Mass spectrum

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Electron ionization mass spectrum of toluene . Note parent peak corresponding to molecular mass M = 92 (C7H8 ) and highest peak at M-1 = 91 (C7H7 , quasi-stable tropylium cation). Toluene ei ms.PNG
Electron ionization mass spectrum of toluene .
Note parent peak corresponding to molecular mass M = 92 (C7H8 ) and highest peak at M-1 = 91 (C7H7 , quasi-stable tropylium cation).

A mass spectrum is a histogram plot of intensity vs. mass-to-charge ratio (m/z) in a chemical sample, [1] 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. [2]

Contents

X-axis: m/z (mass-to-charge ratio)

The x-axis of a mass spectrum represents a relationship between the mass of a given ion and the number of elementary charges that it carries. This is written as the IUPAC standard m/z to denote the quantity formed by dividing the mass of an ion (in daltons) by the dalton unit and by its charge number (positive absolute value). [3] [4] [5] Thus, m/z is a dimensionless quantity with no associated units. [3] Despite carrying neither units of mass nor charge, the m/z is referred to as the mass-to-charge ratio of an ion. However, this is distinct from the mass-to-charge ratio, m/Q (SI standard units kg/C), which is commonly used in physics. The m/z is used in applied mass spectrometry because convenient and intuitive numerical relationships naturally arise when interpreting spectra. A single m/z value alone does not contain sufficient information to determine the mass or charge of an ion. However, mass information may be extracted when considering the whole spectrum, such as the spacing of isotopes or the observation of multiple charge states of the same molecule. These relationships and the relationship to the mass of the ion in daltons tend toward approximately rational number values in m/z space. For example, ions with one charge exhibit spacing between isotopes of 1 and the mass of the ion in daltons is numerically equal to the m/z. The IUPAC Gold Book gives an example of appropriate use: [3] "for the ion C7H72+, m/z equals 45.5".


Alternative x-axis notations

There are several alternatives to the standard m/z notation that appear in the literature; however, these are not currently accepted by standards organizations and most journals. m/e appears in older historical literature. A label more consistent with the IUPAC green book and ISO 31 conventions is m/Q or m/q where m is the symbol for mass and Q or q the symbol for charge with the units u/e or Da/e. This notation is not uncommon in the physics of mass spectrometry but is rarely used as the abscissa of a mass spectrum. It was also suggested to introduce a new unit thomson (Th) as a unit of m/z, where 1 Th = 1 u/e. [6] According to this convention, mass spectra x axis could be labeled m/z (Th) and negative ions would have negative values. This notation is rare and not accepted by IUPAC or any other standards organisation.

History of x-axis notation

Mass spectrum of sodium and potassium positive ions from Arthur Dempster's 1918 publication "A new Method of Positive Ray Analysis " Phys. Rev. 11, 316 (1918) Dempster mass spectrum.gif
Mass spectrum of sodium and potassium positive ions from Arthur Dempster's 1918 publication "A new Method of Positive Ray Analysis " Phys. Rev.11, 316 (1918)

In 1897 the mass-to-charge ratio of the electron was first measured by J. J. Thomson. [7] By doing this he showed that the electron, which was postulated before in order to explain electricity, was in fact a particle with a mass and a charge and that its mass-to-charge ratio was much smaller than the one for the hydrogen ion H+. In 1913 he measured the mass-to-charge ratio of ions with an instrument he called a parabola spectrograph. [8] Although this data was not represented as a modern mass spectrum, it was similar in meaning. Eventually there was a change to the notation as m/e giving way to the current standard of m/z.[ citation needed ]

Early in mass spectrometry research the resolution of mass spectrometers did not allow for accurate mass determination. Francis William Aston won the Nobel prize in Chemistry in 1922. [9] "For his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the Whole Number Rule." In which he stated that all atoms (including isotopes) follow a whole-number rule [10] This implied that the masses of atoms were not on a scale but could be expressed as integers (in fact multiple charged ions were rare, so for the most part the ratio was whole as well). There have been several suggestions (e.g. the unit thomson) to change the official mass spectrometry nomenclature to be more internally consistent.

Y-axis: signal intensity

The y-axis of a mass spectrum represents signal intensity of the ions. When using counting detectors the intensity is often measured in counts per second (cps). When using analog detection electronics the intensity is typically measured in volts. In FTICR and Orbitraps the frequency domain signal (the y-axis) is related to the power (~amplitude squared) of the signal sine wave (often reduced to an rms power); however, the axis is usually not labeled as such for many reasons. In most forms of mass spectrometry, the intensity of ion current measured by the spectrometer does not accurately represent relative abundance, but correlates loosely with it. Therefore, it is common to label the y-axis with "arbitrary units".

Y-axis and relative abundance

Signal intensity may be dependent on many factors, especially the nature of the molecules being analyzed and how they ionize. The efficiency of ionization varies from molecule to molecule and from ion source to ion source. For example, in electrospray sources in positive ion mode a quaternary amine will ionize exceptionally well whereas a large hydrophobic alcohol will most likely not be seen no matter how concentrated. In an EI source these molecules will behave very differently. Additionally there may be factors that affect ion transmission disproportionally between ionization and detection.

On the detection side there are many factors that can also affect signal intensity in a non-proportional way. The size of the ion will affect the velocity of impact and with certain detectors the velocity is proportional to the signal output. In other detection systems, such as FTICR, the number of charges on the ion are more important to signal intensity. In Fourier transform ion cyclotron resonance and Orbitrap type mass spectrometers the signal intensity (Y-axis) is related to the amplitude of the free induction decay signal. This is fundamentally a power relationship (amplitude squared) but often computed as an [rms]. For decaying signals the rms is not equal to the average amplitude. Additionally the damping constant (decay rate of the signal in the fid) is not the same for all ions. In order to make conclusions about relative intensity a great deal of knowledge and care is required.

A common way to get more quantitative information out of a mass spectrum is to create a standard curve to compare the sample to. This requires knowing what is to be quantitated ahead of time, having a standard available and designing the experiment specifically for this purpose. A more advanced variation on this is the use of an internal standard which behaves very similarly to the analyte. This is often an isotopically labeled version of the analyte. There are forms of mass spectrometry, such as accelerator mass spectrometry that are designed from the bottom up to be quantitative.

Spectral skewing

Spectral skewing is the change in relative intensity of mass spectral peaks due to the changes in concentration of the analyte in the ion source as the mass spectrum is scanned. This situation occurs routinely as chromatographic components elute into a continuous ion source. [11] Spectral skewing is not observed in ion trap (quadrupole (this has been seen also in QMS) or magnetic) or time-of-flight (TOF) mass analyzers because potentially all ions formed in operational cycle (a snapshot in time) of the instrument are available for detection.

See also

Related Research Articles

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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">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]]

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<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">Fast atom bombardment</span>

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

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

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A mass chromatogram is a representation of mass spectrometry data as a chromatogram, where the x-axis represents time and the y-axis represents signal intensity. The source data contains mass information; however, it is not graphically represented in a mass chromatogram in favor of visualizing signal intensity versus time. The most common use of this data representation is when mass spectrometry is used in conjunction with some form of chromatography, such as in liquid chromatography–mass spectrometry or gas chromatography–mass spectrometry. In this case, the x-axis represents retention time, analogous to any other chromatogram. The y-axis represents signal intensity or relative signal intensity. There are many different types of metrics that this intensity may represent, depending on what information is extracted from each mass spectrum.

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

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<span class="mw-page-title-main">Time-of-flight mass spectrometry</span> Method of mass spectrometry

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<span class="mw-page-title-main">Mass spectral interpretation</span>

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

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<span class="mw-page-title-main">Direct electron ionization liquid chromatography–mass spectrometry interface</span>

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.

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

Thermal ionization mass spectrometry (TIMS) is also known as surface ionization and is a highly sensitive isotope mass spectrometry characterization technique. The isotopic ratios of radionuclides are used to get an accurate measurement for the elemental analysis of a sample. Singly charged ions of the sample are formed by the thermal ionization effect. A chemically purified liquid sample is placed on a metal filament which is then heated to evaporate the solvent. The removal of an electron from the purified sample is consequently achieved by heating the filament enough to release an electron, which then ionizes the atoms of the sample. TIMS utilizes a magnetic sector mass analyzer to separate the ions based on their mass to charge ratio. The ions gain velocity by an electrical potential gradient and are focused into a beam by electrostatic lenses. The ion beam then passes through the magnetic field of the electromagnet where it is partitioned into separate ion beams based on the ion's mass/charge ratio. These mass-resolved beams are directed into a detector where it is converted into voltage. The voltage detected is then used to calculate the isotopic ratio.

References

  1. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " mass spectrum ". doi : 10.1351/goldbook.M03749
  2. ↑ Turecek, František; McLafferty, Fred W. (1993). Interpretation of mass spectra . Sausalito, Calif: University Science Books. pp.  226-. ISBN   0-935702-25-3.
  3. 1 2 3 IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " mass-to-charge ratio ". doi : 10.1351/goldbook.M03752
  4. ↑ Todd, John F.J. (1995). "Recommendations for nomenclature and symbolism for mass spectroscopy". International Journal of Mass Spectrometry and Ion Processes. 142 (3): 209–240. Bibcode:1995IJMSI.142..209T. doi:10.1016/0168-1176(95)93811-F.
  5. ↑ "TOC_cha12.html". iupac.org.
  6. ↑ Cooks, R. G. and A. L. Rockwood (1991). "The 'Thomson'. A suggested unit for mass spectroscopists." Rapid Communications in Mass Spectrometry 5(2): 93.
  7. ↑ "J. J. Thomson 1897". lemoyne.edu.
  8. ↑ "Joseph John Thomson". lemoyne.edu.
  9. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 13 May 2006. Retrieved 18 April 2006.{{cite web}}: CS1 maint: archived copy as title (link)
  10. ↑ "F. W. Aston". lemoyne.edu.
  11. ↑ Watson, J. THrock, Sparkman,O David.Introduction to Mass Spectrometry.John Wiley & Sons, Inc. 4th Edition,2007. Page:113
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