Gas chromatographic inlet

Matrix-assisted laser desorption ionization MALDI 

Many different techniques have been developed to effect the production of ions, the most common of which are shown in Table 1.4. Mass spectra that are generated by electron (El) are discussed in detail in Chapter 2. Some of the other ionization modes are discussed in Chapter 4. 

Ions may be positively charged by the removal of one or more electrons or negatively charged by the addition of one or more electrons. In addition, smaller mass ions are formed by various fragmentation processes discussed in the next chapter. 

These ions are then accelerated by electrostatic fields into a mass filter to be separated and detected. The entire process takes place under high vacuum to minimize collisions between ions. 

The ions formed in the mass spectrometer are separated according to their masses or, more correctly, according to their mass-to-charge ratios (m/z), as shown in Equation 1.3  

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Because differential mobility spectrometers like IMS analyzers function at ambient pressure in air or nitrogen, combinations with gas chromatograph inlets are technically... 

The most common method of introducing liquid samples into a gas chromatographic inlet is by means of a microsyringe. Typically, this consists of a calibrated glass barrel... 

Methyl esterification of vegetable oils for GC analysis (12) was carried out as follows Sodium (0.5 g) was dissolved in 100 ml methanol. The sodium methoxide solution (0.3 g) together with 2 g of the specific vegetable oil was placed in a vial and heat sealed. The heat sealed vial was left for 2 hours at 85-90 °C in an oil bath, with occasionally shaking. GC analysis of macadamia nut and avocado pear oil samples was carried out to determine their fatty acid content in order to compare with the C NMR spectroscopy data obtained. 20 pi of each sample was diluted with 1 ml of dichloromethane and 1 pi of the solution was inserted in the gas chromatograph inlet. Analysis was performed on a HP 5890 Series 2 Gas Chromatograph. The column employed was a fused silica capillary (30 m x 0.25 mm i.d., 0.2 mm film-thickness) coated with a 100% cyanopropylpolysiloxane non-bonded phase. The column temperature was programmed from 40 to 240 C at a rate of 4 C/min. 

When a direct inlet system is used, spectra can be obtained of the free bile acids. This is not possible with a gas chromatographic inlet. The simplest derivative which can be analyzed with the combination instrument is the methyl ester. Valuable information on the nature of the fragment ions can be obtained by analysis of both the methyl and ethyl esters. Particularly for gas chromatographic reasons it is better, however, to protect hydroxyl groups by acetylation, trifluoroacetylation, or trimethylsilylation. 

A solution to this problem is to replace the electronic pressure controller with a low-dead-volume computer-controlled valve and a source of carrier gas at some preset pressure. Normally, the valve is closed, and the column junction-point pressure is the natural pressure that occurs at the column junction point in the absence of any additional connections. When the valve is opened, the junction-point pressure assumes the preset value of the additional carrier gas source. Usually, the valve is open for only a few seconds, to enhance the resolution of a particular component pair. Thus, the carrier gas in the two columns undergoes a pulsed-flow modulation (28,29). A particularly attractive version of pulsed-fiow modulation uses the gas chromatographic inlet pressure as the preset pressure (30,31). Thus, when the valve is opened, both ends of column Ca are at the same pressure, and carrier gas flow in Ca stops (stop-flow operation). Stop-flow operation is used to enhance the resolution of a targeted component pair without significantly changing the elution pattern and resolution of other components in the mixture. The concept is illustrated by the band trajectory plots shown in Figure 4.6 for a pair of components labeled 1 and 2 that are completely separated by the first column but coelute from the column ensemble. The solid-line plots are for the case without a stop-flow pulse, and the dashed-line plots for the case with a 5-s-wide pulse occurring at the time indicated by the vertical lines. 

All the samples were submitted to a combined gas chromatograph-mass spectrometer (LKB 9000), and both the direct and gas chromatograph inlets were used for these analyses. The samples were compared with equivalent amounts of methylmercury standards. 

Westland, J., Organtini, K., Dorman, F. (2012) Evaluation of lifetime and analytical performance of gas chromatographic inlet septa for analysis of reactive semivolatile organic compounds. /. Chromatogr. A., 1239, 72-77. 

Microlitre syringes, automatic syringe injectors, and liquid sampling valves have been used successfully for introducing representative samples into the gas chromatographic inlet. 

Sample Introduction System, capable of introducing a representative sample into the gas chromatographic inlet. Microlitre syringes and automatic syringe injectors have been used successfully. 

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