Spectroscopy notes pdf

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Elizabeth Howell. Mass spectroscopy pdf 1. It is used for determining masses of particles, for determining the elemental composition of a sample or molecule. The MS principle consists of ionizing chemical compounds to generate charge molecules or molecule fragments and measurement of their mass-to-charge ratio by using the one of a variety of techniques.

Mass Spectrometry is a powerful technique for identifying unknowns, studying molecular structure, and probing the fundamental principles of chemistry. The relative abundance of positively charged fragments of various mass-to-charge ratios is the characteristic feature of the molecule that serve to identify the substance. Each primary product ion derived from the molecular ion, in turn, undergoes fragmentation, and so on.

The ions are separated in the mass spectrometer according to their mass-to-charge ratio, and are detected in proportion to their abundance. A mass spectrum of the molecule is thus produced.

It displays the result in the form of a plot of ion abundance versus mass-to-charge ratio 6. Separation of the positively charged fragments formed, based on their masses, by using electrical or magnetic field or both. Electron impact EI 2. Chemical Ionisation CI 3. Fast atom bombardment FAB 4. Field ionization 5. Plasma desorption Once produced, the reagent gas ions collide with the analyte molecules producing ions through gas phase reaction processes such as proton transfer.

A high-energy beam of netural atoms, typically Xe or Ar, strikes a solid sample causing desorption and ionization. It is used for large biological molecules that are difficult to get into the gas phase.

FAB causes little fragmentation and usually gives a large molecular ion peak, making it useful for molecular weight determination. The atomic beam is produced by accelerating ions from an ion source though a charge-exchange cell.

The ions pick up an electron in collisions with netural atoms to form a beam of high energy atoms. It is perfectly suited for the analysis of synthetic polymers or man made polymer. Examples: poly siloxane, poly phosphazene backelite, nylon etc. Plasma desorption ionization mass spectrometry PDMS , also called fission fragment ionization, is a mass spectrometry technique in which ionization of material in a solid sample by bombarding it with ionic or neutral atoms formed as a result of the nuclear fission of a suitable nuclide, typically the californium isotope Cf.

Plasma desorption: In this technique, a gas chromatograph is used to separate different compounds. This filament emits electrons which ionize the compounds. Structure elucidation 2. Detection of impurities 3. Quantitative analysis 4. Drug metabolism studies 5. Clinical, toxicological and forensic applications 6. GC MS MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds. Determination of molecular weight: Mass spectrometry serves as the best possible technique for the determination or confirmation of molecular weight of compounds that can be easily volatilized.

Determination of molecular formula : For the determination of molecular formula b mass spectrometry, it is essential to identify the molecular ion peak as well as its exact mass. Determination of partial molecular formula: Generally, atoms are polyisotopic.

Special equipment to study vacuum or far UV is required. Routine organic UV spectra are typically collected from nm. This limits the transitions that can be observed:. Not all transitions that are possible are observed. Unlike IR or later NMR , where there may be upwards of 5 or more resolvable peaks from which to elucidate structural information, UV tends to give wide, overlapping bands It would seem that since the electronic energy levels of a pure sample of molecules would be quantized, fine, discrete bands would be observed — for atomic spectra, this is the case In molecules, when a bulk sample of molecules is observed, not all bonds read — pairs of electrons are in the same vibrational or rotational energy states This effect will impact the wavelength at which a transition is observed — very similar to the effect of H-bonding on the O-H vibrational energy levels in neat samples When these energy levels are superimposed, the effect can be readily explained — any transition has the possibility of being observed.

Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves A functional group capable of having characteristic electronic transitions is called a chromophore color loving Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions.

This transition is also sensitive to substituents on the carbonyl. The attachment of substituent groups other than H can shift the energy of the transition. Substituents that increase the intensity and often wavelength of an absorption are called auxochromes.

Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens. Substituents may have any of four effects on a chromophore:. Conjugation — most efficient means of bringing about a bathochromic and hyperchromic shift of an unsaturated chromophore:. The observed shifts from conjugation imply that an increase in conjugation decreases the energy required for electronic excitation. Extending this effect out to longer conjugated systems the energy gap becomes progressively smaller:.

Similarly, the lone pairs of electrons on N, O, S, X can extend conjugated systems — auxochromes. For acyclic butadiene, two conformers are possible — s-cis and s-trans. The s-cis conformer is at an overall higher potential energy than the s-trans ; therefore the HOMO electrons of the conjugated system have less of a jump to the LUMO — lower energy, longer wavelength. The energy of this transition places it outside the region typically observed — nm. For the more favourable s-trans conformation, this transition is forbidden.

While this band is insensitive to solvent as would be expected it is subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation. The effect of substituent groups can be reliably quantified from empirical observation of known conjugated structures and applied to new systems.

This quantification is referred to as the Woodward-Fieser Rules for eg. This work was distilled by Scott in into an extensive treatise on the Woodward-Fieser rules in combination with comprehensive tables and examples — A.

A more modern interpretation was compiled by Rao in — C. Rao, Ultraviolet and Visible Spectroscopy , 3rd Ed. View More in PDF. Tags B.