High-Precision Determination of Phosphorus and Potassium in Fertilizers by ICP-OES

Significance of the topic

Precise and accurate quantification of phosphorus (P) and potassium (K) in fertilizers underpins agronomic performance, regulatory compliance, and commercial labeling. Small errors in declared NPK values can affect crop yield, lead to mislabeling penalties, and undermine product value. High-precision analytical approaches therefore are essential for routine quality control in fertilizer production and certification laboratories.

Objectives and overview of the study

This application study evaluated a standard-bracketing calibration strategy implemented on an Agilent 5800 Vertical Dual View (VDV) ICP-OES to achieve high-precision determination of P and K in fertilizers. The goals were to (1) adapt ISO-style bracketing procedures for fertilizer matrices, (2) demonstrate method accuracy and precision on a NIST SRM and two commercial fertilizers (KCl, DAP, MKP), and (3) illustrate the advantages of real-time software calculations for reporting results as conventional fertilizer oxides (P2O5, K2O) and NPK ratios.

Methodology

Sample preparation and internal standardization

The workflow used gravimetric sample and standard preparation to enable mass-corrected bracketing. Representative 0.125 g sample aliquots were dissolved to 250 mL (final volumetric solution) by adding 25 g of a 6 mg/L yttrium (Y) internal standard (IS) solution, ~100 mL deionized water, and 12.5 mL concentrated HNO3. Final solutions contained 5% (v/v) HNO3 and represented a nominal dilution factor of 2000×. The gravimetric addition of the IS and careful mass recording supported gravimetric correction during data processing.

Calibration and bracketing strategy

Standards were prepared from certified 10,000 mg/L stocks and targeted to lie within ±5% of expected sample concentrations to optimize bracketing performance. The standard–sample–standard measurement sequence (standard bracketing) was performed in five bracketing cycles per sequence; each sample was measured in three sequences (15 bracketing measurements total). The method combined internal standard correction (Y 371.029 nm) with alternating standard/sample measurements to mitigate short- and long-term signal drift and sample-to-sample matrix effects.

Key experimental settings and measurement strategy

The instrument was run in radial viewing mode with a 1.8 mm one-piece VDV torch injector. Typical operating parameters included RF power 1.3 kW, nebulizer flow 0.7 L/min, plasma flow 12 L/min, auxiliary flow 1.0 L/min, and sampling/read settings that provided rapid replicate acquisition and 30 s rinse times. Results were averaged across replicates and bracketing cycles, with gravimetric correction applied in software.

Used instrumentation

• Agilent 5800 Vertical Dual View (VDV) ICP-OES
• Agilent SPS 4 autosampler
• SeaSpray concentric glass nebulizer and double-pass cyclonic spray chamber
• Easy-fit 1.8 mm one-piece torch (VDV)
• Agilent ICP Expert Pro software (version 7.8 or later) with custom column and Sample Aggregate (SSUM) QC features

Main results and discussion

Precision and accuracy

The bracketing method delivered outstanding precision (reported relative standard deviations below 0.1% across replicate sequences) and high accuracy with recoveries for P2O5 and K2O equivalents in the three test materials between approximately 99% and 102%. The NIST KCl SRM showed excellent agreement with certified values (sequence-averaged K2O equivalent recovery ~99%). Results for DAP and MKP also matched expected NPK compositions within the tight tolerance required for fertilizer labeling.

Advantages demonstrated by the method

Standard bracketing combined with internal standard correction effectively compensated for instrumental drift and matrix effects that can limit the performance of conventional external calibrations. This approach is particularly valuable for NPK verification where very small percent-level deviations have practical and regulatory implications. Real-time software calculations converted elemental concentrations to P2O5 and K2O equivalents and computed NPK ratios automatically, reducing post-run processing and transcription errors.

Data handling and QA features

The ICP Expert Pro custom column capability enabled live conversion of elemental results into oxide equivalents and direct reporting of NPK ratios. The Sample Aggregate (SSUM) QC function summarized sequence-level results, supported user-defined thresholds, and could halt runs if pre-set quality limits were exceeded—streamlining high-throughput QC workflows and minimizing rework.

Practical benefits and potential applications

• Reliable verification of labeled NPK values for fertilizer manufacturing QC and regulatory compliance.
• Reduced analytical uncertainty supports tighter product specifications and more accurate pricing.
• High throughput capability for production-scale testing, enabled by autosampler automation and real-time calculations.
• Robust compensation for drift and matrix variability, improving long-term run stability for routine labs.

Future trends and opportunities

Method standardization and broader adoption

Adapting ISO-style bracketing workflows specifically for fertilizer matrices should be further standardized to allow inter-laboratory comparability. Development of certified reference materials covering a wider range of fertilizer matrices (blends, organic amendments) would strengthen method validation.

Automation and data integration

Continued integration of gravimetric tracking, autosampler automation, and LIMS connectivity can further reduce manual steps and reporting errors. Real-time QA/QC alarms and automated corrective actions will be increasingly important for high-throughput manufacturing environments.

Expanded analytical scope

Extensions of the approach can include multi-element panels for micronutrient and contaminant monitoring (e.g., heavy metals), or coupling bracketing strategies with ICP-MS where lower detection limits are required. Advances in software-driven data processing and chemometric correction for complex matrices are expected to improve accuracy across diverse fertilizer types.

Conclusion

The Agilent 5800 VDV ICP-OES employing a gravimetrically controlled standard-bracketing calibration, internal standard correction, and real-time software processing reliably produced high-precision P and K measurements in fertilizers. Achieved precision (RSD <0.1%) and recoveries (≈99–102%) demonstrate the method's suitability for routine NPK verification and production quality control. The combination of bracketing calibration and modern data-handling tools reduces analytical variability, lowers the risk of mislabeling, and enhances laboratory efficiency.

References

1. Bradford R. Determination of 21 Elements in Fertilizers using Autodilution with ICP-OES. Agilent Technologies publication 5994-8647EN.
2. Oppedisano D. Rapid Multi-Elemental Analysis of Fertilizers using the Agilent 5110 VDV ICP-OES. Agilent publication 5994-0600EN.
3. Agilent Technologies. High Precision Analysis of Major Components in Precious Metals by ICP-OES. Application note 5994-8849EN.
4. Agilent Technologies. Integrated Design: Accuracy and Precision of Agilent ICP-OES Instruments. Application note 5994-8853EN.
5. Agilent Technologies. Agilent ICP Expert Software: Powerful software with smart tools for ICP-OES. Publication 5994-1517EN.
6. ISO 11494:2019. Jewellery and precious metals — Determination of platinum in platinum alloys — ICP-OES method using an internal standard element.
7. ISO 11495:2019. Jewellery and precious metals — Determination of palladium in palladium alloys — ICP-OES method using an internal standard element.
8. NIST SRM 999c. Potassium Chloride. Certificate of analysis. National Institute of Standards and Technology.