A.6.2
Describe the principles of atomic absorption. © IBO 2007
Figure 1.5.1 Schematic diagram of the atomic absorption spectrophotometer.
I I
I I II I I III I I I II II I I I I I II I I I I I I
I I I II I I
prism prism prism light beam
is pulsed
solution of sample is sprayed into
flame hollow
cathode
lamp detector recorder
monochromator and slit
stage 1 stage 2 stage 3
In stage one, a sample of the substance being analysed is sprayed (usually in acid solution) into a fl ame. The fuel that is usually used for this fl ame is acetylene mixed with air, or a nitrous oxide–acetylene mixture is used for a hotter fl ame. A slot-type burner is used to increase the path length, and therefore to increase the total absorbance. Sample solutions are usually aspirated (sucked up) with the gas fl ow into a nebulizing/mixing chamber (the atomizer) to form small droplets before entering the fl ame. The purpose of such a hot fl ame is to provide enough energy to convert the sample to atoms. This atomization is a key part of the instrument. The fl ame used is chosen so that only a small fraction of the atoms undergo excitation by heating. More recent AA spectrophotometers use a graphite furnace atomizer. The graphite furnace has several advantages over a fl ame. It is a much more effi cient atomizer than a fl ame and it can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. The main intention is for the atoms to be excited by light from the selected light source.
The light source is chosen so that it produces the exact wavelength of light required by atoms in the sample for excitation (a monochromatic light source). This is achieved by using a hollow cathode lamp made of the same metal as the one being analysed. In this lamp, high voltages are used to excite metal atoms that are present as a low pressure vapour in the lamp. As these excited atoms return to their ground state, they emit their characteristic wavelengths of light. It is these wavelengths that will be absorbed by the metal atoms in the fl ame.
In stage two, the monochromator is used to select a wavelength to be passed to the detector. The stronger the wavelength of light that is selected, the more accurate the absorption data will be. The detector is set to detect the same wavelength of light as that selected by the monochromator.
In stage three, the intensity of the transmitted light is compared to the intensity of the incident light, and the absorbance is calculated. The greater the concentration of the sample, the greater will be the absorbance.
One problem should be evident in the design. The light coming from the atomized sample will include both the transmitted (unabsorbed) light and the light emitted as excited electrons return to their ground state. To overcome A.6.3
Describe the use of each of the following components of the AA spectrophotometer:
fuel, atomizer, monochromatic light source, monochromatic detector and readout.
© IBO 2007
CHAPTER 1 MODERN ANALYTICAL CHEMISTRY In all forms of spectroscopy, the relationship between the amount of a
substance and the intensity or amount of light must be established in a process known as calibration. Known concentrations of the substance under investigation are analysed. A plot is constructed of the amount of substance versus the absorbance of light. This plot is known as a calibration curve. From the calibration curve, the concentration of an unknown sample is determined by interpolation.
1 Prepare a set of standards of known
concentration.
2 Measure the amount of light
absorbed by each standard.
3 Plot a standard curve (calibration curve).
4 Determine the concentration of the
sample by interpolation.
absorbance (arbitrary units)
concentration (ppm) absorbance of
unknown solution
concentration is determined by interpolation
1.0 0.1
0.2 0.3 0.4 0.5 0.6
2.0 3.0 4.0 5.0
0 0
Figure 1.5.2 Spectrometry relies on the construction of a calibration curve to relate the amount of light absorbed to the amount of substance present.
Atomic absorption spectroscopy is a rapid, sensitive and selective technique with many and varied uses. The selectivity of the technique allows metals to be analysed without having to separate them from other components.
Examples of the use of AA spectroscopy include:
• monitoring metals in the quality control of steel and other alloys
• analysing trace amounts of metals in foods
• determining metal concentrations in iron tablets, vitamins and other nutrient supplements
• testing air, water and soil for heavy metal contamination
• testing the grades of ores used in the mining industry
• testing for excess or defi ciency of metals in body fl uids such as blood and urine
• determining levels of trace elements such as Mn in soils.
A.6.1
State the uses of AA spectroscopy. © IBO 2007
Figure 1.5.3 An analytical chemist using an atomic absorption spectrometer.
Worked example 1
Many sports drinks contain high levels of sodium and potassium ions. To determine the potassium content in a particular sports drink, a 5.0 cm3 sample was diluted to 50.0 cm3 and the absorbance of the diluted solution and of several standard solutions was measured using AA spectroscopy. The results are shown in the table.
Concentration of solution (ppm) Absorbance
0.00 0.010
1.00 0.080
2.00 0.150
3.00 0.220
4.00 0.290
Diluted sports drink 0.185
a Plot the calibration curve for this experiment.
b Calculate the concentration of potassium ions in the sports drink in:
i ppm ii g dm–3
c If there is 600 cm3 of sports drink in an average bottle, calculate the mass of potassium ions that a player would consume if he consumed the whole bottle of drink after a football match.
Solution
a Calibration curve.
b i By interpolating the calibration curve, the concentration of potassium ions in the diluted sports drink is 2.5 ppm.
5.0 cm3 of sports drink was diluted to 50.0 cm3 2.5 × 50 0
5 0 . . = 25 The concentration of potassium ions in the undiluted sports drink was 25 ppm.
ii 1 ppm = 1 mg dm–3 so the concentration of potassium ions in the sports drink is 25 mg dm–3= 0.025 g dm–3
c 25 × 600 1000= 15
If a football player consumed 600 cm3 of the sports drink, he would consume 15 mg of potassium ions.
A.6.4
Determine the concentration of a solution from a calibration curve. © IBO 2007
0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
1 3
concentration (ppm)
Calibration curve for potassium in a sports drink
absorbance
4 5
2
CHAPTER 1 MODERN ANALYTICAL CHEMISTRY
Worked example 2
The lead level in a sample of contaminated soil was determined by AA spectrometry. A 2.0 g sample was dissolved in acid and then diluted to a total volume of 100.0 cm3. The absorbance of this solution, determined in a spectrometer set at a wavelength of 218 nm, was found to be 0.60. Several standard solutions of lead were tested under the same conditions and the calibration curve shown at right was generated. Determine the mass of lead in the 2.0 g sample of soil.
Solution
From the graph, an absorbance of 0.60 corresponds to a concentration of 9.0 ppm (mg dm–3).
We have 9.0 mg of Pb in 1 dm3. 9.0 × 100
1000 mg of Pb in 100 cm3 (containing 2.0 g soil sample) 0.90 mg of Pb was present in 2.0 g of soil.
1 Describe the effect on metal atoms of the absorption of light energy.
2 Explain why a fuel such as acetylene is required for the fl ame in an AA spectrophotometer.
3 The diagram below shows the main components of an AA spectrophotometer in schematic form.
I I
I I II I I I I I I II I I II I I I I I I I II I II I II I I
I I I
detector recorder
A B
a In what state is the sample when analysed using AA spectroscopy?
b Explain the function of component B.
c Describe how component A is chosen.
4 Explain the purpose of pulsing the incident monochromatic light source.
5 Explain the purpose of a calibration graph in AA spectrometry.
6 The sodium content of a sports drink was analysed using AA spectrometry.
A 10.0 cm3 sample was diluted to 100.0 cm3. The absorbance of the diluted solution and of several standard solutions was measured using AA
spectroscopy. The results are shown in the following table.
Concentration of solution (ppm)
0.00 1.00 2.00 3.00 4.00 5.00 6.00 Diluted sports drink Absorbance 0.000 0.070 0.140 0.210 0.280 0.350 0.420 0.330
absorbance
5.0 0.1
0.2 0.3 0.4 0.5 0.6 0.7 0.8
10.0 0
0
concentration of Pb (ppm)
Section 1.5 Exercises
a Plot the calibration curve for this experiment.
b Calculate the concentration of sodium ions in the sports drink in:
i mg 100 cm–3 ii mol dm–3
c If there is 600 cm3 of sports drink in an average bottle, calculate the mass of sodium ions that a player would consume if he consumed the whole bottle of drink after a football match.
7 The potassium content of an apple was measured using AA spectroscopy.
A 6.0 g sample of apple was treated with nitric acid and the resulting solution made up to 100.0 cm3. Using several potassium standard solutions, a calibration curve was constructed. The absorbance of the apple solution was then determined to be 0.3.
a Calculate the mass (in gram) of potassium in the sample of apple.
b Apples contain metal ions other than K+. Explain why these other ions do not interfere with the analysis performed to determine the K+ content of the apple.
absorbance
0.05 0.10 0.15 0.20
0.10 0.20 0.30 0.40 0.50
0 0
concentration of K+ (g dm–3)
8 The mercury content of a fi sh fl esh sample was analysed by AA spectrometry. A 2.00 g sample of fi sh was treated with acid to dissolve the mercury. The solution was then made up to a volume of 100.0 cm3. The absorbance of this solution at 254 nm was found to be 0.650. The absorbances of a series of standard mercury solutions were recorded at 254 nm and used to produce the calibration graph shown.
a Calculate the mass (in milligram) of mercury in the fi sh sample.
b Explain why the wavelength of 254 nm was chosen for this analysis.
concentration of Hg (mg dm–3)
absorbance
0.10 0.20 0.30 0.40
0.20 0.40 0.60 0.80 1.00
0 0
CHAPTER 1 MODERN ANALYTICAL CHEMISTRY The samples provided to an analytical chemist are often complex mixtures.
Consider, for example, biochemical samples such as blood and urine. How many components do these contain? Separating the components of such mixtures is often a crucial part of an analysis. Chromatography is particularly useful for separating complex mixtures. It is not only used as a preparative technique for separating mixtures, but it is also a means of identifying and quantifying the components of a mixture. It is used extensively in the areas of quality control, in testing for the presence of banned substances in urine of athletes, and in forensic testing.
During autumn many green leaves turn various shades of red. In 1906, Russian botanist Mikhail Tsvet (1872–1919) reasoned that leaves must therefore contain at least two different pigments: red and green. Tsvet conducted an experiment to test this idea. Leaves were crushed and their pigments extracted with petroleum ether. This extract was added to the top of a column containing fi ne powdered chalk and washed through with the solvent. Several bands of various shades of green and yellow developed along the column, confi rming Tsvet’s idea that more than one pigment existed. He named the process chromatography (from the Greek khroma, meaning ‘colour’, and graphe, meaning ‘writing’).
The term chromatography is now used to describe a range of techniques used to separate, identify and quantify the components of a mixture. Most chromatography is used for analytical purposes; that is, to answer the
‘What is it?’ (qualitative) and ‘How much is there?’ (quantitative) questions for samples of unknown composition. Some chromatography is preparative;
that is, it is used to obtain pure samples of the components of a mixture.
Many mixtures do not appear to be mixtures at all. Consider black ink. It fl ows from the pen onto the page and shows no sign of separating, yet in chromatography several different colours are found to be present in that black ink (see fi gure 1.6.4). In this case, the chromatography experiment is a qualitative technique. For quantitative experiments, column chromatography is much more appropriate than paper and thin-layer chromatography. It allows each component of the mixture to be collected and the individual amounts measured.
All chromatographic techniques involve two phases: a stationary phase and a mobile phase. In Tsvet’s experiment these phases were the solid (powdered chalk) packed into the column and the solvent (petroleum ether) respectively.
The mobile phase moves over or through the stationary phase. The mixture to be separated is dissolved in the same solvent as the mobile phase and applied to the stationary phase. Separation of the components of a mixture occurs because the components adsorb (form bonds) to a solid stationary phase with different strengths. The stronger the adsorption, the more slowly the component moves as the mobile phase sweeps over or through the stationary phase. Components undergo continuous adsorption and desorption (breaking of the bonds) to differing degrees. The rate of movement of each component therefore depends on the strength of its adsorption to the stationary phase and its tendency to desorb into the mobile phase. Examples of separation by adsorption include column chromatography (see pages 42–43), thin-layer chromatography (see pages 41–42) and high performance liquid chromatography (see pages 66–68).
1.6 CHROMATOGRAPHY
Figure 1.6.1 The changes in leaf colours during autumn suggested to Mikhail Tsvet that leaves must contain at least two different pigments.
A.7.1
State the reasons for using chromatography. © IBO 2007
A.7.2
Explain that all chromatographic techniques involve adsorption on a stationary phase and partition between a stationary phase and a mobile phase.
© IBO 2007
Figure 1.6.2 All types of chromatography involve two phases: one stationary and one mobile.