Lambert Beer and Multi Component Analysis

Question 1

Flame test?

Answer 1

Both emission and absorption can be employed, AES and AAS, thermal excitation by flame gives colour

Question 2

Emission or absorption?

Answer 2

Emission more powerful as technique, dark background so pick out photons is much easier and much more selective and sensitive compared to absorption

Question 3

basic principles of fluorescence spectroscopy?

Answer 3

Emission proportional to absorption

Absorption proportional to concentration (because of concentration Beer Lambert law for absorbance can be used)

Question 4

Emission as a technique in fluorescence spectroscopy?

Answer 4

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

Question 5

How can spectrofluorumetry be used to discover MDMA or ecstasy?

Answer 5

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)

Question 6

What does the strength of fluoresce depend on?

Answer 6

Strength of light, how big the cuvette is, how much solution in sample, how strong absorption of light is in molecule

Question 7

How is luminal used in chemical forensics?

Answer 7

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

Question 8

IR or Raman spectroscopy?

Answer 8

Infra red spectroscopy = transmission or absorption
(transmission is amount of light that foes through sample)
I/I0 = T
A = -logT = log(I0/I)

Question 9

What is important about the spectra from IR and Raman method?

Answer 9

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

Question 10

Whats important about the peak position in the IR or Raman spectra?

Answer 10

Peak position - energy of transition (bond stretch, vibration)

Question 11

Whats important about the peak height in the IR or Raman spectra?

Answer 11

Peak height - intensity of absorption/emission, amount of substance

Question 12

Qualitative use of IR or Raman spectra?

Answer 12

Electric field radiation that interacts with polarity of molecule electric interaction in electric field

Question 13

Quantitative use of IR or Raman spectra?

Answer 13

Take peak height (intensity) and work out concentration

Question 14

How are radiowaves used in spectroscopy?

Answer 14

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)

Question 15

Limitations of Beer Lambert law?

Answer 15

Lambert Beer law only works in certain range too high a concentration can have problems with shadows so not good for everything

Question 16

E units?

Answer 16

mol-1dm3cm-1
Na*cm2/molecule
cm2/molecule

Question 17

What is E?

Answer 17

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

Question 18

What is the minimum absorbance?

Answer 18

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

Question 19

What E value do very highly coloured complexes have?

Answer 19

E = 250 m2mol-1

Question 20

How to increase sensitivity?

Answer 20

Add a complexing agent - forms a coloured complex increases sensitivity greatly (alternatively go to fluorescence or change sampling methodology such a bigger cuvette length)

Question 21

How does the Beer Lamber law apply to each species?

Answer 21

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

Question 22

Pulse oximeter application of the Beer Lamber law?

Answer 22

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)

Question 23

What does absorption spectroscopy offer?

Answer 23

Absorption spectroscopy offers highly chemically selective information which is quantitative with Beer Lambert law

Question 24

What does emission (fluorescence) spectroscopy offer?

Answer 24

Emission (fluorescence) spectroscopy is considerably more sensitive and also quantitative with absorption following Beer Lambert law

Question 25

Multi component analysis?

Answer 25

Multi component analysis requires multiple wavelength data which is mathematically independent, analytical spectroscopy works down to the single molecule level