NEWS & REVIEW

1. Editorial


The red hot news this issue is that your elderly Editor has busted his left leg! He is VERY hurt because everyone laughs. I know you won’t believe it, but I was doing some gardening - very odd in a way because I hate gardening and almost never do it –[and odd as you’d spent the last three weeks fixing the roof and not even broken a fingernail! Louise] fell and finished up in hospital. As I write, the plaster comes off in 12 days and 2 hours! Phew!

In this issue, we explore gases and vapours. Very few infrared people record spectra of gases, but a small group do nothing but gases so I thought it would be an idea to produce an edition to bring the non-users up-to-speed.

We kick off with an introductory piece by yours truly covering most of the field [I hope it doesn't read like one of those undergraduate lectures you quickly learn to fall asleep in!] and then go on to an article by Peter Middleton of Servomex. His company makes gas analysers and he kindly responded to my recent ‘cris-de coeur’ about our urgent need for copy. Tom Klapötke did the same (see his paper on the gases generated by explosive reactions in Section 3) [Comment: Some people do make life difficult for themselves. Fancy making a career out of blowing up the kit!!] and so I changed our plans and decided to go for a gas phase edition.

We have been promised other papers in this field including results on a fascinating study of pollution in city streets. I hope we can publish this very soon.

One of the areas where Raman is making progress is in the analysis identification and dating of ancient artefacts [do you mean you? – Louise]. This fascinating field is being covered in a special one day meeting organised by an old friend [the youthful] Prof Howell Edwards at Bradford University. I plan to attend make notes and report my impressions to you in the next edition to follow the meeting. For details see Announcements.

In the submitted section of this edition, we have Tom Klapötke's paper on gases resulting from explosions preceded by an unusual piece from Simona Badelescu entitled 'Scanning Electron Microscopy & FTIR spectroscopy characterization of Polystyrene Colloidal Crystals on various substrates'. Have a look at it - very interesting indeed. I'm sure some of you will comment about the diagrams in my pieces. Humble apologies but travel is very tricky and I live several miles from the office. So - I drew the figures on tracing paper - sent them to Louise by fax and she scanned them and so that's what you get.

2. Feature Article


Infrared Spectroscopy
on Gases

The Editor

Almost every user of infrared spectroscopy has recorded spectra in transmission at some time or another. The overwhelming fact is that infrared is very strongly absorbed hence thin films of liquids or polymer are always used. Relatively few non- specialists have ever recorded spectra of gases and most labs don't have a gas cell. In gas phase spectroscopy, the overwhelming fact is that infrared is absorbed weakly hence path lengths are long. Let's try a few sums.

A typical liquid spectrum would be recorded on a film ~ 20µ thick. The density of the sample if vapourised at 1 atmosphere would be around 10 ^-3, that of the liquid hence to 'see' the same number of molecules as it passes through the vapour, as it did in the liquid, the infrared radiation should pass through ~ 20mm of vapour. If the pressure is below one atmosphere, or, of course, the partial pressure of the sample in a mixture is less than 1 bar the path length would have to be longer than 20mm to do the job adequately. As we shall see MUCH longer paths can be involved.

We are all familiar with vibrational behaviour in molecules and its resultant infrared absorption. Those vibrational modes which give rise to a change in dipole will give rise to an absorption in the infrared at the vibrational frequency. Since molecular vibrational behaviour is usually complex we see the familiar fingerprint in the infrared for our molecules.

Gas phase molecules also freely rotate. The rotation gives rise to absorption if again a dipole is involved. Thus HCl will show features due to rotations but Chlorine will not (or rather, not in the I.R. Raman lines DO occur). The rotational spectrum of a molecule consists of a set of equispaced absorption bands, the spacing being inversely proportional to the moment of inertia of the molecule. Most molecules have 3 different moments, a few have only two and linear molecules like CO₂ or HCl only one. Thus, most molecules generate a very complex rotational spectrum. The moment of inertia values of all, but the tiniest molecules are sufficiently high that the entire rotational spectrum lies well outside the range of our instruments - in fact the bands are found in the microwave.

Dear oh dear, I hear you all sigh - why does the old duffer bother us with it then? Read on!

In vapours, the molecules both vibrate and rotate and can suffer changes of both simultaneously i.e. in a liquid we see vibrational features, but in the vapour vibration-rotation features. The appearance is shown diagrammatically below

Several points need noting.

1. Vibration rotation spectra are symmetrical about either a band (known as the Q branch) or a gap - see A & B respectively above.

2. The Q branch whether present or not corresponds to a vibrational transition ONLY. All the other bands are due to vibrational and rotational changes occurring at the same time.

However….

3. The frequencies of the Q branches are NOT identical with the equivalent vibrational bands in the liquid. The reason is that molecules in the liquid are squeezed together and hence are perturbed by inter-molecular forces. In the vapour, they are isolated and just do their own thing oblivious of all around them (ALMOST!).

You can see some of these effects by running a background spectrum on your FTIR. The complex band system around 1600cm^-1 is due to the bending motion of water undergoing rotation as well (Oh yes - water has 3 moments of inertia - thus the complexity).

Now look at the two CO₂ bands at ~2350 and 670cm^-1. If you can run a background at 1cm^-1 resolution, you will see finely spaced lines on both. The band system near 2350 is due to the asymmetrical stretch

And the rotation
The 667 feature is the bend and again rotating.

CO₂ has only one moment of inertia because rotation about the X axis is meaningless, whilst I _y= I _z.

I said in the beginning that only rotations causing a dipolar effect would give rise to absorptions so how come we see what we do in CO₂. Carbon Dioxide has no rotational spectrum, but it does show rotational features on the infrared active vibrational modes. So, we see no vibrational or vibrational rotation features due to the symmetric stretch

But see both vibrational and rotational features on the two infrared active vibrational modes.

So we see that the infrared spectrum of a gas – even a simple one can be very complex indeed. To see an example or two – have a look at the article which follows this one, where Peter Middleton shows us a spectra of NO (Figure 2), HCl (Figure 4) and SO₂ (Figure 6). The bands are very sharp indeed, at reproducible frequencies and hence make excellent diagnostic features in analysis.

As the temperature of a vapour is increased, the relative intensity of the individual bands change – the bands further from the Q branch increase at the expense of the others.

As you raise the pressure, the bands get broader – the reason is that collisions become more numerous and the gas molecules dash about more rapidly. A Doppler shift occurs hence the individual sharp bands become broader.

The simplest experiment is to pass the gases of interest into a cell and examine the spectrum in transmission. Comparison of the vibrational and vibration rotational bands with standards yields the desired result. A good up-to-date example will be found in our submitted paper section of this edition, where Tom Kaplötke and his colleagues are analysing the gases resulting from explosions.

Sometimes, the gases of interest are at low concentrations e.g. contaminants in air. Clearly, a gas cell that will fit into the sample area of a spectrometer (say 200mm long) is far too short to detect traces. The trick is to use a multi-reflection gas cell. I draw the principles below in Figure G.

These cells can involve many traces of the gas and hence build up effective pathlengths of more than 10m. From the simple minded calculation I used at the start you can see that it must be possible with these cells to observe gases at partial pressures around a few torr and a good deal better. In fact, you can do even better by running the spectra at high resolution. Let me explain.

If the bands are naturally very sharp, improving the resolution will make the bands appear more intense until the resolution of the instrument matches the width of the bands at half height. Further improvement will lose sensitivity. So, since the gas phase bands have widths around ½- ¼ cm^-1, good instrumentation is certainly an advantage.

People have extended the path length much further than a few metres. Atmospheric pollution can be monitored using ‘open beam’ working. In this method, the source is set up with a mirror at one site and projects a beam to a collection system/FTIR at another site. One might want to monitor the pollution of the air in a city street – the source will then be on one side of the road, the instrument on the other. Some experiments have been carried out to detect extremely low concentrations of pollutants by mounting the components on hill-tops separated by distances as large as a few kilometres.

Perhaps the most sensitive way to detect atmospheric pollution involves a bit of a cheat. If the air of interest is passed through a tube cooled with liquid nitrogen, it will condense and of course consist of liquid N₂, O₂, Argon and solid CO₂ and solid pollutants. Once enough liquid has been collected, its temperature is raised slightly and the O₂, N₂ and Ar will boil away leaving the CO₂ and pollutants. These are then warmed and examined as mixed gases in a long path gas cell. This method has been used on planes to monitor pollution levels at high altitudes.

All of these methods, clever though they may be suffer from one handicap – expense. The equipment is elaborate and must involve the use of a really good FTIR capable of recording high resolution spectra. There is an enormous and increasing demand for high sensitivity gas analysis on a continuous basis for example, the analysis of flue gases at power stations, incinerators or industrial furnaces. The nature of the gases is well known, their concentration is the analytical problem. A very clever method exists to tackle this problem, which avoids interferometers – in fact it avoids the use of any dispersion technique. The techniques involve gas filters and Peter Middleton of Servomex Ltd has written a paper for us to explain the method. It follows this one.

Gas phase analysis in atmospheric pollution has been explained already, but in what other areas is it important? Gas phase analysis of decomposition processes can be particularly informative (not only from explosions!). The nature of the gases generated during thermal analysis e.g. Differential Thermal Analysis or Thermogravometric analysis can provide really valuable additional data to help to explain the thermal results. Pyrolis of components such as polymers can also be particularly informative. Just as in the explosion products paper in Section 3, the gases can be monitored by several techniques at once – mass spectroscopy and FTIR for example.

Chromatographic techniques rely on a detector whose output varies with time as it responds to the separated components. Very often, the detector is non-specific and hence simply records the presence of the separated analyte not indicating its nature. The qualitative identification must rely on the retention time. Much greater specificity is available if the chromatographic system is coupled to a compound specific detector. FTIR and mass spectroscopy are both popular.

The problem in both methods is that the separated material, the analyte, is present in only a tiny quantity and to prevent condensation, the entire system coupling the chromatograph to the detector (or in our case an infrared cell) and the detector itself must be kept hot. Also, the analyte will be present in the gas stream for only a few seconds at most. For mass spectroscopy there is a relatively small problem because the sensitivity and speed of MS is so high. For FTIR there is a problem. The carrier gas can be chosen to provide no infrared absorption itself e.g. nitrogen but the quantity of carrier gas/analyte can be very small (much less than a millilitre). There is no question of using large gas cells or multi-reflection. The instrument makers have responded by developing really small cells passing the light through them by multi-reflection. The cells are often called ‘light pipes’.

In some industries, vapours and gases are involved in synthesis and for more than 40 years infrared has been used on-line. The petroleum industry frequently uses vapour steams whose purity or composition need continuous monitoring. Infrared systems are often permanently fitted into the production plant and the putput is used to control the system i.e. they operate as detectors in feedback circuits. The instruments used often involve dispersion or interferometric units or sometimes filters but as they are dedicated to specific analyses and must be made robust, rugged, ultra reliable and inherently safe, they differ a great deal from our laboratory systems.

REF: P.J. Hendra, Internet J. Vib. Spec.[www.ijvs.com] 5, 3, 2 (2001)


3. Feature Article

Peter Middleton Servomex Ltd., Crowborough, East Sussex, TN6 3DU, UK

Infrared gas filter correlation techniques have been developed to allow highly selective measurement of a range of gases (NO, CO, HCl, CO₂, SO₂ , N₂O and CH₄). The methodology is described in detail together with the various ways of optimising the design parameters to meet the performance requirements of the application. The advantages over other techniques are described and the resulting performance figures presented.

The improved performance that gas filter correlation (GFC) conveys has allowed infra red (IR) photometry to displace more complex measurement technologies over the past decade, at lower cost. The earliest description of the technique was by Goody [1] and its early practical use for pollutant analysis with folded path cells was typified by the work of Chaney and McClenny [2]. A review of the various established techniques used in absorption photometer design has been presented by Hanst [3]. This paper reports the development of a practical GFC photometer without the need for a folded path White cell [4] as shown below.

Broad band IR radiation from a hot element source passes alternately through one of two gas filters mounted on a chopper wheel (Figure 1). One contains nitrogen and the other contains the gas of interest, for example nitric oxide (NO) for a NO analyser. When the nitrogen gas filter is in position, no absorption takes place. When the NO gas filter is in position, absorption takes place reducing the intensity in the beam at the characteristic wavelengths for NO.

The temporally spaced beams then pass through a narrow band pass filter which limits the IR region to a specific part of the absorption spectrum and then into the sample cell (Figure 2).

When radiation that has passed through the nitrogen gas filter passes through the sample gas containing NO, absorption occurs according to Beer's law and produces a reduction in the detector signal. The transmitted energy of the measure beam can be described:

M describes the optical path relating transmitted intensity to energy throughput. If we assume it is independent of wavelength and equal for both beams, it becomes a constant.

Radiation that has passed through the NO gas filter has had the intensity at the characteristic wavelengths significantly reduced and so further absorption in the sample cell is small. However, the energy which is transmitted makes an excellent reference signal as it has the same spectral distribution as the measure signal.

Assuming Beer's law is obeyed in the gas filter, the energy falling on the detector can be described:

The spectra of the two beams at the detector with 300vpm NO in the sample cell are shown in Figure 3. The resulting detector signals can be represented by the total energy from each beam and an output proportional to NO concentration can be determined from the difference between these energies. Dividing by the reference energy provides the necessary normalization:

The factor a is effectively a coarse instrument zero which may be effected optically or electronically. However an optical method, such as masking the nitrogen gas filter, may detract from the spectral similarity of the two beams.

Calibration and linearization provide the necessary mapping to experimentally measured values to give the NO sample cell concentration from the GFC photometer.

One of the features of GFC that conveys many advantages over other photometric techniques is that only a single IR region is required for the measurement. That is, a single narrow band IR filter defines the region of interest. Any changes in spectral content due to source ageing, window contamination and any other broad band interference occur equally for both measure and reference beams and thus cancel out in the differential measurement.

The greater the spectral similarity of the two beams, the greater the immunity from interferences, as both would be affected equally by any absorption. As one beam must pass through the gas filter, this is best achieved by gases which exhibit widely spaced rotational fine structure of which NO is a prime example (Figure 2). This feature is usually referred to as the common mode rejection ratio and figures approaching 1:10⁴ have been measured for variations in source energy.

Heterogenous diatomic gases exhibit rotational fine structure from the splitting of their vibrational transitions giving the desired optimum spectra for GFC. Fortuitously these gases (CO, NO, HCl, HF) are also the gases we are interested in for CEM applications in particular, where insensitivity to large varying background concentrations of water vapour and carbon dioxide is required. This gives GFC its suitability for CEM applications where high selectivity and inherent immunity to interferences are major advantages of this technique over more complex compensating spectroscopic techniques.

A background gas with an absorption in the same region of interest may interfere only if there is direct overlap with the narrow gas filter absorption lines of the target gas. This immediately reduces the likelihood of interferences. Then by careful selection of the IR interference filter parameters to exclude such regions of overlap where possible, very low cross sensitivity can be achieved. This is demonstrated for the case of HCl (Figure 4) where the spectrum allows a choice of P or R branches of the absorption. By choosing the lower energy P branch, avoiding the water vapour absorption, a suitable IR filter can be chosen. For maximum sensitivity the gas in the gas filter should be in a similar dynamic molecular state (temperature, pressure) as the gas in the sample cell when maximum correlation between the gas spectra occurs. This is because both line intensities and line widths are affected by the molecular dynamics of the absorbing gas.

A further method of reducing cross sensitivity is to utilise a zero crossing point for cross sensitivity with respect to centre wavelength (CWL) of the IR filter. For example, for NO there are a large number of water vapour lines across the 5.3µm region. Some of these lines overlap the NO lines exactly, giving a positive cross sensitivity and some fall exactly between the NO lines giving a negative sensitivity to water vapour. As the CWL of the IR filter is changed from one such region to another, the cross sensitivity falls to zero where the contributions from overlapping and non-overlapping lines cancel. By including the absorption term product for the cross interfering gas in both integrals (1) and (2),

the relationship between cross sensitivity and IR filter CWL can be found which allows the estimation of the value of CWL for zero cross sensitivity. Figure 5 shows the results of a theoretical determination of NO sensitivity and H₂O cross sensitivity as the CWL of the filter is varied across the region. This helps determine the optimum filter parameters for maximum sensitivity and selectivity. It can be seen that for a particular set of filter parameters a maximum in sensitivity occurs near a CWL at which positive and negative contributions of sensitivity to water vapour cancel. By finely specifying the filter parameters it is possible to manufacture 0-100vpmNO transducers with sensitivity to water vapour less than 2 vpmNO / 0.5%v/v H₂O.

A similar approach is necessary for SO₂ where again water lines cross the 7.5µm region (Figure 6). Moreover, the triatomic SO₂ molecule gives many closely spaced absorption lines which is not as ideal as the spectrum from a heterogenous diatomic molecule. This results in less inherent immunity to interference requiring more precise optimisation of the filter parameters.

Figure 7 shows a simulated cross sensitivity over an expanded region near the zero crossing point together with experimentally measured values for several different IR filters. It can be seen that agreement is good. Slight deviations are expected due to different filter line widths and symmetry etc. From this work we can reduce the sensitivity to water vapour to values which become limited by the manufacturing tolerance of the IR filters.

For CO analysis in CEM applications the largest interfering component is CO₂ (typically 15vol.% in a flue gas) as can be seen in Figure 8. Similar careful selection of filter parameters allows a transducer to detect 0-50vpmCO with an excellent cross sensitivity to CO₂ of less than 1vpm/20%CO₂.

The measurement of nitrous oxide (N₂O) without cross interference from CO or CO₂ which both overlap the N₂O absorption presents additional problems (Figure 9). Optimising the filter parameters for zero CO cross sensitivity do not give the same values as for CO₂. A different approach is thus adopted and that is to remove the CO₂ characteristic energy from both measure and reference beams by placing a static gas filter in the beam containing a high concentration of CO₂. Alternatively, the measure gas filter can be filled with CO₂ and the balance of the reference gas filter can be CO₂. This is the approach we take and results in a cross sensitivity of <0.5vpmN₂O for 500vpmCO₂, 10vpmCO or 2%H₂O for an instrument measuring 0-50vpmN₂O minimum range. For CEMs applications 20%CO₂ gives ~ +3.5vpmN₂O and 100vpmCO gives ~ -3.5vpmN₂O.

The measurement requirement for monitoring low concentrations of methane demonstrates how we can take advantage of the low absorption of CO₂ and H₂O in this region and so trade off immunity from interfering gases for maximum sensitivity. By filling the reference gas filter to a pressure and optical density equal to or greater than that expected in the sample cell we can increase the line widths and intensities of the absorption lines and so maximise the degree of correlation, or overlap, between the spectra. The filter parameters were optimised for maximum sensitivity whilst maintaining a reasonably low figure for cross sensitivity. This results in a minimum range of 0-50vpmCH₄ and cross sensitivities of +2.5vpmCH₄ / 2%H₂O and +1vpmCH₄ / 10%CO₂. The cross sensitivity to other hydrocarbons is expectedly larger due to the common CH stretch absorption characteristic at this wavelength. (Eg 0.2%propane gives a reading ~50vpmCH₄).

Good mechanical and optical design should also ensure that the common mode rejection ratio is as high as possible for a variety of other influences, including ambient temperature, source output, window obscuration, etc. This is most easily brought about by making the optical paths of both measure and reference beams identical, for example by alternately interrupting a single beam with the gas filters. This is the design adopted by Servomex where a single chopper wheel serves three functions; to produce temporally spaced measure and reference beams, to separate the beams with periods of darkness to provide a zero energy reference and to allow the use of a low cost pyroelectric LiTaO₄ detector. Any design which uses spatially separated measure and reference beams will compromise performance, as changes in one path will not affect both beams equally and so cause the output to change.

As most sources of instability and drift are associated with instrumental effects, removing these with a design with high common mode rejection will mean that the major limit on sensitivity is instrument noise. With increased instrument stability, the absorption path length can be reduced and the gain increased until the instrument becomes noise limited. For example a 0.5m single path design can achieve a 0-50 vpm CO measurement range with <1%fsd peak to peak noise.

One source of drift which is not common mode is the stability of the gas inside the gas filters. Any loss of gas due to leakage, permeation, chemical reaction or adsorption will cause a positive zero drift and a reduction in sensitivity. The gas filter design must be able to transmit IR radiation in the region of interest and be physically and chemically stable. Servomex currently use various gas filter designs; one used to contain very reactive gases such as HCl uses a simple one piece IR quartz envelope which has proved extremely stable. Another design for less reactive gases uses a metal to glass sealed construction which is designed to be leak-proof over a wide storage temperature range (-20 to 70C). Earlier attempts to overcome the difficulties in making gas filters by using adhesive window bonding frequently resulted in leakage causing drift and instability, and these are no longer favoured.

Although heterogenous diatomic gases such as CO, NO and HCl exhibit ideal absorption spectra for gas filter correlation, many other IR absorbing gases exhibit a suitable region of adequate fine structure making the GFC method a far more widely applicable technique. Absorption line intensities and widths can vary significantly with ambient conditions for polyatomic gases and so care must be taken to account for these changes (e.g. with NH₃, CO₂, SO₂, CH₄, etc).

The high stability and high sensitivity of this technique also conveys a further advantage of a wide measurement, or dynamic, range. This allows for example, without electrical range changing, the monitoring of CO from 0 to 1000vpm with 1% of reading resolution or 0.5vpm, whichever is larger. This is again particularly useful for CEM applications where occasional high excursions from a normally low value can be accurately monitored.

The optimization of the various design parameters of an infrared gas filter correlation photometer allows this technique to be used for many difficult applications involving measurement of low level gases with high levels of interfering background gases. Over the past decade it has often been the technology of choice for gas analysis of the infrared absorbing gases providing the most cost effective performance for many applications.

References

1. Goody R., ‘Cross-correlation spectrometer’, Opt.Soc.Am., 1968, 58(7), 900

2. Chaney L. W., McClenny W. A., 'Unique Ambient Carbon Monoxide Monitor Based on Gas Filter Correlation: Performance and Application', Environmental Science and Technology, 1977, 11(13), 1186

3. Hanst P. L., 'Spectroscopic Methods for Air Pollution Measurement', in Advances in Environmental Science and Technology, 1971, Vol.2, 91, Ed. Pitts, J. N., Metcalf R. L., John Wiley & Sons (New York)

4. White J. U., 'Long Optical Paths of Large Aperture', Journal of the Optical Society of America, 1942, 32, 285

REF:  P. Middleton, Internet J. Vib. Spec.[www.ijvs.com] 5, 3, 3 (2001)

4. Feature Article


Hot Samples in
Raman Spectroscopy

The Editor

From the earliest days in the development of Raman spectroscopy it was appreciated that it was very easy to examine samples at high temperatures. When Mercury discharge lamps were the source of choice it was hard to examine powders but liquids and melts were much easier. When the laser came along in the 1960’s, nothing much changed except that it became much, much easier to build heated cells for use with a laser and of course, the spectra were a good deal better.

Some people went to quite extreme conditions. For example I remember Ian Beattie at Southampton and his group studying molten inorganic materials using He/Ne and later Ar⁺ sources in the 1960’s and early 1970 showing that Group III halides found as M₂X₆ in the solid state became M⁺MX₄^- in the melt. Ian used quartz cells electrically heated and Spex or Cary 82 Raman instruments.

Now, as you raise the temperatures of a sample it increasingly emits so-called blackbody radiation. By the time you reach ~ 600° some of this emission is in the visible – the sample is red hot. The equation governing the black body emission is –

Hence it turns out that if you are using red or green excitation (remembering that for all practical purposes one is ‘always’ operating on the Stokes side of the existing line) the sample emission becomes obtrusive by 700° C. Clearly, this is not a problem if ones’ aim is to study organics or polymers when temperatures between ambient and 300° C are required but molten salts or molten inorganics are a different matter. They may well need heating to 1000° C or more. See Figure 1.

Fortunately, there is a way of discriminating the sample emission from the Raman scattering – phase sensitive detection. If the source (laser) is interrupted with an electronic or mechanical chopper at frequency n _chop the Raman radiation, which is essentially instantaneously excited will appear at this frequency and exactly in-phase with the chopper. The sample emission will be continuous, see Figure 2. The detector (usually a photo-multiplier) out-put is then fed through a phase-sensitive detector an electronic device which is tuned to look for the chopped rather than the continuous signal and hence the Raman spectra can be detected against a heavy emission background. That the method works is perhaps best demonstrated by the Raman spectra of flames! Back in 1971 we had a look at flames and were able to discriminate between the Raman spectra of the gases within the flame (CH₄, CO, CO₂, N₂ etc) and the very intense emission from the flame itself. See Figure 3.

Nowadays UV, visible and deep red spectra are almost invariably detected using CCD detectors. Although the technology exists to make these devices discriminate between AC and DC signals, it is complex and rarely encountered hence most current Raman systems do not have the facilities to discriminate against backgrounds. On the other hand, they do have very considerably enhanced sensitivity versus their historical counterparts based on photo-multipliers. It is possible to trade-off this sensitivity against discrimination but the results don’t really compare with the old methods. The logic is explained in Ref 1, however the point to remember is that if you need to examine a heated sample with a visible instrument you are OK up to 600° C or so. As a result, no problem is encountered with organics and polymers.

The emission problem in F-T Raman spectroscopy is identical to that in the visible. The problem is much more severe because in F-T Raman we use a near infrared laser source. The normal source is the NdYAG laser operating at 1.064m (9398cm^-1) hence the spectrum lies between 1.064 and 1.7m (9398-5800cm^-1). If you apply equation 1 you then find that severe emission will appear at Du = 3000cm^-1 or 1.7m at temperatures as low as 160° C not 600° C when using a green source. 160° C is a bit low for many applications. Although polyethylene and polypropylene melt below 200° C many others don’t, many pure organics melt or undergo interesting phase transitions between 200 and 300° C. Many dehydration or irreversible chemical transitions don’t occur below 200° C so users of F-T Raman complain a great deal about sample emission.

The spectrum of a thermally emitting sample shows a high background at high shifts falling steadily towards the exciting frequency. See Figure 4 You might think it is OK to study Raman spectrum over the smooth background. Since the formula of this background follows a well-defined equation you could arrange to subtract it. The problem is noise. The noise in an F-T derived spectrum is proportional to the energy in the total spectrum (including of course, the background). As a result, as the sample is heated, the noise gets much worse and hence you lose sensitivity.

Problems should be tackled, as we all know. The simplest method is to use a filter to drastically attenuate the background. If you insert a filter cutting out radiation to the red of 7398cm^-1 you can remove most of the blackbody emission yet still see the Raman bands from Du = 20000cm^-1. You lose the hydrogenic stretches but keep the rest. Using such a filter you can then wind up the temperature to around 275ŗC. See Figure 5. There are other things you can do.

Phase sensitive detection is feasible in F-T Raman but you must chop at a very high frequency. David Cutler and colleagues at Perkin Elmer, UK showed the feasibility of doing this [2]. The problem is that a detector and its electronics optimised to record F-T spectra will be set up to span the frequency range up to only 5Khz. If the chopper is to run at say 25KHz then the performance of the detector/electronics has to be compromised.

There is another very clever way of doing the job, a technique developed and described by Bob Bennett again at Perkin Elmer, UK [3].

The interferometer scans and generates an interferogram. The plot is signal vs. optical delay. As the delay is scanned, the signal appears as an AC function with a frequency domain lying between zero and 5000hz (in a sense this statement is not accurate because the frequency range depends on both the wavelength of the radiation AND the scan speed. The range I quote is typical of current practice). If the laser is sinusoidally varied in intensity at an intermediate frequency – say 500hz then mixing occurs and three interferograms are produced – the normal one and two others shifted upwards or downwards in frequency by 500hz. If the sample is emitting (a continuous process) only the central one contains information about the emission, the shifted ones ignore the emission. In Figure 6, I show a spectrum taken from Bob’s paper. As you see, it really works. Oddly however, the instrument manufacturers haven’t taken it up and to my knowledge no-one currently offers the kit and software to take advantage of what seems to me a really clever idea.

References

1. P.J. Hendra, C. Jones & G. Warnes, F-T Raman Spectroscopy, Ellis Harwood 1991, Chichester UK. Pages 66-72.
2. D.J.Cutler & C.J. Petty, Spectrochim. Acta 50A, 1159 (1994).
3. R. Bennett, Spectrochim. Acta 50A 1813 (1994)

REF: P.J. Hendra, Internet J. Vib. Spec.[www.ijvs.com] 5, 3, 4 (2001)


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