the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Sep 2024
A solid-state IR laser for two-step desorption/ionization processes in single-particle mass spectrometry
Abstract. Recent advancements in single-particle mass spectrometry (SPMS) have enabled the detection of aromatic hydrocarbons at the individual particle level in conjunction with inorganic/refractory particle components. However, the laser desorption (LD) of organic material from particles prior to their ionization in a two-step process necessitates pulsed infrared lasers with adequate pulse energy that can be irregularly triggered on detected particles. Pulsed CO2 lasers with a 10.6 µm wavelength have been traditionally utilized, yet these lasers are bulky, costly, and require regular maintenance, including gas exchange or a continuous laser gas supply. In this study, we present the application of a prototype solid-state laser based on an erbium-doped yttrium aluminum garnet (Er:YAG) crystal, emitting long pulses of 200 µs at 3 µm wavelength as a compact, cost-effective, and user-friendly alternative for LD. We directly compared the new laser with a commonly used CO2 laser and found similar performance in LD for both laboratory particles and ambient air experiments. With the exception of slightly increased fragmentation observed with the CO2 laser due to its beam profile, no qualitative differences were noted in the resulting mass spectra. Additionally, we compared the novel two-step ionization scheme for the combined detection of aromatic molecules and inorganics with conventional single-step laser desorption/ionization (LDI) for the detection of polycyclic aromatic hydrocarbons (PAH) in laboratory and field experiments. The combined methods demonstrated superior performance in the detection of PAHs, for both the CO2 and the new Er:YAG laser. In addition to its higher sensitivity and lower fragmentation for PAHs when compared to single-step LDI, it is less dependent on the particle matrix, sharing the benefits of traditional two-step methods but extending its capability to combine PAH measurements with the LDI-based detection of inorganic particle compounds.
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RC1: 'Comment on egusphere-2024-2587', Anonymous Referee #3, 19 Sep 2024
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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
Citation: https://doi.org/10.5194/egusphere-2024-2587-RC1
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