Reassessment of the influence of carrier gases He and Ar on signal intensities in 193 nm excimer LA-ICP-MS analysis

Abstract

Helium (He) is often used as an aerosol carrier gas in 193 nm ArF excimer laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis because it increases the signal intensity compared with that obtained using argon (Ar). In this study, the influence of two carrier gases (He and Ar) on signal intensities was reassessed using a local aerosol extraction strategy and a commonly used cylindrical ablation cell. The experimental results showed that the sampling position in a common ablation cell strongly affected the signal enhancement factors of refractory lithophile elements (Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Th and U) when using He instead of Ar as the carrier gas. The signal enhancement factor of the refractory elements was about 2–3 times in the center position of the common cylindrical ablation cell, which has an ablation location-related carrier gas flow rate of 1.5 m s⁻¹. Enhancement of 3–4 times was obtained at the edge position with an ablation location-related gas flow rate of 0.2 m s⁻¹. Furthermore, the signal intensities obtained using the local aerosol extraction strategy at a sample distance of 1 mm with an extremely high ablation location-related gas flow rate (approximately 10 m s⁻¹) were enhanced only slightly (by approximately 1–1.3 times) for all measured elements when using He instead of Ar as the carrier gas. The similar signal intensities obtained at high-velocity of the carrier gas on the ablation site using Ar as the carrier gas instead of He indicate that the size of the aerosol particles or agglomerates produced by laser ablation decreased considerably as the ablation location-related gas flow rate increased. To gain information on the mechanisms that are involved in the improvement of signal sensitivity by using Ar as the carrier gas at a high-velocity ablation location, a small amount of water vapor was also added into the plasma in both a normal ablation cell and local aerosol extraction strategy. The addition of small amounts of water vapor into the plasma increased signal sensitivity by a factor of 3 using Ar as the carrier gas, while the signal enhancement in wet plasma was about 1.7 fold under a He atmosphere. In addition, similar signal intensities of the entire mass range elements were obtained in wet plasma using Ar and He as carrier gases. These results demonstrate that the difference of elemental signal intensities obtained using He or Ar as the carrier gas can be dramatically alleviated under wet plasma conditions. Our results illustrate that the pronounced enhancement in sensitivity using He as the carrier gas is not only because of the enhanced transport efficiency of small particles, but mostly caused by the more effective vaporization of the aerosol particles in the ICP. Zircon is the most widely used mineral for U–Pb geochronology to provide the timescales, origin and history of rocks. Under wet plasma conditions or with a high ablation location-related gas flow rate, U–Pb age dating of zircon with similar accuracy was obtained using Ar instead of He as the carrier gas.