Surface desorption atmospheric pressure chemical ionization mass spectrometry for direct ambient sample analysis without toxic chemical contamination

Abstract

Ambient mass spectrometry, pioneered with desorption electrospray ionization (DESI) technique, is of increasing interest in recent years. In this study, a corona discharge ionization source is adapted for direct surface desorption chemical ionization of compounds on various surfaces at atmospheric pressure. Ambient air, with about 60% relative humidity, is used as a reagent to generate primary ions such as H(3)O(+), which is then directed to impact the sample surface for desorption and ionization. Under experimental conditions, protonated or deprotonated molecules of analytes present on various samples are observed using positive or negative corona discharge. Fast detection of trace amounts of analytes present in pharmaceutical preparations, viz foods, skins and clothes has been demonstrated without any sample pretreatment. Taking the advantage of the gasless setup, powder samples such as amino acids and mixtures of pharmaceutical preparations are rapidly analyzed. Impurities such as sudan dyes in tomato sauce are detected semiquantitatively. Molecular markers (e.g. putrescine) for meat spoilage are successfully identified from an artificially spoiled fish sample. Chemical warfare agent stimulants, explosives and herbicides are directly detected from the skin samples and clothing exposed to these compounds. This provides a detection limit of sub-pg (S/N > or = 3) range in MS2. Metabolites and consumed chemicals such as glucose are detected successfully from human skins. Conclusively, surface desorption atmospheric pressure chemical ionization (DAPCI) mass spectrometry, without toxic chemical contamination, detects various compounds in complex matrices, showing promising applications for analyses of human related samples.

What can we learn from ambient ionization techniques?

“…Although the chemical composition and origins of samples can change dramatically in different studies, solid surfaces, solutions, and liquid mixtures are usually the states to be characterized by ambient mass spectrometry. Solid surfaces are classic samples for analysis by DESI [2] and similar techniques such as DART [3], desorption-atmospheric pressure chemi-cal ionization (DAPCI) [45,46], dielectric barrier discharge ionization (DBDI) [47], and ionization by a lowtemperature plasma (LTP) [48]. Theoretically, there are no restrictions with respects to sample geometry, size, and weight.…”

“…For example, the LODs of most explosives were in the sub-picogram range on various surfaces tested [46,91]. The difference in sensitivity can be ascribed to the different formation mechanisms of analyte ions by these techniques, convoluted by differences in ion transmission from the source to the mass spectrometer.…”

“…On-line single-particle aerosol mass spectrometry (SPAMS) detected deprotonated TNT ions as the base peak [96], with no or low abundance of the radical anions (m/z 227). Both signals of TNT (i.e., m/z 226 and 227) ( Figure 2b) were detectable using DAPCI [46] and DART [93] as well; the ratio of these two signals varied with the working conditions of the source. In contrast, the radical anion (m/z 227) was detected as the base peak of TNT using DESI ( Figure 2d), and EESI (Figure 2e).…”

“…Obviously, solutions containing a high percentage of organic solvents are not perfectly compatible for in vivo characterization of biological surfaces, especially for diagnosis of patients with allergies. Other techniques such as DAPCI utilize water to efficiently generate the reagent ions [45,46,59,106] and, thus, have been proposed for in vivo characterization of biological surfaces. However, people might be afraid and thus behave abnormally once they see that a "bright fire" (i.e., a high voltage plasma) contacting their skin.…”

Molecular Ionization-Desorption Analysis Source (MIDAS) for Mass Spectrometry: Thin-Layer Chromatography

“…DAPCI type sources are based around a high voltage corona discharge electrode [14][15][16]. Reagent or solvent molecules near the electrode are ionized and directed at the sample surface.…”