Table 1.
Freshwater invertebrates obtained from 14 NEON sites and used to evaluate the comparability of using ICP-MS and MBC to determine total %P (n = 106).
Table 2.
Total phosphorus as percent dry mass for the three standard reference materials (SRMs) used to evaluate method accuracy.
Fig 1.
Calibration curves for molybdenum blue colorimetry (MBC) and inductively coupled plasma mass spectrometry (ICP-MS).
A) Calibration curves for MBC were measured using 1-, 5-, and 10-cm pathlength cells, with signal (y-axis) reported as blank-corrected Absorbance (885 nm). B) Calibration curves for ICP-MS (right) were measured in KED mode with the signal reported as the ratio of blank-corrected counts-per-second (cps) for m/z 31 (P) and 45 (Sc, internal standard). In both plots, filled markers indicate points that satisfied quality control criteria and were considered to be within the linear quantification range.
Table 3.
Linear quantification range for the molybdenum blue colorimetry (MBC) and inductively coupled plasma mass spectrometry (ICP-MS) kinetic energy discrimination (KED) methods used.
Fig 2.
Total P recoveries for standard reference materials by MBC and ICP-MS.
P recoveries (percent) in split digests of scallop (n = 6), spinach (n = 17 (MBC); n = 20 (ICP-MS)), and cuttlefish (n = 4) reference materials (see Table 2). The dashed line represents 100% P recovery. P recovery was higher for ICP-MS (F1,53 = 6.9, p = 0.011) and differed by SRM (F2,53 = 7.6, p = 0.0013).
Fig 3.
Total %P determined for freshwater invertebrates (n = 101) by MBC and ICP-MS.
The dashed and solid lines represent the Deming regression (equation on plot) and 1:1 line, respectively. Purple markers indicate invertebrate samples that were additionally used to evaluate analytical interferences.
Fig 4.
Comparison of ICP-MS %P measured using KED and STD modes.
%P measured in A) invertebrates (n = 24) and B) spinach SRM (n = 12). The dashed line in the SRM plot shows the certified %P (0.52%). %P was higher when measured in STD mode than in KED mode (W = 159, two-tailed p = 0.0053), but the absolute pair-wise differences were small (0.056 ± 0.051%P).
Fig 5.
Comparison of P-spike recoveries for MBC and ICP-MS.
Recoveries of P spikes for A) invertebrates (n = 10) and B) spinach SRM (n = 9). The dashed line shows 100% recovery of total P. Overall, %P values were higher than ICP-MS values (t18 = 6.3, two-tailed p < 0.0001).