As cited in the paper, the use of alternative gases for DART has previously been discussed in several articles. Because nitrogen DART can result in oxidation, it isn’t well suited for the identification of unknown compounds, but it is a reasonable alternative for the detection of targeted compounds. The specific focus of this article is the determination of detection limits for target compounds by using DART and a single quadrupole MS. The title should state this explicitly.
I was pleased to see that the authors addressed in the conclusions the concerns I had when reading about argon DART in the results and discussion. Specifically, the addition of the dopant for argon DART from a reservoir is inefficient. Relying on vapor from a reservoir is poorly reproducible, and it probably the major reason for poor reproducibility stated on page 8 in the second paragraph.
The choice of dopant and a more efficient dopant introduction method could certainly improve detection limits. Although ethanol is a suitable dopant for proton transfer DART, it is weakly ionized and therefore results in lower detection limits than N2 or He DART. Other dopants are more efficiently ionized (page 9 lines 47-48, plus chlorobenzene – ref 12) but have different proton affinities, and thus different selectivity.
Page 6 discussion. The function of the dopant in negative-ion argon DART is to produce electrons in the Penning ionization step. Using a dopant with a lower ionization energy that is more efficiently ionized should certainly improve negative-ion detection limits.
Page 1, lines 5-52. The description needs to be corrected. The DART gas passes though a heated region BEFORE passing through the grid electrode.
Page 1 line 57 and Page 2 line 51 Omit the apostrophe for the possessive “its”.
Page 6, lines 22-23. It’s equally likely that the very weak ion signal in the argon DART spectrum simply means that the m/z 324 peak is below the detection limits.
Page 6 discussion: Detection limits for high explosives (RDX) in helium and perhaps nitrogen mode can be significantly enhanced by adding a dopant such as dichloromethane or ammonium nitrate that produces abundant anions that will form adducts such as Cl- and NO3-. Without dopant addition, the formation of NO2- and NO3- from the explosives is strongly dependent on DART gas temperature and limited by analyte concentration.
Page 7 lines 3-6 and Figure S5. A peak at m/z 304 appears in the positive-ion He and N2 DART background mass spectra. Because protonated cocaine is also m/z 304, it is important for the authors to state which was(were) the abundant ion(s) chosen for the determination of detection limits for cocaine and the other target compounds. This information could be added to the supporting data.
Supporting data comment:
The positive-ion and negative-ion background spectra do not show low-m/z atmospheric background peaks such as NH4+, O2- or NO3- that could be significant. Is this a limitation of the QDa? If so, just state it in the experimental section. In any case, reactive atmospheric background peaks will be masked in favor of atmospheric contaminants when using the Vapur interface. For example, the positive-ion background spectra show phthalate peaks at m/z 149, 167, 279, etc.