3.1 Introduction
Section 3 gives information about the instrumentation referred to in various sections of the Leaving Certificate Chemistry syllabus. This material is being introduced for the first time, as it is a necessary component of a modern chemistry course. It is clearly not envisaged that schools will equip themselves with any of the instruments referred to here, other than a pH meter and a colorimeter. It is, however, likely that students will encounter the more advanced instruments during recommended visits to an industrial chemical plant or a water treatment plant. As indicated in the syllabus, a knowledge of only the basic principles of each instrument/technique is required. The interpretation of spectra etc. is not required.
3.2 Instrumentation
Syllabus Section 1.2
Instrument: Mass Spectrometer
Ordinary level
The mass spectrometer is an instrument that is capable of separating and recording the relative amounts of the isotopes of an element. These amounts can be used to calculate the relative atomic mass of an element.
Higher level
In a mass spectrometer (Fig. 1 and Fig. 2), the sample to be analysed is "injected" into the instrument, where it is vaporised, and then ionised by bombardment with a beam of high-energy electrons. The positive ions produced in this way are then accelerated through a slit, using an electric field, and deflected along a circular path (the degree of curvature of which depends on the mass of the ion), using a magnetic field. In this way they are separated into beams of ions of similar masses, and then they are detected electronically.
****Fig. 1****
Syllabus Section 3.3
Instrument: Mass spectrometer
Higher level
A mass spectrometer can be used to determine accurately the relative molecular mass of a substance composed of molecules. In a mass spectrometer, molecules are ionised and broken into positively charged fragments with different masses (Figs. 3(i), 3(ii), 4 (i) and 4 (ii)). These are separated and the relative amounts recorded, giving the mass spectrum of the molecule. Usually, the mass spectrum of a molecule M contains the parent molecular ion M+.
The mass of this ion is the same as the relative molecular mass of the molecule. (Note that in Fig. 4 (i) there is a peak at 17 relative atomic mass units, even though the relative molecular mass of methane is 16. This is due to methane containing the isotope 13C, which constitutes about 1.1% of natural carbon.)
(Note that the interpretation of mass spectra is not required.)
**** Fig. 2 ****
Syllabus Section 7.5
Instrument: Mass spectrometer
Ordinary level
In a mass spectrometer, molecules are broken into fragments with different masses. These are separated and the relative amounts recorded, giving the mass spectrum of the molecule. Different molecules have different mass spectra. Mass spectrometry can therefore be used to analyse materials such as gases from a waste dump and to trace organic pollutants in water.
(Note that the interpretation of mass spectra is not required.)
**** Fig 3(i), 3(ii), 4(i), 4(ii) ****
Syllabus Section 1.4
Instrument: Atomic absorption spectrometer
Higher level
The amount of an element in a sample can be estimated from the absorption of radiation by free atoms of that element in an instrument called the atomic absorption spectrometer (fig. 5).
The sample to be analysed is dissolved and introduced into the instrument as a fine spray. It is converted in a flame into atoms of the element to be analysed. A special lamp, whose cathode contains the element of interest, generates light characteristic of the element. This light is passed through the atomised sample and is absorbed by it. The amount of light absorbed depends on the amount of the element present.
Atomic absorption spectrometry (AAS) is particularly useful for the analysis of trace metals. It has many applications, for example the analysis of lead in the blood of workers in lead mines.
Syllabus Section 9.4
Instrument: Atomic absorption spectrometer
Higher level
An atomic absorption spectrometer (see syllabus section 1.4) is used to analyse heavy metals, such as lead and cadmium, in water.
**** Fig. 5 ****
Technique: chromatography
Ordinary level
Chromatography is the term used to describe a separation technique in which a mobile phase carrying a mixture is caused to move in contact with a selectively absorbent stationary phase. There are a number of different kinds of chromatography, which differ in the mobile phase and stationary phase used.
In paper chromatography the mobile phase is a solvent and the stationary phase is water held in the fibres of chromatography paper. In column chromatography the mobile phase is again a solvent and the stationary phase is a finely divided solid, such as silica gel or alumina. In thin- layer chromatography the mobile phase is also a solvent and the stationary phase is a thin layer of a finely divided solid, such as silica gel or alumina, supported on glass or aluminium.
Solid-phase extraction devices are inexpensive forms of column chromatography that allow for particularly fast separations of materials.
Thin-layer chromatography is particularly useful in forensic work, for example in the separation of dyes from fibres.
Gas chromatography and high-performance liquid chromatography are more advanced chromatographic techniques.
Syllabus Section 7.5
Technique: gas chromatography
Ordinary level
In gas chromatography (figs. 6(i) and 6(ii)), the mobile phase is a gas and the stationary phase is a non-volatile liquid (which is coated on fine particles of an inert solid). Gas chromatography has many applications, for example drug tests on athletes and blood alcohol tests. In analytical work a gas chromatograph is sometimes coupled with a mass spectrometer. The gas chromatograph separates the different components of the mixture being analysed, and each component is then separately analysed by the mass spectrometer.
**** Fig. 6(i), 6(ii) ****
Syllabus Section 7.5
Technique: High-performance liquid chromatography (HPLC)
Ordinary level
In high-performance liquid chromatography (fig. 7) the mobile phase is a solvent and the stationary phase is very fine particles of silica. High pressure has to be applied to the solvent to force it through the column. HPLC is used in the separation of mixtures that are less volatile than those amenable to separation by gas chromatography. HPLC has many applications, for example analysis of growth promoters in meat and of vitamins in foods.
Fig. 8 shows a HPLC chromatogram of a mixture of compounds.
In HPLC, detection is usually by means of an ultraviolet absorption spectrometer, but, as in gas chromatography, a mass spectrometer is sometimes used. The mixture is separated by HPLC, and each component can then be individually analysed.
**** Fig. 7, 8 ****
Syllabus Section 7.5
Technique: Infrared absorption spectrometry (IR)
Higher level
Infrared absorption spectrometry is a technique involving the absorption of infrared radiation. It is a "fingerprinting" technique, as each compound has its own almost unique infrared spectrum. For this reason, IR is particularly useful in the identification of substances, for example plastics and drugs. The infrared spectra of methanol and methanoic acid are shown in fig. 9 (i) and fig. 9 (ii), respectively.
(Note that the interpretation of spectra is not required.)
**** Fig. 9(i), 9(ii) ****
Syllabus Section 7.5
Technique: Ultraviolet absorption spectrometry (uv)
Higher level
Ultraviolet absorption spectrometry is a technique involving the absorption of ultraviolet radiation. This is a quantitative technique and is used in the quantitative determination of organic compounds, for example drug metabolites and plant pigments. Fig. 10 shows the ultraviolet spectrum of benzene.
(Note that the interpretation of spectra is not required.)
**** Fig. 10 ****
Syllabus Section 9.4
Technique: colorimetry
Ordinary level
Colorimetry is a technique involving the comparison of the colour of a solution with that of a range of standard solutions. There is a linear relationship between concentration and absorbance of light of a specific wavelength. Calorimetry is therefore a quantitative technique, and examples of its use include the analysis of lead in water and of fertilisers. A simplified diagram of a colorimeter is shown in Fig. 11.
**** Fig. 11 ****
Instrument: pH meter
Ordinary level
A pH meter (Fig. 12) consists of a glass electrode half-cell, a reference half-cell (both usually contained in the same housing), and a sensitive meter which measures voltage. The electrode potential of the glass electrode depends on the concentration of H+ ions in the solution in which the electrode is placed. The voltmeter is designed to read the pH of the solution directly, rather than the voltage.
**** Fig. 12 ****
Syllabus Section Option 2A.1
Technique: x-ray Crystallography
Ordinary level
The structure of crystals may be determined by analysing the way X-rays are scattered by the crystal (fig. 13). This technique was developed by William H. Bragg and his son, William L. Bragg, and has been used to determine the structure of many crystals. Dorothy Hodgkin used this technique to determine the structures of vitamin B12 and penicillin.
**** Fig. 13 ****
X-rays scattered from a crystal produce a pattern of dots on a photographic plate. (In modern instruments the scattered beams are detected electronically.)