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

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 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 C
5-40 seconds
Atomisation
temperatures used in FAAS ( 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 Analyte Matrix Drying time N Y Drying temp. Ashing time
Ashing temp.
Atomisation time
Atom. temp.

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