Retention of ionisable compounds on high-performance liquid chromatography XVI. Estimation of retention with acetonitrile/water mobile phases from aqueous buffer pH and analyte pKa

In agreement with our previous studies and those of other authors, it is shown that much better fits of retention time as a function of pH are obtained for acid-base analytes when pH is measured in the mobile phase, than when pH is measured in the aqueous buffer when buffers of different nature are used. However, in some instances it may be more practical to measure the pH in the aqueous buffer before addition of the organic modifier. Thus, an open methodology is presented that allows prediction of chromatographic retention of acid-base analytes from the pH measured in the aqueous buffer. The model presented estimates the pH of the buffer and the pKa of the analyte in a particular acetonitrile/water mobile phase from the pH and pKa values in water. The retention of the analyte can be easily estimated, at a buffer pH close to the solute pKa, from these values and from the retentions of the pure acidic and basic forms of the analyte. Since in many instances, the analyte pKa values in water are not known, the methodology has been also tested by using Internet software, at reach of many chemists, which calculates analyte pKa values from chemical structure. The approach is successfully tested for some pharmaceutical drugs.     

Prediction of measurement uncertainty in isotope dilution gas chromatography/mass spectrometry

An equation is theoretically derived which describes the relative standard deviation (RSD) of the amount ratios of analyte to its isotope-labeled variant in gas chromatography/mass spectrometry (GC/MS) using the stable isotope dilution method. The determination of methyltestosterone is taken as an example. The uncertainty equation proposed is justified by comparing the theoretical RSD values with the experimental RSD values obtained by replication over a wide range of analyte amount. The detection limit and quantitation limit are estimated from the continuous plot (precision profile) of the theoretical RSD against analyte amount.     

Influence of graphite furnace tube design on vapour temperatures and chemical interferences in ETA-AAS

A two-line atomic absorption method for determination of lead was used for calculation of the temperatures experienced by analyte atoms in the gas phase after wall atomization with modified Philips SP-9 graphite tubes. For each tube, the influence of the temperature gradient on the vapour phase temperature and chemical interferences experienced by Cd, Mn and Pb in ETA-AAS was investigated. A higher vapour temperature and lower chemical interference by chlorides were observed when the tube temperature gradient was reversed through a reduction in the wall thickness towards the ends of the tube.     

Behaviour of the thermospray nebulizer as a system for the introduction of organic solutions in flame atomic absorption spectrometry

The results obtained in the evaluation of the thermospray nebulizer for the introduction of organic solutions in atomic spectrometry are described. To this end, the influence of the nebulization variables (i.e., liquid flow, control temperature and inner diameter of the capillary) and of the nature of the solvent on the fraction of solvent vaporized, on the drop size distribution of the primary aerosol, on the rates of analyte and solvent transport to the atomization cell and on the analytical signal has been studied. Experimental fraction of solvent vaporized values obtained under different nebulization conditions are reported for the first time. The results show that the characteristics of the aerosol generated strongly depend on the nebulization variables since they determine the amount of energy available for surface generation. The median of the volume drop size distribution of the primary aerosol decreases when the control temperature or the liquid flow is increased or when the inner diameter of the capillary is decreased. As regards the physical properties of the solvent, the so-called expansion factor (i.e., the volume of vapour produced per unit volume of liquid solvent) is the most influential. Surface tension and viscosity are much less significant here than in ordinary pneumatic nebulization. The volatility of the solvent and the characteristics of the primary aerosol determine the solvent transport efficiency which reaches values close to 100% in many cases. The analytical signal is mainly determined by the analyte transport rate, although a severe negative effect appears which is related to the high solvent load to the flame. Due to this fact, the use of organic solvents instead of water in thermospray nebulization for Flame Atomic Absorption Spectrometry does not provide clear advantages, at least without desolvation. A new modified Nukiyama-Tanasawa equation has been presented and evaluated in order to predict the Sauter mean diameter of the thermal aerosols. The results show that, under the conditions tested, this equation can not be applied to predict the characteristics of the primary aerosols generated with this type of nebulizer.     

Direct mercury determination in aqueous slurries of environmental and biological samples by cold vapour generation-electrothermal atomic absorption spectrometry

Direct cold vapour generation from aqueous slurries of environmental (marine sediment, soil, coal) and biological (human hair, seafood) samples have been developed using a batch mode generation system coupled with electrothermal atomic absorption spectroscopy. The effects of several variables affecting the cold vapour generation efficiency from solid particles (hydrochloric acid and sodium tetrahydroborate concentrations, argon flow rate, acid solution volume and mean particle size) have been evaluated using a Plackett-Burman experimental design. In addition, variables affecting cold vapour trapping and atomisation efficiency on Ir-treated graphite tubes (trapping and atomisation temperatures and trapping time) have been also investigated. Atomisation and trapping temperatures, trapping time and hydrochloric acid concentration were the significant variables. The 2²+star and 2³+star central composite designs have been used to obtain optimum values of the variables selected. The accuracy of methods have been verified by using several certified reference materials (PACS-1, GBW-07410, NIST-1632c, CRM-397 and DORM-2). A characteristic mass of 390 pg were achieved. The detection limits of methods were in the range of 40-600 ng g⁻¹. A particle size less than 50 μm is adequate to obtain total cold vapour generation of Hg content in the aqueous slurry particles.     

Chemical vapour generation of transition metal volatile species for analytical purposes: determination of Zn by inductively coupled plasma-optical emission spectrometry

Volatile species of Zn were generated by merging acidified aqueous samples and sodium tetrahydroborate(III) solution in a continuous flow system. The gaseous analyte was subsequently introduced via a stream of Ar carrier into the inlet tube of the plasma torch. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used for detection. The operating conditions (chemical and physical parameters) and the concentrations of different inorganic (hydrochloric, nitric) and organic acids (acetic, citric, oxalic, tartaric) were evaluated for the efficient generation of Zn vapour. A detection limit (3sigma(blank)) of 4.6 ng mL(-1) was achieved. The precision (RSD) of the determination was 6.9% at a level of 250 ng mL(-1) and 4.7% for 1000 ng mL(-1) (n = 10). The efficiency of the generation process was estimated to be close to 50% in HCl. The possible interferent effect of transition metals (Cd, Co, Cu, Cr, Fe, Mn, Ni), hydride forming elements (As, Ge, Pb, Sb, Se) and Hg on Zn signal was examined. The method was validated by the determination of Zn in a certified reference material (CRM), NIST 1643d trace elements in water.     

On the role of defects and surface chemistry for surface-assisted laser desorption ionization from silicon

The generation of ions from silicon substrates in surface-assisted laser desorption ionization (SALDI) has been studied using silicon substrates prepared and etched by a variety of different methods. The different substrates were compared with respect to their ability to generate peptide mass spectra using standard liquid sample deposition. The desorption/ionization processes were studied using gas-phase analyte deposition. Mass spectra were obtained from compounds with gas-phase basicities above 850 kJmol and with molecular weights up to 370 Da. UV, VIS, and IR lasers were used for desorption. Ionization efficiencies were measured as a function of laser fluence and accumulated laser irradiance dose. Solvent vapors were added to the ion source and shown to result in fundamental laser-induced chemical and physical changes to the substrate surfaces. It is demonstrated that both the chemical properties of the substrate surface and the presence of a highly disordered structure with a high concentration of "dangling bonds" or deep gap states are required for efficient ion generation. In particular, amorphous silicon is shown to be an excellent SALDI substrate with ionization efficiencies as high as 1%, while hydrogen-passivated amorphous silicon is SALDI inactive. Based on the results, a novel model for SALDI ion generation is proposed with the following reaction steps: (1) the adsorption of neutral analyte molecules on the SALDI surface with formation of a hydrogen bond to surface Si-OH groups, (2) the electronic excitation of the substrate to form free electron/hole pairs (their relaxation results in trapped positive charges in near-surface deep gap states, causing an increase in the acidity of the Si-OH groups and proton transfer to the analyte molecules), and (3) the thermally activated dissociation of the analyte ions from the surface via a "loose" transition state.     

Tandem on-line continuous separation and determination of arsenic by inductively coupled plasma atomic emission spectrometry

The principle of tandem on-line continuous separation techniques as an alternative means of introducing samples into plasmas was applied to the development of a sensitive, selective and convenient method for the determination of arsenic by inductively coupled plasma atomic emission spectrometry (ICP-AES). Arsenic is continuously extracted as AsI₃ into xylene from the sample dissolved in 0.1 M potassium iodide solution in 7.2 M hydrochloric acid. The xylene phase (containing the analyte) is continuously mixed on-line with NaBH₄ in dimethylformamide and acetic acid solutions. Arsine is thus continuously generated directly from the organic phase and is separated in a gas—liquid separation device which prevents most of the xylene phase vapour from reaching the ICP. The system was optimized for the continuous extraction of AsI₃, the direct generation of arsine from xylene and the final ICP determination of arsenic. Finally, the tandem on-line continuous separation ICP detection system was applied to the determination of arsenic in real samples (white metal, cast iron, cupro-nickel and orchard leaves standard materials). Very good agreement between the experimental results and the certified values was obtained.     

Fast methods for screening of trichothecenes in fungal cultures using gas chromatography-tandem mass spectrometry

The excess adsorption isotherms of acetonitrile, methanol and tetrahydrofuran from water on reversed-phase packings were studied, using 10 different columns packed with C1-C6, C8, C10, C12, and C18 monomeric phases, bonded on the same type of silica. The interpretation of isotherms on the basis of the theory of excess adsorption shows significant accumulation of the organic eluent component on the adsorbent surface on the top of "collapsed" bonded layer. The accumulated amount was shown to be practically independent of the length of alkyl chains bonded to the silica surface. A model that describes analyte retention on a reversed-phase column from a binary mobile phase is developed. The retention mechanism involves a combination of analyte distribution between the eluent and organic adsorbed layer, followed by analyte adsorption on the surface of the bonded phase. A general retention equation for the model is derived and methods for independent measurements of the involved parameters are suggested. The theory was tested by direct measurement of analyte retention from the eluents of varied composition and comparison of the values obtained with those theoretically calculated values. Experimental and theoretically calculated values are in good agreement.     

Tandem on-line continuous separations for atomic spectroscopic indirect analysis: Iodide determination by ICP-AES

A sensitive and selective indirect determination of iodide by inductively coupled plasma emission spectrometry (ICP-AES) based on the principle of tandem on-line continuous separations as an alternative means of introducing samples into plasmas is proposed. Iodide is continuously extracted as an ion-pair into xylene by mixing the sample with Hg(II) and dipyridil solutions. The organic phase (containing the analyte in [Hg(Dipy)₂]I₂ form) is on-line continuously mixed with NaBH₄ (in DMF) and acetic acid solutions. Mercury vapour continuously generated from this organic phase is separated in a classical U-type gas-liquid separation device. The system has been optimized for the continuous extraction of KI, for the direct generation of cold mercury vapour from xylene and for the final ICP-AES determination of mercury. The optimised method has been applied to the determination of iodide (detection limit 20 ng/ml of iodide) in table salt and in synthetic samples. Very good agreement between found and certified results was observed. The usefulness and convenience of such alternative sample chemical pretreatment/presentation to the ICP is thus demonstrated for indirect determinations to be carried out by atomic spectroscopy methods.     

Limiting activity coefficients of triethylamine in 30 solvents by a simple gas-liquid chromatographic method

Limiting values of the Raoult's-law activity coefficients of triethylamine in 30 solvents at 298.15 K have been obtained by a simple procedure based on the use of gas-liquid chromatography to determine the mole fraction of triethylamine in the vapour above dilute solutions. Where comparisons can be made, there is good agreement with literature values and with values derived from liquid-liquid distribution coefficients measured in this work.     

Diffusion vapour transfer modelling for an end-capped atomizer. Part 1. Atomizer with closed injection port

For end-cap equipped transverse-heated graphite atomizers (THGA) with integrated contacts used for analytical atomic spectrometry, a model equation describing the diffusional losses of analyte atomic vapour through the tube ends was constructed. The model assumes that the atomic density distribution is stepwise linear along the tube axis and the absence of a sample injection hole. With a quartz tube system, providing controlled experimental conditions at room temperature, the time constant of the diffusion removal function (T_R) of mercury vapour was determined for various open and end-capped tube geometries. These results were also described by an empirical multiple regression equation with a residual standard deviation of 5%. The theoretically predicted T_R values, corrected with an empirical factor of 1.33, agreed well (correlation coefficient = 0.996) with the experimentally obtained T_R values for the endcapped quartz tubes. For the Perkin-Elmer THGA tubes, the diffusional transfer model was evaluated using the integrated atomic absorbance ratio between various end-capped and open tubes. This is meaningful because the signal ratio for graphite atomizers is closely equal to the corresponding T_R ratio. For recommended atomization temperatures the average deviation between these experimental signal ratios and the theoretically predicted ratios for the elements Ag, In, Cd, Co, Hg and Cu was 1–5% for various end-capped tube geometries. The results for the individual elements deviated more from the theoretically predicted ratios mainly because of small differences in the mean gas-phase temperature between open and end-capped tubes. For elements which tend to form molecules in the gas phase at low temperatures and for which the atomization efficiency is increased with the atomization temperature, the experimental ratios tended to be higher than the theoretically predicted values (In, Al, Se, Sn, As), whereas experimental ratios were slightly lower for other elements (Cd, Co, Cu).     

Determination of mercury by furnace atomic nonthermal excitation spectrometry

The determination of Hg using different variants of the Furnace Atomic Nonthermal Excitation Spectrometry (FANES) is described. In the direct analysis of micro volumes of solutions, the results could be improved by working with chemical modifiers for the stabilization of Hg during the thermal pretreatment. The best results were obtained using Ir and Pd as modifiers, with absolute detection limits of 4 and 12 pg, respectively. The determination of mercury in sample volumes up to 10 ml could be achieved by coupling a cold vapour generation system and an amalgam attachment to the FANES source. A detection limit of 22 was obtained with this technique. The best results were obtained by using the cold vapour generation technique and in situ enrichment of Hg onto the modified inner surface of the graphite tube of the FANES source. Using Ir for permanent impregnation of the tube a detection limit of 0.0009 was found. The influence of hydride forming elements on the determination of mercury by the technique of vapour generation and in situ enrichment was studied. A reduction of the concentration of NaBH₄ to 0.002% m/v made it possible to determine traces of mercury in presence of a high excess of hydride forming elements without any depression of the Hg emission intensity.The results were validated using standard reference materials.     

13C NMR study of dilute ternary solutions: Acetonitrile,silver nitrate and other electrolytes in water at 25°C

The ¹³C NMR chemical shifts have been measured for dilute aqueous solutions of acetonitrile in presence of various electrolytes including silver nitrate. The two formation constants of the silver ion/acetonitrile complexes have been calculated assuming an additive contribution of each possible complex configuration. Under these conditions the values obtained for the formation constants are very close to those deduced from vapour pressure or electromotive force techniques. The other systems studied are discussed in relation to the salting phenomenon in aqueous electrolyte solutions.     

Bio-hybrid materials for immunoassay-based sensing of cortisol

Sol–gel encapsulation has been used as the basis for detecting cortisol by an immunoassay approach. Previous research showed that antibodies immobilized in the pores of a sol–gel derived silica were able to bind cortisol and be used as an immunosensor. However, this approach was not effective when measuring cortisol levels in human serum because of interference from other fluorescence sources. The present paper describes a protocol which overcomes these limitations and enables sol–gel immunoassays to effectively measure cortisol in human serum over the physiological range of cortisol blood concentrations in an adult (2–28 μg/dL). The method involves a standard additions approach in which various amounts of cortisol are added to the serum. The cortisol concentration values obtained with our sol–gel immunoassay were typically within 10% of the values obtained by traditional analytical methods. The protocol presented here represents a significant contribution to sol–gel sensing and immunoassays in particular, because of the ability to detect an analyte in human serum. In addition, this work reports the first comparison between results from a sol–gel immunosensor and an alternative immuno-binding method for analyte detection.     

压电晶体传感器阵列用于混合有机溶剂蒸气中官能团的识别

为模拟生物化学传感体系, 提出了可用于识别有机官能团的传感器阵列, 用作人工气味识别系统。该阵列由八个压电晶体传感器组成, 每个传感器涂以具有广谱响应性能的不同吸附活性材料, 阵列对常见小分子有机溶剂混合蒸气的响应频移数据采用逐步判别分析(SDA)处理, 选出五个供信能力最佳的判别变量, 以此构成的阵列用于小分子有机溶剂混合蒸气中醇羟基、羰基与其它官能团的识别, 并采用主成分分析(PCA)法降维投影, 在二维空间含相同官能团的物质聚为一类; 阵列可用于酒类、软饮料的识别。     

Development and test of a new Knudsen effusion apparatus for the measurement of low vapour pressures

A new Knudsen effusion apparatus based on the mass-loss technique was developed. The apparatus was designed for the measurement of vapour pressures below 1 Pa, at temperatures between 250 K and 470 K, using simultaneously up to seven effusion cells. The quality of the results obtained with this new apparatus was verified through the study of two reference compounds, anthracene and phenanthrene. Both the measured vapour pressures and the derived standard molar enthalpies of sublimation are in excellent agreement with values available in the literature for these compounds.     

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source.     

The molecular structure of selenium dibromide as determined by combined gas-phase electron diffraction-mass spectrometric experiments and quantum chemical calculations

The vapour over solid SeBr(4) at 10 degrees C was investigated with a combined gas-phase electron diffraction/mass spectrometric (GED/MS) method. The composition of the vapour derived from the mass spectra (43% SeBr(2), 56.7% Br(2) and 0.3% Se(2)Br(2)) was in agreement with the composition obtained from the analysis of the simultaneously recorded GED intensities (41(3)% SeBr(2), 59(3)% Br(2)). The GED study results in the following geometric parameters (r(g), angle(g) values with total uncertainties): Se-Br = 2.306(5) A and Br-Se-Br = 101.6(6) degrees . Most quantum chemical approximations (B3LYP, MP2, CCSD and CCSD(T) with relativistic effective core potentials and cc-pVTZ as well as aug-cc-pVTZ basis sets for the outer shells) overestimate the Se-Br bond length by 0.01 to 0.03 A. All methods reproduce the bond angle correctly, except for the B3LYP method. Gas phase vibrational frequencies estimated from experimental vibrational amplitudes agree well with those measured by Raman spectroscopy in acetonitrile solutions. All computational methods overestimate vibrational frequencies, especially that for the symmetric stretch vibration, by about or 8 to 13%.