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Top-of-the-line MS FT-ICR MS is finding applications in areas where high resolution and exacting structural determination are required. Gerald Keller
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n the world of mass spectrometers, Fourier transform ion cyclotron resonance MS (FT-ICR MS, or simply FTMS) is often considered the top end of performance. Providing meaningful spectra for much larger analytes than any other type of mass spectrometer, the big superconducting magnet FT-MS instrument is a valuable tool for genomics, proteomics, and pharmaceutical research. Alan Marshall, director of the ICR Program in the National High Magnetic Field Laboratory at Florida State University and a pioneer in developing FT-MS instruments, notes, “FT-ICR MS resolving power is 10–100 times higher than TOF or quadrupole ion traps, both in one-dimensional and tandem MS/MS experiments.” It lends itself to exact mass work in smaller molecules with m/z values of 1 billion theoretical plates (in chromatographic terms), often allows skipping one or more separation steps. FT-MS spectra can be used to determine the composition of the analyte, which is more exact than the fingerprint that chromatography offers. And finally, mass spectra are obtained and analyzed in seconds. This is appreciated nowhere more than in the combinatorial chemistry and drug development area. However, with price tags ranging from just over $300,000 to $1.5 million, the FT-MS market is limited.
Thus, manufacturers tend to specialize in certain applications to fill individual sales niches and avoid redundancy between companies. Marshall estimates that three companies have just about equally divided sales, with another 10% of all FT-MS instruments being homemade.
Only two companies, Bruker Daltonics and IonSpec Corp., currently market FT-MS instruments with superconducting magnets. Finnigan discontinued sales of their FT-MS system in 1999. The discontinued instrument, the Dual Cell Finnigan FT-MS, actually evolved from
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Table 1. Selected features of FT-MS instruments. Company
System
Magnets (T) Ionization sources
Vacuum pumps Special features
Applications
Bruker Daltonics, Inc. Manning Park Billerica, MA 01821 978-663-3660 fax: 978-667-5993 www.daltonics.bruker.com
APEX III
3.0, 4.7, 7.0, and 9.4
Standard turbomolecular, optional cryopump
Rapid ion source changeover, complete automation packages, IR multiphoton speeds MS/MS
Combinatorial $500,000 libraries, drug and up development, proteomics, and genomics
IonSpec Corp. 16 Technology Dr. Ste. 122 Irvine, CA 92618 800-GET-FTMS fax: 949-753-9121 www.ionspec.com
HiResMALDI 4.7, 7.0, and 9.4 for all models
MALDI, ESI, EI, and Turbomolecular Vacuum pumps nanospray or cryopump decouple and can be swapped for servicing without downtime
Exact mass and $310,000 drug-structure confirmation, proteomics, and genomics $368,000
ESI, MALDI, zero adjust nanospray, APCI, EI, and CI
HiResESI Ultima
Approximate cost (USD)
$498,000 Prices for 7.0 T with active shielding
an instrument sold by Nicolet Instruments Corp., which marketed the first commercial FT-MS instrument in 1980. The same year that Finnigan stopped selling the system, the company moved its FT-MS facility to Bremen, Germany. According to Finnigan, they are developing a new FT-MS instrument; however, the company will neither confirm nor deny rumors that it is close to being unveiled. Because the March Pittcon meeting is a traditional time to introduce new products, readers should stay tuned. FT-MS technology is not limited to large, expensive superconducting magnets. Several permanent magnet “benchtop” units are now being sold by Nikkiso; Polychip, Inc.; Siemens; and other firms. These systems use magnets with fields ranging from ~0.4 to 1.0 T and can handle m/z values from 1 to ~1000 Da. These are mostly used for process control, such as monitoring a flow stream for specified components, and will not be covered in this review. Table 1 lists specifications for the Bruker Daltonics and IonSpec systems. Interested readers should contact these companies for further information about their instruments.
Playing the magnetic field Most of the cost of an FT-MS system is due to the superconducting electromagnet. Each company offers a range of magnets, and price increases dramatical100 A
ly with the strength of the field. For instance, IonSpec’s HiResMALDI FT-MS system costs ~$310,000 with a 7.0-T magnet. For the same model with a 9.4T magnet, expect to pay an additional $300,000. But Richard Hunter, cofounder of IonSpec, points out that their 4.7-T and 7.0-T systems are now close in price. The 7.0-T magnet now dominates IonSpec’s sales. As superconductor technology improves, stronger magnets will arrive on the market. The once state-of-the-art 7.0-T magnet no longer demands the premium price of a 9.4-T. Bruker Daltonics and IonSpec both offer commercial systems with up to a 9.4-T magnet. As the magnetic field strength increases, the power of the instrument increases on many fronts. Resolution improves linearly with the magnetic field. The upper mass limit increases with the square of the magnetic field, as do the length of time that an ion can be trapped and the number of ions that can be trapped. “Everything gets better [with stronger magnets],” says Marshall. What can be achieved with an FT-MS system really depends on advances in superconductor magnet technology. These state-of-the-art magnets must be cooled by liquid helium to operate efficiently and, thus, have an operating temperature of ~4.2 K. However, by pumping the liquid helium in the magnet’s dewar, the helium boiling point can be lowered
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to a truly frosty ~2 K. As a result, says Marshall, the field strength can be increased by 10–15%, because more current can be run through the magnet coil. He expects to soon receive from Oxford Instruments the first pumped magnet built for an ICR instrument. Another improvement is a closed refrigeration system, which recirculates helium, instead of the usual method, which boils it off. This will lead to more economical systems. Finnigan delivered a closed cooling system magnet to Eli Lilly last year.
Shielding Strong magnetic fields must be shielded for two key reasons. First, they pose a hazard to electronic media, equipment, and people. Second, outside fields can interfere with the experiment in the mass spectrometer’s ICR cell. There are two classes of magnetic shielding: passive and active. Marshall claims the best shielding is passive, which he describes as >10 tons of soft iron. But with all that weight, it is good to have a sturdy, ground-floor laboratory, or at least one with a reinforced structure. Thus, it is no surprise that all commercial FT-MS instruments are offered with active shielding. These magnets contain “opposite windings” that counter the inner magnetic field, which results in a physically shorter magnet, and make ion introduction more efficient.
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But active shielding is still undergoing improvements and may not completely contain the field.
Data-handling systems One big limitation to FT-MS systems is data analysis. According to David C. Muddiman of Virginia Commonwealth University, FT-MS instrument builders are “far behind” in developing data analysis systems. “As data analysis goes, I think both [companies’] software packages fall extremely short compared to [data packages with other types of] commercial mass spectrometers,” says Muddiman. “When you get into FT-ICR, it’s pretty much manual. That’s its limitation right now, but it’s being taken care of.” In the meantime, research groups are often faced with creating unique analysis software for their applications. Muddiman is currently working with IonSpec to create an analysis package. He is also working with Marshall to develop a beta version of software that will provide genomic information from FT-MS data. IonSpec is also marketing a special version of their OMEGA FTMS data system for older Nicolet, Extrel, and Finnigan machines. Recent increases in computing power have allowed FT-MS data stations to be Pentium PC-based. Bruker just released the XMASS software package for managing a wide range of experiments with their APEX line. They now cover applications such as automation, chromatography, and proteomics. In addition, Bruker BioTools for proteomics is compatible with its FT-MS system.
The commercial landscape J. Paul Speir, an FT-MS product manager, notes Bruker is marketing a thirdgeneration instrument, APEX III, which boasts a reduced-size vacuum cart, a smaller footprint, and new electrospray ionization (ESI) and MALDI sources. More software for automation, autotuning, and interfacing with an HPLC instrument is also available with the APEX III. According to the manufacturer, the APEX system offers quick changes between ESI and MALDI sources. IonSpec sells the HiResMALDI,
HiResESI, and Ultima FT-MS systems. The two HiRes systems are specific to either MALDI or ESI sources, respectively. They differ because the vacuum pumps are on tracks that roll in and out of the magnet; the MALDI system rolls in from the left side, electrospray from the right. Although each of these systems is limited to one method of ionization, they may be the most cost-effective choices. If both sources are needed, IonSpec offers the Ultima for ~$498,000. IonSpec says it is currently working on interfacing its FTMS instrument with sample automation.
Ionization MALDI has been considered the more reliable and easier-to-use source for FTMS instruments. However, Muddiman says, “For the analysis of large molecules and noncovalent complexes [such as protein–protein interactions], it is, and has been, clear that ESI will remain the ionization source for FT-MS.” MALDI, he says, is not as “soft” as ESI. “[MALDI] precludes analysis of fragile complexes and primarily produces singly charged ions,” says Muddiman. “The [singly charged ions] wind up in the cyclotron’s low-kilohertz region. These require a longer observation time in order to achieve high resolution, which limits the applications to smaller molecules to
