6 / 31
T
hiex
:
J
ournal of
aoaC i
nTernaTional
V
ol
.
99, n
o
.
4, 2016
917
only for calibration within the batch of test solutions with which
they were extracted. These standard extract solutions have the
same shelf life (i.e., approximately 16 h) as the other fertilizer
extracts and must be prepared fresh with each run. Calculations
for converting the percentage P
2
O
5
in these materials to
milligrams per liter P are provided in the
Calculations
section
(
see Alternative A
, section
H
). Fertilizer materials below 40%
P
2
O
5
(approximately 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
and
2015.18B
. Empirical calibration is not the preferred option and
should be used as a last resort.
The test solution and internal standard/ionic buffer solution
[
see Alternative A
, section
C(o)
] are blended using aY-connector
(Part No. 30703-90; Cole-Parmer, Bunker, CT) or T-connector
(Part No. 116-0522-01; Bran+Luebbe, Mequon, WI) just before
the nebulizer, using the conditions described in Table
2015.18D
.
E. Sample Preparation
Collect a primary field sample using one of the recommended
AOAC sampling procedures (i.e., Method
929.01
,
969.01
, or
992.33
) or other recognized protocol. Prepare solid fertilizer
materials by riffling [
see Alternative A
, section
B(g)
] the entire
laboratory sample to select an approximate 100 g subsample.
Grind the entire 100 g subsample [
see Alternative A
, section
B(h)
] to pass through a 0.50 mm mesh screen. Place the
ground analytical sample into a one-quart (0.946 L) glass jar
Table 2015.18B. ICP calibration standards from commercial custom blend stock standard solutions
Standard
ID
Volume,
mL
Citrate,
mL
Stock 1,
mL
a
Stock 2,
mL
b
P concn,
μg/mL
P
2
O
5
,
μg/mL
P
2
O
5
solution, %
P
2
O
5
fertilizer, %
K concn,
μg/mL
K
2
O,
μg/mL
K
2
O
solution, %
K
2
O
fertilizer, %
Blank
100
40
0
0
0
0
0
0
0
0
0
0
1
100
38 5 of Std 6
c
12
12
27.5 0.00275
1.4
360
434 0.04336 21.7
2
100
36 10 of Std 6
c
10
24
55
0.00550
2.7
300
361 0.03614 18.1
3
100
36 10 of Std 9
c
8
50
115
0.01146
5.7
240
289 0.02891 14.5
4
100
40
5
6
100
229 0.02291 11.5
180
217 0.02168 10.8
5
100
40
8
4
160
367 0.03666 18.3
120
144 0.01441
7.2
6
100
38
12 5 of Std 6
c
240
550 0.05499 27.5
63
76 0.00759
3.8
7
100
40
15
NA
d
300
687 0.06874 34.4
NA NA
NA
NA
8
100
38
20 5 of Std 6
c
400
917 0.09165 45.8
36
43 0.00434
2.2
9
100
38
25 5 of Std 6
c
500
1146 0.11457 57.3
18
22 0.00217
1.1
10
100
40
NA
15
NA
NA
NA
NA
450
542 0.05421 27.1
11
100
40
NA
20
NA
NA
NA
NA
600
723 0.07227 36.1
12
100
40
NA
25
NA
NA
NA
NA
750
903 0.09034 45.2
13
100
40
NA
30
NA
NA
NA
NA
900 1084 0.10841 54.2
14
100
40
NA
35
NA
NA
NA
NA 1050 1265 0.12648 63.2
a
Stock 1 = 2000 μg/mL P as PO
4
custom stock standard [
see Alternative
A, section
C(p)
].
b
Stock 2 = 3000 μg/mL K from KCl custom stock standard [
see Alternative A
, section
C(q)
].
c
Serial dilution from another standard (e.g., 5 of Std 6 = add 5 mL from Standard 6).
d
NA = Not applicable.
Table 2015.18C. Calibration criteria for direct-available P and soluble K by ICP-OES
Element ID
Wavelength, nm
a
Calibration range, μg/mL
Standards used
b
Curve fit
Spectral deconvolution
P
177.434 (
1
)
0–100
Blank, 1, 2, 3, 4
Linear
None
P
177.434 (
2
)
100–500
4, 5, 6, 7, 8, 9
Linear
None
P
178.222 (
1
)
0–100
Blank, 1, 2, 3, 4
Linear
None
P
178.222 (
2
)
100–500
4, 5, 6, 7, 8, 9
Linear
None
P
213.618 (
1
)
0–100
Blank, 1, 2, 3, 4
Linear
Cu 213.598
P
213.618 (
2
)
100–500
4, 5, 6, 7, 8, 9
Linear
Cu 213.598
P
214.914 (
1
)
0–100
Blank, 1, 2, 3, 4
Linear
Cu 214.898
P
214.914 (
2
)
100–500
4, 5, 6, 7, 8, 9
Linear
Cu 214.898
K
766.485 (
1
)
0–126
Blank, 1, 2, 3, 4
Quadratic
None
K
766.485 (
2
)
50–379
3, 4, 5, 6, 7
Quadratic
None
K
769.897
126–505
4, 5, 6, 7, 8
Quadratic
Possible LiNO
3
K
404.721
505–1046
8, 9, 10, 11, 12
Quadratic
None
a
The designators (
1
) and (
2
) are used to distinguish between the same wavelength selected twice to cover two separate concentration ranges.
b
The standards correspond to those listed in Table
2015.18A
.