Analytical Instruments

1. Analytical Instruments Area Study 1 Unit 3 2010

2. Key Terms • Spectroscopy – energy is used. Energy can be absorbed or emitted. When atoms or molecules absorb energy, can move to higher energy level. Radiation is emitted when returning to original energy level (falling to lower energy level). • Emission – this occurs when excited electrons return to their ground states and radiate energy of fixed wavelengths. Emission spectra have sets of coloured lines on a black background.

3. Terms continued • Absorption – molecules can absorb energy of certain wavelengths. In the visible and UV spectrum, electrons surrounding the atoms may gain energy. If no energy is absorbed, the substance appears white. If coloured, then it has absorbed light complementary to that being seen. Absorption spectra are the opposite of emission spectra, the wavelengths of light absorbed by the electrons appear as dark lines on a rainbow spectra. • Molecules can also absorb energy from the infrared region of the electromagnetic spectrum. This energy is insufficient to disrupt electrons but functional groups, C-H bonds, double and triple bonds can absorb energy, stretch, bend and can be measured. • In Nuclear Magnetic Radiation spectrometry, protons or 13C nuclei absorb energy from the radio frequency,

4. UV visible spectroscopy makes use of the UV visible region, IR spectroscopy the IR region and NMR the radio wave region. UV = short wavelength, high energy Radio = long wavelength, low energy

5. Absorption & Emission Spectra

6. Higher energy level • Each spectrometric techniques, the atom or molecules absorbs a specific amount of energy which causes a move to a higher energy level. For atoms, this is the movement of electrons where in molecules as well as electrons moving to higher energy levels we can also observe the movement of molecules in term of their spin, vibration or rotations.

7. Flame Tests • Used to identify certain metal cations that after excitation emit light within the visible region of the electromagnetic spectrum. • Expected colours for common metal cations:

9. Feature of flame test • Inexpensive • Very fast to perform but unreliable because colours can be masked • Used to identify metal ion in some salts • Qualitative only • Limited to few metals. Many common metals like Al and Mg do not colour flame (only some of group 1 & 2) • Other limitations: colours can be alike and therefore hard to identify unknown, also samples can be impure and mask colours of component • Emission spectroscopy • Can be observed by the naked eye • More qualitative result: use a prism, hotter flame – called atomic emission spectroscopy

10. AAS – Atomic Absorption Spectroscopy • Expensive, costs thousands of dollars. Lamps themselves are very expensive. • Fast, once standard solutions made up • Quantitative, a hollow cathode lamp for the metal being measured must be available • Can be used for all metals and metalloids – can identify & measure a much wider range of metals • Calibration curve constructed, absorbance vs. concentration – dilution is likely to be required • Visible spectrum

11. AAS • Australian invention, sample sprayed into flame • Very sensitive -ppm • Used to detect amount of mercury in shellfish (0.5ppm), lead in soil, trace metals in mineral water, forensic analysis of hair samples from heavy metal poisoning • Absorption spectroscopy – light absorbs indicates quantity of element in sample

12. Example AAS Question p.83 Example 7.3 Lead in oysters ppm = one millionth gram 8.3 ppm- i.e. there are 8.3 g in 1.0 x 106 g of solution 2.5ppm = 2.5 µg per g of sample 2.5mg per kg 2.5ppm =2.5 µg per mL Or 2.5 mg per L

13. UV-visible spectroscopy • Simple UV visible spectrophotometers are available from $1000 to $5000 • Fast once a set of standard solutions set up • Qualitative and Quantitative – mainly used quantitatively to determine concentration of substance in a sample • Used for coloured solutions or organic molecules that absorb in the UV region – energy emitted is not in visible region therefore cannot be seen with the naked eye! • Absorption spectroscopy • Wavelength of light selected for maximum absorbance and a calibration curve constructed of absorbance plotted against concentration

14. UV • Used to find concentration of food colourings, analysis of aspirin and paracetamol, concentration of some coloured pharmaceuticals in blood

15. Example UV Question

16. IR – Infrared Spectroscopy • Expensive, but simple are available at lower cost • Similar to UV spectrophotometer but operates in IR region – lower energy than UV light • Not enough energy to promote electrons to higher level, but causes changes in bond • Fast once sample is prepared • Gases, liquids & solids can be used • Absorption spectroscopy • Qualitative, rarely quantitative • Plot of transmittance against wave number

17. IR • Transmittance: measures how much light has been transmitted. 100% transmittance means no energy has been absorbed. Absorbance is the intensity of light remaining after some has been absorbed. • Used to identify organic compounds, in particular functional groups, design of new drugs, protein analysis, checking quality of wine products, tea leaves that have a smell, forensic identification of oils, fibres, paint flakes

18. IR Spectrophotometer

19. IR

20. NMR – Nuclear Magnetic Resonance Spectroscopy • Expensive • Hazardous because of the strong magnetic field • Operates in the radio region • Very fast (milliseconds) • Qualitative • Absorption spectrum • Spectra can be low resolution or high resolution

21. NMR • Used for fossil fuel analysis, identification of organic molecules, protein & nucleic acid structures, forensic analysis (this type of analysis in non destructive of the sample) • MRI – medical diagnosis. Detects differences in water balance of tumour abnormalities.

22. Colorimetry • Simple calorimeters are available for $100 • Fast once standard solutions made up • Qualitative and Quantitative • Limited to coloured solutions or compounds • Absorption spectroscopy • Calibration curve constructed, absorbance vs. concentration • Used to find copper (II) sulfate in garden sprays, any other coloured solution

23. Mass Spectrometry (MS) • Expensive, but portable models are cheaper • Very sensitive – uses µg samples • Qualitative and Quantitative • Operates at low pressures • Sample is vaporised and high energy electrons knock off electrons of molecules & cause fragmentation. The different species are separated on passing though an electric field and a magnetic field.

24. MS • Used to determine relative atomic mass on periodic table!! • Also used to identify organic molecules, qualitative and quantitative analysis of proteins, determine protein structure

25. Summary • We will study organic families first before talking reading these spectra. However it is important to remember that we have not had the opportunity to work with each instrument in the time given for Unit 3, so the exam may only test broad structural features. You should be able to identify the type of chemical to be identified by the instrument. You do not need to know all of the uses.

26. Analytical Chemistry VCE • With chromatography, you will need to refer to the peak areas, retention times and perhaps which substance would emerge first. You may to construct a calibration curve of area against concentration, then use the graph to determine the concentration of an unknown sample.

27. Analytical Chemistry VCE • With UV and AAS, you are often required to plot a calibration curve of absorbance against concentration. You would then be asked to determine the concentration of an unknown in the correct figures. • IR, NMR & MS – you are likely to be given a spectrum and asked to determine the molecule it represents. The necessary data will be in your data book. They should be a simple molecule like propanol or ethanoic acid. We are going to come back to interpreting spectra. • For MS you may also have to find the relative atomic mass if given the relative isotopic masses.

28. Summary Term 1 • Wet methods: • Slow but necessary. Used in remote areas or labs that do not have expensive equipment. These methods rely on a chemical reaction. • Acid-base volumetric analysis • Preparation of standard solutions to determine concentration in consumer product • Redox volumetric analysis • Gravimetric analysis: a) Analysis of water content b) Analysis of content using precipitation, filtering and drying

29. Summary Term 1 Continued • Instrumental methods: • Are fast and able to accurately determine very small quantities in small samples They play a large role in identification of compounds in forensics, IR NMR and MS are powerful tools in protein analysis and development of new medicines. Although some machines are very expensive, the labs can save labour costs with the speed of analysis and are therefore justifiable. With the exception of MS, they rely on absorbance.

30. Summary Term 1 Complete • AAS – used to determine metals • UV & Colorimetry – used to determine coloured metal complexes as well as coloured organic compounds. • Difference: • IR & NMR – Identification of organic compounds • Difference: • MS – Calculation of relative atomic mass and identification of organic compounds • USE p.120, Table 8.1 –Great for revision!!!