Microwave Calibration
Brenda R. Cleveland
Plant/Waste/Solution/Media Section
NCDA&CS Agronomic Division
Presentation Today
Microwave Calibration
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Procedure
But first . . .
June 21, 2011
NCDA&CS Agronomic Division
PWSM Section
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Legislature enacted the service in 1974
Analyze about 30,000 up to 35,000
samples annually
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18,000 waste
9-10,000 plant (this year >11,300)
2-3,000 solution and
2-300 media
June 21, 2011
NCDA&CS Agronomic Division
PWSM Section
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Analyses (N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B,
Na)
Fees
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Routine $5
Research $12
Out-of-State (2005) $25
Plus fees for additional tests
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NO3-N/NH4-N, heavy metals, CCE ($10 each)
Petiole NO3-N, Mo, Cl ($2 each)
June 21, 2011
NCDA&CS Agronomic Division
PWSM Section
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Six full-time laboratory staff plus 2-3
additional staff during busy season
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Chemistry Lab Supervisor (Mark Buchanan)
Chemist I (Jan Queen)
3 Chemistry Technician III (Ying Kritalugs,
Donta Baker and Kyle Crump)
3 Research Technicians (Karla Barefoot, Ma
Kpa, Brenda Woodall)
June 21, 2011
NCDA&CS Agronomic Division
PWSM Section
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One Agronomist (Dr. Michelle McGinnis)
One Processing Assistant (Darlene
Carpenter)
One Section Chief (Brenda Cleveland)
June 21, 2011
NCDA&CS Agronomic Division
Microwave Procedure for ICP
Analysis
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Add 10 ml of nitric acid to each sample;
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0.5 g plant/solid waste or 5 ml liquid waste
Space samples evenly in microwave
Digest for 30 minutes at 210 watts
Flush sides of the flasks with hydrogen peroxide
Digest for 4 minutes more at 390 watts
Actual time varies depending upon the # of samples
June 21, 2011
NCDA&CS Agronomic Division
Quality Control
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To verify that the samples
have been cooked at
the correct power?
(210 then 390 watts)
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calibrate
adjust
NIST sample
June 21, 2011
NCDA&CS Agronomic Division
Calibrate
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Calibration is the process of adjusting
the output or indication on a measurement instrument to agree with value of
the applied standard, within a specified
accuracy
Assures that microwave power is accurate
and samples are digested according to the
SOP
June 21, 2011
NCDA&CS Agronomic Division
Calibration
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Monthly calibration of all microwaves
Followed by analysis of a NIST sample to
verify that our procedure from start to
finish
June 21, 2011
NCDA&CS Agronomic Division
Calibration - Preliminary
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Microwave is warmed
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June 21, 2011
Remove turntable
Cook 1000 ml of water
in a beaker at full
power (100%) for 5
minutes
Use room temperature
DI water
NCDA&CS Agronomic Division
Calibration - Second
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Watts determined at
four locations
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June 21, 2011
front right
front left
back right and
back left
NCDA&CS Agronomic Division
Determine Power for 210 Watts
1. Measure initial temp of
1000 ml DI water
2. Place beaker in the front
left section of cavity
3. Cook 2 minutes at 210
watts
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36% power
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front right, left rear and
right rear
4. Remove and stir then
measure final temp
5. Repeat steps 1-4:
June 21, 2011
NCDA&CS Agronomic Division
Watts Calculation
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Calculate the watts for
each of these locations
and runs
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T [final]-T [initial] * 35
Calculate the average
If the average does ≠
210 watts OR
If the watts of any of
the locations is very
different from the others
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Repeat the steps
June 21, 2011
NCDA&CS Agronomic Division
Determine Power for 390 Watts
1. Same steps
 49% power
2. And calculations
June 21, 2011
NCDA&CS Agronomic Division
Quality Control

To verify that the samples
have been cooked at
the correct power?
(210 then 390 watts)
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calibrate
adjust
NIST sample
June 21, 2011
NCDA&CS Agronomic Division
That’s It
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Any Questions
Thanks
June 21, 2011
NCDA&CS Agronomic Division