CZ306804B6 - An atomizer of volatile species based on an external dielectric barrier discharge with capillary electrodes - Google Patents

Abstract

The technical solution relates to a new structure of the atomizer of volatile species based on an external dielectric barrier discharge with capillary electrodes. The advantage of the technical solution comparing to the previously used atomizers based on dielectric barrier discharge is the detection of free analyte atoms by atomic spectral methods directly in the plasma area. By a suitable choice of the arrangement of the technical solution, the filamentary discharge, which is necessary for achieving efficient atomization, is formed. The plasma area is shielded from the surrounding atmosphere by an inert gas stream that prevents diffusion of oxygen into the atomization area and thus does not reduce atomization efficiency. Due to the thin-walled quartz electrodes of a small outer diameter, the plasma reaches low performance, and there is no further significant turbulence in the plasma or shielding gas that destabilize the discharge or significantly increase the base line noise. Another advantage is the unrestricted plasma through the walls of the atomizer and therefore there is no loss of radiation through the walls of the atomizer, which results in a multiple reduction of the base line noise of the spectrometer signal and thus in the improvement of the signal noise ratio.

Description

The present invention - a capillary electrode dielectric barrier discharge volatile atomizer can be used to determine trace metal concentrations in conjunction with an atomic absorption or atomic fluorescence spectrometer.

BACKGROUND OF THE INVENTION

Generation and atomization of volatile species using atomic absorption or atomic fluorescence spectrometers is an established and extended method for determination of most elements of the periodic table. The generation of volatile species consists in converting the analyte into a volatile form suitable for transfer to an atomizer for atomization. The advantages of this technique over the introduction of a liquid sample into the atomizer by misting are its high transport efficiency (100%, compared to 5-10% misting) and the separation of the analyte from the sample matrix by gas phase transfer to minimize interference from the matrix. Generation technique is most often used for hydride-forming elements mostly As, Se, Sb, Bi, Pb, Sn or for Hg and Cd. (Dedina, J., Enc. Of Anal. Chem., Wiley, 1999-2014, 1-39).

The atomization of the generated analyte in gaseous form is carried out in conjunction with an atomic absorption spectrometer (AAS) mainly in quartz atomizers. In conjunction with an atomic fluorescence spectrometer (AFS), a diffusion flame atomizer or a flame-gas-shield atomizer is used, which, thanks to its design, allows to significantly reduce detection limits for hydride-forming elements. (Marschner, K. et al., Spectrochim. Acta Part B, 109, 2015, 16-23). For all mentioned types of atomizers, the atomization of the analyte occurs in the same area in which the free atom signal is detected by both AAS and AFS. The spectrometer beam therefore passes through the region in which the analyte is atomized and where the free atom density is highest.

In recent years, work has been published describing the use of another type of atomizer based on the so-called dielectric barrier discharge (DBD) (Meyer, C. et al., Analyst 136, 2011, 24272440). This plasma discharge has proven to be a useful tool in analytical spectrometry as it can serve as a source of free atoms, excited species or ions. As a result, DBD shocks can be combined with AAS (ZL Zhu et al., Spectrochim. Acta B, 61, 2006, 916-921; ZL Zhu et al., Anal Chem, 78, 2006, 865-872), with AFS (ZL Zhua et al., Spectrochim Acta B, 63, 2008, 431-436; ZL Zhu et al. Anal Chim Acta, 607, 2008, 136-141.), But also by atomic emission spectrometry techniques (YL Yu et al., Anal Methods 7, 2015, 1660-1666), or mass spectrometry (C. Guo et al., Anal. Bioanal. Chem. 407, 2015, 2345-2364).

DBD volatile compound atomizers with AAS or AFS detection utilize atomization in low temperature miniature plasma generated in the carrier gas at atmospheric pressure. It is a partially ionized environment in which thermal equilibrium is created separately for the electron component and separately for ions and neutral particles. Most of the energy exposed to the discharge is used to heat the electron component. Typical mean electron energy ranges from 2 to 10 eV. The temperature of ions and neutral particles in the discharge is increased by only a few tens to hundreds of Kelvin. The resulting high energy electrons collide with gases to form various ions and radicals, thereby promoting molecular dissociation or radical production and other chemical reactions. DBD plasma is formed between two planar parallel (or cylindrically arranged) metal electrodes typically spaced several mm apart, at least one of which is covered by a dielectric layer and between which a high alternating voltage is interposed. DBD atomizers have advantages over conventional atomizers in small dimensions, in the need of low power, low working temperature (up to 50 ° C) and in the possibility of use in the field.

- 1 GB 306804 B6

In conjunction with AAS, DBD constructions are used in planar configuration, while tubular constructions are used with AFS. Planar The DBD atomizer has the shape of a cuboid shell without a base horizontally placed on the axis of the spectrometer, with flat electrodes attached to larger outer walls. The electrode gap is as short as possible (typically 3 mm) to maximize the field strength. So far, there are 7 works in the literature using DBD atomizers with AAS detection, always in the planar arrangement of atomizers for atomization of the hydrides As, Se, Sb, Bi and cold vapor Hg. The free atoms of the analyte are formed in the region of the atomizers, which are directly passed through the spectrometer beam, ie the region of atomization and detection is identical. The analytical characteristics achieved in planar DBD atomizers (detection limit, resistance to interference) are comparable to those achieved in quartz atomizers for AAS. It straightens out the construction of DBD atomizers as used in AAS cannot be used in AFS because the geometry of this atomizer is not compatible with beam path and AFS measurement principle.

While in the AAS the radiation source, atomizer and detector lie in one line, in the AFS fluorescence radiation is detected perpendicular to the direction of the excitation radiation. In order not to block the walls of the atomizers in the AFS radiation from the source, a cylindrical arrangement of the DBD atomizers is used. The atomizer consists of two concentric glass or quartz tubes and an internal electrode (cylinder shape, located in the center of the tubes). The space between the inner electrode and the inner tube forms the region of the atomizer itself, which has the shape of an annulus. The volatile compound is fed into this space and atomized. The other of the cylindrical DBD electrodes is in the form of a ring located outside the inner tube. Walls of inner tube and event. the inner electrodes, when the shell is glass or quartz, function as a dielectric barrier. A disadvantage of the cylindrical arrangement of DBD atomizers is the plasma generated inside the atomizers, in the annulus between the inner and outer electrodes. In this region, the volatile compound is atomized, while the detection of free atoms occurs above the mouth of concentrically arranged tubes of this DBD atomizer outside the active plasma region. The active region of the plasma is impenetrable to the spectrometer measuring beam due to an external electrode covering the wall of the tube supplying the volatile species and the plasma gas. In the region above the mouth of the DBD atomizers, the concentration of free atoms is already lower than in the active region of the plasma, since the free atoms gradually disappear by chemical reactions, especially with oxygen from the surrounding atmosphere. The extinction of free atoms is minimized by the use of an inert gas shielding unit, which is realized by an outer tube concentrically positioned around the outer electrode tube. Feed in the lower part of the outer tube is supplied with an inert gas flow rate of about 1-3 1 min ^-1 so as to be above the mouth of the tube, created a laminar flow of inert gas, which protects free carbon from contact with the surrounding atmosphere and prolongs their lives. Even with shielding gas, however, the amount of free atoms above the mouth of the tubes is lower than in the plasma core within the DBD atomizers. In conjunction with AFS detection, DBD atomizers were used to determine hydrides of As, Se, Pb, Sb, Bi and cold mercury vapors, a total of 7 works: ZL Zhu et al., Spectrochim Acta B, 63, 2008, 431-436 ; ZL Zhu et al., Anal Chim Acta, 607, 2008, 136-141; Z. Xing et al., Spectrochim Acta B, 65, 2010, 1056-1060; YL Yu, et al., J Anal Atom Spectrom, 2008, 493-499; Xing Z. et al., Talanta 80, 2009, 139-142; Wu Q. et al., J Anal At Spectrom 27, 2012, 496-500; Mao X. et al., Anal Chem 88, 2016, 4147-4152.

Six of the seven cited works use the above-described cylindrical DBD atomizer arrangement, wherein the atomization of the volatile compounds occurs within the DBD atomizers and the detection of free atoms occurs in the free space above the atomizer, in an area shielded from the surrounding atmosphere by an inert gas stream. Only one of the works (YL Yua et al., J Anal Atom Spectrom, 23, 2008, 493-499) uses a modified arrangement where above a vertically placed flame DBD atomizer is a quartz-shaped block-shaped box for the protection of free atoms of the analyte in front of the ambient atmosphere instead of the shielding gas stream, while allowing the fluorescence spectrometer beam to pass from the source through the space above the atomizer and subsequent emission of fluorescence radiation to the detector at an appropriate angle. It should be emphasized that even this construction is not ideal, since the condition that the area of atomization and detection is identical is still not met. In addition, even when made of quartz glass, the walls of the detection chamber exhibit undesirable optical phenomena such as radiation loss, diffraction and reflection. All of these events worsen the signal / noise ratio and therefore negatively affect the detection limit.

-2GB 306804 B6

To achieve the maximum possible volatile species signal in AAS and AFS spectrometric detectors, two main conditions must be met:

1) the atomization and analyte detection sites should be identical so that signal detection occurs at the site with the highest free atom density, the reasons being discussed in detail above.

2) atomization of volatile species in the plasma of the DBD discharge should be as efficient as possible, since only the free atoms generated by the decomposition (atomisation) of the molecular structure of the volatile species are available for detection by AAS and AFS. In the plasma there can be different processes that compete with each other. It involves atomization, excitation and “soft” ionization of analyte molecules. In the case of using atomic spectral methods, it is necessary that the preferred processes taking place in plasma are atomization (AAS) resp. atomization and excitation (AFS) at the expense of ionization. Conversely, ionization is the preferred process when mass spectrometry is used as a detector. The properties of the plasma are influenced by the construction of electrodes (shape, distance, material) and the properties of the excitation source (frequency, power, shape of modulation of high alternating voltage, eg sinusoidal or rectangular pulses). Due to the combination of these properties, the DBD plasma can have either a homogeneous or filamentary discharge character, and they differ significantly in their ionization and dissociative abilities. While the homogeneous plasma is the predominant event of analyte ionization, the dissociation of the analyte molecules into atoms and excited states of atoms predominates in parliamentary discharges. The issue is described in more detail in F.D. Klute et al., Analytical Chemistry, 88, 2016, 4701-4705; S. Brandt et al. Anal. Chim. Acta, 951, 1017, 16-31.

In addition to AAS and AFS, DBD discharges, in similar designs, are also used to excite non-volatile analytes whose solutions are misted into DBD discharges and detected by atomic emission spectrometry (AES). The use of DBD discharges in AES is reviewed in S. Brandt et al., Spectrochim Acta B, 123, 2016, 6-32. Another construction of a DBD excitation source for anaytes misted in liquid form and detected by AES is described in a patent («Zfc et al., Northeast University, CN 105 717 092, Jun 29, 2016). It should be emphasized that both the design of the DBD and the experimental conditions used may vary considerably depending on the purpose of the DBD discharge and the manner in which the sample is introduced into the plasma. Liquid samples are introduced into the DBD plasma (usually Ar, He) by misting in the form of an aerosol, which must be dried, evaporated and the analyte subsequently dissociated. All of these events utilize a significant amount of plasma energy, since DBD discharges are low power plasma (units up to tens of W only). In the case of generation of volatile compounds, it is sufficient to dissociate the gas analyte, but it should be taken into account that the chemical generation of volatile compounds generates a significant amount of hydrogen (units up to tens of ml.min -1 ^), ie hydrogen fraction in plasma gas. actually reaches units up to tens of percent). Hydrogen significantly changes plasma behavior, it extinguishes excited and metastable states that carry energy in the plasma, which can lead to the extinguishment of the discharge. The robustness of the plasma to hydrogen is absolutely crucial in the determination of hydride-forming elements by chemical generation of volatile species. It does not only depend on the total power inserted into the plasma, respectively. nor the power value per unit of plasma volume, but it also depends on the parameters of the excitation source (frequency, modulation) and the nature of the resulting plasma (filamentary, homogeneous).

The aim of the technical solution is the construction of an atomizer allowing a high degree of atomization of volatile analyte molecules, further creating a plasma not bounded in space by solid walls of an atomizer, while allowing the detection of free analyte atoms by atomic spectrometry methods in the active plasma in the highest density zone. The aim is therefore to create a stable filamentary discharge of millimeter thickness, which does not cause undesirable plasma turbulences in the vertical axis between the electrodes and where the spectrometer beam can be focused, and is sufficiently robust to by-products of generating volatile species of analytes that are cogenerated hydrogen, water vapor and aerosol.

-3 CZ 306804 B6

SUMMARY OF THE INVENTION

The drawbacks of the prior art are solved by the device according to the invention - an external dielectric capillary electrode barrier volatile atomizer based on a dielectric barrier electrode barrier discharge, which uses the already described part: excitation voltage generator and high voltage transformer (J. Kratzer et al. Utility model CZ 28 416 U1).

The atomizer itself consists of an atomizer base made of an insulator (preferably Teflon), an inner atomizer tube, an outer shielding gas tube and two electrodes, an internal electrode and an external electrode. The atomizer base allows a stable attachment of the inner electrode, the atomizer tube, and the shielding gas tube. Moreover, the mounting is gas-tight and the structure of the base of the atomizer further allows the displacement of the inner electrode in the range of several millimeters in the vertical direction due to the sliding part of the inner electrode. The inner and outer electrodes consist of a thin-walled quartz capillary of small external diameter with a sealed end - tip - filled with conductive material here, powdered aluminum sealed with quartz cotton. A conductor in the form of a wire supplying alternating voltage between the two electrodes is embedded in the tip filled with aluminum. The electrode walls serve as dielectric barriers. The top view of the atomizer tube and the shielding gas tube form concentric circles and their orifices lie in the same plane. The inner electrode is located in the center (in the axis) of the atomizer tube and its tip extends by a few millimeters over the mouth of the tubes. The outer electrode is a mirror image of the inner electrode, their tips facing each other and the position of the electrodes can be changed in vertical and horizontal directions. The arrangement of the tubes and electrodes makes it possible to create a thin filamentary plasma discharge between the electrodes outside the walls of the atomizer tubes, ie above their mouth. The discharge thus produced is capable of effectively atomizing the analyte without significant losses, as is the case with electrodes of the same or similar outer diameter, but with a wall thickness greater than 0.3 mm and hence a larger dielectric layer. Necessary condition for preventing significant turbulence of plasma event. of the shielding gas is a small outer diameter of the electrodes. The volatile species of the analyte is fed into the filament discharge through the atomizer tube. In the discharge, the volatile species of the analyte are atomized and a cloud of free analyte atoms is formed. The shielding gas passing through the shielding gas tube prevents the diffusion of oxygen from the surrounding atmosphere, which destabilizes the discharge and causes the free analyte atoms to disappear.

The most significant advantage of the invention, unlike the published arrangements, is the formation of a filamentary thin discharge between thin-walled capillary electrodes without turbulence caused by plasma or shielding gas. This arrangement allows achieving high sensitivity and low measurement noise resulting in a significant reduction in the detection limit for a given analyte.

Clarification of drawings

1. DBD atomizer arrangement - longitudinal section

2. arrangement of tubes and inner electrode of DBD atomizer - plan view

DETAILED DESCRIPTION OF THE INVENTION

Example 1

An exemplary embodiment of the invention is illustrated by FIG. 1, which is a schematic illustration of a dielectric barrier discharge capillary electrode atomizer.

The main components of the atomizer are: a cylindrical atomizer teflon base 1, an inner electrode 2 and an outer electrode 3, an atomizer tube 4, and a shielding gas tube 5. Teflonová zá

-4806 306804 B6 Atomizer Positive 1 (diameter 21 mm, length 50 mm) also acts as an electrical insulator and has holes for the atomizer tube 4 (diameter 5.7 mm, length 70 mm) and the inner electrode 2 anchored by the slider 6 of the inner electrode ( diameter 5.0 mm, length 10 mm, diameter of the part set in the base 1 of the atomizer 5.0 mm, length 5 mm). Shielding gas tube 5 (length 66 mm, outer / inner diameter 12,0 / 9,0 mm with side entry length 20 mm, outer / inner diameter 6,0 / 3,0 mm, distance from the mouth of the tube 55 mm) is mounted on an outer ring (outer / inner diameter 9.0 / 5.7 mm, length 3 mm), which is part of the Teflon base 1 atomizer. Both tubes are made of quartz glass and their mouth lies in one plane. The tip of the inner electrode 2 extends over this plane in the range of 2.0 to 8.0 mm (if the mouth of the atomizer tube 4 extends by less than 2.0 mm, the discharge is not stable). The plan view of the inner electrode 2, the atomizer tube 4 and the shielding gas tube 5 forms concentric circles (see FIG. 2). The outer electrode 3 is a mirror image of the inner electrode 2, the electrode tips facing each other. Teflon cylindrical block - external electrode holder 7, which is fixed in a metal holder (not part of the schemes or patent application) allowing vertical and horizontal movement in units of millimeters, ie. the distance between the electrode tips and the axis of the outer electrode 3 relative to the axis of the inner electrode 2 is infinitely adjustable. Both the outer electrode 3 and the inner electrode 2 consist of a thin-walled quartz capillary (outer / inner diameter 2.0 / 1.7 mm, length 115 mm), whose sealed end (tip) is filled with powdered aluminum (approximately 5 mm long), and sealed with quartz cotton. A conductor (Ni-Cr wire, diameter 0.6 mm, 200 mm, resistivity 5.3 ohm.m- ¹ ) passes through the quartz wool, which is fixed at the end of the capillary with hot-melt adhesive to prevent the wire from twisting or moving and is connected to the excitation source.

The position of the atomizer is adjusted relative to the beam of the spectrometer so that no part of the beam of the spectrometer is shielded. The horizontally oriented spectrometer beam is focused above the mouth of the vertically oriented atomizer tube 4 and the shielding gas tube 5 and on the side closer to the axially offset outer electrode 3 from the inner electrode 2.

The functionality of the atomizer of the invention and its analytical characteristics were demonstrated using arsenic as an analyte by determining trace arsenic concentrations by the HG-AAS technique. A hydride generator was used in the flow injection analysis mode. Arsenic hydride (arsine) was generated from a standard solution of As (III) under optimal conditions, i.e. in an HC1 concentration of about 1 mol of Γ ^1, serving as a reducing agent of 1% NaBH ₄ in 0.1% KOH. The generated hydride was separated from the liquid matrix in the forced withdrawal phase separator and fed to a volatile species atomizer based on an external dielectric barrier discharge through an analyte inlet 8 and then an atomizer tube 4 where it was atomized and detected by AAS. The flow rate of argon as carrier gas through the phase separator was 60 ml min ^-1 . Optimal experimental conditions for the atomizer were found as follows: the distance of the inner electrode 2 and the outer electrode 3 was 15 mm, the outer electrode 3 is offset axially relative to the inner electrode 2 by 2 mm and the distance of the inner electrode 2 end from the mouth of the atomizer tube 4 and the shielding tube 5 the gas was 6 mm; the argon flow through the analyte inlet 8 and the atomizer tube 4 was 600 ml min ¹ , the shielding gas (argon) flow rate was 2.5 1 min ^-1 . The inner electrode 2 and the outer electrode 3 were excited at 25 kHz alternating voltage, the input power being 0.9 W. Under optimal atomization conditions, the dependence of the signal peak area on arsenic concentration was linear over a concentration range of 0.2 to 8 mg Γ ¹ , the limit of determination was 186 pg Γ ¹ As and the limit of detection was 56 pg Γ ¹ As.

Industrial applicability

The device of the invention, an external dielectric barrier discharge capillary electrode volatile atomizer, can be used to effectively atomize volatile species of hydride forming elements and detect them at ultra-trace concentration levels by atomic absorption and fluorescence spectrometry.

The above described atomizer of volatile species based on external dielectric barrier discharge with capillary electrodes as a whole is usable for atomization of analytically usable bodies

These compounds exhibit high robustness to the by-products of the chemical generation of volatile species, i.e. hydrogen, water vapor and aerosol. In particular, the present invention can be used in the determination of hydroforming elements in biological, environmental, industrial, etc. samples. The invention has the potential to become part of commercial atomic absorption and atomic fluorescence spectrometers instead of the atomizers used hitherto.

PATENT CLAIMS

Claims (2)

An atomizer of volatile species based on an external plasma dielectric barrier discharge, characterized in that it comprises an internal electrode (2) mounted in a sliding part (6) of the internal electrode which is mounted in the atomizer base (1) and an external electrode (3) The atomizer comprises an atomizer tube (4) supplying a volatile species of analyte and a shielding gas tube (5), both mounted in the atomizer base (1). 2. A dielectric barrier discharge capillary electrode atomizer with capillary electrodes, characterized in that both the inner electrode (2) and the outer electrode (3) consist of a thin-walled quartz glass capillary (maximum wall thickness of 0.15 mm) forming a dielectric layer; their tip is filled with powdered aluminum.