Miniature Flame Research Articles

Mechanism of atomization interference by oxygen at trace level in miniature flame hydride atomizers

The mechanism of atomization interference by oxygen at trace level in gases supplied to miniature argon hydrogen diffusion flame (MDF), flame-in-gas-shield (FIGS) and flame-in-flame (FIF) hydride atomizers, was investigated for hydrogen selenide, arsine and stibine. A continuous flow hydride generation system coupled with atomic absorption detection was employed. The effect produced by oxygen contamination, intentionally added to gases as contaminant (0–1290 μl l − 1 ), on spatial distribution of free atoms, on sensitivity and signal shape observed for MDF, FIGS and FIF in different atomization conditions and on the shape of calibration graphs was investigated. The oxygen contaminant reacts with hydrogen radicals reducing their concentration and the size of hydrogen radical cloud. Simultaneously, the rate of free analyte atom reactions is enhanced. Both these factors lead to the decrease of free analyte atom population.

Catalyzed combustion of hydrogen–oxygen in platinum tubes for micro-propulsion applications

The present investigation addresses the need to understand the physics and chemistry involved in propellant combustion processes in micro-scale combustors for propulsion systems on micro-spacecraft. These spacecraft are planned to have a mass less than 50 kg with attitude control estimated to be in the 1–10 mN thrust class. Micro-propulsion devices behave differently than macro-scale devices because of the differences in magnitude of flow rates and heat transfer. Reducing the combustor size increases the relative surface area, increasing the heat loss, and as combustors are continuously reduced in size, they approach the quenching dimensions of the propellants. Combustors of this size are expected to significantly benefit from surface catalysis processes. A miniature flame tube apparatus is chosen for this study because microtubes can be easily fabricated from known catalyst materials, and their simplicity in geometry can be used in fundamental simulations for validation purposes. Experimentally, we investigated the role of catalytically active surfaces within 0.4 and 0.8 mm internal diameter microtubes, with special emphases on ignition processes in fuel rich gaseous hydrogen and gaseous oxygen. Calculations of flame thickness and reaction zone thickness predict that the diameters of our test apparatus are below the quenching diameter of the propellants in most atmospheric test conditions. The temperature and pressure rise in resistively heated platinum microtubes and the exit hydrogen concentration were used as an indication of exothermic reactions. Data on imposed heat flux/preheat temperature required to achieve ignition versus mass flow rate are presented. With a plug flow model, the experimental conditions were simulated with detailed gas-phase chemistry and surface kinetics. Computational results, in general, support the experimental findings.

Effect of contamination by oxygen at trace level in miniature flame hydride atomizers

A new type of atomization interference, caused by traces of oxygen in gases supplied to the atomizer, has been identified in three different types of miniature flame hydride atomizer for atomic fluorescence spectrometry, namely argon–hydrogen diffusion flame (MDF), flame-in-gas shield (FIGS) and double flame (DF), which is a combination of MDF and FIGS. The interference magnitude is dramatically dependent on the analyte element (Se ≪ As ≤ Sb), on the type of atomizer and on the atomizer operating parameters. The thresholds of interference could be as low as a few parts per million (μl l−1) of oxygen in the carrier gas. The main sources of interfering oxygen are, in order, the oxygen stripped out from sample and reagent solutions in the hydride generation step followed by the oxygen diffusing inside the gas circuit of hydride generation apparatus.

Micro flame spectrometer

This paper presents a micro atomic emission flame spectrometer fabricated by standard micromachining technologies. The main component is a micro burner unit which consumes a minimum of oxyhydrogen to produce a stable miniature flame. The oxyhydrogen is generated in a miniaturized electrolysis cell which can be operated by battery. Via a sample gas injection system, which is integrated into the micro burner unit, gaseous samples are injected into the flame. Liquid samples are atomized by a miniature piezo driven ultrasonic atomizer and injected directly. An optical micro spectrometer system is used to investigate the flame emission. Because of the minute scale of all components and the low consumption of oxyhydrogen, which is generated as-required, rather than stored as in conventional systems, the micro flame spectrometer is easily portable and completely safe in operation. Furthermore, at this early stage detection limits just 1½ magnitude above most sophisticated systems are demonstrated for alkali metals and comparable detection limits appear within reach.

Micro flame ionization detector and micro flame spectrometer

In this paper we present a miniaturized flame ionization detector and flame spectrometer fabricated using conventional micromachining technologies. The main component of both devices is a micro burner unit, which uses minimal oxyhydrogen to produce a stable miniature flame. The oxyhydrogen is generated at low energy consumption by a miniaturized electrolysis cell, which can be operated by battery. Because of the low oxyhydrogen consumption and the minute scale of the burner unit and electrolyzer the oxyhydrogen is generated as-required, rather than stored as in conventional systems. Thus, there is no explosion hazard and the devices are not only made easily portable, but also safe. Furthermore, these systems possess sensitivity and selectivity that is comparable to conventional systems. Concentrations down to 1 ppm have been demonstrated with the micro flame ionization detector and a detection limit in the ppb range appears within reach. The micro flame spectrometer is undergoing initial development, but measurements based on atomic emission spectrometry demonstrate already a detection limit only 100-fold above levels observed in conventional systems.

Photon detection based on pulsed laser-enhanced ionization and photoionization of magnesium vapour: experimental characterization

A resonance ionization detector (RID) based on the two-step laser-enhanced ionization of Mg in a miniature air–acetylene flame is described. The detector utilizes the 285.213 nm resonance absorption of Mg as the signal transition, i.e., photons to be measured. Magnesium atoms excited by absorption of photons at 285.2 nm were further excited to higher lying energy states by absorption of photons at 571.2 or 435.2 nm, from which collisional ionization could occur, or by absorption of photons at 300.9 nm, which directly ionized the excited Mg atoms via an autoionizing level above the ionization continuum. The miniature flame used (about 2 mm in diameter) had very low noise characteristics and the minimum detectable number of photons (MDP) was limited, in some instances, by the noise of the transimpedance amplifier used. The lowest MDP, obtained for the excitation scheme 3s21S (285.2 nm)→3p 1P0(435.2 nm)→6d 1D, was experimentally determined to be 1 × 103(7 × 10–16 J). The quantum efficiency of this excitation scheme, defined as the number of electrons created per photon absorbed at the signal transition (285.2 nm), was found to be 0.75. The spectral response bandwidth of the detector in the signal transition was determined to be mostly Lorentzian in nature, owing to pressure broadening in the atmospheric pressure flame, with a stray light rejection ratio of approximately 1 × 10–5 at 100 cm–1 displacement from the absorption maximum of the RID at 285.213 nm. An application of the RID is given in the detection of weak Raman scatter of carbon tetrachloride, chloroform and dimethyl sulfoxide.

Simultaneous detection of alkylselenide, alkyllead and alkyltin compounds by gas chromatography using a multi-channel non-dispersive atomic fluorescence spectrometric detector and a miniature flame as the atomiser

A multi-channel non-dispersive atomic fluorescence spectrometer with a miniature argon-hydrogen flame as the atomiser was interfaced with a gas chromatograph using a wide-bore capillary column. The system was tested for the simultaneous detection of alkylselenides (M2Se and Me2Se2), alkylleads (PbMe4 and PbEt4) and alkyltins (SnMe4 and SnEt4). A phototransistor detector was also used to detect organic substances reaching the flame atomiser. Elimination of the interference effects amongst the analystes and those due to the organic compounds themselves was successful. Large volumes of solvent may affect the signal of a given analyte by quenching if they have the same retention time. This can be overcome by the choice of a suitable solvent. Detection limits (3σ) of 10 pg of Se, 30 pg of Pb and 50 pg of Sn were obtained.

Miniature High Temperature Laminar Diffusion Flames for Analytical Atomic Spectrometry

Miniature high temperature laminar diffusion flames are described for analytical atomic spectrometry. Fuel flow rates below 0.2 liters/min are used and temperatures in excess of 2500 K are maintained in a stable, fuel-rich combustion environment of low background emission. The internal atomizer chamber volume is 1.5 ml (below the burner top). A graphite cup atomizer is used for sample introduction and is “fired” inside the miniature burner head in the presence of “fuel only.” Oxidant diffuses into the fuel zone above the burner top. Compared to conventional electrothermal atomizers, the miniature N2O → H2 diffusion flame greatly reduced particulate light scattering in continuum excited atomic fluorescence spectrometry. In atomic absorption spectrometry, the hot miniature flame serves to effectively reduce the molecular spectral interference due to gaseous diatomic alkali halides. The enhanced gas phase concentration of atomic products presently results in a concentration sensitivity improvement of 30 times over conventional premixed flames used in Eimac continuum excited atomic fluorescence. The improvement in absolute weight sensitivity is 140 times for atomic fluorescence.