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Analytical Issues

Analytical Issues. The measurement of any physical quantity (e.g., mass, concentration, length) is subject to some uncertainty - i.e. the error in the measurement. In general, the average of several measurements will be considerably more reliable than a single measurement.

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Analytical Issues

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  1. Analytical Issues • The measurement of any physical quantity (e.g., mass, concentration, length) is subject to some uncertainty - i.e. the error in the measurement. • In general, the average of several measurements will be considerably more reliable than a single measurement. • However, the fact that the measured values agree well among themselves is no guarantee of their accuracy.

  2. Analytical Issues (Glossary) accuracy- a measure of the departure of the measurement from the true value bias (related to systematic error) - in error by a constant amount (or direction)

  3. Analytical Issues (Glossary) precision (related to indeterminate errors) - a measure of the reproducibility of a measurement absolute error - error in the measurement (in the same units as the measurement)

  4. PRECISE BUT INACCURATE ACCURATE BUT IMPRECISE Precision vs Accuracy

  5. Analytical Issues (Glossary) relative error - error expressed as a fractional part of the value measured significant figures - for reporting; use relative error (e.g., for accuracy of 1 in 1000, use four significant figures); however, for computing purposes, more significant figures may be carried to avoid round-off errors.

  6. Measures of Deviation • range - interval between the greatest and least measurements • average deviation - sum of deviations (without regard to sign) divided by number of measurements • standard deviation - square root of the sum of the square of deviations divided by the number of measurements X  1 = 68.3% X  2 = 95.5% X  3 = 99.7% • coefficient of variation: • std. dev./mean x100 = %

  7. Analytical Issues (Glossary) • quantitation limit - lowest concentration reliably measured • detection limit - lowest concentration at which presence of analyte can be reliably confirmed • recovery rate - amount measured relative to amount added, or known to be present, usually as a percentage • known addition - used to test “recovery” on an actual sample

  8. Quality Assurance/Control (QA/QC) • protocol set up to insure accuracy of results within some set limit. • Document all handling and processes • Monitor all storage conditions • Account for the buoyancy of air in all weightings

  9. Weight Correction • Wapp = apparent weight, uncorrected for buoyancy of the air • Wvac = weight in a vacuum • dair = density of air • dsub = density of the substance • dwt = density of the measuring weights

  10. Analytical Techniques • Chromatographic Methods of Analysis • Analysis of Pesticides in ppb range • Gas Chromatography (GC) • High Pressure Liquid Chromatography (HPLC) • Colorimetric Methodsof Analysis • Analysis of Nutrients (eg nitrate) in ppm range • Spectrophotometry • Flow Injection Analysis (FIA) • Additional Analytical Techniques • Ion specific electrodes, Immunoassay

  11. Chromatographic Methods of AnalysisHistoryTsweet - 1910 First coined the term chromatography which means color mapping from his separation of plant pigments on magnesium silicate columns Planar chromatography -1930’s to 1950’sMartin and Synge - 1959 Development of gas chromatography Modern liquid chromatography - developed in the early 1970’s with the introduction of efficient columns, high pressure metering pumps and flow through detectors

  12. What is a Chromatograph ??? Pump Injector Column Detector Recorder

  13. Schematic of a Gas Chromatograph Sawyer and McCarty, 1967 Fig.10.22, p 278

  14. Chromatograph Detectors • FID: Flame Ionization Detector • ECD: Electron Capture Detector • NPD: Nitrogen-Phosphorus Detector • MS: Mass Spectrometer • UV: Ultra Violet Light Detector • Fluorescence Detector

  15. Chromatographic Subsystems • Pump • GCHPLC • Gas Cylinder He, H2 High Pressure Pump • Column • Open Capillary Packed Column • 30 m x 0.25mm 2 m x 4.6 mm • Detectors • FID, ECD, NPD, MS UV, Fluoresence, MS

  16. Chromatographic Columns • “Better Chromatography through Silicon Chemistry” • Developments pushed by analytical needs • Environmental Analysis • Hundreds to thousands of compounds • Method 524 for Drinking Water 84 compounds • Method 525 for Drinking Water Pesticides • 53 Nitrogen containing herbicides • 36 Organochlorine herbicides and insecticides • 26 organophosphate insecticides • Difficult analysis required by federal regulations pushed development of better columns

  17. Column Improvements • Gas chromatography • Fused Silica Capillaries • Consistent, inert, and relatively unbreakable • Bonded Stationary Phases • low bleed, solvent resistant, long lasting, rinseable • HPLC • Bonded Phases C18, C8, glycol • Inert uniform silica particles

  18. Nitrogen Containing Herbicides 8-TBA 9-Acetochlor 10-Alachlor 11-Metribuzin 12-Metolachlor 13-Chlorpyrifos 14-Cyanizine 13 1-Propachlor 2-Trifluralin 3-DEA 4-DIA 5-Propazine 6-Atrazine 7-Simazine 5 8 6 1 11 7 10 4 9 12 14 3 2 10 20 Time, minutes

  19. Solid lines represent third order best fit equations area peak Peak area Peak height Nanograms of alachlor

  20. Pesticides (herbicides and insecticides) GLC (gas-liquid chromatography) • Extract sample (water, solid, biological material) with an organic solvent (e.g., 1500 mL of water with 100 mL of CH2Cl2) • Possibly concentrate, make derivative, and/or “clean-up,” e.g., concentrate organic solvent from 100 mL to 2 mL) • Inject into a GLC (0.5 to 10 µL) • Compare peak height (or area) and retention time with standard(s) 5. Compute concentration

  21. Example: peak height of 1.5 units with injection of 2.5µl (2.5x10-6L) from graph p.h. of 1.5 = 0.30 ng (i.e. 3 x 10-9g) amount in extracted sample = 0.30 ng x 2 mL/2.5 µL = 0.24 µg conc. = 0.24 µg/1500 mL = 0.16 µg/L = 0.16 ppb

  22. Legend Pesticide Conjugated with Enzyme Pesticide Chromogen/Substrate Colored Product Pesticide Immunoassay Magnetic Particle with Antibody Attached

  23. Pesticide Immunoassay 1).Immunological Reaction The pesticide-containing sample is incubated with enzyme conjugate (same pesticide conjugated to enzyme horseradish peroxidase) and magnetic particles with attached antibody that is specific for this pesticide. Both pesticide and pesticide conjugate compete for the same antibody sites on magnetic particles.

  24. magnet Pesticide Immunoassay 2). Separation The magnetic field is applied to hold the magnetic particles (with pesticide and pesticide conjugate attached in proportion to their original concentration) in the tube and to allow excess reagents to be decanted.

  25. Pesticide Immunoassay 3). Color Development The presence of labeled pesticide is detected by adding enzyme substrate, hydrogen peroxide, and chromogen, 3.3’, 5.5’- Tetra Methylbenzidine, and measuring the colored product at 450 nm. Since labeled pesticide (enzyme conjugate) was in competition with unlabeled pesticide (sample), the color developed is inversely proportional to the concentration of pesticide in the sample.

  26. Pesticide Immunoassay • Only works with certain organic compounds including some pesticides • Not very accurate; used more for screening than for quantitative analysis. • Only works over a narrow range of concentrations. • Reacts with families of compounds rather than a specific compound

  27. Photoelectric Colorimeter Sawyer and McCarty, 1967 Fig.4.8, p 77

  28. Colorimeter Schematic Sawyer and McCarty, 1967 Fig.4.7, p 76

  29. COLORIMETRIC METHODS Nutrients (nitrogen and phosphorus) Soil or biological samples 1. Extract (or digest) with water or solutions 2. Measure absorbance 3. Compute concentration from calibration constant or curve

  30. COLORIMETRIC METHODS Nutrients (nitrogen and phosphorus) Beer’s law: A = a b c where A = absorbance = log (incoming light/outgoing light), a = extinction coefficient (cm2/µg), A unique value for each compound b = path length (cm), c = concentration (mg/1000 cm3)

  31. COLORIMETRIC METHODS Nutrients (nitrogen and phosphorus) Example: A = 0.301 a = 0.462 cm2/mg b = 1cm A = abc = 0.301 = 0.462 x 1 x c c = 0.301/0.462 = 0.65 ppm

  32. COLORIMETRIC METHODSAdvantages and Disadvantages • Relatively easy to use • Results will be influenced by impurities in the solution

  33. Ion-Selective Electrodes

  34. Ion-Selective ElectrodesNernst Equation • E = Electrode Potential (volts) • n = valence (equivalents/mole) • F = Faraday (coulombs/equivalent) • R = the gas constant • T = The absolute Temperature (K) • [A] = activity of the selected ion

  35. Ion-Selective ElectrodesExample Analytes • pH (most common use) • Dissolved Oxygen • Ammonia • Fluoride • Nitrate • Potassium • Chloride

  36. Ion-Selective ElectrodesAdvantages and Disadvantages • Relatively inexpensive per sample analyzed • Relatively simple to use • Not influenced by impurities in the solution • Relatively easy to make in-situ field measurements • Need a separate probe for each analyte

  37. Stable Isotope Nitrogen Tracing Atmospheric N2 14N 99.6337 atom %; 15N 0.3663 atom %; 13N 0 atom %; (Th= 603sec) Typical Material Costs: 15N depleted – 99.99% 14N 0.01% 15N NH4NO3 1 kg (365 g N) $230 15N enriched – 95.0% 14N 5.0% 15N NH4NO3 1 kg (365 g N) $2,500 200 kg 14N equivalent/ha ~ $1,400,000/ha

  38. Labeled N-tracingAnalytical Procedure • Conversion of sample N to NH4-N by means of acid digestion • Oxidation of NH3 to N2 2NH3+3NaOBr = 3NaBr + 3H2O+N2 • Determination of isotope composition (+-0.001atom% of15N) mass spectroscopy 14N2+,14N15N+,15N2+ (automated total N, 15N analysis less than $10/sample)

  39. Labeled N-tracingCalculations • As=15N atom% in the sample • Am=15N atom% in applied material • ref = atom% non-labeled N in system

  40. Labeled N-tracingExample Calculations • Given: 200 kg/ha applied nitrogen fertilizer containing 5 atom% 15N • 0.367 atom% non-labeled N in system • 5 cm runoff depth drain sample; Cw = 24.0 mg/L N; As = 3.64 atom% 15N • Find: Percent of fertilizer lost in this runoff event.

  41. Labeled N-tracingExample Calculations Loss from fertilizer Drainage water loss Percent of applied fertilizer lost

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