908-788-5550

Note 96: Reducing Warping in Mass Spectrometer Filaments, with SISAlloy® Yttria/Rhenium Filaments

Home

By John J. Manura, Christopher Baker & Ronald Shomo
Scientific Instrument Services, Ringoes, NJ
Presented at the ASMS Meeting in Denver, CO, June 2011

Introduction

Filament warping is a significant problem in the mass spectrometer especially in Chemical Ionization (CI) sources where the ion entrance hole is small. Different filament materials and geometry have been used to reduce or eliminate this problem. Various wire geometries including ribbon filaments, straight wire filaments, coils and pin shaped filaments were studied using a variety of filament materials including Rhenium, Tungsten, Rhenium/Tungsten alloy and Yttria/Rhenium * (Yttrium Oxide/Rhenium) alloy materials to evaluate the effect of these parameters on filament warping.

Previous studies have shown that round wire filaments are less likely to warp as compared to thin ribbon shaped filaments. Additional studies show that the straight wire filaments demonstrate less warping than the coil or pin shaped filaments.

Rhenium is a very ductile material compared to Tungsten or the Tungsten/Rhenium materials. However Rhenium has been the preferred material for filament construction, since it does not oxidize and form oxide crystals on the filament like the Tungsten filament materials. However because it is soft, Rhenium filaments easily warp especially in a coil or pin shaped geometry.

Rhenium has been alloyed with several other refractory materials at low concentrations to improve the rigidity of the Rhenium. Tungsten was tried first, but the wire was difficult to draw. Low concentrations of Yttria were alloyed with Rhenium. The Yttria/Rhenium alloy proved to have identical electrical characteristics to the pure Rhenium, but filament warping was dramatically reduced with this material.

Method

Filaments were constructed from pure Rhenium, Tungsten, 3% Rhenium-Tungsten, and a Yttria/Rhenium alloy. A variety of filament shapes including straight filaments, coils and pin shaped filaments were tested using a multi-filament DC supply filament testing station and also in several different manufacturers™ mass spectrometers to monitor their performance and to determine filament lifetime cycles and to monitor filament warping. The filaments were tested by cycling the filaments ON and OFF every 60 seconds until the filament sensitivity declined or the filament burned out.

The crystal structure of Yttria/Rhenium alloy was studied by Electron microscopy and demonstrated that the yttria oxide particles intermixed with the rhenium particles reducing the size of the Rhenium crystals in the filament material. In comparison, the pure rhenium filaments exhibited larger and longer grain sizes. This smaller grain size in the alloy filament appears to strengthen the filament wire to provide a more stable filament that displays less sag or warping than pure rhenium filaments. The property of reduced warping has been demonstrated for straight wire filaments, multiple coiled filaments and pin shaped filaments when used in both AC and DC filament operating circuits.

Straight Wire CI Filaments

Straight wire filaments were constructed from 0.0055” diameter pure Rhenium, Yttria/Rhenium alloy and 3%Rhemium/97% Tungsten. Filaments were tested in a mass spectrometer under CI conditions using Methane as the gas and helium as the GC carrier gas. Themass spectrometer utilized a high frequency AC filament power supply. Both the Y/Re and W/Re filaments exhibited longer filament life with minimal warping.

Coil Filaments - 2 Coils

Two coil filaments were constructed from 0.0055” diameter Yttria/Rhenium alloy and pure Rhenium. Testing was done in an EI mass spectrometer using a high frequency AC filament power supply. Multiple Re filaments demonstrate the consistent warping seen with pure rhenium filaments, where as the Y/Re alloy filaments did not exhibit this warping.

Coil Filaments - 3 Coils

Three coil filaments were constructed from 0.003” diameter Yttria/Rhenium alloy and pure Rhenium. Testing was done in both the SIS filament testing station using a DC power supply and also in a standard Mass spectrometer which also operated with a DC filament power supply. Two different lots of Y/Re were tested to verify the repeatability of the alloy properties.

A second Y-Re filament was run to confirm that that the filament did not warp. The filament was stopped after 7,000 cycles due to a power failure in the facility. A new lot (lot 3) of Y/Re wire was made and allowed to run until the filament broke at 25,175 cycles.

Coil Filaments - 4 Coils

Three coil filaments were constructed from 0.0055” diameter 3%Re/97% Tungsten alloy and pure Rhenium. Testing was done in an EI mass spectrometer using a high frequency AC filament power supply. The Re filaments demonstrate the consistent warping seen with pure rhenium filaments, where as the Re/W alloy filaments did not exhibit this warping.

Four Coil Filaments constructed from SISAlloy®. These filament compare to the Tungsten/Rhenium filament above with minimal warping of the filament, unlike the pure rhenium filament.

Coil Filaments - 4 Coils with Long Leads

Four coil filaments were constructed from 0.0055” diameter Yttria/Rhenium alloy, and pure Rhenium. The filaments were constructed with long leads which would theoretically exhibit the greatest degree of warping.Testing was done in an EI mass spectrometer using a high frequency AC filament power supply. The Re filaments demonstrate the consistent warping seen with pure rhenium filaments, where as the Re/W alloy filaments did not exhibit this warping, even with the long leads.

Pin Shaped Filaments

Pin shaped filaments were constructed from 0.004” diameter Yttria/Rhenium alloy and pure Rhenium. Testing was done in the SIS filament testing station using a DC power supply. The filament voltage and current profiles were near identical for both materials as reported above. In both testing systems the Yttria/Rhenium alloy filament demonstrated less filament warping and increased filament lifetimes.

Crystal Structure of Rhenium alloys

Electron microscopy studies demonstrated that the yttria oxide particles intermixed with the rhenium particles which minimized the grain growth in the alloy filament. In comparison, the pure rhenium filaments exhibited larger and longer grain sizes. This smaller grain size in the alloy filament appears to strengthen the filament wire to provide a more stable filament that displays less sag or warping than pure rhenium filaments. The property of holding its shape has been demonstrated for straight wire filaments, multiple coiled filaments and pin shaped filaments when used in both AC and DC filament operating circuits.

Conclusion

The purity of Rhenium appears to have improved over the last twenty years. This has resulted in rhenium wire with large grain size, often approaching the diameter of the filament wire. This large grain size is not optimal for filament production and results in premature filament failure in warping or breaking of the filament.

In this study Tungsten/Rhenium and Yttria/Rhenium alloys have demonstrated improved filament performance, by reducing or eliminating filament warping.

Historically Tungsten/Rhenium alloy filaments have been widely used in mass spectrometers, but tungsten is subject to tungsten oxide build-ups on the filaments when used in the mass spectrometer, which may not be desirable for most applications.

A Yttria/Rhenium alloy has been developed for use in filaments for mass spectrometers and other scientific instruments. Studies on the Yttria/Rhenium alloy filaments in a number of commercial mass spectrometers has demonstrated increased cycle lifetime as compared to the standard rhenium filaments.

The enhanced properties attributed to the addition of Yttria to the Rhenium alter the crystal structure of the Rhenium and result in the reduction of sagging and warping of the filaments. This permits the use of this alloy to produce not only straight wire filaments but also multiple coil shaped filaments and pin shaped filaments that are more stable and less likely to warp or change shape when used at elevated temperatures.