1 WHAT IS SPECTROSCOPY?

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1 1 WHAT IS SPECTROSCOPY? 1.1 The Nature Of Electromagnetic Radiation Anyone who has been sunburnt will know that light packs a punch: in scientific terms, it contains considerable amounts of energy. All light - the colours of the rainbow visible to us - and many other types of radiation, such as ultraviolet, infrared, X-rays, microwaves, radiowaves and gamma rays (but not sound waves), are all forms of energy, and make up the electromagnetic spectrum of radiation, which is depicted in Figure 1.1 below. Increasing energy Micro- Far Mid * Near Visible * Near * Vacuum X- Gamma waves Infrared (IR) Ultraviolet (UV) rays rays Red Orange Yellow Green Blue Violet FIGURE 1.1 The electromagnetic spectrum (the regions marked with * are those which are used in instruments studied in this module) The three common terms used to characterise radiation are energy, frequency and wavelength. The later two are more commonly used, with different regions referred to using one or the other measures, eg the visible region is usually referred to in nanometres, a measure of wavelength. The relationship between the three measures is shown in Figure 1.2. Energy Frequency Wavelength FIGURE 1.2 The relationship between energy, wavelength and frequency Therefore, the frequency of radiation increases, so does the energy contained in each photon. Thus, by looking at Figure 1.2, it should be clear why ultraviolet radiation and X-ray even more are considered dangerous to human. On the other hand, as the wavelength of radiation increases, the energy decreases. CLASS EXERCISE 1.1 Which has the higher frequency? (a) (b) mid infrared or X-rays orange or green visible Which has the higher wavelength? (a) near UV or visible (b) red or violet visible Which has the higher energy? (a) gamma rays or X-rays (b) green or red visible

2 1.2 The Interaction Of Matter With Radiation Everything we see around us is a consequence of visible radiation of different colours (wavelengths) reaching our eyes. The visible light coming from the sun is a combination of all the wavelengths in the visible region, giving us what we call white light. When that light hits something, it will either be reflected (bounce off) or be transmitted (pass through). Depending on the chemical substances contained in the material that the light is hitting, some of the light will be lost, in that it will not be reflected or transmitted. This light is absorbed by the atoms and molecules in the matter. Spectroscopy is the study of the interaction of radiation and matter. Figure 1.3 shows what happens to white light when it passes through (is transmitted) through a solution of copper sulfate, which is of course blue-green. For simplicity, we will assume that white light is an equal combination of all the colours (which isn t too far from the truth). Violet Blue Green Yellow Orange Red Violet Blue Green Yellow Orange Red FIGURE 1.3 The interaction of light and matter The copper sulfate in the water preferentially absorbs some of the wavelengths of the white light (the red, orange and yellow). What reaches our eye is this altered mix, which is dominated by the violet, blue and green. Different chemical substances absorb different wavelengths of radiation. CLASS EXERCISE 1.2 Complete the diagram below for a piece of red glass. Violet Blue Green Yellow Orange Red Copper Red sulfate Copper sulfate solution (bluegreen) solution glass (bluegreen) Not all the light is absorbed; how much depends on a number of factors, including how many atoms or molecules are in the path of the light. CLASS EXERCISE 1.3 What do think might happen to the transmitted light in Figure 1.3 if: (a) the solution was more concentrated? (b) the container was wider (the light passed through more liquid)? Sci Inst Analysis (Spectro/Chrom) 1.2

3 What happens to the absorbed light? We will try to avoid being too technical here, since your main interest in spectroscopy is using spectroscopic instruments to analyse samples. Physicists have worked out that atoms and molecules normally exist in a state known as the ground state, which means the substance contains the lowest energy that it can exist with (think of the ground floor of an building). When the substance is treated with excess energy from radiation, heat or electricity, for example it can take in that energy absorption and move to a higher energy state, known as an excited state (it takes the elevator to a higher floor in the building). These excited states only exist at certain heights above the ground state (you can t go to the seven-and-a-halfth floor, only the seventh or the eighth!). Different substances have different combinations of ground states and excited states. This is why copper sulfate is blue and potassium dichromate is orange. Different colours (different energies) are absorbed. This is shown in Figure 1.4. excited states ground states Substance A Substance B FIGURE 1.4 Ground and excited states Various parts of an atom or molecule are capable of absorbing energy. For example, outer shell (valence) electrons can absorb visible or ultraviolet radiation and move to a different energy shell, while the covalent bond between atoms, which is vibrating, can absorb infrared radiation and vibrate differently. Substances are not stable in an excited state and will quickly (often within microseconds) escape back to the ground state. This means that they must get rid of the absorbed energy before they can return. Depending on the substance and the surroundings, this can happen in either or both of two ways: loss by collision with other substances (like taking the lift back to ground, losing the energy gradually) releasing it in one burst (like jumping out the window); this is known as emission These processes are summarised in Figure 1.5. absorption loss by collision emission FIGURE 1.5 Energy changes between ground and excited state Sci Inst Analysis (Spectro/Chrom) 1.3

4 If the substance releases its excess energy as emission, then it will produce radiation. Thus, when salt water containing sodium is placed in a flame, the sodium atoms absorb the heat energy and then release it as visible (orange) radiation. How do we know whether absorption or emission is occurring? Emission is characterised by a relatively intense energy source, and the production of radiation that wasn t there different wavelengths - in the energy source. Absorption occurs when radiation from a source decreases in intensity (brightness). This difference may not be altogether clear at this stage, but don t despair, time and use of the various type of instruments will help. Table 1.1 attempts to summarise the various possibilities. TABLE 1.1 Emission and absorption processes Energy In Energy Out Process Radiation Radiation of same wavelength but Absorption decreased intensity Radiation Radiation of different wavelength Emission Heat or electricity Radiation Emission CLASS EXERCISE 1.3 In your vision are two objects. One is a bottle of Fanta, the other a flame with salt water being drawn into it. Each object is a similar orange colour orange. What process is happening to give the orange colour in each case? The colour of an object whether it is emitting or absorbing light is due to the wavelengths of the visible region of the spectrum that are coming from that object: emission the colours that are being seen are the colours being emitted, transmission/reflection the colours that being seen are the colours that are not being absorbed The colour of an object that we perceive is a result of its interaction with the visible region of the electromagnetic spectrum. This region covers the wavelength range nm, and is divided into 6 main colours, as shown in Table 7.1 below. (Seven if you count indigo) TABLE 7.1 Colours in the visible regions of the spectrum Wavelength (nm) Colour violet blue green yellow orange red Sci Inst Analysis (Spectro/Chrom) 1.4

5 White light is a combination of all the colours (wavelengths) of the visible region. If a substance absorbs some of the wavelengths, it will appear coloured, since the unabsorbed radiation will no longer "add up" to being white. EXAMPLE What colours are being absorbed by a solution of copper sulfate, which appears blue-green to the eye If the blue and green colours are being transmitted to the eyes, then the other colours yellow, orange and red are being absorbed. CLASS EXERCISE 1.4 What colours are being absorbed by (a) purple permanganate, (b) orange dichromate, and (c) green chlorophyll. How is spectroscopy used in analysis? In this subject, we will examine FOUR different spectroscopic techniques, involving the ultraviolet, visible and infrared regions of the electromagnetic spectrum. They are: molecular ultraviolet-visible absorption molecular infrared absorption atomic absorption atomic emission You will note that a distinction between atomic and molecular species is made. While atoms and molecules both absorb and emit in the same basic way, there are important difference which mean that the instruments and the procedures for analysis are quite different. How spectroscopy is used will be covered more in later chapters, but very briefly, spectroscopic analysis exploits two key properties of the interaction of light and matter: different species absorb/emit at different wavelengths this means that identification can be made based on the particular wavelengths absorbed/emitted by a sample the amount of radiation absorbed/emitted depends on how much of the species is present this means that the concentration of analyte in a sample can be measured 1.3 Terms Used In Spectroscopy As should already be clear, a particular substance will absorb/emit at a particular wavelength (or group of wavelengths), determined by its chemical composition and the set of ground and excited states it possesses. This set of wavelengths is known as its spectrum, and is most commonly shown as a graph with the amount of absorption/emission on the vertical axis and the wavelength (or frequency) on the horizontal axis. Thus the absorption spectrum is the amount of radiation absorbed at different wavelengths, and the emission spectrum is the amount of radiation emitted at different wavelengths. Figure 1.6 shows the visible absorption spectrum for copper sulfate solution, and Figure 1.7 the ultraviolet-visible emission spectrum for sodium atoms. Sci Inst Analysis (Spectro/Chrom) 1.5

6 FIGURE 1.6 Absorption spectrum of copper sulfate solution in the visible region Intensity Wavelength (nm) FIGURE 1.7 Emission spectrum for sodium atoms in the ultraviolet-visible regions Atomic and molecular spectra In spectroscopic terms, the dividing line between atoms and molecules is clear: the atomic state is characterised by single atomic species (neutral or ionic) which are totally free of any interaction with other species. This will only occur in the gas phase, such as occurs in a flame. Molecular species are any others, i.e. solids, liquids, compounds in solution, including apparently monatomic ions. Thus, when copper (II) sulfate is dissolved in water, dissociating to produce Cu 2+, the cation is nevertheless considered to be a molecular species, since it is surrounded by water molecules. It is not free of interaction with other species. CLASS EXERCISE 1.5 What difference does the distinction between atomic and molecular species make? Look again at Figures 1.6 and 1.7. What is the difference in appearance between the two spectra? Which species is atomic and which is molecular? Sci Inst Analysis (Spectro/Chrom) 1.6

7 The joining of atoms in strong bonds to form molecules makes for a much more complicated series of energy levels than for free atoms. As a consequence of the much more complex series of transitions possible, molecular absorption spectra are not single lines, but rather a closely-spaced set of lines over a relatively wide range of energies/wavelengths/frequencies. This spectrum is known as a band spectrum, while that for an atomic species is called a line spectrum. Measures of radiation We have already met the terms energy, wavelength and frequency, which are used to characterise the nature of a particular type of radiation. Intensity might sound like another word for energy, but it is very different. Intensity is the amount of a particular type of radiation. To use a fruit analogy, 6 apples and 20 oranges use two different measurement: the type of fruit (equivalent to energy in the case of radiation) and the number of each type of fruit (equivalent to intensity). Absorption relationships Consider a liquid sample contained in a cell through which a light beam passes, as shown in Figure 1.8. pathlength I in I out FIGURE 1.8 Passage of light through an absorbing material This light beam has an intensity of I in before it passes through the sample. At wavelengths where the sample absorbs, the intensity of the light beam will be decreased (to a value of I out ), as a result of absorption by the sample of a proportion of the photons of that wavelength. Transmittance (T) is the ratio of the intensity of light at a particular wavelength after it passes through a sample to the light intensity before the sample, as shown in Equation 1.1. Iout T Eqn 1.1 I in The greater the proportion of light absorbed by the sample, the lower the transmittance value. Note that transmittance cannot be greater than 1 Percent transmittance (%T) is simply the transmittance converted to a percentage, as in Equation 1.2. %T = 100T Eqn 1.2 Absorbance (A) is the negative logarithm (base 10) of transmittance, as in Equation 1.3. It is used because it has a direct, simple relationship to concentration, which transmittance does not. A = 2 - log 10 %T Eqn 1.3 Sci Inst Analysis (Spectro/Chrom) 1.7

8 EXAMPLE The measured intensity of radiation reaching a sample is 92 units. After passing through the sample, its intensity is reduced to 45 units. Calculate the transmittance, percent transmittance and absorbance of the sample. Transmit tance %transmittance = 48.9% Absorbance = 2 - log 10 (48.9) = CLASS EXERCISE 1.6 Complete the following table. I in I out T %T A (a) (b) (c) Absorption of radiation occurs at the atomic level, so the more atoms or molecules in the path of the radiation., the more will be absorbed. CLASS EXERCISE 1.7 (a) For the following pairs of absorbing materials, indicate which will have the higher absorbance. (i) (ii) A B C D (b) Express the observations in words. As the concentration increases, the absorbance. As the pathlength increases, the absorbance. Sci Inst Analysis (Spectro/Chrom) 1.8

9 It has been found that the absorbance is directly proportional to both the concentration and the pathlength. This is expressed in Equation 1.4, which is known as Beer's Law. A = abc Eqn 1.4 where A is the absorbance, a is a constant, b the pathlength (b is used for no obvious reasons) and c the concentration. The constant term a is known as the absorption coefficient, and is constant for a particular species at a particular wavelength. It is simply an indication of the chance that a species will absorb a particular wavelength of radiation; the greater the chance of absorbing radiation (i.e. energy equal to a energy level transition), the greater the value of the absorption coefficient. EXAMPLE The absorbance of a 50 mg/l solution of potassium permanganate is found to be 0.44 at a wavelength of 525 nm in a 1 cm cell. Calculate the absorption coefficient, a. A = abc 0.44 = a x 1 x 50 a = Quantitative spectroscopic analysis is based on Beer's Law, since it indicates that there is a linear relationship between the measured absorbance of a sample and its concentration. If a series of standards are prepared, and their absorbances measured, a plot should yield a straight line, and can be used to make a calibration graph for the analysis of samples containing the same absorbing species. The use of spectroscopy for quantitative analysis will be covered in more detail in Chapter 2. CLASS EXERCISE 1.8 (a) Calculate the absorption coefficient for a species which has an absorbance of 0.22 for a solution of 1.5 mg/l in a 2 cm cell. (b) Calculate the expected absorbance of a 10 mg/l solution in a 1 cm cell, where the species has an absorption coefficient of Sci Inst Analysis (Spectro/Chrom) 1.9

10 1.4 Basic Instrumentation Requirements How do instruments that measure absorption or emission work? description of what happens. The following box is a general Absorption measurement A beam of radiation passes through the sample area. A single wavelength of the transmitted beam is selected and sent to a detector for measurement. This is done for a blank (no analyte) first to zero (100%T, 0 absorbance) the instrument, and then for samples. Emission measurement The sample is exposed to an energy source to cause emission. The radiation that is produced is passed to a wavelength selector and then to a detector for measurement. This is done for a blank (no analyte) first to zero the instrument, and then for samples.type of instruments Spectroscope: A simple device which splits incoming radiation into its individual wavelengths, but does not measure intensity of the radiation. Photometer: A device for the measurement of intensity of radiation, generally relying on a simple filter system to remove radiation not associated with analyte absorption or emission. Spectra cannot be recorded. Spectrophotometer: A device capable of recording both intensity of radiation at any wavelength (using a complex optical system to split the radiation into its component wavelengths). It is capable of recording spectra as a function of absorbance (or transmittance) against wavelength. Spectrometer: A term which can used either incorrectly as an abbreviation of spectrophotometer, or to describe an instrument which does not measure electromagnetic radiation, but which does produce a form of spectrum. An example is a mass spectrometer which measures the mass of particles against their intensity. Single-beam instrument: Only a single pathway for radiation to pass through the sample area. Any reference (blank) solution must be used first, generally to zero the instrument (no absorption). Double-beam instrument: Splits the radiation before the sample into two exactly equal beams, one of which passes through the sample and the other passes through a reference (usually the solvent). The intensities of the two beams are compared at each wavelength, and in this way, absorption by the solvent can be subtracted. Each of these instruments fulfils the same basic role, and the differences between them arise from different regions of the electromagnetic spectrum being measured, or extra functions, capabilities or expense. General instrumental components Radiation (energy) source this is used in absorption instruments to provide the radiation that the sample absorbs. In emission instruments, it is an energy source, which could be radiation, but is more often heat. Beam focussing optics - often called a collimator. Its purpose is to make the beam of radiation concentrated as it passes into and/or out of the sample. Sample holder an obvious requirement, but its form varies; in some instruments, it is a small openended box called a cell, in others it is a flame. Wavelength selector there are two important types monochromators and filters. When measuring a spectrum, we need the intensity for one wavelength at a time so that the overall picture is built up like a graph. If no wavelength selector was present, then the detector would measure the intensity of all wavelengths, and we could not tell which wavelengths were being absorbed or emitted, and which were not. Sci Inst Analysis (Spectro/Chrom) 1.10

11 Detection system measures intensity of radiation; in general a detector cannot distinguish between different wavelengths of radiation Calculations device - converts the intensity measurements into transmittance and absorbance Readout device we need to see the results; this can be on a computer screen, on paper, digital readout or dial. CLASS EXERCISE 1.9 (a) Draw a block diagram of a single-beam absorption spectrophotometer. (b) Draw a block diagram of an emission spectrophotometer. What You Need To Be Able To Do list the common regions of the electromagnetic spectrum indicate the relative energies of these regions describe the nature of electromagnetic radiation describe the processes by which an atom or molecule absorbs energy describe the processes by which excited state atoms or molecules lose energy define important terms describe the difference between atomic and molecular spectra carry out conversions between the absorption units of transmittance, percent transmittance and absorbance perform calculations using Beer s Law show how a basic single-beam absorption spectrophotometer is constructed using a block diagram show how a basic emission spectrophotometer is constructed using a block diagram explain the need for the components of basic absorption and emission spectrophotometers Sci Inst Analysis (Spectro/Chrom) 1.11

12 Terms And Definitions Match the term with the definition. 1. wavelength 2. white light 3. transmission 4. ground state 5. excited state 6. absorption 7. emission 8. spectrum 9. intensity 10. transmittance 11. Beer s Law A. the passage of radiation through a substance B. the process of releasing excess energy C. a substance containing the lowest energy possible D. a measure of radiation E. the relationship between absorbance and concentration F. a combination of all colours of the spectrum G. the process of taking in extra energy H. the ratio of the radiation intensity released to received I. a substance with increased energy J. the amount of radiation K. a graph of intensity against wavelength Sci Inst Analysis (Spectro/Chrom) 1.12