Anal. Chem. 1998, 70, 4907-4914
Articles
Development of a Digital Micromirror Spectrometer for Analytical Atomic Spectrometry James D. Batchelor and Bradley T. Jones*
Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109
A digital micromirror device (DMD) has been incorporated into a novel spectrometer for use in analytical atomic spectrometry. The device can be taken from a commercial computer projector. A protective glass window covering the DMD chip limits the viewable wavelengths to the visible range. The DMD is used to project an image of the light source onto the exit plane of a flat-field spectrograph. A single photomultiplier tube is used for detection. The high switching rate of the micromirrors (15 µs) enables rapid full-spectrum capture, wavelengthmodulation, source-modulation, fast narrow-wavelength window scans, and rapid-wavelength “jumping.” Calcium, sodium, and potassium have been determined in several standard reference materials (tomato leaves, bovine liver, rice flour, total diet) by flame atomic absorption and emission spectrometry. Absorption sensitivities for each element are near the 0.02 µg/mL level, and detection limits for both absorption and emission are near the 0.01 µg/mL level. Elemental recoveries were within 10% of certified values for most reference materials. The digital micromirror device (DMD) has been produced in a variety of configurations for specific applications, such as jointtransform correlation systems and optical neural networks.1-3 Most recently, these devices have become commercially available as portable computer projectors, sold under the name Digital Light Projector. The DMD is a two-dimensional array of aluminum micromirrors (pixels) which are 16 × 16 µm placed on 17-µm centers (Figure 1). The DMD is available with a variety of aspect ratios: VGA (640 × 480), SVGA (800 × 600), and SXVGA (1280 × 1080). The fill ratio for the device is ∼90%, so it has found use in many high-brightness applications. Upon receiving the appropriate control signal, each mirror is capable of flexing either +10 or -10° away from normal. In this manner, the photon flux striking each mirror may be reflected toward the optical path or away from it, so each mirror acts as an optical light switch. A single mirror can be switched “on” and “off” independently of the others during a time period as small as 15 µs. Each DMD pixel is a microelectromechanical system (MEMS) fabricated on a CMOS substrate (Figure 2). The mirror is rigidly (1) Florence, J. M. Opt. Lett. 1989, 14, 341-343. (2) Cohn, R. W.; Sampsell, J. B. Appl. Opt. 1988, 27, 937-940. (3) Collins, D. R.; Sampsell, J. B.; Hornbeck, L. J.; Florence, J. M.; Penz, P. A.; Gately, M. T. Appl. Opt. 1989, 28, 4900-4907. 10.1021/ac980597p CCC: $15.00 Published on Web 10/24/1998
© 1998 American Chemical Society
Figure 1. SEM photomicrograph of nine micromirrors. The center mirror was removed to show the underlying components (courtesy of Texas Instruments).
connected at the center to a yoke that is in turn connected to two support posts via two torsion hinges. When the memory cell for a given pixel is in the on state, electrostatic fields develop between the mirror and its address electrode, and the yoke and its address electrode. This produces an electrostatic torque, and the mirror rotates in the positive direction until the landing tip strikes its landing site. Since geometry determines the rotation angle, it is precisely determined (+10°). An electron micrograph showing a diagonal cutaway view of a single DMD pixel is given in Figure 3. The torsion hinge is shown in orange. McDonald and Yoder give a detailed technical explanation of the DMD.4 Many modern spectrometers use solid-state detectors, such as the photodiode array (PDA) and the charge-coupled device (4) McDonald, T. G.; Yoder, L. A. Laser Focus World, 1997, (Aug), S5-S8.
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Figure 2. Exploded view of an individual micromirror and underlying components. The mirror is tilted 10° from horizontal (courtesy of Texas Instruments).
(CCD).5-7 These devices allow full-spectrum capture and have specifications (dark current, quantum efficiency, read-out noise) comparable to or better than the photomultiplier tube (PMT).5 Solid-state detectors, however, may have read times on the order of milliseconds, making applications involving rapid source or wavelength modulation difficult. Modulation techniques are often desirable in atomic spectroscopy for the correction of background absorption or emission associated with the atomizer.8-14 Wavelength modulation is often accomplished with a refractor plate placed inside a commercial spectrometer. Typically the plate is oscillated with a torque motor. This changes the angle of incident light striking the grating, thus changing the wavelength of light striking the detector (PMT). Background correction is therefore performed in atomic emission spectrometry by measuring the analyte peak apex and subtracting, via lock-in amplifier, the signal measured at the base of the peak.8,9 Wavelength modulation has been employed regularly in atomic absorption spectrometry (AAS), especially in continuum source AAS.10-14 In this case, the refractor plate rapidly moves the wavelength in and out of the atomic absorption profile. Only background absorption is measured outside of the profile, since this wavelength is insensitive to analyte absorption. On the other hand, single-detector spectrometers employing the PMT, while capable of detecting modulated signals, require (5) Epperson, P. M.; Sweedler, J. V.; Bilhorn, R. B.; Sims, G. R.; Denton, M. B. Anal. Chem. 1988, 60, 327A-335A. (6) Harnly, J. M.; Fields, R. E. Appl. Spectrosc. 1997, 51, 334A-351A. (7) Becker-Ross, H.; Florek, S. V. Spectrochim. Acta 1997, 52B, 1367-1375. (8) Koirtyohann, S. R.; Glass, E. D.; Yates, D. A.; Hinderberger, E. J.; Lichte, F. E. Anal. Chem. 1977, 49, 1121-1126. (9) Bezur, L.; Marshall, J.; Ottaway, J. M. Spectrochim. Acta 1984, 39B, 787805. (10) Harnly, J. M.; O’Haver, T. C. Anal. Chem. 1977, 49, 2187-2193. (11) Zander, A. T.; O’Haver, T. C.; Keliher, P. N. Anal. Chem. 1976, 48, 11661175. (12) Zander, A. T.; O’Haver, T. C.; Keliher, P. N. Anal. Chem. 1977, 49, 665666. (13) Harnly, J. M.; O’Haver, T. C.; Golden, B.; Wolf, W. R. Anal. Chem. 1979, 51, 2007-2014. (14) Elser, R. C.; Winefordner, J. D. Anal. Chem. 1972, 44, 698-709.
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time-consuming sequential-scanning procedures in order to monitor even small wavelength ranges. The grating and grating mounting hardware often are too massive to move at rates needed to monitor large spectral windows in time periods comparable to those typical with solid-state detectors. One alternative is to employ multiple PMTs with a single spectrometer, usually following the Rowland Circle or two-dimensional echelle spectrograph design.5,15-16 These spectrometers take advantage of the sensitivity of the PMT and the speed of “scanless” CCD systems, but they do so at the increased cost and complexity associated with multiple detectors requiring separate signal-processing electronics. Spudich et al. have used an acoustooptic deflector (AOD) to achieve precise wavelength modulation with a single detector using a Rowland Circle spectrometer.17 Varying the radio frequency applied to the acoustooptic device varies the wavelength of light striking the detector (PMT). The system was only usable in the visible region of the spectrum because of the dense flint glass AOD material, but quartz and magnesium fluoride could produce wider spectral ranges if they prove to be suitable AOD materials for this application. According to the authors, possible applications of this device include “spectral profiling, spectral interference characterization, reduction of flicker noise, and derivative spectrometry”.17 Wagner et al. constructed a spectrometer having a DMD at the focal plane of a flat-field spectrograph.18 The DMD consisted of only two rows of micromirrors. The radiant flux exiting the spectrograph fell upon the DMD. If a pixel mirror was switched “on,” then the light falling upon it was directed to a PMT. Wavelength scanning was achieved by sequentially turning on adjacent micromirrors and progressing across the DMD in a linear fashion. The spectrograph was configured to cover a 360-nm range (limited by the size of the DMD and the groove density of the grating), and it was used to scan the absorption spectrum of K2CrO4 and KMnO4. The DMD driving routine limited scan times to 4-5 min or longer. The arrangement of micromirrors also limited the spectral band-pass of the system to ∼1 nm. A vertical line of two mirrors could not be addressed with this system because the mirrors were placed at diagonals to each other instead of in vertical rows. The DMD used in the current experiment was taken directly from a commercial digital light projector, and it has a micromirror array that is 640 × 480 (VGA). This allows the DMD to be accessed as if it were the computer’s monitor, making programming quick and easy (using simple draw or paint commands). It also has micromirrors arranged in rows both vertically and horizontally, so a rectangular “slit” of adjustable size can be obtained by switching on several adjacent micromirrors, or pixels, simultaneously. By placing an image of the DMD at the focal plane of a flat-field spectrograph, the spectral band-pass of the system can be adjusted by varying the width of the slit. The optical throughput can be increased by increasing the height of (15) Lajunen, L. H. J. Spectrochemical Analysis by Atomic Absorption and Emission; The Royal Society of Chemistry: Cambridge, 1992; Chapter 5. (16) Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis; Prentice Hall: Englewood Cliffs, NJ, 1988; Chapter 8. (17) Spudich, T. M.; Pelz, B. A.; Carnahan, J. W. Appl. Spectrosc. 1997, 51, 765769. (18) Wagner, E. P.; Smith, B. W.; Madden, S.; Winefordner, J. D.; Mignardi, M Appl. Spectrosc. 1995, 49, 1715-1719.
Figure 3. Electron microscope closeup of a dissected micromirror. The torsion hinge can be seen in orange. The yoke is highlighted in blue (courtesy of Texas Instruments).
the slit. The detection wavelength can be adjusted by changing the position of the slit. Conventional scanning can be obtained by progressively moving the slit from one end of the DMD to the other. Source modulation can be obtained by turning the slit on and off in rapid succession. And wavelength modulation can be obtained by changing the position of the slit repetitively in rapid succession. Such a device will find many applications in various fields of spectroscopy. Below, the performance of the DMD spectrometer in the area of analytical atomic spectroscopy is evaluated. EXPERIMENTAL SECTION DMD Spectrometer. Figure 4 is a schematic diagram of the analytical atomic DMD spectrometer. The DMD chip and control board were removed from a commercial computer projector (Texas Instruments model DLP) and mounted on a home-built platform which allowed subsequent mounting to standard optical positioning equipment. For atomic absorption measurements, the emission from a hollow cathode lamp was collected with a 50mm focal length lens and brought to a one-to-one image over a Perkin-Elmer premixed burner head assembly. The unabsorbed radiation exiting the opposite end of the flame was collected with a second 50-mm focal length lens and brought to an image at the entrance aperture (7-mm diameter) of a laboratory-constructed light-tight enclosure. For atomic emission measurements, this same lens was used to form an image of the flame on the entrance aperture. Inside the enclosure, a third lens (50-mm focal length) collimated the light and directed it to a flat aluminum mirror. This mirror directed the collimated light toward the DMD at an angle such that any DMD mirrors that were switched on would project
Figure 4. Block diagram of DMD spectrometer.
an image onto the focal plane of a flat-field spectrograph (TJA Mono-Spec18, 1200 grooves/mm grating blazed at 500 nm, 4.5 nm/mm reciprocal linear dispersion, 50-µm exit slit). The offposition mirrors directed light away from the spectrograph. The image generated by the on mirrors on the DMD was magnified by a factor of 2.2 using a 150-mm focal length achromatic lens. This allowed the DMD image (which is originally 11 mm in width) to fill the focal plane of the spectrograph (25 mm). An individual micromirror would thus generate an image that is 35 µm square. Analytical Chemistry, Vol. 70, No. 23, December 1, 1998
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Figure 5. Block diagram demonstrating the relationship between a computer image, a DMD image, and the image projected onto the exit plane of the monochromator. Arrows indicate direction of movement. Figure 7. Scan of a sodium, neon-filled, hollow cathode lamp. Two neon lines and the sodium doublet are labeled. Spectral band-pass is 0.30 nm.
Figure 6. Block diagram of all equipment used in experiment.
A vertical DMD line that is 100 pixels in length by 1 pixel in width generates a vertical slit at the focal plane that is 3.7 mm high by 35 µm wide (the height dimension includes the 1-µm spacing between micromirrors). As mentioned above, the position of the line on the DMD corresponds to a position at the focal plane and thus to a specific wavelength that falls upon the photomultiplier tube (Hamamatsu R955). DMD Control. The DMD acts as a second computer monitor, so any image that is sent to the video screen is also sent to the DMD (Figure 5). This makes control of the DMD simple; shapes can be drawn with QuickBasic programming. Vertical slit lines can be drawn, moved, and altered with a LINE command. The width, height, and position of the slit can be changed by varying the parameters of the LINE command [Line (vertical position, horizontal position) - (vertical position, horizontal position), color code)]. Also, the lines can be turned on and off at a fixed rate (for source modulation) or moved across the monitor at a fixed rate (wavelength scanning) using a Do loop and a counter. A digital I/O card (Computer Boards DIO-24) was programmed to send either a reference signal to a lock-in amplifier (Stanford Research SR-510) for absorption measurements or a trigger pulse to an A/D converter (Sable Systems Datacan V) to signal the beginning of an emission scan (Figure 6). These signals were generated by the same QuickBasic program driving the DMD using an Out I/O BaseAddress, 0-255 command. Emission scans were recorded by moving a vertical DMD slit at a rate of 0.5 s/pixel across the DMD in the horizontal direction (Figure 7). The PMT was operated with a 0.25-s time constant. Two data points were collected for each vertical line position. Modulation was performed in two ways: by flashing a single vertical line on and off (source modulation) or by alternating two 4910 Analytical Chemistry, Vol. 70, No. 23, December 1, 1998
Figure 8. Effect of “slit” height on the spectral band-pass of the chromium 425-, 427-, and 429-nm emission lines. “Slit” is 480, 200, 100, and 50 pixels in height.
different vertical lines on and off (wavelength modulation). Modulation was performed for both absorption and emission measurements. Instrumental Parameters. A Perkin-Elmer premixed burner with a 4-in., three-slot burner head was used for atomic absorption measurements. Nebulizer, fuel, and oxidant flow rates were optimized for each element to give the highest sensitivity. The appropriate matrix modifier was used (for releasing or ion suppression) as described in the Perkin-Elmer flame manual. Lanthanum (0.2% La as La(NO3)3‚6H2O) was used as a releasing agent for calcium determinations. Sodium (0.2% Na as NaCl) was added as an ion suppressor to each solution for determinations of potassium. Potassium (0.2% K as KCl) was added to each solution for determinations of sodium. Calcium, sodium, and potassium were also determined by flame emission spectrometry. Calcium required a nitrous oxide/ acetylene flame (2-in., single-slot burner head) and lanthanum (0.2% La) as a releasing agent. Sodium and potassium were determined with an air/acetylene flame and a round capillarytype burner head (1-cm-diameter with 35- × 1-mm-i.d. capillaries).19 Potassium required sodium (0.2% Na) for ion suppression. Sodium did not require matrix modification for atomic emission measurements. (19) Fraser, L. M.; Winefordner, J. D. Anal. Chem. 1972, 44, 1444-1451.
Figure 9. Changes observed in the chromium triplet. Wavelength (nm) at pixel 320 is above each triplet.
Figure 10. S/N relationship with modulation frequency.
Calcium, sodium, and potassium reference standards were made from serial dilutions of 1000 ppm stock solutions (Spex). Distilled/deionized water was used for all dilutions. The matrix modifiers were prepared at an initial concentration of 2% (w/v) of the appropriate element (La, Na, K) in 1-L flasks from the salt:
La(NO3)3‚6H2O, GFS Chemicals; NaCl, Fisher Scientific; KCl, J. T. Baker. Sample Preparation. Four NIST standard reference materials were used to evaluate the performance of the instrument. These were Rice Flour (SRM 1568a), Tomato Leaves (SRM 1573a), Bovine Liver (SRM 1577b), and Total Diet (SRM 1548). For the elements present at weight percent levels, 0.3 g of sample was digested in nitric acid. For the elements present at the microgram per gram level, 1 g of sample was digested. In all cases, the samples were placed in Teflon microwave bombs (CEM, Inc.) with 10 mL of Trace Metal Grade HNO3 (Fisher). The bombs were sealed and placed in the microwave oven (CEM 81D, 600 W). The heating program was as follows: step 1, 5% power/ bomb for 20 min; step 2, 10% power/bomb for 10 min. These vessels self-vent when they reach a pressure of 830 kPa (120 psi). The vessels were allowed to cool, were opened, and then were placed back in the microwave. The open vessels were then heated with 5% power at 20-min cycles until near dryness (to remove most of the acid). The remaining solutions were quantitatively transferred to 100-mL volumetric flasks and diluted with distilled/ deionized water. These solutions were then diluted and modified (ion suppressor or releasing agent) as needed. RESULTS AND DISCUSSION Spectrometer Performance. Spectrometer characteristics were determined with a hollow cathode lamp (HCL) as a line source. The emission spectra for several HCLs were recorded by scanning a vertical slit (16 × 1700 µm, 1 × 100 pixels) across the DMD (Figure 7). The spectral band-pass was calculated as the full width at half-maximum (fwhm) for an HCL emission line. The measured spectral band-pass for the 585.25-nm Ne line emitted by the Na HCL was 0.30 nm. This compares well with the theoretical spectral band-pass for the spectrograph. The image of the DMD line at the focal plane should be 35 µm wide if properly in focus (2.2 magnification × 16-µm pixel width). Therefore, the theoretical spectral band-pass should be limited
Figure 11. Nitrous oxide/acetylene flame emission spectrum of 300 µg/mL Mn, 30 µg/mL Ca, 250 µg/mL Cr, 50 µg/mL In, 30 µg/mL Ba, and 30 µg/mL Sr.
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Table 1. Modulation and Scanning Wavelengths modulation wavelength (nm)
calcium sodium potassium
scanning wavelength (nm)
peak
background
begin
end
422.7 589.6 766.5
422.2 589.1 766.0
421.2 588.1 765.0
424.2 591.1 768.0
Table 2. Limits of Detection and Sensitivitiesa atomic absorption
calcium sodium potassium a
LOD
sensitivity
ref sensitivity19
0.06 0.003 0.01
0.11 0.018 0.047
0.092 0.012 0.043
atomic emission scanning LOD
modulation LOD
0.05 0.01 0.01
0.04 0.006 0.01
All values in micrograms per millileter.
by the 50-µm PMT slit. The calculated spectral band-pass is therefore 0.23 nm (50 µm × 4.5 nm/mm). This suggests that the DMD image is only slightly out of focus. The spectral band-pass was determined for various DMD line lengths (slit heights) using a Cr HCL. Figure 8 demonstrates the change in spectral band-pass with slit height measured in pixels. Little or no change is observed when the slit height is increased from 50 to 100 micromirrors. A slight broadening occurs when the height is increased to 200 pixels, and the resolution is degraded badly at 480 pixels (full DMD height). To maintain the optimum resolution while maximizing the optical throughput, a slit height of 100 pixels was employed for all subsequent measurements. The width of the spectral window was also determined. The width of the DMD is 10.88 mm (640 pixels × 17 µm/pixel). Since the DMD image is magnified by a factor of 2.2, the used portion of the focal plane of the spectrograph has a width of 23.94 mm. Given the reciprocal linear dispersion of 4.5 nm, a spectral window of 108 nm can be viewed without moving the grating. The spectral window can be adjusted by changing the grating and/or the magnification of the DMD. In molecular absorption spectrometry,
for example, one might employ a 300 grooves/mm grating to obtain a 400-nm window with a spectral band-pass of 1.2 nm. The optical throughput is highest when the wavelength of interest is centered on the DMD (and centered on the focal plane of the spectrograph). Figure 9 demonstrates the change in intensity of the 427-nm Cr triplet when the spectrograph wavelength is centered on 384, 414, 444, and 474 nm. The reduction in intensity observed at the edges of the DMD can be compensated by varying the slit height during a scan, so that a height of 200 pixels is used at the edges and 100 or fewer pixels is used at the center. Such compensation was deemed unnecessary for the current study, so a constant slit height of 100 pixels was employed throughout. Signal-to-Noise Ratio. The optimum modulation frequency was selected based upon its signal-to-noise ratio (S/N). A Cr hollow cathode lamp was used as the light source. A single vertical DMD slit (1 pixel wide by 100 pixels high) with the 425nm emission line centered on the DMD (pixel 320) was flashed at frequencies ranging from 3 Hz to the video hardware’s limit of 40 Hz. The signal output from the lock-in amplifier (τc ) 1 s) was recorded at 1-s intervals for 30 s. S/N was calculated as the average of those 30 signals divided by their standard deviation. Figure 10 shows the variation of S/N with modulation frequency. As indicated by the figure, S/N is nearly independent of source frequency. The magnitude of the signal was higher at low frequency, so all subsequent measurements were performed at a frequency of 5.56 Hz. This frequency is suitable for the steadystate signals associated with flame atomic absorption and emission measurements. Transient signals, like those associated with graphite furnace atomic absorption spectrometry, would demand higher frequency. The limiting source of noise for flame atomic absorption measurements was also determined using the Cr hollow cathode lamp. As described above, the 425-nm Cr was flashed on the DMD at a frequency of 5.56 Hz. The magnitude of the lamp signal (S) was varied by inserting neutral density filters of various transmittance between the lamp and the DMD. A log-log plot of S vs S/N (measured as described above) had a slope of 0.5, indicating that the system is shot noise limited.
Table 3. Element Recoveries for Flame Emission Spectrometry calibration curve certified value
scanning method
modulation method
standard additions scanning method
modulation method
calcium sodium potassium
5.05 ( 0.09 wt % 136 ( 4 µg/g 2.70 ( 0.05 wt %
Tomato Leaves (SRM 1573a) 4.98 ( 0.53% 4.88 ( 0.15% 126 ( 18 µg/g 126 ( 3 µg/g 2.57 ( 0.03% 2.64 ( 0.03%
4.98 ( 0.45% 115 ( 11 µg/g 2.76 ( 0.13%
5.05 ( 0.76% 120 ( 10 µg/g 2.83 ( 0.11%
calcium sodium potassium
118 ( 6 µg/g 6.6 ( 0.8 µg/g 0.1280 ( 0.0008 wt %
Rice Flour (SRM 1568a) 119 ( 7 µg/g 120 ( 8 µg/g BDLa BDL 0.121 ( 0.001% 0.122 ( 0.001%
119 ( 6 µg/g BDL 0.136 ( 0.019%
127 ( 15 µg/g BDL 0.139 ( 0.004%
calcium sodium potassium
116 ( 4 µg/g 0.242 ( 0.006 wt % 0.994 ( 0.002 wt %
Bovine Liver (SRM 1577b) 134 ( 20 µg/g 112 ( 32 µg/g 0.235 ( 0.026% 0.228 ( 0.008% 0.962 ( 0.059% 0.934 ( 0.009%
128 ( 36 µg/g 0.237 ( 0.019% 0.970 ( 0.090%
116 ( 12 µg/g 0.225 ( 0.01% 1.04 ( 0.04%
calcium sodium potassium
0.174 ( 0.007 wt % 0.625 ( 0.026 wt % 0.606 ( 0.028 wt %
Total Diet (SRM 1548) 0.166 ( 0.003% 0.188 ( 0.004% 0.598 ( 0.038% 0.620 ( 0.014% 0.600 ( 0.044% 0.587 ( 0.008%
0.196 ( 0.032% 0.609 ( 0.077% 0.600 ( 0.060%
0.197 ( 0.010% 0.594 ( 0.025% 0.641 ( 0.025%
a
BDL, below detection limit.
4912 Analytical Chemistry, Vol. 70, No. 23, December 1, 1998
BDL, below detection limit. a
standard additions calibration curve
Total Diet (SRM 1548)
certified value standard additions calibration curve
Bovine Liver (SRM 1577b)
certified values standard additions calibration curve
Rice Flour (SRM 1568a)
certified value standard additions calibration curve
Tomato Leaves (SRM 1573a)
certified value
calcium 5.05 ( 0.09 wt % 4.64 ( 0.13% 5.11 ( 0.28% 118 ( 6 µg/g 123 ( 15 µg/g 123 ( 12 µg/g 116 ( 4 µg/g 116 ( 13µg/g 126 ( 2 µg/g 0.174 ( 0.007 wt % 0.166 ( 0.003% 0.181 ( 0.01% sodium 136 ( 4 µg/g 119 ( 2 µg/g 130 ( 8 µg/g 6.6 ( 0.8 µg/g BDLa BDL 0.242 ( 0.006 wt % 0.234 ( 0.034% 0.236 ( 0.01% 0.625 ( 0.026 wt % 0.582 ( 0.06% 0.696 ( 0.34% potassium 2.70 ( 0.05 wt % 2.60 ( 0.10% 3.09 ( 0.14% 0.1280 ( 0.0008 0.119 ( 0.004% 0.136 ( 0.064% 0.994 ( 0.002 wt % 0.994 ( 0.024% 1.09 ( 0.52% 0.606 ( 0.028 wt % 0.585 ( 0.013% 0.671 ( 0.098% wt %
(20) Robinson, J. W. Atomic Spectroscopy; Marcel Dekker: New York, 1990; p 207. (21) Long, G. L.; Winefordner, J. D. Anal. Chem. 1983, 55, 712A-724A.
Table 4. Element Recoveries for Flame Atomic Absorption Spectrometry
For flame emission measurements, the limiting source of noise was determined while aspirating a 20 ppm solution of Ca into a nitrous oxide/acetylene flame. The flame emission line at 422.7 nm was centered on the DMD. A slit 1 pixel by 100 pixels was repetitively scanned (10 times) at a rate of 0.5 s/pixel across a 3-nm window. The DMD was not modulated at 5.56 Hz for this experiment. Neutral density filters were again used to attenuate the signal. The resulting log-log plot of S vs S/N had a slope near zero, indicating that the measurement is flicker noise limited. Flame Atomic Emission Spectrometry. Figure 11 shows a nitrous oxide/acetylene flame emission spectrum resulting from an aqueous solution of 300 µg/mL manganese, 30 µg/mL calcium, 250 µg/mL chromium, 50 µg/mL indium, 30 µg/mL barium, and 30 µg/mL strontium. The CN emission bands are observed at 388 and 418 nm.20 The intensities of the emission peaks below 420 nm are lower than expected due to the glass window present on the DMD chip. As a result, the manganese emission line at 402 nm is weak, and the 388-nm CN band appears no stronger than the 418-nm band. To perform measurements in the ultraviolet region, the glass window on the DMD must be replaced with fused silica. Texas Instruments can perform this replacement by special order. In the current work, elements with absorption or emission bands in the visible region were chosen to evaluate the performance of the DMD spectrometer. Flame atomic emission measurements were performed by two methods. In the first method (wavelength scanning), a small spectral window (20 pixels baseline to baseline) centered on each emission wavelength of interest was scanned (Table 1). This was accomplished by scanning a DMD slit (1 × 100 pixels) across the window at a rate of 0.5 s/pixel. The scan was repeated four times (total analysis time 40 s), and the average baseline emission signal was subtracted from the peak apex. These values were then used in calibration curve and standard addition measurements for each SRM. Table 3 shows the flame emission “wavelength-scanning” results. The limit of detection (LOD) was calculated using the IUPAC definition (3σ).21 LODs are shown in Table 2. Precision data are presented as 2σ for six replicates (three replicates of two separate sample digestions). In most cases, the amount of each element found is within 10% of the certified value. Sodium in Tomato Leaves and calcium in Bovine Liver had consistently low recoveries. This could result from incomplete digestion of the material, although the standard addition method did correct for the Bovine Liver matrix. The second flame atomic emission measurement method was wavelength modulation. In this case, the DMD was used to flash alternating wavelengths. The first wavelength was at the emission peak apex and the other was at a background wavelength near the base of the peak, three pixels away from the apex (Table 1). DMD slits of 1 pixel by 100 pixels were employed for both the peak apex and background wavelengths. The DMD was modulated between the two wavelengths at 5.56 Hz. In this case, the lock-in amplifier automatically subtracts the background value from the peak signal. The signal from each solution was recorded every 1.0 s for 20 s. The wavelength-modulation procedure was much faster than the wavelength-scanning procedure.
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Flame Atomic Absorption Spectrometry. The same SRMs were analyzed by flame atomic absorption spectrometry (Table 4). A line 1 pixel in width by 100 pixels in length, centered on the DMD, was modulated as described above. LODs (3σ) and sensitivities (analyte concentration which gives 0.0044 absorbance) are given in micrograms per milliliter (Table 2). Precision values were calculated as described above. Sensitivities agree well with the values given in the Perkin-Elmer flame atomic absorption manual.22 The ability to modulate between adjacent wavelengths allows the use of near-line background correction (Figure 7).23,24 This technique uses the absorbance measured at a nonanalyte specific wavelength as a way to monitor broad wavelength background absorbances. Equation 1 shows the absorbance calculated at the
Abscorr ) log(INa/IoNa) - log(INe/IoNe)
(1)
Na wavelength (589.6 nm) can be corrected by subtracting the absorbance at the Ne wavelength (588.2 nm). Rearrangement of eq 1 yields
Abscorr ) log(INa/INe) - log(IoNa/IoNe)
(2)
If IoNa ) IoNe, then the second term in eq 2 drops out. Since the Na and Ne lines from the HCL do not naturally have the same intensity, their measured intensities were equated by giving each a different DMD slit height. The slit height of 70 pixels at the Na wavelength gave rise to the same signal as a slit height of 200 pixels at the Ne wavelength. The background-corrected absorbance can then be measured by
Abscorr ) log(INa) - log(INe)
(3)
This calculation was performed electronically by employing a log (22) Analytical Methods for Atomic Absorption Spectrophotometry; Perkin-Elmer: Norwalk, CT, 1982. (23) Sanford, C. L.; Thomas, S. E.; Jones, B. T. Appl. Spectrosc. 1996, 50, 174181. (24) Sneddon, J. Spectroscopy (Eugene, Oreg.) 1987, 2, 38-45.
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amplification circuit that converted the log of the PMT current (pA) to voltage. The log signal was then processed by a lock-in amplifier, operating at the frequency of DMD wavelength modulation between the Na and Ne lines. As a result, backgroundcorrected absorbance was read as the direct output of the lock-in amplifier. Neither calcium nor potassium has near lines that can be used for near-line background correction, but most elements with ultraviolet analytical wavelengths have such lines listed in tabular form.24 CONCLUSION A digital micromirror device has been successfully incorporated into an analytical atomic spectrometer. The instrument uses only one detector (PMT), yet has the capability of rapidly monitoring specific wavelengths within a 100-nm range on the millisecond time scale. The DMD can be removed from a commercial computer projector and driven with simple draw and paint programs. For flame atomic emission and absorption spectrometry, analytical figures of merit are similar to those observed with single-wavelength systems. Future improvements to the system include the incorporation of a fused-silica window on the DMD chip to allow determinations at ultraviolet wavelengths. Also, increasing the size of the DMD array may lead to higher resolution and broader spectral coverage. Such a device should find application in many areas of analytical spectroscopy. ACKNOWLEDGMENT The authors thank Texas Instruments, Inc. and Michael A. Mignardi for the digital micromirror device and color pictures and the NFS-GOALI program (NSF-CHE 9710218) for funding.
Received for review June 1, 1998. Accepted September 21, 1998. AC980597P
