Comparative study of direct immersion and headspace single drop microextraction techniques for BTEX determination in water samples using GC-FID

In the present work the determination of benzene, toluene, ethylbenzene and o-xylene (BTEX) in environmental sample solutions using gas chromatography with flame ionisation detection (GC-FID) combined with three different sampling techniques, such as; direct single drop microextraction (DI-SDME), headspace single drop microextraction (HS-SDME) and ultrasonic assisted HS-SDME, were compared. In all of these techniques, for the determination of BTEX, the experimental parameters such as organic solvent effect, extraction time, agitation speed and salting effect were optimised. At their optimised conditions of operation the detection limits, times of extraction and precision for the three techniques are established. A detailed comparison of the analytical performance characteristics of these techniques for final GC-FID determination of BTEX in water samples was given. The technique provided a linear range of 50–20000?ng?mL^–1 for DI-SDME and 10–20000?ng?mL^–1 for HS-SDME methods, good repeatability (RSDs <4.72–7.74% for DI-SDME and 1.80–7.05% for HS-SDME (n?=?5), good linearity (r?≥?0.9978) and limits of detection (LODs) in the range of 0.006–10?ng?mL^?1 for DI-SDME, 0.1–3?ng?mL^–1 for HS-SDME methods (S/N?=?3). Then the optimised techniques were also applied to real samples (river and waste waters) containing BTEX and similar precision (RSD?<?8.2,?n?=?3) was obtained.     

Critical evaluation of microextraction pretreatment techniques – Part 1: Single drop and sorbent‐based techniques

Sample pretreatment techniques or preconcentration constitute a very important step before the analysis of environmental, clinical, pharmaceutical, and other complex samples. Thanks to extraction techniques it is possible to achieve higher method sensitivities and selectivities. Miniaturization microextraction methods make them more environmentally friendly and only small amounts of samples are required. In the past 30 years, a number of microextraction methods have been developed and used and are documented in thousands of articles. Many reviews have been written focusing on their use in specified professional fields or on the latest trends. Unfortunately, no uniform nomenclature has been introduced for these methods. Therefore, this review attempts to classify all the essential microextraction techniques and describes their advantages, disadvantages, and the latest innovations. The methods are divided into two main groups: single drop and sorbent‐based techniques according to the type of extraction phase.     

Determination of anti‐malaria agent chloroquine using single drop liquid‐liquid‐liquid microextraction

A simple, sensitive, and inexpensive single drop liquid‐liquid‐liquid microextraction combined with isocratic RP‐HPLC and UV detection was developed for the determination of anti‐malaria drug, chloroquine. The target compound was extracted from alkaline aqueous sample solution (adjusted to 0.5 mol/L sodium hydroxide) through a thin layer of organic solvent membrane and back‐extracted to an acidic acceptor drop (adjusted to 0.02 mol/L phosphoric acid) suspended on the tip of a 25 μL HPLC syringe in the organic layer. This syringe was also used for direct injection after extraction. The linear range was 1–200 μg/L. The LOD and LOQ were 0.3 and 1.0 μg/L, respectively. Intra‐and inter‐day precisions were less than 2.0 and 2.3%, respectively. The real samples were successfully analyzed using the proposed method. The recoveries of spiked samples were more than 94.6%.     

Rapid analysis of the essential oil components of dried Zanthoxylum bungeanum Maxim by Fe2O3‐magnetic‐microsphere‐assisted microwave distillation and simultaneous headspace single‐drop microextraction followed by GC–MS

In this work, microwave distillation assisted by Fe₂O₃ magnetic microspheres (FMMS) and headspace single‐drop microextraction were combined, and developed for determination of essential oil compounds in dried Zanthoxylum bungeanum Maxim (ZBM). The FMMS were used as microwave absorption solid medium for dry distillation of dried ZBM. Using the proposed method, isolation, extraction, and concentration of essential oil compounds can be carried out in a single step. The experimental parameters including extraction solvent, solvent volume, microwave power, irradiation time, and the amount of added FMMS, were studied. The optimal analytical conditions were: 2.0 μL decane as the extraction solvent, microwave power of 300 W, irradiation time of 2 min, and the addition of 0.1 g FMMS to ZBM. The method precision was from 4 to 10%. A total of 52 compounds were identified by the proposed method. The conventional steam distillation method was also used for the analysis of essential oil in dried ZBM and only 31 compounds were identified by steam distillation method. It was found that the proposed method is a simple, rapid, reliable, and solvent‐free technique for the determination of volatile compounds in Chinese herbs.     

Determination of bisphenol A in thermal printing papers treated by alkaline aqueous solution using the combination of single‐drop microextraction and HPLC

A method for the quantitative determination of bisphenol A in thermal printing paper was developed and validated. Bisphenol A was extracted from the paper samples using 2% NaOH solution, then the extracted analyte was enriched using single‐drop microextraction followed by HPLC analysis. Several parameters relating to the single‐drop microextraction efficiency including extraction solvent, extraction temperature and time, stirring rate, and pH of donor phase were studied and optimized. Spiked recovery of bisphenol A at 20 and 5 mg/g was found to be 95.8 and 108%, and the method detection limit and method quantification limit was 0.03 and 0.01 mg/g, respectively. Under the optimized conditions, the proposed method was applied to the determination of bisphenol A in seven types of thermal printing paper samples, and the concentration of bisphenol A was found in the range of 0.53–20.9 mg/g. The considerably minimum usage of organic solvents (5 μL 1‐octanol) and high enrichment factor (189–197) in the sample preparation are the two highlighted advantages in comparison with previously published works.     

Single drop microextraction—Development,applications and future trends

Single drop microextraction (SDME) has emerged over the last 10–15 years as one of the simplest and most easily implemented forms of micro-scale sample cleanup and preconcentration. In the most common arrangement, an ordinary chromatography syringe is used to suspend microliter quantities of extracting solvent either directly immersed in the sample, or in the headspace above the sample. The same syringe is then used to introduce the solvent and extracted analytes into the chromatography system for identification and/or quantitation. This review article summarizes the historical development and various modes of the technique, some theoretical and practical aspects, recent trends and selected applications.     

Ionic liquid for single‐drop microextraction followed by high‐performance liquid chromatography‐ultraviolet detection to determine carbonyl compounds in environmental waters

A sensitive method based on ionic liquid for single‐drop liquid microextraction coupled with HPLC‐UV was developed for the determination of carbonyl compounds in environmental waters using 1‐octyl‐3‐methylimidazolium hexafluorophosphate [C₈min][PF₆] as extraction solvent and 2,4‐dinitrophenylhydrazine as derivatizing agent. The extraction parameters affecting the enrichment factors such as solvent volume, pH, extraction time and salt concentration were investigated. A homemade funnel form polytetrafluoroethylene sleeve was fixed at the tip of the syringe needle and this allowed the use of 10 μL drop of ionic liquid for direct immersion extraction. Under the optimal conditions, the remarkable enrichment factors up to 150‐fold were obtained depending on the target analytes. The method has been validated when rectilinear relationship was obtained between the concentrations of analytes and peak area in the range of 5–100 ng/mL, the correlation coefficients were from 0.995 to 0.998, and the limit of detection was in the range of 0.04–2.03 ng/mL. The method was applied to monitor the concentration of carbonyl compounds in environmental waters with spiked recovery in the range of 84.2–106.9%.     

Recent developments and applications of liquid phase microextraction in fruits and vegetables analysis

The sample preparation step has been identified as the bottleneck of analytical methodology in chemical analysis. Therefore, there is need for the development of cost‐effective, easy to operate, and environmentally friendly miniaturized sample preparation technique. The microextraction techniques combine extraction, isolation, concentration, and introduction of analytes into analytical instrument, to a single and uninterrupted step, and improve sample throughput. The use of liquid‐phase microextraction techniques for the analysis of pesticide residues in fruits and vegetables are discussed with the focus on the methodologies employed by different researchers and their analytical performances. Analytes are extracted using water‐immiscible solvents and are desorbed into gas chromatography, liquid chromatography, or capillary electrophoresis for identification and quantitation.     

Use of volatile organic solvents in headspace liquid‐phase microextraction by direct cooling of the organic drop using a simple cooling capsule

A low‐cost and simple cooling‐assisted headspace liquid‐phase microextraction device for the extraction and determination of 2,6,6‐trimethyl‐1,3 cyclohexadiene‐1‐carboxaldehyde (safranal) in Saffron samples, using volatile organic solvents, was fabricated and evaluated. The main part of the cooling‐assisted headspace liquid‐phase microextraction system was a cooling capsule, with a Teflon microcup to hold the extracting organic solvent, which is able to directly cool down the extraction phase while the sample matrix is simultaneously heated. Different experimental factors such as type of organic extraction solvent, sample temperature, extraction solvent temperature, and extraction time were optimized. The optimal conditions were obtained as: extraction solvent, methanol (10 μL); extraction temperature, 60°C; extraction solvent temperature, 0°C; and extraction time, 20 min. Good linearity of the calibration curve (R² = 0.995) was obtained in the concentration range of 0.01–50.0 μg/mL. The limit of detection was 0.001 μg/mL. The relative standard deviation for 1.0 μg/mL of safranal was 10.7% (n = 6). The proposed cooling‐assisted headspace liquid‐phase microextraction device was coupled (off‐line) to high‐performance liquid chromatography and used for the determination of safranal in Saffron samples. Reasonable agreement was observed between the results of the cooling‐assisted headspace liquid‐phase microextraction high‐performance liquid chromatography method and those obtained by a validated ultrasound‐assisted solvent extraction procedure.     

In‐line coupled single drop liquid–liquid–liquid microextraction with capillary electrophoresis for determining fluoroquinolones in water samples

A simple in‐line single drop liquid–liquid–liquid microextraction (SD‐LLLME) coupled with CE for the determination of two fluoroquinolones was developed. The method is capable to quantify trace amount of analytes in water samples and to improve the sensitivity of CE detection. For the SD‐LLLME, a thin layer of organic phase was used to separate a drop of 0.1 M NaOH hanging at the inlet of the capillary from the aqueous donor phase. By this way, the analytes were extracted to the acceptor phase through the organic layer based on their acidic/basic dissociation equilibrium. The drop was immersed into the organic phase during 10 min for extraction and then it is directly injected into the capillary for the analysis. Parameters such as type and volume of organic solvent phase, aqueous donor, and acceptor phases and extraction time and temperature were optimized. The enrichment factor was calculated, resulting 40‐fold for enrofloxacin (ENR) and sixfold for ciprofloxacin (CIP). The linear range were 20–400 μg/L for ENR and 60–400 μg/L for CIP. The detection limits were 10.1 μg/L and 55.3 μg/L for ENR and CIP, respectively, and a good reproducibility was obtained (4.4% for ENR and 5.6% for CIP). Two real water samples were analysed applying the new method and the obtained results presented satisfactory recovery percentages (90–100.3%).     

Single‐drop microextraction combined in‐line with capillary electrophoresis for the determination of nonsteroidal anti‐inflammatory drugs in urine samples

This study describes a method to determine nonsteroidal anti‐inflammatory drugs (NSAIDs) in urine samples based on the use of single‐drop microextraction (SDME) in a three‐phase design as a preconcentration technique coupled in‐line to capillary electrophoresis. Different parameters affecting the extraction efficiency of the SDME process were evaluated (e.g. type of extractant, volume of the microdroplet, and extraction time). The developed method was successfully applied to the analysis of human urine samples with LODs ranging between 1.0 and 2.5 μg/mL for all of the NSAIDs under study. This method shows RSD values ranging from 8.5 to 15.3% in interday analysis. The enrichment factors were calculated, resulting 27‐fold for ketoprofen, 14‐fold for diclofenac, 12‐fold for ibuprofen, and 44‐fold naproxen. Samples were analyzed applying the SDME–CE method and the obtained results presented satisfactory recovery values (82–115%). The overall method can be considered a promising approach for the analysis of NSAIDs in urine samples after minimal sample pretreatment.     

Single‐drop liquid phase microextraction accelerated by surface acoustic wave

A single‐drop liquid phase microextraction method is presented, in which surface acoustic wave (SAW) is used for accelerating extraction speed. A pair of interdigital transducers with 27.5 MHz center frequency is fabricated on a 128° yx‐LiNbO₃ substrate. A radio frequency signal is applied to one of interdigital transducers to excite SAW. Plastic straw is filled with PDMS, leaving 1 mL for holding sample solution. Plastic straw with sample solution droplet is then dipping into extractant, into which SAW is radiated. Mass transportation from sample solution to extractant drop is accelerated due to acoustic streaming, and extraction time is decreased. An ionic liquid and an acid green‐25 solution are used for extraction experiments. Results show that the extraction process is almost finished within 2 min, and extraction speed is increased with radio frequency signal power.     

Recent development in liquid phase microextraction for determination of trace level concentration of metals—A review

As the drive towards green extraction methods has gained momentum in recent years, it has not always been possible to eliminate organic solvents completely. However, the volumes employed have been reduced remarkably, so that a single microdrop is sufficient in some cases. This effort has led to the development of various liquid phase microextractions namely single drop microextraction (SDME), hollow fiber liquid phase microextraction (HF-LPME), dispersive liquid-liquid microextraction (DLLME) and solidified floating organic drop microextraction (SFODME). In this review, the historical development and overview of these miniaturized liquid phase extraction methodologies have briefly been discussed and a comprehensive collection of application of the these methods in combination with different analytical techniques for preconcentration and determination of ultra trace amounts of metals and organometal ions in various matrices have been summarized.     

Single‐drop microextraction followed by in‐syringe derivatization and GC‐MS detection for the determination of parabens in water and cosmetic products

A single‐drop microextraction (SDME) method followed by in‐syringe derivatization and GC‐MS determination has been developed for analysis of five parabens, including methyl, ethyl, isopropyl, n‐propyl and n‐butyl paraben in water samples and cosmetic products. N,O‐Bis(trimethylsilyl)acetamide (BSA) was used as derivatization reagent. Derivatization reaction was performed inside the syringe barrel using 0.4 μL of BSA. Parameters that affect the derivatization yield such as temperature and time of the reaction were studied. In addition, experimental SDME parameters such as selection of organic solvent, addition of salt, extraction time and extraction temperature were investigated and optimized. The RSD of the method for aqueous samples varied from 8.1 to 13%. The LODs ranged from 0.001 (n‐butyl paraben) to 0.015 (methyl paraben) μg/L, and the enrichment factors were between 23 and 150.     

Double sample preconcentration by in‐line coupled large volume single drop microextraction and sweeping in capillary electrophoresis

Single drop microextraction (SDME) is a convenient and powerful preconcentration method for CE before injection. By simple combination of sample‐handling sequences without modification of the CE apparatus, a drop of an aqueous acceptor phase covered with a thin organic layer was formed at the tip of a capillary; 10 min SDME of fluorescein and 6‐carboxyfluorescein from a donor phase of pH 1 to an acceptor phase of pH 9 provided 110‐fold enrichments without stirring the donor phase. To improve the concentration effect further, SDME was coupled with an on‐line (after injection) sample preconcentration method, sweeping, in which analytes in a long sample zone are accumulated at the boundary of a pseudostationary phase penetrating into the sample zone. It is thus necessary to inject a sample of much larger volume than that of a drop in typical SDME. A Teflon sleeve over the capillary inlet allowed a large volume drop to be held stably during extraction. By in‐line coupling 10 min SDME and sweeping of a 30 nL sample using a cationic surfactant dodecyltrimethylammonium, enrichment factors of the double preconcentration were increased up to 32 000.     

Development of single‐drop microextraction and simultaneous derivatization followed by GC‐MS for the determination of aliphatic amines in tobacco

In this work, for the first time, headspace (HS) single‐drop microextraction and simultaneous derivatization followed by GC‐MS was developed to determine the aliphatic amines in tobacco samples. In the HS extraction procedure, the mixture of derivatization reagent and organic solvent was employed as the extraction solvent for HS single‐drop microextraction and in situ derivatization of aliphatic amine in the samples. Fast extraction and simultaneous derivatization of the analytes were performed in a single step, and the obtained derivatives in the microdrop extraction solvent were analyzed by GC‐MS. The optimized experiment conditions were: sample preparation temperature of 80°C and time of 30 min, HS extraction solvent (the mixture of benzyl alcohol and 2,3,4,5,6‐pentafluorobenzaldehyde) volume of 2.0 μL, extraction time of 90 s. With the optimal conditions, the method validations were also studied. The method has good linearity (R² more than 0.99), accepted precision (RSD less than 13%), good recovery (98–104%) and low limit of detection (0.11–0.97 μg/g). Finally, the proposed technique was successfully applied to the analyses of aliphatic amines in tobacco samples of seven different brands. It was further demonstrated that the proposed method offered a simple, low‐cost and reliable approach to determine aliphatic amines in tobacco samples.     

Application of pneumatic nebulization single‐drop microextraction for the determination of organophosphorous pesticides by gas chromatography–mass spectrometry

Organophosphorous pesticides (OPPs) including dichlorvos, diazinon, malathion, phenamiphos and chlorpyrifos, in water samples were extracted by pneumatic nebulization single‐drop microextraction (PN‐SDME) and then determined by gas chromatography–mass spectrometry (GC‐MS). Experimental parameters affecting the performances of PN‐SDME, such as flow rate of carrier gas, extraction time and microdrop volume, were examined and optimized. The limits of detection for the analytes were in the range of 0.0014–0.0019 μg/mL. The linear range was 0.0050–0.50 μg/mL, except dichlorvos (0.0070–0.50 μg/mL). Water samples were analyzed and the recoveries of the analytes in the spiked water samples were from 75.2 to 105.3%. The relative standard deviations were lower than 12.7%.     

Determination of musk fragrances in sewage sludge by pressurized liquid extraction coupled to automated ionic liquid‐based headspace single‐drop microextraction followed by GC‐MS/MS

A method for the quantitative determination of ten musk fragrances extensively used in personal care products from sewage sludge was developed by using a pressurized liquid extraction (PLE) followed by an automated ionic liquid‐based headspace single‐drop microextraction and gas chromatography‐tandem mass spectrometry. The influence of main factors on the efficiency of PLE was studied. For all musks, the highest recovery values were achieved using 1 g of pretreated sewage sludge, H₂O/methanol (1:1) as an extraction solvent, a temperature of 80°C, a pressure of 1500 psi, an extraction time of 5 min, 2 cycles, a 100% flush volume, a purge time of 120 s, and 1 g Florisil as in‐cell clean‐up extraction sorbent. The use and optimization of an in‐cell clean‐up sorbent was necessary to remove fatty interferents of the PLE extract that make the subsequent ionic liquid‐based headspace single‐drop microextraction difficult. Validation parameters, namely LODs and LOQs, ranged from 0.5–1.5 to 2.5–5 ng/g, respectively. Good levels of intra‐ and interday repeatabilities were obtained analyzing sewage sludge samples spiked at 10 ng/g (n = 3, RSDs < 10%). The method applicability was tested with sewage sludge from different wastewater treatment plants. The analysis revealed the presence of all the polycyclic musks studied at concentrations higher than the LOQs, ranging from 6 to 530 ng/g. However, the nitro musk concentrations were below the LOQs or, in the case of musk xylene, was not detected.     

Magnetic ionic liquid assisted single‐drop microextraction of ascorbic acid before its voltammetric determination

For the first time, we coupled a microextraction technique using a magnetic ionic liquid with voltammetric determination. A hydrophobic magnetic ionic liquid that contains the tetrachloromanganate(II) anion, namely, aliquat tetrachloromanganate(II), was synthesized and used for the extraction of ascorbic acid from aqueous solutions followed by voltammetric determination. The extraction procedure was carried out using a single drop microextraction technique in which the ascorbic acid containing magnetic ionic liquid was separated with a magnet and then cast onto the surface of a carbon paste electrode modified with TiO₂ nanoparticles for the voltammetric quantification of the extracted ascorbic acid. Electrochemical quantification was carried out in a blank phosphate buffer solution. After optimizing different experimental conditions, a linear concentration range of 1.50–40.0 nM with a detection limit of 0.43 nM was obtained for the determination of ascorbic acid. The presented approach was successfully applied to the determination of ascorbic acid in samples of vitamin C effervescent tablets and orange juice.     

Sorbent- and liquid-phase microextraction techniques and membrane-assisted extraction in combination with gas chromatographic analysis: A review

Approaches are described for on-line and off-line sample pretreatment of liquid samples utilising liquid- and adsorbent- and sorbent-phase microextraction methodologies with GC analysis. Solid-phase microextraction (SPME), stir-bar sorptive extraction (SBSE), on-line solid-phase extraction (SPE), liquid-phase microextraction (LPME) and membrane-assisted methods are critically evaluated and the applicability of each technique is demonstrated with examples.