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.
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: [email protected].