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 . 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.
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 
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) .
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  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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