Page 264 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32

152 Mass Spectrometry

MALDI is carried out in a mass spectrometer that uses a
time-of-flight (TOF) mass analyzer (described in the next
section). The relevant characteristics of the TOF are its
unlimited mass range, its inherent compatibility with a
pulsed ionization source, and the fact that all ions of all
masses are recorded in the mass spectrum generated by
each laser pulse (i.e., there is no scanning of the mass ana-
lyzer). The TOF is a high-transmission mass analyzer that
allows useful mass spectra to be measured even with the
small absolute number of ions that leave the irradiated sur-
face. The relatively low mass resolution of the TOF is ren-
dered irrelevant by the broad isotopic envelope of higher FIGURE 1 General diagram for a double focusing sector mass
mass ions, and the unlimited mass range (determined sim- analyzer.
ply by how long the analyst waits for the ions to make their
waydownthe flight tube) has moved MALDI-TOF mass ion detector, and a double focusing instrument combines
spectrometry into the regime of sample molecular masses an electric sector and a magnetic sector into an analyzer
6
4
of 10 –10 Dalton. Finally, TOF is a simple device, and operated as a single unit (Fig. 1).
costs are low. Benchtop MALDI-TOF instruments at low An ion beam of constant kinetic energy (but contain-
cost proliferate into a large number of laboratories, where ing ions of different masses) is created in the ion source.
new uses for the data they provide are found. The kinetic energy is derived from the potential differ-
The “matrix assistance” in MALDI is a matrix effect ence (V ) between the source and the flight tube of the
like any other. A matrix effect is a deviation in measured mass spectrometer, which is usually held at ground po-
2
response in either direction from that expected from the tential. Therefore, zeV = 1/2 mv , where z is the number
analyte in the absence of the matrix. The usual assumption of charges on the ion, e is the magnitude of the charge,
is that the deviation is in a negative direction (analytical m is the mass of the ion, and v is the velocity. All pa-
signal is diminished) or that signal to noise, at least, is rameters in the equation must be expressed in proper mks
reduced in the presence of a matrix. Following this as- units (volts, coulombs, kilograms, and meters per second).
sumption, chemists routinely strive for high sample purity A charged particle passing through a magnetic field ex-
before analysis. However, purification of sample for trace periences a force that is directed at right angles to both
level analysis may leave the sample more reactive or more the velocity vector and the magnetic field vector. Cen-
sensitive to loss processes. MALDI is a superb example in tripetal and centrifugal forces are balanced as the ion fol-
which the matrix supports the ability to perform the anal- lows a circular path of radius r while in the magnetic
2
ysis in the first place, and the deviation is advantageous. field. Therefore mv /r = zevB, where B is the magnetic
field strength. Solve both equations for v, and then set-
ting the equations equal to each other yields the classic
C. Mass Analyzers 2 2
equation of m/ze =r B /2V . Since e is a constant, it is
The mass analyzer is the heart of the mass spectrome- usually not listed specifically in the equation. If one solves
ter. In the mass analyzer, some aspect of ion response to for r instead (the radius of the magnetic field sector), the
electric or magnetic fields is exploited so that ions of dif- mathematic expression that results, r = mv/zeB, shows
ferent masses can be differentiated. Salient characteristics explicitly that the radius of curvature of the path of an ion
of mass analyzers are mass range, mass resolution, ion passing through the magnetic field is proportional to its
transmission, and as discussed in a previous section, scan momentum (mv). Since r is fixed, at any given B, only
times. ions with one momentum will pass through the ion optics
to make it through to the ion detector of the instrument.
Other ions collide with the walls of the flight tube and are
1. Magnetic and Electric Sectors
neutralized there.
Ions of different mass but a constant kinetic energy will be Sincer is fixed, there are two parameters that can be var-
dispersed in velocity direction by passage through a mag- ied to change the mass of the ions that passes through the
netic field. The term “sector” derives from a nomenclature magnetic sector mass analyzer to the ion detector. These
that divides a magnetic field into sectors of various angu- are B, the magnetic field strength, and V , the accelerat-
lar dimensions. Magnetic sector mass spectrometers are ing potential of the source. To maximize the transmission
of two general types. Single focusing instruments consist of ions out of the ion source into the mass analyzer, and
of a single magnetic sector between the ion source and the then out of the mass analyzer into the ion detector, the