Mass analyzers separate the ions according to their
mass-to-charge ratio. The following two laws govern the dynamics of charged particles in electric and magnetic fields in vacuum: :\mathbf{F} = Q (\mathbf{E} + \mathbf{v} \times \mathbf{B}) (
Lorentz force law); :\mathbf{F}=m\mathbf{a} (
Newton's second law of motion in the non-relativistic case, i.e. valid only at ion velocity much lower than the speed of light). Here
F is the force applied to the ion,
m is the mass of the ion,
a is the acceleration,
Q is the ion charge,
E is the electric field, and
v ×
B is the
vector cross product of the ion velocity and the magnetic field Equating the above expressions for the force applied to the ion yields: : (m/Q)\mathbf{a} = \mathbf{E}+ \mathbf{v} \times \mathbf{B}. This
differential equation is the classic equation of motion for
charged particles. Together with the particle's initial conditions, it completely determines the particle's motion in space and time in terms of
m/Q. Thus mass spectrometers could be thought of as "mass-to-charge spectrometers". When presenting data, it is common to use the (officially)
dimensionless m/z, where z is the number of
elementary charges (
e) on the ion (z=Q/e). This quantity, although it is informally called the mass-to-charge ratio, more accurately speaking represents the ratio of the mass number and the charge number,
z. There are many types of mass analyzers, using either static or dynamic fields, and magnetic or electric fields, but all operate according to the above differential equation. Each analyzer type has its strengths and weaknesses. Many mass spectrometers use two or more mass analyzers for
tandem mass spectrometry (MS/MS). In addition to the more common mass analyzers listed below, there are others designed for special situations. There are several important analyzer characteristics. The
mass resolving power is the measure of the ability to distinguish two peaks of slightly different
m/z. The mass accuracy is the ratio of the
m/z measurement error to the true
m/z. Mass accuracy is usually measured in
ppm or
milli mass units. The mass range is the range of
m/z amenable to analysis by a given analyzer. The linear dynamic range is the range over which ion signal is linear with analyte concentration. Speed refers to the time frame of the experiment and ultimately is used to determine the number of spectra per unit time that can be generated.
Sector instruments A sector field mass analyzer uses a static electric and/or magnetic field to affect the path and/or
velocity of the
charged particles in some way. As shown above,
sector instruments bend the trajectories of the ions as they pass through the mass analyzer, according to their mass-to-charge ratios, deflecting the more charged and faster-moving, lighter ions more. The analyzer can be used to select a narrow range of
m/z or to scan through a range of
m/z to catalog the ions present.
Time-of-flight The
time-of-flight (TOF) analyzer uses an
electric field to accelerate the ions through the same
potential, and then measures the time they take to reach the detector. If the particles all have the same
charge, their
kinetic energies will be identical, and their
velocities will depend only on their
masses. For example, ions with a lower mass will travel faster, reaching the detector first. Ions usually are moving prior to being accelerated by the
electric field, this causes particles with the same
m/z to arrive at different times at the detector. This difference in initial velocities is often not dependent on the mass of the ion, and will turn into a difference in the final velocity. This distribution in velocities broadens the peaks shown on the count vs
m/z plot, but will generally not change the central location of the peaks, since the starting velocity of ions is generally centered at zero. To fix this problem, time-lag focusing/
delayed extraction has been coupled with TOF-MS.
Quadrupole mass filter Quadrupole mass analyzers use oscillating electrical fields to selectively stabilize or destabilize the paths of ions passing through a
radio frequency (RF)
quadrupole field created between four parallel rods. Only the ions in a certain range of mass/charge ratio are passed through the system at any time, but changes to the potentials on the rods allow a wide range of
m/z values to be swept rapidly, either continuously or in a succession of discrete hops. A quadrupole mass analyzer acts as a mass-selective filter and is closely related to the
quadrupole ion trap, particularly the linear quadrupole ion trap except that it is designed to pass the untrapped ions rather than collect the trapped ones, and is for that reason referred to as a transmission quadrupole. A magnetically enhanced quadrupole mass analyzer includes the addition of a magnetic field, either applied axially or transversely. This novel type of instrument leads to an additional performance enhancement in terms of resolution and/or sensitivity depending upon the magnitude and orientation of the applied magnetic field. A common variation of the transmission quadrupole is the triple quadrupole mass spectrometer. The "triple quad" has three consecutive quadrupole stages, the first acting as a mass filter to transmit a particular incoming ion to the second quadrupole, a collision chamber, wherein that ion can be broken into fragments. The third quadrupole also acts as a mass filter, to transmit a particular fragment ion to the detector. If a quadrupole is made to rapidly and repetitively cycle through a range of mass filter settings, full spectra can be reported. Likewise, a triple quad can be made to perform various scan types characteristic of
tandem mass spectrometry.
Ion traps Three-dimensional quadrupole ion trap The
quadrupole ion trap works on the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected. Ions are trapped in a mainly quadrupole RF field, in a space defined by a ring electrode (usually connected to the main RF potential) between two endcap electrodes (typically connected to DC or auxiliary AC potentials). The sample is ionized either internally (e.g. with an electron or laser beam), or externally, in which case the ions are often introduced through an aperture in an endcap electrode. There are many mass/charge separation and isolation methods but the most commonly used is the mass instability mode in which the RF potential is ramped so that the orbit of ions with a mass are stable while ions with mass
b become unstable and are ejected on the
z-axis onto a detector. There are also non-destructive analysis methods. Ions may also be ejected by the resonance excitation method, whereby a supplemental oscillatory excitation voltage is applied to the endcap electrodes, and the trapping voltage amplitude and/or excitation voltage frequency is varied to bring ions into a resonance condition in order of their mass/charge ratio.
Cylindrical ion trap The
cylindrical ion trap mass spectrometer (CIT) is a derivative of the quadrupole ion trap where the electrodes are formed from flat rings rather than hyperbolic shaped electrodes. The architecture lends itself well to miniaturization because as the size of a trap is reduced, the shape of the electric field near the center of the trap, the region where the ions are trapped, forms a shape similar to that of a hyperbolic trap.
Linear quadrupole ion trap A
linear quadrupole ion trap is similar to a quadrupole ion trap, but it traps ions in a two dimensional quadrupole field, instead of a three-dimensional quadrupole field as in a 3D quadrupole ion trap. Thermo Fisher's LTQ ("linear trap quadrupole") is an example of the linear ion trap. A toroidal ion trap can be visualized as a linear quadrupole curved around and connected at the ends or as a cross-section of a 3D ion trap rotated on edge to form the toroid, donut-shaped trap. The trap can store large volumes of ions by distributing them throughout the ring-like trap structure. This toroidal shaped trap is a configuration that allows the increased miniaturization of an ion trap mass analyzer. Additionally, all ions are stored in the same trapping field and ejected together simplifying detection that can be complicated with array configurations due to variations in detector alignment and machining of the arrays. As with the toroidal trap, linear traps and 3D quadrupole ion traps are the most commonly miniaturized mass analyzers due to their high sensitivity, tolerance for mTorr pressure, and capabilities for single analyzer tandem mass spectrometry (e.g. product ion scans).
Orbitrap Orbitrap instruments are similar to
Fourier-transform ion cyclotron resonance mass spectrometers (see text below). Ions are
electrostatically trapped in an orbit around a central, spindle shaped electrode. The electrode confines the ions so that they both orbit around the central electrode and oscillate back and forth along the central electrode's long axis. This oscillation generates an
image current in the detector plates which is recorded by the instrument. The frequencies of these image currents depend on the mass-to-charge ratios of the ions. Mass spectra are obtained by
Fourier transformation of the recorded image currents. Orbitraps have a high mass accuracy, high sensitivity and a good dynamic range.
Fourier-transform ion cyclotron resonance Fourier-transform mass spectrometry (FTMS), or more precisely
Fourier-transform ion cyclotron resonance MS, measures mass by detecting the
image current produced by ions spiralling in the presence of a magnetic field. Instead of measuring the deflection of ions with a detector such as an
electron multiplier, the ions are injected into a
Penning trap (a static electric/magnetic
ion trap) where they effectively form part of a circuit. Detectors at fixed positions in space measure the electrical signal of ions which pass near them over time, producing a periodic signal. Since the frequency of an ion's cycling is determined by its mass-to-charge ratio, this can be
deconvoluted by performing a
Fourier transform on the signal.
FTMS has the advantage of high sensitivity (since each ion is "counted" more than once) and much higher
resolution and thus precision.
Ion cyclotron resonance (ICR) is an older mass analysis technique similar to FTMS except that ions are detected with a traditional detector. Ions trapped in a Penning trap are excited by an RF electric field until they impact the wall of the trap, where the detector is located. Ions of different mass are resolved according to impact time. ==Detectors==