Using ICP-AES for Quantitative Analysis of Soil Extracts

The ICP technique is widely accepted in the agricultural industry as a means of determining major and minor compositions of soils and plants or heavy metals in soils. It is also useful in monitoring soils for environmental or industrial waste and research and development, as well as in quality control measures for pollution control. This article will focus on the application of ICP-AECs through a quantitative analysis of the ammonium acetate extracts of soil. Depending on elements of interest, various extracts are used to stimulate plant uptake. A review of different digestion and extraction procedures shall also be presented.

Principle

Technique Used

The elemental analysis of solutions was made possible through Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). After the sample was nebulized, it was transferred to an argon plasma where it was decomposed, atomized, and ionized, fully exciting the atoms and ions. Light was emitted when atoms or ions returned to lower levels of energy. The intensity of this process was measured. Every element emits light at a particular wavelength. This enables users to quantitatively analyze the acquired data after calibration.

Wavelength Choice

The use of either the profile function or the Win-IMAGE (a rapid semi-quantitative analysis mode utilizing multiple wavelengths) enables users to choose a certain wavelength in a given matrix. In either option, the device records the scans of the matrix and of the analytes at low concentration. By superimposing the spectra, possible inferences may be seen.

Sample Preparation

Acidic Ammonium Acetate EDTA Soil Extracts

Reagents:

50 mL of extractant solution per determination is prepared as follows: 38.5 g NH4CO2CH3, 25 ml CH3CO2H (96%) and 29.225 g EDTA were diluted to 1 L.

Sample preparation:

5 g of dried soil sample weighed into a 250 mL polypropylene bottle is mixed with 50 ml of extractant solution. The suspension is shaken for one hour at 19-21 °C in a thermostat water bath, laboratory shaker. The sample solution is filtered through folded filter paper (Mg free) and then stored in a polypropylene bottle for ICP analysis. To enable reliable and reproducible results, extraction conditions—such as soil to extractant ratio, temperature, and duration of extraction—must remain constant.

HCl and HNO3 Soil Extracts

Reagents:

Concentrated HCl (37%) and HNO3 (65%) analytical reagent grade acids.

Sample preparation:

2 g of milled soil (< 150 µ sieve) is digested with 15 mL (20 mL, if calcareous) of HCl (18.5% w/v) and 5 mL HNO3 (conc.). The samples were digested in tubes using a heated block over a 9-hour period using temperatures up to 130 °C. After the process, the samples were left to dry. They were then redissolved in HCl and made to 100 mL with deionized water. The final solution was 5% HCl (v/v). It is also possible to make a microwave digestion.

Standards

Two distinct sets of standards were used, one for the major elements and the other for traces. The table below presents these standards.

Table 1. Standards prepared in Acetic Ammonium Acetate EDTA

Concentration in g/kg
Element Std0 Std1 Std2 Std3 Std4
CaO 0 0.56 2.80 5.60 11.20
MgO 0 0.066 0.166 0.664 1.33
K2O 0 0.060 0.120 0.600 1.200
Na2O 0 0.0135 0.027 0.054 0.108

 

Concentration in mg/kg
Element Std0 Std1 Std2 Std3 Std4
Zn 5 10 50 100 150
Mn 10 50 100 200 300
Cu 5 10 50 100 150

 

Table 2. Standards prepared in HCL/HNO3

Concentration in mg/L for major elements
Element Std0 Std1 Std2 Std3
Fe 0 70 100 150
Mg 0 30 100 150
Al 0 200 250 300
Ca 0 700 500 150

 

Concentration in mg/L for minor elements
Element Std0 Std1 Std2 Std3
Zn 0 0.5 1 2
Ni 0 0.1 0.2 0.5
Cr 0 0.3 0.5 1.5
Cu 0 0.1 0.3 1.5

 

Meanwhile, the following approximate matrix is used for the minor standards to improve accuracy in soils: 100 ppm Fe, 300 ppm Al, 400 ppm Ca, 100 ppm K, and 50 ppm Mg.

Instrument Specification

All quantitative work was done using a PANORAMA. The specifications of this instrument are listed in the following tables.

Table 3. Specification of spectrometer

Parameters Specifications
Mounting Paschen Runge
Focal length 0.5 m
Thermoregulation Yes
Nitrogen purge Yes
Grating number of grooves 2400 gr/mm
1st order resolution 0.025 nm
2nd order resolution 0.012 nm
Order 2nd order

 

Table 4. Specification of RF generator

Parameters Specifications
Type of generator Solid state
Observation Radial
Frequency 40.68 MHz
Control of gas flowrate By computer
Control of pump flow By computer
Cooling Air

 

Operating Conditions

Table 5. Operating conditions

Parameter Condition
RF Generator power 1400 W
Plasma gas flowrate 18 L/min
Auxiliary gas flowrate 0.8 L/min
Sheath gas flowrate 0.15 L/min
Nebulizer gas flowrate 0.35 L/min
Nebulizer flowrate 1.2 bars (18 psi)
Sample uptake 0.3 mL/min
Type of nebulizer Concentric
Type of spray chamber Cyclonic
Argon humidifier Yes
Injector tube diameter 3.0 mm

 

Despite high dissolved salts, a trouble-free analysis was enabled through the appropriate use of the argon humidifier, cross flow nebulizer, and the large internal diameter of the injector tube. This tube also minimized any interference that may have confounded the data. Because of high dissolved salts, an initial conditioning of the spray chamber should be done for maximum stability. It is also important to use matched standards due to the viscosity of the solutions.

Discussion

Background correction is a technique that has been heavily utilized in both major and minor elements to ensure improved accuracy. This is due to the empirical evidence suggesting that the matrix better increases the background in comparison to water samples. An argon humidifier and a cross flow nebulizer were utilized to avoid clogging at the tip of the nebulizer, thus improving long-term stability and performance.

Acidic Ammonium Acetate EDTA Soil Extracts

Table 6. Sample results

Analyte Wavelength (nm) Background correction (nm) Sample Concentration
CaO 315.887 -0.068 1.41 g/kg
MgO 279.079 +0.055 0.113 g/kg
K2O 766.490 -0.071 0.130 g/kg
Na2O 589.592 +0.071 0.017 g/kg
Zn 213.856 -0.030 66.88 mg/kg
Mn 257.610 -0.030 41.34 mg/kg
Cu 324.754 +0.041 1.74 mg/kg

 

HCl/HNO3 Acid Soil Digestion

Table 7. Results for BCR Soil Sample 141

Analyte Wavelength (nm) Background Correction (nm) Measured Value (mg/l) True Value (mg/l) Accuracy (%)
Fe 259.940 - 0.030 126.6 131 3.3
Mg 279.079 + 0.055 36.3 36 0.8
Al 396.152 - 0.030 268.1 279 3.9
Ca 317.933 - 0.071 620.9 642 3.3
Zn 213.856 - 0.030 0.390 0.410 4.9
Ni 231.604 - 0.030 0.152 0.160 5
Cr 267.716 - 0.030 0.345 0.370 4.8
Cu 324.754 + 0.041 0.244 0.228 7.3

 

Summary

The results showcased a consistency with the study hypothesis, however, a certain degree of care must be undertaken in preparing samples, especially with soil extracts. In order to reduce matrix effects, radial viewing, rather than axial viewing, is better. Standards were prepared with the same nebulization efficiency between standards and samples; meanwhile, background correction was used to nearly remove spectral matrix effects. Two different sets of standards were utilized to yield accurate results: one for major elements and another for minor elements. High sample throughput was recorded with ICP spectrometers such as the PANORAMA, with rates going at 60 samples per hour.

This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.

For more information on this source, please visit HORIBA Scientific.

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