice), maintaining inventory, and related activities. While the exact cost of using nitrogen gas from tanks for a given user depends on a broad range of local parameters and the amount of gas used, considerable savings can be obtained by using an in-house nitrogen generator.

Environmental benefits

An in-house nitrogen generator should be considered a “green” solution because it dramatically reduces the amount of energy required to provide the gas to the spectrometer compared to the use of a tank or Dewar. The energy requirements for the use of an in-house nitrogen generator are very low; the generator itself does not require any power (the oxygen analyzer uses 25 W) and the only power that is needed is for the generator. In contrast, when nitrogen tanks are employed, the gas is normally obtained by the fractionation of liquid air and is purified and then compressed to approximately 2000 psi. Once the tank is filled, it must be transported to the end user’s site, and the empty tanks must be returned to the supplier. While the amount of energy required for transportation of the tanks depends on the distance between the end user and the supplier, it is clear that a significant amount of energy is expended when gas tanks are employed. The use of an in-house generator can contribute to the reduction of overall energy in the laboratory.

Reliability

An important benefit of an in-house nitrogen generator is its reliability— It has no moving parts and is very robust. This reliability is important because it minimizes the amount of time and effort that must be spent to support the NMR system, allowing more time for the acquisition of spectra. According to Dr. Cavanaugh, “it is a very simple device; if we didn’t have it our lives would be much more complicated.”

Conclusion

In-house generation of nitrogen for NMR spectrophotometers provides the analyst with a significant benefit in terms of safety, convenience, reliability, and cost. In addition, the use of an in-house generator reduces the overall environmental impact of supplying the gas compared to the use of tank gas. Nitrogen is generated from laboratory air using membranes that allow water vapor and oxygen to diffuse while porting the nitrogen directly to the NMR spectrometer. An in-house nitrogen generator eliminates the substantial cost of purchased tanks. Many users have found that it pays for itself very quickly because tanks do not need to be purchased. The generator requires essentially no user interaction except for periodic filter replacement. It can operate 24 hr/7 days a week, and eliminates the need to replace tanks on a periodic basis.

Mr. Kriwoy is District Manager, Parker-Hannifin Corp., Haverhill, MA, U.S.A. Dr. Froehlich is President, Peak Media, 10 Danforth Way, Franklin, MA 02038, U.S.A.; tel./fax: 508-528-6145; e-mail: pfpeakmedia@msn.com.

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