Table 2.

An evaluation of M²⁺ interference-correction approaches on m/z 78 in reagent and regional tap water matrices fortified with Gd

Reagent Water Fortified with Low and High Rare Earth Spikes	m/z 78, No Correction1,2	m/z 78, No Correction1,3	m/z 78, External Correction1,3,4	m/z 78, In-Sample Correction5	m/z 94, 8800 ICP-MS6	High Resolution7
	4.5mL He ([ppb] ± 2σ)	2.5mL He ([ppb] ± 2σ)	2.5mL He ([ppb] ± 2σ)	2.5mL He ([ppb] ± 2σ)	78→94 Se [O2] ([ppb] ± 2σ)	78Se ([ppb] ± 2σ)
2 ppb Gd	2.3 ± 0.7	0.7 ± 0.6	−0.1 ± 0.6	0.0 ± 0.8	0.01 ± 0.04	−0.03 ± 0.02
2 ppb Gd + spike8	3.1 ± 1.3	1.6 ± 0.9	0.8 ± 0.9	1.5 ± 1.3	1.0 ± 0.1	0.924 ± 0.004
50 ppb Gd	53.0 ± 5.7	19.1 ± 1.9	−1.0 ± 2.0	−0.1 ± 1.2	−0.04 ± 0.03	−0.01 ± 0.12
50 ppb Gd+ spike8	54.0 ± 7.0	20.3 ± 1.8	0.0 ± 1.8	0.6 ± 2.3	1.1 ± 0.1	0.94 ± 0.06
Regional tap waters9
Water A + 50 ppb Gd, Nd, Sm	51.2 ± 7.3	15.5 ± 2.9	−9.6 ± 3.2	−1.0 ± 2.6	0.18 ± 0.10	0.36 ± 0.03
Water B + 50 ppb Gd, Nd, Sm	51.1 ± 7.8	16.2 ± 2.0	−9.3 ± 1.8	−1.0 ± 3.9	1.35 ± 0.24	1.33 ± 0.12
Water C + 50 ppb Gd, Nd, Sm	51.8 ± 4.2	17.9 ± 4.3	−7.3 ± 4.6	1.2 ± 4.2	2.29 ± 0.23	2.17 ± 0.26
Water D + 50 ppb Gd, Nd, Sm	53.3 ± 5.8	15.8 ± 2.5	−9.6 ± 2.9	−2.4 ± 3.5	−0.03 ± 0.05	0.15 ± 0.02
Water E + 50 ppb Gd, Nd, Sm	53.2 ± 6.3	15.8 ± 3.6	−10.3 ± 4.3	−2.8 ± 4.2	−0.01 ± 0.09	0.21 ± 0.01
Water F + 50 ppb Gd, Nd, Sm	52.5 ± 6.7	16.3 ± 3.8	−9.6 ± 3.9	−2.1 ± 1.9	0.33 ± 0.07	0.46 ± 0.05

^1.Data were collected in normal resolution mode (0.8 amu at 10% peak height) using In as an internal standard with a 0.3-sec/amu integration time in selective ion monitoring mode. All estimates are based on 10 instrument replicates.

^2.The calibration blank and 1 ppb standard associated with the analysis of reagent water (4.5 mL He) produced 9 cps and 67 cps, respectively. The calibration blank and 5 ppb standard associated with the analysis of regional waters (4.5 mL He) produced 12 cps and 169 cps, respectively.

^3.The calibration blank and 1 ppb standard associated with the analysis of reagent waters (2.5 mL He) produced 23 cps and 87 cps, respectively. The calibration blank and 5 ppb standard associated with the analysis of regional waters (2.5 mL He) produced 31 cps and 279 cps, respectively.

^4.The M²⁺ correction factors were estimated in narrow resolution mode (0.4 amu at 10% peak height) in an external rare earth standard and applied as a fixed correction factor to all samples. This correction also incorporated a narrow to normal conversion factor (see Figure 5).

^5.Data were collected in narrow resolution mode (0.4 amu at 10% peak height) using In as an internal standard with a 0.3-sec/amu integration time in selective ion monitoring mode. All estimates are based on 10 instrument replicates. The M²⁺ correction factors were estimated in each sample. The solutions fortified with 2 ppb rare earth produced about 30 cps at the corresponding half-mass with %RSD across the 10 replicates of about 25%. The solutions fortified with 50 ppb rare earth produced about 300–500 cps at the corresponding half-mass with %RSD across the 10 replicates of 12%. A 1 ppb Se standard in narrow resolution mode using 2.5 mL He produces 51 cps with a 19 cps blank. All in-sample M²⁺ estimates required manual abundance-sensitivity evaluations at the adjacent masses to the M²⁺ ion.

^6.Data were collected using m/z 94 in oxygen mode at 0.5 mL/min with a 0.99-sec/amu integration time in selective ion monitoring mode. All estimates are based on n = 5 using Y as an internal standard (YO⁺, 89→105). The calibration blank and 1 ppb Se standard produced 7 cps and 351 cps, respectively.

^7.Selenium data were collected with an EScan over the mass range of 77.912–77.922 amu with a sampling time of 0.200 sec, 40 samples per peak, and 30% search and 60% integration windows over 3 runs and 2 passes (n = 6). All data were collected in high-resolution mode, and concentrations were determined relative to the ¹¹⁵In internal standard. The Auto Lock Mass feature was active during analysis.

^8.This sample was fortified with 0.050 ppb As and 1 ppb Se.

^9.All regional waters were prepared to reflect the 0.4% HCl concentration that results from an acid digestion according to Method 200.8.