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olong
et al
.:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
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Table 4. Precision in SPIFAN matrixes and NIST SRM 1849a
Matrix
a
Results
b
Interday
mean
Mean
RSD
r
, % RSD
iR
, %
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Adult RTF, high fat
c
Mean, mg/100 g
159
156
161
166
169
160
162
RSD
r
, %
1.66
1.82
1.02
1.82
1.26
0.44
1.34 3.04
SRM 1849a (701±17 mg/100 g)
Mean, mg/100 g
716
706
713
705
705
711
709
RSD
r
, %
0.23
1.22
0.56
0.44
0.70
0.20
0.56 0.80
Pediatric powder
Mean, mg/100 g
342
342
340
350
347
344
344
RSD
r
, %
0.98
1.24
0.13
0.55
1.22
0.82
0.82 1.23
Adult powder, milk-protein-based
Mean, mg/100 g
321
312
311
325
318
319
318
RSD
r
, %
1.05
0.89
0.26
1.80
1.11
0.89
1.00 1.77
Infant powder, soy-based
Mean, mg/100 g
516
521
509
524
517
518
517
RSD
r
, %
0.76
0.23
1.09
0.10
0.14
0.27
0.43 1.01
Infant RTF, milk-based
Mean, mg/100 g
44.9
45.3
46.5
45.5
45.6
46.3
45.7
RSD
r
, %
0.16
1.58
1.14
1.03
1.24
0.46
0.94 1.45
Adult powder, low-fat
Mean, mg/100 g
354
339
347
366
355
359
353
RSD
r
, %
0.13
1.69
1.14
0.43
1.80
0.99
1.03 2.66
Adult RTF, high protein
Mean, mg/100 g
153
157
156
160
160
157
157
RSD
r
, %
0.35
1.47
1.45
1.93
0.44
1.36
1.17 1.78
Infant powder, elemental
Mean, mg/100 g
360
353
354
373
367
371
363
RSD
r
, %
0.49
1.26
0.60
0.99
0.96
0.38
0.78 2.34
Infant powder, partially hydrolyzed soy-based
Mean, mg/100 g
419
415
413
427
428
421
421
RSD
r
, %
0.21
1.57
0.24
0.42
0.66
0.67
0.63 1.53
a
Two samples each of the 10 SPIFAN matrixes were tested.
b
Concentrations on an “as-is” basis.
c
RTF = Ready-to-feed.
Accuracy/Recovery
Accuracy was demonstrated in the precision study with the
use of SRM 1849a (Table 4). All results obtained fell within
the certified concentration range of the method. The accuracy
values ranged from 99.9 to 102.3%, meeting the requirements
of SMPR 2014.015.
Samples of 10 SPIFAN matrixes spiked at two levels (50
and 100% overspikes) were prepared and tested in duplicate
on 3 days for recovery. Results are presented in Table 5.
Within-day recoveries across the matrixes varied between 100.3
and 103.3%, and among-day recoveries varied between 100.9
and 102.5%, all within the 95 to 105% requirement.
LOQ
The minimum silver nitrate consumption end point volume
that can be evaluated by the Metrohm potentiometric titration
system is in the range 0.0 to 0.05 mL, depending on the
instrument sensitivity conditions and day-to-day variation. A
0.05 mL minimum silver nitrate consumption end point volume
is more stable than a 0.02mLcondition and is able to be validated
via appropriate dilution of the NaCl SS. This translates to an
estimated LOQ of 20 mg/100 g chloride on a powder basis and
a 2.2 mg/100 mL chloride result on a ready-to-feed basis (25 g
milk powder reconstituted to total 225 g).
83