Krishna K. Verma

Recent advances in applications of single-drop microextraction: A review

During the past fifteen years since its introduction, single-drop microextraction has witnessed incessant growth in the range of applications of samples preparation for trace organic and inorganic analysis. This was mainly due to the array of modes that are available to accomplish extraction in harmony with the nature of analytes, and to use the extract directly for analysis by diverse instrumental methods. Whilst engineering of novel sorbent materials has expanded the sample capabilities of rival method of solid-phase microextraction, the single-drop microextraction - irrespective of the mode of extraction - uses common equipment found in analytical laboratories sans any modification, and in a much economic way. The recent innovations made in the field, as highlighted in this review article in the backdrop of historical developments, are due to the freedom in operational conditions and practicability to exploit chemical principals for optimum extraction and sensitive determination of analytes. Literature published till July 2011 has been covered.

Determination of ammonia and aliphatic amines in environmental aqueous samples utilizing pre-column derivatization to their phenylthioureas and high performance liquid chromatography

Pre-column conversion of ammonia and a number of aliphatic amines into phenylthiourea or its derivatives by reaction with phenyl isothiocyanate, followed by HPLC, has been used for their determination in environmental waters. Optimum conversion was found when the reaction was carried out in sodium hydrogencarbonate–carbonate medium at 40°C for 15 min. Well separated peaks were obtained on a C18 column with an acetonitrile–water gradient (1 ml min–1) of 30% acetonitrile for an initial 5 min which was increased linearly to 100% over 15 min and then maintained isocratic for 5 min, the acetonitrile ratio finally being returned to 30% in 5 min. The derivatized analytes were subjected to off-line solid phase extraction on C18 sorbent. A linear calibration graph was obtained for 0.01–10 mg l–1 analytes with a correlation coefficient of 0.9954 for ammonia and in the range 0.9982–0.9996 for amines. The limit of detection for ammonia was 0.2 µg l–1 and for amines in the range 0.3–0.6 µg l–1. The method was applied to tap, underground, river and aquarium waters, the recovery being in the range 97–106% (RSD 1.8–4.5%). Many of the samples were found to contain more than the permissible limit of ammonia. Phenyl isothiocyanate is stable for long periods in aqueous medium over wide ranges of pH and temperature, and the resulting phenylthioureas have adequate retention on C18 sorbent and strong UV absorption, making this reagent suitable for the determination of amines in water.

Simultaneous determination of ammonia, aliphatic amines, aromatic amines and phenols at μg l−1 levels in environmental waters by solid-phase extraction of their benzoyl derivatives and gas chromatography-mass spectrometry

Low concentrations of phenols and amines in environmental waters and their low breakthrough volume during solid-phase extraction (SPE) hinder the detection of phenols and aromatic amines, whereas ammonia and aliphatic amines are not suitable for SPE. Pre-column derivatization to arylbenzoates and N-alkyl- or N-arylbenzamides and their GC-MS is proposed to separate and determine phenols and amines in aqueous samples in the range 0.1–100 μg l−1 with correlation coefficients in the range 0.9910–0.9992. The limit of detection ranged from 7 to 39 ng l−1 for most analytes (90 ng l−1 for 2,3,6-trichlorophenol and 20 μg l−1 for ammonia) when 80 ml of sample were preconcentrated, after derivatization, on a styrene–divinylbenzene copolymer sorbent. The developed method was applied to spiked drinking water, groundwater and river water samples, and was used to detect halo-phenols in paper mill effluents. The average recovery ranged from 96 to 110% with RSD of 4–12%. The described method is rapid and can be applied to control the water quality of environmental waters with respect to three important classes of organic pollutants and ammonia.

Liquid-phase microextraction and GC for the determination of primary, secondary and tertiary aromatic amines as their iodo-derivatives

Presence of iodine in aromatic amines, introduced by their reaction with iodine, and other electron withdrawing substituents such as chlorine and nitro, has been found to afford excellent liquid-phase microextraction (LPME) in toluene and separation by gas chromatography in the determination of primary, secondary and tertiary aromatic amines. The effect is due to decreased basic nature of amines when electronegative substituents are present. Single drop microextraction (SDME) of the amines in 2mul of toluene and injection of the whole extract into GC, or LPME into 50mul of toluene and injection of 2mul of extract, were used. LPME has been found more robust and to give better extraction in shorter period than SDME. In SDME-GC-FID, the average correlation coefficient was 0.9939 and average limit of detection 25mugl(-1) (range 12-61mugl(-1)) whereas the corresponding values in LPME-GC-MS were, respectively, 0.9953 and 33ngl(-1) (range 18-60ngl(-1)). The method has been applied to determine aromatic amines in river water, dye factory effluents and food dye stuffs. The LPME was found as robust, rugged and simple extraction method.

Salt-assisted liquid-liquid microextraction with water-miscible organic solvents for the determination of carbonyl compounds by high-performance liquid chromatography.

A simple and rapid method has been reported for the determination of carbonyl compounds involving reaction with 2,4-dinitrophenylhydrazine and extraction of hydrazones with water-miscible organic solvent acetonitrile when the phase separation occurs by addition of ammonium sulphate, a process called salt-assisted liquid-liquid microextraction. The extract was analyzed by high-performance liquid chromatography with UV detection at 360 nm. The procedure has been optimized with respect to solvent suitable for extraction, salt for phase separation between water and organic solvent, reaction temperature and reaction time. The method has been validated when a linear dynamic range was obtained between the amount of analyte and peak area of hydrazones in the range 7 microg-15 mg L(-1), the correlation coefficient over 0.9964-0.9991, and the limit of detection in the range 0.58-3.2 microg L(-1). Spiked water samples have been analyzed with adequate accuracy, and application of the method has been demonstrated in the analysis of benzaldehyde formed as oxidation product in pharmaceutical preparation where benzyl alcohol is used as preservative, and for a keto drug dexketoprofen.

Determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals by flow injection spectrophotometry: Matrix absorbance correction by treatment with sodium hydroxide

Two flow injection systems for the spectrophotometric determination of ascorbic acid at 245 nm have been described. On treatment with sodium hydroxide a fraction of the ascorbic acid was decomposed into substances, which do not absorb in UV region, and the decrease in signal measured. This was directly related to the amount of ascorbic acid present. The calibration graph was linear over the range 1-25 and 1-50 microg/ml in the two methods with a correlation coefficient of 0.9981 and 0.9994, respectively. The detection limit (2sigma) was 0.5 and 0.2 microg/ml, respectively. The RSD for 1 microg/ml standard was 2.5 and 1.8% (n = 6) in the two methods, and the sampling throughput 30/hr. The methods permitted the use of 6 microg/ml of 2-mercaptoethanol as an anti-oxidant and stabilizer for ascorbic acid, which is difficult to handle at its microg/ml level. Upon matrix absorbance correction, spiked samples that are known to contain UV-absorbing substances produced an average recovery of 101% with a RSD of 1.2%. The methods were used for the rapid and simple determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals and the results thus produced compared with those obtained by previously checked methods involving titration with iodine, chloranil 2,6-dichlorophenolindophenol, and HPLC. When there was a disagreement between the results, this was traced to the presence of substances which are known to interfere in comparison methods.

Headspace in-drop derivatization of carbonyl compounds for their analysis by high-performance liquid chromatography-diode array detection

A simple and rapid method has been reported for the determination of carbonyl compounds involving sample preparation by headspace single drop microextraction using 1-butanol as extraction solvent containing 2,4-dinitrophenylhydrazine for hydrazone formation, and direct transfer of the drop into the injector for high-performance liquid chromatography with diode array detection. An angle-cut polytetrafluoroethylene sleeve, 3mm x 0.5mm, was fixed at the tip of the syringe needle and this allowed the use of 7 microL drop of solvent drop for extraction and derivatization. The procedure has been optimized with respect to suitable solvent for headspace drop formation, drop volume, concentration of reagent, sample temperature, reaction time, and headspace-to-sample volume ratio. The method has been validated when rectilinear relationship was obtained between the amount of analyte and peak area ratio of hydrazones in the range 0.01-15 mg L(-1), the correlation coefficient over 0.996-0.999, and the limit of detection in the range 1.7-24.1 microg L(-1). Spiked real samples have been analyzed with adequate accuracy, and application has been demonstrated of the method for analysis of carbonyl compounds formed as oxidation products.

Solid-phase extraction combined with headspace single-drop microextraction of chlorophenols as their methyl ethers and analysis by high-performance liquid chromatography-diode array detection

Solid-phase extraction (SPE) of phenol and chlorophenols, their derivatization to methyl ethers, headspace single-drop microextraction (HS-SDME) of methyl ethers using 1-butanol as extraction solvent, and direct transfer of the drop into the injector for high performance liquid chromatography with diode array detection (HPLC-DAD) have been reported. A flanged-end polytetrafluoroethylene sleeve, 3 mm × 0.5mm, placed at the tip of the syringe needle, allowed the use of 10 μL solvent drop for extraction. The procedure has been optimized for variables involved in SPE and HS-SDME. A rectilinear relationship was obtained between the amount of chlorophenols and peak area ratio of their methyl ethers/internal standard (4-methoxyacetophenone) in the range 0.01-10 mg L(-1), correlation coefficient in the range 0.9956-0.9996, and limit of detection in the range 1.5-3.9 μg L(-1) when HS-SDME alone was used for sample preparation. When using coupled SPE and HS-SDME, the linear range obtained was 0.1-500 μg L(-1), correlation coefficient in the range 0.9974-0.9998, and the limit of detection in the range 0.04-0.08 μg L(-1). Spiked real samples have been analyzed with adequate accuracy, and application of the method has been demonstrated for the analysis of chlorophenols formed upon bamboo pulp bleaching.

Determination of iodide by derivatization to 4-iodo-N,N-dimethylaniline and gas chromatography–mass spectrometry

A real-time determination of iodide is proposed which involves the oxidation of iodide with 2-iodosobenzoate in the presence of N,N-dimethylaniline. The reaction is completed within 1 min to yield 4-iodo-N,N-dimethylaniline, which is extracted in cyclohexane and determined by GC-MS. It was also possible to determine iodine by derivatization in the absence of 2-iodosobenzoate, and iodate by its reduction with ascorbic acid to iodide and subsequent derivatization. A rectilinear calibration graph was obtained for 0.02-50 micrograms l-1 iodide with a correlation coefficient of 0.9998. The limit of detection was 8 ng l-1 iodide. The method was applied to the determination of iodate in iodized table salt and free iodide and total iodine in sea-water, and to spiked samples when the recovery was in the range 96.8-104.3% (RSD 1.9-3.6%). A sample clean-up by solid-phase extraction with a LiChrolut EN cartridge is proposed.

Liquid-phase microextraction and fibre-optics-based cuvetteless CCD-array micro-spectrophotometry for trace analysis.

Liquid-phase microextraction (LPME) has been investigated for trace analysis in the present work in conjunction with fibre-optic-based micro-spectrophotometry which accommodates sample volume of 1 microL placed between the two ends of optical fibres. Methods have been evolved for the determination of (i) 1-100 microM and 0.5-20 microM of thiols by single drop microextraction (SDME) and LPME in 25 microL of the organic solvent, respectively, involving their reaction with the Ellman reagent and ion pair microextraction of thiolate ion formed; (ii) 70 microg to 7 mg L(-1) of chlorine/chlorine dioxide by headspace in-drop reaction with alternative reagents, viz., mixed phenylhydrazine-4-sulphonic acid and N-(1-naphthyl)ethylenediamine dihydrochloride, o-dianisidine, o-tolidine, and N,N-diethyl-p-phenylenediamine; (iii) 0.2-4 mg L(-1) of ammonia by reaction with 2,4-dinitro-1-fluorobenzene to give 2,4-dinitroaniline which was diazotized and coupled with 1-naphthylamine, the resulting dye was subjected to preconcentration by solid-phase extraction and LPME; and (iv) 25-750 microg L(-1) of iodide/total iodine by oxidation of iodide by 2-iodosobenzoate, microextraction of iodine in organic solvent, and re-extraction into aqueous starch-iodide reagent drop held in the organic phase. LPME using 25-30 microL of organic solvent was found to produce more sensitive results than SDME. The cuvetteless spectrophotometry as used in combination with sample handling techniques produced limits of detection of analytes which were better than obtained by previously reported spectrophotometry.