Temperature effects on resolution in ion mobility spectrometry

The separation efficiency of ion mobility spectrometry (IMS) may be measured in terms of either resolving power, based on a single-peak definition, or peak-to-peak resolution, based on the separation of pairs of adjacent peaks. Usually resolving power decreases with temperature. However, the experimental results show that the peak-to-peak resolution may be increased in some cases. Negative ion mobility spectra of halide ions are better resolved at elevated temperatures. In addition, the peaks corresponding to protonated monomer of amylacetate and the proton-bound dimer of ethylacetate are well separated at 100 °C while they fully overlap at 18 °C. This paper focuses on the effect of temperature on peak-to-peak resolution. It was also observed that in some cases peak-to-peak resolution decreases with temperature. Examples are the spectra of cyclohexanone and methyl-iso-butyl ketone (MIBK) as well as dimethyl methyl phosphonate (DMMP) and MIBK. The increase or decrease in resolution at elevated temperatures has been attributed to the changes in separation factor (α) which is governed by the different hydration and clustering tendency of ions.     

Chemical standards in ion mobility spectrometry

Positive ion mobility spectra for three compounds (2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine), dimethyl methylphosphonate (DMMP) and 2,6-di-t-butyl pyridine (2,6-DtBP)) have been studied in air at ambient pressure over the temperature range 37-250 °C with (H₂O)_nH⁺ as the reactant ion. All three compounds yield a protonated molecule but only 2,4-dimethylpyridine and dimethyl methylphosphonate produced proton-bound dimers. The reduced mobilities (K₀) of protonated molecules for 2,4-dimethylpyridine and DMMP increase significantly with increasing temperature over the whole temperature range indicating changes in ion composition or interactions; however, K₀ for the protonated molecule of 2,6-di-t-butyl pyridine was almost invariant with temperature. The K₀ values for the proton-bound dimers of 2,4-dimethylpyridine and DMMP also showed little dependence on temperature, but could be obtained only over an experimentally smaller and lower temperature range and at elevated concentrations. Chemical standards will be helpful as mobility spectra from laboratories worldwide are compared with increased precision and 2,6-di-t-butyl pyridine may be a suitable compound for use in standardizing reduced mobilities. The effect of thermal expansion of the drift tube length on the calculation of reduced mobilities is emphasized.     

Determination of cyanide, in urine and gastric content, by electrospray ionization tandem mass spectrometry after direct flow injection of dicyanogold

A rapid and sensitive electrospray ionization tandem mass spectrometric (ESI-MS-MS) procedure was developed for the determination of cyanide (CN⁻). CN⁻ in biological fluids was reacted with NaAuCl₄ to produce dicyanogold, Au(CN)₂⁻, which was extracted with methyl isobutyl ketone (MIBK). One microliter of the extract was injected directly into an ESI-MS-MS instrument. Quantification of CN⁻ was performed by selected reaction monitoring of the product ion CN⁻ at m/z 26 that derived from precursor ion Au(CN)₂⁻ at m/z 249. CN⁻ could be measured in the quantification range of 10⁻⁷ to 5 × 10⁻⁵ M with the limit of detection at 4 × 10⁻⁸ M using 10 μL of urine within 10 min. A victim's urine and gastric content were diluted with water to 4-fold and 500-fold and measured, respectively.     

Laser desorption-ionization of polycyclic aromatic hydrocarbons from glass surfaces with ion mobility spectrometry analysis

Polycyclic aromatic hydrocarbons (PAHs) were analyzed as adsorbates on borosilicate glass at levels from 40 pg (5.5 pg mm⁻²) to 7 μg (1 μg mm⁻²) using laser desorption-ionization (LDI) in air at ambient pressure and 100 °C with ion characterization by mobility spectrometry. Gas-phase positive ions with distinctive mobilities were produced from six PAHs using an unfocused beam at 266 nm, 6 mJ pulse⁻¹ and 10 Hz from a Nd-YAG laser. The ions produced were identified as M⁺ using mass spectrometry (MS) with a LDI source at atmospheric pressure. The mobility spectrometry drift tube provided low memory effects and allowed observation of time-resolved intensity profiles for ion signals, and changes in this behavior with loading level suggested intermolecular interactions from multilayer formation. Mobility peaks were broader than those seen in gas-phase reactions, and this was attributed to Coulombic repulsion caused by the small volume near the surface where ionization would take place. An ion shutter in the drift tube could be synchronized with the laser pulse to offer additional specificity using tandem mobility separation; further, resolution was improved in mixtures of PAHs with similar mobilities. Negative ions were also detected, though these were mass-identified as ions formed from air through the capture of electrons released from the PAHs; no M⁻-ions were observed in air. Limits of detection ranged from sub-pg to low-ng for individual PAHs.     

Selective extraction, separation and speciation of iron in different samples using 4-acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid

A method for speciation, preconcentration and separation of Fe(II) and Fe(III) in different matrices was developed using solvent extraction and flame atomic absorption spectrometry. 4-Acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid (AMPC) was used as a new complexing reagent for Fe(III). The Fe(III)-AMPC complex was extracted into methyl isobutyl ketone (MIBK) phase in the pH range 1.0-2.5, and Fe(II) ion remained in aqueous phase at all pH. The chemical composition of the Fe(III)-AMPC complex was determined by the Job's method. The optimum conditions for quantitative recovery of Fe(III) were determined as pH 1.5, shaking time of 2 min, 1.64 × 10⁻⁴ mol L⁻¹ AMPC reagent and 10 mL of MIBK. Furthermore, the influences of diverse metal ions were investigated. The level of Fe(II) was calculated by difference of total iron and Fe(III) concentrations. The detection limit based on the 3σ criterion was found to be 0.24 μg L⁻¹ for Fe(III). The recoveries were higher than 95% and relative standard deviation was less than 2.1% (N = 8). The validation of the procedure was performed by the analysis of two certified standard reference materials. The presented method was applied to the determination of Fe(II) and Fe(III) in tap water, lake water, river water, sea water, fruit juice, cola, and molasses samples with satisfactory results.     

Construction of a new Cu2+ coated wire ion selective electrode based on 2-((2-(2-(2-(2-hydroxy-5-methoxybenzylidene amino)phenyl)disufanyl)phenylimino)methyl)-4-methoxyphenol Schiff base

In this article a new coated platinum Cu²⁺ ion selective electrode based on 2-((2-(2-(2-(2-hydroxy-5-methoxybenzylideneamino)phenyl)disufanyl)phenylimino) methyl)-4-methoxyphenol Schiff base (L₁) as a new ionophore is described. This sensor has a wide linear range of concentration (1.2 × 10⁻⁷-1.0 × 10⁻¹ mol L⁻¹) and a low detection limit of 9.8 × 10⁻⁸ mol L⁻¹of Cu(NO₃)₂. It has a Nernstian response with slope of 29.54 ± 1.62 mV decade⁻¹ and it is applicable in the pH range of 4.0-6.0 without any divergence in potentioal. The coated electrode has a short response time of approximately 9 s and is stable at least for 3.5 months. The electrode shows a good selectivity for Cu²⁺ ion toward a wide variety of metal ions. The proposed sensor was successfully applied for the determination of Cu²⁺ ion in different real and environmental samples and as indicator electrode for potentiometric titration of Cu²⁺ ion with EDTA.     

No more conventional reference electrode: Transition time for determining chloride ion concentration

Ion selective electrodes (ISE) are used extensively for the potentiometric determination of ion concentrations in electrolytes. However, the inherent drift in these measurements and the requirement of a stable reference electrode restrict the feasibility of this method for long-term in-situ applications. This work presents a chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode for measuring chloride ion concentration. An anodic current pulse is applied to a Ag/AgCl working electrode which initiates a faradaic reaction that depletes the chloride ions near the electrode surface. The rate of change in potential at the Ag/AgCl electrode, due to chloride ion depletion, reaches an inflection point once the chloride ions deplete completely near the electrode surface. The moment of the inflection point, also known as the transition time, is a function of the chloride ion concentration and is described by the Sand equation. It is shown that the square root of the transition time is linearly proportional to the chloride ion concentration. Drift in the response over two weeks is negligible: 59 μM/day when measuring 1 mM of Cl⁻ ions using a 10 A m⁻² current pulse. The transition time at a specific ion concentration can be tuned by the applied current pulse, e.g., in a solution containing 5 mM chloride ions, the transition times with current pulses of 10 and 20 A m⁻² are 1.56 and 0.25 s, respectively. The moment of inflection determines the response, and thus is independent of the absolute potential of reference electrode. Therefore, any metal wire can act as a pseudo-reference electrode, enabling this approach for long-term and integrated-sensor applications such as measurement inside concrete structures.     

Comparative studies of mercapto thiadiazoles self-assembled on gold nanoparticle as ionophores for Cu(II) carbon paste sensors

Comparative studies of the potentiometric behavior of three mercapto compounds [2-((5-mercapto-1,3,4-thiadiazol-2-ylimino)methyl)phenol] (MTMP), [5-(2-methoxy benzylidene amino)-1,3,4-thiadiazole-2-thiol] (MBYT) and [5-(pyridin-2-ylmethyleneamino)-1,3,4-thiadiazole-2-thiol] (PYTT) self-assembled on gold nanoparticles (GNPs) as ionophores in carbon paste electrodes (CPEs) have been made. These mercapto thiadiazole compounds were self-assembled onto gold nanoparticles and then incorporated within carbon paste electrode. The self-assembled ionophores exhibit a high selectivity for copper ion (Cu²⁺), in which the sulfur and nitrogen atoms in their structure play a role as the effective coordination donor site for the copper ion. These carbon paste electrodes were applied as indicator electrodes for potentiometric determination of copper ions. The sensor based on PYTT exhibits the working concentration range of 4.0 × 10⁻⁹ to 7.0 × 10⁻² M and a Nernstian slope of 28.7 ± 0.3 mV decade⁻¹ of copper activity. The detection limit of electrode was 1.0 × 10⁻⁹ M and potential response was pH independent across the range of 3.0-6.5. It exhibited a quick response time of <5 s and could be used for a period of 45 days. The ion selectivity of this electrode for Cu²⁺ was over 10⁴ times that for other metal cations. The application of prepared sensors has been demonstrated for the determination of copper ions in spiked water and natural water samples.     

Highly selective electrogenerated chemiluminescence (ECL) for sulfide ion determination at multi-wall carbon nanotubes-modified graphite electrode

In the present work, a novel method for immobilization of carbon nanotubes (CNTs) on the surface of graphite electrode was proposed. We further found that superoxide ion was electrogenerated on this CNTs-modified electrode, which can react with sulfide ion combing with a weak but fast electrogenerated chemiluminescence (ECL) emission, and this weak ECL signal could be enhanced by the oxidative products of rhodamine B. In addition, the rate constant of this electrochemical reaction k⁰ was investigated and confirmed that the speed of electrogenerating superoxide ion was in accordance with the subsequent fast CL reaction. Thus, the fast CL reaction of superoxide ion with target brought in the possibility of high selectivity based on time-resolved, relative to other interferences. Based on these findings, an excellently selective and highly sensitive ECL method for sulfide ion was developed. Under the optimum conditions, the enhancing ECL signals were linear with the sulfide ion concentration in the range from 6.0 × 10⁻¹⁰ to 1.0 × 10⁻⁸ mol L⁻¹, and a 2.0 × 10⁻¹⁰ mol L⁻¹ detection limits (3σ) was achieved. In addition, the proposed method was successfully used to detect sulfide ion in environmental water samples.     

p-Dimethylaminobenzaldehyde thiosemicarbazone: a simple novel selective and sensitive fluorescent sensor for mercury(II) in aqueous solution

A novel and simple fluorophore, p-dimethylaminobenzaldehyde thiosemicarbazone (DMABTS), was prepared in order to find available fluorescent chemosensor for mercuric ion in aquesous solution. DMABTS emitted fluorescence at 448 nm in aqueous solution and its fluorescence intensity was completely quenched upon interaction with Hg²⁺ ions, which should be attributed to the 1:1 complex formation between DMABTS and Hg²⁺. The binding constant of the complex was determined as 7.48 × 10⁶ mol l⁻¹. The linear range of quantitative detection of 0 to 5.77 × 10⁻⁶ mol l⁻¹ and the detection limit of 7.7 × 10⁻⁷ mol l⁻¹ for Hg²⁺ in the 6.3 × 10⁻⁶ mol l⁻¹ DMABTS aqueous solution were obtained from a calibration curve. The coexistence of several transition metal ions and anions did interfere the fluorometric titration of Hg²⁺ ion by less than 4% in the emission change.     

Copper(I)-bathocuproine complex as carrier in iodide-selective electrode

A PVC membrane electrode for iodide ions based on Cu(I)-bathocuproine as ionophore in membrane composition is prepared. The electrode exhibits a linear response over a wide concentration range 5.0×10⁻⁶ to 2.0×10⁻¹ mol l⁻¹ with a detection limit 1.0×10⁻⁶ mol l⁻¹. The proposed membrane electrode shows Nernstian behavior with a slope of −56.8 mV/decade, a fast response time 10 s and a lifetime at least 3 months. Iodide-selective electrode reveals good selectivities for iodide ion over a wide variety of the other anions and can be used in pH range of 3-9. It can also be used as an indicator electrode in potentiometric titration of iodide ion.     

Spectrophotometric determination of trace ionic and non-ionic surfactants based on a collection on a membrane filter as the ion associate of the surfactant with Erythrosine B

The spectrophotometric method for the determination of trace surfactants with Erythrosine B (EB) based on the aqueous reaction and the collection on a membrane filter by filtration was studied. Cationic surfactants (CS⁺), such as a quaternary ammonium ion, and polyoxyethylene non-ionic surfactants (NS) in the presence of potassium ion, containing a long-chain alkyl group associate with EB buffered at pH 5.5. CS⁺ associates with anionic surfactants (AS⁻). For the determination of CS⁺, four methods were employed: the collection of the ion associate of CS⁺ with EB on a mixed cellulose ester (MCE) or PTFE membrane filter, the collection of the ion associate of CS⁺ with AS⁻ on a PTFE membrane filter followed by the ion exchange of AS⁻ with EB, and the first collection of CS⁺ followed by the second collection of EB on a PTFE membrane filter. For the determination of AS⁻, the collection of the ion associate of AS⁻ with CS⁺ on a PTFE membrane filter followed by the ion exchange of AS⁻ with EB was done. For the determination of NS, the ion associate of NS with EB was collected on a MCE membrane filter. The MCE membrane filter with the analyte was dissolved in methyl cellosolve. The analyte on the PTFE membrane filter was eluted with ethanol. The CS⁺ up to 5×10⁻⁷ M can be determined by the absorbance at 542 nm of the methyl cellosolve solution or the absorbance at 535 nm of the ethanol solution. The AS⁻ up to 5×10⁻⁷ M can be determined by the absorbance at 536 nm of the ethanol solution. The NS up to 2.53×10⁻⁶ M can be determined by the absorbance at 537 nm of the methyl cellosolve solution. This is the sensitive method for the determination of 10⁻⁸ to 10⁻⁷ M order of ionic surfactants and 10⁻⁷ to 10⁻⁶ M order of NS without toxic organic solvents.     

Multiplex sensor for detection of different metal ions based on on–off of fluorescent gold nanoclusters

In this study, a multiplex fluorescence sensor for successive detection of Fe³⁺, Cu²⁺ and Hg²⁺ ions based on “on–off” of fluorescence of a single type of gold nanoclusters (Au NCs) is described. Any of the Fe³⁺, Cu²⁺ and Hg²⁺ ions can cause quenching fluorescence of Au NCs, which established a sensitive sensor for detection of these ions respectively. With the introduction of ethylene diamine tetraacetic acid (EDTA) to the system of Au NCs and metal ions, a restoration of fluorescence may be found with the exception of Hg²⁺. A highly selective detection of Hg²⁺ ion is, thus, achieved by masking Fe³⁺ and Cu²⁺. On the other hand, the masking of Fe³⁺ and Cu²⁺ leads to the enhancement of fluorescence of Au NCs, which in turn provides an approach for successive determination of Fe³⁺ and Cu²⁺ based on “on–off” of fluorescence of Au NCs. Moreover, this assay was applied to the successful detection of Fe³⁺, Cu²⁺ and Hg²⁺ in fish, a good linear relationship was found between these metal ions and the degree of quenched fluorescent intensity. The dynamic ranges of Hg²⁺, Fe³⁺ and Cu²⁺ were 1.96 × 10⁻¹⁰–1.01 × 10⁻⁹, 1.28 × 10⁻⁷–1.27 × 10⁻⁶ and 1.2 × 10⁻⁷–1.2 × 10⁻⁶ M with high sensitivity (the limit of detection of Fe³⁺ 2.0 × 10⁻⁸ M, Cu²⁺ 1.9 × 10⁻⁸ M and Hg²⁺ 2 × 10⁻¹⁰ M). These results indicate that the assay is suitable for sensitive detection of these metal ions even under the coexistence, which can not only determine all three kinds of metal ions successively but also of detecting any or several kinds of metal ions.     

Novel thiocyanate-selective membrane sensors based on di-, tetra-, and hexa-imidepyridine ionophores

Potentiometric thiocyanate-selective sensors based on the use of three synthesized di-, tetra-, and hexa-imidepyridine derivatives as novel anionic neutral ionophores in plasticized poly(vinyl chloride) (PVC) membranes are described. The sensors exhibit significantly enhanced response towards thiocyanate ions over the concentration range 5×10⁻⁶ to 1.0×10⁻² M with a lower detection limit of 0.3 μg ml⁻¹ and slopes ranging from −55.6 to −58.3 mV per decade. Fast and stable response, good reproducibility, long-term stability, applicability over a wide pH range (2-8) and high selectivity for SCN⁻ ion in the presence of 18 common anions are demonstrated. The sensors are used for direct potentiometric measurements of thiocyanate ions over the concentration range 0.2-580 μg ml⁻¹ and for monitoring sequential titration of some metal ions (e.g. Ag⁺, Tl⁺, Cu²⁺, Pb²⁺) in binary and ternary mixtures. Sequential binding of these metal ions with SCN⁻ ensures share stepwise titration curves with consecutive end point breaks at the equivalent points. Recoveries of 98.5-99.1±0.3% are obtained for metal ion concentrations of 0.06-4 mg ml⁻¹.     

Quantitative analysis of morphine and noscapine using corona discharge ion mobility spectrometry with ammonia reagent gas

Morphine and noscapine were determined using corona discharge ion mobility spectrometry. The detection limits were 5.6 × 10⁻¹¹ and 6.7 × 10⁻¹¹ g for morphine and noscapine, respectively. The linear dynamic ranges of the calibration plots for the compounds were about three orders of magnitude. The method has also been successfully applied for simultaneous determination of the compounds using the standard addition method.     

Retention, kinetics and thermodynamics profile of cadmium adsorption from iodide medium onto polyurethane foam and its separation from zinc bulk

The sorption behaviour of 2.5 × 10⁻⁵ M solution of Cd(II) on polyurethane foam (PUF) from iodide medium have been investigated. The conditions were optimized from aqueous solutions of different pH (1-10) and of acids of varied concentration (0.01-1.0 M). The maximum concentration of KI was found to be 0.24 M and equilibration time was established to be 20 min. The data successfully followed the Freundlich and Dubinin-Radushkevich (D-R) isotherms at low metal ion concentration while Langmuir isotherm followed at higher metal ion concentration. The Freundlich parameter 1/n = 0.66 ± 0.02 have been evaluated whereas D-R isotherm yields the sorption free energy E = 10.5 ± 0.1 kJ mol⁻¹ indicating ion exchange type chemisorption. The monolayer coverage (XL) constant of Langmuir isotherm was found to be 23.7 ± 0.4 mg g⁻¹. The numerical values of thermodynamics parameters enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) indicated the endothermic and spontaneous nature of sorption. The Scatchard plot analysis was tested to evaluate the binding sites of the PUF and stability constants of sorption were determined. On the basis of these parameters, the sorption mechanism was discussed. Among the foreign ions tested, Pb(II), Hg(II), cyanide and nitrite should be absent. The clean separation of Cd(II) from Zn(II) ions in the ratio 1:250, respectively, was achieved by column chromatography.     

Heavy metal adsorptivity of calcium-alginate-modified diethylenetriamine-silica gel and its application to a flow analytical system using flame atomic absorption spectrometry

This study aimed to evaluate the heavy metal adsorptivity of calcium-alginate-modified diethylenetriamine-silica gel (CaAD) and incorporate this biosorbent into a flow analytical system for heavy metal ions using flame atomic absorption spectrometry (FAAS). The biosorbent was synthesized by electrostatically coating calcium alginate onto diethylenetriamine (dien)-silica gel. Copper ion adsorption tests by a batch method showed that CaAD exhibited a higher adsorption rate compared with other biosorbents despite its low maximum adsorption capacity. Next, CaAD was packed into a 1 mL microcolumn, which was connected to a flow analytical system equipped with an FAAS instrument. The flow system quantitatively adsorbed heavy metals and enriched their concentrations. This quantitative adsorption was achieved for pH 3–4 solutions containing 1.0 × 10⁻⁶ M of heavy metal ions at a flow rate of 5.0 mL min⁻¹. Furthermore, the metal ions were successfully desorbed from CaAD at low nitric acid concentrations (0.05–0.15 M) than from the polyaminecarboxylic acid chelating resin (Chelex 100). Therefore, CaAD may be considered as a biosorbent that quickly adsorbs and easily desorbs analyte metal ions. In addition, the flow system enhanced the concentrations of heavy metals such as Cu²⁺, Zn²⁺, and Pb²⁺ by 50-fold. This new enrichment system successfully performed the separation and determination of Cu²⁺ (5.0 × 10⁻⁸ M) and Zn²⁺ (5.7 × 10⁻⁸ M) in a river water sample and Pb²⁺ (3.8 × 10⁻⁹ M) in a ground water sample.     

A novel Mn PVC membrane electrode based on a recently synthesized Schiff base

A new PVC membrane electrode for manganese(II) ion based on a recently synthesized Schiff base of 5-[(4-nitrophenylazo)-N-hexylamine]salicylaldimine is reported. The electrode exhibits a Nernstian response for Mn²⁺ ions over a wide concentration range (4.0 × 10⁻⁷ to 1.8 × 10⁻² mol L⁻¹) with a slope of 30.1 (±1.0). The limit of detection is 1.0 × 10⁻⁷ mol L⁻¹. The electrode has a fast response time (∼10 s), a satisfactory reproducibility and relatively long life time. The proposed sensor revealed good selectivities over a wide variety of other cations include hard and soft metals. This electrode could be used in a pH range of 4.5-7.5. It was used as an indicator electrode in potentiometric titration of manganese(II) ions with EDTA solution.     

An optical redox chemical sensor for determination of iodide

A novel optical sensor based on a redox reaction for the determination of iodide has been developed. The optode membrane is constructed by immobilization of methyltrioctylammonium chloride on triacetylcellulose polymer. The exchange of chloride as counter ion with iodate in the membrane changes the color to yellow, when it is placed in acidic solution of iodide. The sensor can readily be regenerated by 0.1 mol L⁻¹ NaOH in less than 15 s. The optode has a linear range of 3.94 × 10⁻⁶ to 5.51 × 10⁻⁵ mol L⁻¹ of iodide ions with a limit of detection 7.44 × 10⁻⁷ mol L⁻¹. The relative standard deviation for eight replicate measurements of 3.94 × 10⁻⁶ and 1.57 × 10⁻⁵ mol L⁻¹ of iodide was 2.83 and 1.38%, respectively. The sensor was successfully applied to the determination of iodide in tablet, powdered milk and urine samples.