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

Abstract

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 150 000 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.