W. L. Clevenger

Trace Determination of Mercury: A Review

Abstract Methods used for trace (sub-ppm) detection of mercury are reviewed. Included are spectrometric methods such as atomic absorption, fluorescence, emission, and mass spectrometry; electrochemical methods; and as radiometric methods as well as other common and novel techniques. Limits of detection are reported where given, and advantages and disadvantages are compared.

Laser Enhanced Ionization of Mercury Atoms in an Inert Atmosphere with Avalanche Amplification of the Signal.

A new method for laser-enhanced ionization detection of mercury atoms in an inert gas atmosphere is described. The method, which is based on the avalanche amplification of the signal resulting from the ionization from a selected Rydberg level reached by a three-step laser excitation of mercury vapor in a simple quartz cell, can be applied to the determination of this element in various matrices by the use of conventional cold atomization techniques. The overall (collisional + photo) ionization efficiency is investigated at different temperatures, and the avalanche amplification effect is reported for Ar and P-10 gases at atmospheric pressure. It is shown that the amplified signal is related to the number of charges produced in the laser-irradiated volume. Under amplifier noise-limited conditions, a detection limit of ∼15 Hg atoms/laser pulse in the interaction region is estimated.

Distortion-Free Microchannel Plate Mercury Atomic Resonance Ionization Image Detector

An atomic mercury resonance image detector with microchannel plate amplification and charge-coupled device (CCD) detection is evaluated. A thin Pt film on the surface of the input window of the resonance ionization image detector (RIID) eliminated the surface charge on the input window. Image spatial resolution of better than 120 μm was achieved. The linearity of the image intensity vs. imaging signal energy is currently limited by the linearity of the CCD. In addition, the image quality (spatial resolution and dynamic range) is improved by using a microchannel plate (MCP) in front of the input window of the RIID. The RIID has a much improved contrast ratio. Several noises, including multiphoton photoionization noise, noise due to parasitic luminescence of the phospher screen, and MCP noise, were minimized. The main noise source was the photoelectric effect of the metal input electrode of the CCD when illuminated by 254 nm radiation.

Analytical time-resolved laser enhanced ionization spectroscopy I. Collisional ionization and photoionization of the Hg Rydberg states in a low pressure gas

The temporal behavior of the laser enhanced ionization signal of mercury was studied in a quartz cell under low buffer gas pressure. Using fast electronics and a short (34 ns) laser pulse, it was possible to distinguish, in one single time-resolved ionization waveform, the non-selective photoionization component of the signal from that which was due to collisional ionization from selected levels. Experimental results were shown to agree with those obtained by computer simulation, and optimal conditions for deconvolution of the two components were studied.

Laser-enhanced ionization spectroscopy of mercury Rydberg states

The spectral characteristics of mercury Rydberg states (n = 10–42) were observed and studied. Each principal quantum number was observed as a triplet, and after observing a quartet of lines for n = 10, 11, and 12, these lines were assigned, from red to blue, as belonging to upper levels 1P10, 3P20, 3P10, and 3P00. These levels were studied as a function of variable applied high voltage (between the electrodes) and variable buffer gas pressure. In this way, the broadening and splitting caused by the influence of Stark effects and of increasing buffer gas pressure were observed for different n values. These observations will allow one to choose the optimal level for excitation, as well as the optimal operating conditions in terms of pressure and applied high voltage, for obtaining the best sensitivity and limit of detection by analytical laser-enhanced ionization spectroscopy.

Optical Emission Detection of Charged Particles after Selective Laser Ionization of Mercury Atoms in a Buffer Gas

A method for the optical detection of the resonance ionization signal of mercury atoms in a buffer gas is described that is based on the emission from buffer gas atoms that are collisionally excited by interactions with electrons in a strong electric field. The first observations of this phenomenon are reported here, along with comparisons between optical and electrical detection. Advantages of a pulsed electric field over a continuous field are described. A wide range of possible applications for this type of gas phase detector are suggested.

Laser-enhanced ionization detection of magnesium atoms by a combination of electrothermal vaporization and flame atomization

A system for the electrothermal vaporization flame atomization laser-enhanced ionization (ETV-FL-LEI) detection of Mg was optimized and completely characterized. The vaporization, transport, atomization, probing, and detection efficiencies were all determined experimentally. The overall efficiency of the system was found to be 0.00251%. The experimental detection limit of 2 ng ml–1(20 pg) for Mg was limited by noise due to the blank signal. A detection limit of 590 fg ml–1(5.9 fg) could be achieved in the absence of the blank and a reduction of radiofrequency noise.

Plasma emission in a pulsed electric field after resonance ionization of atoms

A technique is presented which is based on the measurement of the emission of excited buffer gas atoms which are created by interactions with electrons in a strong pulsed electric field after laser assisted ionization of analyte atoms. In principle, this emission method can be applied for the detection of single atoms, molecules, or photons.

Temporal behavior of mercury LEI signal in a buffer gas

The temporal behavior of the laser enhanced ionization signal of mercury was studied in a quartz cell under low buffer gas pressure. Using fast electronics and a short (34 ns) laser pulse, it was possible to distinguish between the non-selective photoionization component of the signal and that which was due to collisional ionization from selected levels in one time-resolved ionization waveform.

Resonance ionization detection of 253.7 nm photons from mercury atoms

Avalanche detection of a laser enhanced ionization (LEI) signal has been studied in a resonance ionization detector (RID) cell containing mercury vapor at room temperature. Single photoelectron events were detected with an avalanche multiplication factor of more than 8000.