Anal. Chem. 1981, 53, 1129-1130
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Performance of an Emissionless Infrared Diffuse Reflectance Spectrometer Sir: It has been said that the intensity of diffusely reflected IR radiation from powder samples is too low to measure spectra, and infrared diffuse reflectance spectrometry has been largely ignored for that reason (1). We have constructed an emissionless infrared diffuse reflectance spectrometer (EDR) to make in situ determinations of reacting species on catalyst surfaces at elevated temperatures (2). We used the instrument to follow the reaction of adsorbed species (3). In this paper we compare the efficiency of the EDR with a conventional diffuse reflectance spectrometer (DR).
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EXPERIMENTAL SECTION The details of the EDR system have been described in a previous paper (2). Since si chopper was installed between a light source and a cell (prepositioned chopper), the spectra can be measured without interference from the emission of samples. Two elliptical mirrors are used to focus the IR radiation from a light source onto a powder sample with high efficiency. Conventional DR spectra were recorded by use of a Japan Spectroscopic Co. infrared spectrometer (IR-G)equipped with a diffuse reflectance attachment (DR-3). The sample holder of the DR-3 is the same as that of EDR. Thus, the difference between DR and EDR spectra can be considered as representing the difference in the optical unit. The optical unit of DR-3 was schematically shown elsewhere (4). In the IR-G spectrometer, as the IR beam is chopped after being reflected by the sample, the interference from the emission is not cancellled. The DR-3 includes only one mirror to focus the IR radiation from the light source onto the sample. The mirror has the same geometry as those used in the EDR.
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Figure 1. Diffuse reflectance spectra of KBr (A and B) and CaC03 mixed with KBr (CaCO,/KBr = 1/4 by weight: C and D) measured with EDR (a)and DR-3 (b). Sample temperature was 90 O C . The scales of reflectance of curves C and D were expanded by factors of 1.7 and 4, respectively. 1001
RESULT8 AND DISCUSSION Curves A and B of Figure 1show the spectra of KBr powders by the EDR and DR, respectively. The difference in the signal level arises from the difference in the optical unit, because the sample hollder and the light source are identical. The signal level in the E:DR spectrum represents the intensity of the reflected beam, while that in the DR spectrum represents the sum of the intensities of the reflected beam and emitted radiation. The signal level in the DR is about 25%, while that in the EDR is about 75%. Thus, the efficiency, i.e., the intensity of reflected beam, is, at least, 3 times as high as that of the DR. The spectra of KBr by the EDR shown in Figure l a were measured without any expansion of scale of reflectance. This fact indicates that the efficiency of EDR is sufficiently high to measure IR spectra of powder samples. The prepositioned chopper improved the intensity of the absorption peaks. Curvies C and D of Figure 1are the spectra of CaC03 mixed with KBr obtained by measuring with the EDR and DR, respectively. The absorption bands at 873 and 711 cm-’ were ascribalble to characteristic absorptions of carbonate ion (5). The rielative intensities of absorption peaks, defined as 1- I/Io, where I and Io are the reflectance at the peak minimum and the background, respectively, in the EDR spectrum were 0.64 and 0.65 at 873 and 711 cm-l, respectively, while those in the DR apectrum were 0.34 and 0.23. Thus, the relative intensities of absorption peaks in the EDR were 2 or 3 times as large as those in the DR spectrum. As described in the previous paper (2), the intensities of emission bands were as large as or even higher than those of absorption bands in the DR spectra. Since the absorption intensity in the DR can be considered as the difference in the intensity between the true absorption and the emission, the intensity of the true absorption should be, at least, twice as large as the intensity of the apparent absorption. This results in the large relative intensities of EDR absorption bands which represent 0003-2700/81/0353-1129$01.25/0
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Figure 2. Diffuse reflectance spectra of colored sample measured with EDR (B, B’, B”, and C) and with DR-3 (A and A’). Sample: A, A’, B, B’, and B“, CaCO, mixed with active carbon and KBr (4:1:16 by weight): C, CaC03 mixed with KBr (1:4 by weight). Sample temperature was 100 O C . The scale of reflectance of curve A was expanded by a factor of 4. Arrows Indicate additional expansions of
the reflectance scale.
the true absorption, though the difference in the collecting efficiency also would improve the intensity of absorption band. Spectra of CaC03mixed with active carbon and KBr shown in Figure 2 indicate that the EDR is effective for the measurement of IR spectra of colored samples, too. The reflectance in the DR spectrum shown by curve A in Figure 2 increased as the wavenumber decreased. The signal level at low wavenumber was higher than that in the spectrum of KBr shown by curve B in Figure 1. These results indicate that the high signal level shown by curve A in Figure 2 is not due to the reflectance but due to the emission. With decreasing wavenumber, this emission is more intense, because the intensity of emission is proportional to the number of molecules in the excited state, and the number increases, as expected from the Boltzmann distribution, with decreasing wavenumber which is proportional to the energy difference between the excited and the ground states. At high wavenumber, a small absorption band was observed in the DR spectrum after the 0 1981 American Chemical Society
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Anal. Chem. 1981, 53, 1130-1132
further scale expansion of reflectance as shown by curve A' in Figure 2. But, at low wavenumber, the large emission from the active carbon probably obstructs the observation of the absorption bands. On the other hand, absorption bands can be observed in the EDR spectrum shown by curve B in Figure 2, though the relative intensities of absorption peaks are much less than those observed with curve C in Figure 2 where active carbon is not included. According to the Kubelka-Munk theory (6),the reflectance of a powder sample is a function of the absorption constant ( K ) and of the scattering constant (8)of the powder sample. Thus
IR spectra of colored samples.
LITERATURE CITED (1) Fuller, M. P.; Grlffiths, P. R. Anal. Chem. 1078. 50, 1906. (2) Nlwa, M.; Mttorl, T.; Takahashi, M.; Shirai, K.; Watanabe, M.; Murakami, Y. Anal. Chem. 1070, 51, 46. (3) Hattori, T.; Shlral, K.; Nlwa, M.; Murakami, Y. React. Klnet, Catel. Lett. 1080, 15, 193. (4) Takezawa, N. Chem. Commun. 1071, 1451. (5) Mlller, F. A.; Carlson, Q. L.; Bentiey, F. F.; Jones, W. H. Spectrochlm. Acta 1080, 16, 135. (6) Hecht, H. G. J. Res. Natl. Bur. Stand. Sect. A 1976, 80A, 567.
Tadashi Hattori* Kenji Shirai Miki Niwa Yuichi Murakami
(1- rl2 - K --
(1) 2r S where 1- r is, at the first approximation,equal to the intensity of absorption peak, 1- I/&,. The low intensity of absorption peak, i.e., large r, would be explained by the large scattering constant of a sample including active carbon. After the further scale expansion of reflectance, the absorption peaks become obvious, as shown by curve B' and B" in Figure 2, indicating that the EDR spectrometer is useful for the measurement of
Department of Synthetic Chemistry Faculty of Engineering Nagoya University Chikusa, Nagoya 464, Japan RECEIVED for review March 25, 1980. Accepted March 13, 1981.
AIDS FOR ANALYTICAL CHEMISTS Emltter Current Controller for Field Desorption Mass Spectrometry David L. Smith and James A. McCioskey" Departments of Medicinal Chemlstry and Biochemistry, University of Utah, Salt Lake Cify, Utah 84 112
Jerry K. Mitchell Electronics Laboratory, Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
Field desorption mass spectrometry (FDMS) is experiencing increased use for analysis because of the small quantities of material that are required and because of ita applicability to substances of low vapor pressure or thermal instability. Theory and applications of FDMS have recently been reviewed by Schulten (1). Ions produced by field desorption are often of low abundance, and characteristically persiet for only a short time once a critical emitter temperature has been reached. The selection or manipulation of emitter current during acquisition of the resulting mass spectra is therefore critically important from the standpoint of experimental technique. Several electronic devices for controllingthe current through the emitter wire linearly with time, without feedback control, have been described (2-5). Schulten et al. (6,7)have reported results from a unit which uses feedback control to maintain a constant ion current from the cathode. More recently, Bursey and co-workers described a device based on measurement of resistivity of the emitter wire which permits control of the average temperature across the wire, so that the role of temperature in relation to current can be studied by using different instruments which may employ wires of different lengths and diameters (8). We presently describe a unit which may be used in either the linear program mode or total ion current (TIC) feedback control mode to automatically regulate the emitter current. In addition, the ion signal at any mass value may be substituted for the TIC. Since this device is capable of electrical 0003-2700/81/0353-1130$01.25/0
isolation up to 15 kV, it may be used on all commercially available mass spectrometers.
EXPERIMENTAL SECTION Circuit Description. A block diagram illustrating the functional components of the unit is given in Figure 1. In the feedback mode of operation, an ion current signal (TIC, @-slit collector, or electron multiplier collector) is amplified by an electrometer in the mass spectrometer and is presented to the emitter current controller. In the case of Varian MAT instruments, the 10-mV recorder output may be used as a source for feedback control. This signal is further amplified by the instrumentation amplifier (see Figure 1)AD522 which is operated at a gain of 1000. The difference between the feedback signal and a 5-V reference voltage is used to drive the emitter current up or down as required to keep the difference voltage at a minimum. The absolute value of the difference voltage is amplified by the error amplifier while the polarity of the difference voltage is sensed by the error sign sensor. The amplified error signal determines how fast the voltage-controlledoscillator provides stepping pulses to the up-down counter which ultimately determines how fast the emitter current changes. The error polarity sewor tells the up-down counter in which direction to step, Le., to increase or decrease the emitter current. The parallel digital signal from the up-down counter is changed to an analog signal and is then converted to a serial digital signal by the voltageto-frequency converter. A 15-kVoptical isolator is used to couple this signal to the remainder of the circuit which is typically at 8 kV in our instrument. In the high-voltage section the signal is converted to an analog signal by the frequency-to-voltage converter and is now used to control a current-programmabledc 0 1981 American Chemical Society
