Source conditions

Figure 9. Action spectra acquired in the F Cl B—X, 3-0 spectral region and with the probe laser tuned to the F Cl E—B, 9-1 transition. Both spectra were recorded using the same source conditions, but with the lasers intersecting the expansion at Z = 8.8, (a), and Z = 19.1, (b). Monomer rotational temperatures of 2.34(3) K and 1.09(10) K were measured at the two distances [62].

For each of the hexanes, more than one singly substituted I3C positional isomer is possible there are five for 2-methylpentane, four for 3-methyl-pentane, and three for n-hexane. Clearly, if only Re, Rs, and R are determined, so that only o5 and o4 are available, Eqs. (5) cannot give the proportions of all the positional isomers in 2- and 3-methylpentane. This could, in principle, be achieved by extending (5) to include a3 and 02. However, it turns out that a2 is very dependent on source conditions, and thus is unreliable, while, particularly with 3-methylpentane, 03 is very insensitive to positional isomer composition. The result, in practice, is that one is limited to determining... 

In the case of n-hexane, the three positional isomers can, in principle be determined from the three equations in (5). However, the results in this case are very sensitive to ion source conditions, presumably because of ion rearrangement processes, so the evaluated proportions are subject to greater errors than with 2- or 3-methylpentane. 

Other source conditions. The source height and the source area are also important source characteristics. The greatest impact is typically associated with ground-level sources, so elevated sources are not considered in the model discussion below. 

To achieve a more stable mode of operation, in modem instruments the heating current for the filament is emission-controlled, i.e., the current of the electron trap is used to keep emission comparatively independent from actual ion source conditions. Typical emission currents are in the range of 50-400 pA. 

In Figure 1 are shown three negative ion APIMS spectra of perfluoromethylcy-clohexane (C7F,4) that were obtained under three different ion source conditions. It is well known that perfluorinated molecules of this size attach thermal electrons very rapidly to form molecular anions by the resonance electron capture (EC) mechanism (e + M M"), and this is what is observed in Figure la where pure nitrogen buffer gas has been used. In the use of certain analytical instruments for the detection of such compounds, however, protic molecules such as water or... 

For this purpose, the flow tube emptied directly into a high-pressure ion source. This source was essentially a sealed box with a gas inlet for the Cl reagent gas, a 0.58 mm hole to allow injection of a magnetically collimated electron beam, and a 0.99 mm hole to allow ions to exit into the mass spectrometer. The flow tube was coupled to the source using a 0.1 mm annular gap that thermally isolates the source from the flow tube, but allows little of the gas flow to escape. Even at a flow tube temperature of 1000 K, the source temperature increased no more than 50 K. To avoid any variations in source conditions with flow tube temperature, the source was thermostated to a constant temperature of 100 K. 

An alternative to observing sample and reference together at high resolution involves the use of a double-beam mass spectrometer such as the AEI MS-30. If closely spaced mass multiplets need not be resolved, one can determine the exact mass of each peak in the spectrum at resolutions of 1500, the computer acquiring data from the independent sample and reference beams simultaneously (226). Thus, source conditions can be optimized for compound and reference at the higher sensitivities available at lower resolutions, the mass measuring accuracy still being in the order of 10 ppm. This is possible, since if mass doublets are not involved, the centroid of a peak is as well defined at resolution 1500 as at 10,000. 

Equation (31) is directly consistent with the magnetic source condition... 

Name Source Conditions Nature Action Pure tension stoff... 

Source conditions for both positive and negative ion mass spectrometry are usually discussed in terms of source pressure and style of ionization. The source conditions that have been used for negative ion formation are in Table I. 

Meroney and Neff 1984 To calculate the behavior of heavy and cold gas clouds subject to dilution by a water spray curtain, using a numerical model. Work included model validation. Variations in curtain position, spray entrainment velocity, wind speed and source conditions resulted in a wide range of spray-curtain performances. 

Visible chemiluminescence was observed from the electronically excited HNO product of the crossed-molecular beam reaction of H + (NO)2 [277]. Variation of the nitric oxide beam source conditions confirmed that the reaction was with dimers, (NO)2, rather than with NO. The cross section for chemiluminescence was estimated to be in the range 3.5—9A2. Emission was seen from the (010), (020), (001) and (100) levels of HNO ( A"). 

Graedel TE, Farrow LA, Weber TA. 1978. Urban kinetic chemical calculations with altered source conditions. Atmos Environ 12 1403-1412. 

Single, double and triple filaments have been broadly used in thermal ionization sources. In a single filament source, the evaporation and ionization process of the sample are carried out on the same filament surface. Using a double filament source, the sample is placed on one filament used for the evaporation while the second filament is left free for ionization. In this way, it is possible to set the sample evaporation rate and ionization temperature independently, thus separating the evaporation from the ionization process. This is interesting when the vapour pressure of the studied elements reaches high values before a suitable ionization temperature can be achieved. A triple filament source can be useful to obtain a direct comparison of two different samples under the same source conditions. 

The ion guide produces an ion beam independently of the original parameters of the beam delivered by the source and with highly favourable properties for the oa-TOF mass spectrometer. As a result of the decoupling of the ion source and mass analyser, the performance of the instrument is independent of source conditions and this leads to improved resolution and sensitivity. 

The errors due to the mass spectrometer are also numerous, such as the variation in the source conditions and the instability of the mass scale. For statistical reasons, every measurement of a signal intensity carries a minimal intrinsic error. This error is inversely proportional to the square root of the number of ions detected for that signal. In order to optimize the reproducibility or the precision of the measurements, a maximum number of ions must thus be detected for every ionic species. 

This method is based on a comparison of the intensities of the signal corresponding to the product that has to be quantified with the one of a reference compound called the internal standard. This method allows the elimination of various error sources other than the minimal intrinsic error due to statistical reasons. In fact, if we choose as an internal standard a molecule with chemical and physical properties as close as possible to the properties of the molecule to be measured, the latter and the internal standard undergo the same loss in the extraction steps and in the derivative or the same errors in the introduction of the sample into the mass spectrometer, when the source conditions are varied. As both... 

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