Access Agilent eNewsletter August 2016

High sensitivity analysis of silicon-based nanoparticles using new Agilent 8900 ICP-QQQ

Michiko Yamanaka
Agilent Applications Engineer

Takayuki Itagaki
Agilent Software R&D Engineer

Steve Wilbur
Agilent Software Product Manager,
ICP-MS Systems

Silicon dioxide (SiO₂) nanoparticles (NPs) are used in a range of applications from paints, coatings, and adhesives, to food additives, cosmetics and polishing microelectronic devices. With increasing use, concerns have been raised about the impact of NPs on the environment, in food safety, and on human health. Clearly, there is a requirement to monitor NPs in a range of sample types, at low concentrations.

More options with triple quadrupole ICP-MS

ICP-MS can be used to measure individual NPs using a technique called Single Particle ICP-MS (spICP-MS), which allows simultaneous determination of the number, concentration, and size of particles present, as well as the dissolved element concentration. This type of measurement requires very fast Time Resolved Analysis (TRA) capability, which the Agilent 8900 ICP-QQQ achieves using a new detector with very fast integration time (0.1 ms dwell).

Low-level Si measurement using conventional quadrupole ICP-MS is difficult, due to intense background polyatomic ion interferences from CO and N₂, whichoverlap the major isotope of Si at m/z 28 (²⁸Si – 92.23% abundance). The interferences can be addressed using reaction chemistry in the collision/reaction cell of an ICP-MS, but for controlled and consistent reaction processes, a tandem mass spectrometer instrument, such as the Agilent 8900 ICP-QQQ, is required. Furthermore, the elemental background signal for Si is typically high, due to the presence of Si in many common polymers, sealants, cleaning agents and other laboratory reagents. The Agilent 8900 Advanced Applications configuration used in this work includes a new argon gas flow system that utilizes special inert materials to minimize the signal due to Si (and S) contamination from the ICP-MS hardware. In this way, a very low detection limit specification of < 50 ppt for Si can be achieved.

Double mass selection capability solves even the most challenging spectral overlaps

The 8900 ICP-QQQ uses two quadrupole mass filters (Q1 and Q2), with an octopole reaction cell located between them, to provide a double mass selection capability, commonly termed MS/MS. In our example, the ²⁸Si signal was measured on-mass in MS/MS mode, where Q1 and Q2 were both set to m/z 28. Hydrogen cell gas was used to eliminate any on-mass polyatomic interferences, such as ¹²C¹⁶O and ¹⁴N₂, allowing accurate measurement of Si at low levels, while MS/MS prevented the formation of any new, overlapping reaction product ions.

Figure 1. Data view from the Single Nanoparticle Application Module software.

Figure 2. A: Measured signal, B: frequency distribution, and C: particle size for 50 nm SiO₂ NPs (40 ng/L).

Figure 3. Size distribution results of 100 nm and 200 nm SiO₂ particles in 1% ethanol.

Automated acquisition and calibration approaches support NP characterization

The optional Single Nanoparticle Application Module of the Agilent ICP-MS MassHunter software was used for method setup and data analysis. Sample results for an entire batch are summarized in the ‘Batch at a Glance’ pane, which displays detailed graphical results for the selected samples, permitting visual confirmation and optimization of particle threshold and method settings if needed (Figure 1).

Accurate analysis of silica nanoparticles

Analyses were performed using the Agilent 8900 ICP-QQQ in fast TRA mode, using a dwell time of 0.1 ms (100 µs) per point with no settling time between measurements. Fast TRA allows multiple measurements to be made during the signal peak from a single particle, so the shape and duration of the ion plume from each NP can be identified. Small 50 nm SiO₂ particles provided clear peaks that were easily distinguished from the baseline signal and the low background signal observed in DI water (Figure 2A).

Figure 2C confirms that 50 nm SiO₂ particle sizes could be determined accurately, as a result of the low Si background and effective removal of polyatomic interferences in MS/MS mode with H₂cell gas, combined with the high sensitivity of the 8900 ICP-QQQ.

Real sample analysis requires removal of carbon interference

Samples such as biological fluids and tissues, food matrices, pharmaceutical ingredients, and organic solvents contain carbon matrices which cause a ¹²C¹⁶O polyatomic ion interference on ²⁸Si. The Agilent 8900 ICP-QQQ in MS/MS mode with hydrogen cell gas can eliminate this interference effectively. The particle size distribution for a mixed solution of the 100 nm and 200 nm SiO₂ particles, measured in a sample containing 1% ethanol is shown in Figure 3. Despite the high concentration of carbon, the size distribution for each group of particle sizes was consistent with the result obtained by Transmission Electron Microscopy (TEM), and the different particle sizes were separated with excellent resolution. This result suggests that the spICP-QQQ technique can resolve matrix interferences to allow accurate determination of particle sizes for SiO₂ nanoparticles in real sample matrices.

Characterize a wider range of NPs with Agilent 8900 ICP-QQQ

The Agilent 8900 ICP-QQQ with dedicated MassHunter spICP-MS software provides fast analysis times, excellent detection limits for particle size and concentration, and accurate results for SiO₂ particles less than 100 nm.

Learn how to leave interferences behind with MS/MS, then download Agilent publication 5991-6596EN for more details on this application.

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