Mass spectrum
A mass spectrum is a
X-axis: m/z (mass-to-charge ratio)
The
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
History of x-axis notation
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
Y-axis: signal intensity
The
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
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
See also
References
- ISBN 0-935702-25-3.
- ^
- .
- ^ "TOC_cha12.html". iupac.org.
- ^ Cooks, R. G. and A. L. Rockwood (1991). "The 'Thomson'. A suggested unit for mass spectroscopists." Rapid Communications in Mass Spectrometry 5(2): 93.
- ^ "J. J. Thomson 1897". lemoyne.edu.
- ^ "Joseph John Thomson". lemoyne.edu.
- ^ "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) - ^ "F. W. Aston". lemoyne.edu.
- ^ Watson, J. THrock, Sparkman, O David.Introduction to Mass Spectrometry.John Wiley & Sons, Inc. 4th Edition, 2007. Page:113
External links
- Quantities, Units and Symbols in Physical Chemistry (IUPAC green book)
- An introductory video on Mass Spectrometry The Royal Society of Chemistry
- NIST Standard Reference Database 1A v17