briefly about mw digestion and aqua regia Ülis sõukand...
TRANSCRIPT
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
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