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

Applications – A1
Transmission - Absorption – Reflection, static, dynamic / kinetic

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.

A1.0 Introduction
The interaction between light and matter, may lead to the following effects:
1) Transmission without interaction, Reflection without absorption effects

2) Transmission/Reflection, where the light changes the energetic state of the sample

3) Transmission/Reflection with light scattering effects at the sample

The law of preservation of energy says, that the light transmitted through any sample, plus
the sum of reflections by the sample results in the factor 1, the energy introduced:

T + R = 1

A1.0.1 Principles
If one measures the integrated or wavelength-dependant light flux in front and after a sample, he will find no difference in case 1). From the practical view, that does not always apply, as the following graph demonstrates.
 
Graph A1-1 origin of light loss
Graph A1-1
A1.0.2 Absorption Measurements
are almost always used to characterize liquids and gases, while reflection measurements in the overwhelming number are used for solids and colloidal samples. The Transmittance (the normalized transmission) is

T = [(e1 – BG) / (e0 – BG)]

The calculation of Absorbance F37:

A = -log10 [(e0 – BG) / (e1 – BG)]

A1.0.3 The Reflection Measurement
is based on the theoretical case, that light does not penetrate into the sample surface, thus no absorption takes place.
Hence, the resulting value is linear. The result, if normalized to 1, is called reflectance.

The equation for Reflectance F38:

R = [(e1 – BG) / (e0 – BG)]

A1.0.4 The technical Realization of an ideal Spectro Photometer for Absorption and Reflection
A1.0.4.1 The detection Range in the Scales of Wavelength and Signal.

A1.0.4.2 The Data Acquisition

 
Chopper and Intervals
Graph A1-2
shows a three phase chopper system, rotating counter-clockwise. It creates three intervals: “R“ to illuminate the reference, “BG“ to capture the background with no light at all, and “P“ to illuminate and collect the sample light. Because of the size of the light beam at the chopper, there are times between the phases with crosstalk and overlap. The active time of the signal integrator is adjusted such, that data are only taken during the doubtless time of each phase. In the example, 25% of the theoretical channel time are lost by overlap plus some safety time. A 3-phase chopper runs asymmetric, yet is harder to balance. Four phase choppers are symmetric, and easier to balance. Their drawback is, that the total loss in integration time is higher, in our example it would be 33% per phase.

A1.0.4.3 The Light Path and spectral Disturbance
A1.1.1 The optimum Spectro Photometer

 
optimum Spectro Photometer System
Graph A1-3
The optimum spectro photometer,

A 1.1.2 A standard High Performance Spectro Photometer
If the disturbance, by luminescence or other effects, which would appear at other wavelengths than the one selected, can be neglected or do really not appear, the third monochromator, between sample compartment and detectors, is obsolete. That will streamline the whole system.
 
typical High Performance Spectro Photometer
Graph A1-4

Spectro Photometer with Prism-Grating Mono
Graph A1-4-B
A 1.1.3 Spectro Photometer with parallel Wavelength Detection
 
Graph A1-5: configuration of a fibre coubled dual beam system
Graph A1-5
A1.1.4 Proposal on a universal Sample Chamber for dual Beam Spectro Photometry.
 
Flexible Sample Cahmber
Graph A1-6
A 1.1.5 Calibration and the Definition of Stray Light
 

 

Applications – A2:
Transmission - Absorption – Reflection,
dynamic / kinetic

A2.0 Introduction
In addition to the static measurement, it often is of interest to learn, how a sample behaves spectrally under external stress, or what happens upon mixing two reagents. The spectral changes between the event and the end of the reaction are called kinetics.

A2.1 Some typical Experiments
In arbitrary order, without claim of completeness

A2.1.1 Stopped Flow

In Stopped Flow experiments, two reagents are mixed in the measurement cell, by flowing them through the mixing volume of the cell. After the mixture is assumed representative, the flow is suddenly stopped, where the name comes from. Exactly that moment the data acquisition of absorption data at multiple wavelength starts. At least two wavelengths are needed: the maxima of absorption of both reacting partners. Eventually the created product may have another, third maximum, and / or an isosbestic wavelength exists. The isosbestic place is a wavelength with constant absorption during the kinetic process. If it exists, it is an excellent reference to prove that chemistry and data collection work well. Stopped Flow experiments require time resolution in the micro to millisecond range. The photometric systems used are mainly single beam spectrometers with optics for micro cells. Diode arrays and CCD are used for detection. The spectro photometric limitations (stray light, precision of data, spectral order overlay) are generally accepted.

A2.1.2 Jump Functions
describe experiments, where an external parameter changes ist state quickly, and the spectral change following is recorded. Typical changes are applied to the thermal status, to the externally applied magnetic or electric field, or the pH value and so on.
The optical system may be identical with the one before (A2.1.1), even micro cells are often used. The kinetic measurements vary in the time range of µs to seconds.

A2.1.3 Optically induced Effects, Pulse-Probe-Measurements
If the sample is disturbed by a strong external light source, and a spectral response follows, special requirements apply to the spectro photometric system. Optically induced effects typically run fast, much faster than the above described. They reach down into the nanosecond range. Therefore, the measurement will need to use ns gating functions. The disturbance of the sample in most cases is realized by a short, intense, laser or xenon lamp pulse. To make sure, that the measured absorption spectrum is loaded with a minimum (at best: none) light from the external source, the optical path needs to be optimized. The detector´s time control requires fast triggering and cleaning modes.

A2.2 Technical Requirements and Configurations
While static measurements in research rely on optimum precision and accuracy, it is more important in dynamic applications, to recover the data fast enough and accept the upcoming limits in precision. Thus, the configuration will become different from the static system.

Spectro Photometer for fast Acquisition
Graph A2-1
A typical fast, single beam spectrometer to measure kinetics.

A2.4.2 Measurements with Time Resolution and Illumination Times > 1 ms
A2.4.3 Measurements with Time Resolution and Illumination Times < 1 ms
A realized Example of a milli-second dual Beam Spectro Photometer System:
Dual Beam Absorption Spectro Photometer
for 220…1000 nm and dynamic data acquisition.
Graph A1-7: a real system for fast  photometry
Photo A1-7 shows the modular setup. From left to right, the combined UV-Vis-NIR light source with fibre cable output, in the centre, cream coloured, the also fibre coupled sample/reference station, on right the 300 mm spectrometer, equipped with a CCD camera (hard to see – because black – reaching abaft), in background the CCD controller (also black) and the PC monitor.

The special task: A spectro photometer was required, comprising the following functions:
- Measurement rep rates of < 10 ms, for kinetic absorption spectroscopy
- The system must deliver linear absorbance values, even in the presence of scattering effects from turbid samples
- The sample station shall have a stirrer option for sample and reference cell, and both cells shall be thermostatted by an external bath thermostat
- An extra light port for “side illumination” of the sample position, including the synchronisation of pulsed light sources with the CCD camera is needed.

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Status April 2012