Using the AVIO 200 ICP-OES for Analysis of Trace Elements in Fertilizer

The main benefit of fertilizer lies in increasing crop productivity. Fertilizer production is a key industry for the agricultural sector of a country. The major elements of interest in fertilizers are classified as: primary nutrients, secondary nutrients, micronutrients, and trace elements. The representative elements in each of these classes are shown in Table 1.

Table 1. Elemental Categories in Fertilizer

Class Elements
Primary Nutrients N, P, K
Secondary Nutrients Ca, Mg, Fe, Mn, Cu, Zn, Mo, B, S
Micronutrients Al, Co, V, Se, Ni
Trace Elements As, Cd, Cr, Pb

Chemical fertilizers are widely utilized in agriculture at present, and this has led to increased demand for them. However, this demand has also led to the production of some inferior quality fertilizers. The primary and secondary nutrients and the trace elements need to be analyzed and monitored to ensure that the fertilizers' quality conforms to local regulations. The Thailand Agricultural Department’s regulatory levels of the trace elements present in fertilizers is listed in Table 2. Although various techniques may be employed to measure trace elements in fertilizer, ICP-OES provides the best combination of simplicity, cost, accuracy, and ruggedness. The ICP-OES also has a dynamic range which is suitable for the concentrations expected in fertilizer samples.

Table 2. Regulatory Limits of Elements in Fertilizers in Thailand

Element Regulatory Concentration (mg/kg)
As 50
Cd 5
Cr 300
Pb 500

This article shows the ability of the Avio™ 200 ICP-OES to satisfy the Thai regulatory limits for trace elements in fertilizers by the analysis of reference materials.

Experimental Conditions

Samples and Sample Preparation

The method was validated using two reference materials: BCR-032 Trace Elements in Moroccan Phosphate Rock and NIST 695 Trace Elements in Multi-Nutrient Fertilizer. The first reference material was chosen as it has a high phosphate concentration comparable to the high phosphorus concentrations commonly seen in fertilizers.

The Titan® MPS Microwave digestion system was employed to prepare the samples by microwave digestion. 0.15 g of sample was added to each vessel, after which 6 mL concentrated nitric acid and 2 mL concentrated hydrochloric acid were also added. The vessels were left uncovered for 10 minutes in order to allow any early safe reactions to occur. Then the vessels were capped, kept in the Titan®, and digested using the temperature program, as shown in Table 3. After digestion, the samples were shifted to 100 mL volumetric flasks, and 100 mL of deionized water was used for dilution in performing the analysis.

Table 3. Titan MPS Temperature program

Temperature Program
Step Target Temp (°C) Pressure Limit (bar) Ramp Time (minutes) Hold Time (minutes) Power (%)
1 160 30 5 5 90
2 180 35 2 30 100
3 50 35 1 15 0
4 - - - - -

Instrumental Parameters

The Avio™ 200 was used to analyze all samples by using the parameters given in Table 4, including the standard sample introduction system. Thanks to Avio’s Flat Plate plasma technology, only 9 L/min of argon was used. The plasma technology along the vertical torch improves the ability to analyze high-matrix samples. Table 5 shows the wavelengths and analytes. All quantitative measurements were carried out against external calibration curves constructed from 0.05, 0.1, 1, 2, and 5 ppm standards.

Table 4. Instrumental parameters and sample introduction components for Avio™ 200 ICP-OES

Parameter Value
Nebulizer/spray chamber Meinhard/Cyclonic
Injector Alumina Injector 2.0
Read Time 1 sec (min) – 2 sec (max)
Plasma Gas 8 L/min
Auxiliary Gas 0.2 L/min
Nebulizer Gas 0.7 L/min
Power 1400 W
Plasma View Axial

Table 5. Elements and Wavelengths

Elements Wavelength (nm) Plasma View
Arsenic (As) 188.979 Axial
Lead (Pb) 220.353 Axial
Cobalt (Co) 228.616 Axial
Cadmium (Cd) 228.802 Axial
Nickel (Ni) 231.604 Axial
Chromium (Cr) 267.716 Axial

Results

The reference materials were measured to determine the method accuracy. Initially, calibration curves were established, which are illustrated in Figure 1. In all the curves, the correlation coefficients are greater than 0.9999. Tables 6 and 7 show the results for the reference materials for NIST 695 and BCR-032, respectively. The tables demonstrate the accuracy of the technique as all recoveries are within 10% of the certified values.

Figure 1. Calibration curve for all elements.

Table 6. Result of SRM 695

Analyte Certified Values (mg/kg) AVIO™ 200 Values (mg/kg) % Recovery
As 200 211 107
Pb 273 295 108
Co 65.3 62.9 96
Cd 16.9 18.0 107
Ni 135 139 103
Cr 244 243 100

Table 7. Result of CRM BCR-032

Analyte Certified Values (mg/kg) AVIO™ 200 Values (mg/kg) % Recovery
As 9.5 < DL -
Pb 5.4 < DL -
Co 0.59 0.613 104
Cd 20.8 21.8 105
Ni 34.6 35.0 101
Cr 257 261 102

The detection limits (DLs) of this technique were calculated by analyzing a matrix blank. The matrix blank contained the same reagents and quantities as those employed for sample preparation. The DLs were calculated by multiplying the standard deviation of ten replicate measurements by three (Table 8).

Table 8. Detection Limits in Fertilizer Samples

Analyte DLs (mg/kg) Thai Regulated Levels (mg/kg)
As 3.01 50
Pb 3.37 500
Co 0.31 Not specified
Cd 0.21 5
Ni 0.59 Not specified
Cr 0.79 300

Conclusion

This article has shown the ability of the Avio™ 200 ICP-OES to accurately measure trace elements and secondary nutrients present in fertilizers at or below the regulated levels in Thailand.

Consumables Used

Avio™ 200 ICP-OES
Component Part Number
Red-Red PVC Pump Tubing 09908585
Black-Black PVC Pump Tubing 09908587
Autosampler Tubes B0193233 (15 mL)
B0193234 (50 mL)
PerkinElmer Pure XVI (QC 21 Elements) N93002812 (125 mL)

Titan® MPS Microwave
Component Part Number
Consumable Kit for Standard 75 mL Digestion Vessels N3132000
Rupture Disks for Standard 75 mL Digestion Vessels (25 pieces) N3132001
Pressure Seal for Standard 75 mL Digestion Vessels (10 pieces) N3132002
End Cap Plug for Gas Containment Manifold N3134004
Single Lip Seal Forming Tool for Standard 75 mL Digestion Vessels N3132015
8-Position Lip Seal Forming Tool for Standard 75 mL Digestion Vessels N3132014

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer Inc.

For more information on this source, please visit PerkinElmer Inc.

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