The He-Ne laser (632.8 nm) and photoacoustic spectroscopy (PAS), a variant of the photothermal methods, were combined with the Folin-Ciocalteu colorimetry method to determine total phenolics in wine lees. Utilising this method we found that the total polyphenol content of the nine selected wine lees varied from 1305 to 3907 mg/L. The gallic acid equivalent was determined by means of spectrophotometry using 765 nm as the analytical wavelength. The original Folin-Ciocalteu colorimetry assay was modified for the PAS measurements. Since the PAS does not need dilution, the filtration steps and the Folin-Ciocalteu reagent were used directly on the wine lees samples. Using the original colour reaction process and the modified reaction, the PAS showed linear behaviour between the total polyphenol content and the PA signal; the results of which gave determination coefficients of 0.9946 and 0.9936, while the limit of detection was 232.6 mg/L in both cases.

The analytical performance of the proposed laser-based photoacoustic spectroscopy (LBPAS) and optothermal window (OW) method for quantification of β-carotene content in sea buckthorn juices is compared to that of the conventional high-performance liquid chromatography (HPLC). Both photothermal techniques (laser-based photoacoustic spectroscopy (LBPAS) and optothermal window (OW)) are direct methods that unlike HPLC obviate the need for the extraction of the analyte. The outcome of the study leads to the conclusion that LBPAS and OW are both suitable for quick screening of ß-carotene in sea buckthorn juices. The β-carotene content of the samples changed due to the added sea buckthorn pomace and storage time. The correlation between the two methods and HPLC is linear with R2=0.9897 for LBPAS and R2=0.9928 for OW, respectively. Both methods are capable of the rapid and non-destructive determination of β-carotene in sea buckthorn juices without a need for sample pre-treatment.