AOA C Of fi cial Method 2011.14 AOAC Calcium, Copper, Iron, Magnesiu m, Manganese, Potassium, Phosphorus, Sodium, and Zinc in Forti fi ed Food Products
C. Reagents
(a) High-grade water water,, (18 M Ω ) ).—For .—For slurry preparation and/ or dilution. (b) Nitric acid (HNO3 ), 65% (w/v) (w/v).—Trace .—Trace metal grade throughout. (c) Hydrochloric acid (HCl), 37% (w/v) (w/v).—Trace .—Trace metal grade throughout. (d) Hydrogen peroxide (H 2O2 ), 97% (w/v).—Trace (w/v) .—Trace metal grade throughout.
Microwave Digestion and Inductively Coupled Plasma-Optical Emissio Emissio n Spectrometry First Actio n 2011 2011 Final Action 201 2013 3
(Applicable to analysis of calcium, copper, iron, potassium, magnesium, manganese, phosphorus, sodium, and zinc in fortified food products.) Upper limits (mg/kg): Ca (24 000); Cu (210); Fe (850); K (32 000); Mg (7500); Mn (20); Na (16 000); P (16 000); Zn (320). Limit of quantitation (LOQ; mg/kg): Ca (150); Cu (2); Fe (10); K (200); Mg (50); Mn (0.05); Na (100); P (100); Zn (5). See Tables 2011.14A – I for the results of the interlaboratory See study supporting acceptance of the method. [ Note Note:: Results shown in Tables 2011.14A – I are slightly different from those shown in J. AOAC Int . 92, 1484(2009) due to classical statistics applied instead of robust statistics applied in the tables included in J. in J. AOAC AOAC Int . 95, 177(2012).]
D. Determination
[Caution Caution:: Before using chemicals, refer to the supplier guide for chemical safety and/or other adequate manuals or safety data sheets approved by local authorities. Use fume hood and wear full personal laboratory protective clothing, gloves, and appropriate eye protection (safety glasses) when using glassware and preparing standards or test portions with acid solutions.] (a) Sample preparation.—( preparation.—(1 1) Test sample preparation.— preparation.— Homogenize a representative sample by grinding as finely as possible and/or by preparing a slurry with H2O. For example example:: Infant cereals and fortified milk powders, preheat water at 50°C. Prepare the slurry by weighing 10.0 ± 0.1 g test sample and place into a 100 mL Erlenmeyer flask; add 90.0 ± 0.1 g H 2O. Mix well with stopper. (2) Test portion preparation.—Accurately preparation.—Accurately weigh 0.50 ± 0.01 g test portion or sample mass on a dry weight basis in the prepared slurry to MDC vessel (1.00 ± 0.01 g into a 100 mL volumetric flask for MDO). [ Note: Note: An optimal analytical test portion mass of 0.5 g (1.0 g for MDO) is b ased on an empirical maximum energy release by the food of 3 kcal and 90–110% 90–110% recovery.] recovery.] Line the MDC vessel walls or Pasteur pipet with weighing paper during sample transfer to keep sample from adhering to sides of vessel or use a Pasteur pipet to transfer liquid samples. (Weigh fluid samples or test portion from slurr y test sample directly after mixing.) ( Note: Note: Remove weighing paper from sample prior to next step.) Carefully add 5.0 ± 0.1 mL HNO 3 into MDC/MDO vessel (and then 5 mL H 2O2 only into MDO vessel). Loosely cap MDC vessel without sealing.
Digestion A. Pri nci pl e
Test portion is heated at 200°C either with nitric acid in a closedTest vessel microwave digestion system (MDC) or with a combination of hydrogen peroxide, nitric acid, and hydrochloric acid in an openvessel microwave digestion system (MDO). B. Apparatus
Microwave. ____ Commercial MDC or MDO designed for Microwave laboratory use at 200 ± 20°C, up to 600 psi, and controlled temperature or pressure ramping capability. It is recommended that vessel design be selected that will withstand the maximum possible pressure (600 psi) since organic residues of rich-fat or rich-carbohydrate samples, if not given suf ficient time to predigest, will generate signi ficant pressure during digestion. Table 2011.14 2011.14A. A.
Interlaboratory study results for calcium Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
9732.17
99
152.10
1.56
264.03
2.71
0.68
9
NSg
Dietetic milk powder 2
5678.58
98
75.93
1.34
184.11
3.24
0.74
9
NS
Infant cereal
5983.54
100
210.58
3.52
251.67
4.21
0.97
9
NS
Pea Pe anu nutt but buttter NIS IST T SR SRM 23 2387
409 09..90 (411)h
100
13.37
3.26
28.20
6.88
1.06
9
NS
Wheat gluten NIST RM 8418
367.56 (369)h
100
3.51
0.95
12.90
3.51
0.53
7
2, 4C
Matrix
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
No. of labse Outlier labsf
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
Value Val ue in parentheses = Certified or reference values.
© 2013 AOAC INTERNATIONAL
Table 2011.14 2011.14B. B.
Interlaboratory study results for copper Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
5.92
NAh
0.09
1.57
0.29
4.97
0.41
9
NSg
Dietetic milk powder 2
6.19
101
0.19
3.06
0.38
6.12
0.50
9
NS
Infant cereal
2.01
NA
0.10
4.98
0.24
11.72
0.81
9
NS
Peanut butter NIST SRM 2387
4.98 (4.93)i
101
0.19
3.84
0.35
7.08
0.56
9
NS
Wheat gluten NIST RM 8418
5.64 (5.94)i
95
0.13
2.27
0.27
4.77
0.39
9
NS
Matrix
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
NA = Not available.
i
No. of labse Outlier labsf
Value Val ue in parentheses = Certified or reference values.
Predigest for at least 10 min at room temperature or until vigorous foaming subsides. Close MDC vessels and distribute onto microwave carousel to ensure uniform microwave power application on all samples. (3) Food-grade salt sample preparation.—W preparation.—Weigh eigh 0.20 ± 0.01 g food-grade salt into a 100 mL volumetric flask. Add deionized water and 10 mL HNO3. Dissolve salt and dilute to volume with deionized water. (b) Test portion digestion.—[ digestion.—[Caution Caution:: Application of microwave digestion systems involves hot pressurized acid solutions and concentrated acids. Use fume hood and wear full personal laboratory protective protective clothing, gloves and appropriate eye protection (safety glasses) when using glassware and preparing standards or samples with acid solutions. Follow manufacturer’s directions for safety risk and safety environment of microwave
systems. Never remove hot vessels from microwave; wait until they are near room temperature. Keep microwave door closed while vessels are hot. The door is the primary safety device if a vessel vents.] (1) Sample digestion.— With With power setting appropriate to MDC (maximum power of 1600 W) and MDO models (maximum power of 600 W), and number of v essels used, heat MDO vessels at 200 ± 20°C for 20 min or ramp MDC temperature from ambient to 200 ± 20°C in 15 min and hold at 200°C for 25 min. ( Note: Yellow vapors will be emitted during the hydrolysis in MDO vessels.) Carefully remove the MDO vessels. Allow the vessels to cool down to room temperature. Add 5 mL HCl 35% (w/v) into MDO vessels and heat MDO vessels at 200 ± 20°C for 20 min.
Table 201 2011.1 1.14C. 4C. Interlaboratory study results for iron Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
164.77
97
6.72
4.08
11.25
6.83
0.92
9
NSg
Dietetic milk powder 2
83.11
102
2.51
0.02
4.62
5.56
0.68
9
NS
Infant cereal
75.18
94
1.36
1.81
3.32
4.42
0.53
9
NS
Pea Pe anu nutt but uttter NIS IST T SR SRM M 23 2387 87
15..65 15 (16.4)h
95
0.65
4.17
1.95
12.45
1.18
8
6C
Wheat gluten NIST RM 8418
49.29 (54.3)h
91
0.93
1.89
1.54
3.12
0.35
8
2C
Matrix
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
No. of labse Outlier labsf
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
Value Val ue in parentheses = Certified or reference values.
© 2013 AOAC INTERNATIONAL
Table 201 2011.1 1.14D. 4D. Interlaboratory study results for potassium Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
8996.74
99
135.82
1.51
208.98
2.32
0.57
9
NSg
Dietetic milk powder 2
6443.79
100
65.17
1.01
130.04
2.02
0.47
9
NS
Infant cereal
6438.19
99
119.83
1.86
135.78
2.11
0.49
8
6SG
Pean Pe anut ut bu butt tter er NI NIST ST SR SRM M 238 2387 7
6200.2 6200 .24 4 (6070)h
102
148.84
2.40
187.59
3.03
0.70
9
NS
411.08 (472)h
87
10.19
2.48
31.31
7.62
1.18
9
NS
Matrix
Wheat gl gluten NI NIST RM RM 84 8418 a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
No. of labse Outlier labsf
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
Value Val ue in parentheses = Certified or reference values.
Cool vessels to room temperature before venting (MDC vessels). Transfer the MDC digests to 50 mL (100 mL for MDO) volumetric Note: A digestion is judged judg ed flasks. Dilute to volume with H 2O and mix. ( Note: complete when clear to yellow analytical solutions are produced.) Filter the digested solution using an ashless filter paper for turbid samples containing fat. Discard the first 20 mL filtrate and collect the remaining filtrate for analysis. [ Note Note:: Membrane disc filters (0.45 μm) are not recommended as they are generally not metal-free.] Transfer to polyethylene containers within 2 h. Dilute the samples that are found to be above the standard curve range or have total content of minerals higher than 1000 mg/L with H2O. [ Note: Note: Final dilutions require the addition of appropriate amounts of HNO3 for MDC digests (or appropriate amounts of combined acids for MDO) to maintain the proportion of 10% (v/v) HNO3 (15%, v/v, combined acids and hydrogen peroxide) in the final solution to be analyzed.]
(2) Digestion vessel decontamination decontamination.—Decontaminate .—Decontaminate the used MDC vessels with 5 mL HNO3 according to a program close to that in b(1). Wash the MDO glass volumetric flasks in a laboratory washing machine or soak overnight in a water bath with HNO3 (30%, v/v). Rinse with H 2O to avoid or to minimize risk of iron contamination. Dry MDC vessels at 60°C in drying oven for 1 h. Detection E. Principle
Digested test solution or an appropriate dilution is presented to the inductively coupled plasma-optical emission spectrometry (ICP-OES) instrument. The instrument is calibrated with acid(s) matched standard calibrant solutions. An ionization buffer (cesium) and internal standard(s) (chromium, indium, strontium, and yttrium) are used with or without appropriate in-line addition to
Table 201 2011.1 1.14E. 4E. Interlaboratory study results for magnesium magnesium Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
1746.74
98
38.46
2.20
51.03
2.92
0.56
8
2SG
Dietetic milk powder 2
439.31
99
5.57
1.27
13.37
3.04
0.48
9
NSg
Infant cereal
859.48
98
33.62
3.91
35.38
4.12
0.71
7
2, 6DG
1714.0 1714 .02 2 (1680)h
102
56.83
3.32
73.68
4.30
0.82
8
2SG
482.05 (510)h
95
8.52
1.77
16.53
3.43
0.54
9
NS
Matrix
Pean Pe anut ut bu butt tter er NI NIST ST SR SRM M 238 2387 7 Wheat gluten NIST RM 8418 a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
No. of labse Outlier labsf
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
Value Val ue in parentheses = Certified or reference values.
© 2013 AOAC INTERNATIONAL
Table Ta ble 2011.14F 2011.14F..
Interlab orato ry stud y resul ts for manganes e Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
7.54
NAh
0.14
1.88
0.67
8.82
0.75
9
NSg
Dietetic milk powder 2
0.40
99
0.01
2.56
0.0197
4.89
0.27
8
2C
Infant cereal
16.74
NA
1.14
6.79
1.44
8.61
0.82
9
NS
Pea Pe anu nutt but uttter NIS IST T SR SRM M 23 2387 87
15..85 15 (16.0)i
99
0.45
2.86
1.32
8.31
0.79
9
NS
Wheat gluten NIST RM 8418
12.87 (14.3)1
90
0.23
1.75
1.13
8.75
0.80
9
NS
Matrix
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
NA = Not available.
i
No. of labse Outlier labsf
Value Val ue in parentheses = Certified or reference values.
minimize easily ionizable element (EIE) effects and to correct for physical and chemical chemical interferences.
to each sample solution. A thermostated cyclonic spray chamber equipped with a micro-concentric nebulizer or other components designed to optimize aerosol and maximize precision was used. Follow manufacturer’s recommendations. Select peristaltic pump rotation speed, sample, and internal standard pump tubes of similar size to maximize mixing accuracy while maintaining the required detection levels. Ionization buffer (cesium chloride) is combined with the internal standard solution to compensate EIEs effects (e.g., K, Na, and Ca) in the plasma since certain food materials can contain substantial concentrations of these elements. This provides a signi ficant source of electrons in the plasma. The presence of ionization buffer in all samples and standards will minimize the effects of varying concentrations of EIEs in the sample. The solution presented to the nebulizer contains a maximum of 5000 mg/kg cesium for high-
F. Instrumentation
( Note: Note: ICP-OES equipments must be used by quali fied laboratory personnel who are familiar with the operation and maintenance of analytical systems and must follow manufacturer’s directions for safety risk, safety environment, and use of instruments.) (a) ICP-OES spectromet spectrometer er .—Instrument .—Instrument with axial, radial, or dual view grating con figurations and autosampler, capable of determining multiple wavelengths for each element of interest with the required sensitivity. A 3-channel peristaltic pump with or without appropriate in-line addition system (e.g., T connector) are linked between the peristaltic pump and nebulizer to avoid having to manually add ionization buffer and internal standards Table 2011.14 2011.14G. G.
Interlaboratory study results for sodium Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
1927.84
97
43.30
2.25
51.07
2.65
0.52
9
NSg
Dietetic milk powder 2
1769.19
96
18.64
1.05
33.39
1.89
0.36
9
NS
992.41
95
37.67
3.80
39.72
4.00
0.71
8
6SG
Pean Pe anut ut bu butt tter er NI NIST ST SR SRM M 238 2387 7
4924.1 4924 .12 2 (4890)h
101
158.61
3.22
158.61
3.22
0.72
9
NS
Wheat gluten NIST RM 8418
1346.82 (1420)h
95
19.56
1.45
26.85
1.99
0.37
8
6SG
Matrix
Infant cereal
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
No. of labse Outlier labsf
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
Value Val ue in parentheses = Certified or reference values.
© 2013 AOAC INTERNATIONAL
Table 2011.14 2011.14H. H. Interlaboratory study results for phosph orus Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
4138.12
101
84.53
2.04
115.22
2.78
0.61
9
NSg
Dietetic milk powder 2
3729.50
100
46.37
1.24
101.72
2.73
0.59
9
NS
Infant cereal
4341.81
101
60.56
1.39
111.79
2.57
0.57
9
NS
Pean Pe anut ut bu butt tter er NI NIST ST SR SRM M 238 2387 7
3444.0 3444 .06 6 (3378)h
102
98.10
2.85
111.97
3.25
0.69
9
NS
Wheat gluten NIST RM 8418
2050.02 (2190)h
94
37.86
1.85
67.15
3.28
0.65
9
NS
Matrix
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
No. of labse Outlier labsf
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
Value Val ue in parentheses = Certified or reference values.
salted food raw materials (e.g., culinary products or tastemakers) and a minimum of 500 mg/kg cesium (for main food samples); 20 mg/kg indium and 5 mg/kg strontium, yttrium, and chromium; less than half of each element concentration of the higher working standard Std6 and less than 0.5 g/kg total dissolved minerals. Power settings and nebulizer gas flow must be optimized to ensure that the Mg280.271:Mg285.213 ratio (Mermet principle of robust plasma) demonstrates robust operating conditions in accordance with the ratio established by the instrument manufacturer. manufacturer. At least three replicate readings of the same sample are performed, with relatively long integration times to minimize noise. (b) ICP wavelengths wavelengths.. ____ A number of recommended and alternative wavelengths may be used for the nine elements to be determined and internal standards. As a minimum, select one recommended and one alternative wavelength for each element
corrected by one recommended wavelength for appropriate internal standard. All responses responses for both recommended and alternative wavelengths for each element are corrected using only one internal standard line. The following is a list of wavelengths for each element (and its appropriate internal standard) in priority order that have been found acceptable for main foodstuffs: Wavelength (nm): Ca: 317.933 (In: 303.936); Cu: 324.754 (In: 303.936); Fe: 259.94 (Sr: 338.071); K: 766.491 (Sr: 460.733); Mg: 285.213 (In: 303.936); Mn: 257.610 (Sr: 338.071); Na: 589.592 (Sr: 460.733); P: 213.618 (In: 303.936); Zn: 213.857 (Sr: 338.071). Other wavelengths that are acceptable for both elements and internal standards could be used as con firmatory analytical lines or alternative wavelengths as certain recommended lines may not be available on some ICP-OES systems: Wavelength (nm): Ca: 317.933 (Y: 371.028); Cu: 324.754 (Y: 371.028), 327.395 (In:
Table 201 2011.1 1.14I. 4I. Interlaboratory study results for zinc zinc Mean, mg/kg
Rec., %
S r a
RSDr b, %
SRc, ng/g
RSDRd, %
HorRat
Chocolate milk powder
18.06
NAh
0.396
2.19
0.95
5.24
0.51
8
5SG
Dietetic milk powder 2
53.65
105
1.43
2.66
2.36
4.40
0.50
9
NS
Infant cereal
36.51
NA
0.92
2.25
1.27
3.44
0.37
9
NS
Pea Pe anu nutt but uttter NIS IST T SR SRM M 23 2387 87
26..64 26 (26.3)i
101
1.06
3.96
1.78
6.69
0.69
9
NS
Wheat gluten NIST RM 8418
49.30 (53.8)i
92
0.96
1.95
3.19
6.46
0.73
9
NS
Matrix
a
Sr = Standard deviation for repeatability (within laboratory).
b
RSDr = Relative standard deviation for repeatability repeatability..
c
SR = Standard deviation for r eproducibility (among laboratories).
d
RSDR = Relative standard deviation for reproducibility.
e
Reporting laboratories minus outliers.
f
C = Cochran outlier; outlier; SG = Single Grubbs outlier; DG = Double Grubbs outlier.
g
NS = Not significant.
h
NA = Not available.
i
No. of labse Outlier labsf
Value Val ue in parentheses = Certified or reference values.
© 2013 AOAC INTERNATIONAL
303.936 or Y: 371.028); Fe: 259.94 (Y: 371.028 or Cr: 283.563); Mg: 285.213 (Y: 371.028), 279.028 (In: 303.936); Mn: 257.610 (Sr: 460.733 or Y: 371.028); P: 178.222 (Sr: 460.733 or Y: 371.028); Zn: 213.857 (Sr: 460.733 or Y: Y: 371.028). Other lines of appropriate sensitivity, free of interferences or corrected for interferences in specific matrixes, may be just as acceptable. G. Reagents
(a) Ionization buffer/internal standard standard solution.—Weigh solution.—Weigh 1.27 g cesium chloride into a 1000 mL acid-washed volumetric flask [ Note: Note: This cesium 0.1% (w/v) solution was tested as the minimal recommended concentration required for element analysis in most food matrixes. Cs solution 1% (w/v) is recommended if an element is present at low concentration in high-salted food raw materials, e.g., culinary products or tastemakers, or if it is analyzed as an impurity in food-grade salts.] Add 40 mL indium 1000 mg/kg and 10 mL each of strontium, yttrium, and chromium 1000 mg/kg stock standard solutions, as internal standards. Add 10 mL HNO 3. Dilute to volume with H 2O, mix, and transfer to an acid-washed polyethylene bottle. ( Note: Note: Reagent concentrations assume the use of same pump tubing internal diameter for both internal standard/ ionization buffer and sample pump tubes using automatic addition.) (b) Stock standard solution solution.—W .—Working orking standards can be prepared from ICP-grade individual element 1000 mg/kg (e.g., for copper, iron, manganese, and zinc) and 10 000 mg/kg (e.g., for calcium, magnesium, phosphorus, potassium, and sodium) commercial stock standard solutions. However, it is also acceptable to use commercially prepared, custom blended stock standard mixtures containing all of the nine elements at appropriate concentrations. A number of companies provide this stock standard service. (c) Intermediate stock solution solution.—(Suggested .—(Suggested composition of the intermediate stock standard solution, in mg/kg: Ca = 1500; Cu = 10; Fe = 50; K = 2000; Mg = 500; Mn = 0.25; Na = 1000; P = 1000; Zn = 20.) Add into a 500 mL volumetric flask, 75 mL calcium 10 000 mg/kg, 5 mL copper 1000 mg/kg, 25 mL iron 1000 mg/kg, 100 mL p otassium, 25 mL magnesium 10 000 mg/kg, 0.125 mL manganese 1000 mg/kg, 50 mL sodium 10 000 mg/kg, 50 mL phosphorus 10 000 mg/kg, and 10 mL zinc 1000 mg/kg. Add 10 mL mL HNO3 and dilute to volume with H 2O. (d) Working standard solutions.—Standards solutions.—Standards prepared from intermediate stock standard solution are designed to have the same acid concentration as digested test solutions ( i.e., 10%, v/v, HNO 3) for MDC or 15% (v/v) for MDO using combined acids (HNO3, H2O2, and HCl). (1) Std6 .—Pipet .—Pipet 15.0 mL intermediate stock standard solution into a 100 mL acid-washed volumetric flask. Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO), dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle. (2) Std5 Std5.—Pipet .—Pipet 10 mL intermediate stock standard solution into a 100 mL acid-washed volumetric flask. Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO), dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle. (3) Std4 Std4.—Pipet .—Pipet 5.0 mL intermediate stock stock standard solution into a 100 mL acid-washed volumetric flask. Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO), dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle. (4) Std3 Std3.—Pipet .—Pipet 2.0 mL intermediate stock standard solution into a 100 mL acid-washed volumetric flask. Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO), dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle.
Table 201 2011.1 1.14J. 4J. Suggested concentration of the six standard solutions, mg/kg Element
Std0
Std1
Std2
Std3
Std4
Std5
Std6
Ca
0
7.5
15
30
75
150
225
Cu
0
0.05
0.1
0.2
0.5
1.0
1.5
Fe
0
0.25
0.5
1.0
2.5
5
7.5
K
0
10
20
40
100
200
300
Mg
0
2.5
5
10
25
50
75
Mn
0
0.005
0.0125
0.025
0.0375
Na
0
5
10
20
50
100
150
P
0
5
10
20
50
100
150
Zn
0
0.1
0.2
0.4
1.0
2.0
3.0
0.00125 0.0025
(5) Std2 Std2.—Pipet .—Pipet 1.0 mL intermediate stock standard solution into a 100 mL acid-washed volumetric flask. Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO), dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle. (6 ) Std1 Std1.—Pipet .—Pipet 0.5 mL intermediate stock standard solution into a 100 mL acid-washed volumetric flask. Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO), dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle. (7 ) Blank .—Add .—Add 10 mL HNO3 (MDC) or 15 mL combined acids (MDO) into a 100 mL acid-washed volumetric flask, dilute to volume with H2O, mix, and transfer to acid-washed polyethylene bottle. All calibration calibration solutions when made are stable stable for 1 week week in glass volumetric flasks. (e) Sampler wash solution, 10% HNO3 (v/v) (v/v).—Dilute .—Dilute 100 mL trace metal-grade HNO 3 to 1000 mL with H 2O. H. Determination
Make a calibration curve using either weighted linear or quadratic regression with correlation coef ficients of at least 0.9999 from seven standards prepared from intermediate standard solution, including a blank (Std0) and six suggested concentrations of the standard solution (Std1–Std6) shown in Table 2011.14J and expressed in mg/kg. Analyze test solutions using an ICP-OES instrument calibrated with the working standard solutions. Insert a working standard or other suitable quality control solution every 10 test portions to monitor for instrument drift. The inclusion of a digestion blank, a sample duplicate, and known reference materials materials is highly encouraged. I. Calculations
The concentration (C) of each element, in mg/kg, is calculated as follows: C
aVF m
where C = concentration in the test portion sample (mg/kg); a = concentration (mg/L) of the element in the digest solution as obtained from instrument; V = volume (mL) of the test solution after being made up (i.e., 50 mL for MDC and 10 0 mL for MDO); F = dilution factor of the test solution; m = weight of the test portion (g). Reference: J. Reference: J. AOAC Int . 95, 177(2012) © 2013 AOAC INTERNATIONAL