Lara Lobo

Pulsed radiofrequency glow discharge time of flight mass spectrometer for the direct analysis of bulk and thin coated glasses

Direct solid analysis of bulk and thin coated glasses by pulsed radiofrequency (rf) glow discharge time-of-flight mass spectrometry (GD-TOFMS) is investigated. Modulated low pressure plasma created by pulsed-rf-GD has been coupled to a fast TOFMS in order to obtain complete mass spectra information from the different GD pulse domains (pre-peak, plateau and afterglow). In particular, it was observed that the analytes show the highest atomic ion signals in the afterglow region some hundred microseconds after the maximum of the Ar ion signal. However, it should be highlighted that the analyte ions exhibited their peak maxima at different delay times, depending on the element, after the end of the GD pulse. Such ion signal delays have been measured for different selected isotopes, covering a mass spectrum from light to heavy isotopes at different conditions of pressure and applied power. The results showed that ion signal delays are influenced by both the isotope mass and the pressure of the GD. Furthermore, GD operating conditions (pressure and applied power) were optimised in terms of sensitivity, using a bulk glass certified reference material (NIST 1411). The best analytical performance was observed at low pressure (150–200 Pa) and high applied power (135 W). Moreover, different pulse GD duty cycles (relationship between pulse duration and pulse period) were investigated. An optimum value of 50–65% duty-cycle was selected considering the signal stability and the signal intensity. The previously optimized pulsed-rf-GD-TOFMS system was then evaluated for qualitative in-depth profile analysis of thin coatings deposited onto thick glass substrates. High depth resolution (nm range), comparable to that obtained using rf-GD-OES was achieved. However, the observed depth resolution using the ToF-SIMS system is still superior. In this sense, analytical figures of merit observed in our pulsed-rf-GD-TOFMS demonstrate its great analytical potential for high depth resolution analysis of coated glasses.

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.

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.

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.

Quantitative depth profiling of boron and arsenic ultra low energy implants by pulsed rf-GD-ToFMS

In very recent years particular effort is being devoted to the development of radiofrequency (rf) pulsed glow discharges (GDs) coupled to time of flight mass spectrometry (ToFMS) for depth profile qualitative analysis with nanometre depth resolution of technological materials. As such technique does not require sampling at ultra-high vacuum conditions it facilitates a comparatively high sample throughput, related to the reference technique secondary ion mass spectrometry (SIMS). In this work, pulsed rf-GD-ToFMS is investigated for the fast and sensitive characterization of boron and arsenic ultra low energy (ULE) implants on silicon. The possibility of using a simple multi-matrix calibration procedure is demonstrated for the first time for quantification of this type of samples and the validation of the proposed procedure has been carried out through the successful analysis of a multilayered sample with single and couple 11B delta markers. Results obtained with the proposed methodology for boron and arsenic ULE implants, prepared under different ion doses and ion energy conditions, have proved to be in good agreement with those achieved by using complementary techniques including SIMS and grazing incidence X-ray fluorescence. Thus, although further investigations are necessary for more critical evaluation of depth resolution, the work carried out demonstrates that rf-GD-ToFMS can be an advantageous tool for the analytical characterization of boron and arsenic ULE implants on silicon.

Pulsed glow discharge time of flight mass spectrometry for the screening of polymer-based coatings containing brominated flame retardants

There is an increasing concern regarding the toxicity, environmental distribution and impact of organic compounds employed today as flame retardants. Along these lines, the potential of radiofrequency pulsed glow discharge (rf-PGD) time-of-flight mass spectrometry for mass spectrometric fingerprinting and quantification of brominated flame retardants (BFRs) into polymers is investigated here. Tetrabromobisphenol A was chosen as a “model” BFR compound for system optimisation and analytical characterisation purposes. After optimisation of power and pressure operation conditions of the rf-PGD, 15 W forward power and 200 Pa were chosen with the aim of attaining good sensitivity for elemental and polyatomic ions and long-term signal stability. Then, analytical performance of micro- and milli-second GD pulse regimes was compared and better detection limits were obtained using the millisecond pulse regime (a higher number of polyatomic ions was observed as well). Under the selected optimum conditions, linear calibration graphs were obtained for elemental bromine as well as for several polymer fragments. Limits of detection well below 0.1% were obtained both for elemental bromine and for the fragments investigated. Finally, a comparison of the mass spectra obtained for some polymeric samples containing different BFR compounds was carried out. The signals of polyatomic ions, related to the BFRs, can be successfully applied to the identification and discrimination among the different polymers.

A purged argon pre-chamber for analytical glow discharge—time of flight mass spectrometry applications

Aiming at minimizing microleaks in the seal between the sample and the GD, a surrounding chamber which warrants a continuous isolating argon flow around the solid sample during the analysis, has been investigated for radiofrequency (rf) glow discharge (GD) coupled to time of flight mass spectrometry (TOFMS) applications. Three standard reference materials and two thin coatings were analysed comparing the influence of the purging pre-chamber in the analytical results, both in non-pulsed and pulsed rf-GD operation modes. Results show that the external argon flow seems to reduce the microleaks between the sample and the GD source since a diminished level of polyatomics appeared in the recorded mass spectra. Additionally, using the rf non-pulsed mode better signal reproducibility, less polyatomic interferences and, in some cases, higher ion signals could be achieved. In terms of in-depth profile capabilities, a faster penetration rate was observed when the pre-chamber was used and such difference was dependent on the sample composition. Comparatively, less benefit was apparent using the rf-pulsed mode as the temporal discrimination in such mode allows to separate analytical ion signals from interfering contaminants.

Depth profile analysis with glow discharge spectrometry

Nowadays, glow discharge-optical emission spectrometry (GD-OES) and glow discharge-mass spectrometry (GD-MS) can be considered as two well established techniques for depth profiling, offering practical interest to assist the synthesis optimization process and the quality control of materials coated with thin or thick layers. In this article, actual commercial instrumentation and depth profile quantification methods with these two analytical tools are first briefly reviewed. Afterwards particular attention is paid to the description of recent applications which show the almost unique capabilities of GD-OES and GD-MS for fast elemental quantitative depth profiling of films going from an atomic layer up to more than a hundred micrometres. Moreover, some illustrative applications are shown for the characterization of organic films resorting to GD-OES and GD-MS.

Capabilities of radiofrequency pulsed glow discharge-time of flight mass spectrometry for molecular screening in polymeric materials: positive versus negative ion mode

Negative ionization mode in the recent commercialized “PP-TOFMS, Plasma Profiling Spectrometer” instrument from Horiba has been examined in detail for the analysis of different polymeric materials and the results were compared with those of positive ion mode. Three compounds (tetrabromobisphenol A, tris-2-chloroethyl-phosphate and polytetrafluoroethylene) embedded in a polyurethane matrix were employed for such a purpose. Both, elemental information from the heteroatoms (F, Cl, Br and P) and/or molecular information provided, using either Ar or Ar + 4% O2 as discharge gases, were investigated. Also, the analytical potential of the negative ionization mode for integral speciation of polymers was explored, both in Ar and Ar + 4% O2. The screening of polymeric materials with similar elemental composition, in particular, four brominated flame retardants (BFRs), was performed to investigate this point. The results showed that elemental sensitivity for halogens in the negative mode of the instrument was higher as compared to positive detection. Sensitivity was observed to be directly dependent on the electron affinity of the analyte. Polyatomic information measured in negative mode using Ar as the discharge gas has demonstrated to be promising for a successful identification of the four different BFRs investigated. The adverse effects of reactions occurring in the plasma in the presence of oxygen have proved to be a serious drawback to be tackled for polymer characterization using this novel PP-TOFMS glow discharge-based technology.

Depth profile analysis of rare earth elements in corroded steels by pulsed glow discharge – time of flight mass spectrometry

Depth profiling of 304L stainless steel surfaces exposed to rare earth nitrate solutions over varying time lengths has been undertaken using the recently commercialized “PP-TOFMS Plasma Profiling Spectrometer” by Horiba. This combines a pulsed glow discharge source with a time-of-flight mass spectrometer (TOFMS) which allows elemental depth profiling with nanometric resolution of almost all elements of the periodic table. In this work special attention is paid first to the optimization of the new PP-TOFMS for depth profile analysis of low concentrations of rare earth elements (REE) in the samples. Sensitivity given in [(cps X × sputtering rate in μg s−1)/(μgX g−1)], being the concentration of the element X (μgX g−1) corrected by the abundance of the measured isotope, was in the order of 20 for all REEs under investigation. Moreover, a quantification strategy for these samples resembling the relative sensitive factors concept has been developed. Validation was done using a homogeneous reference material containing certified concentration values for La (6 ± 1 μg g−1), Ce (14 ± 1 μg g−1) and Nd (6 ± 1 μg g−1). Concentrations of 4 ± 1 μg g−1, 13 ± 1 μg g−1 and 5 ± 1 μg g−1 were calculated for La, Ce and Nd respectively with the proposed strategy. Quantitative depth profiles of the investigated samples have shown that the contaminants did not penetrate the substrate deeper than 80 nm even after a month of exposure to 12 M HNO3.