Optimizing the detection, ablation and ion extraction efficiency of a single particle laser ablation mass spectrometer for application in environments with low aerosol particle concentrations

Abstract

<jats:p>Abstract. The aim of this study is to show how a newly developed aerodynamic lens system (ALS), a delayed ion extraction (DIE) and better electric shielding improve the efficiency of the Aircraft-based Laser ABlation Aerosol MAss spectrometer (ALABAMA). These improvements are applicable to single particle laser ablation mass spectrometers in general. To characterize the modifications extensive size-resolved measurements with spherical polystyrene latex particles (PSL; 150–000 nm) and cubic sodium chloride particles (NaCl; 400–1700 nm) were performed. Measurements at a fixed ALS position show an improved detectable particle size range of the new ALS compared to the previously used Liu-type ALS, especially for super-micron particles. At a lens pressure of 2.4 hPa, the new ALS achieves a PSL particle size range from 230 nm to 3240 nm with 50 % detection efficiency and between 350 nm and 2000 nm with 95 % detection efficiency. The particle beam divergence was determined by measuring the detection efficiency at variable ALS positions along the laser cross sections and found to be minimal for PSL at about 800 nm. Compared to measurements of SPMS instruments using Liu-type ALSs, the minimum particle beam divergence is shifted towards larger particle sizes. However, there are no disadvantages compared to the Liu-type lenses for particle sizes down to 200 nm. Improvements achieved by using the DIE and an additional electric shielding could be evaluated by size-resolved measurements of the hit rate, which is the ratio of laser pulses yielding a detectable amount of ions to the total number of emitted laser pulses. In particular, the hit rate for multiply charged particles smaller 500 nm is significantly improved by preventing an undesired deflection of these particles in the ion extraction field. Moreover, it was found that by using the DIE the ion yield of the ablation, ionization and ion extraction process could be increased, resulting in up to seven times higher signal intensities of the cation spectra. The enhanced ion yield results in a larger effective width of the ablation laser beam, which in turn leads to a hit rate of almost 100 % for PSL particles in the size range from 350 nm to 2000 nm. Regarding cubic NaCl particles the modifications of the ALABMA result in a up to two times increased detection efficiency and a up to five times increased hit rate. The need for such instrument modifications arises in particular for measurements of particles that are present in low number concentrations such as ice nucleation particles (INP) in general, but also aerosol particles at high altitudes or in pristine environments. Especially for these low particle number concentrations improved efficiencies help to overcome statistical limitations of single particle mass spectrometer measurements. As an example laboratory INP measurements carried out in this study show that the application of the DIE alone increases the number of INP mass spectra per time unit by a factor of two to three for the sampled substances. Overall, the combination of instrument modifications presented here resulted in an increased measurement efficiency of the ALABAMA for different particle types, particles shapes and for highly charged particles. </jats:p>

Design, characterization, and first field deployment of a novel aircraft-based aerosol mass spectrometer combining the laser ablation and flash vaporization techniques

“…The setup of the optical single particle detection module for ERICA-LAMS consists of the two particle detection units PDU1 and PDU2 (see Fig. 1), based on the design of the ALABAMA (Brands et al, 2011;Clemen et al, 2020). Each of these particle detection units (PDU1 and PDU2) contains a continuous wave laser (LD1 and LD2), an ellipsoidal reflector, and a PMT (PMT1 and PMT2).…”

“…This design collects approximately 75 % of the total scattered light, not considering the losses at the 20 pinholes. The acquired PMT signals are processed by an in-house built electronic board, hereafter referred to as trigger card (TC) following the design from the ALABAMA (Brands et al, 2011;Clemen et al, 2020).…”

“…Upon connection to ground, the electric field decays with an RC constant of 20 approximately 10 ms. The HV switch was implemented, since the electric extraction fields cause charged aerosol particles to deviate from their straight flight direction (e.g., Chen et al (2020) and Clemen et al (2020)) and as a result, they might not hit the vaporizer in the ERICA-AMS part. In order to also reduce particle deflection caused by an electric field forming outside the ion optics, in addition the particle flight path through the ERICA-LAMS part is shielded by grounded plates.…”

“…The PALMS (Particle Analysis by Laser Mass Spectrometry; Murphy et al (1998)) operated at altitudes of up to 20 km. Other aircraft-based, online single-particle laser ablation aerosol mass spectrometers, which are operated at lower altitudes, are for example the A-ATOFMS (Aircraft Aerosol Time-Of-Flight Mass Spectrometer; Pratt et al 15 (2009)), the ALABAMA (Aircraft-based Laser ABlation Aerosol MAss spectrometer; Brands et al (2011) and Clemen et al (2020)), and the miniSPLAT (miniaturized version; Single Particle Laser Ablation Time-of-flight mass spectrometer; Zelenyuk et al (2015). The thermal vaporization and electron impact ionization technique were deployed on research aircraft using a C-ToF-MS (Compact Time-of-Flight Mass Spectrometer) beside others by Bahreini et al (2009), Morgan et al (2010), Schmale et al (2010), Brito et al (2018), Schulz et al (2018), and Haslett et al (2019), while a mAMS (mini Aerosol Mass 20 Spectrometer) was used for example by Vu et al (2016) and Goetz et al (2018).…”