The Application of Resonance-Enhanced Multiphoton Ionization Technique in Gas Chromatography Mass Spectrometry

Gas chromatography resonance-enhancedmultiphoton ionization time-of-flight mass spectrometry (GC/REMPI-TOFMS) using a nanosecond laser has been applied to analyze the 16 polycyclic aromatic hydrocarbons (PAHs).The excited-state lifetime, absorption characters, and energy of electronic states of the 16 PAHs were investigated to optimize the ionization yield. A river water sample pretreated by means of solid phase extraction was analyzed to evaluate the performance of the analytical instrument. The results suggested that REMPI is superior to electron impact ionization method for soft ionization and suppresses the background signal due to aliphatic hydrocarbons. Thus, GC/REMPI-TOFMS is a more reliable method for the determination of PAHs present in the environment.


Introduction
The ionization technique for the analyte molecules is of particular importance to mass spectrometry (MS); the application of varied ion sources of the MS introduces selectivity features of the analysis result, that is, species, isomers, or state-selective ionization and control of fragment intense [1][2][3].Electron impact (EI) represents one of the most useful ionization methods used in the current studies of gas chromatography/mass spectrometry (GC/MS) analysis.The method, however, suffers from the mass information caused by the hard ionization, which results in the difficulty of identifying the complicated matrix of samples.The typical soft ionization including chemical ionization (CI), field ionization (FI), and photoionization (PI) has been developed to overcome the weakness of the hard ionization method.PI is superior to CI and FI on the mass spectrometric analysis because there is no limitation caused by the chemical reaction in CI and oxidation of the emitter in FI.Laser based PI methods, such as resonance-enhanced multiphoton ionization (REMPI) and single-photon ionization (SPI), have been very successfully applied to research and practical applications [4][5][6][7].REMPI is more elective and sensitive than SPI for the ionization of aromatic hydrocarbons using UV wavelengths that are readily accessible with standard pulsed lasers.Most applications of REMPI-MS involve direct-inlet MS, but successful couplings of REMPI-MS to GC and LC as well as laser desorption have also been reported [5,7].The coupling of such techniques with mass-spectrometry has expanded considerably the realm of analytical capabilities of MS.A laser with different pulse durations has been employed for ionization [8][9][10].For example, a femtosecond laser has been successfully used for efficient ionization before relaxation process of internal conversion and intersystem crossing of some chlorinated, brominated compounds [11,12].Unfortunately, a high cost, a large dimension, and difficulties in the maintenance prevent practical use of this method in environmental analysis.In addition, if the laser pulse width is much shorter than the time scale of relaxation, the use of such high intensive laser leads to unfavorable dissociation of both neutrals and ions, thus making sensitive and reliable analysis more difficult [13].To date, a number of studies have been reported to investigate the importance of laser parameters on the ionization yield for various chemical species [9,10,[14][15][16].
In this study, a combination of gas chromatography (GC) and REMPI/MS (GC-REMPI/MS) using a nanosecond laser was utilized for trace analysis of 16 polycyclic aromatic hydrocarbons (PAHs) in the priority list of US Environmental Protection Agency (EPA) to investigate the best performance of this technique.With REMPI, the molecule absorbs the first photon for excitation and the second photon for subsequent ionization.Since only a molecule absorbing the first photon can be ionized, interference arising from aliphatic hydrocarbons can be reduced.Thus, this technique provides superior selectivity since each congener can be selectively ionized via resonance excitation.A river water sample was also analyzed after pretreatment by solid phase extraction (SPE) method using this system to demonstrate the advantage especially in selectivity in environmental analysis.

Materials and Methods
2.1.Apparatus.Figure 1 shows the experimental setup in this study.The fourth harmonic emission of a Nd:YAG laser (Crylas, 266 nm, 1 ns, 1 kHz) was employed as an ionization source.One L of the analyte was injected into a GC system (Agilent Technologies, 6890N) using an autosampler (Agilent Technologies, 7683B) followed by a DB-5 (30 m × 0.32 mm I.D.) capillary column.Helium was used as a carrier gas at a constant flow rate of 1 mL/min.The temperature program for analysis of PAHs was set at a rate of 20 ∘ C/min from 40 to 120 ∘ C. It was further increased at a rate of 5 ∘ C/min from 120 ∘ C to 250 ∘ C and held for 3 min and was then increased at a rate of 5 ∘ C/min to 280 ∘ C and held for 10 min.The temperatures of the injection port and the transfer line were maintained at 300 ∘ C. The sample eluting from GC was introduced into a linear-type TOF-MS as an effusive molecular beam.A microchannel plate (Hamamatsu, F4655-11) was utilized for the detection of the ions induced by multiphoton ionization.The signal of the mass spectrum was optimized using a digital oscilloscope (Tektronix DPO7104, 1 GHz, 20 GS/s).The twodimensioned data of GC/REMPI-TOFMS was recorded by a digitizer (Agilent Technologies, 8 bit PCI High-speed Signal Analyzers, Acqiris AP240, 1 GHz, 1-2 GS/s).The final results of the data were analyzed and displayed using Labview software.

SPE.
A surface water sample was collected from a river located in the northern part of Kyushu area in Japan.A river water sample was spiked with 0.5 mL (200 pg/L) of I.S. to investigate the recovery of PAHs in the SPE process.The 500 mL of water sample was passed through an SPE cartridge (Sep-Pak Plus C18 with 10 mL of dichloromethane, 10 mL of methanol, and 10 mL of Milli-Q water) at a flow rate of 10 mL/min.The PAHs and dioxin were eluted with 5 mL of dichloromethane from the Sep-Pak Plus C18 cartridge connected with the Sep-Pak Dry cartridge.The solvent of the extract was evaporated under a nitrogen flow and filled with 0.5 mL of acetonitrile for analysis by GC/REMPI-TOFMS.

Results and Discussion
3.1.Excited-State Lifetime.Figure 2 shows a total ion chromatogram (TIC) of the standard mixture sample that contained 16 PAHs (200 pg/L).Generally, the excited-state lifetimes (  ) have an important influence on the ionization      efficiency of PAHs, because an important feature of the PAHs molecules considered is the close-to-unity sum of the quantum yields of fluorescence Φ  and of triplet formation Φ  due to the intersystem crossing transition  1 →  1 .The photophysical properties of the PAHs and the experimental results are shown in Table 1.There are only a few reports on the lifetime of isolated PAHs in the gas phase, although the lifetimes of PAHs in the condensed phase are well known [17].
The lifetimes of PAHs in solution are similar to, or slightly shorter than, those in the gas phase due to an additional relaxation pathway from collisions between the analyte and solvent molecules [18].Therefore, the lifetimes of the 16 PAHs (8-1400 ns in the gas phase and 6-650 ns in the condensed phase, except for ACY and IND) were much longer than the pulse width of the laser used in the present study (1 ns).Thus, no obvious loss of ionization efficiency could be observed for most of the 16 PAHs except of ACY.In ACY molecule, the major deactivation pathway from the  1 state was an efficient  1 →  0 internal conversion.The lifetime of the  1 state of ACY had been determined to be 345 ps and 0.2 ns, which were shorter than the pulse width of the laser in the present study [24,25].In the other cases, the   was not the determination factor to the ionization efficiency.For example, the   of PYR is the longest in all the analytes in this research, but the ionization efficiency of it is even lower than that of BaP.Thus, only when the   of the PAHs is shorter than the pulse width of the laser, the ionization efficiency can be increased using a more intensive laser.

Energy of Electronic States.
The schematic diagram of the photochemical conversion in the one-color two-photon ionization process is shown in Figure 3.When the gas molecules absorb photons, the electrons can be elevated to higher energy state to form the excited state.The PAHs molecules having absorbed the first photon in the excited state could absorb another photon to the ionization potential (IP) to accomplish the 1 + 1 REMPI process.Simultaneously, it could lose energy through the intersystem crossing or the internal conversation to the triplet state or the ground state because the crossing and conversation from the upper to the lower excited state are normally fast and very efficient; the states mentioned here are the lowest singlet-state 1 and lowest triplet-state  1 .The photochemistry properties of the 16 PAHs are shown in Table 2.The energy from triplet state to IP was shown as  − in Table 2.It should be noticed that, in the case of ANT,  1 may cross to  2 , which is nearly isoenergetic with  1 [26].Thus, in ANT, a small energy gap and consequently a favorable Frank-Condon factor exist for intersystem crossing, thus leading to a higher limit of detection (LOD) (/ = 3) of it.From Table 2, it was obvious that the two-photon energy applied in this study (266 nm + 266 nm = 9.32 eV) was qualified for a 1 + 1 REMPI for all the 16 PAHs in this research, but one-photon energy could not provide the subsequent exciting from the triplet-state.

Absorption Character.
During 1 + 1 REMPI absorption process, the ionization efficiency of a molecule depended primarily on two factors:  1 , the cross section for absorption of the first photon which excites the molecule from the ground electronic state into an electronically excited intermediate state;  2 , the cross section for absorption of the second photon which pumps the excited molecule into the ionization continuum.In the theory when the first photon absorption event is the rate-limiting step of the ionization process (i.e.,  1 ≪  2 ), then similarities are expected between the absorption or fluorescence excitation spectra and the 1+1 REMPI spectra.If, on the other hand, the second photon absorption event is the rate-determining step (i.e.,  1 ≫  2 ), then the spectroscopy of the molecules in the excited state is expected to dominate the spectrum.As to  1 , it can be derived from the distinction coefficiency () as follows: There were few reports about the gas phase UV absorption spectra of PAHs because in most cases the UV absorption spectra were given in solution phase.But the shape and width of these two phases were the same and they shift generally between 6 and 18 nm [27].Thus the extinction coefficients absorption discussed here was the data obtained from the solution phase.
From the peak area of PHE and ANT, shown in Table 1, it was clear that the ionization efficiency of PHE was about 11 times that of ANT.The dominant ionization efficiency at 266 nm of PHE would change if a wavelength was more selective to ANT.For example, at 310 nm, ANT was preferentially ionized in the presence of PHE due to the different  1 absorption character [28].On the other hand, the  2 could determine the intensity of the REMPI spectra as well.As shown in Table 1, the  of NAP and ACY was the same, but the REMPI intensity of NAP was 15 times that of ACY, which resulted in the lower ionization efficiency of ACY compared to NAP.

Measurement of a Water Sample.
A sample derived from surface water was spiked with three deuterated compounds, which served as internal standards, and was analyzed by GC/REMPI-TOFMS.The TIC was displayed by extracting the data between m/z = 100 and 400 in Figure 4, in which 12 PAHs on the U.S. EPA list and the internal standards were evident.We noticed that several of the chromatograph peaks had m/z values that were identical to those of the PAHs on the U.S. EPA list.These peaks might have been due to isomers of the PAHs.Therefore, the signal peaks should be carefully identified, even though the REMPI technique had superior selectivity.For example, several NAP isomers with the chemical formula C 10 H 8 were detected, which might be either 1Hindene-1-methylene, azulene, or 2-methylene-2H-indene.A more serious case was observed for FLT; there were several peaks with larger signal intensities than that of FLT.These isomers could be either acephenanthrylene, aceanthrylene, or their analogues.However, for all the compounds detected in this research, the mass chromatogram was much simplified and generally no fragments except the parent ion could be detected.Compared to the conventional ionization technique of GC/EI-MS, the background of the mass spectra was much reduced and the selectivity was improved greatly.

Conclusions
The forth harmonic emission of a nanosecond Nd:YAG laser emitting at 266 nm was employed to detect 16 PAHs of US EPA both in standard samples and in a water sample.The detection limits of the 16 PAHs were determined to be 0.18∼ 18 pg.The absorption characters at 266 nm can influence the ionization efficiency obviously, owing to the longer lifetime of the 16 PAHs compared to the pulse width of the laser.As to ionization potential, the photon energy of the laser applied in this study is enough to induce the REMPI process but not an ionization process from triplet state.In the analysis of the river sample, few compounds except PAHs can be detected in the mass chromatogram due to selective ionization of REMPI.The GC/REMPI-TOFMS based on nanosecond laser ionization is suitable for the trace detection of PAHs in environmental sample due to the high selectivity and sensitivity.

Figure 1 :
Figure 1: Experimental setup of the GC/REMPI-TOFMS based on the forth harmonic emission of a nanosecond Nd:YAG laser.

Table 1 :
Quality parameter obtained using REMPI-GC/MS and determination of PAHs in a spiked water sample obtained from the artificial lake.

Table 2 :
Electronic states energy of the 16 PAHs in U.S. EPA list.