Sample valve injection

The introduction of the sample valve, however, helped establish radial equilibrium early in the separation but, unless some special sample spreading device is employed at the front of the column, equilibrium will not necessarily occur at the point of injection. The stream splitting process is depicted in Figure 2. 

It is seen that columns having diameters less than 2 mm will only tolerate a maximum sample volume of a fraction of a microliter. Although larger volume valves can be used to inject sample volumes of this size, the dispersion from the valve is still likely... 

The basic difference between this type of valve and the normal external loop sample valve is the incorporation of an extra port at the front of the valve. This port allows the injection of a sample by a syringe directly into the front of the sample loop. Position (A) shows the load position. Injection in the front port causes the sample to flow into the sample loop. The tip of the needle passes through the rotor seal and, on... 

Sample injection system. Introduction of the sample is generally achieved in one of two ways, either by using syringe injection or through a sampling valve. 

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... 

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

It must be pointed out that (Vm) refers to the volume of mobile phase in the column and not the total volume of mobile phase between the injection valve and the detector (Vo)- In practice, the dead volume (Vo) will include all the extra column volumes (Ve) involved in the sample valve, connecting tube detector cell etc. 

It should be stressed that only those surfaces that actually come in contact with the sample need to be bio-compatible and the major parts of the valve can still be manufactured from stainless steel. The actual structure of the valve varies a little from one manufacturer to another but all are modifications of the basic sample valve shown in figure 13. The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal. 

The products resulting from the reaction were injected vrith an automatic sampling valve into a GIRA GC 181 gas phase chromatograph and analyzed with a flame ionization detector. The separation was carried out in a capillary column DB5 (J and W Scientific). 

The equipment consisted of two Waters (Waters Corp. Milford, MA) M-45 pumps, a Waters 481 UV detector, a six-port Valeo sampling valve (A2L6P) with 0.08" holes in the valve body and rotor, a Rheodyne Model 7413 injection valve with a 1-pl loop, a valve interface box, and a Digital Equipment LSI-11/23-based microcomputer system. The microcomputer was used to control all valves, collect raw data from the UV detector, integrate the chromatogram, and store and plot results. 

The equipment used in this application included two Waters M-45 pumps, a Waters 481 UV detector with microbore cell, an air-actuated Rheodyne 7413 injection valve with a 1-pl injection loop, an air-actuated Valeo four-port sampling valve (A2CI4UW2) with no groove in the injection entry ports, an air-actuated Valeo three-port switching valve (AC3W), and a Digital Equipment LSI-11/23 microcomputer. The LC system was located in a purged cabinet suitable for use in Class I, division 2 areas. The cabinet was in a heated room about 40 feet from the reactor column. The two Valeo valves were mounted next to the reactor column, while the microcomputer was located in the control room. 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

The instrumentation used to implement this comprehensive sampling is the main topic of this chapter. The most common comprehensive mode uses a sampling valve so that second dimension elution can begin as soon as a sampling loop has stored the necessary amount of first column solute. In this case, there is no need for storing the effluent from the first column it is continuously allocated to a sampling loop with subsequent injection into the second-dimension column. 

This is the equivalent of the injection port for the GC technique. With GC you could inject through a rubber septum directly onto the column. With HPLC it s very difficult to inject against a liquid stream moving at possibly 1000 psig. That s why they invented injection port valves for HPLC you put your sample into an injection loop on the valve that is not in the liquid stream, then turn the valve, and voila., your sample is in the stream, headed for the column. 

Figure 30.4, displays the operation of a sample loop in two different modes i.e., (a) sampling mode and (b) injection mode. Here, the sample is loaded at atmospheric pressure into an external loop in the microvolumesampling valve, and subsequently injected into the mobile-phase by a suitable rotation to the valve. However, the volume of sample introduced usually ranges between 2 p/ to over 100 p/ but can be varied either by altering the volume of the sample loop or by employing specific variable-volume sample valves. 

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