Referee comments for egusphere-2024-2587

A solid-state IR laser for two-step desorption/ionization processes in single-particle mass spectrometry

General comments:

The authors discuss the advantages and disadvantages of using a new type of IR-laser for laser desorption followed by REMPI and LDI by an UV laser. The use of a solid-state Er:YAG instead of the widely used CO2 laser offers a more compact design with less maintenance required. However the prototype character of the Er:YAG still leads to instabilities in laser energy and less reproducibility.

The authors show on a variety of aerosol species, that the new laser leads to reasonable and qualitatively good spectra with similar quality compared to the established method, still pointing out the existing problems and differences.

The comparison of the LD/REMPI on three species and LD/REMPI+LDI on the same three species seems very repetitive and does not provide the necessary insight justifying for 6 plots. I suggest, reducing this part to 3 plots, shifting Fig. 2,4 and 6 along with its corresponding text into the supplement. In the main article a short note to the results of Fig. 2,4, and 6 with reference to the supplement should suffice.

Instead the description of the Er:YAG laser falls short, since it is the main news of the publication. Please consider adding more info, e.g. about the dimensions, pulse shape and pumping.

All-in-all the writing and the structure of the paper is very clear, enabling a good understanding of the topic and what the authors are trying to get across. The supplement is useful and offers a reasonable amount of extra information.

I suggest publication in AMT after some minor to major revision.

Specific comments:

Consider shifting some figures and text for the spectral comparisons into the supplement (eg. Fig. 2,4, and 6)

Consider a more detailed description of the Er:YAG laser, e.g. about the dimensions, pulse shape and pumping.

The spectra are compared in detail, however, it is not clear, whether minor differences state a problem. Please state at some point, that due to the missing quantitativeness of the measurement method, minor differences in the acquired spectra do not undermine the applicability of the new laser.

line 100: “(Zhuo, Z., Su, B., Xie, Q., Li, L., Huang, Z., Zhou, Z., Mai, 2021).”

This should be abbreviated as Zhuo, Z. et al., 2021. Name of Mai is missing the Z. and last author Tan, G. is not mentioned, rework the reference.

line 108f: “a 14-bit digitizer card (ADQ14…“

To my knowledge the ADQ14 provides 4 channels. If this instrument uses all four channels, similar to PALMS-NG, ALABAMA, ERICA and others, clarify here: “2 channels per polarity”

line 124: “This is due to the lack of Q-switching. Within the 200 μs pulse duration,”

It is not 100% clear, that the lack of Q-switching and the pulse duration both refer to the Er:YAG laser. Please rephrase.

line 129: “, see Fig. S2)”

This can be more specific, like “a histogram of laser energies can be found in Fig. S2)”

line 144: “no LD“

strictly LDI is a form of LD, thus rather write “no prior LD”

line 153: “was not exposed to LD”

compare comment above, rather write “was not exposed to the IR LD laser”

Fig. 1

The distance between LD and REMPI/LDI seems pretty large. This is totally ok for a sketch, but somewhere it should be clarified, how small the distance is in reality

line 251: “resulting in cluster #4 with dominant fragments”

I am not convinced by the interpretation of a very small peak at ~178 as the only parent molecule and everything else as fragments. All other spectra show more parent molecules than fragments. Can the peaks not be explained by a different parent molecule, possibly hydrogenated or methylated PAH?

line 258f: “The results indicate that the compact Er:YAG laser can replace the more commonly used CO2 lasers for single-particle LD also in ambient air studies.”

This conclusion is not part of the graph description and should only be mentioned in the plain text, not in the caption.

line 263f: “We investigated the capability of the Er:YAG laser for its implementation in this ionization technique”

This sounds like the Er:YAG does the REMPI/LDI, which is not the case. Rephrase like “We investigated the capability of LD by the Er:YAG laser in combination with this ionization technique”

line 281f: “but there are some differences of unknown origin, e.g., 280 enhanced phosphate signals when the CO2 laser is used for LD.”

In the LD scheme without LDI, only cations have been investigated. Do the wood ash LD/Rempi spectra show the phosphate signal in the anions as well?

Since only the LD laser is changed between 7a and 7b, a difference in the LDI spectrum would indeed be astonishing.

line 320: “the vast majority of mass spectra were dominated by fragments”

At this point I am not sure, whether I know what you mean. Since you differentiate between LDI spectrum and PAH spectrum within the same spectrum and you use the term fragment mainly in the context of PAHs, I would guess you mean most of the PAH spectra (mz>140) were dominated by fragments.

Technical comments:

line 161: “isobar substances”

I think the correct term is isomeric

line 161f

several parts of the sentence are written in italic for no obvious reason

line 162f

replace above and below by upper panel and lower panel or similar (top panel, bottom panel)

Table 2

Some lines are not separated by an empty line, in some lines the m/z are not aligned with the species

line 181: “of each 500 diesel soot particles”

The expression sounds unfamiliar, shouldn’t it be “of 500 diesel soot particles, each”

line 235: “from each 500 PAH-containing particles”

What does each 500 mean in the context of 1450 measured spectra? Should it be “from a representative set of 500 PAH-containing particles, each”

line 280, line 292, line 305: “Each n = 500.”

n=500 each.

line 370: “LDI ionization”

This is a pleonasm. Rather write LDI method or LDI scheme.

line 536f: “…Con- centrator… Spectrome- try…”

Please check spelling

line 543

The title of the reference is in capitals, not in agreement with the other references

Review file is attached.