Proton-transfer-reaction mass spectrometry

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PTR-TOF mass spectrometer PTR-TOF1000 Ultra Ionicon mass spectrometer, Faculty of Chemistry, Gdansk University of Technology Poland.jpg
PTR-TOF mass spectrometer

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. [1] 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. [2] A PTR-MS instrument consists of an ion source that is directly connected to a drift tube (in contrast to SIFT-MS no mass filter is interconnected) and an analyzing system (quadrupole mass analyzer or time-of-flight mass spectrometer). 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. [1]

Contents

Theory

With H3O+ as the reagent ion the proton transfer process is (with R being the trace component)

Fig. 1: Evolution of reagent ion yields and sensitivities of PTR-MS instruments taken from peer reviewed journal articles Evolution of ptr sensitivities.PNG
Fig. 1: Evolution of reagent ion yields and sensitivities of PTR-MS instruments taken from peer reviewed journal articles

(1)

Reaction ( 1 ) is only possible if energetically allowed, i.e. if the proton affinity of R is higher than the proton affinity of H2O (691 kJ/mol [3] ). As most components of ambient air possess a lower proton affinity than H2O (e.g. N2, O2, Ar, CO2, etc.) the H3O+ ions only react with VOC trace components and the air itself acts as a buffer gas. Moreover, due to the low concentrations of trace components one can assume that the total number of H3O+ ions remains nearly unchanged, which leads to the equation [4]

(2)

In equation ( 2 ) is the density of product ions, is the density of reagent ions in absence of reactant molecules in the buffer gas, k is the reaction rate constant and t is the average time the ions need to pass the reaction region. With a PTR-MS instrument the number of product and of reagent ions can be measured, the reaction rate constant can be found in literature for most substances [5] and the reaction time can be derived from the set instrument parameters. Therefore, the absolute concentration of trace constituents can be easily calculated without the need of calibration or gas standards. Furthermore, it gets obvious that the overall sensitivity of a PTR-MS instrument is dependent on the reagent ion yield. Fig. 1 gives an overview of several published (in peer-reviewed journals) reagent ion yields during the last decades and the corresponding sensitivities.

Technology

Hydronium ions generated from water vapor in an ionizer are reacting with analytes in a drift chamber. Ions are separated in an analyzer based on the mass-to-charge ratio, and are subsequently transferred to a detector where identification takes place. PTR mass spectrometer with quadrupol mass analyzer, schematics.png
Hydronium ions generated from water vapor in an ionizer are reacting with analytes in a drift chamber. Ions are separated in an analyzer based on the mass-to-charge ratio, and are subsequently transferred to a detector where identification takes place.

In commercial PTR-MS instruments water vapor is ionized in a hollow cathode discharge:

.

After the discharge a short drift tube is used to form very pure (>99.5% [4] ) H3O+ via ion-molecule reactions:

.

Due to the high purity of the reagent ions a mass filter between the ion source and the reaction drift tube is not necessary and H3O+ can be injected directly. The absence of this mass filter in turn greatly reduces losses of reagent ions and leads eventually to an outstandingly low detection limit of the whole instrument. In the reaction drift tube a vacuum pump is continuously drawing through air containing the VOCs one wants to analyze. At the end of the drift tube the protonated molecules are mass analyzed (quadrupole mass analyzer or time-of-flight mass spectrometer) and detected.

As an alternative to H3O+ already in early PTR-MS related publications the use of NH4+ reagent ions has been suggested. [4] Ammonia has a proton affinity of 853.6 kJ/mol. [6] For compounds that have a higher proton affinity than ammonia proton transfer can take place similar to the process described above for hydronium:

.

Additionally, for compounds with higher, but also for some with lower proton affinities than ammonia a clustering reaction can be observed

*

where the cluster needs a third body to get collisionally stabilized. The main advantage of using NH4+ reagent ions is that fragmentation of analytes upon chemical ionization is strongly suppressed, leading to straightforward mass spectra even for complex mixtures. The reason why during the first 20 years after the invention of PTR-MS NH4+ reagent ions have only been used in a very limited number of studies is most probably because the NH4+ production required toxic and corrosive ammonia as a source gas. This led to problems with handling the instrument and its exhaust gas, as well as to increased wear of vacuum components. In 2017 a patent application was submitted where the inventors introduced a novel method of NH4+ production without the need of any form of ammonia. [7] In this method N2 and water vapor are introduced into the hollow cathode ion source and by adjusting electric fields and pressures NH4+ can be produced at the same or even higher purity levels than H3O+. It is expected that this invention, which eliminates the problems connected to the use of NH4+ so far, will lead to a widespread use of NH4+ reagent ions in the near future. [8]

Advantages

Advantages include low fragmentation – only a small amount of energy is transferred during the ionization process (compared to e.g. electron ionization), therefore fragmentation is suppressed and the obtained mass spectra are easily interpretable, no sample preparation is necessary – VOC containing air and liquids' headspaces can be analyzed directly, real-time measurements – with a typical response time of 100 ms VOCs can be monitored on-line, real-time quantification – absolute concentrations are obtained directly without previous calibration measurements, compact and robust setup – due to the simple design and the low number of parts needed for a PTR-MS instrument, it can be built in into space saving and even mobile housings, easy to operate – for the operation of a PTR-MS only electric power and a small amount of distilled water are needed. Unlike other techniques no gas cylinders are needed for buffer gas or calibration standards.

Disadvantages

One disadvantage is that not all molecules are detectable. Because only molecules with a proton affinity higher than water can be detected by PTR-MS, proton transfer from H3O+ is not suitable for all fields of application. Therefore, in 2009 first PTR-MS instruments were presented, which are capable of switching between H3O+ and O2+ (and NO+) as reagent ions. [9] This enhances the number of detectable substances to important compounds like ethylene, acetylene, most halocarbons, etc. Furthermore, particularly with NO+ it is possible to separate and independently quantify some isomers. [9] In 2012 a PTR-MS instrument was introduced which extends the selectable reagent ions to Kr+ and Xe+; [10] this should allow for the detection of nearly all possible substances (up to the ionization energy of krypton (14 eV [11] )). Although the ionization method for these additional reagent ions is charge-exchange rather than proton-transfer ionization the instruments can still be considered as "classic" PTR-MS instruments, i.e. no mass filter between the ion source and the drift tube and only some minor modifications on the ion source and vacuum design.

The maximum measurable concentration is limited. Equation (2) is based on the assumption that the decrease of reagent ions is negligible, therefore the total concentration of VOCs in air must not exceed about 10 ppmv. Otherwise the instrument's response will not be linear anymore and the concentration calculation will be incorrect. This limitation can be overcome easily by diluting the sample with a well-defined amount of pure air.

Sensitivity enhancing measures

As it is the case for most analytical instruments, also in PTR-MS there has always been a quest for sensitivity improvement and for lowering the detection limit. However, until 2012 these improvements were limited to optimizations of the conventional setup, i.e. ion source, DC drift tube, transfer lens system, mass spectrometer (compare above). The reason for this conservative approach was that the addition of any RF ion focusing device negatively affects the well-defined PTR-MS ion chemistry, which makes quantification complicated and considerably limits comparability of measurement results obtained with different instruments. Only in 2016 a patent application providing a solution to this problem was submitted. [12]

Ion funnel

Ion funnels are RF devices which have been used for decades to focus ion currents into narrow beams. In PTR-MS they have been introduced in 2012 by Barber et al. [13] when they presented a PTR-MS setup with a PTR reaction region incorporating an ion funnel. Although the focusing properties of the ion funnel improved the sensitivity of the setup by a factor of >200 (compared to operating in DC only mode, i.e. with the ion funnel turned off) for some compounds, the sensitivities of other compounds were only improved by a factor of <10. [13] That is, because of the highly compound dependent instrumental response one of the main advantages of PTR-MS, namely that concentration values can be directly calculated, is lost and a calibration measurement is needed for each analyte of interest. Furthermore, with this approach unusual fragmentation of analytes has been observed [14] which complicates interpretation of measurement results and comparison between different types of instruments even more. A different concept has been introduced by the company IONICON Analytik GmbH. [15] (Innsbruck, AT) where the ion funnel is not predominantly part of the reaction region but mainly for focusing the ions into the transfer region to the TOF mass spectrometer. [16] In combination with the above-mentioned method of controlling the ion chemistry [12] this enables a considerable increase in sensitivity and thus also an improvement of the detection limit, while keeping the ion chemistry well-defined and thus avoiding problems with quantification and interpretation of the results.

Ion guide

Quadrupole, hexapole and other multipole ion guides can be used to transfer ions between different parts of an instrument with high efficiency. In PTR-MS they are particularly suitable for being installed in the differentially pumped interface between the reaction region and the mass spectrometer. In 2014 Sulzer et al. [17] published an article about a PTR-MS instrument which utilizes a quadrupole ion guide between the drift tube and the TOF mass spectrometer. They reported an increase in sensitivity by a factor of 25 compared to a similar instrument without an ion guide. Quadrupole ion guides are known to have high focusing power, but also rather narrow m/z transmission bands. [18] Hexapole ion guides on the other hand have focusing capabilities over a broader m/z band. Additionally, less energy is put into the transmitted ions, i.e. fragmentation and other adverse effects are less likely to occur. Consequently, some latest high-end PTR-MS instruments are equipped with hexapole ion guides for considerably improved performance [16] or even with a sequential arrangement of an ion funnel followed by a hexapole ion guide for even higher sensitivity and lower detection limit. [19]

Add-ons

As a real-time trace gas analysis method based on mass spectrometry, PTR-MS has two obvious limitations: Isomers cannot be easily separated (for some it is possible by switching the reagent ions [9] or by changing the reduced electric field strength in the drift tube) and the sample has to be in the gas phase. Countermeasures against these limitations have been developed in the form of add-ons, which can either be installed into the PTR-MS instrument or operated as external devices.

FastGC

Gas chromatography (GC) in combination with mass spectrometry (GC-MS) is capable of separating isomeric compounds. Although GC has been successfully coupled to PTR-MS in the past, [20] this approach annihilates the real-time capability of the PTR-MS technology, because a single GC analysis run typically takes between 30 min and 1 h. Thus, state-of-the-art GC add-ons for PTR-MS are based on fastGC technology. Materic et al. [21] utilized an early version of a commercially available fastGC addon in order to distinguish various monoterpene isomers. Within a fastGC run of about 70 s they were able to separate and identify: alpha-pinene, beta-pinene, camphene, myrcene, 3-carene and limonene in a standard mixture, Norway spruce, Scots pine and black pine samples, respectively. Particularly, if the operation mode of a PTR-MS instrument equipped with fastGC is continuously switched between fastGC and direct injection (dependent on the application, e.g. a loop sequence of one fastGC run followed by 10 min of direct injection measurement), real-time capability is preserved, while at the same time valuable information on substance identification and isomer separation is acquired.

Aerosol and particulate matter inlet

Researchers at the Leopold-Franzens University in Innsbruck invented a dedicated PTR-MS inlet system for the analysis of aerosols and particulate matter, [22] which they called "CHemical Analysis of aeRosol ON-line (CHARON)". After further development work in collaboration with a PTR-MS manufacturer, CHARON has become readily available as an add-on for PTR-MS instruments in 2017. [23] The add-on consists of a honeycomb activated charcoal denuder which adsorbs organic gases but transmits particles, an aerodynamic lens system that collimates sub-μm particles, and a thermo-desorber that evaporates non-refractory organic particulate matter at moderate temperatures of 100-160 °C and reduced pressures of a few mbar. So far, CHARON has predominantly been used within studies in the field of atmospheric chemistry, e.g. for airborne measurements of particulate organic matter [24] and bulk organic aerosol analysis. [25]

Inlet for liquids

A now well established setup for the controlled evaporation and subsequent analysis of liquids with PTR-MS has been published in 2013 by Fischer et al. [26] As the authors saw the main application of their setup in the calibration of PTR-MS instruments via aqueous standards, they named it "Liquid Calibration Unit (LCU)". The LCU sprays a liquid standard into a gas stream at well-defined flow rates via a purpose-built nebulizer (optimized for reduced probability of clogging and high tolerance to salts in the liquid). The resulting micro-droplets are injected into a heated (> 100 °C) evaporation chamber. This concept offers two main advantages: (i) the evaporation of compounds is enhanced by the enlarged surface area of the droplets and (ii) compounds which are dissociated in water, such as acids (or bases), experience a shift in pH value when the water evaporates from a droplet. This in turn reduces dissociation and supports total evaporation of the compound. [26] The resulting continuous gas flow containing the analytes can be directly introduced into a PTR-MS instrument for analysis.

Applications

The most common applications for the PTR-MS technique are environmental research, [27] [28] [29] waste incineration, food science, [30] biological research, [31] process monitoring, indoor air quality, [32] [33] [34] medicine and biotechnology [35] [36] [37] [38] and Homeland security. [39] [40] Trace gas analysis is another common application. Some other techniques are Secondary electrospray ionization (SESI), Electrospray ionization (ESI), and Selected-ion flow-tube mass spectrometry (SIFT).

Food science

Fig. 2: PTR-MS measurement of vanillin dissemination in human breath. Isoprene is a product of human metabolism and acts as an indicator for breath cycles. (The measurement was performed utilizing a "N.A.S.E." inlet system coupled to a "HS PTR-MS".) Ptr measurement of vanillin in breath.png
Fig. 2: PTR-MS measurement of vanillin dissemination in human breath. Isoprene is a product of human metabolism and acts as an indicator for breath cycles. (The measurement was performed utilizing a "N.A.S.E." inlet system coupled to a "HS PTR-MS".)

Fig. 2 shows a typical PTR-MS measurement performed in food and flavor research. The test person swallows a sip of a vanillin flavored drink and breathes via his nose into a heated inlet device coupled to a PTR-MS instrument. Due to the high time resolution and sensitivity of the instrument used here, the development of vanillin in the person's breath can be monitored in real-time (please note that isoprene is shown in this figure because it is a product of human metabolism and therefore acts as an indicator for the breath cycles). The data can be used for food design, i.e. for adjusting the intensity and duration of vanillin flavor tasted by the consumer.

Fig. 3: PTR mass spectrum of laboratory air obtained using a TOF based PTR instrument. Ptr mass spectrum of lab air.png
Fig. 3: PTR mass spectrum of laboratory air obtained using a TOF based PTR instrument.

Another example for the application of PTR-MS in food science was published in 2008 by C. Lindinger et al. [42] in Analytical Chemistry . This publication found great response even in non-scientific media. [43] [44] Lindinger et al. developed a method to convert "dry" data from a PTR-MS instrument that measured headspace air from different coffee samples into expressions of flavor (e.g. "woody", "winey", "flowery", etc.) and showed that the obtained flavor profiles matched nicely to the ones created by a panel of European coffee tasting experts.

Air quality analysis

In Fig. 3 a mass spectrum of air inside a laboratory (obtained with a time-of-flight (TOF) based PTR-MS instrument), is shown. The peaks on m/z 19, 37 and 55 (and their isotopes) represent the reagent ions (H3O+) and their clusters. On m/z 30 and 32 NO+ and O2+, which are both impurities originating from the ion source, appear. All other peaks correspond to compounds present in typical laboratory air (e.g. high intensity of protonated acetone on m/z 59). If one takes into account that virtually all peaks visible in Fig. 3 are in fact double, triple or multiple peaks (isobaric compounds) it becomes obvious that for PTR-MS instruments selectivity is at least as important as sensitivity, especially when complex samples / compositions are analyzed. One methods for improving the selectivity is high mass resolution. When the PTR source is coupled to a high resolution mass spectrometer isobaric compounds can be distinguished and substances can be identified via their exact mass. [45] Some PTR-MS instruments are despite of the lack of a mass filter between the ion source and the drift tube capable of switching the reagent ions (e.g. to NO+ or O2+). With the additional information obtained by using different reagent ions a much higher level of selectivity can be reached, e.g. some isomeric molecules can be distinguished. [9]

See also

Related Research Articles

In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the cation [H3O]+, also written as H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: the Eigen cation, which is a tetrahydrate, H3O+(H2O)3, the Zundel cation, which is a symmetric dihydrate, H+(H2O)2, and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.

<span class="mw-page-title-main">Inductively coupled plasma mass spectrometry</span> Type of mass spectrometry that uses an inductively coupled plasma to ionize the sample

Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It is known and used for its ability to detect metals and several non-metals in liquid samples at very low concentrations. It can detect different isotopes of the same element, which makes it a versatile tool in isotopic labeling.

<span class="mw-page-title-main">Mass spectrometry</span> Analytical technique based on determining mass to charge ratio of ions

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.

<span class="mw-page-title-main">Ion source</span> Device that creates charged atoms and molecules (ions)

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.

<span class="mw-page-title-main">Electron ionization</span> Ionization technique

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">Tandem mass spectrometry</span> Type of mass spectrometry

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

<span class="mw-page-title-main">Gas chromatography–mass spectrometry</span> Analytical method

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, food and flavor analysis, 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.

<span class="mw-page-title-main">Selected-ion flow-tube mass spectrometry</span>

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. 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.

<span class="mw-page-title-main">Chemical ionization</span> Ionization technique used in mass [[spectroscopy]]

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.

<span class="mw-page-title-main">Liquid chromatography–mass spectrometry</span> Analytical chemistry technique

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.

In chemistry, molecular autoionization is a chemical reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. In most cases the oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic, or non-protic.

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

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.

<span class="mw-page-title-main">Atmospheric-pressure chemical ionization</span> Ionization method

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.

Sample preparation for mass spectrometry is used for the optimization of a sample for analysis in a mass spectrometer (MS). Each ionization method has certain factors that must be considered for that method to be successful, such as volume, concentration, sample phase, and composition of the analyte solution. Quite possibly the most important consideration in sample preparation is knowing what phase the sample must be in for analysis to be successful. In some cases the analyte itself must be purified before entering the ion source. In other situations, the matrix, or everything in the solution surrounding the analyte, is the most important factor to consider and adjust. Often, sample preparation itself for mass spectrometry can be avoided by coupling mass spectrometry to a chromatography method, or some other form of separation before entering the mass spectrometer. In some cases, the analyte itself must be adjusted so that analysis is possible, such as in protein mass spectrometry, where usually the protein of interest is cleaved into peptides before analysis, either by in-gel digestion or by proteolysis in solution.

<span class="mw-page-title-main">Ion-mobility spectrometry–mass spectrometry</span>

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.

<span class="mw-page-title-main">Triple quadrupole mass spectrometer</span> Type of mass spectrometer

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.

Electron capture ionization is the ionization of a gas phase atom or molecule by attachment of an electron to create an ion of the form . The reaction is

<span class="mw-page-title-main">Atmospheric-pressure photoionization</span> Soft ionization method

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.

References

  1. 1 2 Andrew M. Ellis; Christopher A. Mayhew (17 December 2013). Proton Transfer Reaction Mass Spectrometry: Principles and Applications. Wiley. pp. 15–. ISBN   978-1-118-68412-2.
  2. ↑ A. Hansel, A. Jordan, R. Holzinger, P. Prazeller W. Vogel, W. Lindinger, Proton transfer reaction mass spectrometry: on-line trace gas analysis at ppb level, Int. J. of Mass Spectrom. and Ion Proc., 149/150, 609-619 (1995).
  3. ↑ R.S. Blake, P.S. Monks, A.M. Ellis, Proton-Transfer Reaction Mass Spectrometry, Chem. Rev., 109, 861-896 (2009)
  4. 1 2 3 Lindinger, W.; Hansel, A.; Jordan, A. (1998). "On-line monitoring of volatile organic compounds at pptv levels by means of Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS): Medical applications, food control and environmental research, Review paper". Int. J. Mass Spectrom. Ion Process. 173 (3): 191–241. Bibcode:1998IJMSI.173..191L. doi:10.1016/s0168-1176(97)00281-4.
  5. ↑ Y. Ikezoe, S. Matsuoka and A. Viggiano, Gas Phase Ion-Molecule Reaction Rate Constants through 1986, Maruzen Company Ltd., Tokyo, (1987).
  6. ↑ "Ammonia". webbook.nist.gov.
  7. ↑ 20181220 WOapplication WO2018EP86332 20181220,Hartungen, Eugen,"Method for Producing Gaseous Ammonium for Ion-Molecule-Reaction Mass Spectrometry",published 2019-06-27, assigned to IONICON Analytik GmbH.
  8. ↑ Müller, Markus; Piel, Felix; Gutmann, Rene; Sulzer, Philipp; Hartungen, Eugen; Wisthaler, Eugen (2019). "A novel method for producing NH4+ reagent ions in the hollow cathode glow discharge ion source of PTR-MS instruments". Int. J. Mass Spectrom. 447: 116254. doi: 10.1016/j.ijms.2019.116254 . hdl: 10852/83907 .
  9. 1 2 3 4 Jordan, A.; Haidacher, S.; Hanel, G.; Hartungen, E.; Herbig, J.; Märk, L.; Schottkowsky, R.; Seehauser, H.; Sulzer, P.; Märk, T.D. (2009). "An online ultra-high sensitivity proton-transfer-reaction mass-spectrometer combined with switchable reagent ion capability (PTR+SRI-MS)". International Journal of Mass Spectrometry. 286 (1): 32–38. Bibcode:2009IJMSp.286...32J. doi:10.1016/j.ijms.2009.06.006.
  10. ↑ Sulzer, P.; Edtbauer, A.; Hartungen, E.; Jürschik, S.; Jordan, A.; Hanel, G.; Feil, S.; Jaksch, S.; Märk, L.; Märk, T. D. (2012). "From conventional Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) to universal trace gas analysis". International Journal of Mass Spectrometry. 321–322: 66–70. Bibcode:2012IJMSp.321...66S. doi:10.1016/j.ijms.2012.05.003.
  11. ↑ "Krypton". webbook.nist.gov.
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