Graphite Furnace Analysis

Graphite Furnace Atomization

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 – – – – 2

Flame gases Matrix component - analyte interaction Chemical interferences Dissociation of analyte molecular species

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

3

Flame vs Furnace Sensitivity

100 µg/L Pb @ 217.0 nm

Absorbance

0.936

0.004

4

Furnace Signal for 10 µL

Flame Signal

Flame vs Graphite Furnace AAS

Criteria Elements Sensitivity Precision Interferences Speed Simplicity Flame Hazards Automation Operating Cost 5

Flame 67 ppm - % Good Few Rapid Easy Yes Yes Low

Furnace 48 ppt - ppb Fair Many Slow More complex No Yes (unattended) Medium

Detection Limit Comparison (µg/L)

Element Ag Al As Cd Cr Ni Pb Tl

6

Flame 1.7 20.0 42.0 1.5 5.0 5.8 14.0 15.0

Furnace 0.020 0.10 0.22 0.010 0.04 0.40 0.20 0.25

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

7

Furnace Thermal Stages

Clean Out Atomize Ash Dry

TIME 8

Cool Down

T E M P

Typical Graphite Furnace Atomization Peak

A DDITION 3

A bs 0.78 0.60 0.40 0.00 46.0 Zoom

9

48.0 Ove rla y

50.0 Time

52.0 Autos c a le

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

10

Platform Atomization Solid pyrolytic graphite Central depression to hold sample • Up to ~40 µL

Installed inside graphite tube Minimum physical contact with tube Maximum distance between tube and wall

11

Universal Platform

12

Comparison of Signals – Wall vs Platform Atomization The peaks from the platform are delayed

13

Wall Delay Platform

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

14

Elements Best Determined by Platform Atomization Ag As Be Bi Cd

15

Ga Pb Sb Se Sn

Te Tl Zn

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

16

Chemical Modifiers Used extensively in graphite furnace analysis Control chemistry of ashing and atomization Volatilize matrix components Stabilize analyte

17

Benefit of Modifier – Pb in Waste Water (Atomization at 2400 oC)

18

Ashing Temperature with Pd: Transition Metals

Recommended Ash Temperature Change Ash Temperature with Pd Modifier Element o C o o C C Au 700 1100 +400 Ag 500 950 +450 Co 900 1200 +300 Ni 900 1200 +300 Mn 800 1200 +400 Fe 800 1300 +500 Cr 1100 1300 +200 Cu 900 1100 +200 Zn 400 900 +500 19

Modifiers Selected - Low Level Determinations

20

 

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

Classical Optimization – 1 Variable at a Time

Absorbance

Background

Ash

Atomize

21

Temperature

Practical Example Antimony, SRM NIST 1640

Platform atomization with Mg(NO3)2 as modifier Characteristic mass, peak area : 15.1 pg* Detection limit : 0.13 µg/L Based on a 20 µL sample injection

NIST 1640, Certified Value : 13.79 ± 0.42 µg/Kg Found : 13.54 ± 0.18 µg/l (5 measurements)

* 22

Theoretical value calculated for Varian GTA 110, 2200K according L’Vov : 12.6 pg

Steps In Running SRM Wizard 1. Determine the size of the steps for the Ash & Atomize temperatures

23

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

24

Typical Calibration (Pb)

25

Typical Signal Graphics (Pb)

Standard 2

CRM 786 R Mussel Tissue

SRM Lobster

26

Sample Results

SRM Tort-2 Lobster (NRC, Canada)

Element

Certified Value mg/kg Determinations

27

Found Value mg/kg

Cd

26.7 + 0.6 45

25.7 + 0.9

Cu

106 + 10 50

109 + 4

Pb

0.35 + 0.13 47

0.36 + 0.04

Co

0.51 + 0.09 49

0.55 + 0.02

Ni

2.5 + 0.19 49

2.3 + 0.05

No. of

Soil & Sediments ~ Sample Preparation

Various elements by gfaas

28

•

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

Soil & Sediment Analysis Se by Zeeman gfaas High Fe matrix 0.25 0.2 0.15

Normal Improve

0.1 0.05 0 0ppb

29

2ppb

4ppb

Soil & Sediment Analysis Se by Zeeman gfaas High Fe matrix Modifier 5uL 1000 ppm palladium chloride 5uL 0.1% magnesium nitrate

30

Ash

1400 degrees

Atomise

2600 degrees

Soil & Sediment Analysis Se by Zeeman gfaas High Fe matrix STANDARD 2 Abs 2.00

1.50

1.00

0.50

0.00 65.0 Zoom 31

68.0 Overlay

70.0 Time

71.8 Autoscale

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

32

Transverse Zeeman Background Correction Magnet “Off” With the magnet OFF the TOTAL absorption is measured Energy Absorbed

Analyte Atomic Absorption

33

Transverse Zeeman Background Correction With Polariser - Magnet “On”

With the magnet ON the BACKGROUND ONLY ABSORBANCE is measured Energy Absorbed

34

Real World Examples of Spectral Interferences

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 35

D2 - 30 ppb As in HIGH Al

No aluminium

36

100 ppm aluminium

Varian Zeeman - 30 ppb As in HIGH Al

No aluminium

37

50 ppm aluminium

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

38

Questions

39