Fluorescence-based method for determining the number of ions trapped in a FT-ICR mass spectrometer

International Journal of Mass Spectrometry 338 (2013) 11–16

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Fluorescence-based method for determining the number of ions trapped in a FT-ICR mass spectrometer P. Sagulenko, V. Frankevich, R. Steinhoff 1 , R. Zenobi ∗ Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland

a r t i c l e

i n f o

Article history: Received 12 December 2011 Received in revised form 16 November 2012 Accepted 19 November 2012 Available online 7 January 2013 Keywords: FT-ICR mass spectrometry Laser-induced fluorescence Gas-phase rhodamines Number of trapped ions

a b s t r a c t A novel technique for determining the number of ions trapped inside a FT-ICR mass spectrometer is proposed. This technique is based on the detection of the spectral power radiated by fluorescing ions trapped inside the mass spectrometer. A pre-calibrated tungsten lamp was used to perform a calibration of the optical system. Using the absolute values of the energy radiated during an experiment, the number of ions trapped inside the cell was estimated. As reference ions, cations of rhodamine 6G and rhodamine B, whose gas-phase properties have been thoroughly studied, were chosen. The rhodamines have very high absorbance and high quantum yield in the gas phase. The method developed was compared to a previously published technique based on induced current measurements. A good agreement between the two techniques was found. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Fourier transform ion cyclotron resonance mass-spectrometry (FT-ICR MS) has become a popular technique for the analysis of gas-phase ions. The popularity of the FT-ICR MS is based on its extremely high resolving power, mass accuracy, and the possibility of implementing a number of techniques for isolation, fragmentation, and excitation of the trapped ions. Among the applications of FT-ICR mass spectrometry are exact mass measurements, the study ion-molecular reactions, complex oil mixtures (“petroleomics”), biological systems such as proteins and their complexes, noncovalent interactions and many more. Most of the studies being performed by FT-ICR mass spectrometry are qualitative. However, there are a few quantitative problems that FT-ICR mass spectrometry can in principle successfully approach. One example is the determination of ion/neutral binding energies [1], which requires high-accuracy ion abundance measurements. Reaction rate coefficient and equilibrium constant measurements are also affected by the uncertainty in measured peak magnitudes. A key requirement for quantitative studies is the precise determination of the peak abundance which is directly related to the number of ions confined inside the ICR cell of a mass spectrometer. The knowledge of the number of trapped ions is also important for characterization of an

∗ Corresponding author. Tel.: +41 44 632 43 76; fax: +41 44 632 12 92. E-mail address: [email protected] (R. Zenobi). 1 On leave from the TU München, D-85747 München, Germany. 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2012.11.007

instrument, which includes the efficiency of ionization, transmission and trapping of ions, and determination the detection limit, as well as the dynamic range of the instrument. The present work proposes a novel method to determine the number of ions confined in the ICR cell. Several works dedicated to the problem of determination of the number of ions have appeared. Two methods that have been proposed have been applied for FT-ICR mass spectrometry. The first is based on space-charge effects. An increase in space charge conditions effectively reduces the observed cyclotron frequency [2]. From the reduction in the cyclotron frequency, one can calculate the effective space charge and estimate number of trapped ions [3]. Another technique has been to measure the image current induced in the detector electrodes [4,5]. An ion cloud trapped inside the ICR cell of FT-ICR mass spectrometer orbits in the magnetic field and induces a current in the detection circuit [5–9]. The procedure for determining the number of ions consists of detecting the induced current and comparing this current with that calculated for one orbiting ion. Here we provide a novel technique for determining the number of trapped ions. Our technique is based on laser-induced fluorescence (LIF) of trapped fluorescent molecular ions. LIF studies of ions trapped inside a mass spectrometer have become popular for probing the optical properties of gas-phase ions. The first LIF signal of ions confined inside a mass spectrometer was obtained in the late 1970s, when a fluorescence signal from trapped Ba+ ions was observed [10]. In the beginning of the 1980s, the group of Drullinger observed a LIF signal of trapped Mg+ ions [11]. Fluorescent studies of trapped ions were restricted to ions of small

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m/z until 2001, when Marshall and co-workers published the fluorescence spectrum of trapped hexafluorobenzene cations [12]. In the present decade, LIF probing of trapped ions have become more widely used. Several groups have published on the fluorescent lifetimes and photo-physical properties of gas-phase molecular ions [13–17]. It is worth to note specially the pioneering works of Parks and co-workers, who extensively used laser-induced fluorescence and FRET for studying biomolecular conformations and dynamics in the gas phase [18–22] The characterization of an instrument was shown also to be possible with the help of laser-induced fluorescence. For example, a laser tomography method was proposed for determining the spatial distribution of the ions inside the penning trap of a FT-ICR mass spectrometer was reported in [23]. In this work we use laser-induced fluorescence of ions trapped inside a penning trap of a FT-ICR mass spectrometer for directly determining the number of ions. This goal was achieved by registering the spectral power of the fluorescence signal and comparing it to that calculated for an ion cloud confined inside the cell. 2. Experimental 2.1. Mass spectrometer description Experiments were performed on a FT-ICR mass spectrometer with an electrospray ionization (ESI) source (IonSpec, Lake Forest, CA, USA) and a 4.7 T superconducting magnet (Cryomagnetics Inc., Oak Ridge, Tennessee, USA). A schematic of the experimental setup is shown in Fig. 1. The ICR cell of the FT-ICR instrument was modified to enable optical access to trapped ions [13]. For the experiments, a 5 cm diameter, closed cylindrical cell was used. A 6 mm diameter hole in the detection plates existed in order to allow the excitation laser beam to enter the cell. The laser beam was introduced into the high vacuum region through a vacuum glass viewport (PF610010-X; Pfeiffer Vacuum, Asslar, Germany) and was guided into the ICR cell through the hole in the detection plate by a deflector (74-90-UV; OceanOptics, Dunedin, FL, USA) which consisted of a mirror installed at an angle of 45◦ with respect to the direction of the incident laser beam. In order to reduce scattering and reflections of the laser beam, a home-made baffle with two aligning slits was introduced between the deflector and the viewport. After passing through the ICR cell, the laser beam exited through a second hole in the detection plate and finally entered a cone laser beam dump (PL15; Newport, Irvine, CA, USA). A continuous wave Ar-ion laser at a wavelength of 488 nm was used for exciting the trapped ions.

To collect the fluorescence signal, one of the trapping plates was made from aluminum mesh. The signal was collected and focused onto one end of a 3 mm-core diameter plastic fiber with a NA of 0.51, by a system of two lenses. The opposite edge of the fiber was connected to an optical spectrometer. A notch filter was installed inside the spectrometer in order to suppress scattered light. The filter reduced the intensity of the scattered light by a factor of 106 , which had a bandwidth of 700 cm−1 . The spectrometer was coupled to a liquid nitrogen-cooled CCD camera (OMA-V, Princeton Instruments, Trenton, NJ, USA). The camera was optimized to perform long-exposure measurements. 2.2. Materials In the current experiments we used two homologous laser dyes – rhodamine 6G and rhodamine B. The gas-phase optical properties of different compounds that contain the xanthene chromophore, including rhodamine 6G and rhodamine B, have been thoroughly studied by our group [13–15] and by several other groups [24–26]. The compounds have high quantum yield and high absorbance at 488 nm. These factors simplify the analysis of the experimental data and the adjustment of instrumental parameters. Rhodamine 6G was purchased from Radiant Dyes (Wermelskirchen, Germany) as a chloride salt. Rhodamine B was purchased from Sigma–Aldrich Gmbh (Buchs, Germany) as a chloride salt. For the experiments the rhodamines were dissolved in methanol. The concentrations of the solutions were 1 ␮M. 2.3. Experimental protocol A typical experimental cycle started with production of ions in the ESI source. The ions were accumulated in a collision cell for 800 ms. Then the ions were guided through a hexapole mass filter into the ICR cell. In the ICR cell the ions were stored for a time interval of 5.45 s. During the storage time the ions were continuously irradiated by the Ar+ laser beam at a wavelength of 488 nm. The power of the laser beam was maintained at a constant value of 54 mW. The fluorescence signal was collected during the entire storage time. After storage, the cyclotron motion of the ions was excited and their mass spectrum was registered. The total time of the experimental cycle was texp = 7.15 s, while the fluorescence signal was collected only during the storage time, tst = 5.45 s. Due to the low number of ions inside the cell, the intensity of the optical signal acquired during a single storage period was very low. In order to increase the signal level it was for acquired many consecutive cycles, for a total of 10 min. In principle, the setup presented in this paper allows one to store ions for as long as several hours. However when the storage time was increased, photo-fragmentation and photo-bleaching decreased the number of fluorescing ions significantly. In each experiment, 10 optical spectra were acquired and averaged. The resulting spectrum was smoothed by Savitsky–Golay interpolation and fitted with log-normal functions. This procedure was applied to each experimental fluorescence spectrum. Further processing of the acquired fluorescence spectra consisted of converting the intensity scale to spectral power and integrating the result over the emission band to find the total emitted energy. Using the value of the total energy, the number of ions was calculated. The result was then compared to the signal intensity derived from the mass spectrum. 3. Results and discussion

Fig. 1. Scheme of the modified ICR cell used for optical probing of the trapped ions. 1, 4.7 T magnet; 2, ICR cell excitation/detection plates; 3, ICR cell trapping plates; 4, laser beam deflector; 5, baffle; 6, beam dump; 7, laser beam guiding mirrors; 8, Ar+ laser; 9, collecting optics; 10, optical fiber and 11, CCD camera coupled to a spectrometer.

The spectral power of the light emitted by the trapped ions had to be determined in absolute values (W/nm) in order to calculate the number of ions inside the ICR cell. Given that the CCD camera registered the intensity of the optical signal in arbitrary units

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Fig. 2. Experimental set-up for calibrating the CCD camera. A pre-calibrated tungsten lamp was used to mimic the ion cloud in the center of the ICR trap. The radiation of the lamp was registered by the CCD camera and then used for converting the arbitrary units given by the CCD camera into the absolute irradiance.

Fig. 3. Emission spectrum of the pre-calibrated tungsten lamp (HLX 64342, 100 W power, OSRAM, Munich, Germany) (a), and the spectrum of the calibration lamp, recorded by the CCD camera for the calibration (b).

(counts) and that each part of the optical system had a characteristic spectral transmission that strongly influenced the intensity which was registered by the CCD camera, a calibration of the complete optical system was required. For this purpose a pre-calibrated tungsten lamp (HLX 64342, 100 W power, OSRAM, Munich, Germany) was used. The light beam from the lamp was focused into the center of the ICR trap in order to mimic the ion cloud (Fig. 3). The magnitude of the energy delivered from the light source to the center of the ICR trap was determined as follows. First, the calibration lamp emission was converted to the values of the emission intensity per unit of solid angle. This value was then multiplied by the value of the solid angle used in the calibration experiment (Fig. 2). The resulting emission spectrum in W/nm is given in Fig. 3(a). After the light propagated through the optical system, its spectrum was recorded by the CCD camera (Fig. 3(b)). By relating the emission spectrum to that, recorded by the CCD camera, we obtained the calibration curve, which is to be used for converting the arbitrary units of CCD camera (counts) into the absolute irradiance (W/nm). This calibration was used for determining the energy emitted by the ion cloud confined in the ICR trap followed by determining the number of the confined ions. It is worth noting here that the emission intensity of the calibration lamp is much higher than that of the confined ion cloud. Thus, the calibration had to be performed at much lower exposure time than that used in the real experiment, from milliseconds to seconds. This difference in the exposure times required an additional set of calibration experiments to ensure the linearity of the CCD camera. The spectra of the calibration lamp were collected for a wide range of exposure times, from 1 ms to 2 s. It was shown that the CCD camera response is linear over the whole range of the exposure times and the whole range of the recorded signal intensities (data not shown). The linearity of the CCD camera allowed us to extrapolate the calibration to the large exposure times. The final calibration curve is presented in Fig. 4. This curve represents the relation between arbitrary CCD

camera units (counts) and the spectral power (W/nm) in the range from 450 nm to 700 nm. This covers the emission of rhodamine 6G ions, which is in the range of 490–530 nm and the emission of rhodamine B ions, which is in the range of 550–610 nm. To calculate the total energy emitted by the trapped ions, the experimentally registered spectrum has to be multiplied by the curve presented in Fig. 4. For relating the MS signal abundance to the number of ions, we determined the emission intensity from the trapped ion cloud at different numbers of ions. For each data point, the emission spectrum was recorded, processed according to the described procedure, converted into the absolute values and integrated over the

Fig. 4. Calibration curve of the optical system. Converting raw spectra acquired by the CCD camera into spectral power values implies multiplication of the raw spectra by the calibration curve.

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wavelength range 400–800 nm. Taking into account the CCD camera exposure time and the real time of the fluorescence radiation, we can obtain to the values of energy emitted by the trapped ion cloud. For determining the number of ions from the known value of the emitted energy, one should also know the rates of excitation and relaxation of the trapped fluorescent ions. During the storage time, ions were continuously irradiated by the Ar+ laser beam. The irradiation causes continuous absorption of photons by ions and population of the first singlet excited state. This is followed by relaxation processes that return the ions to the ground state: k1

A  A∗ , k−1

where A represents a trapped ion, and A* represents the excited state of the ion. The rate constants of excitation and relaxation processes are denoted as k1 and k−1 . These constants are the inverse of the lifetimes of the ground and excited states, respectively. Knowing these constants, it is possible to calculate the duration of one excitation–relaxation cycle. Given the duration of the cycle and the emitted energy, the number of ions can be calculated directly.The rate constant of the relaxation process consists of a radiative (k−1,r ) and the non-radiative (k−1,nr ) part: 1 = k−1 = k−1,r + k−1,nr , u

where N is the number of ions in the ion cloud, and E0  is the average energy of one emitted photon, which is given by

where u is the lifetime of the excited (upper) state. It should be noted that non-radiative process may be significant both in solution and in the gas phase. For example, the relaxation processes of rhodamine 6G ions consists of direct radiative relaxation to the ground state and non-radiative transfer to a long-living triplet state, which was a subject to detailed study in [24]. The radiative part of the constant is due to spontaneous emission of photons with the transition of ions to the ground state (i.e. fluorescence). The relaxation constant of gas-phase rhodamine B was recently reported to be 2.50 × 108 s−1 [26]. The rate constant of rhodamine 6G equals 1.68 × 108 s−1 [24]. For calculating the lifetime in the ground state l , we need the excitation cross section of rhodamine dyes. In this work we used the absorption cross sections for the condensed-phase rhodamines, which we found to be reasonable because of the cross sections are largely independent of external parameters [27]. The molar extinction coefficient for rhodamine 6G and for rhodamine B as reported elsewhere [28–30], were converted to the cross sections as follows:  = 1000 ln(10)(ε/Na ) = 3.82 · 10−21 ε. For the particular case of 488 nm excitation, we found the absorption cross sections of rhodamine 6G and rhodamine B to be 2.4 × 10−16 and 1.43 × 10−16 cm2 respectively. The ground state lifetime was then calculated according to the following: l = (h)/(P), where P is the laser irradiance, which was maintained at a constant value of 2.2 W/cm2 in all of the experiments. At this value of the irradiance, we found the excitation rate constants of the rhodamines to be 1.4 × 103 s−1 for rhodamine 6G and 0.83 × 103 s−1 for rhodamine B. Due to the fact that the excitation rate constant is several orders of magnitude lower than the emission rate constant, which means the lifetime of the rhodamines in the ground state greater than that in the excited state, the latter does not contribute any significant value to the time of excitation–de-excitation cycle of a rhodamine ion, and therefore, was omitted in the following manipulations. The energy emitted by the fluorescing ion cloud was then determined as follows: Etot = NE0  ×

tst T · = 1.01 × 106 NE0 , texp l

Fig. 5. Mass spectra of rhodamine 6G: (a) the laser was switched on for 5.45 s as in the experiment. (b) The laser was off. Signal from the fragment ion signal with m/z = 415.20 appears only when the laser is on. The fragment ion signal intensity did not exceed 10% of the parent ion signal intensity. Noise signal was at the level of 10 arb.u., hence the presented intensity of the MS signal corresponds to the signalto-noise ratio of 7000.

(1)

E0  =

 hI()d  . I()d

Here I() is the spectral power of the emission with the integral being taken over the whole emission spectrum. Comparing the energy measured by the calibrated CCD camera to the theoretical value of the emitted energy given by Eq. (1), one can determine the number of fluorescing ions trapped inside the ICR trap. For each experiment both the mass spectrum and the fluorescence signal of trapped rhodamine 6G and rhodamine B ions were registered. An important issue was the loss of fluorescing ions during the experiment. In our case, the loss occurred primarily due to the photo-fragmentation. The laser power was set to a value at which photo-fragmentation did not play a significant role, although the value of the laser power had to be maintained high enough to achieve a suitable intensity of the fluorescence signal. Fig. 5 provides evidence that the abundance of rhodamine 6G ions did not decrease a lot when switching the laser on. The mass spectrum of the rhodamine 6G with the laser on is shown in Fig. 5(a). The laser power was 54 mW as in all of the experiments. A slight decrease of the parent ion signal at m/z = 443.23 and the appearance of the first child ion signal at m/z = 415.20 can be observed in the collected mass spectrum. For comparison, a mass spectrum with the laser beam off is shown in Fig. 5(b). There is no fragment observed. In all of the experiments the abundance of the fragment ions was controlled to be well below 10% of the rhodamine 6G or rhodamine B ion abundance. A set of experiments was carried out with MS signal abundances within the range of (50–300) × 103 . The dependence of the fluorescence signal on the MS signal intensity was established. The number of trapped ions was calculated for each value of the measured fluorescence intensity. Like this, the calculated number of ions was related to the MS signal abundance over the whole experimental range. Fig. 6 shows the results for trapped rhodamine 6G ions (a) and rhodamine B ions (b). At lower numbers of trapped ions, the fluorescence intensity increases linearly with the MS signal intensity both for rhodamine 6G and rhodamine B ions. However,

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Fig. 6. Correlation between the number of ions inside the ICR cell, the intensity of the fluorescence signal and the MS signal intensity (a) for rhodamine 6G and (b) for rhodamine B ions. The right Y-axis represents the dependence of the fluorescence intensity on the MS signal intensity. The left Y-axis shows the dependence between the corresponding number of ions and the MS signal intensity. The noise level of the MS signal was 10 arb.u., so that the MS intensity of 150 × 103 corresponds to the signal-to-noise ratio of 15,000. Such high signal abundances used to maximize the number of trapped ions.

at higher numbers of trapped ions (corresponding to a MS signal intensity above 1.3 × 105 ), the dependence deviates from linear. In both curves the two last two points were excluded from the linear fitting. The reason for the deviation from the linearity will be discussed below. The number of ions calculated based on the rhodamine B fluorescence is approximately 10–15% lower than that for rhodamine 6G. The reason for the difference is probably due to the difference in quantum yield of the ions and the difference in the photofragmentation efficiencies. The method developed for determination the number of trapped ions was validated by comparing the results with those calculated using techniques developed earlier [5,7–9,31]. One of the techniques involves measuring the voltage induced in the detection circuit by an orbiting ion cloud. Based on the value of the voltage, the number of ions can be determined using Eq. (2): N=

CVp−p , qA(r)

(2)

where C is the capacitance of the detection circuit, Vp–p is the induced peak-to-peak voltage, q is the charge of an ion and A(r) is a function of the normalized cyclotron radius r. Once the capacitance of the detection circuit is known, the cyclotron radius and the induced voltage are determined, the number of ions trapped inside the cell is readily calculated. The procedure for determining the capacitance was described previously [7]. Following this procedure, we obtained a detection circuit capacitance of 117 pF for our setup. The Vp–p value was calculated based on the mass spectral intensity. For this purpose the calibration equation for the detection circuit was applied: I = 2308.9 × Vp−p + 627.2, in which I is the measured peak height, and Vp–p is the induced voltage in ␮V. In the experiments we used an ICR cell with a radius of 2.54 cm and an aspect ratio of 1.28. The function A(r) calculated for the given aspect ratio was: A(r) = 0.792r + 0.032r 3 . Determination of the cyclotron radius was similar to that reported in [6,7]. Substituting the value of the cyclotron radius we calculated the value of the function A(r). From these numbers, we obtained the number of trapped ions. The highest intensity of the ion signal, which corresponds to the reasonable fluorescent data, equals to 1.5 × 105 , which leads to a Vp–p of 34 ␮V, and one gets a value for the number of ions of 0.6 × 106 . The largest difference between the two independent measurements is about a factor of 2, i.e. the agreement between the results obtained by the two independent methods are reasonable. Both the fluorescence-based and capacitance-based methods have some specific sources of errors. In the capacitance-based method, the

most significant systematic error emerges from the cell capacitance measurement. Another error is due to the uncertainty of the postexcitation radius and thus of function A(r). The estimated error of the fluorescence-based measurements was estimated to be within 30–45%. The main sources of error are (i) a high value of the optical noise. For instance, at a signal level of 300 counts, the noise was 40–50 counts, which influences the precision of the fluorescence intensity measurements. (ii) The CCD camera calibration procedure contributed to the error of the conversion counts to the absolute units. This error mainly emerged because of introducing an additional optical element (a lens) between the lamp and the center of the ICR cell. Another source of this error is the uncertainty in measuring the parameters of the system and a difference between the ion cloud size and the size of the lamp image in the center of the cell. The overall precision of the calibration procedure is believed to be about 20–30%. (iii) The collection efficiency from different parts of the confined ion cloud also contributes to the error, which result in two effects. A first one is due to inefficient collection of the optical signal from the boundaries of a stored ion cloud. The reason is that the ion cloud can expand beyond the focal spot of the collection optics with increasing numbers of ions in the cloud. This, in turn, leads to less efficient acquisition of the fluorescence signal from the periphery of the ion cloud and a less rapid growth of the optical signal intensity in comparison with that of the MS signal, which does not depend on the dimensions of the ion cloud. This results in a systematic error in determining the slope of the fluorescence-based calculation of the number of ions being different. The difference is not significant at low numbers of ions, but increases as the cell is filled up reaching a factor of 2 at the highest reliable level of MS signal. A second effect is that above a critical number of ions, the ion cloud expands beyond the dimensions of the laser beam, so that the edge of the ion cloud is not excited by the laser. This leads to a loss of the efficiency in the optical excitation process while the MS signal still increases. In this case, the oscillations of the ions inside the ICR cell rather than the absorption of photons become the limiting factor for the excitation–relaxation cycle. This causes less efficient excitation in addition to less efficient collection of the optical signal. This explains the deviation from linearity at high number of ions for both curves. We thus found it reasonable to exclude the last points from the linear fitting, reliable results are obtained for a MS signal intensity up to 2 × 105 . Therefore, the developed method gives a good estimation for the number of the trapped ions. Moreover, it proves the validity of the conventional method based on induced voltage measurements.

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