Response in Ambient Low Temperature Plasma Ionization Compared to Electrospray and Atmospheric Pressure Chemical Ionization for Mass Spectrometry

Abstract

Modern technical evolution made mass spectrometry (MS) an absolute must for analytical chemistry in terms of application range, detection limits and speed. When it comes to mass spectrometric detection, one of the critical steps is to ionize the analyte and bring it into the gas phase. Several ionization techniques were developed for this purpose among which electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are two of the most frequently applied atmospheric pressure methods to ionize target compounds from liquid matrices or solutions. Moreover, recent efforts in the emerging field of “ambient” MS enable the applicability of newly developed atmospheric pressure techniques to solid matrices, greatly simplifying the analysis of samples with MS and anticipating, to ease the required or even leave out any sample preparation and enable analysis at ambient conditions, outside the instrument itself. These developments greatly extend the range of applications of modern mass spectrometry (MS). Ambient methods comprise many techniques; a particular prominent group is, however, the plasma-based methods. Although ambient MS is a rather new field of research, the interest in further developing the corresponding techniques and enhancing their performance is very strong due to their simplicity and often low cost of manufacturing. A precondition for improving the performance of such ion sources is a profound understanding how ionization works and which parameters determine signal response. Therefore, we review relevant compound characteristics for ionization with the two traditional methods ESI and APCI and compare those with one of the most frequently employed representatives of the plasma-based methods, i.e., low temperature plasma ionization. We present a detailed analysis in which compound characteristics are most beneficial for the response of aromatic nitrogen-containing compounds with these three methods and provide evidence that desorption characteristics appear to have the main common, general impact on signal response. In conclusion, our report provides a very useful resource to the optimization of instrumental conditions with respect to most important requirements of the three ionization techniques and, at the same time, for future developments in the field of ambient ionization.

Surface acoustic wave nebulization improves compound selectivity of low-temperature plasma ionization for mass spectrometry

“…In this context, it is expected that the droplet size and thus the overall size of the air–liquid interface is indeed much more reproducible with SAWN than with spontaneous desorption from a paper target. This assumption is supported by the fact that the repeatability was also better in similar experiments employing electrospray and atmospheric pressure chemical ionization 27 with pneumatically assisted nebulization in comparison to Paper-LTPI.…”

“…The median repeatability (assessed as median of the relative standard deviation of each analyte in replicate analyses) of LTPI analyses with paper targets was 46%, though decreased to 24% upon using SAWN. For signal response of dissolved analytes in mass spectrometry, desorption plays a crucial role 27 and the occupation of the air–liquid interface is prerequisite to successful desorption of a molecular species. In this context, it is expected that the droplet size and thus the overall size of the air–liquid interface is indeed much more reproducible with SAWN than with spontaneous desorption from a paper target.…”

“…Consequently, we suggest desorption of the neutral analyte, improved by nebulization and followed by gas phase ionization, to be the favored mechanism in SAWN-LTPI resulting in a higher relative sensitivity for polar analytes. However, results from a comparison of LTP and atmospheric pressure chemical ionization 27 imply that mere enhanced nebulization is not solely responsible for the lower dependence of signal response after SAWN on the polarity as well as on the molar volume of a compound; type and number of available charge carriers could become more important as desorption improves. Here, a comparison of SAWN with other nebulizing techniques such as nanospray 31 a.o.…”

“…As such, the obtained mass spectra are easy to interpret. On the other hand, the minimal sample preparation frequently leads to serious matrix effects hampering quantification with ambient techniques 24 , 27 and the achieved sensitivity with the setups described in the literature is still rather poor.…”

“…Making the ionization process independent of the embedding matrix while increasing the sensitivity of ionization could effectively overcome signal suppression and improve the analytical accuracy in applications of LTPI. A low vaporization enthalpy considerably enhances the ionization efficiency of analytes in LTPI 27 , 28 ; therefore, an additional supply of heat could increase sensitivity 29 , 30 . For thermally labile analytes, however, heating is not desirable and alternative approaches are required to improve desorption.…”