Introduction to LA-ICP-MS
Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry, or LA-ICP-MS for short, is an analytical technique for determining the chemical and isotopic composition of solid samples, especially prominent in the fields of geology and petrology, but also materials science and increasingly, due to its imaging capability, the life sciences.
As the name suggests, the sample is ablated using a finely focused laser and the ablated material, the analyte, is swept by a carrier gas into an ICP-MS for analysis. The laser spot size is very small, on the order of 10-100um in diameter and the ablation occurs in-situ with a collection time of less than one minute. The analytical capability varies dramatically depending on the mass spectrometer being used, but most elements can be detected at the parts per billion (PPB) level. Most notable is that the technique is destructive and does result in the removal of sample material, albeit a very small amount!
The laser source used for LA-ICP-MS needs to be of high power and appropriate wavelength so the sample material can be cleanly ablated. For an analytical laboratory it is also important that the laser source is reliable and easily maintained. Because of these requirements, it is the opinion of ASI and Laurin Technic that a 193nm ArF excimer laser is the best choice for most applications of LA-ICP-MS. Future developments may illuminate the advantages of femtosecond lasers, in which case ASI has the engineering experience to extend the RESOlution to a femto source.
Many different materials can be analyzed by LA-ICP-MS, although the majority of applications are concentrated in the fields of petrology and geochemistry. However, practically any solid material can be analyzed, with samples including rocks, minerals, glass, gemstones and metals. Minerals of organic and low pressure origin can also be analyzed, including corals, bones, teeth, stalagmites and stalactites (speleothems) and even small fish-ear-bones called otoliths. Some users have also applied LA-ICP-MS to medical applications and the ablation of soft tissue as a means of diagnostic imaging of trace elements, though careful sample preparation is vitally important. It is even possible to ablate ice, as one of our users has been doing quite successfully with a special cryo-cell developed by Laurin Technic.
The Ablation Cell
The heart of the LA-ICP-MS instrument is the ablation cell. The ablation cell holds the samples and defines the gas flow conditions at the ablation site. As such, its design is critical to achieving the best results. Studies have shown that the ablation event is intrinsically violent and dynamic, and the material blasted out of the ablation crater by the laser needs to create a suitable aerosol suspended in the carrier gas. The carrier gas of choice is helium; it is constantly sweeping through the cell at a steady rate, carrying the sample aerosol to the mass spectrometer. For correct operation of the plasma, it is also necessary to mix argon with the helium, and in some cases a very small amount of nitrogen will improve sensitivity and reduce fractionation.
To some extent fractionation occurs in all LA-ICP-MS setups. Under ideal conditions, all of the sample would be removed and turned into a fine aerosol before being transported through to the ICP-MS, which would in turn ionize all of the atoms equally. It is observed in the laboratory that this is certainly not possible. While it is not necessary for every one of the ablated atoms to be transported to the ICP-MS, it is important that they do so in equal proportions. The sample interaction between the ablation plume and the carrier gas is extremely complex and some elements will preferentially condense en route to the ICP-MS. It is vitally important that the ablation cell is designed to minimize element fractionation, and it is our belief that the ablation cells from Laurin Technic are the best available on the market today. However, fractionation inside the ICP-MS is also present and must be taken into account when undertaking analysis.
The vaporized sample is swept through to the ICP-MS by the carrier gas. The ICP is a high-temperature plasma driven by a high-power RF generator. The plasma temperature is on the order of 10,000K, which is twice as hot as the surface temperature of the sun! This plasma ionizes the small particles of sample and tears molecules apart, rendering the sample down to individual ionized atoms. This is ideal for analysis by the mass spectrometer, which uses a number of different designs to separate the atoms by their mass-to-charge ratio. In this way it is possible to analyse the sample for individual isotopes, as well as for chemical composition. The exact nature of the analysis depends significantly on the type of mass spectrometer being used, but typically the instruments employed can be separated into single-collector quadruple systems, and multi-collector field-sector systems. Each has advantages over the others, though there is generally a direct relation between price and performance: commonly assessed by mass resolution and sensitivity.