Section 7 Last Resort Procedures to Restore Column Performance

How to Fill a Column Void

Column voids can occur under several different conditions. First, a void could develop because a column is packed poorly. Even a well packed column, however, will occasionally develop a void due to age or analysis conditions.  For example, mobile phase or sample pH conditions greater than pH 7 can cause silica to dissolve and a void to form. When this happens, some of the column's performance may be restored by simply filling in the void. Although you probably will not be able to restore the column to its original performance, you will have about a 50/50 chance of achieving at least acceptable performance. If you think this is worth the effort, follow the procedure below.

For Reversed Phase Columns

1. Prepare a packing material slurry.
   Make a 10% weight:volume slurry mix of column packing material in methanol. Use the same type of stationary phase (C18, C8 etc.) that is in the column to be restored.
2. Place the column in a ring stand or a bench vice with the column inlet pointing up.
3. Use a marking pen to mark the position of the end fitting relative to the column nut. A vertical mark directly across the end fitting and column nut is recommended.  When you tighten the end fitting after replacing the inlet frit, these marks can help you torque the nut and end fitting back to the same position.
4. Allow the column to come to room temperature before removing the end fitting. Do not hold the column while replacing the frit. Heat from your hand can cause expansion of solvent in the column and may extrude packing material from the open end of the column.
5. Hold the column end fitting steady with one wrench while loosening the column nut counterclockwise with another wrench until the nut drops away from the end fitting. Lift the end fitting off the column.
6. Remove the column inlet frit by carefully sliding it to the side. If the frit is lodged in the end fitting, remove it by tapping it on a lab bench until the frit drops out.
7. Slowly add the packing slurry to the column void in a dropwise fashion. Continue to add until the void is completely filled. Let the packing air dry.
8. Level the packing with the top of the column. Use a razor blade or a flat spatula to level the packing. Make sure that the top of the column is clean so that the inlet frit will make good contact with the top of the column wall.
9. Place a new frit on the cleaned column tube.
10. Remove all residual packing material from the column end fitting, tube ferrule, and column nut. This is easily done with isopropanol. A squirt bottle works well for this purpose.
11. Replace the column end fitting and tighten the column nut 1/8 to 1/4 turns past finger tight. Align the vertical marks that were placed on the end fitting and column nut in Step 2 so that the nut is tightened back to the proper torque.
12. Reconnect the column to the chromatograph.
13. Start now through the column and check for leaks. If the column leaks, turn the flow off, allow the pressure to bleed off, and then tighten the nut slightly more. If the column still leaks, some residual packing material probably remains on a sealing surface (column ferrule, end fitting, threads of the column nut). Remove the end fitting again and reclean the surfaces.

Preventing Column Voids

If you prefer not to attempt this procedure, focus on preventing the problem. If your sample solvent pH is above 7, and your mobile phase pH is significantly lower and buffered, a guard column will offer some protection. As a void forms the guard column can be replaced. If the mobile phase pH is greater than pH 7, a silica saturator column may offer some protection. This is a short silica column, placed between the pump and the injector. The mobile phase will become saturated with dissolved silica from this short column, reducing the rate of dissolution of silica in the analytical column. If you find you must operate with a mobile phase pH above 7, then select a densely bonded, double-endcapped HPLC column, such as the Eclipse XDB, to protect the silica surface from the solubilizing effects of the mobile phase.

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