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- Goitom Asfaha, Daniel, 1971-
(författare)
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Fundamental Studies on Direct Injection Nebulisers for Sample Introduction in ICP Spectrometry : Aerosol Properties, ICP Characteristics and Analytical Performance
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The performance of different types of nebulisers: Vulkan direct injection nebuliser (Vulkan DIN), direct injection high efficiency nebuliser (DIHEN), microconcentric nebuliser coupled to cyclonic or double pass spray chamber (MCN-C or MCN-DP, respectively) was investigated and compared when used for sample introduction to ICP-MS or ICP-OES. With ICP-OES, in axial viewing mode, intensity distributions across the radius of the plasma (radial intensity profiles) were determined for different spectral lines with Esum 1.85-15.41 eV to determine fundamental plasma properties for various matrices using Vulkan DIN and MCN-C. The results showed that with the MCN-C the ionisation temperature (Tion) was about the same across the measured region of the plasma (±3.0 mm) whereas with the Vulkan DIN the Tion was significantly lower in the centre of the plasma. A large deviation from local thermodynamic equilibrium, as well as deteriorated stability, was observed for the plasma when using the Vulkan DIN.With ICP-MS noise power spectra (NPS) were generated to identify sources of noise. NPS showed that the magnitude of white noise for the tested sample introduction systems decreased in the following order: Vulkan DIN > DIHEN > MCN-C > MCN-DP. This order follows the decrease of mean droplet size and span of the size distribution, indicating that the white noise is caused by spatial and temporal non-uniform desolvation and ionisation. Another source of noise arose from the peristaltic pump and the magnitude of pump interference noise decreased in the following order: DIHEN > MCN-C/DP > Vulkan DIN. Mains power interference noise and 1/f noise were lower for the direct injection nebulisers compared to the spray chamber systems. The contribution or effects of these noise components on relative standard deviations of steady-state ion-count rate and isotope ratio measurements is discussed in this thesis.Aerosols generated by the Vulkan DIN and the DIHEN were also directly characterised using Particle Dynamic Analysis. The Vulkan DIN produced particles with a mean diameter of ~30 µm and a size distribution between 2-80 µm. With the DIHEN the corresponding values were ~11 µm and 1-40 µm, respectively, with a few particles at 55-78 µm. The mean velocity of particles from the Vulkan DIN was ~10 m s-1 and from the DIHEN ~18 m s-1. The lower velocity allows longer residence time counteracting the effects of the larger droplet size.
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| 2. |
- Goitom, Daniel, et al.
(författare)
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Comparison of aerosol properties and ICP-MS analytical performance of the Vulkan direct injection nebuliser and the Direct Injection High Efficiency Nebuliser
- 2007
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Ingår i: Journal of Analytical Atomic Spectrometry. - : Royal Society of Chemistry (RSC). ; 22, s. 250-7
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Tidskriftsartikel (refereegranskat)abstract
- Monodisperse polymer particles consisting of DVB and styrene-copoly-DVB were successfully synthesized by photoinitiated precipitation polymerization with 2,2'-azobis(2-methylpropionitrile) (AIBN) as initiator. A focused 150 W xenon short arc lamp was used to radiate the sample mixtures. The size of the particles ranged from 1.5 to 4 m and could be varied by changing the monomer concentration between 2 and 10% (v/v) and by means of the time of polymerization. By using photoinitiation instead of thermal initiation, it was possible to avoid coagulum and arrive at spherical particles with an exceptionally high monodispersity for particles of this size range (polydispersity index < 1.02) and with monomer loadings well above 5%.
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| 3. |
- Goitom, Daniel, et al.
(författare)
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Noise characteristics and analytical precision of inductively coupled plasma mass spectrometry using a Vulkan direct injection nebuliser for sample introduction
- 2006
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Ingår i: Journal of Analytical Atomic Spectrometry. - London : Royal Society of Chemistry. - 0267-9477 .- 1364-5544. ; 22:2, s. 168-176
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Tidskriftsartikel (refereegranskat)abstract
- Noise and precision of ion count rate measurements were characterised for ICP-MS using a Vulkan direct injection nebuliser (Vulkan DIN) for sample introduction. For comparison, experiments were also carried out using a direct injection high efficiency nebuliser (DIHEN). The Vulkan DIN gave considerably higher relative white noise compared with the DIHEN, −40.3 compared with −45.8 dB, and measurements of aerosol characteristics showed that this was a result of broader size distribution and larger mean size of droplets for the Vulkan DIN. At a liquid flow rate of 85 μL min−1, the aerosol volume fraction constituted by droplets with a diameter smaller than 10 μm was only 5% for the Vulkan DIN compared to 38% for the DIHEN, and the Sauter mean diameter was 30.2 and 11.1 μm, respectively. Compared with the DIHEN, interference noise from a peristaltic pump was very small with the Vulkan DIN because at the liquid flow rate giving maximum sensitivity, the analyte signal intensity was not much affected by changes in liquid flow rate. Although this had a positive effect on precision, it indicated that the plasma is cooled by large droplets that do not positively contribute to the analyte signal intensity. For analytical precision data it was evident that with the Vulkan DIN, precision was limited by the high white noise in the frequency range 0.4–500 Hz, but for the DIHEN there were relatively large contributions from 1/f and/or interference noise to the observed precision. When using integration times of 1000 ms or longer, the DIHEN did, however, appear to give slightly better precision for most of the isotopes tested. For comparison, precision data was also generated when using a conventional spray chamber sample introduction system and it was found that, with a 3000 ms integration time, both direct injection nebulisers gave better precision for most isotopes. Although analytical precision is impressive, the results in this paper also show that both direct injection nebulisers produce aerosols that are far from ideal for the types of ICPs currently used in commercial instruments.
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| 4. |
- Goitom, Daniel, et al.
(författare)
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Radial ICP characteristics for ICP-AES using direct injection or microconcentric nebulisation
- 2005
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Ingår i: Journal of Analytical Atomic Spectrometry. - London : Royal Society of Chemistry. - 0267-9477 .- 1364-5544. ; 20:7, s. 645-651
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Tidskriftsartikel (refereegranskat)abstract
- Radial analyte signal intensities, fundamental plasma parameters, and non-spectral interference effects were characterized for inductively coupled plasma atomic emission spectrometry (ICP-AES) using a direct injection nebuliser (Vulkan DIN) or a microconcentric nebuliser (MCN) coupled to a cyclonic spray chamber for sample introduction. Radial analyte signal intensity profiles for atomic and ionic lines with energy sum (Esum) between 1.85 and 15.41 eV were used. When using the MCN system, for all lines the signal intensity profiles were parabolic with maxima at the axial centre of the plasma. For the Vulkan DIN, the shapes of the profiles were dependent on their Esum showing minimum intensities at the axial centre of the plasma for lines with high Esum values. The ionisation temperature, electron number density and magnesium ion-atom line intensity ratio determined indicated that ionisation and excitation capabilities were deteriorated at the centre of the plasma when using the Vulkan DIN compared with the MCN. This was found to be not simply a result of high aerosol load when using the Vulkan DIN, but because of poor plasma–aerosol interaction, possibly caused by the confined distribution of aerosol in the plasma and high nebuliser gas velocity. Indeed, for the Vulkan DIN, electron number density and ionisation temperature increased with liquid flow rate, which could be explained by plasma shrinkage, or the thermal pinch effect, which increased the aerosol–plasma interaction at increased liquid flow rates. As a consequence, when increasing the liquid flow rate in the range 20–90 µL min–1: (i) analyte sensitivity increased linearly; (ii) the plasma became unstable; (iii) the magnitude of matrix effects remained almost unaffected. These results are partly in contrast to observations made with other types of direct injection nebulisers and ICP instruments.
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