Extracted from
http://www.impublications.com/discus/messages/5/213.html?1060210133
"Then there is the detector selection. If we take the visible/NIR cutoff as where the optimum detector choice changes, it is about at 1100 nm. In the wavelength range below 1100, there is limited vibrational information available, so many scanning instruments have that as the lower end of the wavelength range. If you scan only below 1100, many of us would say you are not really doing NIR Spectroscopy, but rather visible.
For data collected in the reflection mode, the data above 1800 nm or so is of limited utility because absorptions tend to be so strong as to make quantitation in the presence of surface reflection less reliable.
So, in my experience, 1100 to 1800 nm winds up being the critical range for many analysis, and I recommend to my clients that they make sure that the instrument they buy operates reliably in this range. However, the adjoining wavelengths can add versatility that at times can be very important for specific applications.
By DJ Dahm "
"Generally, 700 -1800 nm covers almost all applications. If you are considering 700 – 1100 nm then more caution would be advised There will be some calibrations which cannot be replaced with Herschel wavelengths.
By Tony Davies (Td)"
วันพฤหัสบดีที่ 2 กรกฎาคม พ.ศ. 2558
Correlation is not the most important statistic in regression analysis.
Extracted from
http://www.impublications.com/discus/messages/5/6974.html?1280931124
"Correlation is not the most important statistic in regression analysis. The standard error of prediction (SEP) which is the error in predicting the independent validation samples is the first test of any NIR method. You can compare this to the SER for your reference analysis.
Hope this helps,
Best wishes,
Tony"
"The correlation coefficient is influenced by the range. The most important information is given to us by the SEP which answers the question "How variable are the answers from this model"? If the SEP is less than that required for the analysis then you have a method. R and r^2 are important but not the most important or useful statistics.
Best wishes,
Tony"
(Tony Davies (td) )
http://www.impublications.com/discus/messages/5/6974.html?1280931124
"Correlation is not the most important statistic in regression analysis. The standard error of prediction (SEP) which is the error in predicting the independent validation samples is the first test of any NIR method. You can compare this to the SER for your reference analysis.
Hope this helps,
Best wishes,
Tony"
"The correlation coefficient is influenced by the range. The most important information is given to us by the SEP which answers the question "How variable are the answers from this model"? If the SEP is less than that required for the analysis then you have a method. R and r^2 are important but not the most important or useful statistics.
Best wishes,
Tony"
(Tony Davies (td) )
วันเสาร์ที่ 30 พฤษภาคม พ.ศ. 2558
Effect of stray light on NIR spectrum (e.g. flattened peak)
From NIR Discussion Forum » ICNIRS
(http://www.impublications.com/discus/messages/43/9407.html?1298057637)
"The second source of instrumental error is stray light. This problem can be inherent in the spectrometer, but it can also be caused by the operator. Stray light is any light reaching the detector without passing through the sample. Thus, if a sample were completely opaque at a certain wavelength, any photons that were detected would be due to stray light. These photons could be passing around the sample, through holes in the sample as might be caused by air bubbles in a liquid, or they "might by the result of poor shielding, permitting room light to reach the detector from some external light source, for example, room lights. The effect of stray light is to add a constant power (intensity) of light, Ps, to both the numerator and denominator in the absorbance expression:
As the concentration increases, P approaches zero, and A asymptotically approaches a maximum level given by log[(P0 + Ps)/Ps]. For Ps equal to 10% of P0, the maximum value of A is 1.04. As this is a log relation, A approaches 1.04 asymptotically with the concentration. Thus, stray light should be suspected any time when nonlinear data is encountered.
Stray light in a spectrophotometer can be measured by inserting an opaque blocking filter into the optical path. A signal observed by the detector under these condition is due solely to stray radiation. A 10 g/L solution of potassium iodide does not transmit appreciably below 259 nm, but is essentially completely transparent above 290 nm when observed in a 10 nm cuvet (Poulson, 1964). If a spectrophotometer is set at a lower wavelength, say 240 nm, any signal that is observed must originate from stray radiation. In determining the stray light below 259 nm, the cuvet holding the solution should be placed in the spectrophotometer and scanned from the longer wavelength, transparent region to the shorter, opaque region. In this way the detector signal will be gradually decreased to its lowest level. False readings can sometimes be obtained by inserting a cuvet containing the sample in the spectrophotometer at an opaque wavelength, as the abrupt decrease in signal may not register correctly."
(http://www.impublications.com/discus/messages/43/9407.html?1298057637)
     Alisha (agnosus) Member Username: agnosus Post Number: 14 Registered: 1-2009 |
Thanks Howard & Karl, It sure makes a lot more sense now. I am using an FT system for tablet analysis in transmission mode using a narrow InGaAs detector. What I have noticed is that I never get absorbance values as high as 7 AU (usually 3 AU is the max) and also I usually don't get much information above 1400 nm. I am trying to understand if this is the detector sensitivity (dark noise) or stray light that is putting a cap on my absorbance values? Also note the spectral shape I am getting that is very different to that of Foss machine (this is a different formulation so comparison might not be right). I have attached a spectrum below: |
Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 399 Registered: 9-2001 |
Alisha - the "flattopping" above 1500 nm is certainly characteristic of stray light. On the other hand, FTIR is known to be resistant (not immune, but definitly resistant) to stray light, so there's somewhat of a contradiction here. One way you can easily tell how much of the signal is noise is to run the spectrum two times in a row, and then compare the two spectra. The parts that are different represents noise, the parts that are the same represent actual signal. It's possible that the limit you're running into is digital, in the number of bits in the instrument's A/D converter. Usually that shows up differently, but when you've got a "mystery" you have to consider all possibilities, as Sherlock Holmes famously said in different words. |
| Karl Norris (knnirs) Senior Member Username: knnirs Post Number: 45 Registered: 8-2009 |
Alisha, Your spectrum indicates you are not using an air reference. What is your reference,and can you provide the spectrum of your reference? I would guess that your reference has a Log(1/T) of about 3, so that your sample has a Log(1/T) of from 2.5 to 6. Stray light in measuring a sample such as a tablet is often from radiation going around the tablet and reaching the detector without going through the tablet. I will be glad to give you the benifit of my many years of experience in NIR, but I think we should switch to e-mail. My address is: knnirs@gmail.com. Karl |
From http://www.impublications.com/discus/messages/5/348.html?1106275130
A quick determination that may indicate the level of stray light would be to measure a highly absorbing sample, say 5mm of water or more in transmission, or a very absorbing sample in reflection, and note where the peaks are flattened and appear "saturated". That is the limit Tony refers to, and the percent stray light is given approximately by the conversion from the log function, as the light measured is essentially all stray light. If the peak "saturates" at 2 AU, the stray light is 1% of the NIR at that region. The stray light you thus determine at 1940 or 2130 n(From m (for example) is also present at other measurement wavelengths.
Best wishes,
Dave
(From Brown, C.W. (2004) Ultraviolet, Visible, Near-Infrared Spectrophotometers. Analytical Instrumentation Handbook, Third Edition edited by Jack Cazes, p.127 - 140)
Best wishes,
Dave
(From Brown, C.W. (2004) Ultraviolet, Visible, Near-Infrared Spectrophotometers. Analytical Instrumentation Handbook, Third Edition edited by Jack Cazes, p.127 - 140)
"The second source of instrumental error is stray light. This problem can be inherent in the spectrometer, but it can also be caused by the operator. Stray light is any light reaching the detector without passing through the sample. Thus, if a sample were completely opaque at a certain wavelength, any photons that were detected would be due to stray light. These photons could be passing around the sample, through holes in the sample as might be caused by air bubbles in a liquid, or they "might by the result of poor shielding, permitting room light to reach the detector from some external light source, for example, room lights. The effect of stray light is to add a constant power (intensity) of light, Ps, to both the numerator and denominator in the absorbance expression:
A = log ((P0 + Ps)/(P + Ps))
Stray light in a spectrophotometer can be measured by inserting an opaque blocking filter into the optical path. A signal observed by the detector under these condition is due solely to stray radiation. A 10 g/L solution of potassium iodide does not transmit appreciably below 259 nm, but is essentially completely transparent above 290 nm when observed in a 10 nm cuvet (Poulson, 1964). If a spectrophotometer is set at a lower wavelength, say 240 nm, any signal that is observed must originate from stray radiation. In determining the stray light below 259 nm, the cuvet holding the solution should be placed in the spectrophotometer and scanned from the longer wavelength, transparent region to the shorter, opaque region. In this way the detector signal will be gradually decreased to its lowest level. False readings can sometimes be obtained by inserting a cuvet containing the sample in the spectrophotometer at an opaque wavelength, as the abrupt decrease in signal may not register correctly."
วันอังคารที่ 7 เมษายน พ.ศ. 2558
Noise in regression coefficients: an indicator of data overfitting
"One well-known sign of over-fitting is the appearance of noise in regression coefficients; this often takes the form of a reduction in apparent structure and the presence of sharp peaks with a high degree of directional oscillation, features which are usually estimated subjectively.
"
(Preventing over-fitting in PLS calibration models of near-infrared (NIR) spectroscopy data using regression coefficients
(Preventing over-fitting in PLS calibration models of near-infrared (NIR) spectroscopy data using regression coefficients
A.A. Gowen, G. Downey, C. Esquerre,
C. P. O'Donnell
Journal
of Chemometrics
Special
Issue: WSC-7: 7th Winter Symposium on Chemometrics
Volume 25, Issue 7, pages
375–381, July
2011)
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