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...

• ELECTRON IMPACT:

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 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 one of the analyte. Because of this large difference the electron beam nearly reacts exclusively with 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 of order of

DESORPTION SOURCES:

The ionisation 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 (CH₃)CH₂N₃• 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 peaks are strong due to the stable structure of the molecules.

• ESTERS

Fragments appear due to bond cleavage next to C=O (alkoxy group loss, -OR) and hydrogen rearrangements.

• ETHERS

Fragmentation tends to occur on the C-C bond next to the oxygen atom.

• HALIDES

The presence of chlorine and bromine is recognizable from isotopic peaks in the spectrum. For bromine the peaks appear with similar intensities, while for chlorine one is more intense than the other.

• KETONES

Major fragmentation peaks result from cleavage of the C-C bonds adjacent to the carbonyl.

INFRARED SPECTROSCOPY

Radiation from the visible part of the electromagnetic

Spectrum interacts with a chemical species to cause an electron to move from its ground state (GS) to a much higher energy orbital (an electronic excitation state (EE)). Radiation corresponding to this electron transition is absorbed, creating a peak at a corresponding energy (wavelength) in the absorbance spectrum.

Vibrational spectroscopic methods use infrared or near infrared (the low energy end of the visible spectrum) to create vibrations (bond stretching or bending) in chemical species. The vibrational excitation states are lower in energy than electronic excitation states.

Vibrational and rotational transitions occur with polyatomic species since molecules have a multitude of quantized energy levels (or vibrational states) associated with the bonds that hold the molecule together. When an infrared photon is absorbed by a molecule, it passes from its fundamental vibrational state to an excited vibrational state. The near-, mid- and far-infrared, named for their relation to the visible spectrum.

The higher-energy near-IR, approximately -114000-4000 cm (0.7-2.5 μm wavelength) can excite overtone or harmonic molecular-1vibrations. The mid-infrared, approximately 4000-400 cm (2.5-25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared,-1approximately 400-10 cm (25-1000 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy.

In order for IR absorbance to occur two conditions must be met:

1. There must be a change in the dipole moment of the molecule as a result of a molecular vibration (or rotation). The change (or oscillation) in the dipole moment allows interaction with the alternating electrical component of the IR radiation wave. Symmetric molecules (or bonds) do not absorb IR radiation since there is no dipole moment.
2. If the frequency of the radiation matches the natural frequency of the vibration (or rotation),

1. The IR photon is absorbed, and the amplitude of the vibration increases.
2. We can compare the bond between two atoms to a spring. The vibrational frequency for a diatomic molecule is given by:
3. This means that for stronger bonds (higher k) the signal will be located at higher wavenumbers, while for larger vibrating masses (higher μ) the signal will be at lower wavenumbers.
4. There are different types of molecular vibrations:
• If the stretch is symmetric, opposite and equal magnitudes of the dipole cancel each other out completely and the overall net dipole change is zero. However, if the stretch is asymmetric, the bond dipoles do not cancel each other out, resulting in a net dipole and an IR active mode.
• Therefore, polar molecules are always IR active and nonpolar molecules must vibrate in a non-symmetrical way to be IR active.
5. IR SPECTRA OF ORGANIC COMPOUNDS:
• ALKANES - Characteristic peaks for the C-H stretch just under 3000 cm^-1.
• ALKENES - Similar to alkanes, with a shoulder above 3000 cm^-1.

1. ALKYNES:
   • In the IR spectrum of a disubstituted alkyne only a signal at ca. 2100 cm^-1 is present.
   • The bands at ca. 3300 cm^-1 and 630 cm^-1 together are fingerprints of monosubstituted alkynes.
2. ETHERS:
   • Ethers' characteristic peaks correspond to two kinds of stretching: symmetric stretching at 890-820 cm^-1 (both C_sp2 and C_sp3) and asymmetric stretching at 1150-1000 cm^-1 if not aromatic (C_sp3), at 1300-1000 cm^-1 if aromatic ether (C_sp2).
3. ANISOLES:
   • Four characteristic peaks: symmetric C-O stretch peak at 883 cm^-1; asymmetric C-O stretch at 1293 cm^-1 (aromatic); asymmetric C-O stretch at 1077 cm^-1 (aliphatic); C-H stretching peak at 2830.0 ± 10 cm^-1. Usually we find this peak at values of wavenumber higher than 3000 cm^-1, but when the methyl group is linked to an oxygen this peak is shifted to lower values.
4. ALCOHOLS:
   • The first diagnostic peak is located at high values of wavenumber because it is characteristic of the O-H

stretching (> 3000 cm ). This peak is broad because there is astrong interaction. Other indicative peaks for O-H are the ones related to the bending: in-1 -1plane OH bending at 1330-1430 cm and out of plane OH bending at 650-770 cm .

• KETONES

The high electronegativity of oxygen gives the C=O bond a large dipole moment, thecorresponding signal is then very intense. For saturated ketones the C=O stretch is-1found around 1715 cm . For aromatic ketones the peak shifts to wavenumbers around-130 cm lower. The C-C-C stretch that is usually not visible appears between 1230 and-1 -11100 cm for saturated ketones and between 1300 and 1230 cm for aromatic ketones.This is due to the large electronegativity of the carbonyl group.

-1Methyl ketones also show a signal around 1360 cm due to the umbrella mode.

• ALDEHYDES

Because the oxygen in the C=O bond is very electronegative, it pulls electron densityaway from the aldehydic C-H bond, weakening the bond and lowering its force

constant.-1The net effect is a lowering of the aldehydic C-H stretch to the 2850–2700 cm range.The combination of this C-H stretch and the C=O stretch gives the characteristic spectraof aldehydes.• CARBOXYLIC ACIDSThe IR peaks of carboxylic acids are unusually broad. The O-H stretch is so broad that -1there isn’t really a peak position for it rather than a range between 3500 and 2500 cm .The peaks corresponding to the C-H stretches fall on top of the O-H broad band. The C-H-1stretch for aromatic carboxylic acids corresponds to a sharp shoulder at 3071 cm .• ESTERS -1Three characteristic peaks with positions at approximately 1700, 1200, and 1100 cm .The three vibrations involved are the C=O stretch, a C-C-O stretch, and an O-C-C st