| Author | Message | ||
     Gerry Downey (Downey) |
I have a need to record spectra of a viscous biological material over an extended time period > 2 years. The outcome will be a qualitative model. To guard against problems arising from longterm instrument drift or repair, I need to run an appropriate qc sample alongside real samples. I intend to use honey as a qc sample. Since this changes over time, I will freeze aliquots and use one of them each time I record new, actual sample spectra. Question 1. Does anyone foresee difficulties with this freeze-thaw step? The qc step will involve comparison of sequential honey spectra with a spectrum (the mean of 10?) taken at time zero. Question 2. Will the mean of say 10-20 sub-samples of a given honey sample suffice for this or should I use a number of honey samples? Question 4. How will I decide that any given qc sample has deviated sufficiently from the time zero spectrum to warrant standardisation? Question 5. What is the best way to do the standardisation? Thanks in advance. | ||
| Bruce H. Campbell (Campclan) |
Gerry, There are standards that have been evaluated for long term stability that you may consider using, as I don't see a large advantage to using honey. You may also want to consider using a pure solvent, subdivided into containers that can be sealed. These can be used as a check on other standards if desired. If you detect a significant difference in standards, however, then you must decide which, if either, standard is unchanged. Bruce | ||
| hlmark |
Gerry - well, you've been around a long time, so you know that the question of instrument standardization is one of the knottiest ones we have. If you're planning to do transmittance through a clear liquid, then Bruce's idea of solvents is about the best you can do. Solvents can be obtained in VERY pure forms, and therefore you can rely on the consistency of the fundamental physics to insure a reproducible sample over long periods - as long as it does not become contaminated. The usual contaminant being moisture, of course. PURE water (i.e., Fisher Scientifc distilled water, or better, at least) would also made a good solvent to try. Dry box technique for transferring the standard to a sealed measurement cell would be appropriate - (even for water, to keep other airborne contaminants out). If the standard becomes suspect, you can buy more, and it will have the exact same spectrum as the original. This is one area where results from classical spectroscopy can be applied - if they can be applied. And don't break the cell! If your sample is not a clear liquid, then let us know that, too. But what I suspect is that you want to use honey because it is more like your actual samples (or some sort of honey IS your actual sample). But then you're relying on the stability of the honey, for which we have no a priori expectation or knowledge, excpet that it will pick up moisture from the air. You might try to check the literature to find out what is known about honey's stability. I've read that it is stable against microbial attack, since no bacteria can survive the osmotic effects, but that's only if moisture is rigorously excluded. Also, honey can crystallize over long time periods. I have a jar that is 10-15 years old in my refrigerator, and every couple of years I have to put the jar into a pot of water and being it ot a boil, to melt the crystals again. It think it remains edible, but I don't know what effect that would have on the spectrum - I didn't measure it when I bought the jar. We didn't expect it to last this long, it's just that we eat so little of it! But freezing might make crystallization happen faster. Also, I don't know what other autocatalytic, oxidative or other processes honey might undergo, either. Freezing should help against chemical reaction, though. Seems to me that what you need to do, along with your main experiment, is a study on the long-term stability of honey, and/or whatever else you might ry to use as the long-term standard. For that matter, you might just want ot try keeping several different materials, both for the scientific interest (maybe we could all finally learn what would make a good long-term standard) and because something unexepcted might turn out to be best for your own work. One of the problems I perceive with the whole issue of standardization is that we know that the instruments are good to parts-per-million, but attempts to standardize them are subjected to the external constraint of doing all work using only the ordinary laboratory methods in ordinary laboratory environments, and not being willing to take the extra pains and go to the extra lengths needed to obtain the extra precision and stability. But doing that handicaps your efforts, and practically ensures failure. "A chain is only as strong as the weakest link." You need to pay attention to hardware, procedures, environment, and everything else. Let us know what happens, when the time comes. Good luck. Howard \o/ /_\ | ||
| Michel Coene (Michel) |
The main influence on the spectrum (and pathlength usuable) will be the concentration of OH groups in your solvent. I suggest you try solvents with different CH/OH ratios until you find something remotely resembling your (honey) samples. Normally the correction will involve dividing transmission values by each other, so you would probably want to avoid using a background that has almost zero transmission in certain wavelength areas. | ||
| kevin |
We analyse molasses samples by NIR. The calibration database has been developed over many years. Analytes include fructose, glucose & sucrose (lab method = HPAEC), brix, dry solids, pol and ash. We use an old NIRSystems 5000 with a flowcell and autosampler. A few molasses samples, previously scanned by the 5000 are kept frozen. In 2001, the encoder system packed up and was rebuilt. Predictions were extremely poor after the rebuild. Mahalanobis values (GH) jumped from 1-2 to 50. The frozen molasses samples were thawed, well-mixed and re-scanned. Average spectra before and after were used with ISI's Standardisation software. After STD predictions were once again excellent and GH values were down to 1-2. So one can keep viscous aqueous samples for STD if kept frozen and use the GH concept to monitor instrument drift. Only certain parts of the spectra were affected by the rebuild. If anyone wants a copy of this work, please email me. Of course analysing shredded cane and STD instruments is not this easy and for those of us involved, this is a big worry. KEVIN J SCHAFFLER | ||
| W. Fred McClure (Mcclure) |
TO: Kevin Schaffler Kevin I find this very interesting. If you will send your Email address, I would like to make an "off line" comment. Fred PS. I would be very interested in what you can share with me about this work. Send to: [email protected]. | ||
| Nuno Matos (Nmatos) |
If your equipment is a FT-NIR you should try use purified water. You'll not need to stock any reference sample. Any regards? Nuno | ||
| DR.K.balasubramanian (Rrfoundation) |
TO: Kevin Schaffler We are interested in having the Molasses standardisation data using NIR and Publications for research Interest.We may look forward for associating for areas of Interest. Pl send details to Regds. DR.K.Balasubramanian |