Elements, plasma sensitivity

Figure 1 Outline of the multiple-collector inductively coupled plasma mass spectrometer (VG Elemental Plasma 54) at the University of Michigan (U-M). The U-M instrument is equipped with an extra 30-cm energy filter for high-abundance sensitivity measurements. (From Halliday et al., 1998b.)...

What happens in a low-pressure plasma process cannot be determined in an a priori manner based only on the nature of the plasma gas or on the objective of the process. The plasma sensitivity series of elements involved, in both the luminous gas phase and the solids, that make contact with the luminous gas phase seems to determine the balance between ablation and polymerization by influencing the fragmentation pattern of molecules in the luminous gas environment. 

The decrease of Si due to F-containing contaminants and the role of the oxygen plasma treatment can be explained by the principle of CAP. The key factor to explain the change of elementary composition at the interface is the plasma sensitivity of elements involved on the surface and in the plasma phase. The ablation of materials exposed to plasmas appears to follow the plasma sensitivity series of the elements involved, which is in the order of the electronegativity of the elements, i.e., elements with higher electronegativity in the condensed phase are more prone to ablate in plasma that contains elements with lower electronegativity [5]. 

Air plasma treatment was used to make one surface hydrophilic, and CF4 plasma treatment was used to make the other hydrophobic. Such a fabric with a different set of surface characteristics on each side can be made however, the success of this undertaking is contingent on which treatment is applied first. The sequence dependency of plasma treatments may be explained by the concept of plasma sensitivity of the elements involved in the two steps. Results are summarized in Tables 10.1 and 10.2. 

As is evident in Table 10.1, two characteristically different sides can be obtained only when the air plasma treatment is applied first. This can be explained by the plasma sensitivity of the elements involved. Fluorine is the most electronegative element involved in this experiment. Consequently, it can be removed relatively easily by exposure to the plasma, which consists of less plasma sensitive elements (elements of lower electronegativity). 

Excepting refractory elements, the sensitivity of inductively coupled plasma spectroscopy is comparable with that of flame atomic absorption spectroscopy (Table 2). Additionally, interactions that occur between the atmosphere and an arc or flame are absent due to the inertness of the argon plasma. Since ioniza-... 

ICP-MS Nebulization pneumatic or ultrasonic High- temperature argon plasma Ion separation, m/z measured by mass spectrometer ppt with few exceptions, ppq for some elements High sensitivity— better than GFAAS Multielemental High sample throughput Expensive Matrix interferences Low dissolved solids tolerance... 

Element sensitivity depends on the emission intensity at the measured wavelength. Each element has a number of possible wavelengths for determination and the best must be chosen also taking into account selectivity. Different emission lines have different sensitivities in diflfer-ent plasmas. Sensitivity, defined by the slope of the response curve, is less often used in C-AED than detection limit , expressed as absolute values of element mass (in a resolved peak) or in mass flow rate units. Detection limits for different elements vary by two or three orders of magnitude this affects inter-element selectivity if spectral overlap is present. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

Inductively Coupled Plasma-Optical (ICP-optical) methods and ICPMS are extremely sensitive elemental survey techniques that also are described in this volume. ICP methods, however, require a solution for analysis, so that the direct... 

Fig. 3.36. Experimental, Fe-related HF- calculated according to [3.74] from plasma SNMS sensitivity factors S(pe)x Ref [3.71] (salts) [3.72] alloys, [3.73] with elements X ordered according to round robins (r.r.). their post-ionization probabilities...

The absolute sensitivity factors Sx must be determined for this procedure by integrating intensities over time while sputtering suitable pure element samples and determining the crater volume for HF-plasma SNMS the weight loss can also be measured. 

It is necessary to supply a spray into the plasma jet, in order to excite the elements. This has presented some difficulty and it is a n stery to the author as to why the plasma jet has not been more widely applied for the analysis of sodium potassium, calcium and magnesium. It has the advantage of safety, in that one inert gas (argon) would be used, and high sensitivity. In addition, four elements, calcium, magnesium. 

New developments are, however, needed to make a major step forward in the field of speciation analysis. The first part, isolation and separation of species, may be the easiest one to tackle. For the second part, the measurement of the trace element, a major improvement in sensitivity is needed. As the concentration of the different species lies far below that of the total concentration (species often occur at a mere ng/1 level and below), it looks like existing methods will never be able to cope with the new demands. A new physical principle will have to be explored, away from absorption spectrometry, emission spectrometry, mass spectrometry, and/or more powerful excitation sources than flame, arc or plasma will have to be developed. The goal is to develop routine analytical set-ups with sensitivities that are three to six orders of magnitude lower than achieved hitherto. 

The weakness of MC-ICPMS lies in the inefficiency by which ions are transferred from the plasma source into the mass spectrometer. Therefore, despite very high ionization efficiencies for nearly all elements, the overall sensitivity (defined as ionization plus transmission efficiencies) of first generation MC-ICPMS instruments is of the order of one to a few permil for the U-series nuclides. For most, this is comparable to what can be attained using TIMS. 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

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