You’ve all been very quiet lately, with questions. However we’ve had a result with some queries raised by Richard Duerst regarding the FTIR paper in the last edition’s Dear Readers, although Patrick knew he knew some of the answers, he decided he needed some expert help. Mike Ford, who has written the second feature article in this edition, with advice from Dr Andy Turner from Perkin Elmer has responded. To make more sense, Richard’s query again was….
Your article (please correct me if I’m incorrect) on “How does FTIR work?” in the latest Internet Journal of Vibrational Spectroscopy was very enjoyable. It brought back memories of two old questions. Could you send me your perspectives on the following questions?
- What are the trade-offs in gong from an 8- to a 16- to a 20-bit A/D convertors? and
- What are the non-random noise contributions (and what equations are used to mix the random with the non-random noise sources) which limit the n (superscript: 0.5) improvement in the S:N?
Mike and Andy’s answer is as follows…..
- Trade offs in A/D convertors. In most FTIR systems, the dynamic range from the noise level to the maximum signal is greater than 64,000 to 1, i.e greater than 16 bits. There are two effects if the noise is not adequately digitised, a) the full signal to noise capability will not be achieved and, b) there can be quite severe distortion due to the higher frequencies (wings of the interferogram) being suppressed or not represented at all – as would tend to be the case with only 8 bits. Depending upon the particular system, 18 to 20 bits is usually enough to adequately digitise the noise and give the full potential S:N. An effective 18 or 20 bit conversion is often achieved by ‘gain rangeing’ i.e. using a 16 bit convertor with high gain for the ‘wings’ and reducing the gain in binary steps to prevent overflow at the centre burst.
- Non-random ‘noise’. Probably the most frequently encountered source of non-random noise is inadequate digitisation of the noise. Under such conditions, the digitised signal will tend to be ‘steppy’ with the steps being largely reproduced in successive scans. This will lead to the spectral distortions mentioned above, with no improvement being achieved by summing multiple scans. Some degree of cancellation of these effects can be achieved by deliberately adding random ‘noise’ (‘dithering’ the signal) at the level of the one or two least significant bits.
A few comments have come in to keep us on our toes regarding the web site itself. It is a great help as it continues to grow, so please if you find something that’s not right, tell us.
John Twilley from Hawthorne, NY USA commented..
Your spectra library is very nice but the link to Zirconium silicate (last entry on the list) doesn’t work. Your journal is the most useful of the online journals that I receive free access to.
Thanks John, its sorted now. Another bad link was advised by Richard Duerst
The Calculation of IR and Raman Intensities (Netherlands) This is not a good link. Can you help?
Another one sorted. Here’s something that always worries me, someone picking up English grammar errors – fortunately I can say that this bit was done by me (ha ha!). As the page was started by the design agency I took over from. I hadn’t noticed it, but Jack Feinberg had.
You misspelled “its” in your Table of Contents. “Its” is the possessive form. “It’s” means it is. Your web site says this:
Volume 1, Edition 3: Features atmosphere and it’s infrared absorption, methods of solving atmospheric interference and a description of a very novel Raman spectrometer.
Volume 1, Edition 2: Features atmosphere and it’s infrared absorption.
Its always nice to have good things said about the journal. Karine Gigant from Wuerzburg, Germany emails….
It’s a very good site, articles are presented in a comprehensive way, what is very valuable for students. Practical aspects are well and simply described. One can find a lot of useful information like spectral databases, WWW links.I hope IJVS will find a new sponsor otherwise I will miss it a lot.
Thanks Karine, we hope so too! Now a comment from Mike McGraw from Oregon, USA.
Use more lateral space for the publication. Your journal is one long slender column.
Great web-journal. I look forward to more issues!
Well Mike, IJVS is produced like that for a reason. When it was being developed it was designed so anyone could print it down from the web on any printer, also the ‘white space’ does make things stand out. But as long as you finding it useful and entertaining bear with us!
|Please keep sending your comments and questions, sometimes it may take a while for us to answer, but we will.
All your questions and our replies are published here in Dear Readers.