AVS2013 Session AS+BI+IS-WeM: Ambient Ionization Mass Spectrometry

Microplasma ionization sources have been shown to be simple, effective tools for ambient mass spectrometry. Microplasmas have the advantages of being cheap to operate and manufacture, and require only modest gas flows and power sources. One of the challenges in ambient mass spectrometry is to produce sample ions from a wide variety of sample molecules without excessive fragmentation. Due to the chemically-specific nature of ionization, this remains a challenge for any new source. In this work we attempt to mitigate both of these problems by using high energy photons to ionize our samples. We have created a microplasma source that employs a mixture of neon and hydrogen to produce vacuum ultraviolet (VUV) light which can ionize samples at atmospheric pressure. Traditionally, VUV light has been both difficult and expensive to produce, which limits its use for applications like ambient mass spectrometry. With the development of our microplasma VUV source, we are able to take advantage of the VUV photon’s capability as a broadly-applicable, low-fragmentation photoionization source, while retaining the low cost and simple operation which makes ambient mass spectrometry so appealing. By combining this ionization technique with laser desorption, we investigate the use of this source for mass spectrometric imaging.

This work was supported by the National Science Foundation under award number 0923179.

The potential benefits of using atmospheric pressure “cold” plasma sources as a means of both desorption and ionisation of material in ambient mass spectrometric analysis of surfaces has been recognized by a number of researchers worldwide. Plasma sources include dielectric barrier jets (PADI [1] and LTP [2]) operating at mid-frequency and also RF corona needle discharges.

Here we develop further strategies to pulse modulate such sources (jet and needle) and investigate the influence of duty cycle on the production of chemical species in the source plasma and the composition of the ionised desorbed species from a range of test surfaces (polymeric, pharmaceutical and biological). For the plasma plume study we use a time resolved (5 µs) Hiden Analytical molecular beam MS (HPR-60) unit, while detailed information on the spatial distribution of surface species from the chosen material samples was done using a Thermo Velos LTQ Orbitrap MS.

The influence of source-sample and sample-instrument distances on the signal intensities is investigated for both sources. We observe that as the duty cycle of the pulse is decreased the positive ion yield shifted towards higher mass clusters, due to a decrease in gas temperature enabling increased hydration reactions. The negative ions also display similar trends with the yield of larger negative ions increasing with shorter duty cycles. From time-resolved ion intensities it is clearly seen that positive ions are produced in the on-phase of the discharge, and decay in the off-phase. Negative ions, in contrast, are produced mainly in the off-phase of the discharge and decay during the on-phase. With increased distance between source and instrument we observe that the yield of large positive ions increases, at the expense of smaller positive ions which are lost before they reach the orifice. Negative ions, on the other hand, show an increase in yield as distance is increased.

Preliminary analysis of common pharmaceutical products suggest that a decreased duty cycle gives improved identification of negative ion surface compounds however, where surface compounds undergo ionisation to form positive ions, larger duty cycles allowed for better identification. The gas-phase plasma chemistry for different arrangments will be discussed.

1] Ratcliffe LV, Rutten FJ, Barrett DA, Whitmore T, Seymour D, Greenwood C, Aranda-Gonzalvo Y, Robinson S, McCoustra M, Anal Chem. (2007) 79 (16) 6094

2] Zhang, JI, Costa, AB,Tao, WA, Cooks, RG, Analyst, (2011), 136, 15, 3091

The detection of illicit drugs and trace explosives represents one of the most significant challenges for law enforcement and forensic communities. Of particular interest to the forensic analyst is the ability to rapidly identify suspected illicit drug materials and explosives residues in their native state (powder, tablet or liquid form), under atmospheric conditions and with a high level of specificity and sensitivity. To that end there are numerous ambient pressure ionization techniques such as atmospheric pressure glow discharge (APGD), low temperature plasma (LTP), and desorption electrospray ionization (DESI) that can be used to interrogate forensic compounds. However, a method of comparing between the different ambient ionization sources does not exist. Here, useful yields are determined for a series of forensic compounds using different ambient pressure ionization sources as a means of comparison. Using a precision inkjet deposition system a well-defined array of microdrops is produced with each deposit containing a known number of analyte molecules. The individual deposits were removed or desorbed until consumed while monitoring the integrated characteristic molecular secondary ions for each analyte. The ratio of integrated counts to the number of molecules in the deposit defines the useful yield of the experiments. The results here highlight a way to compare the different ambient ionization sources for the analysis of forensic compounds of interest.

Almost for a century now, mass spectrometric instrumentation is generally designed to establish linear analyte concentration – ion signal responses. Deviations from linear relationships are tolerated to a certain extent but such observations usually raises doubts on the performance of the method applied. Surprisingly, Atmospheric Pressure Ionization (API) techniques have swiftly led to the frequent usage of terms such as “matrix effects”, “ion suppression”, or “non-linear response”. This is particularly true for ambient ionization methods. In fact, the various API methods not only target different analyte properties, e.g. gas phase acidities, they also frequently generate new ion signals which are then somehow related to the neutral analyte precursor. On the other hand API mass spectra may also show unexpected fragmentation patterns – despite the often claimed “soft” nature of the ionization processes involved.

In this contribution we are attempting to highlight some of the key processes operating on the primarily generated analyte ion population en route from the origin of ion formation to the collision free analyzer region. For this purpose, we are defining chemical domains such as the initial reagent ion generation domain (in ambient ionization often plasmas), the chemical ionization domain, or the thermal ion transport domain, among others. These domains exhibit distinct features, which are significantly changing the chemical matrix in which ions and neutrals are moving towards the collision free - and thus chemically non-reactive - analyzer region.

Primary ionization pathways, subsequent thermal ion molecule chemistry, and electrical fields are discussed as potential drivers of chemical transformation processes, which may render the interpretation of API mass spectra quite difficult. This is particularly true for the analysis of complex mixtures using API MS without chromatographic pre-separation of the neutrals. Ambient ionization methods are generally designed to do exactly that: Analyze samples (e.g., human urine, drug tablets, surface contaminations, to name but a few) without sample preparation and preferentially in real time.

Examples are given to illustrate the extent of selected transformation processes. Among other issues it is discussed why the generation of Helium metastable atoms (He^M) results in protonation in API MS, or why aromatic hydrocarbons ionized with the exact same primary excitation scheme yield almost exclusively radical cations in classical mass spectrometers but often deprotonated molecules in API systems.