You are here:
Analytical considerations associated with implementing M2+ correction factors to address false positives on As and Se within U.S. EPA method 200.8
Citation:
Smith, S., N. Hanks, P. Creed, K. Kovalcik, R. Wilson, K. Kubachka, J. Brisbin, J. Landero Figueroa, AND J. Creed. Analytical considerations associated with implementing M2+ correction factors to address false positives on As and Se within U.S. EPA method 200.8. JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. Royal Society of Chemistry, Cambridge, Uk, 34(10):2094-2104, (2019). https://doi.org/10.1039/C9JA00086K
Impact/Purpose:
Currently EPA Method 200.8 is used to determine metals concentrations in environmental compliance monitoring programs. The most recent version of the method was last updated in 1994 and does not address the use of helium collision cell as an interference reduction technology for Inductively Coupled Plasma Mass Spectrometry. This paper address some of the issues associated with adopting helium collision cell technology within the context of minimizing false positives associated with the method. False positives from polyatomic ions are almost universally reduced by the collision cell technology while false positives from M2+ ions are increased. The increase of M2+ has a negative effect on Arsenic and Selenium detection using the method and for this reason most of the paper outlines procedures that attempt to mitigate the bias associated with the inclusion of M2+corrections in the method for these two analytes.
Description:
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, 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 30cps at the unit mass can induce bias relative to the <300cps signals associated with the M2+ from a 50ppb 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 150,000cps before it bleeds over on the ½ 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 a ICP-QQQ in mass shift mode and a high-resolution ICP-MS.
URLs/Downloads:
DOI: Analytical considerations associated with implementing M2+ correction factors to address false positives on As and Se within U.S. EPA method 200.8