Separation and Enrichment of Selected Polar and Non-Polar Organic Micro-Pollutants—The Dual Nature of Quaternary Ammonium Ionic Liquid

Abstract

In this study, the dual nature of quaternary ammonium ionic liquid–didecyldimethylammonium perchlorate, [DDA][ClO₄], was evaluated. A novel and sensitive in situ ionic liquid dispersive liquid–liquid microextraction method (in situ IL-DLLME) combined with magnetic retrieval (MR) was applied to enrich and separate selected organic micro-pollutants, both polar and non-polar. The magnetic support relied on using unmodified magnetic nanoparticles (MNPs) prepared by the co-precipitation of Fe²⁺/Fe³⁺ (Fe₃O₄). The separation technique was on-lined with high-performance liquid chromatography (HPLC–DAD) verified by inverse gas chromatography. An anion exchanger, NaClO₄, was added to form an in situ hydrophobic IL. The fine droplets of [DDA][ClO₄], molded in aqueous samples, functioned as an extractant for isolating the studied compounds. Then the carrier MNPs were added to separate the IL from the water matrix. The supernatant-free sample was desorbed in acetonitrile (MeCN) and injected into the HPLC system. The applicability of [DDA][ClO₄] as an extraction solvent in the MR in situ IL-DLLME method was checked by the selectivity parameters ($S_{ij}^{\infty }$) at infinite dilution. The detection limit (LOD) ranged from 0.011 to 0.079 µg L⁻¹ for PAHs and from 0.012 to 0.020 µg L⁻¹ for benzophenones. The method showed good linearity with correlation coefficients (r²) ranging from 0.9995 to 0.9999.

1. Introduction

Rapidly growing industries and human activities are among the primary sources of organic micro-pollution. Micro-pollutants are durable compounds that affect water conditions and purity and accumulate in living organisms [1]. Among these compounds, one may enumerate benzophenone and phenolic compounds. Prolonged exposure to even trace amounts of these compounds can cause irreversible changes in the body. These compounds can accumulate in body tissues, potentially impairing the hormonal system. They can also cause several skin allergies and increase the risk of cancer. Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), benzophenones, chlorinated pesticides, and compounds of high biological activity and high bioaccumulation potential are considered a severe threat to living organisms.

Analysis of PAH residues in the aquatic environment has aroused considerable interest [2]. Prolonged exposure to PAHs may cause undesirable changes in the human body. PAHs originated from oil spills, fossil fuels, and domestic and industrial wastewater discharge. They are found as widespread contaminants harming the environment [3]. They have been classified as potential organic pollutants due to the presence of two or more fused aromatic rings of the carbon and hydrocarbon atoms that may account for their danger to the environment and living creatures [4,5,6,7]. According to the EU’s EEA (European Environmental Agency) and the US EPA (US Environmental Protection Agency), PAHs are the most hazardous organic pollutants responsible for mutagenic, carcinogenic, and endocrine-disrupting effects [7,8]. The occurrence of these pollutants in anthropogenic and industrial waste has caused deep concern about their effects on the environment and public health [9].

Directly determining PAHs is fundamentally problematic, for they occur in trace amounts in very complex biological matrix samples. For this reason, it is essential to develop simple yet efficient methods for monitoring PAHs. Numerous researchers have developed interesting sample pretreatment techniques involving liquid–liquid extraction (LLE) and solid-phase extraction (SPE) [10]. Their proposed solutions are, however, burdened with numerous imperfections, which is why a simple and efficient sample preparation procedure is still required. Notably, one should also not ignore the possibility of the mutual reinforcement of PAHs due to the co-occurrence of other synergistic substances in the environment [11]. According to the already published data, PAHs have been detected in wastewater, marine waters, lakes, rivers, sewage sludge, and soil [12,13]. Due to UV radiation, they can chemically transform into metabolites with potentially even stronger adverse effects on the environment [1].

Benzophenones are efficient UV filters. They are commonly used in sunscreen to protect against carcinogenic ultraviolet radiation. Therefore, they are used as cosmetics and synthetic packaging additives, protecting the human skin or extending the shelf life of stored products sensitive to solar radiation. They are also added to pharmaceuticals and everyday products such as body lotions, shampoos, bubble baths, hair sprays, and many more.

The most known and commonly used UV filters are 2,4-dihydroxybenzophenone (BP1) and 2-hydroxy,4-methoxy benzophenone (BP3). These act as chemical filters protecting against the adverse effects of ultraviolet radiation. UV radiation is responsible for the destruction of collagen fibers, which reduces the immune response. They also control the formation of free radicals, which causes structural damage to proteins and leads to tumor formation [1,11].

It is known that even a trace amount (ng·L⁻¹) of PAHs and BPs can pose a severe risk to human and animal health. That is why it is imperative to monitor their presence in the environment at extremely low levels. However, quantifying chemicals on the ppt level is often challenging and poses a difficult analytical problem. Thus, sensitive, environment-friendly, and effective extraction and pre-concentration methods for organic micro-pollutants are still in demand.

The analytical monitoring of trace compounds still requires adequate strategies for pre-concentration and extraction [14,15,16,17], which indisputably form the possible bottleneck of any analytical technique. Even if progress in detection systems is visible nowadays, an efficient trace analysis of complex matrices is rarely achieved [17,18].

An appropriate sample pretreatment method should be characterized by adequate sensitivity to enrich the level of analytes over the detectable limits and remove potential interferences from the sample matrix [14,19]. Thus, sample preparation should minimize the sample complexity and eliminate most interferences while guaranteeing proper pre-concentration. According to this idea, efforts are concentrated on improving and proposing new sample preparation methods, particularly miniaturization, with low energy requirements, simplification, and time reduction [20].

Over the past decade, ionic liquids have been used in multiple scientific and engineering applications, which is evident from the many publications and citations regarding ionic liquids in analytical chemistry. Among numerous extraction methods, those based on ionic liquids are worthy of particular attention.

Ionic liquids (ILs) [21,22], at a temperature below 100 °C, are liquid molten salts. They generally consist of large organic cations and organic or non-organic anions [23]. ILs are characterized by superior thermal, chemical, and electrochemical stability, negligible volatility, and a designable structure. Applied as non-molecular solvents, ILs have achieved much success in applications involving enzyme reaction catalysis, transition metal catalysis solutions [24], the chemical industry [25], extraction, separation [26], and biological sensors [27]. The use of ionic liquids as effective extractants is also associated with the possibility of their recycling, which significantly reduces application costs [28,29,30,31]. The unique physicochemical properties of ionic liquids, i.e., low vapor pressure, high thermal stability, a wide range of liquid states, relatively high viscosity, and the ability to dissolve a broad spectrum of organic compounds, distinguished them as a new class of organic solvents. Additionally, some ionic liquids’ capacity to create electrostatic, hydrophobic, and π–π interactions is responsible for their dualistic nature, resulting in selectivity for polar and non-polar analytes.

This paper proposes a microextraction method using ionic liquids belonging to the quaternary ammonium salts (quats)—so-called quat-based ILs—for the extraction and enrichment of both polar and non-polar compounds. The in situ dispersive liquid–liquid dispersion method (MR in situ IL-DLLME) using ionic liquids modified with iron oxide nanoparticles has been optimized and validated for the extraction and enrichment of selected polycyclic aromatic hydrocarbons (PAHs) and UV benzophenone filters. The analysis is completed with HPLC–DAD detection. The compounds selected in this work are a group of six polycyclic aromatic hydrocarbons (PAHs): fluorene (F), anthracene (Ant), fluoranthene (FL), pyrene (Pyr), benzo(a)pyrene (BaP), benzo(a)anthracene (BaA), and benzophenone. Among the selected UV filters, the following compounds were determined: 2,4-dihydroxybenzophenone (BP1), 2,2′, 4,4′-tetrahydroxybenzophenone (BP2), and 2-hydroxy-4-methoxybenzophenone (BP3).

2. Materials and Methods

2.1. Reagents and Chemicals

All reagents (

Table 1. Chemical structure and typical chromatograms of targeted polycyclic aromatic hydrocarbons and benzophenones. Chromatograms: X axis—minutes, Y axis—AU units.

PAHs
Fluorene (F)		(a) fluorene, (b) anthracene, (c) pyrene, (d) fluorantene, (e) benzo(a)anthracene, (f) benzo(a)pyrene
Anthracene (Ant)
Fluoranthene (FL)
Pyrene (Pyr)
Benzo(a)pyrene (BaP)
Benzo(a)anthracene (BaA)
Benzophenones
2,4-dihydroxybenzo-phenone (BP1)
2,2′,4,4′-tetrahydroxybenzo-phenone (BP2)
2-hydroxy-4-metoxybenzo-phenone (BP3)

) were of analytical grade or better. Two of the benzophenones (Figure 1), 2,4-dihydroxybenzophenone (BP1) and 2,2′,4,4′-tetrahydroxybenzophenone (BP2), were purchased from Sigma-Aldrich (Poznań, Poland) and the third, 2-hydroxy-4-metoxybenzophenone (BP3), from Fluka (Poznań, Poland). The selected PAHs (Figure 1), pyrene (Pyr), fluorene (F), fluoranthene (Fl), anthracene (Ant), benzo(a)pyrene (BaP), and benzo(a)anthracene (BaA), were purchased from Sigma-Aldrich (Poznań, Poland).

Standards of BPs and PAHs (1000 mg L⁻¹) were dissolved in methanol and acetonitrile and stored in a refrigerator to avoid degradation. All analytes’ standards were prepared freshly by diluting the stock solutions with ultrapure water, MeOH, or acetonitrile. HPLC-grade methanol and acetonitrile (≥99.9%) were purchased from Sigma-Aldrich (Poznań, Poland). Didecyldimethylammonium chloride [DDAC] was purchased from Lonza (Mapleton, IL, USA). NaClO₄, FeCl₃ 6H₂O, and FeSO₄ 7H₂O were purchased from Sigma-Aldrich (Poznań, Poland). The 25% NH₃ was purchased from POCH S.A. (Gliwice, Poland). Argon of 99.9992% purity was purchased from Air Products (Poznań, Poland). The deionized water was purified by a water deionizer purchased from Hydrolab (Gdańsk, Poland).

2.2. Preparation of Fe₃O₄ Magnetic Nanoparticles

Fe₃O₄ magnetic nanoparticles were synthesized via the chemical co-precipitation method adopted from a method described by Liu et al. [32]. We dissolved 1.5 g of FeCl₃∙6H₂O and 1.5 g of FeSO₄∙7H₂O in 40 mL of deionized water at 80 °C, then argonized and stirred for 45 min. After that, 25 mL of 25% NH₃ was slowly pipetted into the solution, vigorously stirred under argon for 30 min, and cooled. The orange solution turned black, and magnetite nanoparticles were immediately formed. The black solid was washed with deionized water and further with 0.02 M sodium chloride and twice with methanol. Magnetic nanoparticles separated with a neodymium magnet were dried at room temperature (Figure 1). A scanning electron microscope SU3500 from Hitachi with a BSE-3D detector was used to verify the structure and size of the Fe₃O₄ nanoparticles. Figure 2 shows SEM photos of the Fe₃O₄ MNP aggregates caused by their magnetic features. Single nanoparticles showed almost spherical morphology, an average diameter of 150 nm, and good dispersity.

2.3. Instrumentation

The HPLC separation was carried out using a Shimadzu LC Workstation equipped with an LC-20AD pump, SPD-M20A diode array detector, and CTO-10AC column oven. Samples of 20 µL were injected into a NUCLEOSIL^®100-5 C18 (250 mm × 4.6 mm; 5 μm) column. An acetonitrile/water mixture 85/15 (v/v) at a flow rate of 1.5 mL min⁻¹ and a methanol/water mixture 70/30 (v/v) at 1 mL min⁻¹ flow rate were used as the mobile phases, respectively, for the selected PAHs and BP-type UV filters. The HPLC column temperature was set at 30 °C and 35 °C. The DAD detection for each of the selected benzophenones was carried out at 290 nm, while the selected PAHs, i.e., F, Ant, FL, Pyr, BaA, and BaP, were analyzed at multiple wavelengths, i.e., 254, 251, 235, 239, 286, and 295 nm, respectively. The Hitachi SU3500 scanning electron microscope (SEM) was used for estimating the size and observing the morphology of the iron oxide’s magnetic nanoparticles.

2.4. Procedure for In Situ IL-DLLME with Magnetic Retrieval of IL

We mixed 400 mg of ionic liquid and 5 mg of magnetic nanoparticles (Fe₃O₄) in a glass tube. A water sample of 30 mL was poured into the vial. The mixture was vigorously shaken until the IL was fully dissolved. Then, 800 µL of NaClO₄ solution was added to initiate the metathesis reaction. After this, a hydrophobic IL, didecyldimethylammonium perchlorate [DDA][ClO₄], was formed in situ and the system was stirred for 2 min. A neodymium magnet was held around the tube to collect the MNPs with analyte-loaded ionic liquid. Once the water phase was removed, 200 µL of methanol was added. Finally, the MNPs were isolated by magnetic decantation. The target analytes were separated and quantified using HPLC–DAD analysis (Figure 3).

3. Results and Discussion

Due to the possible high enrichment factors (EFs) and recovery rates, a step-by-step optimization scheme was applied, which took into account the dosage of MNPs, the concentration of ionic liquid, the molar ratio of DDAC to NaClO₄, and the sample volume. After determining the optimum conditions, the MR in situ IL-DLLME technique was evaluated based on linearity, the limit of detection (LOD), the limit of quantitation (LOQ), and the recovery rates. The tests of the calibration curve range were performed at five levels. The LOD was calculated in the traditional manner as three times the signal-to-noise ratio (S/N = 3), and LOQ was calculated as ten times the signal-to-noise ratio (S/N = 10). A blank test was carried out to verify the possibility of contamination. The extraction efficiency was calculated as average recovery at different concentration levels.

3.1. Optimization of the IL Concentration

The concentration of DDAC for polar and non-polar analytes was optimized to achieve the highest recovery of the analytes under study (Figure 4). For both polar and non-polar analytes, the optimization was carried out in the range of 0.5 to 5%. The optimization process was performed with the sample volume equal to 10 mL. The amount of the MNPs entered into the sample was equal to 5 mg. On this basis, it was found that the concentration of IL significantly impacted the extraction efficiency. Recovery values, obtained for a 0.5% IL concentration, were low for both groups of analytes—not exceeding 75% for benzophenones and 30% for PAHs. For the 1% DDAC, recovery values were much higher—above 90% for benzophenones and above 60% for PAHs. Similar recovery values were obtained for the IL concentrations equal to 3% and 5%, for both polar and non-polar analytes. A DDAC concentration of 1% was assumed optimal and used in further tests.

3.2. The Effect of DDAC to NaClO₄ Molar Ratio

The following solutions with molar ratios of DDAC to NaClO₄ were prepared at 5:1, 2:1, 1:1, 3:5, and 1:2 compositions (Figure 5). The highest recovery values were noted when the molar ratio was equal to 1:1 or 1:2 for most of the studied polar and non-polar analytes. Double excess of the ion exchanger ensured the double decomposition reaction, and, as a consequence, the transformation of hydrophilic DDAC into a hydrophobic [DDA][ClO₄]. Regardless of the analytes’ polarity, the best recovery results were obtained when a double surplus of NaClO₄ was fixed. Therefore, the extraction phase composition of 1 to 2 was used in further experiments.

3.3. The Effect of the MNP Amount

MNPs of Fe₃O₄ were used to separate hydrophobic IL from an aqueous solution efficiently. In the primary in situ IL-DLLME method, the IL usually stuck to the vial walls and was difficult to separate from the supernatant.

The optimization range of the Fe₃O₄ MNP amounts was set from 0 to 15 mg. A sample with 5 mg of MNPs and with no IL added was also prepared. In the latter case, the recovery was below 5%—much lower in comparison to when IL was used. We also found that the results obtained without Fe₃O₄ added were not as precise. As presented in Figure 6, in the range from 10 to 15 mg, recovery values slightly decreased with the increased amount of MNPs. The least manageable amount of iron oxide (5 mg) was used for further studies. The amount of iron oxide was tested for selected benzophenones and PAHs. The results for both groups correlated with each other, confirming the correct quantitative selection of the MNPs for the MR in situ IL-DLLME method.

3.4. Method Validation

The calibration curve range, the linearity of the response calibration curves, the correlation coefficient (r²), the limits of detection (LOD) and quantification (LOQ), the recovery rate, and the enrichment factors (EFs) were investigated under the optimized conditions to validate the proposed MR in situ IL-DLLME method (

Table 2. Validation parameters for target PAHs and BPs determination using the MR in situ IL-DLLM method.

PARAMETER	PAHs						BPs
	F	Ant	FL	Pyr	BaA	BaP	BP1	BP2	BP3
Calibration curve range [µg L−1]	1–100	1–100	1–100	1–100	1–100	1–100	1–1000	1–1000	1–1000
Correlation coefficient (r2)	0.9998	0.9999	0.9995	0.9998	0.9999	0.9999	0.9999	0.9995	0.9998
Limit of detection (LOD) [µg L−1]	0.035	0.011	0.061	0.079	0.025	0.024	0.020	0.012	0.016
Limit of quantitation (LOQ) [µg L−1]	0.115	0.036	0.203	0.263	0.083	0.081	0.067	0.041	0.054
Recovery (%) (RSD (%))	77.1–88.5 (0.9–4.8)	26.6–42.0 (1.3–9.9)	34.6–60.4 (2.7–11.0)	52.2–102.7 (1.0–8.8)	28.2–44.0 (2.3–8.3)	26.2–45.3 (6.2–9.5)	70.5–90.3 (3.4–2.9)	68.0–92.5 (7.8–5.4)	79.5–91.3 (3.5–1.3)
Enrichment factor (EF)	164.3	48.0	94.2	117.8	76.7	69.6	135.5	138.8	137.0

). The linearity was examined for hybrid working solutions of benzophenones and PAHs in concentrations ranging from 1 to 1000 µg L⁻¹ and 1 to 100 µg L⁻¹, respectively. Good linearity, with correlation coefficients (r²) ranging from 0.9995 to 0.9999, was obtained for all the analytes. The limits of detection (LODs) ranged from 0.007 to 0.011 µg L⁻¹ for benzophenones and 0.011 to 0.079 µg L⁻¹ for PAHs. The enrichment factors ranged from 135.5 to 137.0 for benzophenones and from 48.0 to 164.3 for the tested PAHs. Extraction recoveries for BPs were high and reached 68.0% to 92.5%, while for PAHs they reached 26.2% to 102.7%.

3.5. Selectivity Parameters Determined by Inverse Gas Chromatography

Before proceeding with MR in situ IL-DLLME optimization, the applicability of the didecyldimethylammonium perchlorate as an extraction solvent was verified. The selectivity parameters $(S_{ij}^{\infty })$ at infinite dilution for hexane/benzene, hexane/diethyl ether, hexane/tetrahydrofuran, hexane/acetonitrile, hexane/ethanol, and hexane/methanol mixtures were calculated from the experimental activity coefficient (${\gamma }_{13}^{\infty }$). The ${\gamma }_{13}^{\infty }$ values of 29 polar and non-polar organic solutes, alkanes, alkenes, alkynes, cycloalkanes, alcohols, ethers, ketones, and aromatic hydrocarbons in [DDA][ClO₄] were determined by inverse gas chromatography (IGC). Four different temperatures were established.

Interactions between ionic liquids and different polar and non-polar solutes are valuable in indicating practical applications of IL in the extraction process. Knowledge of the separation factors allows for the designing of separation processes in various extraction systems [33]. The selectivity values for [DDA][ClO₄] at 50 °C for different extraction systems are gathered in

Table 3. The $S_{ij}^{\infty }$ values for selected separation systems of [DDA][ClO₄] at 50 °C.

Binary Mixture	[DDA][ClO4]
	${\mathit{\gamma }}_{13}^{\mathit{\infty }}{(}_{\mathbf{i}})$	${\mathit{\gamma }}_{13}^{\mathit{\infty }}{(}_{\mathbf{j}})$	Sij∞
Hexane(i)/benzene(j)	2.477	0.499	4.96
Hexane(i)/diethyl ether(j)		1.029	2.41
Hexane(i)/tetrahydrofuran(j)		0.478	5.18
Hexane(i)/acetonitryle(j)		0.543	4.56
Hexane(i)/ethanol(j)		0.497	4.98
Hexane(i)/methanol(j)		0.351	7.06

.

The $S_{ij}^{\infty }$ values for the hexane/acetonitrile, hexane/ethanol, and hexane/methanol binary mixtures were higher than for hexane/diethyl ether. The $S_{ij}^{\infty }$ values obtained for binary mixtures indicated that didecyldimethylammonium perchlorate can play an essential role in the separation of, e.g., aromatics and alcohols from alkanes, both polar and non-polar compounds. However, the differences between the selectivity values of hexane/acetonitrile, hexane/ethanol, hexane/methanol, hexane/benzene, and hexane/tetrahydrofuran were insignificant.

3.6. Comparison with Similar Work

The exploitation of IL as an efficient extraction medium for complex sample matrices is visible in many extraction–enrichment methods such as single-drop microextraction (SDM), liquid-phase microextraction (LPM), solid-phase microextraction (SPME), dispersive liquid–liquid microextraction (DLLME), hollow fiber-supported liquid membrane extraction (HFSLME), and solid-phase extraction (SPE) [34,35,36,37].

Due to its simplicity, low time consumption, rational use of reagents, and environmentally benign sample preparation, DLLME has been gaining popularity in recent years. In particular, it has attracted much interest from scientists working in separation science. Many improvements have been made since its introduction in 2006.

Currently, growing attention has been paid to the new enhancements in the IL-DLLME method. Novel techniques based on in situ IL-DLLME include META IL-DLLME (magnetic effervescent tablet-assisted ionic liquid dispersive liquid–liquid microextraction), MR-IL-DLLME (magnetic retrieval ionic liquid dispersive liquid–liquid microextraction), and MIL-DLLME (magnetic ionic liquid-based dispersive liquid–liquid microextraction) [38].

The idea of in situ IL-DLLME modification that applies magnetic nanoparticles (Fe₃O₄) to separate the in situ created ionic liquid [39] is a new-fangled variant of the IL-DLLME method. To date, the MR in situ IL-DLLME method has been developed and used to analyze five benzoylurea insecticides (BUs) in environmental water samples. Nie et al. proposed to analyze selected contaminants in food samples by the combination of IL-DLLME and SPME [39].

MNPs of iron oxides were used as sorbents to regain the analytes contained in the IL, which were desorbed in further steps. Using an external magnetic field, magnetic nanoparticles characterized by a large surface area can easily isolate the IL from a solution [40,41,42]. The large interfacial area between the IL and the sample solution contributes to a faster mass transfer [43,44]. Therefore, applying MNPs in in situ IL-DLLME makes this technique a sensitive, rapid, simple, effective, and satisfactory microextraction.

Musururwa and Tavengwa proposed a new method for the extraction of micro-pollutants from complex matrices, so-called homogenous liquid–liquid extraction (HLLME). The method aimed at an application of air flotation during the separation phase [45].

Dong et al. proposed a new type of nano-adsorbent composed of inorganic magnetic nanoparticles based on graphene oxide, amino-silanized shell, and ionic liquids. The method was named ionic liquid-coated amino-silanized magnetic graphene oxide (MGO@SiO2-APTES-IL) [46]. The adsorbent was for the separation of heavy metal ions in complex matrices.

Boon et al. proposed poly (β-cyclodextrin functionalized ionic liquid) immobilized magnetic nanoparticles (Fe₃O₄@βCD-Vinyl-TDI) as a sorbent for magnetic SPE [47]. Similar to our research, they concentrated on the determination of selected PAHs, but in rice samples. Moreover, Erdem et al. offered a new IL-based method for the determination of PAHs in coffee and tea [48]. A new method based on the application of ILs was described by Hui et al. [49]. This method also concentrates on the selection of PAHs from food samples. Zhang et al. proposed a method for the extraction of five typical PAHs in meat samples [50], and Zhou et al. [51] in milk samples.

As it may be seen, different modifications of IL-based separation are of growing interest to the research community [52,53]. While several new methods have been proposed based on different applications of ionic liquids, none of them is (and probably never will be) sufficiently generic and efficient for the determination of a wide set of PAHs. Instead, the methods concentrate on selected PAHs and given applications, such as food samples. Our research is sufficiently generic to be used, for example, in the analysis of natural environment pollutants, such as the hoarfrost analysis described in the next section. To our best knowledge, there is no similar proposal already published.

3.7. Analysis of Hoarfrost Samples

Hoarfrost is an atmospheric-rich deposit and a comprehensive source of information about the atmospheric organic and inorganic compounds in a given geographical area. Hoarfrost affects the natural cleaning processes of the atmosphere. It is a reservoir introducing organic and inorganic pollutants to waters and soils, changing their equilibrium. Harmful atmospheric substances trapped in the sediment can be transferred from plants to humans along the food chain. For this reason, knowledge of the quality and level of atmospheric pollution is essential. One can control the degree of air contamination, among others, by monitoring precipitation and atmospheric sediment composition.

Among hazardous substances that can accumulate in hoarfrost are polycyclic aromatic hydrocarbons. PAHs are commonly known for their severe toxicity. They are mainly formed and emitted into the atmosphere due to incomplete combustion of organic energy sources like coal, oil, gas, and wood [4]. Researchers are highly interested in PAHs due to their debilitating impact on humans and the environment. It has been proven that even trace amounts of PAHs (ng L⁻¹) can lead to irreversible health problems or lethal changes in living organisms.

The verification of optimized MR in situ IL-DLLME was carried out to analyze PAHs in samples of hoarfrost. The samples were collected during the winter months (January and February 2022) at seven different locations: 1. parking lot I—Konin city, 2. parking lot II (over the lake)—Konin city, 3. Piątkowo district I—Poznań city, 4. Piątkowo district II— Poznań city, 5. Umultowo village—Poznań suburbs, 6. Umultowo green area I—Poznań suburbs, and 7. Umultowo green area II—Poznań suburbs. Depending on the civil pressure of the sampling site, a wide variety of the total amount of PAHs was observed (

Table 4. Concentrations of selected PAHs in hoarfrost deposited across Wielkopolska district, Poland.

Sampling Place	Concentration [µg·L−1]
	F	Ant	Fl	Pyr	BaA	BaP
1	0.079 ± 0.012	0.0504 ± 0.010	0.066 ± 0.011	0.013 ± 0.005	0.062 ± 0.012	<LOD
2	0.071 ± 0.014	0.2479 ± 0.034	0.126 ± 0.022	<LOD	0.113 ± 0.019	0.026 ± 0.002
3	0.059 ± 0.010	<LOD	0.060 ± 0.011	<LOD	<LOD	<LOD
4	0.044 ± 0.007	<LOD	<LOD	<LOD	<LOD	<LOD
5	0.037 ± 0.004	<LOD	<LOD	<LOD	0.025 ± 0.005	<LOD
6	0.054 ± 0.009	<LOD	<LOD	<LOD	0.032 ± 0.004	<LOD
7	0.040 ± 0.008	<LOD	<LOD	<LOD	<LOD	<LOD

). There was also a great diversity of identified PAHs among the controlled deposits.

Generally, the hoarfrost from urban areas was richer in micro-pollutants than slush waste from non-urban sites. The difference was substantial and confirmed the correlation between the origin of hoarfrost and the presence of micro-pollutants.

Fluorene was the most commonly found compound in the tested samples, regardless of where the sample was taken. The least frequently found compound was pyrene. The obtained data showed that sample one, although loaded with fewer PAHs than sample two, was characterized by similar PAH diversity. The difference was that it was free from BaP within a specified LOD, but, unlike other samples, it contained pyrene. Deposit two, coming from a parking lot almost in a city center, although close to a green area with a lake, contained almost all PAHs except pyrene, the amount of which was below LOD. This sample included a significant quantity of Ant (247.9 ng L⁻¹), F1 (126 ng L⁻¹), and BaP (113 ng L⁻¹) at the level regarded as endangering the health of living organisms. Figure 7 presents a sample chromatogram showing the presence of the above-mentioned compounds in comparison with the standard solution.

The PAH-free samples (excluding fluorine, 40 ng L⁻¹), 4 and 7, were deposited a long distance from the downtown and industrial areas of Poznań, at a place rich in trees and other green zones.

The optimized and validated MR in situ IL-DLLME method allowed for the analysis of trace amounts of selected polycyclic hydrocarbons. Therefore, we can assume that it is a sensitive and efficient tool for the quantification of PAHs and BPs collected from presented environmental matrices.

4. Conclusions

The present study evaluated the dual nature of quaternary ammonium ionic liquid–didecyldimethylammonium perchlorate.

The in situ dispersive liquid–liquid microextraction technique based on magnetic retrieval was proposed as a new approach for separating and enriching selected polar and non-polar organic micro-pollutants. A group of three benzophenones and six PAHs were isolated from the environmental probe and quantified using MR in situ IL-DLLME lined with an HPLC–DAD instrument. The results were verified by inverse gas chromatography.

The MR in situ IL-DLLME method was used with the optimal extraction parameters set out in this work. The extraction parameters were as follows: sample volume—30 mL, the amount of magnetic NPs—5 mg, DDAC concentration—1%, and the molar ratio DDAC to NaClO₄—1:2.

The results show that the proposed separation technique is an effective tool for extracting and enriching the target polar and non-polar analytes. It provides high recoveries for all the determined benzophenones and PAHs. Calculated recoveries ranged from 68.0% to 92.5% for BPs, 77.1% to 88.5% for fluorene, and 52.2% to 102.7% for pyrene. The enrichment factors reached 135.5 to 137.0 for benzophenones and 48.0 to 164.3 for the tested PAHs.

The MR in situ IL-DLLME technique based on the use of didecyldimethylammonium perchlorate ionic liquid as an extraction medium, combined with a selective analytical technique, i.e., HPLC–DAD or, better, LC–MS, offers a universal and efficient separation and enrichment method for polar and non-polar organic micro-pollutants present in environmental matrices. This paper presented the practical application of the described technique on real hoarfrost samples collected in various urban and non-urban locations.