| Author | Message | ||
     Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 59 Registered: 9-2001 |
I have to add something to what David Hopkins said: in the early days of NIR (say, from the mid-1970s to the mid-1980s) the major applications were in the agricultural areas. The absorbance bands corresponding to the key materials in those types of samples were in fact, inherently broad: moisture, starch, and protein all have absorbances that are intrinsically broad due to the effects Dave mentioned. Therefore there was little or no incentive for the instrument manufacturers to design instruments with high spectral resolution, because the applications of them, at that time, did not justify expending the engineering time and resources to improve the resolution. Nowadays, of course, many more different types of applications exist, some of which include measurements of materials with the sharp bands that Dave also described. It is the change in the measurement requirements that drove the technologies in the direction of high spectral resolution devices. \o/ /_\ | ||
| Tony Davies (td) Moderator Username: td Post Number: 134 Registered: 1-2001 |
Eric, I have not much to add to what Michel and David have said. I think we want to say that NIR absorptions are NARROW except that 1) Isolated peaks are rare and 2) they are very strongly affected by their immediate local environment. (By immediate local environment I am thinking in terms of neighbouring molecules not more than one or two from the absorption site; so in a sample there will be many sites which can have subtle differences). You said �let�s take a 1st overtone� so lets take the first overtone of a C-H stretch vibration. There will be three absorptions: the overtone of the symmetrical stretch, the overtone of the anti-symmetrical stretch and the combination absorption due to the combination of the sym and a-sym vibrations. These will all be quite close so with a low resolution instrument you would see one broad peak but as David said if you use a high resolution instrument you would see three peaks. You will find isolated peaks in gases and crystalline materials. In gases there are no neighbouring molecules and crystalline materials are highly organised so local environments can be identical. Talc is one of the best examples; it has a very narrow peak at 1392nm which, according to Karl Norris, has a bandwidth of less than 2nm. In their epic wavelength/wavenumber calibration study Peter Griffiths�s student Husheng Yang investigated the 3,768 lines in the first overtone H-O-H stretching mode of water vapour using a high resolution instrument to find lines that were not overlapped when measured at normal resolutions. He found only TWO! Best wishes, Tony | ||
| David W. Hopkins (dhopkins) Senior Member Username: dhopkins Post Number: 95 Registered: 10-2002 |
Eric, There are 2 more mechanisms of broadening I can suggest. 1) Molecular associations may cause the frequency of an absorber to fall due to weakening of the bond. This causes a shift lower frequency in the band. The classic case is liquid water, where the combination band at about 1900 nm is broadened. A range of strengths of the effect gives rise to a pronounced asymmetry of the band. If the water is bound to starch and protein, as in wheat flour, the band is shifted to about 1940 nm, and the range of interactions gives a high slope to higher wavelengths (lower frequencies). 2) Measurements of samples that are essentially mixtures of chromophores in many different environments will show the same broadening that Michel mentions. This lead to the early understanding that the �starch� band in flour at 2100 nm is very broad, and that the protein band at 2180 nm is quite broad. As we now understand, the OH vibrations have a wide range of frequencies or energies due to the wide range of environments in the polymer. Similarly, because the amide band in a protein occurs in about 20 amino acids, each of which occurs in different environments, the band due to the summation of all those amide transitions, that are referred to as a protein band, is broadened. It should be pointed out that when crystalline materials of high purity are scanned with a spectrometer of high enough resolution, the bands are observed to be quite sharp. Even the region at 1680 in polystyrene appears to be a broad band at 10 nm resolution, but is seen to be a superposition of many bands when measured in an FTNIR instrument at 2cm-1 resolution. This is the first overtone of the phenolic OH of the styrene. It seems to me that the NIR region got a bad reputation as having broad, uninteresting bands in the early days, that is really undeserved. Now, with highly capable instruments, we can use the resolution appropriate to particular questions to make very useful measurements. Hope this helps. Regards, Dave | ||
| Michel Coene (michel) Senior Member Username: michel Post Number: 39 Registered: 2-2002 |
Each molecule in your mixture vibrates at a slightly different frequency due to "local" circumstances (temperature, surrounding molecules...). For the first overtone, you double the frequency and therefore also the variation on that frequency. The peak becomes twice as broad. | ||
| Eric LALOUM (elaloum) New member Username: elaloum Post Number: 2 Registered: 5-2006 |
Dear All, NIR bands are larger than MIR bands because modes of vibrations which occur in NIR are overtones and combinations. I would like your help (some theory) in explaining why an overtone band (lets say 1st order) should be broader than the fundamental one. It's quite straightforward to explain why peak height is smaller for an overtone than for a fundamental (transition probability), but explaining peak broadening sounds more tricky. Thanks, Eric |