Continuing on forms of direct analysis from Part 9: Liquid Extraction Surface Analysis (LESA), in this article we take a look at direct analysis in real-time (DART)  and desorption electrospray ionisation (DESI).

DART & DESI are the most common direct analysis techniques [1][2]. Whilst these direct analysis techniques may seem to be solventless systems there is a degree of ablation/excitation of the surface using either a solvent or an excited gas to allow for the analysis of species on or near the surface of a material.

A list of possible ambient ionisation techniques and their year of introduction is given in Table 1‑3 at the bottom of the article. As can be seen from the table there is a wide and very varied list of techniques with some showing more relevance to the analysis of materials. Only two of the most commonly used techniques will be discussed in more detail.

Technique Acronym Year of Introduction
Desorption electrospray ionisation DESI 2004
Surface sampling probe SSP [4] 2004
Direct analysis in real time DART [5] 2005
Atmospheric solids analysis probe ASAP [6] 2005
Electrospray laser desorption ionization ELDI [7] 2005
Fused droplet electrospray ionization FD-ESI [8] 2005
Direct atmospheric pressure chemical ionization DAPCI [9] 2005
Matrix-assisted laser desorption electrospray ionization MALDESI [10] 2006
Jet desorption electrospray ionization JeDI [11] 2006
Extractive electrospray sonization EESI [12] 2006
Desorption sonic spray ionization DeSSI [13] 2006
Atmospheric pressure thermal desorption ionization APTDI [14] 2006
Helium atmospheric pressure glow discharge ionization HAPGDI [15] 2006
Plasma-assisted desorption ionization PADI [16] 2007
Dielectric barrier desorption ionization DBDI [17] 2007
Neutral desorption extractive electrospray ionization ND-EESI [18] 2007
Laser diode thermal desorption LDTD [19] 2007
Laser ablation electrospray ionization LAESI [20] 2007
Desorption atmospheric pressure photo-ionization DAPPI [21] 2007
Infra red laser ablation electrospray ionization IR-LAESI [22] 2008
Flowing atmospheric-pressure afterglow FAPA [23] 2008
Easy ambient sonic spray ionization EASI [24] 2008
Remote analyte sampling transport and ionization relay RASTIR [25] 2008
Laser ablation flowing atmospheric-pressure afterglow LA-FAPA [26] 2008
Low temperature plasma LTP [27] 2008
Desorption electrospray metastable-induced ionization DEMI [28] 2009
Liquid micro-junction surface sampling probe/electrospray ionization LMJ-SSP/ESI [29] 2009
Surface activated chemical ionization SACI [30] 2009
Single particle aerosol mass spectrometry SPAMS [31] 2009

DESI involves the spray of a charged microdroplets from a pneumatically-assisted electrospray needle, as per standard electrospray ionisation (ESI) mass spectrometry.  The spray is directed towards the surface of object, where it impacts the surface, desorbing the analytes into the gas phase where it is ionised and subsequently sampled by the mass spectrometer.  A number of factors can affect the analyte response and selectivity, such as capillary tip to sample and sample to collector distances as well as angles of incidence.

Figure 1 – Example of DESI set up [32]

DART relies on the formation of a plasma discharge in a heated helium gas stream to give atmospheric pressure chemical ionisation (APCI). The helium atoms react with water molecules via chemical ionisation processes and subsequent downstream ionisation of the sample occurs by thermal desorption into the hot gas stream and then into the mass spectrometer. For more details on the ionisation techniques see Ambient ionisation mass spectrometry: current understanding of mechanistic theory; analytical performance and application areas (Daniel J Weston Analyst 2010, 135 p 661-668) [33].

In general DART is used in fit for purpose applications as it is more geometrically independent when compared to a technique like DESI. DART has been used to identify common stabilisers used in polypropylene [34]  Other surface analytical techniques such as EDX or TOF-SIMs will not be discussed but they too have their potential niche area of analysis.


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