Kin C. Ng

Electrothermal Vaporization for Sample Introduction in Atomic Emission Spectrometry

The importance of trace elements in environmental, nutritional, clinical, forensic, toxicological, and other fields has been well recognized. Among the variety of analytical techniques available for trace element determinations, atomic spectrometry is one of the most popular. This technique may further be divided into atomic absorption, atomic emission, and atomic fluorescence, with the latter two more amenable to multielement analyses. These atomic techniques require the introduction of samples into a high-temperature atom reservoir where atomic (or ionic) vapors are produced. These high-temperature atom sources include flames; the inductively coupled plasma, ICP; microwave-induced plasma, MIP; direct current plasma, DCP; and the graphite furnace.

Microliter sample introduction into an inductively-coupled plasma by electrothermal carbon cup vaporization

Abstract An atomic emission spectrometric system is described for quantifying trace elements in microvolume samples. The system involves vaporizing the sample by electrothermal carbon cup vaporization followed by the atomization and excitation of the vapor cloud in an inductively coupled plasma (i.c.p.). The detection limits for 21 elements in 10-μl samples are at ng ml -1 and sub-ng ml -1 levels with linear dynamic ranges of over four orders of magnitude. Carbon cups coated with pyrolytic graphite are overcoated with tantalum carbide. These cups have resulted in improved detection levels (performances) for Al, As, Bi, Co, Cu and Sn relative to those not containing tantalum. However, cups not treated with tantalum are superior for Au, Cd, Ge, Hg, K, Li, Mg, Mn, Rb and Zn. Comparisons between the two types of carbon cups are presented and discussed. Results also are compared with literature values available for other electrothermal vaporization systems.

Volatilisation of zirconium, vanadium, uranium and chromium using electrothermal carbon cup sample vaporisation into an inductively coupled plasma

Zirconium, vanadium, uranium and chromium react with ammonium chloride (7%m/V) when heated in an electrothermal carbon cup to form their corresponding chlorides. These metal chlorides are subsequently vaporised into an inductively coupled plasma for optical emission spectroscopy. The preferential halide formation of these refractory elements in the electrothermal carbon cup has allowed their determinations to proceed with sub-nanogram detection limits and adequate precision of about 6% relative standard deviation for 5-µl samples. Linear dynamic ranges span about three orders of magnitude.

Laser-induced plasma atomic emission spectrometry in liquid aerosols

Abstract Atomic emission spectrometry was performed in a laser-induced plasma in air. The plasma was produced by focusing the beam of an Ar-F (193 nm) excimer laser into a liquid aerosol nebulizer system. A liquid aerosol was generated with a concentric glass nebulizer-spray chamber assembly and carried into the plasma by the argon (0.5 1 min −1 nebulizer gas. In general, the emission signal lasted ca. 35−40 μs after the laser pulse. The excitation temperature of the plasma decreased from 3994 K at 1 μs to 3607 K at 35 μs after the laser pulse. The solution limits of detection (3σ of the blank) were determined for Na, Li, In, Al, Ga, Ca, Mg, K and Sr to be 0.9, 0.3, 10, 3, 3, 8, 3, 2 and 20 μg ml −1 , respectively. The sensitivity obtained with this system is similar to that of a previous Nd: YAG wet droplet system reported in the literature.

The Applicability of a Multiple-Mode Diode Laser in Flame Atomic Absorption Spectroscopy

A multiple-mode laser diode is used to supply the primary radiation for atomic absorption spectroscopy (AAS). The multiple wavelength characteristics of the laser allow peak ratio measurements and possible background correction by the monitoring of the absorption spectrum with a diode-array spectrometer. In this demonstration, lithium is used as the analyte and air/acetylene flame is used as the atom reservoir. The detection sensitivity (1% absorption) is 20 ng/mL and the detection limit (2 σ of blank) is 4 ng/mL; these values are comparable to those of the conventional flame AAS with the use of a hollow cathode lamp as the source radiation.

Microwave-Induced Plasma Atomic Absorption Spectrometry with Solution Nebulization and Desolvation-Condensation

Electrical discharge (plasma)-based analytical instruments are commercially available. These include the inductively coupled plasma atomic emission spectrometer (ICP-AES), the ICP-mass spectrometer (MS), the gas chromatograph/microwave-induced plasma (MIP) AES, the glow-discharge MS, the direct-current plasma AES, and the ICP atomic fluorescence spectrometer (ICP-AFS). High-temperature inert-gas plasmas are believed to be excellent sources for excitation spectrometry—namely, atomic emission, and ionization for mass spectrometry. The success of ICP-AFS, however, indicates that there also is an abundance of ground-state species for sensitive monitoring. The ICP-AFS technique offers parts-perbillion detection limits.

ATOMIC EMISSION DETECTION OF NON-METALS IN VAPORS INJECTED INTO HELIUM HOLLOW CATHODE DISCHARGE

In this note, we report the direct injection of organic solution (vapor) into a running HCD for the atomic emission detection of Cl, Br, F, I and S. We plan to use this system for GC detection

Direct solid sampling in capacitively coupled microwave plasma atomic emission spectrometry

A capacitively coupled microwave plasma operating in the range between 500 and 700 W is used as an excitation source for the analysis of solid samples. National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) Tomato Leaves (SRM 1573a) and Coal Fly Ash (SRM 1633a) are used for the evaluation of the technique. The system contains a graphite electrode–cup in which the solid sample is deposited. Heating of the electrode–cup vaporizes the analyte into the plasma for atomic emission spectrometry. The detection limits (defined as 3σ of the background) for Mn, Ca, Mg, Zn, Cu, As, Rb and Pb in Coal Fly Ash, and Cd, Fe, Cu, Zn, Zn, Sr, Rb, Mg and Pb in Tomato Leaves were determined. The plasma gas used in this study was 20% nitrogen and 80% helium.

Development of a scanning surface probe for nanoscale tip-enhanced desorption/ablation

We report on the development of a versatile scanning apparatus for nanoscale surface sampling that utilizes the interaction of laser radiation at a sharp probe tip to effect desorption/ablation on opaque substrates. The process, which currently yields surface craters as small as approximately 50 nm diameterx5 nm deep, has been demonstrated with both metal-coated and bare silicon tips. Desorption/ablation under the tip occurs at illumination intensities below the corresponding optical far-field threshold, suggesting that the latter process should not degrade the spatial resolution attainable for proposed chemical imaging methods based on the scanning surface probe.

Combined Apertureless Near-Field Optical Second-Harmonic Generation/Atomic Force Microscopy Imaging and Nanoscale Limit of Detection

A dual function atomic force/near-field scanning optical microscope (AFM/NSOM) with an ultrafast laser excitation source was used to investigate apertureless, tip enhanced second-harmonic generation (SHG) of ZnO nanowires with spatial resolution below the optical diffraction limit. Single-wire SHG spectra show little to no contribution from bandgap or other emission. Polarization data established values for X33/X31 close to previous estimates and confirm the SHG process. Experimental results indicate that the SHG signal was reduced for nanowires after exposure to an atmosphere of carbon dioxide and water vapor. An equation was derived for estimating the minimum X(2) detectable using apertureless SHG NSOM.