Femtosecond laser ablation inductively coupled plasma mass spectrometry: achievements and remaining problems

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is considered to be one of the most versatile methods for trace elemental and isotopic analyses of solid material. Ever since the first feasibility studies initiated by A. Gray during the early 1980s [1], the performance characteristics of LA-ICP-MS have continuously been enhanced. Various operational parameters such as laser wavelength, pulse duration, fluence, carrier gas, and cell volume and/or geometry have been examined to specify conditions favorable for suppressing all effects, namely laser-, transport-, and ICP-induced ones, that result in inaccurate analyses [2–6]. The sum of these effects is commonly be referred to as “elemental fractionation”. In addition, efforts have been made towards the development of alternative calibration [7] and ICP tuning strategies [8, 9] to further improve the quantification capabilities. As a result, LA-ICP-MS using deep UV nanosecond (ns) laser systems emitting at 193 nm has become a well-accepted technique for the “selective” trace element analysis of geological samples. The term “selective” refers to the persistent problem of quantifying elements such as Cd, Zn, Ag, or Pb with sufficient accuracy and precision. Apparently, these elements show a high degree of thermal volatility, low melting points, and partially different ionization potentials and are therefore extremely sensitive to any changes of the LA and ICP conditions. Considering, for instance, the Pband U-specific isotope analysis of zircons, which is often used as a chronometer for geological events (geochronology), temporal drifts of the Pb/U ratio were found to restrict the accuracy [9, 10]. It should be emphasized that such fractionation-related inaccuracies can largely be avoided if matrix-matched standards are used for calibration. However, due to the lack of adequate reference materials, presently, only a few analytical problems can be solved on the basis of matrix matching. In recent years, the aim of performing more comprehensive multi-element analyses including the abovementioned category of elements and increasing standards of precision and accuracy for a growing spectrum of matrices has required a permanent optimization of the instrumentation and analytical procedures as well as the development of strategies to suppress elemental fractionation. In this context, the utilization of ultra-short, i.e., femtosecond (fs) laser pulses probably represents the most promising instrumental advancement enabling the production of ultra-fine aerosol particles from a wide variety of samples, whose compositions exactly comply with that of the bulk material and, thus, allowing one to perform analyses which are less affected by elemental fractionation. In contrast to this, the design and operational characteristics of the ICP as a source of aerosol vaporization, atomization, and ionization have been nearly unchanged even though recent findings indicate that the present configuration does not generally fulfil the requirements for the analysis of laserproduced aerosols. This article reports on the prospects of fs-LA as a way to improve the quantification capabilities of LA-ICP-MS using non-matrix-matched calibration. It also addresses some of the remaining problems related to ICPinduced elemental fractionation which affect the overall accuracy of analysis. Anal Bioanal Chem (2007) 387:149–153 DOI 10.1007/s00216-006-0918-z