Gas chromatographic behaviour of urea herbicides

Summary Gas chromatographic conditions for determining eight phenylurea (chlortoluron, diuron, fluometuron, isoproturon, linuron, metabenzthiazuron, metobromuron and monuron) and one sulfonylurea (chlorsulfuron) herbicides were assessed. Degradation products of the herbicides formed in the injector were used for identification. Most phenylureas formed their respective carbamic acid methyl esters, metabenzthiazuron formed an aminobenzothiazol and chlorsulfuron formed an aminotriazine plus a phenylsulfonamide. On-column injection of standards using a BP10 capillary column was evaluated to identify the chromatographic behaviour. Detection limits ranged from 0.05 ng for chlorsulfuron to 3 ng for monuron with the NPD and, from 0.01 ng for chlorsulfuron to 5 ng for metabenzthiazuron with the ECD. The RSDs (n=4) were lower than 4% at the 12–25 ng level. The method was applied to the analysis of surface waters extracted with C18 Empore disks with recoveries higher than 85%. Each herbicide could be determined in water down to 0.1 μg·L⁻¹. Chlortoluron was found (11.4 μg·L⁻¹) in a water sample and its presence was confimed by gas chromatography-mass spectrometry.     

Critical study of fluorimetric determination of selenium in urine

Different steps for the fluorimetric determination of Se in urine have been investigated. A HNO₃---HClO₄ (4:1) mixture is useful for urine digestion, and reduction of Se(VI) to Se(IV) is effectively carried out with HCl (6M). Selenium(VI) present after the digestion process constitutes 14.5–36.6% of total Se. An optimum pH of 1.80±0.05 and the addition of 1 ml of 2,3-diaminonaphthalene (DAN) (0.1%, w/v) are established in the formation of Se—DAN complex. Heating to 60°C, a time of incubation of 15 min is recommended to assure the complete formation of Se—DAN complex. A volume of 5 ml of cyclohexane and vigorous shaking for 45 sec is necessary for the extraction process. With this optimized method, the detection limit of selenium was 0.82 μg/l., within-day precision for a 50.0 μg/l. standard solution and urine (27.3 μg/l.) were 2.4 and 2.7% and between-day for the urine was 3.9% (33.9 μg/l.). Analytical recovery of 0.5 ml of Se standard (250 μg/l.) added to 1 ml of urine was 99.9±2.9% (95.8–104.4, n = 12). Normal levels of selenium excretion in urine obtained from healthy people were 27.9±8.7 μg/day (13.2–44.1), not observing significant differences (P < 0.05) between sexes.     

Influence of the solvent on the gas chromatographic behaviour of urea herbicides

Summary Degradation products of chlorsulfuron, chlortoluron, diuron, fluometuron, isoproturon, linuron, metabenzthiazuron, metobromuron, and monuron formed in the gas chromatographic injector have been used for identification of the respective herbicides. Mass spectra of the derived compounds were obtained with a quadrupole mass spectrometric detector working in scan mode (20–450 amu). The compounds generated often depended on the solvent used for phenylurea herbicide injection (ethanol, methanol, dichloromethane, and acetonitrile). When methanol and ethanol were used as solvents the major products formed from phenylureas were carbamic acid esters. When acetonitrile or dichloromethane were used the main derivatives were phenylisocyanates. Chlorsulfuron and metabenzthiazuron, however, generated a triazine plus a phenylsulfonamide and a benzothiazolamine, respectively, irrespective of the solvent used. Linuron and diuron behaved similarly and gave degradation products with the same mass spectra. The thermal reactions occurred instantaneously in the injector block and were promoted by the high temperature selected (300°C). Detemination of the compounds derived from urea herbicides, by use of a 30 m BP10 column and a selected ion registering (SIR) program based on two or three ions, can be used for sensitive detection of the presence of urea herbicides in environmental extracts. With standards in methanol instrument detection limits ranged from 0.1 pg for chlorsulfuron (detected as 2-chlorobenzensulfonamide) to 1 pg for monuron and metobromuron (both detected as their carbamic acid methyl esters).RSD were below 9% at the 5 ng L⁻¹ level. The response was linearly dependent on quantily (r>0.9986) in the 5 ng L⁻¹ to 25 μg L⁻¹ range. Unequivocal identification of some phenylurea herbicides was not always possible because some herbicides with similar structures, for example diuron and linuron, gave the same derivative.     

Evaluation of solid-phase micro-extraction coupled to gas chromatography-mass spectrometry for the headspace analysis of volatile compounds in cocoa products

The aroma profile of cocoa products was investigated by headspace solid-phase micro-extraction (HS-SPME) combined with gas chromatography–mass spectrometry (GC–MS). SPME fibers coated with 100 μm polydimethylsiloxane coating (PDMS), 65 μm polydimethylsiloxane/divinylbenzene coating (PDMS-DVB), 75 μm carboxen/polydimethylsiloxane coating (CAR-PDMS) and 50/30 μm divinylbenzene/carboxen on polydimethylsiloxane on a StableFlex fiber (DVB/CAR-PDMS) were evaluated. Several extraction times and temperature conditions were also tested to achieve optimum recovery. Suspensions of the samples in distilled water or in brine (25% NaCl in distilled water) were investigated to examine their effect on the composition of the headspace. The SPME fiber coated with 50/30 μm DVB/CAR-PDMS afforded the highest extraction efficiency, particularly when the samples were extracted at 60 °C for 15 min under dry conditions with toluene as an internal standard. Forty-five compounds were extracted and tentatively identified, most of which have previously been reported as odor-active compounds. The method developed allows sensitive and representative analysis of cocoa products with high reproducibility. Further research is ongoing to study chocolate making processes using this method for the quantitative analysis of volatile compounds contributing to the flavor/odor profile.     

Solid-phase microextraction coupled with high-performance liquid chromatography for the determination of phenylurea herbicides in aqueous samples

Solid-phase microextraction coupled with high-performance liquid chromatography was successfully applied to the analysis of nine phenylurea herbicides (metoxuron, monuron, chlorotoluron, isoproturon, monolinuron, metobromuron, buturon, linuron, and chlorbromuron). Polydimethylsiloxane-divinylbenzene (PDMS-DVB, 60 microm) and Carbowax-templated resin (CW-TPR, 50 microm) fibers were selected from four commercial fibers for further study because of their better extraction efficiencies. The parameters of the desorption procedure were studied and optimized. The effects of the properties of analytes and fiber coatings, carryover, duration and temperature of absorption, pH, organic solvent and ionic strength of samples were also investigated. External calibration with an aqueous standard can be used for the analysis of environmental samples (lake water) using either PDMS-DVB or CW-TPR fibers. Good precisions (1.0-5.9%) are achieved for this method, and the detection limits are at the level of 0.5-5.1 ng/ml.     

Ultra-trace-level determination of polar pesticides and their transformation products in surface and estuarine water samples using column liquid chromatography-electrospray tandem mass spectrometry.

A method is developed for the determination of polar pesticides and their transformation products [atrazine, deethylatrazine, deisopropylatrazine, hydroxyatrazine, diuron, 3,4-dichlorophenylmethylurea, 3,4-dichlorophenylurea (DPU), monuron, bentazone, anthranil-isopropylamide, chloridazon, metolachlor] in surface, estuarine and sea water samples at the low ng/l level. Solid-phase extraction is combined off-line with column liquid chromatography-electrospray ionization tandem mass spectrometric detection (LC-ESI-MS-MS). The applicability of two solid-phase materials, i.e., LiChrolut EN cartridges and graphitized carbon black extraction disks, is evaluated. The influence of the organic solvent used in gradient LC, as well as the amount of co-extracted humic material on the ESI process is studied. The eluotropic strength of the organic solvent was found to have a distinct effect on the sensitivity of ESI-MS if coupled with LC gradient separations. Methanol gave much better results than acetonitrile and phenylurea compounds are more susceptible to solvent changes than triazines. Co-extracted humic material causes signal suppression in ESI-MS-MS detection. The degree of suppression depends upon the sample pH and the nature of the samples, i.e., surface or estuarine water. Detection limits in LC-ESI-MS-MS ranged from 0.2 to 2 ng/l, with the exception of DPU (8 ng/l). The applicability of the procedure was demonstrated by analyzing surface and estuarine water.     

Penicillamine determination using a tyrosinase micro-rotating biosensor

Tyrosinase [EC 1.14.18.1], immobilized on a rotating disk, catalyzed the oxidation of catechols to o-benzoquinone, whose back electrochemical reduction was detected on glassy carbon electrode surface at −150 mV versus Ag/AgCl/NaCl 3 M. Thus, when penicillamine (PA) was added to the solution, this thiol-containing compound participate in Michael type addition reactions with o-benzoquinone to form the corresponding thioquinone derivatives, decreasing the reduction current obtained proportionally to the increase of its concentration. This method could be used for sensitive determination of PA in drug and human synthetic serum samples. A linear range of 0.02–80 μM (r = 0.999) was obtained for amperometric determination of PA in buffered pH 7.0 solutions (0.1 M phosphate buffer). The biosensor has a reasonable reproducibility (R.S.D. < 4.0%) and a very stable amperometric response toward this compound (more than 1 month).     

Analysis of phenylurea herbicides from plants by GC/MS

Seven phenylurea herbicides (chlorbromuron, fluometuron, diuron, linuron, metobromuron, monolinuron, monuron) were isolated from weed plant materials by extraction with acidic aqueous solutions-the aqueous extract preconcentrated and cleaned-up by using a continuous SPE module. Elution with ethyl acetate resulted in >95% recovery of the herbicides from the LiChrolut-EN sorbent column. Separation of the seven herbicides was easily achieved working under fixed gas-chromatographic conditions and, although other peaks were also detected (corresponding to isocyanate degradation products), they showed different retention times and lower intensity than the parent compounds; in addition, as reproducible chromatograms in terms of herbicide-to-isocyanate product peak areas ratio were obtained, no derivatization was required. The analytical figures of merit of the GC/MS method in the selective ion monitoring mode are as follows: limits of detection from 0.5 to 5.0 ng ml(-1); precision (R.S.D.) approximately 7.0%; and preconcentration factor 100 (using 10 ml of sample extract). The proposed GC/MS method offers better resolution and selectivity than the conventional LC-UV method.     

On-line coupling of sequential injection extraction with restricted-access materials and post-column derivatization for sample clean-up and determination of propranolol in human plasma

The presented paper deals with a new methodology for direct determination of propranolol in human plasma. The methodology described is based on sequential injection analysis technique (SIA) coupled with solid phase extraction (SPE) column based on restricted access materials (RAM). Special RAM column containing 30 μm polymeric material—N-vinylacetamide copolymer was integrated into the sequential injection manifold. SIA–RAM system was used for selective retention of propranolol, while the plasma matrix components were eluted with two weak organic solutions to waste.

Due to the acid–basic and polarity properties of propranolol molecule and principles of reversed-phase chromatography, it was possible to retain propranolol on the N-vinylacetamide copolymer sorbent (Shodex MSpak PK-2A 30 μm (2 mm × 10 mm)). Centrifuged plasma samples were aspirated into the system and loaded onto the column using acetonitrile–water (5:95, v/v), pH 11.00, adjusted by triethylamine. The analyte was retained on the column while proteins contained in the sample were removed to waste. Interfering endogenous substances complicating detection were washed out by acetonitrile–water (15:85), pH 11.00 in the next step. The extracted analyte was eluted by means of tetrahydrofuran–water (25:75), pH 11.00 to the fluorescence detector (emission filter 385 nm). The whole procedure comprising sample pre-treatment, analyte detection and column reconditioning took about 15 min. The recoveries of propranolol from undiluted plasma were in the range 96.2–97.8% for three concentration levels of analyte. The proposed SIA–RAM method has been applied for direct determination of propranolol in human plasma.     

An improved bonded-polydimethylsiloxane solid-phase microextraction fiber obtained by a sol-gel/silica particle blend

A novel procedure for solid-phase microextraction fiber preparation is presented, which combines the use of a rigid titanium alloy wire as a substrate with a blend of PDMS sol–gel mixture/silica particles, as a way of increasing both the mechanical robustness and the extracting capability of the sol–gel fibers. The 30 μm average thick fibers displayed an improvement in the extraction capacity as compared to the previous sol–gel PDMS fibers, due to a greater load of stable covalently bonded sol–gel PDMS. The observed extraction capacity was comparable to that of 100 μm non-bonded PDMS fiber, having in this case the advantages of the superior robustness and stability conferred, respectively, by the unbreakable substrate and the sol–gel intrinsic characteristics. Repeatability (n = 3) ranged 1–8% while fiber production reproducibility (n = 3) ranged 15–25%. The presence of the silica particles was found to have no direct influence on the kinetics and mechanism of the extraction process, thus being possible to consider the new procedure as a refinement of the previous ones. The applicability potential of the devised fiber was illustrated with the analysis of gasoline under the context of arson samples.     

Use of self-plasticizing EVA membrane for potentiometric anion detection

A method based on solid-phase microextraction and gas chromatography flame photometric detector for the determination of organophosphorous pesticides (OPPs) in aqueous samples was described. Five kinds of commercially available fibers-7, 30 and 100 mum PDMS, 85 mum PA and 65 mum PDMS-DVB-were compared and 100 mum PDMS and 85 mum PA were the most sensitive fiber coatings for the analytes. The extraction time, extraction temperature, pH and content of NaCl were found to have significant influence on extraction efficiency. The optimized conditions were 100 mum PDMS fiber, 30 min extraction time at 40 degrees C, with 3% NaCl content and no pH adjustment. The linear range was 0.5-100 mug l(-1) for most of the analytes. The limits of detection (LODs) ranged from 0.049 mug l(-1) (for parathion) to 0.301 mug l(-1) (for carbophenothion) and RSD% of repeatability at the 10 mug l(-1) level were all below 8%. Environmental water samples were analyzed, but none of the analytes was detected. The recovery of spiked water samples was from 75.3 to 102.6%.     

Use of on-line liquid chromatography-nuclear magnetic resonance spectroscopy for the rapid investigation of flavonoids from Sorocea bomplandii

A HPLC method, using photochemically-induced fluorescence detection, is described for the separation and determination of four phenylurea herbicides including diuron, isoproturon, linuron and neburon. A post-column photoreactor, consisting of a reactor knitted around a 4 W xenon lamp, has been included between the column and the detector, in order to transform the non-fluorescent herbicides into fluorophors. The influence of mobile phase composition, flow-rate, pH, and buffer concentration has been studied. An acetonitrile–buffer solution of potassium phosphate dibasic of pH 7 and 0.01 M concentration (60:40, v/v), was selected as optimum. For the fluorimetric detection, optimal excitation/emission wavelengths 324/403, 301/433, 335/411 and 326/385 nm were selected for the determination of diuron, isoproturon, linuron and neburon, respectively. The detection limits ranged between 0.07 and 0.46 μg/ml, according to the compound.     

Rapid and sensitive determination of total vanadium inairborne particulates by an extraction—spectrometric method with N-benzoyl-N-phenylhydroxylamine

A method is described for the rapid and sensitive determination of 0.06 μg of total vanadium in airborne particulates collected in the filter bag of an air-cleaner or on a membrane filter attached to an air sampler. The method is based on decomposition of organic matter in the sample and membrane filter by successive use of nitric, sulfuric and perchloric acids, and decomposition of the suspended silicate residue in a sulfuric—hydrofluoric acid mixture, followed by the extraction of a vanadium—N-benzoyl-N-phenylhydroxylamine complex into chloroform for spectrophotometric measurement. A comparison with the troublesome fusion method for the residue is included.     

Trace analysis of chlorobenzenes in water samples using headspace solvent microextraction and gas chromatography/electron capture detection

In the present work, a rapid method for the extraction and determination of chlorobenzenes (CBs) such as monochlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene and 1,2,4-trichlorobenzene in water samples using the headspace solvent microextraction (HSME) and gas chromatography/electron capture detector (ECD) has been described. A microdrop of the dodecane containing monobromobenzene (internal standard) was used as extracting solvent in this investigation. The analytes were extracted by suspending a 2.5 μl extraction drop directly from the tip of a microsyringe fixed above an extraction vial with a septum in a way that the needle passed through the septum and the needle tip appeared above the surface of the solution. After the extraction was finished, the drop was retracted back into the needle and injected directly into a GC column. Optimization of experimental conditions such as nature of the extracting solvent, microdrop and sample temperatures, stirring rate, microdrop and sample volumes, the ionic strength and extraction time were investigated. The optimized conditions were as follows: dodecane as the extracting solvent, the extraction temperature, 45 °C; the sodium chloride concentration, 2 M; the extraction time, 5.0 min; the stirring rate, 500 rpm; the drop volume, 2.5 μl; the sample volume, 7 ml; the microsyringe needle temperature, 0.0 °C. The limit of detection (LOD) ranged from 0.1 μg/l (for 1,3-dichlorobenzene) to 3.0 μg/l (for 1,4-dichlorobenzene) and linear range of 0.5–3.0 μg/l for 1,2-dichlorobenzene, 1,3-dichlorobenzene and from 5.0 to 20.0 μg/l for monochlorobenzene and from 5.0 to 30 μg/l for 1,4-dichlorobenzene. The relative standard deviations (R.S.D.) for most of CBs at the 5 μg/l level were below 10%. The optimized procedure was successfully applied to the extraction and determination of CBs in different water samples.     

On‐line extraction and determination of two herbicides: Comparison between two modes of three‐phase hollow fiber microextraction

Two different modes of three‐phase hollow fiber liquid‐phase microextraction were studied for the extraction of two herbicides, bensulfuron‐methyl and linuron. In these two modes, the acceptor phases in the lumen of the hollow fiber were aqueous and organic solvents. The extraction and determination were performed using an automated hollow fiber microextraction instrument followed by high‐performance liquid chromatography. For both three‐phase hollow fiber liquid‐phase microextraction modes, the effect of the main parameters on the extraction efficiency were investigated and optimized by central composite design. Under optimal conditions, both modes showed good linearity and repeatability, but the three‐phase hollow fiber liquid‐phase microextraction based on two immiscible organic solvents has a better extraction efficiency and figures of merit. The calibration curves for three‐phase hollow fiber liquid‐phase microextraction with an organic acceptor phase were linear in the range of 0.3–200 and 0.1–150 μg/L and the limits of detection were 0.1 and 0.06 μg/L for bensulfuron‐methyl and linuron, respectively. For the conventional three‐phase hollow fiber liquid‐phase microextraction, the calibration curves were linear in the range of 3.0–250 and 15–400 μg/L and LODs were 1.0 and 5.0 μg/L for bensulfuron‐methyl and linuron, respectively. The real sample analysis was carried out by three‐phase hollow fiber liquid phase microextraction based on two immiscible organic solvents because of its more favorable characteristics.     

Implementation of flow-through multi-sensors with bead injection spectroscopy: fluorimetric renewable surface biparameter sensor for determination of berillium and aluminum

For the first time a new, sensitive, and simple bead injection spectroscopy–flow injection analysis (BIS–FIA) system with spectrofluorimetric detection is described for the sequential determination of two metals. The sensor is based on the alternate use of two carriers and a commercially available flow cell (Hellma 176-QS). The flow cell is filled by injecting in the flow system 500 μl of a homogeneous bead suspension of an appropriate solid support (Sephadex QAE A-25) previously loaded with the fluorogenic reagent morin (2′,3,4′,5,7-pentahydroxyflavone). A sequential reaction of Al(III) and Be(II) with morin (immobilized on beads) to form their fluorescent complexes is performed on the bead sensing support and their respective fluorescence emission monitored, after doing two successive injections from the mixture solution. Firstly, Al(III) could be determined in the sample using 0.5 M NaCl/HCl, pH 6 as carrier. Then, the carrier solution was changed (0.3 M NaCl/NaOH, pH 12) making possible the elution of Al(III) and the restoration of the baseline, then allowing the reaction of Be(II). At the end of the analysis, beads are automatically discarded from the flow cell, by reversing the flow, and transported out of the system. The analytical signals are measured at an excitation wavelength of 440 nm and an emission wavelength of 520 nm. Using a sample volume of 600 μl, the analytical signal showed a very good linearity in the range 0.1–8 ng ml⁻¹ and 0.1–1 μg ml⁻¹ with detection limits of 0.024 ng ml⁻¹ and 0.010 μg ml⁻¹ for Be(II) and Al(III), respectively. R.S.D.s (%) lower than 5% were obtained for both analytes and the selectivity was improved using EDTA as masking reagent. The sensor was satisfactorily applied to the determination of these metals in waters and simulated alloy samples.     

Separation and determination of amitriptyline and nortriptyline by dispersive liquid-liquid microextraction combined with gas chromatography flame ionization detection

Dispersive liquid–liquid microextraction (DLLME) coupled with gas chromatography–flame ionization detection (GC–FID) was applied for the determination of two tricyclic antidepressant drugs (TCAs), amitriptyline and nortriptyline, from water samples. This method is a very simple and rapid method for the extraction and preconcentration of these drugs from environmental sample solutions. In this method, the appropriate mixture of extraction solvent (18 μL Carbon tetrachloride) and disperser solvent (1 mL methanol) are injected rapidly into the aqueous sample (5.0 mL) by syringe. Therefore, cloudy solution is formed. In fact, it is consisted of fine particles of extraction solvent which is dispersed entirely into aqueous phase. The mixture was centrifuged and the extraction solvent is sedimented on the bottom of the conical test tube. 2.0 μL of the sedimented phase is injected into the GC for separation and determination of TCAs. Some important parameters, such as kind of extraction and disperser solvent and volume of them, extraction time, pH and ionic strength of the aqueous feed solution were optimized. Under the optimal conditions, the enrichment factors and extraction recoveries were between 740.04–1000.25 and 54.76–74.02%, respectively. The linear range was (0.005–16 μg mL⁻¹) and limits of detection were between 0.005 and 0.01 μg mL⁻¹ for each of the analytes. The relative standard deviations (R.S.D.) for 4 μg mL⁻¹ of TCAs in water were in the range of 5.6–6.4 (n = 6). The performance of the proposed technique was evaluated for determination of TCAs in blood plasma.     

Digestion treatments and risks of Cr(III)–Cr(VI) interconversions during Cr(VI) determination in soils and sediments—a review

Threshold values for Cr(VI) in various types of solid matrices have been set up to protect human health and biota. To ascertain the compliance of solids with these limits different types of extractants and different conditions of pH and temperature have been proposed in the literature. These extraction procedures are reviewed and their potentialities in quantitatively extracting Cr(VI) from solids without inducing undesired Cr(VI)–Cr(III) interconversions during the extraction are carefully evaluated. This evaluation takes into account the knowledge of the kinetics of most important redox reactions of chromium gathered in recent years. Among possible Cr(VI) reductants made available during the digestion, a number of species including Fe(II), sulphide, sulfite and humic matter were considered, while oxidants included hydrogen peroxide, dissolved oxygen, manganese oxides. Theoretical calculations suggest that pH higher than 10, high temperature and high concentrations of carbonate and magnesium ions minimize Cr(III)–Cr(VI) interconversions. The EPA Method 3060A meets these basic requirements. However, the applicability of this method to the analysis of Cr(VI) in soil and sediment samples, whose extracts may suffer from the interference by humic matter, is questionable.     

Removal of humic matter interference in the determination of Cr(VI) in soil extracts by the diphenylcarbazide method

Alkaline digestion of soil samples, which is recommended for minimizing Cr(III)–Cr(VI) interconversions during the extraction of Cr(VI), may also solubilize humic matter (HM). The latter is responsible for both positive and negative interference in the analysis of Cr(VI) in the extract by the diphenylcarbazide (DPC) method. Humic compounds indeed absorb light at 540 nm as the Cr-DPC product and are also able to rapidly reduce Cr(VI) under the pH conditions of the standard DPC method. To prevent any risk of interference and make the DPC method applicable to soil extracts, a new protocol is proposed. This consists of three successive steps: (1) extraction of Cr(VI) by the U.S.EPA method 3060A in the presence of Mg²⁺; (2) batch removal of solubilised HM by the XAD-7 sorbent at pH 3.0 ± 0.1 with 3–5 min contact time; (3) analysis of Cr(VI) with the DPC method at pH 3.0 ± 0.1. The application of this new protocol to the soil product SQC-012 Lot 4 certified by R.T. Corporation (RTC, USA) gave significantly lower Cr(VI) concentrations and smaller variability compared to certified values (46.5 ± 2.3 instead of 153 ± 32.6 mg/kg). The new protocol was validated by Cr(VI) and Cr(III) spikes either to the soil/extractant suspension or to the exctract. Cr(VI) results in the wide range of acceptance limits (104–202 mg/kg) reported by RTC were only obtained when the U.S.EPA method 3060A without the addition of Mg²⁺ and DPC analysis at pH 1.0 were applied. The latter procedure appears questionable since it leads to largely variable results which reflect the complex role played by humic matter in the determination of Cr(VI) and the instability of Cr(III) during the extraction.     

Use of an open-tubular trapping column as phase-switching interface in on-line coupled reversed-phase liquid chromatography—capillary gas chromatography

The applicability of open-tubular traps for phase switching in coupled RPLC—GC was studied. The phase-switching process involves sorption of the analytes of interest from a methanol—water mobile phase into the stationary phase of an open-tubular column, removal of the aqueous phase by purging the trap with nitrogen and desorption of the analytes with hexane. Water elimination carried out in this manner appears to be highly efficient. In the sorption step the sampling flow-rate and the capacity factors of the analytes in the trap are critical parameters. Using a 2 m × 0.32 mm I.D. trap with a swollen 5-μm stationary phase at flow-rates not exceeding 100 μl/min, polycyclic aromatic hydrocarbons are trapped quantitatively from 300 μl of aqueous phases containing up to 65% (v/v) of methanol. For desorption 70–125 μl of hexane are needed. These volumes are easy to handle in solvent elimination carried out using a PTV injector prior to transfer of the analytes to a GC column.