Development of a Portable, Low Cost, Plasma Ionization Source Coupled to a Mass Spectrometer for Surface Analysis B. L. Smith1,2, F. P. M. Jjunju1,2, S. Taylor1,2, I. S. Young3, and S. Maher1* 1. Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK.
2. Q-Technologies Ltd, University of Liverpool, Liverpool, UK.
3. Institute of Integrative Biology, University of Liverpool, Liverpool, UK.
*Email:
[email protected] Abstract— This paper details the development of a handheld battery operated ambient ionization source that when coupled to a mass spectrometer is capable of in-situ, solid phase chemical analysis. There exists a plethora of proven techniques for ionizing liquid and gas phase analytes for mass analysis but due to a lack of volatility, solid phase analytes can be difficult to ionize and introduce directly from a surface to a mass spectrometer. Various ambient technologies have been developed for this purpose such as low temperature plasma (LTP), desorption electrospray ionisation (DESI) and direct analysis in real time (DART), but each of these technologies has associated drawbacks for miniaturization. The ionization device described weighs less than 600 g, can be operated for 6 hours continuously from battery power, requires no other inputs or sample preparation and has a touch screen interface for improved ease of use for non-specialist operators. This paper details the design considerations in its development as well as testing and procedures for integration with a mass spectrometer for chemical sensing. Keywords—Handheld DAPCI; Ambient Ionization; Portable Mass Spectrometry; Chemical Sensing; Plasma; Corona Discharge; Analytical Instrumentation; Surface Analysis
I.
power and/or high gas flow rates limiting their usage to a laboratory setting. Herein we describe a novel hand portable DAPCI source [8] that has undergone significant developments in terms of miniaturization, reduced power consumption and improved ionization efficiency afforded by the addition of thermal assistance with integrated temperature control. II.
DESIGN OF HANDHELD DAPCI
DAPCI is a plasma-based technique that is relatively underutilized compared to DART and LTP despite its inherent simplicity of operation. It is similar in many respects to APCI but with the fundamental difference that it produces ions from solids in the open environment instead of from vapors or solution. DAPCI involves the generation of a corona discharge generated by applying a high DC voltage to a sharp needle from which reagent ions are produced and directed towards the surface to be chemically interrogated by provision of a high velocity carrier gas (e.g., Nitrogen). The analyte is both desorbed from the surface and ionized bringing sample ions in to the gas phase and suitable for MS analysis.
INTRODUCTION
Mass spectrometry (MS), as a general purpose analytical tool, is unsurpassed in terms of the sensitivity, selectivity, speed and chemical insight it provides [1]. The operation of mass spectrometers involves three basic steps: ionization of the sample (or “analyte”) under scrutiny, mass analysis (separation of ions according to mass-to-charge ratio (m/z)) and detection. Sensitivity, selectivity, speed and simplicity of analysis depend crucially upon the ionization step, with MS able to draw from a wide variety of ionization techniques. Recent developments in ambient ionization (AI) technology have increased the simplicity, efficiency and variety of samples that can be analyzed by mass spectrometers [2]. AI-MS is capable of providing solid phase analysis in real time with low limits of detection via multiple ionization techniques such as DESI [3], DART [4], LTP [5] and desorption atmospheric pressure chemical ionization (DACPI) [6, 7]. However, these AI sources usually require bulky equipment, high electrical
Figure 1: Simplified functional block diagram and interconnections of handheld DACPI electronics. A. Power The power source for all the DAPCI electronics is a 12V 1800mAh LiPo battery. The battery has an integrated charging circuit and ON/OFF switch. To avoid the effects of battery discharge curve characteristics, high efficiency DC to DC converters are used to ensure well-regulated +12V and +/-5V power rails.
B. Control The heart of the ECU is a 32-Bit ARM processor. The controller interfaces to the end user via 2.4” touch screen (uLCD-24PTU, 4D systems, AUS) allowing the user to change output parameters and monitor temperatures. The controller sets and regulates a series of control loops to keep output operation stable. The high voltage (HV) applied to the needle electrode is set via a DAC and stability is ensured via an attenuated high voltage monitor. The miniature diaphragm pump is controlled via a PWM signal sent from the controller. A motor driver IC ensures motor drive current regulation based on the PWM mark space ratio for a desired flow rate, calibrated using an industrial flow meter. The output gas heating element is regulated by implementing a software PID algorithm running within the firmware of the micro-controller. The temperature feedback is achieved via a K-type thermocouple and precision amplifier. C. Outputs The ionization probe itself is constructed from a Swagelok PTFE 1/8” tee fitting with a non-metallic tube inserted into one end and a tapered stainless steel needle into the other. The gas supply enters via the third leg (or ‘trunk’) of the tee and a gas tight seal is made on all three sides to safeguard maximum flow rate. The probe is directly mounted onto the HV PCB thus making the high voltage connection much simpler and easier for EM shielding, in addition to aiding an overall reduction of the device footprint. The HV, necessary to produce a corona discharge at the tip of the needle, is generated by a miniaturized HV DC-DC converter (XP-EMCO, Q-50/Q-50N). The air flow is generated using a Parker miniature diaphragm pump capable of delivering 3L/min of gas flow from a 12V, 0.35A power supply. A custom flexible heating element is coiled around the ceramic exit nozzle of the probe and held in position with a potting compound. This ensures maximum thermal transfer and confines the active heating to the escaping carrier gas only.
position for trigger finger operation. The enclosure is predominantly constructed from 3-D laser sintered polyamide plastic that has high strength and stiffness, and good chemical resistance. The front nozzle section (white section of Fig 2) is machined from PTFE due to its chemical inert properties and ability to withstand the higher temperatures in this region generated from the heating element. Double insulation techniques, potting compounds and circuit level safety features ensure complete isolation between the operator and any area potentially under HV. III.
EXPERIMENTAL
Experiments were conducted on a commercial triple quadrupole mass spectrometer (Waters Xevo TQ MS, Waters, Wilmslow, UK) with an atmospheric pressure interface. Fig 3 shows the handheld DAPCI being used to interrogate a surface with the ions generated presented to the inlet of the mass spectrometer. The probe tip that protrudes slightly from the unit is directed towards the compound of the surface under investigation. The sample is positioned approx. 2-3mm away from the sample inlet and the tip of the probe is ~3-5mm away from the sample. The desorbed ions enter the mass spectrometer through the atmospheric pressure inlet due largely to the pressure gradient between the ambient atmosphere and the vacuum environment of the mass spectrometer. The operating parameters for the source are optimized for the analyte(s) under investigation. The HV applied to the needle can be varied between 1 and 5kV. The flow rate of the carrier gas can also be varied between 0.5 L/min - 3 L/min. The temperature of the exiting gas can be varied between room temperature and 120oC. IV.
To demonstrate the efficacy of the handheld device preliminary experiments were carried out for the analysis of tetrahydrocarbazole (molecular weight 171 Daltons) direct from a filter paper surface in the open environment. Carbazoles are aromatic organic compounds, typically found in crude oil that can poison the activity of catalysts used during oil processing and refining. The mass spectra acquired are shown in figure 4 with and without heated carrier gas. When thermally assisted, the ion signal intensity rose by ~30% with a relative increase in the protonated molecular ion peak ([M+H]+ at m/z 172) and increased ion dissociation. Being able to adjust insource ion dissociation via heating can aid in identification, if tandem analysis is not available. V.
Figure 2: Photograph of Handheld DAPCI device. D. Enclosure and UI The handheld DAPCI enclosure and UI have been designed for ergonomic comfort, ease of use and to be aesthetically pleasing (Fig. 2). The unit fits in the palm of the hand in a similar fashion to a pistol. This guarantees the touch screen interface is in direct sight of the user and removed from the ion beam where HV is applied. A consistent weight distribution is achieved by placing the heaviest item, the battery, in the handle section of the enclosure and the ON/OFF switch is in the natural
RESULTS
CONCLUSION
A portable handheld DAPCI has been described with improved performance and ionization capabilities. The provision of thermal assistance by virtue of a heated carrier gas can aid in surface desorption due to increased energy transfer. This leads to improved sensitivity and preliminary experiments suggest that the temperature should be optimized for the analyte in question due to the potential for ion dissociation. It has been demonstrated that the lightweight, power efficient and ergonomic design described herein, when coupled to a mass spectrometer, is a valuable ionization source enabling ambient, chemical sensing and surface analysis.
Figure 3: Cut away view of handheld DAPCI interfaced to atmospheric pressure inlet of a mass spectrometer. current and additional doping of the reagent ion beam to increase selectivity and broaden the range of compounds that can be identified. VI.
ACKNOWLEDGMENT
The authors would like to acknowledge the gracious support of this work through the EPSRC and ESRC Centre for Doctoral Training on Quantification and Management of Risk & Uncertainty in Complex Systems & Environments (EP/L015927/1). In addition the authors gratefully acknowledge support received towards this work from an EPSRC Impact Acceleration Award (EP/K503952/1). VII. REFERENCES [1]
[2]
[3]
Figure 4: DAPCI surface analysis of 10 ppm of 1,2,3,4tetrahydrocarbazole deposited on to filter paper directed at an area of ~1 cm2, (a) with heating control turned off and (b) with carrier gas temperature raised to 100° Celsius. The full range and class of compounds that can be “sensed” via the handheld DAPCI is still being explored and quantified. Several application areas have been identified such as environmental monitoring of aquatic conditions, drugs and explosives detection at safety critical locations and polymer identification. We are actively seeking new sensing applications to deploy the handheld DAPCI and are engaged in continuous improvement to address some of the current limitations of the device. These include: a comprehensive thermal investigation, improved focusing of the ion beam to improve spatial resolution for imaging mass spectrometry applications, novel electrode designs to enhance the ion beam
[4] [5]
[6]
[7]
[8]
S. Maher, F. P. Jjunju, and S. Taylor, "Colloquium: 100 years of mass spectrometry: Perspectives and future trends," Reviews of Modern Physics, vol. 87, p. 113, 2015. M. E. Monge, G. A. Harris, P. Dwivedi, and F. M. Fernández, "Mass spectrometry: recent advances in direct open air surface sampling/ionization," Chemical reviews, vol. 113, pp. 2269-2308, 2013. Z. Takats, J. M. Wiseman, B. Gologan, and R. G. Cooks, "Mass spectrometry sampling under ambient conditions with desorption electrospray ionization," Science, vol. 306, pp. 471-473, 2004. J. H. Gross, "Direct analysis in real time—a critical review on DARTMS," Analytical and bioanalytical chemistry, vol. 406, pp. 63-80, 2014. J. D. Harper, N. A. Charipar, C. C. Mulligan, X. Zhang, R. G. Cooks, and Z. Ouyang, "Low-temperature plasma probe for ambient desorption ionization," Analytical chemistry, vol. 80, pp. 9097-9104, 2008. F. P. Jjunju, S. Maher, A. Li, A. K. Badu-Tawiah, S. Taylor, and R. G. Cooks, "Analysis of polycyclic aromatic hydrocarbons using desorption atmospheric pressure chemical ionization coupled to a portable mass spectrometer," Journal of The American Society for Mass Spectrometry, vol. 26, pp. 271-280, 2015. H. Chen, J. Zheng, X. Zhang, M. Luo, Z. Wang, and X. Qiao, "Surface desorption atmospheric pressure chemical ionization mass spectrometry for direct ambient sample analysis without toxic chemical contamination," Journal of mass spectrometry, vol. 42, pp. 1045-1056, 2007. F. P. Jjunju, S. Maher, A. Li, S. U. Syed, B. Smith, R. M. Heeren, et al., "Hand-Held Portable Desorption Atmospheric Pressure Chemical Ionization Ion Source for in Situ Analysis of Nitroaromatic Explosives," Analytical chemistry, vol. 87, pp. 10047-10055, 2015.
