Area Study 1 Unit 3 2010

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.

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,

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

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.

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:

Metal cation Flame colour

Lithium Crimson

Sodium Golden Yellow

Potassium Lilac

Calcium Brick Red

Strontium Scarlet

Barium Yellow Green

Copper Green Blue

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

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

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

p.83Example 7.3Lead in oystersppm = one millionth gram8.3 ppm- i.e. there are 8.3 g in 1.0 x 106 g of solution

2.5ppm = 2.5 µg per g of sample2.5mg per kg

2.5ppm =2.5 µg per mLOr 2.5 mg per L

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

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

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

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

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

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.

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

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.

Used to determine relative atomic mass on periodic table!!

Also used to identify organic molecules, qualitative and quantitative analysis of proteins, determine protein structure

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.

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.

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.

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

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.

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!!!