Application of dispersive liquid-liquid microextraction based on solidification of floating organic drop for simultaneous determination of alachlor and atrazine in aqueous samples

Rapid analysis of triazine herbicides in fruit juices using evaporation-assisted dispersive liquid-liquid microextraction with solidification of floating organic droplets and HPLC-DAD

A rapid and convenient analytical procedure (evaporation-assisted dispersive liquid-liquid microextraction with solidification of floating organic droplets) is advanced for determining the concentrations of triazine herbicide residues (e.g. simazine and atrazine) in fruit juices via HPLC-DAD. The technique involves adding 1-dodecanol (low density) and dichloromethane (high density) to the test solution to act as the extraction and volatile solvents, respectively. Calcium oxide is added to generate heat to accelerate the evaporation of dichloromethane, whereupon the 1-dodecanol quickly disperses into small droplets to complete the microextraction process. Thus, there is no need to use a dispersive solvent and heating equipment is also not required. The floating 1-dodecanol is subsequently frozen using an ice bath to facilitate its separation from the sample. Under optimal conditions (250 μL of 1-dodecanol (extraction solvent), 150 μL of CH₂Cl₂ (volatile solvent), 1250 mg of CaO, and an extraction time of 60 s) the detection procedure is linear over the range 0.05-5 μg mL^-1 (with R > 0.99). The limits of detection (LOD) and quantification (LOQ) were determined to be 0.0022-0.0034 μg mL^-1 and 0.0073-0.0113 μg mL^-1, respectively. The recovery of simazine and atrazine in three fruit juices ranged between 78.5% and 96.4% with a relative standard deviation <8.2%. Therefore, the proposed approach can be effectively adopted to analyze the triazine herbicide content in fruit juices. The method has been proved to be simple, reliable, and remarkably efficient.

Sensitive determination of vincristine in plasma of children with leukaemia using vortex-assisted dispersive liquid-liquid microextraction based on hydrophobic deep eutectic solvent

Vincristine has a wide spectrum of clinical activity and is currently used in the treatment of leukemia. Despite its high therapeutic properties, vincristine has common side effects. Accordingly, it is desirable to determine vincristine in plasma for the use of the drug with strict monitoring. In the present research, for the first time a hydrophobic deep eutectic solvent (DES) composed of methyltrioctylammonium chloride (MTOAC) and n-butanol in a molar ratio of 1 : 3 was used as the extractant in dispersive liquid–liquid microextraction (DLLME) for the extraction and determination of vincristine in the plasma of children with leukemia prior to its analysis by high-performance liquid chromatography-ultraviolet detection (HPLC-UV). Under optimal experimental conditions, the method showed good linearity with a correlation coefficient (R²) of 0.9986 in the linear range of 0.06–300 μg L⁻¹, low limit of detection of 0.02 μg L⁻¹ and acceptable extraction efficiency (EE) of 88.4%. In the final stage of the study, this proposed technique was successfully applied to determine vincristine in real plasma, and the obtained results demonstrated the ability of the synthesized DES to extract drugs from biological fluids.

Vincristine has a wide spectrum of clinical activity and is currently used in the treatment of leukemia.

Extraction and determination of strobilurin fungicides residues in apple samples using ultrasound-assisted dispersive liquid-liquid microextraction based on a novel hydrophobic deep eutectic solvent followed by H.P.L.C-U.V

In this study, several novel and natural hydrophobic deep eutectic solvents (D.E.Ss) were prepared using methyl trioctylammonium chloride (M.T.O.A.C) as H.B.A and different types of straight chain alcohols as H.B.Ds. One of the D.E.Ss composed of M.T.O.A.C and n-butanol was advantageously used to develop an ultrasound-assisted dispersive liquid-liquid microextraction (U.A-D.L.L.M.E) method combined with high-performance liquid chromatography-ultraviolet detection (H.P.L.C-U.V) for the determination of some strobilurin fungicides in apple samples. Several important parameters influencing extraction efficiency were investigated and optimised, including the type and volume of extractant in ultrasound stage, sonication time, the type and volume of D.E.S, sample solution pH and effect of salt addition. Under optimal experimental conditions, the method showed good linearity with correlation coefficients (R²) of 0.9985 - 0.9991 in the linear range of 4-1500 μg kg^-1, low limits of detection of 1.5-2 μg kg^-1 and acceptable extraction recoveries in the range of 76-92%. Enrichment factor was in the range of 95-115. The proposed method was successfully applied for the extraction and preconcentration of trace fungicides in apple samples, and the results demonstrated the potential of the synthesised D.E.S for the extraction and determination of contaminants in aqueous samples.

Novel hydrophobic deep eutectic solvent for vortex-assisted liquid phase microextraction of common acaricides in fruit juice followed by HPLC-UV determination

In the present research, several novel and natural hydrophobic deep eutectic solvents (DESs) were prepared using methyltrioctylammonium chloride (MTOAC) as the hydrogen bond acceptor (HBA) and different types of straight chain alcohols as hydrogen bond donors (HBDs). One of the DESs composed of MTOAC and n-butanol was advantageously used to develop a vortex-assisted liquid phase microextraction (VALPME) method combined with high-performance liquid chromatography-ultraviolet detection (HPLC-UV) for the determination of common acaricides in fruit juice samples. Several important parameters influencing extraction efficiency were investigated and optimized, including the type and volume of DES, sample solution pH, effect of salt addition and, extraction and vortex time. Under optimal experimental conditions, the method showed good linearity with the correlation coefficients (R²) of 0.9986–0.9991 in the linear range of 2–300 μg L⁻¹, low limits of detection of 0.5–1 μg L⁻¹ and acceptable extraction recoveries in the range of 85–93%. The proposed method was successfully applied for the extraction and preconcentration of trace acaricides in real fruit juice samples, and the results demonstrated the potential of the synthesized DESs for the extraction and determination of contaminants in aqueous samples.

In this research, several novel hydrophobic deep eutectic solvents (DESs) were prepared using methyltrioctylammonium chloride (MTOAC) as the hydrogen bond acceptor (HBA) and different types of straight chain alcohols as hydrogen bond donors (HBDs).

Cyclodextrin-based dispersive liquid-liquid microextraction for the determination of fungicides in water, juice, and vinegar samples via HPLC

Cyclodextrin-based dispersive liquid-liquid microextraction (CD-DLLME) was developed for the determination of triazole and strobilurin fungicides in water, juice, and vinegar samples using high-performance liquid chromatography-diode-array detection (HPLC-DAD). Undecanol, which is a green solvent, was selected as the extraction solvent. A cyclodextrin aqueous solution was chosen as the dispersion solvent and demulsifier to avoid the use of a toxic dispersion solvent and eliminate the centrifugation step. Dispersion and phase separation were completed within 1 and 60 s, respectively. The linear range of this method was 1 to 100 µg L^-1. The limits of detection were 0.3 μg L^-1 along with the preconcentration factor of 133 and enrichment factor of 124. The recovery was 83.2% to 103.2%. This pretreatment method was fast, simple, and environmentally friendly and was successfully applied to the analysis of triazole and strobilurin fungicide residues in water, juice, and vinegar samples.

Determination of eight endocrine disruptor pesticides in bovine milk at trace levels by dispersive liquid-liquid microextraction followed by GC-MS determination

Endocrine disrupting chemicals are chemicals that interfere with any aspect of the endocrine system. Several natural and synthetic chemicals, including pesticides, have been identified as endocrine disruptors, which potentially inhibit the reproductive activity of the hormonal system. The pervasive occurrence with trace level concentrations and extensive variety are the reported characteristics of these chemicals. In this study, a dispersive liquid-liquid microextraction method coupled with gas chromatography and mass spectrometry for the determination of eight potential endocrine disruptor pesticides (Lindane, Diazinon, Fenitrothion, Malathion, Aldrin, α-Endosulfan, β-Endosulfan, Methoxychlor) in bovine milk samples was developed. Several parameters that can influence the extraction efficiency were studied. Under optimized conditions, the calibration curves of all eight analytes presented coefficient of determination higher than 0.998 (range level of 2.0-1000 ng/mL). The limits of detection and quantification ranged from 0.90 to 5.00 ng/mL and 2.50 to 15.0 ng/mL, respectively.

Sensitive determination of methotrexate in plasma of children with acute leukemia using double-solvent supramolecular systemas a novel extractant for dispersive liquid-liquid microextraction

Methotrexate, as a folate antagonist, is one of the first anti-neoplasm drugs offered and is still used as an effective drug in the treatment of various malignancies. Methotrexate has a narrow treatment index and is associated with numerous side effects.In thisresearch, for the first time a double-solvent supramolecular system (DSS) was developed as an extractant without disperser solvent for dispersive liquid-liquid microextraction (DLLME). DSS - DLLME was applied to the extraction of methotrexate in plasma of children with acute leukemiaprior to itsdetermination by high-performance liquid chromatography-ultraviolet detection (HPLC - UV). In the present method, two long normal chain alcohols are mixed in a particular ratio, and then it is injected into the sample solution, which is on the magnetic stirrer. In this case, the mixture of the two alcohol changes to new supramolecular aggregate. This new supermolecule is used as an extractant, which has a higher extraction power than any of its components alone. Under the optimum conditions, the calibration graph was linear in the rage of 0.1-150 µg L^-1 with detection limit of 0.03 µg L^-1. Relative standard deviations (RSDs) including intra-day and inter-day of method based on7 replicate determinations of 100.0 µg L^-1of methotrexate were 2.6% and 4.8%,respectively. The results proved that DSS - DLLME is a sensitive, very simple, inexpensive, environmental friendly, rapid and efficient method for the preconcentration of trace amount of drugs in biological samples.

Determination of pyridaphenthion in soybean sprout samples by gas chromatography mass spectrometry with matrix matching calibration strategy after metal sieve linked double syringe based liquid-phase microextraction

A novel analytical strategy for the trace determination of pyridaphenthion pesticide was developed in this study. Gas chromatography-mass spectrometry (GC-MS) was used for the accurate, feasible and precise determination of this analyte. Liquid phase microextraction (LPME) was performed with a metal sieve linked double syringe (MSLDS) system, which eliminated the need for a disperser solvent. In order to increase extraction efficiency for the analyte, all variable parameters were optimized and the system analytical performance of the proposed method was determined. Limit of detection and quantification (LOD and LOQ) values of pyridaphenthion were found to be 0.8 and 2.7 µg L-1, respectively. Compared to GC-MS system's analytical performance, the developed method provided approximately 273-folds improvement in the detection limit of the analyte. Applicability/accuracy of the developed analytical strategy was checked by recovery experiments carried out with soybean sprouts, and the results obtained were satisfactory.

LC-MS analysis of nitroguanidine compounds by catalytic reduction using palladium modified graphitic carbon nitride catalyst

The analysis of compounds of the nitroguanidine family at trace level poses an analytical challenge. Nitroguanidine, 1-methyl-3-nitroguanidine, and 1-methyl-3-nitro-1-nitrosoguanidine, which are addressed in this article, have low lipophilicity, with log(K_ow) equal to -0.89, - 0.84, and 0.68, respectively, and as such are not amenable for preconcentration from water. Liquid-liquid extraction and SPE fail to concentrate them from water and it is also not possible to extract them by ion exchange resin even after a pH change. Nitroguanidine and 1-methyl-3-nitroguanidine nitramines are explosives of growing use and thereby growing environmental concern due to lower detonation sensitivity compared to RDX. A sensitive method for the determination of nitroguanidine, 1-methyl-3-nitroguanidine, and 1-methyl-3-nitroso-1-nitroguanidine by reduction to the respective amines and subsequent hydrophobization by derivatization with 4-nitrobenzaldehyde followed by LC-ESI-MS analysis is described. Reduction by sodium borohydride using palladium modified graphitic carbon nitride (Pd/g-C₃N₄) provided improved sensitivity compared to the traditional palladium modified activated carbon due to the lower adsorption of the reduction products on the carbon nitride substrate. The limit of detection of the method was 10 ng L^-1 for nitroguanidine, and repeated analyses of spiked effluents and contaminated spring water gave relative standard deviations of 8.8% and 6.5%, respectively. The findings illuminate the great promise of Pd/g-C₃N₄ as a reduction catalyst for the determination of challenging hydrophilic organic contaminants.

Microcystin in source water: pollution characteristics and human health risk assessment

Frequent cyanobacterial blooms in eutrophic waters produce a variety of toxins such as microcystins (MCs), which are seriously harmful to waterbodies and human health. The spatiotemporal distribution characteristics of the MC-LR concentration in drinking water sources in seven river basins in China were investigated in this study. The removal rate of MC-LR in the purification process of water treatment plants and the human health risk of MC-LR in drinking water are also discussed. The results show that the detection frequency of MC-LR in source water was 55.46% and its concentration ranged from 0.06 × 10-3 to 52 × 10-3 μg L-1 (mean of 12.47 × 10-3 μg L-1), which are both below China's drinking water quality standard for algal toxins. The MC-LR concentration in lakes and reservoirs was higher than that in rivers, and exhibited an obvious spatiotemporal variation. The mean removal rate of MC-LR varied with river basin, and was also slightly higher for the advanced water treatment process (97.46%) in comparison to that of the conventional process (96.74%). The concentration of MC-LR in 8.26% of treated water samples was higher than that of raw water, thus indicating that MC-LR may be further released during the purification process. The risk index of MC-LR in treated water samples ranged from 2.29 × 10-3 to 8.40 × 10-3 (mean of 4.73 × 10-3), which corresponded to an extremely low level of risk. However, intensive monitoring should still be carried out in some high-concentration watersheds during the summer to ensure the safety of public drinking water.

Determination of common organophosphorus pesticides in the blood of children with acute leukaemia using a double-solvent system as a novel extractant for dispersive liquid-liquid microextraction

In this research, a new mode of dispersive liquid-liquid microextraction based on a double-solvent system (DLLME-DSS) was developed for the extraction and preconcentration of organophosphorus pesticides (OPPs) in the blood of children with acute leukaemia prior to determination by high-performance liquid chromatography-ultraviolet detection (HPLC-UV). In the present method, two long normal chain alcohols are mixed in a particular ratio, and then injected into the sample solution, which is magnetically stirred. In this case, the mixture of the two alcohols changes to a new aggregate extractant. This new double-solvent is used as an extractant, which has a higher extraction power than any of its components alone. Under the optimum conditions, the calibration graph was linear in the rage of 3-600 μg L-1 with detection limits of 1 to 2 μg L-1. Relative standard deviations (RSDs) including intra-day and inter-day of the method based on 7 replicate determinations of 100.0 μg L-1 for each analyte were in the range of 2.9-4.7% and 3.8-6.1%, respectively. The results proved that DLLME-DSS is a sensitive, very simple, inexpensive, environmentally friendly, rapid and efficient method for the preconcentration of trace amounts of OPPs in blood samples.

Characterization of the taste compounds in 20 pungent spices by high-performance liquid chromatography

High-performance liquid chromatography was used to determine the important taste compounds in 20 pungent spices commonly used in food, including seventeen free amino acids, four 5′-nucleotides and twelve organic acids. The equivalent umami concentration (EUC) and taste activity value (TAV) of the analyzed samples were calculated. The results showed that the content of total free amino acids ranged from 0.57 to 46.67 g/kg in 20 pungent spices. The content of total free amino acids in horseradish was the highest. The content of total 5′-nucleotides ranged from 0.80 to 4.30 g/kg, and chive contains the highest 5′-nucleotide content. Inosine 5′-monophosphate was detected in all 20 pungent spices. The content of total organic acids ranged from 9.37 to 339.58 g/kg. The total organic acids content of fieldmint was the highest (339.58 g/kg). Oxalic acid was detected in 18 pungent spices, except white pepper and chilli. The EUC of fieldmint (37.1 g MSG/100 g) was the highest in all 20 pungent spices, followed with peppermint (24.5 g MSG/100 g), and horseradish (18.4 g MSG/100 g). The TAVs of malic acid, lactic acid and 5′-AMP were higher than 1 in more than 10 spices. Lactic acid were higher than 1 in 13 spices, implying these compounds contributed greater to the flavor of pungent spices. The results of this work will provide references for the application value of pungent spices.

Occurrence and risk assessment of emerging contaminants in a water reclamation and ecological reuse project

Water reclamation and ecological reuse is gradually becoming a popular solution to address the high pollutant loads and insufficient ecological flow of many urban rivers. However, emerging contaminants in water reuse system and associated human health and ecological risks need to be assessed. This study determined the occurrence and human health and ecological risk assessments of 35 emerging contaminants during one year, including 5 types of persistent organic pollutants (POPs), 5 pharmaceutical and personal care products (PPCPs), 7 endocrine disrupting chemicals (EDCs) and 18 disinfection by-products (DBPs), in a wastewater treatment plant (WWTP) and receiving rivers, as well as an unimpacted river for comparison. Results showed that most of PPCPs and EDCs, especially antibiotics, triclosan, estrogens and bisphenol A, occurred frequently at relatively high concentrations, and they were removed from 20.5% to 88.7% with a mean of 58.9% via WWTP. The highest potential noncarcinogenic and carcinogenic risks in different reuse scenarios were assessed using maximal detected concentrations, all below the acceptable risk limits, with the highest total combined risk value of 9.21 × 10^-9 and 9.98 × 10^-7, respectively. Ecological risk assessment was conducted using risk quotient (RQ) method and indicated that several PPCPs, EDCs and haloacetonitriles (HANs) pose high risk (RQ > 1) to aquatic ecology in the rivers, with the highest RQ up to 83.8. The study suggested that ecological risks need to be urgently addressed by updating and optimizing the process in WWTPs to strengthen the removal efficiencies of emerging contaminants. The study can serve as a reference for safer water reuse in the future, while further studies could be conducted on the health risk of specific groups of people, exposure parameters in water reuse, as well as more emerging contaminants.

Sensitive determination of deferasirox in blood of patients with thalassemia using dispersive liquid-liquid microextraction based on solidification of floating organic drop followed by HPLC-UV

Deferasirox is an oral iron chelator that has been on the market since 2005 and has been a suitable replacement for injectable chelators. It is important to check the amount of this drug in the blood of patients due to side effects. In this study, a new dispersive liquid-liquid microextraction based on solidification of floating drganic drop (DLLME - SFO) was applied to the extraction of deferasirox in the blood of patients with thalassemia prior to its analysis by high-performance liquid chromatography-ultraviolet detection (HPLC - UV). In this method, two long alcohols of the normal chain are mixed in a particular ratio, and then it is injected into the sample solution, which is on the magnetic stirrer. In this case, the mixture of the two alcohol changes to new double-solvent aggregate. This new double-solvent system is used as an extractant, which has a higher extraction power than any of its components alone. Under the optimum conditions, the calibration graph was linear in the rage of 0.2-200 μg L^-1 with detection limit of 0.06 μg L^-1. Repeatability (intra-day) and reproducibility (inter-day) of method based on seven replicate measurements of 100.0 μg L^-1of deferasirox were 3.8 % and 5.7 %, respectively. The results showed that DLLME - SFO is a very simple, inexpensive, environmental friendly, sensitive and efficient analytical method for the determination of trace amount of drugs in biological samples and suitable results were obtained.

High-Throughput Liquid-Liquid Extractions with Nanoliter Volumes

Current methods for liquid-liquid extractions generally require microliter to milliliter volumes of solvents and sample, long equilibration times, and manual procedures. Extraction methods for samples in microfluidic systems have been limited as this tool is difficult to implement on the nanoliter or smaller scale. We have developed slug-flow nanoextraction (SFNE), a method based on droplet microfluidics that allows multiple liquid-liquid extractions to be performed simultaneously in a capillary tube, using only 5 nL of sample and extraction solvent per extraction. Each two-phase slug is segmented from the others by immiscible carrier fluid. We found rapid extractions (<5 s), partition coefficients matching those achieved at larger scale extractions, and potential to preconcentrate samples through volume manipulation. This method was used to accurately and rapidly determine octanol-water partition coefficients (Kow), determining identical Kow as the shake-flask method for acetaminophen, Kow = 2.48 ± 0.02. The measurement, along with calibration and 12 replicates, was complete within 5 min, consuming under 150 nL of solvent and sample. The method was also applied to extract analytes from complex biological samples prior to electrospray ionization-tandem mass spectrometry (ESI-MS/MS) at a rate of 6 s per sample, allowing for simultaneous determination of five different drugs spiked into human plasma, synthetic urine (SU), and artificial cerebral spinal fluid (aCSF) using ethyl acetate as the extraction phase. The signal-to-noise (S/N) for analytes improved up to 19-fold compared to direct ESI-MS of single-phase droplets (aqueous plugs segmented by carrier fluid), with limits of detection down to 7 nM (35 amol).

Development of an efficient sample preparation method for the speciation of Se(iv)/Se(vi) and total inorganic selenium in blood of children with acute leukemia

In this study, a new solidified deep eutectic solvent microextraction (SDES-ME) method was applied for the speciation of Se(iv), Se(vi) and total inorganic selenium in the blood of children with leukemia from Kermanshah, Iran, prior to analysis by iridium-modified tube graphite furnace atomic absorption spectrometry (GFAAS).

Data on concentrations of polycyclic aromatic hydrocarbons (PAHs) in roasted and fried chicken - A case study: Bushehr, Iran

In the present data article, the presence and amount of 16 polycyclic aromatic hydrocarbons (PAHs) were surveyed in Iranian roasted and fried chicken in Bushehr restaurants. For data collection, 73 samples (including 36 Roasted chicken and 37 Fried chicken samples) were collected from local restaurants and various retail outlets of Bushehr, Iran. The concentrations of 16 PAHs were determined by gas chromatography-mass spectrometry (GC–MS). Results indicated that the total PAHs level was ranged from 4.20 to 32.29 mg/kg and 2.06–19.65 µg/kg in Roasted and Fried chicken, respectively. The differences in PAHs levels were observed among charcoal and gas-roasted samples as well as between chicken fried with different oil types. benzo[a] pyrene(BaP), an appropriate marker for occurrence and potential toxicity of PAHs food matrixes was found in all roasted samples ranged from 1.41–5.71 mg/kg and all fried samples in range of 0.9–3.32 µg/kg. Moreover, unsaturated fatty acid (UFA) and saturated fatty acid (SFA) ratios in the in the used vegetable oils had an important role in the generation of PAHs in fried chicken.

Development of a liquid-phase microextraction based on the freezing of a deep eutectic solvent followed by HPLC-UV for sensitive determination of common pesticides in environmental water samples

In this research, a new extraction method based on liquid-phase microextraction and the freezing of deep eutectic solvent has been developed for the determination of pesticides in water prior to their analysis by HPLC-UV.

Optimization of a new methodology for trace determination of elements in biological fluids: Application for speciation of inorganic selenium in children's blood

The continuous sample drop flow microextraction (CSDFME) joined with the iridium-modified tube graphite furnace atomic absorption spectrometry (GFAAS) has been developed as a highly sensitive technique for the speciation of selenium in blood samples. In this method 32.0μl carbon tetrachloride is transferred to the bottom of a conical sample cup. Then the 5.0ml of aqueous solution transforms to fine droplets while passing through the organic solvent. At this stage, Se(IV)-APDC hydrophobic complex is extracted into the organic solvent. After extraction, the conical sample cup is transferred to the GFAAS and 20μl of extraction solvent was injected into the graphite tube by the aim of autosampler. Under the optimum conditions, the calibration graph was linear in the range of 0.06-3.0μgl^-1 with detection limit of 0.02μgl^-1. The enrichment factor and enhancement factor were 106 and 91, respectively. Repeatability (intra-day) and reproducibility (inter-day) of method based on seven replicate measurements of 2.5μgl^-1 of selenium were 3.7% and 4.2%, respectively. Total inorganic Se(IV, VΙ) was measured after reduction of Se(VΙ) with gentle boiling in 5M HCl medium for 50min and adjusting pH to 3, and the concentration of Se(VΙ) was calculated by subtracting the Se(IV) concentration from the total selenium concentration.

Sensitive determination of atorvastatin in human plasma by dispersive liquid-liquid microextraction and solidification of floating organic drop followed by high-performance liquid chromatography

A novel and sensitive dispersive liquid-liquid microextraction method based on the solidification of the floating organic drop combined with high-performance liquid chromatography and ultraviolet detection was used for the determination of atorvastatine in blood serum samples. The chromatographic separation of atorvastatin was carried out using methanol as the mobile phase organic modifier. Various parameters affecting the extraction efficiency were optimized, such as the kind and volume of extraction solvent (1-undecanol) and disperser solvent (acetonitrile), pH, and the extraction time. The calibration curve was linear in the range of 0.2-6000 μg/L of atorvastatin (r(2) = 0.995) with a limit of detection of 0.07 μg/L. The relative standard deviation for 100 μg/L of atorvastatin in human plasma was 8.4% (n = 4). The recoveries of plasma samples spiked with atorvastatin were in the range of 98.8-113.8%. The obtained results showed that the proposed method is fast, simple, and reliable for the determination of very low concentrations of atorvastatin in human plasma samples.