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; 150–6000 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 supermicron particles. At a lens pressure of 2.4 hPa, the new ALS achieves a PSL particle size range from 230 to 3240 nm with 50 % detection efficiency and between 350 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 by single-particle mass spectrometry (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 than 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 7 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 to 2000 nm. Regarding cubic NaCl particles the modifications of the ALABAMA result in an up to 2 times increased detection efficiency and an up to 5 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-nucleating particles (INPs) 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 the 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 2 to 3 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 and particles shape as well as for highly charged particles.

中文翻译：

---

优化单粒子激光烧蚀质谱仪的检测，烧蚀和离子提取效率，以用于气溶胶颗粒浓度低的环境

这项研究的目的是展示新开发的空气动力学透镜系统（ALS），延迟离子提取（DIE）和更好的电屏蔽功能如何提高基于飞机的激光烧蚀气溶胶MAss光谱仪（ALABAMA）的效率。这些改进通常适用于单粒子激光烧蚀质谱仪。为了表征这些修改，对球形聚苯乙烯乳胶颗粒（PSL; 150–6000 nm）和立方氯化钠颗粒（NaCl; 400–1700  nm）进行了尺寸分辨的广泛测量 。与以前使用的Liu型ALS相比，在固定的ALS位置进行的测量显示，新ALS的可检测粒径范围有所改善，尤其是对于超微粒子而言。在2.4 hPa的镜头压力下 ，新型ALS的PSL粒径范围在230至3240  nm之间，检测效率为50  ％，在350至2000 nm之间， 检测效率 为95  ％。通过测量沿激光横截面的可变ALS位置处的检测效率来确定粒子束发散度，发现对于约800 nm处的PSL而言，其发散最小 。与使用Liu型ALS的单颗粒质谱（SPMS）仪器进行的测量相比，最小的粒子束发散朝向较大的粒径转移。但是，与Liu型透镜相比，对于低至200 nm的粒径没有任何缺点 。通过使用DIE和附加的电屏蔽实现的改进可以通过对命中率进行大小分辨的测量来评估，命中率是产生可检测离子量的激光脉冲与发射的激光脉冲总数之比。尤其是，通过防止这些粒子在离子提取场中发生不希望的偏转，可以显着提高小于500 nm的多电荷粒子的命中率 。此外，发现通过使用DIE，可以提高烧蚀，电离和离子提取过程的离子产率，从而使阳离子光谱的信号强度提高多达7倍。提高的离子产率会导致更大的消融激光束有效宽度，进而导致命中率接近100  ％PSL颗粒的粒径在350至2000  nm之间。关于立方氯化钠颗粒，对阿拉巴马州的修改导致检测效率提高了2倍，命中率提高了5倍。尤其是对于测量以低浓度存在的颗粒（例如通常为冰核颗粒（INP））以及在高海拔或原始环境中的气溶胶颗粒进行测量时，需要进行此类仪器修改。特别是对于这些低粒子数浓度，提高的效率有助于克服单粒子质谱仪测量的统计限制。例如，在这项研究中进行的实验室INP测量表明单独使用DIE，对于采样的物质，每时间单位的INP质谱数会增加2到3倍。总体而言，此处介绍的仪器改进的组合提高了ALABAMA对不同颗粒类型和颗粒形状以及高电荷颗粒的测量效率。