Optical Spectroscopy with dispersive Spectrometers
Basics - Building Blocks - Systems - Applications

This page summarizes chapter 1 of the book
"Applications of Dispersive Optical Spectroscopy Systems",
ISBN 9781628413724, SPIE monographs, Bellingham, WA, USA

Special Absorption Techniques
AAS - CD - ORD - ST - PAS / OAS

This page presents the directory, the signs and symbols, conversions, and equations of the book, while the details are an exclusive part of  the book.

A3.0 Introduction

This page adds to the page A1/A2 (Transmission - Absorption – Reflection), and assumes their content as known.
Here we review several special techniques of absorption measurement.
Beside the measurement of molecular absorption, are also of interest
A3.1) the atomic absorption by Atomic Absorptions Spectroscopy, AAS
A3.2.1) the measurement of the dynamic, polarized, circular absorption vectors, called the Circular Dichroism, CD
A3.2.2) the measurement of the static, polarized, rotational absorption vector, the Optical Rotational Dispersion, ORD
A3.3) the techniques to measure scattered transmission, and absorbing samples, ST
A3.4) the measurement of absorption through non-optical parameters by Photo(Opto) Acoustic Spectroscopy, PAS (OAS).

A3.1. Atomic Absorption Spectroscopy - AAS
A3.1.1 The principle
A3.1.2 Nomenclature
A3.1.3 The principle of an Atomic Absorption Spectrometer
Graph A3.1_1 Priciple  of an AAS nstrument
Graph A3.1-1, the basic construction of an AA spectrometer.

T = [(e0 – BG) / (e1 – BG)], which in AAS is modified to lead to:
T = [(e0 – N) / (e1 – N)]
Alike "normal" absorption measurements, F37 is applied and leads to

The absorption equation, valid in AAS environment is

F37: 
A = -log10 [(e0 – N) / (e1 – N)].
The absorption equation, valid for compensated AAS modifies to
F37-AA: A =-log10 ([(e0 – N)-(BG – N)] / [(e1 – N)-(BG – N)]).
A3.1.4 The Atomization
A3.1.5 Applicable Elements for AAS
A3.2 Polarized Transmission - CD and ORD
A3.2.1 The origin of circular polarized light, with alternating circulation
Grafik A3.2.1 zeigt die Entstehung der zirkularen Polarisation
Graph A3.2-1 demonstrates the behaviour of polarized light after a photo-elastic modulator, PEM.


Note: Circular polarization a recommendable homepage exists, written by András Szilágyi: http://www.enzim.hu/~szia/emanim/emanim.htm. It is available in hungarian, german, and english, and can be downloaded.

A3.2.2 Set-up and functionality of a CD Spectrometers, and ORD Option.
graph 3.2-2  CD set-up
Graph A3.2-2 CD spectrometer description

ORD: The CD covers the angular information, which is the ORD signal. By an algorithm developed by Kramers and Kronig, it is possible to reduce the ORD from a measured CD spectrum, if enough data points are provided. If ORD shall be analyzed by direct recording, it is necessary, to have an analyzer between sample and PMT, positioned to 90° in relation to the polarizer. If the sample did not turn the polarization angle, a zero signal will appear at angles, where the PEM modulation crosses that angle. Otherwise, the rotation created by the sample is easily found by the PEM angle of minimum transfer.
"Unpolarized Absorption": Recording "normal" absorption spectra will only lead to approximate results, because the CD set-up is not built for that. If, on the other hand, the system is designed to exchange the PEM by a chopper, or a Pockel´s cell, or a similar modulation device, it would act as any other single beam spectro photometer.
MCD: Materials with chiral structures or optically active crystal patterns, often respond not only on polarized light, but also on magnetic impact. That may enhance the CD effect. It can also make it happen at all, or quench it. Thus, a combination of CD and programmed magnetic field does increase the application range of the method. Special electromagnetic systems are on the market, which fit to the sample room, and enclose the sample laterally, without disturbing the light beam. The systems program will combine spectrometer and magnet. The user can create scans of [magnetig field / wavelength] or sequences of spectra with stepped magnetic field.
Thermal Variation: It is obvious, especially for biological samples, that the CD effect may depend on temperature. Thermostat controlled sample holders or sample changers are standard accessories therefore, and also stirring options to avoid sedimentation of the sample.
agains

Instrumental Considerations: CD spectra on biological materials are of special interest in the wavelength range between 160 and 400 nm.
A3.3 Spectrometers for scattering and transmitting samples, ST
A3.3.1 Absorption Spectro Photometer with extra large Detector.
Graph 3.3-11 Principle of a Photometers for scattering Samples
Graph A3.3-1 The principle of a special spectro photometers for scattering samples.

A3.3.2
Absorption Spectro Photometer with Fibre Optic collection


Graph A3.3-2 Setup for kinetic, and scattered transmission.
The photo on left shows the set-up, the graph on right is a duplicating block diagram.
A3.3.3
Absorption Spectro Photometer with integrating Sphere

graph A3.3-3 SPM  with integrating sphere
Graph A3.3-3 presents the principle of a "normal" dual beam spectro photometer with the integrating sphere accessory installed
.
A3.4 Photo-Acoustic Spectroscopy - PAS,
also called Opto-Acoustic Spectroscopy - OAS

A3.4.1 Basics
A3.4.2 Parameters, having impact on the PAS signal
A3.4.3 Set-up of a PAS system
Graph A3.4-1 key parts of a PAS system
Graph A3.4-1 Photo acoustic spectrometer
A3.4.4 Preferred PAS/OAS applications and referencing
 

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Status March 2012