INSTRUMENTAL ANALYSIS (I)

1 INSTRUMENTAL ANALYSIS (I) INTRODUCTION Classification of Analytical Methods Qualitative INSTRUMENTAL ANALYSIS is that measured property indicates presence of analyte in matrix Quantitative INSTRUMENTAL ANALYSIS is that magnitude of measured property is proportional to concentration of analyte in matrix Species of interest: All constituents including analyte and Matrix-analyte (concomitants) Often need pretreatment - chemical extraction, distillation, separation, precipitation (A) Classical: Qualitative - identification by color, indicators, boiling points, odors Quantitative - mass or volume ( gravimetric, volumetric) (B) INSTRUMENTAL .

2 Qualitative - chromatography, electrophoresis and identification by measuring physical property ( spectroscopy, electrode potential) Quantitative - measuring property and determining relationship to concentration ( spectrophotometry, mass spectrometry) Often, same INSTRUMENTAL method used for qualitative and quantitative ANALYSIS Types of INSTRUMENTAL Methods Property Example MethodRadiation emission Emission spectroscopy - fluorescence, phosphorescence, luminescence Radiation absorption Absorption spectroscopy - spectrophotometry, photometry, nuclear magnetic resonance, electron spin resonance Radiation scattering Turbidity, Raman Radiation refraction Refractometry, interferometry Radiation diffraction X-ray, electron Radiation rotation Polarimetry.

3 Circular dichroism Electrical potential Potentiometry Electrical charge Coulometry Electrical current Voltammetry - amperometry, polarography Electrical resistance Conductometry Mass Gravimetry Mass-to-charge ratio Mass spectrometry Thermal Thermal gravimetry, calorimetry Rate of reaction Stopped flow, flow injection ANALYSIS Radioactivity Activation, isotope dilution (Often combined with chromatographic or electrophoretic methods) Example: Spectrophotometry Instrument: spectrophotometer Stimulus: monochromatic light energy Analytical response: light absorption Transducer: photocell Data: electrical current Data processor: current meter Readout.

4 Meter scale Data Domains: way of encoding analytical response in electrical or non-electrical signals. Interdomain conversions transform information from one domain to another. Detector (general): device that indicates change in environment Transducer (specific): device that converts non-electrical to electrical data Sensor (specific): device that converts chemical to electrical data Non-Electrical Domains Electrical Domains Physical (light intensity, color) Chemical (pH) Scale Position (length) Number (objects) Current Voltage Charge Frequency Pulse width Phase Count Serial Parallel Time - vary with time (frequency, phase, pulse width) Analog - continuously variable magnitude (current, voltage, charge) Digital - discrete values (count, serial, parallel, number*) Digital Binary Data Advantages (1) easy to store (2)

5 Not susceptible to noise How to choose an analytical method? How good is measurement? How reproducible? - Precision How close to true value? - Accuracy/Bias How small a difference can be measured? - Sensitivity What range of amounts? - Dynamic Range How much interference? - Selectivity Precision - Indeterminate or random errors Accuracy - Determinate errors (operator, method, INSTRUMENTAL ) Sensitivity (larger slope of calibration curve m, more sensitive measurement) Detection Limit Signal must be bigger than random noise of blank From statistics k=3 or more (at 95% confidence level) Dynamic Range At detection limit we can say confidently analyte is present but cannot perform reliable quantitation Level of quantitation (LOQ): k=10 Limit of linearity (LOL).

6 When signal is no longer proportional to concentration Selectivity: No analytical method is completely free from interference by concomitants. Best method is more sensitive to analyte than interfering species (interferent). k's vary between 0 (no selectivity) and large number (very selective). Calibration methods Basis of quantitative ANALYSIS is magnitude of measured property is proportional to concentration of analyte Calibration curves (working or analytical curves) Calibration expression is Absorbance = slope [Analyte (ppm)] + intercept SPECTROCHEMICAL TECHNIQUES INTRODUCTION Methods of ANALYSIS based on the interaction of LIGHT with matter.

7 LIGHT is an Electromagnetic (EM) radiation: Speed of light, c = x 108 m / s .Frequency is always fixed but velocity can vary! H = Planck's Constant = J s 3. Atoms, ions and molecules exist in certain energy states only E0 = ground state E1, E2 , E3 .. = excited states Excitation can be electronic, vibrational or rotational Energy levels for atoms, ions or molecules different. 2. When an atom, ion or molecule changes energy state, it absorbs or emits energy equal to the energy difference E = E1 E0 3.

8 The wavelength or frequency of radiation absorbed or emitted during a transition is proportional to E EMISSION SPECTRA Have you noticed the bright yellow light emitted when crystals of NaCl fall in the flame in your cooker at home? This is emission Plot of emission intensity vs. or called emission spectrum Atom: line emission spectra Inner shell (core) electrons (1s 2p) x-rays photons Outer shell (valence) electrons (3d 4p) UV/vis photons Molecule: vibrational and rotational transitions - band emission spectra Continuum Spectra: A piece of iron in the fire: first become red then white, Why?

9 Very broad band spectra in Emission from solids produced by blackbody radiation thermal excitation and relaxation of many vibrational (and rotational) levels. Blackbody Spectrum Absorption Spectra Plot of Absorbance vs. or called absorption spectrum just as in emission spectra an atom, ion or molecule can only absorb radiation if energy matches separation between two energy states Atoms: No vibrational or rotational energy levels - sharp line spectra with few features For example: Na 3s 3p , nm (yellow) Na 3s 5p , nm (UV) Visible enough energy for valence (bonding) excitations UV and x-ray enough energy for core (inner) excitations Molecules.

10 Electronic, vibrational and rotational energy levels - broad band spectra with many features E = Eelectronic + Evibrational + Erotational For each electronic state - many vibrational states For each vibrational state - many rotational states many features Absorption spectra affected by (1) number of atoms in molecule more features (2) solvent molecules blurred features Effect of Chemical State Emission produces emission at same wavelength as absorption (common for atoms) Excitation methods: EM radiation SPECTROSCOPY AND INSTRUMENTATION (IR, visible and UV) Examples: T= (100 %T), A= T= (10 %T), A= T= ( %T), A= Determination of concentration from Absorbance measurements; Absorbance is directly proportional to the concentration, c, and the path length, b, of the absorbing species.