Minimizing Matrix Effect by Femtosecond Laser Ablation and

Article pubs.acs.org/ac

Minimizing Matrix Effect by Femtosecond Laser Ablation and Ionization in Elemental Determination Bochao Zhang,† Miaohong He,† Wei Hang,*,†,‡ and Benli Huang† †

Department of Chemistry, Key Laboratory of Analytical Sciences, College of Chemistry and Chemical Engineering, and ‡State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Fujian, 361005 China ABSTRACT: Matrix effect is unavoidable in direct solid analysis, which usually is a leading cause of the nonstoichiometric effect in quantitative analysis. In this research, experiments were carried out to study the overall characteristics of atomization and ionization in laser−solid interaction. Both nanosecond (ns) and femtosecond (fs) lasers were applied in a buffer-gas-assisted ionization source coupled with an orthogonal time-of-flight mass spectrometer. Twenty-nine solid standards of ten different matrices, including six metals and four dielectrics, were analyzed. The results indicate that the fs-laser mode offers more stable relative sensitivity coefficients (RSCs) with irradiance higher than 7 × 1013 W·cm−2, which could be more reliable in the determination of element composition of solids. The matrix effect is reduced by half when the fs-laser is employed, owing to the fact that the fslaser ablation and ionization (fs-LAI) incurs an almost heat-free ablation process and creates a dense plasma for the stable ionization.

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series of matrix-matched standards is indispensable,22,25 which is not only time-consuming but also high-cost. In some extreme conditions, finding a series of matrix-matched solid standards is very difficult, which makes the quantitative determination infeasible for direct solid analysis. Since then, diminishing or even eliminating matrix effect has become a principal task.26,27 Many investigations, including experiments and theoretical calculations, suggest that the employment of a short pulse duration laser, especially an ultrafast laser [usually referred to as femtosecond (fs) laser], might be one of the best solutions to reduce the matrix effect in the ablation process.14,25,28,29 There exist some significant fundamental differences between nanosecond (ns) and fs-laser ablation processes.9,30,31 For fs-laser ablation, the high irradiance (on the order of 1013−1014 W·cm−2) and short duration incur the Coulomb explosion and thereby minimize heating of the lattice.32,33 Also, the fspulse laser will not interact with the mass leaving the sample surface.34−36 The ns-laser (with irradiance on the order of 109− 1010 W·cm−2) causes phase explosion.37,38 Its duration is far longer than the time scale of thermal transmission from thermal electron to lattice (about 10 ps).39 There is enough time for thermal wave to transfer to lattice and propagate into target, which causes the target surface to melt and then to vaporize.20,21 The following plasma shielding has been considered a mechanism of energy loss and a possible contributor to elemental fractionation.22,34

aser ablation (LA) is a popular direct solid sampling method.1−5 It avoids time-consuming sample pretreatment procedures and the risk of bringing pollution into the sample, compared with solution methods that call for digesting solids into solution.6,7 Since LA was born, different analytical techniques based on it have been developed. The two most famous techniques are laser-induced breakdown spectroscopy (LIBS)8−11 and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).12−14 Despite its widespread research applications, there are intensive discussions on the laser ablation processes, including fractional evaporation, limitation in spatial and depth resolution due to melting and resolidification, and matrix dependence of the analytical signal (so-called “matrix effect”).15,16 Among these, matrix effect is thought to be one of the most troublesome drawbacks.3 The matrix effect during the laser ablation process was first discussed in 1962 and was deemed to exist in the vaporizing process.17,18 Additional research showed that not only the vaporization process but also the laser−material interaction process can be affected by matrix. Meanwhile, all of the physical properties of solid and laser parameters could affect the laser− material interaction process, which makes the reduction of matrix effect a huge challenge.19−21 For solids, any physical properties, including absorption efficiency, melting point (MP), boiling point (BP), crystal structure, and ablation ambience might affect the ablation process more or less; while for the laser, influencing factors are wavelength, laser pulse duration, laser irradiance, and incident angle.22−24 In order to adjust the matrix effect during elemental determination of a material, a © XXXX American Chemical Society

Received: January 11, 2013 Accepted: April 8, 2013

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dx.doi.org/10.1021/ac400072j | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

Article

(SRM629), tungsten (GBWQB200335 and GBWQB292436), soil [GBW(E)070043 and GBW(E)070045], ore (GBW07123 and GBW07124), zinc sulfide (GBWYSS2006), and copper sulfide (GBW07268). All the samples were treated following a similar procedure: bulk samples were cut into discs of 6 mm diameter and 1.5 mm thickness, whereas the powder samples were loaded into a die and pressed into the same shape with 1.5 × 107 Pa of pressure for 5 min by a hydraulic press machine. Since iron is presented in all samples, it was chosen as the reference element for calculation of RSCs of other elements:

Much work has been done to investigate the matrix effect during the laser ablation process when the ns-laser is replaced with an fs-laser.9,21,30,40,41 However, most of them are focused on the ablation but not the ionization process. The laser can fulfill the tasks of both ablation and ionization, which is the fundamental basis of high-irradiance laser ionization mass spectrometry. In our work, an ns-laser and an fs-laser were applied in parallel for comparison in the experiment. Samples of different matrices were used, and relative sensitivity coefficients (RSCs) of the elements in different matrices were measured and evaluated. The results indicate that when the fs-laser is employed, the matrix effect is reduced by half and stable RSCs can be achieved with irradiance larger than 7 × 1013 W·cm−2, which could be reliable in the determination of elemental composition of solids.

RSCx = (Ix /IFe)/(cx /c Fe)

(1)

where Ix and cx are the signal intensity and concentration of a specific element, while IFe and cFe are the signal intensity and concentration of iron, respectively.

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EXPERIMENTAL SECTION Instrumental. All experiments were carried out with an inhouse-built buffer-gas-assisted high-irradiance laser ionization orthogonal time-of-flight mass spectrometer (LI-O-TOFMS).42 An fs-laser (S-Pulse HP, Amplitude System, France) with wavelength of 1030 nm and duration of 500 fs was employed. The laser energy can be varied from 0.6 to 1000 μJ by adjusting a continuous laser beam splitter (ABSO-6.35-1030, CVI Melles Griot). Similar experiments were also carried out through an Nd:YAG laser (NL303G, Ekspla) with pulse width of 4.4 ns and wavelength of 1064 nm in the same condition. Both fs- and ns-laser beams were focused on the solids with the same optical geometry, leading to an ablated spot diameter of 40 μm. The pressure of high-purity helium injected into the ion source was 800 Pa (Table 1). All the electronic parameters of the mass

RESULTS AND DISCUSSION To evaluate the differences between ns- and fs-laser ablation and ionization, the laser craters produced by the two lasers were compared (as shown in Figure 1). The diverse features of the

Figure 1. Craters on iron (GBW01399) surface after (a) 10 ns-laser shots with irradiance of 9 × 1010 W·cm−2 and (b) 10 fs-laser shots with irradiance of 9 × 1013 W·cm−2.

Table 1. Ion Source Parameters of ns- and fs-LI-O-TOFMS laser pulse frequency, Hz laser wavelength, nm laser pulse duration typical laser irradiance, W·cm−2 laser incident angle, deg aperture diameter, mm spot diameter, μm source pressure, Pa shots per sample

nanosecond laser

femtosecond laser

10 1064 4.4 ns 9 × 1010 0 4 40 800 50

10 1030 500 fs 9 × 1013 0 4 40 800 50

craters indicate different mechanisms of material removal in nsand fs-laser ablation. The ns-laser ablation leaves a raised rim around the crater perimeter, indicating the existence of thermal diffusion, melting, and resolidification of molten metal during the phase explosion. For the fs-laser ablation, the high irradiance and short duration incur the Coulomb explosion and thereby minimize heating of the lattice.32,33,43 As shown in Figure 1b, melting and splashing are greatly reduced by use of femtosecond pulses, consistent with the absence of a rim. Since laser irradiance is a crucial factor in the laser−material interaction, the influence of laser irradiance on the RSCs has been investigated by ns- and fs-LI-O-TOFMS individually; the results of representative elements are shown in Figure 2. In the ns-laser ablation, most elements, including the elements not shown in Figure 2a, have relatively stable RSCs with variation of the laser irradiance. However, Pb shows different characteristics in that its RSC value increases along with increasing irradiance. The relatively low MP and BP of Pb and the thermal diffusion to the surrounding area of the ablation spot result in an excessive amount of Pb presented in the vaporized mass plume at high irradiance,15 which makes its RSC value much higher than those of other elements. The discrepancy of RSCs is greatly alleviated when the fs-laser is employed (as shown in Figure 2b). Most elements have relatively stable RSCs verging to 1 in the presence of variable laser energies. It can be elucidated that the ignorable thermal diffusion and high irradiance of fs-laser lead to unified ablation and ionization and result in the stable RSCs. Obviously, high-irradiance fs-LAI could possess remarkable advantages in reducing matrix effect during ablation and ionization, compared with the ns-LAI.

spectrometer were the same for both ns- and fs-LI-O-TOFMS during the experiments. Pulse train data acquisition mode, with pulse width of 3 μs and pulse frequency of 40 kHz, was used to reduce the interferences of poly atomic ions.42 A digital storage oscilloscope was employed with a recording length of 500 μs in order to match the ion packets of all the elements. Sample Preparation. Twenty-nine solid standards of 10 different matrices, including six metals and four dielectrics, were used to evaluate the matrix effect of ns- and fs-laser ablation and ionization. These standards were obtained from National Institute of Standards and Technology (SRM series) and Chinese National Standards Center (GBW series), which include ten matrices: aluminum (GBWE921b, GBWE922b, GBWE923b, GBWE925b, and GBWE926a), iron (SRM1762, SRM1264a, GBW01396, GBW01398, GBW01399, GBW01400, GSBH-40115-2, GSBH-40115-5, and GSBH40115-6), copper (SRM1112, SRM1114, SRM1116, and SRM1117), nickel (SRM1244 and SRM1248), zinc B



Article

The RSCs of 36 elements in different matrices were investigated and the results are shown in Figure 4. For most of the elements, except nonmetallic elements, the RSCs are close to 1 in both ns- and fs-laser mode. Since the nonmetallic elements have high ionization potentials (IP), their ions are much likeliier to catch electrons and become neutral particles in the laser plume due to their higher electron binding energy,44 which results in the low RSC values, such as C, O, P, and S shown in Figure 4.45 The RSC variation of a specific element in different matrices is a proper indication for the matrix effect. The more the RSC varies, the severer the matrix effect is. As shown in Figure 4, RSCs for the elements of interest are more diverse in the nslaser mode, compared with those in the fs-laser mode. For example, variation of the RSC for copper is ±0.24 in the fs-laser mode, while the variation widens to ±0.45 in ns-laser mode. The average RSC variation for all the elements is ±0.13 for fsLI-O-TOFMS and ±0.25 for ns-LI-O-TOFMS, which means that the matrix effect is diminished by half in the fs-laser mode. When the RSCs are plotted against the ionization potentials of different elements (in Figure 5), it can be found that the RSC distributions are quite similar in both the ns- and fs-laser mode. RSCs of the elements with IPs < 10 eV are close to 1, but they have a sudden drop for the elements with IPs > 10 eV. The reason for the steep RSC decline at 10 eV is still unclear. The RSC distribution similarity for the two modes is unforeseen, since the ionization locations are considered to be quite different for ns-LAI and fs-LAI (shown in Figure 6). In ns-LAI, the ablation is basically vaporization through phase explosion.3,37,45 Vaporized mass can be atomized and ionized by absorbing the incoming laser beam (Figure 6a), forming a plasma. Once the plasma forms at the irradiance of 9 × 1010 W·cm−2, about 90% of the laser energy in the later portion of the pulse is absorbed by the ablated matter, primarily through the inverse Bremsstrahlung (Figure 6b),1,38,46 accompanied by multiphoton ionization, electron impact ionization, and threebody recombination.46,47 The ionization potentials of elements play a key role, leading to the variation of the degree of ionization. In fs-LAI, due to the ultraintense laser irradiance, atoms are atomized and ionized through multiphoton ionization, tunnel ionization, electron impact ionization, and inverse Bremsstrahlung/avalanche ionization, leading to a dense plasma in the bulk with electron density on the order of 1022 cm−3, which could be at least 2 orders of magnitude higher than that in the bulk for the ns-laser mode (Figure 6c).45,48,49 This dense plasma means a high degree of ionization in the bulk, in which the matrix effect can be largely diminished. The high degree of ionization in the bulk is an important factor causing the Coulomb explosion, which results in direct ejection of ions from the bulk to the gas phase. The Coulomb explosion usually requires several hundred femtoseconds,45 while the inverse Bremsstrahlung and multiphoton ionization mechanisms can barely exist in the gas phase because of the ultrashort duration of the fs-laser (Figure 6d). Due to the relatively low electron density (1017−1018 cm−3) and plasma temperature compared with those of ns-laser,9,50,51 ionization processes in the gas phase are not the principal ways to produce ions. As indicated in Figure 5, although the ionizations occurred mainly in the gas phase for the ns-laser and in the bulk for the fs-laser mode, the ionization mechanisms and efficiencies should be similar in both modes. It can also be expected that the plasma temperatures are comparable in the bulk for fs-LAI and gas phase for ns-LAI. The dense plasma in fs-LAI leads to stable

Figure 2. Dependence of RSC values on laser irradiance for representative elements in GBW01399 with (a) ns- and (b) fs-laser irradiance.

A further investigation on the relationship between copper signal intensities and its concentrations in different matrices and under different fs-laser irradiance was carried out. Fe was selected as the internal reference element in order to eliminate source fluctuation and instrument shift. As shown in Figure 3a,

Figure 3. Logarithmic plot showing the ratio of Cu and Fe intensities as a function of the ratio of their compositions in different matrices (indicated in lower-right corner) at fs-laser irradiances of (a) 3 × 1013, (b) 6 × 1013, and (c) 9 × 1013 W·cm−2.

at low laser irradiance (3 × 1013 W·cm−2), the relationship between signal intensities and elemental compositions is clearly diverse for samples of different matrices. However, the influence of matrix effect is mitigated by increasing laser irradiance, indicated by the improved uniformity of the signal− concentration responses in Figure 3b,c. C



Article

Figure 4. RSC values determined in different matrices by use of (a) ns-LI-O-TOFMS at an irradiance of 9 × 1010 W·cm−2 and (b) fs-LI-O-TOFMS at an irradiance of 9 × 1013 W·cm−2. Average RSC values of each element in different matrices and their variations are listed below the element tags.

Figure 6. Schematic diagram of ns- and fs-laser ablation and ionization process: (a) early and (b) late stage of ns-LAI; (c) early and (d) late stage of fs-LAI.

Figure 5. Relationship between ionization potentials and RSC values for 36 elements determined in different matrices by use of (a) ns-laser at an irradiance of 9 × 1010 W·cm−2 and (b) fs-laser at an irradiance of 9 × 1013 W·cm−2.

RSC differences will exist due to the different ionization potentials and ionization cross sections for different elements. However, with the reduction of matrix effect, true composition values can be approached with the correction of RCS values regardless of sample matrices, which makes the quantitative determination of element composition possible for direct solid analysis through high-irradiance fs-laser ionization mass spectrometry.

ionization efficiency for the specific element related to its ionization potential and ionization cross-section, which helps reduce its RSC variation.

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CONCLUSION The comparison of ns- and fs-laser in the buffer-gas-assisted laser ionization orthogonal time-of-flight mass spectrometry was carried out by analyzing 29 solid samples with 10 different matrices. The matrix effects are reduced by half in the fs-laser mode, compared with that of the ns-laser. One reason is that there is almost no thermal diffusion during the fs-laser ablation process, and another important factor is that the fs-laser creates a dense plasma in the bulk and leads to stable ionization. The

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. D



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Article

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China Financial (20775063 and 21027011) and Research Fund for the Doctoral Program of Higher Education of China (20120121110011). This work has also been supported by NFFTBS (J1210014).

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Minimizing Matrix Effect by Femtosecond Laser Ablation and

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