we're going to start by looking at the general configuration of ir um and then general terms we are going to start with a source we'll go into each of these and then a wavelength selector there's two options for a wavelength selector we have either dispersive or non-dispersive um your dispersive is going to be for double beam and this is going to be more of a reflection grading whereas non-dispersive is going to be a filter or a filter wedge then we proceed to the sample and the samples we'll talk about can be liquids solids or gases depending on the type of holders you have available in the IR to a detector either single Channel or multi-channel and then the readout so starting with the source first um the most common source used for IR is going to be something called black body radiation or black body radiators and these are very high temperature they range anywhere from as high as 1500 Kelvin to 200 Kelvin depending on the the um IR available or the type of source available and with this type of source we get wavelengths from the maximum of about 5500 wave numbers which is about 1.7 to 2 micrometer um and we get a range of anywhere from 1 to 15 micrometers so it spans a very very wide range of about 700 wave numbers to 10,000 wave numbers and we'll look at one of those Spectra later in this lecture black body radiation is an example of a Continuum Source or Continuum radiation and it's produced when solids are heated to incandescent it is very characteristic of the idian source not the material of which the emitter is composed this graph on the right is shown an example of wavelength in micrometer compared to energy and this one is at about 220 Kelvin just showing the difference of the energy associated with the different wave lengths or ultimately wave numbers as we will talk about them there are three types of black body radiators the first one is called a ner glor this is a cylinder of rare earth oxides these are going to range from a [Music] temperature of about 12200 Kelvin to 22200 Kelvin so we got a big range there temperature and as you increase the temperature you have an increase resistance of the source so the circuit must be designed to limit the current that you have so you must be you're dealing with an increase in resistance when you increase the temperature the Second Source type is is glowbar um this is a type that is better at shorter wavelengths uh the temperature with these is about, 1300 to, 1500 Kelvin with this one you get a positive coefficient of resistance but this does require water cooling of the electrical com um components because of the Heat last type is a high pressure Mercury Arc with a fire far IR region this is looking at wavelengths uh greater than 50 micrometer so really big wavelengths this is a thermal Source uh they don't provide sufficient radiant power for convenient detection the sample cell is going to depend on the sample itself um so the sample cell or the cell windows are made of an alkal metal halide a salt so potassium bromide sodium chloride Etc they are largely transparent in ir and these are chosen primarily on the basis of range of transparency cost and solubility in the solvents um so you have to take take into account if you make a solvent or make a solution what solvent you're using and then also reactivity with the sample and the solvent so you have to take into consideration what solvents you're using when picking your cell window solids or particulates um if we looking at particulates that should be smaller than the wavelength of the IR this is to a avoid scattering that we might see um there's two types of solids or two ways we can run a solid the first is to mix it with a salt so KBR and press it into a pellet this is what is commonly used in organic chemistry you're going to mix your material with KBR make it into a pellet it is sometimes advantageous to run it as a solid because it's difficult or even impossible to often find a solvent to make a solution whose IR spectrum does not overlap significantly with the absorption bands of your sample so it's sometimes hard to find a solvent that you won't get interference from so mixing with KBR and pressing it a pellet is one alternatively you can grind it with something called a heavy H hydrocarbon oil or halogenated polymer um and the viewing res View resulting mole that you get from grinding that is a film that lays between two Salt plates and then you can analyze it with that method you can also analyze a gaseous sample this used long path length to get adequate sensitivity gases are often more difficult for analysis there are different instrumentation and techniques use to analyze gases as composed to IR solvent or liquids is alternatively another way you want to choose your solvent carefully um and you choose it based on the IR absorption and reactivity with the salt windows for instance water which we want to avoid water shows several strong absorption bands in the mid IR region and these are going to often overlap or cover up your peaks of interest same with alcohols so here's looking at some different potential solvents the horizontal lines indicate the regions in which the solvent is useful so the blue is where it's useful there is no single solvent that is transparent through the entire mid IR region so you're going to pick a solvent that has minimizes the interference that you're going to see so here's an example of a holder for IR you have a plate on the back with a gasket and if you did this in organic chemistry this should look really familiar on top of that gasket is going to be a window and a spacer another window another gasket and then the front plate your sample is going to sit right on the spacer so whether that sample be a liquid or that sample is a pellet that was pressed but that's going to sit right on there with the front plate on front and then there's going to be these quick acting nuts that go on to attach and squeeze it to make it a very tight fit ultimately what you're looking for is you need to find a narrow cell path length between 10 micrometer and 1 millimeter is to minimize the absorption due to the solvent and that's going to be your primarily concern with the solvent next we're going to look at how you determine the path length so when you're look at at a path length you get interference pattern due to constructive and destructive interferences of electromagnetic radiation waves that are reflected between the salt plates you want to get maximum constructive interference occurring when the radiation reflected from the two internal surfaces of the cell have traveled that distance that is an integral in integral multiple n of the wavelength of the radiation transmitted without reflection so what you're looking for is n where n is going to be the multiple time Lambda is equal to 2 b b is your path length so you can use this to figure out where you can get maximum constructive waves and ultimately this will help you determine the distance between the salt plates so here we have n equals Delta n = 12 can use that to solve for the path length and on the bottom we're going to get two times the wave number of one minus the wave number of two remember that wave number is just 1 over cm of your position of your wave so in this example when n equals 12 with our wave number going from 3230 to 2080 so about here 2080 we get a path length needed of 0 0052 CM detectors there's multiple different detectors we can use the first one's the thermal one this is going to be based on a temperature detection these often have poor sensitivity and they are relatively slow they have a slow response time you're talking a few millisecond response time this is not suitable when you're looking at foyer transform IR next we have TGs which is Tri glycine sulfate this is going to be based on the hyro electric effect which is temperature dependent capacitance so just as a refresher a capacitor is a device comprising a pair of conductors or electrodes that are separated by a thin layer of an electrical insulator here a pyroelectric crystal replaces the insulator these are fast so they're fast enough for forier transform the less sensitive and these are the most common detectors for forier transform infrared the last is a photoconductive these are comprised of various semiconductors the resist resistance decreases with an increased Photon flux this is due to the promotion of electrons to the conduction band and these type are slow we're looking at milliseconds and limited to the visible and near IR region unless there's two stipulations here you can cool it with nitrogen this reduces the thermal noise um and you get microsc response in the IR as opposed to millisecond so here's a general schematic of the instrumentation so we have our IR source and then our sample compartment we're looking at so what we see here is where you have a laser in this situation it's going to come in hit a mirror and that mirror is going to come up to the beam splitter that beam splitter will split the beam as it entails with some of it going to these movable mirrors on the side some going to the detector um we're going to see a movable mirror up top in the inner perometer a fixed mirror and then ultimately that beam will be split back down to the sample compartment we see more mirrors going through to get our IR transducer and then ultimately back to our laser detector this is almost exclusively based on foyer transform inter perometry some advantages um first one is you get signal to noise enhancement and rabid scanning and this is due to fgets and drut jonuts advantages so Multiplex Advantage is what we're looking at for the fets this is where all resolution elements for a spectrum are measured simultaneously so because we're measur everything simultaneously uh we get an increased signal noise ratio the jaut advantage or throughput Advantage foyer transform instruments have few Optical elements and no slits to attenuate therefore the radiance power reduces the detector is much greater than a dispersive instruments thus signal Tois increases second Advantage is precise way number calibration and this is due to laser reference and this fail facilitates signal averaging spectral subtraction and computer-based spectral ID we also see high resolution capability and no stray radiation problems so each IR frequency has a unique modulation frequency so we're not going to deal with any stray radiation problems that we've seen in some of the other instrumentation right so this is looking at an IR spectrum kind of looking at what you would see um and you will notice that it's based on wave numbers versus transmittance or the percent transmittance on the left hand side from about 4,000 down to approximately 1,600 or so somewhere in this region this is called the group frequency region and this is going to be where it's very specific for different groups on your compound also down towards the 2000 3000 is where you get very strong water bands and which is why you have to be careful of water as a solvent down on the far right hand side we have called this the fingerprint region and this is where we see small differences in molecular structure that lead to significant changes in the absorption bands this is as the name applies with fingerprints it's going to be very unique to a compound but you also see less um this is harder to identify a compound from this region as composed to the group frequency region