Simple Automatic Valve for Constant Volume Collection in Column Chromatography Gary 1. Nelson, Donner Laboratory of Biophysics and Medical Physics, University of California, Berkeley, Calif. WORK on the chromatographic behavior of serum phospholipides on silicic acid columns (4, 6) it was necessary to undertake extended chromstographic analysis with large columns and automatic fraction collectors with which fractions of constant volume were desired. The standard methods of fraction cutting with automatic fraction collectors (1, 8, 6, 7) often give unsatisfactory results in column chromatography, especially when used with organic solvents. Included , in this group are volmetric siphoning, drop countings, and constant time and weight collectors. When alternative methods were investigated, the design of Dutton and Castle (2) appeared to offer the beat method of constant volume fraction collecting. However, certain difficulties arose in the application of the above unmodified design. primarily, the steel ball-ground glass seal in the valve was unsatisfactory for organic solvent of low surface tension. Secondly, contact between the eluent and the steel ball was undesirable, especially with reactive eluents. A modification of D u b ton and Castle’s design by Wechter, McCarty, and Fisher (8) could not be adapted simply to small volumes, and changes in the volume collected in each fraction during a run were not easily accomplished. To overcome these dXiculties a modification was developed to be o p erated in conjunction with any auto-
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matic fraction collector. It operates with any solvent and delivers a constant volume regardless of flow rate of the column or surface tension of the eluent. The float used to interrupt the light beam in the original design was eliminated, thereby producing better accuracy and reproducibility in the resulb ing fractions. The operating scheme of the fract.ion cutter is shown in Figure 1. A drop counting attachment was utilized with minor modifications for indexing the turntable. However, this valve system can be constructed and operated independently as long as a photocell and means of indexing the turntable are provided. The accuracy of the valve depends on the seal which a tapered Teflon plug, carefully ground and polished, makes with a groundglass seat of the same taper as the plug. Details of construction are given in Figure 2. The valve stem is constructed entirely of Teflon and encloses a soft iron core which opens the valve when the coil is activated by the photocell. The spring is stainless steel and prevents the valve from lifting too high and obstructing the flow through the outlet. The eluent collecting tube is made of a constant bore glass tubing 1 cm. in outside diameter. Thus, the volume collected in each fraction may be continuously varied, simply by raising or lowering the position of the photocell along the tube. Figure 3 is a photographic view of the components of the valve.
The sequence of operational events is as follows: When the meniscus reaches the level of the phototube, the light beam is interrupted. The turntable motor is activated and the turntable indexed, which takes 1.5 seconds in our instruments. While the turntable is moving, the photocell circuit is inactivated by the time delay switch to prevent double indexing. Then the valve opens for 4 seconds, also actuated by the time delay switch. Flow rates are adjusted to that complete drainage from the lower parts of the valve will occur between indexings of the turntable. Factors which affect the volume collected are the length of time the valve is open, flow rate of the column, drain time of the collecting tube, viscosity of the eluent, and retention of the eluent on the lower portion of the valve stem. Obviously, the valve must remain open long enough for the eluent collected to drain out. There will be some retention of eluent on the sides of the collection tube and, if the valve is operated as shown, solvent will continue to elute into the collector while the valve is open. The latter occurrence can be prevented by attaching a second identical valve to the top of the collector which is closed when the lower valve is open. Practically, this is not necessary when the flow rate is slow and valve is open for less than 5 seconds, in which case the controlling factor becomes the drainage rate off the walls of the collecting tube. With a flow rate of 1 ml. per minute, a time constant of 4 seconds
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Figure 1. valve
Circuitry and operating scheme of solenoid
Significant additions to original circuit are time delay switch, A, and coil, 6
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ANALWICAL CHEMISTRY
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Figure 2. Working drawing of solenoid valve A. 8. C.
Ground-glass m l v e seat Teflon plug Soft iron core
