Detection limit, PIXE

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. 

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). 

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. 

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. 

In conventional PIXE the beam diameter is a few millimetres, which gives detection limits of the order of 10 11g. With p-PIXE and a spatial resolution of about 1 pm, detection limits as low as 10 —16 g can be achieved. 

The PIXE microbeam technique has a spot size in the range 1-10 pm, and this enables a study of the spatial distribution of elemental concentrations. The advantage of p-PIXE over EPMA is a very much increased analytical sensitivity due to the much lower Bremsstrahlung background generated by the proton beam. The detection limits are of the order 0.1% for EPMA and 0.001% using the p-PIXE technique. 

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. 

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

High-resolution compositional measurements are possible through use of a variety of microanalytical methods. Ideally, these should be non-destructive, can be targeted on small areas of sample, and have low minimum detection limits. Electron-probe X-ray microanalysis (EPXMA) and proton-induced X-ray emission (PIXE) techniques have both been used successfully on archaeological sediment thin sections (19, 20). Both techniques yield elemental composition data for a range of elements. EPXMA has the advantage of being nondestructive, whereas PIXE when used on thin-section samples is typically destructive conversely the detection limit for PIXE is lower than EPXMA. 

A major advantage of LA-ICP-MS as a microprobe analytical technique is the ability to obtain data for virtually any element in the periodic table. LA-ICP-MS also can be used to quantify elements that are present in the low parts-per-million (ppm) to parts-per-trillion (ppt) range. In contrast, other surface techniques such as, SEM, XRF, and PIXE are limited by the number of elements detectable and have higher detection limits than ICP-MS. 

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... 

X-radiation can also be induced by high eneigy (several MeV) proton beams from ion accelerators. Such partide-induced x-ray emission (PIXE) (284) is useful for thin samples and particulates, having detection limits of 10 g. Intense synchrotron x-ray sources have found applications in chemical investigations (285), using toroidal holographic gratings for dispersion. 

PIXE (Johansson and Campbell, 1988) and SRIXE (Jones and Gordon, 1989) have similar imaging capabilities and detection limits but both suffer from the drawback that they rely on major pieces of hardware, an accelerator in the PIXE experiment and a synchrotron X-ray source for SRIXE. 

Mercury was included in a thorough review of XRF techniques in clinical studies (Leyden and Noddy, 1977). For normal XRF analysis of 1 g of dried soft tissue, the lower limit of detection was found to be a few mg/kg. For PIXE, the relative detection limits in biological material are in the order of a few tenths of mg/g. For soft tissue analysis, this is normally orders of magnitude too high, compared with "normal" levels of mercury. Thus, the X-ray techniques must be combined with a preconcentration stage. Preconcentration techniques prior to mercury determination means serious risks of losses due to the volatility. To minimize such losses, lenient concentration procedures, e.g. low temperature ashing (Pallon and Malmqvist, 1981), are required. 

Particle-induced XRF (PIXE) has many advantages over conventional XRF. Irradiations are performed in ion accelarators (Muminov and Haidarov, 1980). It ensures up to 100 times better detection limits and values of 0.1-1 mg/kg can be obtained even in routine analysis. Accuracy and precision normally lie in the 1-5% range. The proton beam can be focused to a extremely small spot (d I p.m) which permits microdistribution analysis of elements with Z = 25 80. In this interval lies the best sensitivity... 

Microbeam Particle Induced X-ray Emission (PIXE), often called micro-PIXE, was used for single particle analysis. The greatest advantage of this system is excellent detection limits in the order of 10 -10 g. It also has the merit of a multielement non-destmctive technique with a wide range of elements for various samples. 

PIXE should achieve sensitivities in the range 0.1 1 gg g but this depends on the target and measurement arrangement. PIXE has a very low detection limit 10 —10 g in standard practice. 

PIXE is the preferred method for such applications as the analysis of 15 to 20 elements in a thin sample such as air filters, or for automated analysis of large numbers of geological or archaeological samples, due to its short measurement time. The low absolute detection limit and good sensitivity for elements such as S, P, Cl, K and Ca, Fe make PIXE of great importance in biological and medical applications. 

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