| Author | Message | ||
     question |
Hi All, Thanks a lot for all your input and help. It was indeed very useful to oreint my experimentation. The need is just to evaluate amount of copper sulfate. I talked to a professor in the chemistry dept. and he said since all the sulfate is comming from copper sulfate only we can jsut do a simple FTIR on sulfate and hence know how much copper is in there. I hope its going to get me started. Thanks a lot again. | ||
| David W. Hopkins (Dhopkins) |
Hi Bighjc, Some color agents extend their effects into the NIR. Examples are carbon black, whose effects extend across the entire NIR, and some conjugated systems, like chlorophyll, whose electronic transitions are seen in the visible and whose CH, CH2 and CH3 contributions are measured in the NIR. Many brown or yellow materials have absorbance shoulders that extend far into the NIR, up to 1400 or 1500 nm or more. It is not likely that the blue color of CuSO4 extends into the NIR. However, as a salt, increasing concentrations interfere with the hydrogen-bonding of water to an increasing extent, and so the concentration is measured by the secondary effect on the water spectra in various samples. As temperature also affects the hydrogen bonding of water, you can often get very good measurements of pure salt solutions by using spectra taken at constant temperatures. Delwiche et al have shown that you can even measure salts at various temperatures, with the proper experimental design. I am away from my resources, so I cannot cite the references just now. I hope this helps. Best regards, Dave Hopkins | ||
| Gabi Levin |
To Hejincheng - Good work on copper sulphate - 2 questions: 1. What was the concentration range, was it around 0.1%? 2. What wavelengths gave you high correlations - for exapmple, if you are using software such as Unscrambler you can plot the "Loading Weights" for each PC, (a measure of the correlataion vs. wavelengths) and you can see in what wavelengths you have high correlation to the reference data. This could help determine the nature of the absorption that yielded the correlation. About the color - if the color variation is a result of significant changes in concentration of NIR absorbing entities, then, yes, it will affect the NIR spectrum, if the color variation is a result of small changes of very powerful colorant agents, which in very low concentrations casue large color changes, possibly you will not observe NIR changes. Many such situations exist with pharmaceutical tablets where the different dosage levels are color coded, and the colorant is a very strong colorant, and it takes ~0.05% or less to obtrain the color effect, you don't normally see NIR changes. Thanks, Gabi Levin Brimrose Corp. of America | ||
| hejincheng (Bighjc) |
Could the color variations affect the absorbance of the NIR spectrum that wavelength more than 1100nm. | ||
| David W. Hopkins (Dhopkins) |
Hi Bighjc, Good job! I think it is good to have an experimentalist in the group. What was the absorbance range in the visible? I will theorize that you would obtain somewhat better results using a 5 or 10 mm pathlength cell. This has been an interesting discussion, and thanks for your input. Best regards, Dave | ||
| hejincheng (Bighjc) |
I have done a test using NIR to quantifying copper sulphate with a mixture glucose and glutamic acid.Spectral range from 1100nm to 2498nm. PbS detector at a 2-nm resolution using 1-mm transmission pathlength. r2=0.8856 | ||
| Gabi Levin |
Hi guys, I ,love this discussion - we haven't heard almost a word from the originator, and we fly on the wings of our imagination. I am still convinced that the need is just to determine the Copper sulphate. The sugar and salts possibly simulate some living organism (I am flying on the wings of imagination now) situation. Back to 0.1% constituents in a mixed matrix - my suggestion - forget NIR, even if it has hydration water - although it might improve the situation because usually hydrated water absorb below 1400nm and mostly sharp peaks. I was going back with my memory to the 60's - Copper sulphate is a hydrated complex of the Copper with the electrons of the oxygen - the blue is an asorption of other wavelngths in the visible. They must have some old Beckman photometer in schools, don't they, or maybe they have some more modern ones, anyway, simple single wavelgnth (go back to some textbooks on inorganic quantitative analysis and you will find the recommended wavelength)calibration will give you a good way to measure the copper sulphate with little interference, and since neither sugar, nor salts such as NaCl absorb in the visible interference should be negligible, and anyway, if you include them in the calibration you are even better of. Hope this helps a little bit. Gabi Levin Brimrose Corp. of America | ||
| Bruce H. Campbell (Campclan) |
First of all, I assume the assay only needs to be done within about a 5% relative error. If that is so, you don't need instrumental methods. If you have a solution, gently dry it to a solid. Measure its mass. Then fire it in a muffle furnace. Measure the mass which is the sodium chloride. Subtract that amount from the solids obtained by gentle heating. That result is the sugar. Then make standards of 0.05 to 0.15 % copper sulfate with the known amounts of sugar and salt (just to make sure there is no interaction with the copper). Visually compare the colors - unless you are color blind. | ||
| hlmark |
What Emil said is true enough. If you have the necessary equipment. Otherwise you might need to come up with creative ways to use what's available. And Emil knows better than most the advantages of having your answer in electronic/digital form. So at this point, Question, it's up to you to come up with the answers to the questions that have been developed in this discussion: more information about the nature of the samples, what limitations are there on your ability to expand your sights beyond NIR, how many samples will you be needing to analyze on an ongoing basis, do you have to measure the sugar and salt as well as the CuSO4, and whatever other information might be needed to come up with a sensible analytical method? | ||
| Emil Ciurczak (Ciurczak) |
Instead of looking for esoteric means of doing simple analyses, I would recommend: UV for the sugar, Visible for the sulphate, and a simple titration for the chloride. As Gabi suggested, go back to a general analytical text. NIR (despite my making a living with it) cannot do everything. | ||
| hlmark |
Question - Gabi brought up a good point that I hadn't considered: whether you're dealing with the materials in solution or not. I had assumed that they were in solution. In either case, Gabi, in turn, seems to forgotten that copper sulphate, unless specially prepared, normally exists as the hydrate: CuSO4*5H2O. And it certainly is hydrated in solution. That's why most copper salts, and solutions, are green or blue; in the case of copper sulphate a deep royal blue. As for doing analysis: if you are in fact working in solution and there are no other, unknown interfering contaminants, you could make up standards gravimetrically. This also flies in the face of "normal" NIR procedure, but clear solutions are one of the areas where you can get away with that. The hard part about doing that will be to generate a suitable set of samples for the calibration; they should comprise a balanced set of concentrations of all the known components. You may want to get the assistance of a statistician to help. If you want to contact me off the discussion group, I can talk to you about that. If, indeed, you are working with a mixture of powders, then you could certainly do something along the lines of what Gabi recommended regarding application of one form or other of atomic spectroscopy, to serve as the "primary" method. However, it would also be possible to use NIR alone, in conjunction with one of the more advanced external and/or internal standard techniques that have been developed for classical analytical chemistry (sometimes we forget that analytical chemistry existed before NIR!) Howard \o/ /_\ | ||
| Gabi Levin |
To the question of copper sulphate - 1. Assumption 1 - the measurement is to be done in a mixture of solids, and there is sugar there, and the concentration of the sulphate is 0.1% by weight. Since the copper sulphate has no significant absorption by itself, no chance of measuring 0.1% in powder mixture with organic compounds that dominate the NIR spectrum 2. Assumption 2 - measurement is to be done in solution. - With the sulphate dissolved, in 0.1%, no chance measuring with the water absorption dominating the spectrum. 3. To measure trace 0.1% copper sulphate in mixture with salts and sugar - go to textbooks on analytical chemistry, and start assessing what is the best way to do that. Most likely you will find that you need to prepare soltuions and use something like atomic absorption or similar method that will be based on copper specific wavelngth of absorption or emission spectroscopy for this low level, in such complex media. The atomic absorption spectroscopy is using high temperature to cause the nebulization of a copper containing solution, and send a wavelgth of light that is absorbed by excitation of an electron to a higher energy level. These are very sharp absorption peaks, and the method is very sensitive and accurate. Atomic emission is using high tempertaure such as in a plasma to cause electrons in the nebulized copper atoms to be excited to higher energy states, and when they drop back to the ground state, they emit light in very specific wavelegths, that are unique to copper. The intensity of the emitted light is a measure of the concentration. I hope this helps in any ay Gabi Levin Brimrose Corporation of America | ||
| Question |
Thanks a lot Howard, could you suggest any other method I can use for quantifying (~ 0.1 %) copper sulphate with a mixture NaCl/Sugar. One of my friends suggested that I can assay Sulphate with IR so I can evaluate Cu from there. | ||
| hlmark |
Hello, "Question"! Answer is (or starts off) like this: Most "Common knowledge" and "generally accepted Principles" have exceptions. Also, even besides the exceptions, they have limitation, which if you know and understand what they are, you may be able to work around. In all situations in spectroscopy, the question is whether there are absorbance bands a suitable wavelengths and intensities to be able to measure. We usually consider inorganic salts as being "unmeasureable" because they have no -OH, -CH, or -NH functions, which are generally considered required for NIR analysis. However, that requirement is usually based on the assumption of analysis of organic samples, for which those functional groups are the main ones that create absorbance in the NIR. However, if you put an inorganic salt into warer, there is a possibility for interaction between the salt and the water that surrounds it, that perturbs the absorbance bands not of the sample but the surrounding water. If this can be done consistently and reproducibly, it may well serve as the basis for a measurement. In this cae you would be working (if indeed it "works") around a limitation. In the case of copper sulphate, we know that in water solution it is blue, again because of interactions between the salt and the water. In this case it is the electrons in the copper sulphate that are affected, though, potentially makin it a more specific measurement than when depending on the effects of water structure. Furthermore, since the blue color means that the longer visible waves are being absorbed, it may very well work out that the absoprtion regions of the solution will overlap with the measurement capability of a nominally "NIR" instrument (especially if the instrument has "short wave" NIR capability) and again result in a doable measurement. In this case you would be working with one of the excpetions to the rule that inorganics have no NIR absorption. Howard \o/ /_\ | ||
| question |
hey, my prof aksed me to assay copper sulphate in mixture of copper sulphate (~0.1%)with salt and sugar and instantly I thought of using NIRS but it seems the NIRS cannot work with Metals, whys that ??? | ||
| Nancy Bedore |
Help Our Sales force keeps asking me for a simple explanation on the difference between FT-IR and NIR. I have never worked with FT and only support our NIR instrumentation. Does someone have an explanation for the common man? Thanks in advance | ||
| Bruce H. Campbell (Campclan) |
Nancy, Think of it in this way. Imagine you are in a pond, beating it with a paddle. The waves you create head for the side of the pond. When the waves get there, some parts of the waves hit stone, some hit wood, others sand, and yet others hit dirt. The waves that hit stone are reflected back strongly, the ones that hit wood are less reflected, and so on. You also have a device that measures the amount of interaction of the returned waves with respect to those you are creating. There is math that can tell, from the strength of the interactions of the returning and sent waves, what is causing the reflection. All of this becomes a steady response. The longer the interactions are observed, the better is the definition of the interactions, i.e., better precision. Replace the water waves with light waves and the reflecting surfaces with the sample of interest, wherein the sample has different interactions, dependent on what is in the sample, such as various functional groups. This is perhaps too simple of an explanation. I am sure others will add to it. | ||
| djdahm |
Hope one of the answers captures the sense of your question. If not maybe you can rephrase it. I will comment on the differences between FTIR and NIR by dividing the question into two parts? What is the difference between NIR and IR? The difference, of course, is the wavelength of light used: but What are the practical differences? The absorption bands in the IR region tend to be more intense, more spread out, and sharper for a given compound. It is generally harder to do quantitative analysis in the IR region because frequently just a little bit of a compound absorbs almost all the light. IR is much better than NIR for qualitative analysis. For quantitative analysis, IR analysis was generally limited to dilute solutions. A main advantage of NIR has been the ability to do quantitative analysis on samples as they occur, with minimum of pretreatment. What is the difference between FT and other (e.g. grating) techniques? Again on a practical note: Historically, it was very hard to get useful data in the NIR range from an spectrometer designed for the IR wavelength region. This is changing with the FT method. The region of reliable data from FTIR instruments has been sneaking down into the NIR wavelength range. Consequently, the question like �What is the difference between NIR and FT-IR?� would have made little sense in the past, but it is very relevant now. For a particular set of applications encountered in a given setting, it may well be that purchasing an FTIR instrument and using it in the NIR region may be the most cost effective path. It may also turn out that a simple filter NIR instrument is the most cost effective choice. However, I think most of us who use NIR in a research setting believe that the grating instruments still do a better job than the FT instruments in the NIR range. But �Instrument manufactures reserve the right to be smarter tomorrow than they are today�, and that opinion may change sometime. | ||
| Stephen Medlin (Medlin) |
I thought that was a good survey answer on the differences between IR and NIR. I'm going to share your answer with my colleagues here at work as a starting point to the differences. In fact, I had to give a quick explanation to one of our guys who didn't realize that NIR IR. I do have a question for you re. your statement of "...most of us who use NIR in a research setting believe that the grating instruments still do a better job than the FT instruments in the NIR range." My personal experience has always been with FT-NIR (that is, those designed for the NIR and not an extended range Mid-IR) in the process world. I would be interested to know what you see as the advantages of a grating instrument in the laboratory as compared/contrasted to the process application. I look forward to a non-vendor specific discourse. | ||
| hlmark |
Let me add a third point of view. The practical differences between Mid-IR and "modern" NIR grew largely out of their histories, as Don alluded to. What Don said about the difficulty of getting good data from grating instruments is only partly true: before the advent of FTIR, much good work in the mid-IR was, in fact, done using grating instruments. It is true that with the possible exception of some very special applications, FTIR is almost universally preferred nowadays, due to its well-known advantages. But NIR was developed independently, and was born practically "full grown" from the work of Karl Norris, who put together the three conceptual pieces that were necessary to make it work: diffuse reflectance measurements, use of the NIR spectral region, and multivariate math to make it all work, and the rest of "modern" NIR grew from that basis. Historically, the first commercial instruments to implement Karl's work used interference filters, and much good work was done with those. "Scanning" instruments worked by tilting the interference filters to change the wavelengths, but this provided only a very limited wavelength range. Grating instruments came later. While grating-based UV-Vis-NIR instruments were available (from Varian P-E, Beckman and other manufacturers) they were not useful for implementing Karl's concepts. The reason for this was that those instruments all emphasized their spectral resolution capabilities (0.1-1.0 nm), but their S/N (on the order of 0.1-1.0%, which translates to about 1,000 - 10,000 microabsorbance units of noise) was not sufficient to support the requirements for performing the calibration mathmatics. The initial "modern" instruments did not have such good spectral resolution (10 nm) but the S/N capabilities were 3-4 orders of magnitude better than the UV-Vis-NIR instruments of that time. In fact, you could say that the improvement in noise performance was the distinguishing feature between the "classical" and the "modern" instruments. Had the original instruments had sufficient S/N, it seems likely that their potential for doing quantitative analysis would have been discovered much earlier. | ||
| djdahm |
Thank you, Howard. You are nothing, if not exact. (I wonder if non-native English speakers realize that that is a compliment rather than an insult.) Of course, there was much good work done on grating instruments in the IR region. I didn't wish to say otherwise. In fact, all my work in IR was done on a grating instrument, but then I don't know how "good" it was. And that leads to Stephan's question as to "the advantages of a grating instrument in the laboratory as compared/contrasted to the process application". I used terms like "I think...most.. believe" because I don't know for certain. All my experience is on grating instruments. When it comes to instruments, I tend to think that what I am familiar with is better until someone shows me a clear-cut advantage to something else. To do otherwise tends to lead one into a career of never ending instrument comparisons. And instruments are not really my bag. I assume that there may well be advantaes to FT-NIR in process applications related to speed of data collection. Does anyone have opinions on the advantages in current instrumetns? I will happily sit on the sidelines and listen, especially to people who might actually have collected comparitive data. | ||
| djdahm |
Thank you, Howard. You are nothing, if not exact. (I wonder if non-native English speakers realize that that is a compliment rather than an insult.) Of course, there was much good work done on grating instruments in the IR region. I didn't wish to say otherwise. In fact, all my work in IR was done on a grating instrument, but then I don't know how "good" it was. And that leads to Stephan's question as to "the advantages of a grating instrument in the laboratory as compared/contrasted to the process application". I used terms like "I think...most.. believe" because I don't know for certain. All my experience is on grating instruments. When it comes to instruments, I tend to think that what I am familiar with is better until someone shows me a clear-cut advantage to something else. To do otherwise tends to lead one into a career of never ending instrument comparisons. And instruments are not really my bag. I assume that there may well be advantaes to FT-NIR in process applications related to speed of data collection. Does anyone have opinions on the advantages in current instruments? I will happily sit on the sidelines and listen, especially to people who might actually have collected comparitive data. | ||
| MPDC |
I once compared a grating to an AOTF (crystal). In an airco-room on a polystyrene cushion, I was pretty amazed by how sharp a detail the grating analyser delivered. After moving both of them outside on top of a silo however, S/N on the grating took a serious hit. The solid state AOTF spectrometer was clearly superior. So I guess it is (like always) down to application, application, application... | ||
| hlmark |
Our anonymous "MPDC" makes a good point: in practice you have to evaluate an instrument in the light of the application it's intended for. Theoretical studies of instrument performance are all based on the "pure" instrument, without consideration for external factors. I suppose that to some extent, this is necessary and inevitable, since there are infinite possible "external factors" that might affect it, and it would be impossible to account for them all - and too complicated to do a rigorous study even if you could. There's another point we sort of glossed over, too: the question of NIR versus FTIR really separates into two questions: NIR vs Mid-IR, and FTIR technology versus grating (and other) technology; the question as stated really mixes these two questions together. The question of FTIR vs gratings is relatively straightforward in the mid-IR region, at least. Fellgett's (multiplex) advantage and Jacquinot's (throughput) advantage really do provide better S/N performance in the mid-IR, where the blackbody curve is way down, and the signal enhancements of these two advantages really do provide benefit. The S/N advantage can be traded off for other benefits, such as shorter measurement times. Other technologies (e.g., interference filters) could also provide equivalent benefit, but filters do not measure a full spectrum, so that becomes a matter of apples and oranges even though it would be possible to build an instrument based on filters that would be competitive with FTIR for a particular chemical analysis. In fact, filters could even beat FTIR in special cases: if only a very small number of wavelengths needed to be measured (2-10, say), then a filter-based instrument could make the necessary measurements with equal S/N in less time than even an FTIR. But that, AGAIN, is application-dependent. In the NIR, the comparison is not so clear. The stronger sources and more sensitive detectors available in the NIR region obviate the Fellget and Jacquinot advantages to a large extent. In fact, the energy throughput can actually become a disadvantage: causing saturation of amplifiers and A/D detectors, overheating and (in the extreme case) burning samples, and similar effects. I believe that many, if not most, FTIR instruments used the NIR region have A/D converters that do not have sufficient digital resolution (18-24 bits) to utilize the full potential dynamic range inherent the optical beam, making the S/N become limited by the A/D converter capabilities. Another complication is the fact that in the NIR region we do enjoy other technolgies that also provide full spectral measurement, AOTF is an example. So a thorough comparison would have to take into account these other techniques as well. And then, of course, the evaluation would have to compare them in the light of the intended application: what other characteristics are important? Is milli- or micro-second measurement speed important? Instrument size? Portability? Field of view? Sample size? Flexibility? Mechanical ruggedness? Hostile environment? Power consumption? etc. Howard \o/ /_\ | ||
| Eigil Dabakk (Eigil) |
Just a comment on speed: Modern grating instruments measure spectra with approximately the same speed as FT instruments (or rather; interferometers). Depending on resolution it my be even quicker but high resolution is rarely an issue. On the other hand, and what might be an important discrimator between interferometers and grating instruments, is that interferometers measure all wavelengths simultaneously while grating instruments do not. If samples are moving this may introduce serious noise and artifacts, especially upon co-adding since one part of the spectrum "sees" one part of the sample while the end of spectrum is measuring something different. Artifacts are also found in interferograms but after Fourier transformation they end outside the NIR spectral region (see e.g. Ola Berntsson et al. Analytica Chimica Acta 431 (2001), pp 125-131). | ||
| hlmark |
Sample noise is one of those "external factors" I was alluding to before. Sample noise due to a moving sample will affect both FTIR and grating measurements. Depending on the nature of the noise, you might be able to find a way to nullify its effect using one or the other technology, and that would then be the deciding factor. The general solution, averaging (coadding) spectra, could be applied as well, but if the noise is severe this is likely to require averaging a lot of scans together, whichever technique is used. Here the instrument is not the limiting factor, though, so arguments one way or the other would mostly be moot. Howard \o/ /_\ | ||
| Art Springsteen (Artspring) |
Howard Mark wrote: "Sample noise due to a moving sample will affect both FTIR and grating measurements. Depending on the nature of the noise, you might be able to find a way to nullify its effect using one or the other technology, and that would then be the deciding factor." That's a very good point but I'm guessing the noise would manifest itself in different ways with a moving sample. I would think that with an FT-IR, you would simply see a photometric offset type of noise 9after a lot of signal averaging), while with a scanning/grating type of instrument, you would see what I call 'slope noise'- ie. both shot noise in the individual scans and a moving of the spectrum up and down over the entire band but not symmetically. Art S. | ||
| hlmark |
Art - with enough signal averaging the noise would go away in either case (theoretically, at least) as long as it was random at least from one scan to the next. If it's random and affects one data point to the next, then it will give a random contribution to the spectrum in either case. This is the same as detector/instrument noise, and averaging will also take care of it, depending on its severity. I was thinking more of a situation where the noise had some sort of intermediate frequency, then averaging over several data points might help, but would give different results on the spectrum in the two cases: for a grating instrument you would lose spectral resolution; as we all know, bands broaden with averaging. In the case of an FTIR, if you did the averaging on the interfergram you would retain spectral resolution but lose the short-wavelength end of the spectrum. Another type of "noise" would be some sort of non-random, cyclic effect. Conceivably you could then synchronize the measurement cycle to the noise cycle and the "noise" would then become a constant offset at each data point, and the cyclic phenomenon would appear superimposed on the data. This sort of stuff is done with current instruments, where rotating sample cups are available for some instruments, and the readings are synchronized to the cup rotation to reduce or eliminate the rotational dependence of the data from some types of samples. In an FTIR, such a cyclic effect on the data would appear as a "spike" (probably at the low frequency/long wavelength end of the spectrum) in the spectrum at a characteristic wavelength, where it could be ignored or even edited out of the spectrum. Howard \o/ /_\ | ||
| Art Springsteen (Artspring) |
Howard Mark wrote: "with enough signal averaging the noise would go away in either case (theoretically, at least) as long as it was random at least from one scan to the next. If it's random and affects one data point to the next, then it will give a random contribution to the spectrum in either case. This is the same as detector/instrument noise, and averaging will also take care of it, depending on its severity. " Ah, but that's the rub, isn't it? For an on-line or at-line system, you really don't know how random it is. Furthermore, how many scans DO you need to assure that it is random? In a spinning cell type instrument, that's likely not a problem but there are a lot of instruments out there that may not have that type of sample handling. Cheers! Art S. | ||
| hlmark |
Well, here we are, again: that depends on the application, doesn't it! But you're correct: someone in that situation would have to make that part of their research study. \o/ /_\ | ||
| hlmark |
Well, here we are again: that depends on the application! But you're correct. Someone in that situation would have to include that factor in their research. \o/ /_\ | ||
| Edward Stark |
Art, Howard, and all. I believe there is a subtle effect in FT due to sample variation. The modulation of the signal not only appears at low frequencies (outside the spectral region of interest, but probably also causes a small increase in the bandwidth response of the transformed spectrum due to amplitude modulation of the interferogram. However, I suspect the effect is so small at NIR frequencies that it can be ignored. To mention another type of instrument, diode array instruments effectively measure simultaneously at all wavelengths so they exhibit Fellgett's advantage without the dynamic range limitations of FT. The essentially simultaneous measurements together with short scan times prevent the spectral distortions that may result from sample changes during the scan. AOTF systems lack the simultaneous measurement although short scan times help. | ||
| Tony Davies (Td) |
Hi, I was going to say �In a recent article in NIR news, Karl Norris wrote �� But actually it was 9.4, 3 (1998)! So, in an article in NIR news in 1998 (9.4 p3-5) Karl Norris wrote about interactions among instrument parameters. He used a computer simulation and towards the end of the article he said �So now the question is: is it easier to construct a 10 nm dispersive instrument with a noise less than 50 muau or an 8 wavenumber interferometer with less than 100muau? Interferometers may be measuring all wavelengths �simultaneously� but they need a complete mirror cycle to obtain an interferogram. In most FTs the mirrors do not move very fast so there must be some disturbance from a moving sample. Sometime ago I suggested that we ought to collaborate on a �complete� mathematical model instrument; this topic would be a very useful exercise for it! While we are on the subject of moving samples, I remember (I think) that Mats Josefson gave a very good lecture at EAS (2000?) showing how to identify artefacts caused by moving samples and how to remove them. I don�t know if he has published it anywhere. Tony | ||
| hlmark |
Tony - I could ask him. I think I have his contact information \o/ /_\ | ||
| Tony Davies (Td) |
It's OK Howard, I've done it. Tony | ||
| hlmark |
Tony - so have I! \o/ /_\ | ||
| hlmark |
Tony - re the mathematical model of instruments - the job turns out to be considerably more than you might expect. If you get Spectroscopy (the American one) you'll know that it took me 14 columns just to analyze the behavior of three types of noise, representing (although indpendent of) the behavior of three types of (idealized) instrument. Not that your proposal is undoable, but you should realize what you're proposing getting into. If you don't get SPectroscopy you can download the list of columns at: http://www.nearinfrared.com/columns.htm and the "Noise" set is #40-53 in the "Chemometrics in Spectroscopy" group (#78 - 91 in the list). \o/ /_\ | ||
| Mats Josefson |
Dear Tony and Howard, Tony Davis wrote "...lecture at EAS (2000?) showing how to identify artefacts caused by moving samples and how to remove them. I don�t know if he has published it anywhere." Yes, we are publishing this work in Chemometrics and Intelligent Laboratory Systems with the title: "NIR spectroscopy on moving solids using a scanning grating spectrometer - Impact on multivariate process analysis" authors M. ANDERSSON, O. SVENSSON, S. FOLESTAD, M. JOSEFSON, K.-G. WAHLUND. We are now sending our revised manuscript back after the review and I hope that it will be published soon. Mats J | ||
| Mats Josefson |
Dear Tony and Howard, Tony Davis wrote "...lecture at EAS (2000?) showing how to identify artefacts caused by moving samples and how to remove them. I don�t know if he has published it anywhere." Yes, we are publishing this work in Chemometrics and Intelligent Laboratory Systems with the title: "NIR spectroscopy on moving solids using a scanning grating spectrometer - Impact on multivariate process analysis" authors M. ANDERSSON, O. SVENSSON, S. FOLESTAD, M. JOSEFSON, K.-G. WAHLUND. We are now sending our revised manuscript back after the review and I hope that it will be published soon. Mats J | ||
| newguy |
Hi, this is a very simple question to you big guys, I just saw an NIRS instrument in a company, they said its a reflectance spectroscopy, I read the literature in the book, also the beer law and everything is related with the absorbance like the formula T(transmittance)=i/i0 then taking the log of it to convert it to Absorbance, so how is reflectance come into picture ? Thanks very much. | ||
| Tony Davies (Td) |
Hi Newguy! It is reflectance because for many NIR measurements that is how is how the measurement is made! We shine NIR energy on a solid sample and measure what is reflected i.e. what was not absorbed. The conversion of reflection (R) to absorbtion (A) by A= 1/log(R) was invented by Karl Norris about 40 years ago. Don't worry about the theory, it is still the subject of debate, but NIR analysis is a very useful practical tool for many industries. Hope you are going to join us! Best wishes, Tony | ||
| hlmark |
Hi, newguy: welcome to the clubhouse!! You're correct that originally, absorbance was defined as log (Io / I) only for measurements made by transmission through a sample. In the late 1960s - early 1970s Karl Norris, who was working for the USDA at the time, discovered that you could also make quantitative measurements by reflection from the surface of a sample that was ground up. This was an enormous advance, since many materials of huge physical and economic importance could not reasonably be measured by transmission. These materials include many natural products, and in particular, wheat: you cannot separate out the components (the protein, content, for example) of these natural products and have anything resembling their original form. Nor can you make any form of synthetic sample mixture that resembles a wheat kernel in any meaningful way. So the ability to measure wheat, and other agricultural and natural products in their unchanged form by this means was an enormous advantage. Some theoretical diffculties arise, but as Tony said, much useful work is done via reflection measurements despite them. For performing quantitative measurments, however, in practice it was found that the reflectance values has the same limitation as transmission measurements: in addition to whatever other characteristics it has, reflection is non-linear with respect to the constituent concentrations. A good number of transformations have been tried as a way to linearize that relationship, but for both theoretical and practical reasons, the same log (Io / I) calculation as is used for transmission measurements turns out to be the best overall way. Sometimes other transformations are also applied to the log (Io / I) values, but log (Io / I) is still the generally-accepted starting point. So the bottom line is that you can use it even if you don't understand it fully. Howard Mark \o/ /_\ | ||
| W. Fred McClure (Mcclure) |
To the New Guy on the block: What the guru's have not said is that the reason the theory of reflection is still "up in the air" is that we have not found a way to determine the pathlength followed by the NIR energy in the sample. In addition, we do not yet have a good handle on the way NIR energy interacts with the sample at particle interfaces. Thus, absorbance in reflection [i.e. Log (1/R)] is a "tongue in cheek" approach that allow you to do work in the absence of sound theory. It's kinda like jumping off a bridge into unknown waters. You hope its deep enough to handle your first mistake (jumping off). About the only person working on the theory of the interaction of NIR energy and matter is Don Dahm, Rowan Uni., USA Email: [email protected] | ||
| newguy |
Thank you all, it was really helpful. sincerly, -Amit. | ||
| Ed Stark |
Hi newguy Just wanted to add 2 cents worth to the discussion of log 1/R. I think that a primary reason that log 1/R works well is that it converts multiplicative factors into offsets which are easier to handle in the calibration. There are many multiplicative factors, such as lamp intensity, in spectroscopy. | ||
| Sunil |
What is Basic difference between FTIR and IR? | ||
| hlmark |
Sunil - the most generic meaning of the term "IR" means any part of the infrared region, i.e., wavelengths from about 700 nm (or wherever the "Visible" part ends - I don't want to open that argument) to roughly 50 - 200 microns (or wherever "Infrared" ends and "submicron" or "terahertz" begins). Different groups using different technologies tend to have somewhat different definitions as to where the dividing lines fall. There are offical definitions for these terms, but few people adhere to the official definitions "religiously", I think. Among chemists and spectroscopists, however, the unmodified term "IR" has tended to come to mean mid-infrared, generally starting at about 2.5-3 microns (2500-3000 nm, or 3300-4000 wavenumbers) and ending at roughly 50 microns (200 wavenumbers) to 200 microns (50 wavenumbers). Again, there are still some diffrences of opinion, and often someone will use a looser definition based on what his equipment is capable of measuring. Sometimes the region beyond 50 microns is called "far IR", especially by spectroscopists studying molecular rotations which fall in that region. The key point that I think you're looking for, however, is that regardless of the wavelength region, the term "IR" refers to the spectral region (or measurements made in that spectral region) and means that the measurement is independent of the type of instrument used to make the measurement. "FTIR" (Fourier Transform Infrared), on the other hand, is rather specific in that it means the use of an interferometer to produce what is called an "interferogram", followed by the application of the Fourier Transform computation to convert the interferogram to a spectrum (that's where the name and abbreviation come from). Initially this type of measurement was used almost exclusively in the mid-infrared region and the term "FTIR" has come to be associated with that spectral region so closely that nowadays they are almost synonomous; "FTIR" has practically come to mean "mid-IR measurements using an interferometer" in common usage. Using interferometers to make measurements in the near-IR, for example, is nowadays specified by a term such as "FT-NIR" or something similar, to indicate the wavelength region as well as the measurement technology. There is further blurring of the meanings in that often a person using an interferometer-based instrument will simply call his measurement an "IR" measurement, which is also legitimate since in principle any properly-operating instrument should produce essentially the same spectrum (and no arguments, please, about the differences between interferometer and grating-based measurements; I know about them but am trying to not confuse Sunil further in what is already a confusing area despite the suface simplicity of the question). Howard \o/ /_\ | ||
| ashishkadam |
where i will find information about milk analysis by NIR spectrometer. And what is optical sensors? | ||
| Nancy Bedore (Nancy) |
I'm so embarrassed! I asked the question and then never looked for the answers. I hate getting old. I was also wrong with my first question. What I meant was what is the difference between NIR and FTNIR. Both use the same wavelength region, it's wavelength v wave numbers, interferometers v gratings and so on. But, is it a personal choice or is there an advantage of using one over the other? I promise I will stick around this time to find out the answers. Thanks for the input above. Nancy | ||
| Tony Davies (Td) |
Dear Nancy, Welcome back! You stimulated an interesting debate! If you plan to market a new instrument the instrument designer has to decide if it is going to be a grating, interferometer, or some other technology. At the dawn of the modern era of NIR spectroscopy (in the '60s) Karl Norris and Dave Massie learned how to make a very, very good dispersive spectrometer so that people who followed in their path copied them and used gratings. When NIR was beginning to make a name for rapid analysis (mid'80's?) the "Big" instrument companies decide that they would like a share of this new market. Their expertise was in producing FT instrumentation so for them it was natural to use interferometers to measure NIR spectra. As you will gather from the previous debate (above) FT tends to have better resolution but worse signal-to-noise, (S/N), while grating based instruments can have very good S/N but not so good resolution. NIR spectroscopy needs very good S/N and at first the FTNIR producers did not appear to understand this and their first instruments were not very good. Nowdays there are very good FTNIR instruments. You do not often need high resolution in NIR spectroscopy; if you do then you would use FTNIR. See above for other considerations. If none of these apply then you would consider price, and the manufacturer's reputation for the general quality of their instrument, their experience in NIR spectroscopy and their servicing record. i.e the general considerations when buying instruments. Does this help? Best wishes, Tony | ||
| Nancy Bedore (Nancy) |
Tony Thank you very much. It does help. It is just one of those things I never understood since I have always used a grating instrument. This does clear things up. Thanks again Nancy |