Fluorescence

Topics

• Sensitivity (nM-pM)– Contrast UV-vis measurements

• Derivatization

• Laser-Induced Fluorescence

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

• 1-100 ppm (mg/L)

• varies with element

Hollow cathode lamp

PMT

Flame

sample

nebulizerAir

Acetylene

Coating of element

AA Spectrometer

monochrometer

Experiment

• Aspirate blank, set 100 % T

• Aspirate sample

• Matrix effects are common– 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 , 10000 K– Abundance of e- inhibits ionization– Lack of O2 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-0.001 nm

– Not limited to a set of specific elements

• Echelle grating/prism– Higher orders, 2-D plane, better dispersion– Slew-scan mode

Multi-channel Instruments

• Polychromators– 60 photomultipliers in fixed positions

• Array-based– Echelle and prism for 2-D dispersion plane– Charge injection detector with 94672 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 =163840 individual

detectors• Charge of impinging photons are stored • The charge accumulated is registered one

by one by sending the charge to a preamplifier and readout, emptying the registrar

• Very sensitive

Performance

• Sensitivity: Ranges from 1-300 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

• n1>n2>n3

Lasers

• Qualities– High intensities– Narrow band width– coherence

• Lasing medium– Solid state, gas, dye

• Lasing mechanism– Figure 7.5