Isolation methods for the volatile components of grapefruit juice. Distillation and solvent extraction methods

Summary Five different methods including solvent extraction, distillation and simultaneous distillation-solvent extraction (SDE) have been compared for the isolation of the volatile components of grapefruit juice. The search for an adequare procedure was directed to obtaining aroma concentrates with an odour resembling that of the original grapefruit juice. The methods have also been compared in terms of some experimental parameters. The concentrates have been analyzed by fused-silica, capillary gas chromatography and the GC patterns have been compared in terms of the recovery efficiencies for high-, medium-, and low-volatility components. The SDE methods gave the best results, particularly when using the apparatus proposed by Godefrootet al.     

Comparison of two methods based on dynamic head-space for GC-MS analysis of volatile components of cheeses

Summary Two methods based in dynamic headspace sampling have been compared for GC/MS analysis of volatile components in hard cheeses (Manchego and other ewe’s milk varieties). In the first approach a purge & trap concentrator allowed volatile on-line determination with reduced sample handling. The second method consisted of a manual device for trapping dinamically purged volatiles, which were then anaysed by using an automatic thermal desorption system, coupled on-line with a GC-MS. The influence of the most significant operating parameters (desorption times, flows and temperatures) on recovery and repeatability was studied for both methods. Automatic purge & trap gave the best sensitivity and repeatability for high valatility components, probably because its on-line operation mode, while the second procedure allowed the determination of a greater number of volatile components and gave better yields for fatty acids and other medium volatility components.     

A simple method for concentrating volatile flavor compounds in aqueous systems using RP-HPLC

Summary Volatile flavor compounds in aqueous model systems, distillates of coffee extract and juices such as apple, tomato, pineapple and grape were adsorbed on an ODS column during HPLC and monitoring absorption at 280nm. The adsorbed volatile compounds were eluted with a small volume of ethanol. Volatile compounds in the eluates were concentrated 22 to 100 times those in the distillates. Recovery of compounds was 50–90%. Breakthrough of specific compounds such as -valerolactone, furfuryl acetate and t-2-hexenal in the model systems and several compounds in the juice distillates was detected by capillary GC. Pattern similarity calculated between the GC profiles of a juice distillate and the corresponding eluate gave a qualitative comparison. This novel and versatile method utilizing conventional RP-HPLC may be widely applicable for concentrating volatiles in aqueous systems, at levels from ppm to ppb.     

Chromatographic determination of D-amino acids as native constituents of vegetables and fruits

Summary Free D-amino acids (D-AA) were detected as native constituents in juices of vegetables (cultivars of cabbage, tomato, carrot, garlic) and fruits (organes, clementine, grapefruit, lemon, apples, pear, grapes) using gas chromatography (GC) or high-performance liquid chromatography (LC). For investigation by GC, AA enantiomers were converted into theirN(O)-pentafluoropropionyl 2-propyl esters and resolved on a Chirasil-L-Val capillary column. For determination by LC, precolumn derivatization of AA enantiomers usingo-phthaldialdehyde together with the chiral thiolsN-isobutyryl-L-cysteine orN-isobutyryl-D-cysteine and fluorescence detection of the diastereomeric isoindole derivatives, resolvable on an octadecylsilyl stationary phase, were used. D-Ala (0.6–3.8%) was detected in all freshly pressed plant juices usually in the highest relative amounts. Other D-AA detected were D-Asx (0.1–1.9%), D-Glx (0–1.3%), D-Ser (0–1.7%), D-Arg (0.4–1.2%, in grapes, orange, grapefruit, and clementine) and D-Leu and D-Val (1% in cabbage). Absolute amounts of native D-AA were totally 28–57 mol L^–1 in fruit juices, 14.5 mol L^–1 in a tomato juice and 8.5 mol L^–1 in a carrot juice.Presented in part as lecture at 3rd International Congress on Amino Acids, Peptides and Analogues, August 23–27, 1993, Vienna; and as posters at 31st Scientific Meeting of German Society of Nutrition, Giessen, March 17th and 18th, 1994 [19]; and at Analytica Conference, April 19–22, 1994, Munich [20].     

Headspace solid-phase microextraction (HS-SPME): a microscale sampling technique for determination of monoterpene hydrocarbons in coniferous needles by gas chromatography/mass spectrometry (GC/MS)

A headspace solid-phase microextraction (HS-SPME) technique has been applied for the microscale sampling (single needle for Picea omorica, two needles of Picea abies) of volatile monoterpene hydrocarbons (MTHs) from conifer needles. A simple device consisting of a closed headspace vial equipped with an integral cutting device was used for the collection, homogenisation, and HS-SPME sampling. The highly sensitive gas chromatographic/mass spectrometric (GC/MS) analyses (LODs in tenths of ng g^–1 FW were obtained for 3×S/N criteria) of individual needles confirmed the space distribution of MTHs in different parts of a single sprout (base, centre, apex) and among the left, central and right sprout of a whorl. The highest concentrations of MTHs were found in the apical (leading) sprout (S) of a whorl. The serious increase of MTHs content near the base of the left (SNL) sprout and nearly zero concentrations in the central parts of both the side sprouts of P. abies corresponded to the morphology of the whorl. On the other hand, similar concentrations of MTHs were found in the apex of the side (SNL and SNR) sprouts. The distributions of MTHs obtained after HS-SPME and solvent extraction followed the similar trends for both species. HS-SPME is more suitable for the evaluation of distribution of volatiles in microscale experiments due to the more precise separation of needles into correct categories and elimination of averaging effects of the larger quantities of needles needed for the solvent extraction. The precision was improved by one order of magnitude due to the reduction of uncontrolled losses of volatile species during the sample treatment.     

Cholinesterase sensors based on screen-printed electrodes for detection of organophosphorus and carbamic pesticides

Cholinesterase sensors based on screen-printed electrodes modified with polyaniline, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and Prussian blue have been developed and tested for detection of anticholinesterase pesticides in aqueous solution and in spiked grape juice. The influence of enzyme source and detection mode on biosensor performance was explored. It was shown that modification of the electrodes results in significant improvement of their analytical characteristics for pesticide determination. Thus, the slopes of the calibration curves obtained with modified electrodes were increased twofold and the detection limits of the pesticides were reduced by factors of 1.6 to 1.8 in comparison with the use of unmodified transducers. The biosensors developed make it possible to detect down to 2×10^–8 mol L^–1 chloropyrifos-methyl, 5×10^–8 mol L^–1 coumaphos, and 8×10^–9 mol L^–1 carbofuran in aqueous solution and grape juice. The optimal conditions for grape juice pretreatment were determined to diminish interference from the sample matrix.Abbreviations ChE Cholinesterase - TCNQ 7,7,8,8-Tetracyanoquinodimethane - ChO Choline oxidase - AChE Acetylcholinesterase - BChE Butyrylcholinesterase - BSA Bovine serum albumin - 2-PAM 2-Pyridine aldoxime methiodide     

Analysis of High-Purity Volatile Compounds

The current status of the analysis of high-purity volatile substances is considered. Two types of impurities in high-purity volatile substances were distinguished: molecularly dissolved substances and suspended particles. The main factors that restrict the limiting capabilities of analytical techniques were revealed. The attained detection limits were 10^–8–10^–10% for metal impurities, 10^–4–10^–8% for organic substances, 10^–5–10^–9% for water, and 10^–5–10^–7 for permanent gases. Suspended particles of 0.04–0.003 m in size were determined by light scattering.     

Ionic interactions in the system HBr + BaBr2 + H2O at 25°C

Electromotive-force measurements are reported for mixtures of HBr and BaBr₂ in water at 25°C at constant total ionic strengths. Activity coefficients for HBr were analyzed using Pitzer's equations and Scatchard's neutral-electrolyte treatment. Both methods gave comparable results if the effects of higher-order electrostatic terms on the H⁺–Ba⁺² interactions were included in Pitzer's treatment. Furthermore, the value of the H⁺–Ba⁺² interaction parameter, (s), which should be independent of the anion, is indeed close to that for binary mixtures of these ions having chloride as common ion.     

Solid-phase microextraction and gas chromatography–mass spectrometry for rapid characterisation of semi-hard cheeses

A headspace solid-phase microextraction (HS-SPME) method in combination with gas chromatography–mass spectrometry (GC–MS) has been used for extraction and identification of components of the volatile fraction of Provola dei Nebrodi, a typical semi-hard Sicilian cheese. Cheese samples from different producers and at different ripening stages have been examined. The effects of various conditions (e.g. sample volume, sample headspace volume, sample heating temperature, extraction time, etc.) on extraction efficiency were studied in order to optimise the technique. The technique used made it possible to identify 61 components: fatty acids from C₂ to C₁₄ and their esters, aldehydes, alcohols, methyl ketones, -lactones, aromatic compounds, hydrocarbons and terpenes. The main components were butanoic, hexanoic and octanoic acids. The linear free fatty acids (FFA) from C₂ to C₁₀ were quantified by using the standard addition method. Calibration curves constructed for the FFA spiked into cheese were highly linear with a correlation coefficient R²>0.998. Significant statistical differences (P0.05) were evident for the even-carbon-number fatty acids during ripening.     

Matrix effects of leachates in the determination of volatile priority pollutants by headspace analysis

Summary In order to investigate the matrix effects of a hazardous waste landfill leachate in the analysis of priority pollutants by headspace gas chromatography, spiking experiments were performed (a) with the untreated leachate, (b) with the leachate after separation of oil and (c) with blanc water samples as a reference. A standard mixture containing volatile chlorinated hydrocarbons, benzene and alkylated benzenes (100–200 g) were spiked into 50 ml-samples of (a)–(c). The samples were thermostated at 40°C for 20 min and analyzed by headspace GC. Serious matrix effects were observed for all reference compounds, resulting in significant enhancement or depression of the corresponding peak areas compared with the spiked water control samples. The data sets are discussed with respect to the contents of leachate oil and chloride. From the findings of this study, recommendations for the application of headspace analysis to leachates are given.     

Spectrophotometric determination of ciprofloxacin in pure form and in tablets through charge-transfer complexation reactions

Three simple, quick and sensitive spectrophotometric methods are described for the determination of ciprofloxacin. The methods are based on the reaction of this drug as ann-electron donor with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), 7,7,8,8,-tetracyanoquinodimethane (TCNQ) andp-chloranil (CL) as -acceptors to give highly coloured complex species. The coloured products are quantitated spectrophotometrically at 460, 843 and 550 nm for DDQ, TCNQ and CL, respectively. Optimization of the different experimental conditions is described. Beer's law is obeyed in the concentration ranges 5–50, 1.5–15 and 20–200 g ml^–1 ciprofloxacin, but the concentration ranges for best accuracy are 10–48, 2.5–15 and 35– 195 g ml^–1 of drug for DDQ, TCNQ and CL, respectively. The relative standard deviations are less than 1.5%. Applications of the suggested methods to ciprofloxacin tablets are presented and compared with the USP method. The stability constants of the 11 DDQ and CL complexes were 1.086 × 10⁴ and 2.581 × 10⁴ lmol^–1, respectively, whereas for the 12 TCNQ complex it was 3.62 × 10⁸1. mol^–1.     

Determination of Aniline in Water by Microwave-Assisted Headspace Solid-Phase Microextraction and Gas Chromatography

Determination of aniline in wastewater was investigated by microwave-assisted headspace solid-phase microextraction (MA-HS-SPME), for one-step in-situ sample preparation, and gas chromatography. Aniline in the water was evaporated into the headspace under the action of microwave irradiation and adsorbed directly by the SPME fiber. After desorption in the GC injection port and gas chromatography aniline was detected by FID. Conditions affecting the extraction efficiency, for example the pH of the water, addition of salt, microwave power and irradiation time, and desorption conditions were investigated. Experimental results indicated that adjustment of the pH of the water sample to 12 and headspace SPME sampling with a PDMS-DVB fiber under medium–high power irradiation (345 W) for 3 min resulted in the best extraction efficiency. Desorption of aniline was optimum when the SPME fiber was heated at 230 °C for 3 min. The detection limit was approximately 0.01 g mL^–1. The proposed method is a simple, fast, and organic-solvent-free procedure for analysis of aniline in water. Application was illustrated by analysis of aniline in wastewater from a polymer factory.     

Determination of pyrimethanil and kresoxim-methyl in soils by headspace solid-phase microextraction and gas chromatography-mass spectrometry

A method using headspace solid-phase microextraction (HS-SPME) then gas chromatography–mass spectrometry with selected ion monitoring (GC–MS, SIM) has been developed for determination of trace amounts of the fungicides pyrimethanil and kresoxim-methyl in soil and humic materials. Both fungicides were extracted on to a fused-silica fibre coated with 85 m polyacrylate (PA). Response-surface methodology was used to optimise the experimental conditions. For soil samples the linear dynamic range of application was 0.004–1.000 g g^–1 for pyrimethanil and 0.013–1.000 g g^–1 for kresoxim-methyl. The detection limits were 0.001 g g^–1 and 0.004 g g^–1 for pyrimethanil and kresoxim-methyl, respectively. HP-SPME–GC–MS analysis was highly reproducible—relative standard deviations (RSD) were between 6.7 and 12.2%. The method was validated by analysis of spiked matrix samples and used to investigate the presence of pyrimethanil and kresoxim-methyl above the detection limits in soil and humic materials.     

Detection of 2,4,6-trichloroanisole in chlorinated water at nanogram per litre levels by SPME–GC–ECD

A method involving solid-phase micro extraction (SPME) and gas chromatography with electron capture detection (SPME–GC–ECD) has been optimised for identification and quantification of 2,4,6-trichloroanisole (TCA) at ng L^–1 concentrations in disinfected (chlorinated) water samples. A central composite design was used for factorial analysis of four factors, three factors related to the SPME (PDMS fibre) procedure (adsorption time, temperature of the sample during headspace sampling, and desorption time) and one related to the GC operation (the rate of increase of the temperature of the GC oven). Good linearity (linear correlation coefficient greater than 0.999) was observed for TCA concentrations up to 50 ng L^–1, limits of detection and quantification of 0.7 and 2.3 ng L^–1, respectively, and good precision (relative standard deviation 2.8% and 3.4% for 5 and 30 ng L^–1 of TCA, respectively). Besides TCA, this system also enables the detection and quantification of the four trihalomethanes in the g L^–1 concentration range with limits of detection and quantification of approximately 0.3 g L^–1 and 1 g L^–1, respectively.     

A gas chromatographic assay for quantitative analysis of volatiles in solid materials by discontinuous gas extraction

Summary A method has been developed for the quantitative analysis of volatile compounds in solid samples. The method is based on a stepwise gas extraction of the volatiles with subsequent analysis of the extracted material and is termed discontinuous gas extraction. Any quantitative analysis requires an exhaustive extraction, which, however, is often too time-consuming for routine analysis. It is shown how the total amount of each volatile compound can be calculated from only a few extractions. Such a calculation is possible because for analytical purposes it is the information of the extraction process and not the extracted material that is needed. This method is useful for samples which are insoluble, such as certain polymers or residual solvents in printed foils, and which cannot be analyzed quantitatively by headspace gas chromatography, since no calibration solution can be prepared. It is further shown how discontinuous gas extraction can also be used to calibrate headspace analysis. Thus, both methods combine well together in that discontinuous gas extraction provides the accuracy while the headspace analysis gives convenience and speed of sample throughput, particularly if carried out with an automated headspace analyzer.     

Spectrophotometric determination of nitrite in environmental waters using column preconcentration on biphenyl

Column preconcentration methods have been established for the spectrophotometric determination of trace nitrite with sulfanilic acid (SA) orp-aminoacetophenone (AAP) as the diazotizable aromatic amine andN, N-dimethylaniline (DMA) or 1-aminonaphthalene (AN) as the coupling agent, using differention-pairs co-precipitated with biphenyl. Nitrite ion reacts with SA in the pH range 2.0–3.0 and AAP in the pH range 1.7–3.0 in HCl medium to form water-soluble colourless diazonium cations. These cations are subsequently coupled with DMA in the pH range 3.7–6.1 for the SA-DMA system and AN in the pH range 1.7–2.3 for the AAP-AN system to be retained on microcrystalline biphenyl packed in a column. The solid mass is dissolved out from the column with 5 ml of DMF and the absorbance is measured by a spectrophotometer at 420 nm for the SA-DMA system and at 517 nm for the AAP-AN system. The calibration was linear over the concentration ranges 0.3–6.0 g of nitrite in 5 ml of DMF solution (i.e., 0.02–0.40 g/ml in the sample solution) for the SA-DMA system and 0.5–7.0 g of nitrite in 5 ml of DMF solution (i.e., 0.03–0.47 g/ml in the sample solution) for the AAP-AN system. The molar absorptivity and Sandell's sensitivity were respectively 2.63 × 10⁴lmol^–1cm^–1 and 1.75 × 10^–3 g cm^–2 for SA-DMA and 3.28 × 10⁴lmol^–1 cm^–1 and 1.40 × 10^–3 g cm^–2 for AAP-AN. The concentration factors were 4 and 16 for SA-DMA and AAP-AN, respectively. The detection limits were 0.0138 and 0.0175 g/ml NO₂ ^– for SA-DMA and AAP-AN, respectively. Seven replicate determinations of a solution containing 3.5 g of nitrite gave mean absorbances of 0.410 and 0.500 with relative standard deviations of 0.51 and 0.55% for SA-DMA and AAP-AN, respectively. Interference from various foreign ions has been studied and the methods have been applied to the determination of nitrite in environmental samples.     

Very empirical treatment of solvation and entropy: a force field derived from Log Po/w

A non-covalent interaction force field model derived from the partition coefficient of 1-octanol/water solubility is described. This model, HINT for Hydropathic INTeractions, is shown to include, in very empirical and approximate terms, all components of biomolecular associations, including hydrogen bonding, Coulombic interactions, hydrophobic interactions, entropy and solvation/desolvation. Particular emphasis is placed on: (1) demonstrating the relationship between the total empirical HINT score and free energy of association, G _interaction; (2) showing that the HINT hydrophobic-polar interaction sub-score represents the energy cost of desolvation upon binding for interacting biomolecules; and (3) a new methodology for treating constrained water molecules as discrete independent small ligands. An example calculation is reported for dihydrofolate reductase (DHFR) bound with methotrexate (MTX). In that case the observed very tight binding, G _interaction–13.6 kcal mol^–1, is largely due to ten hydrogen bonds between the ligand and enzyme with estimated strength ranging between –0.4 and –2.3 kcal mol^–1. Four water molecules bridging between DHFR and MTX contribute an additional –1.7 kcal mol^–1 stability to the complex. The HINT estimate of the cost of desolvation is +13.9 kcal mol^–1.     

气相色谱/质谱法分析内蒙古蜂胶挥发性成分

以内蒙古蜂胶为实验材料,采用传统水蒸气蒸馏法(HD)、同时蒸馏萃取法(SDE)和动态顶空进样(DHS)提取制备挥发性成分,以气相色谱-质谱(GC-MS)法检测.对获得的蜂胶挥发性成分进行了分析,从HD所得样品中检测出了12种化合物,主要是3-甲基-2-丁烯-1-醇(26.81%)、苯乙醇(17.16%)、1,2,3,4,4a,5,6,7-八氢-α,α,4a,8-四甲基-2-萘甲醇(14.53%)、2-甲氧基-4-乙烯基苯酚(9.47%)、α-没药醇(4.29%)等.从SDE法所得样品中检测出了40种化合物,主要是α-没药醇(20.19%)、2-甲基-3-丁烯-2-醇(10.76%)、3-甲基-2-丁烯-1-醇(8.28%)、薁(5.21%)等.从DHS法所得样品中检测出了70种化合物,主要是十七烷(6 96%)、菲(3.99%)、芳姜黄酮(3.80%)、1-(1,5-二甲基-4-己烯基)-4-甲基-苯(3.41%)、十八烷(3 23%)、1-甲氧基-4-(1-丙烯基)-苯(2.59%)、十六烷(2.47%)等.研究表明:内蒙古蜂胶挥发物与北京蜂胶及唐山蜂胶的组分相近,原因可能是它们具有相似的胶源植物.DHS得到的挥发性物质种类最多,SDE次之,HD最少.如需获得生物活性检测样品,SDE是一个更好的方法.     

Variation of the detection efficiency of a Ge detector with the height of the sample in Marinelli beaker

Several two and three parameter analytical functions were fitted to the measured detection efficiencies () of a Ge detector for a Marinelli beaker filled with samples up to different heights (H). From these, the expression =308E ^–0.82/((H–6.1)²+107), gave the best fit. The measured and calculated efficiencies were compared and for 70 determinations, a relative standard deviation of only 7.6% was obtained.     

Effect of Sodium Thiosulfate on the Kinetics of Electron–Ion Processes in Powdered Silver Chloride

The results of a microwave photoconductivity study on the effect of pretreatment of a silver chloride powder with sodium thiosulfate (STS) aqueous solutions on the kinetics of electron–ion processes in silver chloride are reported. At an UV light fluence of <10¹⁴ photon/cm^–2 per pulse, photoresponse decay after the end of the pulse consisted of two exponential components over the entire concentration range 10^–8–10^–2 mol STS/mol AgCl examined. The amplitude of the components and the ratio between the slow ^s and fast ^f components depended on the sodium thiosulfate concentration, which was due to the formation of an additional number of new electron traps in AgCl. An analysis of the kinetics in terms of models taking into account various types of decay of an excess electron made it possible to obtain data on the depth of both old traps (0.45 eV) and new STS-created traps (0.45–0.63 eV) in AgCl.