product review
Versatile TOFMS TOFMS is extending its reach into areas traditionally dominated by other MS instruments. Katie Cottingham
T
OF instruments (TOFs) have become real contenders in the MS market. Although the instruments’ share of the life sciences market is currently small, experts say that scientists are beginning to realize that TOFMS has many advantages, including exact mass determinations. “I think people are just waking up to the power of exact mass and the extra dimension that it brings to problem solving and structure elucidation and confirmation,” says Steve Smith at Waters Corp. (U.K.). “Only TOFs can deliver that.” Exact mass measurements are particularly important when one is trying to identify an unknown molecule. Knowing an ion’s m/z to four decimal places, for example, allows researchers to narrow down the list of possible identities, say experts. On the basis of this information, some computer programs can even pinpoint the exact combination of elements that must be present in the molecule or the chemical formula. TOFs are also less expensive and easier to use than some other MS instruments, such as sector and FT devices. According to Smith, TOFs “are now perceived as not being frightening.” He says, “We’re seeing people buy TOF technology now who, five years ago, wouldn’t have touched it at all.” And unlike scanning quadrupole or sector mass spectrometers, TOFs record all the ions in a batch of sample at once. “You get the entire spectrum every time you fly a batch of ions, and you can do this literally thousands of times per second,” says Chris Enke at the University of New Mexico. “The rate of spectral production by a TOF mass spectrometer is completely unmatched by any other technique.” Researchers also do not have to decide in advance which m/z range to analyze with a TOF instrument. The advantages of TOFs are so numerous, says Enke, that these instruments have replaced sector mass spectrometers for many applications. Experts say that researchers are starting to turn to TOFs for biological applications, such as biomarker discovery and metabolomics, which were traditionally studied with FTMS and triple-quadrupole instruments. Tables 1, 2, and 3, which are meant to be representative rather than comprehensive, list several commercially available TOF and TOFTOF tandem MS instruments. © 2005 AMERICAN CHEMICAL SOCIETY
Flying ions Molecules that are injected into a TOF mass spectrometer are first ionized by an ionization source. An electric field within the source accelerates the ions to a constant kinetic energy. The ions then fly through a field-free drift tube to a detector. If the charges are equal, lighter ions arrive at the detector before heavier ones. A reflectron, or ion mirror, is often used to enhance the resolution of TOFs. Ions encounter the reflectron at the end of the drift tube and are reflected back toward their starting point in a V-shaped path. Typically, the path length in a TOF device with a reflectron is longer than in a linear instrument. Experts say that the main effect of a reflectron, however, is to refocus ions of different energies. Robert Cody of JEOL USA describes an experience during a bicycle race that illustrates this principle: “The really fast guys missed the turn, realized that they had gone too far, and turned around. The rest of us had already made the turn,” he says. “At one point, the fast people and the slow people all were together for just a moment. That’s how a reflectron works.” The most common reflectrons have one or two constantfield regions separated by grids and are known as single- and dual-stage reflectrons, respectively. Others, such as the quadratic- and curved-field reflectrons, have more complex fields. “The dual-stage reflectron has higher inherent mass resolution than J U N E 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
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product review
Table 1. Selected orthogonal TOFs.1 Product
LC/MSD TOF
micrOTOF and micrOTOF focus
AccuTOF-GC
AccuTOF-LC
Company
Agilent Technologies 5301 Stevens Creek Blvd. Santa Clara, CA 95051 800-227-9770 www.agilent.com
Bruker Daltonik Fahrenheitstr. 4 D-28359 Bremen Germany, +49-421-2205-0 www.bdal.com
JEOL USA 11 Dearborn Rd. Peabody, MA 01960 978-535-5900 www.jeol.com
JEOL USA 11 Dearborn Rd. Peabody, MA 01960 978-535-5900 www.jeol.com
Price (U.S.D.)
$220,000
$200,000 micrOTOF; $225,000 micrOTOF focus
$179,900
$149,900
Ionization sources
Dual-ESI, dual-nanospray for trans- ESI, APCI, APPI, nanospray, parent introduction of reference AP-MALDI mass compounds, AP-MALDI, APCI, APPI, multimode ESI/APCI
EI; optional: CI, DCI, FD/FI
ESI; optional: APCI, Dual ESI, corona ESI, DART, Mass Tech AP-MALDI, nanoESI, ColdSpray
9
7
7
Total accelera- 6.5; optional 10 tion potential (kV) Fragmentation
In-source CID
In-source CID
EI
In-source CID
Mass range (m/z)
50–8000
50–20,000
4–2000
10–10,000
10,000 micrOTOF focus: >15,000
5000
6000
Software
Fully integrated LC/MS instrument control and acquisition and industry-standard data analysis; Protein Confirmation and Open Access software; Empirical Formula Generation with true isotopic pattern matching; Empirical Formula Confirmation; Confirmation Screening Report
Compass software for fully integrated LC/MS instrument control and data processing, including SigmaFit for molecular formula generation; full automation for LC/NMR/MS; Compass OpenAccess, Compass Security Pack, MetaboliteTools, and BioTools are options
MassCenter software with fully integrated GC/MS instrument control and data processing; software for elemental composition determination based on exact masses and isotopic patterns
MassCenter software with fully integrated LC/MS instrument control and data processing; software for elemental composition determination based on exact masses and isotopic patterns
Special features
Autotune and autocalibration for superior ease of use; 8-bit analogto-digital acquisition system optimized for both small-molecule and proteomics applications with wide dynamic range; integrated CE/MS system; ion polarity switching
True isotopic pattern with SigmaFit algorithm for analysis of the isotopic profile in addition to the accurate mass determination; full implementation in Metabolic Profiler with optional implementation of Avance NMR spectrometer (Bruker Biospin); micrOTOF-Q model is also available
Standard high-dynamic-range detection system with analogto-digital converter and vacuum isolation valve; 3 CI gas connections with electronic flow control; open access capability and U.S. FDA 21CFR11 software options
Standard high-dynamic-range detection system with analog-to-digital converter; open access capability and U.S. FDA 21CFR11 software options; optional DART ion source for rapid analysis under ambient conditions
AP: atmospheric pressure; CI: chemical ionization; CID: collision-induced dissociation; DART: Direct Analysis in Real Time; DCI: desorption chemical ionization; manufacturers; PI: photoionization 1 Some companies offer multiple instruments. Contact the vendors for their full product lines.
the single-stage, but the single-stage can focus a somewhat broader range of energies,” explains Robert Cotter of the Johns Hopkins University. Cotter’s group developed the curved-field reflectron, which he says is similar to the single-stage reflectron, except that the curved field gives it a higher bandwidth. He says that the curved-field reflectron is typically used to focus fragment ions. Although many reflectrons have grids, others are gridless. Experts say that the decision to have grids is a trade-off. “The problem with having grids on a reflectron is there is an abrupt 228 A
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change in the field when you come to the end of the drift tube. It’s like hitting a curb,” says Cotter. “There’s some scattering and some distortion.” But gridless systems are not without their problems. Cotter says that gridless reflectrons are less forgiving if an ion enters at an angle or off-axis.
Sources and detectors TOFMS is often mentioned in the same breath as MALDI because both are pulsed techniques. In MALDI, a laser shoots at a spot of sample on a plate. Ions produced by MALDI are
product review
prOTOF 2000
C-TOF and H-TOF
LCT Premier
Micromass GCT
PerkinElmer Life and Analytical Sciences 710 Bridgeport Ave. Shelton, CT 06484, 800-762-4000 www.perkinelmer.com
TOFWERK Feuerwerkerstr. 39, CH-3602 Thun Switzerland +41-33-228-5019 www.tofwerk.com
Waters Corp. 34 Maple St. Milford, MA 01757 800-252-4752 www.waters.com
Waters Corp. 34 Maple St. Milford, MA 01757 800-252-4752 www.waters.com
INA
$80,000–150,000
$210,000; depends on configuration
$185,000; depends on configuration
High-precision MALDI
Standard EI or coupled to any other ion source on customer demand
ESI, LockSpray and ESCi; optional: APCI, APPI, dual APPI/APCI, MUX (multiplexed LC/MS), nano-LockSpray
EI; optional: CI, FI/FD, DCI
16
5 –10
5.6
5.3
CID
INA
In-source CID
None
100–300,000
Only limited by acceleration voltage
1–10,000
1500
