Mass spectrometer sample inlet

There are several means of introducing samples into the mass spectrometer. The inlet system to be used depends mainly on the nature of the sample (volatility, molecular weight, polarity and thermal stability), and the ionisation method (in particular, the gas pressure in the ion source). The most common inlet systems are ... 

A mass-spectrometer sampling probe is used to measure major species profiles for a variety of surface temperatures. For all cases shown in Fig. 17.23 the inlet mixture is lean methane air at an equivalence ratio of

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Mass Spectrometric Analysis Combusted sample tubes were attached to a purification vacuum line connected to the inlet system of the mass spectrometer. Sample tubes were opened under vacuum using a tube-cracker (35), and the gases were passed through a dry ice trap to remove water vapor and a liquid nitrogen trap to collect CO 2. Noncondensible gases were pumped away. The purified CO2 was thawed and admitted into the inlet system of the mass spectrometer for determination of isotopic composition ... 

A block diagram of a typical molecular-mass spectrometer is shown in Figure 31-10. Sample molecules enter the mass spectrometer through an inlet system. In the case of GC, the sample is in the form of a vapor, and the inlet must interface between the atmospheric pressure GC system and the low-pressure (10 to 10 torn) mass spectrometer system. An elaborate vacuum system is needed to maintain the low pressure. In the mass spectrometer, sample molecules enter an ionization source, which ionizes the sample. The ionization sources for molecular mass spec-... 

Several different techniques can be employed to introduce the sample into the mass spectrometer. These inlets must transition the sample from the higher pressure of the sample line to the vacuum required for the mass spectral analysis, yet this transition must occur without affecting the concentrations of the analytes of... 

Figure 1 Typical mass spectrometer and inlet system for isotopic comparison of an unknown sample with a reference gas of precisely known isotopic composition. Clean gases are needed tor such systems.

The inlet system is used to introduce the sample into the mass spectrometer, to convert it into the gas phase, and to reduce its pressure before ionization. Forensic samples are often impure, so the analytes, have to be separated from the matrix before being inserted into the mass spectrometer. The inlet system is most often an interface between a chromatographic device and the mass spectrometer. By this approach, the analytes are separated from one another and from the contaminants by either gas chromatography (GC) or high-performance liquid chromatography (HPLC), and the isolated compounds in the effluents from the column flow directly into the mass spectrometer. 

The advent of atmospheric-pressure ionization (API) provided a method of ionizing labile and nonvolatile substances so that they could be examined by mass spectrometry. API has become strongly linked to HPLC as a basis for ionizing the eluant on its way into the mass spectrometer, although it is also used as a stand-alone inlet for introduction of samples. API is important in thermospray, plasmaspray, and electrospray ionization (see Chapters 8 and 11). 

The thermospray inlet/ion source does not produce a good percentage yield of ions from the original sample, even with added salts (Figure 11.2). Often the original sample is present in very tiny amounts in the solution going into the thermospray, and the poor ion yield makes the thermo-spray/mass spectrometer a relatively insensitive combination when compared with the sensitivity attainable by even quite a modest mass spectrometer alone. Various attempts have been made to increase the ion yield. One popular method is described here. 

Of course, some substances are sufficiently volatile that a heated inlet line can be used to get them into a mass spectrometer. Even here, there are practical problems. Suppose a liquid or solid is sufficiently volatile, that heating it to 50°C is enough to get the vapor into the mass spectrometer through a heated inlet line. If the mass spectrometer analyzer is at 30°C, there is a significant possibility that some of the sample will condense onto the inner walls of the spectrometer and slowly vaporize from there. If the vacuum pumps cannot remove this vapor quickly, then the mass... 

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. 

Although this system is simple with no moving parts, unfortunately not many ions from the original dissolved sample are produced, and the thermospray inlet/ion source is not very sensitive considering the achievable sensitivities of standard mass spectrometers. 

Continuous inlet. An inlet in which sample passes continuously into the mass spectrometer ion source, as distinguished from a reservoir inlet or a direct-inlet probe. 

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

Sample introduction system. A system used to introduce sample to a mass spectrometer ion source. Sample introduction system, introduction system, sample inlet system, inlet system, and inlet are synonymous terms. 

Vacuum-lock inlet. An inlet through which a sample is first placed in a chamber the chamber is then pumped out, and a valve is opened so that the sample can be introduced to the mass spectrometer ion source. A vacuum-lock inlet commonly uses a direct-inlet probe, which passes through one or more sliding seals, although other kinds of vacuum-lock inlets are also used. 

A mass spectrometer consists of four basic parts a sample inlet system, an ion source, a means of separating ions according to the mass-to-charge ratios, ie, a mass analyzer, and an ion detection system. AdditionaUy, modem instmments are usuaUy suppUed with a data system for instmment control, data acquisition, and data processing. Only a limited number of combinations of these four parts are compatible and thus available commercially (Table 1). 

It has been reported that exchange of protons activated by enolization can be performed directly in a glass inlet system of the mass spectrometer prior to analysis by heating the sample at about 200° with deuterium oxide vapor for a few minutes. " Exchange has been observed with 2-, 3-, 6-, 11- and 17-keto steroids, but the resulting isotopic purity is usually poor,... 

It is possible to carry out a chromatographic separation, collect all, or selected, fractions and then, after removal of the majority of the volatile solvent, transfer the analyte to the mass spectrometer by using the conventional inlet (probe) for solid analytes. The direct coupling of the two techniques is advantageous in many respects, including the speed of analysis, the convenience, particularly for the analysis of multi-component mixtures, the reduced possibility of sample loss, the ability to carry out accurate quantitation using isotopically labelled internal standards, and the ability to carry out certain tasks, such as the evaluation of peak purity, which would not otherwise be possible. 

Mass Spectrometry. The mass spectra were obtained on a CEC 21-llOB mass spectrometer with the batch inlet system maintained at 250°C to assure complete vaporization of the samples. Sensitivity factors for quantitative analysis were obtained from standards of di-, tetra-, hexa-, and octa-chlorodibenzo-p-dioxin. The factors for the intermediate chlorinated species were estimated by interpolation. The analyses were based... 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

Cl and El are both limited to materials that can be transferred to the ion source of a mass spectrometer without significant degradation prior to ionisation. This is accomplished either directly in the high vacuum of the mass spectrometer, or with heating of the material in the high vacuum. Sample introduction into the Cl source thus may take place by a direct insertion probe (including those of the desorption chemical ionisation type) for solid samples a GC interface for reasonably volatile samples in solution a reference inlet for calibration materials or a particle-beam interface for more polar organic molecules. This is not unlike the options for El operation. 

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