Pengyuan Yang

Flow injection–electrochemical hydride generation technique for atomic absorption spectrometry. Invited lecture

A flow injection–electrochemical hydride generation technique for atomic absorption spectrometry has been developed in order to avoid the use of sodium tetrahydroborate, which is capable of introducing contamination. A specially designed thin-layer electrolytic flow cell for hydride generation was used in a normal flow injection system coupled to an electrically heated T-tube atomizer for the atomic absorption measurements. About 200 µl of sample were injected into the electrolyte carrier stream flowing to the electrolytic cell, where hydride-forming elements were reduced to gaseous hydrides. The effects of various factors, e.g., electrode material, electrolyte, current density and carrier stream flow rate, on rate of formation of the hydride and interferences have been studied. The technique has been applied to the determination of As, Se and Sb in various samples. The detection limits for these elements in aqueous solutions were 0.45, 0.62 and 0.92 ng ml–1, respectively, with precisions of 1.2–1.8%.

On-line flow injection cobalt–ammonium pyrrolidin-1-yldithioformate coprecipitation for preconcentration of trace amounts of metals in waters with simultaneous determination by inductively coupled plasma atomic emission spectrometry

The technique of on-line flow injection (FI) cobalt-ammonium pyrrolidin-1-yldithioformate (Co–APDC) coprecipitation for the preconcentration of trace amounts of the heavy metals Cd, Cu, Fe, Ni, Pb and Zn in rain water samples with simultaneous determination by inductively coupled plasma atomic emission spectrometry (KP-AES) has been developed. A precipitate collector system, consisting of a poly(tetrafluoroethylene)(PTFE) membrane on a polypropylene support filtering device combined with a 1.5 m reaction coil, was selected. An inorganic solution of concentrated nitric acid and hydrogen peroxide was applied as the dissolution reagent. The technique is characterized by high retention efficiency (which ranged from 77 to 99% for the six elements of interest), good enrichment factors (ranging from 10 to 50 for 100 s preconcentration depending on the elements studied) and satisfactory accuracy and precision (recoveries from two standard additions to a rain sample ranged from 92 to 104%, with relative standard deviations ranging from 1.9 to 5%). The sample throughput is 20 per hour.

Study of a pulsed glow discharge ion source for time-of-flight mass spectrometry

Abstract This paper describes a new type of glow discharge (GD) ion source coupled to a time-of-flight mass spectrometer (TOFMS). The GD is operated in the microsecond pulse (μs-pulse) mode. The operational parameters of the μs-pulse GD were optimized against the ion signals, giving 180 Pa for the discharge pressure, 3 A for the transient discharge current, 1.75 kHz for the discharge frequency and 2 μs for the discharge pulse duration. Experimental results show that the discharge current in the μs-pulse mode can be one order of magnitude higher than that obtained in the d.c. mode. The structure of the interface between the μs-pulse GD and the mass spectrometer was found to be critical, and a Macor disc must be applied in front of the sampling orifice in order to shield the sampling plate from the anode of the GD to achieve both a good vacuum and the best sputtering. A transient sputtering rate of 24.4 μs s−1 mm−2 was reached in the μs-pulse mode and was significantly higher than that for the d.c.-GD. Typical mass spectra of brass and nickel samples were studied and are discussed.

On-line electrolytic dissolution of alloys and multi-element determination by inductively coupled plasma atomic emission spectrometry

Abstract A method for the direct simultaneous multi-element analysis of solid metal samples by flow-injection on-line electrolytic dissolution and inductively coupled plasma atomic emission spectrometry was developed and applied to the determination of Zn, Si, Fe, Mn, Cr, Mg and Cu in aluminium alloys. The suitable range for the determination was 10 μg g−1–100 mg g−1. The precision of the entire analysis, including electrolysis and determination, was in the range 1–6%, depending on the element and its concentration in the alloys and on the current density applied in electrolysis. The analysis of a sample can be accomplished in a few minutes.

Thermospray nebulizer as sample introduction technique for microwave plasma torch atomic emission spectrometry

Abstract A thermospray nebulizer was used as a sample introduction device for microwave plasma torch (MPT) atomic emission spectrometry (AES). Experimental parameters, including the power supplied to the MPT, the flow rates of support and carrier gases, the observation height, the sample uptake rate, the thermospray working temperature, the temperature of the aerosol spray chamber and cooling water were optimized. Under optimum conditions, the relative standard deviation (RSD) of 10 measurements for 21 elements is in the range 0.3–2.0%. The detection limits were improved in comparison with the ultrasonic nebulizer as sample introduction technique for MPT–AES. The inclusion of 20% methanol into the MPT showed there is no effect on the stability of MPT discharge. The technique can thus be held to have the potential for interface to reverse-phase HPLC systems.

Preliminary study on the use of palladium as a chemical modifier for the determination of silicon by electrothermal atomic absorption spectrometry

This paper describes the use of palladium salt as chemical modifier for the determination of silicon by electrothermal atomic absorption spectrometry. It is found that the optimal atomization temperature of silicon appeared to be flat above 2500 °C and that the sensitivity was significantly improved in the presence of palladium. The method was validated by the analysis of National Bureau of Standards (now National Institute for Standards and Technology) Standard Reference Materials 361, 362 and 363 Low Alloy Steels.

Microwave-induced plasma boosted microsecond-pulse glow discharge optical emission spectrometry

A microsecond-pulse (µs-pulse) glow discharge (GD) source boosted by na microwave-induced plasma (MIP) has been developed and studied for noptical emission spectrometry (OES). The excitation processes of the ntandem GD source were investigated. The analytical characteristics of the nGD-OES source in the presence and absence of the MIP were compared, nincluding the operating parameters, signal-to-background ratios (S/B) and nrelative standard deviation (RSD). The results show that under a nrelatively low discharge pressure (<180 Pa), the µs-pulse GD can ncouple fairly well with the MIP and emit intense analytical lines. When nthe GD source is operated under a pressure higher than 200 Pa, two nemission peaks appear, independent in time, for a given resonance atomic nline, because sample atoms are independently structurally excited, first nby the µs-pulse GD and then by the MIP. The time interval between the ntwo peaks for Zn I 213.8 nm is longer than that for Cu I 324.7 nm, which nis believed to be due to the faster diffusing velocity of copper atoms. nWhen the µs-pulse GD lamp is operated under a gas pressure higher than n220 Pa, the ion population is so high that Cu II ionic line at 224.7 nm n‘becomes’ two peaks because of a possible self-absorption. The nresults show that the supplementary use of an MIP can eliminate the nself-absorption of ionic and atomic lines. When the µs-pulse GD source nis coupled with the MIP, S/Bs are improved by a factor of more than one norder of magnitude for several analytical lines. A short-term RSD of 0.2% nis achieved for the ‘µs-pulse GD+MIP’ configuration ncompared with that of 1.0% for ‘µs-pulse GD only’ mode. The nexperimental results show that the MIP boosted µs-pulse GD is a npromising technique for solid sample and surface nanalysis.

High-resolution spectra of selected rare earth elements and spectral interferences studied by inductively coupled plasma-atomic emission spectroscopy (ICP-AES)

The rare earth elements (REEs) play very important roles in industrial manufacturing, technology development and biological processes. Due to their complex emission spectra, trace levels of REEs are difficult to analyze by conventional ICP-AES techniques. The present study investigates possible spectral interferences of matrices (rare earth oxides of Ce, Pr, Nd, Sm and Dy) on the analytical lines (± 0.1 nm) of a target REE. Detailed and well-resolved spectra for selected REEs are presented, and procedures used to rectify the problem of spectral interferences caused by REE matrices are discussed. A computer-assisted system (CAS) for spectral recognition has been developed and used to assist in the study of matrix interference. To determine directly trace rare earth elements in REE matrices without sample pre-separation, the application potential is demonstrated with a one meter sequential instrument retrofitted with a 3600 grooves/mm grating.

Suppression of gas species signals in direct current glow discharge time-of-flight mass spectrometry

The possibility of suppressing gas species in direct current glow discharge mass spectrometry (dc-GDMS) with a linear time-of-flight mass analyzer was investigated. With this dc-GD ion source, a ‘clean’ mass spectrum rich in analyte could be obtained when the dc-GD was operated under a discharge current of 15–30xa0mA and a gas pressure of 300–500xa0Pa, in contrast to the strong signals of gas species in conventional dc-GDMS, which operates at lower currents and pressures (typically 1–5xa0mA and 100xa0Pa). Such an experimental result is believed to be due to increased sputtering at higher pressures and currents, and the different ionization mechanisms of analyte and gas species. For a possible GD design to eliminate the background gas ions, a new discharge configuration was developed by attaching a TM n 010 n microwave resonator to the GD ion source. The mass spectrum of the cathode sample showed a low gas species background when the microwave-induced plasma (MIP) discharge was ‘off’ under different dc-GD parameters. The mass spectra of analyte and gas species obtained with ‘MIP+dc-GD’ and ‘MIP only’ modes are also compared and discussed. It was found that the analyte signals decrease and the gas species signals increase in the presence of the MIP, and that the analyte signals nearly disappear in the ‘MIP only’ mode. Preliminary results suggest that, for specific discharge conditions and with a suitable design of the GD source, an efficient suppression of gas species in dc-GDMS detection could be realized.

Prediction of spectral interference in inductively coupled plasma-atomic emission spectrometry by primary computer expert system

Spectra and spectral interferences in inductively coupled plasma-atomic emission spectrometry, ICP-AES, have been simulated and predicted by a primary expert system under contrasting conditions of local temperature equilibrium (LTE) and non-LTE. In this expert system a comprehensive computer model has been applied to provide expert knowledge on an non-LTE ICP discharge, analyte ionization and excitation, and spectral line shape. The system also includes several databases to supply the calculation with spectral and elemental parameters. Some typical examples are illustrated, with satisfactory outcomes. It is found that predictions under non-LTE conditions are much closer to the reality than those under LTE conditions.