Septum problems

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Although the problems associated with septum injectors can be eliminated by using stop-flow septumless injection, currently the most widely used devices in commercial chromatographs are the microvolume sampling valves (Fig. 8.3) which enable samples to be introduced reproducibly into pressurised columns without significant interruption of the mobile phase flow. The sample is loaded at atmospheric pressure into an external loop in the valve and introduced into the mobile phase by an appropriate rotation of the valve. The volume of sample introduced, ranging from 2 piL to over 100 /iL, may be varied by changing... 

We could also refer to this as reduced sensitivity. The peaks are smaller than expected based on previous observations when equal or greater quantities of a particular sample were injected. Such an observation usually means a problem with injection (less injected than assumed) or a problem with the detector such that a smaller electronic signal is sent to the recorder. One should check for a leaky or plugged syringe, a worn septum, a leak in the pre- and postcolumn connections, or a contaminated detector. Of course,... 

Unexpected peaks can arise from components from a previous injection that moved slowly through the column, contamination from either the reagents used to prepare the sample or the standards, or a contaminated septum, carrier, or column. Solutions to these problems include a rapid bakeout via temperature programming after the analyte peaks have eluted, use of pure reagents, and replacement or cleaning of septa, carrier, or column. 

As mentioned previously, introducing the sample to the flowing mobile phase at the head of the column is a special problem in HPLC due to the high pressure of the system and the fact that the liquid mobile phase may chemically attack a rubber septum. For these reasons, the use of the so-called loop injector is the most common method for sample introduction. 

Describe in detail how the loop injector works and tell how it overcomes the problems that would be encountered with an injection port-septum system. 

Leaks in the flow system are one of the most common sources of problems. The fact that a rotameter indicates that the flowrate is satisfactory does not assure that there is gas flowing through the column, it merely means that there is gas going through the rotameter. A leaking septum or connection at the head of the... 

One of the biggest problems with present-day septum inlet systems is that of septum bleed. Kolloff (13) was the first to note the bleed of monomers and short-chain polymers (used in the production of the synthetic elastomers from which septa are derived) from gas chromatographic septa. Another problem is sorption of solvents and sample components on the septa. A thorough study of this phenomena was made by Adler (14) related to the use of selfsealing elastomer septa for quantitative operations with volatile laboratory solvents. It was found that a silicone septum could absorb over twice its weight of carbon tetrachloride and chloroform and more than its weight of benzene at 25°C. 

A third problem is leakage of gases in or out through the septum. Improvements or solutions to the problems associated with septa are the following ... 

Cooling the septum prolongs its life and can reduce the bleed problem (but never eliminate it). If a septum is always going to bleed, better solutions than cooling are required. 

Septum bleed is worse than in split mode because all the flow goes through the column. Most of the splitless inlet systems that are presently available use a vented septum technique to avoid this problem. 

Two problems with all septa are their sensitivity to solvents and chemicals and leakage through the punctured septum. Flat composite septa minimize these problems. These consist of a central core of a compliant rubber, such as a soft... 

Thus there are two first-order differential equations (Equations 9.1 and 9.2) and five algebraic equations (Equations 9.4 - 9.6) with which to determine the two integration constants and the five variables. Different numerical techniques can be used to solve the problem. One way is to linearize Equations 9.1 and 9.2 and apply the iteration procedure described by Kerkhof [5]. An equation describing the variation of the total pressure inside the septum,... 

One remark should be made. According to all the available theories the total pressure inside the septum should be uniform if the total pressures at its surfaces are equal. This is not the case for the BFM. However, the numerical solution of the problem shows that this total pressure variation can be neglected in the calculation of the fluxes and the assumption of a constant pressure over the septum made by Kerkhof [5] is correct. 

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