Carsten Engelhard

Fast separation, characterization, and speciation of gold and silver nanoparticles and their ionic counterparts with micellar electrokinetic chromatography coupled to ICP-MS.

In this study, a method for separation, size characterization, and speciation of gold and silver nanoparticles was developed through the use of micellar electrokinetic chromatography (MEKC) coupled to inductively coupled plasma-mass spectrometry (ICP-MS) for the first time. Figures of merit in this proof-of-principle study include peak area precision of 4-6%, stable migration times (1.4% with internal standard), and capillary recoveries on the order of 72-100% depending on species and nanoparticle size, respectively. Detection limits are currently in the sub-microgram per liter range. For example, a total of 1500 50-nm-sized gold nanoparticles were successfully detected. After careful optimization, MEKC-ICP-MS was used to separate engineered nanoparticles (ENPs) of different composition. Speciation analysis of ENPs and free metal ions in solution was feasible using a complexing agent (penicillamine). Gold speciation analysis of a dietary supplement, which contained approximately 6-nm-sized gold nanoparticles, was demonstrated.

Elucidation of Reaction Mechanisms Responsible for Afterglow and Reagent-Ion Formation in the Low-Temperature Plasma Probe Ambient Ionization Source

The development of ambient desorption/ionization mass spectrometry has shown promising applicability for the direct analysis of complex samples in the open, ambient atmosphere. Although numerous plasma-based ambient desorption/ionization sources have been described in the literature, little research has been presented on experimentally validating or determining the desorption and ionization mechanisms that are responsible for their performance. In the present study, established spectrochemical and plasma physics diagnostics in combination with spatially resolved optical emission profiles were applied to reveal a set of reaction mechanisms responsible for afterglow and reagent-ion formation of the Low-Temperature Plasma (LTP) probe, which is a plasma-based ionization source used in the field of ambient mass spectrometry. Within the dielectric-barrier discharge of the LTP probe, He(2)(+) is the dominant positive ion when helium is used as the plasma supporting gas. This helium dimer ion (He(2)(+)) has two important roles: First, it serves to carry energy from the discharge into the afterglow region in the open atmosphere. Second, charge transfer between He(2)(+) and atmospheric nitrogen appears to be the primary mechanism in the sampling region for the formation of N(2)(+), which is an important reagent ion as well as the key reaction intermediate for the formation of other reagent ions, such as protonated water clusters, in plasma-based ambient ionization sources. In the afterglow region of the LTP, where the sample is usually placed, a strong mismatch in the rotational temperatures of N(2)(+) (B (2)Σ(u)(+)) and OH (A (2)Σ(+)) was found; the OH rotational temperature was statistically identical to the ambient gas temperature (~300 K) whereas the N(2)(+) temperature was found to rise to 550 K toward the tail of the afterglow region. This much higher N(2)(+) temperature is due to a charge-transfer reaction between He(2)(+) and N(2), which is known to produce rotationally hot N(2)(+) (B (2)Σ(u)(+)) ions. Furthermore, it was found that one origin of excited atomic helium in the afterglow region of the LTP is from dielectronic recombination of vibrationally excited He(2)(+) ions.

Single particle inductively coupled plasma mass spectrometry: evaluation of three different pneumatic and piezo-based sample introduction systems for the characterization of silver nanoparticles

Engineered nanomaterials are used increasingly around the world. In recent years, environmental concerns have being raised that call for risk assessment, toxicity studies, and nanosafety policies. It is therefore important to provide analytical tools that are able to characterize various types of nanomaterials in a sensitive and fast fashion. In this study, the analytical performance of three sample introduction systems, a PFA micronebulizer with a Peltier-cooled cyclonic spray chamber (PC3), a PFA micronebulizer with a heated cyclonic spray chamber and three-stage Peltier-cooled desolvation system (APEX Q), and a monodisperse droplet generator (Microdrop) with an in-house built spray chamber, was compared for the characterization of silver nanoparticles (Ag NP) with different sizes (20–100 nm) using single particle inductively coupled plasma mass spectrometry (SP-ICP-MS). With continuous polydisperse and pulsed monodisperse droplet sample introduction, single 30 nm NP (PC3) and 20 nm NP (APEX Q, Microdrop), respectively, were successfully detected. Detection efficiencies (20–100 nm Ag NP) were in the range of 5.8 × 10−5 to 8.2 × 10−5 counts per atom, cpa (PC3), 7 × 10−5 to 9.5 × 10−5 cpa (APEX Q), and 8.1 × 10−5 to 1.2 × 10−4 cpa (Microdrop). For a given nanoparticle sample, the variation of the relative standard deviation (RSD) of the size distribution width among the three systems was found to be 1–4% (e.g. 1.3% for 100 nm Ag NP with RSDs of 16.3%, 16.8%, and 17.6% for Microdrop, APEX Q, and PC3, respectively). Size measurements performed by SP-ICP-MS were validated by transmission electron microscopy measurements. Because silver toxicity depends on the silver species in the sample, simultaneous detection of Ag NP and free Ag(I) ions was studied with droplet sample introduction.

Plasma-based ambient desorption/ionization mass spectrometry: state-of-the-art in qualitative and quantitative analysis

AbstractAmbient desorption/ionization mass spectrometry (ADI-MS) aims to enable direct analysis of gaseous, liquid, and/or solid samples under ambient conditions. In ADI-MS, different types of desorption/ionization sources are classified according to their basic method of operation, namely spray-based, laser-based, or plasma-based. This review discusses many of the plasma-based techniques coupled to mass spectrometry in terms of their current performance in fast qualitative screening and quantitative analysis. Critical aspects, for example sample preparation and introduction, quantification, and matrix effects, are addressed. Furthermore, the applicability of plasma-based sources to portable mass spectrometers and their capabilities in imaging experiments are summarized. The applications discussed are of two types. In one, direct screening is performed without any or with minimal sample pretreatment. Samples with low matrix content are qualitatively analyzed without interferences. The other, more challenging applications, namely samples with high matrix content and most quantitative analysis, typically require sample preparation ranging from simple dilution to extensive multi-step procedures. Figᅟ

Characteristics of Low-Temperature Plasma Ionization for Ambient Mass Spectrometry Compared to Electrospray Ionization and Atmospheric Pressure Chemical Ionization

Ambient desorption/ionization mass spectrometry (ADI-MS) is an attractive method for direct analysis with applications in homeland security, forensics, and human health. For example, low-temperature plasma probe (LTP) ionization was successfully used to detect, e.g., explosives, drugs, and pesticides directly on the target. Despite the fact that the field is gaining significant attention, few attempts have been made to classify ambient ionization techniques based on their ionization characteristics and performance compared to conventional ionization sources used in mass spectrometry. In the present study, relative ionization efficiencies (RIEs) for a large group of compound families were determined with LTP-Orbitrap-MS and compared to those obtained with electrospray ionization mass spectrometry (ESI-MS) and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). RIEs were normalized against one reference compound used across all methods to ensure comparability of the results. Typically, LTP analyte ionization through protonation/deprotonation (e.g., 4-acetamidophenol) was observed; in some cases (e.g., acenaphthene) radicals were formed. Amines, amides, and aldehydes were ionized successfully with LTP. A benefit of LTP over conventional methods is the possibility to successfully ionize PAHs and imides. Here, the studied model compounds could be detected by neither APCI nor ESI. LTP is a relatively soft ionization method because little fragmentation of model compounds was observed. It is considered to be an attractive method for the ionization of low molecular weight compounds over a relatively wide polarity range.

Rapid characterization of lithium ion battery electrolytes and thermal aging products by low-temperature plasma ambient ionization high-resolution mass spectrometry.

Lithium ion batteries (LIBs) are key components for portable electronic devices that are used around the world. However, thermal decomposition products in the battery reduce its lifetime, and decomposition processes are still not understood. In this study, a rapid method for in situ analysis and reaction monitoring in LIB electrolytes is presented based on high-resolution mass spectrometry (HR-MS) with low-temperature plasma probe (LTP) ambient desorption/ionization for the first time. This proof-of-principle study demonstrates the capabilities of ambient mass spectrometry in battery research. LTP-HR-MS is ideally suited for qualitative analysis in the ambient environment because it allows direct sample analysis independent of the sample size, geometry, and structure. Further, it is environmental friendly because it eliminates the need of organic solvents that are typically used in separation techniques coupled to mass spectrometry. Accurate mass measurements were used to identify the time-/condition-dependent formation of electrolyte decomposition compounds. A LIB model electrolyte containing ethylene carbonate and dimethyl carbonate was analyzed before and after controlled thermal stress and over the course of several weeks. Major decomposition products identified include difluorophosphoric acid, monofluorophosphoric acid methyl ester, monofluorophosphoric acid dimethyl ester, and hexafluorophosphate. Solvents (i.e., dimethyl carbonate) were partly consumed via an esterification pathway. LTP-HR-MS is considered to be an attractive method for fundamental LIB studies.

Spectroscopic plasma diagnostics on a low-temperature plasma probe for ambient mass spectrometry

Since the inception of ambient desorption/ionization mass spectrometry, plasma ionization sources have played an increasing role in molecular mass spectrometry. Although a variety of discharge designs and geometries, along with a range of applications, have been introduced, very little published work has focused on the characterization and fundamental examination of these discharges, especially on the desorption/ionization processes they employ. In the present work, a simple yet effective ambient desorption/ionization source based on a dielectric-barrier discharge, the low-temperature plasma (LTP) probe, was optically characterized. By means of a spatially selective detection system, maps of reactive species created in both the plasma and the afterglow regions were recorded. From these maps, the origin of impurities important in mass spectrometric analyses, such as H2O, N2, and O2, was deduced. Electron number densities and rotational temperatures for the LTP were found to be similar to those reported for other dielectric-barrier discharges. Lastly, the effect of plasma parameters on emission spectra was correlated with mass spectral results previously reported for the same ionization source.

Inductively coupled plasma mass spectrometry: recent trends and developments

AbstractThis year inductively coupled plasma mass spectrometry (ICP-MS) moves into the fourth decade of development. In this article, some recent trends and developments in ICP-MS are reviewed, with special focus on instrumental development and emerging applications. Some key trends include a novel mass spectrometer for elemental and speciation analysis in Mattauch–Herzog geometry with a focal-plane-camera array detector. The reason for this development is the possibility to record the full elemental mass range simultaneously and all the time. Monitoring fast transient signals in chromatography or laser ablation is now possible and will become an important asset in future studies, e.g., for isotope ratio analysis. In addition, there is a lot of new activity and interest in the area of nanosciences and medicine. Here, instrumental developments are reported that allow the direct analysis of microparticles and single cells. FigureICP-MS: Recent trends and developments are reviewed with special focus on instrumentation

Rapid and quantitative analysis of pesticides in fruits by QuEChERS pretreatment and low-temperature plasma desorption/ionization orbitrap mass spectrometry

Ambient desorption/ionization high-resolution mass spectrometry (ADI-HR-MS) is a powerful method for the analysis of complex samples. Recently, direct analysis in real time (DART) MS and low-temperature plasma probe (LTP) MS demonstrated potential in direct qualitative pesticide residue screening and quantitative analysis of pesticides in liquid extracts. In the present study, a LTP-HR-MS method for quantitative pesticide residue analysis in fruit extracts was developed and evaluated with respect to the European Union (EU) legislation on pesticides. In particular, this study focused on pesticides in different fruit matrices that were reported to often exceed legal maximum residue levels (MRLs) in Germany in the past (namely acetamiprid, cyprodinil, fenhexamid, and fludioxonil; see report on of the German Federal Office of Consumer Protection and Food Safety in 2009). After method optimization, pesticides in spiked and unspiked fruit QuEChERS extracts were identified successfully by LTP-Orbitrap-MS via accurate mass measurements (<4 ppm). The method is considered useful for MRL verification. Matrix-matched calibration was applied for quantification because it was found that the fruit matrix (still present during extract analysis) has a significant effect on analyte ion abundance. Linear working ranges greater than four orders of magnitude were achieved. Limits of quantification ranged from 0.001 mg kg−1 to 0.07 mg kg−1 (which is significantly below permitted MRLs). Measurement precision was below 15% and method precision was typically close to 14% relative standard deviation. Finally, the validated LTP-HR-MS method was tested with unspiked fruit samples bought from a local grocery store. Pesticide residues of cyprodinil and fludioxonil (0.003–0.03 mg kg−1) were readily detected. These results were directly compared to a standard liquid chromatography electrospray HR-MS method and found to be in good agreement.

New torch design for inductively coupled plasma optical emission spectrometry with minimised gas consumption

A new torch design for inductively coupled plasma optical emission spectrometry was developed. The torch was made of one quartz tube with a ball-shaped excitation zone. Under optimised working conditions of the 27.12 MHz Ar plasma with 1.1 kW RF power, the total argon gas flow could be reduced to only 0.6 L min−1 with external air cooling integrated in the fastening of the torch. Fundamental studies of selected element emissions, emission profiles, the effect of auxiliary plasma and sample carrier gas flow rates were carried out. The attainable excitation and rotational temperatures in the new torch were determined at different sample carrier gas flow rates. For 1.1 kW RF power and total gas flows between 50–800 mL min−1 they were found to be in the order of 6000–10 000 K for the excitation temperatures and 3700–4100 K for the rotational temperatures. The new torch was coupled with an ultrasonic nebuliser and detection limits between 0.03–2.9 μg L−1 could be determined for the elements Ba, Li, Mg, Ca, Cd, K and Al. The new torch delivered promising results that raise hopes of having a new generation of torches allowing low argon consumption.