graphite furnace analysis
TRANSCRIPT
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Graphite Furnace Atomization
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Limitations of Flame Atomization
Sensitivity is generally limited to mg/L concentrations• Relatively poor nebulization efficiency
– Only ~ 10 % of sample reaches flame• Short residence time of atoms in the optical path (~10-4 sec.)
– Large dilution of the aerosol with flame gases– Dilution factor ~ 10,000 times
Sample volume required is mLs
Requires use of flammable gases• Unattended operation is not recommended
Sample must be a solution with a viscosity similar to water• Must not contain excessive amounts of dissolved solids
Ground state atom formation subject to many interacting variables– Flame gases– Matrix component - analyte interaction– Chemical interferences– Dissociation of analyte molecular species
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Benefits of Graphite Furnace Atomization
Entire sample is atomized at one time
Free atoms remain in the optical path longer
Enhanced sensitivity
Reduced sample volume
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Flame vs Furnace Sensitivity
Ab
sorb
ance
100 g/L Pb @ 217.0 nm
0.936
0.004
Flame Signal
Furnace Signalfor 10 L
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Flame vs Graphite Furnace AAS
Criteria Flame Furnace
Elements 67 48
Sensitivity ppm - % ppt - ppb
Precision Good Fair
Interferences Few Many
SpeedRapid Slow
Simplicity Easy More complex
Flame Hazards Yes No
Automation Yes Yes (unattended)
Operating Cost Low Medium
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Detection Limit Comparison (g/L)
Element Flame Furnace
Ag 1.7 0.020
Al 20.0 0.10
As 42.0 0.22
Cd 1.5 0.010
Cr 5.0 0.04
Ni 5.8 0.40
Pb 14.0 0.20
Tl 15.0 0.25
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Principles of Graphite Furnace Atomization
Flame replaced by graphite tube in argon chamber• Functions of argon
– Protect graphite from oxidation– Remove interfering species during early thermal stage
Small volume of sample dispensed directly into pyrolytically coated graphite tube
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Furnace Thermal Stages
DryDry
AshAsh
AtomizeAtomizeTTEEMMPP
T I M ET I M E
Clean Clean OutOut
CoolCoolDownDown
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Typical Graphite Furnace Atomization Peak
ADDITION 3
Time
Abs
0.00
0.78
0.40
0.60
46.0 52.048.0 50.0Zoom AutoscaleOverlay
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Advantages of Graphite Furnace Atomization (1)
All analyte in tube is atomized
Atoms retained in tube (light path) slightly longer than in flame
Atoms NOT diluted by flame gases or matrix• Lower sensitivity
• Lower detection limits
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Platform Atomization
Solid pyrolytic graphite
Central depression to hold sample• Up to ~40 L
Installed inside graphite tube
Minimum physical contact with tube
Maximum distance between tube and wall
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Universal Platform
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The peaks from the platform are delayed
Wall
PlatformDelay
Comparison of Signals – Wall vs Platform Atomization
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Benefits of Platform Atomization
• Reduction in vapor phase chemical interferences
• Reduction in background interferences
• Increase in tube lifetime for corrosive matrices
• Possible elimination of need for method of standard additions
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Elements Best Determined by Platform Atomization
Ag Ga Te
As Pb Tl
Be Sb Zn
Bi Se
Cd Sn
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Challenges of Graphite Furnace AAS
• Background
– Molecular absorption or scatter
– Requires accurate background corrector
• Matrix Interferences
– Chemical competition for analyte
– Results in analyte loss or retention
– Requires optimized methods
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Chemical Modifiers
Used extensively in graphite furnace analysis
Control chemistry of ashing and atomization
Volatilize matrix components
Stabilize analyte
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Benefit of Modifier – Pb in Waste Water (Atomization at 2400 oC)
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Element
Recommended Ash Temperature
oC
Ash Temperature with Pd Modifier
oC
Change oC
Au 700 1100 +400Ag 500 950 +450Co 900 1200 +300Ni 900 1200 +300Mn 800 1200 +400Fe 800 1300 +500Cr 1100 1300 +200Cu 900 1100 +200Zn 400 900 +500
Ashing Temperature with Pd: Transition Metals
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Modifier Used
As 1000 ppm Pd + 2% Citric Acid
Sb 1000 ppm Pd + 2% Citric Acid
Pb 500 ppm Pd + 2% Citric Acid
Cd 500 ppm Pd + 2% Citric Acid
Ag 1% Ammonium Phosphate Monobasic
Se 1000 ppm Pd + 2% Citric Acid
Modifiers Selected - Low Level Determinations
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Ab
sorb
ance
Temperature
Background
Ash Atomize
Classical Optimization – 1 Variable at a Time
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Steps In Running SRM Wizard
5. Determine the size of the steps for the Ash & Atomize temperatures
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Marine Invertebrates ~ Sample Preparation
• Samples freeze dried• Homogenized using mortar & pestle (or ball mill)
• Not required for certified reference materials
• 10 mg sample weighed out
• Add 100 uL HNO3
• Heat for 3 Hrs at 80 oC in 2 mL reaction tubes
• Cool and dilute to 2 mL with de-ionized water
• Adjust acid conc. to 3.25 % HNO3 in final solution
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Typical Calibration (Pb)
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Typical Signal Graphics (Pb)
Standard 2
CRM 786 R Mussel Tissue
SRM Lobster
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Element Certified Value Found Value No. of
mg/kg mg/kg Determinations
Cd 26.7 + 0.6 25.7 + 0.945
Cu 106 + 10 109 + 450
Pb 0.35 + 0.13 0.36 + 0.0447
Co 0.51 + 0.09 0.55 + 0.0249
Ni 2.5 + 0.19 2.3 + 0.0549
Sample ResultsSRM Tort-2 Lobster (NRC, Canada)
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Soil & Sediments ~ Sample Preparation
Various elements by gfaas
• Weigh aliquot of soil sample into a teflon beaker
• Add c. HNO3 (6 ml), and heat to 200 deg (0.5 h)
• Cool. For 5 mins. Add c. HF (6 ml) and c. HClO4 (2 ml). Heat to white fumes
• Repeat the addition of HF and HClO4. Cool for 5 mins
• Add HClO4 (2 ml), and heat to white fumes
• Cool to 100 deg, and add c. HNO3 (1 ml)
• Add distilled water (10 ml), warm at 100 deg until residues dissolved
• Cool and make up to volume with distilled water
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Soil & Sediment Analysis
Se by Zeeman gfaas
High Fe matrix
0
0.05
0.1
0.15
0.2
0.25
0ppb 2ppb 4ppb
Normal
Improve
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Soil & Sediment Analysis
Se by Zeeman gfaas
High Fe matrix
Modifier
5uL 1000 ppm palladium chloride
5uL 0.1% magnesium nitrate
Ash 1400 degrees
Atomise 2600 degrees
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Soil & Sediment Analysis
Se by Zeeman gfaas
High Fe matrix
STANDARD 2
Time
Abs
0.00
2.00
0.50
1.00
1.50
65.0 71.868.0 70.0Zoom AutoscaleOverlay
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Zeeman Background Correction
Limitations of deuterium background correction• Intensity of continuum inadequate at high wavelength
• Cannot accurately correct for structured background
• Spectral interferences can occur
– Rare
Zeeman background correction overcomes these limitations
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With the magnet OFF the TOTAL absorption is measured
Energy Absorbed
Transverse Zeeman Background Correction - Magnet “Off”
Analyte AtomicAbsorption
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Energy Absorbed
With the magnet ON the BACKGROUND ONLY ABSORBANCE is measured
Transverse Zeeman Background Correction With Polariser - Magnet “On”
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Determination of LOW Levels of As in the Presence of HIGH CONCENTRATIONS of Al
Determination of LOW Levels of Se in the Presence of HIGH CONCENTRATIONS of Fe
US EPA Se Check Standards• High Levels of Fe Added to Samples????
Others are Possible but do not occur Naturally
Real World Examples of Spectral Interferences
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No aluminium 100 ppm aluminium
D2 - 30 ppb As in HIGH Al
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No aluminium 50 ppm aluminium
Varian Zeeman - 30 ppb As in HIGH Al
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Zeeman Background Correction Summary
Good For difficult samples• High background
• Unknown interferences
Good when spectral interferences occur• Se in the presence of high Fe
• As in the presence of high Al or phosphate
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Questions