Proteomics Mass Spectrometry Facility Saves Time, Money and Space By Using a Nitrogen Generator
The Johns Hopkins School of Medicine Mass Spectrometry Facility is saving time, money and space by generating its own nitrogen gas, according to Robert N. Cole, Ph.D., facility director. "When we first opened this facility, we obtained nitrogen gas from cylinders and discovered that we were changing tanks daily to prevent interruption in the nitrogen supply" Cole said. "So we invested in a nitrogen generator and compressor that produces all of our nitrogen requirements. It takes up less space than a single cylinder and, best of all, almost never needs any attention. Based on cylinder costs alone the gas generator and compressor will pay for itself in less than two years. In addition, it saves me from spending time checking gas levels and setting up tanks. The unit provides very high purity gas and has run reliably since we put it into operation."
Of all the medical schools in this country with associated basic science research, Hopkins has perhaps the longest-standing reputation. It was the first institution, in the late 1890s, to separate teacher-researchers from clinicians and offer them a salary for their work. Many of this country’s first true basic scientists gathered at Hopkins then, eager to set up laboratories. William Henry Howell, for example, sparked physiology research and first laid down a clear explanation of blood clotting. Today, researchers in the School of Medicine bring to Hopkins more National Institutes of Health research funding than to any other medical school. For the 11th consecutive year, The Johns Hopkins University School of Medicine was one of the top two medical schools in the nation, according to U.S. News & World Report’s annual ranking. Hopkins was a close second to Harvard with a score of 94, up dramatically from 73 points last year.
New mass spectrometry facility
Johns Hopkins recently opened a new mass spectrometry facility designed to service the entire university. Most of their work involves the identification of proteins or peptides that researchers have isolated and are trying to identify. The facility has two mass spectrometers, the most advanced is the QSTAR hybrid liquid chromatograph/mass spectrometry system from Applied Biosystems, Inc. For scientists engaged in identifying metabolites or potential-lead compounds, the QSTAR system offers high mass accuracy to determine the compound’s elemental composition and fragmentation analysis to obtain structural information for compound identification. Parent molecules or fragment ions from the quadrupole section enter the ion accelerator and are pulsed into the flight tube. The ion mirror reverses the direction of the ions and corrects for small energy differences in ions. The detector records the precise arrival time of each ion and generates the signal to form the mass spectrum.
Nitrogen is a critical requirement for the QSTAR as well as nearly all state-of-the-art mass spectrometers. It is used to form a curtain of gas behind the inlet of the instrument that prevents air from entering along with the sample. Curtain gas must be maintained at higher than atmospheric pressure so it continually seeps out of the inlet and requires continual replenishment. Nitrogen is also used as a collision gas. After ions enter the instrument, they are accelerated and made to collide with a second reservoir of nitrogen. The purpose of the collision gas is to break up sample molecules to determine their composition.
Solving the nitrogen supply problem
When Johns Hopkins first purchased the QSTAR, they were faced with the issue of how to provide nitrogen gas to their new instrument. Cole decided to try purchasing nitrogen cylinders from a local gas supplier. He soon discovered that the cylinders needed to be changed at least once and sometimes twice a day. This meant that he had to pay close attention to the amount of gas in the cylinder and, when it was nearly empty, take the time to change the cylinder and order a replacement. The time spent dealing nitrogen supply subtracted from the amount time that he had to set up analysis runs and manage the facility. In addition, he had to keep two or three cylinders on hand at all time to avoid a supply disruption that could shut down the laboratory. The problem was that the cylinders are quite bulky and the laboratory is small, creating a substantial space problem.
Cole investigated the new breed of nitrogen generators that eliminate the inconvenience and cost of cylinder gas supplies. He made the decision to purchase the Balston 75-A74 membrane nitrogen generator and AGS-L189 compressor from Parker Hannifin Corporation, Filtration and Separation Division, Tewksbury, MA. The system utilizes proprietary membrane separation technology. The generator separates air into its component gases by passing inexpensive, conventional compressed air through bundles of individual hollow fiber, semi-permeable membranes. Each fiber has a perfectly circular cross-section and a uniform bore through its center. Because the fibers are so small, a great many can be packed into a limited space, providing an extremely large membrane surface area.
Three stages of pre-filtration
Three stages of coalescing pre-filtration are incorporated into the Balston 75-A74 nitrogen generator to protect the membrane module from contamination. These filters are located behind the filtration access panel, and they remove liquids, airborne organics and particulate matter from the incoming air supply. The filters are equipped with float drains, which automatically open to empty any accumulated liquid inside the filter housing. The drains are connected to ¼" O.D. plastic tubing which discharges to atmosphere at the back of the nitrogen generator.
Air separation takes place in the membrane module. The module consists of bundles of hollow fiber membranes. The inlet air enters the center bore of these fibers and travels the length of the fibers. As the air passes through these hollow fibers, oxygen and water molecules pass through the membrane at a higher rate than nitrogen molecules. This results in a high purity, dry LC/MS grade nitrogen gas exiting the membrane module. The oxygen enriched permeate stream exits the membrane module through ports on the side of the module at very low pressure.
Final membrane filtration
The final filter, a 0.01 micron absolute membrane filter, provides clean, commercially sterile supply of high purity nitrogen. The controls on the nitrogen generator consist of an operating pressure gauge, a flow meter and flow control valve, an outlet pressure regulator and final gauge. Proper use of these controls assures the user of a 99% to 99.95% LC/MS grade nitrogen outlet stream, depending on operating pressure and flow rate. The pressure gauges, which are mounted on the front panel, measure operating pressure and outlet pressure. The flow meter measures the flow rate of nitrogen exiting the membrane module.
The generator requires virtually no attention because it uses simple electromechanical components such as pressure vessels, and valves with a history of reliability in laboratory applications. A key factor in the increased reliability provided by the generator is its elimination of the logistics of the gas supply chain. Since the nitrogen generator simply separates air into its constituent parts, it has no adverse environmental effects. Both the nitrogen produced by the unit and the oxygen mixture generated as a byproduct can be released into the atmosphere. Gas generators are also much safer than high-pressure cylinders. The generator typically operates at a low pressure in the neighborhood of 100 psig and stores small volumes of compressed gas. The stored volume is much less than 1 cubic foot, compared to about 300 cubic feet stored in a typical high-pressure gas cylinder. Gas generators also eliminate the need to handle cylinders, which presents a risk of injury caused by dropping, lifting, or asphyxiation.
"The nitrogen generator now supplies all curtain and collision gas required by the QSTAR mass spectrometer," Cole said. " It basically runs by itself without requiring any attention from me. It eliminates the need to keep track and change gas cylinders. Most important, it eliminates having to worry about whether we have enough gas to continue in operation until new cylinders are delivered. It saves a considerable amount of space. The gas generator is less than the size of a single cylinder. The gas generator also saves about $1,000 per month in cylinder costs. Including the compressor and storage tank, it cost about $20,000, so it will pay for itself in cylinder costs alone in less than two years. The unit has provided trouble-free operation since it was installed. The only interruption of supply that I have had was when someone accidentally turned off the compressor, which is located in a different room. All in all, it has met all my requirements, providing high quality gas without disrupting my work."