Wavelength Dependence of fs Laser Ablation Ionisation Mass Spectrometry: a Dedicated Study on NIST SRM 664

Laser ablation mass spectrometry provides fast and direct chemical information of solids with high spatial resolution without the need for complex sample preparation. It has been shown that reducing the laser pulse length below picoseconds improves the quantification of chemical composition measurements of solids. This study compares the impact on chemical quantification of applying femtosecond laser ablation from IR to UV wavelengths to a steel alloy sample from NIST.

A compact laser ablation ionisation mass spectrometer (LIMS), coupled to a fs laser ablation ion source with a fundamental laser wavelength of 775 nm, is used for the presented mass spectrometric results. The fundamental wavelength is frequency doubled or tripled to 387 and 258 nm, respectively. An extensive mass spectrometric campaign, comprising various laser pulse energies, was conducted using the NIST iron alloy standard reference material 664. The recorded element composition was compared with the NIST certified values.

Chemical composition analysis demonstrated the presence of significant chemical inhomogeneity of the NIST reference samples at spatial scales of about 10 μm, particularly in Ti and S. Large deviations identified were avoided for the presented study. A score was calculated for each of the accumulated spectra, indicating how close the measured composition reflected the certified values. The results show only minor differences between the wavelengths applied for sufficiently high irradiances.

These studies conducted on steel alloy NIST SRM 664 demonstrate that the impact of laser wavelength on the quantification becomes only noticeable for low irradiances. At sufficiently high laser irradiances at around 3.1 TW/cm² and beyond, comparable calibration coefficients can be observed.