Glufosinate in Plant-Based Matrices: Analytical Challenges and Solutions

Glufosinate is a non-selective herbicide widely used in agriculture worldwide. Its high water solubility and pronounced polarity clearly distinguish it analytically from more lipophilic compounds.

After application, glufosinate can be partially metabolized in plants—particularly in tolerant or resistant crops—into N-acetyl-glufosinate (NAG). MPPA occurs both as a plant metabolite and as a degradation product in the environment.

From a regulatory perspective, glufosinate, NAG, and MPPA are assessed collectively in the European Union under Regulation (EC) No. 396/2005 as part of a defined residue definition and evaluated against the applicable maximum residue levels (MRLs).

The current residue definition is:

“Glufosinate (sum of glufosinate isomers, their salts and their metabolites 3-[hydroxy(methyl)phosphinoyl]propionic acid (MPP) and N-acetyl-glufosinate (NAG), expressed as glufosinate).”

For analytical laboratories, this requires the reliable and quantitative determination of all three compounds to ensure correct compliance assessment.

Analytical Challenges in Dried Plant Matrices

Dried plant matrices such as herbs, teas, spices, or dietary supplements represent particularly challenging sample types.

The drying process leads to a significant concentration of matrix constituents, including secondary plant metabolites, pigments, and inorganic salts. This enrichment often results in pronounced matrix effects in LC-MS/MS analysis, especially ion suppression or enhancement, which can significantly impact the quantification of polar pesticides.

At the same time, glufosinate, NAG, and MPPA are typically present at very low concentrations in these matrices, often in the low µg/kg range.

Additionally, the high polarity of the target analytes complicates chromatographic retention on conventional reversed-phase columns. Phosphorus-containing compounds such as glufosinate and MPPA may also interact with metallic surfaces, leading to adsorption, signal loss, and consequently underestimated results.

Sample Preparation as a Critical Success Factor

Against this background, sample preparation plays a central role.

Classical multi-residue methods such as QuEChERS are primarily designed for less polar pesticides and often reach their limits when applied to glufosinate and its metabolites.

For polar pesticide analysis, the QuPPe method (Quick Polar Pesticides) has become a well-established standard. It is continuously refined and enables reliable extraction of highly polar analytes.

For dried plant matrices, thorough homogenization is essential to ensure representative and reproducible results. Depending on the matrix, additional clean-up steps may be required to remove interfering substances without causing losses of the highly polar target compounds.

LC-MS/MS as the Core Analytical Technique

LC-MS/MS offers high sensitivity and selectivity, making it ideally suited for the determination of glufosinate, NAG, and MPPA.

To improve retention, HILIC columns or specially modified reversed-phase materials are often used to achieve adequate chromatographic separation from matrix components.

Detection is typically performed in negative electrospray ionization mode using compound-specific mass transitions, allowing for unambiguous identification and quantification.

Matrix effects, however, remain a key challenge. The use of isotopically labeled internal standards is therefore essential in routine analysis of highly polar pesticides, as they compensate for signal suppression or enhancement and significantly improve analytical accuracy.

Insights from Method Development and Routine Analysis

Experience from method development and routine analysis shows that glufosinate residues are most frequently detected in black tea.

Measured concentrations typically range from approximately 0.01 mg/kg to 0.1 mg/kg. In individual cases, higher levels may occur, potentially reaching or exceeding the applicable regulatory limits for this matrix.

In addition to black tea, fennel, chamomile, paprika, and derived products are among the matrices where glufosinate residues are comparatively often detected.

Conclusion

The analytical determination of glufosinate, NAG, and MPPA in dried plant matrices is challenging but can be reliably achieved using modern LC-MS/MS methods.

A combination of optimized sample preparation, tailored chromatographic conditions, and sensitive mass spectrometric detection forms the basis for robust analysis.

Given the complexity of plant matrices and increasing regulatory requirements, careful method development remains essential to ensure reliable residue monitoring and quality control of plant-based raw materials.