Studies On The Diagnoses And Application Of Oxygen-shielded Argon Microwave Plasma Torch (OS-ArMPT) Excitation Source

Microwave plasma torch (MPT) is one of microwave plasmas (MWPs)that is a kind of important source for analytical atomic spectrometry. Theoxygen shielded argon microwave plasma torch (OS-ArMPT) is the newestresult, which has experienced practicing, and modifying for more than 10years since Prof. Jin and his coworkers developed the MPT in 1985. MPTtorch has a similar configuration to that of ICP, and its central channelenhances the interaction between the analyte and plasma, therefore, thematrix effect reduces. As a result, MPT has higher tolerance to theintroduction of samples and its lower background emission increases thesignal-to-background ratios (S/Bs) of analytical lines of some elements, andimproves precision of measurement. When making use of O2 to shield theArMPT plasma, a stable oxygen atmosphere is formed around the plasma.Thus the convection and diffusion of the air outside the plasma towards theplasma is prevented. The OS-ArMPT plasma becomes whiter and brighter.Meanwhile, plasma appears to be longer and the "plasma core" becomesclearer and moves a little bit upward. This thesis focuses on the low poweredOS-ArMPT discharge, and some results on characteristics and applications ofthe OS-ArMPT discharge were obtained.Temperature and electron number density (n_e) are important parametersto characterize plasma. Different temperatures describe the behavior ofdifferent species in a plasma. In this thesis, the excitation temperature (T_exc)was measured by Boltzmann slope with Fe as thermo-species, the rotationaltemperature (T_rot) was measured by lope method with Q₁ and R₂ branches ofOH (0,0) band, and the electron number density (ne) was measured withSaha-Eggert equation. The function of oxygen in OS-ArMPT was studied bymeasuring important parameters of the plasma, Texc, Trot and ne, that is shownall these parameters changed with the change of O2 flow rate. The resultsshow that Texc and Trot increase differently when O2 flow rate raises and theyreach a maximum when O2 flow rate goes from 1400 ml min-1 to 1600mlmin-1, but there are much difference between them. ne increases also when O2flow rate increases. These facts prove that OS-ArMPT is still far away fromthe state of local thermodynamic equilibrium (LTE). Because some O2 isdrawn into the plasma, the thermal conductivity of the plasma is improved toa certain extent. Different kinds of temperature of the plasma approach thesame value, that is to say, plasma is better approaching to the LTE state. Thus,in the OS-ArMPT, intensities of ionic lines of almost all analytes decrease,but the intensities of atomic lines increase.As a shielding gas, besides preventing ambient air from entering into theplasma, O2 plays also a "thermal pich" role. The result is that the dischargechannel is further depressed and makes the plasma of high energy density,and thus Texc, Trot and ne increase all. The characteristics of Ar emissionspectra change also with the entering of O2, The most evident change is thationic emission lines of Ar weaken apparent or even disappear completely.These changes show that O2 affects the energy distribution in plasma, whichreduces the number of Ar metastable atoms, hence, the probability to formionic lines reduces to a certain extent. But when the flow rate of O2 is toohigh, Texc, Trot and ne of plasma drop. The apparent reason for this is that theO2 flow carries away a lot of energy and reduces the power density of theplasma.ArMPT has a lot of metastable Ar atoms and also a huge amount of highenergy electrons that makes the plasma more powerful excitation ability andvery high electronic temperature as an ionizing plasma. But as much cold O2is entrained into the plasma, although the density of total electrons in theMPT increases, the number of high energy electrons decreases. Which resultsin the intensities of ionic lines of Ar and other elements decrease evidently.This fact proves that the main cause of the high excitation level ionicemission lines, in the OS-ArMPT, is the high level electrons. The electronswith lower energy work effectively only on the excitation collisions of lowerexcitation levels, the increase of these electrons only make the intensities ofatomic emission lines with lower excitation potential increase.The excitation and ionization processes of metal elements with lowerexcitation potentials are completely different from that of the Ar atomic,because Ar has higher resonance excitation potential. As easily ionizedelements (EIE) are introduced into the plasma, because of the shift ofionizing equilibrium and so on, the number density of the Ar ion numberdeceases;excitation and ionization of the nonmetal and halogen elementswith high excitation energy became more difficult.Since samples have to complete the vaporization, atomization and excitationin the OS-ArMPT, and the temperature of the plasma gas is relative lower, thematrix effect in the plasma is quite complicated. For any analytical lines, theeffect is tightly related to the excitation potential of the line, that is the energyneeded by atoms to transit from basic or a lower energy state to a high energystate. The analytical lines having higher energy are influenced morepronounced than those having lower energy. This kind effect seems to haverelation with ionization potentials of matrix elements in the plasma. Theapparent interference was coming from easily ionization elements belong tothis effect. However this process is not a simple ionization buffer process, italso has close relation to the selective energy distribution in the plasma. Inthe process of energy transfer, easily volatilized and easily excitationelements get energy easier.By introducing O2 as shielding gas, the tolerance of MPT to the sampleintroduction is improved and the interfering effect from background emissionreduced. The characteristic that it can be combined with various separationtechniques has attracted much attention of many experts. It has beenrecognized as a promising new source.The main results and contributions obtained in the thesis are as follows:Firstly, some parameters including Texc, Trot and ne of lower powerOS-ArMPT were measured for the first time with spectral method and theeffects of the O2 flow rate on the parameters of OS-ArMPT were studied;Secondly, the effects of some operating conditions, such as microwavepower, flow rates of support gas, carrier gas and shielding gas, as well asobservation height, on the parameters of OS-ArMPT were investigated anddiscussed, the possible mechanisms in OS-ArMPT were discussed;Detailedreports of the parameters were used to provide a better understanding of theexcitation mechanism in OS-ArMPT;Thirdly, the effects of EIE on signals of some elements (atomic lines andionic lines) and plasma parameters (rotational temperature, excitationtemperature and electron density) in the OS-ArMPT were investigated. Thedischarge characteristics of plasma were evidently changed owing to theintroduction of EIE and the effect of EIE on excitation mechanism inOS-ArMPT was discussed;Fourthly, the effects of organic reagents on signals of some elements(atomic lines and ionic lines) and plasma parameters (rotational temperature,excitation temperature and electron density) in the OS-ArMPT wereinvestigated. It was shown that discharge characteristics of plasma, especiallyexcitation temperature, were evidently changed owing to the introduction oforganic reagents and the effect of organic reagents on excitation mechanismin OS-ArMPT was discussed;Finally, a novel, simple and economic extraction device that combines acircumfluent unit with a extraction device was developed for thedetermination of wear metals in used aviation lubricating oil by using asimultaneous MPT-AES, and therefore the applied field of OS-ArMPT wasexploited.