- High accuracy mass measurements by mass spectrometry are often used to determine the chemical composition of compounds.Standard practice is to select the mono-isotopic peak because it is the only peak for which the accurate mass can be conveniently calculated from the chemical formula.
- The percentage of abundance and isotopic mass is used to calculate the average isotopic mass. For example, two isotopes of Nitrogen are N-14 and N-15 and average isotopic mass of Nitrogen is 14.007. The percentage abundance of both isotopes can be calculated as given below. (14.003074) (x) + (15.000108) (1 – x) = 14.007.
- The total number of neutrons and protons (symbol A), or mass number, of the nucleus gives approximately the mass measured on the so-called atomic-mass-unit (amu) scale. The numerical difference between the actual measured mass of an isotope and A is called either the mass excess or the mass defect (symbol Δ; see table).
- The mass number is. The sum of the protons and neutrons in the nucleus. Protons in the nucleus of each atom; if it had anything else it would. The isotope Iodine-131 also has 78 neutrons in the. Nucleus, giving a total of 131 nucleons. To find the isotopic mass, which is the same as the mass.
An isotope and/or nuclide is specified by the name of the particular element (this indicates the atomic number) followed by a hyphen and the mass number (e.g. Helium-3, helium-4, carbon-12, carbon-14, uranium-235 and uranium-239).
'Isotopes' | |
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Monoisotopic and Average Mass | |
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-Figure A. | Isotopes Neogeo bios for retropie download. Figure A. shows a simulated isotopic distribution of the [M+H]+ ion of a compound with the following elemental composition, C48 H82 N16 O17 (Poly-Alanine) The Program IsoPro ver 3.1 was used for the simulation. The resolving power was set to 10,000. |
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Monoistopic Mass = 1155.6 Average Mass = 1156.3 (calculated) As shown in Figure B. the monoisotoptic mass of this compound is 1155.6. For a given compound the monoisotopic mass is the mass of the isotopic peak whose elemental composition is composed of the most abundant isotopes of those elements. The monoisotopic mass can be calculated using the atomic masses of the isotopes. The average mass is the weighted average of the isotopic masses weighted by the isotopic abundances. The average mass can be calculated using the atomic weights of the elements. The calculated average mass is shown in Figure B with the red line. | Figure B. |
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Figure C.- | Average mass can be measured by attempting to centroid a low resolution profile of a compound. The better the centroiding program the closer the measured mass will be to the theoretical average mass. The yellow envelope, shown in figure C., depicts how the peak might look at low resolution (perhaps with a low res. quadrupole instrument). A good centroiding program will be able to find the center of mass of this peak (yellow blob). A not so good program (that's a euphemism) will only pick the peak top. So with bad software, the reported mass for this compound would be closer to the monoisotopic mass. As the A+1 peak grows with increasing mass the peak top of the low resolution peak will inch closer to the theoretical average mass, and the program will begin to report values closer to the average mass. One might attempt to measure the masses and the abundances of the isotopes (at a higher resolution) then calculate the average mass of the compound but this exercise is seldom performed in routine analysis. |
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| Once again we would like to reiterate that at lower resolution a peak labeling program might report masses closer to the monoisotopic mass when the compound mass is below 1500 amu and may report masses closer to the calculated average mass when the compound is larger than 1500 amu. | |
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This web application calculates the molecular mass (average,monoisotopic, and nominal), the elemental composition, and the massdistribution spectrum of a molecule given by its chemical formula, relativeelement weights, or sequence.
Isotopic Mass Unit
Calculations are based on theisotopic composition of the elements.Mass deficiency due to chemical bonding is not considered.
Examples
- Simple chemical formulas:H2O orCH3COOH
- Specific isotopes:D2O or[30Si]3O2
- Abbreviations of chemical groups:EtOH orPhOH
- Simple arithmetic:(COOH)2 orCuSO4.5H2O
- Relative element weights:O: 0.26, 30Si: 0.74
- Nucleotide sequences:CGCGAATTCGCG ordsrna(CCUU)
- Peptide sequences:MDRGEQGLLK orpeptide(CPK)
Formulas are case sensitive and ’+’ denotesthe arithmetic operator, not an ion charge.
Disclaimer
Isotopic Mass Table
Because this service is provided free of charge, there is nowarranty for the service, to the extent permitted by applicable law.The service is provided 'as is' without warranty of any kind,either expressed or implied, including, but not limited to, the impliedwarranties of merchantability and fitness for a particular purpose.The entire risk as to the quality and performance is with you.
Isotopic Mass Questions
About
Molecular Mass Calculator version 2020.1.1 byChristoph Gohlke.Source code is available atpypi.org.