Analytical performance and optimization of figures of merit of a portable nano liquid chromatograph (NanoLC) with UV detection at 255 nm have been established for in place analysis. Methylxanthines: caffeine, theophylline and theobromine were selected as target analytes. A fast lab method based on IT-SPME coupled on line with capillary liquid chromatograph (CapLC) with diode array detection (DAD) was employed for comparative studies. IT-SPME and solid phase extraction were coupled off-line to NanoLC for improving instrumental parameters, mainly detection capacity and selectivity. IT-SPME or SPE/portable NanoLC based methods were superior in terms of chromatographic resolution and organic solvent consumption per sample, around 200 μL vs 10 mL for IT-SPME-CapLC-DAD. Limits of detection (LODs) obtained with the SPE/portable NanoLC were 2–10 ng/mL, which can be suitable for testing the presence of the analytes in several environmental waters in the field. As predictable, the lab method provided better LODs, between 0.1 and 0.5 ng/mL. Good linearity was achieved for both methods and precision was similar for them (≤7%). Both systems were tested for the analysis of real water samples with suitable results.
Because of the complexity of modern society, results in the field are needed. In this context, progress in the miniaturization of analytical instrumentation has led the development of portable versions of the bench top instruments for many analytical techniques. However, progresses made in liquid chromatography (LC) have been slow (Campíns-Falcó et al., 2019), most probably due to the difficulty of manufacturing portable miniaturized high pressure pumping systems compatible with high efficient columns (Sharma et al., 2014; Chatzimichail et al., 2019). Although the first attempts to develop portable LC instruments date back from the 1980s, the development of fully portable LC has gained interest during the past decade due to the progresses in areas such as microfluidics, electronics or battery technology. Different attempts have been made to develop chromatographs that fulfill the general requirements for field portable instruments, namely, fully portability, low size, low power consumption, robustness, reliability and adequate analytical performance. Table 1 summarizes portable LC equipments developed during the past decade, along with some of their features and their respective applications. As observed in this table, most of the systems reported were homemade and, therefore, not marked for general use (Elkin, 2014; Sharma et al., 2014, Sharma et al., 2015; Ishida et al., 2015; Li et al., 2015; Zhao et al., 2017; Chatzimichail et al., 2019; D. Li et al., 2020; S. Li et al., 2020). Only very recently, assays with portable NanoLC instruments commercially available have been reported (Abinamah et al., 2019; Foster et al., 2020). Table 1 also shows that most of the proposals reported so far are focused on the evaluation of the functionality of the LC system itself. Only in a few cases the systems have been tested for real samples (Sharma et al., 2015; Abinamah et al., 2019; Foster et al., 2020). The potential utility of portable LC for real applications remains to be seen and this paper is focused to advance in this field.
One of the application fields that may benefit from portable LC is water analysis (Gałuszka et al., 2015). On-site tests could provide valuable information about the presence of water pollutants in real time, reducing the risks associated with the alteration of the sample during the time elapsed between sampling and analysis in the lab. In this context is developed this work and its finality is showing a new possible option for following pollution due to given contaminants, but on site. Determination of trimethylxanthines was selected as a use case. However, in environmental water pollutants are usually present at low concentrations, and therefore, simple and efficient sample treatment is necessary to reach the required analyte detectability. To date, a wide variety of extraction and microextraction techniques are available that can be applied in the labs to enhance analyte detectability. However, only a few of these sample treatment techniques are amenable for on-site tests.
In view of the above, the present study was aimed at evaluating the performance and potential utility of a portable NanoLC with UV detection at 255 nm in the analysis of trimethylxanthines, and taking into account different options for sample treatment adapted for field tests. Achieving a good compromise between the quality of analytical results and greening analytical operations is also a challenge. To this end, capillary liquid chromatography (CapLC) coupled on-line to in-tube solid-phase microextraction (IT-SPME) with diode array detection (DAD) has been selected as lab reference technique (Moliner-Martinez et al., 2015, Moliner-Martinez et al., 2020; Serra-Mora et al., 2017, Serra-Mora et al., 2018, Serra-Mora et al., 2019). Caffeine and theobromine are naturally occurring alkaloids present in coffee, tea, cola and energy drinks, and in many cocoa-containing foods. In addition, caffeine and theophylline are ingredients of a wide variety of pharmaceutical products. Because of the high consumption of these compounds, the above methylxanthines are increasingly found in wastewater and environmental water samples. The concentration levels of caffeine and some of its metabolites (including theophylline and theobromine) in wastewater have been proposed to investigate consumption habits of caffeine-containing products (Loos et al., 2010; Teijon et al., 2010; Gracia-Lor et al., 2017), and even as an indicator of the life-style (Naidu et al., 2016). Caffeine levels in wastewater have been also used as a marker to investigate population size of specific areas (Rico et al., 2017), and to check the efficiency of wastewater treatments (Lin et al., 2018).
In the present work, figures of merit and operability of a portable NanoLC were established for the determination of several methylxanthines. Next, possible strategies for water sample treatment suitable for on-site analysis with the portable NanoLC system have been explored, namely off-line IT-SPME and solid phase extraction (SPE) with cartridges. Resolution, precision, selectivity, analyte detectability and analysis time have been studied and optimized. Finally, the technique has been applied to the analysis of real water samples and the results were compared with those obtained with a fast lab IT-SPME coupled on line with CapLC-DAD. The option shown in this paper is an alternative more; it is not intended to replace lab methods.
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Originally published in Science of The Total Environment 2020, 747, 140966; doi.org/10.1016/j.scitotenv.2020.140966.