Dear Readers (Volume 3 Edition 1)
Don’t forget if you contact anyone with solutions to any queries highlighted on this page, then PLEASE copy to me as well.
Janusz Ryckowski of the Marie Curie-Sklodowska University in Lublin sent us a list of no less than eight new additions to the Spectroscopists Bookshelf and some updates on existing entries.
Thanks a million Janusz. Now you readers – let Uncle Patrick draw your attention to New Year Resolutions – Can you all please emulate Dr Ryczkowski and resolve to do something for IJVS this year!!
Hurray – our first response from a reader. Ian Wesley at the CSIRO Labs in Australia sent this response to a question raised by Dominic Mikulin in the last Edition [/volume2/edition4/enquiries.htm].
He emailed: In response to your question in IJVS about the spectrum of water, the near infrared spectrum of water is discussed in detail in the following reference:
The Near Infrared Absorption Spectrum of Liquid Water, J.A. Curcio and C.C. Petty, Journal of the Optical Society of America, Vol 41, 5, 302-304 (1951).
Dr. Jerome Sallo of Sallo Consulting Services has asked me to recommend an IR Polymer atlas, my response was as follows:
The best available set of standard spectra – the set prepared by Dieter. O. Hummel. The series was published in 2 volumes – Atlas of Polymer and Plastic Analysis, published by Hanser Verlag in German and an English version published by Wiley Interscience. Sadtler also produced the set in digital form.
Don’t forget the bookshelf section of IJVS.
Dr. Ruelle Ghislain of Belgian Railways has written from their Polymer lab to enquire if we can give any advice on far infrared sampling. Ruelle uses a P-E 2000 and has experience of mid IR, but wants to know what tricks are required in the longer wavelength region. Ruelle’s email address is ghislain.ruelle.042 [at] b-rail.be . My response was:
Far infrared means different things to different people, but most chemists consider it is the frequency range 400-100cm-1. The range is this because most instruments don’t scan below 400 cm-1 and there is little data of interest to chemists below 100 cm-1
Now – I presume you have the appropriate beam splitters and a detector with a window that transmits over the range 4000-100 cm-1. Another problem with all FTIRs is that the window in the interferometer case must transmit the infrared or be removed. Removal leads to problems unless you have a reliable flushing gas. Replacement is a problem. Standard kit uses KBr – obviously useless below 400 cm-1. A thin Mylar window will do but it must be thin or you will have problems in the mid i.r. 2.5µ thick film is available but a special holder would be required and it is fragile and you will get problems with interference fringes.
It is essential to pass dry N2/Air through your instrument. If you look at ijvs, Vol 1, Ed II you will see that the use of liquid N2 blow off will reduce the water vapour to an acceptable level. The real killer in the far infrared is the absorption caused by the rotational spectrum of water vapour. Water has 3 axes of rotation all with different moments of inertia hence the rotational spectrum is very complex. Further, it spreads from zero to ~350 cm-1. Absorption is very intense and the path length for the absorption process is the total distance from the source to the detector. [If you unfold the path in the 2000 this is much more that a metre!]. So you must reduce the water level to almost nothing. Back in the 60’s and 70’s most practitioners used evacuated instruments but really dry N2 works just as well.
Sampling: its 30 years since I was a far infrared enthusiast and even built my own spectrometers. I guess the laws of physics haven’t changed much since 1970!
You can’t use KBr discs because the KBr doesn’t transmit. You can use high-density polyethylene discs but it is hard to lay on a reliable supply of finely powdered HDPE. The polymer is produced ex-reactor as a powder but immediately melted and pelleted so you can usually only get hold of ex-reactor powder by personal contact with a petroleum company. Perhaps some chemical suppliers can help. Aldrich can supply powder but it is described as surface modified so it may be of little use.
An alternative is to use paraffin mulls and squeeze these between high-density polyethylene windows 3 or 4mm thick. The problem is that polyethylene isn’t flat or rather won’t stay flat. By careful design of the holder the problem can be contained.
When you load the sample into the sample area, the sample chamber itself and detector housing will fill with wet air and you will have to wait ages for the nitrogen to replace it. If the window in the interferometer is removed, the interferometer casing will become contaminated. What to do I hear you ask?
I suggest removing the sample chamber lid and taping over the opening a 1m length of 300mm(1ft) wide lay flat polyethylene bag material. You need good wide tape – say 50mm. The brown packaging tape available everywhere works well. It doesn’t look very neat, but it works if the tube completely seals around the open sample chamber. Now seal the tube itself by folding it over close to the instrument. A couple of washing pegs or large paper clips will hold it closed. Put your sample into the open section of the tube and fold the outer end shut, then grip with paper clips. Cut the corner off with scissors leaving a really small hole and squeeze all the air out around your sample holder. Then release the inner fold. Nitrogen will now flow from the sample chamber towards the hole you have cut. Leave it like this for 10 minutes and then slide the sample into position. Removal is the reverse of course. In this way you will keep the wet air out of your machine. During sample changes I would use 10l/minute flow or even a little more. Overnight ~5l/minute. Consumption of liq. N2 – 1 litre yields about 800l of gas so the cost won’t break the bank.
Interpreting the spectra can be a nightmare, because the bands you see are due to fundamental modes plus lattice modes but this may not bother you. Of course, the real way to analyse polymers and inorganics in this part of the spectrum is to use RAMAN. Buy a 2000R accessory and you are home and dry. Perhaps the biggest advantage of Raman is that you don’t have to prepare samples and you get 3500-100cm-1 in one go – but then I suppose I’m biased!!