instrumental methods for materials anaysis

Instrumental methods for material analysis

Molecular mass spectrometry

Mass spectrometry provides information about:

• The elemental composition of samples of matter.
• The structure of inorganic, organic and biological molecules.
• The quantitative and qualitative composition of complex mixtures.
• The structure and composition of solid surfaces.
• Isotopic ratios of atoms in samples (Cl and Br give spectra with isotopic peaks).

The basic mechanism of molecular mass spectrometry consists in the formation of a molecular ion M starting from a compound in the vapor phase. The molecular ion is a radical ion that has the same mass as the molecule. To produce a molecular ion, the molecules are often left in an excited state. Relaxation occurs by fragmentation of part of the molecular ion to produce ions of lower mass, giving the characteristic spectra of a molecule.

The sample is usually vaporized and introduced in the spectrometer through an inlet system. The sample is then allowed to leak into the evacuated ionization region. Solids can be sublimated directly into the chamber. The positive ions produced are attracted through the slit of the mass spectrometer where they are sorted, thanks to the magnetic field in the chamber, according to mass-to-charge ratios and displayed in the form of a mass spectrum.

Methods to produce ions

• Electron impact:

  The sample is brought to a temperature high enough to produce molecular vapor, which is then ionized by bombarding the resulting molecules with a beam of energetic electrons. Electrons are emitted from a heated tungsten filament and then accelerated through approximately 70 V. Electron impact is not very efficient as only 1 molecule in 1 million undergoes ionization. EI is considered a hard ionization source and often the parent peak is not visible. This phenomenon could lead to ambiguity in the determination of the molecular mass of the analyte. Despite certain disadvantages, EI is one of the most common techniques.

• Chemical ionization:

  Gaseous samples are ionized by collision with ions produced by electron bombardment of an excess of a reagent gas. Usually, positive ions are used, but sometimes also negative ions can be produced. The pressure of the reagent in the ionization area is about 1 torr, whereas the concentration of the analyte is below 10^-5 torr. It means that the concentration of the reagent is nearly 10⁴ times that of the analyte. Because of this large difference, the electron beam nearly reacts exclusively with the reagent molecule. Collision between the reagent ions and the analyte can cause proton transfer or hydride transfer, which gives to the generation of (M+1) and (M-1) as the molecular ion peak, respectively.

• Field ionization:

  Ions are formed under the influence of a large electric field (10⁸ V/cm) obtained by applying voltage as high as 10-20 kV. The parent peak is often not detectable, and the sensitivity is lower than the EI by an order of magnitude.

• Desorption sources:

  The ionization methods above mentioned require gaseous samples, hence not applicable to non-volatile or thermally unstable samples. In desorption methods, energy is introduced into the solid or liquid sample in such a way as to cause direct formation of gaseous ions. Spectra are usually greatly simplified and often consist only in the molecular ion or protonated molecular ion.

Fragmentation

• Alcohols:

  Molecular ion small or non-existent. Cleavage of the C-C bond next to the hydroxyl usually occurs. A loss of water may occur (H₂O, molecular ion minus 18).

• Aldehydes:

  Cleavage of bonds next to the carboxyl group results in the loss of hydrogen (molecular ion minus 1) or the loss of CHO (molecular ion minus 29).

• Carboxylic acids:

  In short-chain acids, peaks due to the loss of OH (molecular ion minus 17) and COOH (molecular ion minus 45) are prominent due to cleavage of bonds next to C=O.

• Alkanes:

  Molecular ion peaks are present, possibly with low intensity. The fragmentation pattern contains clusters of peaks 14 mass units apart, which represent loss of (CH₂)_nCH₃.

• Amides:

  Primary amides show a base peak due to the McLafferty rearrangement: Molecular ion peak in amides is an odd number, alpha-cleavage usually occurs in aliphatic amines.

Aromatic molecules molecular ion...