A method of detecting a material comprising a thiol group in a sample, wherein the material is homocysteine (HCys) and glutathione, the method comprising: providing an electrode comprising a carbon material; contacting said electrode with said sample in the presence of an optionally substituted catechol (1,2-dihydroxybenzene); carrying out an electrochemical procedure to convert catechol to ortho-quinone and wherein said ortho-quinone reacts with the material comprising a thiol group to form an electrochemical reaction product, wherein the method comprises determining the concentration of homocysteine in the presence of glutathione, wherein the electrochemical procedure comprises cyclic voltammetry to determine the presence of the electrochemical reaction product, wherein the homocysteine is distinguished from other materials which may be detected by a voltammogram by varying the scan rate, wherein the concentration of homocysteine is calculated and the concentration of glutathione is calculated in the sample by: obtaining a first cyclic voltammogram at a first scan rate, obtaining a second cyclic voltammogram at a second scan rate, wherein the two scan rates are different so that there is provided a higher and a lower scan rate, determining the concentration of HCys using the peak current obtained at the higher scan rate using an analytical curve for this scan rate, using the determined concentration of HCys to calculate the peak current at the lower scan rate using an analytical curve obtained for the lower scan rate, wherein the peak current calculated for the lower scan rate is subtracted from a peak corresponding to a total adduct peak current at the lower scan rate thereby obtaining the current for the glutathione-catechol reaction and determining the concentration of glutathione using an appropriate calibration curve and the obtained current for the glutathione-catechol reaction; wherein the electrode comprising a carbon material is a carbon nanotube modified glassy carbon electrode (CNT-GCE), wherein the higher scan rate is at least 500 mVs -1 and the lower scan rate is about 50 mVs -1 or less, and optionally wherein the concentrations of glutathione and homocysteine are determined in the presence of cysteine and/or ascorbic acid. providing an electrode comprising a carbon material; contacting said electrode with said sample in the presence of an optionally substituted catechol (1,2-dihydroxybenzene); carrying out an electrochemical procedure to convert catechol to ortho-quinone and wherein said ortho-quinone reacts with the material comprising a thiol group to form an electrochemical reaction product, wherein the method comprises determining the concentration of homocysteine in the presence of glutathione, wherein the electrochemical procedure comprises cyclic voltammetry to determine the presence of the electrochemical reaction product, wherein the homocysteine is distinguished from other materials which may be detected by a voltammogram by varying the scan rate, wherein the concentration of homocysteine is calculated and the concentration of glutathione is calculated in the sample by: obtaining a first cyclic voltammogram at a first scan rate, obtaining a second cyclic voltammogram at a second scan rate, wherein the two scan rates are different so that there is provided a higher and a lower scan rate, determining the concentration of HCys using the peak current obtained at the higher scan rate using an analytical curve for this scan rate, using the determined concentration of HCys to calculate the peak current at the lower scan rate using an analytical curve obtained for the lower scan rate, wherein the peak current calculated for the lower scan rate is subtracted from a peak corresponding to a total adduct peak current at the lower scan rate thereby obtaining the current for the glutathione-catechol reaction and determining the concentration of glutathione using an appropriate calibration curve and the obtained current for the glutathione-catechol reaction; wherein the electrode comprising a carbon material is a carbon nanotube modified glassy carbon electrode (CNT-GCE), wherein the higher scan rate is at least 500 mVs -1 and the lower scan rate is about 50 mVs -1 or less, and optionally wherein the concentrations of glutathione and homocysteine are determined in the presence of cysteine and/or ascorbic acid. obtaining a first cyclic voltammogram at a first scan rate, obtaining a second cyclic voltammogram at a second scan rate, wherein the two scan rates are different so that there is provided a higher and a lower scan rate, determining the concentration of HCys using the peak current obtained at the higher scan rate using an analytical curve for this scan rate, using the determined concentration of HCys to calculate the peak current at the lower scan rate using an analytical curve obtained for the lower scan rate, wherein the peak current calculated for the lower scan rate is subtracted from a peak corresponding to a total adduct peak current at the lower scan rate thereby obtaining the current for the glutathione-catechol reaction and determining the concentration of glutathione using an appropriate calibration curve and the obtained current for the glutathione-catechol reaction; wherein the electrode comprising a carbon material is a carbon nanotube modified glassy carbon electrode (CNT-GCE), wherein the higher scan rate is at least 500 mVs -1 and the lower scan rate is about 50 mVs -1 or less, and optionally wherein the concentrations of glutathione and homocysteine are determined in the presence of cysteine and/or ascorbic acid.