Eesti Keskkonnauuringute Keskus

Briefly about MW digestion and aqua regia

Ülis Sõukand PhD

Main ideas

• The microwave digestion is an appropriate method for the treatment of soil samples for the trace metal analysis

• Pretreatment methods are in continuous development

Our results in 8th Soil Interlaboratory Test Programme (2015)

Samples passed Parameter

Mean recovery of samples; %

1

2

3

4

5

+ + - + + Al 116

+ + + + + Ca 104

+ + + + + Cr 117 + + + + + Cu 114

+ + + + + Fe 101 + + - + + K 122

+ + - - + Mg 112 + - + + + Mn 105

+ + + + + Na 114

+ + + + + Ni 112 + + + + + P 111

+ + + + + Pb 96 + + + + - Zn 100

+ + + + + S 106

+ + + + + Cd 102 + + + + + Hg 107

Comments to the table

• all results of analysed elements passed the test

• only 8,8% from total results are out of range

• most of the results are a bit higher compared to the average recovery

• higher results might be due to MW digestion, but the real reason is unclear

Our results of other soil ringtests

Parameter Mean recovery of samples; %

2015 2012 2009 2007

Al 116 100 76 118 Ca 104 93 86 126

Cr 117 106 87 115 Cu 114 122 103 110

Fe 101 93 89 104

K 122 104 74 135

Mg 112 100 114 104

Mn 105 100 92 103 Na 114 105 66 126

Ni 112 101 93 101 P 111 110 103 104

Pb 96 107 93 94

Zn 100 101 93 101 S 106 95 100 -

Comments to the table

• determination of Al with ICP is difficult

• K, Na, Ca give ionization interferences by determination with ICP

• in digestion stage either loss or contamination may occur

• sample inhomogeneity may be a problem

• some concentrations of elements are very low

Conclusion

• results of analysed elements passed the ringtest since 2002 and therefore MW is an appropriate method for the pretreatment of soil samples

Eesti Keskkonnauuringute Keskus

Do you love smell of nitrogen dioxide in morning?

The second idea

Pretreatment methods are in continuous development

Starting with soil analysis (1996)

• Time – 2 weeks per batch

• Number of hot plate positions – 10

• Reagent composition – mixture of conc. HNO3, HClO4 and H2O2 (very flammable)

• Analysis – FAAS (problems with sensitivity)

Later... (1997-2007)

• Time – 30 min per batch

• Number of CEM MW oven positions – 12

• Reagent composition – aqua regia

• Analysis – ICP-OES

...and today

• Time – 30 min per batch

• Number of Berghof MW oven positions – 24 (+40 in reserve)

• Reagent composition – reverse aqua regia

• Analysis – ICP-OES and ICP-MS, AF for Hg

Aqua regia and...

• 3 HCl : 1HNO3

• Contains not enough HNO3 to decompose high C content (up to 60%) in samples

• therefore forms resin-like residue, hard to clean up

• vapor pressure of HCl is about 10x higher than vp. of HNO3, so it is good to have less HCl in vessel

• high HCl conc. act like an „unneccesary ingredient” of mixture

reverse aqua regia

• European st. method EN 16174:2012 for soil digestion with aqua regia recommends to add more HNO3 to common aqua regia:

„Then add an extra 1ml of nitric acid (6.3) only to every 0,1 g of organic carbon above 0,5 g.“

• We use: 1 HCl : 3HNO3, 2ml HCl : 6ml HNO3,

0.5g or 0.25g sample

Eesti Keskkonnauuringute Keskus

Thank you!