MS TIP No. 7 - Repairing Clogged Thermospray Probes

Mass Spec Tips is a collection of tips relating to the operation and service of mass spectrometers. Some of these Tips have been reported in our newsletter, "The Mass Spec Source." If you have a question relating to the operation or service of mass spectrometers or would like to add your comments to the below tip and would like to have it included in this forum, please contact us. Input to this database of MS Tips is needed from our readers to complete this section.

Christopher Baker

Scientific Instrument Services

Probe inserts for the thermospray interfaces used by Vestec, Hewlett-Packard and others become clogged by salts, polymers and particles. Use of an in-line filter before the probe removes the particles, but will not completely eliminate the problem. The probe, with its small inner diameter, typically 100 microns, is still susceptible to clogging.

We have designed a probe insert in which clogged capillaries are easily replaced by the user. There is a guide tube inside the 1/4Ó thermospray insert which has an inner diameter which allows a new 100 micron stainless steel (s.s.) capillary to be inserted. The thermocouples are spotwelded to the outside of this guide tube. See the schematic on page 16 of this newsletter. Replacement of the clogged 100 micron s.s. capillary is fast and simple because of this design. When a capillary becomes clogged, it is cut at the back of the insert using a tubing cutter. Use of a tubing cutter is necessary so as to obtain a smooth surface, important because the capillary has to slide through the guide tube which is closely matched to the outside diameter of the capillary. The swagelok fitting is loosened at the tip of the probe and the clogged capillary slid out the end of the probe. A new 100 micron s.s. capillary is then inserted through the guide tube, and a new stainless steel ferrule placed at the tip of the probe and tightened. The probe is then ready for use.

Operation parameters of the probes are similar to the original manufacturers probes, especially when using filament or discharge modes. These probes have been shown to produce a very stable baseline with similar or better sensitivity. Take-off temperatures may be different than those observed with the original manufacturers probes, perhaps important for those doing ion evaporation experiments where no filament or discharge electrode is being used as a source of electrons. However these new probes have been used successfully on both the Hewlett-Packard 5988 and 5989 mass spectrometers as well as the Vestec 201 LC/MS system.

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