SIFT-MS selected ion flow tube mass spectrometry
Encyclopedia
Selected ion flow tube mass spectrometry, SIFT-MS, is a quantitative mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 technique for trace gas analysis which involves the chemical ionization
Chemical ionization
Chemical ionization is an ionization technique used in mass spectrometry. Chemical ionization is a lower energy process than electron ionization. The lower energy yields less fragmentation, and usually a simpler spectrum...

 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 ratio of the precursor and product ion signal ratios, without the need for sample preparation
Sample preparation (analytical chemistry)
In analytical chemistry, sample preparation refers to the ways in which a sample is treated prior to its analysis. Preparation is a very important step in most analytical techniques, because the techniques are often not responsive to the analyte in its in-situ form, or the results are distorted by...

 or calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

 with standard mixtures.

The instrument is an extension of the selected ion flow tube, SIFT, technique, which was first described in 1976 by Adams and Smith. 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
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...

. Its application to ionospheric
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 and interstellar ion chemistry
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 over a 20-year period has been crucial to the advancement and understanding of these interesting 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
Diagnosis
Diagnosis is the identification of the nature and cause of anything. Diagnosis is used in many different disciplines with variations in the use of logics, analytics, and experience to determine the cause and effect relationships...

. It has since shown potential for use across a wide variety of fields, particularly in the life sciences
Life sciences
The life sciences comprise the fields of science that involve the scientific study of living organisms, like plants, animals, and human beings. While biology remains the centerpiece of the life sciences, technological advances in molecular biology and biotechnology have led to a burgeoning of...

, such as agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 and animal husbandry
Animal husbandry
Animal husbandry is the agricultural practice of breeding and raising livestock.- History :Animal husbandry has been practiced for thousands of years, since the first domestication of animals....

, environmental research
Environmental science
Environmental science is an interdisciplinary academic field that integrates physical and biological sciences, to the study of the environment, and the solution of environmental problems...

 and food technology
Food technology
Food technology, is a branch of food science which deals with the actual production processes to make foods.-Early history of food technology:...

.

Instrumentation

In the selected ion flow tube mass spectrometer, SIFT-MS, ions are generated in a microwave plasma
Microwave plasma
Microwave plasma is a type of plasma, that has high frequency electromagnetic radiation in the GHz range. It is capable of exciting electrodeless gas discharges.-Properties of microwave-excited 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
Quadrupole mass analyzer
The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...

 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
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 at a pressure of 1 Torr) via a Venturi orifice
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

  (~1 mm diameter) where they travel along the reaction flow tube by convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

. 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
Proton affinity
The proton affinity, Epa, of an anion or of a neutral atom or molecule is a measure of its gas-phase basicity. It is the energy released in the following reactions:-Acid/base chemistry:...

 or ionization energy
Ionization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...

.

The newly formed "product ions" flow into the mass spectrometer chamber, which contains a second quadrupole mass filter, and an electron multiplier
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...

 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
Collision theory
Collision theory is a theory proposed by Max Trautz and William Lewis in 1916 and 1918, that qualitatively explains how chemical reactions occur and why reaction rates differ for different reactions. For a reaction to occur the reactant particles must collide. Only a certain fraction of the total...

), the coefficient for which, kc, is calculable using the method described by Su and Chesnavich, providing the polarizability
Polarizability
Polarizability is the measure of the change in a molecule's electron distribution in response to an applied electric field, which can also be induced by electric interactions with solvents or ionic reagents. It is a property of matter...

 and dipole moment
Bond dipole moment
The bond dipole moment uses the idea of electric dipole moment to measure the polarity of a chemical bond within a molecule. The bond dipole μ is given by:\mu = \delta \, d....

 are known for the reactant molecule. NO+ and O2+ reactions proceed at kc 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 precursor ions to that of H3O+ as the sample flow of reactant molecules is increased. 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.

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
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...

, ppb, level.

It is an important point that the reactions of the chosen precursor ions, H3O+, NO+, and O2+, with the trace analyte molecules occur at relatively low energies (compared to electron ionization
Electron ionization
Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions...

), and therefore tend to cause relatively little fragmentation. This greatly simplifies the resulting spectra and thereby facilitates the analysis of complex mixtures of gases, such as human breath.

Another key feature of SIFT-MS is the upstream mass quadrupole, which allows the use of multiple precursor ions. On the one hand, this means that the precursor ion counts can be limited, compared to say PTR-MS, which has no upstream quadrupole, and as a result is capable of sensitivities to parts-per-trillion, ppt, levels, as opposed to the ppb levels that are possible with SIFT-MS. However, 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+. 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 sulphide (C2H6S, 62amu) 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.
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