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Ion distribution profiling in an ion mobility spectrometer by laser induced fluorescence Kaitai Guo, Kai Ni, Xiangxiang Song, Kunxiao Li, Binchao Tang, Quan Yu, Xiang Qian, and Xiaohao Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04912 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Analytical Chemistry

Ion distribution profiling in an ion mobility spectrometer by laser induced fluorescence Kaitai Guo1†, Kai Ni1†,*, Xiangxiang Song1, Kunxiao Li1, Binchao Tang1, Quan Yu1, Xiang Qian1, Xiaohao Wang1,2,* 1 Division of Advanced Manufacturing, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China 2 State Key Laboratory of Precision Measure Technology and Instruments, Tsinghua University, Beijing 100084, China ABSTRACT: Measuring the ion distribution pattern in a drift tube under atmospheric pressure is very useful for studies of ion motion and design of ion mobility spectrometers (IMS); however, no mature method is available for conducting such measurements at present. We propose a simple and low-cost technique for profiling the two-dimensional ion distribution in any cross section of a drift tube. Similar to particle-image velocimetry, we first send sample ions with fluorescence properties into the drift tube and use a receiving plate to collect and accumulate them. Then, the receiving plate is illuminated by exciting light and the ion distribution appears as a fluorescence image. In this study, Rhodamine 6G was selected as a typical fluorescence-tracer particle. Electrospray ionization (ESI) was chosen as an ionization source to keep the fluorophore undamaged. A plasma-cleaned coverslip was placed at the detection position as a receiving plate. When a layer of ions was collected, the slide was placed under the exciting light with a wavelength of 473 nm. A camera with a 490-nm high-pass light filter was used to capture the fluorescence image representing the ion distribution. The measured-ion detection efficiency of the method was 156 ion/dN, which is equivalent to the level of IonCCD. In addition, we studied the ionpassing characteristics of a Bradbury-Nielsen (BN) ion shutter and the ion-focusing effect in the drift tube using this method. The two-dimensional ion-distribution images behind the ion shutter and the images of the focused ion spot were first observed experimentally. Further theoretical analysis yielded the same conclusions as the experimental results, proving the feasibility of this method and producing a deeper understanding of ion motion in the IMS. This method has promising prospective application to the design, debugging, and optimization of IMS instruments and hyphenated systems.

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INTRODUCTION In an atmospheric environment, ion mobility spectrometry (IMS) separates various compounds based on their mobility rate under a weak1, static electric field which is associated with the collision cross section, quality and charge of the sample ions. This technique was first used independently to detect drugs and explosive materials2-4. It can also be used to detect food additives5, 6, air quality7 and breathing gas8. In addition, the IMS technology can be combined with other analytical instruments including mass spectrometry9, and chromatography10 (for sample transfer, detection and pre-separation purpose) to improve sample analysis ability. Ion-distribution profiling in the IMS is important for better understanding the ion-mobility rules. Simulation is the dominating strategy for predicting and analyzing the behavior of ions in the IMS under atmospheric pressure. The SIMION software is packaged with a statistical-diffusion-simulation module that is capable of solving the Poisson equation. The ion loss and resolving power of IMS can be calculated using this software11. LORENTZ is another simulation software package with a particle-trajectorymodeling module12. However, sometimes simulations are still not enough or satisfied. For example, as there are always errors between practical systems and ideal simulation models, experimental ways are more useful for system optimizing and troubleshooting. Another example, as there are often complex processes of physical and chemical changes in the ionization region, it is difficult to build an accurate and complete simulation model for this region. As a result, experimentally profiling methods have received widespread attention for years13, 14 in exploring the ion motion and optimizing the simulation accuracy. Ion distributions of

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Ni15, ESI16, nano-ESI17, 18, corona discharge19 were measured in experi-

ment. Detectors for measuring ion distributions can be traced back to J. J. Thompson’s work20. The images of luminescent screens were used to visualize and display ions in the mass spectra. Later, Giffin et al.21 improved the photo-plate detector into an electro-optical-imaging detector (EOID). The microchannelplate detector is another type of detectors with a signal-magnifying function. These methods perform well in detecting the ions’ location information under the vacuum condition, however, the methods do not work properly in atmosphere (where they will either be restricted by the ion’s kinetic energy or too sensitive). Only a few strategies other than simulation for profiling the ion distribution exist in the atmosphere. Kanu et al.18 utilized a target-rings Faraday plate detector to probe the radial distribution of ions moving in a drift tube. Tang et al.16 promoted this method and profiled ion transport, and Zhou et al.22 evaluated the focus performance of the drift tube. The Faraday plate has zero amplification and is confined by Johnson noise23, 24; therefore, each Faraday-ring plane should possess a large enough area to receive a sufficient number of ions to produce a detectable signal, thereby limiting the spatial resolution 2


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of this profiling method. Thus, the capacitive-transimpedance amplifier has been developed as a highgain detector with accumulation ability. It builds the charge up for a period of time to somewhat reduce the minimum required area of the receiving units25, 26. Eiceman et al.13, 15 used an IonCCD to profile the ion distribution in the drift region to improve the ion-distribution measurement. However, the IonCCD yielded only one-dimensional ion-distribution images and the detection position was limited to the end of the drift tube. In this study, a two-dimensional ion-distribution-profiling method was proposed for which the detection position was not limited to the end of the drift tube. The whole system was built using an ESI ion source and a fluorescence-detection system in atmosphere. Using this method, some experiments were designed to demonstrate the detection effect and to study the corresponding ion-drift characteristics. METHOD DESCRIPTION

Figure 1. (a) Sampling procedure. ESI is used to generate sample ions with ion-fluorescence characteristics. (b) Receiving procedure. A clean receiving plate is placed at the detection position to catch ions. (c) Development procedure. The plate with ions on its surface is pulled out from the drift tube and put into the developing optical system to show the ion distribution. (d) From left to right: schematic of ions being excited on the receiving plate; the receiving plate with ions under natural light; and the receiving plate with ions under exciting light. This method basically aims to ionize samples with fluorescence properties and direct them into the drift tube to show their ion distribution, similar to that of particle-image velocimetry (PIV). However, the fluorescence intensity is too weak to be detected directly because the sizes and fluorescence quantum yields of ions are much smaller than those of the tracer particles used in the PIV. Thus, a receiving and 3



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developing scheme is used to solve this problem. A receiving plate is used to collect and accumulate the fluorescence ions for a certain period of time to increase the intensity of the fluorescence image; the whole process can be divided into the following three steps: (a). Sampling. Sample ions with the fluorescence characteristic in a gaseous state can be used as tracer particle. In our experiments, Rhodamine 6G (R6G) was selected as the sample ion because its fluorescence characteristic in gas phase has been widely studied27-31. The sample was dissolved in methanol (Aladdin M116119 Shanghai, China) with a concentration of 0.5 mM. Electrospray ionization (ESI) was chosen as the ionization technique because it will not damage the fluorescence fluorophore of the sample27, 31-35. The sample injector was driven by a mechanical pump (America KD Scientific, KDS Syringe Pump, America, Holliston Massachusetts) at a rate of 10 µL/min. The diameter of the ESI capillary was 75 µm, and the ionization voltage was 4,500V. (b). Receiving. A receiving plate is used to record the ion distribution at the position of interest. The moving ions in the drift tube are captured when they hit the plate. In our experiment, a coverslip (24*50 mm, Ultra Clear Glass, 0.13-0.16mm-thickness, Citotest Labware Manufacturing Co., Ltd, Jiangsu, China) was used as the receiving plate because of its good light transmission, small scattering behavior, and low price. The electric field analysis based on COMSOL showed that the influence of such a thin receiving plate was negligible. The receiving plate was cleaned to improve its ion-capturing capacity before receiving ions; the cleaning process included ultrasonic and plasma washing. The receiving plate was first bathed in water and ethanol via ultrasonic washing for 5 min at 30°C; then, it was dried until no residual liquid remained on its surface. Finally, it was cleaned with plasma for 5 min to eliminate organic residues. A special insulation ring with a slot (receiving plate holder) was used to hold the receiving plate at the detecting position, as shown in Figure 1b. The cleaned receiving plate was inserted into the slot. The center part of the slot was wider so that plate’s surface could be prevented from any scratches when the plate is insert or withdrawn from the holder. The design of the outer shape of the receiving plate holder was similar to an ordinary insulation ring allowing its placement at any position between two drift rings. When the receiving plate was fixed, the ion swarm was opened for a period of time (5 s in our experiments), after which the plate surface was coated with the ion-distribution pattern. (c). Developing. As shown in Figure 1c, the plate with the distribution pattern is withdraw from the drift tube and inserted into the developing optical system. The optical system comprises a laser, a beam expander, a developing holder, and a camera with filters and two beam dumps. In our system, a 473-nmwavelength laser (MBL-S-473-A, Changchun New Industries Optoelectronics Tech, Co., Ltd, Changchun, China) passed through the beam expander to obtain an exciting beam with a 22-mm diameter. The 4


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plate was fixed on the developing holder and illuminated by this beam, thereby exciting the fluorescent particles and allowing the fluorescence image to be collected from the other side. A camera (C1144022CU digital camera, Hamamatsu Photonics K.K., Hamamatsu, Japan) photographed the plate via a high pass filter (490 nm long-wave pass filter, Beijing Jingnan applied technology research laboratory, Beijing, China) for an exposure time of 100 ms. EXPERIMENTS AND RESULTS (1) Experiment System The drift length of the IMS was 123.2 mm with 17 metal rings (Figure 1b); the drift ring was 16-mm inner-diameter and 1.6-mm thick. The whole drift tube was equipped with a high-voltage power supply (HV1) to form an electric field of about 50 V/mm in the drift region; The electric field strength could be easily changed by adjusting the output voltage of HV1. Resistances (1/2W, 0.1%, 1MΩ) in series were used to divide the HV1 to generate voltages for each drift ring. The focusing electric field used in the third experiment was generated by replacing the original resistances between rings 14-17 with higher resistances. Ions were produced continuously by a ESI ionization source through a 75-µm-innerdiameter capillary. The ESI voltage was supplied by a second high-voltage power supply (HV2). The potential difference between the spray emitter and the first drift ring electrode was 4.5 kV. The sample injector was driven by a mechanical pump at a rate of 10 µL/min. Data were collected at 25 °C and 1.01×105 Pa. (2) Ion-detection Performance The purpose of this section is to measure the detection accuracy and efficiency of the method. In our system, the size of every single pixel of the CCD camera corresponds to a 48 × 48-µm2 area on the plate. If higher spatial resolution is required, we can either simply increase the optical magnification or use a CCD with a smaller pixel size.

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Figure 2. Diagram of the ion distribution in (a) stereo view; (b) top view; (c) ion-intensity profiles along the X axis of the top view (black line) for various drift voltages. The receiving plate was placed between the last two drift rings as shown in Figure 1b. The ion intensity was studied as a function of drift tube bias in 1 kV increments. The profile of the ion distribution with a total drift voltage of 10 kV is shown in Figure 2a and b. We can see that the ions were distributed within a circle slightly smaller than the cross-sectional area of the drift tube (16-mm diameter). This clearly shows the edge effect of the drift tube: ions were neutralized on the shell surface because of the uneven electric-field distribution near the edge. Figure 2c shows the ion-intensity profile of different drift voltages (1-10 kV) along the X axis (Y = 0, Figure 2b). We can see that the intensity increases with the voltage. To measure the detection efficiency, the total fluorescence intensity was compared to the current detected by a Faraday plate detector (166


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mm diameter) using a traditional transimpedance ampliﬁer (DLPCA-200, Femto, Berlin, Germany). The Faraday plate was fabricated out of stainless steel and its installation position was designed to be the same as that of the receiving plate. The corresponding experimental conditions were kept constant. The ion current detected by the Faraday plate at each drift voltage increased from 56 pA to 7.6 nA. The total fluorescence intensity is calculated by accumulating the intensity of all pixels in the 16-mmdiameter circle. Figure 3 shows the curve between the fluorescence intensity and the corresponding ion current; this relation can be linearly fitted as the red line in the figure. The goodness of fit (R2) is 0.9972.

160

Fitting Line Fluorescence Intensity

140

Intensity(107dN)

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120 100 80 60 40

R2 = 0.9972

20 0 0

1

2

3

4

5

6

7

Ion Current (nA)

Figure 3. The relation between fluorescence intensity and ion current. The detection efficiency is defined as the number of ions required to generate unit intensity in the image; it can be calculated as eq 113 : ∙

= ∙

(1)

where η is the ion-detection efficiency, t is the receiving time (5 s), I is the ion current detected by the Faraday plate, e is the charge on an electron (1.6 × 10-19 C) and S is the total fluorescence intensity. As S/I is the slope of the fitted line (2 × 108 dN/nA), η is calculated to be 156 ion/dN. A smaller value of η indicates higher sensitivity. The experimental results show that the fluorescence intensity is linearly correlated with the ion current, and the detection efficiency is at the same level as the IonCCD (125 ion/dN13), indicating that the meth7



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od exhibits good accuracy and sensitivity. (2) Ion Distribution downstream of an Ion Shutter An ion shutter comprises two groups of wires that are parallel and insulated from each other on a plane. When the shutter is closed, a potential difference between two groups is formed. Ions are consumed on the lower group by the influence of the electric-field force. When the shutter is opened, the electric potentials of the two wire sets are equal. Most ions can pass through the ion shutter; however, some ions are consumed on the wires. Because the ions passing through the shutter are responsible for the resolution and signal intensity of the analytical results, the ion-transmission characteristics have received widespread attention36-40. However, most of these reports have studied this characteristic by simulation; none have measured the actual two-dimensional distribution image of ions when they first enter the drift tube. The objective of this section is to detect this distribution downstream of the shutter using our method. A Bradbury Nielsen (BN) shutter was placed between the 9th and 10th drift rings. The wire distance was 1 mm, and the wire diameter was 0.1 mm. The receiving plate was 1.5 mm behind the shutter; the total drift voltage of the tube was 6,080 V (50 V/mm). The voltages of the 9th and 10th drift rings were V9 = 3,040 V and V10 = 2,660 V, respectively. The shutter was always kept open. We defined ΔVr to be the difference between the actual voltage on the ion shutter and (V9 + V10) / 2 and used different values of ∆Vr as -25 V, -15 V, 0 V, 15 V and 25 V separately in our experiment. The voltage applied on the shutter wires was set and varied with a third high-voltage power supply (HV3). The experimental results are shown in Figure 4, where a-e are the measured ion-distribution images, and f-j are the intensity curves along the X axis. As can be seen in each image, the ion swarm was cut into 16 stripes and their spatial period is about 1 mm; this parameter is consistent with the physical structure parameters of the ion shutter.

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Figure 4. Ion distribution when different ∆Vr values were applied to the ion shutter. (a), (f) ∆Vr = -25 V; (b), (g) ∆Vr = -15 V; (c), (h) ∆Vr = 0 V; (d), (i) ∆Vr = 15 V; and (e), (j) ∆Vr = 25 V. The trends of ion intensity and contrast change versus ∆Vr are shown in Figure 5. The blue square illustrates the fluorescence intensity of Figure 4a-e, whereas the red dot is the half-peak width of Figure 4f-j, which is the average value of half-peak width of all peaks in each figure. Both of these quantities were normalized by the corresponding result for ∆Vr = 0 V. We can see that the intensity increases, and the 9



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half-peak width decreases with an increasing value of ∆Vr; this means that the total number of passed ions, which corresponds to the brightness of the stripes, become higher when we increase ∆Vr. The dispersity of the passed ions, which correspond to the width of the stripes, decreases as ∆Vr increases.

Figure 5. Intensity of the fluorescence images and half-peak widths of the stripes with Vr from -25 V to 25 V. Results for more samples can be found in supplementary information Figure S-1. Figure 6 explains this phenomenon. The shutter is in the open state and the direction of ion movement is from left to right in the figure. The uniform distribution of the equipotential lines (red lines) will be distorted by the metal wires because the diameter of the shutter wire is not zero. This influences the direction of the electric-field lines, thereby changing the ion trajectory. The black and blue lines in the figure correspond to the trajectories of the ions that are blocked and passed respectively. L is the passing-ionchannel width upstream of the shutter, and W is the downstream. Obviously, L determines the total number of ions that can pass through the shutter, and W is the width of the stripes in the experimental ion-distribution image.

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Figure 6. Ion distribution around the ion shutter with (a) ∆Vr = -25 V, (b) ∆Vr = 0 V, and (c) ∆Vr = 25 V in the simulation. When ∆Vr = 0 V, the twisted equipotential line is symmetric about the central plane of the shutter, indicating L0 = W0. When ∆Vr decreases (Figure 6a), the equipotential lines near the shutter stretch in the direction opposed to the ion movement, such that a large range of electric-field lines converge on the shutter to decrease the range of ions passing through the shutter (L decreases). Correspondingly, when ∆Vr increases (Figure 6c), the result is reversed (L increases). The relation between L and W among these different ΔVr result satisfies L-25W25. The experimental results indicate that an appropriate increase in the open-state reference voltage of the shutter helps to enhance the utilization ratio of the sample ions and compress the ion swarm downstream of the shutter. This feature is useful when using IMS for sample pre-separation, while the influence of the electric field distortion on the drift time difference needs to be studied further when measuring K0. (3) Ion Focusing The drift tube can be used to focus ions. When the IMS is combined with other instruments such as mass spectrometers, ion focusing can enhance the efficiency of ion transport. In a traditional drift tube, the uniform drift electric field is generated by drift rings that are arranged at equal intervals with the same voltage difference between each adjacent pair (∆Vd). If the voltage difference between the last several drift rings (∆Vf) is increased, a focusing electric field can be formed at the end of the drift tube22, 41

. Although simulations with SIMION have been used to study the behavior of ions in a drift tube with

this focusing effect, the real distribution has never been observed. The object of this section is to detect the actual focused ion-distribution using our method. Further theoretical analysis will be conducted based on the results. 11



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The experimental setup was similar as that shown in Figure 1 b. Rings 1-14 were used as normal drift rings with ∆Vd = 200 V; rings 14-17 were used as focusing rings with ∆Vf ≥ ∆Vd. The electric field was focused between rings 13-15 and remained uniform again until the last ring. The receiving plate was placed just upstream of the last ring. We used ten different focusing voltages (∆Vf = 200, 400, 600, 800, 1,000, 2,000, 3,000, 4,000, 5,000, and 6,000 V) in the experiment. The results are shown in Figure 7. The intensities of the ten spots are on the same scale and normalized by the maximum value of the 10th spot (Figure 7 j). We can clearly see that the ion-spot size becomes smaller and the intensity becomes stronger with the increasing ∆Vf.

Figure 7. Image of the ion distributions for different focusing voltages ∆Vf. (a) 200 V, (b) 400 V, (c) 600 V, (d) 800 V, (e) 1,000 V, (f) 2,000 V, (g) 3,000 V, (h) 4,000 V, (i) 5,000 V, and (j) 6,000 V. The size of the ion spot is calculated from the result (Figure 7), and the relation curve between the normalized size and the focusing-voltage ratio (∆Vf /∆Vd) is obtained, as shown in Figure 8. In logarithmic coordinates, the size linearly decreases with increase in ∆Vf /∆Vd; thus, it can be supposed that there exists an inversely proportional relation between these quantities. The reason is explained below.

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1

Normalized Area Size (Sf / Sd)

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Theory Normalized Area Size Normalized Area Size

1.000

0.5

0.520 0.359

0.3 0.2

0.250 0.190

0.1 0.106

0.071

0.05 0.03

0.046 0.037 0.030

1

2

3

5

10

20

30

Focusing Voltage Ratio (∆Vf / ∆Vd) Figure 8. Experimentally measured ion-spots size (red squares) and theoretical expectation (blue line) when the focusing-voltage ratio (ΔVf /ΔVd) increased from 1 to 30. Results for more samples can be found in supplementary information Figure S-2. Since the ion trajectory at atmospheric pressure is mainly determined by the electric-field strength, the drift trajectory coincides to a large extent with the electric-field lines, as shown in Figure 9. As the electric-field intensity in the focusing zone is stronger than that in the drift zone, the density of the electricfield lines increases in proportion of the intensity. Therefore, the distribution area of the electric-field lines from the drift zone will be reduced accordingly when they enter the focusing zone.

Figure 9. Ion trajectory and equipotential lines in the SIMION (ΔVd = 200V, ΔVf = 1000V). Thus, the relation between the sizes of the ion-distribution area at the drift and the focusing zone can be summarized as 13



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∆ ∙ = ∆ ∙

(2),

where ΔVd and ΔVf are the voltage differences between each two adjacent rings in the drift and the focusing zone, respectively, and Sd and Sf are the sizes of the cross-sectional areas of the ion distribution in the drift and focusing zones, respectively. The relationship curve calculated by eq 2 is also shown in Figure 8 as the blue curve. The goodness of fit between the experimental results and the theoretical curve is 0.9986, indicating that the focusing formula was correct. The equation can be further expanded for a long drift tube with several focusing zones of different voltages (from ΔVf1 to ΔVfn). When the ion passes through the drift tube, the size of the ion-distribution cross-sectional area can be summarized as follows: S = S ∙

∆ ∆

∙

∆ ∆

∙…

∆ ∆

= S ∙

∆ ∆

(3),

It can be seen from eq 3 that the final focusing effect of the ion distribution in the drift tube is determined only by the ratio between the initial and final electric-field intensities, regardless of the change in the middle part. Furthermore, we find that the ion-spot size still agrees well with the theoretical expectation when the focusing voltage increases to 6,000 V, where the ion-spot was focused to only 3% of its original size; this means that effect of the coulomb repulsion at this focusing voltage is still not obvious. We did not further test the focusing limit of the system in the experiment because of our limited power supply. However, from the existing data trends, we can confirm that the system still has the potential for a smaller convergence. This feature is desirable for a variety of applications, particularly when combined with other instruments to improve ion-transport efficiency. An in-depth study of this issue will be made in subsequent studies. CONCLUSION This study has presented a simple, low-cost method for profiling the two-dimensional ion distribution in the cross-section of a drift tube for IMS at ambient pressure. The method first uses a receiving plate to collect sample ions with fluorescence characteristics, and then excites the fluorescence image with a laser to obtain the ion distribution. The ion detection efficiency of the method was 156 ion/dN, which is similar to the level of the IonCCD. Moreover, using this method, we studied the ion-passing characteristics of a BN ion shutter and the ion-focusing effect in the drift tube. The two-dimensional iondistribution images behind the ion shutter and the images of the focused-ion spot were observed experimentally for the first time. Further theoretical analysis yielded the same conclusions as the experimental results, providing the feasibility of this method and offering a deeper understanding of ion mo14


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tion in the IMS. This method may help to improve the simulation model of ions at atmospheric pressure and may have application to the design, debugging, and optimization of IMS instruments and hyphenated systems. ASSOCIATED CONTENT Supporting Information Results of the ion shutter and the ion focusing experiments with six different samples. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Kai NI). *E-mail: [email protected] (Xiaohao WANG). †The authors wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors. ACKNOWLEDGMENT This work is supported by grants from National Natural Science Foundation of China (Grant No. 21205067), Science and Technology Planning Project of Guangdong Province (Grant No. 2015A030401015) and Shenzhen fundamental research funding (Grant No. JCYJ20160531195459678).

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(25) Knight, A. K.; Sperline, R. P.; Hieftje, G. M.; Young, E.; Barinaga, C. J.; Koppenaal, D. W.; Denton, M. B. International Journal of Mass Spectrometry 2002, 215, 131-139. (26) Babis, J. S.; Sperline, R. P.; Knight, A. K.; Jones, D. A.; Gresham, C. A.; Denton, M. B. Analytical and bioanalytical chemistry 2009, 395, 411-419. (27) Uteschil, F.; Kuklya, A.; Kerpen, K.; Marks, R.; Telgheder, U. Analytical and Bioanalytical Chemistry 2017, 409, 62796286. (28) Nagy, A. M.; Talbot, F. O.; Czar, M. F.; Jockusch, R. A. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 2012, 244, 47-53. (29) Sagulenko, P.; Frankevich, V.; Steinhoff, R.; Zenobi, R. International Journal of Mass Spectrometry 2013, 338, 11-16. (30) Frankevich, V.; Martinez-Lozano Sinues, P.; Barylyuk, K.; Zenobi, R. Analytical chemistry 2012, 85, 39-43. (31) Guo, K.; Ni, K.; Ou, G.; Zhang, X.; Yu, Q.; Qian, X.; Wang, X. 2015; International Society for Optics and Photonics; 96210W. (32) Chingin, K.; Balabin, R. M.; Frankevich, V.; Chen, H. W.; Barylyuk, K.; Nieckarz, R.; Fedorov, A.; Zenobi, R. PHYSICAL CHEMISTRY CHEMICAL PHYSICS 2010, 12, 14121-14127. (33) Law, W. S.; Wang, R.; Hu, B.; Berchtold, C.; Meier, L.; Chen, H. W.; Zenobi, R. ANALYTICAL CHEMISTRY 2010, 82, 4494-4500. (34) Talbot, F. O.; Rullo, A.; Yao, H.; Jockusch, R. A. Journal of the American Chemical Society 2010, 132, 16156-16164. (35) Chingin, K.; Frankevich, V.; Balabin, R. M.; Barylyuk, K.; Chen, H.; Wang, R.; Zenobi, R. Angewandte Chemie International Edition 2010, 49, 2358-2361. (36) Kirk, A. T.; Zimmermann, S. International Journal for Ion Mobility Spectrometry 2014, 17, 131-137. (37) Tadjimukhamedov, F. K.; Puton, J.; Stone, J. A.; Eiceman, G. A. Review of Scientific Instruments 2009, 80, 103103. (38) Puton, J.; Knap, A.; Siodłowski, B. Sensors and Actuators B: Chemical 2008, 135, 116-121. (39) Salleras, M.; Kalms, A.; Krenkow, A.; Kessler, M.; Goebel, J.; Müller, G.; Marco, S. Sensors and Actuators B: Chemical 2006, 118, 338-342. (40) Puton, J. Sci. Instrum 1989, 4, 29-41. (41) Han, F.; Li, H. International Journal for Ion Mobility Spectrometry 2014, 17, 11-18.

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Figure 1. (a) Sampling procedure. ESI is used to generate sample ions with ion-fluorescence characteris-tics. (b) Receiving procedure. A clean receiving plate is placed at the detection position to catch ions. (c) Development procedure. The plate with ions on its surface is pulled out from the drift tube and put into the developing optical system to show the ion distribution. (d) From left to right: schematic of ions being excited on the receiving plate; the receiving plate with ions under natural light; and the receiving plate with ions under exciting light. 322x136mm (120 x 120 DPI)



Figure 2. Diagram of the ion distribution in (a) stereo view; (b) top view; (c) ion-intensity profiles along the X axis of the top view (black line) for various drift voltages. 243x216mm (120 x 120 DPI)


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Figure 3. The relation between fluorescence intensity and ion current. 114x73mm (96 x 96 DPI)



Figure 4. Ion distribution when different ∆Vr values were applied to the ion shutter. (a), (f) ∆Vr = -25 V; (b), (g) ∆Vr = -15 V; (c), (h) ∆Vr = 0 V; (d), (i) ∆Vr = 15 V; and (e), (j) ∆Vr = 25 V. 142x205mm (96 x 96 DPI)


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Figure 5. Intensity of the fluorescence images and half-peak widths of the stripes with △Vr from -25 V to 25 V. Results for more samples can be found in supplementary information Figure S-1. 200x83mm (120 x 120 DPI)



Figure 6. Ion distribution around the ion shutter with (a) ∆Vr = -25 V, (b) ∆Vr = 0 V, and (c) ∆Vr = 25 V in the simulation. 242x126mm (96 x 96 DPI)


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Figure 7. Image of the ion distributions for different focusing voltages ∆Vf. (a) 200 V, (b) 400 V, (c) 600 V, (d) 800 V, (e) 1,000 V, (f) 2,000 V, (g) 3,000 V, (h) 4,000 V, (i) 5,000 V, and (j) 6,000 V. 293x127mm (96 x 96 DPI)



Figure 8. Experimentally measured ion-spots size (red squares) and theoretical expectation (blue line) when the focusing-voltage ratio (∆Vf /∆Vd) increased from 1 to 30. Results for more samples can be found in supplementary information Figure S-2. 95x51mm (96 x 96 DPI)


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Figure 9. Ion trajectory and equipotential lines in the SIMION (∆Vd = 200V, ∆Vf = 1000V). 318x189mm (96 x 96 DPI)


Ion Distribution Profiling in an Ion Mobility ... - ACS Publications

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