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A novel driving mode for ion shutter based on alternating current superposition and its application to ion mobility spectrometry
Sensors and Actuators B 211 (2015) 102–110
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel driving mode for ion shutter based on alternating current superposition and its application to ion mobility spectrometry Sheng Liu a,b , Chaoqun Huang a,∗ , Chengyin Shen a , Haihe Jiang a , Yannan Chu a,∗∗ a Laboratory of Medical Optical and Mass Spectrometry, Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, No. 350 Scientific Road, Hefei 230031, China b College of Computer Science and Technology, Huaibei Normal University, 235000, Anhui, China
a r t i c l e
i n f o
Article history: Received 16 September 2014 Received in revised form 11 December 2014 Accepted 5 January 2015 Available online 28 January 2015 Keywords: Ion mobility spectrometry Ion shutter Driving mode Alternating current superposition
a b s t r a c t In order to simplify the design of an ion shutter power supply and decrease its cost, a simple ion shutter driving mode based on alternating current (AC) superposition mode was designed. A power supply with AC superposition was fabricated and applied to the ion mobility spectrometry instrument successfully. The effect of the AC superposition mode on the drift time, ion intensity, and resolution of the ion mobility spectra was studied. Combined with electric field simulation and numerical calculation, a reasonable explanation for the effect of the AC superposition mode on the performance of ion mobility spectrometry is given. Two equations have been fitted by exponential fitting to correct the offset of the drift time. Cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) has been investigated to validate the effectiveness of the correction by using corona discharge IMS combined with the new ion shutter driving mode. The results show that the new low-cost power supply based on AC superposition mode is feasible. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ion mobility spectrometry (IMS) is a trace chemical substances analytical technology that appeared in the late 1960s to the early 1970s [1,2]. Compared to the mass spectrometer, IMS is capable of being operated at atmospheric pressure, and has the advantage of low cost and ease of use. Recently, IMS has been widely used to detect many trace compounds such as chemical warfare agents [3,4], illegal drugs [5,6], explosives [7,8], and various other chemicals [9,10] due to its high sensitivity, analytical flexibility, and real time monitoring capability. The ion shutter is one of the key parts in an IMS instrument. In general, the Bradbury-Nielson (BN) ion shutter has been widely used as an electronic switch. The BN ion shutter is composed of two arrays of wires, which are insulated and separated from each other in an interdigital form fixed at the bracket. Since the ion shutter is perpendicular to the direction of the movement of ions, ions are prevented from entering the drift region when the electric field is applied to the wires. Such electric field is removed transiently in each cycle, therefore ion packet can enter the drift region, and a stable mobility spectrum is formed.
∗ Corresponding author. Tel.: +86 551 65592429. ∗∗ Corresponding author. Tel.: +86 551 65595311. E-mail addresses: [email protected] (C. Huang), [email protected] (Y. Chu). http://dx.doi.org/10.1016/j.snb.2015.01.061 0925-4005/© 2015 Elsevier B.V. All rights reserved.
The phenomena occurring in the vicinity of the ion shutter are important and have attracted extensive attention. Several researchers have investigated how the change of the electrical field around the ion shutter will influence the performance of the IMS [11–17]. Eiceman et al. systematically studied the effect of the influence and the importance of optimizing electric fields in each region of an IMS drift tube by experimental and simulation methods. They concluded that the non-homogeneous fields in the drift region resulting from electric field between ion source and ion shutter will influence the mobility time of ions, leading to a broadening of the observed peak and a decrease in peak intensity. Moreover, the effect on signal intensity from electric fields arising from the ion shutter and drift tube has been investigated in detail by experimental and modeling methods [12]. In addition, the modeling of the penetration of ions through an ion shutter has been done with the aid of computer calculation. The electric field in the neighborhood of the ion shutter has a distinctive effect on the shape of the ion peak and ion intensity as well as the resolution of IMS spectrometer [17]. The design and fabrication of the ion shutter have also attracted many researchers’ attention. Salleras et al. have made an ion shutter for miniaturized ion mobility spectrometers by bulk silicon micromachining and optimized its parameters, such as electrode geometry and applied voltage [14]. Zhou et al. have constructed a novel ion shutter for electrospray-ionization atmospheric pressure ion mobility spectrometry. A chopper wheel with two windows was used to push the ions to the drift region periodically as an ion
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Fig. 1. Schematic of the IMS operation principle.
shutter. The experimental resolution was around 70 in this report [18]. An etching method has also been used to fabricate the ion shutter. Brunner et al. designed and constructed a novel and large BN ion shutter with flexible wires based on photo-etched stainless steel grids. This ion shutter, which was etched by 50 m stainless wire, has good mechanical tolerance and is easy to assemble [19]. Apart from the design and fabrication of ion shutters, the electronic driving mode of ion shutters also deserves to be studied. The conventional electronic diagram of the driving mode for the BN ion shutter is complicated. Also, the cost of a commercial power supply for an ion shutter is several thousand dollars, which increases the cost of the whole IMS instrument too much. Hence, it is necessary to study simple and feasible electronic driving modes and power supplies for the ion shutter to replace the conventional ones. However, to date few reports have focused on the study of the electronic driving mode and power supply for ion shutters. This study proposes a novel design for the driving mode of the BN ion shutter based on an alternating current (AC) superimposed mode. A simple and low-cost power supply for driving the BN ion shutter was fabricated and applied to the IMS apparatus successfully. The influence of the new driving mode on the drift time, ion intensity, and resolution of IMS is discussed combined with experiment and numerical calculation. The novel driving AC superposition mode is proved to be feasible according to these results. In contrast to previous equipment, the cost of the new power supply is much lower, costing less than one hundred dollars due to its simple electronic design and ease of assembly.
In general, the periodical pulse applied to the ion shutter is provided by the ion shutter power supply. The principle of the conventional driving mode of the ion shutter in IMS is shown in Fig. 2(a). A pulse amplified by the driving power supply is sent directly to the ion shutter which is connected to the high voltage source through resistors. It is important to note that the pulse voltage should be floating and the pulse driving power must be insulated from the ground. Because R1 is equal to R2 , the voltage of the two adjacent wires is UA + Vd /2 and UA − Vd /2 respectively when the ion shutter is closed, where UA is the potential of point A on the ion shutter and Vd is the pulse voltage which is applied to the ion shutter. Without taking into consideration the influence of UA , the electric field around the ion shutter generated by the two sets of wires is equal in amplitude but opposite in its polarity, leading to the nearly uniform electric field on both sides of the ion shutter. Fig. 2(b) and (c) shows the simulated electric field distribution using SIMION software during the opening and closing of the ion shutter respectively. As shown in Fig. 2(b) and (c), the electric
2. Driving principle of the ion shutter 2.1. The principle of the conventional driving mode of the ion shutter in IMS Fig. 1 is the schematic of IMS operation principle. When a gas phase sample is introduced into the reaction region by the carrier gas, product ions are formed using different ion sources such as photon, corona discharge, electrospray ionization (ESI), or radioactive ionization. These ions can enter the drift region under the action of the ion shutter pulse and are separated due to their different velocities. The velocity of the ions depends on their weight, charge, and shape. The ions in the drift region move toward the Faraday plate and create a current signal. The output of the amplified signal is synchronized with the ion shutter pulse, yielding a mobility spectrum, i.e., a plot of ion current versus time.
Fig. 2. (a) Conventional drive mode of the ion shutter and the electric field distribution simulated by SIMION when the ion shutter is (b) open and (c) closed.
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Fig. 3. The principle of AC superposition driving mode of the ion shutter in IMS.
field in the drift region is nearly uniform in the conventional driving mode. Hence the voltage applied to the ion shutter has no effect on ion behaviors in the uniform electric field, no matter whether the ion shutter is opened or closed. 2.2. The principle of AC superposition driving mode of the ion shutter in IMS In the IMS instrument, a potential difference Vd is also needed to apply to the ion shutter to prevent ions from entering the drift region without the narrow pulse signal using the AC superposition driving mode when the ion shutter is closed. However, the potential of the narrow pulse compensates for the potential difference of the ion shutter during the narrow pulse duration. This means that the ion shutter is opened and ions are able to enter the drift region. The principle of the circuit diagram of the AC superposition driving mode in IMS is seen in Fig. 3. A set of resistors in series is connected to one end of the ion shutter. The value of the resistors is three times more than the value of the divider resistors which are connected to the drift rings. The purpose of using the high-value resistors is to reduce the load of the high voltage power supply. The amplitude of Vd is adjustable by adjusting the value of potentiometer W1 . Without the narrow pulse signal, the amplitude of Vd is adjusted to high enough to completely prevent ions traveling from the reaction region to the drift region. The ion shutter stays closed while the pulse signal is absent. Once the narrow pulse voltage is applied, its amplitude and polarity are able to compensate for the Vd . The potential at two metal wires of the ion shutter is equal. The ion shutter stays open. Therefore the function of IMS is realized using the new driving mode. The AC pulse is superimposed by a capacitance which is connected to one end of the ion shutter to control the behavior of ions. When the ion shutter is closed, the potentials of both metal wires of the ion shutter are UA and UA + Vd , respectively. So the electric field distribution at both sides of the ion shutter is different, which means that there is a non-uniform electric field around the ion shutter region. The behavior of ions, such as the drift time, ion intensity and resolution of the IMS will be influenced due to the existence of the non-uniform electric field in the vicinity of the ion shutter. 3. Experimental 3.1. Apparatus A homemade IMS experimental device is set up in our laboratory at the Hefei Institutes of Physical Science, Chinese Academy
of Sciences. A detailed description of the apparatus appears in our previous report [20]. Briefly, the whole IMS cell consists of twelve metal guards which are insulated from each other using Teflon rings. The whole IMS cell is divided into three parts: ionization region, reactant region, and drift region. The length of the drift region is 6.3 cm. A continuous corona discharge ion source is applied in the IMS apparatus. A BN ion shutter is installed between the reaction region and the drift region. The electric field strength in the drift region is 300 V/cm. All the experiments were conducted at room temperature (295 K) except the ones with additional description below. The current signal was collected by a Faraday plate which is located at the end of the drift region, then fed into the computer data processing system. The IMS works in negative detection mode and the spectrum is averaged every 30 continuous acquisitions. 3.2. Design and fabrication of the power supply of AC superposition Designed pulse generating and amplifying circuits for AC superposition driving mode are shown in Fig. 4. The amplified pulse signal is applied to the ion shutter through capacitance. The supply voltage V+ is adjustable from 70 V to 500 V. As shown in Fig. 4, U2 and its subsidiary circuits generate a periodic square wave. The frequency of the square wave is adjusted using potentiometer of R14 . The square wave is sent to U1A . Then the narrow pulse signal is generated by U1A . The width of the narrow pulse signal is adjustable using potentiometer R6 . After amplification by the amplifier circuit, the output pulse signal of U1A is superimposed to one end of the ion shutter through high voltage capacitor C7 for controlling the ion shutter. While the ion shutter is closed, there are two cases when the AC superimposition driving mode is applied to IMS. Case 1 is (C1 , |UB | > |UA |), where the absolute value of the potential of UB is higher than that of UA . Here UA and UB are the potentials at points A and B on the ion shutter. The ion shutter will be opened when the polarity of the narrow pulse is opposite to the polarity of the high voltage after the narrow pulse has been applied. In case 2, (C2 , |UB | < |UA |), the absolute value of the potential of UB is lower than that of UA . The ion shutter will be opened when the polarities of the pulse and the high voltage are the same after the narrow pulse has been applied. It is noted that the function of the ion shutter could be realized with different performance of IMS in the two cases. The reaction ion peak (RIP) was recorded at the different voltage Vd under C1 , C2 , and the conventional driving mode. The results measured by the C1 and C2 mode were compared with the results measured by the conventional driving mode.
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Fig. 4. Pulse generation and amplifier circuits of the ion shutter based on AC superposition.
4. Results and discussion 4.1. The spectrum of IMS under C1 , C2 , and the conventional driving mode As shown in Fig. 5(a), the peak position is almost the same at the different voltages Vd when the conventional driving mode was applied. However, the RIP peaks shift to the left and their widths are narrowed as the signal intensity decreased gradually with the increase of the value of Vd under the C1 driving mode as shown in Fig. 5(b). On the contrary, the peaks shift to the right and their widths are broadened with the increase
of the value of Vd under the C2 driving mode as shown in Fig. 5(c). In order to further evaluate and compare the performance of the AC superimposition driving mode, the drift time, ion intensity, and resolution of the RIP measured under C1 , C2 , and the conventional driving mode were calculated and plotted against different Vd . All the results are shown in Fig. 6. 4.2. Drift time of the reaction ions The center of the peak, i.e., the drift time of the RIP was extracted and plotted against the Vd. As shown in Fig. 6(a), the drift time which
Fig. 5. The reaction ion peaks (RIP) attained under (a) the conventional driving mode, (b) the C1 mode, and (c) the C2 mode.
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Fig. 6. The effects of the ion shutter potential Vd on (a) the drift time, (b) the ion intensity, and (c) the resolution under the conventional driving mode, the C1 mode, and the C2 mode.
was measured under the different Vd is almost the same when the conventional driving mode was applied. This means that the Vd has little influence on the peak position because there is a uniform electric field throughout the drift region. However, the drift time measured under the C1 driving mode decreased with the increase of the Vd . On the contrary, the drift time became longer with the increase of the Vd when the C2 driving mode was applied. As mentioned above, the potential in both sides of the ion shutter is asymmetric in the AC superimposition driving mode. The electric field between the two adjacent metal wires of the ion shutter was strengthened with the increase of the Vd . The distortion of the electric field near the ion shutter region became more serious with the enhancement of the potential near the ion shutter. The velocity of ions was influenced by the distorted electric field under the AC superimposition driving mode. The behavior of the ions in the non-uniform electric field under the C1 and C2 driving modes was clarified with the aid of simulation and calculation. To study the C1 driving mode, the electric field in the drift region was simulated by SIMION with the ion shutter open and with it closed. Fig. 7(a) shows the distribution of the electric field in the drift region with the ion shutter open. There is a nearly uniform electric field along the axis of drift tube because Vd is zero. Fig. 7(b) shows the distribution of the electric field in the drift region when the ion shutter is closed (Vd = 400 V). The electric field was distorted around the ion shutter region. It is obvious that the electric field on the right side of the ion shutter was strengthened, whereas the electric field on the left side of the ion shutter was weakened. The electric field in on the right side of the ion shutter changed from strong to weak along the axis of the drift region from left to right as shown in Fig. 7(b).
The Laplace equation was utilized to calculate the amplitude of the electric field along the axis of the drift tube when the ion shutter was closed. Fig. 7(c) shows that the electric field strength along the axis of the drift tube increased with the increase of Vd from 100 V to 400 V in 100 V intervals when the ion shutter was closed under the C1 driving mode. Also, the electric field strength on the right side of the ion shutter decreased from left to right at a certain Vd voltage. This agrees with the simulated results very well.
Fig. 7. The electric field distribution simulated by SIMION when the ion shutter is (a) open, and (b) closed, and (c) the distribution as calculated by Laplace equation under different ion shutter potentials Vd when using the C1 driving mode.
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Fig. 8. The electric field distribution (a) simulated by SIMION when the ion shutter is closed, and (b) as calculated by Laplace equation under different ion shutter potentials (Vd ) when using the C2 driving mode.
When the ion shutter is open, ion packets will enter the drift region and move to the downstream of the drift region in the distorted electric field. The electric field on the right of the ion shutter was stronger than that in the uniform electric field. Therefore the velocity of the ions is faster in the distorted electric field, so less drift time is needed for the ions in the distorted electric field. The offset time increases with the increase of Vd , because there is stronger electric field strength at higher values of Vd . Contrary to the C1 driving mode, the drift time of the ions under the C2 driving mode is longer than that of the ions under the conventional driving mode at a certain Vd value. The simulated and calculated results in Fig. 8 show that the distorted electric field on the right side of the ion shutter was weakened whereas the electric field on the left side of ion shutter was strengthened. The electric field on the right side of the ion shutter changed from weak to strong along the axis of the drift tube from left to right. The reduced electric field downstream of the drift region led to a longer drift time of the ions.
Fig. 9. The ratios of the drift times measured under the conventional driving mode and the C1 (a) and C2 (b) driving modes, plus their fitted curves.
The ratio R of the drift times which are determined in a uniform electric field and a non-uniform electric field is given as: uniform
R=
td
non−uniform td
=
E non−uniform
(3)
E uniform
where td uniform is the drift time measured in a uniform electric field which is determined under conventional driving mode and td non−uniform is the drift time measured in a non-uniform field which is determined under the C1 and C2 driving modes. Euniform and Enon−uniform correspond to the uniform electric field strength and non-uniform electric field strength, respectively. The ratio R was plotted against the voltage Vd and fitted by exponential as shown in Fig. 9. The fitted equations are expressed as equations (4) and (5). uniform
R(
td
R(
td
C
td 1
4.3. The correction of drift time and reduced mobility
) = exp(0.00463 + 1.43 × 10−4 x + 3.47 × 10−8 x2 )
(4)
) = exp(−0.012 − 5.51 × 10−5 x − 5.55 × 10−7 x2 )
(5)
uniform
The drift time of ions is an important parameter for IMS measurement and can be expressed by Eq. (1): Ld
L td = = d vd KE
(1)
where td is the drift time, Ld is the length of drift region, vd is the drift velocity of ions, K is the ion mobility, and E is the electric field strength in the drift region. The drift time is always used to calculate the ion mobility K by Eq. (2) because the electric field is uniform under the conventional driving mode.
where tdC1 and tdC2 are the drift times which are measured under C1 and C2 driving modes and x is the voltage which is applied to the ion shutter. Using the fitted Eqs. (4) and (5), the drift times tdC1 and tdC2 were corrected and tabulated in Table 1. The corrected values agree with the drift time which is measured by the conventional driving mode as shown in the table. Combined with the Eqs. (2) and (3), the ion mobility under the C1 and C2 driving modes can be calculated by Eq. (6): K=
L K= d td E
(2)
However, there is a non-uniform electric field in the drift region when the AC superposition driving mode was utilized as mentioned above. The electric field is no longer constant but rather its strength is a function of the distance between the ion shutter and the location of the ions in the drift region under the C1 and C2 driving mode. The drift time has an offset under the C1 and C2 driving mode as described above. In these conditions, Eq. (2) is no longer utilized to calculate the ion mobility.
C
td 2
Ld td E non−uniform
=
Ld td E uniform
×
1 R
(6)
Cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) was investigated using the AC superposition ion shutter power supply under the C1 and C2 driving modes. About 20 pg RDX was introduced into the reaction region of the IMS apparatus by thermal desorption. The current signal which is generated by ions was collected by a Faraday plate located at the end of the drift tube. The voltage Vd applied on the ion shutter was 170 V and the electric field strength used in the drift region was 300 V cm−1 . The temperature of the IMS cell was 345 K and drift region was 6.3 cm long. Fig. 10 shows the mobility spectra of RDX under the C1 and C2 driving modes. The
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Table 1 The drift time of the RIP under the C1 and C2 modes, and the conventional driving mode; and the corrected values by exponential fitting. Vd (V)
100
150
200
250
300
350
400
Drift time under the Conventional driving mode (ms) Drift time under the C1 driving mode (ms) Drift time under the C2 driving mode (ms) Corrected drift time under C1 driving mode Corrected drift time under C2 driving mode
10.24 10.04 10.48 10.24 10.24
10.26 10.00 10.60 10.27 10.26
10.26 9.92 10.74 10.27 10.27
10.28 9.82 10.90 10.25 10.26
10.24 9.74 11.10 10.25 10.26
10.24 9.68 11.30 10.27 10.23
10.26 9.58 11.60 10.25 10.26
4.5. Resolution The resolution of the IMS was used to judge the ion separation capability of IMS which has been investigated intensively [13,24–28]. Equation 8 is given to calculate the resolution, where Res is the resolution of IMS, td is the drift time of the ion, Wg is the initial ion shutter width broadening, and Wdiff is the diffusional broadening of IMS peak [24,27,28]. Res =
td Wg2
(8)
2 + Wdiff
Eq. (8) can be expressed as Eq. (9) by combining it with the Nernst-Einstein equation [24]. Res =
td
Wg2 + Fig. 10. The mobility spectra of RDX ions carried out under the C1 and C2 driving modes.
drift times of the product ions are 10.96 ms and 11.66 ms, respectively. The reduced ion mobility of RDX ions can be calculated by Eq. (7): K0 (RDX) =
Ld td E non−uniform
=
Ld td E uniform
×
1 273 P ×( )( ) R T 760
4.4. Ion intensity The ion intensity of the RIP was calculated and plotted against the different Vd as shown in Fig. 6(b). The ion intensity under the three driving modes decreased with the increase of Vd . This agrees with the results by Tadjimukhamedov et al. in 2009 [16] and Tabrizchi et al. in 2010 [23]. Moreover, the ion intensity under the C2 mode was the highest while the ion intensity under the C1 mode was the lowest at a certain Vd . As shown in Fig. 7(b) and (c), the electric field in the left region of the ion shutter became weaker under the C1 driving mode when the ion shutter was closed. Fewer ions accumulated in this region near the ion shutter because of the weaker potential. Also the number of ions extracted from the reaction region became smaller than that of the ions under the conventional driving mode. Therefore, the ion intensity under the C1 mode was less. On the contrary, the electric field in the left region of the ion shutter was stronger, as shown in Fig. 8(a) and (b). That led to the enhancement of the ion intensity.
(9)
where k is Bolztmann’s constant, T is the absolute temperature, q is the ion charge, E is the electric field strength at the drift region, and L is the length of the drift region. If Wg td , then equation (9) can be rewritten as: Res = (
1 ⁄2 qELd ) 44.2kT
(10)
The electric field was distorted and was not constant under the AC superposition driving mode. According to
Ld
(7)
where T is the temperature of the drift tube in Kelvin (K) and P is the pressure in the drift region (mm Hg). The reduced ion mobility of RDX ions under the C1 and C2 driving modes were calculated as 1.48 cm2 V−1 s−1 and 1.44 cm2 V−1 s−1 , respectively, which are very close to the reported values of 1.47 cm2 V−1 s−1 [21], 1.46 cm2 V−1 s−1 [22] as measured by corona discharge IMS.
44.2kT td 2 qELd
E
non−uniform
=
E(i)dL
(11)
0
where E(i) is the electric field strength of the position where the ions is located in the drift region, Eq. (11) can be changed to: 1⁄2
L qLd Res = (
E(i)dL 0
44.2kT
)
(12)
As shown in the results calculated by Laplace equation in Fig. 7(c), the value of Enon−uniform in the drift region under the C1 driving mode is higher than that of E value under the conventional driving mode, leading to higher resolution according to equation (10). The reason for the resolution increase under the C1 driving mode also can be tentatively explained. For any particular ion packet in the drift region, the ions on the left side of the center have a higher electric field than those on the right side according to both simulated and calculated results. As shown in Fig. 11, the electric field strength E1 is a little bit higher E2 . The velocity of ions in the left part of the ion packet was greater than that of the ions in the right part of the ion packet. Hence, the width of the ion peak was compressed which means the resolution was improved in this situation. On the contrary, the value of the Enon−uniform in the drift region under the C2 driving mode was lower than that of E in the conventional driving mode. The resolution was decreased in this mode.
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References
Fig. 11. An ion packet measured under the non-uniform electric field.
Moreover, the ions on the left side of center had lower electric field strength than those on the right side. The velocity of ions in the left part of the ion packet was smaller than that of the ions in the right part of the ion packet. Hence, the width of the ion peak was broadened which means the resolution became worse under this situation. In summary, both of the driving modes (C1 and C2 ) were proved to be feasible and they have different influences on the performance of the IMS. The offset of the drift time can be corrected and the reduced mobility of RDX can be calculated according to the fitted equations. Higher resolution and lower ion intensity were attained by using the C1 driving mode. On the contrary, the C2 driving mode produced lower resolution and the higher ion intensity. The preferred driving mode depends on practical requirements. 5. Conclusions A novel AC superposition mode was designed for controlling the ion shutter in an IMS device. A new power supply utilizing this mode was designed and fabricated. The power supply of the ion shutter through AC superposition mode used a one-side feeding method, which formed a non-uniform electric field when the ion shutter was closed. In this study, two AC superposition modes are discussed in detail combined with the experiment and simulation results, and both were proved to be feasible and promising. The offset of the drift time was corrected by exponential fitting. The reduced ion mobility of RDX was calculated by using the fitted equations and the results agreed with previously reported values well. An appropriate between the two driving modes will improve the resolution of IMS or enhance ion intensity. Moreover, the cost of the new power supply is much lower than that of previous devices. Using this novel AC superposition power supply for an IMS instrument would greatly reduce the cost of the instrument. Acknowledgments This work is supported by the Anhui Provincial Program for Science and Technology Development (No.1301042095), the Direction Program of Hefei Center of Physical Science and Technology (No. 2012FXCX009), the State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF13024), and the Natural Science Foundation for Colleges and Universities of Anhui Province (No. KJ2014A225). Authors would like to give their thanks to Ms. Lanlan Li for polishing the language.
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Siodlowski, Modelling of penetration of ions through a shutter grid in ion mobility spectrometers, Sensor. Actuat. B-Chem. 135 (2008) 116–121. [18] L. Zhou, D.C. Collins, E.D. Lee, M.L. Lee, Mechanical ion gate for electrosprayionization ion-mobility spectrometry, Anal. Bioanal. Chem. 388 (2007) 189–194. [19] T. Brunner, A.R. Mueller, K. O’Sullivan, M.C. Simon, M. Kossick, S. Ettenauer, A.T. Gallant, E. Mane, D. Bishop, M. Good, G. Gratta, J. Dilling, A large Bradbury Nielsen ion gate with flexible wire spacing based on photo-etched stainless steel grids and its characterization applying symmetric and asymmetric potentials, Int. J. Mass Spectrom. 309 (2012) 97–103. [20] H.Y. Han, G.D. Huang, S.P. Jin, P.C. Zheng, G.H. Xu, J.Q. Li, H.M. Wang, Y.N. Chu, Determination of alcohol compounds using corona discharge ion mobility spectrometry, J. Environ. Sci. 19 (2007) 751–755. [21] T. Khayamian, M. Tabrizchi, M.T. Jafari, Analysis of 2,4,6-trinitrotoluene, pentaerythritol tetranitrate and cyclo-1,3,5-trimethylene-2,4,6-trinitramine using negative corona discharge ion mobility spectrometry, Talanta 59 (2003) 327–333. [22] K.M. Roscioli, E. Davis, W.F. Siems, A. Mariano, W.S. Su, S.K. Guharay, H.H. Hill, Modular ion mobility spectrometer for explosives detection using corona ionization, Anal. Chem. 83 (2011) 5965–5971. [23] M. Tabrizchi, H.R. Shamlouei, Relative transmission of different ions through shutter grid, Int. J. Mass Spectrom. 291 (2010) 67–72. [24] S. Rokushika, H. Hatano, M.A. Baim, H.H. Hill, Resolution measurement for ion mobility spectrometry, Anal. Chem. 57 (1985) 1902–1907. [25] G.A. Eiceman, V.J. Vandiver, T. Chen, G. Ricomartinez, Electrical parameters in drift tubes for ion mobility spectrometry, Anal. Instrum. 18 (1989) 227–242. [26] P. Watts, A. Wilders, On the resolution obtainable in practical ion mobility systems, Int. J. of Mass Spectrom. Ion Processes 112 (1992) 179–190. [27] W.F. Siems, C. Wu, E.E. Tarver, H.H. Hill, P.R. Larsen, D.G. McMinn, Measuring the resolving power of ion mobility spectrometers, Anal. Chem. 66 (1994) 4195–4201. [28] A.B. Kanu, M.M. Gribb, H.H. Hill, Predicting optimal resolving power for ambient pressure ion mobility spectrometry, Anal. Chem. 80 (2008) 6610–6619.
Biographies Sheng Liu obtained his Ph. D degree in physics in 2012 from Graduate University of Chinese Academy of Sciences. His current research interests include the development of chemical sensors and the application of ion mobility spectrometer. Presently
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he is working in College of Computer Science and Technology at Huaibei Normal University, Anhui, China as an associate professor. Chaoqun Huang obtained his Ph. D degree in physical chemistry in 2006 from University of Science and Technology of China. His is an associate professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research interests are development of the ion mobility spectrometer and mass spectrometer. Chengyin Shen obtained his Ph. D degree in physics in 2010 from Graduate University of Chinese Academy of Sciences. His is an associate professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research interests are development of mass spectrometer and its application.
Haihe Jiang received his B.S. degree in physics and M.S. degree in physics in 1983 and 1991 from North China Electric Power University and Graduate University of Chinese Academy of Sciences respectively. His is a professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research fields are laser physics, development of laser instrument. Yannan Chu received his M.S. degree and Ph. D degree in physics in 1991 and 2003 from Graduate University of Chinese Academy of Sciences respectively. His is a professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research fields are development and application of ion mobility spectrometer and mass spectrometer.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel driving mode for ion shutter based on alternating current superposition and its application to ion mobility spectrometry Sheng Liu a,b , Chaoqun Huang a,∗ , Chengyin Shen a , Haihe Jiang a , Yannan Chu a,∗∗ a Laboratory of Medical Optical and Mass Spectrometry, Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, No. 350 Scientific Road, Hefei 230031, China b College of Computer Science and Technology, Huaibei Normal University, 235000, Anhui, China
a r t i c l e
i n f o
Article history: Received 16 September 2014 Received in revised form 11 December 2014 Accepted 5 January 2015 Available online 28 January 2015 Keywords: Ion mobility spectrometry Ion shutter Driving mode Alternating current superposition
a b s t r a c t In order to simplify the design of an ion shutter power supply and decrease its cost, a simple ion shutter driving mode based on alternating current (AC) superposition mode was designed. A power supply with AC superposition was fabricated and applied to the ion mobility spectrometry instrument successfully. The effect of the AC superposition mode on the drift time, ion intensity, and resolution of the ion mobility spectra was studied. Combined with electric field simulation and numerical calculation, a reasonable explanation for the effect of the AC superposition mode on the performance of ion mobility spectrometry is given. Two equations have been fitted by exponential fitting to correct the offset of the drift time. Cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) has been investigated to validate the effectiveness of the correction by using corona discharge IMS combined with the new ion shutter driving mode. The results show that the new low-cost power supply based on AC superposition mode is feasible. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ion mobility spectrometry (IMS) is a trace chemical substances analytical technology that appeared in the late 1960s to the early 1970s [1,2]. Compared to the mass spectrometer, IMS is capable of being operated at atmospheric pressure, and has the advantage of low cost and ease of use. Recently, IMS has been widely used to detect many trace compounds such as chemical warfare agents [3,4], illegal drugs [5,6], explosives [7,8], and various other chemicals [9,10] due to its high sensitivity, analytical flexibility, and real time monitoring capability. The ion shutter is one of the key parts in an IMS instrument. In general, the Bradbury-Nielson (BN) ion shutter has been widely used as an electronic switch. The BN ion shutter is composed of two arrays of wires, which are insulated and separated from each other in an interdigital form fixed at the bracket. Since the ion shutter is perpendicular to the direction of the movement of ions, ions are prevented from entering the drift region when the electric field is applied to the wires. Such electric field is removed transiently in each cycle, therefore ion packet can enter the drift region, and a stable mobility spectrum is formed.
∗ Corresponding author. Tel.: +86 551 65592429. ∗∗ Corresponding author. Tel.: +86 551 65595311. E-mail addresses: [email protected] (C. Huang), [email protected] (Y. Chu). http://dx.doi.org/10.1016/j.snb.2015.01.061 0925-4005/© 2015 Elsevier B.V. All rights reserved.
The phenomena occurring in the vicinity of the ion shutter are important and have attracted extensive attention. Several researchers have investigated how the change of the electrical field around the ion shutter will influence the performance of the IMS [11–17]. Eiceman et al. systematically studied the effect of the influence and the importance of optimizing electric fields in each region of an IMS drift tube by experimental and simulation methods. They concluded that the non-homogeneous fields in the drift region resulting from electric field between ion source and ion shutter will influence the mobility time of ions, leading to a broadening of the observed peak and a decrease in peak intensity. Moreover, the effect on signal intensity from electric fields arising from the ion shutter and drift tube has been investigated in detail by experimental and modeling methods [12]. In addition, the modeling of the penetration of ions through an ion shutter has been done with the aid of computer calculation. The electric field in the neighborhood of the ion shutter has a distinctive effect on the shape of the ion peak and ion intensity as well as the resolution of IMS spectrometer [17]. The design and fabrication of the ion shutter have also attracted many researchers’ attention. Salleras et al. have made an ion shutter for miniaturized ion mobility spectrometers by bulk silicon micromachining and optimized its parameters, such as electrode geometry and applied voltage [14]. Zhou et al. have constructed a novel ion shutter for electrospray-ionization atmospheric pressure ion mobility spectrometry. A chopper wheel with two windows was used to push the ions to the drift region periodically as an ion
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Fig. 1. Schematic of the IMS operation principle.
shutter. The experimental resolution was around 70 in this report [18]. An etching method has also been used to fabricate the ion shutter. Brunner et al. designed and constructed a novel and large BN ion shutter with flexible wires based on photo-etched stainless steel grids. This ion shutter, which was etched by 50 m stainless wire, has good mechanical tolerance and is easy to assemble [19]. Apart from the design and fabrication of ion shutters, the electronic driving mode of ion shutters also deserves to be studied. The conventional electronic diagram of the driving mode for the BN ion shutter is complicated. Also, the cost of a commercial power supply for an ion shutter is several thousand dollars, which increases the cost of the whole IMS instrument too much. Hence, it is necessary to study simple and feasible electronic driving modes and power supplies for the ion shutter to replace the conventional ones. However, to date few reports have focused on the study of the electronic driving mode and power supply for ion shutters. This study proposes a novel design for the driving mode of the BN ion shutter based on an alternating current (AC) superimposed mode. A simple and low-cost power supply for driving the BN ion shutter was fabricated and applied to the IMS apparatus successfully. The influence of the new driving mode on the drift time, ion intensity, and resolution of IMS is discussed combined with experiment and numerical calculation. The novel driving AC superposition mode is proved to be feasible according to these results. In contrast to previous equipment, the cost of the new power supply is much lower, costing less than one hundred dollars due to its simple electronic design and ease of assembly.
In general, the periodical pulse applied to the ion shutter is provided by the ion shutter power supply. The principle of the conventional driving mode of the ion shutter in IMS is shown in Fig. 2(a). A pulse amplified by the driving power supply is sent directly to the ion shutter which is connected to the high voltage source through resistors. It is important to note that the pulse voltage should be floating and the pulse driving power must be insulated from the ground. Because R1 is equal to R2 , the voltage of the two adjacent wires is UA + Vd /2 and UA − Vd /2 respectively when the ion shutter is closed, where UA is the potential of point A on the ion shutter and Vd is the pulse voltage which is applied to the ion shutter. Without taking into consideration the influence of UA , the electric field around the ion shutter generated by the two sets of wires is equal in amplitude but opposite in its polarity, leading to the nearly uniform electric field on both sides of the ion shutter. Fig. 2(b) and (c) shows the simulated electric field distribution using SIMION software during the opening and closing of the ion shutter respectively. As shown in Fig. 2(b) and (c), the electric
2. Driving principle of the ion shutter 2.1. The principle of the conventional driving mode of the ion shutter in IMS Fig. 1 is the schematic of IMS operation principle. When a gas phase sample is introduced into the reaction region by the carrier gas, product ions are formed using different ion sources such as photon, corona discharge, electrospray ionization (ESI), or radioactive ionization. These ions can enter the drift region under the action of the ion shutter pulse and are separated due to their different velocities. The velocity of the ions depends on their weight, charge, and shape. The ions in the drift region move toward the Faraday plate and create a current signal. The output of the amplified signal is synchronized with the ion shutter pulse, yielding a mobility spectrum, i.e., a plot of ion current versus time.
Fig. 2. (a) Conventional drive mode of the ion shutter and the electric field distribution simulated by SIMION when the ion shutter is (b) open and (c) closed.
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Fig. 3. The principle of AC superposition driving mode of the ion shutter in IMS.
field in the drift region is nearly uniform in the conventional driving mode. Hence the voltage applied to the ion shutter has no effect on ion behaviors in the uniform electric field, no matter whether the ion shutter is opened or closed. 2.2. The principle of AC superposition driving mode of the ion shutter in IMS In the IMS instrument, a potential difference Vd is also needed to apply to the ion shutter to prevent ions from entering the drift region without the narrow pulse signal using the AC superposition driving mode when the ion shutter is closed. However, the potential of the narrow pulse compensates for the potential difference of the ion shutter during the narrow pulse duration. This means that the ion shutter is opened and ions are able to enter the drift region. The principle of the circuit diagram of the AC superposition driving mode in IMS is seen in Fig. 3. A set of resistors in series is connected to one end of the ion shutter. The value of the resistors is three times more than the value of the divider resistors which are connected to the drift rings. The purpose of using the high-value resistors is to reduce the load of the high voltage power supply. The amplitude of Vd is adjustable by adjusting the value of potentiometer W1 . Without the narrow pulse signal, the amplitude of Vd is adjusted to high enough to completely prevent ions traveling from the reaction region to the drift region. The ion shutter stays closed while the pulse signal is absent. Once the narrow pulse voltage is applied, its amplitude and polarity are able to compensate for the Vd . The potential at two metal wires of the ion shutter is equal. The ion shutter stays open. Therefore the function of IMS is realized using the new driving mode. The AC pulse is superimposed by a capacitance which is connected to one end of the ion shutter to control the behavior of ions. When the ion shutter is closed, the potentials of both metal wires of the ion shutter are UA and UA + Vd , respectively. So the electric field distribution at both sides of the ion shutter is different, which means that there is a non-uniform electric field around the ion shutter region. The behavior of ions, such as the drift time, ion intensity and resolution of the IMS will be influenced due to the existence of the non-uniform electric field in the vicinity of the ion shutter. 3. Experimental 3.1. Apparatus A homemade IMS experimental device is set up in our laboratory at the Hefei Institutes of Physical Science, Chinese Academy
of Sciences. A detailed description of the apparatus appears in our previous report [20]. Briefly, the whole IMS cell consists of twelve metal guards which are insulated from each other using Teflon rings. The whole IMS cell is divided into three parts: ionization region, reactant region, and drift region. The length of the drift region is 6.3 cm. A continuous corona discharge ion source is applied in the IMS apparatus. A BN ion shutter is installed between the reaction region and the drift region. The electric field strength in the drift region is 300 V/cm. All the experiments were conducted at room temperature (295 K) except the ones with additional description below. The current signal was collected by a Faraday plate which is located at the end of the drift region, then fed into the computer data processing system. The IMS works in negative detection mode and the spectrum is averaged every 30 continuous acquisitions. 3.2. Design and fabrication of the power supply of AC superposition Designed pulse generating and amplifying circuits for AC superposition driving mode are shown in Fig. 4. The amplified pulse signal is applied to the ion shutter through capacitance. The supply voltage V+ is adjustable from 70 V to 500 V. As shown in Fig. 4, U2 and its subsidiary circuits generate a periodic square wave. The frequency of the square wave is adjusted using potentiometer of R14 . The square wave is sent to U1A . Then the narrow pulse signal is generated by U1A . The width of the narrow pulse signal is adjustable using potentiometer R6 . After amplification by the amplifier circuit, the output pulse signal of U1A is superimposed to one end of the ion shutter through high voltage capacitor C7 for controlling the ion shutter. While the ion shutter is closed, there are two cases when the AC superimposition driving mode is applied to IMS. Case 1 is (C1 , |UB | > |UA |), where the absolute value of the potential of UB is higher than that of UA . Here UA and UB are the potentials at points A and B on the ion shutter. The ion shutter will be opened when the polarity of the narrow pulse is opposite to the polarity of the high voltage after the narrow pulse has been applied. In case 2, (C2 , |UB | < |UA |), the absolute value of the potential of UB is lower than that of UA . The ion shutter will be opened when the polarities of the pulse and the high voltage are the same after the narrow pulse has been applied. It is noted that the function of the ion shutter could be realized with different performance of IMS in the two cases. The reaction ion peak (RIP) was recorded at the different voltage Vd under C1 , C2 , and the conventional driving mode. The results measured by the C1 and C2 mode were compared with the results measured by the conventional driving mode.
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Fig. 4. Pulse generation and amplifier circuits of the ion shutter based on AC superposition.
4. Results and discussion 4.1. The spectrum of IMS under C1 , C2 , and the conventional driving mode As shown in Fig. 5(a), the peak position is almost the same at the different voltages Vd when the conventional driving mode was applied. However, the RIP peaks shift to the left and their widths are narrowed as the signal intensity decreased gradually with the increase of the value of Vd under the C1 driving mode as shown in Fig. 5(b). On the contrary, the peaks shift to the right and their widths are broadened with the increase
of the value of Vd under the C2 driving mode as shown in Fig. 5(c). In order to further evaluate and compare the performance of the AC superimposition driving mode, the drift time, ion intensity, and resolution of the RIP measured under C1 , C2 , and the conventional driving mode were calculated and plotted against different Vd . All the results are shown in Fig. 6. 4.2. Drift time of the reaction ions The center of the peak, i.e., the drift time of the RIP was extracted and plotted against the Vd. As shown in Fig. 6(a), the drift time which
Fig. 5. The reaction ion peaks (RIP) attained under (a) the conventional driving mode, (b) the C1 mode, and (c) the C2 mode.
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Fig. 6. The effects of the ion shutter potential Vd on (a) the drift time, (b) the ion intensity, and (c) the resolution under the conventional driving mode, the C1 mode, and the C2 mode.
was measured under the different Vd is almost the same when the conventional driving mode was applied. This means that the Vd has little influence on the peak position because there is a uniform electric field throughout the drift region. However, the drift time measured under the C1 driving mode decreased with the increase of the Vd . On the contrary, the drift time became longer with the increase of the Vd when the C2 driving mode was applied. As mentioned above, the potential in both sides of the ion shutter is asymmetric in the AC superimposition driving mode. The electric field between the two adjacent metal wires of the ion shutter was strengthened with the increase of the Vd . The distortion of the electric field near the ion shutter region became more serious with the enhancement of the potential near the ion shutter. The velocity of ions was influenced by the distorted electric field under the AC superimposition driving mode. The behavior of the ions in the non-uniform electric field under the C1 and C2 driving modes was clarified with the aid of simulation and calculation. To study the C1 driving mode, the electric field in the drift region was simulated by SIMION with the ion shutter open and with it closed. Fig. 7(a) shows the distribution of the electric field in the drift region with the ion shutter open. There is a nearly uniform electric field along the axis of drift tube because Vd is zero. Fig. 7(b) shows the distribution of the electric field in the drift region when the ion shutter is closed (Vd = 400 V). The electric field was distorted around the ion shutter region. It is obvious that the electric field on the right side of the ion shutter was strengthened, whereas the electric field on the left side of the ion shutter was weakened. The electric field in on the right side of the ion shutter changed from strong to weak along the axis of the drift region from left to right as shown in Fig. 7(b).
The Laplace equation was utilized to calculate the amplitude of the electric field along the axis of the drift tube when the ion shutter was closed. Fig. 7(c) shows that the electric field strength along the axis of the drift tube increased with the increase of Vd from 100 V to 400 V in 100 V intervals when the ion shutter was closed under the C1 driving mode. Also, the electric field strength on the right side of the ion shutter decreased from left to right at a certain Vd voltage. This agrees with the simulated results very well.
Fig. 7. The electric field distribution simulated by SIMION when the ion shutter is (a) open, and (b) closed, and (c) the distribution as calculated by Laplace equation under different ion shutter potentials Vd when using the C1 driving mode.
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Fig. 8. The electric field distribution (a) simulated by SIMION when the ion shutter is closed, and (b) as calculated by Laplace equation under different ion shutter potentials (Vd ) when using the C2 driving mode.
When the ion shutter is open, ion packets will enter the drift region and move to the downstream of the drift region in the distorted electric field. The electric field on the right of the ion shutter was stronger than that in the uniform electric field. Therefore the velocity of the ions is faster in the distorted electric field, so less drift time is needed for the ions in the distorted electric field. The offset time increases with the increase of Vd , because there is stronger electric field strength at higher values of Vd . Contrary to the C1 driving mode, the drift time of the ions under the C2 driving mode is longer than that of the ions under the conventional driving mode at a certain Vd value. The simulated and calculated results in Fig. 8 show that the distorted electric field on the right side of the ion shutter was weakened whereas the electric field on the left side of ion shutter was strengthened. The electric field on the right side of the ion shutter changed from weak to strong along the axis of the drift tube from left to right. The reduced electric field downstream of the drift region led to a longer drift time of the ions.
Fig. 9. The ratios of the drift times measured under the conventional driving mode and the C1 (a) and C2 (b) driving modes, plus their fitted curves.
The ratio R of the drift times which are determined in a uniform electric field and a non-uniform electric field is given as: uniform
R=
td
non−uniform td
=
E non−uniform
(3)
E uniform
where td uniform is the drift time measured in a uniform electric field which is determined under conventional driving mode and td non−uniform is the drift time measured in a non-uniform field which is determined under the C1 and C2 driving modes. Euniform and Enon−uniform correspond to the uniform electric field strength and non-uniform electric field strength, respectively. The ratio R was plotted against the voltage Vd and fitted by exponential as shown in Fig. 9. The fitted equations are expressed as equations (4) and (5). uniform
R(
td
R(
td
C
td 1
4.3. The correction of drift time and reduced mobility
) = exp(0.00463 + 1.43 × 10−4 x + 3.47 × 10−8 x2 )
(4)
) = exp(−0.012 − 5.51 × 10−5 x − 5.55 × 10−7 x2 )
(5)
uniform
The drift time of ions is an important parameter for IMS measurement and can be expressed by Eq. (1): Ld
L td = = d vd KE
(1)
where td is the drift time, Ld is the length of drift region, vd is the drift velocity of ions, K is the ion mobility, and E is the electric field strength in the drift region. The drift time is always used to calculate the ion mobility K by Eq. (2) because the electric field is uniform under the conventional driving mode.
where tdC1 and tdC2 are the drift times which are measured under C1 and C2 driving modes and x is the voltage which is applied to the ion shutter. Using the fitted Eqs. (4) and (5), the drift times tdC1 and tdC2 were corrected and tabulated in Table 1. The corrected values agree with the drift time which is measured by the conventional driving mode as shown in the table. Combined with the Eqs. (2) and (3), the ion mobility under the C1 and C2 driving modes can be calculated by Eq. (6): K=
L K= d td E
(2)
However, there is a non-uniform electric field in the drift region when the AC superposition driving mode was utilized as mentioned above. The electric field is no longer constant but rather its strength is a function of the distance between the ion shutter and the location of the ions in the drift region under the C1 and C2 driving mode. The drift time has an offset under the C1 and C2 driving mode as described above. In these conditions, Eq. (2) is no longer utilized to calculate the ion mobility.
C
td 2
Ld td E non−uniform
=
Ld td E uniform
×
1 R
(6)
Cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) was investigated using the AC superposition ion shutter power supply under the C1 and C2 driving modes. About 20 pg RDX was introduced into the reaction region of the IMS apparatus by thermal desorption. The current signal which is generated by ions was collected by a Faraday plate located at the end of the drift tube. The voltage Vd applied on the ion shutter was 170 V and the electric field strength used in the drift region was 300 V cm−1 . The temperature of the IMS cell was 345 K and drift region was 6.3 cm long. Fig. 10 shows the mobility spectra of RDX under the C1 and C2 driving modes. The
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Table 1 The drift time of the RIP under the C1 and C2 modes, and the conventional driving mode; and the corrected values by exponential fitting. Vd (V)
100
150
200
250
300
350
400
Drift time under the Conventional driving mode (ms) Drift time under the C1 driving mode (ms) Drift time under the C2 driving mode (ms) Corrected drift time under C1 driving mode Corrected drift time under C2 driving mode
10.24 10.04 10.48 10.24 10.24
10.26 10.00 10.60 10.27 10.26
10.26 9.92 10.74 10.27 10.27
10.28 9.82 10.90 10.25 10.26
10.24 9.74 11.10 10.25 10.26
10.24 9.68 11.30 10.27 10.23
10.26 9.58 11.60 10.25 10.26
4.5. Resolution The resolution of the IMS was used to judge the ion separation capability of IMS which has been investigated intensively [13,24–28]. Equation 8 is given to calculate the resolution, where Res is the resolution of IMS, td is the drift time of the ion, Wg is the initial ion shutter width broadening, and Wdiff is the diffusional broadening of IMS peak [24,27,28]. Res =
td Wg2
(8)
2 + Wdiff
Eq. (8) can be expressed as Eq. (9) by combining it with the Nernst-Einstein equation [24]. Res =
td
Wg2 + Fig. 10. The mobility spectra of RDX ions carried out under the C1 and C2 driving modes.
drift times of the product ions are 10.96 ms and 11.66 ms, respectively. The reduced ion mobility of RDX ions can be calculated by Eq. (7): K0 (RDX) =
Ld td E non−uniform
=
Ld td E uniform
×
1 273 P ×( )( ) R T 760
4.4. Ion intensity The ion intensity of the RIP was calculated and plotted against the different Vd as shown in Fig. 6(b). The ion intensity under the three driving modes decreased with the increase of Vd . This agrees with the results by Tadjimukhamedov et al. in 2009 [16] and Tabrizchi et al. in 2010 [23]. Moreover, the ion intensity under the C2 mode was the highest while the ion intensity under the C1 mode was the lowest at a certain Vd . As shown in Fig. 7(b) and (c), the electric field in the left region of the ion shutter became weaker under the C1 driving mode when the ion shutter was closed. Fewer ions accumulated in this region near the ion shutter because of the weaker potential. Also the number of ions extracted from the reaction region became smaller than that of the ions under the conventional driving mode. Therefore, the ion intensity under the C1 mode was less. On the contrary, the electric field in the left region of the ion shutter was stronger, as shown in Fig. 8(a) and (b). That led to the enhancement of the ion intensity.
(9)
where k is Bolztmann’s constant, T is the absolute temperature, q is the ion charge, E is the electric field strength at the drift region, and L is the length of the drift region. If Wg td , then equation (9) can be rewritten as: Res = (
1 ⁄2 qELd ) 44.2kT
(10)
The electric field was distorted and was not constant under the AC superposition driving mode. According to
Ld
(7)
where T is the temperature of the drift tube in Kelvin (K) and P is the pressure in the drift region (mm Hg). The reduced ion mobility of RDX ions under the C1 and C2 driving modes were calculated as 1.48 cm2 V−1 s−1 and 1.44 cm2 V−1 s−1 , respectively, which are very close to the reported values of 1.47 cm2 V−1 s−1 [21], 1.46 cm2 V−1 s−1 [22] as measured by corona discharge IMS.
44.2kT td 2 qELd
E
non−uniform
=
E(i)dL
(11)
0
where E(i) is the electric field strength of the position where the ions is located in the drift region, Eq. (11) can be changed to: 1⁄2
L qLd Res = (
E(i)dL 0
44.2kT
)
(12)
As shown in the results calculated by Laplace equation in Fig. 7(c), the value of Enon−uniform in the drift region under the C1 driving mode is higher than that of E value under the conventional driving mode, leading to higher resolution according to equation (10). The reason for the resolution increase under the C1 driving mode also can be tentatively explained. For any particular ion packet in the drift region, the ions on the left side of the center have a higher electric field than those on the right side according to both simulated and calculated results. As shown in Fig. 11, the electric field strength E1 is a little bit higher E2 . The velocity of ions in the left part of the ion packet was greater than that of the ions in the right part of the ion packet. Hence, the width of the ion peak was compressed which means the resolution was improved in this situation. On the contrary, the value of the Enon−uniform in the drift region under the C2 driving mode was lower than that of E in the conventional driving mode. The resolution was decreased in this mode.
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References
Fig. 11. An ion packet measured under the non-uniform electric field.
Moreover, the ions on the left side of center had lower electric field strength than those on the right side. The velocity of ions in the left part of the ion packet was smaller than that of the ions in the right part of the ion packet. Hence, the width of the ion peak was broadened which means the resolution became worse under this situation. In summary, both of the driving modes (C1 and C2 ) were proved to be feasible and they have different influences on the performance of the IMS. The offset of the drift time can be corrected and the reduced mobility of RDX can be calculated according to the fitted equations. Higher resolution and lower ion intensity were attained by using the C1 driving mode. On the contrary, the C2 driving mode produced lower resolution and the higher ion intensity. The preferred driving mode depends on practical requirements. 5. Conclusions A novel AC superposition mode was designed for controlling the ion shutter in an IMS device. A new power supply utilizing this mode was designed and fabricated. The power supply of the ion shutter through AC superposition mode used a one-side feeding method, which formed a non-uniform electric field when the ion shutter was closed. In this study, two AC superposition modes are discussed in detail combined with the experiment and simulation results, and both were proved to be feasible and promising. The offset of the drift time was corrected by exponential fitting. The reduced ion mobility of RDX was calculated by using the fitted equations and the results agreed with previously reported values well. An appropriate between the two driving modes will improve the resolution of IMS or enhance ion intensity. Moreover, the cost of the new power supply is much lower than that of previous devices. Using this novel AC superposition power supply for an IMS instrument would greatly reduce the cost of the instrument. Acknowledgments This work is supported by the Anhui Provincial Program for Science and Technology Development (No.1301042095), the Direction Program of Hefei Center of Physical Science and Technology (No. 2012FXCX009), the State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF13024), and the Natural Science Foundation for Colleges and Universities of Anhui Province (No. KJ2014A225). Authors would like to give their thanks to Ms. Lanlan Li for polishing the language.
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Biographies Sheng Liu obtained his Ph. D degree in physics in 2012 from Graduate University of Chinese Academy of Sciences. His current research interests include the development of chemical sensors and the application of ion mobility spectrometer. Presently
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he is working in College of Computer Science and Technology at Huaibei Normal University, Anhui, China as an associate professor. Chaoqun Huang obtained his Ph. D degree in physical chemistry in 2006 from University of Science and Technology of China. His is an associate professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research interests are development of the ion mobility spectrometer and mass spectrometer. Chengyin Shen obtained his Ph. D degree in physics in 2010 from Graduate University of Chinese Academy of Sciences. His is an associate professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research interests are development of mass spectrometer and its application.
Haihe Jiang received his B.S. degree in physics and M.S. degree in physics in 1983 and 1991 from North China Electric Power University and Graduate University of Chinese Academy of Sciences respectively. His is a professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research fields are laser physics, development of laser instrument. Yannan Chu received his M.S. degree and Ph. D degree in physics in 1991 and 2003 from Graduate University of Chinese Academy of Sciences respectively. His is a professor in the Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Science. His current research fields are development and application of ion mobility spectrometer and mass spectrometer.