Radical theory of hydride atomization confirmed after four decades - determination of H radicals in a quartz hydride atomizer by two-photon absorption laser-induced fluorescence

Atomization of antimony hydride and in-situ preconcentration of antimony in a dielectric barrier discharge atomizer: A mechanistic study by laser induced fluorescence

BACKGROUND

Dielectric barrier discharge (DBD) ambient plasma has been recently used as hydride atomizers for atomic absorption spectrometry (AAS). DBD performance in terms of sensitivity has been proven comparable with the most common hydride atomizer, heated quartz tube (QTA), for some analyte hydrides while being significantly worse for the others. Moreover, a simple approach to analyte hydride preconcentration directly in the DBD (in-situ) prior to AAS detection has been reported with preconcentration efficiency significantly analyte-dependent. Detailed insights into the mechanisms of analyte hydride atomization and preconcentration are thus essential to utilize the full potential of DBD atomizers in analytical routine.

RESULTS

Using SbH3 as a model analyte hydride and laser induced fluorescence (LIF) as a detector, absolute concentration of Sb free atoms was quantified and their spatial distribution in the DBD discharge was visualized. The atomization efficiency of SbH3 reaches (75 ± 20) % with homogeneous distribution of Sb free atoms in the whole DBD discharge area indicating long life of ground state free Sb atoms. In addition, the mechanisms of in-situ preconcentration of antimony in the DBD were investigated using LIF. The release of preconcentrated antimony from the inner quartz surface of the DBD walls was visualized and temporally resolved formation of free Sb atoms was acquired. Free atoms are firstly observed in the gas phase in the central part of the DBD, where they had been preconcentrated, having the character of a wave traveling towards the atomizer edges within approximately 2 s.

SIGNIFICANCE

Both, high atomization efficiency and long life of free Sb atoms found by LIF prove perfect compatibility of DBD atomizer with AAS detection. This agrees well with high sensitivity reached in DBD atomizer in AAS which is comparable to that achieved in QTA. In preconcentration mode, spatio-temporally resolved LIF measurements revealed analyte trapping in a narrow spot in the central part of the DBD and enabled to study the dynamics of its subsequent release.

Fluorescence of Atomic Germanium - Solution of Excitation Transfer

Laser induced fluorescence was used for detection of free germanium atoms in a miniature diffusion flame atomizer. Two excitation schemes were tested, which were based on excitation wavelengths around 250 nm and 205 nm. The second scheme suffered from a fast collisionally induced excitation transfer to another radiative state, which could led to an erroneous overestimation of Ge concentration by an order of magnitude. The excitation transfer was quantified and the problem was solved by means of measurement of temporally resolved fluorescence spectra. The equations for evaluation of fluorescence measurements affected by an excitation transfer to radiative states are presented. The obtained Ge concentration profiles revealed a strongly inhomogeneous distribution of free Ge atoms and a low germane atomization efficiency in the flame.

Unified analysis method for total and inorganic As determination in foodstuffs by hydride generation high-resolution continuum source quartz tube atomic absorption spectrometry

A unified analytical method applicable to common foodstuff matrices was developed and characterized for total and inorganic arsenic determination by hydride generation high-resolution continuum source quartz tube atomic absorption spectrometry, which was established based on different sample preparation procedures. This new method was found to be interference-free and cost-effective in terms of reagents consumption for sample preparation and derivatization to arsine for the inorganic arsenic fraction. Microwave-assisted digestion in HNO3-H2O2 for total arsenic and extraction in 0.28 mol L-1 HNO3 by mechanical stirring in a water bath or ultrasound-assisted extraction in 0.01 mol L-1 HCl without separation of inorganic As, all coupled with arsine generation in 0.01 mol L-1 HCl medium with 0.6% NaBH4 in 0.01% NaOH in the presence of 0.2% L-cysteine was found to be suitable for all matrices. The results were statistically compared by applying Tukey's and Dunnett's multiple comparison methods (p > 0.05). The use of external calibration with As(III) standards and standard addition method for quantification showed the lack of non-spectral interferences from the multimineral matrices, resulting in a reliable method for total/inorganic As determination in various foodstuffs. The limits of detection for total/inorganic As using peak height measurement were 0.0044 ± 0.0005/0.0022 ± 0.0003 mg kg-1 (n = 25 days). The overall recovery for total/inorganic As in the certified reference materials was in the range of 98% ± 22%, and 99% ± 24% (k = 2). The extraction of inorganic As in 0.01 mol L-1 HCl and 0.28 mol L-1 HNO3 provided the recovery of 106% ± 25% and 100% ± 25% (k = 2), which was better than in 10 mol L-1 HCl. The precision of measurements in real samples of fish muscle, meat and organs, rice and rice-based baby foods with contents of 0.052-5.29 mg kg-1 total As and 0.005-0.063 mg kg-1 inorganic As was 9.8-18.8% and 8.7-32.0%, respectively, which was calculated based on the combined uncertainty.

Atomization of As and Se volatile species in a dielectric barrier discharge atomizer after hydride generation: Fate of analyte studied by selected ion flow tube mass spectrometry

Atomization of hydrides and their methylated analogues in a dielectric barrier discharge (DBD) plasma atomizer was investigated. Selected ion flow tube mass spectrometry (SIFT-MS) was chosen as a detector being capable of selective detection of non-atomized original volatile species allowing thus direct quantification of atomization efficiency. Selenium hydride (SeH₂) and three volatile arsenic species, namely arsenic hydride (AsH₃), monomethylarsane (CH₃AsH₂) and dimethylarsane ((CH₃)₂AsH), were selected as model analytes. The mechanistic study performed contributes to understanding of the atomization processes in atomic absorption spectrometry (AAS). The presented results are compatible with a complete atomization of arsenic hydride as well as its methylated analogues and with atomization efficiency of SeH₂ below 80%. Using AsH₃ as a model analyte and a combination of AAS and SIFT-MS detectors has revealed that the hydride is not atomized, but decomposed in the DBD atomizer in absence of hydrogen fraction in the carrier gas. Apart from investigation of analyte atomization, the SIFT-MS detector is capable of quantitative determination of water vapor content being either transported to, or produced in the atomizer. This information is crucial especially in the case of the low-power/temperature DBD atomizer since its performance is sensitive to the amount of water vapor introduced into the plasma.

Effect of additives on cadmium chemical vapor generation and reliable quantification of generation efficiency

Chemical vapor generation (CVG) of cadmium was optimized based on response from atomic absorption spectrometry (AAS) with a heated quartz tube atomizer (QTA). Effect of several modifiers on analytical performance was studied. These additives were: inorganic salts of Cr³⁺, Ti⁴⁺ and Co²⁺ and their on-line synthesized complexes with KCN and thiourea, respectively. The use of these additives resulted in sensitivity enhancement, better repeatability and correspondingly in improvement of overall CVG efficiency. The latter was quantified by two independent approaches: a) by means of ^115mCd radioactive indicator, b) from comparison of sensitivities obtained with conventional solution nebulization and with CVG, both coupled simultaneously to inductively coupled plasma mass spectrometry. Both approaches provided comparable results. The highest efficiency, between 60 and 70%, was reached in the presence of Cr³⁺/KCN and Ti⁴⁺/KCN while 19% was achieved in Co²⁺/ascorbic acid/thiourea environment. Highly irreproducible results with low CVG efficiency ranging from 2.5 to 15% were reached in the absence of any additives. The generated cadmium species were identified to be mostly free atoms regardless of the additives presence or their absence. Cr³⁺/KCN environment was selected as the most robust for CVG of Cd reaching sensitivity of 6.6 s ng^-1 Cd and limit of detection of 60 pg mL^-1 Cd (9 pg Cd absolute) with detection by QTA-AAS.