Electrothermal AAS

1. Electrothermal AAS AIT

2. Limitation of flame AAS • advantages of flame atomisation are: • simplicity • speed • most metals atomise in readily available flames • sensitivity suitable for general analyses (mg/L) • need for lower concentrations became apparent in the 60s (heavy metal pollution)

3. Exercise 3.1 Limiting factors for FAAS sensitivity • 90% of sample not analysed • rapid passage through flame • dilute atomic vapour in flame • flame chemistry problems • matrix problems

4. Non-flame atomisation • an AA system based around a flame and nebuliser will never achieve ug/L • vapour generation AAS doesn’t use a flame • atomisation occurs without significant temperature input • for most elements, this will not work • electrothermal AAS – electrical heating; same temperatures as flame

5. Basic principles • the flame and the nebuliser are replaced in EAAS • sample is delivered as a single aliquot into a sample holder • heated electrically • the absorption beam passes through the sample holder to the monochromator and detector

6. Exercise 3.2 How are problems in Ex 3.1 dealt with in EAAS? 90% of sample not analysed • all sample analysed as single aliquot rapid passage through flame • sample held within sample holder dilute atomic vapour in flame • small volume of sample holder increases concentration flame chemistry problems • no flame matrix components • matrix removed or reduced in effect

7. Equipment • burner/nebuliser replaced by workhead • most instruments allow the two units to be interchangeable • rest of the instrument is identical: • hollow cathode lamp • monochromator • photomultiplier detector • signal processing and readout • EAAS instruments must have computer control • automate the sample delivery • monitor the heating • signal measurement.

8. Workhead and cell • workhead contains the sample cell and provides the services for controlled heating of the sample • sample cell is a hollow graphite tube approximately 28mm in length by 8mm in diameter • EAAS also called graphite furnace AAS • interior of the cylinder is coated with a layer of pyrolytic graphite • greater resistance to heat than normal graphite • sample is delivered into the graphite tube through a small hole in the side of the tube

9. the tube rests on electrodes • graphite is only a moderate conductor of electricity, so the tube heats up • metal housing around the furnace is water cooled • enables rapid restoration of the furnace to ambient temperature after atomisation • inert gas (generally argon) is flushed through and around the tube from the ends: • to remove steam and smoke vaporised during the heating • to prevent oxidation of the graphite • to help with cooling

10. Ar flows through and around tube cooling water coils HCL beam quartz windows Workhead

11. Autosamplers • use an autosampler to place the sample in the graphite tube • volumes involved are typically 5-20 uL • also allow multiple samples to be run without an operator in attendance • different solutions (blank, sample, standard, matrix modifier) are used in combination for the analysis • kept apart in the plastic tubing by air gaps

12. The heating process • does not simply involve an instant transition to 1800-3000C. Class Exercise 3.3 • What do you expect would happen to a droplet of milk inside the graphite furnace it is was suddenly exposed to 2200C? • spatter everywhere • analyte would be lost in the spray

13. Heating stages Drying • removal of all volatiles • temperatures slightly above the boiling point of the solvent • 30-60 seconds Ashing • organic matter is burnt away • temp. increased to 500-1000C • 5-40 seconds Atomisation • temperatures used in FAAS (1800-3000C) • 3-4 seconds • measurement of absorbance

14. Temperature cycle Atomising Cooling Ashing Drying

15. Designing the temperature program • time and temperature conditions must be carefully selected • analyte and the matrix will affect the times and temperatures in each step • instrument manual provides a standard program for all elements (in pure water or dilute acid • can be used as a starting point for a real sample

16. Exercise 3.4

17. Drying • designed to remove all volatiles • without losing analyte by over-vigorous boiling Exercise 3.5 Factors affecting drying time • volume • viscosity of liquid Factors affecting temperature • bp of solvent

18. Drying • divided into a number of steps: • increase from room temperature to boiling point of solvent • remain at this temperature for a time • increase up to about 50ºC above b.p. to remove last volatiles • main problem: • incomplete drying • leads to splattering of the remaining material • listen for a fizz as the temperature goes up

19. Exercise 3.6 • differences in drying stage, compared to the standard program for lead in water, where the matrix is: • petrol • Time: no change • Temp.: decrease • milk • Time: increase • Temp.: no change

20. Ashing • non-volatile organic components of the matrix by burning them away Exercise 3.7 Factors affecting ashing time • amount of organic matter • temperature being used Factors affecting temperature • volatility of analyte (eg lead)

21. Ashing • also into three stages • quick increase from the final drying temperature to the ashing temperature • remain at ashing temperature for required time • in the last few seconds before atomisation, turnoff gas flow • atmosphere inside the tube is still for atomisation

22. Chemical modifiers • used where the analyte is relatively volatile • would be lost during the ashing stage at normal temps >600C • eg lead which is volatile above 500ºC; • ashing temperature would have to be quite low • a much longer time required • modifier used to reduce volatility of analyte • eg modifier for lead is phosphate • lead phosphate has a much higher boiling point than other lead salts • modifier must not hold onto the analyte during atomisation! • other elements needing modifiers include Cd, Hg, As & Sb

23. Ashing problems • generation of smoke during atomisation • causes a high background reading due to scattering • observe this at the start of the atomisation step • a puff of smoke coming out through the delivery hole

24. Exercise 3.8 • differences in ashing stage, compared to the standard program for copper in water: • lead in water • Time: no change • Temp.: decrease or use modifier • aluminium in milk • Time: increase • Temp.: increase

25. Atomisation • convert analyte to atomic vapour Exercise 3.9 Factors affecting atomisation time • nothing Factors affecting atomisation temp. • volatility of analyte

26. Signal measurement • more complex than in flame AAS • analyte atoms are only present for a few seconds once per run • rather than being present whenever the nebuliser is in the flask. • signal is a short-lived peak • signal capture by computer is essential • data only recorded in atomisation steps • ignores “absorbance” (actually scattering) during drying and ashing due to steam and smoke

27. Checking instrument performance • in flame AAS, sensitivity is the concentration giving an absorbance of 0.0044 • in EAAS, the characteristic mass is the mass giving that absorbance • used in the same way to check performance

28. Calculating mc • mc and ms are in picograms (10-12 g) • pg = uL x ug/L • eg 15 uL of 10 ug/L is 150 pg

29. Exercise 3.10 • Calculate the expected absorbance for 10 uL of 20 ug/L Fe, if the characteristic mass for Fe is 1.2 pg. • Comment on the instrument performance, if a 10 uL aliquot of 50 ug/L Pb gives an absorbance of 0.24, given the characteristic mass is 5.5 pg.

30. Memory effect • with flame AAS, if you put too a high a conc. soln in, the absorbance reading goes off scale and all you do is dilute it • with EAAS, these things happen • as well you temporarily wreck the tube • lots of the excess metal doesn’t atomise and stays in the tube – memory effect • if it is very hard to atomise (eg Al, Ca) throw the tube away • try a number of tube cleans • not just the analyte (think sea water)

31. Exercise 3.11 • How could you check whether the tube is ready to be used after a number of tube cleans? • run a blank and check its absorbance

32. Solutions • sample or standard • blank • standard addition • modifier • should be between 15 and 30 uL • should be consistent for all solutions

33. Tube life • 500 cycles with very simple solutions Exercise 3.12 • in your own time • is examinable

34. Background correction • a means of correcting for sources of non-analyte reduction in beam intensity (things that increase Abs): • smoke particles • molecular species • not all the ash smoke is removable • measured absorbance is the sum of analyte and background absorbance • correction systems measure total and background • analyte is difference

35. Correction systems Deuterium lamp • signal reaching the detector alternates between: • the HCL, which measures At, and • a deuterium lamp which only detects Ab • a simple system • requires that the HCL and D2 beams are exactly aligned through the tube (!!! not flame)

36. Exercise 3.13 • Why would the analyte absorption not be detected in the deuterium beam? • same reason HCL lamps were developed • atomic absorption line too narrow to be detectable in continuous source

37. Correction systems Zeeman effect • magnetic effect which changes the characteristics of the analyte absorption line • when the magnet is on, the analyte no longer absorbs the HCL • alternate between off (total) and on (background) • only requires the HCL lamp • much more expensive

38. Vapour generation • relies on the formation of a volatile species containing the analyte metal • the pure metal itself, eg mercury (cold vapour) • the readily decomposed hydride of metals, eg arsenic, antimony and tin (hydride generation) • volatile form of the metal is created by chemical reaction • a stream of gas (eg nitrogen) passes through it • transferred to cell in HCL beam • heating if required (hydrides) • gain in sensitivity (ug/L) Amendment– not an aliquot – continuous uptake

39. heated quartz tube Hollow Cathode Lamp Monochromator & Detector Gas stream out (with analyte) Gas stream in Reaction vessel (sample + reagents)