An ion source for radiofrequency-pulsed glow discharge time-of-flight mass spectrometry

Spectrochimica Acta Part B 76 (2012) 159–165

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An ion source for radiofrequency-pulsed glow discharge time-of-flight mass spectrometry☆ C. González Gago a, L. Lobo b, J. Pisonero a, N. Bordel a, R. Pereiro b, A. Sanz-Medel b,⁎ a b

Department of Physics, Faculty of Science, University of Oviedo, 33007 Oviedo, Spain Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, 33006 Oviedo, Spain

a r t i c l e

i n f o

Article history: Received 9 May 2012 Accepted 26 June 2012 Available online 3 July 2012 Keywords: Glow discharge Ion source GD-MS

a b s t r a c t A Grimm-type glow discharge (GD) has been designed and constructed as an ion source for pulsed radiofrequency GD spectrometry when coupled to an orthogonal time of flight mass spectrometer. Pulse shapes of argon species and analytes were studied as a function of the discharge conditions using a new in-house ion source (UNIOVI GD) and results have been compared with a previous design (PROTOTYPE GD). Different behavior and shapes of the pulse profiles have been observed for the two sources evaluated, particularly for the plasma gas ionic species detected. In the more analytically relevant region (afterglow), signals for 40Ar+ with this new design were negligible, while maximum intensity was reached earlier in time for 41(ArH)+ than when using the PROTOTYPE GD. Moreover, while maximum 40Ar+ signals measured along the pulse period were similar in both sources, 41(ArH)+ and 80(Ar2)+ signals tend to be noticeable higher using the PROTOTYPE chamber. The UNIOVI GD design was shown to be adequate for sensitive direct analysis of solid samples, offering linear calibration graphs and good crater shapes. Limits of detection (LODs) are in the same order of magnitude for both sources, although the UNIOVI source provides slightly better LODs for those analytes with masses slightly higher than 41(ArH)+. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Glow discharges (GD) are being increasingly used as atomization and ionization sources for mass spectrometry (MS). Their capability to generate ionic populations from a solid sample allows the elemental quantification of a wide range of materials. The increases in the application of GD-MS for direct solid analysis are closely related to crucial advantages of this direct solid analysis tool, including multi-element capabilities (most of the elements of the periodic table could be determined), isotopic information, low matrix effects, low limits of detection (in the range of μg/g-ng/g), good depth resolution, and ease of use [1]. Additionally, the use of radiofrequency (RF) power sources offers the possibility of analyzing both conductive and insulating samples [2]. GDs can be operated in continuous or pulsed power mode. Nowadays, pulsed GDs have gained importance because this mode of operation allows the application of higher instantaneous power, enhancing the excitation and ionization efficiencies while reducing the thermal stress effect on fragile samples. Moreover, the pulsed GD source behaves as a dynamic plasma, showing different ionization processes along the power pulse period. Three main time regimes can be observed in the pulsed GD due to different ionization processes occurring along ☆ This paper is dedicated to Gary M. Hieftje, on the occasion of his 70th birthday, in recognition of his boundless contributions to spectroscopy and analytical chemistry. ⁎ Corresponding author. E-mail address: [email protected] (A. Sanz-Medel). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.06.043

the temporal distribution of power in the discharge: prepeak, plateau and afterglow [3]. Analytical GDs have been coupled to mass spectrometers such as quadrupoles [4] and double-focusing [5–7] spectrometers; however, those types of mass analysers are of sequential nature and therefore they are not the most adequate for certain applications such as depth profiling of thin films. This limitation has been overcome with the coupling of the GD to time of flight mass spectrometry (TOFMS) [8], capable of fast collection of a complete mass spectrum within one single measurement with high precision and sensitivity. Concerning the source configuration, the pin-type GD sources have been the most commonly employed with GD-MS [9,10] versus the Grimm-type GDs which have been mainly applied for optical emission spectrometry [11,12]. The Grimm-type GD has not been frequently used in MS till rather recently, due to difficulties for ion extraction. However, it has been shown that after proper modifications, this design can be used as ion source for MS [13,14]. The Grimm-type design results advantageous because the flat sample itself serves as the vacuum sealing part, enabling in this way fast sample changing and source cleaning. In addition, such configuration allows depth profile analysis [15]. The GD Grimm-type source coupled to TOFMS has been the subject of several investigations and, in fact, a modified Grimm-type source was already designed in our group and coupled with a commercial on-axis TOFMS analyzer [16]. Considering the promising capabilities of GD-TOFMS, research on GD designs aiming to obtain the best performance is still welcomed

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[10]. Therefore, in this work, a new design (“UNIOVI GD”) based on the Grimm-type GD has been constructed and interfaced to TOFMS. In this UNIOVI GD design, and following previous investigations [10,16,17], it has been searched for a short distance between sample and MS skimmer cone as well as for a simpler design without flow tube. Studies with the UNIOVI GD chamber investigated here are compared with those observed for the previous GD design (“PROTOTYPE GD”) of the RF-GD-TOFMS under investigation [18], paying special attention to the RF-pulsed GD working mode. 2. Experimental The RF-GD-TOFMS system consists of a RF-GD bay unit (RF-generator, matching box, RF-connector, refrigerator disk and mounting system with a pneumatic piston to press the cathode against the GD) from Horiba Jobin Yvon (Longjumeau, France), coupled to a fast orthogonal time-offlight mass spectrometer (Tofwerk, Switzerland) with a MCP detector (2 micro channel plates in chevron configuration) [19]. The ion response was monitored along 100 successive TOF extractions, with a time resolution of 33 μs (1 TOF extraction each 33 μs). The prototype allows the use of GDs operated either in continuous or pulsed RF modes. For the pulsed mode, a pulse length of 2 ms and a period of 4 ms were selected in our studies. The PROTOTYPE GD ion source (Fig. 1a) was a copper-based modified Grimm-type chamber similar to those used in the commercial GD-OES instruments from Horiba Jobin Yvon, with a 4 mm diameter anode and 15.5 mm thickness. A ceramic piece placed on the anode

avoids electrical arcing anode-sample. The base of the anode consists of 16 concentric holes through which the Ar goes into the region between the anode and the ceramic piece. Then, the Ar is introduced inside the anode tube through four holes located on the side of such anode tube. In addition, for MS, this design includes a 2.5 mm inner diameter flow tube (EMPA, Switzerland) inserted from the back of the anode to face the gas flow towards the cathode surface favoring sputtering and ion extraction [19]. The UNIOVI GD design, based upon reports of previous Grimm type sources developed for mass spectrometry [13], pursued a comparatively short distance between the sample and the extraction cones. The new UNIOVI GD ion source is shown in Fig. 1b and based upon a design of a previous GD prototype of our group [16]. The source consists of a stainless steel disk of 45 mm diameter and 7 mm thickness (Fig. 1b) allowing a short distance from the sample surface to the sampler cone (approximately 8.8 mm). These distances are based upon investigations of the spatial distribution of analyte and plasma species conducted by optical emission for a similar RF-pulsed plasma carried out by Valledor et al. [17]. In these emission experiments, a GD source of the same design and characteristics of the UNIOVI GD was used and it was observed that (at the frequency of our experiments in pulsed mode, 250 Hz) the plasma was confined close to the anode, showing a rather round shape. Moreover, the region of maximum emission intensity in the plasma axis was observed at about 1 cm from the sample (cathode) position, which very nearly matches the actual distance between the solid sample in the UNIOVI GD source and the sampler of the GD-TOFMS.

Fig. 1. Schematic view of the evaluated GDs (PROTOTYPE and UNIOVI). Most important differences between both sources are related with the absence of flow tube in the UNIOVI GD along with the shorter distance between cathode and skimmer cone with this chamber. a) The PROTOTYPE source. b) The designed GD source (UNIOVI source). Inner channels for Ar entrance can be seen in the cross-section. The final UNIOVI GD chamber consists of eight of these channels symmetrically distributed.

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In a first approach during the UNIOVI GD developing, the discharge gas was driven into the anode cylinder through just four concentric inner channels (Ø 1.5 mm) which constrained the GD plasma in a symmetrical way. The cylindrical orifice of the anode restricts the “sputtering spot” of the discharge to a well-defined circular area of 4 mm diameter on the sample surface. For both designs the sample is externally placed against the GD ion source (a pneumatic holder presses the sample against the GD ion source). A ceramic piece which maintains the distance anode–cathode at around 0.2 mm was included to avoid arcing effects between anode and cathode. A disk located in the back side of the cathode is used to refrigerate the sample and to fix the rf connector. Concerning the coupling of the corresponding GD chamber to the TOFMS, an interface consisting of two extraction orifices (sampler and skimmer) is used to extract the ions from the GD into the lower pressure mass analyser. The sampler and skimmer orifice diameters are 500 μm and 1000 μm, respectively. The distance between these cones is fixed to 7 mm. The GD source is vacuum evacuated using a dry pump (Triscroll 300, Varian Inc., Palo Alto, USA), and the interface and TOFMS regions by a multi-stage turbo pump (TMH 261, Pfeiffer Vacuum, Germany). High purity Ar (99.999% minimum purity) from Air Liquide (Oviedo, Spain) was employed as discharge gas. Certified materials with iron matrix (NIST 1262B, NIST 1263A, NIST 1264A, NIST 1265A) were used to evaluate the performance of the GD sources and to obtain the calibration curves. The detailed composition of those reference materials is given in Table 1. The samples were polished using metallographic grinding papers.

3. Results and discussion 3.1. UNIOVI GD source design The performance of the UNIOVI GD was first evaluated operating in continuous RF mode. For this purpose the reference material NIST 1262B (Table 1) was measured with different GD applied power conditions. As expected, signals registered for elements in the NIST 1262B showed a linear behavior with the applied power, confirming proper operation of the proposed source [20]. However, it was observed that the sensitivity was poor, around two orders of magnitude lower than that obtained with the PROTOTYPE GD operated also in the continuous mode. The lack of sensitivity was attributed to a low transfer efficiency of the Ar flow into the source. Therefore, in order to improve the operation of the new chamber, the prototype was modified by machining four additional argon inlets into the anode cylinder. The diameter of the eight channels was also increased on the back of the source to improve the flow of Ar into the discharge region. The linearity of signals with applied power was again observed for this modified GD prototype and a noticeable improvement in signal to noise ratio (S/N) of the measured signals was achieved. The observed sensitivity of the UNIOVI GD was found to be comparable to that observed with the PROTOTYPE ion source.

Fig. 2. Effect of pressure on pulse profiles of 40Ar+. The plotted surface consists of one point every 33 μs (x axis) and one point every 100 Pa (y axis). a) With the UNIOVI GD. b) With the PROTOTYPE GD.

3.2. Temporal pulse profiles of ions obtained with both GDs coupled to TOFMS As already mentioned, analytical GDs can be operated in continuous or pulsed powering mode. The pulsed mode is being increasingly used as it offers certain advantages, particularly when coupled to TOFMS analysers [21]. Thus, the performance of the UNIOVI GD source coupled to TOFMS was investigated in this operation mode. The temporal distribution of the ion signals along the pulse can be affected by the source geometry (i.e. source design) and the operation

Table 1 Elemental composition (% w/w) of reference materials NIST 1262B, 1263A, 1264A and 1265A.

1262B 1263A 1264A 1265A

1262B 1263A 1264A 1265A

B

Mg

Al

P

S

Ti

V

Cr

Mn

Co

Ni

Cu

0.0025 0.0009 0.011 0.0013

0.0006 0.00049 0.00015 –

0.081 0.024 – 0.0007

0.044 0.02 0.010 0.0011

0.037 0.005 0.025 0.0055

0.100 0.050 0.24 0.0001

0.041 0.31 0.10 0.0006

0.30 1.31 0.06 0.007

1.05 1.50 0.25 0.0057

0.30 0.048 0.15 0.007

0.59 0.32 0.14 0.041

0.51 0.09 0.25 0.0058

As

Zr

Nb

Mo

Ag

Sn

Sb

Ta

W

Pb

0.096 0.010 0.05 0.0002

0.22 0.050 0.069 –

0.30 0.049 0.15 –

0.070 0.030 0.49 0.0050

0.0011 0.0037 – –

0.016 0.10 0.008 –

0.012 0.002 0.034 –

0.20 – 0.11 –

0.20 0.046 0.10 –

0.0004 0.0022 0.024 0.00001

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Fig. 3. Influence of applied power on pulse profiles of 80(Ar2)+. The plotted surface consists of one point every 33 μs (x axis) and one point every 10 W (y axis). a) With the UNIOVI GD. b) With the PROTOTYPE GD source.

conditions (e.g. pressure and power). Therefore, the pulse shapes of plasma ionic species were investigated as a function of pressure and power for the UNIOVI GD and then compared to those provided by the PROTOTYPE GD. It should be noted that pulse profiles showed graphically in Figs. 2, 3 and 4 have been normalized to the maximum intensity for each GD source under the studied conditions and over a range pressures, or powers as noted. 3.2.1. Discharge gas (Ar) derived ions Pulse profiles of 40Ar + were studied first as this ion plays the predominant role in the sputtering and the ionization of the atoms sputtered from the sample surface. According to previous studies [22], the presence of 40Ar + and 80(Ar2) + can give rise to the formation of highly excited argon through different recombination processes, which by radiative decay will produce metastable argon. For this reason, 80(Ar2) + profiles were measured in this investigation as well. Fig. 2 depicts the normalized 40Ar + signal profiles for both GD designs. Pulse profiles of 40Ar + measured with the UNIOVI GD at 80 W and varying the pressure (between 300 and 1200 Pa) are shown in Fig. 2a, while Fig. 2b shows the corresponding profiles using the PROTOTYPE design. In the investigated range of pressures, Ar flow varies between 0.4 L/min and 2.6 L/min. It is interesting to note that the maximum intensity for 40Ar + signals was rather similar using both sources being investigated (the area of maximum intensity in both plots of Fig. 2 corresponds to about 10,000 counts per second).

Fig. 4. Effect of pressure on pulse profiles of 59Co+. The plotted surface consists of one point every 33 μs (x axis) and one point every 100 Pa (y axis). a) With UNIOVI GD. b) With the PROTOTYPE GD.

The 40Ar + signals in the plateau are also higher at low pressures for both chambers. However, important differences can be observed for each design in the afterglow region. The PROTOTYPE source shows higher signals than the UNIOVI GD, particularly at pressures above 700 Pa. For the PROTOTYPE GD, intensities in the plateau and the afterglow are quite similar at lower pressures. However, the UNIOVI GD exhibited the highest intensities at the beginning of the power pulse, then decreasing over the duration of the pulse at a rate depending on the pressure (for pressures higher than 700 Pa the signals decayed quicker than at low pressures giving rise to a pronounced “prepeak”). The PROTOTYPE chamber always showed longer afterglows (particularly at low pressures): at about 500 Pa two afterglow maxima are observed, while at higher pressures only one maximum of the afterglow (closer to the end of the pulse) was detected. In fact, the plot in Fig. 2 for the afterglow region looks like it depicts two possible peaks, the later of which (at 2.4 ms) strongly decreases with increasing pressure. These different two regions in the afterglow could be produced by distinct ionization mechanisms. It has been reported [22] that electrons are mainly responsible for Ar ionization at the beginning of the afterglow, while electrons are already thermalized at times later in the pulse cycle. Ionization at later times and 40Ar + ion production could be attributable to metastable-metastable Ar collisions, although this Penning ionization seems to be less relevant in the UNIOVI GD source as can be observed in Fig. 2a.

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Fig. 5. Effect of pressure on 41(ArH)+ signals in the afterglow for UNIOVI and PROTOTYPE GD sources. a) Intensity of afterglow maximum signal. b) Temporal position of the maximum signal in the afterglow.

Fig. 6. Normalized pulse profiles measured for 40Ar+, 41(ArH)+ and 48Ti+. a) With the UNIOVI GD source operated at 550 Pa and 100 W. b) With the PROTOTYPE source operated at 700 Pa and 100 W.

Similarly, temporal profiles of 40Ar + were evaluated as a function of the applied power (20–120 W) for both GD cells (not shown). The principal effect of increasing power, observed for both GD designs, was an enhancement of 40Ar + signals along the pulse period. No changes in the profiles (shape) of the pulses were observed in either case. These results are in full agreement with previous observations for pulsed dc-GD-TOFMS [10]. In contrast, the shape of the 80(Ar2) + profiles measured seemed to be more affected by the forward power than by the pressure. Pressure did influenced the 80(Ar2) + detected intensities in both chambers, showing higher observed 80(Ar2) + signals in the afterglow at lower pressures in both cases. Also, it was observed that the position of the maximum intensity of 80(Ar2) + in the afterglow shifted slightly towards shorter times with increasing pressure in both systems. The temporal profiles of 80(Ar2) + obtained under different power conditions at 700 Pa with the UNIOVI and the PROTOTYPE GD sources have been plotted in Figs. 3a and 3b, respectively (and normalized to 100 in each case). It is important to note that the value of maximum 80 (Ar2) + intensities was close to one order of magnitude greater when using the PROTOTYPE source than the UNIOVI GD design. For both GD chambers, the most intense argon dimer signals were registered in the afterglow region. However, different behaviors were observed in the plateau period for the pulsed GD. The PROTOTYPE GD showed a single maximum at approximately 2.5 ms after the discharge initiation. The ratio of the plateau region signal to the maximum signal observed in the afterglow was found to be above 0.5 with the UNIOVI source while for the PROTOTYPE GD the signals in the plateau were negligible. The different profiles obtained for both GD designs could be attributed to the farther MS sampling distance when using the PROTOTYPE GD as compared with the UNIOVI design.

3.2.2. Temporal profiles of sputtered species (analyte ions) The temporal behavior of analytes was similarly evaluated for both GD sources. As an example, Fig. 4 plots the effect of pressure on pulse profiles for 59Co +. As could be expected, changes of the afterglow shape with pressure were observed with both GD designs. For the UNIOVI GD (Fig. 4a) a single maximum occurs around 180 μs after the end of the power pulse and for pressures below 800 Pa. This maximum is attributed to Penning collisions since it is expected that at this rather long time the electrons are already thermalized. At higher pressures the maximum decreases noticeably while a new maximum arises in the early afterglow (around 90 μs after the end of the pulse), turning to be dominant at pressures above 900 Pa. The early maximum could be due to collisions with electrons which density increases right at the end of the power. For the PROTOTYPE GD source (Fig. 4b) signals of analytes showed a double peak in the afterglow, with the first of these maxima predominant in the entire range of studied pressures. The position of the first maximum intensity also slightly shifts to shorter times with pressure increases. The effect of power on analyte response was also investigated (not shown). In the plateau region, analyte signals obtained with the UNIOVI chamber decreased with increasing power. By contrast, the maximum intensities in the afterglow region were observed at higher values of applied power for the UNIOVI GD source. As mentioned previously, the PROTOTYPE GD source shows two maxima in the afterglow. At 700 Pa two maxima of intensity were observed for the PROTOTYPE GD in the afterglow: the first maximum at around 90 μs after the end of the power pulse increased with power, becoming predominant above 100 W. The second maximum also increased with increasing applied power but to a lesser extent. Considering that the temporal overlapping of highly intense signals from 40Ar+ and 41(ArH)+ could negatively affect the detectability of

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C. González Gago et al. / Spectrochimica Acta Part B 76 (2012) 159–165 Table 2 Detection limits (μg/g) obtained in the RF-pulsed mode GD-TOFMS using the in-house (UNIOVI GD). Isotopes are ordered according to their mass. Results are compared with those for the PROTOTYPE source [17].

11

B Mg Al 31 P 32 S 48 Ti 51 V 52 Cr 55 Mn 59 Co 60 Ni 63 Cu 75 As 90 Zr 93 Nb 98 Mo 107 Ag 118 Sn 121 Sb 181 Ta 184 W 208 Pb 24 27

UNIOVI GD DLs (μg/g)

PROTOTYPE GD DLs (μg/g)

0.2 0.3 1.1 6.5 6.3 17 19 3.7 3.9 18 11 1.5 1.7 3.5 2.5 0.1 0.3 1.8 1.1 0.6 0.8 0.5

0.8 0.5 2.1 6.4 2.4 46 25 8.9 7.8 46 19 1.2 0.8 2.2 1.6 0.2 0.5 0.6 0.8 0.5 0.9 0.4

configuration. At these lower pressures, the afterglow is observed at longer times delayed from the GD pulse. These longer delays permit almost complete decay of the 40Ar+ and 41(ArH)+ signals. That is, their contribution as potential interferences upon analytes of slight higher masses would be negligible. 3.3. Analytical performance characteristics of UNIOVI GD source

Fig. 7. Calibration curves of 11B+, 63Cu+ and 184 W+ obtained using the UNIOVI GD in the RF-pulsed mode. Four reference iron matrix materials (collected in Table 1) were measured at 550 Pa and 100 W.

analytes with masses slightly higher than these ions [18], the pulse shape of 41(ArH)+ was investigated with both GD designs was also carried out. Fig. 5 compares the effect of pressure on the intensity and temporal position of the maximum signal of 41(ArH)+ in the afterglow region for both sources. As can be observed in Fig. 5a with the UNIOVI GD the signal from 41(ArH)+ signal becomes negligible for pressures higher than 500 Pa. In the PROTOTYPE GD source, rather higher pressures are needed to decrease 41(ArH)+ signals. Furthermore, the argon hydride signal is observed to decrease more rapidly with the UNIOVI GD source. As shown in Fig. 5b, the 41(ArH)+ afterglow maximum appears delayed using the PROTOTYPE GD as compared to the new design. The differences observed between both GD sources could be attributed to the distinct plasma regions sampled with the MS. From an analytical perspective, the UNIOVI GD offers a significant benefit for analytes with ions in close proximity to the argon species. As can be clearly seen in Fig. 6 the temporal profiles for 40Ar+, 41(ArH)+ and 48 + Ti differ significantly between the two cells. The temporal overlapping of analyte ions in the afterglow with 40Ar+ and 41(ArH)+ observed for the PROTOTYPE source would cause a significant limitation. From Figs. 4–6 it can be stated that, for analytical applications, the UNIOVI GD should be operated at lower pressures than the PROTOTYPE

Considering that analytes commonly reach maximum ion intensities in the afterglow of the pulsed GD, the analytical performance of the UNIOVI source (calibration graphs, detection limits and crater shapes) was evaluated in this temporal region. Experiments carried out above showed best S/N for analytes at 550 Pa (Ar flow around 1 L/min). Concerning power, though recorded intensities increased with power for the interval observed (20–120 W), 100 W was selected for further experiments in order to reduce signal instability. As in previous experiments, the power pulse width has been fixed at 2 ms with a period of 4 ms. Four reference materials with an iron matrix and including different elements at low concentrations (Table 1) were measured using the selected operation conditions. The intensities registered around the afterglow maximum for such elements were plotted, obtaining calibration curves with good linearity. Fig. 7 shows the calibration curves for B, Cu and W within those iron matrix materials as an example. Detection limits (DLs), calculated as three times the standard deviation of ten measurements of the background divided by the slope of the calibration curve, for the UNIOVI GD are collected in Table 2. For comparison purposes, DLs for the PROTOTYPE source [18] have been also included in Table 2. As can be seen, most values obtained for the UNIOVI GD (ranging from hundreds of ng/g to low μg/g) are similar to those previously obtained with the PROTOTYPE chamber of the instrument. Besides, special attention should be paid to the elements with DLs collected in bold and italics in Table 2 (i.e. Ti, V, Cr, Mn, Co, Ni). These observed DLs show significant improvements, which can be ascribed to the fact that 41(ArH) + in the afterglow does not temporally overlap with these analytes when using the UNIOVI GD (Fig. 6a). Finally, Fig. 8 shows a typical crater shape obtained with the UNIOVI GD in the reference material NIST 1262B at 550 Pa and 100 W. As can be

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References

Fig. 8. Crater profile measured on the reference material NIST 1262B with the UNIOVI GD operated in the RF-pulsed mode at 550 Pa and 100 W (after 30 min).

seen, under these conditions, the UNIOVI source produces a layer by layer sputtering resulting in a flat bottom crater. The sputtering rate calculated for the UNIOVI GD under its optimum conditions (550 Pa 100 W) was 7.33 ± 0.7 μg/s which does not differ significantly from the value obtained in a previous work with the PROTOTYPE at 800 Pa 100 W (6.80 ±0.9 μg/s) [18]. 4. Conclusions These experiments have illustrated the different pulse time profiles for 40Ar +, 41(ArH) + and 80(Ar2) + between the two GD sources investigated. The maximum intensity of the 40Ar + ion when using the UNIOVI GD was similar to those obtained with the PROTOTYPE source. However, dimer profiles were strikingly different in terms of temporal shape and pressure dependence. Moreover, signals for 41(ArH) + were much less prevalent in the UNIOVI GD source and, consequently, the temporal overlap of hydride on some analytes was alleviated with the UNIOVI GD design. These studies contribute to show how the GD design plays a crucial role in the temporal characteristics and composition of plasma gas ions detected. Analytical detection limits achieved with the UNIOVI GD were comparable to the PROTOTYPE source for pulsed RF-GD-TOFMS measurements. Crater shapes provided by UNIOVI GD (flat bottoms and perpendicular walls) are suitable for the analysis of thin films. Furthermore, from a practical point of view, the UNIOVI GD ion source can be easily cleaned between analyses as it does not require of an inner flow tube and UNIOVI chamber, evaluated in detail in this work has a simpler design, being simpler to manufacture. This feature is of great practical importance as it affords constructing a specific source for each kind of sample matrix avoiding any cross-contamination effects. Acknowledgments Financial support from “Plan Nacional de I+D+I” (Spanish Ministry of Science and Innovation, and FEDER Program) through MAT201020921-C02 is gratefully acknowledged. C. Gonzalez Gago acknowledges the financial support from Ferroatlantica through FUO-EM-115. J. Pisonero acknowledges support from the “Ramon y Cajal” Research Program. We also thank the contract with Horiba Jobin Yvon for the loan of the GD-TOFMS.

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