Michiko Yamanaka

Inductively coupled plasma–tandem mass spectrometry (ICP-MS/MS) is a relatively new technique, with the first triple quadrupole ICP-MS (ICP-QQQ) being introduced commercially in 2012. However, the technique has quickly been accepted for a wide variety of applications in industries including, but not limited to, manufacturing, contract analysis, government, research, and academia. In this article, we explain the key steps to setting up an ICP-MS/MS method and address some of the common misconceptions surrounding the technique.

Compared to conventional single quadrupole inductively coupled plasma–mass spectrometry (ICP-MS), the main benefits of ICP-MS/MS include:

the unprecedented ability to resolve spectral interferences using controlled reaction chemistry in the collision reaction cell (CRC);

significantly better separation of adjacent mass, “peak tail” overlaps because of two mass filtering steps; and

improved detection limits through higher sensitivity and lower backgrounds.

The flexibility of ICP-MS/MS has led some people to assume that method development must be more difficult on ICP-MS/MS than on single quadrupole ICP-MS. However, existing single quadrupole methods can usually be transferred to ICP-MS/MS without significant modifications. Also, reaction gas methods run more consistently on ICP-MS/MS, so methods are more universal and operation is simpler.

The following six steps help guide method setup for ICP-MS/MS (Figure 1):

FIGURE 1: Method development steps for ICP-MS/MS.

When developing an ICP-MS/MS method, users may focus on a specific interference that needs to be resolved. However, the starting point for method development on ICP-MS/MS—as with conventional ICP-MS—should consider the wider analytical needs. These needs include several factors that can be easily overlooked.

First, can the instrument handle the sample matrix? Matrix tolerance is important for many ICP-MS applications. Optimizing the plasma to give low CeO

 (<1.5%) ensures that the instrument will easily tolerate typical sample matrix levels. Low CeO/Ce indicates better matrix decomposition, which reduces deposits on the interface and leads to better stability and less maintenance. Low CeO/Ce conditions also increase analyte ionization, improving sensitivity for poorly ionized elements, such as beryllium, arsenic, cadmium, and mercury. Low CeO/Ce indicates better decomposition of molecular species that would otherwise form polyatomic ion overlaps (1).

Second, is the sample composition unknown or variable? The major element levels and sample-to-sample variability can be important factors. Variable sample matrices can give rise to unexpected polyatomic ion overlaps, which a given reaction gas mode may not resolve effectively. In such cases, helium (He) collision mode may provide a simpler, more universal, and more reliable solution (2,3).

Third, which analytes must be measured and at what level? Most ICP-MS applications can be run successfully using He collision mode, but lower analyte levels may require a reaction gas method to further reduce backgrounds and improve detection limits. Similarly, analysis of high-level major elements may require custom tuning or sample dilution if the detector cannot measure the full concentration range.

Finally, how many samples will be analyzed? Many parameters affect analysis speed for large sample batches. ICP-MS/MS systems are compatible with discrete sampling devices to shorten sample analysis times. However, the number of cell gases used—and the time required to switch between cell modes—will also impact throughput.

Step #2: Identify Critical Method Needs—Why is ICP-MS/MS Needed?

Once the basic analytical requirements have been addressed, specific, critical aspects of the analysis should be considered. This stage usually focuses on the most difficult sample types, unusual analytes, or extremely low (or high) concentration levels. In the context of ICP-MS/MS method development, this step should identify analytes that are affected by spectral overlaps.

Step #3: Apply the Simplest Approach to Address Interferences 

ICP-MS/MS offers performance benefits because of the additional quadrupole, Q1, before the CRC, which controls reaction processes in the cell. Reaction gas modes can give extremely effective interference reduction, useful for ultralow level analysis in high purity reagents. However, reaction gas methods may be specific for each overlap being resolved, so multielement methods may require several sets of cell conditions. These conditions may not always be applicable to other sample types.

As mentioned above, many ICP-MS interferences can be addressed using He collision mode. Most spectral overlaps in ICP-MS are caused by polyatomic (molecular) ions. For example, 

 75. Helium mode uses kinetic energy discrimination (KED) to reduce the transmission of polyatomic ions relative to the monatomic analyte ions they overlap (4). As a result, He mode supports multi-element analysis and allows access to secondary (confirmatory) isotopes for many elements, while also practically eliminating the need for unreliable mathematical correction equations.

The major appeal of He mode is that the same cell conditions can be used for multiple analytes across a range of sample types, so method setup is simple and consistent. He mode is the default mode for single quadrupole ICP-MS methods, and can be used in the same way on ICP-MS/MS. Using He mode for most analytes makes it much simpler to develop methods for the remaining analytes that need a reaction gas approach.

Step #4: Identify Analytes or Overlaps that Cannot Be Addressed Using He Mode

Many users employ ICP-MS/MS precisely because their application cannot be performed successfully using single quadrupole ICP-MS, even with well optimized He mode. Usually, the requirements relate to specific spectral interferences or adjacent mass overlaps that can only be resolved using a reaction gas or double mass selection (MS/MS).

ICP-MS/MS can use reaction gas modes to reduce intense spectral overlaps, such as 

S, allowing these previously difficult elements to be measured at a low level. Reaction gas modes can also reduce matrix-based overlaps to lower levels, enabling ultratrace level analysis.

He mode cannot resolve isobaric overlaps, such as 

Pb, and so on. In typical, routine ICP-MS analysis, these overlaps are avoided by measuring an alternative mass (

Pb, in the examples above). However, measurement of the overlapped isotope is required for some applications and provides performance benefits, such as higher sensitivity, in others. In these cases, a reaction gas mode may be able to resolve the overlap.

Doubly charged ion interferences form when an atom loses two electrons in the plasma and forms a doubly charged (M

) ion. A quadrupole mass filter separates ions based on mass to charge ratio (

 ions appear at half their true mass. Most elements form insignificant levels of M

 ions, but some, such as the rare earth elements (REEs), do form a small percentage of M

. Single quadrupole ICP-MS can correct for normal levels of M

 overlap, but this approach is not effective at high levels of interference. For analyses such as trace arsenic and selenium in REE minerals, a reaction gas method is more effective.

Peak tail overlaps affect trace analytes measured next to intense major element peaks. An example is trace manganese (

 55) analysis in an iron or steel matrix (the major isotopes of iron are at 

 54 and 56). Peak tail overlaps are resolved using the double mass selection of MS/MS, as the abundance sensitivity (AS) of a tandem MS is the product of the AS of the two mass filters, so Q1 AS x Q2 AS in the case of ICP-QQQ.

Step #5: Select the Preferred Reaction Gas Mode

For overlaps that cannot be addressed using He mode, an appropriate reaction gas mode is selected, which is not as daunting as new users often assume, because ICP-MS/MS ion–molecule reaction chemistry is well understood and documented. Applications can often be run using the predefined method settings provided with the instrument software. Manufacturers’ application notes (5) and users’ publications (6) document proven methods for many more applications.

ICP-MS/MS users also benefit from unique method development tools, including product ion and precursor ion scans. These are useful for identifying suitable analyte product ions and potential matrix-based product ion overlaps (7).

Step #6: Ensure Control of Cell-Formed Reaction Product Ions 

When reaction gases are used in ICP-MS, new product ions can be formed in the CRC. The creation of potentially interfering product ions is a major downside of reaction gases compared to helium KED mode, but control of product ion formation is a key benefit of a tandem MS configuration. The Q1 mass filter, before the CRC, ensures that only ions at the target mass—that is, the analyte and any on-mass interferences—enter the CRC.

Q1 therefore prevents the formation of product ions from analytes or matrix elements present at other masses. However, product ions can still be created from the reaction gas itself. For example, when 

 is measured in organic matrices, Q1 is set to 

 product ions could react further with the NH

The sequential reactions that form the NH

 product ions can be prevented using either mass discrimination or ion energy discrimination, depending on the ICP-MS/MS cell configuration and operating conditions. The energy discrimination approach is effective because product ions created from the cell gas only have the kinetic energy they derive from the collision. These low-energy product ions have insufficient energy to pass through the gas-filled cell, so they do not appear in the mass spectrum (Figure 2).

FIGURE 2: (a) Overlaid ICP-MS/MS spectra (blank and 1 ppb chromium (Cr) in 1% methanol). (b) Calibration plot for 

Cr. Background <1 ppt confirms cell-formed NH

 52) are removed using energy discrimination.

(2) N. Yamada, J. Takahashi, and K. Sakata, 

(4) E. McCurdy, N. Sugiyama, and S.M. Wilbur, 

(5) “Handbook of ICP-QQQ Applications” (Agilent Technologies, publication 5991–2802EN, 2020).

(6) Agilent ICP-QQQ bibliography https://www.agilent.com/en/promotions/8900icp-qqq-trust.

(7) E. McCurdy, G. Woods, and N. Sugiyama, 

 are with Agilent Technologies Inc. Direct correspondence to: 

Articles

Method development for ICP-MS/MS should not be difficult. The six steps here will guide you to success.

Key Steps in Method Development for Inductively Coupled Plasma–Tandem Mass Spectrometry (ICP-MS/MS)

) nanoparticles (NPs) are widely used in industry, manufactured goods, and consumer products, such as food additives. However, there are several significant challenges associated with the measurement of silicon by inductively coupled plasma–mass spectrometry (ICP-MS). 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. In addition, background polyatomic ions affect all three silicon isotopes, 

Si, with the most intense interferences (

Si isotope. 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. Consequently, maintaining a low background for silicon in the analytical laboratory can be highly problematic. When characterizing silica in nanoparticle form, the analysis is especially difficult. 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. The method provides high sensitivity to ensure accurate analysis at the small (~50 nm) particle sizes of interest in environmental and biological applications.

	Nanomaterials are increasingly used in industrial processes, manufactured goods, medicines, and as ingredients in consumer products such as cosmetics, sunscreen, and food. 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.

	Some of the most common types of NPs are difficult to measure accurately using conventional single-quadrupole inductively coupled plasma–mass spectrometry (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. 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. 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.

	Nanostructured synthetic amorphous silica (SAS) is one of the most commonly used nanomaterials. 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. 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). Reliable analytical methods are needed to allow monitoring of compliance with upcoming regulations (4) and for the assessment of nano-safety (5). However, Si measurement by ICP-MS is not easy because of the combination of relatively low sensitivity and high spectral background. Silicon is only about 70% ionized and the major isotope (

Si – 92.23% abundance) is interfered by the intense background polyatomic ions CO

. These limitations mean that conventional single-quadrupole ICP-MS is unable to detect silica particles with diameters smaller than about 200 nm (6). 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).

 can be resolved using a reactive cell gas such as hydrogen (H

) in the collision–reaction cell (CRC) of an ICP-MS system. 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. This extra mass filter enables MS/MS operation, where nontarget ions are prevented from entering the cell. This approach ensures that reaction processes and the resulting reaction product ions are consistent, even when the sample composition is complex or variable. The general layout and performance capability of a triple-quadrupole configuration of tandem ICP-MS is described in the literature (8).

 NPs were measured by single-particle ICP-MS (spICP-MS), using MS/MS mode and H

 reaction gas on a triple-quadrupole ICP-MS system. The instrument software included an optional single nanoparticle application module for method development, signal processing, and data interpretation. The practical applications capabilities of the instrument for the measurement of a range of SiO

	An Agilent 8900 ICP-QQQ system (model #100, Advanced Applications Configuration) was used for the analysis of silica nanoparticles. The instrument was equipped with a glass concentric nebulizer, a quartz spray chamber, a quartz torch with a 1-mm injector, and standard nickel sampling and skimmer cones. Samples were introduced with a standard peristaltic pump and pump tubing (1.02 mm i.d.). The configuration of the instrument used has a novel argon gas delivery path that uses special inert materials to minimize contamination of the gas supply through the instrument, reducing the elemental background for common contaminant elements Si and S. 

Si was measured in continuous fast time-resolved analysis (fast TRA) mode, using a dwell time of 0.1 ms (100 μs) per point with no settling time between measurements. The 

Si signal was measured on-mass in MS/MS mode, with both quadrupoles (Q1 and Q2) set to 

 28. Hydrogen cell gas was used to resolve the on-mass polyatomic interferences, N

 and CO. The main operating conditions of the instrument are shown in Table sI online; please visit 

www.spectroscopyonline.com/supplementary-information-accurate-measurement-ultrafine-silica-nanoparticles-using-icp-msms

Reference Materials and Sample Preparation 

 NP reference materials with nominal diameters of 50, 60, 100, and 200 nm determined by transmission electron microscopy (TEM) were purchased from nanoComposix. The reference materials were diluted with deionized (DI) water to give particle concentrations of 40, 40, 100, and 1000 ng/L (ppt) for the 50-, 60-, 100-, and 200-nm particles, respectively. The solutions were sonicated for 5 min before analysis to ensure sample homogeneity. To allow the elemental response to be calculated, a Si ionic standard of 5 μg/L (ppb) was prepared in DI water. MS/MS with H

 cell gas should be effective in removing the CO

 polyatomic overlap. To check this interference removal capability, a SiO

 NP solution containing a matrix of 1% ethanol was also prepared.

	NPs measured using TRA with ICP-MS give a narrow peak or signal plume lasting around 0.5 ms for each particle that passes through the plasma; the peak signal area of each particle plume is proportional to the particle mass. The signals measured in DI water (without particles) and in solutions containing SiO

 particles with diameters of 50, 60, and 100 nm are shown in Figure 1. Fast TRA mode allows multiple measurements to be made during the signal peak from each individual particle. The shape and duration of the ion plume from each NP can be measured, and the total ion count can be converted to a particle mass and therefore diameter (assuming spherical particles). The Single Nanoparticle Application module of the ICP-MS MassHunter software includes the calculations to convert signal intensity to particle diameter, along with other key parameters, such as minimum detectable particle size and background equivalent diameter. Because of the high sensitivity, low elemental background, and effective interference removal of the ICP-MS instrument used, the smallest (50-nm) SiO

 particles provided clear signal peaks (Figure 1b) that were easily distinguished above the low background signal seen in DI water (Figure 1a). As expected, the 60-nm and 100-nm particle solutions gave proportionally higher signal peaks as shown in Figures 1c and 1d, respectively.

	Frequency distributions were calculated automatically by the instrument software from the signals measured for the four SiO

 nanoparticle solutions shown in Figure 1; the frequency distribution plots are shown in Figure 2. The particle signals were clearly separated from the background caused by the dissolved  ionic component, although the background and particle signals were partially overlapped for the 50-nm particles (shown in Figure 2b). From these results, we can estimate that the practical detection limit for the particle diameter was below 50 nm. The background equivalent diameter (BED) calculated by the instrument software from the 50-nm particle solution was 22 nm.

	The particle size distribution results for the different SiO

 NP solutions are shown in Figure 3. The plots confirm that all the SiO

 particle sizes could be determined accurately, including the 50 nm NPs. The high sensitivity and low background of triple-quadrupole ICP-MS combined with the effective removal of polyatomic interferences in MS/MS mode with H

 cell gas, enabled the smaller NP sizes to be clearly distinguished from the background signal. All the particle size plots demonstrated a Gaussian distribution, which agrees with the information provided by the supplier of the NP solutions. The triple-quadrupole ICP-MS results obtained for median, mode, and mean particle sizes also agreed well with the reference sizes obtained by TEM (Table I). Also, the measured particle concentrations agreed well with the theoretical particle concentrations, although the concentration result for the 50-nm solution was slightly high (49 ng/L compared to an expected value of 40 ng/L) because of the overlap with the background signal.

	Figure 4 shows the mean signal intensity measured by spICP-MS/MS plotted against the reference particle diameter determined by TEM. The logarithmic plot has a slope of 2.84, which is in good agreement with the theoretical cube relationship between diameter and mass indicated in equation 1: 

	Equation 1 is used to calculate particle mass (

Resolving Carbon Interference Using MS/MS and H

	Real samples of interest-such as biological materials, food matrices, polymers, pharmaceutical ingredients, and organic solvents-may contain a high level of carbon, which causes a 

Si. Triple-quadrupole ICP-MS in MS/MS mode with H

 cell gas can eliminate this interference effectively. A mixed solution of the 100-nm and 200-nm SiO

 NPs was prepared in a sample containing 1% ethanol. The particle size distribution for this sample is shown in Figure 5. Despite the high concentration of carbon, the size distribution for each group of particle sizes was consistent with the result obtained by TEM, and the different particle sizes were separated with excellent resolution. This result suggests that the spICP-MS/MS technique can resolve the carbon interference successfully, allowing accurate determination of particle sizes for SiO

 nanoparticles in natural sample matrices.

	Triple-quadrupole ICP-MS operating in MS/MS mode with H

 cell gas was used for the successful determination of SiO

 nanoparticles at sizes smaller than 100 nm. The triple-quadrupole ICP-MS method delivered exceptionally low background signals and high sensitivity, allowing the smallest (50-nm) SiO

 particles to be distinguished from the background signal. MS/MS operation gave effective control of the H

 reaction chemistry, eliminating polyatomic interferences from N

 and CO, even in the presence of a high level of carbon matrix. Dedicated software was used to calculate the particle sizes, providing accurate results for individual NP reference materials and mixed solutions. The spICP-MS/MS method provided fast analysis times, excellent detection limits for particle size and concentration, and accurate results for SiO

		J. Tuoriniemi, G. Cornelis, and M. Hassellöv, 

		C. Degueldre, P.-Y. Favarger, and C. Bitea, 

		D. Napierska, L.C.J. Thomassen, D. Lison, J.A. Martens, and P.H. Hoet, 

		L. Calzolai, D. Gilliland, and F. Rossi, 

		S. Dekkers, A.G. Oomen, E.A. Bleeker, R.J. Vandebriel, C. Micheletti, J. Cabellos, G. Janer, N. Fuentes, S. Vázquez-Campos, T. Borges, and M.J. Silva, 

		M.D. Montaño, B.J Majestic, Å.K. Jämting, P. Westerhoff, and J.F. Ranville, 

		International Organization for Standardization, 

ISO TS 80004-1:2010: Nanotechnologies - Vocabulary - Part 1: Core terms,

		L. Balcaen, E. Bolea-Fernandez, M. Resano, and F. Vanhaecke, 

 is with Agilent Technologies in Stockport, UK. 

 is with Agilent Technologies in Tokyo, Japan. 

 is with Agilent Technologies in Bellevue, Washington. Direct correspondence to: 

A method to measure ultrafine silica nanoparticles using ICP-MS/MS to control the elemental and polyatomic ion backgrounds is described here.

Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS

	This table is supplementary information to the article "Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS," which was published in the September 2017 supplement issue 

Applications of ICP & ICP-MS Techniques for Today’s Spectroscopists

		E. McCurdy, M. Yamanaka, and S. Wilbur, 

This table is supplementary information to the article "Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS," which was published in the September 2017 supplement issue "Applications of ICP & ICP-MS Techniques for Today’s Spectroscopists" to Spectroscopy. 

Supplementary Information: Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS

Characterization of nanoparticles (NPs), including samples containing different sized NPs is described using ICP-MS. New integrated data analysis software is capable of quickly and automatically performing the complex calculations required for the analysis of single particles, while permitting simple, visual optimization and validation of results. The method provides a combination of particle size distribution and sample concentration information that is generally not available with other techniques.

Michiko Yamanaka

Articles

Key Steps in Method Development for Inductively Coupled Plasma–Tandem Mass Spectrometry (ICP-MS/MS)

Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS

Supplementary Information: Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS

Automated, High Sensitivity Analysis of Single Nanoparticles Using the Agilent 7900 ICP-MS with Single Nanoparticle Application Module