Mass spectroscopy is different from other forms of spectroscopy. In UV-Visible, I.R., ESR, NMR etc one irradiates the sample and determine the effect of sample upon radiation. In mass we observe the effect of a source of ionizing energy upon the sample molecule.
In Mass spectrometry, a substance is bombarded with an electron beam having sufficient energy to fragment the molecule. The positive fragments which are produced (cations & radical cations) are accelerated in a vacuum through a magnetic field and are sorted on the basis of mass-to-charge ratio. Since the bulk of the ions produced in the mass spectrometer carry a unit positive charge, the value m/e is equivalent to the molecular weight of the fragment.
The sample is introduced into the ionization chamber very slowly to produce the positive ions. Then the accelerating voltage is adjusted to highest value (4000 V). At this time the ions with low m/e ratio will acquire highest velocity & will be deflected by the magnet & finally recorded as a peak. The voltage V is then decreased progressively so that the ions with increasing m/e value will be brought into focus simultaneously. The scanning of mass spectrum is completed by decreasing the accelerating voltage slowly at zero.
The mass spectrum of an organic compound is the plot of relative abundance or intensity on ordinate versus m/e ratio on abscissa. The most intense peak in the mass spectrum is the base peak & is given the value of 100 & the intensities of other peaks are expressed relative to this. The base peak in most of the cases is due to the molecular ion and is helpful in determining the molecular weight of the compound.
(1) MOLECULAR ION PEAK:
When a sample substance is bombarded with electrons of 9 to 15 eV energies, the molecular ion (M+) is produced by loss of a single electron.
M+ e → M+ +2e-, where M represents molecule & M+ is the molecular ion /parent ion.
The first ion appear from the removal of one electron from the molecule, hence called as parent ion (M+) & this (M+) ion is not fragmented before collection at the detector and is recorded at m/e value corresponding to the molecular weight of the material, hence called as molecular ion and the peak due to this ion in the mass spectra is called the molecular ion peak (or) parent ion peak.
If some of the molecular ions/parent ions remain intact long enough (about 10-6 seconds) to reach the detector, a molecular ion peak is observed. This peak gives the molecular weight of the compound.
The molecular ion peak is usually the peak of highest mass number except for the isotope peak.
The molecular ion M+ has a mass corresponding to the molecular weight of the compound from which it is generated, shows mass of molecular ion M+, which is an important parameter in the identification of a compound
The intensity of a molecular ion peak depends upon the stability of the ionized particle & also on the stability of the molecular ion. The molecular ion is stabilized by the presence of electron systems, which are capable of accommodating a loss of one electron more easily. Cyclic structures also give large molecular or parent peaks.
Generally the stability of molecular ion decreases in the following order:
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The most stable molecular ions are of purely aromatic system. The detection of molecular ion is difficult in aliphatic alcohols,nitrites, nitrates, nitro compounds, nitriles & highly branched compounds.
Important features of molecular ion peak are:
1. The molecular ion peak in aromatic compounds is relatively much intense due to the presence of -electron system.
2. Conjugated olefins show more intense molecular ion peak as compared to the corresponding non-conjugated olefins with the same number of unsaturation. Conjugated olefins are more stable than the corresponding non-conjugated olefins.
3. Unsaturated compounds give more intense peak as compared to the saturated or the cyclic molecule.
4. The relative abundance of the saturated hydrocarbon is more than the corresponding nranched chain compound with the same number of carbon atoms.
Example: Molecular ion peak for n-pentane is more intense than that of neopentane.
5. The substitute groups like -OH, -OR, -NH2, etc., which lower the ionization potential increase the relative abundance in case of aromatic compounds.
Also groups like -NO2, -CN, etc., which increase ionization potential, decrease the relative abundance of the aromatic compounds.
6. Absence of molecular ion peak in the mass spectrum means that the compound under examination is highly branched or tertiary alcohols.
Primary and secondary alcohols give very small molecular ion peaks.
7. In case of Chloro or Bromo compounds, isotope peaks are also formed along with the molecular ion peak.
In case of Bromo compounds, M+ & (M++2) peaks are formed in the intensity ratio 1:3.
(2) FRAGMENT IONS PEAK:
When potential and energy is given to the molecular ion during electron impact , further cleavage takes place and ions of lower mass number (fragment ions) are produced which gives the fragment peak.
Many of the peaks in the mass spectrum are due to fragment ions. Fragment peaks in the spectrum give valuable information regarding molecular structure, because fragmentation is specific to the structure of the molecule.
The greater the fragmentation of the molecular ion or parent ion, the greater is the loss in its intensity.
The probability of fragmentation decreases in the following order:
ALCOHOLS > BRANCHED HYDROCARBONS > CARBOXYLIC ACIDS > ETHERS > ESTERS > AMINES > KETONS >THIOLS > STRAIGHT CHAIN HYDROCARBONS > SULPHIDES > ALICYCLIC COMPOUNDS > CONJUGATED DIENES>AROMATIC COMPOUNDS.
Fragmentation is initiated by electron impact. Only a small part of the driving force for fragmentation is energy transferred as the result of the impact.
(3) REARRANGEMENT ION PEAK:
These are probably due to the recombination of fragment ions & known as rearrangement peaks.
In most cases, rearrangement takes place w.r.to hydrogen.
A prominent peak in the spectrum of Diethylether occurs at m/e =31. This peak is due to the ion CH3O+ which is formed by rearrangement of the C2H5O+ ion.
Involves the migration of γ-hydrogen atom followed by the cleavage of a β-bond. The rearrangement leads to the elimination of neutral molecules from aldehydes, ketones, amines, unsaturated compounds, anic compounds, viz. ketones, amines, alcohols, esters, acids which contain a γ-hydrogen atom forms a McLaferrty rearrangement ion.
(4) METASTABLE ION PEAK:
The ions resulting from decomposition between the source region & magnetic analyzer are called as metastable ions.These appear as broad peaks called metastable ion peaks.
Characteristics of metastable peaks are:
(a) These peaks are much broader, i.e., they spread over several mass units.
(b) These peaks appear in the mass spectrum usually at non-integral m/e values.
(c) These peaks are of relatively low abundance or low intensity.
(d) The metastable ions can be detected by a double focusing mass spectrometer.
Formation of metastable ions:
Suppose M1+ is the parent ion & m1+ is the daughter ion and if the reaction M1+→m1+ takes place in the source, then the daughter ion, m1+ may travel the whole analyzer region & is recorded as m1+ ion.
On the other hand, if the transition M1+ to m1+ occurs after the source exist & before arrival at the collector, then m1+ is called a metastable ion.
In double focusing mass spectrophotometer, there are two field free regions (drift regions). The ions pass through these regions after acceleration.
The first free region refers to the portion of the ion path immediately before the electrostatic analyzer.
The second field free region lies between electrostatic analyzer & magnetic analyzer.
An ion with charge e after being accelerated by a potential, V volts will possess kinetic energy equal to eV. Thus, all ions arriving at A will have transnational energy equal to eV & thus energy of the ion will be independent of its mass.
Now suppose the reaction, M1+ → m1+ occurs in the second field free region, then the daughter ion (m1+) will have kinetic energy equal to m2/m1eV. Thus, m1+ ion formed in the second field free region will have less kinetic energy than it would have possessed if it were formed in the source.
Hence, the peak of daughter ion (m1+) will not appear at the normal position for m1+ on the mass scale and instead, the signal will appear at m12/M1.
Hence, the position of the metastable peak (m*) due to the reaction M1+→m1+ occurring in the second field free region is like that: m*= m12/M1
1) P-amino anisol:
The position of the metastable peak,m* =(m1)2/M1=108x108/123=94.8
The position of metastable peak due to following fragmentation in second field free region can also be determined as:
The position of metastable peak, m*=(m1)2/M1=80x80/108=59.2
Suppose the transition C7H7+(91) to C5H5+(65) occur in second field free region, then a metastable peak is formed. The position of metastable peak is determined as, m*= (m1)2/M1=65*65/91=46.4
Importance/significance of metastable peaks:
The metastable peaks in the mass spectrum greatly contribute in structure elucidation.
From the positions of the parent ion & the daughter ion, the position of the metastable ion is calculated as given above & confirmed in the spectrum.
Why metastable peaks are broadened?
The broadening of the metastable peak is the possibility that some of the excitation energy leading to bond capture may be converted into additional kinetic energy.
(5) MULTICHARGED IONS:
Ions may exist with 2 or 3 charges instead of the usual singal charge. These are known as doubly or triply charged ions & the peaks due to these charged ions are known as multicharged ion peaks.
They are represented as:
M + e- → M++ + 3e-
or M +e- → M+++ +4e-
When M is an even number, M++ will be recorded as half this value in the spectrum & will be indistinguishable from any other ion of mass M/2.
When M is an odd value, M++ will be recorded as peak halfway between the whole mass numbers.
Formation of multicharged ions are common in heteroaromatic compounds. They are also common in organic mass spectrum. Fixed gases such as CO, N2, CO2 & O2 have measurable peaks corresponding to CO+2, N+2,O+2.
Doubly charged ions are often helpful in confirming the molecular weight.
(6) BASE PEAK:
The largest peak in the mass spectrum corresponding to the most abundant ion or the most intense peak in the spectrum is called the base peak.
Depending on the nature of the compound, it may be either a fragment ion peak or the parent peak.
Sometimes the molecular ion peak may be the base peak.
In the mass spectrum of toluene, the molecular ion peak if m/e=92 & the base peak is m/e=91.
The most intense peak in the spectrum, called the base peak , is assigned a value of 100% and the intensities (height x sensitivity factor) of the other peaks, including molecular ion peak, are reported as percentages of the base peak.
(7) NEGATIVE ION PEAK
In addition to positive ions, negative ions may be formed from electron bombardment of sample. These results due to the capture of electron by a molecule during collision of molecules.
These are not observed with the usual mass spectrometer unless some modifications are made. These are generally ignored during studies.
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