A method of determining a concentration of a glycopeptide antibiotic containing a phenol moiety in a complex sample matrix, the method comprising: extracting the glycopeptide antibiotic from a metered portion of the complex sample matrix by: exposing said metered portion to an extraction material having an affinity with the glycopeptide antibiotic; and exposing the extraction material to a metered portion of a first eluent being a first organic solvent or a second eluent being a methanol/water mixture for releasing the glycopeptide antibiotic from the extraction material; and determining a concentration of the glycopeptide antibiotic by: adding an excess amount of Gibbs reagent (2,6 dichloroquinone-4-chloroimide) relative to the amount of the glycopeptide antibiotic to the metered portion of the first or second eluent; activating the Gibbs reagent under alkaline conditions and, after the reaction between the activated Gibbs reagent and the antibiotic has stabilized; detecting the reaction product of the activated Gibbs reagent and the glycopeptide antibiotic in said first or second eluent; and determining the concentration of the glycopeptide antibiotic in the complex sample matrix from the detected reaction product, said concentration being: - the concentration of unbound glycopeptide antibiotic in the complex sample matrix in case of the first eluent; and - the concentration of bound glycopeptide antibiotic in the complex sample matrix in case of the second eluent. extracting the glycopeptide antibiotic from a metered portion of the complex sample matrix by: exposing said metered portion to an extraction material having an affinity with the glycopeptide antibiotic; and exposing the extraction material to a metered portion of a first eluent being a first organic solvent or a second eluent being a methanol/water mixture for releasing the glycopeptide antibiotic from the extraction material; and exposing said metered portion to an extraction material having an affinity with the glycopeptide antibiotic; and exposing the extraction material to a metered portion of a first eluent being a first organic solvent or a second eluent being a methanol/water mixture for releasing the glycopeptide antibiotic from the extraction material; and determining a concentration of the glycopeptide antibiotic by: adding an excess amount of Gibbs reagent (2,6 dichloroquinone-4-chloroimide) relative to the amount of the glycopeptide antibiotic to the metered portion of the first or second eluent; activating the Gibbs reagent under alkaline conditions and, after the reaction between the activated Gibbs reagent and the antibiotic has stabilized; detecting the reaction product of the activated Gibbs reagent and the glycopeptide antibiotic in said first or second eluent; and determining the concentration of the glycopeptide antibiotic in the complex sample matrix from the detected reaction product, said concentration being: - the concentration of unbound glycopeptide antibiotic in the complex sample matrix in case of the first eluent; and - the concentration of bound glycopeptide antibiotic in the complex sample matrix in case of the second eluent. adding an excess amount of Gibbs reagent (2,6 dichloroquinone-4-chloroimide) relative to the amount of the glycopeptide antibiotic to the metered portion of the first or second eluent; activating the Gibbs reagent under alkaline conditions and, after the reaction between the activated Gibbs reagent and the antibiotic has stabilized; detecting the reaction product of the activated Gibbs reagent and the glycopeptide antibiotic in said first or second eluent; and determining the concentration of the glycopeptide antibiotic in the complex sample matrix from the detected reaction product, said concentration being: - the concentration of unbound glycopeptide antibiotic in the complex sample matrix in case of the first eluent; and - the concentration of bound glycopeptide antibiotic in the complex sample matrix in case of the second eluent. - the concentration of unbound glycopeptide antibiotic in the complex sample matrix in case of the first eluent; and - the concentration of bound glycopeptide antibiotic in the complex sample matrix in case of the second eluent.