Mass spectrometer, detectors operation

Figure 19.3. A Varian 3400 gas chromatograph with a Tekmar 3000 purge-and-trap concentrator and a Varian Saturn 2000 mass spectrometer detector operated by Varian Saturn GC/MS Workstation Software Version 5.41 k.

Thermal-ionization mass spectrometers (TIMS) combine a hot-filament source with a magnetic-sector mass spectrometer. The mass spectrometers are operated at low to moderate mass-resolving power. A large number of elements can be measured with thermal ionization mass spectrometry. Special care is taken to purify the samples using ion exchange columns. Samples are loaded onto the filaments along with an emitter, and a typical run may take several hours. Modem systems have multiple collectors so that several isotopes can be measured simultaneously. High-precision measurements are done with Faraday cup detectors, but low-abundance isotopes can be measured on electron multipliers. Modem machines are capable of precisions of 0.1 to 0.01 permit. 

Irrespective of the detection method used, the analysis of VOCs often involves the presence of interferences in the chromatogram. This is normally due to VOCs present in the atmosphere, such as normal laboratory solvents (acetone, dichloromethane, chloroform, acetonitrile, or methanol) which can be detected, especially when using a universal detector such as a mass spectrometer which operates under vacuum. This problem is eliminated when the laboratory of analysis is free of solvents and is isolated, especially from urban areas. 

NMR and GC/MS Analyses. Reaction products were analyzed by GC-mass spectrometry (MS) and NMR. For GC/MS, silylation of methyl-esterified extract was achieved with a mixture of TMSI + pyridine (1 4, vol/vol) for 30 min at room temperature. GC/MS analysis was performed with a Hewlett-Packard Model 5890 gas chromatograph interfaced with a Model 5971 mass selective detector operating at 70 eV. The capillary column used was a Hewlett-Packard HP-5-MS cross-linked 5% phenylmethyl silicone, 30 m x 0.25 mm i.d., film thickness 0.25 xm. The carrier gas was helium and its flow rate was 0.65 mL/min. The GC column was programmed from 65 to 260°C at a rate of 20°C/min and then kept at 260°C for 20 min. NMR spectra were obtained with a Bruker model ARX-400 spectrometer (Billerica, MA) equipped with a 5-mm dual probe ( C NMR, 100... 

Interesting results were obtained by using the combined HPLC-MS technique (120). Mass spectrometry is very specific and appeared as sensitive as UV detection (Table 2). After treatment with diazomethane, biotin has been identified in biological samples as its methyl ester, using a C8 reversed-phase column and an isocratic system methanol-water (4 6) as mobile phase. The mass spectrometer was operated in positive-ion chemical ionization mode, and the limit of detection was <10 ng. The regression between the detector response and the injected sample was found to be linear up to 300 ng. 

Modem electrospray detectors are capable of handling flows up to several 100 j1 min" and can therefore be coupled easily with existing conventional LC equipment. A parallel array of concentration detector(s) (e.g., UV, RI, and ELSD) and mass spectrometer allows operating all detectors under... 

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

Quadmpole mass spectrometers (mass filters) allow ions at each m/z value to pass through the analyzer sequentially. For example, ions at m/z 100, 101, and 102 are allowed to pass one after the other through the quadmpole assembly so that first m/z 100 is transmitted, then m/z 101, then m/z 102, and so on. Therefore, the ion collector at the end of the quadmpole unit needs to cover only one point or focus in space and can be placed immediately behind the analyzer (Figure 30.1). A complete mass spectram is recorded over a period of time (temporally), which is set by the voltages on the quadmpole analyzer. In this mode of operation, the ions are said to be scanned sequentially. The resolution of m/z values is dependent solely on the analyzer and not on the detector. The single-point collector is discussed in detail in Chapter 28. 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

High-pressure pumps operating at up to 6000 psi are required to force solvent through a tightly packed HPLC column, and electronic detectors are used to monitor the appearance of material eluting from the column. Alternatively, the column can be interfaced to a mass spectrometer to determine the mass spectrum of every substance as it elutes. Figure 12.18 shows the results of HPLC analysis of a mixture of 10 fat-soluble vitamins on 5 jam silica spheres with acetonitrile as solvent. 

Morristown, NJ) for the ion source. No carrier gas separator was used. For determination of nitrosamines and TBDMS derivatives of hydroxy-nitrosamines, columns and operating conditions were identical to those for GC-TEA analyses For most work, the He flow rate was 15 cc/min and the column effluent was split 1 1 between a flame ionization detector and the mass spectrometer. The stainless steel splitter, solvent vent valve (Carle Instruments, Fullerton, CA), and associated plumbing were... 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

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