Overview of ICP-MS

Prepared by Robert J. Speakman, University of Missouri Archaeometry Laboratory

 

Introduction

In 1999, scientists at the Archaeometry Laboratory at the University of Missouri Research Reactor (MURR) received funding from the National Science Foundation to acquire a high-resolution, magnetic-sector inductively coupled plasma-mass spectrometer (ICP-MS) and a laser ablation (LA) system. The equipment was installed in our newly remodeled laboratory in June 2000. In July 2002, the Archaeometry Lab acquired a second ICP-MS system, a VG Plasma Quad 3.

Since 2000, researchers at the Archaeometry Laboratory have been using ICP-MS to address archaeological questions. We believe our newest analytic method, ICP-MS, holds great promise for chemistry-based provenance research. In fact, it is quite possible that ICP-MS will eventually replace INAA as the preferred method for chemistry-based compositional analyses of archaeological materials as research reactors become increasingly less available. ICP-MS is a highly sensitive analytic method, which permits measurement of elements at the parts-per-billion and part-per-trillion level. Whereas INAA typically provides data for about 35 different elements, ICP-MS easily provides data for 120+ isotopes. Some elements such as Pb, Li, P, Nb, Y, and Bi which cannot be measured by INAA but can be measured by LA-ICP-MS may prove important for separating materials into different compositional groups. For many elements ICP-MS has lower detection limits than INAA (e.g., Sr, Ba, Zr).

We continue to offer INAA as our primary analytical method for provenance-based research, however we encourage researchers to take advantage of our newest analytical techniques. The Archaeometry Laboratory welcomes the submission of collaborative research and CRM projects in which elemental characterizations would be beneficial to the goals of the project. Support for academic-based projects is available and interested researchers should contact the lab for full information regarding submission of projects.

An Overview of Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Prepared by Barry Higgins, Spectometrist, University of Missouri Research Reactor

An excellent overview and description of ICP-MS as conducted at MURR can be found here.

An Overview of Laser-Ablation ICP-MS

 

[Simplified schematic of high-resolution magnetic sector laser ablation ICP-MS]

Since the development of inductively coupled plasma most applications have required digestion of solid samples with heat and/or strong acids, which is both time consuming and unpleasant. An alternative sample-introduction technique for ICP-MS, laser-ablation, was developed by Gray (1985) and became commercially available in the early to mid-1990s. The coupling of laser-ablation (LA) with state-of-the art inductively coupled plasma-mass spectrometers (ICP-MS) has resulted in the development of extremely sensitive microprobes capable of determining most elements of the periodic table. The potential of LA-ICP-MS has been recognized in various scientific fields, including the earth sciences (e.g., Jackson 2001; Jackson et al. 2001; Jeffries et al. 1995; Norman 2001; Sylvester 2001), zoology (e.g., Campana 1999; Sinclair et al. 1998; Thorrold et al. 1997), botany (Hoffman et al. 1994; Watling 1998; Watmough et al. 1998), ecology (e.g., Garbeschonberg et al. 1997; Raith et al. 1996), and archaeology (e.g., Devos et al. 2000; Kennett et al. 2001; Neff 2003; Pollard and Heron 1996; Speakman et al. 2002; Tykot 2002). As a result, LA-ICP-MS has become one of the most exciting new fields of research in materials science. Future developments in instrumentation, applications, and data calibration methods may only serve to generate additional interest in this technique.

In LA-ICP-MS, a sample(s) is placed inside a sample holder or laser cell where ablation takes place. A video camera inside the laser ablation unit projects a digital image of the laser cell onto a computer screen. With the aid of the camera, the analyst can magnify areas of interest on the computer screen and identify loci on the sample to be targeted for ablation by the laser. Using the computer software integrated with the laser ablation system, the analyst can superimpose a series of spots, lines, or rasters over an area of interest that the laser will target during ablation. Laser parameters such as the amount of power applied to the sample (laser intensity), how many shots the laser will fire per second (repetition rate, hz), and how fast the laser will scan across the surface of the sample (scan speed) are also selected to optimize the analysis. As sample matrices differ in composition and texture, experimentation with different laser parameters is necessary to optimize the analysis.

[Merchantek 213 nm Nd YG laser ablation system and glass slide sample holder.]

During analysis, high-energy UV or IR photons are emitted by the laser to produce a localized thermal event on a sample surface. Ablation particles released as a fine dense aresol with particle diameters finer than 1 mm are then transported from the laser cell using an argon or an argon-mixed carrier gas (i.e., helium or nitrogen) and introduced into the ICP-MS torch. The ablation area varies in size depending on the sample matrix, but is usually smaller than 500 X 500 mm and less than 20 mm deep.

At MURR, LA-ICP-MS has been used to characterize a variety of archaeological materials (Speakman et al. 2002). Some of these projects include characterization of pigments on pottery from the Mesa Verde region (Speakman and Neff 2002), Mexico (Rodriguez 2002), and Peru (Vaughn et al 2002); analysis of glazes on Mesoamerican Plumbate pottery (Neff 2003) and historic Euro American pottery; characterization of glass beads (Glascock and Speakman 2002), Midwestern cherts (Speakman et al. 2001), obsidian (Sall et al. 2001), turquoise (Zedeño et al 2002); and determination of inclusions in pottery (Larson et al. 2002). These studies have consistently demonstrated the potential of LA-ICP-MS for archaeological provenance and technological studies.

Unlike INAA or ICP-MS of solutions, which produces a bulk chemical data representative of the analyzed sample, LA-ICP-MS is point specific, that is, only the area ablated is subject to analysis. Attempting to obtain bulk compositional data on heterogeneous samples such as pottery is challenging due to spatial variation in the sample (e.g., clay and temper particles). On the other hand, homogeneous samples such as obsidian and to a certain extent chert are ideally suited for this type of analysis because spatial variation is minimal in these materials. ICP-MS can provide compositional data for 130+ isotopes, including rare earth elements, whereas INAA typically provides compositional data for about 30–35 different elements. Some elements, such as Li, Pb, Bi, Mg, and P, which cannot be measured by INAA but can be measured by LA-ICP-MS, may prove important for separating materials into different compositional groups. For certain elements LA-ICP-MS has superior detection limits to INAA (e.g., Ni, V, Sr, Sb, Ba, Zr).

LA-ICP-MS is unique compared to other analytical techniques. Like instrumental neutron activation analysis (INAA) and X-ray fluorescence, LA-ICP-MS can be used to obtain bulk compositional data for a variety of materials including: obsidian, chert, metals, minerals, glass beads, etc. Unlike, INAA and XRF LA-ICP-MS can also be used to analyze surfaces of artifacts such as ceramic pigments, slips, and glazes. The size and intensity of the laser is variable and can be adjusted to less than 10 microns diameter which makes laser ablation a powerful microprobe. For archaeologists, this means that in-situ analyses of temper and other nonplastics in ceramic pastes and other materials is possible. LA-ICP-MS can also be used for in-situ determination of lead isotopes and to analyze materials difficult or impossible to analyze by other techniques.

Microwave Digestion

[Laser ablation system at work]

Scientists at the University of Missouri Research Reactor (MURR) were the first researchers to develop and apply microwave ovens for acid digestion of samples (Abu-Samra et al. 1975). The early microwave digestion systems were little more than residential microwave ovens converted for use in the laboratory. As a result, these ovens were unsafe, particularly, when used for closed-vessel acid digestions involving perchloric acid. Safety concerns and the need to closely monitor and control heat and pressure in closed-vessels digestions facilitated the development of microwave systems specifically designed for laboratory use. Most ICP laboratories use these newer microwaves to put solid sample matrices into solution for ICP analyses. Microwave digestion continues to be one of the major sample preparation methods used at MURR for ICP-MS research.

Weak-acid extraction does not equal digestion. Several researchers have published on a method that advocates weak-acid extraction for ICP sample preparation (e.g., Burton and Simon 1993). Weak-acid (sometimes referred to as strong acid) extraction and microwave digestion are two separate approaches to ICP sample preparation that should not be confused. The weak acid approach involves placing a sample into a weak HCl (or EDTA) solution. The sample is left in the solution for several weeks during which time some of the constituents in the sample are leeched out by the acid. The resulting leechate is then filtered and analyzed by ICP. The most basic problem with this approach is that the leechate is not representative of the sample matrix and thus not a bulk analytical technique (e.g., Neff et al. 1996; Triadan et al. 1997). For example, HCl will dissolve more material in a calcareous clay than a clay with high iron. The original firing temperature of pottery, its porosity, amount of non-plastics, and other factors will also affect how much material enters into solution. It is therefore possible to obtain clusters of data that may or may not represent different compositional groups using this approach. Similar groups could probably be obtained by soaking a sherd in water for several weeks and analyzing the water. Researchers that use the acid-extraction approach do so at their own risk—archaeological interpretations based on data obtained using this method are tenuous at best. With microwave digestion the entire sample is digested by a combination of heat and acids. All samples are therefore on equal footing for the analysis. ICP-MS analysis of the digested material permits fully quantitative data to be obtained that has been shown to be comparable to other bulk analytical techniques (e.g., Sakai 2001; Kennett et al 2002).

At MURR, we have a Milestone and two CEM microwave ovens we use for sample digestion. Samples are cleaned, powdered (when possible), and weighed into a digestion vessel. High purity acids which minimize background noise are added to the vessel. The vessels are sealed, placed in to the oven. Computers monitor the internal temperature and pressure of the vessels during the digestion process. After digestion samples are diluted, spiked with a multielement internal standard and analyzed by ICP-MS. An auto-sampler contained within a Class 100 chemical station facilitates sample introduction while maintaining a clean environment. Analysis of multielement standard solutions permits blank-subtracted counts obtained from the analysis to be transformed to abundance data.

Complete microwave digestion systems range in price from $5000–$30,000. In most cases, analyses that do not require high temperature and pressure can be conducted in one of the more inexpensive models. Researchers desiring to conduct some of the more difficult digestions such as samples requiring high pressure, heat, or HF acid would be well advised to purchase one of the more "sturdier" units. Researchers also need to be aware that replacement parts for digestion vessels are highly variable in cost. Laboratories anticipating high volume through-put of samples should therefore consider the cost of replacement parts when purchasing an oven.

Dr. Skip Kingston has devoted a significant amount of research to microwave chemistry. His books (Kingston and Jassie 1988; Kingston and Haswell 1997) and the Center for Microwave and Analytical Chemistry Web page contains important information for researchers concerning acid chemistry, digestion, and safety.