The equipment used share any common features regardless of the! being measured. Each will have a light source sample cell! selector 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. 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 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 Common arrangement for UV/Vis readout! Source Sample readout selector Common arrangement for IR source! selector sample This is an emission method. All three of these work together as our source and sample.! selector readout 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 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. sample cell for gases 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. D 2 lamp D 2 + electrical energy D 2 * D 2 + h"! range: 160-380 nm
This source produces emission lines specific for the element used to construct the cathode. 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 nonconducting materials. This source is used with atomic absorption and fluorescence methods. 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 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.
! selected 50% effective bandwidth Absorbance filters Interference filters light allowed to pass material 1 material 2 Colored glass plates are used to absorb the! that are of no interest. Thin coating of CaF 2 or MgF 2 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. n! = d(sin i + sin r)! r i b! 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 is to convert our response into a measurable signal. The approach taken varies based on the type of light that is being used. cathode anode 90 V + -
anode dynode conversion dynode window 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. top view Amplifications of 10 6-10 7 are obtained. p region - - - - - - - - - - n region - - - - - - - - -
thermistor or thermocouple focusing mirror mirror i wafer slit - + pneumatic chamber absorbing film flexible mirror 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.
3 1 2 4 5 6 1 - light source 4 - sample cell 2 - wavelength selector 5-3 - shutter 6 - readout W lamp Another view grating This type of instrument works with a single light path. One must account for variations in response and source output for each!. cuvette! selector It is best when working with single! methods and individual analytes.. DB in time. recombining mirror A DB in time instrument works by splitting the light at regular intervals using a chopper. source s Half of the light passes through your sample, the other through a reference (blank).! selector chopper r The ratio of the sample to reference is used to measure absorbance and account for other variations.
This approach will account for variations in and source response since it is the ratio that is measured. Noise spikes are also reduced by using a lock-in R amp that only measures a signal with the right frequency - based on our chopper. S 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. Double beam in space. beam splitter With a typical instrument - no kinetic studies are possible. 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 s 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. sample absorption light source excitation monochromator emission monochromator fluorescence
source moving mirror source sample monochromator beam splitter fixed mirror sample data processing A complex signal is produced by passing light through an interference filter and varying the path length. sample