Mixed-mode ion-exchange polymeric sorbents in environmental analysis

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Mixed-mode ion-exchange polymeric sorbents in environmental analysis Núria Fontanals, Francesc Borrull, Rosa Maria Marcé∗ Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Marcel·lí Domingo, s/n, Campus Sescelades, building N4, 43007 Tarragona, Spain

a r t i c l e

i n f o

Article history: Received 2 July 2019 Revised 3 September 2019 Accepted 5 September 2019 Available online xxx Keywords: Mixed-mode ion-exchange polymeric sorbents Solid-phase extraction Water samples Solid samples Environmental analysis Organic contaminants

a b s t r a c t When determining contaminants from environmental samples, sample treatment plays an important role. Among the different extraction techniques available, solid-phase extraction (SPE) has been commonly used for liquid samples, as well as for solid samples after an extraction technique amenable for solids. The broad applicability of SPE is attributed to the availability of different sorbents. Among these sorbents, the mixed-mode ion-exchange polymeric ones have been widely exploited since they suitably combine both capacity (to concentrate the compounds) and selectivity (to clean-up the interferences from complex matrices). This review overviews the application of mixed-mode ion-exchange polymeric sorbents in environmental field when water and solid samples are analyzed. The sample analyzed, the compounds determined, the type of mixed-mode sorbent selected, the approach used and the SPE protocol applied are comprehensively discussed through different examples. This review intends to summarize the role of mixed-mode ion-exchange polymeric sorbents in the environmental field. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The determination of organic contaminants in environmental samples is always a challenging task not only because of the complexity of the samples, but also because of the low concentrations at which the contaminants are present in them. Therefore different sample treatment techniques may be used to obtain an extract for further analysis using an instrumental technique, usually a chromatographic separation followed by mass spectrometry (MS) detection. When handling complex samples, matrix effect (ME), which could take the form of signal suppression or enhancement, might occur during the ionization in MS-based detectors mainly when using liquid chromatography (LC). Therefore, apart from enriching the analytes, it is also important to simplify the matrix to make it possible to appropriately quantify the contaminants present in environmental samples [1,2]. As far as water samples are concerned, different extraction techniques such as liquid–liquid extraction, microextraction techniques including solid-phase microextraction, liquid-phase microextraction and dispersive liquid–liquid microextraction have been applied in environmental analysis [3–6]. Nevertheless, solidphase extraction (SPE) is the most commonly used sample preparation technique both to enrich the contaminants and to clean up

∗

Corresponding author. E-mail address: [email protected] (R.M. Marcé).

any interferences from the sample matrix [1,5]. This broad applicability is attributed to the availability of different sorbents for SPE that make it feasible to retain compounds with a wide range of properties and features. Moreover, depending on the choice of sorbent, either the selectivity and the capacity or both can be exploited [7–9]. The extraction of organic contaminants from solid samples typically includes an extraction with solvent that can be assisted in different ways. This is the case with ultrasound assisted extraction (UAE), pressurized liquid extraction (PLE) and microwave assisted extraction (MAE). Other extraction techniques for solid samples are the classical Soxhlet, matrix solid-phase dispersion (MSPD) and Quick, Easy, Cheap, Effective, Rugged and Safe (QuEChERS) [5,10,11]. Due to the complexity of these solid samples, in most instances the extract obtained should be further cleaned of interferences. Of the different clean-up steps applied, SPE plays an important role [10,11] and, depending on the selectivity of the sorbent, the cleanup step will be more or less exhaustive. As well as the clean-up, a concentration of the extract can be also achieved. The various different sorbents available in SPE are generally classified as being high capacity or selective. Polymeric-based sorbents that can be functionalized with hydrophilic moieties are an example of high-capacity sorbents and interact nonspecifically with the polar compounds. Molecularly-imprinted polymers belong to the selective sorbent group, since they interact specifically with a compound or class of compounds [12,13] but usually lack

https://doi.org/10.1016/j.chroma.2019.460531 0021-9673/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: N. Fontanals, F. Borrull and R.M. Marcé, Mixed-mode ion-exchange polymeric sorbents in environmental analysis, Journal of Chromatography A, https://doi.org/10.1016/j.chroma.2019.460531

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Fig. 1. Classification of the mixed-mode sorbents commercially available and in-house prepared. Footnote: AMPSA: 2-acrylamido-2-methylpropane sulfonic acid; DEAEMA: 2-diethylaminoethyl methacrylate; DVB: Divinylbenzene; EDCs: Endocrine disruptor compounds; EDMA: Ethylenedimetracrylate; HEMA: hydroxyethylmethacrylate; HXLPP: hypercrosslinked resin prepared with particles from precipitation polymerization; PETRA: pentaerythritol triacrylate; SAX: Strong anionic exchange; SCX: Strong cationic-exchange; TEA: Triethylamine; VBC: Vinylbenzylchloride; WAX: Weak anionic-exchange; WCX: Weak cationic-exchange.

capacity. Mixed-mode ion-exchange polymeric sorbents (henceforth mixed-mode sorbents), which are functionalized with ionic moieties, compromise capacity (reversed-phase interactions with the polymeric backbone) and selectivity (ionic interactions with the ion-exchange group attached) if the compounds are ionizable. Due to this appealing combination, in recent years mixed-mode sorbents have been successfully introduced in various types of applications including the analysis of environmental samples [12,14]. Depending on the ion-exchange group that functionalizes the polymer skeleton, mixed-mode sorbents can be classified as strong cation exchange (SCX), strong anion exchange (SAX), weak cation exchange (WCX) or weak anion exchange (WAX). Fig. 1 shows the typical moiety of each group and lists different examples of each type of sorbent whether commercially available or in-house prepared. The main difference between strong- and weak-ionexchange sorbents arises from the chargeability of the ionic groups. Thus the strong ion-exchange groups always remain charged regardless of the pH, whereas the weak groups may or may not be charged depending on the pH. Therefore, to fully exploit mixedmode sorbent performance, the selection of suitable SPE conditions (pH in the different steps and washing solvent) is key [14]. Fig. 2 shows an example of the recommended protocol for each type of mixed-mode sorbent as well as the amenable compounds in each case. It can be seen that the pH in the different steps and for each particular type of mixed-mode sorbent is crucial since it promotes the chargeability and therefore the ionic interactions.

The role of mixed-mode sorbents in improving the selectivity of the method used to analyze complex samples such as those of environmental origin has been demonstrated. There are several examples in the literature, most of them included in this review, where these sorbents have improved the selectivity (reducing the ME), and other examples where both the selectivity and capacity (improved sensitivity) have improved when mixed-mode polymeric sorbents have been included in the analytical method used to analyze environmental samples. The aim of this review is to present the current state of the art of mixed-mode sorbents in environmental analysis. As regards the samples, our intention is to cover all environmental samples, and based on the application and aim of mixed-mode sorbents, the review has been organized into two sections: water samples and solid samples. 2. Water samples Mixed-mode sorbents have been applied in different types of environmental water samples including surface water, influent and effluent wastewater, and also less complex samples such as tap water, ground water and well water. Table 1 lists different examples of the mixed-mode sorbents applied to extract different organic contaminants from different types of environmental water samples. The aim of mixed-mode sorbents in most instances is both to concentrate the target analytes (which are present at very low concentration levels in such samples) and to clean up the sample

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Table 1 Commercial and in-house prepared mixed-mode sorbents and the SPE protocol applied to environmental water samples.

SPE sorbent

SPE conditions

Compounds

Water sample

Instrumental technique

Ref.

L: 100 mL pH 2 W: 5 mL MeOH pH 2 + drying 10 min E: 5 mL 2%NH4 OH MeOH L: 100 (IWW), 100 (EWW) and 250 (river) mL pH 3 W: 5 mL MeOH E: 15 mL 5%NH4 OH in MeOH L: 500 mL W: E: 3 mL 2.5%NH4 OH in MeOH L: 50 mL (EWW) and 25 mL (IWW) pH 3 W: 5 mL MeOH E: 3 mL 5%NH4 OH in MeOH L: 500 mL (EWW) and 200 mL (IWW) pH 7 W: 6 mL 2%HCOOH in H2 O + 4 mL MeOH + 10 min drying E: 8 mL 5%NH4 OH in MeOH L: 20 mL pH 2 W: E: 2 mL MeOH + 2 mL 2%NH4 OH in MeOH

10 cathinones

IWW

LC-(ESI)QqQ

[31]

Illicit drugs and cathinones

River, EWW and IWW

LC-(HESI)Orbitrap

[29]

Drugs

Surface and EWW

LC-(ESI)QqQ

[37]

Drugs

EWW and IWW

LC-(ESI)QqQ

[47]

Fluoxetine and norfluoxetine

EWW and IWW

LC-(ESI)QqQ

[46]

Illicit drugs

IWW

LC-(ESI)QqQ

[27]

PPCPs, EDCs and artificial sweeteners

Surface, ground and IWW

LC-(ESI)QqQ

[15]

Illicit drugs

EWW and IWW

LC-(ESI)QqQ

[26]

B-blockers and b-agonists

Tap

GC-(EI)Q

[69]

68 pyschoactive drugs, illicit drugs and its related metabolites Antimycotic drugs

EWW and IWW

LC-(ESI)QqQ

[41]

River, EWW and IWW

LC-(ESI)QToF

[34]

SCX sorbents Oasis MCX (150 mg)

Oasis MCX (500 mg)

Evolute CX (200 mg) Sulfonated HEMA/DVB AMPSA/HEMA/PETRA (200 mg) Evolute CX-50 Oasis MCX (200 mg) Oasis MCX Oasis MAX Oasis WCX (150 mg) Chromabond HR-X Chormabond HR-AW (500 mg) Oasis MCX Oasis HLB (150 mg) Strata X-C Oasis MCX Among other sorbents (200 mg) Strata XC Strata X (200 mg) Oasis MCX Oasis HLB (150 mg)

L: 500 (surface, groundwater) and 250 (IWW) mL pH 2 W: 6 mL H2 O + 30 min drying E: 2 × 5 mL MeOH L: 20 mL pH 4.5 W: 10 mL H2 O pH 4.5 E: 2 mL MeOH + 4 mL 5%NH4 OH MeOH L: 250 mL pH 3 W: 3 mL 5%HCOOH in H2 O + 3 mL MeOH E: 3 mL 5%NH4 OH MeOH L: 50 mL (IWW) pH 2.5 W: 3 mL H2 O pH 2.5 + 1 h drying E: 3 × 2 mL 2%NH4 OH in MeOH L: 150 (IWW), 300 (EWW) and 500 (river) mL pH 3 W: 5 mL MeOH/H2 O (10/90, v/v) + 2.5 mL 0.1%HCOOH E: 2 mL 2%NH4 OH in MeOH

SAX sorbents Oasis MAX Bond Elut Plexa SAX (500 mg) Oasis MAX Envi-carb Plexa Envi-carb/Plexa (150 mg) Strata XA Oasis MAX NVI-EDMA (150 mg) HXLPP-SAX (200 mg) DEAEMA-DVB func TEA (200 mg)

VBC-EGDMA func TEA Imidazole Piperidine Pyrrolidone (200 mg)

L: 250 mL W: 10 mL 50 mM sodium acetate + 2 min drying E: 12 mL MeOH + 12 mL 5%HCOOH in MeOH L: 100 mL pH 7 W: 5 mL H2 O + 5 mL ACN E: 5 mL 0.8%HCl in ACN

Herbicides

Stormwater

LC-(ESI)QqQ

[21]

Water soluble organic compounds (dicarboxylic acids)

Aerosol water samples

GC-(EI)Q

[33]

L: 300 mL W: 10 mL MeOH+ 2 mL H2 O + 3 mL THF E: 2 mL 0.2%HAc MeOH + 2 mL 1%HAc in MeOH + 2 mL 2%HCOOH in MeOH L: 500 (river) and 100 (EWW) mL pH 7 W: 10 mL MeOH E: 10 mL 10%HCOOH in MeOH L: 500 mL pH 7 W: 3 mL 10%HAc H2O E: 9 mL MeOH + 3 mL 1%HCOOH in MeOH L: 200 mL pH 7 W: 1 mL H2 O + 15 min drying E: 4.5 mL MeOH +4.5 mL 2%TFA in MeOH

Acidic oil degradation products

Harbor water

GC-(EI)Q

[18]

Drugs

River and EWW

LC-UV

[70]

Drugs

EWW

LC-UV

[45]

Estrogens + NSAIDs

Tap and river

LC-UV

[42]

(continued on next page)

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Table 1 (continued) SPE sorbent

WCX sorbents Oasis WCX (on-line 20 × 2 mm) Strata X-CW (200 mg) Oasis WCX Oasis MCX (500 mg)

SPE conditions

Compounds

Water sample

Instrumental technique

Ref.

L: 10 mL pH 7 W: 0.5 mL MeOH E: mobile phase: 0.5% HCOOH aq/CAN (90/10, v/v) L: 10 mL pH 7 W: 2 mL H2 O E: 5 mL 2% HCOOH in MeOH/ACN (20/80, v/v) L: 500 (river), 250 (EWW) and 100 (IWW) mL pH 7 W: 10 mL MeOH E: 5 mL 5%NH4 OH in MeOH

Illicit drugs

River and EWW

LC-(ESI)Q

[49]

Metformin (drug)

Tap, surface and EWW

LC-(ESI)QTRAP

[71]

Cathinones

River, EWW, IWW

LC-(HESI)Orbitrap

[30]

L: 3 mL W: 2 mL H2 O E: 0.05 mL 0.1%NH4 OH in MeOH L: 50 mL pH 3.9 W: E: 2 × 3 mL 0.2% NH4 OH in MeOH L: 400 mL pH 6–8 W: 5 mL MeOH/H2 O (50/50, v/v) E: 2 mL 2%NH4 OH in MeOH L: 100 mL pH 10 W: E: 6 mL MeOH + 6 mL 5%HCOOH in MeOH

14 PFAAs

Drinking water

LC-(ESI)QqQ

[23]

PFCAs

Surface, groundwater and drinking

LC-(ESI)QqQ

[25]

Sartan drugs

Tap, river, EWW and IWW

LC-(ESI)QqQ

[22]

Cytostatic drugs

Surface, EWW and IWW

UHPLC-(ESI)QqQ

[24]

NSAIDs

River and EWW

LC-UV

[44]

Drugs

EWW and IWW

UHPLC-(ESI)QToF

[38]

Micropollutants and its TP from ozonization

Tap, surface and WW

LC-(ESI)QqQ

[54]

Bisphenols

River, EWW and IWW

LC-(ESI)QqQ

[55]

Drugs

River, EWW and IWW

LC-(HESI)Orbitrap

[48]

26 polar (log D<0) contaminants

Tap, surface and EWW

HILIC-(ESI)QqQ

[51]

WAX sorbents Oasis WAX (On-line 20 × 2.1 mm) Oasis WAX Strata X-AW (150 mg) Oasis WAX Oasis MAX (150 mg) Oasis WAX Oasis HLB, Strata X, Isolute ENV+, Oasis MCX, Oasis WCX, Oasis MAX, Strata-XL-AW (500 mg) HXLPP-WAX (200 mg)

L: 500 (river) and 250 (EWW) mL pH 7 W: 4 mL MeOH E: 2 mL 2%NH4OH in MeOH/ACN (25/75, v/v) Multi-layered and tandem approaches Tandem Oasis MCX – L: 50 mL pH 2 W: 2 mL 2% HCOOH in H2 O (MCX) MAX 2 mL 0.5%NH4 OH in H2 O (MAX) (60 mg) E MCX: 2 mL MeOH (neutrals & acids) + 2 mL 2%NH4 OH in MeOH (bases) E MAX: 2 mL MeOH (neutrals) + 2 mL 2%HCOOH in MeOH (acids)

Tandem Oasis MAX-MCX (100 mg)

Tandem Oasis HLB-MAX (150 mg)

Combining SCX/SAX SCX/WAX (500 mg in total)

Multi-layered GCB/Oasis WCX/Oasis WAX (30 mg each)

L: 1000 mL W: 2 mL 5% NH4 OH aq. E: 6 mL 2% HCOOH in MeOH/EtAc (70/30, v/v) L: 1000 mL W: 2 mL 2% HCOOH aq. E: 6 mL 5% NH4 OH in MeOH/EtAc (70/30, v/v) Oasis HLB L: 300 mL (river and EWW) and 100 mL (IWW) pH 3–7 W: 6 mL H2 O/MeOH (50/50, v/v) E: 6 mL 2%NH4 OH in MeOH Oasis MAX L: 1.5 mL H2 O + 6 mL 2%NH4 OH in MeOH W: E: 6 mL 2% HCOOH MeOH L: 100 mL (river, EWW) and 50 (IWW) pH 7 W: 15 mL MeOH E (SCX/SAX): 5 mL 10% HCOOH in MeOH + 5 mL 5% NH4 OH in MeOH E (SCX/WAX): 5 mL 5% NH4 OH in MeOH L: 100 mL pH 5.5 W: - + drying 15 min E: 3 mL MeOH + 3 mL 5%NH4 OH in MeOH + 3 mL 2% HCOOH in MeOH+ 1.5 mL MeOH/DCE 80/20, v/v

In bold, the sorbent selected when different sorbents are compared. ACN: Acetonitrile; AMPSA: 2-acrylamido-2-methylpropane sulfonic acid; DEAEMA: 2-diethylaminoethyl methacrylate; DVB: Divinylbenzene; EDCs: Endocrine disruptor compounds; EGDMA: Ethyleneglycoldimetracrylate; EI: Electron impact; ESI: Electrospray ionization; EtAc: Ethylacetate; EWW: Effluent wastewater; GC: Gas chromatogrpahy; H2 O: Water; HAc: Acetic acid; HCOOH: Formic acid; HEMA: hydroxyethylmethacrylate; HESI: Heated electrospray ionization; IWW: Influent wastewater; LC: Liquid chromatography; MeOH: Methanol; NH4 OH: Ammonium hydroxide; NSAIDs: Non-steroidal anti-inflamatori drugs; PETRA: pentaerythritol triacrylate; PFAAs: Perfluoroalkylacids; PFCAs: Perfluorinated carboxylic acids; PMOCs: Persistent and mobile organic compounds; PPCPs: Pharmaceuticals and personal care products; Q: Quadrupole; QqQ: Triple quadrupole analyzer; QTRAP: Quadrupole linera ion trap analyzer; SAX: Strong anionic exchange; SCX: Strong cationic-exchange; TEA: Triethylamine; TFA: Trifluoroacetic acid; TP: Transformatin product; UHPLC: Ultrahigh performance LC; UV: Ultraviolate detector; VBC: Vinylbenzylchloride; WAX: Weak anionic-exchange; WCX: Weak cationicexchange. L: loading; W: washing; E: elution.

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Fig. 2. Recommended SPE protocols and analytes for each type of mixed-mode sorbents. In brackets, an example of the most usual conditions.

from interferences of the matrix. This clean-up step is more critical when the samples for analysis are more complex. The selection of the appropriate mixed-mode sorbent and the protocol to be applied is crucial in order to enhance both the capacity and selectivity of the sorbent. In the following sections we explore the possibilities of applying an appropriate protocol and selecting the best mixed-mode sorbent and approach in order to achieve the sensitive and selective determination of organic contaminants from environmental water samples. This is done by describing several different examples in which mixed-mode sorbents were applied to environmental samples. 2.1. Optimization of the protocol The selection of a suitable SPE protocol and, in particular, the pH in each step is one of the most critical aspects in mixedmode sorbents. As mentioned in the introduction, Fig. 2 presents the guidelines for the recommended protocol and the compounds amenable for each type of mixed-mode sorbent. This optimization is closely related to the correct displaying of the ionic interactions between the sorbent and the analytes in each SPE step. However, in some studies this optimization is not fully developed, and therefore the results derived from the mixed-mode evaluation are not conclusive [15–17]. In addition, Table 1 gives details of the protocol used in each study. Unless otherwise indicated, the examples discussed in the water samples section are detailed in Table 1. Readers should therefore consult this table for the details of each study. 2.1.1. Loading conditions The sample’s pH should be carefully controlled in order to fully exploit the ionic interactions between the analytes and the sorbent. Acidic pH (2–3) is necessary when mixed-mode SCX sorbents are used, while with other mixed-mode sorbents (SAX, WAX, WCX) neutral pH (7) or weak acidic pH (6) are commonly used to pro-

mote ionic interactions. Moreover, the pKa of the target compounds should also be taken into account. In most studies (as specified in Table 1) the pH is adjusted as necessary according to the mixedmode sorbent used, but in some examples [18–23] where the sample should be loaded at neutral pH, no mention is made of the sample’s pH being adjusted. However, this would not affect the achievement of ionic interactions since the pH in environmental water samples is about 7. Nevertheless, in one study Oasis WAX performed better when the sample was adjusted at pH 10 rather than when it was not adjusted or adjusted at pH 2, since two (cyclophosphamide and 5-fluoroucil) of the target compounds (a group of cytostatic drugs) were charged at this pH (10) [24]. In another study [25], Strata X-AW and Oasis WAX were compared for the extraction of a group of perfluorinated carboxylic acids (PFCAs). Different sample pHs (from 3 to 6) were tested. The best results were achieved when the sample was adjusted at pH 3.9, which was attributed to the minimization of competition effects with the biocarbonate anions present in the sample (spring water). Although the recommended sample pH for these sorbents is 7, in this particular case, since the pKa of the compounds is lower than 4, they are also deprotonated at the working pH and able to ionically interact with mixed-mode WAX sorbents. The volume of sample percolated is another variable that should be optimized and depends on the complexity of the sample and the amount of sorbent. Most of the environmental applications used an off-line SPE format in which large amounts of sorbent are preferred so as to percolate the large sample volumes used in environmental applications. As detailed in Table 1, most of the studies used 150–500 mg cartridges, with just a few examples using lower bed-volume cartridges (30–60 mg) or on-line approaches. In one study, the comparison of the sorbents and the optimization of the variables was carried out using a 60 mg format, whereas the final protocol was adapted to a 150 mg cartridge in order to exploit the capacity of the method [18].

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Fig. 3. % ME when 10 mL of river water sample (A) and effluent wastewater (B), either with a washing or without a washing step involving 0.5 mL of MeOH, were on-lineSPE-LC-MS analyzed using Oasis WCX as sorbent. ME = 100% denotes no matrix effect. Reproduced from [49] with permission of Elsevier.

Regarding sample volume, in the studies that use a 150–500 mg cartridge, the usual volume of surface waters is between 100 and 500 mL, whereas for more complex samples such as effluent and influent wastewater samples, the volume usually drops to 100– 250 mL and 50–100 mL respectively. There are other studies where, despite using a similar bed volume, the sample volume is lower [26,27]. As usual, when the on-line format is used, the volumes percolated are lower (i.e. from ∼5 mL to 250 mL). Another aspect that should be taken into account during the optimization of the loading conditions is the ionic strength of the sample since ionic species present in an ionically rich sample may show competition with the ionic sites of the mixed-mode sorbent. In one study [28], Oasis WAX failed to retain a group of perfluorinated compounds (acidic) from sea water. The experiments revealed that the weak retention of the sorbent was attributed to the competition of the ionic species with the anion-exchange sites. 2.1.2. Washing One of the main benefits of this type of sorbent is the inclusion of an effective washing step based on organic solvent [18,22,29– 35] if the selectivity is to be exploited. However, some studies did not include it [20,25,27,36–39] or it was based only on aqueous solution [15,19,23,40–42], which is not as effective as organic solvent. In one interesting example [39], Oasis MCX was selected to extract two herbicides from water and soil samples. Only in the case of the soil samples did the protocol include washing with 10 mL of methanol (MeOH), whereas no washing was included in the case of the water samples. Other studies [31,43–45] add an acidic or basic additive to the washing solution, which can be aqueous [44– 46] or organic solvent [31,43] based, in order to maintain the desired ionization state of the analyte and the sorbent, even though they should have remained charged because of the sample’s pH. Some studies compare the performance of mixed-mode sorbents when different washing solvents are tested [22,29,31,46,47]. For instance, Castro et al. [22] tested different washing solvents (water, MeOH, acidic water and 1% HCOOH in MeOH) in the extraction of a group of drugs using Oasis WAX. As expected when using acidified solution, the less acidic compounds were partially lost since, in the presence of the acidic additive, they were protonated and the ionic interactions with the sorbent were already broken. Finally, 5 mL of water/MeOH (1/1, v/v) was chosen as the washing solution. In another study [29], during the optimization of the extraction of a group of basic illicit drugs using Oasis MCX, different washing solvents including 5 mL of acidified water, 5 mL of water/MeOH (1/1, v/v) and increasing volumes (from 2 to 10 mL) of MeOH were tested. It was found that with 5 mL of MeOH the recoveries were maintained (75–80%), while the values of ME decreased when using pure MeOH in comparison to the aqueous-

based solvents. A similar conclusion was reached in the extraction of fluoxetine and norfluoxetine from wastewater using Evolute CX cartridges [46]. In this study, when a washing step consisting of 4 mL of MeOH was added to the SPE protocol, it was found that cleaner extracts were obtained, and these in turn helped to increase the sensitivity of the method [46]. As expected, the inclusion of an effective washing step promotes selectivity and reduces ME when complex samples are analyzed using LC-MS. Different studies [26,30,47–50] associated the effect of including the washing step with an improvement in terms of ME. A reduction in ME was reported when Oasis WCX was used in the on-line SPE to extract a group of illicit drugs from wastewater samples with a washing step consisting of 0.5 mL of MeOH [49]. Fig. 3 compares the percentage of ME in river and effluent wastewater samples when the washing step was or was not included. Similarly, better results in terms of ME were reported when a cleanup based on 5 mL of acidified MeOH was included in the protocol where Oasis MCX was used to extract a group of illicit drugs from influent wastewater samples [31]. Apart from MeOH, other organic solvents have also been tested as washing solvents. These include acetonitrile (ACN) [33,47] and less polar solvents such as tetrahydrofuran (THF) [18]. For instance, in the extraction of acidic oil degradation products using Strata XA, apart from MeOH, it was claimed that the addition of THF reduced undesired non-polar neutrals-sorbent interactions. Thus the final washing step consisted of 10 mL of MeOH, followed by 2 mL of water and 3 mL of THF [18]. In general, the behavior of ACN and MeOH should be similar, as was demonstrated when both solvents were compared in the washing step during the extraction of drugs using an in-house prepared SCX [47]. Other studies [19,24,26,27,45] added a methanolic fraction before elution, although this did not act as a washing solvent since it eluted some of the compounds. In the end, the analytes eluted in the elution fraction may appear less affected by the ME because of the previous methanolic fraction that also elutes some of the interferences present. This was the case when Oasis MCX was selected for the determination of a group of illicit drugs from effluent wastewater samples with a protocol that included an elution based on 2 mL of MeOH followed by 4 mL of 2% NH4 OH in MeOH [26]. The authors found that the ME was considerably reduced when this 2 mL of MeOH was applied. However, this was not considered a washing fraction, since two of the target compounds (cannabinoids) were already eluted in this fraction as they were neutral and only retained to Oasis MCX trough reversed-phase interactions [26]. In a similar approach, Oasis MAX and Bond Elut Plexa were tested in the extraction of a group of herbicides from stormwater [21]. Both sorbents enabled the complete extraction of all the compounds, but the clean-up step was not actually included in the

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protocol. Nevertheless, the authors found that the ME can be tackled depending on the type and amount of acid added during the elution. In conclusion, if trifluoroacetic was the acidic additive, it could induce the elution of matrix interferences and increase the ME. Thus 5% HCOOH in MeOH was selected as the elution solvent [21]. Some studies adopted the drying step before elution, with timing ranging between 2 [21], 10 [27,31,46], 15 [20,51] and 30 [40] minutes. Indeed, since the washing solvent and the elution solvent are compatible, the drying step might be senseless. In spite of this, the effect of drying the cartridge after the washing step was explored in one study [31] which, apart from testing different washing solvents, also evaluated the influence of drying or not drying the cartridge for 10 min before the elution step. The optimal SPE procedure (details in Table 1) with Oasis MCX involved a washing step with 5 mL of acidified MeOH and drying for 10 min with relative recoveries ranging from 83% to 93% [31]. 2.1.3. Solvent elution The elution solvent plays a crucial role in disrupting the ionic interactions as well as eluting the unretained compounds thanks to its elutropic strength. As shown in Fig. 2, the usual elution solvent is a combination of acidic (for SAX and WCX) or basic (SCX and WAX) additives in pure organic solvent. In strong exchangers, elution is achieved by neutralizing the analyte; whereas, in weak exchangers the sorbent is usually neutralized. During the comparison of the performance of three SAX sorbents (Oasis MAX, Strata XA and an in-house sorbent based on N-vinylimidazoleethyleneglycoldimethaclylate (NVI-EGDMA) that also displays SAX interactions) in a 60 mg format for the extraction of a group of acidic oil degradation products, it was found that in the case of NVI-EGDMA 1 mL of 10% HCOOH in MeOH was needed to elute the compounds, whereas for Oasis MAX and Strata XA 1 mL of 1% HCOOH was enough. The authors decided to increase the volume of MeOH (2 mL) and decrease the percentage of HCOOH (5%) for 60 mg of sorbent, finally using 4 mL of 5% HCOOH for 150 mg format [18]. Mokh et al. [52] studied the percentage of ammonium and elution solvent when they evaluated the performance of Oasis MCX in extracting a group of aminoglycosides from water samples. Of the different percentages (2%, 5%, 7% and 10%) of basic additive, they found that 7% NH4 OH provided the best results when extracting these compounds. Nevertheless, different percentages of basic and acidic additive have been applied in the different protocols reported (examples are given in Table 1). The amount of solvent to elute is also linked to sorbent capacity, insofar as the larger the bed volume, the larger the solvent volume. A 500 mg Oasis MCX cartridge was used to extract a group of illicit drugs from river water (250 mL) and effluent and influent (100 mL) wastewater samples, which needed 15 mL of 5% NH4 OH in MeOH [29]. The volume was then reduced to 5 mL of 2% NH4 OH in MeOH to extract a similar group of compounds from the same volume of influent wastewater using the same sorbent, but in a 150 mg cartridge format [31]. The reduction in elution volume might help in reducing the total analysis time if this volume has to be further concentrated by evaporation. 2.2. Comparison of sorbents As for selecting the most suitable sorbent, different studies have compared the performance of sorbents with different features. Some have compared only sorbents with mixed-mode features, whereas others have also included other types. Table 1 shows some of these studies, in which bold denotes the sorbent selected, and only the optimum protocol for the selected sorbent is detailed. For instance, for the extraction of a group of water-soluble organic

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compounds (that mainly included dicarboxylic acids), different sorbents with different properties such as carbon-based (Envi-Carb), polymeric-based with reversed-phase features (Plexa), a combination of both (Envi-carb and Plexa) and mixed-mode (Oasis MAX) were compared [33]. Oasis MAX was ultimately selected, since it provided the best retention due to the anionic interactions with the acidic compounds, especially those with the smallest carbon chains (composed of C2–C5) [33]. In another study [24], different sorbents (Oasis HLB, Strata X, Isolute ENV+, Oasis MCX, Oasis WCX, Oasis WAX, Oasis MAX, Strata-XL-AW) and different protocols were evaluated for the extraction of a group of cytostatic drugs (most of them with carboxylic acid moieties) from surface and wastewater. Oasis WAX provided similar or enhanced relative recoveries (85–130%) for all the target drugs compared to previous studies, with the exception of 5-fluorouracil (29%) (basic compound), which achieved better recoveries with Isolute ENV+ (51%). Nevertheless, Oasis WAX was selected [24]. For the extraction of a group of sweeteners (pKa from −8.66 to 12.52), different SPE sorbents including reversed-phase (i.e. Oasis HLB, Isolute Env+, Bond Elut Plexa and Strata X) and mixed-mode (i.e. Oasis MAX, Oasis WAX and Bond Elut Plexa PAX) were evaluated. However, according to the authors, none of the mixed-mode sorbents were selected due to either poor (for the non-acidic analytes) or excessive (for the acidic analytes) retention [53]. The performance of strong and weak ion-exchange sorbents was also compared in a number of studies. Oasis MCX and Oasis WCX were compared in the extraction of a group of cathinones from river and wastewater samples [30]. In this case, Oasis WCX outperformed Oasis MCX in terms of SPE recoveries and ME. With both sorbents the SPE method developed included an effective washing step that consisted of 10 mL of MeOH. Oasis WCX was ultimately selected for the method validation and to be applied in the analysis of different samples [30]. Meanwhile, in another study for the extraction of a similar group of cathinones from environmental samples, Oasis MCX was used [31]. Oasis MAX and Oasis WAX along with their optimum protocols were also compared in the extraction of a group of sartan drugs [22]. Oasis WAX was selected since all the target compounds were eluted in the first 2 mL fraction of 2% NH4 OH in MeOH, whereas some compounds displayed a slow elution profile in Oasis MAX, needing more than 10 mL of 2% HCOOH in MeOH to elute. In recent years various studies have emerged [29– 31,35,40,47] in which mixed-mode SCX sorbent is used in the sample treatment for the determination of illicit drugs in complex environmental waters. This is because most of these drugs present basic features, so they are charged at acidic pHs and can interact ionically with the acidic groups of the mixed-mode cationexchange sorbent. Under these conditions, an effective washing step based on organic solvent can be applied, disrupting only the reversed-phase interactions and not the ionic ones. In some of these examples [27,32,41], mixed-mode sorbent was selected after comparing the performance with other sorbents. Oasis HLB, Oasis MAX, Oasis MCX and Oasis WCX were compared for the extraction of cocaine and its main metabolites [27]. Initially, the 60 mg cartridge format was tested and all sorbents failed to retain the most water-soluble metabolite (ECG, ecgonine). However, when the bed volume was increased to 150 mg, Oasis MCX was the only material able to retain this compound (63%). In addition, this sorbent with the optimized protocol was selected in the method developed to monitor these illicit drugs in wastewater samples [27]. In another study [41], 68 psychoactive pharmaceuticals, illicit drugs and related metabolites including not only the basic compounds but also other families (i.e. antiepileptics, cannabinoids and barbiturates) that are neutral at working pH (pH 2.5) were determined. In order to deal with these different physicochemical properties, two cartridges were evaluated (Strata-X and Strata-XC) using

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an optimized protocol (Table 1 details that for Strata-XC). The recoveries obtained using both approaches were similar for most of the compounds. However, the recoveries for EME (ecgonine methyl ester, a very polar cocaine metabolite) and compounds belonging to the cannabinoid family (9-THC and THCA) were better using Strata-XC. This enhanced retention on Strata-XC, despite not displaying ionic interactions with the compounds, is attributed to the influence of the sulfonic group on both the polar retention characteristics and delocalizing the π -π interactions with the divinylbenzene nucleus of Strata-XC compared to the neutral polymer with the same backbone structure, Strata-X [41]. Nonetheless, in the SPE protocol using Strata-XC, the washing step was only aqueous-based so that the reversed-phase interactions with neutral interfering compounds would not be broken. 2.3. Multi-layered and tandem approaches The above section highlighted selectivity (converted into a reduction of ME) when a washing step with pure organic solvent is included to remove interferences. However, this methodology is limited to groups of ionizable compounds with the same charge, and several environmental applications required the simultaneous extraction of basic and acidic compounds with the inclusion of a washing step. One alternative for simultaneous extraction is sequential elution with MeOH (which can be the washing fraction) in addition to the elution fraction consisting of acidic or basic additive in MeOH. These two fractions can later be mixed or treated separately. As detailed in the above section, some authors have adopted this strategy in order to achieve simultaneous extraction of analytes [19,24,26,27], but by doing this, the analytes eluted in the washing fraction are still affected by the ME and the selectivity exploited in mixed-mode technology is canceled because no washing step is included. Another alternative is to place complementary mixed-mode cartridges in tandem (also known as in series). For instance, Oasis MCX was placed in tandem with Oasis MAX for the sequential extraction of a group of acidic, basic and neutral drugs [38]. Specifically, after loading in Oasis MCX 50 mL of sample at pH 2 and washing with 2 mL of acidic water, 2 mL of MeOH was used to elute the neutral and acidic compounds, which were then submitted to Oasis MAX, whereas the basic compounds were readily eluted from Oasis MCX with 2% NH4 OH in MeOH. The neutral compounds were then eluted from Oasis MAX with 2 mL of MeOH, whereas the acidic compounds were eluted with 2 mL of 2% HCOOH in MeOH. Despite not including a real washing fraction, the authors found the elution extracts more simplified when using MCX/MAX in tandem rather than MCX alone, which also resulted in a reduction of ME [38]. In another study [54], different single approaches (including SCX, SAX, WCX and WAX for both Oasis and Strata brands) and tandem approaches (combinations of HLB, X, SAX and SCX) were investigated for the simultaneous extraction of a group of micropollutants [54]. Better recoveries were obtained with all the tandem approaches compared to with each single cartridge, and the best results were obtained when using the SCX and SAX combination with the protocol detailed in Table 1. One advantage of using cartridges in tandem rather than in parallel is that no additional sample treatment is needed only to deal with different elution fractions. However, the proposed protocols are long, involving the collection of compounds in more than one fraction. Another tandem approach is to use one of the cartridges to enrich the compounds and the other to clean up the matrix. This strategy was adopted in the simultaneous determination of seven bisphenols in river and wastewater samples [55]. Fig. 4 shows the schematic protocol used for this tandem approach that was also used during the analysis of solid samples. Initially the authors

found that, as already reported in previous studies, Oasis HLB suitably enriched the target analytes. However, when they dealt with the wastewater samples, some of the compounds showed considerable ion suppression (>90%), even though the Oasis HLB protocol included a clean-up based on 6 mL of MeOH/water (50/50, v/v). In view of this, the authors included a clean-up step based on Oasis MAX. The result of this was that the ME for all the compounds decreased considerably. Thus, in this example, Oasis MAX acted merely as a clean-up cartridge [55]. Multi-layered approaches have also emerged with the aim of extracting compounds with different acidic and basic features in the same cartridge, but using only a single protocol. Specifically, our research group designed four cartridges resulting from the combination of SCX/SAX, SCX/WAX, SAX/WCX and WAX/WCX so that each cartridge (100 mg) contained positive and negative charges together [48]. The four combinations tested strongly retained the acidic and basic target pharmaceuticals without significant differences (recoveries between 76 and 107%) as long as the SPE protocol of each cartridge configuration was carefully optimized after a washing step based on MeOH. Finally, the SCX/SAX and SCX/WAX configurations were selected using the protocol detailed in Table 1 for the validation and application to environmental samples. The recovery (60–104%) and ME (10–48% in the form of ion suppression) results indicated the suitability of this approach to simultaneously determine this group of pharmaceuticals. Another multi-layered approach [51] consisted of a combination of graphitized carbon black (GCB), WCX and WAX (30 mg each) for the simultaneous extraction of a group of very polar (logD < 0) contaminants of environmental relevance with acidic, basic, neutral and zwitterionic properties. In general, all the acidic and basic compounds were amenable to the multilayered SPE protocol, achieving recoveries higher than 76%. However, the recoveries achieved for the neutral compounds were lower. This was attributed to the weaker interactions of these compounds with the sorbents in the multilayered cartridge. In addition, as no clean-up step was included in the SPE protocol, some of the compounds were greatly affected by the ME (more than 80% ion suppression for 6 compounds). In spite of this, the authors claimed that the method presented outperformed the generic and well-established sample treatment method based on Oasis HLB [51]. A similar multi-layered approach (60 mg GCB, 60 mg Oasis WCX and 60 mg Oasis WAX) was applied for the extraction of a group of persistent and mobile organic compounds (PMOCs) that were determined in a monitoring study comprising 24 water sites [56]. A total of 8 different enrichment methods were tested in combination with the instrumental methods. Of the sample treatments, this multi-layered approach proved to be the most effective since it was able to enrich most of the PMOCs, whereas the enrichment method based on Oasis MCX was only successful with a few of the PMOCs, which was attributed to the retention of the cationic compounds in that sorbent being too strong [56]. In short, if the group of compounds includes compounds with acidic and basic properties, either a tandem or a multi-layered approach might be a good alternative for extracting them simultaneously after a suitable washing step that removes interferences and diminishes the ME.

3. Solid environmental samples When solid environmental samples are being dealt with, mainly sludge, sediments, soils and particulate matter from both air and water are included. As mentioned in the introduction, the organic contaminants in solid samples can be extracted using different techniques and SPE is applied in order to clean up and/or concentrate the extract obtained.

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Fig. 4. Example of a protocol using a tandem approach in the SPE for the analysis of water and solid samples. Reproduced from [55] with permission of Elsevier.

Of the different sorbents available, mixed-mode sorbents play an important role in increasing the selectivity of the method, taking into account the complexity of the solid samples. Their use can therefore significantly reduce the ME if a washing step is included as appropriate. Examples of different applications of mixed-mode sorbents will be discussed in this section, although the specific protocol will not be detailed since it is the same as that described for liquid samples. Table 2 summarizes the application of mixed-mode sorbents as a clean-up step for the extract obtained from environmental solid samples using different protocols. The extraction technique and the solvent used are shown in Table 2 because these are important parameters in the subsequent SPE clean-up step. As well as specifying the sorbent used, the loading solution is also included in the table. However, in some cases the authors selected a mixedmode sorbent but the conditions applied were not those suitable for promoting ionic interactions. Some methods for the clean-up of solid samples use the same SPE method as for water samples, but they have to adjust the composition of the extracts to a high percentage of water adjusted to the suitable pH [39,55]. For instance, Yang et al. [55] evaporated the extract (MeOH/acetone, 50:50) obtained by UAE and diluted it to 10 mL of water in order to then apply the method previously described for the determination of bisphenols from water samples (Table 1). Fig. 4 shows the schematic diagram used for water and

river sediment samples. As can be seen in Table 2, this is a typical practice that is extensively used by several authors [57–59]. In other cases, an additional clean-up is added to the water sample protocol when analyzing solid samples. This is the case with the method described by Nanita et al. [39], who determined the herbicide aminocyclopyrachlor and its methyl analogue in water and soil samples, as discussed in Section 2. The authors extracted the analytes by UAE using ACN and water mixtures and, after evaporating an aliquot and diluting the residue with 6 mL of water with 0.2% HCOOH, they loaded it into the Oasis MCX cartridge. They applied the same protocol as for water samples (already discussed), but adding a washing step with 10 mL of MeOH due to the higher complexity of the soil samples. In this study the authors compared different sorbents for reducing the ME and, due to high polarity, aminocyclopyrachlor was not retained in any of the sorbents tested except for the Oasis MCX sorbent. The authors explained that this behavior was due to the sorbent retaining the analytes by mixedmode, i.e. cation-exchange and reversed-phase. However, judging by the pKa of the compound, it should be neutral at the working pH. In other examples, the method applied for water samples using other types of sorbents is not suitable due to the complexity of the extract, and therefore mixed-mode sorbents are demonstrated to be successful in significantly reducing ME. This is the case with the study by Petrie et al. [60], which used Oasis HLB for extracting a

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Table 2 Mixed-mode sorbents applied in SPE during the clean-up step in the analysis of environmental solid samples.

Clean up

Extraction technique

Compounds

Sample

Instrumental technique

Ref.

Oasis MCX Extract evaporated and dissolved in water pH 2 Oasis MCX Extract evaporated and dissolved in acidic water Oasis MCX Extract evaporated and dissolved in water pH 2 Oasis MCX Extract evaporated and dissolved in water Oasis MCX Oasis MAX, Oasis HLB Extract diluted with wàter pH 2 to <5% MeOH Oasis MCX Extract evaporated and dissolved in water pH 2 OASIS MCX C18 , ENV, Phenyl, ENV+, Oasis HLB Extract evaporated and dissolved in 0.2% formic acid Bond Elut SCX (on-line to MSPD syringe) Methanolic extract from MSPD Bond Elut SCX (on-line to MSPD syringe) Bond Elut CBA, Oasis MCX Methanolic extract from MSPD Oasis MAX Strata X Aqueous solution from PILS Oasis WAX Extract evaporated and dissolved in water ENVI-Carb and Oasis WAX Extract through ENVI-Carb, eluted with MeOH, concentrated, dissolved with H2 O and Oasis WAX Bond Elut SCX (neutral and acid species) Bond Elut SAX (neutral and basic compounds) Oasis MAX and Oasis MCX Evaporated extract diluted in water pH 12. Elution from OASIS MAX evaporated and diluted in water pH 2 and loaded into MCX Oasis HLB and Oasis MAX Evaporated extract diluted in water loaded in Oasis HLB. Eluted with MeOH 2%ammonia and loaded into Oasis MAX. MCX-WAX-MAX Aqueous extract

UAE MeOH and acetone

Acidic pharmaceuticals

Sludge

LC-(ESI)QqQ

[57]

UAE MeOH/acetone (1:1)

Parabens

Sludge

LC-(ESI)QqQ

[58]

SLE by orbital shaking MeOH/water (5:3)

Parabens

Sludge Sediment

LC-(ESI)QqQ

[61]

UAE MeOH and acetone

Acidic pharmaceuticals

Sludge

GC-(EI)Q

[72]

MAE MeOH/water pH 2 (50:50)

UV-filters, PBs, estrogenic compounds, BPA, antibacterials/antibiotics, NSAIDs and other pharmaceuticals

Sludge

UHPLC(ESI)QqQ

[60]

SLE by orbital shaking MeOH/water (5:3)

8-bisphenol

Indoor dust

LC-(ESI)QqQ

[59]

UAE ACN/0.2%HCOOH aq (80:20) and ACN/ammonium acetate 0.15 M (70:30) MSPD (Fluorisil and PSA) MeOH

Aminoydopyrachlor

Soil

LC-(ESI)QqQ

[39]

Amidarone and metabolite

Sludge

LC-(ESI)QToF

[64]

Antimycotic drugs

Sludge

LC-(ESI)QToF

[62]

Atmospheric aerosol particles Sludge

LC-(ESI)QqQ

[73]

LC-(ESI)QqQ

[65]

UHPLC(ESI)QTrap

[66]

LC-(ESI)QToF

[63]

MSPD (PSA) MeOH

Particle-into-liquid sampler (sampling) Biogenic acids

UAE 1% HAc in MeOH, MeOH/ACN (50:50)

PCFAs, PFSAs

UAE MeOH/H2 O (8:2) with 100 mM NaOH and MeOH

PFOA, PFOS, BPA

MSPD (fluorisil and PSA) MeOH

Chlorinated azoles (antimycotics)

PLE MeOH/ H2 O (65:35)

Cytostatic drugs

Sludge

UHPLC(ESI)QTrap

[67]

UAE MeOH/acetone (50:50)

Bisphenols

Sludge Sediment

LC-(ESI)QqQ

[55]

SLE and UAE Different solvents

Central carbon metabolites

Lake sediments

LC-(ESI)QqQ

[68]

Sludge Water particulate matter Sludge Biosolid

In bold, the sorbent selected when different sorbents are compared. ACN: Acetonitrile; EDTA: Ethylenediaminetetraacetic acid; EI: Electron impact; ESI: Electrospray ionization; GC: Gas chromatogrpahy; H2 O: Water; HAc: Acetic acid; HCOOH: Formic acid; IT:Ion trap; LC: Liquid Chromatography; MeOH: Methanol; MSPD: Matrix solid phase dispersion; NaOH: Sodium hydroxide; NH4 Cl: Ammonium chloride; PFCAs: Perfluorinated carboxylic acids; PFOAs: Perfluorosulfonates; PFOSs: Perfluorooctanesulfonic acids; PLE: Pressurized liquid extraction; PSA: Primary secondary amine; Q: Quadrupole; QqQ: Triple quadrupole analyzer; QTRAP: Quadrupole linear ion trap analyzer; SLE: Solid–liquid extraction; ToF: Time of Flight; UAE:Ultrasound assisted extraction; UHPLC: Ultrahigh performance LC.

group of 90 emerging contaminants from water samples, although when analyzing sludge the authors proposed the use of Oasis MCX and the separated elution of basic and acidic analytes. Prior to the SPE, the extraction solvent used in MAE – 50:50 H2 O:MeOH (pH 2) – was diluted to <5% of MeOH with water at pH 2. Finally, the method was suitable for determining 63 emerging contaminants from sludge samples. In other examples [59,61], the mixed-mode sorbent is also used to purify the extract. In this cases, the extract from a

solid–liquid extraction (SLE) by orbital shaking (MeOH/water, 5:3) was evaporated and diluted with water at pH 2.5. This solution was loaded in Oasis MCX and the sorbent washed with water/MeOH (75:25) and eluted with MeOH. The same procedure was used to extract parabens from sediment and sewage sludge [61] and bisphenol analogues from indoor dust [59]. In these cases the analytes were retained by reversed-phase interactions because at the working pH the analytes are neutral. However, the basic interferences might be retained by ionic

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interactions and not eluted during either the washing or the elution steps. Another interesting application is that described by Casado et al. [62], who used a Bond Elut SCX (silica-based) cartridge on-line connected to an MSPD syringe for the determination of antimitotic drugs from sludge samples. After passing 10 mL of MeOH through the MSPD syringe, the antimycotic drugs present in the MeOH were retained in the SCX cartridge through cationexchange interactions, while neutral interferences were removed by the MeOH itself. The authors evaluated several strategies for decreasing the ME by using different cosorbents in MSPD and different cartridges for SPE clean-up (Oasis MCX, Isolute CBA (silicabased WCX), and Bond Elut SCX). MSPD followed by a clean-up with SPE using SCX sorbent were those that provided better recoveries and lower ME. A similar procedure was used for the determination of chlorinated azoles [63] and the cardiac drug amiodarone and its N-desethyl metabolite [64] in sludge samples. Although mixed-mode sorbents with strong ionic moieties are the most frequently used, weak sorbents are also used in some studies [65,66] in which a number of perfluoroalkyl compounds are determined. For instance, Man et al. [66] extracted perfluorosulfonic acids (PFOAs) and perfluorosulfonates (PFOSs) from sludge and water particulate matter by UAE and, after a clean-up step with ENVI-CarbTM, the MeOH extracts of this clean-up process were concentrated to 1 mL, diluted in 125 mL of water and further subjected to Oasis WAX clean-up. In the case of Sindiku et al. [65] the clean-up process includes only the Oasis WAX step. In a similar way as with water samples, tandem approaches that include the combination of two mixed-mode sorbents are also used in some studies [67,68]. Seira et al. [67] optimized a method for initially determining three anticancer drugs (ifosfamide, cyclophosphamide and tamoxifen). This method includes a PLE using MeOH:water (65:35) and, for the clean-up, they evaluated sorbents with different characteristics: reversed-phase, hydrophilic– lipophilic balance (HLB) and mixed-mode ion-exchange. The results showed low recoveries of two analytes when reversed-phase was used, which improved when using HLB sorbent although high ME was obtained. Finally, the authors used an Oasis MAX cartridge followed by an Oasis MCX, but the method was only valid for the two neutral drugs (ifosfamide and cyclophosphamide), while the basic tamoxifen displayed strong variability and could not be determined. Although absolute recoveries increased with the use of the clean-up steps, the ME found was still high and greatly dependent on the type of sludge and analyte. Another interesting example is described by S. Yang et al. [68] where the performance of different mixed-mode sorbents are used in the clean-up of the extract. In this study to determine central carbon metabolites from lake sediments, the clean-up of the liquid–solid extract obtained was carefully optimized and the method involved loading the sample consecutively in three different sorbents: Oasis MCX, Oasis WAX, and Oasis MAX. After loading the aqueous extract into an MCX cartridge connected to a WAX cartridge, the loaded fraction was collected and basified at pH 10.2, then loaded into an MAX cartridge. Each cartridge was eluted separately and the method enabled recoveries higher than 68% to be obtained for 51 out of the 56 targeted metabolites using standard solutions. When the sediment was analyzed, the authors had to increase the amount of sorbent (from 30 mg to 60 mg) in order to obtain acceptable recoveries due to the high content of humic substances and other organic materials. In this study, the complementarity of the different mixed-mode sorbents is clearly demonstrated. The possibilities of mixed-mode sorbents along with their combinations are therefore numerous and can be further expanded in the future.

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Please cite this article as: N. Fontanals, F. Borrull and R.M. Marcé, Mixed-mode ion-exchange polymeric sorbents in environmental analysis, Journal of Chromatography A, https://doi.org/10.1016/j.chroma.2019.460531