Photoacoustic infrared spectroscopy method

First, we will examine the various ways of examining samples using the traditional transmission methods of infrared spectroscopy. In the second part of this ch ter we will examine the more modem reflectance methods that arh now available, such as the attenuated total reflectance, diffuse and specular reflectance methods. We ml also look at a number of more specialist techniques which you might encounter, such as photoacoustic spectroscopy, gas chromatography-infrared spectroscopy, and the use of temperature and raicrosampling cells. 

The various reflectance methods which are now widely available, such as attenuated total reflectance, specular reflectance, and diffuse reflectance spectroscopy, were also introduced. Photoacoustic spectroscopy, gas chromatography-infrared spectroscopy, temperature studies and microsampling techniques were also described. 

Routine methods of chemical analysis can be applied to water-based coatings. For example, the nature of the binder can be determined by infrared spectroscopy. Water-resistant plates, such as KRS5 or zinc selenide, must be used if capillary films are cast from the aqueous dispersion. Dried films can be analyzed by surface-sensitive techniques such as attenuated total reflection (ATR) or photoacoustic spectroscopy (PAS). Both these techniques require the use of Fomrier transform infrared (FT IR) instruments to obtain spectra in a reasonable time. 

In order to determine the composition and structure of a biomaterial surface different methods which provide varying degrees of information are commonly used (Fig. 6). Surface-sensitive infrared spectroscopy suppHes the characteristic absorption bands of functional groups with an informational depth of 0.1-10 pm by measurement in attenuated total reflectance (IR-ATR). In the case of samples with rough surfaces photoacoustic spectroscopy (PAS), which allows an informational depth of approximately 20 pm, can be used [72]. The achieved informational depths are usually larger than the thickness of the modified interface, so that the spectra include information on the bulk composition as well. As a consequence, surface-sensitive infrared spectroscopy is often not sensitive enough for the characterization of the modified surfaces. 

Principles and Characteristics Infrared spectroscopy is one of the oldest and most established analytical methods in industry. New technical developments, such as IR microscopy, photoacoustic IR spectroscopy and on-line techniques for process analysis are now routinely being used in many laboratories. Furthermore, chemomet-ric data evaluation, which is very frequently used in near-IR spectroscopy, is often advantageous also in the field of mid-IR spectroscopy and strengthens its outstanding position towards both basic and applied research. 

Gardalla and Grobe compared attenuated total reflectance and photoacoustic sampling for surface analysis of polymer mixtures by Fourier transform infrared spectroscopy. They show that analysis by attenuated total reflectance is more suitable for smooth surfaces and is faster. Photoacoustic methods have shallower sampling depths than attenuated total reflectance but the latter technique is applicable over a range that is more controllable. 

C02 in the gas phase can be determined by means of its significant infrared absorbance (Fig. 10) at wave lengths (A) < 15 pm, particularly at 4.3 pm [289], or by acoustic means. Integrated photoacoustic spectroscopy and magnetoacoustic (PAS/MA) technology for combined C02 and 02 analysis has rapid response time and a small sample volume is sufficient. The acoustic methods are accurate, stable over long periods and very simple to use. 

On the other hand, successful identification of bacterial spores has been demonstrated by using Fourier transform infrared photoacoustic and transmission spectroscopy " in conjunction with principal component analysis (PCA) statistical methods. In general, PCA methods are used to reduce and decompose the spectral data into orthogonal components, or factors, which represent the most coimnon variations in all the data. As such, each spectrum in a reference library has an associated score for each factor. These scores can then be used to show clustering of spectra that have common variations, thus forming a basis for group member classification and identification. 

Modulated infrared energy is absorbed by the sample. The material heats and cools in response to modulated infrared light. This response is converted into a pressure wave, which communicates with a surrounding gas. The pressure change is detected by an acoustic detector in the enclosed sample chamber. The necessary amount of sample material is less than 10 mg. The measuring time is about a few minutes, and the quality of the spectra is similar to that of the DRIFTS and ATR methods. The costs for the equipment are higher. A comparison of photoacoustic spectroscopy with all various FT-IR methods for the use in combinatorial chemistry was made by Yan et al. [12,13]. 

Vibrational spectroscopy represents two physically different, yet complementary spectroscopic techniques IR and Raman spectroscopy. Although both methods have been utilised for many years, recent advances in electronics, computer technologies and sampling made Fourier transform infrared (FTIR) and Raman (FT-Raman) one of the most powerful and versatile analytical tools. Enhanced sensitivity and surface selectivity allows non-invasive, no-vacuum molecular level analysis of surface and interfaces. Emphasis is placed on recent advances in attenuated total reflectance (ATR), step-scan photoacoustic (SS-PA), Fourier transform infrared (FTIR) and FT-Raman microscopies, as utilised to the analysis of polymeric surfaces and interfaces. A combination of these probes allows detection of molecular level changes responsible for macroscopic changes in three dimensions from various depths. 7 refs. 

Step-scan photoacoustic data are presented that prove the ability of the technique to successfully isolate the infrared signature on the top layer from the infrared spectrum of the bulk material, proving the sub-micron resolution capability of the method. Results are shown that underline the fact that the most serious problem in photoacoustic spectroscopy is saturation at high absorptivities. 12 refs. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

Hanh BD, Neubert RHH, Wartewig S, Christ A, and Hentzsch C (2000) Drug penetration as studied by non-invasive methods Fourier transform infrared-attenuated total reflection, Fourier transform infrared, and ultraviolet photoacoustic spectroscopy. Journal of Pharmaceutical Sciences 89 1106-1113. 

Other methods have also been developed but are harder to use routinely (higher size of the equipment, longer measurement time, higher cost, etc.). They will not be described here and the reader is referred to scientific literature for details. They include, for example, near-infirared [106], infrared [107], and optothermal infrared [108] spectroscopy, photoacoustic spectroscopy [109], as well as many other indirect methods taking advantage of the consequences of skin dehydration (D-Squame disk analysis, profilometry, viscoelastic properties of the skin, etc.). 

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