Advanced Spectroscopy

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 • 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 • 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

UV Visible IR Energy Freq. Wavelength Revision • Rank the following regions of the electromagnetic spectrum - ultraviolet, visible and infrared - in terms of increasing energy, frequency and wavelength.

Revision • 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 • 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

Radiation source Sample cell Detector Readout Wavelength selector Revision • Draw a schematic diagram showing the components of a typical absorption spectrophotometer.

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 • consistent across the range • no missing bits or big spikes • consistent over time • so that measurements don’t drift (no need to re-zero all the time) • not too low • detector inaccuracy • 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 • How many units of 500-510 radiation will reach the detector with the sample out? • 100 • How many units of 500-510 radiation will reach the detector with the sample in? • 50 • What should the % transmittance at 500-510 be? • 50%

Exercise 1.4 • What is the total intensity reaching the detector with the sample in (given no wavelength selector)? • 4000 – 50 = 3950 • What is the actual %T that the instrument will display? • 100 x (3950 ÷ 4000) = 98.8%

absorption wavelength range radiation not absorbed radiation absorbed • without the wavelength selector, the detector is swamped by lots of radiation that has nothing to do with the analyte’s absorption

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 Monochromator setting 550 nm 500 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 spectralbandwidth (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

Sample  selector Source  selector Sample Source 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

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 Source Detector Mirror Semi-transparent mirror Design of Chopper transparent sector mirrored sector

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

Array of detectors Radiation source Sample cell Dispersing element Single- or multi-channel

Advanced Spectroscopy