RESUMEN
This paper proposes a novel approach to implement an internal standard (IS) correction in single particle inductively coupled plasma mass spectrometry (SP ICP-MS), as exemplified for the characterization of Au nanoparticles (NPs) in complex matrices. This approach is based on the use of the mass spectrometer (quadrupole) in bandpass mode, enhancing the sensitivity for the monitoring of AuNPs while also allowing for the detection of PtNPs in the same measurement run, such that they can serve as an internal standard. The performance of the method developed was proved for three different matrices: pure water, a 5 g L-1 NaCl water solution, and another water solution containing 2.5% (m/v) tetramethylammonium hydroxide (TMAH)/0.1% Triton X-100. It was observed that matrix-effects impacted both the sensitivity of the NPs and their transport efficiencies. To circumvent this problem, two methods were used to determine the TE: the particle size method for sizing and the dynamic mass flow method for the determination of the particle number concentration (PNC). This fact, together with the use of the IS, enabled us to attain accurate results in all cases, both for sizing and for the PNC determination. Additionally, the use of the bandpass mode provides additional flexibility for this characterization, as it is possible to easily tune the sensitivity achieved for each NP type to ensure that their distributions are sufficiently resolved.
RESUMEN
This paper presents a novel approach, based on the standard addition method, for overcoming the matrix effects that often hamper the accurate characterization of nanoparticles (NPs) in complex samples via single particle inductively coupled plasma mass spectrometry (SP-ICP-MS). In this approach, calibration of the particle size is performed by two different methods: (i) by spiking a suspension of NPs standards of known size containing the analyte, or (ii) by spiking the sample with ionic standards; either way, the measured sensitivity is used in combination with the transport efficiency (TE) for sizing the NPs. Moreover, such transport efficiency can be readily obtained from the data obtained via both calibration methods mentioned above, so that the particle number concentration can also be determined. The addition of both ionic and NP standards can be performed on-line, by using a T-piece with two inlet lines of different dimensions. The smaller of the two is used for the standards, thus ensuring a constant and minimal sample dilution. As a result of the spiking of the samples, mixed histograms including the signal of the sample and that of the standards are obtained. However, the use of signal deconvolution approaches permits to extract the information, even in cases of signal populations overlapping. For proofing the concept, characterization of a 50 nm AuNPs suspension prepared in three different media (i.e., deionized water, 5% ethanol, and 2.5% tetramethyl ammonium hydroxide-TMAH) was carried out. Accurate results were obtained in all cases, in spite of the matrix effects detected in some media. Overall, the approach proposed offers flexibility, so it can be adapted to different situations, but it might be specially indicated for samples for which the matrix is not fully known and/or dilution is not possible/recommended.