Transmission spectra aqueous solutions

The sample absorbs the radiation during this process and an absorption spectrum characteristic of the sample is thus obtained. The spectrum of a sample acquired by the ATR technique is generally comparable to the transmission spectrum, but band shifts have been observed. The ATR technique has been used for materials that are too thick or absorb too strongly to be analyzed by transmission spectroscopy. Aqueous solutions can be analyzed by the liquid ATR technique, which was previously very difficult, using transmission spectroscopy. 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

Fig. 1 (a) Differential transmission spectrum, at magic angle, of a GFPuv sample in aqueous solution (540 pg/ml, pH 6.5). (b) Cross section centered at 508 nm (FWHM of 15 nm). Data are obtained with 400 nm pump (fluence=2 mJ/cm2 duration 200 fs). 

The internal reflectance technique is usually called attenuated total reflection (ATR) spectroscopy. It is especially useful for studying strongly absorbing media, for example, aqueous solutions. When the infrared radiation is absorbed in the test medium, one obtains a spectrum similar to that from a transmission experiment. However, there are distortions in the ATR spectrum, especially in the region of intense bands. One reason for distortion is the fact that the depth of penetration varies with wavelength. The other effect is due to the change of the refractive index of the solution in the region of the intense band. ATR spectra should be corrected for these effects so that they may be compared to normal transmission spectra. 

All spectra were run on a Digilab FTS-10 FTIR system equipped with fast-scan capabilities, a Hycomp 32 data array processor, and a nitrogen-cooled mercury-cad-mium-telluride detector. Transmission spectra were obtained using CaF2 windows with a 6- xm spacer. For each transmission spectrum, 500 scans were co-added at 4-cm-1 resolution. Smoothing was not needed on these spectra. All the spectra in this chapter (both transmission and ATR) are subtracted spectra, that is, they are the resultof subtracting a saline (H20) spectrum from the spectra of the aqueous protein solutions. 

Figure 4 Absorption (transmission) spectrum for a serum film dried onto a barium fluoride window. The specimen was first diluted twofold in aqueous 4gL potassium thiocyanate (KSCN) solution. The absorption of SON" at 2060 cm" was used for subsequent normalization of the spectra as part of the development of quantitation models (see Shaw et al. )-...

The sample techniques just described are designed for collection of transmission (absorption) spectra. This had been the most common type of IR spectroscopy, but it was limited in its applications. There are many types of samples that are not suited to the conventional sample cells and techniques just discussed. Thick, opaque solid samples, paints, coatings, fibers, polymers, aqueous solutions, samples that cannot be destroyed such as artwork or forensic evidence samples, and hot gases from smokestacks—these materials posed problems for the analytical chemist who wanted to obtain an IR absorption spectrum. The use of reflectance techniques provides a nondestructive method for obtaining IR spectral information from materials that are opaque, insoluble, or cannot be placed into conventional sample cells. In addition, IR emission from heated samples can be used to characterize certain types of samples and even measure remote sources such as smokestacks. In reflectance and emission, the FTIR spectrometer system is the same as that for transmission. For reflectance, the sampling accessories are different and in some specialized cases contain an integral detector. The heated sample itself provides the light for emission measurements therefore, there is no need for an IR source. There may be a heated sample holder for laboratory emission measurements. 

Solution filters are readily prepared in any photochemical laboratory. Their compositions and optical properties are available throughout the literature.137,157 158 Figure 3.3 shows, for example, the transmission curve of a band-pass solution filter made of Cr(SC>4)2 dissolved in dilute aqueous H2SO4, which transmits radiation at 325 20 nm. As a result, the 313.9 nm band, available in the emission spectrum of medium-pressure mercury lamps, can be isolated. However, some of the reported solutions have limited photochemical stability and should be regularly checked for transmittance. 

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