Graphite Furnace Analysis

  • Published on
    10-Apr-2015

  • View
    2.567

  • Download
    2

Embed Size (px)

Transcript

<p>Graphite Furnace Atomization</p> <p>Limitations of Flame AtomizationSensitivity is generally limited to mg/L concentrations Relatively poor nebulization efficiency Only ~ 10 % of sample reaches flame</p> <p>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</p> <p>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 solidsGround state atom formation subject to many interacting variables 2</p> <p>Flame gases Matrix component - analyte interaction Chemical interferences Dissociation of analyte molecular species</p> <p>Benefits of Graphite Furnace AtomizationEntire sample is atomized at one time Free atoms remain in the optical path longer</p> <p>Enhanced sensitivity</p> <p>Reduced sample volume</p> <p>3</p> <p>Flame vs Furnace Sensitivity</p> <p>100 g/L Pb @ 217.0 nm 0.936 Absorbance Furnace Signal for 10 L</p> <p>Flame Signal</p> <p>0.004</p> <p>4</p> <p>Flame vs Graphite Furnace AAS</p> <p>Criteria Elements Sensitivity Precision Interferences Speed Simplicity Flame Hazards Automation Operating Cost5</p> <p>Flame 67 ppm - % Good Few Rapid Easy Yes Yes Low</p> <p>Furnace 48 ppt - ppb Fair Many Slow More complex No Yes (unattended) Medium</p> <p>Detection Limit Comparison (g/L)</p> <p>Element Ag Al As Cd Cr Ni Pb Tl</p> <p>Flame 1.7 20.0 42.0 1.5 5.0 5.8 14.0 15.0</p> <p>Furnace 0.020 0.10 0.22 0.010 0.04 0.40 0.20 0.25</p> <p>6</p> <p>Principles of Graphite Furnace AtomizationFlame 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</p> <p>7</p> <p>Furnace Thermal Stages</p> <p>Clean Out Atomize Ash Dry Cool Down T E M P</p> <p>TIME8</p> <p>Typical Graphite Furnace Atomization Peak</p> <p>A bs 0.78 0.60 0.40 0.00 46.0 Zoom Ove rla y 48.0</p> <p>A DDITION 3</p> <p>50.0 Time Autos c a le</p> <p>52.0</p> <p>9</p> <p>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</p> <p>10</p> <p>Platform Atomization Solid pyrolytic graphite Central depression to hold sample Up to ~40 L</p> <p>Installed inside graphite tube Minimum physical contact with tube Maximum distance between tube and wall</p> <p>11</p> <p>Universal Platform</p> <p>12</p> <p>Comparison of Signals Wall vs Platform Atomization The peaks from the platform are delayed</p> <p>Wall Delay Platform</p> <p>13</p> <p>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</p> <p>14</p> <p>Elements Best Determined by Platform Atomization Ag As Be Bi Cd Ga Pb Sb Se Sn Te Tl Zn</p> <p>15</p> <p>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</p> <p>16</p> <p>Chemical Modifiers Used extensively in graphite furnace analysis Control chemistry of ashing and atomization Volatilize matrix components Stabilize analyte</p> <p>17</p> <p>Benefit of Modifier Pb in Waste Water(Atomization at 2400 oC)</p> <p>18</p> <p>Ashing Temperature with Pd: Transition Metals</p> <p>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 +50019</p> <p>Modifiers Selected - Low Level Determinations As Sb Pb Cd Ag Se</p> <p>Modifier Used1000 ppm Pd + 2% Citric Acid 1000 ppm Pd + 2% Citric Acid 500 ppm Pd + 2% Citric Acid 500 ppm Pd + 2% Citric Acid 1% Ammonium Phosphate Monobasic 1000 ppm Pd + 2% Citric Acid</p> <p>20</p> <p>Classical Optimization 1 Variable at a TimeBackground</p> <p>21</p> <p>Absorbance</p> <p>Ash</p> <p>Atomize</p> <p>Temperature</p> <p>Practical Example Antimony, SRM NIST 1640</p> <p>Platform atomization with Mg(NO3)2 as modifier Characteristic mass, peak area : 15.1 pg* Detection limit : 0.13 g/LBased on a 20 L sample injection</p> <p>NIST 1640, Certified Value : 13.79 0.42 g/Kg Found : 13.54 0.18 g/l(5 measurements)</p> <p>*22</p> <p>Theoretical value calculated for Varian GTA 110, 2200K according LVov : 12.6 pg</p> <p>Steps In Running SRM Wizard1. Determine the size of the steps for the Ash &amp; Atomize temperatures</p> <p>23</p> <p>Marine Invertebrates ~ Sample Preparation Samples freeze dried Homogenized using mortar &amp; pestle (or ball mill) Not required for certified reference materials</p> <p> 10 mg sample weighed out Add 100 uL HNO3 Heat for 3 Hrs at 80 oC in 2 mL reaction tubes</p> <p> Cool and dilute to 2 mL with de-ionized water Adjust acid conc. to 3.25 % HNO3 in final solution</p> <p>24</p> <p>Typical Calibration (Pb)</p> <p>25</p> <p>Typical Signal Graphics (Pb)</p> <p>Standard 2</p> <p>CRM 786 R Mussel Tissue</p> <p>SRM Lobster</p> <p>26</p> <p>Sample Results</p> <p>SRM Tort-2 Lobster (NRC, Canada)</p> <p>Element</p> <p>Certified Value mg/kg Determinations</p> <p>Found Value mg/kg</p> <p>No. of</p> <p>Cd Cu Pb Co27</p> <p>26.7 + 0.6 45 106 + 10 50 0.35 + 0.13 47 0.51 + 0.09 49 2.5 + 0.19 49</p> <p>25.7 + 0.9 109 + 4 0.36 + 0.04 0.55 + 0.02 2.3 + 0.05</p> <p>Ni</p> <p>Soil &amp; Sediments ~ Sample Preparation</p> <p>Various elements by gfaas</p> <p>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</p> <p>28</p> <p>Soil &amp; Sediment AnalysisSe by Zeeman gfaas High Fe matrix0.25 0.2 0.15 0.1 0.05 0 0ppb 2ppb 4ppb</p> <p>Normal Improve</p> <p>29</p> <p>Soil &amp; Sediment AnalysisSe by Zeeman gfaas High Fe matrixModifier 5uL 1000 ppm palladium chloride 5uL 0.1% magnesium nitrate Ash Atomise 1400 degrees 2600 degrees</p> <p>30</p> <p>Soil &amp; Sediment AnalysisSe by Zeeman gfaas High Fe matrixSTANDARD 2 Abs 2.00</p> <p>1.50</p> <p>1.00</p> <p>0.50</p> <p>0.00 65.0 Zoom31</p> <p>68.0 Overlay Time</p> <p>70.0 Autoscale</p> <p>71.8</p> <p>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</p> <p>Zeeman background correction overcomes these limitations</p> <p>32</p> <p>Transverse Zeeman Background Correction Magnet Off With the magnet OFF the TOTAL absorption is measuredEnergy Absorbed</p> <p>Analyte Atomic Absorption</p> <p>33</p> <p>Transverse Zeeman Background Correction With Polariser - Magnet On</p> <p>With the magnet ON the BACKGROUND ONLY ABSORBANCE is measuredEnergy Absorbed</p> <p>34</p> <p>Real World Examples of Spectral Interferences</p> <p>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????</p> <p>Others are Possible but do not occur Naturally35</p> <p>D2 - 30 ppb As in HIGH Al</p> <p>No aluminium</p> <p>100 ppm aluminium</p> <p>36</p> <p>Varian Zeeman - 30 ppb As in HIGH Al</p> <p>No aluminium</p> <p>50 ppm aluminium</p> <p>37</p> <p>Zeeman Background Correction Summary Good For difficult samples High background Unknown interferences</p> <p>Good when spectral interferences occur Se in the presence of high Fe As in the presence of high Al or phosphate</p> <p>38</p> <p>Questions</p> <p>39</p>

Recommended

View more >