This paper describes the addition of delayed extraction to the ion source of a single particle mass spectrometer. The authors investigate the hit rate of particles, i.e. the percent of particles that are sized by the mass spectrometer system from which viable mass spectra are generated. The authors make the case that the implementation of delayed extraction is simpler than the use of an external particle neutralizer (see comments below) and that the hit rate improvement is significant. Overall, the paper shows relatively dramatic impacts on hit rates of polystyrene latex spheres (PSLs) and modest impacts on indoor particles of mixed composition. The authors provide no information in this paper about the instrument changes that are required to implement delayed extraction, including addition of power supplies, adjustments to instrument timing circuitry, etc., and, while they make a good case for the well-known impact of particle charge on particle trajectories in electric fields, they do not make their case that delayed extraction is simpler than the use of a neutralizer.
1. The English language usage in the manuscript is not yet grammatically clean enough for publication. There are too many issues to enumerate here, but revision by a native speaker is imperative.
2. The authors refer to figures in the supplementary information (line 161), but no supplementary information was available with this manuscript to review.
3. The authors state (lines 58 – 60) that “the management of radioactive sources is a difficult task, the equipment is relatively expensive, and it is very inconvenient to use it in the [sic] field measurements.” However, they give no explanation of the costs, hardships, or complexity, nor do they provide this information in comparison to their proposed solution. It is not sufficient to state that something is better in specific ways without providing information about both methods. It is likely that the challenges associated with radioactive neutralizers are location (i.e. regulation) specific. The use of X-ray neutralizers, however, is typically more straightforward. Cost comparisons of the neutralizers with the modifications made to the commercial instrument (additional power supplies, software modifications, etc.) are not discussed.
4. The majority of the work done in this paper is using spherical PSL particles, with a small experiment added at the end in which indoor aerosols are sampled. The impact of the delayed extraction on the PSLs is shown to be 1 – 2 orders of magnitude, depending on field strength (shown in Figure 2), whereas it is only 2 - 5x greater for room-air particles below 500 nm and only 0.25x greater for room-air particles ≥ 500 nm in diameter. The authors state that this difference can depend on composition, but make no effort to explain their results using composition information, which they have from the SPMS instrument. This portion of the paper should be significantly enhanced to make a stronger case for the utility of this instrumental modification for the analysis of ambient aerosol particles. A side-note, the use of the term “real” aerosol particles (line 221) is incorrect. All particles sampled are real, but only the ones sampled from the room are “ambient” or “not lab generated.”
5. Throughout the paper, the authors state but do not illustrate that the issue they are trying to solve is “significant.” It is not until the very end of section 3.2 that quantitative information is provided about particle deflection relative to the size of the laser spot. This is the fundamental issue that they are trying to solve and quantitative information about it is not provided to make the case at the beginning. In addition, in line 124, the authors state that this dispersion leads to a “significant” drop in hit rate, without giving any definition of the use of the word. I would recommend that they not use the term “significant” since it carries with it specific criteria that they are not addressing.
6. Figure 1 shows a schematic of particle deflection but it is not clear whether the dispersion illustrated for the particle beams as drawn is based on any calculations, or whether it is just a cartoon to show the possible impact of the electric field on the particle beam. This must be clarified and the figure should show quantitative information if at all possible. Including the laser spot size would also be helpful.
7. The authors mention early on that the beam divergence problem “can be solved” using lasers with shorter delay times, but they give no information about this.
8. In the “Instruments and methods” section, lines 94 – 98, the authors describe a modification made to their instrument that replicates that in use in laboratory and commercial ATOFMS instruments for many years. Citations are necessary.
9. In lines 141 – 143, the authors state that the poor performance for 320 nm PSL particles is due to the aerodynamic lens’ characteristics. However, they provide no data about the performance of the lens to support this.
10. Given the size range that this instrument is stated to cover, and the distribution shown for the room-air particles in Figure 7, it is surprising that the authors didn’t attempt to quantify the impact of delayed extraction on particles between 200 and 320 nm. This is likely because atomization of particles below approximately 300 nm tends to produce artefacts, and therefore they are typically generated using methods that incorporate neutralizers in the system (such as a DMA). However, this should be discussed, as the results presented suggest that the issue addressed by incorporation of delayed extraction is more important for smaller sized particles.
1. Terms are not defined or are used inconsistently:
a. ADC (line 38 and below)
b. Turnaround time (line 146 and below)
c. Hit rate increment (line 152 and below)
d. Constant field (CF) (line 159) is used when DC is the abbreviation defined in the abstract. It should be defined in the body of the text, as well.
2. What the authors refer to as “light laser calipers” (line 35 and below) is typically called aerodynamic particle sizing in the aerosol literature.
3. The authors state that the types of experiments for which SPMS instruments are used typically “requires a high performance instrument” (line 45) but give no indication what they mean by this.
4. The authors claim that their results are “in good agreement” for the charge on the 720nm PSLs, but they do not provide any quantitative information about what is predicted in the source they site. Their value should be compared.