Electrothermal AAS

Electrothermal AASAIT

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)

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

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

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

Exercise 3.2

How are problems in Ex 3.1 dealt with in EAAS?

90% of sample not analysed• all sample analysed as single aliquotrapid passage through flame• sample held within sample holderdilute atomic vapour in flame• small volume of sample holder increases concentrationflame chemistry problems• no flamematrix components• matrix removed or reduced in effect

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.

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

• 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

Workhead

Ar flows through and around tube

quartz windows

HCL beam

cooling water coils

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

The heating process

• does not simply involve an instant transition to 1800-3000C.

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 2200C?

• spatter everywhere• analyte would be lost in the spray

Heating stages

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

Temperature cycle

0

500

1000

1500

2000

2500

0 20 40 60 80 100 120

Drying

Ashing

Atomising

Cooling

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

Exercise 3.4

Analyte Matrix

Drying time N Y

Drying temp. N Y

Ashing time Y Y

Ashing temp. Y N

Atomisation time

N N

Atom. temp. Y N

Drying

• designed to remove all volatiles• without losing analyte by over-vigorous boiling

Exercise 3.5Factors affecting drying time• volume• viscosity of liquid

Factors affecting temperature• bp of solvent

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

Exercise 3.6

• differences in drying stage, compared to the standard program for lead in water, where the matrix is:

a) petrol• Time: no change• Temp.: decrease

b) milk• Time: increase• Temp.: no change

Ashing

• non-volatile organic components of the matrix by burning them away

Exercise 3.7Factors affecting ashing time• amount of organic matter• temperature being used

Factors affecting temperature• volatility of analyte (eg lead)

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

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

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

Exercise 3.8

• differences in ashing stage, compared to the standard program for copper in water:

a) lead in water• Time: no change• Temp.: decrease or use modifier

b) aluminium in milk• Time: increase• Temp.: increase

Atomisation

• convert analyte to atomic vapour

Exercise 3.9Factors affecting atomisation time• nothing

Factors affecting atomisation temp.• volatility of analyte

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

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

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

s

sc

A

m

0044.0

m

Exercise 3.10

a) Calculate the expected absorbance for 10 uL of 20 ug/L Fe, if the characteristic mass for Fe is 1.2 pg.

b) 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.

73.02.1

200x0044.0A

goodnot;4.05.5

500x0044.0A

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)

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

Solutions

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

Tube life

• 500 cycles with very simple solutions

Exercise 3.12• in your own time• is examinable

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

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)

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

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

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

Reaction vessel

(sample + reagents)

Hollow Cathode

Lamp

Monochromator & Detector

heated quartz tube

Gas stream in

Gas stream out (with analyte)