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Page 1: Graphite Furnace Analysis

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


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