Development And Application Of Needle Discharge For Direct Solid Elemental Analysis

The elemental determination in solid sample is playing significant roles in many fields,especially in metallurgy,environment,electronics,geochemistry,space exploration and semiconductor industry.In contrast to solution analysis methods,the direct solid analysis can provide spatial and in-depth information with little sample preparation,avoidable introduction of contamination and low risk of sample loss.Currently,the techniques with the integrating advantages of real-time,in-situ,non-destructive,high throughput,high selectivity and high sensitivity have been pursued by analytical scientists.However,these performances cannot be offered by solution-based methods.It seems necessary and important to develop direct solids techniques.Compared with laser or secondary ion technique,reduced pressure discharge,employed as excitation or ionization source,has been routine methods for elemental analysis due to its easy-to-operate and low cost.However,discharges for solid analysis at atmospheric pressure were not fully explored.Hence,in this dissertation,needle-plane discharge source for direct solid analysis at atmospheric pressure was developed and fully investigated.Related works are briefly described as follows:1.Designing a needle discharge cell at atmospheric pressure.Due to the high non-uniform electric field around the needle tip,it seems easy to achieve a stable discharge at atmospheric pressure.Features such as discharge parameters,optical emission spectra,craters and N2 vibrational temperatures,were studied in nitrogen ambient environment using direct current(DC)needle discharge and pulsed needle discharge.Voltage-current relationship suggests that pulsed needle discharge is in the arc regime while DC needle discharge belongs to glow.Using optical spectrometer,only common emission lines of N2 can be acquired in DC mode,whereas primary atomic and ionic lines of the sample are obtained in the case of pulsed mode.Capability in ablating metal samples demonstrates that pulsed needle discharge is a viable option for direct solid sampling.The diameter of the crater ablated by pulsed needle discharge was about 400 μm.The calculated vibrational temperatures are 4500 and 10000 K for DC and pulsed needle discharge,respectively.2.The discharge system was coupled to inductively coupled plasma mass spectrometry(ICP-MS)for the determination of elements in solid samples.Combined needle-discharge cell with ICP-MS,no signals were detected except for the background in DC mode,suggesting its failure to effectively sputter sample.In contrast,pulsed mode exhibits much better performances in calibration linearity and limits of detection(LOD)in direct analysis of samples with different matrices.To improve transmission efficiency,a mixture of Ar and N2 was employed as discharge gas as well as carrier gas in the follow-up experiments,facilitating that LODs of most elements reached ng/g.3.Development and characterization of a pulsed radio-frequency needle discharge.This technique owns the capability of sampling nonconductive solids effectively and belongs to the traditional radio-frequency discharge regime.Combined with ICP-MS,this technique allows to measure elemental composition of nonconductive solids directly.Capability of quantitative analysis was demonstrated by the use of two series of nonconductive samples.Good calibration linearity and limits of detection(LODs)in range of 10-8-10-9 g/g were achieved for most elements.