Analytical Performance of Printed Circuit Board Ion Trap Array Mass Analyzer with Electrospray Ionization

CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 41, Issue 1, January 2013 Online English edition of the Chinese language journal

Cite this article as: Chin J Anal Chem, 2013, 41(1), 152–158.

RESEARCH PAPER

Analytical Performance of Printed Circuit Board Ion Trap Array Mass Analyzer with Electrospray Ionization CHU Yan-Qiu, XIAO Yu, LING Xin, DING Chuan-Fan* Laser Chemistry Institute, Chemistry Department, Fudan University, Shanghai 200433, China

Abstract: The analytical performance of rectilinear ion trap array made up of printed circuit board material has been investigated. The experimental results indicated the intensity of ions determined were dramatically affected by ion kinetic energy, as in 18 eV, the arginine (Arg) ion (m/z 175.2) dominated in the mass spectrum. The ion trap array can perform multiple sample ion storage, mass-selected ion isolation, ion ejection, collision induce dissociation. The ions of m/z 175.2, 117.3 and 71.8 were isolated by SWIFT notch 50–60 kHz, 75–85 kHz and 130–145 kHz, respectively. The ions of m/z 175.2, 117.3 and 71.8 were selectively ejected using 55, 80 and 135 kHz sine wave. In helium gas, the fragment ions of m/z 157.2, 130.3 and 117.3 were obtained by the fragmentation reaction of Arg precursor ion (m/z 175.2) using 102 kHz sine wave. The ion detection efficiency was investigated by electric current integration method, which indicated that the ion detection efficiency was estimated to be 46.3% in 256 kHz AC resonant ejection and was about 9.7% in boundary ejection. Key Words: Ion trap array; Printed circuit board; High-throughput; Electrospray ionization source; Analytical performance

1

Introduction

Since the invention of three-dimensional (3D)[1] by Paul and Steinwedel, by virtue of simple, fast, and higher sensitivity, ion trap has been widely applied in analysis and characterization of isotope, chemical element, and biological samples. However, Paul three-dimensional ion trap was composed of a hyperbolic ring electrode and two hyperbolical end-cap electrodes. This kind of ion trap has some disadvantages in terms of its requirement of very high mechanical accuracy and being relatively expensive. To overcome these disadvantages, Cooks and co-workers fabricated a cylindrical ion trap (CIT). In comparison to Paul hyperbolic trap, the CIT is easy to be fabricated and has been developed as miniature mass analyzers. However, the lower trapping capacity and lower trapping efficiency limits its further development. Soon, linear ion trap was invented and successfully commercialized[2–6]. Rectilinear ion trap (RIT) is a novel type of ion trap based

on linear ion traps and cylindrical ion trap. Combining the advantages of higher trapping capacity and higher trapping efficiency of linear ion trap with the advantages of the geometrical simplicity of the cylindrical ion trap, RIT becomes to be a promising candidate, which can be used as a new mass analyzer. With the miniaturization of mass spectrometer, other ion traps with different geometries such as halo[7] and toroidal[8] were also built for the purpose of higher ion trapping capacity and efficiency. Recently, high-throughput analysis[9–12] received considerable attentions due to its extensive applications in many fields such as combinatorial chemistry, proteomics, and metabolomics. In these research areas, large numbers of highly similar samples are still analyzed via introduction of one sample into one mass spectrometer at a time. Two ways can be used to realize the high throughput mass spectrometric analysis, one way is an array of ion source sharing a common mass analyzer, the other way is multiple ion sources and multiple mass analyzers are used simultaneously in one mass

Received 8 April 2012; accepted 5 September 2012 * Corresponding author. Email: [email protected] This work was supported by the National Technology Support Project of China (No. 2009BAK60B03). Copyright © 2013, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(13)60625-8

CHU Yan-Qiu et al. / Chinese Journal of Analytical Chemistry, 2013, 41(1): 152–158

spectrometer. For example, a TOF with four-channels applied four TOF mass analyzers to analyzing several samples simultaneously. Cooks and co-workers[13] applied the CIT and RIT to the fabrication of high-thru mass spectrometer. The pioneering work includes fabricating multiplexed and array ion trap. Cooks[14] designed and built a CIT array mass spectrometer with electrospray ionization source, which contained four independent channels and was operated using two fully multiplexed channels (sources, ion optics, ion traps, detectors) capable of analyzing different samples simultaneously. Subsequently, Cooks[15–17] addressed the rectilinear ion trap, and constructed a multichannel mass spectrometer with electron ionization sources. The multiplexed ion trap divided the chamber into four parts, sharing one vacuum system and the same RF coil and control electronics. Due to the independence of four ion traps, the signal “cross talk” can be greatly decreased. Four isomeric and isobaric analysts (m/z 126) including 3-fluoroanisole, 4-fluoroanisole, 2,6-dimethylcyclohexanone, and 2-fluorobenzyl alcohol were chosen and introduced into channels 1–4 for simultaneous MS and MS/MS analysis, respectively. Eletrospray ionization (ESI) source is a typical ionization mode, which has been extensively applied[18–20] in mass spectrometry. Recently, Korhari et al[21] devised a four-channel multiplexed mass spectrometer with rectilinear ion trap (RIT) mass analyzers. The system consists of four parallel ESI or atmospheric pressure ion (API) sources, four RIT mass analyzers, four sets of ion optical elements, and four conversion dynode detectors. The complete instrument is housed in a single vacuum manifold with a common vacuum system. To maintain the vacuum of 10–3 Pa for the measurement of ion trap, the custom-built vacuum manifold was made of aluminum and accommodates four stages of differential pumping. In our lab, we have successfully fabricated a four-channel rectilinear ion trap array (ITA) which is made up of printed circuit board material (PCB). The surface of the printed circuit board electrode was coated with gold layer, and then the

Au-coated PCB electrode was precisely assembled to the ITA. Besides the advantages of higher ion trapping capacity, and relatively smaller size, this ITA mass analyzer is cheaper. By using electron impact (EI) source, Li et al[22] has preliminarily tested the analytical performance of one of the four channels for PCB-ITA mass analyzer. In this work, by electrospray ionization source (ESI), we will measure the analytical performance of one channels for PCB-ITA mass analyzer, including the effect of ion kinetic energy, mass-selected ion isolation, ion ejection and collision induce dissociation, which is helpful to the further research of ITA in the near future.

2

Experimental

PCB-ITA was designed and fabricated by our laboratory, the detail of the structure was described elsewhere[22]. The length, width and height (z axial) for PCB-ITA were 0.55 cm (X0), 0.625 cm (y0), 4.75 cm (Z0), the volume was 6.53 cm3. Adjustable DC potentials were added to skimmer (36 V), ion guide, aperture, the front end cap, the back end cap. The dc voltage on the front end-cap electrode can be pulled up or down, so it acts as an ions gate. The RF power supply refers to a conventional sinusoidal trapping waveform (768 kHz, Sciex, Canada). A supplementary ac signal is used for resonant excitation, the maximum amplitude of AC signal is 10 V. The photograph and schematic diagram for PCB- ITA are shown in Fig.1. The operation procedure of PCB-RIT mass spectrometer is divided into four periods: ionization, cooling, mass analysis (Ramp), and ion clearance. In ionization period, the RF voltage signal loaded on the rectilinear ion trap maintained invariable (300 V). The vacuum system was a homemade two-stage stainless steel vacuum chamber, which was pumped by two turbo molecular pumps. An ESI ion source was used to produce sample ions. The produced ions were introduced into one of the ion trap channels of the ITA through an aperture on the front end-cap electrode. The vacuum in the front chamber was about 0.6 Pa. The background pressure in the PCB-ITA chamber is about 3 × 10–3 Pa. Helium buffer gas was admitted into the ITA area to kinetically cool down the trapped ions during the experiments.

Fig.1 Experimental set up for printed circuit board ion trap array mass spectrometer (a) photograph, (b) schematic diagram

CHU Yan-Qiu et al. / Chinese Journal of Analytical Chemistry, 2013, 41(1): 152–158

The time sequence of collision induced dissociation (CID) includes eight periods: ionization, cooling, isolation, cooling, CID, cooling, mass analysis (Ramp), and ion clearance. A stored waveform inverse Fourier transform (SWIFT) waveform, whose frequency varies from 10 to 550 kHz was used for isolation of precursor. Arginine (simplified as Arg, 99.0%) was obtained from Huasun Bioengineering Co., Ltd (Shanghai, China), methanol was purchased from Lingbo Chemical Reagent Co., Ltd (Shanghai, China).

3 3.1

Results and discussion Effect of ion kinetic energy on peak intensity

Our previous study[23] demonstrated that in mass spectrometric analysis, because the air kinetic energy arising from vacuum pressure difference is relatively smaller (about 0.01 eV), the voltage difference n(R0 – Va) between ion guide (R0) and aperture (refers to interface between first and second vacuum chamber) can be recognized as the kinetic energy of ions entering chamber. When Va is fixed, increasing R0 gradually will lead to an increment of n(R0 – Va). The rising up of ion kinetic energy will lead to increase the fragmentation extent of precursor, resulting in the increment of ion peak intensities in mass spectra. In Fig.2a, six peaks can be observed. They can be attributed to the precursor Arg and its fragment ions [M + H]+. The peaks at m/z 175.2, 157.2, 117.3, 130.3, 72.2 and 59.0 can be attributed to Arg (m/z 175.2), the ion (m/z 157.2) of Arg lost one H2O, the ion (m/z 117.3) of Arg lost guanidyl group, the ion (m/z 130.3) of Arg lost carboxyl group, the ion (m/z 34 72.2) of Arg lost carboxyl and guanidyl group, the ion of CH2CH2CHNH2 (m/z 60.0). Figure 2 shows the ESI mass spectra for Arg ion at different kinetic energy (28, 23 and 18 eV). Decreasing ion kinetic energy will be beneficial to produce more Arg (m/z 175.2). When ion kinetic energy decreased to 18 eV, the obtained mass spectrometric peak was mainly the Arg (m/z 175.2). The

Fig.3

main reason may be that the relatively larger kinetic energy leads to stronger collision inducing fragment. It is seen that adjusting kinetic energy can decrease the fragmentation extent of Arg. 3.2

Z-axis dc trapping potential effects on peak intensity peak full width at half-maximum (fwhm)

The effects of z-electrode dc voltage on RIT performance were investigated by ionizing Arg for 30 ms, using a constant value of the ac amplitude of 9 V and a resonance frequency of 256 kHz, but varying the dc voltage applied to the z-electrode. As shown in Fig.3, the abundance increases significantly with the z electrode dc voltage up to 70 V, presumably

Fig.2 ESI mass spectra for Arginine at various ion kinetic energy (a) 28 eV, (b) 23 eV, (c) 18 eV

z axis dc trapping potential effects on a peak intensity of Arg (m/z 175.2 ) a. Relative abundance of peak; b. peak full width at half-maximum (FWHM)

CHU Yan-Qiu et al. / Chinese Journal of Analytical Chemistry, 2013, 41(1): 152–158

because the deeper dc potential well in the z direction helps to trap more ions. The fwhm of Arg decreases with further increasing in the z electrode voltage, presumably as a result of ions forced to the center of the RIT and ejected within a relatively narrow band along the z axis. At z-electrode dc voltages above 80 V, the ion abundance also starts to decrease and the improvement in resolution due to the loss of ion trapping capacity as the ions are pushed closer to the center of the RIT. The spectral resolution was further optimized as described later in this study. 3.3

Effect of RF scan rate on mass resolution

It is well known that higher mass resolution of ion trap mass analysis can be obtained at a slower scan rate. The main reason is that in sine scan mode, decreasing the mass scan rate suggests increasing scanning steps of RF voltages, thereby providing more plenty of time for ions to leave the stable region of stability diagram. Consequently, the mass resolution will be increased. The scan rate effect on mass resolution was tested in this work. Figure 4 shows an effect of RF scan rate on mass resolution in mass spectra. The ramp time was 85, 135, 185 ms and the calculated scan rate were 2258, 1422, 1038 Th s1, respectively. According to the resolution formula m/m, the mass resolution 208, 310, 360 can be calculated. The resolution increased slightly with the decreasing of scan rate. To further improve resolution, some measurement may be carried out, for example, locking the phase of RF power source and the phase of dipole excitation AC, further decreasing the scan rate, and applying a digital ion trap[24] which exhibits a good performance of locking the phase. 3.4

resonant point, which was a higher efficiency q value point. At this point, the hexpole and octopole excitation took place simultaneously. The SWIFT resonant ejection point was relatively smaller[25], the efficiency of resonant ejection was lower than that of nonlinear resonant ejection.

Fig.4 Mass resolutions at different RF scan speeds for Arg (m/z 175.2) a. 2258 Th s1, b. 1422 Th s1, c. 1038 Th s1

Mass-selected ion isolation and mass-selected ion ejection

The function of mass-selected ion isolation and mass-selected ejection of ITA was tested in “conventional RF mode”. Arginine was used to produce sample ions as well. To perform mass-selected ion isolation, the RF trapping voltage was fixed at 300 V (zero-to peak) while the frequency was 768 kHz; a stored wave form inversed Fourier transform (SWIFT) was used to make notch. The frequency notches of 50–60 kHz, 75–85 kHz and 130–145 kHz and amplitude of 4.5 V was coupled to the RF signal and applied to the electrodes of the ITA for 8 ms, and then the ions that remained in the ITA were mass-analyzed by ramping the voltage of the RF signal. The AC frequency used for resonant remains to be 256 kHz. The result is shown in Fig.5, it can be seen that m/z 175.2, 117.2 and 71.8 were isolated in turn. As shown in Fig. 5b, although m/z 117.2 was isolated, some small peaks were still observed. The main reason probably because the resonant ejection frequency point (AC = 256 kHz) situated in nonlinear

Fig.5 Mass-selected ion isolation in ITA mass analyzer, SWIFT frequency notch is (a) 50–60 kHz, (b) 75–85 kHz, and (c) 130–145 kHz

CHU Yan-Qiu et al. / Chinese Journal of Analytical Chemistry, 2013, 41(1): 152–158

The capability of mass-selected ion ejection using an ac waveform was also tested in the ITA (Fig.6). The selected ion was ejected out of the ion trap, while the others were saved in the ion trap. In mass-selected ion ejection, different ac waveforms were used with different frequencies to eject ions m/z 175.2, 117.1 and 71.8. In our experiment, the amplitude of the RF signal was fixed at 175 V (zero-to-peak) and its frequency was also fixed to 768 kHz. Ions of m/z 175.2, 117.1 and 71.8 were ejected when a supplementary ac waveform with amplitude of 1V and frequency of 55, 80 and 135 kHz, respectively. The duration of ejection was 8 ms. 3.5

Collision induced dissociation (MS2)

Arg (m/z 175.2) was used as the precursor to perform collision induced dissociation experiment. At first, the precursor Arg was isolated using a notched SWIFT, then sine wave (amplitude 2 V) was used to excite the precursor, inducing dissociation of precursor. The q = 0.6, oscillation frequency was 101, 102 and 103 kHz, respectively. The results of MS2 for the precursor Arg was shown in Fig.7. The ions of m/z 175.2 were isolated by SWIFT notch 60–70 kHz, q = 0.5, other ions were ejected out of ion trap. After 8 ms isolation, sine wave (amplitude 2 V) with oscillation excitation frequency of 102, 102 or 103 kHz was adding on a pair of y electrodes for 10 ms, leading to the disruption of Arg precursor. After fragmentation, ion cooling for 10 ms, m/z 157.2, 130.3 and 117.3 fragment ions were obtained. It should be noted that the oscillation frequency could play an important role on the intensity of fragment ions, in which the signal intensity were relative weaker with 101 kHz (Fig.7a) and 103 kHz (Fig.7b), and signal was relative stronger with 102 kHz (Fig.7c). Such phenomena were also observed in other literature. 3.6

integral areas of ions from boundary ejection and resonant ejection are divided by current integral area measured by detector, respectively. The ion detection efficiency for boundary ejection and 256 kHz resonant ejection can be obtained. Ion detection efficiency  (256 kHz, resonant ejection) =

Fig.6

Mass-selected ion ejection in ITA mass analyzer

Ion detection efficiency ()

In the process of measuring ion detection efficiency (), the test device is still the same one. The detector was removed from ion ejection slit into side of the back end cap, and the ions are repulsed to detector by DC voltage. In the experiment, the MS-Lab (ITMS 3.6) software (developed by China) was employed to change the experimental conditions, that is, after ionization and cooling period, voltages of other periods was set to zero, voltages of the front and back end cap were set to zero. All the ions trapped by negative high voltage were attracted to the detector, and thus the total number of ions was determined (Current integral area). In the experiment, the trapped ions are assumed to situate in the center of ion trap. After rescinding the voltages of RF, the front and back end cap, the total number of ions (Current integral area) equals to total number of ions which have been captured by ion trap. Then the total number of ions from boundary ejection and resonant ejection can be calculated. The

Fig.7

Tandem mass spectrometric experiments at different excitation frequency

CHU Yan-Qiu et al. / Chinese Journal of Analytical Chemistry, 2013, 41(1): 152–158

3.11 × 106/6.69 ×106 = 46.3%, ion detection efficiency (Boundary ejection) 6.5 × 105/6.69 × 106 = 9.7%. Therefore, it can be seen that the ion detection efficiency  (256 kHz, resonant ejection) is larger than that the  (Boundary ejection). Simion 8.0 (Commercial software) simulation results indicated when resonant ejection took place, most ions (40% from every directions) could ejected from two y directions, only small amounts of ions stroke the electrodes of ion trap (data are not shown here). The simulation result coincides with the  value 256 kHz resonant ejections (46.3%). Because 256 kHz is the three frequency divider of the main RF drive (768 kHz), which is the nonlinear resonant ejection point, predicting that the ejection efficiency will be relative high.

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The analytical performance of one channel of rectilinear ion trap array made up of printed circuit board material was determined. The experimental results indicated the intensity and variety of ion peak determined by mass spectrometry were affected dramatically by ion kinetic energy. As the kinetic energy was smaller than 18 eV, the Arg precursor ion (m/z 175.2) dominated in the mass spectrum. Increasing kinetic energy, the fragmentation of precursor occurred, producing series of fragment ions such as m/z 157.2, 117.3, 130.3, 72.2 and 59.0. Increasing the z-axial DC voltage from 30 V to 70 V, the intensity of ion peak increased gradually. The ion trap array could perform multiple sample ion storage, mass-selected ion isolation, ion ejection and collision induce dissociation. The ions of m/z 175.2, 117.3 and 71.8 were isolated by SWIFT notch 50–60 kHz, 75–85 kHz, and 130–145 kHz, respectively. The ions of m/z 175.2, 117.3 and 71.8 were ejected selectively using 55 kHz, 80 kHz and 135 kHz sine wave. In helium gas, the fragment ions of m/z 157.2, 130.3 and 117.3 were obtained by the fragmentation reaction of Arg precursor ion (m/z 175.2) using 102 kHz sine wave. The ion detection efficiency was determined by electric current integration method, which indicated that the ion detection efficiency was estimated to be 46.3% in 256 kHz AC resonant ejection and was about 9.7% in boundary ejection.

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