Spectrochimica Acta Electronica Research Articles

Peakfitter — an integrated Excel-based Visual Basic program for processing multiple skewed and shifting Gaussian-like spectral peaks simultaneously: application to radio frequency glow discharge ion trap mass spectrometry

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the SAE homepage at (http://www.elsevier.nl/locate/sabe). The archive contains program and data files. The main article discusses the scientific spectroscopic and instrumental aspects of the subject and explains the purpose of the program and data files. The work deals with a Microsoft Excel Visual Basic program, Peakfitter, which can process multiple Gaussian-shaped spectral peaks quickly and easily. The program employs Microsoft Excel Solver to process any Gaussian-like spectra that can be opened in Microsoft Excel 97. Up to three peaks in one to 225 spectra, each containing up to 2000 data points can be processed per data file to give background corrected peak areas for both raw data and its associated fit data as calculated by the trapezoidal method or by simple successive addition of channel intensities across each peak. Concurrently output also includes fit peak heights for Gaussian-shaped spectral peaks. Use of other statistical distributions such as the Lorentzian model requires only slight modification to a template file. Hence, Peakfitter was actually written as two application programs, ‘Gaussfitter’ and ‘Lorenfitter’ to accommodate spectra of Gaussian or Lorentzian character, respectively. Written initially to process data from a radio frequency glow discharge ion trap mass spectrometer (rf-GD/ITMS), the program is useful for processing sequentially acquired spectra, which have a limited number of data points across each peak. The user may examine and manipulate program variables in cases where the raw data is skewed with respect to the fit data. An assessment of Peakfitter is given using rf-GD/ITMS elemental analysis and ion–molecule reaction data. Peakfitter’s (i.e. ‘Gaussfitter’s) utility in processing rf-GD/ITMS spectra is characterized by a slight enhancement in signal-to-noise ratio for fit data and (possibly more importantly) the ability to reliably process multiple peaks simultaneously. For the rf-GD/ITMS data, peak area calculation by simple summation of raw data channel intensities proved more useful than other combinations of calculating peak area (i.e. trapezoidal method applied to either fit or raw data or summation method applied to fit data).

Digital video clips for improved pedagogy and illustration of scientific research — with illustrative video clips on atomic spectrometry

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the CD-ROM accompanying this issue. The archive contains video clips. The main article discusses the scientific aspects of the subject and explains the purpose of the video files. Short, 15–30 s, digital video clips are easily controllable at the computer keyboard, which gives a speaker the ability to show fine details through the use of slow motion. Also, they are easily accessed from the computer hard drive for rapid extemporaneous presentation. In addition, they are easily transferred to the Internet for dissemination. From a pedagogical point of view, the act of making a video clip by a student allows for development of powers of observation, while the availability of the technology to make digital video clips gives a teacher the flexibility to demonstrate scientific concepts that would otherwise have to be done as ‘live’ demonstrations, with all the likely attendant misadventures. Our experience with digital video clips has been through their use in computer-based presentations by undergraduate and graduate students in analytical chemistry classes, and by high school and middle school teachers and their students in a variety of science and non-science classes. In physics teaching laboratories, we have used the hardware to capture digital video clips of dynamic processes, such as projectiles and pendulums, for later mathematical analysis.

Spectral interferences in the determination of trace elements in environmental materials by inductively coupled plasma atomic emission spectrometry

This paper deals with spectral interferences in inductively coupled plasma atomic emission spectrometry (ICP-AES) encountered with environmental materials. These samples normally contain high concentrations of aluminium, calcium, magnesium, iron, titanium, potassium and sodium. The investigations cover: (a) spectral data for Al, Ca, Mg, Fe, Ti, K and Na as interferents for 200 pm wide windows centred (±100 pm) around the prominent lines of As, B, Ba, Be, Cd, Cr, Cu, Hg, Mn, P, Pb, Sb, Se, Sn, Tl, U and Zn; (b) a data base of Q-values for line interference [ Q I j (λ a)] and Q-values for wing background interference [ Q W j (Δλ a)] for two values of the excitation temperature 6200 K and 7200 K. The lines free or negligibly influenced by line interference were selected for analyte determination. Q-values were used for calculation of correction factors under a spectral line without the measurement of a reference blank at the wavelength of the prominent analysis lines. The accuracy of ICP-AES with the Q-concept as a basic methodology is checked by the analysis of a certified reference material IAEA/Sediment SD-N-1/2/. The precision of the method is characterised by an RSD of 0.6–1.7%. Extraction of trace elements soluble in aqua regia was used as a decomposition method. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta, Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the SAE homepage at http://www.elsevier.nl/locate/sabe. The archive contains the tabular material of this article in electronic form.

An electronic spectroscope for classroom displays of visible spectra

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the SAE homepage at http://www.elsevier.nl/locate/sabe. The archive contains program, data and text files. The hardcopy text describes a program that has been developed to accept data from a compact electronic spectrograph and translate the data into a display on a computer screen. The computer display mimics the view seen through a direct vision spectroscope. Atomic spectra appear as vertical lines of the appropriate color dispersed on a black field. A conventional plot of intensity versus wavelength is simultaneously displayed. The update of the display is sufficiently rapid that the image on the screen is essentially a real-time representation of the light entering the spectrograph. The system was designed for use in conjunction with video projection equipment in demonstrations to large groups of students.

Cumulative indices for Spectrochima Acta, Part A and Spectrochimica Acta, Part B

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta, Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the SAE homepage at http://www.elsevier.nl/locate/sabe. The archive contains data, index and program files. The main article discusses the bibliographical purpose of the program and data files. A collective index for Spectrochimica Acta for volumes since it was split into Parts A and B, and continuing through 1991 for SAA and 1997 for SAB, is presented in DBF format, along with rudimentary data entry and access software.

Spectral interferences in the determination of traces of scandium, yttrium and rare earth elements in “pure” rare earth matrices by inductively coupled plasma atomic emission spectrometry. Part IV. Lutetium and yttrium

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the SAE homepage at http://www.elsevier.nl/locate/sabe. The archive contains data files and text files. The present article is the fourth part of a series of papers discussing the spectral interferences of rare earth elements (REEs) in inductively coupled plasma atomic emission spectrometry (ICP-AES). The spectral interferences for 200-pm wide windows centred (±100 pm) around the prominent lines of the analytes, due to matrix lines and oxide radicals (LuO or YO) that emit band spectra depending on the excitation temperature ( T exc.) in ICP were investigated. The main result is that for T exc.=7200 K, LuO and YO band components can be eliminated so that prominent analysis lines of La, Ce, Pr, Nd, Eu and Sm were observed on a smooth background. T exc..=7200 K was chosen as the optimal excitation temperature in the determination of traces of REEs in Lu 2O 3 and Y 2O 3, respectively. The quantification of the interferences in terms of Q-value was used in accordance with Boumans and Vrakking [Spectrochimica Acta Part B 42 (1987) 819; 43 (1188) 69]. The “best” analysis lines are free of line interferences and negligibly influenced by wing interferences and Lu 2O 3 and Y 2O 3 as matrices do not raise the real detection limits.

An expert system program for ICP-AES system diagnosis

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by two diskettes containing the installation program for the application. The main article discusses the purpose of this work and the Appendix provides instructions on the use of the program. ICP-AES performance can be followed by monitoring a series of atomic lines. QUID Expert is a Windows based program which uses line intensities from a standard solution in conjunction with a series of rules to determine when an ICP-AES is degrading and where the probable fault lies. The software provides other features such as a logbook and automatic recording to allow users to monitor system performance over extended times. In the tradition of modern expert systems, QUID Expert provides guidance as to what possible fault(s) might be present in the system and recommends remedial action(s).

Modelling of the evaporation behaviour of particulate material for slurry nebulization inductively coupled plasma atomic emission spectrometry

This paper is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta, Part B (SAB). This hardcopy text, comprising the main body and an appendix, is accompanied by a disk with programs, data files and a brief manual. The main body discusses purpose, design principle and usage of the computer software for modelling the evaporation behaviour of particles in inductively coupled plasma atomic emission spectrometry (ICP-AES). Computer software has been developed in FORTRAN 77 language in order to simulate the evaporation behaviour of particles of refractory materials such as encountered in the analysis of advanced ceramic powders by slurry nebulization inductively coupled argon plasma atomic spectrometry. The program simulates the evaporation of single particles in the inductively coupled plasma and also enable it to calculate on the base of a given particle size distribution the evaporation behaviour of all the particles contained in a sample. In a so-called “intensity concept”, the intensity is calculated as a function of the observation height in order to determine recovery rates for slurries compared with aqueous solutions. This yields a quick insight whether a calibration with aqueous solutions can be used for analysis of slurries of a given powder by slurry nebulization ICP-AES and also is a help in determining the optimal parameters for analyses of powders by means of slurry nebulization ICP-AES. Applications for the evaporation of Al 2O 3 and SiC powders document the usefulness of the model for the case of a 1.5 kW argon ICP of which the temperature at 8 mm above the load coil has been determined to be 6100 K. The model predicts the maximum particle size for SiC and Al 2O 3 that can be transported (10–15 μm) and evaporated for a given efficiency under given experimental conditions. For both Al 2O 3 and SiC, two ceramic powders of different grain size were investigated. The median particle sizes cover the range typical of ceramic powders. Investigations were made for SiC A 10 (median particle size 2.2 μm), SiC F1200 (4.3 μm) and Al 2O 3 AKP 30 (< 1.9 μm) and Al 2O 3 Cilas 715 (3.0 μm), respectively, in which particles with diameters of up to 23 μm still are found.

Possibilities and limitations of spectral fitting to reduce polyatomic ion interferences in inductively coupled plasma quadrupole mass spectrometry in the mass range 51–88

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta, Part B (SAB). The hardcopy text is accompanied by a diskette containing the fitting program and instructions for use. The work deals with spectral fitting in ICP-quadrupole-MS. The possibilities and limitations of spectral fitting to reduce polyatomic ion interferences in inductively coupled plasma quadrupole mass spectrometry in the mass range 51–88 were investigated. Spectral fitting (simulation of the observed spectrum by a linear combination of the spectra of all individual components, both atomic and polyatomic ions) was carried out using standard spreadsheet software. Isotopic distributions for the polyatomic ions were calculated from the abundances of the individual isotopes. It was observed that in certain mass regions the number of compounds was higher than the number of independent relationships leading to more than one solution. This problem was tackled by establishing relationships between a number of (related) components (variables), all being molecular ions, and setting constraints (limits) for some other components. Spectral fitting was applied to the spectra of a number of environmentally relevant salt solutions showing severe interferences in the mass range investigated. In all cases these interferences could be reduced to negligible or acceptable levels by the application of spectral fitting. Correction for mass discrimination turned out to be an important parameter for the goodness of fit.

Application of inductively coupled plasma atomic emission spectrometry in analytical control of single crystals of potassium titanylphosphate: spectral interferences, line selection, detection limits and trace element determination

The present paper is a new application of inductively coupled plasma atomic emission spectrometry (ICP-AES) to the determination of Na, Cr, Mn, Fe, Co, Ni, Al, Mg, Rh, W, Pt, Ge, Rb, Zr, Nb, Ce, Nd, Eu, Tb, Er and Yb, present as dopants or impurities in single crystals of potassium titanylphosphate (KTP). The investigations cover: (a) spectral data for potassium, titanium and phosphorus as interferents for 400 pm wide windows centred (± 200 pm) around the prominent lines of the analytes; (b) a data base of Q-values for line interference ( Q I) and Q-values for wing (background) interference ( Q W) for two values of the excitation temperature—6200 K and 7200 K; and (c) detection limits for the dopants and impurities in the presence of 8 mg ml −1 KTP in solution, measured by using the “true detection limit” criterion as proposed by Boumans and Vrakking [Spectrochim. Acta, 42B (1987) 819; 43B (1988) 69]. The lines with the lowest values of true detection limits were selected for analyte determination. Using the ICP-AES method, data were obtained for the average dopant content in the single crystals of KTP, the dopant distribution along the growth axis of the crystals and the impurity content. The precision of the method is characterised by an RSD of 2–6%, the accuracy of the method was assessed by comparison with a flame AAS method. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta, Part B (SAB). The hardcopy text is accompanied by a disc with data and text files. The data files comprise in particular the tabular material of this article in electronic form.

Monte Carlo simulation of electrothermal atomization on a desktop personal computer

Monte Carlo simulations have been applied to electrothermal atomization (ETA) using a tubular atomizer (e.g. graphite furnace) because of the complexity in the geometry, heating, molecular interactions, etc. The intense computational time needed to accurately model ETA often limited its effective implementation to the use of supercomputers. However, with the advent of more powerful desktop processors, this is no longer the case. A C-based program has been developed and can be used under Windows TM or DOS. With this program, basic parameters such as furnace dimensions, sample placement, furnace heating and kinetic parameters such as activation energies for desorption and adsorption can be varied to show the absorbance profile dependence on these parameters. Even data such as time-dependent spatial distribution of analyte inside the furnace can be collected. The DOS version also permits input of external temperaturetime data to permit comparison of simulated profiles with experimentally obtained absorbance data. The run-time versions are provided along with the source code. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by a diskette with a program (PC format), data files and text files.

Computer expert system for spectral line simulation and selection in inductively coupled plasma atomic emission spectrometry

This paper is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta, Part B (SAB). This hardcopy text, comprising the main body and an appendix, is accompanied by a disk with programs, data files and a brief manual. The main body discusses purpose, design principle and usage of the computer software for the inductively coupled plasma atomic emission spectrometry (ICP-AES) expert system. The appendix provides a brief instruction on the manipulation of the demonstration program and relevant information on accessing the diskette. The computer software of the expert system has been developed in C++ language to simulate spectra and to select analytical lines in ICP-AES. This expert system is based on a comprehensive model of non-LTE ICP-AES, which includes expertise in plasma discharges, analyte ionization and excitation, and spectral-line shapes. The system also provides several databases in which essential elemental and spectral data are stored. A logic reasoning engine is utilized for selection of the best analytical line with a main criterion of minimizing the true detection limit. The system is user-friendly with pop-up menus, an editor for database operation, and a graphic interface for the display of simulated spectra. The system can simulate spectra and predict spectral interferences with good accuracy.

Spectral interferences in the determination of traces of scandium, yttrium and rare earth elements in “pure” rare earth matrices by inductively coupled plasma atomic emission spectrometry Part III. Europium

This paper is the third part of a series of papers dealing with spectral interferences of rare earth elements (REEs) in inductively coupled plasma atomic emission spectrometry (ICP-AES). The present article shows: (a) the spectral data of europium interferent for 200 pm wide windows centred (±100 pm) about prominent lines of scandium, yttrium and REEs; (b) the database of Q values for line interference ( Q l ) and Q values for (wing) background interference ( Q w); (c) the detection limits measured by using the “true detection limit” criterion as proposed by P.W.J.M. Boumans and J.J.A.M. Vrakking (Spectrochim. Acta Part B, 42 (1987) 819; 43 (1988) 69). Different possibilities for improvement of the true detection limits are discussed: the use of equipment of high resolving power, the application of multicomponent analysis (MCA) techniques and preliminary separation of the matrix component combined with preconcentration of trace REEs. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by a disk with data and text files. The data files comprise in particular the tabular material of this article in electronic form.

The importance of the rollover absorbance and the Zeeman sensitivity ratio related coefficient for the accuracy in linearization of calibration curves in electrothermal atomic absorption spectrometry with Zeeman effect background correction

An improvement in the calculation of the Zeeman effect sensitivity ratio related coefficient R is presented. The importance of accuracy when determining the two factors R and the rollover absorbance A r is discussed. Of these two factors it appears that an accurate value of R is more important for the efficacy of linearization. For Ag, Al, Be, Cd and Pb linearized calibration curves has been constructed where the deviation of sensitivity along the calibration curve showed, for some elements, signs of systematic errors. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by two disks with executable programs, source code files and data files suitable for use on IBM compatible PCs. The manual text file (manual.txt) includes a detailed description of how to use the supplied programs and information concerning source code files and data files.

DensMat: fully time-resolved simulation of two-step pulsed laser excitation of atoms in highly collisional media

A program that simulates and displays the level populations of atomic systems exposed to dual-wavelength (i.e. two-colour) pulsed laser excitation in highly collisional media (such as flames and plasmas) has been developed. The program is based upon a previously published fully time-dependent density-matrix model that describes step-wise excitations of atoms with degenerate states under collision-dominated conditions, and which thus goes beyond the rate-equations formalism. This model can predict such phenomena as Rabi flopping and a.c.-Stark splitting, shifting and broadening. The program can be used as a prediction tool for laser-enhanced ionization, laser-induced fluorescence, fluorescence dip spectroscopy and other two-colour laser-based spectroscopic experiments. The program provides the user with a flexible four-level atomic system, configurable as a one- or two-step excitation ladder, along with an ionization continuum and non-laser-connected level(s) that may act as trap(s) or metastable level(s). Parameters such as level degeneracy, collisional rates and laser pulse widths, shapes, wavelengths, intensities and bandwidths are accessible to the user. The program can display both the time development of the level populations and also level populations versus laser wavelength. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by a disk containing the program DensMat, an associated on-line help file and manual, an installation program, and data files pertaining to the examples illustrated in this article. The program runs under Windows 3.1 on an IBM-compatible computer.

IDAS: a Windows based software package for cluster analysis

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text, comprising the main article and one appendix, is accompanied by two installation diskettes with the software package and data files. The main article discusses the chemometric aspects of the package and explains its purpose. The IDAS software package combines three cluster analysis methods (hierarchical, non-hierarchical and fuzzy) and runs under MS Windows. Modified algorithms for non-hierarchical and fuzzy clusterings are described. The interpretation of the clustering results is facilitated by the extensive use of different types of graph. New approaches to the graphical representation of the results of fuzzy clustering are proposed. Two data sets, the Iris data by Fisher and a data set on the chemical composition of tea, are used to demonstrate the capabilities of the software.

Signal processing of transient atomic absorption signals

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by a disk with a demonstration version of the simulation program, libraries of electronic and optical component blocks, simulation models, manual, and other files. Absorbance signals for electrothermal atomization atomic absorption spectroscopy (ETA-AAS) were generated digitally and the effect of various types and sources of noise upon the precision of the absorbance measurement was evaluated by numerical calculation. Peak area measurement, peak height measurement, and matched filtering were used for processing these signals. The performance of these three techniques in the presence of various types of noises and the sensitivity of each to small variation in the atomization conditions was calculated. It is demonstrated that significant improvement in signal-to-noise ratios can be realized by application of appropriate signal processing methods. The results also indicate that one of the principal causes for loss of precision could be the variation in the heating characteristics of the furnace.

A LabVIEW ® program for determining electron number density from Stark broadening measurements of the hydrogen-beta line

A useful plasma diagnostic is the measurement of electron number density. One way to accomplish such measurements is to determine the contribution to the broadening of a spectral line due to the Stark effect. To simplify and extend such electron density measurements across computer platforms, a program that calculates electron number density from the Stark-broadened hydrogen-beta line has been written for the LabVIEW ® environment. This program calculates electron number densities from the field strength that would be exerted on a hydrogen atom immersed in a plasma. Using the new program, the electron number density in a glow discharge is calculated for two different operating conditions. Not surprisingly, the results indicate that to increase the current density in the discharge, the source electrodes must be reduced in surface area. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by one disk with an executable program (written for Apple Macintosh), data and text files including a manual.

Automated simplex optimization for monochromatic imaging inductively coupled plasma atomic emission spectroscopy

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by a diskette with data files and text files with the hardcopy paper in Word for Windows 2.0 and ASCII format, and a disclaimer. The text details the purpose of the work, and the appendix provides essential information for accessing the diskette and using programs and data. An automated simplex optimization program was written in an attempt to relieve the monotonous human effort required to optimize an inductively coupled plasma atomic emission spectroscopic (ICP-AES) instrument. The program controls the operating conditions of a solid-state ICP while a monochromatic imaging spectrometer collects two-dimensional response images. In the work described in this article, four of the most important ICP operating parameters are optimized automatically: radio frequency (rf) power, intermediate Ar flow rate, nebulizer flow rate and sample uptake rate. Optimization of these parameters is based first on the response of the Ca II net signal and, independently, on the response of the Ca II net signal-to-background noise ratio ( S N B ). Since the best height and off-axis positions of the designated response are known from the optimal two-dimensional response image, two spatial parameters are indirectly optimized through use of a CCD camera. Optimal operating conditions are reported for the CA II net signal and Ca II S N B as well as the Mg II Mg I ratio.

Contributions to computer-aided interpretation of ion sputtering depth profiling

To evade some of the problems that restrict the quantification of sputtering depth profiling, i.e. the conversion of measured signal intensity vs sputtering time profiles into the concentration vs depth scale in the case of changing sputtering rate with depth, algorithms and computer programs are presented for modelling the intensity vs time profile by computer simulation of the measuring process. Required are only rough a priori information on the sample type under investigation, e.g. implanted material or layer systems, some knowledge of the actual sputtering conditions and basic assumptions on origin, transmission and registration of the analytical signals. The modelled profile may be adapted to the measured one by interactive, stepwise variation of parameters assumed to be important. For scanned ion beams, procedures for efficient estimation of the actual ion current density and of the changing sputtering rate with depth are given too. All calculations can be performed at a desk using any IBM compatible personal computer. Owing to its modular structure, the program may be easily extended and completed according to model refinements by users. Agreement between computed and measured profiles indicates that the a priori assumptions on the true depth profile were correct in all probability. Otherwise, in most cases, further experiments have to be performed in order to decide between experimental artefacts and material-inherent deviations considering also atomic mixing, radiation enhanced diffusion, preferential sputtering and similar effects. Moreover, the quantitative relations between sputtering conditions and crater shape obtainable by the model can support optimization of the measuring conditions particularly with respect to depth resolution. At last, the procedures and algorithms given here are a contribution to the final aim of direct quantification of the results of sputtering depth profiling. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by three disks with executable programs, source code files, data files and text files. The manual and tutorial text files include material that is useful for learning the effective handling of the program and for program extension by user-written program parts.