Professor Went: Calibration

Question 1

What are calibration standards?

Answer 1

• Calibration standards contain exactly known concentrations of analyte element ideally in a similar matrix
• Can just plot a calibration curve but signals vary with time due to fluctuations in gas flow rates, aspiration rates

Question 2

How can the precision of emission techniques be improved?

Answer 2

• Internal standards can be used to improve precision of emission techniques
• They use a fixed amount of an internal standard (element A) i.e. independent of the element being measured.
• If aspiration rate fluctuates, both emission by A and B is affected and the ratio remains the same for a given B concentration.

Question 3

What must be considered when using internal standards?

Answer 3

• The choice of A - excitation potentials must be fairly similar
  
  • Use of Li in determination of K or Na in in blood
  • Also a good choice because Li is not naturally present in blood.
• Another problem can be suppression/enhancement of signal by sample matrix due to chemical reactions with analyte.
• This can be countered with standard addition calibration.

Question 4

How is a standard-addition calibration carried out?

Answer 4

1. Measure response of sample and then repeat experiment with addition of known amount of standard solution of element under investigation.
2. Standard is then subjected to the same matrix as the unknown analyte.
3. Increase in signal is due to the standard and the original signal is due to the analyte.

A simple proportionality applies assuming a linear calibration curve is present.

Question 5

What is ‘spiking’ in terms of standard addition?

Answer 5

• Spiking involves adding one or more increments of a standard solution to sample aliquots containing identical volumes
• Each solution is then diluted to a fixed volume
• The sample matrix is nearly identical after each addition

Question 6

What issues can arise due to sample preparation when calibrating?

Answer 6

• An issue with flame/plasma methods is the sample must be introduced into excitation source as a solution.
• Main materials of interest (soils, minerals etc) are not directly soluble in common solvents.
• Decomposition is time consuming and can introduce errors
• Refactory materials require treatment at high temperatures and losses can occur as vapour/smoke
• If looking for trace amounts, reagents used in sample prep may introduce errors.

Question 7

What preparation methods may be used?

Answer 7

• Treatment with hot mineral acids
• Oxidation with sulfuric, nitric and perchloric acids
• Combustion in an oxygen bomb
• High temp fusion with boric oxide, Na2CO3, sodium peroxide etc.
• Makes little difference what chemical form the analyte is in as it will be dissociated to the free element in the flame.

Question 8

What are electrothermal atomisers?

Answer 8

• This can directly atomise a solid as well as liquid. but for solids, calibration can be difficult.
• Less reproducible but more sensitive
• Burners waste sample.
• Electrothermal is better as the entire sample is atomised in a shorter period.

Question 9

How do electrothermal atomisers work?

Answer 9

1. Few microlitres of sample are first evaporated at low temperature (100 to 200 degrees)
2. The sample is ashed at 500-1400 ^oC on an electrically heated surface of carbon/other conducting material
3. The current is increased to 100s of amps to produce 2000-3000^oC
4. Atomisation occurs in few milliseconds
5. AA is measured above the conductor.