Leak detectors with mass spectrometer

Leak detectors with mass spectrometers (MSLD)... 

Mass Spectrometers Leak detectors using mass spectrometers are built to measure the flow rate of helium past the sensor. Helium is relatively rare in the air, so it registers only when there is a leak in the system being tested and when helium is being sprayed near the leak. By using a sensitive system with a very fine and slow helium probe, leaks as small as 1 X 10 ° torrL/s can be located. 

Partial pressure analysis using a mass spectrometer or the pressure rise method may be used to differentiate between these two causes. Since the pressure rise method will only prove the presence of a leak without Indicating Its location In the apparatus. It Is advisable to use a helium leak detector with which leaks can, in general, also be located much more quickly. 

Leak detectors with quadrupole mass spectrometer (ECXDTEC II)... 

INFICON builds leak detectors with quadrupole mass spectrometers to register masses greater than helium. Apart from special cases, these will be refrigerants. These devices thus serve to examine the tightness of refrigeration units, particularly those for refrigerators and air conditioning equipment. 

Helium leak detectors with 180° sector mass spectrometer (UL 200, UL 500)... 

This level of leak detection is not inexpensive A new mass spectrometer helium leak detector with the accompanying equipment can easily cost between 20,000 and 30,000. Purchasing used equipment can significantly reduce the initial costs, but one should not enter the level of helium leak detection because it seems like a good idea. On the other hand, if you need a helium leak detector, you cannot afford not to have one. 

FIGURE 5.2 Flat DMA structure avoiding leaks and discontinuities on wetted surfaces based on an insulating box covered on its top and bottom with the two DMA electrodes. (From Rus J., Fernandez de la Mora J., Resolution improvement in the coupling of planar differential mobility analyzers with mass spectrometers or other analyzers and detectors. US patent application publication 20080251714, October 16,2008. With permission.)... 

The experiments of Kistiakowsky and Kydd [1] were done by single-pulse photolysis with a 500-J flashlamp, the reaction vessel contents being sampled via a pinhole leak into the electron ionization source of a Bendix time-of-flight (TOF) mass spectrometer. Mass spectra were obtained by pulsed extraction of ions from the ion source at 50-fis intervals after the flash. The signal from the electron multiplier detector was displayed on a cathode ray tube, which was photographed with a rotating drum camera. 

Compounds not detected or detected in lower-than-expected concentrations. First, make sure that the problem is definitely due to a problem with the model mouth. For example, the cause of the problem may be due to the analytical equipment (e.g., gas chromatograph or mass spectrometer), inconsistencies in the food sample, and/or extraction errors. If volatile compounds are not detected or are detected in far lower-than-expected concentrations, there may be a gas leak somewhere in the system. All connections should be checked with a leak detector as described for the RAS. 

If pressure-rise measurements indicate that a search for a leak is worthwhile then it is likely that helium will be used as a search gas and a commercial mass-spectrometer leak detector (MSLD) will be used as a selective helium detector. In practice, with a leaking system, air... 

Silicon oils do not break down on contact with air while hot, but on the other hand they can polymerize and develop an insulating film on electronics. Thus, their use is not acceptable for instruments such as mass spectrometers (including He leak detectors). Some specific properties of a spectrum of diffusion pump fluids are shown in Table 7.9. 

Fig. 7.60 Using the inside-out technique, the piece in question is filled with helium and then placed in a covering of some type that can be evacuated. Any helium that is released within this covering will be detected by the helium leak detector. Note that the tested piece can have either end lifted up so that all surfaces are exposed to the vacuum. From Introduction to Helium Mass Spectrometer Leak Detection, Figs. 3.3 and 3.4, by Varian Associates, Inc. 1980, reproduced with permission.

Most helium leak detectors will not operate with pressures above lO"4 torr to 10 5 torr. At these greater pressures, the main element to the mass spectrometer will bum out. Fortunately most, if not all, helium leak detectors have various safety check mechanisms that automatically shut off the current to the main filament if the pressure goes above a set limit. So, you must depend on alternate leak detection methods, or use the detector-probe technique to discover large leaks. Once large leaks have been discovered and closed, you can concentrate on the smaller leaks that can be found with the tracer-probe technique. 

If the leak rate is unacceptable, the leaks have to be located by a helium leak detector (HLD), a mass spectrometer that measures only the amount of helium atoms. The freeze-drying plant is evacuated and the HLD with its pumping system is operated parallel to the main VPS. The HLD can measure He coming from a leak down to 2 X lO" mbar L/s, but its He sensor can only operate at a pressure of 10 mbar. The pressure in a freeze-drying plant will always be higher. Therefore a pressure reducer has to be placed between the HLD and the freeze-drying plant. If the pressure reducer has a conduetance L 5 and L . becomes... 

Mass spectrometers with electromagnets can scan a wide mass range and hence, with suitable inlet systems, can analyse for a number of gases. They are thus more versatile than modified leak detectors, but are more costly and unsuited for field use. 

The working principle of a helium-specific mass spectrometer leak detector is as follows [7], The mass spectrometer mainly consists of three parts an ion source, a deflection system and an ion collector. When the atoms pass the ionisation chamber of the detector, they are ionised to become positive ions. When these ions are then accelerated in the ionisation chamber, they obtain sufficient energy and enter into the magnetic field of the detector. These positive ions travel in a specific mbit in the magnetic field. As different ions have specific orbit radii only ions with the same orbit radius as that of the spectrometer can pass a narrow gap, as indicated by K in Figure 2.13, and reach the ion collector K to form an ion stream. 

After calculations of the desired flow rates and consequent required flow conditions, the entire gas line is purged by alternate pumping and filling with the desired gases. The metering valve V(2) and the flow meter pressure regulating valve V(3) are adjusted to provide the conditions of flow meter pressure PFM and flow meter indication necessary to produce with accuracy the desired flow rates. Leak checks with a helium mass spectrometer leak detector are made before and after the experimental run. Valve V( 1) is a gas flow on-off valve. 

Mass spectrometry has been utilized for on-line measurements for many years. The technique has been widely used in the petroleum industry as well as the chemical industry where the process stream contains primarily gases. This type of sample is the most compatible with the mass spectrometer and requires little sample preparation before introduction into the instrument. Early instruments designed for process monitoring did not have full scanning capability, but rather had several detectors at fixed position to monitor specific ions. Most of these instruments had the capability to monitor 5-7 components in the gas stream. These instruments were also widely used as leak detectors since they had the requisite sensitivity for low level detection and the speed to detect leaks very quickly. 

If none of these approaches succeeds in locating the leak, the next step is to use a helium leak detector, a self-contained El mass spectrometer tuned to respond only to helium m/z 4) and complete with its own vacnnm system with sufficient pumping speed that it can handle inlet pressures (i.e., from the system with the leak) of up to 10 torr. These items are fairly expensive and are certainly not available in every laboratory, so it may be necessary to borrow or rent one. Another possibUity is to use a residual gas analyzer, a small El mass spectrometer mounted on a vacuum flange that can be inserted into the vacuum chamber, e.g., to temporarily replace the ion gauge or other component. These devices record complete El mass spectra typically in the range m/z l- 200, but usually are not supphed with their own independent vacuum system and in fact require a vacuum of better than 10 torr for operation (and at this pressure can operate only with a Faraday cup detector instead of an electron multipher, see Chapter 7). 

The major components of the mass spectrometer are a gas-handling manifold that enables the gas to be transferred from the ovens to the ion source, the ion source that consists of an electron impact source, magnetic mass analyzer with gap field of 6500 G, four different EM detectors, and an ion pump to ensure low pressure for the EMs. The gas handling system also included a controlled leak valve that could obtain Martian gas and feed the gas directly into the mass spectrometer for analysis. All the tubes in the gas handling system were heated to 35 C to ensure that there is no condensation of water and other volatile vapors on the hardware. 

High-pressure photocatalytic reaction studies can be undertaken by incorporating a sapphire window into the reaction cell. This allows the sample to be illuminated by an external light source (U V or visible), when the sapphire window is aligned with another window in the UHV chamber. Product gases may be detected and analyzed by the use of a gas chromatograph attached to the sample loop, By the use of suitable chromatographic columns and detectors (such as flame ionization, thermal conductivity, or photoionization), most likely product combinations may be separated and analyzed. Moreover, a more direct analysis can be made if the product stream is throttled (by means of a leak valve) into a separately pumped mass spectrometer. 

Recommended Practices for Determining Hermeticity of Electron Devices by a Bubble Test, Annual Book ofASTM Standards, ASTM F 98—72. American Society for Testing and Materials, Philadelphia, 1981. Recommended Practices for Determining Hermeticity of Electron Devices by Dye Penetration, Annual Book ofASTM Standards, ASTM F 97-72. American Society for Testing and Materials, Philadelphia, 1981. Recommended Practices for Determining Hermeticity of Electron Devices with a Helium Mass Spectrometer Leak Detector, Annual Book ofASTM Standards, ASTM F 134. American Society for Testing and Materials, Philadelphia, 1981. 

Either split, splitless, or on-column injections can be used in GCMS. Split injections are usually avoided in cases where trace-level components are being analyzed. Splitless or on-column injections are preferred for trace component analysis. Often splitless injections (septum purgeoff) are made with split injection port liners that are packed with a small amount of adsorbent to trap nonvolatile, polar materials that could contaminate the column and/or the mass spectrometer. For the same reasons, a retention gap is recommended for on-column injections, as the initial column section can be replaced with little effect on the chromatogram. When using a retention gap, however, care must be taken to ensure leaktight connections since a small air leak can have greater consequences for the mass spectrometer than for other detectors. 

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