MICRONUTRIENTS
SA09
Last Revised:
June 2008
1. PURPOSE
This exercise measures four important micronutrients – Mn, Zn, Fe and Cu – extraction and analysis
by ICP emission spectrometry.
2. REAGENTS & EQUIPMENT
2.1 DTPA extracting solution (see end for details)
2.2 Air-dried soil (passed through a 2 mm sieve)
2.3 Mixed standards (see end for details)
2.4 ICP emission spectrometer
3. PROCEDURE
3A. Extraction
3.1 A reagent blank with no soil should be included with each batch of samples.
3.2 Weigh accurately about 25 g soil, in duplicate, and transfer into a 250 mL polyethylene bottles.
3.3 Add DTPA extracting solution (50 mL), stopper, and mechanically shake for 2 hours.
3.4 Filter through paper and Millepore.
3B. Standard preparation
3.5 Pipette 5, 10 and 20 mL of the LOW standard into separate 250 mL volumetric flasks and make
up to the mark with distilled water.
3.6 Pipette 25 and 50 mL of the HIGH standard into separate 250 mL volumetric flasks and make
up to the mark with distilled water.
3C. Analysis
3.7 Run all standards and samples through the ICP and record intensities.
5. REPORT
Calculations
1.
Calculate the concentration of each species in the standards.
2.
Plot calibration graphs for the four elements.
3.
Determine the analyte concentrations from the graphs.
4.
Calculate average mg/kg for each analyte.
Questions
1.
Compare your value for each analyte to the value required by grain crops in Table 6.2.
2.
Explain how DTPA works in extracting the metals from the soil.
3.
Compare ICP to AAS for this analysis in terms of accuracy and standard preparation.
Soil Analysis Practical Manual
SA09
Solution Preparation
For use by laboratory staff
DTPA Extracting Solution
This solution is 0.005M with respect to DTPA, 0.01M with respect to CaCl2 and 0.1M with respect to
triethanolamine (TEA).
For 1.0L of extracting solution, dissolve:
•
diethylenetriaminepentaethanoic acid (2.0 g)
•
calcium chloride dihydrate (1.5 g)
•
triethanolamine (15 g)
separately in deionised water and combine.
Add 6 mL of concentrated HCl and dilute to about 990 mL with deionised water.
Check pH and adjust to 7.3 with either dilute HCl or triethanolamine, then make the volume to 1.0 L.
Store in a teflon or low density polyethylene container not previously used to store any of the four
metals under test
The solution remains stable for at least three months if kept cool and away from direct sunlight.
Mixed stock standards
The procedure below is based on 1000 mg/L Cu & Zn standards, and 5000 mg/L Fe & Mn.
Low standard
10 mL of 1000 Cu
10 mL of 1000 Zn
20 mL of 5000 Mn
20 mL of 5000 Fe
Dilute to 1 L.
High standard
40 mL of 1000 Cu
40 mL of 1000 Zn
200 mL of 5000 Mn
200 mL of 5000 Fe
Dilute to 1 L.
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