Acidic herbicides (free acids, esters and conjugates) in grapefruit

It is essential to apply a hydrolysis step for this ring test. An analysis of the esters and the free acids only results in an underestimation of the spiked levels! The test samples MUST be analysed twice - with and without hydrolysis. 

The residue definitions of acidic herbicides according to regulation (EC) No. 396/2005 include the esters, salts and conjugates of the acidic herbicides. The conjugates can contribute significantly to the total amount of the respective herbicides determined in routine samples. An alkaline hydrolysis step is applied to quantify residues of the conjugates in routine samples.

P2106-RT is designed to cover all aspects of the analysis of acidic herbicides, including the hydrolysis step in accordance with the residue definition.

For that purpose the test material is spiked with selection of

  • conjugates of acidic herbicides,
  • commercially available esters of acidic herbicides, and
  • free acids of acidic herbicides.

In order to ensure that the results are unambiguously, either the conjugate or the ester or the free acid is spiked for each of the selected herbicide. In case of one of the herbicides, the conjugate, an ester and the free acid is spiked to simulate the residue situation in routine samples. Acidic herbicides within the scope of the test are summarised here.

The participants are asked to report the results with and without hydrolysis. The results after alkaline hydrolysis (sum of conjugates, esters and free acids) are considered for evaluation of the trueness of results. 

The analytical data are assessed with respect to

  • the comparability (z-score), and
  • the trueness (70 to 120% of the spiked level).


Analytical results without hydrolysis

4-CPA (free acid), 2,4-D (free acid), 2,4-DB (free acid), 2,4-dichlorprop (free acid), 2,4,5-T (free acid), 2,4,5-TP (free acid), clopyralid (free acid), dicamba (free acid), fenoxaprop (free acid), fluazifop (free acid), fluroxypyr (free acid), haloxyfop (free acid), MCPA (free acid), MCPB (free acid), triclopyr (free acid)

Analytical results with hydrolysis

4-CPA (sum), 2,4-D (sum), 2,4-DB (sum), 2,4-dichlorprop (sum), 2,4,5-T (sum), 2,4,5-TP (sum), clopyralid (sum), dicamba (sum), fenoxaprop (sum), fluazifop (sum), fluroxypyr (sum), haloxyfop (sum), MCPA (sum), MCPB (sum), triclopyr (sum)

Expected time schedule

Please register before:
15 September 2021
Dispatch of samples:
15 November 2021
Deadline for reporting results:
07 December 2021

Costs of P2106-RT

Ring test and material Amount Price per unit
Acidic herbicides (free acids, esters and conjugates) in grapefruit 750 €
Test material grapefruit 100 g included
Analytical results without hydrolysis
Free acids and esters
Analytical results with hydrolysis
Sum of free acids, esters and conjugates
Blank material grapefruit
Homogenate, unspiked
100 g 70 €

Note: PROOF-ACS reserves the right to cancel tests in case of less than ten participants.