Tandem Mass Spectrometry
Tandem mass spectrometry is a technique that incorporates multiple spectrum analysis in space or time, according to design of the instrument. The multi stage mass spectrum analysis is also known as MS-MS or MSn where ‘n’ being the number of stages of separation. Many instruments have mass analyzers which are sequentially aligned.
Many common conventional designs used in MS-MS space analysis are known as triple quad. As the name elaborates, a triple quad is used consisting of three quadruple mass spectrometers which are aligned together. The first quadruple (Q1) separates certain m/z ions from the analytes. Varying on ionization methods, the ions can either be molecular ions (EI) or pseudo-molecular ions (ESI, CI, APPI, APCI) which are basically known as pre-cursor ions. The Q2 – second quadruple tends to break/fragment the ions where as the third quadruple (Q3) separates fragment and product ions.
The collision cell, also known as Q2, involves a chamber in which collision gas is inserted inside and fragmentation of ions is done by CID as a process mentioned previously. The conventional quadruple in Q2 is replaced by designs like hexapole and octapole. Better designs result in efficient CID in the Q2 area with the latest designs.
The triple quad, Quad-Ot-Quad and the Quad-Hex-Quad perform in a similar fashion by isolating the precursor ion, fragmenting it and detecting the product ions.
The QIT-MS instruments are also used to carry out MS-Ms procedure by they work under time not space mechanism. The ion trap isolates the precursor ion and accumulates it. Other m/z ion are destabilized and then ejected. Then CID is applied and product ions are distinguished. This process is done a lot of times according to the analyte’s concentration and the scanning potential of the instrument. In many situations while using normal parameters, an ‘n’ equal to 4 or lesser is acceptable but an MS10 has also been accounted.
Basically, 5 scanning methods are used while doing the MS-MS analysis: Simple scanning method, Precursor ion scanning method, Product ion scanning method, Neutral loss scanning method and multiple reactions monitoring (MRM) method which is also called Selected reaction monitoring (SRM). In Figure 3.9, a triple quad along with its different scans modes in shown. However, a simple scan doesn’t use 3 but only 1 quadruple to perform the mass scan. A precursor ion uses the scanning of a precursor ion in Q1 and selects particular product ions in Q2. But, a product scan contradicts the functioning and rather selected the ions in Q1 which are fragmented in Q2 and finally scans them in Q3. This is the method which is used in confirmatory and quantitative analysis. In contrast, the neutral loss mode uses mass analyzers in Q1 as well as Q3 while scanning. But the Q3 is set to according to a specific mass of Q1 which identifies the molecules which lose neutral groups while the CID takes place. The last mode i.e. the MRM is a highly selective and sensitive technique that is used in tandem MS. It is analogous to SIM in the Q-MS analysis mode. The Q1 and Q3 both are set selectively such that they can monitor S/N ratios. This monitoring lets only specific ions to undergo long scanning time and due to their specificity, the higher flow rates are adjusted within.
All of such factors cause MRM to be quite powerful tool in forensic sciences. The GC-MS-MS and L-MS-MS are now being commonly used in referred laboratories due to drug testing economics. Reference laboratories tend to test compounds that are normally not encountered in drug testing and therefore, are quite difficult to analyze via conventional Q-MS or QIT-MS instruments. Therefore, by testing of these compounds once or twice every week, the batches become large enough to be economically reasonable.
Tandem MS is quite popular and economically practical for use. The instruments for tandem MS are becoming cheaper, more user friendly, convenient and quicker to use which makes them quite versatile and solidifies their role. A lot of methods are published to be used for tandem MS analysis and many more researches are being published every year.
Hybrid tandem MS-Ms instruments like QTRAP include linear trap for ions where as the Q-TOF has a time of flight to be used as Q3 analyzer. Even though these instruments are basically used for macromolecular analysis of compounds like proteins, enzymes and amino acids, they are also being commonly used in drug testing fields. These instruments, when used with LC analyzers, are able to detect and quantify drugs of abuse at substantial levels from samples which are injected directly, on order to give a full scan, SIM and exact mass concentrations of the compounds which as comparatively quite lower than the detection capacity of conventional Q-MS systems. Many hybrid instruments are now being more prevalent because the demand for versatile and sensitive instruments is ever increasing.
Magnetic Sector Mass Spectrometry
Magnetic mass spectrometry basically includes ions source, ion optics, magnet, ion detector and a signal amplifier. Figure 3.10 illustrates a magnetic sector MS. Even though these instruments are commonly used for research purposes, they are very important as far as their applications to the sports drug testing regimes are concerned.
The ions which are present in magnetic sector MS are made by ion source which are then focused and propelled by ion optics inside the magnetic field. These ions basically travel in the form of an arc, where their curvature in a fixed magnetic field is directly proportional to the m/z ratio of the ions. Therefore, every m/z forms its own distinctive pathway which is disconnected from other m/z in space. Every m/z is divided and detected via adjustments in their electrical voltage (V) or/and their magnetic field (B) which allows the ions of particular m/z to hit the detector. This can also be simply expressed as:
m/z = (B2r2)/(2V)
In the above mentioned equation, ‘r’ stands for the radius of the curve in which the ions travel while they are in their magnetic field. Only one sector i.e. the magnetic field is used in single focused analyzers where the ions are separated in fixed beam path ranging from 60° to 180°. This result in limited resolution as it doesn’t include minute differences in the kinetic energies of ions which have the same m/z. many modern instruments have the capability of double-focusing which varies with two sectors, there by adding electrostatic analyzers to separate ions which depend on mass and kinetic energies. This also enhances the resolution up to 100,000 and gives mass accuracies up to 6 important figures. Even for the compounds which have smallest differences in their composition can also be identified in this manner.
The magnetic sector MS is used a lot in drug testing programs. Its instruments are costly and therefore, its usage has been limited to identify metabolites and in anti-doping programs.