Optimized DLP Linear Ion Trap for a Portable Non-Scanning Mass Spectrometer
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International Journal of Mass Spectrometry 369 (2014) 30–35 Contents lists available at ScienceDirect International Journal of Mass Spectrometry journa l homepage: www.elsevier.com/locate/ijms Optimized DLP linear ion trap for a portable non-scanning mass spectrometer a, a b c Boris Brkic *, Stamatios Giannoukos , Neil France , Robert Murcott , d a Fabrizio Siviero , Stephen Taylor a Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, United Kingdom b Q Technologies Ltd, 100 Childwall Road, Liverpool L15 6UX, United Kingdom c TWI Technology Centre Yorkshire, Advanced Manufacturing Park, Wallis Way, Catcliffe, Rotherham S60 5TZ, United Kingdom d Vacuum Technology Lab, SAES Getters S.p.A., Viale Italia 77, Lainate 20020, Milan, Italy A R T I C L E I N F O A B S T R A C T Article history: This work reports implementation of an optimized linear ion trap (LIT) mass analyzer manufactured Received 7 April 2014 using digital light processing (DLP), a low cost rapid prototyping technique. To satisfy requirements for a Received in revised form 23 May 2014 portable commercial instrument, our previous DLP LIT has been improved by using polymer material for Accepted 2 June 2014 electrodes and housing with higher heat deflection temperature and much improved metal coating of the Available online 4 June 2014 electrodes. In this way, a polymer-based LIT can operate at higher pressures with more stable voltages, low outgassing and with much longer electrode lifetime. This paper demonstrates non-scanning Keywords: operation of a DLP LIT based on ceramic resin with specially electroplated electrodes. Non-scanning mode Linear ion trap is suitable for applications in which only specific mass fragments are targeted such as security and Rapid prototyping forensics. Experimental results from a DLP LIT are therefore shown for simulants of cocaine, TNT and Portable mass spectrometer Non-scanning mode sarin. ã Security applications 2014 Elsevier B.V. All rights reserved. 1. Introduction quadrupole mass filter (QMF) [9] and linear ion trap (LIT) [10] with hyperbolic rods. DLP is a rapid prototyping (RP) technique that uses For in-field applications that require analysis of specific dynamic masking capability to selectively cure a photosensitive molecules, simple and low cost portable mass spectrometers are polymer resin, which enables fabrication of complicated 3D highly suitable. Such applications can be found within environ- structures. Modern DLP machines also have enhanced resolution mental [1], space exploration [2], automotive [3], medical [4], food mode (ERM) that further improves surface quality and can provide [5] and security [6] sectors. When analysis of application-targeted surface roughness at higher nanoscale levels. Compared to molecules is required, the use of a non-scanning mass spectrome- selective laser ablation (SLA), another RP technique used for ter is highly recommended. This is because a non-scanning mass polymer-based rectilinear ion traps [11–13], DLP provides better spectrometer performs selective ion monitoring with optimal manufacturing accuracy and surface smoothness. This is because voltages for each ion mass without having to obtain full mass DLP has much smaller volumetric pixels (15 mm  15 mm Â25 spectrum [7,8]. A result of such operation mode is higher mm) compared to SLA tracks width (200 mm) and ERM capability sensitivity, simpler control electronics, smaller size, lower power for improving surface quality. Typical arithmetic average (Ra) for consumption and cost. surface roughness of DLP parts with 25 mm voxel size is 749 nm Apart from simplicity and cost savings with electronics for a with ERM off and 434 nm with ERM on [10]. Due to these non-scanning mass spectrometer, manufacturing of mass advantages, our work has focused on rapid manufacturing of analyzers can also be simplified at significantly lower cost than analyzers with hyperbolic electrodes that provide the ideal with conventional techniques. Our previous work has demon- quadrupole electric field and therefore optimal performance of strated design, fabrication and proof of concept for using digital an analyzer [14,15]. light processing (DLP) technique to implement polymer-based Our previous DLP QMF and LIT were made using polymethyl- methacrylate (PMMA), which gave accurate and smooth electro- des. However, some level of outgassing under vacuum was noticed * Corresponding author. Tel.: +44 151 794 4541; fax: +44 151 794 4540. during previous QMF tests from PMMA [9], which resulted in an E-mail address: [email protected] (B. Brkic). unwanted pressure increase. For those ion traps that operate at http://dx.doi.org/10.1016/j.ijms.2014.06.004 1387-3806/ã 2014 Elsevier B.V. All rights reserved. B. Brkic et al. / International Journal of Mass Spectrometry 369 (2014) 30–35 31 higher pressures than quadrupole mass spectrometers and use designed for axial ion injection and ejection, which are done buffer gas for ion cooling, outgassing could be further increased through the holes on entrance and exit endcaps respectively. and affect mass analysis. Therefore, PMMA has been replaced with During ion injection, entrance endcap is switched to zero or low DC ceramic-filled resin (HTM140), which has higher heat deflection to allow ions to enter the trap, while during ejection exit endcap is temperature (HDT) and lower outgassing rate. The HDT for switched to negative DC to extract the ions from the trap. HTM140 is 140 C straight out of the machine without performing any UV and thermal post cure processes that would increase it 2.2. DLP machining further, while it is 75 C for PMMA [16]. Another important issue for a polymer-based analyzer is high quality of metal coating on Typical fabrication procedure in a DLP machine consists of the electrodes, which is essential for a commercial instrument. Types following steps: of coating used on previous analyzer prototypes are evaporative and sputter processes that deposit gold directly on polymer (a) CAD model is loaded into a machine and processed using its electrodes [9,10]. Even though such coatings provided very good software, conductivity on the rods, gold deposits would start to wear out (b) Model is vertically sliced into a series of individual layers of over time on all of our previously made samples. This affects 25 mm thickness, dielectric constant and capacitance between the RF rods, which (c) Direct light projector projects a mask of the slice to be could cause instability of RF voltages and errors in mass analysis. processed using a digital micromirror device (DMD), For these reasons, polymer electrodes were coated using specially (d) Build substrate moves up by 25 mm, adapted electroplating process for plastic structures. Coating (e) Procedure is repeated until all slices are processed. included thick base layer of copper and nickel with thin layer of high gloss gold, which provided very firm attachment of metal The DLP machine used for LIT manufacturing is Envisiontec 2 layers to the rods and excellent electrical conductivity. Perfactory 3 Mini Multi Lens with ERM. It uses 700 mW/dm of The following section describes experimental characteristics of visible light at 25 mm layers. Smallest ERM voxel size is 16 mm. a non-scanning LIT, DLP machining, electroplating process and Maximum build envelope available is 84 mm  63 mm  230 mm. experimental setup for tests with a non-scanning DLP LIT. Results The build time of structures is not dependent on the number/size and discussion are given for optimized DLP LIT prototype, of parts, but only on the height of the parts. Manufacture of outgassing tests with HTM140 polymer, and experiments with multiple DLP LIT parts takes approximately 11 h. DLP LIT for detection of key mass fragments in simulants for cocaine, TNT and sarin. 2.3. Electroplating process TM 2. Experimental The Metalise process from 3DDC Ltd, UK was used for the metallic coating of DLP LIT rod electrodes. This method has been 2.1. Non-scanning linear ion trap specifically developed for plastic components that have been produced via additive manufacturing. The DLP electrodes were Theory of a non-scanning LIT has been described in detail by initially pre-processed using a fine sanding procedure. The surface Brkic et al. [8]. In a non-scanning mode, only one specific targeted roughness of the parts was previously reduced using ERM ion mass is confined within an analyzer and ejected at a time. Mass capability to allow for the electroplating process to be carried isolation is performed by applying suitable RF and DC voltages on out and to achieve the best operational results. This is because LIT rods with values near the tip of the stability diagram for given patch potentials on rough areas of LIT electrodes could increase ion ion mass. Fig. 1 shows a schematic diagram of a non-scanning LIT motional heating, which would cause distortion of ion motion [17]. consisting of four hyperbolic rod electrodes for radial (x,y) ion For the LIT electrode application, a thick layer of copper and nickel confinement and two disc endcap electrodes for axial (z) (130 mm) was first applied before the final coating with high gloss confinement. Positive ions are radially trapped and isolated by gold (0.3 mm). The hyperbolic outer surfaces of the electrodes were applying constant positive RF and DC voltages to y-axis rods then polished smooth for best performance. This coating technique (y-electrodes) and negative DC voltage to x-axis rods (x-electro- is a specially adapted electroplating process, where electrical des). Axial confinement is done by applying identical positive DC current is applied to an anode and cathode both placed into an voltages to entrance and exit endcaps (z-electrodes). Our LIT is electrolytic solution. In this case, the cathode is the DLP electrode and the anode would be either a sacrificial anode or an inert anode.