Two are better than one: Dual electrosprays for desorbed samples in ambient mass spectrometry

Dual spray

In the rapidly expanding world of ambient mass spectrometry, in which ions are created in the open air, there have been many variations on the theme since its introduction in 2005. One set of modifications involves the introduction of an electrospray jet which is fired through the stream of desorbed analytes to ionise them, adding more flexibility to the technique by separating the desorption step from the ionisation step.

The initial desorption step can be initiated by several different process such as laser desorption and laser ablation before the desorbed species are intercepted by the electrosprayed solution to produce ions, in some cases multiply charged ions. Judicious selection of the spray solvent can promote the ionisation of a particular analyte in a mixture and optimise the ion abundances.

In some experiments, analysts will want to spray a second solvent to produce a different set of conditions to analyse the same analytes, or to detect different types of compound in the same sample. While it might be desirable, this decision requires a time-consuming change of the spray solution, including cleaning steps to avoid carryover from the first to the second solvent.

One team of scientists in China has come up with a solution which avoids this cumbersome changeover, which they have illustrated in the Journal of Mass Spectrometry. Hai Luo and coworkers from Peking University simply added a second sprayer to the unit, so that switchovers can be carried out almost instantaneously. They illustrated the concept on a laser desorption mass spectrometry system and have named it laser desorption dual spray post-ionisation mass spectrometry.

Switching sprays

Luo tested the new system on an ion trap mass spectrometer which was modified to allow an IR laser to irradiate the target and to accept two homemade spray ionisation emitters. The sprays were positioned in line with the mass spectrometer inlet while crossing the beam of desorbed analyte species.

Wet samples were placed on a gold plate and desorbed with a Nd:YAG laser operating at 1064 nm and one of the two sprays was switched on to intercept the analytes. Switching from one spray to the other was accomplished in a few seconds simply by switching off the power for one spray and switching on the power for the other. The rapid switching time gives almost simultaneous detection of different compounds in the same sample.

The dual-spray system was demonstrated with a range of compound types, including chocolate, proteins, and magic number clusters.

From triglycerides to proteins to clusters

In the first instance, a portion of melted dark chocolate was analysed. With a spray solution of pure acetonitrile, prominent peaks were observed for the triglycerides which were present mainly as their ammonium adducts. No protonated or alkali metal ion-adducted species were observed. Some non-adducted triglyceride dimer ions were also seen at higher m/z values.

When the spray solution was switched to aqueous methanol, the water-soluble saccharides maltose and sucrose were observed. They appeared as multimers containing at least five saccharide molecules that were adducted with potassium ions. This is a prime example of how the dual spray system can simply and rapidly analyse compounds with different properties in the same sample, aided by the respective solution chemistries.

In another example, the utility of the system was demonstrated by studying the charge-state distribution (CSD) of the protein cytochrome c, which gives insight into its structure. Spraying with aqueous methanol gave a very narrow CSD, with large peaks corresponding to the 7+ and 8+ charge states. Switching to pure acetonitrile produced abundant charge states from 7+ up to 16+ in a bimodal CSD, indicating that some denaturation had occurred.

The addition of acetic acid to the aqueous methanol spray solution changed the CSD again with high charge states due to denaturation clearly observed. Other agents like diethylamine and 3-nitrobenzyl alcohol were also used to manipulate the CSD.

In the third example, the reactivities of different nucleobases with potassium were compared by using the laser to desorb potassium chloride solution and having either thymine or 5,6-dimethyluracil in the spray solutions. The corresponding nucleobase pentamers complexing with potassium ions were observed. When a solution of thymine and dimethyluracil was laser irradiated and thymine was added to the spray solution, mixed clusters were obtained.

These three cases illustrate the usefulness of laser desorption dual spray post-ionisation mass spectrometry when switches of conditions are deemed necessary to investigate reactions, structures and mixtures. In theory, more sprays could be added too, to give even more flexibility.