Mass spectra obtained from the LAAPTOF single-particle mass spectrometer using a conventional 193 nm excimer ns-pulse laser and a 800 or 266 nm Ti:Sapphire fs-pulse laser are presented. The objective appears to be to explore if the higher laser power density of a fs laser produces higher ion signal and more complete ionization of the entire particle. This would improve the quantitative abilities of SP-MS using LDI, which is hampered by particle matrix effects and the complex interactions between individual particles and the high energy laser pulse. This is certainly a worthwhile objective whose results will be of interest to readers of AMT. This main objective could have been more clearly presented upfront, and then discussed again as to its success or failure in the conclusions. While the results presented do not support this hypothesis or indicate good promise in the use of a fs-pulse for single-step LDI, the new findings still provide valuable information. The manuscript has already gone through one round of peer review and has improved during the revisions and rebuttal. There are a few key remaining areas that require better clarification and discussion, as well as some relevant references that should be added and discussed. While the scientific quality of the results and their discussion presented here is not high, these measurements required considerable resources and efforts to obtain. We can let the community decide for themselves if this is a worthwhile study. This manuscript should be acceptable for publication in AMT following minor revisions.
My major comment and confusion concerns the role of the LDI laser wavelength. The fs-pulse laser was operated at 800 or 266 nm. I would think the laser wavelength would have a huge influence on how the photons interact with the particle, since the complex index of refraction is a strong function of wavelength. While a multi-photon ionization process is involved, the 266 and 193 nm photons have much more energy than the 800 nm photons. I realise this issue was already brought up by reviewer #2, but this important topic is still not addressed satisfactorily.
Using 800 nm for LDI seems a poor choice since so many particle components will absorb poorly in the near IR. There are few good chromophores here, and the wavelength is not long enough to access most vibrational modes, as is done using a 10.6 micron CO2 laser for two-step LDI, for example (Morrical et al., 1998; Zelenyuk et al., 2008). So the fact that the mass spectra obtained from 266 and 800 nm LDI are so similar is surprising. I would think that the particle hit rate would be much higher for 266 vs. 800 nm, but this is hard to quantify using the free-fired laser and the unreliable particle detection by light scattering from the laser pulse. Indeed the very different particle detection scheme when using the fs vs. ns laser makes a direct comparison of the performance of these two types of lasers almost impossible, which is most unfortunate. These issues should be discussed more fully, especially in regard to how the laser wavelength influences how the particle interacts with the photons.
Also, it is often not clear what wavelength is being discussed for the fs laser. 800 nm appears to be the default? Please clarify. Also, why is so much more emphasis placed on the 800 nm fs results and not on the 266 nm fs results? Since 266 nm LDI is commonly used for SP-MS by the ATOFMS, for example, a direct comparison at the same wavelength for the fs and a ns 266 nm laser would be possible by comparing to reported ATOFMS spectra. Why was this not done? My main concern here is that the laser wavelength is a critical variable and its important role is not being properly considered and discussed in the manuscript.
There are several earlier reports of using two-step LDI for SP-MS that should be included and discussed. Passig et al. is an interesting and recent approach but is certainly not the first. Note that the Zelenyuk group has using two-step LDI for depth profiling. (Morrical et al., 1998; Smith et al., 2002; Sykes et al., 2002; Whiteaker and Prather, 2003; Woods et al., 2002; Zelenyuk et al., 2008).
As particle detection for laser triggering is an important aspect of SP-MS (the use of a free fired fs laser here results in a low level of mass spectral reproducibility), more information on how the ns excimer laser is triggered would be useful. It is my understanding that the design of the LAAPTOF’s particle detection system by laser light scattering has rather poor performance, with low particle detection rates < 500 nm diameter. There have also been several different attempts to improve the particle detection scheme. Please describe what particle detection scheme is used here (and if it is similar to that used in other LAAPTOF-based papers), and what its typical performance is (i.e. particle detection rate as a function of particle size).
Line 61: While quantification vis SP-MS is challenging, this has been successfully demonstrated several times, especially if you confine the analysis to particles of a similar type, which reduces variability caused by particle matrix effects. The SP-MS response can also be calibrated using other co-located measurements. This paragraph should be expanded to more accurately reflect what can be achieved by LDI SP-MS analysis. Some examples: (Bhave et al., 2002; Fergenson et al., 2001; Gross et al., 2000, 2005; Healy et al., 2013; Saul et al., 2006; Sullivan et al., 2007, 2009)
The emphasis on excimer laser-based SP-MS instruments while ignoring Nd:YAG (266 nm) SP-MS instruments was odd to me, especially since a 266 nm fs laser is used here. There are lots of 266 nm SP-MS spectra you could compare to.
One missing example is the study of LDI of Au metal nanoparticles using a tunable visible laser coupled to an ATOFMS (Spencer et al., 2008). They discussed the importance of the surface plasmon effect for LDI of metal nanoparticles.
Schoolcraft et al. performed MD simulations of the LDI laser ablation process (Schoolcraft et al., 2000, 2001).
Line 300: Is this for 800 nm or 266 nm fs laser? The low hit rate for NH4NO3 particles would be expected using 800 nm laser as these particles are weakly absorbing even in the UV.
Sect. 3.2: The ion signal response versus particle size is certainly worth exploring and important, but you would have to test more than just two particle sizes of the same particle matrix to really conclude anything meaningful here. Why were more particle sizes explored?
Cited References
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