レーザーアブレーション試料導入法(Laser Ablation Technique、以下LA法と略記)では、レーザー光を集束して固体試料の一部を加熱し、気化あるいはエアロゾル化した試料をプラズマイオン源(ICP)に導入する(図1)。固体試料を分解処理することなく迅速な化学組成分析が可能であり、さらにレーザー光を絞り込むことにより固体試料の局所部分の化学・同位体組成分析も可能である (Gray, 1985)。照射するのがレーザー光であることから、試料を研磨処理したり伝導性膜をコーティングする必要がなく、また試料を真空容器内にセットする必要がないため、試料交換も簡便であり、迅速かつ高感度な元素分析が可能である。試料前処理が最小限であることから、ICP質量分析計の最大の特徴である「分析の迅速性」を最大限に引き出すことができる試料導入法の一つである。


プラズマ質量分析法(ICP-mass spectrometry, ICP-MS)の進歩は元素分析の高感度化、分析空間分解能の向上、分析の迅速化の全てに多大な貢献を果たした。質量分析計では分析元素あるいは分子をイオン化し、電場あるいは磁場を用いて分析対象とするイオンまたは分子を重さごとに分離する。分析元素イオンを”数える”ことができるため、他の分析法に比べ非常に高い検出感度が得られる。さらにプラズマイオン源では周期律表の殆どの元素に対して90%以上の高いイオン化効率が得られるため高感度・多元素同時分析法が実現できる。さらに高温アルゴンプラズマの特長はイオン源が大気圧であるため、様々な試料導入法が適用可能である。様々な試料導入法のなかで、レーザーアブレーション試料導入法は、固体試料を直接化学分析できるため、地球化学試料の分析には広く用いられてきた。私たちの研究グループでは、地質試料・固体鉱物試料中の微量元素分析を目的に、これまでレーザーアブレーション試料導入法の開発を続け、2007年にフェムト秒レーザーを応用した次世代レーザーアブレーション法の実用化に成功し、これまで分析が困難であった試料から正確な地球化学データを引き出すことが可能となった。

The laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) technique is now widely accepted as one of the most sensitive and rapid analytical tools for elemental and isotopic analysis of solid materials.  The continuous development of LA-ICPMS techniques has provided more precise elemental and isotopic data.  With a better understanding of the mechanism of the laser ablation process, and with a higher elemental sensitivity of the LA-ICPMS technique, the precision of the isotopic ratio measurement has successively improved (Hirata and Miyazaki, 2007; Hirata, 2007).  However, even with the UV lasers, serious elemental fractionation occurs during laser ablation.  It is recognised that the differences in elemental volatility, or time-dependent changes in the particle size distribution. can cause elemental fractionation.  Moreover, there are several other causes of elemental fractionation, such as differences in the transport efficiency of the particles, mass loading onto the ICP or amount of re-deposition.  Although many efforts have been made to reduce the elemental fractionation, the complexity in the physical and chemical processes related to the laser ablation of solid samples has prevented a detailed understanding of the elemental and isotopic fractionation induced by laser ablation, and therefore the reduction of elemental fractionation through the analysis was still a key issue to improve the data quality.  It is widely recognised that the particle size distribution is a critical parameter controlling both the analytical sensitivity (ionisation efficiency) and the level of elemental fractionation.  The size distribution of the sample aerosol is seriously dependent upon various conditions, including laser irradiance, cell geometry, the type of carrier gas, and laser pulse length.  The difference in size of the laser-induced sample particles can be due to the large difference in thermal conductivity of the materials (1 Wm-1 K-1 for Glass, and 120 Wm-1 K-1 for Si).  The rate of thermal diffusion of the metallic sample (Si wafer) occurs in the order of a pico-second (10-12 s), which is much faster than that of non-metallic samples.  Because of the large loss of laser energy due to faster thermal diffusion and the fact that the samples could not be totally evaporated, the resulting molten phase was released from the ablation site by means of laser-induced shockwave.  The large size particles can survive from the total evaporation and therefore the solid sample can pass through ICP into the sampling orifice.  The introduction of survived sample particles can be a source of signal spikes, the elemental fractionation possibly due to preferential evaporation of the volatile elementals or due to re-deposition of the ablated materials onto the sample surface.  This is particularly serious when metallic or semi-conductor materials, such as Cu, Zn, Fe or Si-wafer are analysed with the conventional laser ablation technique.  To obtain the stable signal intensity and reliable analytical results, both a higher evaporation efficiency and the shift of size distribution of the sample particles toward smaller size is very important.  A new approach to improve the signal stability and to reduce the elemental fractionation is to use femtosecond (fs) lasers.  Using the fs-laser ablation system, laser-induced sample particles can be modified toward smaller sizes, resulting in better analytical precision or repeatability of the measurement with higher analytical sensitivity for silicate minerals, glass and metallic samples (Hirata and Kon, 2008; Ikehata et al., 2008).

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