Mass spectrometer tuning

Mass spectrometer tuning acceptance criteria are different for different regulatory methods. 

Although there are other leak detectors and leak detection techniques that rely on detection of a change in gas, none have enjoyed the success of the mass spectrometer tuned to helium. There are other types of tuned mass spectrometers that specifically look for oxygen or halogen, but they are not as common. Their opera-... 

Fig. 18.13. Mass spectrometric signal as a function of the potential sweep on the electrode as shown in Fig. 18.12, with the mass spectrometer tuned to mass number 44 (C02).

Mass Spectrometer Tuning. A three-step process was used to tune the quadrupole mass spectrometer prior to its use as a detector for SFC. In the first step, perfluorotributylamine (Pfaltz Bauer Inc., Stamford, Conn.) was ionized by electron ionization and used to calibrate the mass axis. In the second step, methane was introduced into the Cl source and the reagent ion profiles were optimized. In the third step, the mass resolution was adjusted for improved sensitivity. This was accomplished by introducing a volatile brominated compound, such as 2-bromopentane, into the Cl source. The mass spectrometer s resolving power was reduced such that the peaks... 

Figure 3. The effect of temperature and WHSV on the distribution of selected products from the conversion of pine wood pyrolysis vapors over HZSM-5 catalyst. (Note % of total ion abundance should not be equated with % of neutral products since mass spectrometer tuning and cross sections favor aromatics.)...

In a search for other reaction channels, a number of time-of-flight spectra were also collected with the mass spectrometer tuned to other mass-to-charge ratios. Time-of-flight data collected at m/e = 99,7244,53,52,50,47,44,40,39.38,36,27,26, and 17 all had the same shape as the m/e = 100 time-of-flight spectrum and are interpreted as being daughter ions from the same channel as that observed at m/e = 100. At m/e = 46 (NO2) a new feature is seen in the time-of-flight data. The... 

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). 

A Figure 2.13 Diagram of a mass spectrometer, tuned to detect ions. The heavier ions are not defiected enough for them to reach the detector. 

SIMS utilizes an impinging ion beam to cause the ejection of charged atoms and molecules (35). The impinging primary ion beams may be Cs", O2", 0 , Ar", and so on. The secondary ions ejected may be H+, D+, C, and so on. The sample surface is commonly rastered by the impinging beam to erode a portion of the sample surface (36). The charged species of the secondary beam are filtered and then detected by a mass spectrometer tuned to the element of interest. 

Every effort was made to standardize the sample size and mass spectrometer tuning to ensure consistent spectra throughout this collection. Where it was necessary to manipulate the spectra, e.g., to remove traces of the injection solvent or column background, due care was exercised to avoid distorting the data. Mass calibration was checked several times a day, and the inertness of the interface was demonstrated daily by the analysis of cholesterol, which produced a 386/368 ion ratio of greater than 2 1. 

Trigger the primary bombardment. Examine the emitted secondary ions from the selected surface to modify the mass spectrometer tuning if required. 

High stability of the mass spectrometer (tuning, temperature, pressure)... 

When macroscopic oscillations occur (as monitored, e.g., with a Kelvin probe or by a mass spectrometer tuned to CO2), the partial pressures change periodically with time. Since the mean free path of the gas molecules is at the applied pressures considerably larger than the vessel diameter, this gives rise to a global coupling mechanism [71]. The coupling is essentially mediated by CO because O2 is present in excess (and CO2 is just an inert product). As... 

Helium leak detector (vacuum technology) A mass spectrometer tuned to the helium peak that is attached to a vacuum chamber and monitors any change in helium concentration in the chamber as helium gas is directed toward the exterior of the chamber. 

Before sample preparation, the laboratory must demonstrate that the mass spectrometer is operating satisfactorily. First, the instrument must be tuned by calibration using one of two compounds. 

Calibration and tuning of the mass spectrometer are achieved using either bromofluorobenzene (BFB) or decafluorotriphenylphosphine (DITPP). 

In the vast majority of GC-MS applications, the chromatographic conditions employed have little or no effect on the operation of the mass spectrometer. This means that the spectrometer may be tuned for optimum performance and a number of samples containing different analytes can be analysed without operator intervention. This is not the case with LC-MS where the chromatographic conditions will invariably have a significant, compound-dependent, effect on the mass spectrometry conditions required to obtain useful analytical data. 

GC/MS. GC/MS is used for separation and quantification of the herbicides. Data acquisition is effected with a data system that provides complete instrument control of the mass spectrometer. The instrument is tuned and mass calibrated in the El mode. Typically, four ions are monitored for each analyte (two ions for each herbicide and two ions for the deuterated analog). If there are interferences with the quantification ion, the confirmation ion may be used for quantification purposes. The typical quantification and confirmation ions for the analytes are shown in Table 4. Alternative ions may be used if they provide better data. 

Geromanos, S., Freckleton, G., and Tempst, P., Tuning of an electrospray ionization source for maximum peptide-ion transmission into a mass spectrometer, Anal. Chem., 72, 777, 2000. 

Mass spectrometers must be regularly tuned or calibrated against a known standard, e.g. perfluorotributy-lamine (PFTBA). The trend is towards miniaturisation (10 x 24 x 14 in.). A concept for a micro mass spectrometer, with potential applications in process monitoring, has been presented [167]. Mass-spectrometry instrumentation (1997) has been reviewed [166]. 

With the FT-ICR mass spectrometer it is possible to obtain high mass accuracy (about 1-5 ppm calibrated externally, 0.5-1-0 ppm if calibrated internally here an external calibration has been carried out using a tune mix containing compounds with m/z from... 

It is often neglected that the first step of de novo sequencing is data acquisition. The quality of the spectrum or spectra used for sequencing is the most critical parameter of the entire procedure. First of all, the mass spectrometer should be well calibrated and tuned. If it can operate in different modes, the one with the highest possible mass accuracy and resolution should be applied. If the experimenter has more spectrometers to choose from, the one with the highest mass accuracy and resolution should be used, provided it shows good fragmentation efficiency. 

Selected ion monitoring (SIM) is when the mass spectrometer is tuned to record spectra foi specific m/z values... 

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