HGA 4 Method Development

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Method Development

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Method Development in GF-AA
 Introduction
- Look up the default temperature program
- Run a series of blanks
- Run a standard using default conditions, assess the
performance of the instrument (e.g.. check characteristic
mass)
- Optimize dry, pyrolysis, and atomization steps for the
particular application
- Analyze spike of representative sample(s) or standard
reference material to assess validity of the method
- Run a Standard Reference Material
- Check precision

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Find the Defaults
Don’t forget the modifier!

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Method Development
Dry Steps
 Optimization of Sample Drying
- Drying should be smooth with no boiling or spattering
- Use pre-heated injection whenever possible
(temperature near or at the beginning of the initial dry
step)
- With platform in place, drying temperature is usually 20-
40o
C above the boiling point of the solvent; can vary
depending on the temperature of the cooling water
- The default programs are set for 25 mL of solution

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Method Development
Dry Steps
 Optimization of Sample Drying (cont.)
- In THGA, verify drying without the extraction
system first, watching for condensation on the
small dental mirror
- A two-step dry is excellent for insuring drying of
the sample
- The sample should be dry with 10 sec. remaining
in the dry step
- A completely dry sample will prolong tube lifetime

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Typical Program
Dry Step(s)
Conditions:
20 mL sample
5 mL modifier
THGA
Two step dry

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Method Development
Sample Pyrolysis
 Optimization of Sample Pyrolysis
- Original guidelines for wall atomization were a 1 sec.
ramp for each 100o
C between the final dry step and the
pyrolysis step. This approach has proven to be useful
using the platform in analyzing diverse environmental
samples.
- Typically, it is sufficient to hold at the pyrolysis
temperature for 15-20 sec. depending on the sample.
- If the samples are relatively free of matrix, it may not be
necessary to pyrolyze the sample at all. If this is the
case, omit the modifier and the pyrolysis step

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Method Development
Pyrolysis Study in a THGA
0
0.05
0.1
0.15
0.2
0.25
200 400 600 800 1000 1200
Temperature o
C
Abs-sec
PO4 + Mg(NO3)2
PO4
Pd + Mg(NO3)2

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Conditions:
20 uL sample
5 uL modifier
THGA
Typical Program
Pyrolysis
Using the
Phosphate/Magnesium
Nitrate Modifier

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Method Development
Pre-Atomization Cool Down Step
 Pre-Atomization Cool Down Step
- Studies indicated it is often advantageous to cool the tube off prior to
atomization in a HGA
- Heating rate is maximized
- Careful measurements indicated a broader isothermal zone is present
within the graphite tube

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Method Development
Atomization Study
 Optimization of Atomization
- Generally, THGA furnaces utilize atomization
temperatures that are 200-300o
C lower than a
HGA
- Many furnace users rely on peak shape and/or
sensitivity to determine the optimum temperature
- Generally, in a HGA, a symmetrical peak (peak
height = peak area) is considered “ideal”

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HGA Platform Atomization
“Ideal” Peak

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HGA Method Development
Pb: Effect of Atomization Temperature
0.610
0
5.00
Time (sec)
1400o
C
1600o
C
1800o
C
2200o
C

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THGA Method Development
Pb: Effect of Atomization Temperature

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Conditions:
20 uL sample
5 uL modifier
THGA
•0 sec ramp
•Stopped gas flow
•Read step
Method Development
Atomization

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Method Development
Clean-out
•Gas on
•Do not exceed
2650 degrees
More aggressive clean-out
may be necessary for
refractory elements

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Furnace Program Temperatures
Furnace
Step
Analyte
Dependent
Matrix
Dependent
Volume
Dependent
Dry No Yes Yes
Pyrolysis Yes Yes No
Atomization Yes No No

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Quality Control Methods
 Precision
 Accuracy
 Spike Recovery
 Reference Samples

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Using the Software for
Quality Control
 Checks
- Calibration Curves
- Spike Recoveries and Duplicates
- Precision
 QC
- After calibration, ongoing, at the end
- Actions based on when it fails
- Use the software to implement your QC policy

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Checks - Calibration

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Checks - Spikes and Duplicates

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Checks - Precision

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QC - Identifying

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QC - Scheduling

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QC - Actions (Calib. and Periodic)

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QC - Actions (End and Retry)

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Reference Materials
 NIST, National Institute of Standards and
Technology
 Environmental Resource Associates
 High Purity Standards
 Brammer Standard Co.
 Conostan Metallo-Organic Standards
 BCR, European Community

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Graphite Furnace
Atomic Absorption
Workbook Exercises