Stop-flow injection

For higher pressures, stopped flow injection has been used in which a valve releases the pressure in the column to atmosphere, at which point syringe injection at the top of the column (without septum) takes place. There has, however, been a trend in recent years to use valve injectors because of their generally high pressure capabilities and their ability to deliver large... 

Figure 6.7 depicts an autosampler employed in a jtPLC system. Figure 6.8 details the autosampler component. Samples are transferred from the desired well in the microtiter plate into the columns of the Brio cartridge. If a 384-well plate is employed, the autosampler will carry out 3 sets of 8 injections into the columns, for a total of 24 columns. The solvent (mobile phase) does not circulate in the cartridge but is diverted into a backpressure regulator located in the waste line (Figure 6.2). This process of injection is known as stop-flow injection. After all samples are placed into the injection pits of the 24 columns in the cartridge (Figure 6.5), a clamp containing a seal...

In HPLC, the mobile phase is constantly passing through the column at a finite rate. The sample injections are made rapidly in the dynamic state in the form of a narrow band or plug (see Fig. 15.1), which provides a significant advantage over older stop-flow injection techniques in HPLC. After a sample has been injected as a narrow band, the separation can then be envisioned as a three-step process shown in the figure ... 

Ideally, a sample is introduced into a chromatograph as a perfect plug. In practice, this is not the case, and diffusion occurs because of the injector. For narrow-bore and microbore applications, injectors capable of introducing the required sample volumes are commercially available and optimized to reduce dispersion. This is not the case for capillary LC, and homemade injection systems include the sample tube technique, in-column injection, stopped-flow injection, pressure pulse-driven stopped-flow injection (PSI), groove injection, split injection, heart-cut injection, and the moving injection technique (MIT). Of the injection techniques, only the split injector, MIT and PSI approaches can introduce subnanoliter sample volumes accu-... 

Other possibilities include stop-flow injection and septumless syringe injection valves, in which the injection into the valve is made off-line under ambient pressure. 

For the analysis of esters by RP-HPLC, it is very hard to avoid water because it will be present in the sample extraction solvent and even in the mobile phase. However, the kinetics of ester hydrolysis in solution can be studied by stopped flow injection experiments to determine compatibility with mobile phase. The same experiment must be repeated with different pH (from pH 1.2 to pH 6.8), since it is known that ester hydrolysis is pH-dependent. An example of pH-dependent ester hydrolysis is methylphenidate HCl. Methylphenidate HCl has an ester functional group, and its hydrolysis is pH-dependent. Refer to Table A15-1 for stability of methylphenidate HCl at different pHs [5]. The hydrolysis product of methylphenidate HCl is the free acid (refer to Figure A15-3) [5]. 

Espinosa-MansUla A, Mimoz de la Pena A, Salinas F. Semiautomatic determination of furanic aldehydes in food and pharmaceutical samples by a stopped-flow injection analysis method. JAOAC Int 1993 76 1255-1261. 

Another injection technique, which involves a sample injection port a small distance downstream of the channel inlet, where the sample was injected and relaxed between two focusing flows with a reversed flow from the channel outlet, was first employed for asymmetrical FIFFF [5]. The theoretical basis of the horizontal transport was derived and fundamental practical aspects were discussed. This procedure is now standard for asymmetrical FIFFF, partly due to the difficulties of adapting the stop-flow injection mode and was further optimized by Wahlund and Litzen [6] to include injection of up to 5 mL of sample in a 0.8-mL channel. 

Stop-flow injection In high-performance liquid chromatography, introduction of the sample at the head of the column while solvent flow is temporarily discontinued. 

C.S. Lim, J.N. Miller, J.W. Bridges, Automation of an energy-transfer immunoassay by using stopped-flow injection analysis with merging zones, Anal. Chim. Acta 114 (1980) 183. 

G.D. Christian, J. Ruzicka, Exploiting stopped-flow injection methods for quantitative chemical assays, Anal. Chim. Acta 261 (1992) 11. 

M. Blanco, J. Coello, H. Iturriaga, S. Maspoch, J. Riba, Modification of a commercial dual channel valve to be used in a stopped-flow injection analysis system, J. Flow Injection Anal. 7 (1990) 3. 

M. Pistonesi, M.E. Centurion, B.S.F. Band, P.C. Damiani, A.C. Olivieri, Simultaneous determination of levodopa and benserazide by stopped-flow injection analysis and three-way multivariate calibration of kinetic-spectrophotometric data, J. Pharm. Biomed. Anal 36 (2004) 541. 

D.M. Magni, A.C. Olivieri, A.L. Bonivardi, Artificial neural networks study of the catalytic reduction of resazurin stopped-flow injection kinetic-spectrophotometric determination of Cu(II) and Ni(II), Anal. Chim. Acta 528 (2005) 275. 

A.N. Anthemidis, D.G. Themelis, J.A. Stratis, Stopped-flow injection liquid-liquid extraction spectrophotometric determination of palladium in airborne particulate matter and automobile catalysts, Talanta 54 (2001) 37. 

Figure 4.15. Three types of flow channel designs for stopped-flow injection analysis allowing options for selecting initial reaction time (/i) between injected sample ( /i) and reagent (B), mixing time (/m), delay time (fd), and stop time Us) of the sample within the flow cell (FC). While the system shown in (a) suffices for assays based on reaction rate measurements (cf. Figs. 4.12-4.14), physicochemical rate studies aimed at determining reaction rate constants require manifolds such as those depicted for slower bj and faster (c) reactions, the delay time required for sufficient radial mixing within the element of study (fm ") being a crucial parameter (d) C represents an inert carrier stream.

H. Kagenow and A. Jensen, Kinetic Determination of Magnesium and Calcium by Stopped-Flow Injection Analysis. Anal. Chim. Acta, 145 (1982) 125. 

F. Lazaro, M. D. Luque de Castro, and M. Valcarcel, Stopped-Flow Injection Determination of Copper(II) at the ng per ml Level. Anal. Chim. Acta, 165 (1984) 177. 

The syringe is removed, the port is closed and the eluent stream is started again. An example of such a stopped flow injector, which is commercially available (Varian), is shown in Fig. 7. For precise retention time measurements, stopped flow injection is less preferable because of the time needed to pressurize the system, which might be of the order of 3-2D sec depending on the capacity of the pumping device. On the other hand, for routine analysis this is less important and stopped flow injection might be a very valuable, cheap and reliable sample introduction technique. 

Septum injection, pneumatic high-pressure syringe injection and v.alve injection are the usual kinds of sampling devices. The stop-flow injection technique is also used. Automated sampling devices based on high-pressure syringe injection or valve injection are already available. 

S. Somnam, K. Grudpan, J.M. Jakmunee, Stopped-flow injection method for determination of phosphate in soils and fertilisers, Int. J. Sci. Tech. 2 (2008) 172-181. 

Kagenow, H. and A. Jensen. 1983. Kinetic determination of magnesium and calcium by stopped-flow injection analysis. Anal. Chim. Acta 145 125-133. 

Tsukatani, T. and K. Matsumoto. 2000. Quantification of L-tartrate in wine by stopped-flow injection analysis using immobilized D-malate dehydrogenase and fluorescence detection. Anal. Sci. 16(3) 265-268. 

Zeng, S. and H. Tan. 2000. Stopped-flow injection analysis method for simultaneous determination of ascorbic acid and L-cysteine. Huaxue Fence 36 439-441. 

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