Reference inlet system

Highly volatile samples cannot be introduced into the ion source by means of a direct insertion probe even when cooling is applied. A reference inlet system or reservoir inlet system is better suited for that purpose. [59] The name of this type of 

Reference inlets serve equally well for the analysis of gases, solvents, and similar volatile samples. They are especially convenient when a continuous signal is desired for instrument tuning or long-lasting MS/MS experiments in ion chemistry. In addition, the components of a mixture are admitted to the ion source without fractionation, i.e., without affecting their partial pressures. This property of reservior inlets has extensively been used in the petroleum industry. 

The liquid introduction system represents another variation of reservoir inlets. Here, the ion source housing serves as the reservoir into which a few microliters of liquid are introduced by means of a kind of micro DIP . [62] 

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Section 6.4 deals with other EI-MS analyses of samples, i.e. analyses using direct introduction methods (reservoir or reference inlet system and direct insertion probe). Applications of hyphenated electron impact mass-spectrometric techniques for poly-mer/additive analysis are described elsewhere GC-MS (Section 7.3.1.2), LC-PB-MS (Section 7.3.3.2), SFC-MS (Section 13.2.2) and TLC-MS (Section 7.3.5.4). 

If high-resolution measurements are performed in order to assign elemental compositions, internal mass calibration is almost always required. The calibration compound can be introduced from a second inlet system or be mixed with the analyte before the analysis. Mixing calibration compounds with the analyte requires some operational skills in order not to suppress the analyte by the reference or vice versa. Therefore, a separate inlet to introduce the calibration compound is advantageous. This can be achieved by introducing volatile standards such as PFK from a reference inlet system in electron ionization, by use of a dual-target probe in fast atom bombardment, or by use of a second sprayer in electrospray ionization. 

Fig. 5.13. Ion source housing of a Autospec magnetic sector instrament. The ion source can be accessed from several directions to allow for simultaneous connection to DIP/DEP, GC and reference inlet system. By courtesy of Waters Corporation, MS Technologies, Manchester, UK.

Aliphatic alcohols show a strong tendency to thermally eliminate a water molecule. This is of special relevance if volatile alkanols are introduced via the reference inlet system or by means of a gas chromatograph. Then, the mass spectra correspond to the respective alkenes rather than to the alkanols that were intended to be analyzed. The water is often not detected, simply because mass spectra are frequently acquired starting from m/z 40 to omit background from residual air. 

A reference compound is leaked continuously from the reference inlet system during the GC run into the ion source. Typically perfluoro-tributylamine (FC43) is used as reference compound in HRGC-HRMS for dioxin analysis. Other reference compounds may be used to suit individual experimental conditions. 

Perfluoroalkane-225 (PGR, Gainesville, FL) was admitted through a glass inlet system to provide reference peaks. Analytical and reference peaks for the nitrosamines studied are shown in Table I. Sample and reference peaks were scanned alternately at a repetition rate of approximately 1 sec and were monitored on an oscilloscope. When the nitrosamine peak appeared, the oscillographic recorder chart drive was engaged and remained on until the peak disappeared. Nitrosamine quantities were estimated by comparing the sum of sample peak heights measured from the chart (usually 10 to 20 values) with values derived from injection of standard solutions. 

To relate the difference between the areas A and B with the enthalpy of reaction 9.10, a calibration had to be performed. As discussed, this calibration can be made by generating a known amount of heat in a resistor in the reaction vessel. Alternatively, we can make a chemical calibration, involving a procedure that mimics the main experiment. For instance, if a known mass of iodine is dropped through the inlet system (and a similar empty capillary is dropped into the reference cell), the thermogram of the following process can be recorded ... 

Many manufacturers now offer other sample injection systems compatible with the vacuum lock used for the solids probe. These include small (e.g., 75-ml) heatable batch inlet systems, usually accessible via syringe (gas syringe or GC microliter syringe for liquids), which can be particularly useful as inlets for mass reference compounds. Other probes are designed as flexible, easily removed connections to a gas chromatograph via some form of interface. 

Several published reports review the basic types of inlets available for capillary gas chromatography [45-50]. Reference 41 can be a useful guide to the proper selection of an inlet system and the parameters for its optimum performance (Table 4.1). 

Both FAB and LSIMS can be used in conjunction with continuous-flow inlet systems to optimise sensitivity, to improve reproducibility and to allow the use of in-line liquid chromatography for LC-MS. In the flow technique an LC pump delivers a continuous flow of solvent (containing a low concentration of the FAB/LSIMS matrix) to the point where the atom or ion beam is applied. The sample is introduced into this solvent flow by an LC injection valve and is delivered to the point of ionisation in a sharp concentrated slug. A good account of the method of continuous flow FAB is given in reference [5]. 

Bombick et al. [3] presented a simple, low cost method for producing thermal potassium metal ions for use as Cl reagents. All studies were performed on a commercial gas chromatography-mass spectrometiy (GC-MS) system. Thermionic emitters of a mixture of silica gel and potassium salts were mounted on a fabricated probe assembly and inserted into the Cl volume of the ion source through the direct insertion probe inlet. Since adduct ions (also referred to as cationized molecular ions or pseudomolecular ion ) of the type (M + K)+ have been observed, molecular weight information is easily obtained. The method is adaptable to any mass spectrometer with a Cl source and direct inlet probe (DIP). In addition, the technique is compatible with chromatographic inlet systems, i.e., GC-MS modes, which will provide additional dimensions of mass spectral information. 

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.

Stainless steel capillaries (1 m length and 0.1 to 0.2 mm i.d.) transport the gas from the variable volumes to the ion source. The changeover valve alternately connects the reference and sample bellows to either the ion source or vacuum and the isotope compositions of the two gases are measured in turn. This method ensures a constant flow of gas through the capillaries at all times. To maintain viscous flow conditions, a pressure of 20 mbar in the inlet system is required. This constitutes a lower limit of gas suitable for analysis with the dual inlet system. The smallest practical volume of an inlet system is -250 pL. This represents 200 nmol of gas at 20 mbar. In some cases, there is not enough sample material available to produce sufficient quantities of gas and alternative analytical methods must be employed, e.g. isotope ratio monitoring. 

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