Inter-element fractionation and correction in laser ablation inductively coupled plasma mass spectrometry
Inter-element fractionation in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis is one of the major challenges for using the technique for in situ trace element determination and isotopic ratio measurement of geological, environmental and biological solid samples. Attempts have been made to reduce inter-element fractionation in LA-ICP-MS analysis. However, this fractionation cannot be eliminated. The mechanism of the fractionation in LA-ICP-MS analysis is not very well understood. This study investigated the inter-element fractionation of seven elements (Ca, V, Zn, Ga, Sr, La and Nd) in three different sample matrices (NIST 613, BCR-2 and SY-4) using a UV 266 nm laser. The study showed that the inter-element fractionation depends on the sample matrices and varies with time. The inter-element fractionation behaviour of V, Zn and Ga in the synthetic silicate glass NIST 613 is different from that in the quenched glass of fused silicate rocks (BCR-2 and SY-4). Relative to Ca, V, Zn and Ga show less fractionation in NIST 613 but larger fractionation in BCR-2 and SY-4. The relative internal standard normalized element intensity (RISNEI) is not linear with time for a laser ablation period of 210 s. Therefore, data acquisition using prolonged laser ablation without a matrix match will not improve the precision and accuracy for elements whose fractionation behavior is different from that of the internal standard element. The RISNEI versus time relationship for the first 100 s laser ablation can be treated as linear to simplify the data calculation. In this paper, the internal standard normalized fractionation factor (ISNFF) is defined as the sum of the second half average RISNEI and the difference between the second and first half average RISNEI, divided by the second half RISNEI of data acquisition, for the analyte concentration calculation. The ISNFF was applied for the correction of the data reduction in LA-ICP-MS analysis. The data accuracy for these seven elements is generally improved, particularly for an element whose calibration standard normalized ISNFF is significantly greater or less than 1 (e.g., Zn and Ga in this study). Good accuracy can be obtained for elements without ISNFF correction and matrix matches only if the calibration standard normalized ISNFF of the elements is close to 1.