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).

 

2. 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.
  2.     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

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.

The automatic distributor cycle enables to perform this calibration on the platform from the selenium solution at 50 μg/l.

4. 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.

5. 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.