interpretation of mass spectroscopy

BASIC PRINCIPLES IN INTERPRETATION OF MASS

SPECTRA

KOMMINENI.VIDYACHOWDHARY

VAAGDEVI PHARMACY COLLEGE

BOLLIKUNTA,WARANGAL

MASS SPECTROMETRY

Mass spectrometry is the most accurate method for

determining the molecular mass of the compound and

its elemental composition.

It is also called as positive ion spectra or line spectra.

Introduction

• Mass spectrometry (Mass Spec or MS) uses high energyelectrons to break a molecule into fragmentation.

• A beam of high-energy electrons breaks the molecule apart.

• The masses of the fragments and their relative abundance reveal information about the structure of the molecule.

• Separation and analysis of the fragments provides informationabout:

– Molecular weight

– Structure

General approach for acquiring information about a sample

• degree of unsaturation

• Structures and functional groups Connectivity of structures

• Reference spectra and comparison to possible structures where unique identification is not possible

GENERAL RULES FOR INTERPRETATION OF MASS SPECTRA

.In order to interpret the mass spectrum , one should attain

an understanding of the ionisation process.

.To observe fragmentation pattern.

1.The exact molecular weight: The molecular weight of a pure compound from the identification of the parent peak. The molecular weight one can determine molecular formula.

2.The isotope effects : Heavy isotopes will exhibit peaks in a mass spectrum at m/e one or more units higher than normal.

i.e.,there will be small peaks at M+1 and M+2.

The nitrogen rule

• In organic compounds , is a relationship between the valance and the mass of the most common isotope for most elements.

• Even elements have an even valance.

• Odd elements have an odd valance.

• This leads to the ‘nitrogen rule.’ It assumes

that we are limiting our elements to C, H, halogens, O and N.

The nitrogen rule

• A compound containing only C, H, O or X will have an even molecular weight. A compound with an odd number of nitrogens will have an odd molecular weight.

• A compound with an even number of nitrogens will have an even molecular weight.

Logical losses

General steps inmass spectral interpretation

What is a mass spectrum

m/z ratio:

Molecular weight divided by the Charge on this protein

All proteins are sorted based on a

mass to charge ratio (m/z)

RELATIVE ABUNDANCE:

• It is a method of reporting the amount of each mass to charge measurement after assigning the most abundant ion 100%.

• Abundance:

. The amount of an isotope of an element that exists in nature , usually expressed as a percentage of the total amount of all isotopes of the element.

A simple mass spectrum

Fragmentation pattern

What is base peak?

• 100% abundance is called as base peak.

• M- is parent ion peak . It is all so called as molecular ion peak.

• Molecular peak of M produced when an electron is ejected from the molecule

M + e- M++2e-

Background of fragmentation

• The impact of a stream of high energy

electrons causes the molecule to lose an

electron forming a radical cation.

– A species with a positive charge and one unpaired

electron.

+ e-C H

H

HH H

H

H

HC + 2 e-

Molecular ionM/z=16

Background

• The impact of the stream of high energy electrons can also

break the molecule or the radical cation into fragments.

(not detected by MS)

m/z = 29

molecular ion (M+) m/z = 30

+ C

H

H

H

+ H

HH C

H

H

C

H

H

H C

H

H

C

H

H

H C

H

H

+ e-H C

H

H

C

H

H

H

m/z = 15

Background

• Molecular ion (parent ion):– The radical cation corresponding to the mass of the

original molecule

• The molecular ion is usually the highest mass in the spectrum– Some exceptions specific isotopes

– Some molecular ion peaks are absent.

H

H

H

HC H C

H

H

C

H

H

H

Background

Mass spectrum of ethanol(MW = 46)(C2H5OH)

M+

Background

M+base peak

The mass spectrum of ethanol

Fragmentation Patterns

• The impact of the stream of high energy electrons often breaks the molecule into fragments, commonly a cation and a radical.

– Bonds break to give the most stable cation.

– Stability of the radical is less important.


• Alkanes– Fragmentation often splits off simple alkyl groups:

• Loss of methyl M+ - 15

• Loss of ethyl M+ - 29

• Loss of propyl M+ - 43

• Loss of butyl M+ - 57

– Branched alkanes tend to fragment forming the most stable carbocations.


• Mass spectrum of 2-methylpentane(M.w-86)

Example:

ASPIRIN (or) SALICYLICACID(m.wt-180)

Mass spectra of toulene:(m/z=91)

Conclusion

• We’ve totally left out any of the mechanisms

of ion fragmentation.

• On the other hand, the assumption was that

you were working with a GC/MS system.

• The chromatographic information will also help solve many problems.

interpretation of mass spectroscopy

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