PIXE analysis limitations

PIXE detection limits for surface layers on bulk specimens are sufficiendy low to permit calibradon of true surfe.ce analysis techniques (e.g., Auger electron spectroscopy). 

The advantage of p-PIXE analysis over the scanning electron microprobe arises from the presence of a strong Bremsstrahlung background in the latter, which tends to mask the characteristic X-ray peaks. There is thus a striking difference in sensitivity between the two techniques the detection limits are of the order of 0.1% for the electron microprobe and 0.001% for p-PIXE. 

Figure 13.5 Detection limits in a pixE analysis of a biological sample. [From anc Morita (1990).]...

Minimum detection limits in PIXE analysis of atmospheric aerosol samples... 

Use of heavy ions like C, Si and still more heavy ions have the following limitation for PIXE analysis ... 

The penetration depths and irradiation areas are totally different in PIXE and XRFS. In XRFS penetration depths are relatively large, of the order of a few millimeters while in PIXE analysis, the analytical depths are ss 10-50 pm because of the limited penetration of particles into the sample. Therefore PIXE analysis is essentially a surface technique even when applied to thick samples. 

In PIXE analysis, using proton beam of 1-3 MeV for most favored elements in low-Z matrices and think targets, the best sensitivities down to 0.1 ppm have been obtained. These levels are achieved for elements near Z = 40 using K lines and Z = 80 using L lines. For elements with Z valves different from 40 and 80 the LLDs increase rapidly to 100 ppm and are >100 ppm for Z < 20. For thick targets and Z < 20 most matrices yield LLDs that are generally lower than 100 ppm and can be as low as 1 ppm under favorable conditions (absolute detection limits down to 10 g and relative detection limit down to 0.1 p.gg ). Compared to XRF, the detection limit offered by PIXE is better by one order of magnitude. 

Typically, PIXE measurements are perfonned in a vacuum of around 10 Pa, although they can be perfonned in air with some limitations. Ion currents needed are typically a few nanoamperes and current is nonnally not a limiting factor in applying the teclmique with a particle accelerator. This beam current also nonnally leads to no significant damage to samples in the process of analysis, offering a non-destmctive analytical method sensitive to trace element concentration levels. 

Compared to EDS, which uses 10-100 keV electrons, PEXE provides orders-of-magnitude improvement in the detection limits for trace elements. This is a consequence of the much reduced background associated with the deceleration of ions (called bremsstrahlun compared to that generated by the stopping of the electrons, and of the similarity of the cross sections for ioiuzing atoms by ions and electrons. Detailed comparison of PIXE with XRF showed that PDCE should be preferred for the analysis of thin samples, surfrce layers, and samples with limited amounts of materials. XRF is better (or bulk analysis and thick specimens because the somewhat shallow penetration of the ions (e.g., tens of pm for protons) limits the analytical volume in PIXE. 

It is apparent that PIXE exhibits its maximum sensitivity or minimum detection limit (MDL) in the two atomic number regions 20 < Z < 35 and 75 < Z < 85. These are attained at relatively low proton energies, which implies that small accelerators are most suitable for PIXE with the corresponding benefits in reliability and economics. Analysis times are typically a few minutes in duration. The MDL is very strongly influenced by the nature of the sample, especially if there are strong X-rays from the matrix visible in the spectrum or if the sample is strongly insulating. 

PIGE is a rapid, non-destructive technique that is employed in the analysis of light elements such as lithium (10-100 ppm limit of detection), boron (500-1000 ppm limit of detection), and fluorine (1-10 ppm limit of detection), which are often difficult to determine by other analytical means. Because the technique is based upon specific nuclear reactions, the sensitivity of PIGE varies greatly from isotope to isotope, and this non-uniformity of sensitivity has limited its widespread use as a complementary technique to micro-PIXE. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

Elemental mass distribution - The aerosol sampled by the LPI for elemental analysis was impacted on coated mylar films affixed to 25 mm glass discs. The mylar had been coated with Apiezon L vacuum grease to prevent particle bound. The LPI samples were sent to Crocker Nuclear Laboratory for elemental analysis by PIXE using a focused alpha particle beam of 3 to 4 mm diameter. Nanogram sensitivities for most elements were achieved with the focused beam. A detailed description of the PIXE focused beam technique applied to LPI samples can be found in Ouimette (13). Based upon repeated measurements of field samples, the estimated measurement error was about 15-20% or twice the minimum detection limit, whichever was larger. 

Typical detection limits for various elements in a biological sample are shown in Figure 13.5. Typically, PIXE has sensitivity at the parts per million level for many elements. About 25% of the applications of PIXE are in biology and medicine. The light-element matrices lead to smaller continuous backgrounds, and many trace and toxic elements are easily detected by PIXE. (There are no holes in detection limits as there are in activation analysis as all the elements emit some X-rays.) Considerable attention has been and must be devoted to the preparation of thin, representative samples. Note that PIXE is only sensitive to the elemental composition of the sample and not to the isotopic composition. 

The PLD target was sintered from 99.9995 at. % ZnO powder. DL is the minimum detection limit of the combined PIXE/RBS analysis with 1.2 MeV protons. Each wafer (32.8 mm diameter) was analyzed at center (c) and edge (e) position. Measured by D. Spemann... 

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