Laser ablation for direct solid sample chemical analysis has advanced over the past 50 years with applications in many disciplines, including environmental, geological, medical, energy, security, and others. Although the choice of laser is still highly dependent on the application requirements, there are distinct fundamental effects attributed to the laser pulse duration that drive the ablation sampling process. An overview of nanosecond and femtosecond laser ablation is presented with respect to analysis based on the optically induced plasma at the sample surface (such as laser-induced breakdown spectroscopy or laser ablation molecular isotopic spectrometry) and transport of the ablated mass aerosol to an inductively coupled plasma.

Laser ablation is widely recognized as a powerful technology for direct solid sampling chemical analysis. Without requiring sample preparation in most cases, any solid sample can be analyzed for elemental and isotopic content within minutes. There are two primary approaches for detecting the ablated mass — either by measuring the photons emitted by the optical induced plasma at the sample surface or by entraining the ablated aerosol into a gas stream with delivery to a secondary source. Laser-induced breakdown spectroscopy (LIBS) and laser-ablation molecular isotopic spectroscopy (LAMIS) are based on measuring optical emission in the plasma (1–10). The inductively coupled plasma (ICP) with mass spectrometry (MS) or optical emission spectroscopy (OES) methods are based on the transport of the ablated aerosol (8,11–20). The sampling process is the same (laser ablation); detection is dictated by the application.

Laser ablation actually dominates in applications other than chemical analysis, such as cutting, welding, micromachining, and laser-assisted in situ keratomileusis (LASIK). Although chemistry is not emphasized in these other applications, their requirements are similar: efficiently use laser photons to remove mass. Over the past 50 years, research has addressed almost every parameter influence on the ablation process. The basis of this column is to provide physical concepts on a critical parameter driving efficient mass removal — the laser pulse duration. This column describes observations from nanosecond and femtosecond ablation research with comments related to chemical analysis based on both plasma emission and particle detection. Picosecond lasers are not addressed mainly because they have not been that prevalent in this field; applications are primarily driven by the availability of commercial lasers. Until about 15 years ago, the majority of research emphasized nanosecond pulsed lasers, which had replaced microsecond and longer pulsed lasers. There are early references on long-pulse laser ablation (21) but for the most part, chemical analysis would be challenging if not for the development of short pulsed (nanosecond and femtosecond) lasers. Based on a series of concepts established from time-resolved measurements, we provide an overview of the processes under which ablation occur. Foremost, we admit upfront that much of the pulsed laser ablation research is empirical — there is no unified theory that predicts the quantity of mass ablated, the chemistry of the plasma, or the aerosol properties from an ablation event. The discussion herein does not delve into specific mechanisms, but instead presents concepts based on observations from reproducible time-resolved measurements.