Laser ablation of soft tissue using a cryogenically cooled ablation cell

Abstract

A cryogenically cooled ablation cell enables the direct analysis of thin sections from fresh soft tissue samples, such as liver or kidney, for trace elements using laser ablation ICP-MS. We show here, for the first time, that reproducibilities of about 2–6% can be achieved if the tissue sample can be ablated at a temperature below −60 °C. All ablation and detection parameters, such as energy, spot size, focus, fire frequency and integration time at the ICP-TOF-MS, were optimised. A calibration method using three different tissue samples (sheep kidney, pig liver and sheep liver), from which the bulk element concentrations were determined, was validated with CRM Pig Liver (LGC 7112), which was pressed and frozen in the form of a thin slice. Good recoveries (86–124% for the certified values) were achieved for Cd (0.31 mg kg⁻¹; 0.25 ± 0.04 certified), Cu (101 mg kg⁻¹; 117 ± 8 certified), Zn (43.0 mg kg⁻¹; 43.0 ± 2.7 certified) and Mo (1.8 mg kg⁻¹; ∼1 indicative value). Therefore this CRM can be used for the quantification of other tissues with similar C-content using a one-point calibration. Detection limits in the lower µg kg⁻¹ range (Cd: 15 µg kg⁻¹, Cu: 50 µg kg⁻¹, Zn: 20 µg kg⁻¹, Mo: 10 µg kg⁻¹ and Pb: 2 µg kg⁻¹) were determined based on 3σ of the blank signal with a spatial resolution of less than 200 μm. Using the CRM Pig Liver, it was shown that the use of an internal standard (¹³C) can account for fluctuations in the ablated material during a line scan. Instead of 12% RSD without internal standard, the stability of the signal was improved using the normalized signal (5.2%) compared to 2.5–3.5% precision when a NIST 610 Glass standard was ablated. Hence, LA coupled to ICP-MS with a cryogenically cooled ablation chamber is the ideal technique for 2D mapping of trace elements in soft tissues. Depending on the concentration of element present, it may be possible to determine trace elements directly in tissue samples at a spatial resolution of <20 µm.