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ICP-MS Sample Preparation
Inductively coupled plasma mass spectrometry (ICP-MS) is applicable to a wide variety of sample types; therefore, it is used in many different industries.
To be processed efficiently in the plasma, samples must be in either gas or vapor (aerosol) form. So, while gases can be analyzed directly by the plasma (e.g., when separated by gas chromatography), solids and liquids have to be converted to aerosol form using either a nebulizer (for liquids) or an ablation device (for solids).
ICP-MS sample processing
In most routine ICP-MS analyses, the sample is introduced as a liquid using a nebulizer and spray chamber. The nebulizer uses supersonic expansion of gas to turn the liquid into a fine mist, and the spray chamber then removes any droplets that are too large to be processed in the plasma. This occurs at the sample interface of the instrument.
Liquid ICP-MS samples
Liquid samples are typically digested and then reconstituted in an aqueous matrix to stabilize elements as an ionic solution. The matrix typically contains 2% nitric acid, and may have 0.5% hydrochloric acid added to stabilize certain elements. The final composition of any matrix is highly dependent on the nature of the analytes being measured.
Liquids can be analyzed directly without dissolution using a standard ICP-MS introduction kit—provided the TDS (total dissolved solids) level is below ~0.5%. Above this level, the TDS may precipitate in the nebulizer or overload the plasma, either of which alter the way the sample is processed in the plasma. This can result in a data collection issue called drift. Specialized nebulizers and spray chambers may be required to counteract drift; more commonly, sample dilution is performed.
Organic liquid samples can be analyzed in the sample with minor modifications to the ICP-MS system. Usually, a smaller injector and platinum tipped cones are used, and oxygen is added to the plasma to prevent carbon deposition and signal drift.
Solid ICP-MS samples
With solid samples, digestion in strong and hot acids is the typical protocol. The acids themselves range from simple nitric acid (for relatively simple matrices) to hydrofluoric acid (for samples containing high silicon dioxide content). Samples containing organic matter may also have hydrogen peroxide added to them during the digestion step because H2O2 breaks down organic matter efficiently.
Solid samples can also be directly turned into aerosols several different ways, the most common being laser ablation (LA), spark ablation, or electrothermal vaporization (ETV). In all of these methods, the samples are converted to an aerosol and transported to the plasma with inert gas.
ETV is a bulk analysis method that interacts with the entire sample and is useful for combustible samples. Spark ablation is a semi-bulk analysis tool and is useful for sampling large spots (1–3 mm diameter) of a conducting sample. LA is a microanalysis technique using high irradiance (UV) lasers to analyze very small spots (2–750 µm in diameter) on virtually all solid samples. Because of the small spot sizes, LA is also useful for tracking element distribution in a sample.
Other considerations during ICP-MS
Internal standards can be added into the sample to monitor and correct for a range of parameters during the experiment, from sample viscosity to analyte precipitation to instrument drift.
The amount of dissolved solid in the sample also has an effect on the analysis. Typically, the maximum percentage of total dissolved solid (TDS) that will allow robust data to be produced is 0.2%, but this also depends on the instrument setup (nebulizer used, cones, and interface setup) and the constituents of the matrix. Heavier elements have a much stronger effect on the ion beam than lighter elements, so a 0.5% TDS for a sodium or calcium-based matrix can be tolerated, whereas only 0.1% of a tungsten or lead-based matrix is possible.
Specialized sample introduction systems can be coupled to an ICP-MS, including spark- or laser-ablation systems, hydride generators, and chromatography systems (GC, IC, HPLC, and UPLC systems). In these cases, sample preparation protocols will vary depending on the sample introduction system being used.