Selected-ion flow-tube mass spectrometry

Last updated March 06, 2023

Picture of SIFT-MS profile 3.

Selected-ion flow-tube mass spectrometry (SIFT-MS) is a quantitative mass spectrometry technique for trace gas analysis which involves the chemical ionization of trace volatile compounds by selected positive precursor ions during a well-defined time period along a flow tube.^[1] Absolute concentrations of trace compounds present in air, breath or the headspace of bottled liquid samples can be calculated in real time from the ratio of the precursor and product ion signal ratios, without the need for sample preparation or calibration with standard mixtures. The detection limit of commercially available SIFT-MS instruments extends to the single digit pptv range.

The instrument is an extension of the selected ion flow tube, SIFT, technique, which was first described in 1976 by Adams and Smith.^[2] It is a fast flow tube/ion swarm method to react positive or negative ions with atoms and molecules under truly thermalised conditions over a wide range of temperatures. It has been used extensively to study ion-molecule reaction kinetics. Its application to ionospheric and interstellar ion chemistry over a 20-year period has been crucial to the advancement and understanding of these topics.

SIFT-MS was initially developed for use in human breath analysis, and has shown great promise as a non-invasive tool for physiological monitoring and disease diagnosis. It has since shown potential for use across a wide variety of fields, particularly in the life sciences, such as agriculture and animal husbandry, environmental research and food technology.

SIFT-MS has been popularised as a technology which is sold and marketed by Syft Technologies based in Christchurch, New Zealand.

The SIFT technique, which is the basis of SIFT-MS, was conceived and developed nearly 40 years ago in Birmingham University UK by N.G. Adams and David. Smith. Professor David Smith passed away 15/02/2023

Instrumentation

In the selected ion flow tube mass spectrometer, SIFT-MS, ions are generated in a microwave plasma ion source, usually from a mixture of laboratory air and water vapor. From the formed plasma, a single ionic species is selected using a quadrupole mass filter to act as "precursor ions" (also frequently referred to as primary or reagent ions in SIFT-MS and other processes involving chemical ionization). In SIFT-MS analyses, H₃O⁺, NO⁺ and O₂⁺ are used as precursor ions, and these have been chosen because they are known not to react significantly with the major components of air (nitrogen, oxygen, etc.), but can react with many of the very low level (trace) gases.

The selected precursor ions are injected into a flowing carrier gas (usually helium at a pressure of 1 Torr) via a Venturi orifice (~1 mm diameter) where they travel along the reaction flow tube by convection. Concurrently, the neutral analyte molecules of a sample vapor enter the flow tube, via a heated sampling tube, where they meet the precursor ions and may undergo chemical ionization, depending on their chemical properties, such as their proton affinity or ionization energy.

The newly formed "product ions" flow into the mass spectrometer chamber, which contains a second quadrupole mass filter, and an electron multiplier detector, which are used to separate the ions by their mass-to-charge ratios (m/z) and measure the count rates of the ions in the desired m/z range.

Analysis

The concentrations of individual compounds can be derived largely using the count rates of the precursor and product ions, and the reaction rate coefficients, k. Exothermic proton transfer reactions with H₃O⁺ are assumed to proceed at the collisional rate (see Collision theory), the coefficient for which, k_c, is calculable using the method described by Su and Chesnavich,^[3] providing the polarizability and dipole moment are known for the reactant molecule. NO⁺ and O₂⁺ reactions proceed at k_c less frequently, and thus the reaction rates of the reactant molecule with these precursor ions must often be derived experimentally by comparing the decline in the count rates of each of the NO⁺ and O₂⁺ precursor ions to that of H₃O⁺ as the sample flow of reactant molecules is increased.^[1] The product ions and rate coefficients have been derived in this way for well over 200 volatile compounds, which can be found in the scientific literature.^[4]

The instrument can be programmed either to scan across a range of masses to produce a mass spectrum (Full Scan, FS, mode), or to rapidly switch between only the m/z values of interest (Multiple Ion Monitoring, MIM, mode). Due to the different chemical properties of the aforementioned precursor ions (H₃O⁺, NO⁺, and O₂⁺), different FS mode spectra can be produced for a vapor sample, and these can give different information relating to the composition of the sample. Using this information, it is often possible to identify the trace compound(s) that are present. The MIM mode, on the other hand will usually employ a much longer dwell time on each ion, and as a result, accurate quantification is possible to the parts per billion (ppb) level.^[1]

SIFT-MS utilises an extremely soft ionisation process which greatly simplifies the resulting spectra and thereby facilitates the analysis of complex mixtures of gases, such as human breath. Another very soft ionization technique is secondary electrospray ionization (SESI-MS).^[5]^[6] For example, even proton-transfer-reaction mass spectrometry (PTR-MS), another soft ionisation technology that uses the H₃O⁺ reagent ion, has been shown to give considerably more product ion fragmentation than SIFT-MS.^[7]

Another key feature of SIFT-MS is the upstream mass quadrupole, which allows the use of multiple precursor ions. The ability to use three precursor ions, H₃O⁺, NO⁺ and O₂⁺, to obtain three different spectra is extremely valuable because it allows the operator to analyse a much wider variety of compounds. An example of this is methane, which cannot be analysed using H₃O⁺ as a precursor ion (because it has a proton affinity of 543.5kJ/mol, somewhat less than that of H₂O), but can be analysed using O₂⁺.^[8] Furthermore, the parallel use of three precursor ions may allow the operator to distinguish between two or more compounds that react to produce ions of the same mass-to-charge ratio in certain spectra. For example, dimethyl sulfide (C₂H₆S, 62 amu) accepts a proton when it reacts with H₃O⁺ to generate C₂H₇S⁺ product ions which appear at m/z 63 in the resulting spectrum. This may conflict with other product ions, such as the association product from the reaction with carbon dioxide, H₃O⁺CO₂, and the single hydrate of the protonated acetaldehyde ion, C₂H₅O⁺(H₂O), which also appear at m/z 63, and so it may be unidentifiable in certain samples. However dimethyl sulfide reacts with NO⁺ by charge transfer, to produce the ion C₂H₆S⁺, which appears at m/z 62 in resulting spectra, whereas carbon dioxide does not react with NO⁺, and acetaldehyde donates a hydride ion, giving a single product ion at m/z 43, C₂H₃O⁺, and so dimethyl sulfide can be easily distinguished.

Over recent years, advances in SIFT-MS technology have vastly increased the sensitivity of these devices such that the limits of detection now extend down to the single-digit-ppt level.^[9]

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.

An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

Electron ionization is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of the first ionization techniques developed for mass spectrometry. However, this method is still a popular ionization technique. This technique is considered a hard ionization method, since it uses highly energetic electrons to produce ions. This leads to extensive fragmentation, which can be helpful for structure determination of unknown compounds. EI is the most useful for organic compounds which have a molecular weight below 600. Also, several other thermally stable and volatile compounds in solid, liquid and gas states can be detected with the use of this technique when coupled with various separation methods.

<span class="mw-page-title-main">Electrospray ionization</span> Technique used in mass spectroscopy

Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. ESI is different from other ionization processes since it may produce multiple-charged ions, effectively extending the mass range of the analyser to accommodate the kDa-MDa orders of magnitude observed in proteins and their associated polypeptide fragments.

A plasma afterglow is the radiation emitted from a plasma after the source of ionization is removed. The external electromagnetic fields that sustained the plasma glow are absent or insufficient to maintain the discharge in the afterglow. A plasma afterglow can either be a temporal, due to an interrupted (pulsed) plasma source, or spatial, due to a distant plasma source. In the afterglow, plasma-generated species de-excite and participate in secondary chemical reactions that tend to form stable species. Depending on the gas composition, super-elastic collisions may continue to sustain the plasma in the afterglow for a while by releasing the energy stored in rovibronic degrees of freedom of the atoms and molecules of the plasma. Especially in molecular gases, the plasma chemistry in the afterglow is significantly different from the plasma glow. The afterglow of a plasma is still a plasma and as thus retains most of the properties of a plasma.

Gas chromatography–mass spectrometry (GC-MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples, including that of material samples obtained from planet Mars during probe missions as early as the 1970s. GC-MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. Like liquid chromatography–mass spectrometry, it allows analysis and detection even of tiny amounts of a substance.

Chemical ionization (CI) is a soft ionization technique used in mass spectrometry. This was first introduced by Burnaby Munson and Frank H. Field in 1966. This technique is a branch of gaseous ion-molecule chemistry. Reagent gas molecules are ionized by electron ionization to form reagent ions, which subsequently react with analyte molecules in the gas phase to create analyte ions for analysis by mass spectrometry. Negative chemical ionization (NCI), charge-exchange chemical ionization, atmospheric-pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) are some of the common variants of the technique. CI mass spectrometry finds general application in the identification, structure elucidation and quantitation of organic compounds as well as some utility in biochemical analysis. Samples to be analyzed must be in vapour form, or else, must be vapourized before introduction into the source.

<span class="mw-page-title-main">Matrix-assisted laser desorption/ionization</span> Ionization technique

In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.

Liquid chromatography–mass spectrometry (LC–MS) is an analytical chemistry technique that combines the physical separation capabilities of liquid chromatography with the mass analysis capabilities of mass spectrometry (MS). Coupled chromatography - MS systems are popular in chemical analysis because the individual capabilities of each technique are enhanced synergistically. While liquid chromatography separates mixtures with multiple components, mass spectrometry provides spectral information that may help to identify each separated component. MS is not only sensitive, but provides selective detection, relieving the need for complete chromatographic separation. LC-MS is also appropriate for metabolomics because of its good coverage of a wide range of chemicals. This tandem technique can be used to analyze biochemical, organic, and inorganic compounds commonly found in complex samples of environmental and biological origin. Therefore, LC-MS may be applied in a wide range of sectors including biotechnology, environment monitoring, food processing, and pharmaceutical, agrochemical, and cosmetic industries. Since the early 2000s, LC-MS has also begun to be used in clinical applications.

Flowing-afterglow mass spectrometry (FA-MS), is an analytical chemistry technique for the sensitive detection of trace gases. Trace gas molecules are ionized by the production and flow of thermalized hydrated hydronium cluster ions in a plasma afterglow of helium or argon carrier gas along a flow tube following the introduction of a humid air sample. These ions react in multiple collisions with water molecules, their isotopic compositions reach equilibrium and the relative magnitudes of their isotopomers are measured by mass spectrometry.

Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry which utilizes gas-phase ion-molecule reactions at atmospheric pressure (10⁵ Pa), commonly coupled with high-performance liquid chromatography (HPLC). APCI is a soft ionization method similar to chemical ionization where primary ions are produced on a solvent spray. The main usage of APCI is for polar and relatively less polar thermally stable compounds with molecular weight less than 1500 Da. The application of APCI with HPLC has gained a large popularity in trace analysis detection such as steroids, pesticides and also in pharmacology for drug metabolites.

In mass spectrometry, direct analysis in real time (DART) is an ion source that produces electronically or vibronically excited-state species from gases such as helium, argon, or nitrogen that ionize atmospheric molecules or dopant molecules. The ions generated from atmospheric or dopant molecules undergo ion-molecule reactions with the sample molecules to produce analyte ions. Analytes with low ionization energy may be ionized directly. The DART ionization process can produce positive or negative ions depending on the potential applied to the exit electrode.

Field desorption (FD) is a method of ion formation used in mass spectrometry (MS) in which a high-potential electric field is applied to an emitter with a sharp surface, such as a razor blade, or more commonly, a filament from which tiny "whiskers" have formed. This results in a high electric field which can result in ionization of gaseous molecules of the analyte. Mass spectra produced by FD have little or no fragmentation because FD is a soft ionization method. They are dominated by molecular radical cations M^+. and less often, protonated molecules $. The technique was first reported by Beckey in 1969. It is also the first ionization method to ionize nonvolatile and thermally labile compounds. One major difference of FD with other ionization methods is that it does not need a primary beam to bombard a sample.$

Desorption electrospray ionization (DESI) is an ambient ionization technique that can be coupled to mass spectrometry (MS) for chemical analysis of samples at atmospheric conditions. Coupled ionization sources-MS systems are popular in chemical analysis because the individual capabilities of various sources combined with different MS systems allow for chemical determinations of samples. DESI employs a fast-moving charged solvent stream, at an angle relative to the sample surface, to extract analytes from the surfaces and propel the secondary ions toward the mass analyzer. This tandem technique can be used to analyze forensics analyses, pharmaceuticals, plant tissues, fruits, intact biological tissues, enzyme-substrate complexes, metabolites and polymers. Therefore, DESI-MS may be applied in a wide variety of sectors including food and drug administration, pharmaceuticals, environmental monitoring, and biotechnology.

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical chemistry technique that uses gas phase hydronium reagent ions which are produced in an ion source. PTR-MS is used for online monitoring of volatile organic compounds (VOCs) in ambient air and was developed in 1995 by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria. A PTR-MS instrument consists of an ion source that is directly connected to a drift tube and an analyzing system. Commercially available PTR-MS instruments have a response time of about 100 ms and reach a detection limit in the single digit pptv or even ppqv region. Established fields of application are environmental research, food and flavor science, biological research, medicine, security, cleanroom monitoring, etc.

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.

A direct electron ionization liquid chromatography–mass spectrometry interface is a technique for coupling liquid chromatography and mass spectrometry (LC-MS) based on the direct introduction of the liquid effluent into an electron ionization (EI) source. Library searchable mass spectra are generated. Gas-phase EI has many applications for the detection of HPLC amenable compounds showing minimal adverse matrix effects. The direct-EI LC-MS interface provides access to well-characterized electron ionization data for a variety of LC applications and readily interpretable spectra from electronic libraries for environmental, food safety, pharmaceutical, biomedical, and other applications.

Extractive electrospray ionization (EESI) is a spray-type, ambient ionization source in mass spectrometry that uses two colliding aerosols, one of which is generated by electrospray. In standard EESI, syringe pumps provide the liquids for both an electrospray and a sample spray. In neutral desorption EESI (ND-EESI), the liquid for the sample aerosol is provided by a flow of nitrogen.

Atmospheric pressure photoionization (APPI) is a soft ionization method used in mass spectrometry (MS) usually coupled to liquid chromatography (LC). Molecules are ionized using a vacuum ultraviolet (VUV) light source operating at atmospheric pressure, either by direct absorption followed by electron ejection or through ionization of a dopant molecule that leads to chemical ionization of target molecules. The sample is usually a solvent spray that is vaporized by nebulization and heat. The benefit of APPI is that it ionizes molecules across a broad range of polarity and is particularly useful for ionization of low polarity molecules for which other popular ionization methods such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are less suitable. It is also less prone to ion suppression and matrix effects compared to ESI and APCI and typically has a wide linear dynamic range. The application of APPI with LC/MS is commonly used for analysis of petroleum compounds, pesticides, steroids, and drug metabolites lacking polar functional groups and is being extensively deployed for ambient ionization particularly for explosives detection in security applications.

<span class="mw-page-title-main">Secondary electrospray ionization</span>

Secondary electro-spray ionization (SESI) is an ambient ionization technique for the analysis of trace concentrations of vapors, where a nano-electrospray produces charging agents that collide with the analyte molecules directly in gas-phase. In the subsequent reaction, the charge is transferred and vapors get ionized, most molecules get protonated and deprotonated. SESI works in combination with mass spectrometry or ion-mobility spectrometry.

References

1 2 3 "Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis" Smith D., Španěl P.; Mass Spectrometry Reviews 24 (2005) pp661– 700. doi : 10.1002/mas.20033
↑ "The selected ion flow tube (SIFT); A technique for studying ion-neutral reactions" Adams N.G., Smith D.; International Journal of Mass Spectrometry and Ion Physics 21 (1976) pp349-359.
↑ "Parametrization of the ion-polar molecule collision rate constant by trajectory calculations" Su T., Chesnavich W.J.; Journal of Chemical Physics 76 (1982) pp5183-5186.
↑ "An index of the literature for bimolecular gas phase cation-molecule reaction kinetics" Anicich, V. G.; JPL-Publication-03-19 Pasadena, CA, USA http://hdl.handle.net/2014/7981
↑ Martínez-Lozano, P; Zingaro, L; Finiguerra, A; Cristoni, S (2011-03-01). "Secondary electrospray ionization-mass spectrometry: breath study on a control group". Journal of Breath Research. 5 (1): 016002. Bibcode:2011JBR.....5a6002M. doi:10.1088/1752-7155/5/1/016002. ISSN 1752-7155. PMID 21383424. S2CID 9881848.
↑ Vidal-de-Miguel, Guillermo; Herrero, Ana (June 2012). "Secondary Electrospray Ionization of Complex Vapor Mixtures. Theoretical and Experimental Approach". Journal of the American Society for Mass Spectrometry. 23 (6): 1085–1096. Bibcode:2012JASMS..23.1085V. doi: 10.1007/s13361-012-0369-z . ISSN 1044-0305. PMID 22528202.
↑ K. Buhr, S. van Ruth, C. Delahunty, Analysis of volatile flavour compounds by Proton Transfer Reaction-Mass Spectrometry: fragmentation patterns and discrimination between isobaric and isomeric compounds, Int. J. Mass Spectrom., 221, 1-7 (2002)
↑ "Quantification of methane in humid air and exhaled breath using selected ion flow tube mass spectrometry" Dryahina K., Smith D., Španěl P.; Rapid Communications in Mass Spectrometry 24 (2010) pp1296–1304.
↑ B.J. Prince, D.B. Milligan, M.J. McEwan, Application of selected ion flow tube mass spectrometry to real-time atmospheric monitoring, Rapid Communications in Mass Spectrometry, 24, 1763-1769 (2010)

Literature

"The selected ion flow tube (SIFT); A technique for studying ion-neutral reactions" Adams N.G., Smith D.; International Journal of Mass Spectrometry and Ion Physics 21 (1976) pp. 349–359.
"Parametrization of the ion-polar molecule collision rate constant by trajectory calculations" Su T., Chesnavich W.J.; Journal of Chemical Physics 76 (1982) pp. 5183–5186.
"Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis" Smith D., Španěl P.; Mass Spectrometry Reviews 24 (2005) pp. 661–700.
"Quantification of methane in humid air and exhaled breath using selected ion flow tube mass spectrometry" Dryahina K., Smith D., Španěl P.; Rapid Communications in Mass Spectrometry 24 (2010) pp. 1296–1304.

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[Selected-1] 1 2 3 "Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis" Smith D., Španěl P.; Mass Spectrometry Reviews 24 (2005) pp661– 700. doi : 10.1002/mas.20033

[2] "The selected ion flow tube (SIFT); A technique for studying ion-neutral reactions" Adams N.G., Smith D.; International Journal of Mass Spectrometry and Ion Physics 21 (1976) pp349-359.

[3] "Parametrization of the ion-polar molecule collision rate constant by trajectory calculations" Su T., Chesnavich W.J.; Journal of Chemical Physics 76 (1982) pp5183-5186.

[Index-4] "An index of the literature for bimolecular gas phase cation-molecule reaction kinetics" Anicich, V. G.; JPL-Publication-03-19 Pasadena, CA, USA http://hdl.handle.net/2014/7981

[5] Martínez-Lozano, P; Zingaro, L; Finiguerra, A; Cristoni, S (2011-03-01). "Secondary electrospray ionization-mass spectrometry: breath study on a control group". Journal of Breath Research. 5 (1): 016002. Bibcode:2011JBR.....5a6002M. doi:10.1088/1752-7155/5/1/016002. ISSN 1752-7155. PMID 21383424. S2CID 9881848.

[6] Vidal-de-Miguel, Guillermo; Herrero, Ana (June 2012). "Secondary Electrospray Ionization of Complex Vapor Mixtures. Theoretical and Experimental Approach". Journal of the American Society for Mass Spectrometry. 23 (6): 1085–1096. Bibcode:2012JASMS..23.1085V. doi: 10.1007/s13361-012-0369-z . ISSN 1044-0305. PMID 22528202.

[7] K. Buhr, S. van Ruth, C. Delahunty, Analysis of volatile flavour compounds by Proton Transfer Reaction-Mass Spectrometry: fragmentation patterns and discrimination between isobaric and isomeric compounds, Int. J. Mass Spectrom., 221, 1-7 (2002)

[8] "Quantification of methane in humid air and exhaled breath using selected ion flow tube mass spectrometry" Dryahina K., Smith D., Španěl P.; Rapid Communications in Mass Spectrometry 24 (2010) pp1296–1304.

[9] B.J. Prince, D.B. Milligan, M.J. McEwan, Application of selected ion flow tube mass spectrometry to real-time atmospheric monitoring, Rapid Communications in Mass Spectrometry, 24, 1763-1769 (2010)

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