Codex Œnologique International

Download document

SELENIUM - DOSAGE PAR SAA

COEI-2-SELENI Determination of selenium by atomic absorption spectrometry

  1. 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).

 

  1. 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.
    1.     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

 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.

 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

  1. 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.

  1. 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.

  1. 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.