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 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. Radiation source Sample cell Wavelength selector DetectorReadout

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. ( nm) each 10 nm range has 100 units of radiation from the source (a total of 4000 units) sample absorbs 50% of radiation in nm band

Exercise 1.4 a) How many units of radiation will reach the detector with the sample out?  100 b) How many units of radiation will reach the detector with the sample in?  50 c) What should the % transmittance at 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 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 spectral bandwidth (or bandpass: the wavelength interval of radiation leaving the monochromator  eg: 500 nm; 1 nm the radiation leaving the exit slit would range from to 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 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 I in I out Reflectance I in I out

Exercise 1.13  Is the filter photometer transmission or reflectance?  transmission

Exercise 1.14 Now categorise all the instruments on the provided sheet.