| Differential mobility analyzer (DMA), also known as electrical aerosol analyzer (EAA), differential mobility particle size analyzer (DMPS), is a kind of ion mobility spectrometry which is used to detect and separate samples based on mobility of the charged particles or ions in the low electrical field. DMA is originally used to detect the charged particles such as aerosol particles and is gradually applied to the field of trace substance. Having the advantage of simple structure, high sensitivity, fast response and continuous detection, DMA is a great potential on-site detection of trace substances technology in the field of explosives, drugs, environmental pollutants. However, application and development of DMA in the field detection are primarily constrained by the following two problems:Firstly, as an analytical technique, DMA is very suitable to be used as a field instrumentation, thus DMA is required to be small, light and portability. Conventional DMA usually has a cylindrical drift tube structure, consisting of two coaxial cylindrical electrode rod. The concentricity accuracy is required to be very high because a small error of electrode will low the performance of DMA when the electric field is non-uniform. The smaller is the electrode size, the more difficult to control the concentricity accuracy, which would greatly increase the difficulty of the miniaturization of DMA. Meanwhile, the inner cylindrical electrode is connected to high voltage power supply and its interior space is wasted. It is unfavorable for portable devices because DMA has strict requirement for the volume.Secondly, DMA is originally used to detect micron or submicron aerosol particles and its detection range has gradually widened to the nanometer order in recent years. However, when the particle is less than50nm, the diffusion of ions in the gas increases significantly and results will deteriorate significantly. The diffusion of charged particles in the gas will be reduced through reducing the resident time with large flow and high voltage. So large flow rate (>100L/min) and high voltage (>1kV) are usually adopted. However, it requires a larger pump and power supply to achieve the large flow rate and high voltage, which not only increases the cost but also complicates DMA. It is as well unfavorable for the futher development of DMA as a field instrumentation.In view of the above problems, the main work of this thesis is: First, improve the structure of DMA drift dube and optimize its structre parameters. There are three kinds of strctures of DMA drift tube:cylindrical, radial and planar. Compared to the other kinds of drift tube, planar tube has the advantage of easy manufacture, assembly and high space utilization. Also, it has higher resolution than a cylindrical drift tube at the same flow rate because it’s internal electric field is more uniform. Modeling the planar DMA drift tube theoretically and model-based analysis and discussion of the impact of the drift tube core structure parameters including electrode parameters and in and out interface parameters on the DMA detection performance, so as to provide the basis for the design and optimization of structural parameters of the drift tube. The range of the internal electrode radius, external electrode radius and electrode length is1～2.5cm,2.5～4cm and1.8～45.52cm, respectively. Based on alumina ceramic and thick film electrode printing technology, this thesis developed two kinds of drift tube:tangential convergent DMA drift tube and normal converged DMA drift tube. The length and width of their electrode are1to2orders of magnitude smaller than that of traditional cylindrical tube’s. Decrease of electrode parameters can not only directly reduce the size of the DMA structure but also reduce the required condition parameters, such as smaller vlatage and flow. In addition, alumina ceramic and thick film technology makes the drift tube has a high precision and smoothness.Then study the effect of measuring and controlling parameters on the detection performance of DMA. Its performance can be affected by many parameters. Some of the most important parameters are:conduct voltage of the lower chip, type of sheath gas, ratio between the sheath gas and the sample gas. According to the experimental results and analysis, select and optimize the measuring and controlling parameters so that the performance of DMA is at its best.Finally, two homemade drift tube were used to detect common VOCs small molecules under the condition of low flow (<7L/min-1) and volatage (<30V). Compare the two drift tube’s respective three sets of drift tube detection performance to achieve a further optimization of the structural parameters. A method is proposed to solve the mobility of sample based on the slope of the fitting line of the total flow rate and the deflect voltage, by which substances are identified based on spectrogram. |
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