Pulse chromatographic method

Figure 3.38 Typical chromatograms obtained in the determination of equilibrium isotherms by pulse chromatographic methods, (a) Injection on a concentration plateau, and response of a selective detector for a tracer, (b) Response of a nonselective detector for the tracer injection made in (a), (c) Individual profiles of the labeled tracer (gray line) and the unlabeled component (black line).

Some investigators have developed methods in which the reaction proceeds directly in the chromatographic system. The most important of these are the so-called pulse chromatographic methods. In such methods of studying the kinetics of chemical transformations, a pulse of a volatile compound whose transformation provides information on the reaction taking place in the reactor column is fed to the inlet of the reactor column in a flow of carrier gas. In the pulse methods the chemical reaction and separation (analysis) are integrated into a single procedure. 

The kinetic characteristics are determined in GC methods both directly and indirectly. In direct pulse chromatographic methods the reaction rate can be established by the direct determination of the amount (concentration) of the reacting component, whereas in indirect methods this is done on the basis of the variations with time of the chromatographic properties of the reacting system, which are usually determined from the relationship between and the retention times of the non-reacting components and the composition of the reaction mixture used as the stationary phase [58]. Pulse chromatography... 

The pulse chromatographic method was also used to study the kinetics of the etherification of alcohols of various structures with acetic anhydride [74]. The reaction kinetics were studied for high-boiling alcohols the volatile reagent (acetic anhydride) was fed into the rdactor column in the form of a pulse, and the involatile one (alcohol) was present in the column reactor as the stationary phase. Unlike the pulse method used in studying the reactions involved in diene synthesis, in the etherification of an alcohol with acetic anhydride one of the reaction products (acetic acid) is eluted from the column reactor after the starting component (acetic anhydride). The reaction of alcohol etherification was examined at 80—130 C. The mixture of acetic anhydride with the standard (benzene) was pulsed into the reactor column in which the alcohol under study served as the stationary phase. Various extents of reaction were achieved by varying the carrier gas flow-rate. Table 2.6 summarizes the kinetic characteristics of the etherification of alcohols with acetic anhydrides [74]. The rate constants decrease in the order primary > secondary > tertiary. 

Another application of pulse chromatographic methods is in studies of the kinetics of isotope-exchange reactions [75]. A deuterium-labelled compound was formed as a pulse of a volatile compound (reagent) passed through a column packed with Gas-Chrom A with 10% of Carbowax 6000 and 10% of KO H applied on its surface. The isotope exchange rate is a characteristic of the nature of the substance under investigation and is of... 

In this work, we present adsorption data for very low partial pressures obtained with the pulse chromatographic method. We studied both a Na-faujasite zeolite with a Si/Al ratio of 2.43 (NaY) and two exchanged form HY and CsY. The same zeolites were impregnated with 0.5 % in weight of palladium (noted Pd-NaY, Pd-HY and Pd-CsY) as to obtain catalysts for VOCs oxidation. The experimental adsorption measurements consist of Henry constants for various alkanes and aromatics in the temperature range 448 - 673K. 

Figure 1 XRD diffractograms for NaY, HY and CsY 3. ADSORPTION STUDY 3.1 The Pulse Chromatographic Method...

Given the Hemp s zone is limited to very low adsorption pressures, it is generally impossible to perform direct measurements of the coverage ratio as a function of pressure [7, 8]. The pulse chromatographic method appears as a fast and convenient technique for die direct measurement of the Henry constants which does not required any coverage ratio and pressure determination. It is based on the mathematical treatment of the concentration profile of the adsorptive in an inert carrier gas at the outlet of an adsorption column which was submitted to an inlet pulse concentration. 

Chromatographic methods are widely used for the study of both physisorption and chemisorption. In its simplest form the technique consists of passing a pulse of the adsorbate through a column of the adsorbent and measuring the retention time and registering the elution curve. Measurement of the variation in the retention time as a function of temperature permits the evaluation of the enthalpy of adsorption, and analysis of the shape of the elution curve provides information about the adsorption isotherm. 

In the chromatographic method a pulse or step change in sor-bate concentration is introduced into the carrier stream at the inlet of a packed adsorption column and the diffusional time constant is determined from the dispersion of the response signal at the column outlet. Since heat transfer in a packed bed is much faster than in a closed system the chromatographic method may, in principle, be used to follow somewhat faster sorption processes. 

Manyanga, V. et al. Improved hquid chromatographic method with pulsed electrochemical detection for the analysis of gentamicin, J. Chromatogr. A. 2008, 1189, 347-354. 

The experimental method used in TEOM for diffusion measurements in zeolites is similar to the uptake and chromatographic methods (i.e., a step change or a pulse injection in the feed is made and the response curve is recorded). It is recommended to operate with dilute systems and low zeolite loadings. For an isothermal system when the uptake rate is influenced by intracrystalline diffusion, with only a small concentration gradient in the adsorbed phase (constant diffusivity), solutions of the transient diffusion equation for various geometries have been given (ii). Adsorption and diffusion of o-xylene, / -xylene, and toluene in HZSM-5 were found to be described well by a one-dimensional model for diffusion in a slab geometry, represented by Eq. (7) (72) ... 

Zawilla, N.H., Li, B., Hoogmartens, J., Adams, E. Improved reversed-phase liquid chromatographic method combined with pulsed electrochemical detection for the analysis of amikacin. J. Pharm. Biomed. Anal. 43, 168-173 (2007)... 

The 10-membered ring zeolites (ZSM-22 and ZSM-23) were kindly provided by Prof. Martens (COK, KULeuven). Both of the zeolites have unidimensional pore structures without any intersection. The crystals are needle-like shaped for both materials. Zeolite ZSM-22 (belonging to the TON fhmily) has free pore dimensions of 0.44 X 0.55 nm and zeolite ZSM-23 (MTT fiimily) has free pore diameters of 0.45 X 0.42 nm. The framework structures are sketched in Figure 2. The low coverage adsorption properties were determined with the pulse chromatographic technique. The details of the experimental method are discussed elsewhere. The Henry constant was determined from the first moment of the response curve on the TCD detector alter injection of an alkane trace. Adsorption enthalpy and entropy were obtained by fitting the temperature dependence of the Henry constant to the van t Hoff equation. 

In order to study the isotherm at lower pressure range, we used a pulse response method with a gas chromatograph. 

The primary use of isotherm data measurements carried out on single-component elution profiles or breakthrough curves is the determination of the single-component adsorption isotherms. This could also be done directly, by conventional static methods. However, these methods are slow and less accurate than chromatographic methods, which, for these reasons, have become very popular. Five direct chromatographic methods are available for this purpose frontal analysis (FA) [132,133], frontal analysis by characteristic point (FACP) [134], elution by characteristic point (ECP) [134,135], pulse methods e.g., elution on a plateau or step and pulse method) [136], and the retention time method (RTM) [137]. 

Gangwall et al. [47] were the first to apply Fourier analysis for the evaluation of the transport parameters of the Kubin-Kucera model. Gunn et al. applied the frequency response [80] and the pulse response method [83] in order to determine the coefficients of axial dispersion and internal diffusion in packed beds from experiments performed at various Reynolds numbers. Bashi and Gunn [83] compared the methods based on the analytical properties of the Fourier and the Laplace transforms for the calculation of transport coefficients. MacDonnald et al. [84] discussed the applications of the method of moments to the analysis of the profiles of skewed chromatographic peaks. When more than two parameters have to be determined from one single run, the moment analysis method is less suitable, because only the first and second moments are reliable (see Figure 6.9). Therefore, only two parameters can be determined accurately. 

SOME MODIFICATIONS OF PULSE GAS CHROMATOGRAPHIC METHODS OF DETERMINING KINETIC CHARACTERISTICS... 

Yet another interesting chromatographic method was developed by Schulz [61], consisting in determination of the kinetic characteristics of a reaction between two compounds A and B, when the pulse of compound B overtakes that of A in the chromatographic column. Unfortunately the equations derived are too complicated to process the results an iterative routine was run on a computer. To test his method Schulz [61] studied the reaction between acetic anhydride and m-xylenol (1-hydroxy-3,5-dimethyl-benzene). First, a pulse of wi-xylenol was fed into a chromatographic column containing squalane, then after a period equal to half of its retention time a pulse of acetic acid was introduced. The measurements were taken at various flow-rates at 120-140°C. The fit between the data derived by classical chromatographic techniques and the method proposed by Schulz is satisfactory (Fig. 2.5). 

Eisen and Ivanov [60] elaborated a pulse, micro-catalytic gas chromatographic method of selective hydrogenation of alkenes and cyclohexenes at 90°C on 5% palladium on silica gel as the catalyst. Under these conditions aromatic hydrocarbons are not hydrogenated, and cyclopentenes are hydrogenated to an insignificant extent. 

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