Lambert Beer and Multi Component Analysis Flashcards
Flame test?
Both emission and absorption can be employed, AES and AAS, thermal excitation by flame gives colour
Emission or absorption?
Emission more powerful as technique, dark background so pick out photons is much easier and much more selective and sensitive compared to absorption
basic principles of fluorescence spectroscopy?
Emission proportional to absorption
Absorption proportional to concentration (because of concentration Beer Lambert law for absorbance can be used)
Emission as a technique in fluorescence spectroscopy?
Much more sensitive than absorption spectrocppsy, emission of light can be seen for a single molecules (eg DNA labelling), but limited range of compounds fluorescence
How can spectrofluorumetry be used to discover MDMA or ecstasy?
If the MDMA molecules fits into another molecule that has the right size cavity for MDMA the larger molecule will no longer be floppy its degree of freedom will be gone it will change configuration and so will fluoresce (if it is still floppy it will de excite and not fluoresce)
What does the strength of fluoresce depend on?
Strength of light, how big the cuvette is, how much solution in sample, how strong absorption of light is in molecule
How is luminal used in chemical forensics?
Luminol is sprayed onto a crime scene in a solution with hydrogen peroxide light emission will be highly sensitive on a dark background, hydrogen peroxide decomposes with catalyst to form OH radical, this reacts with luminal which then emits UV/blue light, Fe (eg from blood traces) catalyses the decomposition of H2O2, this triggers light emission and gives fluorescence, also compared biofluorescence as extremely sensitive detection tool
IR or Raman spectroscopy?
Infra red spectroscopy = transmission or absorption
(transmission is amount of light that foes through sample)
I/I0 = T
A = -logT = log(I0/I)
What is important about the spectra from IR and Raman method?
Peak intensity proportional to concentration
But different peaks respond differently, difficult to get consistent sample length
Difficult but not impossible to measure absolute concentrations, more commonly used to measure relative contraptions (internal standard etc) to follow reactions kinetics
Whats important about the peak position in the IR or Raman spectra?
Peak position - energy of transition (bond stretch, vibration)
Whats important about the peak height in the IR or Raman spectra?
Peak height - intensity of absorption/emission, amount of substance
Qualitative use of IR or Raman spectra?
Electric field radiation that interacts with polarity of molecule electric interaction in electric field
Quantitative use of IR or Raman spectra?
Take peak height (intensity) and work out concentration
How are radiowaves used in spectroscopy?
NMR, emission technique, emission at right angle detected that comes out of magnets, magnetic field interacts with magnetic molecules, absorption of radiation into spins magnetic interactions work only if nucleus has nuclear spin, peak in NMR correlate to concentration, change concentration of sample more or less peaks, a lot of components that change with intensity (can used Beer Lambert law)
Limitations of Beer Lambert law?
Lambert Beer law only works in certain range too high a concentration can have problems with shadows so not good for everything
E units?
mol-1dm3cm-1
Na*cm2/molecule
cm2/molecule
What is E?
Active area for each molecule how big shadow is of a molecule how good it is at absorbing light how big it is in the light, understood as an active area or area blocked off by the molecule, absorption coefficient = optical size, therefore a high concentration limit to Lambert Beer could be for example when more than two molecules are in the light path
What is the minimum absorbance?
Minimum absorbance accurately measurable is A ~ 0.01 (limited in sensitivity) A = 0.01 = Ecl corresponds to
c = 4 x 10-6 moldm-3 for a 1 cm cell (limit in high and low level) instrument becomes unreliable with systematic errors and errors in cuvettes, an estimate of the practical detection limit, here micro molar
What E value do very highly coloured complexes have?
E = 250 m2mol-1
How to increase sensitivity?
Add a complexing agent - forms a coloured complex increases sensitivity greatly (alternatively go to fluorescence or change sampling methodology such a bigger cuvette length)
How does the Beer Lamber law apply to each species?
The Beer lambert law applies to each species provided that they do not react with each other, the total absorbance of the solution is the sum of the absorbances of the individual species
Number of independent points = number of independent parameters
Pulse oximeter application of the Beer Lamber law?
Sources of light at ~650nm (red) and ~910nm (infrared) and two light collecting sensors which measure the intensity that passes through the tissues, oxygenated blood absorbs light at 660nm deoxygenated blood preferentially at 910nm, the light is partly absorbed by oxyhemoglobin (HbO) and deoxyhaemoglobin. By calculating the absorption at the two wavelengths the processor can compute the proportion of haemoglobin which is oxygenated (two unknown concentrations require two independent measurement pint –> determine both unknowns)
What does absorption spectroscopy offer?
Absorption spectroscopy offers highly chemically selective information which is quantitative with Beer Lambert law
What does emission (fluorescence) spectroscopy offer?
Emission (fluorescence) spectroscopy is considerably more sensitive and also quantitative with absorption following Beer Lambert law
Multi component analysis?
Multi component analysis requires multiple wavelength data which is mathematically independent, analytical spectroscopy works down to the single molecule level
