Derivative Spectrophotometry: Application to Trace Sulfur Dioxide Analysis Jerzy W. Strojek,’ Dennis Yates, and Theodore Kuwana Department of Chemistry, Ohio State University, Columbus, OH 43210
A laboratory rapid scanning spectrophotometer employing an oscillating mirror for wavelength scanning was modlfled to provide wavelength modulation for derivative spectrophotometry. The modulation resulted from the superlmposltlon of a small slnusoldal waveform on the main driving trlangular waveform for the oscillating mirror. Also, the results of wavelength modulation and correlation spectrophotometry uslng a new optical deslgn based on a Czerny-Turner monochromator system and applied to the analysls of low-levels of sulfur dloxlde wlll be presented.
Stimulated by the need for spectral monitoring and characterizing of transient intermediates produced during electrolyses a t optically transparent electrodes, a rapid scanning spectrophotometer (RSS) was designed and constructed in 1968 ( I ) . In this RSS, the scanning was made possible by an oscillating mirror in a galvanometer suspension which then allowed the dispersed spectrum from the grating to be swept across the exit slit. By driving the galvanometer with a repetitive triangular waveform, the swept spectra were, in principle, linear in wavelength with time. A width of up to ca. 250-300 nm could be swept per spectrum. This method of scanning can be easily modified to provide a simple and convenient way of modulating t h e wavelength for derivative spectrophotometry. It is only necessary to superimpose a small sinusoidal waveform on the main triangular waveform. In this paper, the results of wavelength modulation using a modified RSS and of a new optical design based on a Czerny-Turner monochromator will be discussed. This latter design has been applied to the analysis of low levels of SO2 which is of interest as an environmental pollutant ( 2 , 3 ) . The usual mode of modulation is amplitude modulation where the intensity of the light beam is mechanically “chopped” or the source itself is modulated through the power supply (4).The dc offset and drift can thus be eliminated. Amplitude and wavelength modulation has been used in atomic absorption spectrometry (5). Bonfiglioli and coworkers (6, 7) have described a self-modulating derivative spectrometer in which a vibrating mirror in front of the exit slit provided wavelength modulation. Hager (8, 9) recently described an instrument where the entrance slit was moved for modulation (marketed by Lear Siegler). This latter instrument has been applied to SO2 analysis with sensitivities reported in the 25-ppb range. In most of the wavelength modulation instruments, the frequency of the modulation has been limited to less than 100 Hz. The principles and advantages of derivative spectrophotometry have been discussed by Bonfiglioli and coworkers (6, 7) and by Hager (8-10). Essentially, the advantages are the improvement of the detectability of overlapping spectral bands a n d of the signal-to-noise ratio by elimination of the low frequency flicker noise (commonly called l / fnoise), a n d interference noise such as 60-Hz noise from the power Permanent address, Gliwice Technological University, 44100 Gliwice, Poland. 1050
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
lines. The l / f noises present in the spectrometer system, particularly those associated with the light source, the vibration of optical components, and those from electronic components can be effectively reduced because of the higher modulation frequency coupled with band-pass filtration. Thus, those noises occurring at a different frequency than the modulation frequency will not appear through the phase rectified lock-in amplifier. Signal-to-noise enhancement due to modulation and correlation has been discussed by Hieftje (11,12). For 1st and 2nd derivative wavelength modulation, the change in the optical intensity, dP, as a function of change in wavelength, dh, is given by ( 1 0 )
dk dP dX = P C l -dX
d2P d2k a n dX d 7 = lPCl;i;\r
(1)
where is the initial radiant power of light, C is the concentration in atm. and E is the path length in cm, and k is the coefficient of absorption in atm-l cm-l. Thus, dP/dX and d2P/dX2are proportional to the concentration. In the application of the galvanometer modulation to the analysis of trace amounts of SOz, the position of the galvanometer was moved and placed before the entrance slit of a Czerny-Turner monochromator. The light throughput and resolution were increased over the previous RSS while the total scan width was decreased. The reason for the change will be understandable by examination of the basis for the SO2 analysis. The sulfur dioxide spectrum in the wavelength range of 285 to 305 nm is characterized by a series of well-defined vibrational bands on the electronic absorption spectrum (See Figure 1).These bands are separated uniformly by about 20 A. Considering the difference between the maximum and minimum of one of the larger bands (e.g., band No. 5 at A,, 300 nm in Figure I), the Ac is equal to about 10 atm-l cm-’. Assuming a 1-meter path length cell and 1 ppm SO2 concentration, the AA is equal to ca. absorbance unit. For most commercially available scanning spectrophotometers, this LA is near the sensitivity limit. T o increase sensitivity and selectiuity of the instrument to SOz, several of the bands can be monitored simultaneously using multiple slits separated by 20 8, at the exit of the monochromator. Barringer (13) in his correlation spectrometer vibrated the multiple slits and processed the ac output from the photomultiplier tube. In the present galvanometer modulation, it is possible to combine the advantages of the Barringer system and derivative spectrophotometry by employing a wide amplitude, low frequency ( f l ) to sweep across the desired number of bands, and by superimposing a small amplitude, higher frequency (e.g., f z = 1000 Hz) for the derivative modulation. The selection of the wavelength width for the derivative modulation is dependent on the resolution of the optical system. For the SO2 spectrum where the wavelength maxima of absorbance of the vibrational bands are separated by ca. 20 A, the width is a compromise between the magnitude of AP/AX when LA is too small, and the broadening of the derivative peaks when AA is too large. Adjustments of the
Figure 2. Optical layout for Scheme II spectrophotometer
Figure 1. Absorbance spectrum
of sulfur dioxide
X = Xenon lamp. 35-75 watts, PEK lamps, Sunneyvale, Calif. (powered by PEK Type 401 A Power Supply). MI = Focusing mirror. SI and S p = Slits, Gaertner, Chicago, IL. Mp = Concave mirror, focal length 10 cm, dla. 5 cm. G, = Galvanometer, Type 0606, General Scanning Corp., Danbury, CT (modified with special UV coated mirror, dia. 5 mm). M3 and M4 = Concave mlrror, focal length 60 cm, dia. 7.5 cm. C, = Cell 1, 88-cm path length, 2.54-cm dia. quartz tube. C p = Ceii 2, 1-cm length, 2.54-cm dia. quartz tube. SP, and SP2 = Beam splitters, thin Pt film on quartz. M5 = Concave mirror, focal length 30 cm, dla. 5 cm. MF = Front surfaced mirror, special UV coating. PMT = Photomultiplier tube, EM1 type 9526BQ
The inset shows the spectrum from 270 and 320 nm taken on a Cary 15 Spectrophotometer.An expanded spectrum Is shown for absorbance bands 1-10 between about 292 and 310 nm
width of AA can be made experimentally. The resolution of the present Czerny-Turner optical system (Figure 2), depending on the slit width, could be adjusted to the order of 0.2 A. For use with a single slit, it is convenient to scan through five of the most similarly shaped and sized bands (e.g., peaks 3-7, Figure 1).However, due to the slight differences in the band shapes a t high resolution and the fluctuation of the midpoint from band to band, rectification of the in-phase modulated components was difficult. To circumvent this, a second lock-in amplifier was employed to process the derivative signal from the first lock-in amplifier. The reference signal for the 2nd lock-in was provided by the output of a 3rd photomultiplier tube that was monitoring light from a 2nd cell containing a high concentration of SOP. By this means, the output of the 1st lock-in could be rectified and correlated exactly according to the SO2 band spectrum.
EXPERIMENTAL Optics a n d Operations. In our previously published optical design (Figure l, Ref. I ) , lens L1 and LZ were replaced by concave mirrors and the optical path was then folded (RSS available from Harrick Scientific Corp., Ossining, NY); henceforth, this optical design will be referred to as Scheme I). The oscillating mirror in galvanometer suspension allowed the incident angle on the grating to change and consequently, the dispersed spectrum from the grating (300 lines/mm) was swept across the exit slit. In Scheme I1 (Figure 2), the focal plane, F P , was normally the location of the entrance slit to a Czerny-Turner type monochromator. Instead, the galvanometer, G,, reflected the incoming light from slit, SI, to a concave mirror, Mp, and focused the image of SI a t FP. The distances Ds,-G, and DG,M*, were the same and were equal to the focal length of Mz. The distance, DM?-FP.was twice the focal length of Mz. The image of S1 thus appeared a t different positions, for example, positions 1 and 2, along the focal plane, FP, DM~-G", as the G, mirror was rotated. The distances, DFP-M~, DG,M~, and D M ~ - S *were , the same and were equal to the focal lengths of M3 and Mq. In this optical design, the mirror of G, was focused on the front plane surface of the grating, G, (1200 lines/ mm). The image of the G, mirror (diam. 5 mm) was linearly magnified by the ratio of the focal lengths of M3 to MP, that is, by a factor of 6 or the area by 36 times. The diameter of the light beam on the surface of G, was, therefore, 30 mm. The slit image of SI which appeared a t focal plane, FP, remained as a 1:l image a t exit slit Sp. The throughput light intensity and optical resolution were greatly enhanced over Scheme I. With rotation of G, mirror, the light beam was angularly displaced along M2 and M3 which resulted in changing the incident angle of the light beam on the grating, G,. The dispersed spectrum
Flgure 3. Block diagram of electronic circuitry
Components: HV = High voltage power supply. Fluke Model 4128 (John Fluke Mfg. Co.. Seattle, WA). Log = Logarithmic amplifier,Phllbrlck-Teledyne Model 4361, bipolar (Phllbrlck-Teledyne, Waltham, MA). Lock-In I = Princeton Applied Research Model 126 with Model 116 preamplifier (Princeton, NJ). E = Princeton Applied Research Corp.. Model TDH-9 Waveform Eductor (Princeton,NJ). Lock-in II = (See Figure 4). PMT = EM1 9526BQ. G, = (See Figure 2) In Scheme I, G, was a Bell and Howell Galvanometer (Bell and Howell, Pasadena, CA). GI= Wave generator, Hewlett-Packard Model 3300A was thus swept across the exit slit, Sp, in focus by Mq. The monochromatic light from SZwas divided by beam splitt,ers, SP1 and SPp, into three separate beams, two of them for sample and reference cells, C1 and Cp, respectively. The other served as a beam for reference output from PMT2. Cell Cp was filled with 80 Torr of SOz. The output of PMT3 then served as reference input signal for lock-in amplifier If, (See electronics section for details.) T o simulate Barringer's correlation experiments, Sz was replaced by a mask containing 5 exit slits. This mask was made from a thin bronze plate with the 5 parallel slits of 0.2 mm width separated each by a distance of 1.52 mm. The width and separations were calculated from the spectral lines of an Argon laser, and SO2 spectra which were taken on Polaroid films located a t the focal plane of SZ. Sulfur dioxide was obtained in two stainless steel tanks from Air Products and Chemicals (Allentown, PA.) with analysis of 5 and 15 ppm, respectively. These were diluted accurately with nitrogen for filling cell, C1, which had a path length of 88 cm. With a flow rate of 10 l./min, C1 could be filled or flushed with Np in about 20 seconds. Electronics. The basic electronics were common to operation of both derivative spectrophotometers (Scheme I and 11). For wavelength modulation, a small amplitude sinusoidal waveform of frequency, f p , was superimposed on the main ramp or triangle waveform of frequency, f l ( f 1
