Capillary columns offer challenges in gas chromatography (GC) work due to their small size and fragile nature. Typical repair techniques to join two capillary columns require cumbersome metal fittings. This solution is not applicable in all situations due to size and mass limitations. Another repair or joint technique involves the use of polyimide glue with a tapered glass union. Without the bulky glass fitting, this technique is difficult to perform without plugging the column; the low-viscosity glue wicks up the capillary column and fills the small opening of the joint. Polyimide is also a problem due to its hydrolysis when trying to analyze water; the polyimide glue interacts with the water in the sample and affects the analysis. These solutions don’t support a repair of capillary columns for water analysis in a small gas chromatograph module.
This innovation describes two methodologies to join capillary columns without the use of a metal fitting. This technique is particularly applicable when very small capillaries, dissimilar capillaries, or size and mass restrictions are present, such as in MEMS GCs. These instances do not allow for the use of the typical metal fittings to join columns.
The purpose of this work was to create techniques for joining capillary columns, including very small pieces of fused silica capillary tubing (below 0.25 mm OD), with minimal size and weight requirements, that could be performed in space-restricted areas. It was specifically applied to the repair of dissimilar size capillary joints where no commercial union was available to produce a gas-tight seal due to the small size of one of the capillaries, sometimes as small as 0.1 mm OD. Two solutions to this problem were created.
The first technique involves the use of a product called Dual Shrink, which typically is used to seal electrical connections by applying heat and encapsulating the connection. The inner layer (FEP) of Dual Shrink melts and flows to encapsulate the part while the outer layer (PTFE) shrinks to form a tight fit around the fitting. While Dual Shrink has been used to provide water-tight seals in electrical applications, it was shown that with a thorough melt of the FEP liner, a gas-tight seal could be achieved. The innovation of using this material in the confined space of a GC module requires a localized heat source that will not damage the surrounding components. This product normally entails heating the entire assembly to 625 °F using a heat gun or oven. These heating techniques will not work in this application due to the tight spacing of components and the lower temperature limits of some of these components.
A modified soldering iron has been developed for wire repair techniques. This type of heat application could be applied to allow the use of Dual Shrink in the confined spaces of the GC module. The tip attached to the soldering iron had to be modified for use in the GC and so it could closely surround the Dual Shrink joint during heating. The tip was also modified to decrease the clearance required near the joint repair, allowing for use in the small space. The soldering iron could be set to a precise temperature to provide control over the time required for heating. Two methods for repair were used. The first method was to heat the device and clamp it onto the joint after reaching the desired temperature. The second method was to apply the device when at room temperature and heat while in place.
This technique required the capillary columns to stay stationary during the heating process. The melt flow of the FEP could move the columns and cause the joint to come apart. To alleviate this problem, vacuum chucks were designed to hold the small capillary tubes in place during the repair. These chucks are adapted from designs used when working with fiber optic cables. The use of these chucks is not required for the repair technique, but it offers additional stability during the repair process.
An additional method for capillary column repair is the use of a highly viscous glue, such as silicone RTV or hot melt glue. These highly viscous glues can be applied to the joint and do not wick up the joint as the polyimide glue does during the repair. Initial tests indicate the glue travels 1-2 mm up the joint, allowing the joint overlap to be minimized. This technique does not require any heat applied to the system for the repair to take place. As such, it can be used to join polymer tubes that could melt if the repair technique reached too high of a temperature (this would depend on the specific polymer). The hot glue melt has a low operational temperature limit so care must be taken when deciding on repair techniques for specific applications. The silicone RTV has a higher temperature limit, and specific formulations can be used to meet the temperature requirements of the application.