D. Kouřilová

Trace analysis by peak compression sampling of a large sample volume on microbore columns in liquid chromatography

Abstract Theoretical analyses and practical examples have shown the advantages of microbore columns when injecting solutes dissolved in a non-cluting solvent. In the determination of polynuclear aromatic hydrocarbons and chlorinated phenols, reversed-phase chromatography with amperometric detection yielded detection limits from 20 to 280 ng 1-1 in the sample. The method works with a small amount of sample (1 ml), with simple injection and without a pump for flushing the injection loop.

Minimization of extra-column effects with microbore columns using electrochemical detection

Abstract The characteristics of a simple two-electrode electrochemical detector (cell volume 20 nl) for microbore columns are described. An injection valve is connected to the microbore columns so as to provide low sample dispersion during injection. The applications described show that the equipment fully exploits the advantageous features of short columns down to 50 × 0.5 mm. The minimum detectable quantity when using small columns is 3 pg of pyrocatechol, the linear dynamic response range exceeding three orders of magnitude.

Measurement of exclusion volumes of packed columns by means of electrokinetic detection

Abstract A method is described for the measurement of the exclusion volumes of packed chromatographic columns by means of electrokinetic detection in liquid chromatography.

Enrichment techniques and trace analysis with microbore columns in liquid chromatography

The advantages of microbore columns for trace analysis by liquid chromatography are identified, with reference to on-column enrichment techniques performed on analytical micro-columns. The selectivity and high sensitivity of the amperometric detector are utilized in combination with a microbore column for a number of pharmaceutical and bioanalytical analyses, including phenothiazines, parabens, sulphonamides, catecholamines, tetracyclines, vitamins, amino acids and dipeptides.

Evaluation of changes in the specific surface area of sorbents based on the volume of stationary phase in gas-liquid chromatography

Summary Specific surface areas of sorbents were studied as a function of the coating volume of the stationary phase. Equations are suggested for the calculation of the distribution of pore volumes according to their diameters, based on the relative surface areas of chromatographic supports loaded with various volumes of the stationary phase. The results are compared with distribution curves obtained experimentally by porosimetric measurements.

Disturbing phenomena in the measurement of small surface areas by the dynamic desorption method

Abstract Thermal diffusion as a disturbing phenomenon in the measurement of small surface areas by the dynamic desorption method is described. The origin of the blank response caused by thermal diffusion is described for the cooling of the measuring column containing the sample in a bath of liquid nitrogen. The influence of the flow rate of the mixture of gases, dead volume of the column and composition of the gaseous mixture on the value of the blank response was studied.

Electrokinetic detection at different points in a narrow-bore glass column in liquid chromatography

Abstract The measurement of the electrokinetic current at different points in a narrow-bore glass column (0.4–0.5 mm I.D.) packed with sorbent is described. The origin of a “derivative” signal and the dependence of its shape on the capacity ratio of the solute and the flow-rate of the mobile phase are explained and verified experimentally. The values of the height equivalent to a theoretical plate, were determined at two places in the column, and a value of Hmin = 0.11 mm (i.e., 3.1 dp) was calculated.

Trace analysis using a competing modifier added to the sample for retention and focusing control in microbore column high-performance liquid chromatography

Abstract A method for trace analysis is microbore column high-performance liquid chromatography (HPLC) is developed. Solute focusing in the zone of a modifier competing with the solute sorption is proposed to increase the sensitivity and speed of HPLC. The elution mechanism of ionic solutes in the zone of the modifier is discussed on the basis of the theory of protolytic and sorption equilibria. The conditions of chromatographic separation are chosen such that the modifier concentration gradually increases. Under these conditions the gradient elution of solutes is achieved. The proposed method is used to advantage in combination with the elution of ionic solutes in the counter-ion zone. Both the modifier and counter ion are injected simultaneously with the sample. The method is applied to the analysis of catecholamine trace concentrations by microbore column HPLC with electrochemical detection. It presents possibilities of increasing the ratio of the sample volume injected to the retention volume.

Modified peak compression sampling in fast micro-HPLC

SummaryThe advantages of short microbore columns packed with a sorbent having a particle diameter of 3µm, such as the speed of analysis, the high solute concentration in the effluent and the detectability of small quantities are experimentally verified. The broadening in the valve is eliminated by the peak compression technique reached by adding a ion-pairing agent into the sample. The examples of catecholamines show the dependence of the height equivalent to a theoretical plate on the velocity of the mobile phase. The optimum is h=3 for a column 30mm long, i.d. 0.7mm, particle diameter 3µm. The minimal detectable quantity with a two-electrode amperometric detector with a gold electrode is from 30 to 40fmol for catecholamines.

Liquid chromatography with a dynamic temperature gradient

Abstract The theoretical principles of chromatography using a dynamic temperature gradient are described in terms of aspects of liquid-solid systems. Along the chromatographic column, an oven moves in the direction of flow of the mobile phase and thus creates a temperature gradient on the column, the temperature of which decreases in the direction of flow of the mobile phase. Each component of the mixture under analysis has a characteristic temperature in the gradient at which it is eluted from the column. The use of the dynamic temperature gradient has some advantages similar to those for the technique using a concentration gradient of the mobile phase (reduction in the time of analysis, linearization of the sorption isotherms, etc.). However, some additional advantages were found that can be used particularly in high-efficiency liquid chromatography. The composition of the mobile phase does not change and therefore the response of the detector can be used for the quantitative evaluation of the chromatogram. The column has the same adsorption properties after the analysis as before the application of the dynamic temperature gradient. An increase in the number of effective plates per second and in the peak capacity of the column were found, simultaneous with a decrease in the capacity ratio. The method can also be used for the enrichment of samples containing trace amounts of admixtures.