Figure 1 illustrates the schematic arrangement of the ...



1 : Diagnostic Techniques

1 Introduction

This chapter reviews the basic instrumentation of mass spectrometry used in Study 2 and tunable diode laser spectroscopy used in Studies 1-3. The instrumentation provided data that could be readily quantified in each of the studies discussed. Details concerning the mass spectrometer sampling apparatus, specific wavelength ranges of diode lasers used, as well as the overall experimental arrangements will be given in the next chapter with respect to each flame investigated.

2 Mass Spectrometry

Mass Spectrometry (MS) is a well known analytical technique that converts analytes to gaseous ions which are separated by their respective mass to charge ratio. The essential elements of the MS instrument are the ion source, mass analyzer, and detector which act in conjunction to obtain a mass spectrum that consists of a record of the relative current or ion abundance verses the mass to charge ratio.

The MS technique is initiated by the introduction of a sample which is ionized by bombardment with electrons. The most common mode of ionization is by electron impact. In this mode, gaseous molecules are introduced into the ionizer region of the instrument where they are bombarded by electrons from a red or white hot metal filament. Upon impact with an electron, e, ions are formed from molecules, M, as

[pic]

Equation 1

The ion [pic]formed is called a molecular ion and the symbol identifies a species with an unpaired electron or ion radical. The mass analyzer separates the ions by their respective mass-to-charge values. The most widely used analyzer is a quadrupole mass filter which consists of four parallel cylinder electrodes arranged at the apices of a diamond geometry. The filter creates a time-varying electric field between opposite electrodes by application of a dc potential to the vertical pair of electrodes and a radio frequency potential between the horizontal pair. As ions enter this field parallel to the length of the electrodes, they are subject to a particular field intensity and frequency from which the ions of a unique mass-to-charge ratio will follow a stable path through the field. Specifically the RF potential field acts to remove low mass ions from the beam while high mass ions are deflected by the dc potential. The mass of ions passed by the filter is then essentially determined by varying the ratio of the RF potential to dc potential. Ultimately the action of the filter causes ions to be directed in a focused beam that moves toward the detector. At the detector the current in the ion beam is measured at all values of the mass-to-charge ratios.

On-line mass spectrometry was used exclusively in Study 2 to investigate heavier molecular weight gas-phase analytes, particularly toluene. One of the limitations of on-line mass spectrometry is the relatively high detection limits for many gas-phase analytes, but the primary difficulty experienced here was from interferences by other analytes at the same molecular mass, e.g., CO and N2. Because of this constraint, which makes quantification difficult, the concentrations of these lower molecular weight compounds can be more reliably measured using TDLAS as described below.

3 Tunable Diode Lasers

1 Background

An infrared diode laser, pictured in Figure 1, is a solid-state, semiconductor diode device composed of a lead (Pb) salt alloy mounted in a gold-plated copper package [[1]]. The diode lasing principle was first demonstrated in the Pb salt family of semiconductors by Butler et al. [[2]] with the observation of radiation at 6.6 (m from a PbTe diode laser. Diode laser operation was also observed in other Pb salts [[3]] where subsequent research ascertained that a spectral range of 2.7 to 33 (m can be covered by lasers constructed from various lead Pb salt stoichiometries [[4]]. The use of diode lasers for high-resolution infrared spectroscopy was first demonstrated by Hinkley [[5]] and Ralston et al.[[6]] of which Hinkley et al. [[7]] established the necessary measurement techniques for general

[pic]

Figure 1 Top: Tunable diode laser with magnified view of crystalline chip [[8]]

Bottom: Schematic diagram of typical diode laser package [[9]]

applications in infrared molecular spectroscopy. However with regard to tunable diode laser measurements in combustion systems, Hanson et al. [[10],[11],[12],[13],[14],[15]] pioneered diagnostic techniques for measuring analyte concentrations, flame temperatures, and the spectroscopic parameters of line strength and collision linewidth which are needed for quantitative analysis of the measured absorption signal.

2 Semiconductor Overview

Semiconductors are crystalline materials that have an electrical resistivity intermediate between conductors and insulators. Diode lasers, like most semiconductor devices, will conduct electricity and emit light when a sufficient voltage is applied. The schematic structure of a typical semiconductor is a pn junction, Figure 2, that is composed of “p-type” material which is doped to produce an excess “hole” concentration (an electron acceptor), and “n-type” material which can contribute excess electron concentration (an electron donor). In this configuration electrons migrate from the n to p regions causing a buildup of negative charge on the p side and a buildup of positive charge on the n side. The buildup of charges in the device creates a barrier potential, or depletion region. The width of the depletion region can be directly controlled to determine the

[pic]

Figure 2 Simplified illustration of electron and hole movement in a forward biased pn junction [[16]].

[pic]

Figure 3 Current-voltage curve for a typical semiconductor diode. Note: In the forward current direction there is a nominal voltage that must be supplied before current begins to flow [16].

resistance and thus the amount of current that can pass through the device. Figure 3 plots a current-voltage (I-V) curve for a representative semiconductor diode. When the device is biased in the forward direction there is a nominal applied voltage (V>0) before current flow begins to increase exponentially. At higher current values the resistance of the junction causes the current characteristics to become more linear. In the reverse direction, the current flow curve appears to be essentially flat until -8V which corresponds to the breakdown voltage where there is no longer any resistance in the device.

3 Diode Lasing

Figure 4a and b present a simplified schematic energy level diagram of a typical diode laser. Forward biasing of the diode device causes a valence electron to jump from the valence band to the conduction band thus creating a hole in the valence band. When the electron returns or falls back across the junction to the valence band, the electron and hole undergo radiative recombination which gives off energy in two forms: heat and the emission of light (h(). The wavelength of light emitted is determined by the energy difference between the energy levels of electrons and holes, which is essentially the band gap of the device. If enough current is applied to the device more electrons will be promoted to the conduction band than exist in the valence band creating a population inversion. Under a population inversion an emitted photon can stimulate the emission of a photon from an excited electron, and another photon may do the same as the process continues as a chain reaction or stimulated emission of radiation. The population inversion exists in a narrow region of the device called the active region or junction which lies between the polished end faces of the crystal. In the active region, the radiation is amplified by multiple reflections from the end faces of the crystal where if the frequency of

[pic]

Figure 4 Energy level diagram of (A) unbiased semiconductor diode and (B) with a forward voltage applied [9].

light falls within the bandwidth of one of the discrete optical frequencies, the growth of the wave continues. If the gain on the repeated passages is sufficient, an oscillating wave of light resonates along the junction. Within this optical resonator there will be several wavelengths which are standing waves and these are known as longitudinal or axial modes. Typical resonator lengths are ( 0.025 cm and emission linewidths are on the order of 10-4 cm-1.

Most infrared lead salt lasers consist of elements with two to six valence electrons from the Group IV B and VI compounds with common stoichiometric schemes of PbxSn1-xTe or PbxEu1-xSeyTe1-y. The emission wavelength of the laser depends on the stoichiometric composition of a diode where most lasers are constructed with a 15 cm-1 spectral window between 2.8 to 30 (m. Typically, mid-infrared diodes exhibit small band gaps ( 0.5 eV which require operation at cryogenic temperatures. That is, with such a small band gap slight changes in the junction temperature can significantly alter the output wavelength. Thus junction temperatures are sensitive, especially when the laser is operated at increased temperatures, to operating conditions that give off excess heat in the junction. For these reasons most mid-infrared semiconductor lasers are operated at liquid nitrogen temperatures or cooler to insure wavelength stability.

4 Summary

A brief overview of the instrumentation used in this dissertation has been provided in this chapter. The next chapter will provide the details of flame conditions and experimental arrangements used to implement the instrumentation reviewed here for measurements in Studies 1-3.

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[1]. Lengyel, B.A. in Introduction to Laser Physics, John Wiley and Sons, Inc., New York, 1966, Chapter 3, 136.

[2]. Butler, J.F., Calana, A.R., Phelan, R.J., Harmon, T.C., Strauss, Rediker, R.H., Appl. Phys.Lett., 5: 75 (1964).

[3]. Butler, J.F. and Calawa, A.R., J. Electrochem. Soc. 112 (1965)

[4]. Melngailis, I., J. Phys. Paris Colloq. C-4 (Suppl). 11 (1968).

[5]. Hinkley, E.D., Appl. Phys. Lett. 16: 351 (1970).

[6]. Ralston, R.W., Walpole, J.N., Calawa, A.R., Harman, T.C., and McVitte, J.P. J. Appl. Phys. 45: 1323 (1974).

[7]. Hinkley, E.D., Nill, K.W., and Blum, F.A, Infrared spectroscopy with Tunable Lasers, in Laser Spectroscopy (H. Walther, ed.), Springer-Varlag, Berlin, 1976.

[8]. Spectra-Physics Operation and Maintenance Manual, Laser Analytics Division, Bedford MA, 1986.

[9]. Demtroder, W., in Laser Spectroscopy: Basic Concepts and Instrumentation, Springer-Verlag, Berlin, 1981, Chapter 7.

[10]. Hanson, R.K., Falcone, P.A., and Kruger, C.H., Appl. Optics 16: 2045 (1977).

[11]. Hanson, R.K, SPIE Conference on Tunable diode laser absorption spectroscopy, lidar, and DIAL techniques for environmental and industrial measurements, 1983.

[12]. Hanson, R.K., Varghese, P.L., Schoenung, S.M., and Falocne, P.K., ACS Symposium Series No. 134, Laser Probes for Combustion Chemistry, 1980, p.227.

[13]. Varghese, P.L. and Hanson, R.K., J. Quat. Spectrosc. Radiat. Transfer, 24: 479-489 (1980).

[14]. Schoenung, S.M. and Hanson, R.K., Appl. Optics 21: 1767 (1981).

[15]. Hanson, R.K., Falcone, P.A., and Kruger, C.H., Combust. Sci. and Technol. 35:81-99 (1983).

[16]. Strobel, H.A. and Heineman, W.R., in Chemical Instrumentation: A Systematic Approach Third Edition John Wiley and Sons, New York, 1989, Chapter 2 p 49.

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