Mohamed Shaheen

Evaluation of the analytical performance of femtosecond laser ablation inductively coupled plasma mass spectrometry at 785 nm with glass reference materials

This study critically evaluates the robustness of near infra red (NIR) femtosecond (fs) LA-ICP-MS as an analytical method, for as large an element suite as feasible and at concentrations ranging from many mg g−1 to 10s of ng g−1, for a range of silicate glass matrices. Sixty three major, minor and trace elements covering the mass range from Li to U in NIST (610, 612, and 614) and USGS basalt glass (BHVO-2, BCR-2G, and BIR-1) reference materials were determined by fs-LA-ICP-MS. NIST 612 was used as an external standard for calibration and 43Ca isotope was used as an internal standard to correct for matrix effect, drift, and the amount of ablated materials. Accurate element concentrations in NIST and USGS SRMs were determined with relative standard deviations (RSD) of less than 10% for most elements in all samples analyzed. The measured concentrations agree within 10% of the previously published values for most elements. Fractionation of almost all elements during extended periods of spot ablations was minimal based on calculated fractionation indices for NIST 610 and measured U/Pb ratios for all reference materials.

Improving the analytical capabilities of femtosecond laser ablation multicollector ICP-MS for high precision Pb isotopic analysis: the role of hydrogen and nitrogen

The effect of nitrogen and hydrogen addition to the central gas flow in femtosecond LA-ICP-MS has been investigated. Enhanced sensitivity of most elements was observed as a result of nitrogen and hydrogen addition to the Ar carrier gas before the ablation cell. Both gases resulted in decreased oxide ratios and increased abundances of doubly charged ions. The effect of nitrogen on sensitivity and mass bias of Pb isotopes in fs-LA-MC-ICP-MS was then investigated. The intensity of Pb and Tl was enhanced by a factor of 3–4 when nitrogen was mixed with Ar carrier gas. At the same time there was a systematic decrease in the mass bias (13%) of Pb and Tl with nitrogen addition. The mass bias of Tl is not identical to that of Pb but accurate and precise Pb isotope ratios were obtained when correcting for the mass bias difference. The results of this work indicate the usefulness of mixed gas plasma to improve signal intensities in ICP-MS in general and MC-ICP-MS in particular where large ion beams and robust and controlled mass bias corrections are necessary for precise and accurate isotopic analysis using Faraday detectors.

Laser ablation of iron: A comparison between femtosecond and picosecond laser pulses

In this study, a comparison between femtosecond (fs) and picosecond (ps) laser ablation of electrolytic iron was carried out in ambient air. Experiments were conducted using a Ti:sapphire laser that emits radiation at 785 nm and at pulse widths of 110 ps and 130 fs, before and after pulse compression, respectively. Ablation rates were calculated from the depth of craters produced by multiple laser pulses incident normally to the target surface. Optical and scanning electron microscopy showed that picosecond laser pulses create craters that are deeper than those created by the same number of femtosecond laser pulses at the same fluence. Most of the ablated material was ejected from the ablation site in the form of large particles (few microns in size) in the case of picosecond laser ablation, while small particles (few hundred nanometers) were produced in femtosecond laser ablation. Thermal effects were apparent at high fluence in both femtosecond and picosecond laser ablation, but were less prevalent at low fluence, closer to the ablation threshold of the material. The quality of craters produced by femtosecond laser ablation at low fluence is better than those created at high fluence or using picosecond laser pulses.

Femtosecond laser ablation behavior of gold, crystalline silicon, and fused silica: a comparative study

The influence of target material on the ablation behavior of femtosecond laser pulses was investigated. Three different materials, representing the spectrum of electrical conductivities, were selected: a dielectric (fused silica), a semiconductor (crystalline silicon), and a metal (gold). Ablation was performed in ambient air using a Ti:sapphire laser, which emits radiation at a wavelength of 785 nm and a pulse width of 130 fs. Surface morphology and ablation depth were evaluated using optical and scanning electron microscopy. Significant changes in surface morphology were observed with variation of the fluence and number of laser pulses. In all materials, two different ablation regimes were distinguished depending on the fluence. Ablation threshold, which was determined from the relationship between crater diameter squared and the logarithm of laser energy, was found to depend on the number of laser pulses incident on the same spot (i.e. incubation phenomenon).

Femtosecond laser ablation of brass in air and liquid media

Laser ablation of brass in air, water, and ethanol was investigated using a femtosecond laser system operating at a wavelength of 785 nm and a pulse width less than 130 fs. Scanning electron and optical microscopy were used to study the efficiency and quality of laser ablation in the three ablation media at two different ablation modes. With a liquid layer thickness of 3 mm above the target, ablation rate was found to be higher in water and ethanol than in air. Ablation under water and ethanol showed cleaner surfaces and less debris re-deposition compared to ablation in air. In addition to spherical particles that are normally formed from re-solidified molten material, micro-scale particles with varying morphologies were observed scattered in the ablated structures (craters and grooves) when ablation was conducted under water. The presence of such particles indicates the presence of a non-thermal ablation mechanism that becomes more apparent when ablation is conducted under water.

Evaluation of ablation efficiency and surface morphology of human teeth upon irradiation with femtosecond laser pulses

This study investigates changes in ablation efficiency and surface morphology induced in human dental enamel and dentin upon interaction with femtosecond laser pulses at variable energies and number of laser pulses. Craters were created using a Ti:sapphire femtosecond laser ablation system operating at a wavelength of 785 nm, pulse width of 130 fs, and repetition rate of 20 Hz. Various techniques, such as optical and scanning electron microscopy and inductively coupled plasma mass spectrometry (ICP-MS), were used to evaluate ablation depth, amount of material ablated, and surface morphology of the craters. Ablation rate (ablation depth per pulse) was found to be lower in enamel than dentin with the maximum rate occurring at fluence of 12.4 J cm−2 in both materials. A drop in ablation rate was observed for fluence greater than 12.4 J cm−2 and was attributed to attenuation of laser energy due to interaction with the laser-generated particles. Above this fluence, signs of thermal effects, such as melting and formation of droplets of molten material at the sample surface, were observed. The response of the ICP-MS indicated that the amount of ablated material removed from dentin is greater than that removed from enamel by a factor of 1.5 or more at all investigated fluence.