Advanced Analysis of Arsenic and Selenium Using TQ-ICP-MS

Accurate determination of arsenic and selenium in environmental samples using triple quadrupole ICP-MS Authors: Marcus Manecki1 , Simon Lofthouse2 , Philipp Boening3 and Shona McSheehy Ducos1 ; 1 Thermo Fisher Scientific, Bremen, Germany; 2 Thermo Fisher Scientific, Hemel Hempstead, UK; 3 Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, Oldenburg, Germany Keywords: Arsenic, interference removal, REE, rock, selenium, soil, sediment Goal To demonstrate the accurate determination of arsenic and selenium in sediments and rocks that contain elevated levels of rare earth elements using triple quadrupole ICP-MS. Introduction Due to the impact arsenic and selenium can have in the environment at low levels, as a toxin or essential nutrient respectively, it is important to be able to quantify them accurately. Selenium for example is an essential element that is necessary for normal thyroid function and due to its antioxidant properties, is associated with several health benefits. Diseases associated with selenium deficiency such as Keshan disease and symptoms of hypothyroidism, are most commonly found in areas where levels of selenium in soil are particularly low. Supplementation as a remedy is common practice and is not isolated to humans. Understanding where soil selenium deficiencies occur for example supports the correct supplementation of cattle grazing in those areas to prevent white muscle disease (a cattle specific selenium deficiency disease). 2 Arsenic on the other hand, in its inorganic forms (the most common forms found in ground water and soils) is classified as carcinogenic. Arsenic can be found at natural, elevated levels or highly enriched in ground waters (e.g. in Bangladesh) and in soils from irrigation with arsenic contaminated ground water. In this case, accurate analysis of arsenic is key to understanding whether crops, such as rice grown in these areas could contain an elevated level of arsenic and be a potential risk for consumption. In addition to assessing the exposure implications of these elements, their accurate analysis is vital to understanding their geochemical cycling processes and impact on the environment. Analysis of these two elements by ICP-MS is challenging due to multiple spectral interferences, and becomes especially challenging in the presence of high amounts of rare earth elements (REEs) such as dysprosium, gadolinium, neodymium, samarium or terbium due to the formation of doubly charged ions. These doubly charged REEs lead to false positive results on arsenic and selenium and as such lead to incorrect conclusions and actions based on that data. Triple quadrupole (TQ) ICP-MS offers improved interference removal for such challenging applications through the use of selective reaction chemistry to produce higher mass ions, which can either mass shift analytes into an interference free region of the mass spectrum or mass shift interferences away from analytes. This application note evaluates the efficiency of TQ-ICP-MS measurement modes and compares them to single quadrupole (SQ) ICP-MS measurement modes with the Thermo Scientific™ iCAP™ TQ ICP-MS for the quantification of arsenic and selenium in the presence of REEs. To test the robustness and the accuracy of the approach, two samples, a deep sea sediment and a geochemical reference standard, were analyzed under optimal conditions. Instrumentation An iCAP TQ ICP-MS was used to analyse all samples. The system was configured with a high matrix interface (Table 1) for improved handling of the high amounts of total dissolved solids (TDS) encountered in the samples and a 200 µL·min-1 free aspirating, glass, concentric nebulizer due to the limited volume of digested sample. Four different measurement modes were evaluated: SQ-STD – single quadrupole mode with no collision/ reaction cell (CRC) gas. SQ-H2 – single quadrupole mode with CRC pressurized with pure hydrogen as a reaction gas. Please note that hydrogen is not available on the Thermo Scientific™ iCAP™ TQe ICP-MS. SQ-KED – single quadrupole mode with CRC pressurized with helium as a collision gas and Kinetic Energy Discrimination (KED) applied. TQ-O2 – triple quadrupole mode with CRC pressurized with oxygen as a reaction gas, Q1 set to analyte mass (M+) and Q3 set to product ion mass (MO+). All parameters within each of the measurement modes were defined automatically by using the autotune procedures provided in the Thermo Scientific™ Qtegra™ Intelligent Scientific Data Solution (ISDS) Software. The autotune functionality ensures that plasma and interface related settings, such as nebulizer flow and extraction lens voltage are automatically applied across all associated measurement modes so that the sample is processed in exactly the same way in the plasma, independent of the CRC and quadrupole settings. Details about the settings used for the different modes are shown in Table 1. 3 Table 1. Instrument parameters for all measurement modes Parameter Value Nebulizer MicroMist quartz nebulizer 0.2 mL·min-1, free aspirating Spraychamber Quartz cyclonic spraychamber cooled to 2.7 ˚C Injector 2.5 mm id, quartz Interface High Matrix (3.5 mm) insert, Ni cones RF Power 1,550 W Nebulizer Gas Flow 1.04 L·min-1 Modes SQ-STD SQ-H2 SQ-KED TQ-O2 Mass shift applied No No No Yes Mass shift over x mass units - - - 16 Gas Flow - 9.0 mL min-1 4.65 mL min-1 0.35 mL min-1 CR Bias -2 -7.55 -21 V - 7.0 V Q3 Bias -1 -12 V -18 V -12 V Scan Settings 0.2 s dwell time per analyte, 10 sweeps The method development assistant in the Qtegra ISDS Software, Reaction Finder, automatically selects the best mode to use for the analyte measurements. In this evaluation exercise, in which the effectiveness of different measurement modes for the same analyte was investigated, replicate analytes were added and the measurement modes selected manually. The formation of doubly charged ions and the resulting interferences in ICP-MS are known issues. There are several ways to mitigate these interferences on the analyte signals, including: • Interference correction equations • Tuning of the instrument to reduce formation of doubly charged ions within the plasma • Mass shift reactions that move the analyte of interest to a different m/z Many laboratories prefer to avoid the approach of using interference correction equations as it is possible that due to small daily changes in plasma conditions, they need to be calculated or checked on a daily basis to verify their accuracy. Mass shift reactions show promise but have limitations with SQ-ICP-MS due to the complex mixture of ions in the CRC that can cause other potential interferences. With TQ-ICP-MS, the pre-selection of the mass of interest in Q1 enables a more controlled reaction for the analytes and removes interferences that could still be problematic in SQ-ICP-MS. The Reaction Finder tool selects TQ-O2 mode automatically for 75As and 80Se. To be able to compare different modes and the results for different isotopes, the same measurement mode was also selected for 78Se and 82Se. This mode uses pure O2 in the CRC to create oxide ions of the arsenic and selenium isotopes. Arsenic was measured at m/z 91 as 75As16O and the selenium isotopes 78Se, 80Se and 82Se were measured at m/z 94 as 78Se16O, at m/z 96 as 80Se16O and at m/z 98 as 82Se16O respectively. Figure 1 demonstrates how Q1 (when set to the analyte mass), effectively removes the singly charged REEs and any ions that would eventually interfere with the product ions, such as 91Zr and 94Mo for 75As and 78Se respectively. Q2 (the CRC) is filled with O2 and creates the product ions 91[AsO]+ and 94[SeO]+ for 75As and 78Se respectively. In Q3, any remaining doubly charged REE are rejected and the product ion is isolated f