Emission-current and electron-energy control of the n-dodecane EI fingerprint in a quadrupole RGA: compensating knobs with non-overlapping ranges

Purpose Intensity ratios in electron-ionization (EI) fingerprints from quadrupole residual gas analyzers (RGAs) are widely used as substance-specific identifiers for library matching, yet their quantitative dependence on emission current and electron energy, and whether the two parameters can compensate each other, have not been established on a single instrument. On the H100, n-dodecane gave 57/43 = 0.70–0.80 at 2000 µA/102 eV (m/z 43 stronger than 57), opposite to the NIST MS-NW-2010 base peak at 57 (43/57 = 0.78); branched isomers would enhance 57 rather than 43. Methods Four campaigns comprising 51 full spectra (m/z 3.0–100.0, 0.2 amu step, 486 points per spectrum) were acquired on one INFICON Transpector 2 H100, varying emission current from 100 to 2000 µA and electron energy from 90 to 130 eV. Triplicate measurements at 90 and 110 eV, three back-to-back 102 eV references, and a 32.7 min continuous monitor constrained repeatability. Peak heights were taken at the apex within ±0.3 amu with three-point parabolic interpolation; ratios are referenced to m/z 43. Results The m/z 43 signal increased sublinearly with current (power-law exponent 0.65–0.91), the 100→2000 µA gain being only 9.9–14.9× (20× expected). The mean 57/43 of two acquisitions fell from 0.932 to 0.760 (−18.4%). Electron energy acted in the opposite direction: 43/57 decreased from 1.359 at 90 eV to 1.139 at 115 eV and then plateaued at 1.144 ± 0.007 (range 1.3%) up to 130 eV. The two knobs compensated each other but their effective ranges did not overlap: raising energy from 102 to 120 eV increased 57/43 by +10.0% at 2000 µA but only +4.5% at 100 µA; lowering current from 2000 to 100 µA increased 57/43 by +19.4% at 90 eV but only +9.7% at 120 eV. Even at the most favorable combination (100 µA/120 eV), 57/43 reached only 0.960, still 33.7% below the NIST MS-NW-2010 value (1.284); the pre-fixed proxy criterion (43/57 ≤ 1.00) was not met and is reported as such. Repeatability budgets were ±5% within a day, ±10%–13% across days, and +3%–14% over 32.7 min; at fixed current, absolute peak heights varied by 7%–41% whereas 57/43 varied by only 1.3%–5.3%. Conclusions EI fingerprint intensity ratios are instrument-and-state quantities, not substance constants. Emission current and electron energy drive the same mass-discrimination scalar in opposite directions with non-overlapping effective ranges; tuning these two knobs cannot make the local spectrum converge to the library. Comparisons should fix the emission current, report the electron energy, use ratios rather than absolute intensities, and establish a daily local reference.