AOAC Final Action Methods in 2017

P ACQUETTE & T HOMPSON : J OURNAL OF AOAC I NTERNATIONAL V OL . 101, N O . 2, 2018 541

Table 2015.06B. Microwave operating parameters: Stages 1 and 2 are operated sequentially, without removing vessels from the oven

Table 2015.06C. Typical ICP-MS parameters for the Agilent 7700x a

RF power, W b RF matching, V b

1600

1.8

Stage 1 sample digest

Sampling depth, mm

9 0

1 2 3 4 5 1 2 3 4 5

Power, W

1600 (100%)

Extract 1 lens, V

Ramp to temperature, min

20 20

Carrier gas, L/min

0.9 0.2

Hold time, min

Make-up gas, L/min

Temperature, °C

180

Nebulizer (glass concentric)

MicroMist

Cool down, min

20

Spray chamber temperature, °C

2

Stage 2 sample digest

Interface cones

Ni

He cell gas flow rate, mL/min H 2 cell gas flow rate, mL/min Nebulizer pump rate, rps

4.5 4.2

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Power, W

1600 (100%)

Ramp to temperature, min

20 20

0.1 (0.5 mL/min)

Hold time, min

Peristaltic pump tubing

White/white, 1.02 mm id Blue/yellow, 1.52 mm id

Temperature, °C

200

Drain tubing

Cool down, min

20

a The isotopes used for analysis are 23 Na, 24 Mg, 31 P, 39 K, 44 Ca, 52 Cr, 55 Mn, 56 Fe, 63 Cu, 66 Zn, 78 Se, and 95 Mo, with 72 Ge and 130 Te as internal standards. b RF = Radiofrequency.

Total, h

2

determined in He collision mode, using KED. Te must be used as the ISTD for Se determinations, and we recommend that low levels of Se be determined in H 2 mode, i.e., reaction mode. Depending on the instrument model, it may not be possible to easily switch between helium and hydrogen mode. In such a case, follow the instructions of the instrument manufacturer for changing from helium to hydrogen mode and analyze Se separately from the other elements. Alternatively, verify in separate experiments that the practical limit of quantification (PLOQ) for Se is at or below 10 ng/g in the sample when using an alternate collision/reaction gas. (c) Typical calibration correlation coefficients are 0.9995 or better for all analytes, but suitability is determined by calibration residuals as follows. Analyze calibration standard (Cal Std) 3 or another suitable QC solution every 10 test portions to monitor for instrument drift and linearity (the result must be within 4% of the standard ’ s nominal concentration). The inclusion of a method blank (run as a sample; its measured concentration must be < half of the lowest calibration standard), and known reference materials serving as control samples (recovery check within control or certified limits) are mandatory for good method performance. Duplicate samples are optional. If used, the mean result is reported and appropriate criteria based upon the data would be a relative percent difference within 10% for Cr, 7% for Se, and 5% for all other elements. If any of these QC checks fail, results should be considered invalid. (d) The order of analysis should be calibration standards, followed by rinse, blank check, check standard, control sample, sample, sample duplicate (if used), and, finally, a repeated check standard. Sample concentrations in nanograms per gram are automatically calculated by the software using a nonweighted least-squares linear regression calibration analysis to produce a best-fit line: y = ax + blank Note that, the sample blank is identical to the Cal Blk in this method and is essentially zero because high-purity reagents are used. G. Calculations

The analyte concentration in the sample is calculated:

y − blank a

x =

× DF

where x = analyte concentration (nanograms per gram); y = analyte-to-ISTD intensity ratio, which is the measured count of each analyte ’ s standard solution data point in the calibration curve divided by the counts of the ISTD at the same level; similarly, blank = analyte-to-ISTD intensity ratio, which is the measured count of the blank standard solution data point in the calibration curve divided by the counts of the ISTD at the same level as the blank standard solution; a = slope of the calibration curve (mL/ng); and DF = the dilution factor, or the volume of the sample solution (milliliters) divided by sample weight (grams). This method is very well characterized. It has undergone a thorough Single Laboratory Validation (SLV) using AOAC guidelines to probe its linearity, LOQ, specificity, precision, accuracy, and ruggedness/robustness. Two independent MLT protocols were carried out to measure the reproducibility of the method, and a special study focused on the performance of the method at very low levels between the lowest calibration standard and the actual LOQ as measured from digested and undigested blanks. Accuracy of the ICP-MS results was confirmed by comparing the mean MLT results from Method 2015.06 to those fromMethod 2011.14 , which employed ICP-AES testing on the same sample set (data to be published). Based upon all these data, Table 2015.06D summarizes the latest figures of merit for this method. The last row gives the recommended analytical ranges of this method to fully meet the SMPR requirements. The minimum limits for Mn, Cu, and Fe are slightly above the SMPR criteria, which were very aggressive for these analytes – well into the inherent level for the kinds of products that apply to this method. Note that the method can be used for analyte concentrations in the sample below this range down to the H. Method Validation