Investigation of Methanol Behavior at the Designed Electrochemical Sensor based on Ni(II) Complex and Graphene Nanosheets

In this work, the electrochemical oxidation of methanol was investigated by different electrochemical methods at a carbon paste electrode (CPE) modified with (N‐5‐methoxysalicylaldehyde, N´‐2‐hydroxyacetophenon‐1, 2 phenylenediimino nickel(II) complex (Ni(II)–MHP) and reduced graphene oxide (RGO), which is named Ni(II)‐MHP/RGO/CPE, in an alkaline solution. This modified electrode was found to be efficient for the oxidation of methanol. It was found that methanol was oxidized by the NiOOH groups generated by further electrochemical oxidation of nickel(II) hydroxide on the surface of the modified electrode. Under optimum conditions, some parameters of the analyte (MeOH), such as the electron transfer coefficient (α), the electron transfer rate constant) k_s), and the diffusion coefficient of species in a 0.1 M solution (pH = 13), were determined. The designed sensor showed a linear dynamic range of 2.0–100.0 and 100.0–1000.0 μM and a detection limit of 0.68 μM for MeOH determination. The Ni(II)‐MHP/RGO/CPE sensor was used in the determination of MeOH in a real sample.     

一系列阳离子性卟啉化合物的基质辅助激光解吸质谱行为

应用基质辅助激光解吸电离飞行时间质谱(MALDI-TOF-MS)研究了一系列阳离子性卟啉化合物的质谱行为.结果表明,阳离子性卟啉与非离子性卟啉化合物的激光解吸电离方式有明显不同.对于四-氮R基(R=甲基,乙基,丙基,苄基)吡啶基卟啉,吡啶基氮上的侧链基团(R)可明显影响该类化合物在MALDI-TOF-MS测试过程中离子形成方式;R基团的增大以及平衡阴离子半径的增大可增加平衡阴离子与卟啉环阳离子之间相互作用力的共价成分,因此在MALDI-TOF-MS测定过程中能够得到卟啉环阳离子与多个平衡阴离子结合在一起的较高质量数的离子峰.另外,还初步探讨了阳离子性卟啉化合物的激光解吸电离机理.     

Effect of chemical modification on behavior of various organic vanadium forms during analysis by electrothermal atomic absorption spectrometry

The behavior of various organic V forms dissolved in xylene during analysis by electrothermal atomic absorption spectrometry (ETAAS) was compared. The investigated analyte forms included compounds with vanadium at the oxidation state III, IV or V, as well as N, O or S atoms in molecules. Another group consisted of petroleum products containing naturally-occurring V species. Although the characteristic mass determined under different analytical conditions was in the very wide range from 11 up to 55 pg, some rules of V behavior were found. In the case of porphyrins and petroleum products, the application of Pd as a chemical modifier (xylene solution of Pd(II) acetylacetonate) seemed to be crucial. It was shown that Pd must be introduced to a furnace together with a sample. Pd injected and thermally pretreated before the sample injection was less effective for porphyrins and the petroleum products, but it increased signals of V compounds containing O as donor atom. The iodine pretreatment followed by the methyltrioctylammonium chloride (MTOACl) pretreatment was advantageous for these V forms. The air ashing in a graphite tube appeared to be important to improve decomposition of the petroleum products. No significant influence of the V oxidation state on the analytical signal was observed. The behavior of V contained in two Conostan oil standards, the single-element and the S21 multielement standard, was different in many situations. Probably, the joint action of other elements is responsible for this effect. In general, chemical modification was applied in the work for two reasons: to reduce the V volatility (in some cases losses at about 300 °C were observed) and to enhance the atomization efficiency. For routine analysis air ashing, modification by Pd introduced into the furnace together with the sample solution and petroleum products with known V content as standard is recommended. Using this procedure the characteristic mass varied from 16 to 19 pg for porphyrins and all the investigated petroleum products of different character and origin. The detection limit of 1.8 μg kg^− 1 obtained for xylene solutions of petroleum products is very beneficial. Additionally, the stability of V in petroleum products xylene solutions allows the use of an autosampler. A more general finding of this work is that the effect of the organic part of an analyte compound molecule in ETAAS analysis can be enormous which should be taken into account in routine analyses of various samples.     

Photoelectron resonance capture ionization (PERCI): A novel technique for the soft-ionization of organic compounds

Photoelectron resonance capture ionization (PERCI) is demonstrated as a sensitive ionization technique involving minimal fragmentation of organic molecules. PERCI has been used successfully to softly and efficiently ionize both strongly UV absorbing and non-absorbing molecules. Tunable low energy (<1 eV) electrons are generated by focusing a pulsed UV laser on an aluminum photocathode in the presence of gas phase analyte. Negative ions are formed through a resonance electron capture process. Mass analysis is done using a reflectron time-of-flight mass spectrometer. PERCI is demonstrated for a number of gas phase compounds and simple mixtures, including sulfur hexafluoride, nitrobenzene, nitrophenol, 2-pentanone, hexanal, heptanal, and octanal. In all cases the molecular ion (or [M - H](-)) was observed to be the dominant peak. The 1sigma limit of detection was estimated to be on the order of 10(6) molecules in the ionization region.     

Comparison of dielectric barrier discharge, atmospheric pressure radiofrequency-driven glow discharge and direct analysis in real time sources for ambient mass spectrometry of acetaminophen

Three plasma-based ambient pressure ion sources were investigated; laboratory constructed dielectric barrier and rf glow discharges, as well as a commercial corona discharge (DART source). All were used to desorb and ionize a model analyte, providing sampling techniques for ambient mass spectrometry (MS). Experimental parameters were optimized to achive highest signal for acetaminophen as the analyte. Insight into the mechanisms of analyte desorption and ionization was obtained by means of emission spectrometry and ion current measurements. Desorption and ionization mechanisms for this analyte appear to be identical for all three plasma sources. Emission spectra differ only in the intensities of various lines and bands. Desorption of solid analyte requires transfer of thermal energy from the plasma source to sample surface, in the absence of which complete loss of MS response occurs. For acetaminophen, helium was the best plasma gas, providing 100- to 1000-fold higher analyte response than with argon or nitrogen. The same trend was also evident with background ions (protonated water clusters). MS analyte signal intensity correlates with the ion density (expressed as ion current) in the plasma plume and with emission intensity from excited state species in the plasma. These observations support an ionization process which occurs via proton transfer from protonated water clusters to analyte molecules.     

Enhancement in MS-based peptide detection by microfluidic free-flow zone electrophoresis

Matrix components are known to significantly alter the ionization of a target analyte in ESI-based measurements particularly when working with complex biological samples. This issue however may be alleviated by extracting the analyte of interest from the original sample into a relatively simple matrix compatible with ESI mass-spectrometric analysis. In this article, we report a microfluidic device that enables such extraction of small peptide molecules into an ESI-compatible solvent stream significantly improving both the sensitivity and reproducibility of the measurements. The reported device realizes this analyte extraction capability based on the free-flow zone electrophoretic fractionation process using a set of internal electrodes placed across the width of the analysis channel. Employing lateral electric fields and separation distances of 75 V/cm and 600 µm, respectively, efficient extraction of the model peptide human angiotensin II was demonstrated allowing a reduction in its detection limit by one to three orders of magnitude using the ESI-MS method. The noted result was obtained in our experiments both for a relatively simple specimen comprising DNA strands and angiotensin II as well as for human serum samples spiked with the same model peptide.     

Towards a second generation of ionic liquid matrices (ILMs) for MALDI-MS of Peptides, proteins, and carbohydrates

Second generation ionic liquid matrices are developed, examined, and tested. They have shown a wide mass detection range (<1000 Da to >270,000 Da) for proteins and peptides with greater S/N ratios than solid matrices. These ionic liquid matrices also exhibit the ability to effectively ionize proteins of large mass without disrupting noncovalent interactions between monomers. Both the anionic and cationic moieties have been varied systematically to find an ionic liquid matrix with the best physical properties, analyte signal intensity, and widest mass detection range. It was determined that both the proton affinity and pK_a of the cation have a large effect on the ionic liquid matrices’ ability to effectively ionize the analyte. The ionic liquid matrices can be used to detect polysaccharides with fewer degradation products than solid matrices. N,N-diisopropylethylammonium α-cyano-4-hydroxycinnamate and N-isopropyl-N-methyl-t-butylammonium α-cyano-4-hydroxycinnamate were the best matrices for proteins and peptides, while N,N-diisopropylethylammonium α-cyano-4-hydroxycinnamate and N,N-diisopropylethylammonium ferulate were the best matrices for carbohydrates.     

Hydrogen chemical ionization mass spectrometry of metalloporphyrins

The hydrogen chemical ionization (H₂ CI) mass spectra of a range of metal(II) (Ni, Cu, Co, Pt), metal (III) (Al, Mn, Ga, Fe (bearing a single axial ligand)) and metal(IV) (Si, Ge, Sn (bearing two axial ligands) and V (as V?O²⁺)) porphyrins have been determined, The spectra are highly dependent on the coordinated metal, rather than the axial ligand(s) (where present). Ni(II), Cu(II), Mn(II or III), Ga(III), Ge(IV), Fe(III) and Sn(IV) porphyrins fragment via hydrogenation and demetallation, followed by cleavage of the resulting porphyrinogens at the meso(bridge) positions to give mono- and di-pyrrolic fragments. Tripyrrolic fragments are also observed in the case of Ni(II), Cu(II) and Sn(IV). Fragmentations of this type are similar to those observed for free-base porphyrins. In the case of Pt(II), Co(II), Al(III), Si(IV) and V(IV) (as vanadyl), the dipyrrolic fragment ions are either very weak or completely absent; hence their H₂CI spectra contain limited structural information. This variable CI behaviour may be related to the relative stabilities of the metalloporphyrins together with the multiple stable valency states exhibited by several metals.     

Performance of a sheathless porous tip sprayer for capillary electrophoresis-electrospray ionization-mass spectrometry of intact proteins

The performance of a prototype porous tip sprayer for sheathless capillary electrophoresis-mass spectrometry (CE-MS) of intact proteins was studied. Capillaries with a porous tip were inserted in a stainless steel needle filled with static conductive liquid and installed in a conventional electrospray ionization (ESI) source. Using a BGE of 100 mM acetic acid (pH 3.1) and a positively charged capillary coating, a highly reproducible and efficient separation of four model proteins (insulin, carbonic anhydrase II, ribonuclease A and lysozyme) was obtained. The protein mass spectra were of good quality allowing reliable mass determination of the proteins and some of their impurities. Sheath-liquid CE-MS using the same porous tip capillary and an isopropanol-water-acetic acid sheath liquid showed slightly lower to similar analyte responses. However, as noise levels increased with sheath-liquid CE-MS, detection limits were improved by a factor 6.5-20 with sheathless CE-MS. The analyte response in sheathless CE-MS could be enhanced using a nanoESI source and adding 5% isopropanol to the BGE, leading to improved detection limits by 50-fold to 140-fold as compared to sheath liquid interfacing using the same capillary - equivalent to sub-nM detection limits for three out of four proteins. Clearly, the sheathless porous tip sprayer provides high sensitivity CE-MS of intact proteins.     

Solid-phase microextraction combined with surface-enhanced laser desorption/ionization introduction for ion mobility spectrometry and mass spectrometry using polypyrrole coatings

The successful application of polypyrrole (PPY) solid-phase microextraction (SPME) coatings as both an extraction phase and a surface to enhance laser desorption/ionization (SELDI) of analytes is reported. This SPME/SELDI fiber integrates sample preparation and sample introduction on the tip of a coated optical fiber, as well as acting as the transmission medium for the UV laser light. Using ion mobility spectrometry (IMS) detection, the signal intensity was examined as a function of extraction surface area and concentration of analyte. The linear relationship between concentration and signal intensity shows potential applicability of this detection method for quantitative analysis. Extraction time profiles for the fiber, using tetraoctylammonium bromide as test analyte, illustrated that equilibrium can be reached in less than one minute. To investigate the performance of the PPY coating, the laser desorption profile was studied. The fiber was also tested using a quadrupole time-of-flight (Q-TOF) mass spectrometer with leucine enkephalin as test analyte. Since no matrix was used, mass spectra free from matrix background were obtained. This novel SPME/SELDI fiber is easy to manufacture, and is suitable for studying low-mass analytes because of the intrinsic low background. These findings suggest that other types of conductive polymers could also be used as an extraction phase and surface to enhance laser desorption/ionization in mass spectrometry.     

Nickel Hydroxide and Intercalated Graphene with Ionic Liquid Nanocomposite‐modified Electrode for Sensing of Glucose

A novel non‐enzymatic glucose sensor based on nickel hydroxide and intercalated graphene with ionic liquid (G‐IL) nanocomposite modified glass carbon electrode was fabricated. Scanning electron microscope, Fourier transform infrared spectra and energy dispersive X‐ray spectroscopy of the nanocomposite confirmed the morphology and ingredient of Ni(OH)₂ as well as G‐IL. Moreover, experimental results of cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry indicated the sensing properties of Ni(OH)₂ at Ni(OH)₂/G‐IL modified electrode towards the typical electrocatalytic oxidation process of glucose at 0.43 V in 0.10 M NaOH. The current response was linearly related to glucose concentration in a range from 0.5 to 500 μM with a detection limit of 0.2 μM (S/N = 3) and sensitivity of 647.8 μA mM^?1 cm^?2. The response time of the sensor to glucose was less than 2 s. This work may be expected to develop an excellent electrochemical sensing platform of G‐IL as a catalysis carrier.     

Using surfactants to enhance the analyte signals in activated carbon, surface-assisted laser desorption/ionization (SALDI) mass spectrometry

The effect of surface activity in surface-assisted laser desorption/ionization (SALDI) mass spectrometry was examined. Several surfactants, including p-tolunensulfonic acid (PTSA), sodium dodecyl sulfate and alkyltrimethylammonium bromide, were used as analytes or additives in the SALDI matrix to demonstrate the surface activity effect. The experimental results demonstrate that analytes that have good surface activity have good sensitivity. Adding suitable amounts of surfactants to the SALDI matrix can dramatically enhance the sensitivity of analytes lacking surface activity. We propose that the enhancement of analyte signals is due to the ionic interaction between ionic surfactants and analytes because non-ionic surfactant additives in the SALDI matrix do not affect the analyte signals. The detection limit of methylephedrine can be as low as 100 pg in the SALDI analysis of 0.5 M PTSA additive in the SALDI matrix. Although other surfactants can also be used as matrix additives to enhance the analyte signal, they do not improve the ion abundance as much as PTSA does.     

Electrospray ionization from a gap with adjustable width

In this paper, we present a new concept for electrospray ionization mass spectrometry, where the sample is applied in a gap which is formed between the edges of two triangular-shaped tips. The size of the spray orifice can be changed by varying the gap width. The tips were fabricated from polyethylene terephthalate film with a thickness of 36 microm. To improve the wetting of the gap and sample confinement, the edges of the tips forming the gap were hydrophilized by means of silicon dioxide deposition. Electrospray was performed with gap widths between 1 and 36 microm and flow rates down to 75 nL/min. The gap width could be adjusted in situ during the mass spectrometry experiments and nozzle clogging could be managed by simply widening the gap. Using angiotensin I as analyte, the signal-to-noise ratio increased as the gap width was decreased, and a shift towards higher charge states was observed. The detection limit for angiotensin I was in the low nM range.     

液相色谱-串联质谱法快速测定水产品中结晶紫及其隐色代谢物残留

采用液相色谱-串联质谱法(LC-MS/MS)测定了水产品中结晶紫以及它的隐色代谢物残留。匀质后的水产品样品用乙腈和乙酸铵缓冲液提取,合并提取液,用二氯甲烷反提取,经中性氧化铝柱和PRS柱固相萃取净化,且不使用氧化铅柱在线氧化,色谱分离后直接进入串联质谱检测器。采用电喷雾正离子,多反应监测(MRM)模式检测。方法的检出限可达0.5 ng/g,线性方程的相关系数r大于0.99,添加样品平均回收率为77.6%~93.8%,相对标准偏差均小于8.2%。     

Rapid analysis of raw solution samples by C18 pipette-tip electrospray ionization mass spectrometry

A C18 pipette-tip electrospray ionization mass spectrometry technique was developed for rapid analysis of raw solution samples. In this technique, a C18 pipette tip was employed for rapid purification and enrichment of analytes in raw sample solutions. The adsorbed analytes were eluted by solvents supplied by a syringe and a syringe pump, and a high voltage was applied onto the syringe needle to induce electrospray ionization at the pipette tip end for mass spectrometric analysis. This technique is simple, easy to assemble, enables generation of stable and reproducible signals, and can be conveniently used for qualitative and quantitative analysis of raw solution samples. Analysis by the technique only involved simple sample preparation procedures followed by direct mass spectrometric detection, all of which could be completed within minutes, while the analytical performances of the technique, including the limit of detection, limit of quantitation, liner range, accuracy and precision, were comparable to those by conventional methods.     

Infrared laser desorption and ionization of polypeptides from a polyacrylamide gel

We observed direct desorption and ionization of angiotensin II and bovine insulin from a frozen polyacrylamide gel without the addition of an exogenous matrix, using picosecond pulses from a tunable, mid-infrared free-electron laser tuned to strong absorption bands of the gel. At 5.7, 5.9, 6.1 and 6.3 microm we were able to desorb and ionize both analyte molecules, with the strongest analyte signal generated at 5.9 microm. However, no analyte signal was observed at 5.5 microm. Consistent with a previous report, we did not observe ions of either polypeptide at 2.9 microm, in spite of strong overall absorption. We discuss the implications of this wavelength-dependent ionization, including possible ablation mechanisms and energy partitioning between competing vibrational modes.     

Activated carbon cloth filled pipette tip for solid phase extraction of nickel(II), lead(II), cadmium(II), copper(II) and cobalt(II) as 1,3,4-thiadiazole-2,5-dithiol chelates for ultra-trace detection by FAAS

A solid phase extraction method is established for preconcentration of nickel, lead, cadmium, copper and cobalt using pipette tip solid phase extraction. The presented process was dependent on chelation of analytes with 1,3,4-thiadiazole-2,5-dithiol, then allowing the solution to flow through an activated carbon cloth packed pipette tip. The adsorbed metal chelates on the surface of activated carbon cloth were eluted by 5 mL of 3 M HNO₃. The concentrations of nickel, lead, cadmium, copper and cobalt were detected using a flame atomic absorption spectrometer (FAAS). The pipette tip solid phase extraction exhibit a preconcentration factor of 120. The limit of detection values were 2.7, 1.7, 1.3, 2.0 and 2.9 µg L^?1 for Ni(II), Pb(II), Cd(II), Cu(II) and Co(II), respectively. Validation of the method was checked by the analysis of TMDA-53.3 and TMDA-64.2 certified reference materials. The method was successfully applied for water and fertiliser samples.     

Direct electrochemistry of double stranded DNA on ionic liquid modified carbon paste electrode

The direct electrochemistry of herring sperm double-stranded DNA (dsDNA) on an ionic liquid N-butylpyridinium hexafluorophosphate modified carbon paste electrode was investigated. The cyclic voltammogram showed two irreversible oxidation peaks at 0.868 V and 1.188 V (vs. SCE), which corresponded to the oxidation of guanine and adenine residues, respectively. Compared to the common carbon paste electrode the electrochemical response was greatly improved. The electrochemical behavior of dsDNA on the modified electrode was carefully investigated with the electrochemical parameters were calculated. Under optimal conditions the dsDNA can be directly determined in the concentration range from 50 to 600 μg mL^?1 with a detection limit of 17 μg mL^?1 (3σ).     

Electrochemically initiated tagging of thiols using an electrospray ionization based liquid microjunction surface sampling probe two‐electrode cell

This paper reports on the conversion of a liquid microjunction surface sampling probe (LMJ‐SSP) into a two‐electrode electrochemical cell using a conductive sample surface and the probe as the two electrodes with an appropriate battery powered circuit. With this LMJ‐SSP, two‐electrode cell arrangement, tagging of analyte thiol functionalities (in this case peptide cysteine residues) with hydroquinone tags was initiated electrochemically using a hydroquinone‐doped solution when the analyte either was initially in solution or was sampled from a surface. Efficient tagging (～90%), at flow rates of 5–10 µL/min, could be achieved for up to at least two cysteines on a peptide. The high tagging efficiency observed was explained with a simple kinetic model. In general, the incorporation of a two‐electrode electrochemical cell, or other multiple electrode arrangement, into the LMJ‐SSP is expected to add to the versatility of this approach for surface sampling and ionization coupled with mass spectrometric detection. Published in 2009 by John Wiley & Sons, Ltd.