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Waters UniSpray Ionization Source

INTRODUCTIONThe first commercial electrospray ionization (ESI) sources for mass spectrometers became available at the end of the 1980s. These early sources broadened the applications of mass spectrometry (MS) to biological compounds such as peptides and proteins which would typically be infused into the source at low flow rates of 1–5 µL/min. Under these low flow conditions, atomisation was achieved via a classical Taylor cone which was formed at the end of the liquid capillary following the application of a few kilovolts between the capillary and the ion inlet cone. More recently, this type of infusion-based analysis for biological applications has been superseded by nanospray ESI capillaries that operate at extremely low flow rates (10–1000 nL/min) and offer high ionization efficiency for small sample volumes. From a commercial viewpoint, the greatest leap in the utilisation of ESI sources came from the marriage of mass spectrometry with liquid chromatography (LC-MS) which enabled analytical chemists to benefit from the enhanced specificity offered by both techniques. In order to adapt ESI sources to the high flow rates (0.1–1.0 mL/min) typically used in LC, or its modern equivalent UPLC, it was necessary to aid the atomisation process with the addition of a concentric flow of high velocity nitrogen gas at the ESI tip. However, when conducting infusion experiments with a fixed analyte concentration from flow rates of 10 nL/min to 1 mL/min, it is common to observe only a 20 times increase in analyte ion signal at the higher flow rate whilst the analyte consumption has increased by a factor of 100,000. This is known to be due to the poor ionization efficiency of high flow rate ESI which is critically dependent on a number of factors such as droplet size distribution, droplet charge per unit volume, droplet evaporation rates and additional factors such as the inlet sampling efficiency. Additionally, ionization efficiency can become particularly challenging when using highly aqueous mobile phases. In this white paper, we introduce a new ionization technique called UniSpray™ which attempts to increase ionization efficiency by interacting a high velocity spray with a high voltage, cylindrical target that is positioned in an off-axis, cross-flow arrangement. A number of physical processes will be described which are believed to be important mechanisms that lead to enhanced UniSpray sensitivity when compared to ESI, viz. high Weber number droplet impacts, the Coanda effect, vortex shedding, and counter-rotating surface microvortices.

S. BajicWaters Corporation, Wilmslow, UK

UniSpray Ionization Source.

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WHAT IS UNISPRAY?UniSpray shares some common features with high flow rate ESI sources in that the liquid flow is nebulised by a high velocity, concentric nitrogen gas flow. However, in comparison to ESI, the high voltage is applied to a cylindrical target electrode that is positioned in close proximity to the grounded nebulizer such that the near-supersonic jet impinges on the target surface. A schematic of the UniSpray source is shown in Figure 1. The source is formed by directing a high velocity nebulised jet from a grounded sprayer onto a cylindrical metal target that is held at a high voltage and is located between the sprayer and the ion inlet orifice of the mass spectrometer. For singly charged analytes, it is conventional to use a positive potential of typically 1 kV for positive ion analysis and vice versa for negative ion analysis. For multiply charged analytes, such as peptides and proteins, it is generally found that higher potentials of 3–4 kV are required for signal optimisation. The pneumatically-assisted nebulizer is formed from a 130 µm I.D. by 220 µm o.d. liquid delivery capillary (stainless steel) that is surrounded by a 330 µm I.D. nebulizer tube (stainless steel) with a restriction length of 10 mm. Nitrogen gas is delivered to the nebulizer tube at a gauge pressure of 7 bar. Under these conditions, the nitrogen jet will be near supersonic at a distance of a few millimetres from the nebulizer tip. The distance over which the inner capillary protrudes from the nebulizer tube is adjustable and is typically found to optimise at very small protrusions (<0.2 mm) where the spray appears highly collimated. In a conventional manner, the nebulizer is also surrounded by an annular heater that delivers hot nitrogen gas at a flow rate of typically 1200 L/hr.

The 1.6 mm-diameter, 35 mm long, cylindrical HV target is constructed from a cold-drawn, 316L stainless steel wire that is polished to a near-mirror finish with 1 µm-grade lapping paper. The target is connected to a 0–5 kV DC power supply via a 47 MΩ current-limiting resistor. Since the target has a low thermal conductance to the source housing, it rapidly reaches an equilibrium temperature that is equal to the local temperature of the heater gas (typically >250 °C for a heater set-point temperature of 500 °C). In order to optimise source sensitivity, it is critically important to adjust the point at which the collimated spray impacts on the cylindrical target. As shown in Figure 1, the maximum signal intensity is typically obtained when the spray is asymmetrically positioned such that it impacts on the upper right quadrant of the target. Under these conditions, the gas flow becomes attached to a portion of the curved surface and results in asymmetric gas streamlines in the wake that are directed towards the ion inlet orifice. This flow phenomenon is known as the Coanda effect. Under the influence of the Coanda flow field, ions and charged droplets are directed towards the ion inlet which is surrounded by a cone gas nozzle that supports a drying gas flow of nitrogen at 150 L/hr.

UniSpray sources are generally found to give enhanced ionization efficiency when compared to high flow rate ESI which, in turn, can lead to enhanced analytical sensitivity. Some general characteristics of the UniSpray source have been studied by Lubin et al1. These authors compared the signal intensity obtained by ESI and UniSpray for small pharmaceutical compounds (16 analytes in positive ion mode and 7 in negative ion mode) with acidic, basic and neutral mobile phase types. Figure 2 shows the signal gains (UniSpray ion intensity divided by ESI ion intensity) obtained at various mobile phase compositions and for three different flow rates. Here, each point is an average of the pooled data for each compound and each mobile phase type. From these plots it can be concluded that the signal intensity observed with the UniSpray source is, on average, higher than the ESI signal for all mobile phase compositions with relatively higher gains observed under higher aqueous conditions. The full data set exhibited a strong compound dependence where, whilst UniSpray gains in excess of x20 were observed under certain conditions, other analytes could occasionally give greater responses with an ESI source.

In view of the performance enhancements outlined above, the following sections will consider some underlying physical mechanisms that are unique to the UniSpray source and may contribute to the observed increase in ionization efficiency or ion sampling efficiency.Figure 1. A schematic of the UniSpray API source.

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DROPLET SIZE DISTRIBUTION: THE LIMITATION OF HIGH FLOW RATE ESIIn the traditional ESI model, charged droplets are formed by positive and negative charge separation in the high electric field region at the tip of the liquid capillary. At very low flow rates (<1 µL/min), this process proceeds with extremely high efficiency to yield highly-charged, sub-micron droplets that almost instantaneously give rise to gas phase ions due to Rayleigh disintegration processes2. However, at the high flow rates typically used in LC and UPLC-MS (100–800 µL/min), it is necessary to use a high velocity nebulizer gas to atomize the liquid flow, a process which is known to produce larger droplets with a lower charge per unit volume and hence lower ionization efficiency3.

Whilst the use of a high velocity gas is advantageous from an atomisation viewpoint, it has the disadvantage of reducing the residence time of droplets between the ESI capillary and the ion inlet orifice of the mass spectrometer, which, in turn, reduces the time available for droplet evaporation which is critical to the Rayleigh disintegration process. In-house measurements, using Phase Doppler Anemometry (PDA) and Laser Diffraction Particle Sizing (LDPS) reveal that the ESI and UniSpray nebulizers typically produce initial droplet size distributions that peak at typical diameters of d0=10 µm in the volume domain and d0=1 µm in the number domain. Figure 3 shows typical LDPS droplet distributions obtained by nebulising a flow of 0.5 mL/min of water under ambient conditions using a nebulizer as described above (no target). These distributions reveal that although the greatest numbers of droplets are produced at micron and submicron diameters, the overwhelming volume of the spray, and hence analyte, is contained within droplets that are significantly greater than a micron in diameter. PDA measurements also reveal that droplets on the spray axis can have average velocities in excess of 100 ms-1 which corresponds to a residence time of <100 µs for the sources used in this study (ignoring recirculation effects). This source residence time represents the time period during which significant droplet evaporation must occur in order to promote the Rayleigh disintegration process. Droplet evaporation rates have been determined for a number of common solvents and

experimental techniques where, in particular, the ping-pong drift cell method4 more closely resembles the dynamics of LC/MS sources. The diameter of a droplet, dp, after an evaporation time, t, an evaporation rate, s, and an initial droplet diameter, d0, is given by

where s has been experimentally determined as 1250 and 6500 µm2s-1 for water and acetonitrile, respectively4. According to equation (i), Figure 4 shows a plot of initial droplet diameter versus droplet diameter after an evaporation period of 100µs for water and acetonitrile. From these data, it becomes apparent that (i) droplets above 2 µm in diameter do not undergo significant evaporation in this time frame and (ii) rapid evaporation only occurs for diameters below the “knee” of the curve, which corresponds to approximately 0.4 and 1.0 µm for water and acetonitrile, respectively. Initial droplets diameters below the value d0(crit.) in Figure 4 will completely evaporate in this time period (approximately 0.35 and 0.8 µm for water and acetonitrile, respectively). In essence, this would suggest that a 100 µs residence time is totally inadequate for efficient evaporation of the larger droplets that dominate the volume distribution in high flow rate ESI. While this laminar-flow evaporation model may be oversimplified, it would suggest that only sub-micron droplets would participate in the production of gas phase ions by the ESI process. Since the sub-micron population represents <1% of the total sprayed

Figure 2. A comparison of the average signal intensity ratios for UniSpray and ESI versus mobile phase composition for 16 positive ion analytes and 7 negative ion analytes (from reference1).

Figure 3. Typical droplet size distribution for a pneumatically assisted ESI probe at a flow rate of 0.5 mL/min of water (Malvern Instruments Spraytec system).

dp2 = d0

2 − st (i)

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volume at high flow rates (Figure 3), this would contribute to the low ionization efficiency typically observed with high flow rate ESI when compared to nanospray ESI (<1 µL/min), where submicron-sized droplets contribute significantly to the volume distribution.

HIGH WEBER NUMBER DROPLET IMPACTSThe above inefficiency argument was the original inspiration for the UniSpray ionization source that compels high velocity droplets to impact onto a hot metallic surface which results in their break-up into smaller secondary droplets that can be evaporated more efficiently at low source residence times. A number of groups have studied the break-up of water droplets on heated stainless steel surfaces5. These workers experimentally determined that the number of visible secondary droplets (Nvis) produced per impact was directly proportional to the droplet Weber number (We), which, in turn, is directly proportional to the droplet diameter (d) and the square of its velocity (v):

where is ρ the droplet density and σ is the liquid surface tension. The Weber number can be regarded as the ratio of droplet kinetic energy to droplet surface energy. Thus, it stands to reason that the collision of a droplet with kinetic energy that far exceeds the droplet surface energy will result in instability and droplet break-up.

To illustrate the break-up mechanism, Figure 5 shows a time lapse sequence of a water droplet (We=630) as it impacts on a planar, polished stainless steel surface which is held at a temperature of 260 °C. In the case of a UniSpray source, it can

be shown from equations (ii) and (iii) that a water droplet with a diameter of 4 µm and a velocity of 100 m/s would have a Weber number of 571 and would give rise to 35 visible secondary droplets on impact with the hot target. If we consider a simple, linear break-up model, this would result in secondary droplets with diameters of the order 1.2 µm. More realistically, a skewed Gaussian distribution for the secondary population would contain sub-micron droplets (equivalent to the invisible droplets that could not be detected by the experimental method used in the above work). As described in the previous section, an increase in the sub-micron droplet population could be expected to significantly increase the droplet desolvation process and aid ionization efficiency.

In addition to shifting the droplet size distribution to smaller diameters, the impact process may play an additional role that may be important for ionising certain analytes that may be weakly ionised by ESI due to their low surface affinity. In ESI, droplets with a high surface affinity (such as detergents) tend to preferentially ionise at the droplet surface and are ejected from the parent droplet as Rayleigh fission processes occur. Less surface active analytes which may previously have been paired with negative counterions in the core of the parent droplet may now populate the surface. However, the surface charge of the parent droplet has become reduced as a result of the fissions which greatly reduces the probability of further fissions and hence ionization of the less surface active analytes. A prompt and violent breakup of initial droplets that have already partitioned due to their relative surface affinity may increase the probability of secondary droplets being formed from the core of the initial droplet provided that adequate charging occurs during this process.

Figure 4. Evaporation rates for water and acetonitrile droplets for a 100 µs evaporation time.

Figure 5. High speed photographs of the break-up of a water droplet on a hot stainless steel plate (from reference5).

Nvis = 0.0427 We + 10.465 (ii)

We = ρv2d⁄σ (iii)

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When a droplet in a flow of gas approaches a target, the necessary conditions for a collision with the target will be determined by the Stokes number, Stk, and the turbulence of the flow where

Here, µ is the dynamic viscosity of the gas, D is the diameter of the target and the other terms are as defined in equation (iii). Flow turbulence is governed by the Reynolds number, Re, which will be described in more detail in the following sections of this paper. Droplets that have Stk≤1 (termed Stokesian particles) tend to follow the gas streamlines whilst those with Stk>1 tend to deviate from the streamlines and strike the target owing to momentum dominance over the drag forces. Figure 6 shows a plot of Stk versus droplet diameter for water droplets with a velocity of 100 m/s and a UniSpray target diameter of D = 1.6 mm. This shows that small droplets with diameters below dlim.= 2.5 µm would be expected to avoid collisions with the target and would follow the gas streamlines. Conversely, larger droplets (d>2.5 µm), which account for the majority of the spray volume in a UniSpray source (Figure 3), would be expected to strike the target, thus creating smaller secondary droplets. As previously mentioned, the impact efficiency will decrease with increasing flow turbulence. Importantly, it should also be noted that the smaller droplets that avoid collisions with the target will still benefit from other unique flow characteristics of the UniSpray source which will be described in the following sections.

Although the impact of high Weber number droplets is believed to be an important feature of the enhanced sensitivity observed with the UniSpray source, it can be experimentally demonstrated that wide area, flat-plate stainless steel targets do not give rise to significant and stable signal enhancements over ESI. In this respect, it is believed that the geometrical form of the UniSpray source, shown schematically in Figure 1, plays an additional role in the enhancement of ionization efficiency. In particular, it is believed that the curved profile of the target and the off-axis, perpendicular cross-flow arrangement between the sprayer and the target give rise to two important gas flow phenomena that may aid the break-up and desolvation of liquid droplets in the source, viz. the Coanda effect6 and surface microvortices7.

THE COANDA EFFECTThe Coanda effect is a phenomenon whereby a fluid flow attaches itself to a nearby surface and remains attached even as the surface bends away from the initial fluid direction. Figure 7(a) shows a schematic illustration of this effect and an actual photographic image of flow attachment to the high-voltage cylindrical target in a UniSpray ionization source. This image shows that the gas and droplets from the off-axis, nebulizer jet are deflected towards the ion inlet orifice as shown schematically in Figure 1. As a water droplet evaporates, the gas surrounding the droplet can rapidly become saturated with water vapour, which ultimately reduces the rate of evaporation due to re-condensation on the droplet surface. In API sources, this effect is minimised by supplying a flow of heated dry nitrogen gas to the nebulised spray where the nitrogen becomes “entrained” into the nebulizer flow due to the low pressure created by the high velocity nebulising jet. In the case of Coanda flow attachment to a curved surface, as occurs at the UniSpray high-voltage target, it is the imbalance of the entrainment flow where flow cannot penetrate from one side that ultimately leads to a deflection of the gas streamlines towards the target surface. This effect is known to create a stronger total entrainment when compared to a free jet which could aid the droplet desolvation process by enhanced mixing and hence dilution of the water vapor.

Figure 6. A plot of Stokes Number versus droplet diameter for 100m/s water droplets in a UniSpray source.

Stk = ρvd2 ⁄ 18µD (iv)

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Figure 7(b) illustrates the effect of “droplet beam steering” that occurs due to the Coanda effect in UniSpray sources. As the nebulizer jet impact point is adjusted from a central to an off-axis position on the upper right hand quadrant of the Ø1.6 mm target, the ion signal intensity rapidly increases until an optimum is reached. This characteristic is found to be compound dependent but is generally a subtle, as opposed to a critical, tuning parameter that allows an acceptable compromise tuning position to be found for multi-polarity analyte mixtures. Conversely, Figure 7(b) shows that impacting on the upper left hand quadrant of the target results in a severe loss of ion signal as the deflected wake is directed away from the ion inlet cone and ion sampling efficiency is adversely affected. It should be noted that the term beam steering is used as opposed to focussing since the gas streamlines will not cross to form a focal point. It is also anticipated that this flow geometry will give rise to a momentum separation effect where smaller droplets will tend to follow the Coanda flow towards the MS ion inlet whilst the larger non-Stoksian droplets will continue in a forward direction and become separated from the Coanda flow.

Effects that occur in the “wake” of the deflected gas flow could also be beneficial in terms of droplet break up and enhanced droplet desolvation as charged droplets are directed towards the ion inlet cone. One such effect is periodic vortex shedding that occurs behind a cylindrical object where the frequency of shedding increases as the velocity of the gas flow increases. Figure 8 shows an example of classic vortex shedding that occurs behind a cylinder as the gas flow moves from the top to the bottom of the image. The transition between laminar (uniform) flow and turbulent flow in these systems is determined by the Reynolds number (Re) where

where ρ and µ are the density and viscosity of the gas. At low Re (<10), the gas will flow uniformly around the cylinder which results in a uniform wake that contains no vortices. As Re is increased to a value of, for example 400, the flow wake will include a number of vortices as shown in Figure 8. This flow regime is not considered to be turbulent since the “vortex street” is relatively two-dimensional, such that the same vortex pattern exists in a

parallel plane in front of, or behind, the plane of Figure 8. However, as Re progressively increases above a value of 2300, the flow becomes turbulent, or three-dimensional. It can be shown that the near-supersonic gas flow at the UniSpray target will have a highly turbulent flow wake with a Re of typically 30,000. Under these conditions, it is proposed that primary or secondary droplets will be subjected to enhanced desolvation due to increased shear forces, increased entrainment (mixing) and an increased residence time in the turbulent flow.

To understand the importance of turbulence in aiding droplet break-up, we should reconsider the expression for the Weber number (equation (iii) above). In pneumatic nebulisers, liquid droplets are created with low initial velocities, vl, and are surrounded by a high speed gas jet with a velocity, vg. It is the difference in these two velocities that gives rise to initial droplet shearing where v2 in equation (iii) is now substituted by (vg−vl)2. In a pneumatically-assisted ESI probe, where the gas flow is significantly less turbulent than the gas flow at a UniSpray target, the initial droplets reach a Figure 7. (a) A schematic and photographic evidence of Coanda gas flow in a UniSpray source;

(b) The dependence of UniSpray ion signal versus the position of the spray impact point.

Figure 8. A typical illustration of van Karmen vortex flow behind a cylinder for moderate Reynolds numbers.

Re = ρvD⁄µ (v)

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terminal velocity at a distance of approximately 5 mm from the tip. At this point, vg−vl becomes zero and the shearing process stops. In comparison, the UniSpray source creates chaotic, turbulent flow over a greater proportion of the droplet in-source trajectory which reduces the probability that vg−vl=0, particularly for the larger, non-Stokesian droplets.

In the case of the UniSpray source geometry, it can be shown that the frequency of shedding (fs) is given by

where v is the freestream gas velocity, D is the cylinder diameter and Sr is the dimensionless Strouhal number. Sr is related to the Reynolds number (Re) by

For a near-supersonic nitrogen gas flow (v=300 ms-1) and a Ø1.6 mm cylinder, it can be shown that the vortex shedding frequency in the wake of the UniSpray target may be as high as 37 kHz (ultrasonic). As an experimental observation, it is found that the point of signal optimisation shown in Figure 7(b) is accompanied by both a visual increase in “wake disturbance” and an audible increase in sound from the UniSpray source. On this latter point, it is not clear whether significant energy is mechanically imparted into the cantilevered target, but experiments in this laboratory with ultrasonic nebulisation via a similar 40 kHz agitation of a target (no gas) reveal enhanced ionization of non-polar analytes that are difficult to ionise by ESI; relatively large sensitivity enhancements over ESI for certain non-polar compounds is also a characteristic of UniSpray under certain conditions.

CROSS-FLOW SURFACE MICROVORTICESAs described in the section above, a gas flow will become attached to the surface of a cylinder at the stagnation point and will detach from the surface at a downstream point known as the separation point. This is shown schematically in Figure 9 where the region bounded by the stagnation and separation points is known as the stagnation zone. For the near supersonic nebulizer conditions encountered with UniSpray, the stagnation zone can be shown to have a radial thickness of the order 10 µm over which a flow velocity gradient exists from zero at the surface to the freestream velocity. Primary or secondary droplets that enter this reduced velocity region would be expected to experience enhanced desolvation due to (i) an increased residence time, (ii) greater thermal transfer from the hot target surface and (iii) increased agitation/entrainment with the hot nitrogen flow due to surface vortex effects.

This latter point is a further feature that is unique to the cross-flow geometry of the UniSpray source and will be considered in more detail in this section.

For a cylinder in cross-flow, a uniform gas flow will become inherently unstable (three dimensional) in the stagnation region where the flow becomes attached to the curved surface. These instabilities take the form of a linear series of counter-rotating vortices whose axes of rotation are aligned with the streamlines of the gas flow. A single vortex pair is shown schematically in Figure 10(a). The disturbance wavelength, λ, between adjacent counter-rotating pairs is known to be directly proportional to the cylinder diameter (D) and inversely proportional to the square root of the local Reynolds number (Re) so that

where κ is a constant and Re is defined in equation (v) above. Kestin and Wood7 have used wind tunnel tests with oil coated cylinders to experimentally verify the relationship between λ and Re and their results are reproduced in Figure 10(b). Using the Tu=4% (tubulence intensity) plot and assuming a nebulizer velocity of Mach-1 and D=1.6 mm, Figure 10(b) would predict a disturbance wavelength (λ) of approximately 37 µm for the surface microvortices on a UniSpray target electrode. Firstly, this disturbance dimension is significant in that it is of the same order as the size of the larger droplets and thus could be expected to efficiently impart energy into the droplets to aid break-up. This efficiency dependence on the size of the perturbation is analogous to, for example, microwave heating where the microwave frequency is chosen to match the natural rotational frequencies of water molecules. Secondly, the highly mobile smaller droplets that enter the vortex region would be highly agitated compared to those in the freestream and would benefit from enhanced mixing and enhanced heat transfer.

Figure 9. A schematic representation of the stagnation zone that is formed as gas flows over a cylinder.

fs = v Sr ⁄D (vi)

Sr = 0.198(1−19.7/Re) (vii)

λ = κD⁄√Re (viii)

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Therefore, in a similar manner to the vortex shedding region described in the previous section, this surface boundary layer region represents an additional desolvation enhancement zone that subjects droplets to an increase in residence time, shearing and entrainment.

Figure 11 shows a scanning electron microscope (SEM) image of a 1.6 mm diameter, stainless steel UniSpray target which was used for the analysis of analytes contained in protein-precipitated human plasma. The granular, circular “halo” is due to the deposition of involatile components of the plasma and is not relevant to the present discussion. The SEM image was taken in the same direction as the impinging droplet stream and nebulizer gas jet. The cross (+) in Figure 11 is an approximation of the point of impact, and hence stagnation point, of the incoming nebulizer jet and was located from an optical micrograph of the same target. A close examination of the circled region of the image reveals a linear series of striation marks which are aligned with the direction of the flow streamlines. These striation marks, which are not visible at optical wavelengths, are evidence of the existence of counter-rotating surface vortices as described above. From this image, we can estimate a disturbance wavelength (λ) of 23 µm which assumes that three striation marks represent the outer extent and centre of one counter-rotating vortex pair. Thus, there would appear to be some correlation between the observed experimental data and the theory of surface vorticity for cylinders in cross-flow.

Figure 10. (a) A schematic of a counter-rotating vortex pair on the surface of a cylinder with a cross flow of gas; (b) The relationship between disturbance wavelength (λ) and the inverse of the square root of the Reynolds number for a cylinder in cross flow (from reference7).

Figure 11. An electron micrograph of an UniSpray target showing evidence for the existence of a linear series of surface microvortices at the flow separation point.

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THE ROLE OF THE TARGET SURFACEIn this paper, we have sought to provide an explanation of how the unique geometry of the UniSpray source gives rise to a number of hydrodynamic and aerodynamic phenomena that may aid in the nebulisation and desolvation of primary droplets from the liquid capillary. Whilst these hypotheses may be supported by the observation of a greater increase in sensitivity over ESI for high flow rates and highly aqueous mobile phases, where nebulisation and desolvation are particularly difficult, it is clear that a full understanding of these effects is far from complete. The processes described thus far are highly interdependent and highly complex from a modelling perspective. In particular, the exact role of the metallic surface in the flow stagnation region is not fully understood.

It is known that any significant damage (gouging) to the target surface in the stagnation region will severely affect the UniSpray source performance. Referring to Figure 11, if we assume that the cross represents the location of the flow stagnation point and the end of the striation marks represent the flow separation point, we can determine from a simple geometric projection that the UniSpray target stagnation zone subtends a radial angle of approximately 46 degrees. For a 1.6 mm diameter target, this equates to a stagnation zone that has a 0.65 mm circumferential length. Since the effects of Coanda steering, surface microvorticity and vortex shedding are all associated with the formation of a stagnation zone, one would assume that any gross interference with this region would have detrimental effects on the performance of the UniSpray source. To test this hypothesis, an experiment was conducted where a surface groove, with an equivalent width to the stagnation length (0.65 mm), was cut longitudinally into the 1.6 mm diameter stainless steel

target. It can be shown that by rotating the position of the groove with respect to the stagnation region, significant sensitivity decreases are observed when the groove overlaps the stagnation zone. Figure 12 shows the effect of target groove position on the relative signal intensity for a UniSpray-MS analysis of busiprone and reserpine which were infused into the source at a concentration of 0.125 pg/µL and a flow rate of 0.8 mL/min. Referring to Figure 12, the highest signal intensity is observed when the groove is positioned well away from the stagnation zone (upper right hand quadrant). The lowest sensitivity is observed when the groove completely overlaps the upper right hand quadrant, where presumably, the stagnation region is overwhelmed by turbulence such that a clear definition between a stagnation zone and freestream flow no longer exists. The two additional reference points for busiprone and reserpine were obtained from a different 1.6 mm-diameter target which contained no groove. Whilst this experiment does not distinguish between the relative importance of the dominant processes described in this paper, it further reinforces the hypothesis that the curved surface is central to the enhanced signal and ionization efficiency observed with the UniSpray source.

Throughout this paper, we have considered the dominant ionization mechanism to be based on an electrospray-type process where gas phase ions are created by evaporating charged liquid droplets. Whilst some droplet charging may originate at the point of nebulisation, it is likely that charging will occur at the point of impact on the high voltage target via a process that resembles (i) electrospray charge separation, (ii) spray electrification8 or (iii) statistical charging such as observed in sonic spray9 or thermospray ionization10. It is plausible that an explanation of all experimental data obtained over a wide range of analyte classes, mobile phases and flow

Figure 12. The effect of a large surface defect on the sensitivity of a UniSpray source.

Figure 13. A schematic representation of the formation of surface liquid filaments and secondary droplets at the flow separation point on a UniSpray target.

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rates cannot be attributed to a single ionization mechanism. Referring to Figure 2, it is known that the general behaviour of the UniSpray source undergoes a transition at flow rates between 100 and 200 µL/min, which results in distinct differences in the gain versus mobile phase composition plots for the low flow rate data. In view of this observation, an alternative model could be considered where the actual generation of charged droplets occurs due to wetting of the target surface and the subsequent effects of the high velocity gas flow on the wetted zones. A schematic of this process is shown in Figure 13 where the size of the wetted region is massively exaggerated. Wetting is known to occur on the underside of the cylindrical target and is particularly prevalent at high flow rates and with highly aqueous mobile phases. The surface liquid could exist just below the flow separation point shown in Figures 9 and 13 where the local surface gas flow is stagnant but the flow velocity is high at a small distance (>10 µm) from the surface. This gas flow could create secondary droplets from the resulting liquid filaments shown in Figure 13. In fact, it is conceivable that the surface liquid at this junction could be “squeezed” between the counter-rotating microvortices (Figure 10(a)) to form a linear series of secondary droplet emitters which may be responsible for the striation marks shown in Figure 11. Although this model differs from one based on high We-number droplet impacts, it does however depend on the same aerodynamic principles outlined in the previous sections.

Although not covered in any detail here, it is also believed that the nebulizer/target gap current and its effect on the charging of both the target surface and gas phase molecules is another important parameter that will influence the performance of a UniSpray source. In fact, experiments with an ambient UniSpray source in this laboratory have shown that gas phase ion/molecule reactions are highly likely to occur under these operating conditions. Furthermore, such reactions can be expected to benefit from the enhanced desolvation processes that occur with the UniSpray geometry, as highlighted in the previous sections.

References1. A. Lubin, S. Bajic, D. Cabooter, P. Augustijns, and F.

Cuyckens, J. Am. Soc. Mass Spectrom., 28, 286-293 (2017).

2. P. Kebarle, J. Mass Spectrom., 35, 804-817 (2000).

3. R. Juraschek, T. Dulcks, and M. Karas, J. Am. Soc. Mass Spectrom., 10, 300-308 (1999).

4. R.L. Grimm and J.L. Beauchamp, Anal. Chem., 74, 6291-6297 (2002).

5. S.W. Akhtar, G.G. Nasr, and A.J. Yule, Atomization and Sprays, 17, 659-681 (2007).

6. A. Dumitrache, F. Frunzulica, and T.C. Ionescu, Mathematical Modelling and Numerical Investigations on the Coanda Effect, Chapter 5 of Nonlinearity, Bifurcation and Chaos - Theory and Applications, ISBN 978-953-51-0816-0 (2012).

7. J. Kestin and R.T. Wood, J. Fluid Mech., 44, 461-479 (1970).

8. L.B. Loeb, Static Electrification, ISBN 978-3- 642-88243-2 (1958).

9. A. Hirabayashi, M. Sakairi, and H. Koizumi, Anal. Chem., 66, 4557-4559 (1994).

10. V. Katta, A.L. Rockwood, and M.L. Vestal, Int. J. Mass Spectrom. Ion Processes, 103, 129-148 (1991).