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Abstract
Mass spectrometry (MS) is an important analytical technique with high sensitivity and selectivity. However, all known MS ionization methods are significantly hindered by ion suppression that is caused by the competition for charge between analytes of interest and other species present in the sample matrix. To approach this key problem without using chromatography and/or sample preparation which can be tedious and time consuming, a portable ionization source was developed by (i) integrating solid phase microextraction with low temperature plasma MS, (ii) characterizing the extent of internal energy deposition during the ionization using novel thermometer ions, and (c) miniaturizing the plasma power source.
Solid phase microextraction is directly integrated with plasma. The SPME fiber serves: (i) to extract molecules (e.g., toxins) from their native environment (e.g., urine) and (ii) as the ionization electrode that is used to desorb and ionize molecules. The SPME fiber consists of stainless steel coated with zeolitic material. Using this method, chemical warfare agent analogues can be detected at less than 100 ppb directly in water and urine in ≤ 2 min. Due to surface polarity differences, Linde Type A coating significantly outperformed the high alumina ZSM-5 coating of comparable thickness. Conversely, by conditioning the Linde Type A coated probe with aqueous CuSO4, the ion abundances can be significantly increased and sample recoveries of near 100 % were obtained.
The internal energy deposition upon ion formation using three different ionization, techniques, low temperature plasma (LTP), atmospheric pressure chemical ionization (APCI), and direct analysis in real time (DART), was investigated by use of benzylammonium thermometer ions. Varying the DART ion source parameters, such as the distance between the plasma ionization region and the capillary entrance to the mass spectrometer and the DART temperature, had a minimal impact on the “softness” of the DART ion source under these conditions. Overall, LTP can be a significantly “softer” ion source than two of the most widely used plasma based ion sources that are commercially available.
The key advantage of this innovation is that analyte of interest can be directly and rapidly sampled from complex mixtures, including urine, without sample preparation or chromatography for detection by MS. This ion source should prove beneficial for portable MS applications because relatively low detection limits can be obtained without the use of additional gadgets and importantly, has softer ionization. Besides, this ion source, when square-wave alternating current is utilised, generates homogeneous microdischarge, consumes 100× less power, and offers high ionization efficiency. Importantly, it was miniaturized using an H-bridge circuit, and when coupled with optimized transformer, it can be continuously powered for ~50 h by 9 V-battery (PP3).