Atmospheric Pressure Chemical Ionization Sources Used in the Detection of Explosives by Ion Mobility Spectrometry

Explosives detection is a necessary and wide spread field of research. From large shipping containers to airline luggage, numerous items are tested for explosives every day. In the area of trace explosives detection, ion mobility spectrometry (IMS) is the technique employed most often because it is a quick, simple, and accurate way to test many items in a short amount of time. Detection by IMS is based on the difference in drift times of product ions through the drift region of an IMS instrument. The product ions are created when the explosive compounds, introduced to the instrument, are chemically ionized through interactions with the reactant ions. The identity of the reactant ions determines the outcomes of the ionization process. This research investigated the reactant ions created by various ionization sources and looked into ways to manipulate the chemistry occurring in the sources. The ionization source most utilized in IMS instruments is Ni. It is a very reliable and well understood source, but due to safety and regulatory concerns non-radioactive sources are being put to use. One non-radioactive source already implemented into IMS instruments is corona discharge. The predominant reactant ion observed in a point-toplane corona discharge occurs at m/z 60 in clean air. There have been multiple references in the literature to the identity of this ion with some disagreement. It was postulated to be either CO3 or N2O2. The identity of this ion is important as it is a key to the ionization of analytes. The ion at m/z 60 was determined here to be CO3 through the use of O labeled oxygen. Further confirmation was provided through MS/MS studies. An example of the importance of knowing the reactant ion identity was the ionization of nitroglycerine (NG) with CO3 which produced the adduct NG·CO3. This adduct formation was similar to the ionization of NG with NO3 and Cl reactant ions that also formed adducts with NG. The fragmentation patterns of these adducts provides insight into the charge distribution. The fragmentation of the NG·NO3 adduct produced the nitrate ion whereas fragmentation of the NG·Cl adduct also produced the nitrate ion indicating that the charge resided predominantly with the nitrate in both complexes. However, the fragmentation of NG·CO3 yielded CO3. This indicates that CO3 has a relatively high electron affinity, higher than that of chlorine and likely close to that of the nitrate ion. As part of this research a new atmospheric pressure ionization (API) source was developed, characterized and compared to commonly used API sources with both mass spectrometry and ion mobility spectrometry. The source, a distributed plasma ionization source (DPIS), consisted of two electrodes of different sizes separated by a thin glass slide. Application of a high RF voltage across the electrodes generated plasma in air yielding both positive and negative ions. The positive ions generated were similar to those created in a conventional point-to-plane corona discharge ion source, being mass identified as solvated protons of general formula (H2O)nH, with (H2O)2H as the most abundant reactant ion. The negative reactant ions produced were mass identified primarily as CO3, NO3, NO2, O3 and O2 of various relative intensities. The predominant ion and relative ion ratios varied depending upon source construction and supporting gas flow rates. A few compounds including drugs, explosives and amines were selected to evaluate the new ionization source. A lifetime experiment was run to test the stability of the source. The source was operated continuously for three months and although surface deterioration was observed visually, the source continued to produce ions at a rate similar that of the initial conditions. The ions created in a discharge were dependent upon experimental conditions. It was postulated that the change in ions was caused by reactions with neutral species O3 and NO2. In an effort to better understand the formation of negative reactant ions in air produced by an atmospheric pressure corona discharge source, the neutral vapors generated by a corona discharge were introduced in varying amounts into the ionization region of an ion mobility spectrometer/mass spectrometer containing a Ni ionization source. With no discharge gas, the predominant ions were O2 , however, upon the introduction of low levels of discharge gas the NO2 ion quickly became the dominant species. As the amount of discharge gas increased, the appearance of CO3 was observed followed by the appearance of NO3. At very high discharge gas levels, NO3 species became effectively the only ion present and appeared as two peaks in the IMS spectrum, NO3 and the NO3·HNO3 adduct, with separate mobilities. RDX was examined in order to investigate the ionization properties with these three primary ions. It was found that RDX forms a strong adduct with both NO2 and NO3 with reduced mobility values of 1.49 and 1.44 cmVs, respectively. No adduct was observed for RDX with CO3 although this adduct has been observed with a corona discharge mass spectrometer. It is believed that this adduct, although formed, does not have a sufficiently long lifetime (greater than 10 ms) to be observed in an ion mobility spectrometer. Many explosives form product ions via adduct formation. Thus, ion identities and subsequent mobility values will change based upon reactant ion identity. Thermal stability of the product ion, sensitivity and selectivity can be altered by the selection of the reactant ion. This study investigated the use of O2, Cl, NO2, and NO3 reactant ions for explosive ionization. It was shown that NG, PETN, RDX and tetryl form adducts with chloride and nitrate reactant ions. RDX forms nitrite adducts with NO2 reactant ions and in the presence of O2 RDX also forms the nitrite adduct via a 2-step process by first forming the nitrite ion. When O2 or NO2 is the reactant ion NG forms the nitrate ion and subsequently produces the NG-nitrate adduct. Tetryl fragments to form (tetryl-NO2) with O2 and NO2 reactant ions. TNT is ionized by proton abstraction with O2, Cl, and NO2 reactant ions. However, when NO3 ions are present TNT is not ionized. Because of the high electron affinity of the nitrate ion, many compounds will not ionize in its presence. The use of nitrate reactant ions result in a reduction of ions created from background interferents while allowing the ionization of selected explosive compounds, making the nitrate ion a good candidate as a selective reactant ion species. The reduced mobility values from mass identified ion mobility peaks are provided for a variety of product ions observed resulting from combinations of the explosives and the reactant ions investigated.