Mass spectrometer-coupled instrumentation

FIGURE 10.20 An illustration of a mass spectrometer coupled to an inductively coupled plasma instrument. 

Arpino, P. J. Guiochon, G. Krien, R Devant, G. 1979. Optimization of the instrumental parameters of a combined liquid chromatograph-mass spectrometer, coupled by an interface for direct liquid introduction. . /. Chromatogr., 185,529-547. 

Mass spectrometers coupled with gas and liquid chromatographs (GC-MS and LC-MS) result m versatile analytical instruments that combine the resolving power of the chromatographs with the exquisite specificity and sensitivity of a mass spectrometer. Such instruments are powerful analytical tools that are used by clinical labs to identify and quantify organic analytes. They provide structural and quantitative information in real time on individual analytes as they elute from a chromatographic column. These coupled techniques are very sensitive and only nanogram or picogram quantities of an analyte are required for analysis. Specific apphcations of these coupled instruments are found in Chapters 33 and 34. 

Figure 1 shows a magnetic-type sector field mass spectrometer coupled with a Knudsen cell. The most important part of the instrument is the Knudsen cell. It can be heated up to temperatures above 2500 K. The temperatures are measured with an optical pyrometer or a thermocouple. There would be thermodynamic equilibrium in the Knudsen cell if it were closed. However, real Knudsen cells have an effusion orifice (typical diameter 0.1 to 1 mm) through which a small fraction of the molecules effuse without practically disturbing the equilibrium in the cell. A molecular beam representing the equilibrium vapor in... 

Arpino, P.J., Guiochon, G., Krien, R, Devant, G. (1979) Optimization of the Instrumental Parameters of a Combined Liquid Chromatograph-Mass Spectrometer, Coupled by an Interface for Direct Liquid Introduction I. Performance of the Vacuum Equipment. J. Chromatogr. 185 529-547. 

As mentioned earlier, mass spectrometers coupled to HPLC (the combination is commonly abbreviated as LC-MS) are becoming more and more common, owing to their increased robustness and increased automation and performance, as well as decreasing costs of the simplest instruments. Usually most compounds are determined by mass spectrometry as long as they can be ionized and transferred to the gas phase. A general schematic representation of an LC-MS is shown in Figure 3.27. 

For GC-MS, spectra were obtained using an updated Finnigan Mat SSQ 70 mass spectrometer coupled to a Varian 3400 gas chromatograph. Ionisation was by electron impact at 70 eV with a source temperature of 180 C. The scan range was 40 - 700 daltons. The GC was equipped with a J W DB5ms capillary column, 30 m x 0.32 mm with a film thickness of 0.52 /tm 5% phenyl / 95% dimethylpolysiloxane. The GC temperature programme was 3 min at 40°C then raised at 20 C/min to 320 C and held at that temperature for 20 min. A solution of 100 ng/mL of the reference substance was prepared for the measurement in the GC-MS. As a check and also for substances that could not be analysed by GC-MS, mass spectra were also obtained in a second laboratory using different instrumentation and conditions. 

Electrospray can be used with sector, time-of-flight, and quadrupole instruments. The technique has been used extensively to couple liquid chromatographs to mass spectrometers. 

Instrumental Interfaces. The basic objective for any coupling between a gas chromatograph (gc) and a mass spectrometer (ms) is to reduce the atmospheric operating pressure of the gc effluent to the operating pressure in the ms which is about 10 kPa (10 torr). Essential interface features include the capability to transmit the maximum amount of sample from the gc without losses from condensation or active sites promoting decomposition no restrictions or compromises placed on either the ms or the gc with regard to resolution of the components and reliability. The interface should also be mechanically simple and as low in cost as possible. 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

Although GC/MS is the most widely used analytical method that combines a chromatographic separation with the identification power of mass spectrometry, it is not the only one. Chemists have coupled mass spectrometers to most of the instruments that are used to separate mixtures. Perhaps the ultimate is mass spectrometry/mass spectrometry (MS/MS), in which one mass spectrometer generates and separates the molecular ions of the components of a mixture and a second mass spectrometer examines their fragmentation patterns ... 

Instrumentation. H and NMR spectra were recorded on a Bruker AV 400 spectrometer (400.2 MHz for proton and 100.6 MHz for carbon) at 310 K. Chemical shifts (< are expressed in ppm coupling constants (J) in Hz. Deuterated DMSO and/or water were used as solvent chemical shift values are reported relative to residual signals (DMSO 5 = 2.50 for H and 5 = 39.5 for C). ESl-MS data were obtained on a VG Trio-2000 Fisons Instruments Mass Spectrometer with VG MassLynx software. Vers. 2.00 in CH3CN/H2O at 60°C. Isothermal titration calorimetry (ITC) experiments were conducted on a VP isothermal titration calorimeter from Microcal at 30°C. 

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