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E.
Procedure
– Select sample weights between 1 and 5 grams based on expected concentration of the
two nutrients in each sample. Adjust the final volume to 10 mL (sample + acid +water = 10 mL)
prior to microwave digestion which makes acid concentration around 3.5 N.
a.
Powder Products
i.
Powder products are reconstituted with water prior to analysis, typically 11.1%
w/w (25g powder added to 200g water)
b.
Sample Analysis
i.
Weigh sample into a tarred microwave reaction vessel
ii.
Add 125 µL of the mixed intermediate internal standard solution
iii.
Add water to the sample to achieve a total sample volume of 10 mL after the
addition of the nitric acid.
iv.
Add 2.2 mL 70% nitric acid and seal the vessels
v.
Vortex the vessels for 20 seconds, then put caps on.
vi.
Complete microwave digestion using the conditions defined in Table 3.
Table 3. Microwave Digestion Parameters
1
Power
100 % (1600 W)
2
Ramp to temperature
10 minutes
3
Hold time
40 minutes
4
Temperature
120 ºC
vii.
Uncap each vessels after cool down (by air) , add 3.5 mL concentrated
ammonium hydroxide to each vessels.
viii.
Vortex each sample for 20 seconds
ix.
Allow the samples to stand in a hood for 30 minutes.
x.
Vortex the samples for 20 seconds
xi.
Transfer 1 mL of the digested sample into a disposable centrifuge tube containing
25 mL laboratory water
xii.
Vortex for 20 seconds
xiii.
Filter ~ 1 mL of the diluted sample using a 0.45 µm nylon syringe filter
xiv.
Transfer 0.1 mL of this filtered solution into an autosampler vial containing 0.9
mL acetonitrile. Mix well. The sample is ready for analysis.
xv.
Samples diluted into acetonitrile are stable for 24 hours when stored on a
refrigerated (8ºC) autosampler. Filtered samples (aqueous) and WS are stable for
7 days stored 2-8ºC.
F.
Instrument Operating Conditions
a.
Mass Spectrometry Conditions
- See Tables 4 and 5
b.
UPLC Conditions
– See Table 6
c.
UPLC analysis
.—Column stability was improved by storing the column in water /
acetonitrile 5/95 (v/v) without additives (recommended by column supplier). After
verifying equilibration of the UPLC system, inject the mid-level working standard 4
times to verify the system suitability. The %RSD of the peak areas from these
injections should be less than 5%. Once system suitability has been established, inject
Candidates for 2016 Method of the Year
328