5 APCI-Related Techniques (DART, DAPCI, ASAP)


The increasing number of ambient ionisation techniques using an electrical discharge to effect ionisation, either before, during, or after sample desorption, can be broadly categorised as relying upon fundamental principles of atmospheric pressure chemical ionisation (APCI). These solvent-free systems include, for example, direct analysis in real time (DART), atmospheric solids analysis probe (ASAP), direct atmospheric pressure chemical ionisation (DAPCI), plasma-assisted desorption/ionisation (PADI), dielectric barrier discharge ionisation (DBDI), helium atmospheric pressure glow discharge ionisation (HAPGDI) or flowing afterglow flowing atmospheric-pressure afterglow (FAPA), surface activated chemical ionisation (SACI), and low temperature plasma (LTP). Typically, when used in LC-MS, APCI techniques are amenable to polar and non-polar, low molecular weight analytes and their transposition to ambient ionisation is no exception. As a general rule, these techniques do not usually extend to the analysis of larger (bio) molecules, much like the traditional APCI methods upon which they are based.


As with many APCI-based techniques, DART relies upon the formation of a (distal) plasma discharge in a heated helium gas stream. These mechanisms have been attributed to possible Penning ionisation or proton transfer from excited solvent (water) clusters, largely understood through early fundamental mass spectrometry, to give simple APCI-like spectra. It is proposed that a series of metastable helium atoms react with water molecules via chemical ionisation processes, and downstream ionisation of the sample occurs by thermal desorption into the hot gas stream, and subsequent sampling by the mass spectrometer.


In ASAP the effect of solvent conditions and source dryness can affect whether a conventional proton-transfer ionisation occurs or a preferential charge-transfer ionisation due to a reduction in available reagent species, such as residual water.


When considering applicability of the DART method, perhaps the main drawback is a lower effective mass range, which, although a general function of APCI-based ionisation mechanisms, is a clear disadvantage compared to DESI, where successful analysis of proteins up to 66 kDa have been reported. That said, DART mass spectra are often more simple to interpret, due to lack of gas-phase adduction from the solvent environment, as seen with ESI-based counterparts such as DESI.


The emergence in the last few years of a series of new plasma-based techniques, such as LTP, PADI and FA-APGD, have demonstrated a great deal of finesse, in particular for the potential removal of a number of issues often precluding a safe and elegant in vivo ambient ionisation experiment (e.g. elevated temperature, exposure to high voltages). These techniques rely upon ionisation at the sample surface using a proximate plasma source, rather than a distal plasma such as used in DART, and effect APCI-like ionisation. PADI uses a radio-frequency generated plasma for desorption and ionisation of target analytes. The temperature of the plasma approximated that of the ambient environment, which could be of use for thermally-labile samples, however the sample was still subjected to the actual discharge. A recently disclosed plasma-based method showing increasing applicability is the low temperature plasma (LTP) technique; here instead of the physical restrictions of placing the sample between two counter electrodes, as in conventional DBDI, the plasma is generated in a probe configuration allowing increased flexibility for nonproximate analysis of bulk samples or large objects, akin to a DESI experiment.


The field of ambient MS has progressed rapidly in the last few years, with key advances in both technological and mechanistic understanding creating new directions, which often seem limited only by the imagination of the scientist. Arguably, this area of science could be described as being in its formative years and recommendations have been made for closer investigation of sampling heterogeneity, ion suppression effects, and a critical evaluation of areas for improvement. It is for these reasons that the BMSS Ambient Ionisation SIG has been formed. The aims of the Ambient Ionisation SIG are to survey the level of use of ambient ionisation across BMSS, to support practitioners of ambient ionisation MS, demonstrate the applicability of the different techniques to different application areas, investigate sampling heterogeneity towards better quantitative analysis and understanding suppression/enhancement effects. The Ambient Ionisation SIG has surveyed BMSS members as to what techniques they are currently using and in what application areas. This information is currently being used to develop an Inter- lab comparison across as many techniques as possible in the most important application areas. The comparison will involve running a series of samples (which will be supplied) under test conditions which we would hope takes less than a day. The results of this will be fed back at a one day meeting to be held later in 2013.


Figure 2. PADI (Plasma Assisted Photon Desorption/Ionisation) (Courtesy Dave Barrett, University Nottingham)


Figure 3. Further example of an ambient ionisation technique. (Courtesy of Peter Stokes, Durham University)


This article was originally published in the British Mass Spectrometry Society’s Mass Matters, 67th Ed. April 2012, pp13-14.


Thanks to Dan Weston and the RSC for permission to use abstracts from The Analyst 2010 Apr. 135(4) 661.


ESI OR SPRAY RELATED TECHNIQUES Desorption electrospray ionisation (DESI)


Extractive Electrospray Ionisation (EESI)


Neutral Desorption Extractive Electrospray Ionisation (ND-EESI) Easy Ambient Sonic Spray Ionisation (EASI) Jet Desorption Electrospray Ionisation (JeDI) Liquid Extraction Surface Analysis (LESA) Paperspray


SPRAY-BASED PHOTON/ENERGY TECHNIQUES Electrospray Laser Desorption Ionisation (ELDI)


APCI-RELATED TECHNIQUES Direct Analysis in Real Time (DART)


Matrix-Assisted Laser Desorption Electrospray Ionisation (MALDESI) Atmospheric Solids Analysis Probe (ASAP) Desorption Atmospheric Pressure Photo Ionisation (DAPPI) Laser Ablation Electrospray Ionisation (LAESI)


Infra Red Laser Ablation Electrospray Ionisation (IR-LAESI)


Direct Atmospheric Pressure Chemical Ionisation (DAPCI) plasma-assisted desorption/ionisation (PADI) dielectric barrier discharge ionisation (DBDI)


Laser Desorption Atmospheric Pressure Chemical ionisation (LD-APCI) helium atmospheric pressure glow discharge ionisation (HAPGDI)


flowing afterglow flowing atmospheric-pressure afterglow (FAPA) surface activated chemical ionisation (SACI) low temperature plasma (LTP) Laser Diode Thermal Desorption (LDTD)


INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2013


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