advanced spectroscopy
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Advanced Spectroscopy. 1. General Aspects. Emission. Absorption. Exc. radiation. Gnd. Revision. What is the difference between absorption and emission of radiation? Abs. – uptake; Em. - release. Revision. - PowerPoint PPT PresentationTRANSCRIPT
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Advanced Spectroscopy1. General Aspects
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Revision1. What is the difference between absorption and
emission of radiation? Abs. – uptake; Em. - release
Exc.
Gnd
radiation
Absorption Emission
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Revision2. Why are the peak wavelengths in the absorption
and emission spectra of the same species identical?
the energy gap is the same going up and coming back down
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Revision3. In spectroscopic terms, what is the difference
between an atom and a molecule? How do their spectra differ in appearance?
atom is totally free of any connection to any other species; doesn’t have be neutral; single wavelength of absorption: LINE spectrum
molecule has bonds (not necessarily covalent) to other species; eg Na+ in water; range of wavelengths: BAND spectrum
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Revision4. Rank the following regions of the
electromagnetic spectrum - ultraviolet, visible and infrared - in terms of increasing energy, frequency and wavelength.
Energy
Freq.
UV
Visible
IR
Wavelength
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Revision5. How do absorbance and transmittance differ?
Which is more useful for analytical purposes? Why?
transmittance is simple ratio of intensity out/intensity in
absorbance is log T
absorbance is more useful because it is linear with concentration
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Revision State Beer’s Law, and explain the meaning of
each term. Under what situations does Beer’s Law not apply?
A = abc A = absorbance a = constant specific to species b = pathlength c = concentration low & high absorbances (for most species < 0.1
& > 1)
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Revision7. Explain how you could determine what
concentration range for a given species obeyed Beer’s Law.
dilute a standard until you get an absorbance <2
use simple proportions to work out the concentration that would give an absorbance of 0.2
round this conc. to a convenient value make stds 1x, 2x and 4x
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Revision8. Draw a schematic diagram showing the
components of a typical absorption spectrophotometer.
Radiationsource
Samplecell
Wavelengthselector
Detector Readout
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1.1 Radiation sources all spectroscopic instruments require one
radiation is measured to provide the analytical measurement: spectrum or absorbance
the type of source varies
particularly between absorption and emission instruments.
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Exercise 1.1 What is the most important difference between
the radiation source in absorption and emission instruments?
absorption: separate lamp source emission: excited sample is source
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Role of radiation sources
to provide radiation that can be absorbed at specific wavelengths by the analyte
allowing a comparison of intensity before and after sample
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General requirements it must produce radiation in the wavelength
range that the instrument is designed to operate most sources are continuous
produce radiation at every wavelength across the range they are designed to work in
Exercise 1.2 One absorption instrument that you are familiar
with does not use a continuous source. Which one is it?
AAS
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Desirable characteristics the intensity should be consistent across the
range (normally it will taper off at the extremes of the range),
the intensity should not fluctuate over time
the intensity of radiation should be not be too low
the intensity of radiation should be not be too high
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Exercise 1.3a) consistent across the range no missing bits or big spikes
b) consistent over time so that measurements don’t drift (no need to re-
zero all the time)
c) not too low detector inaccuracy
d) not too high decompose the sample & detector inaccuracy
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Wavelength selectorsThe role of a wavelength selector
To reduce the range of wavelengths reaching the detector to those near the absorption (or emission) wavelength of the analyte.
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Why is one needed? absorption of radiation at one wavelength is not
affected by the presence of others
it is the detector that creates the need for a selector
it can’t tell the difference between wavelengths and responds to all of them
without a selector: no spectra, since only one measurement is
available totally inaccurate absorbance measurements
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Illustration of absorbance problem assume the following:
visible abs. spectr. (400-800 nm) each 10 nm range has 100 units of radiation
from the source (a total of 4000 units) sample absorbs 50% of radiation in 500-510
nm band
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Exercise 1.4a) How many units of 500-510 radiation will reach
the detector with the sample out? 100
b) How many units of 500-510 radiation will reach the detector with the sample in?
50
c) What should the % transmittance at 500-510 be? 50%
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Exercise 1.4d) What is the total intensity reaching the detector
with the sample in (given no wavelength selector)?
4000 – 50 = 3950
e) What is the actual %T that the instrument will display?
100 x (3950 ÷ 4000) = 98.8%
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without the wavelength selector, the detector is swamped by lots of radiation that has nothing to do with the analyte’s absorption
radiation absorbed
radiation not absorbed
absorption wavelength
range
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Types of wavelength selectors an ideal wavelength selector would allow one
wavelength of radiation only to pass to the detector
actually allow through a range of wavelengths (this is known as the bandpass)
how wide that range is depends on the design of the selector, and also the experimental conditions required
two basic classes of wavelength selector: monochromators filters
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Filter sheet of plastic or glass that absorbs most
radiation cheap simple no moving parts => portable wide range of wavelengths filter must chosen to match absorption peak
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Monochromators a series of optics inside a lightproof box entry and exit slits which allows the radiation of
all wavelengths in and a narrow range of wavelengths out
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Monochromators dispersing medium is either a prism or a
diffraction grating. work by causing the different wavelengths of
radiation to change their direction at different angles depending the wavelength
results in a band of single wavelengths which are directed towards the exit slit
because it is very narrow, only a small range of wavelengths can actually exit and reach the detector
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Monochromators to select a wavelength, the prism or grating
rotates causing the band of radiation to shift, moving a different wavelength over the exit slit
Monochromatorsetting
550 nm500 nm
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Prisms vs gratings prisms are simpler but less accurate gratings the most commonly used
a grooved surface, where the grooves are extremely close together
100’s-1000’s grooves/mm transmission or more commonly reflection better performance in terms of throughput
and consistency
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The significance of the exit slit how wide the exit slit is determines the range of
wavelengths that come through
known as the slit width
usually the exit slit is adjustable
as the slit width decreases, the range of wavelengths that are passed by the monochromator decreases (and vice versa)
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The significance of the exit slit the actual slit width is not important in itself it is not equal to the range of wavelengths that
pass through it the important measure is spectral bandwidth
(or bandpass: the wavelength interval of radiation leaving
the monochromator eg: monochromator @ 500 nm; bandpass @ 1 nm
the radiation leaving the exit slit would range from 499.5 to 500.5 nm
the bandpass affects the appearance of the spectrum
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Positioning the wavelength selector can go either before or after the sample cell better afterwards in some instruments, it must go before determining factor is the energy of the beam
from the source whole UV beam may decompose the sample
=>the selector is placed before the sample
Source Sample selector
Source selector Sample
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if before sample, light from surrounds can enter and reach detector
known as stray light: any radiation that reaches the detector that is not from the source
light-seal doors over the sample compartment are required
if after sample, wavelength selector will block most external light
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Sample holders
shouldn’t absorb where analyte is absorbing
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Detectors respond only to the total intensity of radiation cannot distinguish between different
wavelengths
high sensitivity high signal-to-background ratio constant response across the range of
wavelengths rapid response linear response (i.e. output is proportional to
radiant intensity) minimal response to no radiation (known as dark
current)
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Evolution of detectors human eye photographic plates and film electrical electronic
most generate a current from the radiation energy
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1.5 Instrument configurationsScanning or non-scanning the ability to record a spectrum requires automatic wavelength changes and
many of them two key requirements:
measure the intensity at wavelengths that are very close together
vary the wavelength without human assistance
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Exercise 1.5 An instrument using a filter cannot be a scanning
instrument. True Cannot measure close together wavelengths
An instrument using a monochromator must be a scanning instrument.
False A monochromator doesn’t have to have a motor
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use a stepping motor which rotates the grating or prism by very small angles
other ways exist
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Exercise 1.6 Is the filter photometer scanning or non-
scanning?
non-scanning
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Single or double beam absorption instruments require two intensity
measurements going into the sample (“before”) passing through
before computers, two ways of measuring the “before”
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Single beam only one sample holder “before” measurement is taken using a blank at
the start if the wavelength is changed, the instrument
needs re-zeroing
Double beam two sample holders and a split optical system “before” intensity measured continually really double-path, not double-beam
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Rotating chopper (see below)
Mirror
Mirror
Semi-transparent mirror
transparent sector
mirrored sector
Design of Chopper
Source
Detector
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Double-beam should double-beam configurations have two of
everything: beams, sources, detectors etc cost no way that the two systems could be made
exactly equal extra optics (mirrors) mean that less radiation
goes through the system (known as throughput) more bits and pieces which can get out of
alignment necessary to have two matched cells –
difficult/expensive
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Single-beam (no PC) does not have double-beam problems or
requirements without a computer it can’t record spectra when the wavelength changes, re-zero take the sample out and put the reference back
in
Exercise 1.8 Why is it necessary to re-zero the instrument
when the wavelength changes? source output and detector response vary
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Single-beam (with PC) advent of desktop computers revolutionised
spectrometer design allows the spectrum baseline to be measured at
the start stored in memory a single-beam instrument can scan
double-beam instruments with PCs do exist – they seem to be overkill (one or the other!)
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Exercise 1.9 Is the filter photometer single– or double-beam?
single
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Dispersive or non-dispersive means “dividing up” a component (eg grating) that divides up the
radiation by wavelength doesn’t apply to filter-based instruments ND1: no wavelength selector at all
employed for pollution monitoring in harsh environments
achieve some measure of selectivity by clever use of reference materials
non-scanning mostly in the infrared region
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Dispersive or non-dispersive ND2: use a mathematic function Fourier
transform scanning they are very fast simpler internal configuration also mostly in the infrared region
ND3: a detector that is capable of distinguishing between different wavelengths of radiation only in the X-ray region
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Exercise 1.10 Is the filter photometer dispersive or non-
dispersive?
non-dispersive
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Single- or multi-channel refers to the number of detectors scanning using a stepping-motor monochromator
takes time and the optics can become misaligned alternative is numerous detectors, each
responsible for a range of wavelengths a dispersing medium is still needed no exit slit or motor parts some are not capable of producing a spectrum,
only numerous wavelength measurements others can a continuous bank of very small
detectors
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Single- or multi-channel
Radiation
source
Sample
cell
Dispersing
element
Array
of
detectors
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Exercise 1.11 Is the filter photometer single or multi-channel?
single
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Transmission or reflectance does not apply to emission instruments familiar with spectroscopy involving a semi-
transparent liquid or solid how much radiation passes through the sample not all materials will transmit radiation if opaque, unabsorbed light is reflected (bounces
off)Exercise 1.12 Give examples of materials which transmission
spectroscopy would be unsuitable for. paint, fabric, some plastics
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Transmission or reflectance Reflectance spectroscopy measures radiation
that bounces off the surface of an opaque material
direct reflection by a mirror-like surface (called specular)
scattered at a variety of angles by a rough surface (called diffuse)
basic principles still apply: certain wavelengths will be absorbed, and an
absorption spectrum obtained
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Transmission
Iin Iout
Reflectance
Iin
Iout
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Exercise 1.13 Is the filter photometer transmission or
reflectance?
transmission
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Exercise 1.14Now categorise all the instruments on the
provided sheet.