Residence time in the ion source

Experimental evidence for the distortion of a kinetic growth curve due to a closed ion source was reported by Martinez et al. [42]. In an FPTRMS investigation of the infrared laser multiphoton dissociation of CF2HC1, these authors found that the risetime of the HC1 molecular elimination product, which was expected to be formed on a microsecond time scale, was approximately 2 ms. This they attributed to holdup in the ion source, which was a partially enclosed box. Gas entered the ion source chamber through a hole at one end and exited through several holes at either end or at its sides. 

It is useful to examine the consequences of a closed ion source on kinetics measurements. We approach this with a simple mathematical model from which it is possible to make quantitative estimates of the distortion of concentration-time curves due to the ion source residence time. The ion source pressure is normally low enough that flow through it is in the Knudsen regime where all collisions are with the walls, backmixing is complete, and the source can be treated as a continuous stirred tank reactor (CSTR). The isothermal mole balance with a first-order reaction occurring in the source can be written as 

When 0 k, Eq. 16 reduces to ca(t) =ct0e kt, and the kinetics can be observed unperturbed by the source residence time. However, when this inequality is not fulfilled, (16) predicts an approximately exponential rise of c, which passes through a maximum and then decays away. If 0 is known or can be determined, then (16) can be used to fit data and extract the first-order rate coefficient. The value of 0 could be experimentally determined by introducing an unreactive species into the ion source as a step function. The transient response is given by 

The concentration step might be generated by rapidly introducing a conveniently detectable species into the flow through the reactor, or by a photoreaction that produces a stable molecular product that can be readily determined with the mass spectrometer. Fitting the exponential rise of Eq. 

FLASH PHOTOLYSIS WITH TIME-RESOLVED MASS SPECTROMETRY 

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The sample pressure, electron beam current, auxiliary magnetic field, and ion residence time in the ion source are not stated and cannot be inferred from the data given. As will be seen below, the details of the ion source are quite important in the production of doubly-charged negative ions. 

Low-pressnre Cl (<0.1 Pa) can only be performed in systems that allow for an elongated sample residence time in the ion source, e.g., in ion-cyclotron resonance and ion-trap cells. Low-pressure Cl is hardly used in LC-MS. 

A time-of-flight mass spectrometer (TOFMS) is an excellent instrument for these studies. The TOFMS produces approximately 10 spectra per sec. It is difficult to record individual mass spectra at this rate, but several data acquisition schemes see below) have been developed which record a finite number of discrete spectra. These techniques follow changes in peak intensity over millisecond intervals, thus recording the mass spectra of compounds whose residence time in the ion source is short. 

One of the most intensively examined type of pesticides handled under PBl conditions were the chlorinated phenoxy acids and their esters which were determined in water [82-86] and soil samples [83, 86]. Even results of an interlaboratory comparison study of 10 chlorinated phenoxy acids using PBl or TSP ionisation were published by Jones et al. [37] [87]. Statistically significant differences were observed between the interfaces and under these conditions PBl was found to have a better precision than TSP [87]. Betowsld et al. [88] observed thermal degradation induced by residence time in the ion source and the influence of ion source temperature in the ionisation of the chlorinated phenoxy acid derivatives 2,4-D and MCPA. Several authors successfully examined a large number of different carbamate pesticides [67, 89, 90] and their transformation products by PBl-LC-MS [89, 90], by PBl-FIA-MS (flow injection analysis) [67] or by supercritical fluid chromatography (SEC) PBl-interfaced to MS [91],... 

Since m/z 149 is produced from m/z 205, as the temperature is increased one would expect to observe an increase in the abundance of m/z 149 and a decrease in the abundance of m/z 205. The cations residence time in the ion source are all decreased at higher temperatures. In addition to the effect of... 

Typical results for these three collision mechanisms are shown in Figure 3 where the relative intensities of the primary, secondary, and tertiary ions are plotted against N, the concentration of molecules in the source. In deriving these curves, the parameters used were kp = 2.0 X 10 9 cc./molecule-sec. k8 = 1.0 X 10 9 cc./molecule-sec. tp = 8.5 X 10 7 sec., (the residence time of the ion (jn/e — 33) in a field of strength 9.1 volts/cm. in the Leeds mass spectrometer). In applying this analysis to a system in which the tertiary ion reacts to form quaternary and higher order ions, ITtotal represents the sum of tertiaries, quaternaries, etc. 

Pyridine has also recently been studied in the special type of threshold PIPECO experiment (see Sect. 4.1.1), in which the residence time of the ion in the source is varied [719]. In terms of eqn. (9), the limit t2 on the sampling interval or observation window was variable. Breakdown diagrams for formation of (C4H4)t were determined at two residence times (t2 = 0.805 and 5.925 ps). Detailed analysis of the results suggested that the k(E) vs. E curve for this decomposition was somewhat steeper than indicated by the earlier PIPECO study [274], It was concluded that the transition state for... 

The decompositions of bromobenzene [717] and chlorobenzene ions [716] have been studied by the special PIPECO experiment using variable source residence times. In the case of chlorobenzene, increasing the residence time from 0.7 to 8.9 ps resulted in a shift (kinetic shift) in the breakdown curves by 0.4 eV. Detailed analysis of the effects of varying residence time provided information on the k(E) vs. E curve in the vicinity of 104—106 s-1. The k(E) vs. E curve obtained differed significantly [by almost an order of magnitude in k(E) at some energies] from the curve reported in the earlier PIPECO study of metastable ions [22], The initial analysis [716] placed the critical energy for chlorine loss at 3.40 0.05 eV, but this has subsequently been revised to 3.19 0.02 eV [717]. The transition state was found to be loose . 

The time advantage of MS/MS also is exemplified by the ability to select from a mixture of ions in the source any parent ion, in any order, and to return as necessary to that parent ion for precise measurements. This independence of access persists for the duration of the sample residence time in the source. This is in marked contrast to the situation in GC/MS, where each sample is available for examination by the mass spectrometer only during the retention time window. To repeat the measurement, the entire sample must be reinjected. The source residence time for most samples introduced via the direct insertion pjrbbe is on the order of a minute or two, depending on the temperature of the source and the rate of heating of the prcbe tip itself. 

Integral Mass Spectra of LC-GPC Subfractions. The heavy oil from coal liquids consists of numerous components with wide differences in boiling point. The size of molecular ions changes progressively with the GC retention time or with the residence of the sample in the ion source for the direct injection method. Mass spectra were measured repeatedly at short interval times. The sum of these spectra represents the whole... 

The structure of unknown components is far easier to elucidate by examination of both, El- and Cl- mass spectra. This is typical for many components and implies that no one ionization technique is always superior to another. In order to achieve quantitative data, the abundance of ions has to be measured, since in an ionized compound, the absolute abundance of any of its ions is related to the quantity of that substance. During the residence of the compound in the ion source, repetitive scanning with a mass spectrometer can provide several complete mass spectra. Eor a typical total ion chromatogram (TIC), in which mass spectra may be recorded every second, any single m/e would be sampled 5-30 times, being focused on the electron multiplier detection system each time for a few microseconds only. 

Ammonia is the primary basic gas in the atmosphere and, after N2 and N20, is the most abundant nitrogen-containing compound in the atmosphere. The significant sources of NH3 are animal waste, ammonification of humus followed by emission from soils, losses of NH3-based fertilizers from soils, and industrial emissions (Table 2.8). The ammonium (NH ) ion is an important component of the continental tropospheric aerosol. Because NH3 is readily absorbed by surfaces such as water and soil, its residence time in the lower atmosphere is estimated to be quite short, about 10 days. Wet and dry deposition of NH3 are the main atmospheric removal mechanisms for NH3. In fact, deposition of atmospheric NH3 and NH4" may represent an important nutrient to the biosphere in some areas. Atmospheric concentrations of NH3 are quite variable, depending on proximity to a source-rich region. NH3 mixing ratios over continents range typically between 0.1 and lOppb. 

While ions containing only vibrational and rotational excitation retain this energy in the absence of collisions for times that are long (> 10 sec) compared with typical ion residence times in the usual type of ion sources (e.g., in sources such as those shown in Fig. 1), this is very often not true for electronically excited ions—particularly for those formed by photo-... 

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