Mass spectrometer sensitivity

However, it is the hope that the rapid progress in sample preparation procedures and in proteomic technologies particularly improved mass spectrometer sensitivity will solve these problems. 

Desorption was monitored with mass spectroscopy. The cracking patterns of 2-propanol, acetone, and propene were individually determined ( ). For quantitative analysis, masses 45, 45, 41, 18, and 2 were used for 2-propanol, acetone, propene, water, and hydrogen, respectively, after correction for cracking in a similar procedure as described (52 ) The mass spectrometer sensitivities were determined to be 5.26, 7 88, 5.07, 4 74, and 5.20 amp/torr, and the pumping speeds were 9.5, 15.1, 51.0, 1.7, 56.9 L sec"", respectively for the five species. These two latter quantities were used to convert the mass spectrometer readings into molecular fluxes. 

Values in the brackets are ratios corrected for the mass spectrometer sensitivities and pumping speeds. They represent the ratios of the molecular fluxes for desorbing from the surface. 

The production rate of He is much higher than for any other cosmogenic nuclide. In minerals where it is quantitatively retained, He thus provides the chance to date exceptionally young surfaces. For example, we have derived an age of years for a sample from the 1993 lava flow of Lascar volcano (Chile) taken at 4540 m altitude a few months after eruption (Niedermann et al. 2001b), which illustrates the potential of He on timescales also relevant to archeological studies. New developments in noble gas mass spectrometry, such as the compressor ion source which improves the mass spectrometer sensitivity for He and Ne by two orders of magnitude (Baur 1999), may further increase the precision of He (as well as Ne) determinations in the future. 

Ibrahim, Y., Tang, K., Tolmachev, A.V., Shvartsburg, A.A., Smith, R.D., Improving mass spectrometer sensitivity using a high-pressure electrod3fnamic ion funnel interface. J. Am. Soc. Mass Spectrom. 2006, 17, 1299. 

Water and CO desorption from oxides, carbons, or polymers was studied at 300-900 K by the one-pass temperature-programmed desorption (TPD) with mass-spectrometry control (OPTPD-MS) method (chamber pressure 10" Torr, sample weight 5-7 mg, heating rate 2K/s, with a short distance [ 0.5cm] between sample and MS detector) with a MSC-3 ( Electron, Sumy, Ukraine) time-of-flight mass spectrometer (sensitivity 2.2x10 A/Torr, accelerating voltage 0.5 kV, pulse frequency 3 kHz). 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

A further consequence of the high temperatures is that much of the sample is simply evaporated without producing isolated positive ions. There is a competition between formation of positive ions and the evaporation of neutral particles. Since the mass spectrometer examines only isolated charged species, it is important for maximum sensitivity that the ratio of positive ions to neutrals be as large as possible. Equation 7.1 governing this ratio is given here. 

The Z-spray inlet causes ions and neutrals to follow different paths after they have been formed from the electrically charged spray produced from a narrow inlet tube. The ions can be drawn into a mass analyzer after most of the solvent has evaporated away. The inlet derives its name from the Z-shaped trajectory taken by the ions, which ensures that there is little buildup of products on the narrow skimmer entrance into the mass spectrometer analyzer region. Consequently, in contrast to a conventional electrospray source, the skimmer does not need to be cleaned frequently and the sensitivity and performance of the instrument remain constant for long periods of time. 

The thermospray inlet/ion source does not produce a good percentage yield of ions from the original sample, even with added salts (Figure 11.2). Often the original sample is present in very tiny amounts in the solution going into the thermospray, and the poor ion yield makes the thermo-spray/mass spectrometer a relatively insensitive combination when compared with the sensitivity attainable by even quite a modest mass spectrometer alone. Various attempts have been made to increase the ion yield. One popular method is described here. 

Eventually, multipliers become less sensitive and even fail because of surface contamination caused by the imperfect vacuum in the mass spectrometer and the impact of ions on the surfaces of the dynodes. 

Although this system is simple with no moving parts, unfortunately not many ions from the original dissolved sample are produced, and the thermospray inlet/ion source is not very sensitive considering the achievable sensitivities of standard mass spectrometers. 

A detector is needed to sense when the separated substances are emerging from the end of the column. A mass spectrometer (MS) makes a very good, sensitive detector and can be coupled to either GC or LC to give the combined techniques of GC/MS or LC/MS, respectively. 

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. 

Different combinations of stable xenon isotopes have been sealed into each of the fuel elements in fission reactors as tags so that should one of the elements later develop a leak, it could be identified by analyzing the xenon isotope pattern in the reactor s cover gas (4). Historically, the sensitive helium mass spectrometer devices for leak detection were developed as a cmcial part of building the gas-diffusion plant for uranium isotope separation at Oak Ridge, Tennessee (129), and heHum leak detection equipment is stiU an essential tool ia auclear technology (see Diffusion separation methods). 

Three important parameters for mass spectrometers are mass resolution, mass range, and sensitivity. The resolution, R, required to separate two ions of mass m and (m + Am) is given by equation 1. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

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