Tuning the Mass Spectrometer

Before the mass spectrometer can be used to collect mass spectra, the instrument must be tuned and calibrated. The tuning procedure involves setting voltages associated with the ion source, lenses, and detector (to optimize sensitivity), and selecting values for potentials applied to the quadrupole (to set the instrument resolution). These tasks are accomplished while a calibration standard is continuously added to the instrument. A common calibration standard is perfluorotributylamine (PFTBA), (CF3CF2Cp2CF2)3N. Usually ions at miz 69,219, and 502 are monitored (Fig. 8.50). 

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ESI and MALDI sources have been coupled to many different mass analyzer types. Of these mass analyzers, five will be discussed next and are shown in Figure 4.5. To achieve the highest performance from these analyzers, an experienced operator will tune the mass spectrometer to improve the resolution, sensitivity, and signal-to-noise SIN) level. Tuning is essential to achieve optimum performance, but it differs for each mass spectrometer and will not be covered in this chapter. 

Tuning The mass spectrometer should be tuned prior to the analysis of an analytical series. If any of the checked parameters (mass axis, resolution/peak width, and response/abundance) not comply with the factory limits, then a complete autotune is performed and the new and corrected parameter settings are saved to the tune file. 

Before initiating the calibration procedure, tune the mass spectrometer according to manufacturer s instructions. Set the mass spectrometer data system to acquire data in the full scan (TIC-RIC) mode. 

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. 

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... 

Adjustment or tuning refers to the correct setting of an instrument. For example, setting 0 and 100 % in THERMOVACs or setting the mass spectrometer to mass 4 in the helium leak detector. 

For the most recent LC-MS on the market, an automatic procedure is included in the software package to tune and calibrate in the ESI mode. However, older instruments and/or very specific applications still require manual or semiautomatic procedures to optimize the parameters that affect ion detection. In an LC-MS instrument, the mass spectrometer is tuned and calibrated in three steps (1) ion source and transmission optimization, (2) MS calibration, and (3) fine tuning (detection maximization of one or more particular ions). 

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).

Calibration and tuning of the mass spectrometer are achieved using either bromofluorobenzene (BFB) or dccafluorotriphenylphosphine (DFTPP).-----------------------------------------------------------... 

Particle Beam HPLC/MS. The particle beam HPI.C/MS analysis was performed on a Hewlett-Packard 5988A (Palo Alto, CA) mass spectrometer. The mass spectrometer was operated at 70 eV electron energy with a source temperature ranging from 150-350°C for acquisition of the El spectra. The Cl spectra was obtained using methane reagent gas, at an electron energy of 100 eV, and with a source temperature between 100-300°C. The instrument was scanned from m/z 50-550 in 1 second. The mass spectrometer was tuned and calibrated daily with FC--43. The particle beam (Hewlett-Packard, Palo Alto, CA) was operated at the conditions determined optimal from a previously reported study for the analysis of the various drugs and antibiotics [13]. 

Mass Spectrometer. The mass spectrometer was a Hewlett-Packard 5988A quadrupole mass spectrometer with a dual EI/CI source and positive and negative ion detection. The system was controlled by a Hewlett-Packard 1000 computer. The mass spectrometer was periodically tuned manually using perfluorotributylamine (PFTBA) on ions m/z 69, m/z 214, and m/z 502 in El and PCI modes, and on ions m/z 245, m/z 414, and m/z 633 in NCI mode. 

There has been signihcant effort and progress by the mass spectrometer vendors in the last decade to create software that is user-friendly and automates many of the routine funchons. One particular example is the use of autotuning and calibrahon scripts that are used to tune and caUbrate against a series of reference peaks to be found in a tuning standard. 

Analyte detection in all these methods involved an ultraviolet diode array detector over the range 214-216 nm to minimize data file size. Mass spectrometric data were obtained following a split of the HPLC solvent stream of approximately 100 1. The mass spectrometer was tuned for optimal sensitivity and unit mass resolution. The mass range 180-800 daltons... 

Several experimental variables have an important influence on the SFC-MS experiment using this equipment. These variables include the size of the restrictor orifice, the tuning of the mass spectrometer, the temperature of the MS-interface probe tip, and the pressure in the ion source manifold. All of these variables as well as the class and molecular weight of the analyte influence the sensitivity of the technique. 

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