(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."
