1.2. F ORMULACIÓN DEL PROBLEMA
1.4.5. Geosintéticos
Experiments were carried out over a wide pressure range and the spectra were recorded after each increase in pressure. Care was taken to manually scan the spectrum before recording it. This allowed the correct attenuation to be selected and if possible all product peaks remained on scale, thus making data interpretation easier. The primary ion peak was always off scale during the recording. At the end of each run the peak was attenuated until it was on scale and a recording made.
Great care was taken when calibrating spectra as a mistake in m arking the peaks would make all the assignm ents incorrect. It was essential to calibrate at least in duplicate to ensure correct peak assignment. Great care was also taken when calibrating peaks with m/e>100. It is known that the linearity is lost above m/e 100 due to non linearity of the precision rectifier. The amount of each ion present in
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as tetrachloromethane, methanal and the alcohols tended to condense at the needle valve. This resulted in erratic
pressure fluctuations during the course of an experiment. It i was found that the use of a heating coil wrapped around the
needle valve at a tem perature of approxim ately 40® C m inim ized this problem . H ow ever the warming of the needle valve was undesirable.
the spectrum was determined by measuring the peak height and multiplying by the attenuation. The percentage of each ion was then calculated using the following formula:
For peak one:
%
Total
= Peak height x Attenuationfpeak 11ion flux
peak height x Attenuation(peaks l,2,3..nincluding the primary ion.) This gives a measure of the extent of the reaction:
% Secondary
= peak height x A ttenuation!peak 11ion flux
peak height x A ttenuation(peaksl,2,3..n excluding the primary ion).This gives a m easure of the relative am ount of each individual product ion. A graph of secondary ion current against pressure for all the fragments can then be plotted; a typical plot is shown in graph 2.1, When examining a particular series of ion molecule reactions the primary ion and target gas pressures at which spectra were recorded were as sim ilar as possible to allow direct comparison between systems to be made.
44 I 10 1 4 - 3 5 pressure increase/ x 10 7 1 mbar 6 9
G raph 2.1. Typical plot of pressure increase against ion current for an ion molecule reaction.
It can be seen that an indication of reaction pathw ays leading to secondary and tertiary product ions can be gained from this plot. Taking the above example, as the pressure of target gas is increased the amount of ion of m/e 69 increases and correspondingly the amount of ion of m/e 28 decreases. It can therefore be inferred that ion 69 is derived in some way from ion 28. Further evidence that ion of mass 69 is derived from the reaction of the ion 28 with the neutral gas can be obtained using the following procedure; S O U R C E . QUAD ONE. QUAD TW O . QUAD THREE. GAS X . ION x+ . T arget gas . PRODUCT IONS
Using this set up the the ion m/e 28 is generated in the source chamber by electron impact and selected in the first quadrupole. It is then reacted with the target gas in the second quadrupole and the fragments analysed in the third. If ion m/e 69 is present in the resulting spectrum it is further evidence to suggest that in the particular reaction ion m/e 69 is derived from the reaction of ion m/e 28 with the target gas. It is how ever not unam biguous and information about the relative contribution of the reaction of ion 28 with neutral gas to the total ion flux of 69 cannot be obtained.
2.4 Instrumental problems.
A great advantage of the trip le quadrupole m ass sp ectro m eter over other m ass sp ectro m eters is its durability. Only m inor instrum ental problem s w ere encountered, the m ost common being filam ent burn out (about every 6 weeks) and occasional leaks in the vacuum system .
The other problem s encountered using the triple quadrupole m ass spectrom eter were in the main easily rectified. However it was found that the following points had to be borne in mind for accurate results
A. It was found that the best results were obtained when operating the instrum ent at maximum total pressures of the
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order of 5x1 0“^ mbar. Occasionally reactions were studied at higher pressures above 1x10”^ mbar. In some cases this resulted in the drown out of signals; signal heights dim inished and the m ultiplier began to give spurious resu lts.
B. In order to observe all product ions the instrument was operated at a relatively high gain when recording spectra (1 x 1 0 ’^ A ). Any ions present in a spectrum which were off scale were recorded at a different gain setting.
C . The vacuum line was completely dismantled and cleaned at regular intervals to avoid the build up of vacuum grease in the system, since this build up was found to retain a "memory" of previous gases used. All taps were cleaned and replaced. The ion source was also cleaned periodically when replacing the filament. Another area where system contamination occured was at the needle valves. Build up of grease at the taps resulted in erratic pressure fluctuations. This was rectified by periodically stripping the taps and cleaning the needle valves.
D . At regular intervals the instrument was purged with nitrogen to "flush" out any waste gases remaining.
E . The rotary pumps were dismantled, cleaned and re charged with oil at regular intervals. They were also regularly ballasted.
The triple quadrupole mass spectrometer is shown in figure 2.3 and photograph 1.
primary ion source gas i n l e t '^ N2 cold trap p u m p s p enning h ead
h
quadrupole sy stem target gas in le t controlF ig u re 2 .3 Overview of the triple quadrupole mass spectrom eter including the pumping svstem.
1
1
I
□
quadrupole mass spectrometer.
The reactions of CX+ and CX2+ ions (where x=H, Cl or F) with linear alkanes and alkenes.
1
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3.The reactions of CX+ and CXi+ where X = H, Cl or F with linear alkanes#