Research On Peak Detection Method And Instrument Control Software For TOF-SIMS

Time of flight secondary ion mass spectrometry?TOF-SIMS?is a powerful surface analysis technique,that has many advantages,such as lower sample consumption,micro-area and in situ analysis,higher determination speed and no limit to the mass range.However,to date,TOF-SIMS can be only used for qualitative and semi-quantitative analysis of chemical elements and cannot be used for high-precision quantitative isotope measurement.The development of TOF-SIMS for high-precision isotope measurement will meet the requirements of current geochemistry and cosmochemistry,and promote related disciplinary development.Research on data processing methods and instrument control software is one of the core efforts in the subject of instrument development.The TOF-SIMS instrument requires customized control software for component control and parameter adjustment.Current LabVIEW software development methods have several shortcomings,such as,expansibility and maintainability.Research on software development methods can make up for these shortcomings.TOF-SIMS used for high-precision isotope measurement has no precedent internationally.The existing peak detection method has limited identification ability in the case of overlapping or low-amplitude peaks.Improvements in the peak detection methods will contribute to the performance of the instrument to meet these requirements of practical application.The specific research contents are as follows:?1?Research on peak location detection methods in TOF-SIMS spectraPeak detection is an important preprocessing step in the analysis of mass spectrometry data.The performance of the method directly influences the instrument's analysis results.Most current peak detection methods have limited identification ability when there are overlapping or low-amplitude peaks,that do not meet the requirements of the instrument.We present an improved algorithm that combines the continuous wavelet transform?CWT?with the crazy climber algorithm.Particles move on the CWT coefficient matrix according to certain rules and gradually gather at local maximal points.Peaks are identified by drawing ridges based on the weighted occupation measure of the grid points.The proposed method is evaluated using simulated noisy spectra and real spectra of benzene in nitrogen.The results show that the proposed approach is better at identifying overlapping peaks than existing methods.Receiver operating characteristic curves show that the proposed method can detect more true peaks while maintaining a low false discovery rate.?2?Research on peak area detection methods in TOF-SIMS spectraCommonly,the concentration of certain compounds or element is associated with peak areas in mass spectrometry and chromatography spectra.Therefore,detecting and measuring peak areas has a crucial influence on the analysis result.Curve fitting is the most utilized technique to attain a better result for this calculation.Reliable estimations of peak parameters should be provided.However,there is not currently a reliable method to estimate the half-width of spectral peaks.We present a method for peak detection in TOF-SIMS spectra,incorporating continuous wavelet transform and curve fitting.The half-width was estimated by derivative spectrometry based on the continuous wavelet transform.Combined with peak number,peak heights and peak positions form the initial input for curve fitting.Accurate parameters for each peak were obtained using a subsequent iteration process.The method was evaluated with simulated and real TOF-SIMS spectra of silicon isotopes.The results showed that,compared with the existing method,this proposed method obtains more accurate half-width estimations of spectral peaks when the signal-to-noise ratio?SNR?=5³0.Compared with the existing method and PeakFit software,the proposed method is better at separating overlapping peaks and has better resistance to noise.Nine TOF-SIMS silicon isotope spectra are processed by the proposed method.The results show that:the proposed method can separate the overlapping peaks formed by ²⁹Si and ²⁸SiH.Compared with calculating peak area by added counts of peaks,the relative standard deviation?RSD?of the ³⁰Si/²⁹Si isotope ratio was optimized from 10.4‰to 7.6‰.?3?Research on development method for instrument control softwareThe components of the TOF-SIMS instrument are numerous.Traditional Lab VIEW programming methods will lead to a complicated software structure because the programs share several subroutines,which influences the expandability,reusability and maintainability of the software.These problems can be solved by introducing Lab VIEW Object-Oriented for development of the control software.A layered architecture is chosen for the overall software structure.The class library for controlling the instrument components is built based on the I/O interface types of the components.The programs are developed based on a producer/consumer model.All of these constitute a set of software development methods.According to the hardware structure and control requirements of the self-developed TOF-SIMS instrument,a control software that meets the requirements is developed based on this method.The software structure has the advantages of good scalability,easy development and extension,and a lack of adverse effects in response to hardware change.?4?The experiment of TOF-SIMS lead isotope analysisLead isotopes are an effective tracer in mineral resource exploration that provide important evidence and clues for many applications such as mineralization material source,deposit dating,geological prospecting and exploration,measuring the depth of concealed deposits and resource evaluation.Compared with the existing determination method,TOF-SIMS has the advantages of high spatial resolution,lower sample consumption and wide mass range.Galena from Broken Hill,Australia was chosen as the sample.Lead isotope analysis was completed on an internally developed TOF-SIMS instrument.The experimental procedure verified the effectiveness of the internally developed instrument control software.Nine experimental data sets were processed by the proposed peak location detection method named CWTC and the peak area detection method combined with CWT and curve fitting.The results show that:compared with calculating peak area by added counts of peaks,the proposed method can optimize the RSD of isotope ratio ²⁰⁷Pb/²⁰⁶Pb from 8.6‰to 5.3‰and optimize the RSD of isotope ratio ²⁰⁸Pb/²⁰⁶Pb from 2.6‰to 1.7‰.