MAAC: Analytical Spectrophometry 5 Flashcards
Pharmaceutical aplications of atomic absorbation
Determination of metal residues remaining from the manufacturing process in drugs
Used to measure:
–Zn content of insulin
–Fe in blood or tablets
–Heavy metals (Hg, Cd, etc.) in drinking water
–Cu or Mn in tonics.
What is the basic mechanism of atomic Absorption?
- Measures absorbance of light by ground state atoms or ions.
- Highly specific method useful in some aspects of quality control
What is the difference in flame use in AE and AA?
- In AE – a flame is used to excite electrons from ground state to excited state (Boltzmann Law)
- In AA – flame is only used to atomise the sample.
- Absorption is carried out using a hollow cathode lamp
- Hollow cathode lamp – made of same metal as sample – emits light of exactly the same wavelength as absorbed by sample.
- Flame – 10 cm acetylene flame, acts as absorbance cell. Should not excite sample.
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Modulator – provides lamp with AC current to overcome small number of excited atoms emitting light.
- Only amplifies light from the lamp and not from the flame (see later)
- Modern instruments use graphite furnace.
Describe how the intrumentation of AA works
you’ll see very similar to the flame photometer method we covered before the sample, again, has to be in solution. We have a nebuliser, there’s a fuel, gas and oxidant, those gases are going to pass in front of the capillary that’s gone into the sample. That’s going to create a pressure differential between one end of the capillary to the other. And that’s going to cause the sample to be pulled up into the nebuliser. Once that liquid comes into the nebuliser, it’s going to be broken down into droplets. The larger droplets will be too heavy and will fall down the drain. So only about 10 percent of the actual sample will carry on with the flame gases and head up to the burner. From there, the flame will be used to remove any solvent, the sample, but not cause it to be excited. Instead will now have our light source. The hollow cathode lamp, passing the light of a very specific wavelength through the flame. And that’s going to give us an atomic absorption. reading dependent on how much atoms are in the sample. The rest of the system, is then very similar, a monochromator to pick our wavelength and the detector. Probably a PMT being used and a PC for the readout
Describe the make of the hollow cathode lamp
Describe how the hollow cathode light generates light emission
Highly energetic electrons emitted by cathode
- Ar ionised by collision with electrons (Ar + e- —> Ar+ + 2e-)
- Ar+ accelerated to cathode causing sputtering of metal atoms (M(s) —–> M(g))
- This is when there is phase change directluy from solid to gas
- Metal atoms are excited by collisions with e- and ions (M(g)) —> M*(g))
- The excited metal atoms relax then emit the excess energy in the form of a photon of light which produces the characteristic atomic emission lines (M*(g) —–> M(g) + hν)
Describe the temperature and pressure of the gas in emission region
What does this mean?
Temperature of gas in emission region is only a few hundred oC and is at low pressure
–Little Doppler broadening
–Little pressure broadening
–i.e. very narrow lines
What are the limitations of a hollow cathode lamp?
Limitations:
- One lamp required for each element
- Not good for volatile elements e.g. As, Se
- Use electrodeless discharge lamp
Disuss the need for a monochromator
Several emission lines both from atoms and filler gas for each lamp so a monochromator is still required
What are the requirements for the atom cell (flame)
The flame itself also generates spectral emission as it is generating a light source. It also has lots of free readicals inside of the flame gases and these will produce different emissions.
How methods can this be used to resolve/ correct this?
- Amplifier
- Back ground correction light source
Describe the process of amplification
Both modulated current and using a chopper will produce the same effect ie a pulsed signal generated by the lamp.
Describe how background correction works
- If molecular absorption / scattering is due to a component of the sample
- e.g. measurement of Ba in the presence of Ca (forms CaOH+)
- then problem finding AA when HCL will measure AA + BK
- Solved using Background Correction.
- Most common method is deuterium (D2) lamp
- Broad spectral output Io from D2
- Any atomic absorption has little effect on I
- Molecular absorption or scattering will be broad and cover whole width of monochromator
- Thus reduces D2 lamp intensity
- D2 lamp only measures unwanted broad absorption
- Thus may be subtracted from AA absorption to give true reading
Summary:
Deuterium lamp emits broad spectrum of light. Light passing the sample in the flame is alternated between the HCL and the deuterium lamp. Broad spectrum not affected by narrow absorbance of atom = background. Whilst HCL is affected by absorbance by atoms = signal. Able to subtract the two to leave just the AA signal.
Describe the use of the monochromator in both AA and AE
•In AA
–Used to isolate the AA line from other lines emitted from the HCL
•In AE
–Used to isolate the AE line from other lines due either to element of interest or other elements in the sample matrix
How is GFAAS carried out?
