Linear ion trap

Last updated February 27, 2023

The linear ion trap (LIT) is a type of ion trap mass spectrometer.

History

One of the first LITs was constructed in 1969, by Dierdre A. Church,^[2] who bent linear quadrupoles into closed circle and racetrack geometries and demonstrated storage of ³ He ⁺ and H ⁺ ions for several minutes.

Earlier, Drees and Paul described a circular quadrupole.^{[ citation needed ]} However, it was used to produce and confine a plasma, not to store ions. In 1989, Prestage, Dick, and Malecki described that ions could be trapped in the linear quadrupole trap system to enhance ion-molecule reactions, thus it can be used to study spectroscopy of stored ions.^[1]

How it works

The LIT uses a set of quadrupole rods to confine ions radially and a static electrical potential on the end electrodes to confine the ions axially.^[3] The LIT can be used as a mass filter or as a trap by creating a potential well for the ions along the axis of the trap.^[4] The mass of trapped ions may be determined if the m/z lies between defined parameters.^[5]

Advantages of the LIT design are high ion storage capacity, high scan rate, and simplicity of construction. Although quadrupole rod alignment is critical, adding a quality control constraint to their production, this constraint is additionally present in the machining requirements of the 3D trap.^[6]

Selective mode and scanning mode

Ions are either injected into or created within the interior of the LIT. They are confined by application of appropriate RF and DC voltages with their final position maintained within the center section of the LIT. The RF voltage is adjusted and multi-frequency resonance ejection waveforms are applied to the trap to eliminate all but the desired ions in preparation for subsequent fragmentation and mass analysis. The voltages applied to the ion trap are adjusted to stabilize the selected ions and to allow for collisional cooling in preparation for excitation.

The energy of the selected ions is increased by application of a supplemental resonance excitation voltage applied to all segments of two rods located on the X-axis. This increase of energy causes dissociation of the selected ions due to collisions with damping gas. The product ions formed are retained in the trapping field. Scanning the contents of the trap to produce a mass spectrum is accomplished by linearly increasing the RF voltage applied to all sections of the trap and utilizing a supplemental resonance ejection voltage. These changes sequentially move ions from within the stability diagram to a position where they become unstable in the x-direction and leave the trapping field for detection. Ions are accelerated into two high voltage dynodes where ions produce secondary electrons. This signal is subsequently amplified by two electron multipliers and the analog signals are then integrated together and digitized.

Combination with other mass analyzers

LITs can be used as stand alone mass analyzers, and they can be combined with other mass analyzers, such as 3D Paul ion traps, TOF mass spectrometers, FTMS, and other kind of mass analyzers.

Linear traps and 3D trap

3D ion trap (or Paul trap) mass spectrometers are widely used but have limitations. With a continuous source, such as one utilizing electrospray ionization (ESI), ions generated while the 3D trap is processing other ions are not used, thereby limiting the duty cycle. Furthermore, the total number of ions that can be stored in a 3D ion trap is limited by space charge effects. Combining a linear trap with a 3D trap can help overcome these limitations.^[1]

Recently, Hardman and Makarov have described the use of a linear quadrupole trap to store ions formed by ESI for injection into an orbitrap mass analyzer. Ions passed through an orifice and skimmer, a quadrupole ion guide for ion cooling and then entered the quadrupole storage trap. The quadrupole trap has two rod sets; short rods near the exit were biased so that most ions accumulated in this region. Because the orbitrap requires that ions be injected in very short pulses, kilovolt ion extraction potentials were applied to the exit aperture. Flight times of ions to the orbitrap were mass dependent, but for a given mass, ions were injected in bunches less than 100 nanoseconds wide (fwhm).

Linear traps and TOF

A TOF mass spectrometer can also have a low-duty cycle when coupled with a continuous ion source. Combining an ion trap with a TOF mass analyzer can improve the duty cycle. Both 3D and linear traps have been combined with TOF mass analyzers. A trap can also add MSn capabilities to the system.^[1]

Linear trap and FTICR

Linear traps can be used to improve the performance of FT-ICR (or FTMS) systems. As with 3D ion traps, the duty cycle can be increased to nearly 100% if ions are accumulated in a linear trap, while the FTMS performs other functions. Unwanted ions that can cause space charge problems in the FTMS can be ejected in the linear trap to improve the resolution, sensitivity, and dynamic range of the system, although the system parameters used to optimize such signal characteristics co-vary with one another.^[1]^[7]

Linear trap and triple quadrupole

The combination of triple quadrupole MS with LIT technology in the form of an instrument of configuration QqLIT, using axial ejection, is particularly interesting, because this instrument retains the classical triple quadrupole scan functions such as selected reaction monitoring (SRM), product ion (PI), neutral loss (NL) and precursor ion (PC) while also providing access to sensitive ion trap experiments. For small molecules, quantitative and qualitative analysis can be performed using the same instrument.

In addition, for peptide analysis, the enhanced multiply charged (EMC) scan allows an increase in selectivity, while the time-delayed fragmentation (TDF) scan provides additional structural information. In the case of the QqLIT, the uniqueness of the instrument is that the same mass analyzer Q3 can be run in two different modes. This allows very powerful scan combinations when performing information-dependent data acquisition.

Related Research Articles

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

A mass spectrum is a histogram plot of intensity vs. mass-to-charge ratio (m/z) in a chemical sample, usually acquired using an instrument called a mass spectrometer. Not all mass spectra of a given substance are the same; for example, some mass spectrometers break the analyte molecules into fragments; others observe the intact molecular masses with little fragmentation. A mass spectrum can represent many different types of information based on the type of mass spectrometer and the specific experiment applied. Common fragmentation processes for organic molecules are the McLafferty rearrangement and alpha cleavage. Straight chain alkanes and alkyl groups produce a typical series of peaks: 29 (CH₃CH₂⁺), 43 (CH₃CH₂CH₂⁺), 57 (CH₃CH₂CH₂CH₂⁺), 71 (CH₃CH₂CH₂CH₂CH₂⁺) etc.

<span class="mw-page-title-main">Tandem mass spectrometry</span>

Tandem mass spectrometry, also known as MS/MS or MS², is a technique in instrumental analysis where two or more mass analyzers are coupled together using an additional reaction step to increase their abilities to analyse chemical samples. A common use of tandem MS is the analysis of biomolecules, such as proteins and peptides.

An ion trap is a combination of electric and/or magnetic fields used to capture charged particles — known as ions — often in a system isolated from an external environment. Atomic and molecular ion traps have a number of applications in physics and chemistry such as precision mass spectrometry, improved atomic frequency standards, and quantum computing. In comparison to neutral atom traps, ion traps have deeper trapping potentials that do not depend on the internal electronic structure of a trapped ion. This makes ion traps more suitable for the study of light interactions with single atomic systems. The two most popular types of ion traps are the Penning trap, which forms a potential via a combination of static electric and magnetic fields, and the Paul trap which forms a potential via a combination of static and oscillating electric fields.

The quadrupole mass analyzer, originally conceived by Nobel Laureate Wolfgang Paul and his student Helmut Steinwedel, also known as quadrupole mass filter, is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of four cylindrical rods, set parallel to each other. In a quadrupole mass spectrometer (QMS) the quadrupole is the mass analyzer - the component of the instrument responsible for selecting sample ions based on their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.

<span class="mw-page-title-main">Quadrupole ion trap</span>

A quadrupole ion trap or paul trap is a type of ion trap that uses dynamic electric fields to trap charged particles. They are also called radio frequency (RF) traps or Paul traps in honor of Wolfgang Paul, who invented the device and shared the Nobel Prize in Physics in 1989 for this work. It is used as a component of a mass spectrometer or a trapped ion quantum computer.

<span class="mw-page-title-main">Orbitrap</span>

In mass spectrometry, Orbitrap is an ion trap mass analyzer consisting of an outer barrel-like electrode and a coaxial inner spindle-like electrode that traps ions in an orbital motion around the spindle. The image current from the trapped ions is detected and converted to a mass spectrum using the Fourier transform of the frequency signal.

Electron-transfer dissociation (ETD) is a method of fragmenting multiply-charged gaseous macromolecules in a mass spectrometer between the stages of tandem mass spectrometry (MS/MS). Similar to electron-capture dissociation, ETD induces fragmentation of large, multiply-charged cations by transferring electrons to them. ETD is used extensively with polymers and biological molecules such as proteins and peptides for sequence analysis. Transferring an electron causes peptide backbone cleavage into c- and z-ions while leaving labile post translational modifications (PTM) intact. The technique only works well for higher charge state peptide or polymer ions (z>2). However, relative to collision-induced dissociation (CID), ETD is advantageous for the fragmentation of longer peptides or even entire proteins. This makes the technique important for top-down proteomics. The method was developed by Hunt and coworkers at the University of Virginia.

Time-of-flight mass spectrometry (TOFMS) is a method of mass spectrometry in which an ion's mass-to-charge ratio is determined by a time of flight measurement. Ions are accelerated by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has the same charge. The velocity of the ion depends on the mass-to-charge ratio. The time that it subsequently takes for the ion to reach a detector at a known distance is measured. This time will depend on the velocity of the ion, and therefore is a measure of its mass-to-charge ratio. From this ratio and known experimental parameters, one can identify the ion.

Ion mobility spectrometry–mass spectrometry (IMS-MS) is an analytical chemistry method that separates gas phase ions based on their interaction with a collision gas and their masses. In the first step, the ions are separated according to their mobility through a buffer gas on a millisecond timescale using an ion mobility spectrometer. The separated ions are then introduced into a mass analyzer in a second step where their mass-to-charge ratios can be determined on a microsecond timescale. The effective separation of analytes achieved with this method makes it widely applicable in the analysis of complex samples such as in proteomics and metabolomics.

A triple quadrupole mass spectrometer (TQMS), is a tandem mass spectrometer consisting of two quadrupole mass analyzers in series, with a (non-mass-resolving) radio frequency (RF)–only quadrupole between them to act as a cell for collision-induced dissociation. This configuration is often abbreviated QqQ, here Q₁q₂Q₃.

<span class="mw-page-title-main">Reflectron</span> Time-of-flight mass spectrometer with an ion mirror

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<span class="mw-page-title-main">Hybrid mass spectrometer</span>

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Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions are usually accelerated by applying an electrical potential to increase the ion kinetic energy and then allowed to collide with neutral molecules. In the collision some of the kinetic energy is converted into internal energy which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. These fragment ions can then be analyzed by tandem mass spectrometry.

<span class="mw-page-title-main">Miniature mass spectrometer</span>

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SCIEX is a manufacturer of mass spectrometry instrumentation used in biomedical and environmental applications. Originally started by scientists from the University of Toronto Institute for Aerospace Studies, it is now part of Danaher Corporation with the SCIEX R&D division still located in Toronto, Canada.

References

1 2 3 4 5 Douglas, Donald J.; Frank, Aaron J.; Mao, Dunmin (2005). "Linear ion traps in mass spectrometry". Mass Spectrometry Reviews. 24 (1): 1–29. Bibcode:2005MSRv...24....1D. doi:10.1002/mas.20004. ISSN 0277-7037. PMID 15389865.
↑ Church, D. A. (1969-07-01). "Storage‐Ring Ion Trap Derived from the Linear Quadrupole Radio‐Frequency Mass Filter". Journal of Applied Physics. 40 (8): 3127–3134. Bibcode:1969JAP....40.3127C. doi:10.1063/1.1658153. ISSN 0021-8979.
↑ Douglas DJ, Frank AJ, Mao D (2005). "Linear ion traps in mass spectrometry". Mass Spectrometry Reviews. 24 (1): 1–29. Bibcode:2005MSRv...24....1D. doi:10.1002/mas.20004. PMID 15389865.
↑ Quadrupole; March, Raymond E.; Spectrometry, Mass (2000). "Quadrupole ion trap mass spectrometry: a view at the turn of the century". International Journal of Mass Spectrometry. 2000 (1–3): 285–312. Bibcode:2000IJMSp.200..285M. doi:10.1016/S1387-3806(00)00345-6.
↑ Peng, Ying; Austin, Daniel E. (November 2011). "New approaches to miniaturizing ion trap mass analyzers". TrAC Trends in Analytical Chemistry. 30 (10): 1560–1567. doi:10.1016/j.trac.2011.07.003.
↑ Schwartz, Jae C.; Michael W. Senko; John E. P. Syka (June 2002). "A two-dimensional quadrupole ion trap mass spectrometer". Journal of the American Society for Mass Spectrometry. 13 (6): 659–669. doi: 10.1016/S1044-0305(02)00384-7 . PMID 12056566.
↑ Lemonakis, N.; Skaltsounis, A.L.; Tsarbopoulos, A.; Gikas, E. (2016). "Optimization of parameters affecting signal intensity in an LTQ-orbitrap in negative ion mode: A design of experiments approach". Talanta . 147: 402–409. doi:10.1016/j.talanta.2015.10.009. PMID 26592625.

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[DouglasFrank2005-1] 1 2 3 4 5 Douglas, Donald J.; Frank, Aaron J.; Mao, Dunmin (2005). "Linear ion traps in mass spectrometry". Mass Spectrometry Reviews. 24 (1): 1–29. Bibcode:2005MSRv...24....1D. doi:10.1002/mas.20004. ISSN 0277-7037. PMID 15389865.

[2] Church, D. A. (1969-07-01). "Storage‐Ring Ion Trap Derived from the Linear Quadrupole Radio‐Frequency Mass Filter". Journal of Applied Physics. 40 (8): 3127–3134. Bibcode:1969JAP....40.3127C. doi:10.1063/1.1658153. ISSN 0021-8979.

[pmid15389865-3] Douglas DJ, Frank AJ, Mao D (2005). "Linear ion traps in mass spectrometry". Mass Spectrometry Reviews. 24 (1): 1–29. Bibcode:2005MSRv...24....1D. doi:10.1002/mas.20004. PMID 15389865.

[4] Quadrupole; March, Raymond E.; Spectrometry, Mass (2000). "Quadrupole ion trap mass spectrometry: a view at the turn of the century". International Journal of Mass Spectrometry. 2000 (1–3): 285–312. Bibcode:2000IJMSp.200..285M. doi:10.1016/S1387-3806(00)00345-6.

[5] Peng, Ying; Austin, Daniel E. (November 2011). "New approaches to miniaturizing ion trap mass analyzers". TrAC Trends in Analytical Chemistry. 30 (10): 1560–1567. doi:10.1016/j.trac.2011.07.003.

[6] Schwartz, Jae C.; Michael W. Senko; John E. P. Syka (June 2002). "A two-dimensional quadrupole ion trap mass spectrometer". Journal of the American Society for Mass Spectrometry. 13 (6): 659–669. doi: 10.1016/S1044-0305(02)00384-7 . PMID 12056566.

[lemonakis2016-7] Lemonakis, N.; Skaltsounis, A.L.; Tsarbopoulos, A.; Gikas, E. (2016). "Optimization of parameters affecting signal intensity in an LTQ-orbitrap in negative ion mode: A design of experiments approach". Talanta . 147: 402–409. doi:10.1016/j.talanta.2015.10.009. PMID 26592625.

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