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Introduction: An EI source creates a large flux of energetic helium neutrals. The high ionization energy of helium (24.6 eV) means these excited neutrals can ionize other molecules and create secondary charged particles from surfaces. Non-mass-selected noise results when these secondary charged particles reach the detector. Known as neutral noise, this occurs even with off-axis dynode/electron multiplier detectors. Currently, baseline subtraction is used to improve the signal-to-noise ratio by subtracting out neutral noise. But even so, this steady background of noise precludes the ability to detect a single mass analyzed ion; therefore, low-level detection is compromised and dynamic range is reduced. In this work, we compare neutral noise with on-axis and off-axis ion optics.
Methods: Two prototype dual-stage quadrupole instruments containing the same ion source, lenses, quadrupole mass filter (Q2), and detector were compared. The orientation of the ion source with respect to the mass filter and detector differed by the geometry of the RF-only quadrupole (Q1) in each instrument. The on-axis instrument used a straight Q1 to transmit ions into Q2. The off-axis instrument used a Q1 with an 11.3 bend. The minimum bend required was determined by using SIMION 7.0. The trajectories of neutrals were simulated to assure that neutrals would strike a surface or exit between the rods of the mass filter before reaching the end of Q2. A shield was placed around the detector on each instrument to form a physical barrier to charged particles originating outside the central ion beam.
Preliminary Data: Measurements were performed in EI with the filament set to 70 eV and 250 A emission current. A series of experiments determined that there was no measurable chemical noise between m/z 900 and 1000. Therefore, signals detected in this range were assumed to be non-mass-selected noise. With the dynode and multiplier on, 1 count/min of noise was observed on both instruments. This includes spontaneous emission from the multiplier, stray particles, and field emission from the 10 kV dynode.
With the filament on and no helium flow from the GC, 7 counts/s of noise were observed on the on-axis instrument. When 1.0 mL/min of helium was introduced into the ion source, about 126,000 counts/s were observed. Higher helium flows resulted in a linear increase in noise indicating the helium is the cause of the noise.
With the off-axis instrument, only 1 count/min of noise was observed with the filament on and no helium flow from the GC. This is the same as with the filament off. With 1.0 mL/min of helium the noise was about 20 counts/s or about 6000x lower than the on-axis instrument. This noise was further reduced by about 10x. This will be described in the presentation.
Next, hexachlorobenzene (1 pg to 250 ng) was injected onto the GC. Response factors, linear dynamic range, and isotope ratios will be presented. Overall, the off-axis instrument demonstrated better low-level detection and linear dynamic range. |
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