The gas inlet system

Total pressure, required for detailed interpretation of the mass spectra, is determined with an ionization gauge (S). The gas inlet system (A, B, C) is used for calibration purposes. The relation between measured total pressure and the ion current of an injected specific gas permits calibration of the mass spectrometer in absolute partial pressure units or amps/torr. 

Equip a 2-litre three-necked flask as above the solids addition system is not required but the gas inlet system described below should be assembled for use in the latter stages of the experiment. An appropriate water bath should be used for heating. Place 37.5 g (1.54 mol) of washed and dried magnesium turnings and 300 ml of sodium dried ether in the flask and add a small crystal of iodine. Prepare a solution of 205.5 g (161 ml, 1.50 mol) of pure dry butyl bromide (Expt 5.54) in 300 ml of dry ether in the separatory funnel and introduce about 25 ml of the solution into the flask. As soon as the reaction commences (disappearance of the iodine colour), set the stirrer in motion and add... 

Figure 1 is a flow diagram of the catalyst testing unit. The gas inlet system and the components operating at elevated temperatures and pressures are stainless steel. Feed gas, stored in high pressure cylinders, is metered by an orifice. Orifice pressure, pressure drop, and temperature are recorded. The feed gas is preheated before it enters the bottom of the reactor. Both the reactor and the preheater are electrically heated. 

The position of the pumping intakes is important to obtaining a uniform gas distribution in the processing chamber. At pressures above a few mTorr, the chamber volume in front of the pump intake can have an appreciably lower pressure than elsewhere in the chamber. This is particularly true if the chamber is crowded with fixtures and the gas inlet system is not properly designed. These pressure differentials can affect the deposition process. 

Methylsulfinyl carbanion (dimsyl ion) is prepared from 0.10 mole of sodium hydride in 50 ml of dimethyl sulfoxide under a nitrogen atmosphere as described in Chapter 10, Section III. The solution is diluted by the addition of 50 ml of dry THF and a small amount (1-10 mg) of triphenylmethane is added to act as an indicator. (The red color produced by triphenylmethyl carbanion is discharged when the dimsylsodium is consumed.) Acetylene (purified as described in Chapter 14, Section I) is introduced into the system with stirring through a gas inlet tube until the formation of sodium acetylide is complete, as indicated by disappearance of the red color. The gas inlet tube is replaced by a dropping funnel and a solution of 0.10 mole of the substrate in 20 ml of dry THF is added with stirring at room temperature over a period of about 1 hour. In the case of ethynylation of carbonyl compounds (given below), the solution is then cautiously treated with 6 g (0.11 mole) of ammonium chloride. The reaction mixture is then diluted with 500 ml of water, and the aqueous solution is extracted three times with 150-ml portions of ether. The ether solution is dried (sodium sulfate), the ether is removed (rotary evaporator), and the residue is fractionally distilled under reduced pressure to yield the ethynyl alcohol. 

In gas dispersion systems the gas inlet should normally be direcdy below the impeller inlet, or on a circular pattern at the periphery of the impeller. 

A second gas inlet system (M, N, P) serves to set starting conditions and to vent the system after a test. 

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. 

Between the early 1950s, when the dual viscous-flow mass spectrometer was introduced by Nier and the mid 1980s only minor modifications have been made on the hardware of commercial mass spectrometers. Special efforts have been undertaken in the past years to reduce the sample size for isotope measurements. This has led to a modification of the classic dual inlet technique to the continuous-flow isotope ratio monitoring mass spectrometer in which the gas to be analyzed is a trace gas in a stream of carrier gas, which achieves viscous-flow conditions. Today, the majority of gas mass spectrometers are sold with the continuous flow system, instead of the dual inlet system. 

The coupling of a GLC column with the sample inlet system of a mass spectrometer is relatively easy, as the effluents are already in gaseous form. The main problem is the relatively high pressure at which these effluents reach the spectrometer and the excess of carrier gas in the stream. Several experimental devices now allow separation of the sample from the carrier gas, either by an effusion process or with the help of a thin, semi-permeable membrane222,353. The use of capillary columns permits direct insertion of the GLC effluent into the ion source without overtaxing the pumping capacity of the mass spectrometer 311 3 5 5 >3 5 6. 

---