Sensitivity factors, spectrometer

In principle, therefore, the surface concentration of an element can be calculated from the intensity of a particular photoelectron emission, according to Eq. (2.6). In practice, the method of relative sensitivity factors is in common use. If spectra were recorded from reference samples of pure elements A and B on the same spectrometer and the corresponding line intensities are and respectively, Eq. (2.6) can be written as... 

Mass Spectrometry. The mass spectra were obtained on a CEC 21-llOB mass spectrometer with the batch inlet system maintained at 250°C to assure complete vaporization of the samples. Sensitivity factors for quantitative analysis were obtained from standards of di-, tetra-, hexa-, and octa-chlorodibenzo-p-dioxin. The factors for the intermediate chlorinated species were estimated by interpolation. The analyses were based... 

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. 

XRF nowadays provides accurate concentration data at major and low trace levels for nearly all the elements in a wide variety of materials. Hardware and software advances enable on-line application of the fundamental approach in either classical or influence coefficient algorithms for the correction of absorption and enhancement effects. Vendors software packages, such as QuantAS (ARL), SSQ (Siemens), X40, IQ+ and SuperQ (Philips), are precalibrated analytical programs, allowing semiquantitative to quantitative analysis for elements in any type of (unknown) material measured on a specific X-ray spectrometer without standards or specific calibrations. The basis is the fundamental parameter method for calculation of correction coefficients for matrix elements (inter-element influences) from fundamental physical values such as absorption and secondary fluorescence. UniQuant (ODS) calibrates instrumental sensitivity factors (k values) for 79 elements with a set of standards of the pure element. In this approach to inter-element effects, it is not necessary to determine a calibration curve for each element in a matrix. Calibration of k values with pure standards may still lead to systematic errors for unknown polymer samples. UniQuant provides semiquantitative XRF analysis [242]. 

Secondary ion mass spectrometry (SIMS) was used to characterise the coatings for their Ti, Ru and O stoichiometry on the surface and as a function of depth into the coating. A PHI 6650 Quadrupole mass spectrometer, with Cs+ as the ion source was used in these studies. The conversion of the measured secondary ion counts to concentration was performed using relative sensitivity factors, which were first determined with a standard sample containing known amounts of RuC>2 and TiC>2. All of the SIMS profiles were repeated several times, to determine the measurement precision, which was typically +10%. 

The energy of the primary electron beam was 2.5 keV and the energy of the Ar+ used in the depth profiles was 2 keV. The relative sensitivity factors were determined using pure standard samples and are presented in Table 14.1. Some samples were studied using an Auger spectrometer equipped with a retarding field analyser. The thickness of the films was... 

The XPS data were obtained with an extensively modified AEI ES-100 photoelectron spectrometer. The samples were analyzed at a pressure typically < 10 g Torr. A magnesium anode (1253.6 eV) was used as the excitation source. The analyzed sample area was of the order of 5 mm2. Survey scans from 0 to 1000 eV were first obtained for each sample to confirm that only the expected elements were present on the fiber surfaces. Subsequently, high resolution spectra were obtained by slowly scanning — 20 eV binding energy windows that included the Si 2p, Al 2p, Ca 2p, B Is, O Is, and C Is photoelectrons, respectively. Integrated peak areas of the photoelectron spectra were determined. The sensitivity factors, which were independently obtained on this spectrometer with oxide standards, were then utilized in the determination of the surface atomic percent compositions of the fibers. 

Elemental compositions were calculated from the satellite-subtracted low resolution spectra normalized for constant transmission using the software supplied by the manufacturer. The sensitivity factors employed in these computations were C(ls)=0.34, N(ls) = 0.54, Si(2p) = 0.4, and O(ls) = 0.78, empirically derived for the MAX-200 spectrometer by Leybold. 

A second approach to quantification factors consists of establishing the spectrometer transmission function T, By using the theoretical values for the effective cross section and the mean fiee path, it is possible to calculate the sensitivity factor for any element. Significant recent w ork in standardisation has been used to create reference spectrometers, for which the transmission factor is knowm and can therefore be compensated for in order to obtain the reference spectra of several pure elements (Cu, Au, Ag). The transmission function of the spectrometer being used is obtained by comparing the spectrum obtained on one of these elements with certified spectra. It then becomes possible to calibrate the spectrometer regularly. The results of the sensitivity factor calculation are recorded as a database. 

Figure 5.6 Sensitivity factors ascertained for two XPS spectrometers (logarithmic scale, reference FIs I), For certain elements, the sensitivity levels may differ by a factor of two.

An atomic fraction of a surface element (X ) can be calculated from known its peak intensity (/, ) and sensitivity factor (Si). The sensitivity factor for each element is experimentally determined and varies with conditions of the instrument and sample surface. It is desirable to measure the sensitivity factor for a given spectrometer and sample type. Common practice is to use the sensitivity factors from published handbooks with certain corrections according to the... 

These will Include C4F3, C2F2 and C3F4 (l.e., olefins, acetylenes, dloleflns). The cyclic structures will manifest themselves as small signals at 100 and 131 a.m.u. for hexafluorocyclopropane and at 100, 131 and 181 a.m.u. for octafluorocyclobutane (12). The total sensitivity factor of the spectrometer Itself decreases quite rapidly with increasing mass ntuaber. 

Chemical stoichiometry can be calculated from TPD peak areas after the sensitivity factors of the mass spectrometer are... 

X-ray photoelectron spectroscopy (XPS) measurements were performed using a SSX-100 model 206 Surface Science Instrument Spectrometer operated at 10 kV, 12 mA with a monochromatized A1 Ka radiation (1486.6 eV). The catalysts were pressed into the samples holders of 6 mm and then introduced into the preparation chamber of the spectrometer. The Cu, Mosd, Co2p, Niap and Ou lines were recorded for each sample. All binding energies were referenced to the Cu level at 284.8 eV. Surface composition was determined from the peak intensities and the Scofield sensitivity factors provided by the instrument software. For spectrum deconvolution, a Shirley baseline was used and peaks were considered Gaussian/ Lorentzian ratio of 85/15. 

X-ray photoelectron spectroscopy (XPS) analyses were performed in a VG Scientific spectrometer and sensitivity factors of 4.0 for Cu 2p3/2 and 1.1 for Zr 3ds/2 were used. Electron paramagnetic resonance (EPR) analyses were performed with a modified Varian E-4 X-band spectrometer. Temperature programmed reduction (TPR) was performed in the tubular flow reactor using 5% H2 in argon as the reductant. 

Therefore, only relative values of Q are needed to make quantitative analysis. Usually, sensitivity factors are normalized to fluorine F Is (2(F Is) = 1) or to Cls(2(Cls) = 1) and are included in libraries furnished with spectrometers. 

X-ray photoelectron. spectroscopic analyses were performed on a Surface Science SSX-100 spectrometer. The monochromatic A1 Ka source was focussed to a spot size of 150-1000 pm. A pass energy of 150 eV or 50 eV was employed for the survey or high resolution spectra, respectively. Surface compositions were calculated from spectral peak areas utilizing computed sensitivity factors 2.13. 

This represents an improvement in signal-to-noise by more than a factor of 100 over the most sensitive grating spectrometer that we have used previously (Seliger et aZ., 1974). 

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