The equipment used share any common features regardless of the

The equipment used share any common features regardless of the ! being measured.

Each will have a light source sample cell ! selector detector

We’ll now look at various equipment types.

Electronic detection was not always available.

Early absorption methods were based on using the human eye as our detector.

In some cases, this is still a reasonable approach.

Color comparison

This ‘eye-ball’ method only requires that you compare your unknown to a series of standards.

Depth Comparison

This is an extension of the color comparison method.

The depths of the solutions are adjusted until there is a match between the two.

d_S c_R d_R = c_S

All of the methods have the same general components

Source, Wavelength selector Sample cell, Detector

Read-out

The actual arrangement of the components will vary based on the method.

Source &

Sample

!

selector detector readout

With emission methods, the sample is an integral portion of the source. It is used to produce the EM radiation that will be measured.

Source !

selector Sample detector readout

Source Sample ! detector readout selector

Common arrangement for UV/Vis

Common arrangement for IR

source

! selector

sample !

selector detector readout This is an emission method.

All three of these work together as our source and sample.

Each system will have:

The proper arrangement of components to measure the phenomenon.

Components designed to work together.

Proper slits, lenses, controls, ...

Cell materials

UV quartz, fused silica

Visible glass, plastic (UV cells can be used) IR KBr, NaCl crystals are most common material nm range

silica 150 - 3000 glass 375 - 2000 plastic 380 - 800

standard liquid cuvette

sample cell for gases

liquid sandwiched between two NaCl plates for IR

For a general purpose instrument, we need a way to produce a broad range of ! with reasonably uniform intensity.

We can seldom obtain uniform intensity but most instruments can account for this.

Lets review some of the more common sources.

The tungsten lamp is similar to a normal light bulb.

! range: 350-2200 nm Useful in visible and near IR range.

D₂ lamp

D₂ + electrical energy D₂* D₂ + h"

! range: 160-380 nm

This source produces emission lines specific for the element used to construct the cathode.

This source is used with atomic absorption and fluorescence methods.

Similar to a hollow cathode lamp in its use, it produces spectral lines by RF excitation of a metal salt - used for more volatile on non- conducting materials.

RF coil ‘salt’ containing bulb

We typically only want to look at a single wavelength at any given time.

Only interested in a single !.

Scan a range of !, in sequence.

The goal of a wavelength selector is to only allow a specific ! to reach our detector and any given time.

We can’t really obtain a single

wavelength, regardless of the source.

Line sources are subject to the Doppler effect which causes line broadening.

Our slits allow a range of wavelengths to pass through.

effective bandwidth

! selected

50%

Absorbance filters

material 1 material 2 light allowed to pass

Colored glass plates are used to absorb the ! that are of no interest.

Interference filters Thin coating of CaF₂ or MgF₂

d

!MAX = 2 d n N

where:

d = thickness n = refractive index N = order

Bandwidth is a function of the exit slit width.

Changing the position of the prism will change the ! that will pass through the exit slit.

b

!

! r

n! = d(sin i + sin r) i

i - incident angle of light beam r - reflective angle of light beam d – distance between lines n - order of reflection

! - wavelength

As with a prism, we still need the proper lenses and slits in order for a grating to work as a monochromator.

OK, now we need a way of detecting any light that has made it though our system.

The purpose of a detector is to convert our response into a measurable signal.

The approach taken varies based on the type of light that is being used.

- + 90 V cathode

anode

dynode

conversion dynode

anode window

top view

A single electron is ejected at the conversion dynode.

Subsequent dynodes are

~90V more positive which results in the e^- being accelerated and ejecting additional electrons.

Amplifications of 10⁶-10⁷ are obtained.

-- --- - -- -

-

p region n region

-

- -

- -

- - -

- -

focusing mirror

thermistor or thermocouple

mirror

slit

i

- +

wafer

absorbing film flexible mirror pneumatic chamber

All instruments can be expected to have:

! Proper amplification to produce a measurable signal.

! Signal processing to remove, average data, drive a readout, A/D conversion.

! A readout - meter, digital meter, chart, ...

! It may have some numerical processing capability.

1 2 3

4 5

6 1 - light source 4 - sample cell 2 - wavelength selector 5 - detector 3 - shutter 6 - readout

W lamp

grating

! selector cuvette

detector

Another view This type of instrument works with a

single light path.

One must account for variations in detector response and source output for each !.

It is best when working with single ! methods and individual analytes..

DB in time.

source

! selector

chopper s

r

recombining mirror

detector

A DB in time instrument works by splitting the light at regular intervals using a chopper.

Half of the light passes through your sample, the other through a reference (blank).

The ratio of the sample to reference is used to measure absorbance and account for other variations.

R

S

This approach will account for variations in detector and source response since it is the ratio that is measured.

Noise spikes are also reduced by using a ‘lock-in’

amp that only measures a signal with the right frequency - based on our chopper.

While a double beam in time instrument can reduce much of our noise and make it possible to obtain entire spectra, there are still problems.

The major one is that you can’t look at anything that changes at a rate near or faster that the chopper rate.

With a typical instrument - no kinetic studies are possible.

Double beam in space.

beam splitter

With this approach, we simply split the beam into two identical paths.

No chopper is needed so we can look at time dependent processes.

Not as much noise reduction.

It also requires two detectors that are closely matched.

With current computer technology, some manufacturers offer single beam scanning instruments.

You acquire a blank run which is stored.

Subsequent runs can then be corrected based on the blank.

One problem with traditional scanning instruments is that it can take several minutes to acquire a complete scan.

Your sample can decompose during that time.

Volatile solvent can evaporate.

Also, don’t forget that we all hate to wait. photodiode array

The photodiode array is able to measure a range of ! at once.

You typically have a trade off between resolution and ! range.

A resolution of 1 nm is possible

An entire spectrum can be measured in less than one second.

light source

excitation monochromator

emission monochromator fluorescence

detector absorption

detector sample

source sample monochromator detector

moving mirror

fixed mirror beam

splitter source

sample

detector data

processing

A complex signal is produced by passing light through an interference filter and varying the path length.

sample