Atomic Spectrometers Based On Electrothermal Tungsten Coil/Microplasma And Applications In Analysis Of Oilfield Samples

With rapid industrial progress,developing special analytical apparatuses and methods for site operation is drawing more and more attention of researchers.In oilfield work,element analysis is throughout petroleum exploration and extraction,environmental monitoring and protection,quality and safety of food and drinking water for workers.To be specific,element analysis of crude oil and formation water can be used to study sedimentary history and sedimentary environment of stratums,relation to geologic structures,and tracing petroleum and formation water;and heavy metal pollution,fluorine pollution and organic pollution are key monitoring parameters for drinking water,formation water and surface water.Currently,element analysis for oilfield samples is mainly based on the conventional atomic spectrometers,which have the merits of high degree of automation,good stability and low limits of detection（LODs）,but also have the defects of bulksize,high energy consumption and requiring auxiliary equipment（such as cooling facility）.That makes them unsuitable for on-site analysis,restricting operation efficiency of fieldwork and wasting lots of time and cost.The atomizer and excitation source of atomic spectrometers are the key to restrict miniaturization.For example,graphite furnace,air-acetylene flame,inductively coupled plasma,etc.,all have complex operation and high energy consumption.Tungsten wire and microplasma（such as point discharge,dielectric barrier discharge,etc.）techniques have the advantages of compactness,simple structure,less additional equipment,low energy consumption,simple operation,and also have a higher atomization temperature or high excitation capability,making them becoming popular miniaturized instrument component in recent years.Therefore,the development of miniaturized atomic spectrometers based on tungsten wire and microplasma to realize on-site element analysis of oilfield samples is not only feasible,but also has broad application prospects.Based on tungsten wire and microplasma technology and analysis requirements of oilfield samples,in this dissertation,a series of miniaturized atomic spectrometers and analytical methods were developed.The specific work includes the following six parts:（1）Through a miniaturized electrothermal tungsten wire atomic absorption spectrometer coupled with injection sampling after dilution,the trace amount of nickel in the undigested crude oil was directly analyzed.In the process of sample pretreatment,the crude oil sample was diluted into cyclohexane,and then sampled directly onto the tungsten coil of the atomizer after sonication and shaking,avoiding the complicated and time-consuming operation in sample digestion.By controlling the heating program of the tungsten wire,the steps of drying（removing the solvent）,ashing（removing the matrix）,cooling,atomizing,and cleaning（removing the sample residue）were realized.Experimental conditions including the ashing current,atomization current,working gas flow rate and atomizer height were optimized;and Pd was used as a chemical modifier.Under the optimal conditions,the linear range of the standard curve of this analysis method is 59.0～694μg kg^-1,with a LOD of 10μg kg^-1.In addition,this method has the advantages of easy portability（powered by the built-in battery）,small injection volume（20μL）,fast analysis speed（approximately2 minutes/time）,long lifetime of the tungsten coil（each tungsten wire can be tested300 times）,etc.;it will have certain application prospects in petroleum exploration and development.（2）A tungsten wire electrothermal evaporation coupled with long optical path atomic absorption spectrometry experimental device was constructed and used to simultaneously determine Hg and Cd in deep formation water,surface river water and shallow groundwater from oil fields.The light emitted by the mercury hollow cathode lamp and the cadmium hollow cathode lamp were concentrated to the same optical path through a dichroic mirror and a reflector,and then enters the CCD spectral collector after passing through the folded optical path.The sample solution was dropped onto the tungsten coil using a pipette.By controlling the temperature program of the tungsten wire,the moisture,mercury and cadmium in the sample are evaporated successively.The mercury trap（gold-coated tungsten wire）was used to capture mercury,achieving separation of mercury and moisture and avoiding the interference of water vapor on the analysis results.When the sample volume is 20μL,the linear range of the standard curve of Hg is 1～500μg kg^-1,with a LOD of 0.3μg kg^-1,and that of Cd is 0.5～40μg kg^-1,with a LOD of 0.1μg kg^-1.The accuracy and reliability of the proposed method for real water samples from the oil field were assessed by analytical results in comparison with those by a commercial ICP-MS.Furthermore,the proposed method is of potential application in solid sample analysis.（3）Based on the atomizer technique and the excitation source technique of electrothermal tungsten wire,the simultaneous atomic absorption and atomic emission spectrometer was constructed and applied in measurement of Cr,V,Cu,Mg,Ba,Fe,Mn and Ni in formation water.The device realized three functions:1)the simultaneous measurement of atomic AAS and AES of the same element in the same light path direction based on time resolution,2)the simultaneous measurement of AAS and AES of different elements in the same light path direction based on spectral resolution,and 3)the simultaneous measurement of AAS and AES of the same or different elements in different optical path directions.Compared with a single-function tungsten wire atomic absorption spectrometer or a tungsten wire atomic emission spectrometer,this device can effectively expand the types of simultaneous analysis elements and the measurable concentration range of the elements,improving the analysis efficiency.Analytical resuts of real samples by the proposed method are in accordance with those by ICP-OES.Furthermore,the instruments lay potential potentiality for researching spectral interference and mechanism for atomic absorption spectroscopy and atomic emission spectroscopy.（4）An analytical device based on SiF₄ chemical vapor generation sampling method coupled with point discharge microplasma atomic emission spectrometry was constructed and applied in silicon measurement of oilfield drinking water and reaction fluid after lye eroding rocks.The device mainly includes a sampling unit,a SiF₄chemical vapor generation unit,a point discharge unit and a spectral collector CCD.Under the optimal experimental conditions,60μL sample solution with 0.05%Na F was injected into a PP vessel with 150μL concentrated sulfuric acid through a constant speed syringe pump.A vortex mixer was used to mix the solution quickly and evenly.Gaseous SiF₄ in the PP vessel was transported into the point discharge microplasma,and was excited silicon atomic emission spectra for quantitative analysis.The linear range of the standard curve is 1～20 mg L^-1,with a LOD of 0.2 mg L^-1.The analytical results of real samples by the proposed method is in accordance with those by ICP-OES.The compact instrumentation,low energy consumption and simple construction lay certain application prospects in monitoring of drinking water quality in oilfields in remote areas and evaluating the reaction rate of lye eroding rock during oilfield development.（5）The analytical device based on SiF₄ chemical vapor generation sampling method coupled with point discharge microplasma silicon atomic emission spectrometry was used to indirectly determine fluorine in surface sedimentary water,surface river water,untreated underground water and purified and sterilized underground water（drinking water）.Under the optimal experimental conditions,60μL sample solution with 50 mg L^-1 silicon was injected into a PP vessel with 200μL concentrated sulfuric acid.The gaseous products was transported into the point discharge unit.Fluorine was quantitatively analyzed indirectly according to silicon atomic emission spectra.The linear range of the standard curve is 1～50 mg L^-1,with a LOD of 0.2 mg L^-1,meeting demand of real oilfield sample analysis.The analytical results of surface sedimentary water,untreated underground water and purified and sterilized were lower than GB 8537-2008 drinking natural mineral water standard and water levelⅠof standard GB 3838-2002 surface water environmental quality standard.The analytical results of surface river water was lower than water levelⅣof standard GB 3838-2002 surface water environmental quality standard.（6）A miniaturized TOC（Total organic carbon）analyzer based on DBD for catalyzing oxidation vapor generation of organic carbon by persulfate and point discharge microplasma atomic emission spectrometry was constructed and applied in TOC measurement in oilfield formation water,surface sedimentary water and surface river water.Firstly,phosphoric acid was used to remove inorganic carbon in the samples.And then,mixture solution of sample and persulfate solution was injected into the DBD reactor through a syringe.Under the effect of persulfate and microplasma,organic carbon was oxidized into CO₂ effectively and quickly.The produced CO₂ was transported into the point discharge microplasma and to excite carbon atomic emission spectra for quantitative analysis of TOC.This method has high oxidation efficiency（the oxidation efficiency of KHP standard solution reaches96.4%）,low detection limit（0.02 mg L-1）,wide linear range（0.1～200 mg L-1）,small size,and low energy consumption,meeting the requirement of on-site analysis of oilfields.Electron paramagnetic resonance（EPR）was used to explore the possible mechanism of the catalytic oxidation process,that is:persulfate solution can generate a large number of sulfate radicals and hydroxyl radicals under the action of DBD microplasma,which has a strong oxidizing capacity and oxidizes organic carbon into CO₂ quickly.The analytical results of the real samples by this method is close to that by a commercial TOC analyzer based on high-temperature combustion nondispersive infrared absorption spectroscopy technique,indicating that the this method is reliable.