Compact Ambient Pressure Pyroelectric Ion Source for Mass Spectrometry

We present the construction and implementation of a compact, low-power ambient pressure pyroelectric ionization source. The source comprises a z-cut lithium niobate or lithium tantalate crystal with an attached resistive heater mounted in front of the atmospheric pressure inlet of an ion trap mass spectrometer. Positive and negative ion formation alternately results from thermally cycling the crystal over a narrow temperature range. Ionization of 1,1,1,3,3,3-hexafluoro-2-propanol or benzoic acid results in the observation of the singly deprotonated species and their clusters in the negative ion mass spectrum. Ionization of triethylamine or triphenylamine with the source results in observation of the corresponding singly protonated species of each in the positive ion mass spectrum. Although processes in which ion formation occurs directly on the highly charged crystal surface may contribute to the observed signal, ion formation appears to result mainly from electrical discharges occurring on the surface of the crystal, from one z face to another. This dielectric breakdown originates from the high electric fields generated at the surface of pyroelectric crystals when they are thermally cycled by as little as 30 K from ambient temperature. Ion formation is largely unaffected by contamination of the crystal faces. This robust source might prove particularly useful in applications where unattended operation in harsh environments, long service lifetimes, and durability are desirable characteristics.