Recently, first analyses of single sub-micrometre particles, embedded in liquid droplets, by inductively coupled plasma optical emission spectrometry (ICP-OES) with a size-equivalent detection limit of several hundred nanometres were reported. 1,2 To achieve lower detection limits which might allow for the analysis of particles in the nanometre size range a more sensitive technique such as mass spectrometry (MS) is required. Various modifications of particle delivery and data acquisition systems commonly used were carried out to install a setup adequate for ICP-MS detection. These modifications enabled us to supply droplets generated by a commercial microdroplet generator (droplet size: 30-40 mm) with nearly 100% efficiency and high uniformity to the ICP. Analyses were performed using both standard solutions of dissolved metals at concentrations of 1 (Ag), 2 (Au), 5 (Au), or 10 (Cu) mg L À1 and highly diluted suspensions of gold and silver nanoparticles with sizes below 110 nm. In doing so, detection efficiencies of 10 À6 counts per atom could be achieved while size-related limits of quantification were found to be 21 nm and 33 nm for gold and silver, respectively. Furthermore, the advantages of utilizing microdroplet generators vs. conventional nebulizers for nanoparticle analyses by ICP-MS are discussed.
A prototype inductively coupled plasma time-of-flight mass spectrometer (ICPTOFMS) for time resolved measurements of transient signals in the microsecond regime is described in this work. Analytical figures of merit for the prototype are given for both liquid nebulization and single droplet introduction and are compared to a conventional quadrupole-based ICPMS using the same ICP source and vacuum interface.Quasi-simultaneous detection at a time resolution of 33 ms of the prototype ICPTOFMS allows multiisotope monitoring of short signals (200-500 ms duration) generated from individual droplets and particles. The capabilities of the instrument for the analysis of single nanoparticles are studied using microdroplets consisting of a multi-element standard solution and containing 114 nm Au particles. The detection efficiencies for Ag and Au, calculated from the response of individual droplets and particles, are similar to those of the quadrupole-based instrument and amount to 1.3 Â 10 À6 ions per atom and 3.1 Â 10 À6 ions per atom, respectively. The sizes of the smallest detectable Ag, Au and U metallic nanoparticles are estimated to be 46 nm, 32 nm and 22 nm, respectively. Furthermore, time shifts of the signals of different elements within single droplets were observed. These new results demonstrate the advantage of the temporal resolution of the instrument for studying processes taking place in the plasma on the ms-time scale.
Detecting and quantifying engineered nanoparticles (ENPs) in complex environmental matrices requires the distinction between natural nanoparticles (NNPs) and ENPs.
In
this work, a novel droplet microfluidic sample introduction
system for inductively coupled plasma mass spectrometry (ICPMS) is
proposed and characterized. The cheap and disposable microfluidic
chip generates droplets of an aqueous sample in a stream of perfluorohexane
(PFH), which is also used to eject them as a liquid jet. The aqueous
droplets remain intact during the ejection and can be transported
into the ICP with >50% efficiency. The transport is realized via
a
custom-built system, which includes a membrane desolvator necessary
for the PFH vapor removal. The introduction system presented here
can generate highly monodisperse droplets in the size range of 40–60
μm at frequencies from 90 to 300 Hz. These droplets produced
very stable signals with a relative standard deviation (RSD) comparable
to the one achieved with a commercial droplet dispenser. Using the
current system, samples with a total volume of <1 μL can
be analyzed. Moreover, the capabilities of the setup for introduction
and quantitative elemental analysis of single cells were described
using a test system of bovine red blood cells. In the future, other
modules of the modern microfludics can be integrated in the chip,
such as on-chip sample pretreatment or parallel introduction of different
samples.
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