Mass spectrometers inlet systems

The mass spectrometer inlet system for liquid chromatography, often termed the interface between the two component techniques, must therefore remove as much of the unwanted mobile phase as possible while still passing the maximum amount of analyte into the mass spectrometer. This must be done in such a way that the mass spectrometer is still able to generate aU of the analytical information of which it is capable. 

A similar flow and mass spectrometer inlet system has been developed by Lossing for the investigation of photochemical systems. Mercury photosensitization was used so that the concentration of radicals should be within the limits of sensitivity of the mass spectrometer. With the light intensities used in experiments with acetone 80 % of the reactant could be decomposed with an exposure time of 0.002 sec. Later modifications 1 permitted variation of the exposure time by altering... 

The choice of an ionization source must be consistent with the mass spectrometer inlet system, as described above, and the ionization characteristics of the analyte. Likewise, the appropriate choice of mass analyzer is highly application-, compound-, and matrix-dependent, and each mass analyzer requires certain characteristics from its ion detector The variety of individual ionization, mass analyzer, and ion detector systems that can be integrated creates a myriad of unique mass spectrometer systems, each with certain advantages and disadvantages. To add even more complexity to the situation, modular configuration of these individual components (ionization source, mass analyzer, and detector) is not limited... 

Hoch, G. Kok, B., A mass spectrometer inlet system for sampling gases dissolved in liquid phases, Arc/t. Biochem. Biophys. 1963, 101, 160. 

Hoch, G., Kok, B. (1963) A Mass Spectrometer Inlet System for Sampling Gases Dissolved in Liquid Phases. Arch. Biochem. Biophys. 101 160-170. 

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. 

It might be noted at this stage that some mass spectrometer inlets are also ionization sources. For example, with electrospray ionization (ES) and atmospheric pressure chemical ionization (APCI), the inlet systems themselves also provide the ions needed for mass spectrometry. In these cases, the method of introducing the sample becomes the method of ionization, and the two are not independent. This consideration can be important. For example, electrospray produces abundant... 

A mass spectrometer produces an enormous amount of data, especially in combination with chromatographic sample inlets [42]. Over the years, many approaches for analysis of GC-MS data have been proposed using various algorithms, many of which are quite sophisticated, in efforts to detect, identify, and quantify all of the chromatographic peaks. Library search algorithms are com monly provided with mass spectrometer data systems with the purpose to assist in the identi cation of unknown compounds [43]. 

Furthermore, there is a risk in operating a leak detector on the basis of a mass spectrometer at systems with condensable product. The conductance of the throttle could be changed or the inlet point clogged even completely due to condensed product. This means that the helium leak detector is getting blind to the test gas. 

In early GC with packed columns several tens of milliliters of carrier gas per minute were eluted. Thus, most of the flow had to be separated before entering the ion source to prevent the mass spectrometer vacuum system from breakdown [4,33,38]. Flow reduction was either effected by a simple split to divide the effluent in the inlet system by a factor of about 1 100 or by means of a more elaborate separator, the jet separator being the best-known of those [57,58]. The advantage of separators over a simple split is the enrichment of the analyte relative to the carrier gas in case of the jet separator this effect is based on a principle related to that of the nozzle-skimmer system in ESI (Chap. 12.2.1). 

Polymer samples were placed in a tungsten crucible in a Knudsen cell inlet system of a time-of-flight mass spectrometer. The system was evacuated. Samples were then heated at a linear rate. Spectra were determined at one minute intervals. The thermocouple was welded to a support rod 3 mm below the crucible and was calibrated against... 

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

Liquids that are sufficiently volatile to be treated as gases (as in GC) are usually not very polar and have little or no hydrogen bonding between molecules. As molecular mass increases and as polar and hydrogen-bonding forces increase, it becomes increasingly difficult to treat a sample as a liquid with inlet systems such as El and chemical ionization (Cl), which require the sample to be in vapor form. Therefore, there is a transition from volatile to nonvolatile liquids, and different inlet systems may be needed. At this point, LC begins to become important for sample preparation and connection to a mass spectrometer. 

Apart from ES and APCI being excellent ion sources/inlet systems for polar, thermally unstable, high-molecular-mass substances eluting from an LC or a CE column, they can also be used for stand-alone solutions of substances of high to low molecular mass. In these cases, a solution of the sample substance is placed in a short length of capillary tubing and is then sprayed from there into the mass spectrometer. 

The choice of a mass spectrometer to fulfill any particular task must take into account the nature of the substances to be examined, the degree of separation required for mixtures, the types of ion source and inlet systems, and the types of mass analyzer. Once these individual requirements have been defined, it is much easier to discriminate among the numerous commercial instruments that are available. Once suitable mass spectrometers have been identified, it is then often a case of balancing capital and running costs, reUability, ea.se of routine use, after-sales service, and manufacturer reputation. 

Electrospray is both an atmospheric-pressure (API) liquid inlet system for a mass spectrometer, and, at the same time, it is an ionization source. 

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