Laser ablation-inductively coupled detection limits

Several other microanalytical methods in common use potentially have application on soil and sediments section samples. Laser-ablation inductively coupled plasma mass spectrometery (LA-ICP-MS) has been used on soil thin-sections from a controlled field experiment (21) but required special resins in the preparation. There is presently (May 2006) no reported use of this method on archaeological soil samples. Likewise, for extremely fine-resolution studies (i.e. <10 pm) with low minimum detection limits and despite difficult calibration, secondary ion microscopy (SIMS) has a potential role in examining archaeological soil thin sections. At even higher lateral resolutions ( 100 nm) Auger electron spectroscopy (AES) could also be considered for surface (<5 nm deep) analyses. At present however, the use of these methods in soil systems is limited. SIMS has been focused on biochemical applications (22), whereas AES... 

Laser-ablation, inductively coupled-plasma mass spectrometry (LA-ICPMS) is an instrumental technique in which a laser-ablahon cell and ophcal microscope supplant the spray chamber/nebulizer apparatus of a standard ICP-MS instrument. Subsamples of questioned material are ablated from a solid sample via laser (often a pulsed Nd-YAG tuned to 266 or 213 nm). Ablated specimens are transported in a stream of Ar to a plasma torch for ionization and mass discrimination as per solution ICP-MS. Only minimal sample prep is required, and few restrictions are placed on the nature of questioned solid samples (Brundle et al. 1992 Vickerman 1998). While laser spot sizes can be reduced to several micrometers, sensitivity is degraded as a result, and usual spatial resolutions are on the order of 10-100 pm. Matrix-matched standards are also necessary for accurate trace-element and isotopic quantitative analyses in LA-ICPMS. Depending on the quality of such primary standards, LA-ICPMS accuracies are typically 1-10%, with limits-of-detection in the parts-per-billion (ppb) range (O Table 62.1). 

Inorganic pigments and lakes (organic dyes bonded to an inorganic support) can be recognized by the ratio of elements in their composition, making elemental analysis an important tool in their identification. EDS may facilitate an initial qualitative analysis, but quantitative analysis and the detection of trace elements are needed to identify the inorganic colorant components. Due to sample size restrictions, the methods that can be employed are limited. The techniques of inductively-coupled plasma mass spectrometry (ICP-MS), ICP-optical emission spectroscopy (ICP-OES), and laser ablation ICP-MS are described in the literature (56). 

Speakman and his colleagues describe the application of inductively coupled plasma mass spectrometry (ICP-MS) to the elemental analysis of obsidian, chert, pottery and painted and glazed surfaces. Only a very small area is affected in the laser ablation sampling, usually 1000 pm by 1000 pm by 30 pm. Their method is virtually nondestructive since the ablated area is not visible to the naked eye. Because ICP-MS has a lower detection limit than other... 

Guenther, D, Longerich, H. P., Jackson, S. E., and Forsythe, L. (1996). Effect of sampler orifice diameter on dry plasma inductively coupled plasma mass spectrometry (ICP-MS) backgrounds, sensitivities, and limits of detection using laser ablation sample introduction, fresemus /. Anal. Chem. 355(7-8), 771. 

AES = atomic emission spectroscopy BPTH = l,5-bis[phenyl-(2-pyridyl)-methylene]-thiocarbonohydrazide] DPTH = l,5-bis(di-2-pyridyl)methylene thiocarbohydrazide FI = flow injection ICP = inductively coupled plasma LA = laser ablation LOD = limit of detection MIBK = methyl isobutyl ketone MS = mass spectrometry QMS = quadrupole mass spectrometry SF-MS = sector field mass spectrometry... 

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