What is the difference between atomic and molecular absorbtion?
see table.
Why do you not get any molecular information from atomic spectroscopy?
- Atomic spectroscopy focuses only on free atoms and their electronic transitions, leaving out the vibrational and rotational information associated with molecules.
- Since atomic spectroscopy only deals with atoms the more complex molecular transitions are not observed
Instrumentation of a flame photometer
Sample - must be in aqueous soln
Nebuliser - as sample is sucked up into nebuliser it can strike an impact bead which can create a fine spray
Air + Gas inlet - creates pressure difference between sample and nebuliser to allow sample to be sucked up into the nebuliser
Propellers - mix the sample with the air/gas to create the spray - fine droplits will be sucked up into the flame
U-tube + waste - most of the sample will be seperated into large droplets which goes down th etube into waste
Flame - temperature causes excitation of molecules and give of atomic emission
Filter - to filter out different elements
Photo detector - to detec the light
Limitation of flame photometer
- Only around 10% of the sample will be sucked up into the flame - mostly waste
- limited amount of elements it can be used to detect
Specificity of ICP-OES
- Very specific - creates a plasma as hot as 10,000K, converting a very large proportion of its atoms to an excited state which can identify and determine simualtaneously up to 40 elements.
Principles of ICP-OES
- Ar (argon) gas used
- Radio frequency (rf) electrical current applied to Cu load coil (induction coil)
- rf current has associated magnetic fiels with lines of force passing along the axis of a quartz tube placed inside the coil
- spark applied which ionised a few Ar atoms
- the e- formed are rapidly accelerated in circular orbits by the rapidly alternating electric field
- collisions cause gas to heat up to 10,00K forming a plasma
Sources of interference in atomic emission - Ionisation
- Ionisation:
* at high temperatures, atoms can become ionised reducing th observed emission from the sample
* combat by adding a more readily ionised element to the sample to shift the equilibrium eg; strontium chloride added to surpress ionisation of K
Sources of interference atomic emission - Spectral
- caused by two overlapping spectral lines
- must then select a different non-overlapping emission line
- > 50,000 ICP-OES spectral lines documented
Sources of interference atomic emission - Chemical
- Sulphate or Phosphate present in the matrix, when interacted with the plasma torch can impact the sample and reduce excitability, reducing light emitted, reducing the reading
Sources of interference atomic emission
- calibration graph
- Calibration graph is non-linear at high concentrations due to reabsorption of emitted light by ground state atoms/ions in flame
How do standard additions work?
- Sample Aliquots: Divide the sample into several equal portions (aliquots).
- Add Standard: Add increasing known amounts of a standard containing the analyte (the substance being measured) to the aliquots, leaving one unspiked (no standard added).
- Measure Signal: Measure the absorption (or emission) of each aliquot, including the unspiked one, using atomic spectroscopy.
- Plot Graph: Plot the instrument’s signal (y-axis) against the concentration of the added standard (x-axis).
- Extrapolate: Extrapolate the graph to the x-axis (where the signal is zero). The x-intercept gives the concentration of the analyte in the original, unspiked sample.
This method helps correct for matrix effects.
