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Title: spectrometry instrumentation / UV-vis spectrometer


1
Chapter 25
Instruments for Optical Spectrometry
2
Components of Various Types of Instrument for
Optical Spectroscopy
1) Absorption measurement
2) Fluorescence measurements
3) Emission spectroscopy
3
Instrumentation for spectrometry
1. Light sources continuum source
line spectrum
Spectral source types. The spectrum of a
continuum source (a) is much broader than that of
line source (b).
4
Black Body Radiation Any object surface can
radiate heat to and receive heat from outside, if
an object can absorb all the incident radiation,
regardless of the frequencies and directions,
this object is called Black Body. A ball cavity
with a small hole can be regarded as a black
body, since any radiation entering the ball
cavity can only reflect inside it, thus totally
absorbed.
Spectral distribution of blackbody radiation.
5
Low pressure mercury arc lamp 253.7 nm Hg
line Hollow cathode lamps line sources / AA
spectrometry Laser source
6
H2 Ee ? H2 H2 ? H H h? Ee EH2
EH EH h?
A deuterium lamp
A tungsten lamp
Intensity of a tungsten filament at 3200K and a
deuterium arc lamp.
Light sources.
7
Laser Laser is the acronym of Light
Amplification by Stimulated Emission of
Radiation. A device which produces light with a
narrow spectral width. Laser is light of special
properties, light is electromagnetic (EM) wave in
visible range. Lasers, broadly speaking, are
devices that generate or amplify light, just as
transistors generate and amplify electronic
signals at audio, radio or microwave frequencies.
Here light must be understand broadly, since
lasers have covered radiation at wavelengths
ranging from infrared range to ultraviolet and
even soft x-ray range. A laser is a cavity that
has mirrors at the ends and is filled with
lasable material such as crystal, glass, liquid,
gas, or dye. These materials must have atoms,
ions, or molecules capable of being excited to a
metastable state by light, electric discharge, or
other stimulus. The transition from this
metastable state back to the normal ground state
is accompanied by the emission of photons which
form a coherent beam.
8
Laser construction  A laser system generally
consists of three important parts - An energy
source (usually referred to as the pump or pump
source) - A gain medium or laser medium - A
mirror, or system of mirrors, forming an optical
resonator.
9
Laser cavity. The electromagnetic wave travels
back and forth between the mirrors, and the wave
is amplified with each pass. The output mirror is
partially transparent to allow only a fraction of
the beam to pass out of the cavity.
10
(a) Energy-level diagram illustrating the
principle of operation of a laser. (b) Basic
components of a laser. The population inversion
is created in the lasing medium. Pump energy
might be derived from intense lamps or an
electric discharge.
11
Amplification of light All lasers contain an
energized substance that can increase the
intensity of light passing through it. This
substance is called the amplifying medium or,
sometimes, the gain medium, and it can be a
solid, a liquid or a gas. Whatever its physical
form, the amplifying medium must contain atoms,
molecules or ions, a high proportion of which can
store energy that is subsequently released as
light. In a neodymium YAG (NdYAG) laser, the
amplifying medium is a rod of yttrium aluminium
garnate (YAG) containing ions of the lanthanide
metal neodymium (Nd). In a dye laser, it is a
solution of a fluorescent dye in a solvent such
as methanol. In a helium-neon laser, it is a
mixture of the gases helium and neon. In a laser
diode, it is a thin layer of semiconductor
material sandwiched between other semiconductor
layers. The factor by which the intensity of the
light is increased by the amplifying medium is
known as the gain. The gain is not a constant for
a particular type of medium. It's magnitude
depends upon the wavelength of the incoming
light, the intensity of the incoming light, the
length of the amplifying medium and also upon the
extent to which the amplifying medium has been
energized. http//members.aol.com/W
SRNet/tut/ut1.htm
12
Schematic of a NdYAG laser.
13
Energizing the amplifying medium Increasing the
intensity of a light beam that passes through an
amplifying medium amounts to putting additional
energy into the beam. This energy comes from the
amplifying medium which must in turn have energy
fed into it in some way. In laser terminology,
the process of energizing the amplifying medium
is known as "pumping".
14
There are several ways of pumping an amplifying
medium. When it is a solid, pumping is usually
achieved by irradiating it with intense light.
This light is absorbed by atoms or ions within
the medium raising them into higher energy
states. Xenon-filled flashtubes positioned as
shown below are used as a simple source of
pumping light. Passing a high voltage electric
discharge through the flashtubes causes them to
emit an intense flash of white light, some of
which is absorbed by the amplifying medium. The
assembly of flashtubes is enclosed within a
polished metal reflector (not shown in the
diagram below) to concentrate as much light as
possible on the amplifying medium. A laser that
is pumped in this way will have a pulsed output.
Pumping an amplifying medium by irradiating it
with intense light is referred to as optical
pumping. The source of pumping light can be
another laser. Some types of laser that were
originally pumped using xenon-filled flashtubes
are now pumped by laser diodes.
15
Gaseous amplifying media have to be contained in
some form of enclosure or tube and are often
pumped by passing an electric discharge through
the medium itself. The mechanism by which this
elevates atoms or molecules in the gas to higher
energy states depends upon the gas that is being
excited and is often complex. In many gas lasers,
the end windows of the laser tube are inclined at
an angle and they are referred to as brewster
windows. Brewster windows are able to transmit a
beam that is polarized in the plane of the
diagram without losses due to reflection. Such a
laser would have an output beam that is polarized.
The diagram illustrates pumping by passing a
discharge longitudinally through the gaseous
amplifying medium but, in some cases, the
discharge takes place transversely from one side
of the medium to the other. Many lasers that are
pumped by an electric discharge can produce
either a pulsed output or a continuous output
depending upon whether the discharge is pulsed or
continuous. Various other methods of pumping the
amplifying medium in a laser are used. For
example, laser diodes are pumped by passing an
electric current across the junction where the
two types of semiconductor within the diode come
together.
16
Creating a Population Inversion Finding
substances in which a population inversion can be
set up is central to the development of new kinds
of laser. The first material used was synthetic
ruby. Ruby is crystalline alumina (Al2O3) in
which a small fraction of the Al3 ions have been
replaced by chromium ions, Cr3. It is the
chromium ions that give rise to the
characteristic pink or red color of ruby and it
is in these ions that a population inversion is
set up in a ruby laser.
17
In a ruby laser, a rod of ruby is irradiated with
the intense flash of light from xenon-filled
flashtubes. Light in the green and blue regions
of the spectrum is absorbed by chromium ions,
raising the energy of electrons of the ions from
the ground state level to the broad F bands of
levels. Electrons in the F bands rapidly undergo
non-radiative transitions to the two metastable E
levels. A non-radiative transition does not
result in the emission of light the energy
released in the transition is dissipated as heat
in the ruby crystal. The metastable levels are
unusual in that they have a relatively long
lifetime of about 4 milliseconds (4 x 10-3 s),
the major decay process being a transition from
the lower level to the ground state. This long
lifetime allows a high proportion (more than a
half) of the chromium ions to build up in the
metastable levels so that a population inversion
is set up between these levels and the ground
state level. This population inversion is the
condition required for stimulated emission to
overcome absorption and so give rise to the
amplification of light. In an assembly of
chromium ions in which a population inversion has
been set up, some will decay spontaneously to the
ground state level emitting red light of
wavelength 694.3 nm in the process. This light
can then interact with other chromium ions that
are in the metastable levels causing them to emit
light of the same wavelength by stimulated
emission. As each stimulating photon leads to the
emission of two photons, the intensity of the
light emitted will build up quickly. This cascade
process in which photons emitted from excited
chromium ions cause stimulated emission from
other excited ions is indicated below
18
The ruby laser is often referred to as an example
of a three-level system. More than three energy
levels are actually involved but they can be put
into three categories.These are the lower level
form which pumping takes place, the F levels into
which the chromium ions are pumped, and the
metastable levels from which stimulated emission
occurs. Other types of laser operate on a four
level system and , in general, the mechanism of
amplification differs for different lasing
materials. However, in all cases, it is necessary
to set up a population inversion so that
stimulated emission occurs more often than
absorption. http//members.aol.com/WSRNet/tut/ut5
.htm
19
Properties of laser light Monochromatic one
wavelength Extremely bright high power at one
wavelength Collimated parallel rays
Polarized electric field of waves oscillates in
one plane Coherent all waves in
phase Coherence can be devided into spatial and
temporal coherence. For any em wave, if at time
t0 and t0 the phase diference between two points
in space remains the same, we say the em wave has
spatial coherence If at a point P, the em wave
at t and tdt has same phase difference if dt is
the same, temporal coherence exists.
Disadvantages of a laser High maintenance
Limited wavelengths
20
Common light sources, such as the electric light
bulb emit photons in all directions, usually over
a wide spectrum of wavelengths. Most light
sources are also incoherent, i.e., there is no
fixed phase relationship between the photons
emitted by the light source. By contrast, a laser
generally emits photons in a narrow, well-defined
beam of light. The light is often
near-monochromatic, consisting of a single
wavelength or color, is highly coherent and is
often polarised. Some types of laser, such as dye
lasers and vibronic solid-state lasers can
produce light over a broad range of wavelengths
this property makes them suitable for the
generation of extremely short pulses of light, on
the order of a femtosecond (10-15 seconds). Laser
light can be highly intense able to cut steel
and other metals. The beam emitted by a laser
often has a very small divergence (i.e. it is
highly collimated). A perfectly collimated beam
cannot be created, due to the effect of
diffraction, but a laser beam will spread much
less than a beam of light generated by other
means. A beam generated by a small laboratory
laser such as a helium-neon (HeNe) laser spreads
to approximately 1 mile (1.6 kilometres) in
diameter if shone from the Earth's surface to the
Moon. Some lasers, especially semiconductor
lasers due to their small size, produce very
divergent beams. However, such a divergent beam
can be transformed into a collimated beam by
means of a lens. In contrast, the light from
non-laser light sources cannot be collimated. A
laser can also function as an optical amplifier
when seeded with light from another source. The
amplified signal can be very similar to the input
signal in terms wavelength, phase and
polarisation this is particularly important in
optical communications.
21
The output of a laser may be a continuous,
constant-amplitude output (known as c.w. or
continuous wave), or pulsed, by using the
techniques of Q-switching, modelocking or
Gain-switching. The basic physics of lasers
centres around the idea of producing a population
inversion in a laser medium. The medium may then
amplify light by the process of stimulated
emission, which if the light is fed back through
the medium by means of a cavity resonator, will
continue to be amplified into a high-intensity
beam. A great deal of quantum mechanics and
thermodynamics theory can be applied to laser
action, though in fact many laser types were
discovered by trial and error. Population
inversion is also the concept behind the maser,
which is similar in principle to a laser but
works with microwaves. The first maser was built
by Charles H. Townes in 1953. Townes later worked
with Arthur L. Schawlow to describe the theory of
the laser, or optical maser as it was then known.
The word laser was coined in 1957 by Gordon
Gould, who was also credited with lucrative
patent rights in the 1970s, following a
protracted legal battle. The first maser,
developed by Townes, was incapable of continuous
output. Nikolai Basov and Alexander Prokhorov of
the USSR worked independently on the quantum
oscillator and solved the problem of continuous
output systems by using more than two energy
levels. These systems could release stimulated
emission without falling to the ground state,
thus maintaining a population inversion. In 1964,
Charles Townes, Nikolai Basov and Alexandr
Prokhorov shared a Nobel Prize in Physics "for
fundamental work in the field of quantum
electronics, which has led to the construction of
oscillators and amplifiers based on the
maser-laser principle."
22
The first working laser was made by Theodore H.
Maiman in 1960 at Hughes Research Laboratories in
Malibu, California, beating several research
teams including those of Townes at Columbia
University, and Schawlow at Bell laboratories.
Maiman used a solid-state flashlamp-pumped ruby
crystal to produce red laser light at 694
nanometeres wavelength. The verb "to lase" means
to give off coherent light or possibly to cut or
otherwise treat with coherent light, and is a
back-formation of the term laser. http//www.wordi
q.com/definition/Laser
Laser (U.S. Air Force)
23
Laser types - Gas Laser HeNe (543 nm and 633
nm) Argon(-Ion) (458 nm, 488 nm or 514.5
nm) Carbon dioxide lasers - used in
industry for cutting and welding, up to 100 kW
possible Carbon monoxide lasers - must be
cooled, but extremly powerful, up to 500 kW
possible - Excimer(excited dimer or trimer) gas
lasers, producing ultraviolet light, used in
semiconductor manufacturing and in LASIK eye
surgery 157 nm (F2) 193 nm (ArF)
222 nm (KrCl) 248 nm (KrF) 308
nm (XeCl) 351 nm (XeF)
24
- Commonly used laser types for dermatological
procedures including removal of tattoos,
birthmarks, and hair Ruby (694 nm)
Alexandrite (755 nm) Pulsed diode array
(810 nm) NdYAG (1064 nm) YAG
yttrium/aluminum garnet HoYAG (2090 nm)
ErYAG (2940 nm) - Semiconductor laser
diodes, small used in laser pointers,
laser printers, and CD/DVD players
bigger bigger industrial diode laser are
available used in the industry for cutting and
welding, up to 10 kW possible - Dye lasers -
Quantum cascade lasers
25
- Neodymium-doped YAG lasers (NdYAG), a
high-power laser operating in the infrared, used
for cutting, welding and marking of metals and
other materials - Erbium-doped YAG, 1645 nm -
Thulium-doped YAG, 2015 nm - Holmium-doped YAG,
2090 nm, a high-power laser operating in the
infrared, it is explosively absorbed by
water-bearing tissues in sections less than a
millimeter thick. It is usually operated in a
pulsed mode, and passed through optic fiber
surgical devices to resurface joints, remove rot
from teeth, vaporize cancers, and to pulverize
kidney and gall stones. - Titanium-doped
sapphire - Erbium-doped fiber lasers, a type of
laser formed from a specially made optical fiber,
which is used as an amplifier for optical
communications.
26
2.
Wavelength selectors (Monochromator)

1) Filter a. Absorption filter
o
30
b. Interference
filter



l
l
q

1
2
1

2) Prism a. Transmission prism
Cornu

l
1
b. Reflection prism
Littrow
q
2
l
Cornu prism
2
q
l

Dispersion d
/ d
Approximate transmission limits of prism
materials
m
Flint glass (contains
PbO
) 360nm 2
m
mm

Quartz(crystalline silica) 190nm 3.3
mm

NaCl or
KCl
0.315
m

KBr
0.330
m
m

LiF
0.2 5
m
m

CaF
0.212
m
2
m

AgCl
0.425
m
m

CsBr
0.3 50
m

m

CsI
0.3 70
m
m
KRS-
5(
TlBr-TlI) 130
m

3) Diffraction grating
a. Transmission grating
b. Reflection grating
27
Wavelength selectors for spectrometry
28
Dispersion of radiation along the focal plane AB
of a typical prism(a) and echellette grating (b).
29
Schematic diagram of diffraction from a
grating. n? (a b) d sin ? a d sin ?
b n? d (sin ? sin ? )
30
Diagram of a Czerny-Turner grating monochromator.
31
Interference of adjacent waves that are a) 0o ,
b) 90o and c) 180oout of phase.
32
Choosing the monochromator bandwidth Monochromator
bandwidth should be as large as possible, but
small compared with the width of peak being
measured.
33
stray light In every instrument, inadvertent
stray light (wavelength outside the bandwidth
expected from the monochromator) reaches the
detector. High qulity spectrometers could have
two monochromators in series to reduce stray
light.
Absorbance error introduced by different levels
of stray light. Stray light is expressed as a
percentage of the irradiance incident on the
sample.
34
Nominal wavelength
Output of an exit slit as monochromator is
scanned from l1-Dl to l1Dl.
35
Filters It is frequently necessary to filter
(remove) wide bands of radiation from a signal.
Bandwidths for two types of filters(interference
filter vs absorption filter).
36
(a) Schematic cross section of an interference
filter. (b) Schematic to show the conditions for
constructive interference
37
  • Transmission spectra of interference filters.
  • Wide band pass filter has 90 transmission in
    the 3- 50 5- ?m wavelength range but lt0.01
    transmittance outside this range.
  • Narrow band-pass filter has a transmission width
    of 0.1 ?m centered around 4 ?m.

38
3. Optical Materials and sample containers
Transmittance range for various cell construction
materials.
39
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40
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41
4. Detectors for spectrometry
A transducer is a type of detector that converts
various types of chemical and physical quantities
into electrical signals such as electrical
charge, current, or voltage.
42
Response of several different detectors. The
greater the sensitivity, the greater the output
(current or voltage) of the detector for a given
incident power of photons.
43
Phototube
Schematic diagram of photomultiplier with nine
dynodes.
44
Comparison of spectra recorded in 5 min by a
photomultiplier tube and a charge coupled device.
Absorption spectra of hemoglobin with identical
signal levels but different amount of noise.
45
Silicon photodiode array
46
Charge transfer device (CTD)
47
An operational amplifier current-to-voltage
converter used to monitor the current in a solid
state photodiode. Eout IR kPR kP P
radient power G KP K G electrical response
of the detector in units of current, voltage, or
charge. K dark current
48
Components and materials of spectroscopic
instruments.
49
Instrumentation of UV-visible spectrophotometer Ty
pes of UV-visible spectrophotometer 1) Single
beam spectrophotometer
2) Double beam spectrophotometer
50

Block diagram for a double-beam in-time scanning
spectrophotometer .
51
3) Diode-array spectrophotometer
Block diagram for a diode array spectrometer.
52

Spectronic 20 spectrophotometer
53
  • Procedure
  • Power on
  • Select wavelength
  • 0 T adjustment
  • (Calibration)
  • 4) Blank (Reference cell) is inserted into cell
    holder
  • 5) 100 T adjustment
  • 6) Sample cell is placed in the cell
    compartment
  • Readout absorbance
  • Power off

Scale of spectronic 20 spectrophotometer
LED
Spectronic 20 spectrophotometer
54
Schematic diagram of optical system of Spectronic
20 single beam UV-visible spectrophotometer
55
The dual-beam design greatly simplifies this
process by simultaneously measuring P and Po of
the sample and reference cells, respectively.
Most spectrometers use a mirrored rotating
chopper wheel to alternately direct the light
beam through the sample and reference cells. The
detection electronics or software program can
then manipulate the P and Po values as the
wavelength scans to produce the spectrum of
absorbance or transmittance as a function of
wavelength.
56
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57

HP8452A diode array UV-visible spectrophotometer
58
Optical schematic of the Hewlett-Packard HP-8450A
diode array UV-visible spectrophotometer.
59
Q n A Thanks
Home page http//www.swu.ac.kr/dslee Electro
nic mail dslee_at_swu.ac.kr
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