Note 44: The Design Of a New Direct Probe Inlet For a Mass Spectrometer

INTRODUCTION

Direct introduction of samples into a mass spectrometer continues to be a fast and convenient method to analyze many samples. Applications, as screening methods and molecular weight determination, in areas such as drug development are particularly suited to rapid analyses such as can be achieved using direct probe sample introduction.

One of the difficulties of using probes for the introduction of samples into the mass spectrometer has been the interface or probe inlet which allows the probe to be inserted into the mass spectrometer. These inlets must provide a sealed intermediate pumping, area where a partial vacuum can be pulled before a valve is opened to allow insertion of the probe into the source of the mass spectrometer. This requires that the seals that are used in these inlets are designed to withstand the wear due to repetitive insertion and removal of the probe and that there be some method of insuring that the valve is closed and opened at the proper time so that venting of the mass spectrometer does not occur. One or both of these potential problems exist on many of the probe inlets such as the inlet that is supplied with the Hewlett-Packard 5989 MS Engine.

We have designed a new probe inlet for use on the HP Engine which provides for much longer seal life and minimizes the chance of accidental venting of the mass spectrometer.

Probe Inlet Design

The new probe design consists of three basic parts. These are the probe seal assembly where the probe seals are located, the isolation valve which isolates the high vacuum of the mass spec source, and the probe guide bar and auto-stop assembly which protects against accidental venting of the mass spectrometer. These are shown in the schematic in Figure 1.

Figure 1 - Design Of HP Probe Inlet

The newly designed probe seals and the intermediate pumping area are part of the probe seal assembly. Our design consists of two PTFE vacuum seals which use Viton® o-rings. Previous designs, as those used on the engine, were manufactured from graphitized Vespel® which wear quickly with use. As these seals wear, small particles of graphite are released providing possible contamination of the inlet. The new PTFE seals provide for a clean, contamination free, long lasting seal. The small intermediate pumping region is located between these two seals. This area is small minimizing the additional vacuum pump requirements.

The isolation valve is the valve which isolates the intermediate pumping region from the high vacuum of the mass spec source. This valve is a precision designed ball valve specifically designed for vacuum applications.

Figure 2 - Probe Inserted into MS source

The guide bar and auto-stop assembly is an indexed probe slide which automatically stops at three locations. There is a load position and then stop positions at the first PTFE seal and then another stop at the second PTFE seal. This prevents pushing the probe in too far which could damage the probe tip, and also prevents pulling the probe out past the PTFE seal which would vent the mass spectrometer. A quick release push button must be activated by the user to allow movement of the probe past one of the auto-stops. Figure 1 shows the probe at the high vacuum auto-stop location. This stop is where the isolation valve would be activated. Figure 2 is a schematic drawing showing the probe inserted completely into the mass spec source.

Figure 3 - Probe Inlet System Attached To HP Engine Mass Spec

Figure 3 is a picture of the newly designed probe inlet installed on a Hewlett-Packard 5989 MS Engine. The probe inlet is attached to this mass spec using the standard HP o-ring fitting supplied with the mass spectrometer. The probe inlet could also be modified to mount on other mass spectrometers which use one half inch diameter probes.

Probe Inlet Seal Life Study

Figure 4 - Probe Seal Life Study

A study was done to compare the seal life on the Hewlett-Packard inlet system and the newly designed SIS inlet system. The results are shown in the chart in Figure 4.

The SIS probe inlet and the HP probe inlet systems were mounted onto an automatic probe injection system which was fitted to a vacuum chamber where the vacuum was measured with an ionization gauge tube. The vacuum in the chamber was measured after each injection of the probe through the inlet. This allowed us to monitor the vacuum levels as the probe seals began to wear from repeated injections. The probe inlet seals were considered to have failed when the ionization gauge would not remain on (i.e. the vacuum was greater than 1 x 10-4 torr).

The chart shows that the SIS probe inlet system provided satisfactory performance for over 2500 injections of the probe before it failed at injection number 2649. The HP probe inlet first failed the test at injection number 14. The inlet was removed and inspected. There were fine particles of graphite throughout the probe inlet. The inlet was cleaned and the same seals were installed again in the inlet and the test was continued. The probe inlet failed again at injection 29. The cleaning process was repeated and the test continued with the same seals. This cleaning process was repeated seven times before the probe inlet seals failed completely at injection number 290.

Conclusion

We have designed a new probe inlet system which greatly improves the seal life and reliability of previous probe inlets systems. The new design also greatly reduces the possibility of accidental venting of the mass spectrometer. The probe inlet mounts easily onto a Hewlett-Packard 5989 MS Engine but could be modified to work with other mass spectrometers.

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