Advanced Spectroscopy

1. General Aspects

Revision

1. What is the difference between absorption and emission of radiation?

Abs. – uptake; Em. - release

Exc.

Gnd

radiation

Absorption Emission

Revision

2. 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

Revision

3. 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

Revision

4. 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

Revision

5. 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

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)

Revision

7. 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

Revision

8. Draw a schematic diagram showing the components of a typical absorption spectrophotometer.

Radiationsource

Samplecell

Wavelengthselector

Detector Readout

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.

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

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

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

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

Exercise 1.3

a) 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

Wavelength selectors

The 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.

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

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

Exercise 1.4

a) 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%

Exercise 1.4

d) 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%

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

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

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

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

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

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

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

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)

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

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

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

Sample holders

shouldn’t absorb where analyte is absorbing

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)

Evolution of detectors

human eye photographic plates and film electrical electronic

most generate a current from the radiation energy

1.5 Instrument configurations

Scanning 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

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

use a stepping motor which rotates the grating or prism by very small angles

other ways exist

Exercise 1.6

Is the filter photometer scanning or non-scanning?

non-scanning

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”

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

Rotating chopper (see below)

Mirror

Mirror

Semi-transparent mirror

transparent sector

mirrored sector

Design of Chopper

Source

Detector

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

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

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!)

Exercise 1.9

Is the filter photometer single– or double-beam?

single

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

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

Exercise 1.10

Is the filter photometer dispersive or non-dispersive?

non-dispersive

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

Single- or multi-channel

Radiation

source

Sample

cell

Dispersing

element

Array

of

detectors

Exercise 1.11

Is the filter photometer single or multi-channel?

single

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

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

Transmission

Iin Iout

Reflectance

Iin

Iout

Exercise 1.13

Is the filter photometer transmission or reflectance?

transmission

Exercise 1.14

Now categorise all the instruments on the provided sheet.