Alexander Gundlach-Graham

High-Speed, High-Resolution, Multielemental Laser Ablation-Inductively Coupled Plasma-Time-of-Flight Mass Spectrometry Imaging: Part I. Instrumentation and Two-Dimensional Imaging of Geological Samples

Low-dispersion laser ablation (LA) has been combined with inductively coupled plasma-time-of-flight mass spectrometry (ICP-TOFMS) to provide full-spectrum elemental imaging at high lateral resolution and fast image-acquisition speeds. The low-dispersion LA cell reported here is capable of delivering 99% of the total LA signal within 9 ms, and the prototype TOFMS instrument enables simultaneous and representative determination of all elemental ions from these fast-transient ablation events. This fast ablated-aerosol transport eliminates the effects of pulse-to-pulse mixing at laser-pulse repetition rates up to 100 Hz. Additionally, by boosting the instantaneous concentration of LA aerosol into the ICP with the use of a low-dispersion ablation cell, signal-to-noise (S/N) ratios, and thus limits of detection (LODs), are improved for all measured isotopes; the lowest LODs are in the single digit parts per million for single-shot LA signal from a 10-μm diameter laser spot. Significantly, high-sensitivity, multielemental and single-shot-resolved detection enables the use of small LA spot sizes to improve lateral resolution and the development of single-shot quantitative imaging, while also maintaining fast image-acquisition speeds. Here, we demonstrate simultaneous elemental imaging of major and minor constituents in an Opalinus clay-rock sample at a 1.5 μm laser-spot diameter and quantitative imaging of a multidomain Pallasite meteorite at a 10 μm LA-spot size.

High-Speed, High-Resolution, Multielemental LA-ICP-TOFMS Imaging: Part II. Critical Evaluation of Quantitative Three-Dimensional Imaging of Major, Minor, and Trace Elements in Geological Samples

Here we describe the capabilities of laser-ablation coupled to inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) for high-speed, high-resolution, quantitative three-dimensional (3D) multielemental imaging. The basic operating principles of this instrumental setup and a verification of 3D quantitative elemental imaging are provided. To demonstrate the potential of 3D LA-ICP-TOFMS imaging, high-resolution multielement images of a cesium-infiltrated Opalinus clay rock were recorded using LA with a laser-spot diameter of 5 μm coupled to ICP-TOFMS. Quantification of elements ablated from each individual laser pulse was carried out by 100% mass normalization, and the 3D elemental concentration images generated match well with the expected distribution of elements. After laser-ablation imaging, the sample surface morphology was investigated using confocal microscopy, which showed substantial surface roughness and evidence of matrix-dependent ablation yields. Depth assignment based on ablation yields from heterogeneous materials, such as Opalinus clay rock, will remain a challenge for 3D LA-ICPMS imaging. Nevertheless, this study demonstrates quantitative 3D multielemental imaging of geological samples at a considerably higher image-acquisition speed than previously reported, while also offering high spatial resolution and simultaneous multielemental detection.

Toward faster and higher resolution LA–ICPMS imaging: on the co-evolution of LA cell design and ICPMS instrumentation

AbstractWe describe trends in fast, high resolution elemental imaging by laser ablation–inductively coupled plasma mass spectrometry (LA–ICPMS). Recently developed low dispersion LA cells deliver quantitative transport of ablated aerosols within 10 ms and also provide enhanced sensitivity compared to conventional LA cells because the analyte ion signal becomes less diluted during aerosol transport. When connected to simultaneous ICPMS instruments, these low dispersion LA cells offer a platform for high speed and high lateral resolution shot-resolved LA–ICPMS imaging. Here, we examine the current paradigms of LA–ICPMS imaging and discuss how newly developed LA cell technology combined with simultaneous ICPMS instrumentation is poised to overcome current instrumental limitations to deliver faster, higher resolution elemental imaging. Graphical AbstractOn means for obtaining high-speed, high-resolution, multielemental images is to combine new lowdispersion laser-ablation cell technology with an inductively coupled plasma time-of-flight mass spectrometer (ICP-TOFMS). Here, we show three selected-isotope LA-ICP-TOFMS images of a hetereogeneous Opalinus clay sample

Single-particle multi-element fingerprinting (spMEF) using inductively-coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) to identify engineered nanoparticles against the elevated natural background in soils

The discrimination of engineered nanoparticles (ENPs) from the natural geogenic background is one of the preeminent challenges for assessing their potential implications. At low ENP concentrations, most conventional analytical techniques are not able to take advantage of inherent differences (e.g. in terms of composition, isotopic signatures, element ratios, structure, shape or surface characteristics) between ENPs and naturally occurring nanoscale particles (NNPs) of similar composition. Here, we present a groundbreaking approach to overcome these limitations and enable the discrimination of man-made ENPs from NNPs through simultaneous detection of multiple elements on an individual particle level. This new analytical approach is accessible by an inductively-coupled plasma time-of-flight mass spectrometer (ICP-TOFMS) operated in single-particle mode. Machine learning is employed to classify ENPs and NNPs based on their unique elemental fingerprints and quantify their concentrations. We demonstrate the applicability of this single-particle multi-element fingerprinting (spMEF) method by distinguishing engineered cerium oxide nanoparticles (CeO2 ENPs) from natural Ce-containing nanoparticles (Ce-NNPs) in soils at environmentally relevant ENP concentrations, orders of magnitude below the natural background.

Extension of the focusable mass range in distance-of-flight mass spectrometry with multiple detectors.

RATIONALE Distance-of-flight mass spectrometry (DOFMS) is a velocity-based mass separation technique in which ions are spread across a spatially selective detector according to m/z. In this work, we investigate the practical mass range available for DOFMS with a finite-length detector. METHODS A glow-discharge DOFMS instrument has been constructed for the analysis of atomic ions. This instrument was modified to accommodate two spatially selective ion detectors, arranged co-linearly, along the mass-separation axis of the analyzer. With this geometry, each detector covers a different portion of the distance-of-flight spectrum and ions are detected simultaneously at the two detectors. The total flight distance covered by the two detectors is 106 mm and simulates DOF detection across a broad mass range. RESULTS DOFMS theory predicts that ions of all m/z values are focused at a single flight time, but at m/z-dependent flight distances. Therefore, ions that are detected across a wide portion of the DOF axis should all yield the same peak widths. With a focal-plane camera detector and a micro-channel plate/phosphor-screen detection assembly, we found simultaneous, uniform focus of (40)Ar(2)(+) and of (65)Cu(+) and (63)Cu(+) with the ions spread 82 mm across the DOF axis. This detection length, combined with the current instrument geometry, allows for a simultaneously detectable m/z value of 4:3 (high mass-to-low mass). CONCLUSIONS These results are the first experimental verification that constant-momentum acceleration (CMA)-DOFMS provides energy focus across an extended detection length. Evidence presented demonstrates that DOFMS is amenable to detection with (at least) a 100-mm detector surface. These results indicate that DOFMS is well suited for detection of broader mass ranges.

Characterization of a new ICP-TOFMS instrument with continuous and discrete introduction of solutions

In this work, we evaluate the capabilities of a new commercially available inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) instrument, the icpTOF (TOFWERK AG, Thun, Switzerland), for analysis of liquid samples with a standard pneumatic nebulizer – cyclonic spray chamber sample-introduction system and a microdroplet sample-introduction system. The ICP-TOFMS instrument provides simultaneous, high-speed detection across almost the entire elemental mass range, from around 7–275 amu. The instrument provides a standard mass resolving power (Rm) of ∼2500 and can provide Rm greater than 4000 through the use of collisional cooling prior to the TOF mass analyzer. In standard operation mode (without collision gas), a sensitivity of 60 000 cps ppb−1 is routinely achieved for heavy elements such as U, with two orders of magnitude decrease in sensitivity from high-to-low masses. Limits of detection (LODs) are in the low parts-per-trillion to high parts-per-quadrillion. The data acquisition system of the TOFMS instrument provides a linear dynamic range greater than 106. The native abundance sensitivity of the ICP-TOFMS is 6.5 × 10−4 on an adjacent mass and is characterized by a non-linear sloping baseline beyond the adjacent mass channel. To obtain simultaneous high-dynamic-range detection, post-acquisition peak fitting and baseline subtraction can be used to reduce the effect of peak tailing on adjacent masses. Simultaneous quantification of adjacent isotopes across six orders of magnitude of signal intensity is possible after baseline subtraction. Advantages of ICP-TOFMS are apparent for high-speed transient analysis. Here, we assess the performance of the icpTOF for the multi-elemental analysis of single microdroplets, with specific emphasis on the characteristics of this approach for 100% transport of liquid samples and for the analysis of nanoparticles. Absolute limits of detection are in the attogram range for single droplets and concentration LODs for the detection of ensembles of droplets are competitive with those attained with pneumatic nebulization sample introduction.

First inductively coupled plasma-distance-of-flight mass spectrometer: instrument performance with a microchannel plate/phosphor imaging detector

Here we describe the first combination of a Distance-of-Flight Mass Spectrometry (DOFMS) instrument and an inductively coupled plasma (ICP) ion source. DOFMS is a velocity-based MS technique in which ions of a range of mass-to-charge (m/z) values are detected simultaneously along the length of a spatially selective detector. As a relative of time-of-flight (TOF) MS, DOFMS leverages benefits from both TOFMS and spatially dispersive MS. The simultaneous detection of groups of m/z values improves dynamic range by spreading ion signal across many detector elements and reduces correlated noise by signal ratioing. To ascertain the performance characteristics of the ICP-DOFMS instrument, we have employed a microchannel-plate/phosphor detection assembly with a scientific CCD to capture images of the phosphor plate. With this simple (and commercially available) detection scheme, elemental detection limits from 2–30 ng L−1 and a linear dynamic range of 5 orders of magnitude (10–106 ng L−1) have been demonstrated. Additionally, a competitive isotope-ratio precision of 0.1% RSD has been achieved with only a 6 s signal integration period. In addition to first figures of merit, this paper outlines technical considerations for the design of the ICP-DOFMS.

Interleaved Distance-of-Flight Mass Spectrometry: A Simple Method to Improve the Instrument Duty Factor

AbstractDistance-of-flight mass spectrometry (DOFMS) is a velocity-based, spatially dispersive MS technique in which ions are detected simultaneously along the plane of a spatially selective detector. In DOFMS, ions fly though the instrument and mass separate over a set period of time. The single flight time at which all ions are measured defines the specific m/z values that are detectable; the range of m/z values is dictated by the length of the spatially selective detector. However, because each packet of ions is detected at a single flight time, multiple groups of ions can fly through the instrument concurrently and be detected at a single detector. In this way, DOFMS experiments can be interleaved to perform several mass separation experiments within a single DOF repetition period. Interleaved operation allows the orthogonal acceleration region to be operated at a repetition rate higher than the reciprocal of the flight time, which improves the duty factor of the technique. In this paper, we consider the fundamental parameters of interleaved DOFMS and report first results. Figureᅟ

Capabilities of laser ablation inductively coupled plasma time-of-flight mass spectrometry

In this paper, we characterize an inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) instrument (icpTOF, TOFWERK AG, Thun, Switzerland) in combination with laser-ablation sample introduction. Three sample introduction approaches for LA-based ICP-TOFMS analysis are described: (1) steady-state LA with a conventional high-dispersion LA cell, (2) single-pulse analysis with large spot sizes (44 μm diameter) using a low-dispersion LA cell, and (3) pulse-resolved, high-speed, high-resolution (5 μm spot sizes) elemental imaging with the same low-dispersion LA cell. These sample-introduction schemes span the range of approaches most interesting for users of LA-ICP-TOFMS, from routine bulk quantification, to low-sample-consumption trace-element analysis, to demanding elemental imaging applications. From steady-state signal intensities, element concentrations in NIST SRM 612 and USGS BCR-2G were quantified within the uncertainty range of the preferred values when NIST SRM 610 was used as the external reference material. Relative deviations were less than 10% in most cases. When using a 44 μm diameter spot and a laser repetition rate of 10 Hz, limits of detection (LODs) were in the single digit ng g−1 range for the most sensitive isotopes. Isotope-ratio precision was in the sub per mill regime and governed by counting statistics. Similar accuracies were also achieved in low-dispersion LA-ICP-TOFMS experiments, when NIST SRM 612 and USGS BCR-2G element concentrations were quantified using signal intensities from single 44 μm diameter laser pulses. LODs were in the tens of ng g−1 range for most sensitive isotopes resulting in absolute LODs in the tens of attograms. Capabilities of the icpTOF for elemental imaging are demonstrated with pulse-resolved multi-elemental imaging of a multi-phase geological thin section. With the low-dispersion LA cell and a spot diameter of 5 μm, aerosol plumes were confined to less than 10 ms, which allowed elemental imaging at a laser repetition rate of 100 Hz with minimized pulse-to-pulse mixing and an adjacent-pixel dynamic range of greater than 102. Quantitative results for elements of major, minor and trace concentrations were in agreement with bulk composition of individual regions, which had been determined via petrographic microscopy and high-dispersion laser ablation inductively coupled plasma quadrupole mass spectrometry (LA-ICPQMS). LODs were in the single digit μg g−1 range for most sensitive isotopes.

Distance-of-Flight Mass Spectrometry with IonCCD Detection and an Inductively Coupled Plasma Source

AbstractDistance-of-flight mass spectrometry (DOFMS) is demonstrated for the first time with a commercially available ion detector—the IonCCD camera. Because DOFMS is a velocity-based MS technique that provides spatially dispersive, simultaneous mass spectrometry, a position-sensitive ion detector is needed for mass-spectral collection. The IonCCD camera is a 5.1-cm long, 1-D array that is capable of simultaneous, multichannel ion detection along a focal plane, which makes it an attractive option for DOFMS. In the current study, the IonCCD camera is evaluated for DOFMS with an inductively coupled plasma (ICP) ionization source over a relatively short field-free mass-separation distance of 25.3–30.4 cm. The combination of ICP-DOFMS and the IonCCD detector results in a mass-spectral resolving power (FWHM) of approximately 900 and isotope-ratio precision equivalent to or slightly better than current ICP-TOFMS systems. The measured isotope-ratio precision in % relative standard deviation (%RSD) was ≥0.008%RSD for nonconsecutive isotopes at 10-ppm concentration (near the ion-signal saturation point) and ≥0.02%RSD for all isotopes at 1-ppm. Results of DOFMS with the IonCCD camera are also compared with those of two previously characterized detection setups. Graphical Abstractᅟ