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standard wavelength used for correction should also be from the
atomic state, such as Sc 361.383. Conversely, match ionic sample
lines with ionic internal standard lines. (
Note
: Do not use yttrium as
an internal standard, since it is found native at low levels in some
phosphate ore sources.)
(
b
)
ICP wavelengths.
—A number of wavelengths may be used
for analysis of the 8 elements of interest, depending on the capability
of the analytical instrument used. As a minimum, select at least 2
wavelengths for each element of interest, and report the average
value of closely agreeing results, except for lead and selenium, for
which there is only one reliable wavelength available. Following is a
list of suggested wavelengths, not in any priority order, that have
been found acceptable for most fertilizer materials. Other lines of
appropriate sensitivity, free of interferences or corrected for
interferences, may be just as acceptable. However, it is imperative
that instrument response (both instrument graphic output and
calculated concentration) be reviewed for each sample and element.
Fertilizer materials are extremely variable in composition, and a
wide concentration range of potential interfering elements is
expected, so no single wavelength will work in every instance.
Occasional data with interference will inevitably be found, and must
be eliminated from inclusion in the mean calculation for that
particular element and sample.
Wavelengths (nm): As: 188.980, 193.696; Cd: 214.439, 226.502;
Co: 228.615, 230.786, 235.341; Cr: 205.560, 267.716, 276.653;
Mo: 201.512, 202.032, 203.846, 204.598; Ni: 216.555, 222.295,
222.486, 227.877, 231.604, 239.452; Pb: 220.353; Se: 196.026; Sc:
361.383, 431.408; Be: 234.861, 249.473.
(
c
)
Wavelength interference treatment.
—Interelement
interference can cause substantial error in analytical result. Error
can be minimized by several techniques: (
1
) Three or more
analytical lines may be used for a given element, and when an
interferent is present in a particular line, the result for that line is
omitted from the mean value reported. (
2
) Certain vendors’
instrument software has the capability of mathematically modeling
potential interferents, and deconvoluting the instrument response
into an analytical element portion and an interferent portion. (
3
)
Interelement correction is an alternative mathematical technique to
use with instruments for which mathematic modeling is not
available, or where direct spectral overlap negates use of the
deconvolution technique.
The following lines, if used, must utilize one of the correction
techniques; corrections for other lines may be applied as needed and
appropriate: (
1
) As 188.980: Correct for Cr interference at 188.995,
or verify that Cr is not present in the test portion analyzed. (
2
) As
193.696: Fe affects the arsenic peak. Remove with an Fe model, or
verify that Fe is not present in the test portion analyzed. (
3
) Cd
214.439 and 226.502: Fe, present in many fertilizers, interferes with
both suggested Cd wavelengths. Mathematically correct instrument
Cd response for the interference, or verify analytically that Fe is not
present in the test portion analyzed. (
4
) Pb 220.353: Mathematically
correct instrument Pb response for Fe interference, or verify that Fe
is not present in the test portion analyzed. (
5
) Se 196.026:
Mathematically correct instrument Se response for Fe interference,
or verify that Fe is not present in the test portion analyzed.
(
d
)
ICP instrument calibration
.—Prepare working standard
solutions from commercial stock standards at 1000 mg/kg. Custom
blended multielement stock standard in HNO
3
is acceptable. Prepare a
minimum of 5 working standards at 0.1, 0.5, 1.0, 5.0, and 10.0 mg/kg,
plus blank, of each element, in 10% trace metal grade HNO
3
. Working
standards should be in the linear range, with correlation coefficients
of at least 0.9999.
G. Reagents
(
a
)
Water
.—Use 18 Megaohm water for dilution.
(
b
)
HNO
3
.—Use trace metal grade HNO
3
.
(
c
)
0.5% Triton X100 solution
.—Dilute 0.5 mL Triton X100 to
100 mL with H
2
O.
(
d
)
Ionization buffer/internal standard solution.—
Weigh 8.0 g
CsCl into a 1000 mL acid-washed volumetric flask. Add 3 mL each
of ICP grade scandium and beryllium 1000 mg/kg stock solution, as
internal standards. Also add 1 mL of 0.5% Triton X100, dilute to
volume, and mix. Store in a polypropylene bottle. (
Note
: Reagent
concentrations assume the use of white/white, 1.02 mm id sample
pump tube, and orange/white, 0.64 mm id internal standard pump
tube. If the sample and internal standard solutions are mixed in
different proportions by the instrument’s peristaltic pump, then
adjust the reagent concentrations to meet concentration
requirements of mixed solution nebulized by the instrument, as
outlined in
F
. Note that sample and internal standard solutionmixing
ratio is proportional to pump tube flow rates, not proportional to
pump tube IDs.)
(
e
)
Stock standard solution.
—Working standards can be
prepared from ICP grade individual element 1000 mg/kg
commercial stock standard solutions. However, it is also acceptable
to use commercially prepared custom blended stock standard
mixtures containing all of the 8 elements at 1000 mg/kg. A number
of companies provide this stock standard service.
(
f
)
10 mg/kg
intermediate stock standard solution for
preparation of low-level working standards.
—Dilute 5.0 mL of
stock standard solution to 500mL. Prepare fresh each time standards
are prepared, and use immediately after preparation.
(
g
)
Working standard solutions.
—Standards are designed to have
the same acid concentration as digested test solutions. Date all
calibration solutions when made, which should be stable for at least
1 month, but not longer than 2 months. Monitor standard curve fit
and intensity for signs of change and degradation over time. (
1
)
10 mg/kg elements
.—Pipet 5.0 mL of combined 1000 mg/kg
element stock solution into a 500 mL acid-washed volumetric flask.
Add 50 mL of trace metal grade HNO
3
, dilute to volume with H
2
O,
mix, and transfer to acid-washed polypropylene bottle. (
2
)
5 mg/kg
elements
.—Pipet 5.0 mL of combined 1000 mg/kg element stock
solution into a 1000 mL acid-washed volumetric flask. Add 100 mL
of trace metal grade HNO
3
, dilute to volume with H
2
O, mix, and
transfer to an acid-washed polypropylene bottle. (
3
)
1 mg/kg
elements
.—Pipet 50.0 mL of 10 mg/kg intermediate stock solution
into a 500 mL acid-washed volumetric flask. Add 50 mL of trace
metal grade HNO
3
, dilute to volume with H
2
O, mix, and transfer to
an acid-washed polypropy lene bott le. (
4
)
0.5 mg/kg
elements
.—Pipet 25.0 mL of 10 mg/kg intermediate stock solution
into a 500 mL acid-washed volumetric flask. Add 50 mL of trace
metal grade HNO
3
, dilute to volume with H
2
O, mix, and transfer to
an acid-washed polypropy lene bott le. (
5
)
0.1 mg/kg
elements
.—Pipet 5.0 mL of 10 mg/kg intermediate stock solution
into a 500 mL acid-washed volumetric flask. Add 50 mL of trace
metal grade HNO
3
, dilute to volume with H
2
O, mix, and transfer to
an acid-washed polypropylene bottle. (
6
)
0.0 mg/kg elements
(blank)
.—Add 50 mL of trace metal grade HNO
3
into a 500 mL
ã
2009 AOAC INTERNATIONAL
AOAC Research Institute
Expert Review Panel Use Only