Sinha M P, Neidholdt E L, Hurowitz J, Sturhahn W, Beard B, Hecht M H
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, California 91109, USA.
Rev Sci Instrum. 2011 Sep;82(9):094102. doi: 10.1063/1.3626794.
A laser ablation-miniature mass spectrometer (LA-MMS) for the chemical and isotopic measurement of rocks and minerals is described. In the LA-MMS method, neutral atoms ablated by a pulsed laser are led into an electron impact ionization source, where they are ionized by a 70 eV electron beam. This results in a secondary ion pulse typically 10-100 μs wide, compared to the original 5-10 ns laser pulse duration. Ions of different masses are then spatially dispersed along the focal plane of the magnetic sector of the miniature mass spectrometer (MMS) and measured in parallel by a modified CCD array detector capable of detecting ions directly. Compared to conventional scanning techniques, simultaneous measurement of the ion pulse along the focal plane effectively offers a 100% duty cycle over a wide mass range. LA-MMS offers a more quantitative assessment of elemental composition than techniques that detect ions directly generated by the ablation process because the latter can be strongly influenced by matrix effects that vary with the structure and geometry of the surface, the wavelength of the laser beam, and the not well characterized ionization efficiencies of the elements in the process. The above problems attendant to the direct ion analysis has been minimized in the LA-MMS by analyzing the ablated neutral species after their post-ionization by electron impaction. These neutral species are much more abundant than the directly ablated ions in the ablated vapor plume and are, therefore, expected to be characteristic of the chemical composition of the solid. Also, the electron impact ionization of elements is well studied and their ionization cross sections are known and easy to find in databases. Currently, the LA-MMS limit of detection is 0.4 wt.%. Here we describe LA-MMS elemental composition measurements of various minerals including microcline, lepidolite, anorthoclase, and USGS BCR-2G samples. The measurements of high precision isotopic ratios including (41)K/(39)K (0.077 ± 0.004) and (29)Si/(28)Si (0.052 ± 0.006) in these minerals by LA-MMS are also described. The LA-MMS has been developed as a prototype instrument system for space applications for geochemical and geochronological measurements on the surface of extraterrestrial bodies.
本文描述了一种用于岩石和矿物化学及同位素测量的激光烧蚀 - 微型质谱仪（LA - MMS）。在LA - MMS方法中，由脉冲激光烧蚀产生的中性原子被引入电子碰撞电离源，在那里它们被70 eV的电子束电离。这产生了一个典型宽度为10 - 100 μs的二次离子脉冲，而原始激光脉冲持续时间为5 - 10 ns。然后，不同质量的离子在微型质谱仪（MMS）磁扇区的焦平面上进行空间分散，并由能够直接检测离子的改进型电荷耦合器件（CCD）阵列探测器并行测量。与传统扫描技术相比，沿焦平面同时测量离子脉冲在很宽的质量范围内有效地提供了100%的占空比。与直接检测烧蚀过程中直接产生的离子的技术相比，LA - MMS对元素组成提供了更定量的评估，因为后者会受到基质效应的强烈影响，基质效应会随表面结构和几何形状、激光束波长以及该过程中元素未充分表征清楚的电离效率而变化。通过对烧蚀后的中性物质进行电子碰撞后电离分析，LA - MMS将上述直接离子分析伴随的问题最小化。这些中性物质在烧蚀蒸汽羽流中比直接烧蚀产生的离子丰富得多，因此有望成为固体化学成分的特征。此外，元素的电子碰撞电离已得到充分研究，其电离截面已知且易于在数据库中查找。目前，LA - MMS的检测限为0.4 wt.%。在此，我们描述了LA - MMS对包括微斜长石、锂云母、钠长石和美国地质调查局BCR - 2G样品在内的各种矿物的元素组成测量。还描述了通过LA - MMS对这些矿物中高精度同位素比率的测量，包括(41)K/(39)K（0.077 ± 0.004）和(29)Si/(28)Si（0.052 ± 0.006）。LA - MMS已被开发为一种原型仪器系统，用于在地球外天体表面进行地球化学和地质年代学测量的空间应用。
