AOAC Final Action Methods in 2018

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) and heat MDO vessels at 200 ± 20°C for 20 min. Cool vessels to room temperature. Transfer the digests quantitatively to 100 mL volumetric flasks or 25 mL autosampler tubes. Dilute to volume with H 2 O and mix. [ Note : Final dilutions in volumetric flasks (e.g., 50 mL) or autosampler tubes (e.g., 25 mL) require the addition of appropriate amounts of combined acids for MDO to maintain the proportion of 15%, v/v, combined acids and hydrogen peroxide in the final solution to be analyzed.] ( Note : A digestion is judged complete when a clear to yellow analytical solution is produced.) ( 3 ) 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 : Membrane disc filters (0.45 µm) are not recommended as they are generally not metal-free.] ( 4 ) Transfer to polyethylene containers within 2 h. ( 5 ) If samples are found to be above the standard curve range or have total content of minerals higher than 1000 mg/L, dilute with with H 2 O. [ Note : Final dilutions require the addition of appropriate amounts of HNO 3 for MDC digests (or appropriate amounts of combined acids for MDO) to maintain the proportion of 10% (v/v) HNO 3 (15%, v/v, combined acids and hydrogen peroxide) in the final solution to be analyzed.] ( f )  Digestion vessel decontamination .—Decontaminate used MDC vessels with 5 mL HNO 3 according to a program close to that in e ( 1 ). Dry MDC vessels at 60°C in drying oven for 1 h. Wash the MDO glass volumetric flasks or tubes in a laboratory washing machine or soak overnight in a water bath with HNO 3 (30%, v/v). Rinse with H 2 O to avoid or to minimize risk of iron contamination. E. Determination ( a )  ICP-AES setup .—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 significant 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-salt 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. At least three replicate readings of the same sample are performed, with relatively long integration times to minimize noise. ( b )  ICP wavelengths .—A number of recommended and alternative wavelengths may be used for the nine elements to be determined and internal standards (Table 2011.14L ). As a minimum, select one recommended and one alternative wavelength for each element corrected by one recommended wavelength for appropriate internal standard. All responses for both recommended and alternative wavelengths for each element are corrected using only one internal standard line. The following is

Table 2011.14L. Selected emission wavelengths and second- check wavelengths and interferences for determination by ICP-AES

Emission wavelength, nm

Type of “line”

Element

Interference

Ca

210.324 211.276 317.933 422.673 324.754 327.395 238.204 259.940 239.563 766.491 769.897 279.800 285.213 257.610 293.306 259.372 588.995 589.592 178.222 213.618 214.914 202.548 213.857 360.074 371.028 410.237 303.936 338.071 460.733 283.563

Ion Ion Ion

Y

Atom

Cu

Atom Ti, Fe

Atom

Fe

Ion Ion Ion

K

Atom Atom

Mg

Ion

Fe

Atom

Mn

Ion

Fe, Mo, Cr, Al, Fe

Ion Ion

Na

Atom Atom Atom

Ar

P

Atom Cu, Fe, Mo, Zn Atom Cu, Al, Mg

Zn

Ion

P

Atom

Y (internal standard)

Ion Ion

Atom Atom

In (internal standard) Sr (internal standard)

Ion

Atom

Cr (internal standard)

Ion

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); and Zn: 213.857 (Sr: 338.071). Other wavelengths listed inTable 2011.14L that are acceptable for both elements and internal standards could be used as confirmatory analytical lines or alternative wavelengths as certain recommended lines may not be available on some ICP-AES systems. Other lines of appropriate sensitivity, free of interferences or corrected for interferences in specific matrixes (e.g., milk and milk products), as listed in Table 2011.14L , may be just as acceptable.

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