Note 92: Yttria Coated Mass Spectrometer Filaments

Introduction

Historically most mass spectrometers have used plain uncoated filaments usually constructed of rhenium, tungsten or an alloy of rhenium and tungsten. In the past twenty or so years, several manufactures have tried using thoria (thorium oxide) coated iridium filaments in their mass spectrometers or leak detectors. This thoria and yttria coating technology came out of the vacuum tube industry in the 1940's and early 1950's. The thoria coated iridium filament provides longer filament life because the thoria work function was lower than tungsten, the filament required less power and it operated at lower temperatures. The major disadvantage of thoria is the environmental issues due to the exposure to the alpha particles emitted by thorium. Yttria (Yttrium Oxide) has been used as a replacement by several manufacturers of leak detectors and ion gauge tubes. SIS has been investigating Yttria (Yttrium Oxide) as a replacement for use in mass spectrometer filaments. The work function of Yttrium and Thorium are very similar (3.1 eV versus 3.41 eV respectively) and Yttria does not present the environmental issues associated with Thoria.

Mechanism of Emission

It has been reported by several authors in the early literature that free Yttrium (Y) is released within the Yttrium Oxide coating during the heating of the filament during the activation phase and this free Yttrium (Y) migrates to the surface of the oxide coating. It is this free metal film of Yttrium (Y) on the surface of the oxide coating that results in the emission characteristics of the Yttria coated filaments.

Preparation of Filaments

Figure 1 - Yttria Coated Filament

The filament wires were spot welded to metal body filament presses. (Figure 1) The filaments were thoroughly cleaned and Yttria (Yttrium Oxide) was coated onto the filaments using an electrophoretic process. After coating the Yttria onto the filaments, they were inspected for integrity of coating and the film thickness was measured.

Yttria was coated onto various filaments wires and the emission characteristics were studied. Filaments were tested on the SIS filament testing station shown in Figure 2. This station has been used in filament studies we have published previously and is routinely used to study filament behavior. This filament testing station permits the simultaneous study of up to 12 filaments. The DC voltage and current through each filament can be varied and the emission current between the filament and a collector is measured. The emission voltage between the filament and the collector was set to 70 volts DC, which is the standard emission voltage used in most EI mass spectrometers. For each of the studies as the filament current was increased in 0.1 amp increments, the filament current, the filament voltage and the emission current were measured using 3 digit digital voltage and current meters. Filament temperatures were calculated using the Stefan Boltzman equation using the measured filament current and voltage as well as the physical dimensions of the filament wire.

Figure 2 - SIS Filament Testing Station

Comparison of Thoria and Yttria Coatings

The chart (Figure 3) compares two Iridium filaments, one coated with Thoria (Thorium Oxide) and the other with Yttria (Yttrium Oxide). The filaments consisted of a filament press with a 10.0 mm length of 0.007" diameter Iridium wire. Yttria and Thoria were electrophoretically deposited on the filament wires to a thickness of 0.0005".

Figure 3

As can be seen from the chart (Figure 3) the emission currents of the Yttria and Thoria filaments are very similar. For both the thoria and yttria coated filaments, the filament operates at about 2200°C to achieve 10 mA of emission current. This study and the following studies confirm the feasibility of using Yttria as a replacement for Thoria on filaments for mass spectrometers.

Study of Yttria Coated Rhenium Filaments

Rhenium filaments were coated with Yttria to determine its application for mass spectrometer filament manufacture. Emission plots of both Rhenium and Yttria coated Rhenium are shown below. Rhenium by itself has often been used as a mass spectrometer filament and therefore emits over a broad current range as shown in Figure 4.

Figure 4

As shown in the chart (Figure 4), when the Rhenium filament is coated with Yttria it emits at a significantly lower filament current than the plain uncoated rhenium filament. The Yttria coated filament required about 70% of the current and power to obtain the same emission as the uncoated filament wire. Emission occurred at temperatures between 1850°C and 1950°C for the Yttria coated rhenium filament. The Yttria coated filament provides emission at about 500 lower in temperature than the uncoated filament to achieve the same emission current. This will result in good filament emission at a lower filament temperature and would be expected to produce longer filament life with the Yttria coated filaments.

Reproducibility of Yttria Coated Rhenium Filaments

A set of 9 rhenium (.0055" diameter) filaments were coated with Yttria to a thickness of 0.0007" to 0.0010" to determine the reproducibility of manufacturing these filaments. The results are shown in Figure 5. The filaments were very reproducible in the emission range of 0 to 10 milliamps of emission, but this variation was probably due to the variation in thickness of the Yttria coatings or variation in the distance between the filament and the collector since different mounting positions were used for each of the filaments in the testing station.

Figure 5

In general we have demonstrated that we can reproducibly produce filaments with our processes and that film thicknesses between 0.0005" and 0.0010" thick provide for optimum performance and uniform coatings of the filament surface.

Filament Testing on an Agilent MSD

The next study was done to compare the actual operation of the Yttria coated filaments in a mass spectrometer. Two filaments were installed in an Agilent 6890 MSD. The first filament (Filament #1) was a plain 0.0055" rhenium filament which is the standard filament normally used in the MSD. The second filament was the same style filament, 0.0055" rhenium, coated with Yttria to a thickness of 0.0005". The mass spectrometer was operated in the normal EI mode and was tuned with each filament using the standard Agilent ChemStation Autotune. In both cases the filament emission current was set to 34.96 in the ChemStation tune setup.

The standard uncoated rhenium filament (Filament #1) operated at a measured filament voltage of 2.059 Volts AC. The Yttria coated filament (Filament #2) operated at measured filament voltage of 1.455 volts, which is about 70% of the voltage requirements for the non Yttria coated filament. This data agrees with the previous study above in which filament current and power for the Yttria coated rhenium filament were reduced to 70% of that required for an uncoated filament. This reduction in filament voltage and conversely filament current results in lower filament temperature and it is expected that the filament life will be increased. However, actual lifetimes of Yttria coated filaments in a real world mass spectrometer requires additional testing. It has been reported that these filaments are subject to poisoning by inorganics such as Chlorine and Flourine. The Yttria coated filament has been used in our MSD for about 3 months now and is still working fine.

CONCLUSION

The above studies demonstrate the feasibility of using Yttria coated iridium and rhenium filaments in a mass spectrometer. Yttria is a good replacement for Thoria and eliminates the environmental problems associated with Thoria. Yttria coated filaments require less power than uncoated filaments and the filament operates at lower temperature to achieve the same emission as the uncoated filaments. Historically iridium has been used as a base metal for Thoria and Yttria coated filaments, however we have demonstrated that Rhenium is also a viable alternative to Iridium and may be superior. We have demonstated the SIS can reproducibly manufacture Yttria coated filaments to meet the end users requirements.