Simple and Fast Methods Based on Solid-Phase Extraction Coupled to Liquid Chromatography with UV Detection for the Monitoring of Caffeine in Natural, and Wastewater as Marker of Anthropogenic Impact
Two concentration methods for fast and routine determination of caffeine (using HPLC-UV detection) in surface, and wastewater are evaluated. Both methods are based on solid-phase extraction (SPE) concentration with octadecyl silica sorbents. A common “offline” SPE procedure shows that quantitative recovery of caffeine is obtained with 2？mL of an elution mixture solvent methanol-water containing at least 60% methanol. The method detection limit is 0.1？μg L？1 when percolating 1？L samples through the cartridge. The development of an “online” SPE method based on a mini-SPE column, containing 100？mg of the same sorbent, directly connected to the HPLC system allows the method detection limit to be decreased to 10？ng L？1 with a sample volume of 100？mL. The “offline” SPE method is applied to the analysis of caffeine in wastewater samples, whereas the “on-line” method is used for analysis in natural waters from streams receiving significant water intakes from local wastewater treatment plants. 1. Introduction Caffeine is the world most widely-consumed psychoactive stimulant. It is possible to find this alkaloid in foods, beverages and drug preparations, and the daily average load has been estimated at between 16 and 70？mg？person？1？day？1 [1, 2]. The amounts of caffeine reaching wastewaters in urban areas are considerable since (i) large amounts of caffeine go directly down the drains from unconsumed drinks (e.g., coffee, tea, and soft drinks) and the rinsing of pots and cups [3], and (ii) 0.5–3% of human caffeine intake is excreted in a nonmetabolized form by urine [4, 5]. Caffeine has been found to reach values of around 100？μg·L？1 [2, 6] in the influents of wastewater treatment plants (WWTPs) and is one of the compounds that most contribute to the total mass loads of pharmaceuticals in these plants [7]. On the Costa Brava (Girona, Spain), a large percentage of the wastewater treated by WWTPs is reused for the irrigation of fields, golf courses, and public gardens as well as to feed natural streams for the recovery of their natural flow and ecological quality. Given these uses, it is extremely important that microcontaminants be removed in the WWTPs. In selecting chemical markers of anthropogenic impact, it is necessary to use ones that are able to distinguish wastewater inputs from both treated and non-treated sources [2, 8]. Such a compound would have to be largely eliminated in WWTPs. Caffeine, therefore, may be regarded as a suitable marker given that >99% can be degraded in WWTPs [2, 6, 7, 9, 10]. Henjum et al. [11] used caffeine as an indicator for
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