Report
The New Time-of-flight
Mass Spectrometry Performance results have finally laidtorest TOF's reputation as a low-resolution mass technique.
the duty cycle, hybrid TOF instruments, and the progress that has been made in miniaturizing instruments. Applications of these new TOF instruments to various problems also will be discussed. The TOF and mass resolution
M
ass spectrometers, it seems, are no longer just for mass spectrometrists. Mass spectrometers have been so successful solving structural problems in biochemistry, immunology, genetics, and nearly every other area of biology that scientists in these fields have begun to purchase and operate their own instruments. And it is well known to these new users that the remarkable success of today' s mass spectrometers is the result of two new methods for creating ions from biological macromolecules: electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) What is perhaps less well known is that improvements in the design and performance of the mass analvzer leading to user-friendlv instruments which are annroachino- benchtop and portable dimensions have also contributed to this success.
ble the use of virtually any ionization source. This Report will describe reflectrons for improving mass resolution, orthogonal injection for improving mass resolution and
Remarkably, the TOF begins as a very simple instrument (Figure la). Ions are formed in the source and accelerated to a constant kinetic energy (eV) as they enter the drift region (D), and they arrive at the
This is particularly true of the time-offlight (TOF) mass analyzer (2), which is the means for measuring ion masses with MALDI. Though traditionally regarded as a low-resolution mass analyzer, some extraordinary developments in TOFMS—in some instances borrowing from techniques of the past—have greatly improved mass resolution and made possi-
Robert J . Cotter Johns Hopkins University School of Medicine
Figure 1 . TOF mass spectrometer designs. (a) Linear TOF mass spectrometer showing ions formed with different initial kinetic energies, ions formed at different locations in the ion source, and ions focused at the space-focus plane, (b) Singlestage reflectron mass spectrometer in which the ions are retarded, turned around, and focused by a constant electric field constructed from a series of lenses with linearly increasing voltages. Analytical Chemistry News & Features, July 1, 1999 445 A
Report
Figure 2. Two-stage extraction source and potentials, and pulsed extraction schemes. (a) General diagram for a two-stage ion source. Specific pulsing schemes used by (b) the WileyMcLaren instruments manufactured by Bendix, (c) PerSeptive Biosystems, and (d) BrukerFranzen Analytik.
detector following flight times (t) that are proportional to the square root of their masses, as shown in Equation 1. /
\ 1/2
*=M D
(i)
A mass spectrum is created because ions of different mass arrive at the detector at different times. However, resolution can suffer because ions of the same mass do not all arrive at the detector at exactly the same time. One reason for this is that the ions will generally have some initial kinetic energy distribution (Alf0). As ions enter the flight tube, their kinetic energies (eV+ AUf) are not constant. This energy variation results in different velocities and different flight times for ions with the same mass. Secondly, the field (E) in the source is in units of volts/s and Equation 1 assumes that the ions all travel the distance (s) within the Generally there is some initial SDatial distribution (As ) again causions to enter the drift rerion with
different flight times. These ions all pass one another at the so-called space-focus plane (ions 1 and 3 in Figure la)) but they do not arrive at the detector at the same time. (As shown in Figure la, the detector is generally situated further down the fieldfree drift region.) Time-lag focusing. In 1955, Wiley and McLaren (2) approached the resolution problem by compensating for these two initial distributions simultaneously. In their dual-stage source, space focusing was achieved by using two extraction regions in which the lengths and relative field strengths moved the space-focus plane to the location of the detector. Kinetic energy focusing was achieved by introducing a time delay between ion formation and ion extraction from the source which is called time-lag focusing. During that time the ions rearranged their positions in the according to their initial velocities so that those with higher velocities traveling in the direction of the mass analv7er will train lf*cc f»n**rm7 from thf* p v t r a c t i o n
field when thatfieldis switched on
different enercripQ pK(V2>V,) acquire less energy from the electric field (U3
