**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.

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 =**
[(e_{1} – BG) / (e_{0} – BG)]

**
The calculation of Absorbance F37**:

A = -log_{10}
[(e_{0} – BG) / (e_{1} – 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 = [(e_{1} – BG) / (e_{0} – 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.
**

**
A1.0.4.3 The Light Path and spectral Disturbance**

**
A1.1.1 The optimum Spectro Photometer**

**
Graph A1-3**

**
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.

**
Graph A1-4**

**
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.

**
Graph A2-1**

**
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.

**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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Spectroscopy with dispersive Spectrometers
Basics - Building Blocks - Systems - Applications " are reserved by
Wilfried **Neumann, D-88171 Weiler-Simmerberg.
Status April 2012**