Ciencias conexas de la pedagogía

Mass spectrometry (MS) is a highly sensitive analytical technique used to determine the chemical composition of a substance. It can be used for volatile liquids and gases and can be used for time resolved qualitative and quantitative analysis. In the mass spectrometer an ionised species is detected based on its mass to charge ratio, m/z. A mass spectrometer consists of 3 sections, 1) an ion source, 2) mass separation, and 3) species detection. The mass spectrometer used in this thesis is a Hiden Quantitative Gas Analyser (QGA), which is a type of quadrupole mass spectrometer. An image of the 3 mass spectrometer components that are essential for species detection in the QGA are shown in figure Figure 46.

Figure 46: Mass spectrometer components used for species detection found within the Hiden QGA (Image taken from [111])

Plasma reactor

Mass flow

controllers FTIR

Oxygen _{Chemical Test} Ozone

The mass spectrometer generates a constant stream of charged particles with an ion source, by electron impact ionsation, at the gas inlet to the MS. The ionised particles pass into a quadropole, which consists of 4 long, parallel, cylindrical rods to which a radiofrequency (RF) with a superimposed DC electric field is applied. The direction of motion of the ionised particles is affected by the applied field. By tuning the voltage applied to the quadropole, ions with a specific mass to charge ratio can be allowed to pass to the detector. In the QGA, two different detectors are available, a Faraday detector and a multiplier detector. In each type of detector, when an ion hits the detector it generates a current which is detected by an amplifier. The magnitude of the current is directly proportional to the number of ions striking the detector. In a multiplier detector, the ions striking the detector initiate electron avalanches that effectively magnify the amplitude of the current generated in the detector per incoming ion.

The mass spectrometer operates at a reduced pressure, 3×10-6 _{torr, in order for ionised species}

entering the MS to have a sufficiently long mean free path to not collide with another molecule before reaching the detector. However, due to natural variation in atmospheric pressure, temperature, and cycling of the external pump, the operating pressure of the MS is prone to fluctuation. The concentration of a detected species, determined by its mass to charge ratio, is expressed in terms of partial pressure relative to the QGA operating pressure. Consequently, in order to quantitatively use the mass spectrometer, the concentration of the species of interest must be measured relative to a species with a fixed concentration, i.e. the use of an internal standard is essential. It is desirable for the internal standard used in an MS experiment to have three properties, 1) for it to be monatomic, 2) for its principal ion to have a mass to charge ratio within the same order of magnitude of the species of interest, and 3) for its characteristic m/z signal to not overlap with the species of interest. The carrier gas used in this thesis is Argon, as it fulfills these 3 criteria. The reasons for these criteria is because the m/z peaks detected in a mass spectrometer can appear to be very complex, even in a system with very components. The fragmentation pattern obtained from argon is shown in Figure 47.

Figure 47: Argon QGA fragmentation pattern. Instrument settings used: Electron energy – 70 eV, Emission - 250 µA, Focus – 90 V. Peaks occur at m/z = 40, 36, 20 and 18 due to the 2 most

common isotopes, 40_{Ar and} 36_{Ar, and the respective single and double ionised forms.}

Consider the species that are expected from the experiment, CO, CO2, Ar, O2 and O3. Also

consider that due to the high sensitivity of the mass spectrometer, and the difficulty in completely isolating external gas leaks into the system, that the MS will also detect small concentrations of N2 and water vapour from atmospheric sources. If we include only the

possibility of single ionisation events, with only the most common isotopes considered, we would have 7 different species with 6 independent m/z peaks. This number is reduced to 6 different species with 5 independent peaks, as O3 (which would be found at m/z = 48) cannot be

detected due to its instability. The overlapping peaks are due to CO and N2 sharing a peak at m/z

= 28. This result is already problematic, as CO is the species that we are most interested in detecting. However, this problem is further compounded when we consider the possibility of species cracking, double ionisation events and isotopes. Taking CO2 as an example, the following

cracking reactions are possible:

12_C16_O

2 + e- è 12C16O2+ + 2e- (Peak at m/z = 44)

12_C16_O

2 + e- è 12C16O+ + O + 2e- (Peak at m/z = 28, and formation of unstable O) 12_C16_O

2 + e- è 12C16O + O+ + 2e- (Peak at m/z = 16, and formation of unstable ionised O)

This can also lead to recombination reactions of unstable species:

16_{O +} 16_O+ _è 16_O

2+ (Peak at m/z = 32)

and considering the isotope 13_C:

13_C16_O 2 + e- è 13C16O2+ + 2e- (Peak at m/z = 45) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 10-8 10-7 10-6 10-5 10-8 10-7 10-6 10-5

Mass to charge ratio, m/z

Count

40_{Ar + e}- _è 40_Ar+ _{+ 2e- (Peak at m/z = 40)}

40_{Ar + e}- _è 40_Ar2+ _{+ 3e- (Peak at m/z = 20)}

These examples show how a single species can create multiple peaks. Taking a closer look at the CO2 example, and comparing to the species that we are interested in detecting, CO2 not only

generates a peak at m/z = 44, but also at m/z = 28 and 32, the expected characteristic peaks for nitrogen and carbon monoxide (m/z = 28), and oxygen (m/z = 32) respectively. Although this overlap appears to be a significant problem, in most cases it can be compensated for by considering the relative contribution to any m/z peak for each non-ionised species that is present. Peaks caused by cracking of molecules, double ionization events and isotopes are typically relatively much smaller than the main peak, with their amplitude proportional to the main peak, and therefore predictable. Shown in Figure 48 is the cracking pattern for pure CO2

generated in the QGA, with the electron energy at the ion source set to 70 eV. The possible ions that are causing each peak are labeled.

Figure 48: Pure CO2 cracking pattern obtained from the Hiden QGA. Instrument settings used:

Electron energy – 70 eV, Emission - 250 µA, Focus – 90 V.

The height of each peak is directly proportional to the concentration of ions generated by the fragmentation of CO2 as it passes through the ion source. As expected, CO2+ ions generate the

largest peak (5.30 × 10-6_{), with} 16_O+ _{(4.27 × 10}-7_{) and CO}+ _{(3.36 × 10}-7_{) ions 2}nd _{and 3}rd _largest

peaks respectively. This indicates that for an ion source electron energy of 70 eV, using a pure CO2 stream, that the peak height at m/z = 16 and 28 will be 8.0% and 6.3% of the peak height at

m/z = 44 respectively. Also worth noting is that the peak at m/z = 32, the same expected peak as

oxygen, has a height of 3.00 × 10-8_{, which equates to 0.57% of the peak height at m/z = 44. The}

contribution of CO2 to these peaks could theoretically be accounted for in the mass spectrometer

calibration methodology, as no other species in the gas mixture would have a peak at m/z = 44, which would make removal of secondary CO2 peaks relatively straightforward.

However, the relationship between fragmentation pattern characteristic peak heights of any given compound is non-linear in the presence of other compounds, particularly noble gases. This can be explained by considering that the ion source to the QGA is a non-equilbrium plasma, and therefore similar non-linear behavior that is observed with different gas mixtures in the laboratory plasma also occur in the ion source. In experiments where the gas concentration of noble gases is kept relatively constant, it is reasonable to approximate that the relationship between the heights of the secondary m/z peaks relative to the primary peak is constant. This allows the contribution of the secondary peaks to be accounted for, as in most cases the variation in relative secondary peak height is insignificant compared with the height of a primary peak found at the same m/z.

There are two potential methods to compensate for these problems in non-linearity in the method of measurement:

1. Measure fragmentation patterns of different compounds to be detected for a range of different gas compositions that are similar to those that are expected to be found experimentally. Identify overlapping peaks, and mathematically account for the relative contributions of secondary fragments to primary peaks in order to determine species concentration. This method requires extensive preparation prior to experimental work. 2. Prior to an experimental run, feed two different gas compositions through the reactor to

the mass spectrometer. One with a gas composition consisting of just expected product gases (CO and O2), and carrier gas (Ar) at concentrations close to those expected limit of

the experiment, and the other at the feed composition of gas that is to be used in the experiment, i.e. CO2 and Ar. The concentrations of the compounds of interest obtained

experimentally can then be found by linear interpolation between the measured values at each m/z peak height value. This method is easier to implement, but requires more time spent on post experimental analysis.

As mentioned earlier in this section, O3 concentration cannot be measured using mass

spectrometry due to its instability. Results from FTIR indicate the O3 concentration is

typically very low, and does not exceed 100 ppm during these experiments. The selectivity of CO, compared to other possible products, C and O3, is sufficiently high that it is possible

depending on the other instrument for supporting data. Therefore, for the experiments carried out in this thesis, the 2nd _{method (i.e. linear interpolation) described above is used to}