Selenium- Determination by AAS
COEI-2-SELENI Determination of selenium by atomic absorption spectrometry
- Principle
After mineralisation of the sample by wet process, the selenium is determined by atomic absorption spectrometry without flame (electro-thermal atomisation in the graphite oven).
- Apparatus
2.1. Glassware
- Graduated flasks 50, 100 ml (class A)
- Graduated pipettes 1, 5 and 10 ml (class A)
- Polypropylene tubes 50 ml with screw top.
- Instrumental parameters: (given as an example)
- Atomic absorption spectrophotometer equipped with an atomiser
- with a graphite tube.
- wave length: 196.0 nm
- hollow-cathode lamp (selenium)
- width of slit: 1.0 nm.
- intensity of the lamp: 10 mA
- correction of continuum by the Zeeman effect
- introduction in hot conditions of the samples in the graphite oven with anautomatic distributor (rinsing water contains 2 drops of Tritonper litre).
- measurement of signal: peak height
- time of measurement: 1 second
- number of measurements per sample: 2
Pyrolytic graphite tube:
Pyrolytic graphite oven containing a platform of L’Vov tantalised.
tantalisation of a platform: see given procedure beforehand.
inert gas: argon.
parameters for oven: table I
|
Table I - Parameters for oven for determining selenium |
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|
step |
temperature |
time |
gas flow rate |
type of gas |
reading of signal |
|
(°C) |
(s) |
(l/min) |
|||
|
1 |
85 |
5 |
3.0 |
argon |
no |
|
2 |
95 |
40 |
3.0 |
argon |
no |
|
3 |
120 |
10 |
3.0 |
argon |
no |
|
4 |
1 000 |
5 |
3.0 |
argon |
no |
|
5 |
1 000 |
1 |
3.0 |
argon |
no |
|
6 |
1 000 |
2 |
0 |
argon |
no |
|
7 |
2 600 |
0.8 |
0 |
argon |
yes |
|
8 |
2 600 |
2 |
0 |
argon |
yes |
|
9 |
2 600 |
2 |
3.0 |
argon |
no |
2.3. Automatic sampler parameters (table II)
(given as an example)
|
Table II - Parameters de automatic sampler. |
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|
volumes injected in µl |
|||
|
solution |
blank |
matrix modifier |
|
|
blank |
17 |
3 |
|
|
calibration n°1 50 μg/l |
5 |
12 |
3 |
|
calibration n°2 100 μg/l |
10 |
7 |
3 |
|
calibration n°3 150 μg/l |
15 |
2 |
3 |
|
sample |
15 |
2 |
3 |
- Reagents
3.1. Pure demineralised water for analysis
3.2. Pure nitric acid for analysis at 65%
3.3. Anhydrous palladium chloride (59% in Pd)
3.4. Pure hexahydrated magnesium nitrate for analysis
3.5. Ammonium dihydrogenophosphate
3.6. Matrix modifier: mixture of palladium chloride and magnesium nitrate (dissolve 0.25 g of PdCl2 and 0.1 g of Mg(NO3)2.6H2O in 50 ml of demineralised water) ammonium dihydrogenophosphate at 6% (dissolve 3 g de NH4H2PO4 in 50 ml of demineralised water).
3.7. Selenium reference solution at 1 g/l, off the shelf or prepared as follows: dissolve 1.4052 g SeO2 in a solution of HNO3 0.5 M, adjust at 1 l avec HNO3 0.5 M.
3.8. Selenium solution at 10 mg/l: place 1 ml of the reference solution at 1 g/l in a 100 ml graduated flask; add 5 ml of nitric acid at 65%; complete to volume with pure demineralised water for analysis
3.9. Selenium solution at 50 µg/l: place 0.5 ml of the selenium solution at 10 mg/l, 5 ml of nitric acid at 65% in a 100 ml graduated flask; complete to volume with pure demineralised water for analysis.
3.10. Set of calibration solutions: 0, 50, 100 and 150 μg/l of selenium.
The automatic distributor cycle enables to perform this calibration on the platform from the selenium solution at 50 μg/l.
- Preparation of samples
Weigh with precision a test sample of 1 to 3 g in the graduated tube; add 5 ml of nitric acid at 65%; close with the screw cap; leave 12 hours at room temperature;
place the tube in a water bath at 90°C for 3 hours (the caps are unscrewed during the heating); allow to cool; adjust the volume to 20 ml with pure demineralised water for analysis.
- Determinations
Set up the calibration graph (absorbance depending on the concentration in µg/l of selenium); determine the concentration of selenium in the samples.
Calculate the concentration of selenium in the mineralisate, then in the samplein μg/kg.