Consumption of nitrogen for LC-MS depends on various factors such as the flow rate for the chromatograph, the mobile phase composition and the nature of the detection technique. Various in-house generators are available with flow capacities ranging from 30 L/min to >200 L/min.

In-house generation of nitrogen from ambient air involves separating the gas from oxygen, water vapor and particulate matter and porting it into the LC-MS. The heart of an in-house nitrogen generator is a hollow fiber membrane which permits oxygen and water vapor to permeate the membrane and escape through the sweep port while the nitrogen flows through the tube. While each individual fiber membrane has a small internal diameter, a large number of fibers are bundled together to provide a large surface area for the permeation of oxygen and water.

Compressed air is filtered using a high efficiency activated carbon filter to remove hydrocarbons while a pre-filtration system removes particulates down to 0.1 micron and delivers the gas to the membrane bundle. Oxygen and water that permeate the membrane are vented and purified nitrogen is passed through another membrane filter and delivered to the LC-MS.

Parker-Hannifin’s N2-45 nitrogen generator can continuously produce pure nitrogen. The purity of this nitrogen depends on the operating pressure and the desired flow rate; for example, 67 L/min of 99.5% nitrogen gas can be generated at an operating pressure of 145 psi and 68 F, which is sufficient to supply several LC-MS systems. The gas delivered to the LC-MS by the N2-45 has an atmospheric dew point of -50 C, contains no particulate matter >0.01 µm, and is hydrocarbon- and phthalate-free and commercially sterile.

Benefits of a gas generator

The in-house nitrogen generator provides a number of significant benefits to LC-MS operators including improvements in safety, increased convenience, the possibility of improving the sensitivity of the assay and reduced cost of supplying the gas.

— SAFETY —
When in-house nitrogen generators are used, only a small amount of the gas is present at a low pressure at any given time. The N2-45 generates a maximum of 133 L/min of nitrogen at a maximum pressure of 140 psig. If a generator leak occurred, there would be a small change in the composition of the lab air with only trace nitrogen
dissipating harmlessly.

In contrast, serious hazards exist when nitrogen is supplied in tanks. For example, if the contents of a full tank are suddenly vented into the lab, up to 9,000 L of nitrogen can be released. This volume could displace the air, reduce the breathable oxygen content and create an asphyxiation hazard. Another hazard that’s eliminated by in-house generators is the possibility of injury or damage during transport and installation of the tanks. A standard tank is heavy and can become a guided missile if the valve is compromised during transport.

When a Dewar is used to supply nitrogen, the possibility of user contact with the cryogenic liquid (with a b.p. of -196 C) must be considered. As with tank nitrogen, a leak in the delivery system could create a significant volume of gas in the lab.

CONVENIENCE
With in-house generators, the nitrogen can be supplied continuously and provided 24/7 without any user interaction other than routine maintenance. In contrast, when tank gas is employed, the user must monitor the tank level and periodically replace it. An in-house system obviates the need for replacements.

If tanks are used and replacement is required during an analysis, the analyst must interrupt his work to restart the system, wait for a stable baseline and possibly even recalibrate the system. In addition, if a series of automated analyses is desired, the analyst must ensure that a sufficient volume of gas is on hand before starting the sequence.

Using a gas generator saves a considerable amount of time and increases efficiency since it’s not necessary to re-calibrate the system. A standard sample is analyzed on a periodic basis which takes just a few minutes. An additional benefit is that it’s unnecessary to train each technician in the calibration process.

A large number of labs have converted to in-house nitrogen generation. Chris Beecher, a professor of pathology at the Univ. of Michigan, recently obtained an in-house system for his LC-MS because of the convenience and stability it provides.

Similarly, A. Daniel Jones, director of the Mass Spec Lab at Michigan State Univ. has been using in-house generators for several years and reports that "no maintenance is required with the three systems we use. One system has been operating for more than 26,000 hr and the only maintenance has been annual filter replacement, which takes just a few minutes."

The frequency of tank replacement clearly depends on system usage. Changing tanks is clearly an inconvenience and leads to reduced operating efficiencies. In addition to the actual time required to change tanks, staff must verify there are replacement tanks in storage and order them as needed. The use of an in-house generator eliminates the need to keep track of and change gas cylinders. Similarly, if Dewars are used, the lab must rely on the secure delivery of the LN2, which is questionable in inclement weather or during holidays.

In contrast, the cost for using nitrogen gas from tanks includes the actual cost of obtaining the gas tank as well as the time involved in changing tanks, ordering new tanks, maintaining inventory, and support. The calculation of the precise cost of nitrogen from tanks for a given user depends on a broad range of parameters and the amount used.

It should be noted that there are hidden costs, including transportation costs, demurrage costs and paperwork when tanks are used. In addition, the value of the time required to transport the tank from the storage area, install the tank, replace the used tank in storage and wait for the system to re-equilibrate after replacement also involves costs.

A comparison of the costs of supplying tank gas versus in-house generators is presented in Table 3. In this analysis, we assumed that a single tank of gas is consumed each week and that the cost of each tank is $60 (this approximation ignores the incidental cost of handling the gas tank, down time and ordering tanks). In comparison, the cost of using the in-house generator is approximately $20/wk.

SENSITIVITY
When an atmospheric pressure chemical ionization interface is employed, it’s been found that raising the oxygen content can increase the sensitivity of the detector. If tank gas is used, that gas is normally extremely high purity, and the oxygen content is fixed—changing it may require considerable plumbing.

For example, Dr. Jones routinely uses 99% nitrogen for his LC-MS systems. If he wanted a different concentration for enhanced sensitivity or to eliminate the oxygen, he could simply change the pressure of the input gas.

In conclusion, in-house generation of nitrogen can provide the necessary gas for LC-MS systems with significant improvements in safety and convenience compared to tank nitrogen or LN2 from a Dewar. The overall cost is also lower and the operator can rapidly change the composition of the gas by simply varying the input pressure to optimize sensitivity of the procedure.