Electro-Focusing Liquid Extractive Surface Analysis (EF-LESA) Coupled to Mass Spectrometry

Analysis of the chemical composition of surfaces by liquid sampling devices interfaced to mass spectrometry is attractive as the sample stream can be continuously monitored at good sensitivity and selectivity. A sampling probe has been constructed that takes discrete liquid samples (typically <100 nL) of a surface. It incorporates an electrostatic lens system, comprising three electrodes, to which static and pulsed voltages are applied to form a conical “liquid tip”, employed to dissolve analytes at a surface. A prototype system demonstrates spatial resolution of 0.093 mm2. Time of contact between the liquid tip and the surface is controlled to standardize extraction. Calibration graphs of different analyte concentrations on a stainless surface have been measured, together with the probe’s reproducibility, carryover, and recovery. A leucine enkephalin-coated surface demonstrated good linearity (R2 = 0.9936), with a recovery of 90% and a limit of detection of 38 fmol per single spot sampled. The probe is compact and can be fitted into automated sample analysis equipment having potential for rapid analysis of surfaces at a good spatial resolution.


Measurement of Successive Liquid Tips And Residue Droplets To Assess Their Dimensions. (Tables S1, S2 and S3)
The dimensions of the liquid tip were recorded from successive tip images taken on a x20 USB camera, calibrated against an engineer's scaled ruler with 0.5 mm graduations. For simplicity it shall be considered that the tip has radial symmetry along its length so that it can be characterised by measurements of diameter and height indicated in Figure 1 and the text of the manuscript.

Table S1
(A) (B) n w n /mm Table S1. Measurement of the liquid tip width, w n (at the neck) and w c (at contact) for 28 successive liquid tips formed on a stainless steel surface with h s = 0.30 mm, h g = 0.50 mm (refer to Figure 1). The average values of the widths n w and c w (errors to 1 s.d.) are tabulated for, (A) electro-focusing on, i.e. V 1 (= V 2 ) = +440V and V 3 = -500 V, and (B) electro-focusing off, V 1 = V 2 = V 3 = 0. Table S2. Exemplar data illustrating the residue droplet dimensions, on a stainless steel surface immediately after liquid tip contact is broken, and estimation of the mean width r w , height r h and volume, V g calculated from the width w r and height, h r of 11 successive liquid tips formed with h s = 0.30 mm, h g = 0.50 mm (refer to Figure 1) and with electro-focusing, i.e. V 1 (= V 2 ) = +440V and V 3 = -500 V.  Carry over was measured for an initial sampling of 14.4 pmol/µL LE spot (1µL deposited), followed by five successive samplings of neighbouring blank spots, i.e. spots of blank solvent, 100% water.
The probe was not washed between successive samplings. The signal fall off from 100% (initial analyte signal equivalent to 0.37 pmol) was: 10.7%, 1.3%, 0.3%, 0.3% and 0.3%. The data is shown below in Figure S2, errors are calculated to 2 s.d. (95% confidence). It can be seen that the LE signal falls off exponentially and follows where c is the amount (or concentration) remaining after n samplings, c 0 is the initial amount (or concentration) and k is a constant ≈2.

Calculation of the conical tip height, h t
The volume of liquid in the sessile droplet, V g , will be equal to the volume in the conical tip, V c (refer to Figures 1A and 1B), where and solving gives the height of the conical tip, h t (refer to Figure 1B

Higher Spatial Resolution Operation
It was found after a few days the EF-LESA probe became more stable and reproducible in operation; some probes gave higher performance routinely. It is not clear what process conditioning takes, whether it relates to improved wettability to the circumference of the plastic tip or reducing imperfections of the silver coating to produce more reproducible and finer liquid tips. Two cases of finer liquid tip formation were observed (see Figures S3A and S3B). Both cases were initiated when a higher voltage than normal V 1 voltage is applied i.e. ≈ +500 to +1200 V, inducing other tip modes. The voltage is controlled to be below ESI onset and immediately followed by reducing V 1 quickly to ≈ +450 V. The first case ( Figure S3A), a tip is formed that is <100 µm. A second mode ( Figure S3B) is observed to show a glow discharge established and a liquid tip size ≈ 100 -200 µm. Without adaptation of the apparatus this cannot unambiguously be verified and requires further investigation. These modes were observed to be reproducible.
(A) (B) Figure S3. Pictures of higher spatial resolution operation taken of liquid tips under higher initial voltage conditions for V 1 (=V 2 ) where the probe surface has conditioned to produce fine liquid tips, (A) a stable fine tip is formed by reducing the voltage immediately on liquid tip contact (a reflection of the tip can be seen in this image); (B) the case where a glow discharge appears to be formed with a fine but ill defined liquid tip (the photograph was taken at an angle, the tip was mounted vertically).