The discharge of trace substances into surface waters from industry, agriculture and municipal sewage endangers the aquatic ecosystem [1]. The list of anthropogenic substances identified in the aquatic environment already includes around 1000 different chemicals and is rapidly increasing, due to frequent mass spectrometry screening in environmental studies [2,3]. However, the conventionally applied analytical methods for target analysis and non-target screening have severe limitations due to two aspects.

The first challenge is the relatively low concentration (in the ng/L range) of the analytes often found in the aquatic ecosystem, enforcing the use of enrichment steps for detection. Offline solid phase extraction (SPE) is conventionally used for enrichment but is limited to analytes with sufficient interaction with the solid phase material [4]. Additionally, offline SPE is extremely laborious, resulting in high processing times, low sample throughput and high risk for loss of analytes [5,6]. A promising alternative to offline SPE is the large volume injection (LVI) which increases the sensitivity by applying injection volumes above 10 % of the effective column void volume and thus achieves enrichment of the analytes. Reported LVI methods are distinguished between coupled column LVI (CC-LVI), in which a serially coupled pre-column is used for enrichment, and single column LVI (SC-LVI), in which enrichment is performed directly on the separation column [7]. Further advantages of LVI over offline SPE are the lower risk for loss of analytes, the lower use of materials and solvents and the lower workload, since only the removal of particulate components by centrifugation [8] or filtration [9] is necessary as sample preparation [7,10].

The second challenge is the wide range of polarities of anthropogenic substances discharged to the aquatic ecosystem, enforcing the use of multiple and complex methods to analyze a wide polarity detection range. For instance, separation principles for nonpolar substances, such as hormones and sartans, cannot be applied to polar compounds, such as iodinated X-ray contrast agents [11]. Samples with this broad polarity spectrum can no longer be analyzed by one-dimensional liquid chromatography (1D LC) unless two chromatographic methods with different selectivity are used. As a promising and more advantageous alternative, two-dimensional liquid chromatography (2D LC) enables higher peak capacity, and selectivity. Consequently, the separation space for a 2D LC separation is much higher when compared to 1D LC [12], [13], [14], [15]. For instance, coupling of reversed phase liquid chromatography (RPLC) and hydrophilic interaction liquid chromatography (HILIC) is an interesting approach to separate both polar and nonpolar analytes in a single chromatographic run [16,17]. However, if RPLC is used in the first dimension, a mobile phase with a high fraction of water at the start of the gradient is usually applied. This highly aqueous effluent from the RP column carries the polar compounds and if transferred to the HILIC column in the second dimension, little or no retention in the HILIC phase is obtained [18]. Different column coupling methods for 2D LC have been developed and reported to counteract the solvent mismatch. A comprehensive overview of the different column switching concepts was presented by Chen et al. [19]. One is the valve switching system with double trapping columns. Here, the analytes are focused on columns in the first dimension. Additional trapping columns between the first and second dimension are applied for enrichment of the analytes and reduction of the impact of the strong eluting solvent in the second dimension [19]. Alternatively, the solvent of the first dimension can be evaporated in a fraction loop, reducing the solvent mismatch on the separation in the second dimension [20]. An alternative to reduce the influence of the strong solvent from the first on the second dimension column is to add a high portion of the weak solvent using an additional pump [21], [22], [23], [24]. This can be accomplished by repeated switching between storage loop and 2D solvent [25] or by using a bypass [26].

However, none of these 2D LC technical concepts combines effectively LVI as a solution to improve the detection limit of the analytes. In a few studies, the RP column is used for trapping the analytes from LVI but enrichment of polar analytes is not possible [19,27,28]. Other column switching studies report the HILIC column as the first dimension [23,25]. However, enrichment of aqueous samples, which is often the case for studies of the aquatic ecosystem, is not possible due to the required organic phase for applying HILIC.

Therefore, in this study, a novel and comprehensive column switching method is presented to enrich and separate both polar and nonpolar analytes in one chromatographic run by combining LVI, RPLC, porous graphitic carbon (PGC) and HILIC. For the enrichment of the nonpolar analytes in aqueous samples, the focusing effect of RPLC is used. Compounds that cannot be trapped by the RP phase will be transported by the aqueous mobile phase to the PGC column. Here, polar analytes can be enriched. Using a PGC column downstream of the RP column also guarantees that there is no irreversible adsorption of nonpolar and thus highly retentive substances on the PGC column that might lead to a very fast clogging. After enrichment, nonpolar compounds are separated in the RPLC phase and a HILIC column is placed after the PGC column to separate the enriched polar compounds.