Restek Capillary Column Installation Guide - Section III: Operational Information
- Standby operation
- Column removal and storage
- Protection against dirty samples
- Connecting fused silica tubing
- Connecting stainless steel tubing
- Column maintenance and rinsing
- Injecting water and other solvents
A. Standby Operation
If the column is not going to be used for several days but the instrument must remain on standby, reduce the splitter vent flow rate to 10cc/min. (to conserve carrier gas) and maintain an oven temperature between 100°C and 150°C. Keeping the oven slightly hot minimizes septa bleed from accumulating at the head of the column and reduces the need for excessive conditioning when the column is used. Make sure there is a sufficient carrier gas supply when leaving the column in standby operation. If the column will be unused for three or more days, it is best to shut down all heated zones and leave the column in the GC with a low flow of carrier gas. Never leave the column in the GC without carrier gas flowing.
B. Column Removal and Storage
Remove the column from the GC. Make sure that there are no ferrule fragments in the injector or detector fittings. If a graphite ferrule sticks in the fitting, remove it by using a tapered needle file (cat.# 20106). Insert the file and spin it slightly until it is latched inside the ferrule. Then move the file slightly from side-to-side, while applying force, until the ferrule slips out of the fitting. If the ferrule does not come out in one piece, disassemble the fitting and inspect all parts to make sure that all fragments are removed.
Insert the column ends into a septum to prevent phase oxidation during storage. Always store the column in the original box, away from sunlight or fluorescent lights, to avoid polymer damage. In particular, Stabilwax, Rtx-1301, and Rtx-1701 columns (polyethylene glycol and cyanosilicone polymers) are especially susceptible to uv induced degradation and should be shielded from light.
C. Protection Against Dirty Samples
Samples that contain non-volatile or high molecular weight residue can contaminate the stationary phase, decrease quantitative accuracy, and cause a loss of peak resolution after a limited number of injections. Packing the inlet sleeve and using a guard column are two specific precautions recommended to help increase the number of analyses performed before column and inlet sleeve maintenance is required.
The first precaution involves the use of an inlet sleeve packed with silanized fused silica or glass wool (cat.# 20790, 20789) or highly inert silica beads (cat.# 20791) to trap sample residue and prevent it from entering the capillary column. Insert a loosely packed 2cm wool plug approximately 1cm below the maximum penetration distance of the syringe needle. Too much packing material can detrimentally affect the performance of the column. Refer to the instrument manual for specific inlet packing instructions.
Stationary phase coated packings are not recommended since they tend to be adsorptive and can bleed stationary phase onto the column. This problem is particularly damaging if a methyl silicone inlet packing is used with a Carbowax PEG-type capillary column.
The second precaution involves using a 5-meter deactivated, uncoated guard column at the analytical column inlet (Figure L). The guard column prolongs column life by trapping non-volatile contaminants before they enter the analytical column. Sample components travel at the same rate as the carrier gas on the uncoated guard column inlet, but slow down considerably when they reach the analytical column's stationary phase. Because the sample's rate of travel is faster through a guard column, the sample has a limited interaction time with residue and, therefore, is not as affected by its presence. Without a guard column, sample residue affects chromatographic performance more rapidly due to increased interaction times. A 5-meter guard column also allows contaminated segments to be removed without having to reconnect the column each time. Restek's Integra-Guard is a built-in guard column configuration that eliminates connectors.
D. Connecting Fused Silica Tubing
Connectors are necessary to attach guard columns and restrictors, or to repair broken analytical columns. Both Press-Tight connectors and Vu-Union connectors are effective at making low dead volume, inert connections. Press-Tight connectors are best for standard applications at temperatures below 325°C. Polyimide resin (cat.# 20445) can be used to strengthen and create permanent press-tight connections, preventing the possibility of premature separation. Vu-Union connectors are appropriate for high temperature or high pressure applications, or when attaching an analytical column to a mass spectrometer transfer line. Call Restek's Technical Service at (800) 356-1688 (ext. 4) for more information on the use of these two connectors.
- Connecting Columns with Press-Tight Connectors
- Cut the column ends squarely.
- Wipe the column ends with methanol to lubricate and clean.
- Carefully insert the ends into the Press-Tight Connector.
- Visually inspect (see photo).
- Leak-check using a thermal conductivity leak detector.
E. Connecting Stainless Steel Tubing
In order to connect MXT columns, special connectors must be used. The MXT Low Dead Volume connector (cat.# 20397, 20394) and MXT Low Dead Volume "Y" connector (cat.# 20396, 20395) are Silcosteel-treated and deactivated to make them inert to active compounds. They will not cause peak tailing or affect system inertness and can be used up to 400°C without degrading the deactivation layer. Stainless steel replacement ferrules are available to fit 0.28mm ID MXT columns (cat. # 20398) and 0.53mm ID MXT columns (cat. # 20399).
MXT tubing can be connected to fused silica tubing by using a capillary Vu-Union connector (cat.# 20418) provided the end of the MXT column is properly burnished into a conical shape. The flat side of a ceramic scoring wafer can be used to polish or round the column end into a conical taper. An MXT connector with a special ferrule (cat. # 20259) can also be used with a fused silica column. A fused silica Press-Tight connector cannot be used with MXT columns because without polyimide coating on the outside of the tubing, a leak-free connection cannot be obtained.
F. Column Maintenance and Rinsing
Maintenance is necessary if a column experiences high bleed (that cannot be decreased by overnight conditioning), adsorption of active components, peak tailing, or changes in relative retention times. Cutting two loops (approximately one meter) from the column inlet is often sufficient to restore a contaminated column. However, high molecular weight residue carried deep into the column bore can only be removed by solvent rinsing. To remove pyrolyzed contaminants that are not solvent-soluble, cut two loops from the column's inlet and one loop from the outlet. Closely examine the inlet end of the column to make sure that all contaminated tubing has been removed. Occasionally, it becomes necessary to remove more than two loops from a heavily contaminated column to restore it to acceptable performance. Solvent rinsing removes only soluble contaminants and may not always restore a bonded phase capillary column to its original performance. Figure M illustrates the performance of a column that has been successfully solvent-rinsed. Choose a series of polar and non-polar volatile solvents that will solubilize the suspected contamination. Only use solvents shown in Table VII to rinse Restek columns. If you are using a solvent that is not listed, call Restek BEFORE rinsing to prevent permanent damage to your capillary column.
Figure M: Solvent rinsing removes soluble contaminants
Table V Dead Volume Times (for commonly used capillary columns)
Magic Mix = 20% DI H2O, 60% CH3OH, 20% CH2Cl2
Do not rinse the following columns because they are either partially or not bonded: Rtx-2330, Rt-2330, Rt-2340, Rt-βDEX, Rt-βDEX, Rt-TCEP & Micropacked
Restek's chemists have found that a mixture of 60% methanol, 20% DI H2O, and 20% methylene chloride (i.e. magic mix) is ideal for rinsing and cleaning most bonded stationary phases. (Do not use this mixture unless these solvents are recommended for rinsing.) The methylene chloride tends to swell the polymer which allows the methanol/DI H2O to deeply penetrate into the swollen polymer lattice to remove polar contamination. Without the addition of methylene chloride, polar solvents such as water or methanol are repelled by the polymer and do not clean deeply inside the polymer lattice.
Always rinse the column from the back (detector side) to the front (inlet side) to prevent contaminants lodged on the inlet side from being carried deeper inside the column bore (backflushing). Columns can be rinsed by either forcing solvent through the column under pressure or by pulling solvent through under vacuum. Restek offers a convenient Capillary Column Rinsing Kit (cat.# 20612, shown at left) to rinse columns. This kit contains a glass reservoir with a fritted disk to prevent particulates from damaging the capillary column. The amount of solvent used to rinse a column should equal approximately three to five times the column's internal volume. Table VII lists typical solvent volumes and pressures used to rinse each column ID.
Table VII Typical Solvent Volumes and Pressure Used for Rinsing
Use non-swelling solvents such as methanol or acetone to minimize swelling while solvent-rinsing thick film columns. Sometimes very thick films (>3µm) can swell shut and occlude the column flow while being solvent-rinsed. Do not try to remove the blockage by increasing the rinsing pressure. Attach both column ends to a vacuum system for 24 hours. The vacuum evaporates the solvent and opens the column bore.
After rinsing, allow the column to purge with a dry carrier gas for 4-6 hours to decrease polymer swelling after solvent exposure. If the column is installed in a GC and heated rapidly before the solvent swelling has diminished, extensive damage to the polymer occurs. After purging, install the column in the GC at 40°C and hold for 30 minutes. Then, program slowly (4°C/min.) to the column's maximum operating temperature and condition for two hours before using.
G. Injecting Water and Other Solvents
In general, injecting water and other solvents will not damage bonded or non-bonded phase capillary columns provided that the solvent is vaporized before it reaches the front of the column (i.e. when split, splitless, or direct injections are performed). However, if operating the capillary column in cold or heated on-column modes, some damage may occur. To reduce the risk of damage, avoid water and methanol with Stabilwax, Stabilwax-DB, Stabilwax-DA, Rtx-2330, and Rtx-225 phases when using cold or heated on-column injection techniques. Alternatively, avoid damage from water and methanol by attaching a 5-meter Hydroguard guard column to the capillary column inlet. This allows the solvent to vaporize before reaching the capillary column. Injecting water, methanol, or other solvents will not damage the other phases Restek offers, regardless of the injection mode.