AOACSPIFANMethods-2017Awards

Pacquette & Thompson:

ournal of

AOAC I

nternational

ol.

98, N

6, 2015

1703

were equipped with modern collision/reaction cells that are

thought to be necessary to avoid the Ar/C spectral interferences

on the major Cr and Se isotopes.

Before actual MLT study samples were analyzed, each

participating laboratory was asked to set up the method and

evaluate the linearity and the method LOQ with their given

instrument model. This exercise is identical to what is done to

transfer a mineral method to another site in the authors’ internal

laboratory network, as it quickly identifies problems in procuring

or preparing suitable standards and standardblanks, or inotherwise

setting up the instrument parameters. To check the linearity,

standards were analyzed and the calibration curve prepared on

each of 3 separate days. On each day, working standards at the

lowest concentration level (WS1) and at ½WS1 were analyzed as

samples, and then their calculated concentrations were compared

to their nominal concentrations. The mean recovery of each

standard versus its nominal concentration (i.e., the calibration

residual) had to be within 5%. All laboratories passed this test

except Laboratories 9 and 11, both of which failed only at the

lowest standard level for Se (Table 1). For these laboratories,

the practical LOQ (PLOQ) for Se was therefore equal to WS1,

whereas the other laboratories could analyze as low as ½ WS1

in concentration.

The second setup test was to analyze the sample blank on

5 separate days (done at same time as the linearity study, plus

two more days), calculating the SD of these results, multiplying

that by 10, and then adding that result to the blank mean. This

LOQ was multiplied by the method’s dilution factor of 50 to

arrive at the approximate LOQ in terms of sample weight. The

SMPRs state an LOQ of 20 ng/g Cr and Mo and 10 ng/g Se on

a ready-to-feed (RTF) basis. Table 1 shows the prework results

from the participating laboratories (Laboratories 6 and 7 dropped

out about this time). Note that it is desirable to have low,

consistent blanks for good sensitivity, as well as the linearity, to

avoid excessive calibration bias. These trials immediately pointed

to Laboratories 1 and 10 as having potential problems; they were

allowed to proceed with the MLT, but indeed Laboratory 1’s

data were eventually rejected in total. The prework results for

Laboratory 10 may not have been so ominous because it did not

submit all the data, and the PLOQs could not be calculated.

The final prework for the participating laboratories was to

analyze the NIST Standard Reference Material (SRM) 1849a

sample. All laboratories passed this test by producing Cr, Mo, and

Se results within 5% of the certified means (data not shown, but

similar to data collected during the MLT, which is tabulated later).

The fact that Laboratories 1 and 10 produced good results on the

SRM might be attributed to the relatively high concentration of

these elements in the SRM. It should be noted that six of the nine

laboratories determined Na, K, P, Mg, Ca, Fe, Cu, Zn, and Mn

concurrently with the Cr, Mo, and Se with good precision and

accuracy. This work was done under the direction of the Study

Director. Results on SPIFAN samples are published in this

issue of

J. AOAC Int.

[AOAC First Action Method

2015.06

Thompson, J.J., Pacquette, L., & Brunelle, S.L. (2015)

J. AOAC

Int.

, 1711–1720]. Method

2011.19

appears to be viable as a

12-element method, not just as a method for ultratrace elements.

Each participating laboratory received blind duplicates of

seven of the SPIFAN matrixes (this study used the original

SPIFAN set) for a total of 14 samples to test. NIST SRM 1849a

was not included as a blind sample, but rather the participants

were instructed to analyze it concurrently with the other samples

as if it were a control sample. The seven matrixes tested were an

infant formula partially hydrolyzed milk-based powder, an adult

nutritional low-fat powder, an adult nutritional milk protein-

based powder, a child formula powder, an infant elemental

powder, an adult high protein nutritional RTF liquid, and an adult

high-fat nutritional RTF liquid. Only two infant formula types

were chosen (there were four more in the SPIFAN set) because

they were known to be unfortified in Cr and Mo and would not

yield useful information.

Participants were asked to reconstitute all powders prior to

analysis with the exception of SRM 1849a, which was unblinded

but rather easily identified by its sachet anyway. Participants used

a direct weight of 0.2 g SRM powder, which has proven to be

homogeneous for minerals at this weight through extensive use in

the authors’ laboratories. All other powders were reconstituted by

either dissolving 20 g powder in enough laboratory water to make

200 g solution, i.e., a 10% (w/w) reconstitution, or by following

the official method with the SPIFAN-recommended 25 g sample

+ 200 g water (11.1%, w/w). Some laboratories asked to work

with the 10% reconstitution rates, as this is certainly an easier

Table 1. Set-up tests for participating laboratories

Lab

LOQ Cr (20 ng/g

required), ng/g

PLOQ Cr, µg/L

LOQ Mo (20 ng/g

required), ng/g

PLOQ Mo, µg/L

LOQ Se (10 ng/g

required), ng/g

PLOQ Se, µg/L

0.4

0.2

0.4

0.2

0.4

0.2

0.4

0.2

0.4

0.2

0.4

0.2

0.4

Laboratories 6 and 7 dropped out at this time and Laboratory 10’s data were incomplete. A PLOQ of 0.4 µg/L Cr/Mo and 0.2 µg/L Se along with an

LOQ below 20 ng/g (10 ng/g for Se), was desired.

See

text for details.

158