MASS SPECTRA - THE MOLECULAR ION (M+) PEAK This page explains how to find the relative formula mass (relative molecular mass) of an organic compound from its mass spectrum. It also shows how high resolution mass spectra can be used to find the molecular formula for a compound. Using a mass spectrum to find relative formula mass The formation of molecular ions When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Note: If you aren't sure about how a mass spectrum is produced, it might be worth taking a quick look at the page describing how a mass spectrometer works. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
The molecular ion is often given the symbol M+ or The molecular ions tend to be unstable and some of them break into smaller fragments. These fragments produce the familiar stick diagram. Fragmentation is irrelevant to what we are talking about on this page - all we're interested in is the molecular ion. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Note: If you are interested in a detailed look at fragmentation patterns you could follow this link. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
Using the molecular ion to find the relative formula mass In the mass spectrum, the heaviest ion (the one with the greatest m/z value) is likely to be the molecular ion. A few compounds have mass spectra which don't contain a molecular ion peak, because all the molecular ions break into fragments. That isn't a problem you are likely to meet at A'level. For example, in the mass spectrum of pentane, the heaviest ion has an m/z value of 72.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Note: This mass spectrum has been drawn using data from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan. It has been simplified by omitting all the minor lines with peak heights of 2% or less of the base peak (the tallest peak). | ||||||||||||||||||||||||||||||||||||||||||||||||||||
Because the largest m/z value is 72, that represents the largest ion going through the mass spectrometer - and you can reasonably assume that this is the molecular ion. The relative formula mass of the compound is therefore 72. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Note: This assumes that the charge on the ion is 1+. That's always the case when you are interpreting these mass spectra. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
Finding the relative formula mass (relative molecular mass) from a mass spectrum is therefore trivial. Look for the peak with the highest value for m/z, and that value is the relative formula mass of the compound. There are, however, complications which arise because of the possibility of different isotopes (either of carbon or of chlorine or bromine) in the molecular ion. These cases are dealt with on separate pages. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Note: The presence of the carbon-13 isotope in a molecular ion causes a little peak 1 unit to the right of the M+ peak. This is called the M+1 peak. The presence of a chlorine atom in a compound causes two peaks in the molecular ion region - the M+ peak and the M+2 peak depending on whether the particular molecular ion contains a chlorine-35 or chlorine-37 isotope. Bromine creates a similar problem. Follow these links if you are interested - or explore them later via the mass spectrometry menu. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
Using a mass spectrum to find a molecular formula So far we've been looking at m/z values in a mass spectrum as whole numbers, but it's possible to get far more accurate results using a high resolution mass spectrometer. You can use that more accurate information about the mass of the molecular ion to work out the molecular formula of the compound. Accurate isotopic masses For normal calculation purposes, you tend to use rounded-off relative isotopic masses. For example, you are familiar with the numbers:
To 4 decimal places, however, these are the relative isotopic masses:
The carbon value is 12.0000, of course, because all the other masses are measured on the carbon-12 scale which is based on the carbon-12 isotope having a mass of exactly 12. Using these accurate values to find a molecular formula Two simple organic compounds have a relative formula mass of 44 - propane, C3H8, and ethanal, CH3CHO. Using a high resolution mass spectrometer, you could easily decide which of these you had. On a high resolution mass spectrometer, the molecular ion peaks for the two compounds give the following m/z values:
You can easily check that by adding up numbers from the table of accurate relative isotopic masses above. A possible exam question A gas was known to contain only elements from the following list:
The gas had a molecular ion peak at m/z = 28.0312 in a high resolution mass spectrometer. What was the gas? After a bit of playing around, you might reasonably come up with 3 gases which had relative formula masses of approximately 28 and which contained the elements from the list. They are N2, CO and C2H4. Working out their accurate relative formula masses gives:
The gas is obviously C2H4. In an exam, you would hope that - apart from the most simple cases - you would be given the possible formulae to work from. Trying to work out all the possible things which might add up to the value you want is quite time-consuming - and it's easy to miss an important possibility! Questions to test your understanding If this is the first set of questions you have done, please read the introductory page before you start. You will need to use the BACK BUTTON on your browser to come back here afterwards. questions on the molecular ion peak answers
© Jim Clark 2000 (modified August 2014) | ||||||||||||||||||||||||||||||||||||||||||||||||||||
- the dot in this second version represents the fact that somewhere in the ion there will be a single unpaired electron. That's one half of what was originally a pair of electrons - the other half is the electron which was removed in the ionisation process.
FAQs
Mass spectra - the molecular ion (M+) peak? ›
Molecular Ion Peak (or Parent Peak): This is the second-highest peak that corresponds directly to the compound in question. The m/z ratio directly correlates to the molecular weight. For example, if hexane is the compound, then the m/z ratio is 86 since that is the molecular weight. [44]
What is the m peak in mass spectrometry? ›Mass spectrometry measures the mass to charge ratio of molecules, gives structural information about a compound and can tell us the molecular mass of a compound. The M in M+ represents molecular mass and the + sign indicates a positive charge.
Why is M 1 peak in mass spectra? ›The M+1 peak is caused by the presence of the 13C isotope in the molecule. C is a stable isotope of carbon - don't confuse it with the 14C isotope which is radioactive.
What is the M 2 peak of a mass spectrum? ›mass spectra - the M+2 peak. This page explains how the M+2 peak in a mass spectrum arises from the presence of chlorine or bromine atoms in an organic compound. It also deals briefly with the origin of the M+4 peak in compounds containing two chlorine atoms.
How to know which peak is the molecular ion? ›In the mass spectrum, the heaviest ion (the one with the greatest m/z value) is likely to be the molecular ion. A few compounds have mass spectra which don't contain a molecular ion peak, because all the molecular ions break into fragments.
What is the M +2 peak? ›The peak at 79 is called the M + 2 peak. The ratio of the relative abundance/intensity of the M:M + 2 is about 3:1, which reflecting the isotopic abundance of 35C:37C. With bromine, the isotopic distribution of 79Br and 81Br is more like 50:50.
What does the M-1 peak tell you? ›If you measure the peak height of the M+1 peak as a percentage of the peak height of the M+ peak, that gives you the number of carbon atoms in the compound. We've just seen that a compound with 2 carbons will have an M+1 peak approximately 2% of the height of the M+ peak.
How to identify M+? ›Remember that every peak has an "isotopic shadow". Based on this, when we look at this MS, we see a group of ions at m/z = 83, 84, 85 and even a very tiny peak at 86. The last "big" peak is the one at 84 which we assign as the molecular ion M+.
How do you interpret a mass spectra? ›- Look for the molecular ion peak. ...
- Try to calculate the molecular formula: ...
- Calculate the total number of rings plus double bonds: ...
- Postulate the molecular structure consistent with abundance and m/z of fragments.
Because there are two abundant isotopes of both chlorine (about 75% 35Cl and 25% 37Cl) and bromine (about 50% 79Br and 50% 81Br), chlorinated and brominated compounds have very large and recognizable M+2 peaks.
What does M 2 mean in mass spectrometry? ›
M+2: In mass spectrometry the molecular ion whose mass is two higher than M. Molecular ion region of the mass spectrum of 1-chlorodecane (C10H21Cl). The relative abundance of M is due to 12C101H2135Cl.
What is m/4 peak? ›A compound that contains two chlorines, or two bromines, or one chlorine an one bromine will show a distinct M+4 peak, in addition to the M+2 peak. ➢Three chlorine atoms in a molecule will give peaks at M+2, M+4 and M+6.
What is the molecular ion peak in mass spectrum? ›The molecular ion peak is the peak in a mass spectrum that represents the molecular ion (symbol: M peak). The molecular ion peak is the point with the largest mass-to-charge ratio after excluding any peaks induced by the existence of heavier isotopes.
Why are there two molecular ion peaks? ›There are two molecular ion peaks in the mass spectra of compounds containing a single chlorine atom. This is because chlorine exists as two isotopes, 35Cl and 37Cl. The mass spectrum of 2-chloropropane, CH3CHClCH3, has a peak at m/z ratio 78 due to the molecular ion [CH3CH35ClCH3]+ containing an atom of 35Cl.
What is the base peak of a mass spectra? ›The most intensive peak in a spectrum is called the "Base Peak", whose intensity is taken as 100 percent. This ion exists most abundantly in the ion source and represents the most stable ion, which is useful for identifying the compound.
What does M mean in mass spectrometry? ›BASIC MASS SPECTROMETRY
M stands for mass and Z stands for charge number of ions. In mass analysis, an electron is taken from molecules to create single charged ions. If two electrons are removed, double charged ions are produced. The number of electrons removed is the charge number (for positive ions).
The base peak is always the tallest peak in the spectrum. The base peak represents the most common and most abundant ion. The molecular ion peak is the peak that represents the heaviest ion. This peak has the highest m/z ratio on the mass spectrum but it does not necessarily represent the most abundant ion.