Analysis of 5-hydroxytryptophan in the presence of excipients from dietary capsules: comparison between cyclic voltammetry and UV visible spectroscopy

5-Hydroxytryptophan (5-HTP) is an important neurochemical and is available as a dietary supplement, which is used to regulate eating behaviour and potentially overcome depression. Dietary supplements have undergone a change in their regulation, where improved analytical assessment is required during manufacturing. We compared cyclic voltammetry with UV visible spectroscopy for the detection of 5-HTP in capsules, where an emphasis was placed in understanding if recordings could be conducted in the presence of pharmaceutical excipients. Both voltammetric and spectroscopic methods had linear responses over a concentration range between 50 and 150 mg per 25 ml(-1) of 5-HTP and similar limits of detection. There current response was increased for 5-HTP in the presence of magnesium stearate and silica, however in the presence of all capsule excipients there was no difference on the voltammetric response and current. Following oxidation of 5-HTP, electrode fouling was observed on the carbon composite electrodes. The electrode surface following fouling was partly regenerated following soaking in ethanol and fully recovered when the electrode was polished in alumina slurry for 5 s. Voltammetric responses were capable of detecting 5-HTP content within individual capsules with or without filtration of pharmaceutical excipients. Only following filtration was spectroscopy able to provide stable response of 5-HTP content within an individual capsule. Overall, voltammetric responses provide a means of conducting measurements of 5-HTP dietary supplement capsules without the need for extensive sample preparation, thus providing a total analysis time within 8 minutes.