Injection problems

Usually the problem comes from the injection system  

Replace worn parts of the autosampler, i.e. needle, capillary between needle and valve, rotor seal and/or stator. 

Look for poor capillary joints and replace them. Replace the injector loop. Replace capillaries with rough inner surface. Check the column and replace it if necessary. Overfilled sample vials. Wrong flushing solvent of the autosampler without cleaning effect. 

Check and replace the worn parts as described above. 

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Linear, branched and cyclic paraffins can all be present in fuel. At low temperatures, fuel filtration, pumpability and injection problems are primarily due to paraffinic wax. Refiners can contend with fuel wax through processing changes and blending. Examples include ... 

Mass transfer requirements dictate low propellant viscosity. In addition to the excessive work required to transfer a viscous propellant from the tank to the combustion chamber (either through pumping or pressurization), the injection problems associated with viscous propellants have often compromised efficient combustion. Low propellant vapor pressure allows a more efficient pump design and avoids one problem area in fluid pumping. 

J5 Consider the wall-injection problem in an axisymmetric setting, where a uniform injection velocity flows through the wall of a cylindrical tube. There is a mean velocity U that enters through one end of the tube. Following a procedure analogous to the flow-between-plates problem (Section 5.6), develop a solution for the velocity profiles and the wall shear stress as characterized by the product of a Reynolds number and a friction factor. 

Reproducibility of peak height is also quite dependent on the reproducibility of the sample injection. This is especially important on early and thus normally quite sharp, narrow peaks. On such early peaks the width of the peak is controlled more by the injection time rather than the chromatographic process. A fraction of a second increase in injection time can double the width of these peaks and reduce peak height 50%. The peaks most subject to error in peak height measurement from injection problems are those with retention volumes between one and two times the hold-up volume of the column. Peaks beyond five to ten times the hold-up volume are negligibly affected by injection technique. 

Prior to World War II the majority of experimental work in the field of sprays concerned itself with Diesel engine injection problems. Sauter (21C) has suggested that the efficiency of atomization is determined by its fineness and uniformity. De Juhasz (2C) has determined the effect of many different variables on the process of spraying. The more important physical factors tested include the pressure drop across the orifice, the viscosity of the liquid, and the density of the air. For the many different nozzles tested, the ratio of orifice length to diameter has little effect. To determine the effect of the density of the air into which the liquid was sprayed, the nozzles were allowed to discharge into an evacuated chamber. Under these conditions good results are still obtained, suggesting little influence of air friction on jet breakup. 

Spruce, B., and Bakalyar, S. R., Troubleshooting Guide for HPLC Injection Problems, 2nd Ed. Rheodyne Incorporated, Cotati, California, 1992. 

In the one case where injectivity problems initially occurred, these happened before any dispersion was formed, and later disappeared(3). 

Many samples are more soluble or less prone to decompose in organic mobile phases than in aqueous mobile phases, and do not cause injection problems in NPLC, as is occasionally observed in RPLC. 

An alternative approach is to use a splitless injection system. If the valve in Fig. 1 is closed, then all the sample passes into the column and there is no split ipso facto, the device is a splitless injector. When used in the splitless mode, however, it is usual to employ a somewhat wider capillary column, which will allow the penetration of a small-diameter injection syringe and thus permit on-column injection. Under these circumstances, there can be no differential sampling of the form described. This procedure, however, introduces other injection problems that can affect both resolution and quantitative accuracy that need to be addressed (See the entries Retention Gap Injection Method and Solute Focusing Injector Method). 

The common problems in HPLC concern pressure (high, low, or unstable, or none), leaks, quantitation (detection problems, injection problems, sample problems or data-system problems), chromatogram (peak shape), and hardware. 

Table 10.1 shows a comparison of some of the properties of several common alkalis. Potassium-based alkalis, the price of which is higher than sodium-based alkalis, are not included. They are considered when clay swelling and injectivity problems are expected. Some alkalis are further discussed and compared in the following sections. 

Several others techniques dealing with the injection problems have been developed. Among them the solid-phase microextraction method (SPME) and the full evaporation technique must be mentioned. According to Camarasu, the SPME technique seems to be very promising for RS determination in pharmaceuticals, with much better sensitivity than the static headspace technique. 

Many samples are more soluble or less prone to decompose in organic than in aqueous mobile phases. Such samples do not cause direct-injection problems in NPLC, unlike to RPLC. (5) The recovery of organic-soluble sample components is easy in preparative NPLC. (6) Very large changes in separation selectivity are possible by changing either the mobile phase or the stationary phases in NPLC. 

If an unsteady-state type flow develops in an injection well, and it causes injectivity problems, the injectivity of the well could be restored by periodically shutting down the injection. 

The experiment has supplied encouraging results on the acceleration of plasma liners and on the concentration of their energy in a small volume. In order that meaningful answers to the fusion problem can be obtained, it is necessary to scale up the apparatus, simplify the injection problem and as already mentioned, provide separate deuterium injection . 

To tackle the injection problems a new automated injection system was tested (Fig. 6b). An injection slit etched by Bosch avoids any formation of asymmetric shaped plugs, which typically occurs during loading of the sample in wide channels. The slit is connected to a vacuum system and syringes for the supply of the analyte and the mobile phase. A CFD simulation was first performed to compare different injection well shapes and configurations in order to account for less profitable configurations. 

We have spoken of constant pressure and uniform velocity fracture models. Where are these used When the produced fluid in a fracture is clean, its pressure equalizes almost instantaneously and the former applies. On the other hand, the latter describes injection problems in which a filter cake or extremely low permeability material is left at the sandface, which in turn controls local volume influx rate. The subject of formation invasion will be studied starting with Chapter 16. 

For water injection problems, this assumes that the displacement is fast (or, inertia dominated), so that surface tension can be neglected however, when water breakthrough occurs, the assumption breaks down locally. In formation invasion, this zero capillary pressure assumption may be valid during the early periods of invasion near the well, when high filtrate influx rates are possible, as... 

The borate formulation was not previously optimized using phase behavior or oil/water i.f.t. screening tests and yet still performed reasonably well in terms of tertiary oil recovery, chemical (strong alkali, surfactant, and polymer) loss, and in addition no injectivity problems were encountered. 

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