An improved analytical performance of magnetically boosted radiofrequency glow discharge

Abstract

A magnetically boosted radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES) system has been evaluated using an electromagnet setup that allows the application of transverse magnetic fields up to 70 mT. Such a configuration provides a magnetic field that is independent of the sample thickness. In particular, Cu emission lines, with different upper energy levels, are investigated using GD analysis of a pure copper sample. It is observed that their emission signals are significantly increased when applying magnetic field intensities above a threshold value (∼30 mT). Moreover, it is noticed that the GD pressure conditions as well as the upper energy levels of emission lines are the dominant parameters affecting the enhancement factor of the magnetically boosted Cu emission signals. On the other hand, aluminum matrix samples, with different copper mass contents, have been analyzed using the rf-GD-OES system, both in the normal mode and in the magnetically boosted mode. Linear calibration curves are obtained in both cases, but the slope of the calibration curves (sensitivity) is almost an order of magnitude higher when using the magnetic field. Furthermore, background levels and background noise are not affected by the presence of the external magnetic field; therefore, limits of detection are significantly improved.

---

1. Introduction

Radiofrequency glow discharge plasmas coupled either to optical emission spectrometry (rf-GD-OES) or to mass spectrometry (rf-GD-MS) have shown to be fast and sensitive analytical techniques for the direct chemical characterization of both bulk and coated materials.^1–4 Moreover, during the last few years, their great potential for the analysis of technological materials such as ultra-thin layers, nanowires or solar cells has also been demonstrated.^5–10 Additionally, several methodological strategies, such as use of plasma gas mixtures, deposition of conductive thin layers, or magnetically boosted glow discharges, have been investigated using rf-GD spectroscopy to further enhance the analytical capabilities of this technique.^11–15

Particularly, in magnetically boosted GD, the combination of an external magnetic field and the electric field, applied between the anode and the cathode, causes modifications in the charged particle motion within the GD plasma. At the relatively low magnetic fields employed in glow discharge spectroscopy, only electron trajectories are significantly affected as ions are much heavier and their paths more difficult to alter.^16–18 In this case, the electrons circulate around the magnetic field lines, and thus the electron residence time in the plasma is enhanced, which leads to a substantial increase in the collision probability. Hence, a higher excitation/ionization efficiency is obtained. Previous reports on magnetically boosted glow discharges, for both optical emission spectroscopy and mass spectrometry, were mainly investigated in direct current glow discharges using hollow cathode,^19,20 coaxial cathode^21,22 or Grimm type²³ sources. Nevertheless, the benefits of combining an external magnetic field and a radiofrequency glow discharge have not yet been fully exploited for analytical purposes.

Fundamental studies on magnetically enhanced glow discharge spectroscopy were performed by Li et al.,²⁴ who developed a Monte Carlo model where the number of collisions in a direct current GD argon plasma was calculated. The model showed that the application of a magnetic field increased the number of collisions in the GD plasma, leading to an energy loss and consequently to an electron temperature decrease. Due to the increased number of collisions and the confinement of energetic electrons, the ionization/excitation processes were enhanced, producing a higher plasma density. Furthermore, the gas pressure in the GD chamber was found to play an important role in magnetically boosted GD plasma. In this sense, McCaig et al.²⁵ observed a higher enhancement in the emission/ion signals under lower pressure conditions (of about 1 × 10⁻³ Torr) due to more effective electron trapping. Conversely, a negligible signal increase was observed when the magnetic field was applied at high pressure (of about 1 Torr), as in that case the collisional losses were already high. Additional studies also reported this pressure dependence of the magnetic effect, indicating that valuable signal increases were only obtained in magnetically boosted GDs under low pressure conditions.^19,20

From the analytical point of view, two different magnetic field configurations, axial and transverse, have been reported in the literature in combination with Grimm type glow discharges. In the denominated axial configuration the magnetic field is applied perpendicular to the sample surface and along the axis of the cylindrical anode.^26,27 The magnetic field is usually applied by means of permanent magnets placed behind the sample, which implies that the magnetic field strength generated in the GD plasma is a function of the sample thickness.^11,26 Additionally, the application of axial magnetic fields with this experimental configuration significantly affects the produced crater shapes, which are more crowned, in detriment to the achieved depth resolution.^23,27 On the other hand, in transverse configuration the magnetic field is applied parallel to the sample surface and perpendicular to the axis along the cylindrical anode.^21,22 In a previous work carried out in our laboratory,¹⁴ a transverse magnetic field was applied using permanent magnets that were inserted into the GD chamber, which was coupled to a time of flight mass spectrometer. Such a configuration allows a magnetic field that is independent of the sample thickness. Moreover, the crater shapes were not significantly deteriorated by the magnetic field, whereas the analytical ion signals were still noticeably increased in agreement with previous studies reported by Chen et al.²⁸ Nevertheless, permanent magnets provided a limited magnetic field strength of about 7.5 mT in the plasma.

The goal of the present work has been to investigate the analytical figures of merit achieved through the application of more intense transverse magnetic fields. For that purpose, the use of an in-house rf-GD-OES setup that allows the application of intense magnetic fields in the plasma using an electromagnet is investigated. The electromagnet provides a tunable magnetic field strength by controlling the current intensity circulating along the coils. In this sense, the role of the magnetic field strength in the resulting emission intensities of several lines is studied.

2. Experimental setup

The in-house rf-GD-OES experimental setup used in this work was described elsewhere by Valledor et al.²⁹ It consists of a modified Grimm type GD anode, similar to the GD source previously designed by Pisonero et al.,^30,31 and coupled to a quartz cylinder by means of a flat seal. On the other side of the cylinder a metallic piece provides two symmetric vacuum exits and an additional upper exit for the gauge pressure connection (MKS Baratron capacitance pressure transducer, Model 122B). The argon flow rate was controlled by a mass flow controller (MKS Model 1179B). This metallic piece includes a frontal quartz window to allow the end-on emission measurements carried out in this work. The radiation emitted through the frontal window was focused by a quartz flat-convex lens (f = 7 mm) onto the end of an optical fiber (LG 455-020-3) whose other end was coupled to the entrance slit of a spectrograph (SpectraProR-500, Princeton Instruments, NJ, USA) provided with two ruled gratings: 2400 and 3600 lines per mm. Finally, the light was detected by an intensified charge coupled device (PI-MAX camera, ST-133 controller, Princeton Instruments, NJ, USA). A schematic diagram of the in-house setup used in this work is shown in Fig. 1.

Fig. 1 Diagram of the in-house rf-GD-OES setup developed and scheme of the electrical circuit for the external magnetic field application.

Flat solid samples, acting as the cathode, were externally placed against the anode by means of a macor ceramic spacer and a sealing O-ring. A cooler disc is placed on the sample backside not only to keep the system at low temperature (<15 °C) but also to apply the radiofrequency by the connection to an rf power supply (Dressler CESAR Generator Model 133, CO, USA). Moreover, a matching network system minimizes the reflected power (Advanced Energy ATX Tuner, USA). Argon with 99.999% minimum purity from Air Liquide (Oviedo, Spain) was used as the discharge gas.

The primary system is complemented with an electromagnet formed by two confronted coils (842 turns per coil, 2.66 Ω, ref. 06480-01, PHYWE, Göttingen, Germany) provided with a ferrous nucleus for the magnetic field homogeneity. To create the magnetic field, both coils are connected in parallel and then connected to a tunable power supply (0–12 A, ref. 13531-93, PHYWE, Göttingen, Germany) (see Fig. 1). An electrolytic capacitor (22 000 μF, ref. 06211-00, PHYWE, Göttingen, Germany) is connected in parallel to the power supply to minimize instabilities. Additionally, an amperimeter is used to measure the current intensity through the coils, which is proportional to the applied magnetic field. The coil nuclei are confronted and separated by a distance of about 7 cm to place the GD anode between them. The measurement of the magnetic field intensity produced in the middle of the gap between the nuclei (inside the GD anode) was carried out with a digital teslameter (ref. 13610-93, PHYWE, Göttingen, Germany) and a tangential flat-electrode Hall probe (ref. 13610-02, PHYWE, Göttingen, Germany).

A scheme of the 3 dimensions in which the magnetic field was characterized is also shown in Fig. 1. The component named Y is the most important one because the collocation of the coils generates the most homogeneous and intense magnetic field in this direction. The initial point (X = 0, Y = 0, Z = 0) is taken at the centre of the anode orifice. The Y component of the magnetic field measured at this point results in intensities between 0 and 70 mT depending on the current through the coils. When the maximum current is applied to the coils, the Y component of the magnetic field exponentially decreases along the plasma axis (moving the probe along the X dimension in Fig. 1, keeping constant Y and Z at 0 cm) from 70 mT at an initial point of X = 0 to 0.4 mT when moving 14 cm further. The magnetic field intensity in the component X and in the component Z at the initial point has been also measured, resulting both of them in a magnetic strength of around 5 mT at the maximum current intensity applied.

The study of the magnetic field effects on different copper lines was carried out using a pure Cu sample (CURM no. 09.01-4 certified copper content of 99.82 ± 0.01%). Moreover, a set of reference materials with Al matrices were employed to obtain the calibration curves for low Cu concentration with and without a magnetic field. Table 1 lists the Al reference materials used and their Cu and Al elemental mass content. Table 2 collects the copper emission lines studied in this work. Adequate flat surfaces of the different materials were obtained with a polisher (LaboPol-5, Struers, Denmark) in several steps: samples were first smoothed by two metallographic grinding papers (Buehler 120 and 600 grit), then polished to a mirror finish by means of two diamond suspensions (Struers 9 and 3 μm) and finally cleaned with ethanol.

Table 1 Al reference materials used and their Cu and Al content

Reference material	Company	Cu (mass%)	Al (mass%)
VAW E-2/8	VAW aluminium AG, Germany	0.200	96.210
VAW 3015-4	VAW aluminium AG, Germany	0.620	83.800
VAW 3035-3	VAW aluminium AG, Germany	1.980	84.900
VAW E-3/8	VAW aluminium AG, Germany	4.000	84.280

---

Table 2 Cu(I) transitions studied in this work

λ (nm)	Energylower level (eV)	Energyupper level (eV)	Lower level	Upper level
222.57	0	5.57	3d^104s(^2S1/2)	3d^9(^2D)4s4p(^3P^0)(^4D^01/2)
222.78	1.64	7.21	3d^94s^2(^2D3/2)	3d^9(^2D)4s4p(^3P^0)(^2F^05/2)
223.01	1.39	6.95	3d^94s^2(^2D5/2)	3d^9(^2D)4s4p(^3P^0)(^2F^07/2)
224.43	0	5.52	3d^104s(^2S1/2)	3d^9(^2D)4s4p(^3P^0)(^4D^03/2)
276.64	1.64	6.12	3d^94s^2(^2D3/2)	3d^105p(^2P^03/2)
282.44	1.39	5.78	3d^94s^2(^2D5/2)	3d^9(^2D)4s4p(^3P^0)(^2D^05/2)
296.12	1.39	5.57	3d^94s^2(^2D5/2)	3d^9(^2D)4s4p(^3P^0)(^2F^07/2)
299.74	1.64	5.78	3d^94s^2(^2D3/2)	3d^9(^2D)4s4p(^3P^0)(^2D^05/2)
301.08	1.39	5.51	3d^94s^2(^2D5/2)	3d^9(^2D)4s4p(^3P^0)(^4D^05/2)
303.61	1.64	5.72	3d^94s^2(^2D3/2)	3d^9(^2D)4s4p(^3P^0)(^2D^03/2)
319.41	1.64	5.52	3d^94s^2(^2D3/2)	3d^9(^2D)4s4p(^3P^0)(^4D^03/2)
324.75	0	3.82	3d^104s(^2S1/2)	3d^104p(^2P^03/2)
327.39	0	3.79	3d^104s(^2S1/2)	3d^104p(^2P^01/2)
327.98	1.64	5.42	3d^94s^2(^2D3/2)	3d^9(^2D)4s4p(^3P^0)(^2F^05/2)
510.55	1.39	3.82	3d^94s^2(^2D5/2)	3d^104p(^2P^03/2)
515.32	3.79	6.19	3d^104p(^2P^01/2)	3d^104d(^2D3/2)
521.82	3.82	6.19	3d^104p(^2P^03/2)	3d^104d(^2D5/2)
522.01	3.82	6.19	3d^104p(^2P^03/2)	3d^104d(^2D3/2)

---

3. Results and discussion

3.1. Optimization of the operating conditions

Several papers have reported that the magnetic field effect on the GD plasma strongly depends on the discharge conditions.^19,20,25 Thus, in this work the argon pressure and the forward applied power have been optimized for both, the absence of a magnetic field and the presence of the maximum magnetic field (70 mT).

Particularly, as mentioned above, the application of an external magnetic field produces stronger plasma modifications when a low gas pressure is used, because a more effective electron trapping is achieved. In this work, the pressure-dependence study of the magnetically boosted rf-GD is carried out by changing the argon flow within the chamber. Atomic copper emission has been studied for a forward applied power of 50 W and argon flow rates between 150 and 330 sccm (standard GD conditions), operating with the maximum magnetic field (70 mT) and also without the magnetic field. The results are shown in Fig. 2, for Cu(I) 510.5 nm (Fig. 2a) and Cu(I) 515.3 nm (Fig. 2b). Argon flow values lower than 150 sccm (200 Pa measured in the pressure gauge) did not produce a measurable emission in the absence of a magnetic field, whereas values higher than 330 sccm (650 Pa measured in the pressure gauge) generated a turbulent flow that led to a different and less stable plasma regime.

Fig. 2 Net copper emission intensity at (a) 510.5 nm and (b) 515.3 nm, as a function of the argon flow rate (i.e. argon pressure varying between 200 and 650 Pa) for both magnetically and non-magnetically boosted rf-GD.

It is observed that the non-magnetically boosted analysis produces a linear emission increment with increasing argon flow, for both atomic copper lines shown in Fig. 2a and b, respectively. However, the 70 mT magnetically boosted intensity trend seems to depend on the emission line under study. In Fig. 2a, Cu(I) 510.5 nm shows a sharp rise up to 250 sccm, achieving then a plateau and even decreasing at argon flows higher than 300 sccm. The magnetic field addition results in an improvement in Cu(I) 510.5 nm emission intensities at all the argon flow rates studied. The same is true for the magnetically boosted copper intensity measured at 515.3 nm, as can be seen in Fig. 2b, although the effect is less pronounced than that measured at 510.5 nm, particularly at flow rates lower than 250 sccm. Since Cu(I) 510.5 nm and 515.3 nm seem to present different signal enhancements, the magnetic field dependence on the emission line was further investigated later on in the manuscript.

Furthermore, the 510.5 nm and 515.3 nm copper emission lines have been measured for different forward applied powers between 30 and 70 W. This study was performed with and without a magnetic field at three different argon flow rates (200 sccm, 250 sccm and 300 sccm). Fig. 3a and b represent the evolution of Cu(I) 510.5 nm and 515.3 nm intensities at increasing applied power and constant flow rate (250 sccm), respectively. It is observed that in the absence of a magnetic field, both emission lines are proportional to the applied power within the evaluated interval. However, in the presence of a 70 mT magnetic field, emission lines show two different linear regions at increasing applied power, the linear response (slope) being much higher at applied power values below 45 W. Similar trends were observed for the measurements performed at 200 and 300 sccm.

Fig. 3 (a) Net intensity from Cu(I) 510.5 nm as a function of the forward applied power at a constant argon flow rate of 250 sccm with and without the external magnetic field. (b) Net intensity from Cu(I) 515.3 nm as a function of the forward applied power at a constant argon flow rate of 250 sccm with and without the external magnetic field.

Fig. 4a and b show the ratio between the magnetically and the non-magnetically boosted intensity of both emission lines, versus the forward applied power at the three different argon flow rates. It is observed that for both emission lines, the maximum enhancement factor (relative intensity) is shifted towards a lower applied power when increasing the Ar flow rates. For instance, at 300 sccm the maximum relative intensity is achieved at 30 W, while at 200 sccm it is obtained at 40 W. Furthermore, under high applied power conditions (above 40 W), the enhancement factor continuously decreases at all the evaluated Ar flow rates. The maximum enhancement factor is achieved at the lowest evaluated Ar flow rate, which demonstrates that the improvement due to the magnetic addition is higher at a lower pressure. In particular, a maximum enhancement of ×20 was obtained for the Cu(I) 510.5 nm emission line, operating the rf-GD at 200 sccm, and 40 W. Under the same operating conditions, a maximum enhancement of ×8 was found for the Cu(I) 515.3 nm emission line.

Fig. 4 (a) Cu(I) 510.5 nm relative intensity as a function of the forward applied power at three different argon flow rates (200 sccm, 250 sccm and 300 sccm). (b) Cu(I) 515.3 nm relative intensity as a function of the forward applied power at three different argon flow rates (200 sccm, 250 sccm and 300 sccm).

3.2. Influence of the magnetic field strength on the emission spectra

The magnetic boost of the GD plasma has been induced using an electromagnet (instead of using permanent magnets as most of the published works) that allows the application of a more reliable and uniform magnetic field. The electromagnet used in this work produces magnetic fields from 0 mT (0 A at the coils) up to 70 mT (12 A at the coils). In this sense, the influence of the magnetic field strength on the emission spectra has been evaluated on a pure copper sample analyzed by the rf-GD-OES under fixed operating conditions of 300 sccm, which provides the maximum intensity for the emission lines of analytical interest (i.e. 324.7 or 510.5 nm), and 45 W that is a typical compromise value used for bulk and depth profile analyses. It was observed that the increasing magnetic field on the GD plasma produces increasing intensities on the spectra collected. For instance, Fig. 5 shows the evolution of five atomic copper lines (222.78 nm, 296.12 nm, 299.74 nm, 324.75 nm and 521.82 nm) at increasing magnetic field strengths. The intensities represented in Fig. 5 are normalized to the value obtained at 0 mT (so that values correspond to the magnitude previously named as relative intensity). It is observed that the emission signal enhancement factor reaches the maximum value at 70 mT, the highest magnetic field applied (i.e. the 296.12 nm copper line is increased up to a factor of 20). Additionally, it has been observed that the application of these magnetic fields does not affect the background levels, which remain constant. Therefore, signal to noise ratio is significantly improved and, consequently, the achievable limits of detection.

Fig. 5 Net emission intensities from five copper atomic lines (222.7 nm, 521.8 nm, 299.8 nm, 296.1 nm and 324.7 nm) at 300 sccm argon flow rate and 45 W forward applied power, for a magnetic field strength variable between 0 and 70 mT.

Moreover, it should be highlighted that the application of a magnetic field below a threshold value of about 30 mT leads to a much smaller enhancement of the emission intensities (i.e. the 296.12 nm copper line is increased up to a factor of 3). When a magnetic field higher than 30 mT is applied, the intensities become remarkably more enhanced, obviously depending on the magnetic strength.

It is also noticed in Fig. 5 that the effect of the magnetic field strength on the emission intensity depends on the evaluated emission line (as has been previously suggested in Fig. 3). The signal enhancement due to the magnetic field was found to be related to the upper energy level of the corresponding transition, which is in agreement with previously reported publications.^32,33 In this sense, the relative magnetic-to-non-magnetic intensity under the same discharge conditions (300 sccm and 45 W) is represented in Fig. 6versus the corresponding upper energy level. It is shown that the emission lines from highly excited copper atoms (6 to 7.2 eV) present a light magnetic enhancement up to a factor of 5. The transitions from upper energy levels between 5 and 6 eV (middle zone in Fig. 5) form a transition zone where the intensity enhancement is somewhat linear to the upper energy level. Finally, the emission lines from low excited copper atoms (upper energy level around 3.8 eV) show a medium enhancement of around 15. However, according to their lowest excitation energy, they should be the most enhanced lines. The out of trend behavior of these lines (the resonance lines 324.7 nm and 327.4 nm, together with the 510.5 nm emission line whose lower level is metastable) could be related to the self-absorption processes in the plasma.

Fig. 6 Relative intensity calculated for all the Cu(I) emission lines under study versus the corresponding upper energy level.

The enhancement factors for the atomic copper lines under study, resulting from the application of a 70 mT magnetic field, are given in Table 3 (organized by the upper energy level). The third column compiles the ratio of the emission intensities measured with and without a magnetic field under the same operating conditions (300 sccm and 45 W), which are indeed the values represented previously in Fig. 6. As can be seen, the atomic copper line intensities are enhanced up to 24 times when the magnetic field is included. The emission lines corresponding to a transition from an upper energy level around 5.4–5.5 eV reports the highest enhancements. The fourth column also collects the ratio calculated relative to the intensity measured at 330 sccm because it is the optimal argon flow rate in the rf-GD without the added magnetic field (see Fig. 2). Moreover, the commonly used 324.7 nm copper emission line, even if affected by self-absorption, presented an improvement factor of ×12 when the non-magnetically boosted intensity, measured under analytical conditions, and the magnetically boosted intensity, working under low pressure conditions, are compared.

Table 3 Intensity enhancements obtained for several Cu atomic lines when a magnetic field is applied

Energyupper level (eV)	λ (nm)	I B=70 mT/IB=0 mT	I B=70 mT/IB=0 mT
		Discharge conditions: power = 45 W flow rate = 300 sccm	Discharge conditions: power = 45 W B = 70 mT → flow rate = 300 sccm B = 0 mT → flow rate = 330 sccm
3.79	327.39	15.29	11.65
3.82	324.75	16.38	12.49
3.82	510.55	12.84	12.73
5.42	327.98	24.50	20.48
5.51	301.08	23.49	22.51
5.52	224.43	15.82	15.75
5.52	319.41	18.55	16.53
5.57	296.12	20.16	18.86
5.57	222.57	11.25	7.89
5.72	303.61	11.16	9.00
5.78	282.44	10.04	7.79
5.78	299.74	10.74	8.36
6.12	276.64	4.47	2.91
6.19	515.32	4.24	3.17
6.19	521.82	4.35	3.97
6.19	522.01	4.40	3.98
6.95	223.01	5.36	3.83
7.21	222.78	3.08	1.93

---

Additionally, an estimation of the sputtering rates was carried out at the three different conditions: at 300 sccm and 50 W with and without a magnetic field and at 330 sccm and 50 W (best conditions without a magnetic field). The application of a 70 mT magnetic field resulted in higher sputtering rates (lower than a factor of 2) compared to the values obtained under the same experimental conditions without a magnetic field. Therefore the observed enhancement factors are due to both the higher sputtering rates and the higher excitation efficiency. However, when comparing the sputtering rates measured at the optimal analytical conditions without a magnetic field (330 sccm and 50 W) with those obtained under the optimal conditions with the magnetic field (300 sccm, 50 W and 70 mT) lower values were obtained in the latter case (about 30%). In consequence, the enhancement factor achieved with the magnetic field in this case is due to the improvement of the excitation efficiency, which is even able to counteract the lower sputtering rates.

Finally, the figures of merit achieved with this configuration are compared to other works already published as summarized in Table 4. As can be seen the enhancements obtained in the present work are higher than those reported by previous studies.

Table 4 Previous studies on magnetically boosted rf-GD-OES

GD type	Magnetic field	Emission lines	Enhancement factor	Ref.
Hollow cathode	50 mT on magnet surface (plasma site)	Mg(I) 285.2 nm	up to 3	Raghani et al.^20
Hollow cathode	100 mT on cathode axis (plasma site)	Cu(I) 324.7 nm	up to 4	Simonneau and Sacks^19
Planar cathode	60 mT on cathode surface (sample backside)	Cu(I) 406.2 nm	up to 7	McCaig et al.^25
		Al(I) 396.2 nm	up to 7
Planar cathode	30 mT on cathode surface (plasma site)	Cu(I) 324.7 nm	up to 1.3	Chen et al.^28
		Al(I) 396.2 nm	up to 1.5
		Ni(I) 341.5 nm	up to 1.8
Grimm	10 mT on cathode surface (sample backside)	Al(I) 396.2 nm	up to 1.5	Alberts et al.^11
Grimm	32 mT on cathode surface (sample backside)	Cu(I) 282.4 nm	up to 2	Heintz et al.^23
Grimm	70 mT on cathode surface (plasma site)	Several Cu(I) lines	up to 25	Present work

---

3.3. Calibration curves and limits of detection

In order to study the analytical capabilities of the magnetically boosted rf-GD-OES developed in this work, a set of aluminum matrix reference materials with growing copper concentration from 0.2% to 4% was analyzed (Table 1). The samples were analyzed under two different conditions: the analytical conditions of the equipment (330 sccm) and the optimal conditions for the magnetically boosted analysis (300 sccm with 70 mT of external magnetic field). Although a forward applied power of 45 W has been used in the previous experiments, the calibration curve for the Al samples has been carried out at 50 W since the aluminum samples are harder to sputter than the previously used copper sample.

Fig. 7a shows the spectrum in a region around 324 nm acquired at 330 sccm, 50 W and 0 mT together with the spectrum acquired at 300 sccm, 50 W and 70 mT, both measured in the sample with 1.98% of copper. As can be seen the copper resonance lines (324.7 nm and 327.4 nm) are especially enhanced as well as the iron atomic copper lines present in the spectra (the content of Fe in this sample is 0.65%). The outstanding enhancement observed on the Fe(I) 328.0 nm emission line could be related to the interference with the copper line at 327.9 nm. On the other hand, it is noticed that the ionic argon lines detected in this wavelength range decrease their intensities when the magnetic field is applied and the background and noise levels remain at the same values for both analyses.

Fig. 7 (a) Superposition of 2% Cu sample spectra with and without magnetic field addition. (b) Error weighted calibration curve measured for the aluminum matrix set of samples at both magnetically (300 sccm, 50 W and 70 mT) and non-magnetically (330 sccm, 50 W, 0 mT) boosted analysis.

Fig. 7b depicts the net intensity of Cu(I) 324.7 nm versus the copper concentration of every sample for both magnetically and non-magnetically boosted analysis. The resonance line Cu(I) 324.7 nm was selected for this study because, although it did not present the highest magnetic enhancement (Table 3), it is a widely used line for analytical purposes, is easily recognized in such a complex spectra and no self-absorption is expected in low concentration copper contents. A weighted linear regression was carried out for both data series obtaining a remarkably higher slope when the external magnetic field is applied. In this sense it is interesting to calculate the limits of detection (LOD) using the IUPAC definition:³⁴

LOD = 3σ_B/m

In this equation σ_B represents the blank noise and m symbolizes the calibration curve slope. A similar noise was measured with and without the magnetic field addition (0.48 and 0.41 arbitrary units, respectively) which, taking into account the slope values, means that better limits of detection can be achieved by the magnetically boosted rf-GD. As shown in Fig. 7b, the non-magnetic analysis presents a slope of 67 ± 7 that leads to a limit of detection of about 0.02% (around 200 μg g⁻¹). On the other hand, the magnetically boosted analysis presents a slope of 412 ± 31 that leads to a detection limit of about 29 μg g⁻¹, which is almost one order of magnitude better than the non-magnetic one. It should be highlighted that the absolute limits of detection here obtained are not at the level of the GD-OES state of the art performance; however, this approach might be applied to commercial GD-OES instruments to improve their current limits of detection.

4. Conclusions

A new experimental setup has been developed in order to apply an intense and transverse magnetic field (up to 70 mT) to radiofrequency glow discharge plasma for optical emission spectroscopy, which produces a magnetic field independent of the sample thickness. The magnetic field was applied using an electromagnet that consists of two confronted coils that produce a tunable magnetic field.

The emission signals of several Cu(I) lines have been studied for different magnetic field strengths and discharge conditions. Different emission signal enhancement factors have been obtained depending on the energy levels involved in the emission transition. In particular, the signal enhancement was higher when the emission line corresponded to a transition from a lower upper energy level, with the exception of emission lines that may be affected by self-absorption. A magnetic field strength threshold was observed at approximately 30 mT, achieving an improvement factor of about 20 when higher fields were applied.

Finally, a set of aluminum matrix samples with low copper concentration has been analyzed for both the magnetically and the non-magnetically boosted rf-GD-OES. The higher intensity measured together with the unaffected background and noise levels leads to a limit-of-detection improvement of about one order of magnitude. Therefore, this approach might be applied to commercial GD-OES instruments to improve their current limits of detection. However, more studies should be carried out to check possible matrix effects as well as the effect of the magnetic field in other kinds of samples such as glasses or polymers where soft experimental conditions are usually employed.

Acknowledgements

The authors gratefully acknowledge financial support by the Spanish Ministry of Economy, and FEDER Programme through the project MAT2010-20921-C02. P. Vega acknowledges financial support by the Government of Asturias (Ph.D. fellowship from FICYT, Principado de Asturias, Spain). R. Valledor acknowledges financial support by the Spanish Ministry of Education (Ph.D. “FPU” fellowship). J. Pisonero acknowledges financial support from the “Ramon y Cajal” Research Program of the Spanish Ministry of Economy.

References

1. J. Angeli, A. Bengtson, A. Bogaerts, V. Hoffmann, V. D. Hodoroaba and E. Steers, J. Anal. At. Spectrom., 2003, 18, 670–679.
2. R. K. Marcus, A. B. Anfone, W. D. Luesaiwong, T. A. Hill, D. Perahia and K. Shimizu, Anal. Bioanal. Chem., 2002, 373, 656–663.
3. J. Pisonero, J. M. Costa, R. Pereiro, N. Bordel and A. Sanz-Medel, Anal. Bioanal. Chem., 2004, 379, 17–29.
4. M. R. Winchester and R. Payling, Spectrochim. Acta, Part B, 2004, 59, 607–666.
5. D. Alberts, V. Vega, R. Pereiro, N. Bordel, V. M. Prida, A. Bengtson and A. Sanz-Medel, Anal. Bioanal. Chem., 2010, 396, 2833–2840.
6. M. Bustelo, B. Fernandez, J. Pisonero, R. Pereiro, N. Bordel, V. Vega, V. M. Prida and A. Sanz-Medel, Anal. Chem., 2011, 83, 329–337.
7. M. Hohl, A. Kanzari, J. Michler, T. Nelis, K. Fuhrer and M. Gonin, Surf. Interface Anal., 2006, 38, 292–295.
8. J. Pisonero, B. Fernandez, R. Pereiro, N. Bordel and A. Sanz-Medel, TrAC, Trends Anal. Chem., 2006, 25, 11–18.
9. P. Sanchez, B. Fernandez, A. Menendez, R. Pereiro and A. Sanz-Medel, J. Anal. At. Spectrom., 2010, 25, 370–377.
10. R. Escobar Galindo, R. Gago, D. Duday and C. Palacio, Anal. Bioanal. Chem., 2010, 396, 2725–2740.
11. D. Alberts, L. Therese, P. Guillot, R. Pereiro, A. Sanz-Medel, P. Belenguer and M. Ganciu, J. Anal. At. Spectrom., 2010, 25, 1247–1252.
12. B. Fernandez, A. Martin, N. Bordel, R. Pereiro and A. Sanz-Medel, Anal. Bioanal. Chem., 2006, 384, 876–886.
13. A. Martin, R. Pereiro, N. Bordel and A. Sanz-Medel, Spectrochim. Acta, Part B, 2008, 63, 692–699.
14. P. Vega, J. Pisonero, N. Bordel, A. Tempez, M. Ganciu and A. Sanz-Medel, Anal. Bioanal. Chem., 2009, 394, 373–382.
15. K. Wagatsuma, Spectrochim. Acta, Part B, 2001, 56, 465–486.
16. M. A. Hassouba, Eur. Phys. J.: Appl. Phys., 2001, 14, 131–135.
17. L. Pekker and S. I. Krasheninnikov, Phys. Plasmas, 2000, 7, 382–390.
18. J. Li, Q. M. Chen and Z. G. Li, J. Phys. D: Appl. Phys., 1995, 28, 1121–1125.
19. R. Simonneau and R. Sacks, Appl. Spectrosc., 1989, 43, 141–148.
20. A. R. Raghani, B. W. Smith and J. D. Winefordner, Appl. Spectrosc., 1996, 50, 417–420.
21. B. L. Bentz and W. W. Harrison, Anal. Chem., 1982, 54, 1644–1646.
22. S. Tanguay and R. Sacks, Appl. Spectrosc., 1988, 42, 576–583.
23. M. J. Heintz, K. Mifflin, J. A. C. Broekaert and G. M. Hieftje, Appl. Spectrosc., 1995, 49, 241–246.
24. J. Li, Q. M. Chen and Z. G. Li, J. Phys. D: Appl. Phys., 1995, 28, 681–688.
25. L. McCaig, N. Sesi and R. Sacks, Appl. Spectrosc., 1990, 44, 1176–1182.
26. M. J. Heintz and G. M. Hieftje, Spectrochim. Acta, Part B, 1995, 50, 1109–1124.
27. A. I. Saprykin, J. S. Becker and H. J. Dietze, Fresenius' J. Anal. Chem., 1996, 355, 831–835.
28. M. Chen, J. S. Ren, H. B. Ma and G. S. Zhang, Spectrochim. Acta, Part B, 1997, 52, 1161–1166.
29. R. Valledor, J. Pisonero, T. Nelis and N. Bordel, J. Anal. At. Spectrom., 2011, 26, 758–765.
30. J. Pisonero, J. M. Costa, R. Pereiro, N. Bordel and A. Sanz-Medel, J. Anal. At. Spectrom., 2001, 16, 1253–1258.
31. J. Pisonero, J. M. Costa, R. Pereiro, N. Bordel and A. Sanz-Medel, J. Anal. At. Spectrom., 2002, 17, 1126–1131.
32. A. R. Raghani, M. A. Bolshov, B. W. Smith and J. D. Winefordner, Talanta, 1995, 42, 1817–1825.
33. S. N. Sen, R. N. Gupta and R. P. Das, J. Phys. D: Appl. Phys., 1972, 5, 1260.
34. G. L. Long and J. D. Winefordner, Anal. Chem., 1983, 55, 1432.

---