Spurious bands

In analyzing the spectrum of a substance, one occasionally finds a band or a shoulder that is very difficult to interpret. This band may, upon further consideration, be found to be spurious, that is, it does not belong to the sample under analysis, but is caused by an instrumental effect, the method of handling the sample, or some unexpected phenomenon. For example, it has been pointed out (Inchiosa, 1965) that certain disposable syringes used in clinical and laboratory medicine yielded water-soluble extractives. One of these substances was identified as 2-(methylthio)ben-zothiazole, which has fungistatic and insecticidal activity. Such extractives or their reaction products could show up in the infrared spectrum during a laboratory procedure. 

Sometimes, optically polished materials that one obtains for the laboratory show absorption bands of an unknown origin. These bands could have been caused by impurities within the material itself or by impurities left in the surface from the grinding and polishing process. To help workers identify the source of such absorption bands, McCarthy (1968) has published the spectra of aluminum oxide, bamesite, cerium oxide, glassite, rouge, sodium thiosulfate, stannic oxide, and titanium oxide. 

Band position, cm Compound or structural group Comments 

3704 HjO In the near-infrared, where thick cells are used, a trace of moisture in carbon tetrachloride or hydrocarbon solvents gives an O—H band at 3704 cm .  

3650 H2O Some fused silica windows show a sharp band at 3650 cm produced by occluded water. 

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Because we use total DNA from many species on one filter and hybridize at a reduced stringency (55 °) with cloned cpDNA which is not removed from the vector, we find the technique of Qark and Hanson 6 provides a useful control for occasional spurious bands. A small amount of very pure chloroplast DNA from one member of our study group is radioactively labeled and hybridized to every blot at a higher stringency (65°). A comparison of the films obtained from cloned cpDNA and from pure cpDNA determines if any bands are anomalous. 

Spurious bands can occur readily in infra-red spectra, particularly when a biological sample has undergone several purification procedures. Traces o lasticisers, surfactants, and oils left on glassware can all give rise to artefacts some spurious infrared bands are given below (after H. A. Szymanski, A Systematic Approach to the Interpretation of Infrared Spectra, Buffalo, New York, Hertillon Press, 1971). 

Figure 2. ATR spectra of the same Nafion samples as in Figure 1. The upper spectrum has been moved up the transmittance scale by 40%. Asterisks indicate spurious bands, artifacts of the ATR...

Table 1.3 gives a list of spurious bands, arranged according to the frequency of the band. Table 1.4 presents a list of spurious bands that might occur at any frequency. 

Table 1.3. Spurious Bands That Occur at Specific Frequencies...

Alkali halide windows (particularly KBr) can react with carboxylic acid or metal carboxylates to produce an alkali salt of a carboxylic acid and to give a spurious band due to the carboxylate anion. This sort of thing also happens in KBr disks. Dicarboxylic acids like maleic acid seem to be the worst offenders. 

NaNOj A spurious band at 1355 cm", along with a weaker but sharper band at 836 cm", was traced to a deposit of sodium nitrate on a sodium chloride window near a Nernst glower. The deposit was attributed to the reaction of nitric acid with the window, the nitric acid originating from nitric oxide produced by direct union of nitrogen and oxygen at ca. 1900°C on the Nernst glower. 

Table 1.4. Spurious Bands That Might Occur at Any Frequency (Launer, 1962)...

How do we know if we have over-deconvolved Well, we don t. Neither do we know if our derivative transform produces spurious bands. Both FSD and the derivatization of recorded data are pretty much an art, not a science. One just has to try it and develop a feel for its effect on the data. But here are some guidelines (a) FSD will produce negative lobes when you over-deconvolve a spectrum. If these lobes go below zero, you have probably over-deconvolved, (b) Look at the narrowest bands. If lobe production around the narrow bands look as if they should not be there, then you probably have over-deconvolved, (c) Finally, you can compare deconvolved spectra with derivative spectra. Until you get a feel for FSD you may want to use this security blanket. 

Christiansen effect A spurious band on the high frequency side of a true absorption band may sometimes be observed when examining the mulls of crystalline materials if the particle size is of the same order of magnitude as the infrared wavelength being used. 

Fold-back The maximum frequency that may be measured by an FT Raman spectrometer is governed by the frequency of the excitation radiation. However, radiation of a higher frequency than that of the maximum may still pass through the interferometer. As a result of this, the detector may observe electromagnetic interference due to this higher frequency which it cannot distinguish from that due to radiation that is below the maximum frequency by an equivalent amount. This fold-back below the maximum, by an amount equal to the difference in the frequencies, may therefore result in spurious bands appearing in Raman spectra. Most instruments these days have optical and electronic filters which try to overcome this effect but these devices do not always completely remove the problem. 

Table 1.1 gives the positions of some spurious bands and the reasons for their appearance. 

An alkali halide may react with a carboxylic acid or metal carboxylate to produce a salt and hence give rise to a spurious band due to the carboxylate anion. This may occur in fte preparation of KBr discs or as an interaction with cell windows. 

A spurious band is one which does not truly belong to the sample but results from either the sampling technique used or the general method of sample handling, or is due to an instrumental effect, or some other phenomenon. There are numerous reasons why spurious bands appear in spectra and it is extremely important to be aware of the possible sources of such bands and to be vigilant in the preparation of samples for study. 

Due to the careless handling of cells, pressed discs, plates, films, internal reflection crystals, etc., spurious bands may be observed in spectra due to a person s fingerprints. These bands may be due to moisture, skin oils or even laboratory chemicals. Unfortunately, such carelessness is a common source of error. If an instrument experiences a sudden jolt, a sharp peak may be observed in the spectrum. Similarly, excessive vibration of the spectrometer may result in bands appearing in the spectrum. 

Although not so common these days, when recording a spectrum to magnetic disc, errors in software programmes have lead to spurious bands appearing in spectra or even bands disappearing from a recorded spectrum. 

Numerous laser emission frequencies Some lasers used in Raman spectrometers produce a number of other emissions in addition to their base frequency which are of lesser intensity (i.e. the emission is not monochromatic). Of course, a sample can also reflect or scatter these additional radiations. As a result, spurious bands may be observed in Raman spectra at any position - the positions of bands and tbeir intensities being dependent on the laser and the sample. The problem can be avoided by the use of a pre-monochromator or suitable filter. 

A section dealing with spurious bands that may appear in both infrared and Raman spectra has been included in the hope that confusion may be avoid by prior knowledge of the reasons for such bands and the positions at which they may occur. 

Spectra often display spurious bands. Some of these arise from imperfect compensation of atmospheric absorption bands. Even with good compensation, the spectrometer becomes insensitive when scanning regions of strong atmospheric absorption, so that band contours are distorted. It is worthwhile to purge the spectrometer with dry air or nitrogen. 

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