Kendrick mass scale: Difference between revisions
No OR, since the cited article does define the unit Kendrick with symbol Ke, see discussion |
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: 1 Ke = 14.0156/14.000 Da = 1.00111429 Da = 1.00111429 u |
: 1 Ke = 14.0156/14.000 Da = 1.00111429 Da = 1.00111429 u |
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== Kendrick mass == |
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In the literature |
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<ref name="Marshall2008">{{cite journal |last1=Marshall |first1=A. G. |last2=Rodgers |first2=R. P. |title=Mass Spectrometry Special Feature: Petroleomics: Chemistry of the underworld |journal=Proceedings of the National Academy of Sciences |volume=105 |pages=18090 |year=2008 |doi=10.1073/pnas.0805069105}}</ref> |
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it has been proposed that |
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:''the Kendrick mass scale is obtained by setting the mass of the <sup>12</sup>CH<sub>2</sub> radical to 14.00000. '' |
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However, according to the rules of metrology outlined in the [[IUPAC green book]], the [[IUPAP red book]] and the [[ISO 31]] standards, a mass is not dimensionless and always needs a unit of dimension mass (like when indicating masses in Da). |
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The following conversion has also been suggested by the same authors: |
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: Kendrick mass = SI mass · 14.00000 / 14.01565. |
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It is worth noting that |
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* by "Kendrick mass" the authors mean the mass of a molecule measured in the Kendrick mass scale and not the Kendrick mass unit Ke. |
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* by "SI mass" the authors mean the mass measured in dalton mass units Da even though the Da is explicitly refered to as "a unit outside the SI <ref>The International System of Units(SI), 8th edition 2006, Bureau International des Poids et Mesures </ref>" |
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* according to the rules of metrology outlined in the [[IUPAC green book]], the [[IUPAP red book]] and the [[ISO 31]] standards, above formula is wrong. |
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The correct formula would be: |
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: Kendrick mass = Dalton mass |
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Clearly, the mass of a molecule is a global constant and does not change depending on the units (or mass scale) used. According to the prior formula, a molecule would suddenly become lighter just by changing to Kendrick units (or mass scale) from Dalton units. The following equations should further illustrate this, using the molecule <sup>12</sup>CH<sub>2</sub> as an example: |
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: Kendrick mass of <sup>12</sup>CH<sub>2</sub> = m<sub>Ke</sub>(<sup>12</sup>CH<sub>2</sub>) = 14 Ke |
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: Dalton mass of <sup>12</sup>CH<sub>2</sub> = m<sub>Da</sub>(<sup>12</sup>CH<sub>2</sub>) = 14.01565 Da |
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: m<sub>Ke</sub>(<sup>12</sup>CH<sub>2</sub>) = m<sub>Da</sub>(<sup>12</sup>CH<sub>2</sub>) |
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: 14 Ke = 14.01565 Da |
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It would be NPV to draw conclusions from these facts about the quality of this literature. Therefore, explicitly no conclusions are drawn. |
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==Kendrick Mass Excess/Defect== |
==Kendrick Mass Excess/Defect== |
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Revision as of 01:38, 23 October 2010
 | It has been suggested that this article be merged into Kendrick mass. (Discuss) Proposed since August 2010. |
The Kendrick mass scale defines a unit of mass kendrick (Ke) which is useful in chemistry of hydrocarbons. The kendrick is close to the dalton (=unified atomic mass unit) but it uses the group CH2 as the basis of an integer mass.
When measuring the masses of hydrocarbon molecules in kendricks, all homologous molecules will have the same Kendrick Mass Excess Δm. This unit simplifies the interpretation of a hydrocarbon mass spectrum [1] [2] .
Definition
The Kendrick mass unit is defined as [3]
- m(12CH2) = 14 Ke
In words: "the group 12CH2 has a mass of 14 Ke exactly, by definition."
- 1 Ke = 14.0156/14.000 Da = 1.00111429 Da = 1.00111429 u
Kendrick mass
In the literature [4] it has been proposed that
- the Kendrick mass scale is obtained by setting the mass of the 12CH2 radical to 14.00000.
However, according to the rules of metrology outlined in the IUPAC green book, the IUPAP red book and the ISO 31 standards, a mass is not dimensionless and always needs a unit of dimension mass (like when indicating masses in Da).
The following conversion has also been suggested by the same authors:
- Kendrick mass = SI mass · 14.00000 / 14.01565.
It is worth noting that
- by "Kendrick mass" the authors mean the mass of a molecule measured in the Kendrick mass scale and not the Kendrick mass unit Ke.
- by "SI mass" the authors mean the mass measured in dalton mass units Da even though the Da is explicitly refered to as "a unit outside the SI [5]"
- according to the rules of metrology outlined in the IUPAC green book, the IUPAP red book and the ISO 31 standards, above formula is wrong.
The correct formula would be:
- Kendrick mass = Dalton mass
Clearly, the mass of a molecule is a global constant and does not change depending on the units (or mass scale) used. According to the prior formula, a molecule would suddenly become lighter just by changing to Kendrick units (or mass scale) from Dalton units. The following equations should further illustrate this, using the molecule 12CH2 as an example:
- Kendrick mass of 12CH2 = mKe(12CH2) = 14 Ke
- Dalton mass of 12CH2 = mDa(12CH2) = 14.01565 Da
- mKe(12CH2) = mDa(12CH2)
- 14 Ke = 14.01565 Da
It would be NPV to draw conclusions from these facts about the quality of this literature. Therefore, explicitly no conclusions are drawn.
Kendrick Mass Excess/Defect
Kendrick Mass Excess (or defect) Δm is defined as:
- Δm = m - round(m)
or more rigorously
- Δm = m - A·Ke
where:
- Δm is the Kendrick mass excess
- A is the mass number of the molecule
- Ke is the mass unit kendrick
- m is the mass of the molecule (or isotopologue) in kendricks.
- round(m) and A·Ke are the integer masses of the molecule in kendricks.
Note:
- the Kendrick mass excess Δm is defined different than the mass excess in nuclear physics
Equivalence relation
The Kendrick mass scale was introduced to find an equivalence relation for hydrocarbons. The same relation could be expressed with modular arithmetic using the modulo operation without introducing a new mass scale.
- A ~ B (mod CH2)
The above statement is read: "A is modulo CH2 equivalent to B." Or, when considering the mass of the molecules A and B:
- m(A) ~ m(B) (mod m(CH2))
"A has the same modulo CH2 mass as B."
In a computing code the Kendrick mass defect of a molecule M, Δm(M), would be expressed as the remainder r:
- Δm(M) = r = m(M) mod m(CH2)
or, if the modulo operation nor the remainder operation are defined
- Δm(M) = m(M) - m(CH2)·round(m(M)/m(CH2))
Note that:
- most programming languages implement the modulo operation with trunc or floor instead of round
- this approach with modular arithmetic works independent of the mass units (or mass scale)
- this approach is more generalized and allows for other building blocks than CH2, e.g. in polymer chemistry
History
In 1963 the chemist E. Kendrick suggested an alternative mass scale in the following publication:
Kendrick, E.: A mass scale based on CH2 = 14.0000 for high resolution mass spectrometry of organic compounds. Anal Chem. 1963;35:2146–2154.
See also
Notes
- ^ Kendrick, Edward (1963). "A mass scale based on CH2 = 14.0000 for high resolution mass spectrometry of organic compounds". Anal. Chem. 35: 2146–2154. Retrieved 2010-01-25.
{{cite journal}}: Cite has empty unknown parameter:|coauthors=(help) - ^ Marshall AG, Rodgers RP (2004). "Petroleomics: the next grand challenge for chemical analysis". Acc. Chem. Res. 37 (1): 53–9. doi:10.1021/ar020177t. PMID 14730994.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ http://www.atmos-meas-tech.net/3/1039/2010/amt-3-1039-2010.html
- ^ Marshall, A. G.; Rodgers, R. P. (2008). "Mass Spectrometry Special Feature: Petroleomics: Chemistry of the underworld". Proceedings of the National Academy of Sciences. 105: 18090. doi:10.1073/pnas.0805069105.
- ^ The International System of Units(SI), 8th edition 2006, Bureau International des Poids et Mesures