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but ensure that the curvature is not excessive. Many ICP software programs have algorithms to
detect excessive curvature of second order or quadratic calibration curves. Alternatively, linear
calibration can be achieved by removal of the high concentration K standards; however,
secondary dilution of high concentration test solutions will be required. Dilutions must maintain
the solvent matrix, which is prepared by diluting 400 mL citrate–EDTA extract solution
[Alternative A, B (
m
)] to 1 L.
(d)
Empirical calibration (optional).
–The combination of an organic solvent, high salts and high
Phosphorus in the test portion can result in suppression of signal intensity. This method is
designed to address these issues by matrix and aliquot dilution using the recommended pump
tube configuration, plus the use of robust plasma conditions and an internal standard. However,
if this recommended configuration still produces low phosphorus recoveries for the fertilizer
concentrates (i.e. 40-52% P
2
O
5
), then empirical calibration may be necessary. Fertilizer
concentrates with certified or accepted consensus values can be obtained from Laboratory
Quality Services International (LQSI)
(www.sgs.com/en/mining/Analytical-Services/Proficiency- Testing-Programs-L QSi.aspx), and the Magruder
(www.magruderchecksample.org) or AFPC
(www.afpc.net) check sample programs. Note that calibration solutions obtained from these
certified or consensus reference materials are prepared by following the recommended
extraction procedure (Alternative A, E) and that these standards can only be used for calibration
within the batch of test solutions with which they were extracted. These standard extract
solutions have the same shelf life (i.e. approx. 16 h) as the other fertilizer extracts and must be
prepared fresh with each run. Calculations for converting the % P
2
O
5
in these materials to mg/L
P are provided in the Calculations section (Alternative A, G). Fertilizer materials below 40% P
2
O
5
(or approx. 350 µg/mL P) typically do not experience this suppression issue, so standards below
this concentration can be obtained using those listed in Tables
2015.18A
or
B
. Empirical
calibration is not the preferred option and should only be used as a last resort.
The test solution and internal standard/ionic buffer solution [Alternative A, B (
o
)] are
blended using a Y-connector (Part 30703-90; Cole-Parmer, Bunker, CT) just prior to the
nebulizer using the conditions described in Table
2015.18D
.
D.
Sample Preparation
Collect a primary field sample using one of the recommended AOAC sampling procedures (i.e.
929.01, 969.01
, or
992.33
) or other recognized protocol. Prepare solid fertilizer materials by
riffling [Alternative A, A (
g
)] the entire 5 lb (2300 g) laboratory sample twice before further
Candidates for 2016 Method of the Year
248