Atomic Absorption Spectroscopy Elemental analysis Dissolved metals Source –Hallow Cathode Lamp –Cathode of specific element Sample –Acetylene-air flame, nebulizer Detector –PMT

Atomic absorption Gas-phase elemental atoms Narrow absorption lines (±0.01 nm) Element specific source Flame conditions control sensitivity ppm (mg/L) varies with element

Hollow cathode lamp PMT Flame sample nebulizer Air Acetylene Coating of element AA Spectrometer monochrometer

Experiment Aspirate blank, set 100 % T Aspirate sample Matrix effects are common –Standard addition analysis

Standard Addition Analysis

Removal of Atomic Emission Place a chopper before the flame The signal from the source is modulated by the chopper Thus, a AC signal is produced on top of a DC signal that originates from emission in the flame

Atomic Emission Spectroscopy Emission of light from exited species ICP source Multi-element analysis –Fancy optics Greater sensitivity for most elements

ICP source Ar tourch –Very hot, K –Abundance of e- inhibits ionization –Lack of O 2 inhibits oxide formation –Excited atomic species predominate –5-20 L/min

Sample Introduction Nebulizer Electrothermal vaporization Laser ablation In either case Ar gas in used to carry sample into the ICP

Scanning Instruments Grating monochromator Slew-scan mode –Slew-The detector and slit is quickly moved to a specific wavelength for a given element –Slowly scan across the wavelength window near a line in very small steps nm –Not limited to a set of specific elements Echelle grating/prism –Higher orders, 2-D plane, better dispersion –Slew-scan mode

Diffraction Equation n = d(sin i + sin r) Dispersion D = dy/d = F (n/d cos r)

Multi-channel Instruments Polychromators –60 photomultipliers in fixed positions Array-based –Echelle and prism for 2-D dispersion plane –Charge injection detector with different detector elements (8.7x6.6mm) –A set of 39 elements makes up a read window for a given element –CCD detectors

CCD detectors Array of semiconductor detection elements 512x320 array = individual detectors Charge of impinging photons are stored The charge accumulated on each detector element is registered one by one by sending the charge to a preamplifier and readout (emptying the registrar) Very sensitive

Fig 10-6 and Fig is the slew can instrument 10-7 is the multi-channel instrument

Performance Sensitivity: Ranges from ng/mL Number of useful lines; 1-24 Calibration curves; almost 3 orders of magnitude Internal standards Self-absorption at higher concentrations

Other components of visible spectroscopy Photon Counting –Each time a photon strikes the PMT, a pulse of electrons are produced –Normally, an average current over time is integrated to produce the signal –In photon counting each pulse is counted, and the total number of pulses in a time interval is summed –Filters out noise –Fluorescence and Raman Spec

Fiber Optics Total Internal Reflections n 1 >n 2 >n 3

Lasers Qualities –High intensities –Narrow band width –coherence Lasing medium –Solid state, gas, dye Lasing mechanism –Figure 7.5