General Scheme of a Mass Spectrometer
As can be expected from the basic principle of mass spectrometry, there are several techniques available to serve for the analysis of ionic mass. Whatever type the mass analyzer may be, it always requires that the ions are delivered in the dilute gas phase. They are all based on the prerequisite that the ions can be subjected to acceleration, deceleration, deflection or oscillation without any hindrance by collisions with neutrals. A mean free path long enough to fulfill this criterion is achieved by running mass analyzers in high vacuum. As either type of electric or magnetic field is only able to influence ionic species neutrals are not affected, and thus, neutrals are useless for mass spectral analysis. Neutrals just move statistically while ions can be manipulated in sophisticated ways.
Ions are created by an ion source. The ion source housing provides various means of transferring the sample from the laboratory into the vacuum environment of the ion source where ionization is effected. The ion source also takes care to create a focused ion beam that is injected into the mass analyzer. Finally, there is a detector, often a secondary electron multiplier (SEM), that converts the event of ion impact into an electric signal that can be used for data acquisition.
Despite the large number of ionization methods and types of mass analyzers in use, all mass spectrometers follow the same basic scheme: they comprise of an ion source, a mass analyzer, a detector, and a data system to control the instrument and to acquire the data.
Magnetic sector instruments represent the classic type of a mass spectrometer. So-called double focusing magnetic sector instruments combine a magnetic sector with an electrostatic analyzer to compensate for the spread in ion kinetic energy caused by the ion source. Double focusing mass spectrometers were the standard in MS for many decades.
Modern mass spectrometry laboratories operate various types of mass spectrometers to deal with different applications, some of which demand for mass highest resolving power and mass accuracy while other may require highest speed, optimum reliability in quantitation, or utmost sensitivity. The choice of a mass analyzer is also dictated by the investment and laboratory space requirements. While some instruments are of bench top size others may fill a room alone.
The following table provides a brief overview of mass analyzers currently in use.
|Mass Analysor||Symbol and Alignments||Principle of Operation and Applications|
|Magnetic sector instruments||B, BE, EB, EBE, EBEB, BEBE,…||Magnetic (B) and electric field (E) orthogonal to the ion flight path are used to separate a continuous ion beam by m/z. For decades, double focusing magnetic sector instrument were the most important mass spectrometers. Normally, this type of mass analyzer is comparatively large, heavy, and expensive. Today, they are rarely acquired anymore although there are some dedicated applications where they are still advantageous.|
|Linear quadrupole analyzers||Q, QqQ||Superimposition of constant and oscillating radiofrequency electric quadrupole fields (Q: mass selective, q: only ion-guiding) to separate an ion beam by m/z. Compact instruments for GC-MS and LC-MS. Triple quadrupole instruments (QqQ) are ideal for quantification in trace analysis as they offer both MS/MS and high dynamic range.|
|Three-dimensional quadrupole ion traps||QIT||Superimposition of constant and oscillating radiofrequency electric three-dimensional quadrupole fields to trap, store, and separate a swarm of ions by m/z. QITs are run in batch mode, i.e., packages of ions are treated at a time. QITs may hold mass-selected ions for subsequent ion activation and dissociation to realize MS/MS mode. QITs are very compact and often used in GC-MS, LC-MS, and LC-MS/MS, especially for structure elucidation.|
|Linear quadrupole ion traps||LIT||Superimposition of constant and oscillating radiofrequency electric quadrupole fields in a linear quadrupole to trap, store, and separate a swarm of ions by m/z. LITs are run in batch mode, i.e., packages of ions are treated at a time. LITs may hold mass-selected ions for subsequent ion activation and dissociation to realize MS/MS mode. LITs are compact and often used in GC-MS, LC-MS, and LC-MS/MS, especially for structure elucidation.|
|Fourier transform-ion cyclotron resonance analyzers||FT-ICR||Ions are injected parallel into a strong magnetic field (7–15 T) where they are stored and then excited to orbit as a package at a cyclotron frequency unique to their m/z value. This frequency is measured using image current detection. Fourier transformation is then used to translate the frequency data into m/z data.
FT-ICR instruments offer the highest level of mass resolving power and mass accuracy. They can be operated in MSn mode and are highly variable to run a multitude of experiments that would be impossible on any other mass analyzer.
|Time-of-flight analyzers||TOF||The time-of-flight of ions travelling down a field-free drift tube is taken as a measure of m/z. TOF instruments require very short and well-defined ion packages to be pulsed into the analyzer tube. Thus, MALDI-TOF evolved into a highly successful combination. The capabilities of MALDI-TOF instruments then led to the development of TOFs with orthogonal acceleration and many other designs. Today, TOFs have a high market share in MS instrumentation as they unite compact design, high m/z range, and often high resolving power in a reasonably compact package.|
|Orbitraps||Orbitrap||Orbitrap instruments entered the market in 2005. Ion packages are injected and are simultaneously orbiting around and oscillating along a spindle-shaped central electrode. The axial oscillation is recorded by image current detection and converted into m/z data via Fourier transformation. Orbitraps are strong competitors of high-resolving TOF and FT-ICR instruments.|
|Hybrid instruments||BEqQ, EBE-TOF, QqTOF, QqLIT, Qq-FT-ICR, LIT-FT-ICR…||Combinations of different types of mass analyzers to achieve MS/MS, MSn and eventually combinations thereof for fragment ion analysis with high resolution and accurate mass. The selection of stages is made in a way as to most efficiently achieve the desired performance characteristics. Many modern instruments are hybrids of the one or the other type.|
A quadrupole ion trap (QIT) mass spectrometer is a compact bench top instrument (top) whereas a Fourier transform ion cyclotron resonance (FT-ICR) instrument (bottom) requires a comparatively large amount of laboratory space. On the other hand, the FT-ICR instrument is in almost any respect superior to the QIT.