Tests, spectrometer sensitivity

The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

Temperature consistency between measurements performed on different spectrometers is particularly critical for accurate interpretation of the data (see Refs. [19, 20] for post-acquisition temperature consistency tests). However, temperature control and equalization are also important for the combined analysis of T1, T2, and NOE data measured on the same spectrometer, because of the possible temperature differences between these measurements. Fig. 12.1 illustrates the sensitivity of relaxation parameters to temperature variations. Accurate measurement of protein dynamics requires that all experiments be done at the same temperature. To improve temperature consistency between Tlr T2, and... 

Solubility screens using LC/MS detection do not require an ultra-pure sample of the test compound due to the selective detection of the mass spectrometer. Mass spectrometric detection offers high selectivity and low detection limits, which eliminates the need to develop complex chromatographic methods. The LC/MS-based solubility screen surpasses the traditional HPLC/UV-based equilibrium solubility assay with increased throughput, minimal manual intervention, and high sensitivity and selectivity. 

The detection of a test gas using mass spectrometers is far and away the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending aaoss many powers of ten (see Section 5.2) whereby the lower limit = 10 mbar l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely ( ) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD s ... 

System suitability allows the determination of system performance by analysis of a defined solution prior to running the analytical batch. System suitability should test the entire analytical system, chromatographic performance as well as the sensitivity of the mass spectrometer for the compounds of interest. Some LC-MS SOPs reference analytical methods as the source of operating details for a given analysis. This works particularly well for quantitative analysis, where analytical methods include critical details on instrument parameters and special calibrations that might be required for a particular analyte. Thus, system suitability testing provides the daily [3] checking of the system. 

Note Current (circa 1981) computer search lists the CA Registry No of VCM as [75-01-4] Analysis E. Sistig, Recent Methods and Trends of Automatic Analyzers in the Chemical Industry. .. Vinyl Chloride. .. ppm Range , Proc 5 th IntCongrChemEngrgChemEquipDes-Autom, Prague (1975) CA 36, 8217 (1977) [The instruments with the ability to determine 0 to 20 ppm of VCM with an accuracy > 2 ppm include flame ionization detectors (FID), gas chromatographs using FID, dispersive IR analyzers with spectrometer cells, analyzers based on electrical conduction with upstream combstn chambers, and analyzers with Cl sensitive test paper]... 

Most organic compounds may be best analyzed by GC/MS. Such GC/MS or GC analysis, however, is preceded by either a purge and trap concentration step or a liquid or solid phase extraction step using a suitable organic solvent. The purge and trap method for aqueous samples is applicable for volatile substances that have lower solubility in water. A mass spectrometer should be used wherever possible to identify the compounds more correctly. Although it has a lower sensitivity than other GC detectors, mass spectrometiy is, by far, the most confirmatory test for compound identification. 

Specifications for a probe head/field strength (magnet) combination are given by the spectrometer manufacturer for line shape, sensitivity, resolution, and radio-frequency field strengths. A specific test is considered passed when the specification value is met. 

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