AVS2012 Session VT-MoA: Gas Flow, Leaks, Permeation and Mass Analysis

A porous plug made of sintered stainless steel, which is named as standard conductance element (SCE), has been developed as an open-type standard leak element for in-situ calibration of ionization gauges (IGs) and partial pressure analyzers [1]. Since the pore size of sintered filter is less than 1 micro meter, the molecular flow condition is realized up to 10 kPa of the upstream pressure of SCE. Therefore, four useful characteristics shown in below are available. (1) Flow rate is proportional to the upstream pressure of SCE. (2) Introducing various gas species with known flow rate is available by applying this single leak element. (3) Calibration using mixture gases is available. (4) Dependence of flow rate on the temperature is small and easy to compensate. In addition, the molecular conductance C_S of SCE has good long-term stability of less than 3 %/year typically. No significant influence is observed by introducing water vapor and bake-out. Since SCE with C_S from 1x10^-10 m³/s to 2x10^-9 m³/s is available, the flow rate of less than around 10^-5 Pa m³/s is generated with various gas species.

Three typical applications of SCE are introduced. First is the quantization of the gas desorption rate for thermal desorption spectroscopy (TDS) [2]. The gas desorption rate was measured by a quadrupole mass spectrometer (QMS) which was calibrated by SCE with H₂, H₂O, N₂, CO, and CO₂ gases. Second is the measurement of the effective pumping speed S_eff of cryopump in the range from 10^-9 Pa to 10^-6 Pa [3]. H₂, CH₄, N₂, and Ar gases with known flow rate Q were introduced into the test chamber with the cryopump through SCE, and measured the pressure increment p by extreme high vacuum gauges. Pressure indications of the gauges were compensated by relative sensitivity factors for N₂. The last one is the in-situ calibration of IG and QMS. The standard pressure p was obtained from Q divided by S_eff, where Q and S_eff were determined by using SCE and conductance modulation method, respectively. The sensitivity of IG and QMS were measured for H₂, He, CH₄, H₂O, Ne, CO, N₂, C₂H₄, C₂H₆, O₂, Ar, C₃H₆, CO₂, N₂O, and C₃H₈.

These results show that SCE seems to satisfy almost all of requirements for the quantitative measurements in high and ultrahigh vacuum. SCE will be used as a new vacuum standard device.

[1] H. Yoshida, et.al., Vacuum 86 (2012) 838-842.

[2] S. Inayoshi, et.al, 52th Annual Symposium of the Vacuum Society of Japan (AVSSJ-52), Tokyo, Japan, 2011.

The ISO Technical Committee (TC) 112 is responsible for international standards in the field of vacuum technology. Supporting TC112 are three working groups (WG): WG1 is responsible for vacuum pumps, WG 2 for vacuum instrumentation and WG 3 for vacuum hardware. Hence, the WG 2 is responsible for total and partial pressure measurement in vacuum. In 2006, WG 2 began a project towards standardization for the specification and calibration of quadrupole mass spectrometers (QMS). A new ISO (Draft) standard 14291 “Definitions and specifications for quadrupole mass spectrometers” will be published in 2012. Additionally WG 2 is working on documenting the proper use of a QMS and establish a standard for meaningful calibration procedures. In support of this ISO goal, the European Metrology Research Programme has established a project IND12 “Vacuum metrology for production environments” to open new measurement capabilities for vacuum and to help industry to characterize vacuum in industrial environments.

One of the focuses of this IND12 effort is to address traceability to national measurement standards for partial pressure measurements and outgassing rate measurements for materials characterization in industry. To gather current status on such methods, a workshop on calibration of the QMS for industrial use was held in Bled, Slovenia. Quadrupole mass spectrometers are widely used to measure partial pressures in vacuum although the performance and accuracy of these measurements is the subject of ongoing discussion among users. The wide range of partial pressure measurement needed from UHV/XHV (10^-10 Pa) to 1 Pa emphasizes that one type of QMS does not address all applications. From presentations at this workshop, application-specific data on performance of QMS types gives guidance for selection of a QMS type for an application. Talks were given on setup and ion source operation recommendations; initial calibration methods for an application and in situ calibration methods for verification or recalibration of a QMS during use. The fact that QMS electron energies differ by application and manufacturer leads to the requirement that gas species calibration is needed for each QMS for accurate partial pressure or compositional analysis. Papers on the electron-ion space charge at pressures>10^-4 Pa in ion sources were given together with gas interference where addition of a partial pressure of a new chemical species B gives a different ion current output for species A (when A is known to be at its original partial pressure). Results of these talks at the workshop will be summarized in this presentation.

There are numerous advantages to performing in situ chemical analyses in the field rather than collecting samples for laboratory analysis. These include reducing the possibility for sample contamination or degradation, significantly improving the spatial and temporal resolution of analyses, and providing the ability to implement adaptive sampling strategies by receiving the analytical information in real-time or near real-time. Many analytical instruments, such as mass spectrometers (MSs), require high vacuum for operation. Vacuum systems for these portable instruments must be reasonably small, low-power, and often more rugged than needed for laboratory operation. Underwater instruments of this type have the additional requirements that the vacuum systems must be self-contained (or exhaust to a high-pressure environment) and the sampling interfaces must withstand pressure differentials greater than 1 atmosphere – hydrostatic pressure increases by approximately 1 bar for every 10 m depth in the ocean. Consequently, all of these constraints must be taken into account when selecting or building vacuum systems for these applications.

Several groups around the world have been working on developing and using underwater MSs for in situ chemical measurements in oceans, lakes, and rivers – at times over a mile deep. To date, all of these operational underwater MSs have employed membrane-inlet interfaces to introduce analytes into the vacuum of the MS. Analytes that are soluble in the membrane material (typically polydimethlysiloxane or Teflon) permeate through the membrane and evaporate into the ion source of the MS, where they are ionized by an electron impact source. For operation at depth, the membrane must be supported by a porous frit to withstand the increased hydrostatic pressure. Membrane inlet MSs are very effective at detecting and quantifying dissolved gases, light hydrocarbons, and volatile organic compounds, often at trace levels.

Design considerations for underwater MSs will be discussed in this presentation, along with examples of specific components chosen for the vacuum systems. In particular, the focus will be on problems with operating vacuum systems under these often extreme and harsh conditions, along with solutions to mitigate these problems. In addition, we will present examples of deployments of underwater MSs for a variety of applications on a number of unmanned deployment platforms.

A new generation of residual gas analyzers based on autoresonant ion trap mass spectrometry (ARTMS) technology has recently become commercially available and is rapidly gaining market acceptance for vacuum processing applications. Recent investigations into this novel ion trap technology have focused on a detailed understanding of the role of ion generation and storage on the mass spectrometer’s performance. The rate of ion formation, the initial energetics of the ions stored inside the trap, the shape of the electrostatic trapping potential and the characteristics of the radiofrequency signal sweep used to eject the ions all have a significant impact on the performance of an ART MS instrument. Unit-to-unit variations in ion formation rates and energetics due to mechanical assembly tolerances, can lead to slight variations in ion trap performance which can be easily corrected through adjustment of the electron beam focusing optics and the shape of the electrostatic trapping potential. This presentation describes a straight forward methodology that has been developed in our research laboratory to manually optimize ion trap performance during routine measurements and also an automated tuning procedure (Auto-Tune) that is built into the control software and can be used to restore trap performance each time an ART MS sensor is fitted with a new filament assembly.