![]() ![]() This extra mass filter enables MS/MS operation, where nontarget ions are prevented from entering the cell. However, ion–molecule reaction chemistry can only be controlled effectively using a tandem mass spectrometer, where an additional mass filter is situated before the CRC of the ICP-MS system. The interferences on Si + can be resolved using a reactive cell gas such as hydrogen (H 2) or ammonia (NH 3) in the collision–reaction cell (CRC) of an ICP-MS system. This capability is not sufficient for characterization of the nanoscale particle sizes of interest in emerging applications and for regulatory requirements the accepted definition of a nanomaterial is one with major dimensions of 100 nm or less (7). These limitations mean that conventional single-quadrupole ICP-MS is unable to detect silica particles with diameters smaller than about 200 nm (6). Silicon is only about 70% ionized and the major isotope ( 28Si – 92.23% abundance) is interfered by the intense background polyatomic ions CO + and N 2 +. However, Si measurement by ICP-MS is not easy because of the combination of relatively low sensitivity and high spectral background. Reliable analytical methods are needed to allow monitoring of compliance with upcoming regulations (4) and for the assessment of nano-safety (5). Silica NPs also have great potential for a variety of other uses, including silica aerogels as thermal and electrical insulators, and imaging and therapeutic applications in medicine (3). It is an authorized food additive (E-551) and is also found in a wide variety of industrial and consumer products from polymers, composites, paints, and textiles to toothpastes, detergents, and cosmetics. Nanostructured synthetic amorphous silica (SAS) is one of the most commonly used nanomaterials. However, many of the natural and engineered NPs of interest are composed of elements that are much more difficult to measure by ICP-MS-for example, iron, sulfur, titanium, and silicon. Much of the work reported thus far has concentrated on particles that contain elements such as zirconium, silver, and gold (1,2), which are relatively easy to measure by ICP-MS. Such instruments often cannot achieve the combination of high sensitivity and low background that is required to allow the signal generated from a small particle to be distinguished from the background. Some of the most common types of NPs are difficult to measure accurately using conventional single-quadrupole inductively coupled plasma–mass spectrometry (ICP-MS). The measurement of nanoparticles (NPs) is a subject of interest because the fate of NPs in the environment and the potential for toxic effects once absorbed into biological systems are not yet well understood. Nanomaterials are increasingly used in industrial processes, manufactured goods, medicines, and as ingredients in consumer products such as cosmetics, sunscreen, and food. The method provides high sensitivity to ensure accurate analysis at the small (~50 nm) particle sizes of interest in environmental and biological applications. In this article, we describe a method to measure ultrafine silica nanoparticles using ICP–tandem MS (ICP-MS/MS) to control the elemental and polyatomic ion backgrounds. When characterizing silica in nanoparticle form, the analysis is especially difficult. Consequently, maintaining a low background for silicon in the analytical laboratory can be highly problematic. Finally, silicon-based materials are widely used in manufactured goods and industrial materials ranging from glass, alloys, and air and water filters to organic compounds such as silicone tubing, sealants, adhesives, lubricants, and coatings. In addition, background polyatomic ions affect all three silicon isotopes, 28Si, 29Si, and 30Si, with the most intense interferences ( 14N 2 + and 12C 16O +) occurring on the major 28Si isotope. First, silicon has a moderately high first ionization potential (8.152 eV), which means it is less than 70% ionized in a typical argon plasma, so the sensitivity is significantly lower than for a fully ionized element. However, there are several significant challenges associated with the measurement of silicon by inductively coupled plasma–mass spectrometry (ICP-MS). ![]() Silica (SiO 2) nanoparticles (NPs) are widely used in industry, manufactured goods, and consumer products, such as food additives. ![]()
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