Nerea Bordel

A comparison of non-pulsed radiofrequency and pulsed radiofrequency glow discharge orthogonal time-of-flight mass spectrometry for analytical purposes

The analytical potential of a radiofrequency glow discharge orthogonal time-of-flight mass spectrometer (RFGD-TOFMS) has been evaluated in both pulsed and non-pulsed modes. A certified reference steel was selected for this study. The operating conditions of the GD plasma (pressure and applied power) were optimized in terms of sensitivity. Additionally, duty cycle and pulse width parameters were investigated in the pulsed RF mode. In this case, high analyte ion signals and improved signal to background ratios were measured after the end of the pulse, in the so-called afterglow domain. The analyte ion signals were normalized to sputtering rates to compare different operating conditions. It was found that the sensitivity in the pulsed mode was improved in comparison to the non-pulsed mode; however, the factor of enhancement is element dependent. Moreover, improved analytical performance was obtained in terms of ion separation capabilities as well as in terms of accuracy and precision in the evaluation of the isotopic ratios, using the pulsed RFGD-TOFMS. Additionally, depth profile analyses of a Zn/Ni coating on steel were performed and the non-pulsed and pulsed RFGD-TOFMS analytical performances were compared.

Investigations of the effect of hydrogen, nitrogen or oxygen on the in-depth profile analysis by radiofrequency argon glow discharge-optical emission spectrometry

The effect of adding either hydrogen, nitrogen or oxygen (from 0.5 to 10% v/v) to an argon radiofrequency glow discharge (rf-GD) for optical emission spectrometric measurements has been investigated. Changes to the dc-bias voltage developed for conductive samples, to the crater shapes produced (both in homogeneous conductive materials, as austenitic stainless steels, and in a glass sample), and to the depth resolution for thin films on a glass substrate have been measured, operating the rf-GD at constant pressure and constant delivered power. Concerning the dc-bias voltages, the observed general effect was an increase of the voltage with increasing added hydrogen. For nitrogen and oxygen additions, an enhancement of the dc-bias was also observed, as compared to pure argon, but just in the interval 2–10% v/v of added gas. Experiments related to the crater shapes showed that working at 40 W and 600 Pa the craters produced in stainless steels increased their convexity with increasing percentages of any of the three molecular gases assayed. However, such convexity was reduced by working at lower delivered powers. The studies on adding hydrogen, nitrogen or oxygen to the argon rf-GD on the crater shapes produced in a homogeneous glass sample showed very good crater shapes in the glass at 20 W and 450 Pa, for most of the assayed plasma gas compositions in the interval 0.5–5% molecular gas/argon. Finally, qualitative profiles of two glass samples covered with multilayers, in the order of nanometres (6–27 nm), were measured for different plasma gas compositions and their relative depth resolution calculated. Results show that the plasma gas mixtures under investigation should be considered, at least for qualitative in-depth profile analysis, as they seem to offer a great potential to improve depth-resolution in rf-GD work.

The influence of hydrogen, nitrogen or oxygen additions to radiofrequency argon glow discharges for optical emission spectrometry

The influence of controlled addition of either hydrogen, nitrogen or oxygen (in the interval from 0.5 to 10% v/v) to an argon radiofrequency glow discharge (rf-GD) with optical emission spectrometric detection has been investigated in terms of analyte emission intensities, sputtering rates and analyte emission yields. Considering that plasma characteristics may vary greatly between conductive samples and insulators, both sample types (austenitic stainless steels and a homogeneous glass, respectively) have been employed in this study. Analytes investigated were silicon, iron, nickel and aluminium in the steel, while silicon, sodium, calcium and magnesium were selected in the glass. It was observed that the addition of the assayed molecular gases gave rise to a decrease of the sputtering rates, when comparing with pure argon, at any percentage of the molecular gas investigated. For hydrogen–argon or nitrogen–argon mixtures it was found that the percentage of sputtering rate reduction, with respect to pure argon, was highly similar when comparing the results from both types of samples (e.g., the sputtering rate decrease for 1% H2, as compared with pure argon, was 33% for stainless steel versus 28% for glass). However, when adding oxygen to the argon rf-GD, the sputtering rate decrease, as compared with argon, was much stronger in the conductive matrix than in the glass. Concerning the emission yields, selective enhancements were obtained with the addition of hydrogen (e.g., at the Si I 288.158 nm and the Mg I 383.829 nm lines) or nitrogen (e.g., at the Al I 396.152 nm and the Mg I 383.829 nm line). However, a rather systematic increase of emission yields was found in the presence of 0.5–2% of oxygen in an argon matrix as compared to pure argon.

Pulsed radiofrequency glow discharge time-of-flight mass spectrometry for molecular depth profiling of polymer-based films

We demonstrate the potential of an innovative technique, pulsed radiofrequency glow discharge time-of-flight mass spectrometry, for the molecular depth profiling of polymer materials. The technique benefits from the presence, in the afterglow of the pulsed glow discharge, of fragment ions that can be related to the structures of the polymers under study. Thin films of different polymers (PMMA, PET, PAMS, PS) were successfully profiled with retention of molecular information along the profile. Multilayered structures of the above polymers were also profiled, and it was possible to discriminate among layers having similar elemental composition but different polymer structure.

The concept of plasma cleaning in glow discharge spectrometry

A plasma cleaning procedure to improve elemental depth profiling of shallow layered materials by glow discharge spectrometry is proposed. The procedure is based on two approaches applied prior to depth profiling, either individually or sequentially. The first approach employs a plasma generated at low power, i.e. a “soft” plasma, for removal of contaminants adsorbed on the surface of the target material. In the second approach, sacrificial material is sputtered under normal conditions, e.g. those used for depth profiling, to clean the inner surface of the anode of the glow discharge source. It is demonstrated that plasma cleaning in glow discharge optical emission spectrometry and glow discharge time-of-flight mass spectrometry improves significantly the spectrum of the target material, particularly at the commencement of sputtering due to stabilisation of the plasma as a result of removal of contaminants. Furthermore, modelling and validation studies confirmed that the soft plasma cleaning does not sputter the target material.

Effect of operation parameters on the sputtering and emission processes in radiofrequency glow discharge. A comparison with the direct-current mode

In this paper, a comparison of the direct current (DC) and radiofrequency (RF) operating modes in glow discharge optical emission spectrometry (GD-OES) is carried out using the same discharge chamber, based on the Marcus design, powering alternatively with DC or RF energy. The effect of discharge pressure, the DC bias voltages and the delivered power divided by the DC-bias voltage on the sputtering rates, emission intensities and emission yields achieved for conducting materials was investigated in order to characterize both discharge types. Our results show that the effect of plasma variables on sputtering rates and emission yields using a DC-GD based on the chamber described by Marcus, can be considered to follow trends similar to those of the well-known DC-Grimm source. However, if the effect of plasma variables are compared for a DC-GD and a RF-GD, both generated by the same source as designed by Marcus, the behaviour of the DC and RF operations of the source proved to have some differences. Thus, at a fixed delivered power, the sputtering rate in the DC-GD decreases noticeably with pressure while the reverse effect is observed in the RF-GD. Moreover, under selected operating conditions, using the tin emission line (Sn I 380.102 nm), lower sputtering rates and higher emission yields were observed for the RF-GD than for the DC-GD source. Extension of known theoretical expressions and concepts from analytical DC-GD to RF-GD-OES work appears rather involved and is not yet possible.

Polymer screening by radiofrequency glow discharge time-of-flight mass spectrometry

AbstractThe aim of this work is to optimise and evaluate radiofrequency glow discharge (RF GD) time-of-flight mass spectrometry (TOFMS) for identification of organic polymers. For this purpose, different polymers including poly[methylmethacrylate], poly[styrene], polyethylene terephthalate-co-isophthalate and poly[alpha-methylstyrene] have been deposited on silicon wafers and the RF GD-TOFMS capabilities for qualitative identification of these polymeric layers by molecular depth profiling have been investigated. Although some molecular information using the RF continuous mode is available, the pulsed mode offers a greater analytical potential to characterise such organic coatings. Some formed polyatomic ions have proved to be useful to identify the different polymer layers, confirming that layers having similar elemental composition but different polymer structure could be also differentiated and identified. FigureRadiofrequency glow discharge time-of-flight mass spectrometry can be used for qualitative identification of polymers.

Critical evaluation of the potential of radiofrequency pulsed glow discharge–time-of-flight mass spectrometry for depth-profile analysis of innovative materials

The combination of radiofrequency pulsed glow discharge (RF-PGD) analytical plasmas with time-of-flight mass spectrometry (TOFMS) has promoted the applicability of this ion source to direct analysis of innovative materials. In this sense, this emerging technique enables multi-elemental depth profiling with high depth resolution and sensitivity, and simultaneous production of elemental, structural, and molecular information. The analytical potential and trends of this technique are critically presented, including comparison with other complementary and well-established techniques (e.g. SIMS, GD–OES, etc.). An overview of recent applications of RF-PGD–TOFMS is given, including analysis of nano-structured materials, coated-glasses, photovoltaic materials, and polymer coatings

Microsecond pulsed versus direct current glow discharge as ion sources for analytical glow discharge-time of flight mass spectrometry

Investigations have been carried out into coupling a microsecond pulsed direct current glow discharge to a time-of-flight mass analyser (μs-GD-TOFMS), aiming at studying the effects of the pulse parameters on mass spectra for analytical applications, including signal intensities, crater shapes and sputtering rates of conducting samples. By comparing the results obtained in four model materials (stainless steel, zinc, brass and an aluminium matrix) using pulsed and continuous dc voltages, a strong reduction (3–6 times, depending of the matrix) of the sputtering processes for the pulsed mode has been observed. This can be associated to the low average power available in each period. Conversely, the high voltage reached in a pulse seems to enhance ion generation, allowing for a notable sensitivity increase (about 50 fold) compared with dc continuous voltage operation. Concerning the effect of operating parameters typical of pulsed sources, the repeller delay has been demonstrated to be a critical parameter in achieving good analytical signals.

A double microsecond-pulsed glow discharge ion source

A double pulsed glow discharge was compared to the single pulsed mode as an ion source for atomic mass spectrometry. During the time-window between the first applied voltage pulse and signal collection, the second pulse is applied. The double pulse was used to enhance analyte ionization and study the timing sequences within the pulsed glow discharge, the signals from which were followed using a time-of-flight mass spectrometer. Results showed that the double pulse glow discharge increased ion signals, provided that appropriate pulsing delays and gas flow rates were applied. The effect of the double pulse on methane-enhanced polyatomic cluster formation has also been evaluated.