1. Editorial
This is the last edition of IJVS to appear under the Perkin Elmer banner. At Easter 2002, P-E finish their sponsorshop, a contribution which I pointed out last time is and was unequally valuable. P-E have funded costs for the first 5 years of the Journal and all of us must be thankful. Could I please ask that ALL of you email a "thank you" to Dr. Dave Clarke at David.R.Clark@perkinelmer.com. HOW SUCCESSFUL IS IJVS? There are currently 1600 'registered' readers and many, many more casual ones. Recently, we used web analysis software to analyse the log files on the website. A full report is available on www.ijvs.com/stats . Several points are very clear.
So, there is no doubt at all that the Journal is a success and must
continue. IT WILL if I have anything to do with it!!
Mission The international Infrared and Raman Users' Group (IRUG) is dedicated
to the professional development of its members by providing a forum for the exchange of IR
and Raman spectroscopic information, reference spectra, and reference materials. IRUG is
comprised of individuals within the fields of art conservation and historic preservation
who use IR and Raman spectroscopy to study the world's cultural heritage.
First winners – call for applications 2002 Last year, the Büchi NIR Award for outstanding contributions in the field of NIR spectroscopy and its applications has been awarded for the first time. The value of the award amounts to USD 5'000. The first award went to the group around James Drennen III at the School of Pharmacy of Duquesne University, Pittsburg. The work the international jury deemed best deals with the problem of monitoring powder blend homogeneity in the mixing process of drug material. This process is of high importance in the pharmaceutical industry, its monitoring, however, proves to be difficult. James Drennen and his team devised an NIR-based method for monitoring the process using spectroscopic and imaging techniques at the same time. It enables the optimization of both the mixing quality and duration of the mixing process. Furthermore, the residual inhomogeneities are quantified and their spatial distribution becomes visible.
Interested parties are encouraged to submit their NIR results and apply
for the Büchi NIR Award 2002. Detailed information and application forms can be requested
at nir.award@buchi.com.
tel. +48-81 537-55-96
After nearly 60 years of intensive application, infrared spectroscopy (IR) remains the most widely used, and usually most effective, spectroscopic method for characterization of surface chemistry of heterogeneous catalysts [1,2]. From the historical point of view both catalysis and IR spectroscopy are of the same "age". Scientific bases for those both scientific areas were created at the beginning of XIX century, and since 1940 they are successfully "co-operating" together [1]. In the past few years one can observe a growing interest in the application of IR techniques in catalytic investigations. One of the reasons, among the others, is their wide distribution (nowadays, IR and/or FT-IR spectrometers belong to the standard equipment of every scientific laboratory) and the relatively low costs (compared to the other modern physico-chemical techniques for surface characterization) of the base instrument. A variety of IR techniques has been and can be used in order to obtain information on the surface chemistry of different solids (Figure 1). Special meaning have investigations carried out under the reaction conditions. In principle for in situ measurements, all forms of IR spectroscopy are suitable. For most practical experimental reasons, however, the transmission-absorption and diffuse reflectance techniques are best suited. This is more related to the design of the cells that are to be used as reactor, rather than with the principal problems of the other techniques.
Cells for in situ studies The IR cell in which the catalyst sample is pre-treated and subsequently studied is extremely important in surface studies. The perfect, all-purpose cell has yet to be devised, and cell design is normally chosen to suit the purposes of a particular study. Some features are usually of overriding importance in a given application. If catalytic reactions are to be studied the exposure of catalytic metals must be eliminated in cell construction, and bare heating elements within the cell are ruled out. In some surface characterisation studies such features may be completely acceptable, but even in such studies it is well to avoid any possibility of Ni or other carbonyls being formed from cell components. A variety of relatively simple, but effective, cells has been used for studies. Many of these have been described in the literature and schemes of them have been given [4]. As it was mentioned, starting from the pioneering work of Eischens et al. [5] on supported metal catalysts (adsorption of ammonia and carbon monoxide), the use of IR in surface science and catalysis has grown rapidly. IR, with its high-energy resolution, can be a very appropriate tool to investigate the internal and external modes of adsorbates and their vibration dynamics. The development of in situ vibrational spectroscopies applicable to metal-support interfaces in recent years has exerted a profound influence on our understanding of adsorptive chemistry in heterogeneous systems. Some pertinent information can be obtained from the number of bands in the spectra at a single stage of surface coverage as shown in the original spectra of chemisorbed CO. However, these single stage spectra do not reveal the relative strength of bonding for the chemisorbed CO contributing to each band or the effect of interaction on the band positions. To obtain this information the spectra of chemisorbed CO were studied as a function of surface coverage over silica-supported Pt, Pd and Ni [6]. The authors wrote: "In order to carry out this work efficiently it was necessary to design apparatus in which the IR spectra could be obtained while the samples were subjected to a wide range of temperatures and pressures. Successful development of this in situ apparatus not only makes it possible to study the effect of surface coverage but also opens the way to IR studies of chemisorbed molecules while reactions are in progress" [6]. It was one of the first in situ cells for IR studies published in scientific literature. In catalytic research very often "home-made" IR cell reactors are used for the particular in situ studies. In the following figures there are presented selected examples of the "home-made" and commercially produced such devices. Many of them are successfully used in the research for a considerable period of time. For the figures presented only general information will be provided. Details can be found in the quoted references. Peri and Hannan [7] designed an IR cell for the determination of the surface hydroxyl groups on g-alumina (Figure 2).
Greenler [8] has adopt his cell for IR study of the adsorption of methanol and ethanol on aluminum oxide (Figure 3).
Ryason [9] has described a stainless steel quartz cell for IR transmission spectroscopy of catalyst wafers (Figure 4).
Amenomiya [10] has described a double beam cell for the high temperature infrared spectroscopy of adsorbed species during catalytic reaction (Figure 5).
Hicks et al. [11] have reported on the design and construction of a reactor for in situ IR studies of catalytic reactions (Figures 6 and 7). It seems, that this is a very successful design because it is extensively used by Bell and co-workers for more than 20 years.
Johnson et al. [12] have described two cells (Figures 8 and 9) for IR emission spectroscopy (IRES) studies of metal oxide catalysts.
Arakawa et al. [13] have reported details of a novel high-pressure FT-IR spectroscopic system combined with a specially designed in situ IR cell for studying heterogeneous catalytic reactions (Figure 10).
Prokopowicz et al. [14] have presented a design of a transmission IR cell for the high-temperature study of transient adsorption and reaction in a flow system (Figure 11).
Larkins and Nordin [15] have described a high-temperature IR cell for in situ studies of the catalysts for methane oxidative dehydrogenation (Figure 12).
Echoufi and Gelin [16] have applied IR using the cell shown in Figure 13 to measure CO physisorption on zeolites.
Van Neer et al. [17] have reported on a reactor for IR experiments in a flow system (Figure 14).
Chafik et al. [18] have applied their high-temperature IR cell for transient experiments (Figure 15).
Kardash et al. [19] have reported the design and performance of an insulated IR spectro-electrochemical cell that is capable of operation at temperatures other than ambient (Figure 16).
Burcham et al. [20,21] have utilized a "fixed-bed" IR cell for catalytic studies (Figure 17).
Figures 18-21 show examples of commercial equipment. However, the last one (Figure 21) is not an attachment which can be used for in situ studies. It is the only commercially available detector for FT-IR/photo acoustic spectroscopy measurements, and can be applied for non transparent samples (e.g., carbon deposits, metal catalyst precursors, etc.).
There is no doubt that there is an increasing interest in the application of infrared in catalysis. Two of the classical IR techniques are still the most popular - transmission and diffuse reflectance. This is largely connected to the difficulties encountered with in situ studies, which nevertheless are of increasing significance. Moreover, monitoring for the presence and behaviour of adsorbed molecules on metal surfaces during heterogeneous catalytic reactions is of central importance for elucidating reaction mechanisms. Abstract Infrared spectroscopy undoubtedly represents one of the most important tools in catalysis research. A variety of IR techniques has been and can be used in order to obtain information on the surface chemistry of different solids. Special meaning applies to investigations carried out under reaction conditions. In principle for in situ measurements, all forms of IR spectroscopy are suitable. For most practical experimental reasons, however, the transmission-absorption and diffuse reflectance techniques are best suited. This is more related to the design of cells that are to be used as reactor, than with the principal problems of the other techniques. Material presented here is partially based on the review recently published [1]. References
REF: J. Ryczkowski, Int.J.Vibr.Spec., [www.ijvs.com] 6, 2, 2 (2002)
In this piece we will attempt to explain the enormous range of software options you have available on even the simplest FTIR. If you go to your instrument and open up the settings available, you will invariably find "resolution", "number of scans" and "background parameters", all of which were discussed in the first article [reference] and then a whole mass of confusing options such as "Apodization functions". In addition you are sometimes offered the facilities to include "Zero filling" and its hard work to find out what they all do. Just to confuse you further, you are offered a set of data ‘improvement’ procedures including "averaging" and "KK processing" and often lot’s more. In our explanation of some of these below, we assume you will read the article at your FTIR and play games on your own instrument to demonstrate to yourself what each function does. In all cases, we are going to use the standard polystyrene test spectrum so we assume you have a thin film of polystyrene available. [Most instrument manufacturers provide these when they deliver the instrument]. If you don’t have a film – despair not. Stand a transparent polystyrene ball pen in a small test tube of chloroform until it goes gooey (about 10 minutes). Stir it around and then pipette a generous coating of the chloroform solution onto a horizontal polished KBr flat. Allow it to dry in a fume hood (keeping the flat just a little warm helps). Put the coated flat in your FTIR and run a spectrum and you should see something like this –
Apodization Functions When your FTIR scans, it actually records an interferogram, which is subsequently digitised, and then Fourier Transformed into a spectrum (See Article 1 for details). The form of the interferogram is
The vertical axis is never labelled i.e. it is in arbitrary units BUT in reality it is the output voltage or current of the detector. Unlike a spectrum, which shows peaks against a background, the interferogram shows swings of output about a non-zero mean. At the end of each scan, the machine reverses its direction. Inevitably the reversal means that the scan must be slowed down, stopped, reversed and then sped up again. As the optical delay is slowed, stopped and then accelerated, the output data is corrupted, so the instrument is told to switch off the detector until the reversal is complete and the movement has become nice and smooth. So to put it diagrammatically we have -
And when you display the interferogram you only ‘see’ the good bit, the span between the two switching intervals. Fine – but there is a problem. If you switch off a signal, it will naturally fall rapidly to zero i.e. there will be a ‘step’ in the output value of the detector circuit. Unfortunately, the Fourier Transform processor will process this step and will make spectral nonsense out of it. The trick is to minimise this nuisance by lowering the output along a chosen curve – an "apodization function". Each function has a different shape eg.,
To demonstrate the effect of these, we recorded spectra of polystyrene under various conditions and compared them. All spectra were recorded with a TGS detector at 4cm-1 using 4 co-added scans. The backgrounds and the spectra were each recorded under identical conditions.
The spectra of polystyrene showed little or no effect as one changes the apodization however, where bands are sharp one can see effects. Look at the backgrounds and you will see that reducing or switching off the apodization can cause the sharp vibration rotation bands of water vapour to alter in apparent position and shape. So – if you are going to record spectra of gases especially at higher resolutions or of crystalline solids with very sharp bands you must be careful about apodization. If too low the bands will appear to be better resolved but ‘ringing’ can occur.
In most analytical spectroscopy there is little problem. Since the bands are broad, the effect of using high apodization is not disastrous and it is safer to use it rather than not. Smoothing Older spectrometers offered simple smoothing procedures eg., the instrument took 3, 5 or even more spectral data points, averaged them and plotted out the average. It then moved by one cm-1 increment (usually ½ resolution value) and repeated. These days more sophisticated functions are used but the effect is similar – you reduce the noise by averaging but unfortunately you lose resolution. Let’s try an experiment. Take a piece of Al foil (kitchen quality is just fine) and punch a hole in it 1 to 1.5mm in diameter with a needle of sharp pencil. Place this over your polystyrene film and put the sandwich in the spectrometer after you have run a background. Make sure the hole is roughly in the middle of the IR beam. Run a spectrum. You should see something like this –
Figure 7. That is the whole spectrum is squashed up – very little light passes the hole in the foil. Now expand the spectrum to fill the screen – Figure 8.
Figure 8. The spectrum is fine but the result is noisy. This problem can arise where for some reason the energy is very restricted (examples include the use of some spectral reflection techniques, spectra on samples held in some high or low temperature cells or in IR microscopy). The correct way to deal with it is to co-add many scans [Remember, the S:N ratio – improves as v# of scans]. Either this may not be possible or just take too long, or the experiment is finished and you want to clean up the results you have and not do the job again. The application of smoothing is worth considering. In Figure 9 we show the effect of smoothing the data in Figure 8.
Figure 9. There is a loss of resolution but the noise is indeed reduced. In this case the loss of resolution is acceptable but in some other cases it may not be. Absorbance vs Transmittance In our experience most analytical users of infrared record spectra in transmission (0-100% transmission) and never use absorbance. The theory behind this is –
The negative sign means that the peaks in Absorbance are the negative ones in transmission. In Figure 10 we show part of the spectrum of polystyrene presented in transmission (bands downwards) and absorbance (bands upwards).
Figure 10. Several points are clear –
The band at 1451cm-1 in polystyrene has W½ of ~18cm-1 in transmittance and ~2½cm-1 in absorbance. If you check the vibration rotation gas phase spectrum of water vapour in the background you will find less of an effect eg., the band at 1050cm-1 narrowed from W½ = 6.5 to about 5cm-1 . So the question must arise – why not use absorbance more often as a method of presentation. You will note that in Figure 10b, the absorbance scale goes from 0 to 7 i.e. 100 to about 0.1%. This article is getting a bit long so we will prepare How FTIR Works III for you and include differentiation and other parameters REF: P.J. Hendra & F.
Birembaut, Int.J.Vibr.Spec., [www.ijvs.com] 6, 2, 3 (2002) There are two conventional ways of generating mid infrared frequency specific images on a microscope.
The Array detector method is superb but VERY expensive. Most of the appropriate detector elements have been developed for military hardware where cost is no object. Need we say more? The Perkin Elmer team has addressed this problem using a fairly conventional infrared microscope, a commercial Spectrum One FTIR and a LINEAR array detector resulting in a high performance system at a sensible price.
The System
Light from the Spectrum One interferometer passes into a redesigned version of Perkin Elmers’ Autoimage Microscope.
The radiation path is shown in orange. The optics are of the Cassigrainian Mirror type (no lenses are used). Once imaged onto the sample held on the stepper motor controlled stage, the collected radiation passes through a Z-fold x 4 magnifier (the horizontal gold coloured unit in the centre of Figure 2) – more to follow – and then to the detector – the red/brown unit at the top. This contains 2 elements – a conventional single point detector and a linear array. A conventional visible light path is also supplied so that the user can ‘see what he or she is doing’. The detector is very unusual and is illustrated below.
Within an area of about 1 sq. mm, the two detectors are arranged side by side each other, each pixel being hardwired to the outside of the evacuated and cooled detector enclosure. The microscope can operate in two ways – point by point or strip
by strip, the switch from one to another being carried out using motorised aperture blades
in the optical path. You can see the blades in Figure 2 just above the Z fold magnifier.
To regularise and maximise performance it is obvious that the movement of the stage should occur only at the ends of the interferometer scan where no useful data is being recorded. See Figure 6. Note that by scanning the interferometer rapidly, enormous amounts of data can be accumulated very quickly.
Z Fold 4 x Magnification To improve versatility in the system, a small path extending system is incorporated into the optical path. In one orientation, light goes straight through but rotate the tubular assembly about its axis and the optical path is diverted through a combination of mirrors increasing the microscope’s magnification by four times. The effect is that each pixel views a patch ~6.25µ square at the sample instead of the normal ~25 µ square. So, the user can have high spatial resolution or look at lower resolution over a wider area. See Figure 7.
Examples of application of Spotlight In Figure 8, you will see in grey, a view of a tiny polypropylene specimen containing an inclusion. The identification of bits and pieces of rubbish in otherwise uncontaminated material is frequently encountered in polymer processing. The identity can lead to the origin of the inclusion and is commercially crucial. On the lower right part of Figure 8, you will see a pseudo 3D image of the defect recorded in only 2 minutes. The sequence of operations required to generate the results is illustrated in Figure 9. Note – the ‘background’ must be appropriately recorded (as you always do in FTIR). The coloured picture to the right develops in real time. The x and y axes are distance, the colour goes from blue?red as the absorbance increases.
In Figures 10, 11, 12 and 13 you will see data from a fingerprint on a glass slide. These spectra were recorded in transmission using the region above 2000cm-1. The striking thing about this image is its size, over 400,000 spectra! Measuring this took about one and a half hours. The average spectrum in Figure 11 shows weak CH2 and OH absorptions. In Figure 12 an image at a CH2 frequency follows the ridges of the fingerprint, but an image of the OH highlights isolated spots. Closer examination in Figure 13 reveals that the spectrum at one of the spots has CH3, CH2, NH and OH absorptions typical of proteins, while neighbouring spectra just show finger grease. Forensic applications of this type of analyses are obvious!
Orientation of the polymer in a PET bottle The bottles we all buy containing carbonated drinks like Coke are made of polyethylene terephthalate PET.
Now PET is very strong, particularly if the molecules are
aligned. Like most regular polymers, PET can be extended in one direction
to align most of the chains parallel to one another. Unusually, it
is quite simple to extend the polymer in two orthogonal directions
– the chains align and the benzene rings rotate until they are
parallel to one another and are packed on top of each other. Highly
oriented PET is strong, very resistant to tearing, hard and very impermeable
to CO2 – ideal for a Coke bottle. The bottles are
made by moulding a small bottle (around 20mls capacity and called
a ‘pre-form’), heating it to more than 100şC and then blowing
with heated air. The bottle expands into a heated mould and hey presto
you have your bottle. A complex process like this is fraught with
potential disasters. Uneven heating or blemishes in the pre-form will
produce an unacceptable product with inadequate molecular orientation.
One of the best and most convenient ways to monitor orientation is
to use FTIR with a polarizer [see ijvs – Volume
5, Edition 1] Below, we give an example of an orientation measurement
using a polarizer in the Spotlight system. Figure 14 shows the conventional
spectrum of the wall of a PET bottle. The problem here is that the
wall is far too thick for absorption experiments. PET shows ‘dichroism’
– different results when the electric vector of the IR is ||
or
The bottle wall shows a perfectly reasonable spectrum in the NEAR IR (Remember, the extinction coefficients typical in the NIR are much lower than in the mid IR). See Figure 16. The band areas can be related to orientation. See Figure 17.
Quite obviously, the orientation of a bottle will vary within the bottle, Figure 18 shows the spatial distribution based on Figure 16 and 17. Again this is an enormous image containing over 600,000 spectra that took just over two hours to record..
Sectioning the wall of the bottle to produce thin samples is not very practical, but specular reflection off the surface of the PET is an alternative. The spectral reflectance spectrum can be recorded in polarized light and the spectrum is highly specific just like its transmission analogue. The snag is that the spectral reflectance spectra are weak. In Figure 19, the polarized specular reflectance spectrum of a PET bottle is shown. The inside surface is not the same as the outside – see Figures 19 and 20. Using the Kramers Kronig algorithm, the absorbency component can be extracted so that this spectrum looks like conventional absorption spectra – See Figures 21 and 22. Clearly, the inner surface is better oriented than the outer.
REF: R. Hoult & R Spraggi, Int.J.Vibr.Spec., [www.ijvs.com] 6, 2, 4 (2002)
|
(a)
(b)
to the draw (or molecular) axis. See Figure
15 where spectra are shown in a thin film only 40µ
thick.
