Soo Beng Khoo

Micellar electrokinetic capillary chromatography of vitamin B6 with electrochemical detection

Abstract A system for interfacing electrochemical detection with micellar electrokinetic capillary chromatography is demonstrated. This system couples the separation column to a short length of the same column material together with a section of porous graphite tubing which forms an electrically conductive joint. The joint is immersed in a buffer reservoir together with the ground electrode of the high-power source (15 kV). The reservoir is electrically insulated from the electrochemical cell containing the carbon fibre detector. This configuration effectively separates the detector from the high separation potential applied. Amperometric detection with micellar solutions is demonstrated for a mixture of B 6 vitamers on a 50 μm I.D. column. A detection limit of ca. 4 fmol is obtained. The linear dynamic range of the calibration plot is slightly over two orders of magnitude (from ca. 1 to 200 ppm).

Separation of polycyclic aromatic hydrocarbons by micellar electrokinetic chromatography with cyclodextrins as modifiers

Abstract The use of α-, β- and γ-cyclodextrins as modifiers to the electrophoretic medium containing sodium dodecyl sulphate and a phosphate-borate buffer in the micellar electrokinetic chromatography of polycyclic aromatic hydrocarbons (PAHS) was investigated. The results showed that the migration behaviour of PAHs could be related to their sizes and the cavity sizes of the cyclodextrins. With the addition of γ-cyclodextrin higher selectivity could be achieved, which consequently resulted in better separation of the seven PAHs investigated.

Antimony speciation by inductively coupled plasma mass spectrometry using solid phase extraction cartridges

A novel and simple method for inorganic antimony speciation is described based on selective solid phase extraction (SPE) separation of antimony(III) and highly sensitive inductively coupled plasma mass spectrometric (ICP-MS) detection of total antimony and antimony(V) in the aqueous phase of the sample. Non-polar SPE cartridges, such as the Isolute silica-based octyl (C8) sorbent-containing cartridge, selectively retained the Sb(III) complex with ammonium pyrrolidine dithiocarbamate (APDC), while the uncomplexed Sb(V) remained as a free species in the solution and passed through the cartridge. The Sb(III) concentration was calculated as the difference between total antimony and Sb(V) concentrations. The detection limit was 1 ng L(-1) antimony. Factors affecting the separation and detection of antimony species were investigated. Acidification of samples led to partial or complete retention of Sb(V) on C8 cartridge. Foreign ions tending to complex with Sb(III) or APDC did not interfere with the retention behavior of the Sb(III)-APDC complex. This method has been successfully applied to antimony speciation of various types of water samples.

Simultaneous speciation of inorganic selenium and tellurium by inductively coupled plasma mass spectrometry following selective solid-phase extraction separation

A new method was developed for the simultaneous determination of inorganic tellurium and selenium species in waters by inductively coupled plasma mass spectrometry (ICP-MS) following selective solid-phase extraction (SPE) separation. Under acidic conditions, only selenium(IV) and tellurium(IV) formed complexes with ammonium pyrrolidine dithiocarbamate (APDC), and the complexes were completely retained on a non-polar C18 cartridge. Te(VI) and Se(VI) passed through the cartridge and remained as free species in the solution, thereafter being determined by ICP-MS. Se(IV) and Te(IV) concentrations were obtained as the respective differences between total selenium and Se(VI), and total tellurium and Te(VI) concentrations. The detection limits (3σ) are 7 ng L−1 selenium and 3 ng L−1 tellurium. Factors affecting the separation and detection were investigated. Coexisting ions did not show significant interferences. This method has been successfully applied to the inorganic selenium and tellurium speciation analysis of water samples with spiked recoveries of 82.3–106%.

Substituted metal carbonyls. Part 21. [M(CO)5(dppf-P)][M = Cr, Mo or W; dppf = Fe(C5H4PPh2)2] as a metalloligand in heteropolymetallic aggregates of AuI, PdII and PtII. Crystal and molecular structures of [PtCl2{(µ-dppf)W(CO)5}2] and [Mo(CO)5(dppf-P)]

The complexes [M(CO)5(dppf-P)][M = Cr, Mo or W; dppf = 1,1′-bis(diphenylphosphino)ferrocene] behave like a monodentate phosphine ligand and displace the labile ligands from [AuCl(SMe2)], trans-[PdCl2(PhCN)2] and cis-[PtCl2(dmso)2](dmso = dimethyl sulfoxide) to yield the corresponding dppf-bridged heteropolymetallic complexes of general formula [M′Clx{(µ-dppf)M(CO)5}y](M′= Au, x=y= 1; M′= Pd or Pt, x=y= 2). Only the trans isomers have been isolated for PdII and PtII. Isomerisation of the M′= Pt, M = Cr complex to the cis form, followed by partial elimination of [Cr2(CO)10(µ-dppf)] to form [PtCl2(dppf-P,P′)], after 3 d in CDCl3 was revealed by NMR spectroscopy. The solution characteristics of both geometrical isomers of the representative M′= Pt, M = Cr complex have been established by two-dimensional NMR studies. UV-Photolytic degradation of the M′= Pd or Pt, M complexes generally gave [M(CO)6], [M(CO)4(dppf-P,P′)], [M2(CO)10(µ-dppf)] and [M′Cl2(dppf)]. The molecular structures of trans-[PtCl2{(µ-dppf)W(CO)5}2] and [Mo(CO)5(dppf-P)] have been determined. The former represents a trimetallic pentanuclear aggregate and the latter a metalloligand with a pendant phosphine on a bimetallic complex. Cyclic voltammetry of all the complexes has been examined and generally reveals one chemically reversible phosphinoferrocene-based oxidation, followed by an irreversible oxidation of the complex.

Determination of trace aluminium by differential pulse adsorptive stripping voltammetry of aluminium(II) -8-hydroxyquinoline complex

Abstract A rapid and sensitive differential-pulse adsorptive stripping voltammetric method, based on the formation of an electroactive complex between aluminium(III) and 8-hydroxyquinoline (HOx), is presented for the determination of trace levels of aluminium. The complex showed an oxidative peak potential at +0.876 V vs. Ag/AgCl (saturated KCl) at a glassy carbon (GC) electrode in aqueous ammonium acetate (0.024 M, pH 6.8. A good linear relationship between the peak current and Al(III) concentration in the ranges 4.00 X 10 −8 –5.00 X 10 −6 and 8.00 X 10 −6 –4.00 X 10 −5 M were observed. In the lower Al(III) concentration region, the anodic peak was the result of the stripping of the absorbed Al(III)-HOx complex, whereas in the higher concentration range, the diffusional oxidation current of the solution species predominated. A surface saturation region was observed from 5.00 X 10 −6 to 8.00 X 10 −6 M Al(III). Relatively few interferences were found and these were easily masked. The detection limit for Al(III) for this method is 1.00 X 10 −8 M and ten determinations of Al(III) at 1.00 X 10 −7 M gave a relative standard deviation of 6.2%. The method was employed in the analysis of Al(III) in a US Environmental Protection Agency water pollution quality control sample (WP 386) and excellent agreement with the certified value was obtained.

Electrochemical detection for liquid chromatography using the wall-jet cell/ultramicroelectrode detector

Abstract A carbon fibre array and a platinum thin ring ultramicroelectrode were constructed and used in conjunction with a large volume wall-jet cell as an amperometric detector in liquid chromatography. Electrode characterization studies were performed in 5.00 m M K 3 Fe(CN) 6 + 1 M KCl aqueous solution. It was found that the wall-jet cell/ultramicroelectrode combination has much reduced flow rate dependence compared to the wall-jet cell/conventional electrode system. LC application in the absence of supporting electrolyte was found to be possible and have good sensitivity using the ultramicroelectrode. Detection limits were 100 pg and 10 ng for the platinum thin ring and the carbon fibre array electrodes respectively.

Poly(catechol) Film Modified Glassy Carbon Electrode for Ultratrace Determination of Cerium(III) by Differential Pulse Anodic Stripping Voltammetry

Oxidative electropolymerization of resorcinol, catechol and pyrogallol at the glassy carbon electrode in different media such as 0.10M NaOH, 0.10M phosphate buffer (pH 7.00) or 0.10M NaClO4 all gave water-insoluble films, adherent on the electrode surface. Amongst them electropolymerization of catechol at the GC electrode in 0.10 M NaOH provided a highly sensitive and selective film for Ce(III) and therefore, this poly(catechol) film modified glassy carbon electrode was exploited for the selective preconcentration of Ce(III) at open circuit, followed by its determination by differential pulse anodic stripping voltammetry both in batch and flow systems. Factors affecting the accumulation, stripping and removal steps were investigated and an optimized procedure was then developed. Under optimized conditions, for batch determination, the calibration plot was linear in the concentration ranges 2.00 × 10–9 M–1.00 × 10–8 M and 2.00 × 10–8 M–1.00 × 10–7 M Ce(III). A detection limit of 2.0 × 10–10 M (0.027 ppb) (SN = 3) was found for a 10 min accumulation. For six successive determinations of CE(III) at concentrations of 2.00 × 10–7, 2.00 × 10–8 and 2.00 × 10–9 M, relative standard deviations were 3.36%, 1.76% and 4.08%, respectively. Similar results were obtained for continuous flow analysis. Interference from selected foreign ions and substances were examined. The developed method was applied to Ce(III) determination in human urine, both in batch and continuous flow systems.

Highly Selective and Sensitive Determination of Silver(I) at a Poly(8-mercaptoquinoline) Film Modified Glassy Carbon Electrode

8-Mercaptoquinoline was found to be electropolymerizable by a cathodic process at a glassy carbon electrode. The resulting modified electrode could be used for selective open circuit preconcentration of Ag(I). This, followed by medium exchange and anodic stripping analysis provided a highly selective and sensitive method for Ag(I) determination. Optimized procedures for both batch and flow analysis were developed. For batch determination, a detection limit of 2.7×10–11 M Ag(I) was achieved (S/N=3, 10 min preconcentration). The relative standard deviations (n=11) for 1.00×10–9 M and 2.00×10–10 M Ag(I) were 2.60 % and 4.74 %, respectively. Similar results were observed for flow analysis. Of 44 selected potential interferents studied, only Au(III) interfered.

Determination of trace thallium after accumulation of thallium(III) at a 8-hydroxyquinoline-modified carbon paste electrode

Thallium(III) was selectively accumulated, in an open circuit, from a stirred Britton–Robinson buffer solution (pH 4.56) onto a carbon paste electrode incorporating 8-hydroxyquinoline. The ensuing measurement was carried out by differential pulse anodic stripping voltammetry after reducing the thallium(III) to metallic thallium in ammonia–ammonium chloride buffer (pH 10.00). Factors affecting the accumulation, reduction and stripping steps were investigated and an optimized procedure was developed. Under optimized conditions, a calibration plot for thallium(III) concentration in the range 5.00 × 10–10–1.60 × 10–5 mol l–1 gave two linear regions arising from different controlling factors during the accumulation step. A detection limit of 2.30 × 10–10 mol l–1(0.047 ppb)(S/N = 3) was found for a 2 min accumulation. For 10 successive determinations of 1.00 × 10–6 mol l–1 and 1.00 × 10–7 mol l–1 TIIII, relative standard deviations, sr, of 2.8 and 4.8% were obtained, respectively. Interferences from other ions and organic substances were examined. The developed method was applied to thallium determinations in sea-water and human urine samples.