Concept explainers
(a)
Interpretation:
The factors related to the analyte influencing the emission intensity needs to be identified.
Concept introduction:
X-Ray photoelectron spectroscopy or XPS is a quantitative spectroscopic technique that is surface-sensitive. It measures the composition of materials with respect to elements in the range of parts per thousand. It also helps in determining the empirical formula, chemical state and electronic state of elements present in the materials.
It is now widely used because with the help of this technique not only the element within the films, but the other elements bonded to it can be determined.
For example, with the help of XPS technique, the oxidation state of metal in the metal oxide can be determined.
Explanation of Solution
The equation relating the intensity of emission and atomic concentration (density) is represented as follows:
Here,
In spectroscopy, the analyte is the molecule for which the concentration needs to be measured using spectroscopic techniques. In the above formula, the factors related to analyte are
(b)
Interpretation:
The factors related to the spectrometer influencing the emission intensity needs to be identified.
Concept introduction:
X-Ray photoelectron spectroscopy or XPS is a quantitative spectroscopic technique that is surface-sensitive. It measures the composition of materials with respect to elements in the range of parts per thousand. It also helps in determining the empirical formula, chemical state and electronic state of elements present in the materials.
It is now widely used because with the help of this technique not only the element within the films, but the other elements bonded to it can be determined.
For example, with the help of XPS technique, the oxidation state of metal in the metal oxide can be determined.
Explanation of Solution
The equation relating the intensity of emission and atomic concentration (density) is represented as follows:
Here,
Now, the factors influencing the emission intensity related to the spectrometer or instrument are
(c)
Interpretation:
The relation between the ratio of measured quantity in XPS for analyte to the internal standard and their atomic concentrations needs to be determined.
Concept introduction:
X-Ray photoelectron spectroscopy or XPS is a quantitative spectroscopic technique that is surface-sensitive. It measures the composition of materials with respect to elements in the range of parts per thousand. It also helps in determining the empirical formula, chemical state and electronic state of elements present in the materials.
It is now widely used because with the help of this technique not only the element within the films, but the other elements bonded to it can be determined.
For example, with the help of XPS technique, the oxidation state of metal in the metal oxide can be determined.
Explanation of Solution
In the quantitative XPS, the measured quantity is usually I/S. Here, I is the peak area and S is the sensitivity factor. For the analyte, the quantity measured is
The internal standard is the concentration of a substance present in every sample needed to be analyzed.
The ratio I/S is directly proportional to the concentration on the surface.
Or,
Similarly,
Thus, the relation between the ratio
(d)
Interpretation:
The given equation for the fractional atomic concentration of element A needs to be proved.
Concept introduction:
X-Ray photoelectron spectroscopy or XPS is a quantitative spectroscopic technique that is surface-sensitive. It measures the composition of materials with respect to elements in the range of parts per thousand. It also helps in determining the empirical formula, chemical state and electronic state of elements present in the materials.
It is now widely used because with the help of this technique not only the element within the films, but the other elements bonded to it can be determined.
For example, with the help of XPS technique, the oxidation state of metal in the metal oxide can be determined.
Explanation of Solution
The atomic concentration for an element (say 1) measured by XPS technique can be represented as
If the total number of elements is n, the fractional atomic concentration can be represented as follows:
Or,
Thus, for all the elements measured by XPS technique, the fractional atomic concentration for element A can be represented as follows:
Here,
(e)
Interpretation:
The atomic concentration of three elements that is C, N and O in the polyurethane sample needs to be determined.
Concept introduction:
In the quantitative XPS, the measured quantity is usually I/S.
Here, I is the peak area and S is the sensitivity factor. For the analyte, the quantity measured is
The internal standard is the concentration of a substance present in every sample needed to be analyzed.
The ratio I/S is directly proportional to the concentration on the surface.
Or,
Similarly,
Explanation of Solution
The given sample is polyurethane containing C, N and O elements that need to be detected by XPS.
The sensitivity factor for C, N and O is 0.25, 0.42 and 0.66 respectively.
Also, the peak area for C, N and O is 26550, 4475 and 13222 respectively.
The value of the atomic concentration of C, N and O can be calculated as follows:
Putting the values,
Similarly,
And,
Now, the value of the fractional atomic concentration of C can be calculated as follows:
Putting all the values,
Similarly, for N and O the atomic concentration can be calculated as follows:
And,
Putting the values,
Thus, the fractional percentage concentration of C, N and O will be:
Or,
Or,
(f)
Interpretation:
The limitations of quantitative analysis with XPS needs to be explained. The reason for atomic concentration measured to not correspond to the bulk composition needs to be explained.
Concept introduction:
X-Ray photoelectron spectroscopy or XPS is a quantitative spectroscopic technique that is surface-sensitive. It measures the composition of materials with respect to elements in the range of parts per thousand. It also helps in determining the empirical formula, chemical state and electronic state of elements present in the materials.
It is now widely used because with the help of this technique not only the element within the films, but the other elements bonded to it can be determined.
For example, with the help of XPS technique, the oxidation state of metal in the metal oxide can be determined.
Explanation of Solution
The limitation of quantitative analysis with XPS is as follows:
XPS is a surface technique, thus, it provides a limited amount of the organic information i.e. it is limited to elements with
The reason for the atomic concentrations measured not corresponding to the bulk composition is as follows: -
XPS can determine the quantity of elements present within the top 1-12 nm of the surface of the sample. XPS can only detect those electrons which get escaped from the sample and reach the detector for that photoelectron must travel through the sample. As depth increases, the signal detected from analytes at the surface is much stronger than the signals detected deeper to the surface of the sample.
(g)
Interpretation:
The percentage error in the atomic concentration of C, N and O needs to be determined.
Concept introduction:
For all the elements measured by XPS technique, the fractional atomic concentration for element A can be represented as follows:
Here,
The atomic concentration of C, N and O for the polyurethane sample is given 76%, 8.0% and 16% respectively.
The percentage error from the values calculated in part (e) can be calculated as follows:
Thus, the percentage error in the atomic concentration of C, N and O is 1%, 0.22% and 1.4% respectively.
Explanation of Solution
The atomic concentration of C, N and O for the polyurethane sample is given 76%, 8.0% and 16% respectively.
The percentage error from the values calculated in part (e) can be calculated as follows:
Thus, the percentage error in the atomic concentration of C, N and O is 1%, 0.22% and 1.4% respectively.
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Chapter 21 Solutions
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- Principles of Instrumental AnalysisChemistryISBN:9781305577213Author:Douglas A. Skoog, F. James Holler, Stanley R. CrouchPublisher:Cengage Learning