The LDTD technology has been around for quite some time and yet because it is so far removed from the typical HPLC-MS systems people are used to, everyone keeps comparing it to a standard LC-MS system. Therefore, in order to clarify how similar and different LDTD Technology (paired with an MS) is to traditional LC-MS, we decided to answer the most common questions we get on the technology.
Topic # 1: How can separation be achieved without chromatography?
Simple answer is, we don’t! The LDTD technology doesn’t rely on chromatographic separation to analyze samples. It relies on the parent mass and daughter mass targeted and detected by mass spectrometry to differentiate between analytes. Also, an optimized sample extraction process permits us to get rid of interferents found in the matrix.
But what of isobaric molecules? Well, we have found that we usually can find a specific fragment mass for each molecule, even if their parent mass is identical. When this is not possible, a derivatization step, specific to one of the isobaric molecules, can be added to the sample extraction to change its parent mass.
And what if derivatization is not an option? A good alternative is ion-mobility spectrometry-mass spectrometry (IMS-MS) which was developed to separate similar molecules based on their mobility in an electric field. This technology can also be quite useful to diminish the signal acquired from the interferents in the sample.
Topic #2: Can I use pure Nitrogen as a carrier gas?
No. We use compressed air as a carrier gas because of the presence of water (between 3 ppm and 1800 ppm), nitrogen and oxygen. In positive mode, the nitrogen and traces of water are needed for the ionization of the molecules in the corona discharge region. In fact, the corona discharge takes an electron from what is available, in our case that is nitrogen, which creates H3O+ with the water which eventually interacts with the molecule to give it a proton, hence looking for the parent mass corresponding to M + 1. Since we are in a dry environment, the only thing that can supply water is the carrier gas in which the molecules are being transported.
In negative mode, the corona discharge gives an electron to the oxygen in the air, which creates o2- that will interact with the molecule to take a proton from it.
So theoretically, a nitrogen gas with traces of water could be used with the LDTD technology, but only in positive mode.
 I. Dzidic, D. I. Carroll, R. N. Stillwell, E. C. Horning, Comparison of Positive Ions Formed in Nickel-63 and Corona Discharge Ion Sources Using Nitrogen, Argon, Isobutane, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization Mass Spectrometry, Anal. Chem., 1976, Vol. 48, No. 12, 1763.
 K. Sekimoto, M. Sakai, M. Takayama, Specific Interfaction Between negative Atmospheric Ions and organic Compounds in Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry, J. Am. Soc. Mass. Spec., 2012, 23, 1109.