Helen J. Reid

Determination of iodine and molybdenum in milk by quadrupole ICP-MS

A reliable method for the determination of iodine and molybdenum in milk samples, using alkaline digestion with tetramethylammonium hydroxide and hydrogen peroxide, followed by quadrupole ICP-MS analysis, has been developed and tested using certified reference materials. The use of He+O2 (1.0 ml min(-1) and 0.6 ml min(-1)) in the collision-reaction cell of the mass spectrometer to remove (129)Xe+-- initially to enable the determination of low levels of 129I--also resulted in the quantitative conversion of Mo(+) to MoO2+ which enabled the molybdenum in the milk to be determined at similar mass to the iodine with the use of Sb as a common internal standard. In order to separate and pre-concentrate iodine at sub microg l(-1) concentrations, a novel method was developed using a cation-exchange column loaded with Pd2+ and Ca2+ ions to selectively retain iodide followed by elution with a small volume of ammonium thiosulfate. This method showed excellent results for aqueous iodide solutions, although the complex milk digest matrix made the method unsuitable for such samples. An investigation of the iodine species formed during oxidation and extraction of milk sample digests was carried out with a view to controlling the iodine chemistry.

Single Cell Tracking of Gadolinium Labeled CD4+ T Cells by Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Cellular therapy is emerging as a promising alternative to conventional immunosuppression in the fields of hematopoietic stem cell (HSC) transplantation, autoimmune disease, and solid organ transplantation. Determining the persistence of cell-based therapies in vivo is crucial to understanding their regulatory function and requires the combination of an extremely sensitive detection technique and a stable, long-lifetime cell labeling agent. This paper reports the first application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to perform single cell detection of T cell populations relevant to cellular immunotherapy. Purified human CD4(+) T cells were labeled with commercially available Gd-based magnetic resonance imaging (MRI) contrast agents, Omniscan and Dotarem, which enabled passive loading of up to 10(8) Gd atoms per cell. In mixed preparations of labeled and unlabeled cells, LA-ICP-MS was capable of enumerating labeled cells at close to the predicted ratio. More importantly, LA-ICP-MS single cell analysis demonstrated that the cells retained a sufficient label to remain detectable for up to 10 days post-labeling both in vitro and in vivo in an immunodeficient mouse model.

Quantitative Analysis of Gold Nanoparticles in Single Cells by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry

Single cell analysis has become an important field of research in recent years reflecting the heterogeneity of cellular responses in biological systems. Here, we demonstrate a new method, based on laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), which can quantify in situ gold nanoparticles (Au NPs) in single cells. Dried residues of picoliter droplets ejected by a commercial inkjet printer were used to simulate matrix-matched calibration standards. The gold mass in single cells exposed to 100 nM NIST Au NPs (Reference material 8012, 30 nm) for 4 h showed a log-normal distribution, ranging from 1.7 to 72 fg Au per cell, which approximately corresponds to 9 to 370 Au NPs per cell. The average result from 70 single cells (15 ± 13 fg Au per cell) was in good agreement with the result from an aqua regia digest solution of 1.2 × 10(6) cells (18 ± 1 fg Au per cell). The limit of quantification was 1.7 fg Au. This paper demonstrates the great potential of LA-ICPMS for single cell analysis and the beneficial study of biological responses to metal drugs or NPs at the single cell level.

The use of background ions and a multivariate approach to characterise and optimise the dominant H2-based chemistries in a hexapole collision cell used in ICP-MS

Holistic analysis of ICP-MS spectra modified by the use of H2, He and mixtures thereof in a hexapole collision cell has enabled the identification of the major chemistries affecting cell performance. Correlation analysis has shown that groups of ions exist that have similar behaviours across a wide range of gas conditions. Examination of these groups indicates that the dominant reactions are related to species derived from argon and water, in particular 17OH+, 19H3O+, 21H3O+, 36Ar+, 38Ar+ and 80Ar2+. The ion signals at these m/z values reflect the concentrations of such species and are ratioed to produce indicators that are highly discriminant of the cell conditions. The objective variable used for optimisation was the S/N, calculated from an equation incorporating terms for the Poisson and flicker noise for both the signal and the background intensities. Detection limits for isotopes such as 28Si, 56Fe and 80Se are greatly improved at the expense of a general increase in oxide levels and some loss of sensitivity due to ion scattering for isotopes up to approximately m/z 70. It is further demonstrated that, in this largely charge-conservative system, the terminal fate of the charge occurs in parts of the spectrum that are not detrimental to elemental analysis.

The effect of ion energy on reactivity and species selectivity in hexapole collision/reaction cell ICP-MS

This paper describes experiments aimed at discriminating the different effects of ion energy in collision/reaction cell ICP-MS. It is demonstrated that the input ion energy, as determined by the plasma offset potential and ion energy distribution, is a key determinant of cell reactivity and this is termed the ion kinetic energy effect (IKEE). The ion kinetic energy is varied by alteration of the potential difference between the plasma and the hexapole cell. The plasma offset potential and ion energy distribution are not accurately known but are inferred from “stopping curves” produced by varying the pole bias of the quadrupole analyser. Kinetic energy discrimination (KED), where the difference in bias potentials between the quadrupole mass analyser and the hexapole cell is exploited to reject slow cell-formed ions, is shown to be a different effect. It can be used to change the relative levels of polyatomic ions arriving at the detector. The influence of IKEE and KED on the levels of plasma and analyte oxide (MO+) ions and on the 21(H3O)+/36Ar+ reactivity indicator ratio are considered. It is shown that IKEE can be used to influence the reactive attenuation of argide ions and the production of MO+ in the cell. KED is shown to preferentially reject cell-formed MO+ from the mass analyser.

Speciation of oxaliplatin adducts with DNA nucleotides

This paper describes a set of fast and selective high performance liquid chromatography (HPLC) methods coupled to electro-spray ionisation linear ion trap mass spectrometry (ESI-MS), sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) and UV detection for in vitro studies of the bifunctional adducts of oxaliplatin with mono-nucleotides, di-nucleotides and cellular DNA. The stationary phases and the optimised conditions used for each separation are discussed. Interaction of oxaliplatin with A and G mono-nucleotides resulted in the formation of five bifunctional platinum diaminocyclohexane (DACHPt) adducts. These were two isomers of the A-DACHPt-A and A-DACHPt-G adducts, and one G-DACHPt-G adduct, as confirmed by MS/MS spectra obtained by collision induced dissociation. These adducts were also characterised by UV absorption data and SF-ICP-MS elemental (195)Pt and (31)P signals. Further, interaction of oxaliplatin with AG and GG di-nucleotides resulted in the formation of three adducts: DACHPt-GG and two isomers of the DACHPt-AG adduct, as confirmed by ESI-MS and the complementary data obtained by UV and SF-ICP-MS. Finally, a very sensitive LC-ICP-MS method for the quantification of oxaliplatin GG intra-strand adducts (DACHPt-GG) was developed and used for monitoring the in vitro formation and repair of these adducts in human colorectal cancer cells. The method detection limit was 0.14 ppb Pt which was equivalent to 0.22 Pt adduct per 10(6) nucleotides based on a 10 μg DNA sample. This detection limit makes this method suitable for in vivo assessment of DACHPt-GG adducts in patients undergoing oxaliplatin chemotherapy.

Laser Ablation–Inductively Coupled Plasma Mass Spectrometry: An Emerging Technology for Detecting Rare Cells in Tissue Sections

Administering immunoregulatory cells to patients as medicinal agents is a potentially revolutionary approach to the treatment of immunologically mediated diseases. Presently, there are no satisfactory, clinically applicable methods of tracking human cells in patients with adequate spatial resolution and target cell specificity over a sufficient period of time. Laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) represents a potential solution to the problem of detecting very rare cells in tissues. In this article, this exquisitely sensitive technique is applied to the tracking of gold-labeled human regulatory macrophages (Mregs) in immunodeficient mice. Optimal conditions for labeling Mregs with 50-nm gold particles were investigated by exposing Mregs in culture to variable concentrations of label: Mregs incubated with 3.5 × 109 particles/ml for 1 h incorporated an average of 3.39 × 108 Au atoms/cell without loss of cell viability. Analysis of single, gold-labeled Mregs by LA-ICP-MS registered an average of 1.9 × 105 counts/cell. Under these conditions, 100% labeling efficiency was achieved, and label was retained by Mregs for ≥36 h. Gold-labeled Mregs adhered to glass surfaces; after 24 h of culture, it was possible to colabel these cells with human-specific 154Sm-tagged anti–HLA-DR or 174Yb-tagged anti-CD45 mAbs. Following injection into immunodeficient mice, signals from gold-labeled human Mregs could be detected in mouse lung, liver, and spleen for at least 7 d by solution-based inductively coupled plasma mass spectrometry and LA-ICP-MS. These promising results indicate that LA-ICP-MS tissue imaging has great potential as an analytical technique in immunology.

Tricine-SDS-PAGE

Tricine-sodium dodecyl sulphate-polyacrylamide gel electrophoresis (tricine-SDS-PAGE) is an efficient way of separating low-molecular-mass proteins. However, the standard system is quite complicated and specifically may not be useful when the separated proteins require to be recovered from the gel for quantitative analysis. Here, we describe a simplified system whereby these smaller proteins can be resolved in comparatively low-percentage gels which have high compatibility with modern detectors such as UV and inductively coupled plasma mass spectrometry (ICP-MS).

High-Speed, Integrated Ablation Cell and Dual Concentric Injector Plasma Torch for Laser Ablation-Inductively Coupled Plasma Mass Spectrometry.

In recent years, laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) has gained increasing importance for biological analysis, where ultratrace imaging at micrometer resolution is required. However, while undoubtedly a valuable research tool, the washout times and sensitivity of current technology have restricted its routine and clinical application. Long periods between sampling points are required to maintain adequate spatial resolution. Additionally, temporal signal dispersion reduces the signal-to-noise ratio, which is a particular concern when analyzing discrete samples, such as individual particles or cells. This paper describes a novel, two-volume laser ablation cell and integrated ICP torch designed to minimize aerosol dispersion for fast, efficient sample transport. The holistic design utilizes a short, continuous diameter fused silica conduit, which extends from the point of ablation, through the ICP torch, and into the base of the plasma. This arrangement removes the requirement for a dispersive component for argon addition, and helps to keep the sample on axis with the ICP cone orifice. Hence, deposition of sample on the cones is theoretically reduced with a resulting improvement in the absolute sensitivity (counts per unit mole). The system described here achieved washouts of 1.5, 3.2, and 4.9 ms for NIST 612 glass, at full width half, 10%, and 1% maximum, respectively, with an 8-14-fold improvement in absolute sensitivity, compared to a single volume ablation cell. To illustrate the benefits of this performance, the system was applied to a contemporary bioanalytical challenge, specifically the analysis of individual biological cells, demonstrating similar improvements in performance.

The effect of adventitious water in hexapole collision cell inductively coupled plasma mass spectrometry

This paper considers several aspects of the role of water in collision cell inductively coupled plasma mass spectrometry. Studies with water and deuterium oxide, employed both as samples and as impurities in the cell gas, were used to investigate the origins of the polyatomic ions. It was found that no new species were formed in the cell as water from the plasma provides the basic reactive components; however, some were greatly enhanced when the cell gas was deliberately wetted. The reactivity of the water/deuterium oxide dominated the cell chemistry when an unreactive gas such as He was used, indicating a need for careful control of water content, but the effects were greatly reduced when a reactive gas such as hydrogen was also employed. It is concluded that water could be a useful reagent molecule, if its partial pressure can be adequately controlled. Seeking complete removal of water from the reagent gas is unnecessary as there is a baseline contribution from the plasma and, in any case, when reactive reagent gases are used its effect is minimal.