ICP-MS Ion Research Articles

Analytical considerations associated with implementing M2+ correction factors to address false positives on As and Se within U.S. EPA method 200.8.

Rare earth elements (REE) can produce M2+ ions in ICP-MS and 150Nd2+, 150Sm2+, and 156Gd2+ can produce false positives on 75As and 78Se. Alternative instrumental tuning conditions, that utilize lower He flows within the collision cell, reduce these false positives by a factor of 2 (to 0.8 ppb As and 19 ppb Se in solutions containing 50 ppb Nd and Gd) with comparable 16O35Cl+ reduction (<100 ppt false 51V in 0.4% HCl) and Se sensitivity (DL < 1 ppb). Further reduction of these false positives is achieved by estimating the M2+ correction factors and utilizing them in the interference-correction software. Approaches to estimating the M2+ correction factor were evaluated with an emphasis on techniques that tolerate daily variability in end-user backgrounds and their ability to reduce the initial and ongoing purity requirements associated with the rare earth standards used to estimate the M2+ correction factor. The direct elemental and polyatomic overlaps associated with unit-mass approaches tend to overcorrect as non-rare-earth signals as small as 30 cps at the unit mass can induce bias relative to the <300 cps signals associated with the M2+ from a 50 ppb REE standard solution. Alternatively, shifting the M2+ estimate to a half mass (i.e., m/z 71.5: 143Nd2+) eliminates the direct overlap source of bias and allows the unit mass signal to approach 150000 cps before it bleeds over on the 1/2 mass because of abundance sensitivity limitations. The performance of the half-mass approach was evaluated in reagent water and regional tap waters fortified with Nd, Sm, and Gd at 2 ppb and 50 ppb. In addition, a half-mass in-sample approach was also evaluated. This approach was found to be beneficial relative to the external or fixed-factor half-mass approach as it could compensate for instrument drift and matrix-induced shifts in the M2+ factors. Finally, all results were evaluated relative to the As and Se concentrations determined using an ICP-QQQ in mass shift mode and a high-resolution ICP-MS.

Optimisation and assessment of an innovative high sensitivity 90-degree reflecting ICP-MS ion optics system for the validation of simultaneous determination of arsenic and selenium in water matrices

A routine ICP-MS method for the determination of As and Se in fresh and sewage water has been developed. After a first phase of optimisation, where the torch alignment, flow of gases and ion optic adjustments were calibrated, the method was validated successfully. The parameters limits of detection, linearity, working range, sensitivity and the interferences, precision and accuracy were studied with three procedures for the method: measuring two SRMs (BCR-714 of influent wastewater and NIST 1643e of groundwater); spiking four different matrices at three levels of concentration; and doing a proficiency test. The limits of detection obtained were 0.2 and 0.8 µg L−1 for As and Se, respectively. The percentages of linearity obtained were 99.2% for As and 99.8% for Se. All recovery values were according to the AOAC intervals, from 95.3% for As in the wastewater matrix to 106.4% for Se in the seawater matrix. The accuracy was also studied determined via a proficiency testing resulting in acceptable Z-scores of 0.65 and 0.4 respectively for As and Se. The complete method allowed analysis of water samples according to European Directive 2000/60 EC and the different guides for method validation.

Determination of Pu isotope concentrations and isotope ratio by inductively coupled plasma mass spectrometry: a review of analytical methodology

A number of analytical methods for Pu isotopes based on ICP-MS measurement have been developed and applied to the measurement of ultra-trace levels of Pu in nuclear fuel samples and environmental samples. The endeavour to improve the detection limit to sub-10−18 g (ag) mL−1 has been continued by using efficient sample introduction systems. This review summarizes and critically discusses various types of ICP-MS, sample introduction systems and separation methods applied to the determination of Pu in many sample types. Together with the separation method, the adjustment of the Pu oxidation state in the loading solution preparation step and Pu elution step is summarized. The interference problem from polyatomic ions in ICP-MS coupled with various sample introduction systems is described, especially focused on the UH+ interference. Several factors affecting the trueness and precision of Pu results are discussed.

Collision and reaction cells in atomic mass spectrometry: development, status, and applicationsThe opinions expressed in the following article are entirely those of the authors and do not necessarily represent the views of the Royal Society of Chemistry, the Editor or the Editorial Board of JAAS.

The development and utilization of collision and reaction cells in atomic mass spectrometry is reviewed. These devices have been used for decades in fundamental studies of ion–molecule chemistry and have only recently been applied in the GD-MS and ICP-MS fields. Such cells are used to promote reactive and non-reactive collisions, with resultant benefits in interference reduction, isobar separation, and thermalization/focusing of ions in ICP-MS. Novel ion–molecule chemistry schemes, using a variety of reaction gas reagents selected on the basis of thermodynamic and kinetic principles and data, are now designed and empirically evaluated with relative ease. These chemical resolution techniques can avert interferences requiring mass spectral resolutions of >600000 (m/Δm). Purely physical ion beam processes, including collisional dampening and collisional dissociation, are also employed to provide improved sensitivity, resolution and spectral simplicity. Collision and reaction cell techniques are now firmly entrenched in current-day ICP-MS technology, enabling unprecedented flexibility and freedom from many spectral interferences. A significant body of applications has now been reported in the literature. Collision/reaction cell techniques are found to be most useful for specialized or difficult analytical needs and situations, and are employed in both single- and multi-element determination modes.

Ion stopping experiment to determine the origin of background ions in icp-ms

Ion stopping experiments have been performed using a triple grid placed between the ion extraction and collector lenses of a modified lens stack. Stopping voltages for ions extracted from the plasma were between 6.6 and 13.1 V (Be+), 9.8–13.1 V (Co+, In+, Ce+, Pb+), and 11.8–14.4 V (U+). Controlled contamination of the ion extraction interface was performed, and the stopping voltage for Be+ ions contributing to the subsequent background signal was determined to be between 1.5–5.9 V. This suggests that these ions originated downstream of the ion sampling interface, possibly due to deposition of Be on the skimmer cone, which was subsequently re-volatilised. The low stopping voltage of these ions meant that they would be prevented from entering the quadrupole by applying a small retarding potential on one of the lens elements (or possibly the quadrupole itself), while still admitting those ions with higher energies which were extracted from the plasma.

Mass spectrometric and theoretical investigations into the formation of argon molecular ions in plasma mass spectrometry

The abundance and distribution of argon molecular ions (e.g., ArH+, ArO+, ArN+, Ar2+ and MAr+; M = metal) in plasma MS (ICP-MS, LA-ICP-MS and rf-GDMS) were investigated and compared. In ICP-MS the non-metal argon molecular ions were formed with higher intensity compared with the metal argide ions. This could be explained by theoretical calculations of the binding energies. The ArH+ ion can be viewed as an isoelectronic system, comparable to HCl. The intensities of diatomic metal argide ions relative to metal ions in ICP-MS are less than 10–4. A correlation between the intensities of metal argide ions with the bond dissociation energies of diatomic ions was found. The highest intensity of metal argide ions, of the order of per cent. values, were observed in rf-GDMS. The intensity of the argon molecular ions in an rf-GD varied by up to three orders of magnitude as a function of the plasma parameters (e.g., argon pressure in the GD ion source). The characteristic distribution of diatomic argide ions of REEs in ICP-MS was found to be comparable to the distribution of rare earth oxide ions.

Evaluation of Ethene Addition to the Nebulizer Gas in Inductively Coupled Plasma-Mass Spectrometry for the Removal of Matrix-, Solvent-, and Support-Gas-Derived Polyatomic Ion Interferences

Ethene addition to the nebulizer gas is assessed as a means of reducing the levels of polyatomic ions in ICP-MS. It was found that, while the absolute analyte response was generally poorer with the addition of ethene, the levels of the interferences were greatly reduced, when compared to the all argon plasma. The plasma operating conditions, with the addition of ethene to the nebulizer gas, were optimized with the use of a variable-step-size Simplex procedure in order to remove or reduce the ArO+, ArCl+, ClO+, CeO+, ArNa+, SO2+/S2+, and PO2+ interferences. Complete removal of the ArCl+, ArNa+, SO2+/S2+, and PO2+ was achieved, while the levels of the remaining interferences were very much reduced. Detection limits for As, Se, and V in 10% HCl were excellent (0.27, 2.7, and 2.04 ng cm−3, respectively), as was the detection limit for Cu in 1% H3PO4 at 6.9 ng cm−3. Cu, Ni, and Zn detection limits in the presence of H2SO4 and sodium ions were also improved, and the detection limit for Fe was 0.48 ng cm−3. Levels of CeO+ were reduced to just 0.2% relative to Ce. Recovery tests and the analysis of certified reference materials further demonstrated the utility of the optimal conditions. A set of compromise ethene conditions was identified and shown to be of value in reducing the interferences simultaneously.

The effect of plasma operating parameters on analyte signals in inductively coupled plasma-mass spectrometry

Utilizing the SCIEX ICP-MS an extensive study of the effects that plasma operating parameters have on analyte ion signals in ICP-MS has been carried out. Parameters studied included aerosol flow rate (nebulizer pressure), auxiliary flow rate, power and sampling depth (sampling position from the load coil). The two key parameters are aerosol flow rate (nebulizer pressure) and power. Elements can be grouped into characteristic behaviour patterns based on the overall dependence of their ion count signal on these two parameters. The nebulizer pressure-power behavior patterns allow a sensible selection of compromise operating conditions and significantly clarify single parameter observations which often indicate confusing trends in behavior. In addition to characterizing analyte ion signals the parameter behavior plots have also been used to study oxide species and plus two ions in ICP-MS. While aerosol flow rate and power appear to be the key ICP parameters in ICP-MS, ion signals are dependent on sampling depth and auxiliary flow rate and some data are also presented illustrating the signal dependence on these two parameters.