Scientific Publications by FDA Staff

J Anal Atom Spectrom 2019 Oct 1;34(10):2094-104

Analytical considerations associated with implementing M(2+) correction factors to address false positives on As and Se within U.S. EPA method 200.8.

Smith SW, Hanks N, Creed PA, Kovalcik K, Wilson RA, Kubachka K, Brisbin JA, Figueroa JL, Creed JT

Rare earth elements (REE) can produce M(2+) ions in ICP-MS and (150)Nd(2+), (150)Sm(2+), and (156)Gd(2+) can produce false positives on (75)As and (78)Se. 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 (16)O(35)Cl(+) reduction (<100 ppt false (51)V in 0.4% HCl) and Se sensitivity (DL < 1 ppb). Further reduction of these false positives is achieved by estimating the M(2+) correction factors and utilizing them in the interference-correction software. Approaches to estimating the M(2+) 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 M(2+) 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 M(2+) from a 50 ppb REE standard solution. Alternatively, shifting the M(2+) estimate to a half mass (i.e., m/z 71.5: (143)Nd(2+)) 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 M(2+) 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.