| Author | Message | ||
     Art Springsteen (artspring) Advanced Member Username: artspring Post Number: 24 Registered: 2-2003 |
Hello Gareth, One last post, I promise! Typically, one does NOT use a mirror for a baseline in a Preying Mantis. These devices are made to measure diffuse reflectance. If the PM is well designed, the specular component measured should be zero. Typically one uses a 'white' material such as BaSO4, Spectralon�, Fluorilon�, or packed PTFE as a reference. All have very high reflectance with a very small specular component. Yes, the BaSO4 is much like KBr is used in mid-IR. It's a non-absorbing diluent, although it is opaque, rather than transparent. You get good internal scatter, however. Good luck with your work. If this char is really fully 'char', it's just carbon black and I doubt you will see much. When we looked at a lot of carbon blacks through dilution, we would see some spectral structure in a few of them- most likely the remnants of lignin, which have some poly-cyclic aromatic structure. | ||
| Gareth D. Clay (gdclay) New member Username: gdclay Post Number: 2 Registered: 2-2010 |
Hi everyone, Thanks for all your posts over the weekend - at least I might be in the right ballpark with the ideas! As NIR isn't really my field, I really appreciate all the comments and encouragement. We're using the Cary and PM primarily as it was being moved from another lab, so to save it going into storage and collecting up dust, it's on 'loan' to us. I realise it's an old beast but at least it�s being used. Is there anything I can do to make the most of this set-up? Thanks Art for the idea about BaSO4, I'm assuming it acts in much the same way as KBr that I've read about in some papers? I think I would agree with Art that at the moment its only really qualitative results that are coming through. Baselines - this is still confusing me a bit. Some papers explain in some detail about collecting baselines, whilst others do not (suggesting they didn't use one?). Some reference to a white card or powder whilst the instruction manual suggests you can use a reference mirror. Can you use a reference material that is user defined e.g. a standard char? Currently I have all my scans in the raw data form without reference to a baseline, though I do have scans of the reference mirror. The existing data has given me the overall idea though I realise it is not suitable to be a proper dataset yet. So, before I start in earnest with more work, what material should I be using as a baseline given the low reflectance of the material in the first place? Cheers, Gareth | ||
| Peter Flinn (peterflinn) New member Username: peterflinn Post Number: 3 Registered: 1-2006 |
Hi Gareth, I was very interested in your PhD studies, (a) because I am an NIR user and (b) because I am very involved in the volunteer fire service and a passionate advocate of fuel reduction burning (managed fires)to reduce the impact of the monstrous bushfires we have endured in Australia in recent years (I imagine our vegetation fires are hotter then yours - lots of the white ash that Tony mentioned!) If only it were possible to use NIR spectra of as an indicator of fire intensity. However as others have noted, the spectra become rather "bland" when the fire is very hot. Years ago, I was confronted with a problem caused by a fire in the hold of a ship used to export live sheep to the Middle East (commonly over 100,000 head per voyage). The hold contained a huge amount of the pelleted mixed diet fed to the sheep. On this occasion, the residue of pellets at the bottom of the hold was not cleaned out before a new batch was pumped in, and there was a lot of condensation on the walls of the hold. The new pellets were dry (less than 10% moisture) but the residue on the bottom would have been quite wet. As expected, over time, the contents of the hold became warmer and warmer, until spontaneous combustion occurred and most of the pellets were either destroyed by fire (fortunately when the ship was still in port) or the water used to (try to) extinguish it. Obviously there was an investigation, and also an attempt to salvage at least some of the pellets. I was sent a heap of samples, with varying degrees of fire damage, from slightly singed right through to black! Needless to say, I scanned all samples by NIR (a Pacific Scientific 6250 in those days)- and guess what, the spectra became progressively flatter as fire damage increased. I would be interested to hear further details of your project, if convenient. E-mail: [email protected] Regards Peter Flinn | ||
| Art Springsteen (artspring) Advanced Member Username: artspring Post Number: 23 Registered: 2-2003 |
Tony, I know you didn't- I was just pulling your leg a bit there. You are 100% correct- the Preying Mantis was developed as a diffuse reflectance accessory for the mid-IR, then ported over to the NIR. They are perfectly ok if you have only a small amount of sample in a qualitative sense but you can't really do any quantitative spectroscopy using a PM. | ||
| Tony Davies (td) Moderator Username: td Post Number: 221 Registered: 1-2001 |
Come on Art! I didn't say it was a bad instrument, I said it wasn't the best! I do admit to a long held feeling of irritation with Varian that when they acquired Cary they generally ignored Karl's work in developing the BEST NIR instrument. The Praying Mantis Diffuse Reflectance accessory was designed for mid-IR and is not ideal for NIR Best wishes, Tony | ||
| Art Springsteen (artspring) Advanced Member Username: artspring Post Number: 22 Registered: 2-2003 |
Hi Tony and Gareth, I didn't say Gareth has bad instrumentation (I very much appreciate Cary-5's and my beasts); it's just not the most sensitive for the work he's doing. I'm not totally convinced that Praying Mantiseses (Manti?) are the most sensitive piece of kit for diffuse reflectance measurements, especially in low reflectance situations. Gareth: You might try diluting your samples with a bit of barium sulfate and see if you can see structure. The BaSO4 is spectrally neutral in the NIR (you could also use PTFE powder). As for baseline correction- you should have had some sort of white standard that came with your PM accessory. And yes, they can go out of alignment and are real bear to get back in. basically, you put the Cary-5 in energy mode and just tweak the mirrors to maximize the signal in that beam. And yes, mirrors do attenuate signal- about 5-10% per mirror. | ||
| Tony Davies (td) Moderator Username: td Post Number: 220 Registered: 1-2001 |
Hello Gareth, First of all I am really pleased to hear of someone in the UK (outside of pharma) doing some NIR! Next I have to agree with Art (hello Art!) you do not have the best instrumentation. And I agree with Howard (hello Howard!)that what is happening is that your fires are burning mainly volatiles and that leaves behind a lot of carbon. Carbon is VERY black in the NIR region (apart from diamond of course). If your fires get really hot then this carbon will burn and you will be left with a white ash. Ash will be more reflectively and generally flat. So this seems generally in line with your observations. If you would like to e-mail me with a phone number, I could give you a call so that we could chat about your aims. I'm at: [email protected] Best wishes, Tony | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 305 Registered: 9-2001 |
Gareth - There are some (very broad) similarities between charring and cooking. When you cook food, you cause chemical reactions to occur. The hotter you heat the food and the longer the food is cooked, the more reaction takes place. When wood or vegetation is charred there are also chemical reactions, which can be lumped together as "pyrolysis". Creosote and other volatiles are given off; these basically consist of large organic molecules with lots of aromaticity. For the most part what happens is that these volatiles carry off the hydrogen content of the wood, leaving a residue that is mostly carbon. That's the char. Spectroscopically, NIR is well-known to be particularly sensitive to the absorbance bands of the -CH content of organic materials, so when the hydrogen disappears, so do the bands they give rise to. \o/ /_\ | ||
| Art Springsteen (artspring) Advanced Member Username: artspring Post Number: 21 Registered: 2-2003 |
Hi Gareth, We've done quite a bit of reflectance measurements on 'black stuff'- carbonized wood, etc. over the years. I would have been surprised to see 20-30%R. I think the 5% R you are seeing is right in the ballpark. You are going to have difficulty making out any spectral features in really dark stuff on a Cary-5 (we use Lambda 9/19's- same beast, different manufacturer). It just gets lost in the baseline. | ||
| Gareth D. Clay (gdclay) New member Username: gdclay Post Number: 1 Registered: 2-2010 |
Hi, I've recently started to work with NIRS as a technique and I have a whole series of questions that hopefully the NIRS community may be able to help with. During my PhD I started working with using NIRS as a tool to characterise char samples for natural vegetation fires in the UK uplands (wildfires and managed fires). From a limited set of experimental samples there appears to be a relationship between the intensity of the fire and the NIR spectra produced, both in terms of overall shape and in terms of amplitude of absorbance features. What we saw was a decrease in depth of absorbance features and a general flattening out of the char spectra as fire temperature and duration increased. Does anyone recognise this as an existing idea? Also it�s quite hard to disentangle duration from temperature - any thoughts? In terms of technical questions, I�m using a Varian Cary 5E UV-VIS-NIR Spectrophotometer with a Praying Mantis Diffuse Reflectance accessory scanning in the range 400-2500nm. Char samples are ground into a powder and placed into the sample cell. The overall shapes of many of the spectra match existing literature however the overall % reflectance is lower. As I�m using char samples, I would expect a low degree of reflectance (20-30%) as the sample is dark but typically I�m only getting 5% at best (often much less). Without posting all the set-up details what could be the likely causes for this? Ones I�ve thought of: Poor mirror alignment on the accessory; Dirty mirrors (I read the mirrors themselves can attenuate the signal to a degree); what�s the best/worst method for baseline correction. As you might be able to tell from all that I�m pretty new to all this so any help you could give would be greatly appreciated! I�m really keen to get into NIRS and to develop this technique so any general reading would also be welcome Many thanks, Gareth Clay |