Breaking the PPB Detection Limit Barrier Using Laser Ablation Microprobe ICP-MS

Ingo Horn Memorial University, Department of Earth Sciences, St. John's, NF, A1B 3X, Canada

Henry P. Longerich Memorial University, Department of Earth Sciences, St. John's, NF, A1B 3X5, Canada

Simon E. Jackson Memorial University, Department of Earth Sciences, St. John's, NF, Canada, A1B 3X5, Canada

Laser Ablation Microprobe inductively coupled plasma mass spectrometry (LAM-ICP-MS) is becoming established as an extremely powerful microanalytical tool for high spatial resolution in situ elemental analysis of minerals for a wide variety of trace elements. The LAM at Memorial University is based on a Q-switched, Nd:YAG laser having a fundamental wavelength of 1064 nm, the output of which has been quadrupled to 266 nm. The mass spectrometer is an enhanced sensitivity Fisons PQII+"S" ICP-MS instrument which has a sensitivity of 500 million cps/ppm/abundance for La and U when used with a standard solution sample nebulisation system (Meinhard concentric nebulizer with the VG Scott type spray chamber). This is the largest sensitivity reported for any instrument. Assuming a solution nebulization efficiency of 1%, the ICP-MS efficiency is ca. 0.05%. When the instrument is optimized for laser ablation sample introduction, and the instrumental sensitivity is maximized for the heavy REE, a sensitivity of approximately
7000 cps/ppm/abundance results when ablating the NIST 612 reference glass with a laser energy of 0.5 mJ/pulse. This produces a pit having a diameter of around 40 micro m.

Detection limits for laser ablation sampling are a function of a number of variables, the most fundamental of which are the instrumental sensitivity, the number of elements determined, the isotopic abundance of the analyte element, and the volume of the pit ablated. The number of elements determined can cause a 10 fold change in detection limits. The pit size can be varied from 100 micro m down to less than 8 micro m depending upon the application. If the depth is maintained equal to the diameter, this results in a 1000 fold change in detection limits. Depending upon conditions, therefore, detection limits for heavy elements typically range from 1 ppb to more than 1 ppm. To test the detection capability of the system, reference glass NIST 616, containing nominal concentrations of 20 ppb of the analytes, was analysed under various conditions. When two elements are determined in a pit with dimensions (diameter and depth) of 60 micro m, a signal of slightly more than 100 cps is obtained for a monoisotopic rare earth element (modelled as a 50% transport efficiency of the sample from the sample cell to the ICP and an ICP-MS instrumental efficiency of 0.05%). With a background of less than 10 cps for most of the rare earth elements, limits of detection of less than 1 ppb for a monoisotopic rare earth element are obtained.

The ability to make in situ determinations at ppb levels is opening up numerous new areas of research in the earth sciences. The low level detection power of this facility has been applied to analytical studies of silicate, carbonate and sulphide mineral analysis, instances of which will exemplify the capabilities of the technique.