Direct detection of inorganic nitrate salts by ambient pressure helium-plasma ionization mass spectrometry

Inorganic nitrates in solid deposits were detected directly by ambient-pressure helium-plasma ionization-mass spectrometry (HePI-MS), without the need for extensive sample preparation. Nitrates were detected even from complex matrices such as meats and fruit juices. Any electrospray-ionization mass spectrometer can be modified to perform ambient-pressure HePI-MS by simply passing helium through the metal capillary intended for liquid-sample delivery. Nitrates on paper strips, glass slides, or cotton swabs (sometimes wetted with a mineral acid) were inserted directly into the ambient-pressure HePI source. The spectra acquired under negative-ion generating conditions showed a peak at m/z 62 for the nitrate ion, along with a lower-intensity peak at m/z 125 for the nitrate adduct of nitric acid. Apparently, it is nitric acid that is initially transferred to the gas phase, forming an ion-molecule complex with hydroxyl anions present in the plasma. The ion-neutral complex then dissociates by eliminating water to produce gaseous NO₃^- ions. This hypothesis was supported by the observation that certain solid nitrate salts, which were not readily amenable to HePI (notably the alkali nitrates), were immediately detected as m/z 62 and 125 ions upon acidification by a strong acid. Quantitative evaluations showed that the nitrate-signal response versus the deposited mass is linear for over 3 orders of magnitude. With the use of ¹⁵N-labeled nitrate (m/z 63), the limit of detection was determined to be as low as 200 fg.