Liquid chromatography loop injectors

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

For liquid chromatography, a sample of the mixture solution is injected through a loop injector which allows a quantity of the solution to be placed in a small tubular loop at atmospheric pressure. By manipulating a valve, the high-pressure flow of solvent to the column is diverted through the loop, carrying the sample with it (Figure 35.5). 

Successful use of modern liquid chromatography in the clinical laboratory requires an appreciation of the method s analytical characteristics. The quantitative reproducibility with respect to peak height or peak area is quite good. With a sample loop injector relative standard deviations better than 1% are to be expected. The variability of syringe injection (3-4% relative standard deviation) requires the use of an internal standard to reach the 1% level (2,27). 

The D4 content of the samples was determined by reverse-phase high-pressure liquid chromatography (HPLC) with a Varian 5500 liquid chromatograph. A DuPont Zorbax ODS (Cis) column was used with a Wilmad infrared detector set at 12.45 xm to monitor the Si-CHa vibration. The mobile phase was an 83 17 mixture of acetonitrile and acetone at a flow rate of 0.8 mL/min. A Rheodyne injector valve operating on compressed air was used with a 10-p,L sample loop for reproducible injection volumes. Ethyl acetate was used to dissolve the samples for analysis. 

High Performance Liquid Chromatography. Tissue extracts were analyzed with a Varian model 5020 liquid chromatograph equipped with a Rheo-dyne model 7120 loop injector valve, a Tracer 970 variable wavelength detector set at 257 nm, an automated Hewlett-Packard 3385A printer-plotter system for determining retention times and peak areas, and a Waters /x Bondapak column (3.9 mm i.d. X 300 mm) for carbohydrate analysis. The buffer was eluted isocratically at 1 mL/min with a 1 4 (v/v) mixture of 0.01 M monobasic sodium phosphate (pH 4.46) and methanol. The minimum amount detectable was 10 ng. 

I. Sample loop injector. (From D. C. Scott in Modern Practice of Liquid" Chromatography,... 

Fernandez Otero, G.C. Lucangioli, S.E. Carducci, C.N. Adsorption of drugs in high-performance liquid chromatography injector loops. J.Chromatogr.A, 1993, 654, 87-91... 

The LC system consisted of a Jasco 880 PU pump (JASCO, Tokyo, Japan) for the mobile phase, a Rheodyne Model 7125 injector with a 25 pL loop, and a 0.2 x 15 cm Mightysil RP-18 GP (5 pm) column (Kanto Chemical, Tokyo, Japan). LC was performed using an aqueous solution containing 80% MeCN as the mobile phase at a flow rate of 0.2 ml/min at 26°C. The column was connected to L-7455 diode array detector (Hitachi, Tokyo, Japan) with a flow cell (17.7 pL, 10 mm), and Hitachi D-7000 advanced high performance liquid chromatography (HPLC) system manager was utilized for data acquisition. 

The method of characterization by high-performance liquid chromatography [HPLC] is as follows 25 mg of hemicellulose was put into 50 mL volumetric flask and pure water was added to the mark. The mixture was shaken and then filtered [first few milliliters of filtrate was discarded]. The solution was then filtered through a membrane filter of 0.2 pm cellulose nitrate. Then about 100 mL solution was injected into the HPLC system via a loop injector with a 20 mL, using an isocratic elution system with distilled water with a mobile phase, flow rate 0.8 mL/min. The detection was made using a UV detector at a wavelength of 280 nm [34]. 

Ideally the output of a loop Injector used In liquid chromatography or flow Injection analysis would be a sharp concentration pulse. However, this Is unlikely to be the case because of various dispersive forces which act on the concentration plug. Convective flow under laminar conditions In a circular tube, such as the outlet tube of a loop Injector, tends to be much slower near the walls of the tube than In the center and this will distort the Initial shape of the Injected materials (8,9). In addition, radial and axial diffusion of material In the tube can alter its initial shape. The degree of dispersion can be evaluated (9) by the Peclet nusd>er (P,) and the reduced time (r). These values are defined as... 

The analytical HPLC system was a Waters Model 600S liquid chromatography system (Waters Associates, Milford, Massachusetts, U.S.A.) equipped with a Waters 515 Multisolvent Delivery System with a 486 Tunable Absorbance Analytical Detector, and a Rheodyne injector (50 pi sample loop). The data acquisition system was a Chromate (Ver. 3.0 Interface Engineering, South Korea) installed in a PC. The flow rate of mobile phase was fixed at 4, 2, and 1 ml/min with CIM QA, QlOO, and HiTrap Q, respectively. The wavelength was fixed at 260 and 280 nm and the injection volume was fixed at 20 pi. The experiment was performed at room temperature. 

Loop Injectors for High Performance Liquid Chromatography (HPLC)... 

An injector block used in liquid chromatography injects about 500 pL of sample into the carrier stream. The volume of the carrier stream loop from one side of the ISE membrane to the opposite side including the volume of both membrane sides is also approximately 500 pL. Thus, when the sample segment is fully washed out of the first membrane side the first front of it reaches the backside. Through this a positive and a negative analyte peak are obtained, which look like a second derivative of a peak. 

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