mass spectrometry- structural elucidation

MASS SPECTROMETRY-STRUCTURAL ELUCIDATION : 

MASS SPECTROMETRY-STRUCTURAL ELUCIDATION D.PADMINI M- Pharm (1 st year) Pharm Chemistry Gitam University

BASIC PRINCIPLE: : 

BASIC PRINCIPLE: Mass spectrometry is the most accurate method for determining the molecular mass of the compound and its elemental composition.

Slide 3: 

In order to measure the characteristics of individual molecules, a mass spectrometer converts them to ions so that they can be moved about and manipulated by external electric and magnetic fields. The three essential functions of a mass spectrometer, and the associated components, are: A small sample is ionized, usually to cations by loss of an electron. The Ion Source. The ions are sorted and separated according to their mass and charge.   The Mass Analyzer The separated ions are then measured, and the results displayed on a chart.   The Detector

What the mass spectrometer output looks like: : 

What the mass spectrometer output looks like: The output from the chart recorder is usually simplified into a "stick diagram".

The Nature of Mass Spectra: : 

The Nature of Mass Spectra: A mass spectrum will usually be presented as a vertical bar graph, in which each bar represents an ion having a specific mass-to-charge ratio (m/z) and the length of the bar indicates the relative abundance of the ion. The most intense ion is assigned an abundance of 100, and it is referred to as the base peak. The highest-mass ion in a spectrum is normally considered to be the molecular ion, and lower-mass ions are fragments from the molecular ion. The peak it represents is called as parent peak. The base peak and parent peak can sometimes be the same peak.

Slide 8: 

The stick diagram for molybdenum looks lilke this:

Slide 9: 

In the diagram, the vertical axis is labelled as either "relative abundance" or "relative intensity". The vertical scale is related to the current received by the chart recorder - and so to the number of ions arriving at the detector: the greater the current, the more abundant the ion. the commonest ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97 and 100. That means that molybdenum consists of different isotopes on the carbon-12 scale are 92, 94, 95, 96, 97, 98 and 100.

THE ORIGIN OF METASTABLE IONS: : 

THE ORIGIN OF METASTABLE IONS : For a mass spectrometric fragmentation reaction in which an ion M + of mass m 1 yields a fragment ion A + of mass m 2 and a neutral particle N. M +→A+ + N metastable ion actual position can be computed from the formula m *=m ₂2/m₁

Slide 11: 

The following diagram displays the mass spectra of three simple gaseous compounds, carbon dioxide, propane and cyclopropane .

Slide 13: 

The molecules of these compounds are similar in size, CO ₂ and C ₃ H ₈ both have a nominal mass of 44 Da , and C ₃ H ₆ has a mass of 42 Da. Even though these compounds are very similar in size, it is a simple matter to identify them from their mass spectra.

Slide 17: 

Since a molecule of carbon dioxide is composed of only three atoms, its mass spectrum is very simple. The molecular ion is also the base peak, and the only fragment ions are CO (m/z=28) and O (m/z=16). The small m/z=39 ion in propane and the absence of a m/z=29 ion in cyclopropane are particularly significant in distinguishing these hydrocarbons.

4-nonanone: 

4-nonanone

Slide 19: 

The odd-electron fragment ions at m/z = 86 and 58 are the result of a McLafferty rearrangement, involving the larger alkyl chain, and a subsequent loss of ethene (the "double- McLafferty " rearrangement).Alpha-cleavage leads to the m/z = 99, 71 and 43 ions. The charge is apparently distributed over both fragments.

Slide 22: 

In 1-pentanol the hydroxyl group is at the end of the five-carbon chain. There are two significant odd-electron fragment ions, one at m/z=70 (loss of water), and the other at m/z=42 (loss of water and ethene ). The fragment ion at m/z=55 is probably due to a methyl radical loss from the m/z=70 ion. The m/z=31 ion may be a protonated formaldehyde ion, formed by alpha-fragmentation. 3-Pentanol shows three significant fragment ions. Alpha-fragmentation (loss of an ethyl radical) forms the m/z=59 base peak. Loss of water from this gives a m/z=41 fragment, and loss of ethene from m/z=59 gives a m/z=31 fragment.

PROBLEMS: : 

PROBLEMS: Question 1:Which of the following compounds will give a molecular ion having m/z = an odd number? CH 3 CH 2 OH CH 3 CH 2 NH 2 CH 3 CO 2 H (CH 3 ) 2 NCH 2 C≡ N ANS:ethyl amine Question 2:how will you distinguish between the isomeric alcohols with molecular formula C₄H₁₀O by mass spectroscopy?

REFERENCE:: 

REFERENCE: Y.R. Sharma -Elementary Organic spectroscopy- multicolor edition- S.Chand & company LTD G.Vidya sagar - Instrumental methods of drug analysis- pharma med press

THANK YOU : 

THANK YOU