Electrochemically Induced pH Changes Resulting in Protein Unfolding in the Ion Source of an Electrospray Mass Spectrometer
Bibliographic record
Abstract
The operation of an electrospray ion source in the positive ion mode involves charge-balancing oxidation reactions at the liquid/metal interface of the sprayer capillary. One of these reactions is the electrolytic oxidation of water. The protons generated in this process acidify the analyte solution within the electrospray capillary. This work explores the effects of this acidification on the electrospray ionization (ESI) mass spectrum of the protein cytochrome c (cyt c). In aqueous solution containing 40% propanol, cyt c unfolds around pH 5.6. Mass spectra recorded under these conditions, using a simple ESI series circuit, display a bimodal charge-state distribution that reflects an equilibrium mixture of folded and unfolded protein in solution. These spectra are not strongly affected by electrochemical acidification. An "external loop" is added to the ESI circuit when the metal needle of the sample injection syringe is connected to ground. The resulting circuit represents two coupled electrolytic cells that share the ESI capillary as a common anode. Under these conditions, the rate of charge-balancing oxidation reactions is dramatically increased because the ion source has to supply electrons for both, the external circuit and the ESI circuit. The analytical implications of this effect are briefly discussed. Mass spectra of cyt c recorded with the syringe needle grounded are shifted to higher charge states, indicating that electrochemical acidification has caused the protein to unfold in the ion source. The acidification can be suppressed by increasing the flow rate and lowering the electrolyte concentration of the solution and by using an electrolyte that acts as redox buffer. The observed acidification is similar for sprayer capillaries made of platinum and stainless steel. Removal of the protective oxide layer on the stainless steel surface results in effective redox buffering for a few minutes.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".