Absolute sensitivity factor

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. 

In principle GD-MS is very well suited for analysis of layers, also, and all concepts developed for SNMS (Sect. 3.3) can be used to calculate the concentration-depth profile from the measured intensity-time profile by use of relative or absolute sensitivity factors [3.199]. So far, however, acceptance of this technique is hesitant compared with GD-OES. The main factors limiting wider acceptance are the greater cost of the instrument and the fact that no commercial ion source has yet been optimized for this purpose. The literature therefore contains only preliminary results from analysis of layers obtained with either modified sources of the commercial instrument [3.200, 3.201] or with homebuilt sources coupled to quadrupole [3.199], sector field [3.202], or time-of-flight instruments [3.203]. To summarize, the future success of GD-MS in this field of application strongly depends on the availability of commercial sources with adequate depth resolution comparable with that of GD-OES. 

Where XA is the concentration of element A and dI /dXA is the absolute sensitivity factor of element A in matrix M. A calibration curve is established using known samples for each element in each matrix. Unknown concentrations may then be determined provided all instrumental factors and experimental conditions are held constant, which conditions may be difficult to fulfil. 

Empirical Methods. The empirical methods use calibration standards to derive sensitivity factors that can be used to determine the unknown concentration of given elements in similar matrices [3. The sensitivity factors are derived from calibration curves that plot measured secondary ion intensities versus the known concentration of standards. Three types of sensitivity factors have been used the absolute sensitivity factor, the relative sensitivity factor, and the indexed relative elemental sensitivity factor. 

The element concentration levels are deduced in LEISS from the peak surfaces corresponding to each chemical species using sensitivity factors. If a reference sample with a known atomic density is used (for example, flat monocrystals), it is possible to determine an absolute sensitivity factor relating the atomic density to the peak area for a given primary beam intensity. In practice, ratios between sensitivity factors are most commonly used to establish the relative concentration levels of species because any surface contamination, for example by a hydrocarbon, would considerably influence the overall intensity of the spectrum. 

Ix is the background-corrected net intensity of the principal peak of analyte X, Kx a proportionality factor for the absolute sensitivity of the standard reference, e. g. an Ni plate, and c the concentration of X. Multielement analyses are based on known relative sensitivities S ... 

As XRF is not an absolute but a comparative method, sensitivity factors are needed, which differ for each spectrometer geometry. For quantification, matrix-matched standards or matrix-correction calculations are necessary. Quantitative XRF makes ample use of calibration standards (now available with the calibrating power of some 200 international reference materials). Table 8.41 shows the quantitative procedures commonly employed in XRF analysis. Quantitation is more difficult for the determination of a single element in an unknown than in a known matrix, and is most complex for all elements in an unknown matrix. In the latter case, full qualitative analysis is required before any attempt is made to quantitate the matrix elements. 

Depending on the type of relationships between the measured quantity and the measurand (analytical quantity) it can be distinguished (Danzer and Currie [1998]) between calibrations based on absolute measurements (one calibration is valid for all1 on the basis of the simple proportion y = b x, where the sensitivity factor b is a fundamental quantity see Sect. 2.4 Hula-nicki [1995] IUPAC Orange Book [1997, 2000]), definitive measurements (b is given either by a fundamental quantity complemented by an empirical factor or a well-known empirical (transferable) constant like molar absorption coefficient and Nernst factor), and experimental calibration. 

Isotope Spin Natural abundance (%) Quadrupole moment (10" m ) Relative sensitivity Width Factor Relative Intensity Absolute sensitivity NMR frequency (MHz)... 

Table II summarizes the differences in EIMS and NAA for the detection of %g in biological materials. Sample processing for EIMS analysis is simpler and more rapid than that in preparation for NAA. The absolute sensitivity is increased by about two orders of magnitude and precision by a factor of 2-3. Consequently, lower levels of %g excess can be detected with confidence (16). This comparison suggests that EIMS can be applied to %g measurements in speciments likely to be low in total magnesium and Mg excess, e.g. urine and plasma. Unfortunately, these expectations were not fully realized.

Typical spectra from top and bottom samples of the catalyst bed are shown In Figs. 8-11. The elements detected and approximate sufrace mole percentages are tabulated in Table 3. For comparison, previous results obtained on the fresh catalyst are included also Uncorrected elemental sensitivity factors supplied by VG were used in calculations of surface concentrations for most of the elements. Absolute concentrations may be off by a factor of two, but relative concentrations for a given element should be significantly better than this. 

This use of atomic sensitivity factors will give semiquantitative results, within 10-20% of the value, for most homogeneous materials. While the absolute sensitivity of XPS is high, the actual sample volume analyzed is small, so the amount of an element that can be detected is in the % range with sensitivity of about 0.5 atom%. 

The calibration of a PIXE system, i.e., the determination of sensitivity factors, which assign absolute concentration data to numbers of counts in X-ray peaks, can be performed in two different ways. First, sensitivity factors can be deduced theoretically or in a semiempirical way from calculated cross sections for X-ray excitation and from X-ray absorption data for the absorbents present between the points of emission and detection, in the actual experimental setup. Second, sensitivity factors can be deduced from measurements performed on standard samples consisting of pure elements or pure chemical compounds. The detection solid angle and the energy-dependent detector efficiency should also be determined. 

The ionization cross section is a central quantity in the sensitivity factor in Equation 48.1. Difficulties in obtaining reliable, absolute cross sections result in the use of calculated atomic cross sections [63-66] together with the additive rule for molecular species for absolute pressure calculations with KEMS. More recently, better experimental results have been published for absolute... 

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