WHAT IT IS
Laser ablation (LA) systems coupled to inductively coupled plasma mass spectrometry (ICP-MS) use focused laser pulses to remove material from solid samples for elemental and isotopic analysis. The choice of laser type is critical because wavelength, pulse width, and energy density control how efficiently material is ablated, transported, and ionized. Different laser designs offer distinct advantages in terms of sample compatibility, spatial resolution, and signal stability.
HOW IT WORKS
A pulsed laser beam is focused onto the sample surface, converting a small portion into aerosol. This aerosol is carried by a transport gas (often He or Ar) into the ICP-MS torch for ionization and detection.
Key factors determined by laser type:
Wavelength - Governs absorption efficiency in different materials.
Pulse Duration - Nanosecond vs femtosecond pulses affect the thermal vs non-thermal nature of ablation.
Beam Quality - Defines spot size, resolution, and ablation uniformity.
TYPES OF LASERS USED IN LA-ICP-MS
Nd:YAG Lasers (213 nm, 266 nm):
Solid-state lasers commonly frequency-quadrupled or quintupled to reach UV. Provide stable performance, robust design, and sufficient energy for many materials.
Excimer Lasers (193 nm, ArF):
Gas lasers operating in deep UV. Shorter wavelength improves absorption across most sample types, reducing elemental fractionation. Widely used for high-precision geochemistry.
Femtosecond Lasers (200–800 nm range):
Deliver ultrashort pulses (10⁻¹⁵ s) with minimal heat transfer. Ablation is nearly “cold,” minimizing melting and fractionation. Suitable for delicate materials and ultra-high-resolution work.
Other Wavelengths (355 nm, 1064 nm):
Sometimes used for specialized applications, though less efficient in many solid matrices due to lower absorption.
KEY FEATURES
Nd:YAG (213/266 nm): Durable, cost-effective, moderate absorption, widely used in routine labs.
Excimer (193 nm): High absorption efficiency, minimal fractionation, excellent for geological samples.
Femtosecond: Precise, low thermal effects, highest resolution but expensive and complex.
Longer Wavelengths: Lower absorption, greater matrix dependence.
IMPACT ON PERFORMANCE
Fractionation: Shorter wavelengths and femtosecond pulses reduce elemental fractionation.
Spatial Resolution: Beam quality and spot size control resolution down to a few micrometers.
Material Compatibility: Deep-UV lasers ablate a wider range of matrices effectively.
Signal Stability: Stable beam energy provides reproducible aerosol generation.
Cost vs Benefit: Nd:YAG offers affordability, while excimer and femtosecond lasers support advanced, high-precision applications.
CHALLENGES AND LIMITATIONS
Nd:YAG: More prone to elemental fractionation than 193 nm or femtosecond sources.
Excimer: Higher maintenance, gas handling requirements, and operating costs.
Femtosecond: Very high cost, complex optics, and lower robustness for routine use.
General: Laser choice must balance precision, cost, sample type, and laboratory needs.