Anal. Chem. 1990, 62, 1983-1988
1983
Continuum-Source Atomic Absorption Spectroscopy with an Echelle Spectrometer Adapted to a Charge Injection Device C h u n m i n g Hsiech, Steven C. Petrovic, a n d H a r r y L. P a r d u e *
Department of Chemistry, Purdue University, West Lafayette. Indiana 47907
An instrumental system for continuum-source atomic absorp tlon spectroscopy has been developed for simultaneous muItieiement determinations. The system consists of an electrothermal atomizer and a charge Injection device adapted to an echelle spectrometer to achieve multiplex detection. A continuous 40-nm spectral range in the two-dimensional echelle spectrum was acquired simultaneously through the capabiilty of the charge injection device to integrate signals in Its MOS capacltors. Novel methods were developed to compute absorbances by "scanning" through all orders in the entire echelle spectrum or selecting absorption lines randomly. I n the range 300-430 nm, characteristic concentrations ( 1 % absorption) were 1.6, 2.6, 2.9, and 3.8 ng mL-' respectively for Cu, Mn, and two Cr lines; these values are slmllar to those (1.3, 2.2, 1.2, and 3.6 ng mL-') obtained for slngle-element detection wIth an Imagedissector system.
INTRODUCTION Since the early work of Fassel et al. ( I ) there have been many studies of continuous-source atomic absorption spectroscopy (CS-AAS), most of which have been discussed in recent reviews (2-5). Although many of these studies have involved the use of conventional optics with conventional detectors, some more recent studies have combined conventional and echelle spectrometers with one- and two-dimensional imaging detectors (6-8). In the work of Masters et al. (6, 7), an image dissector was adapted to an echelle spectrometer to obtain high resolution with moderately broad spectral range. Although good sensitivity was obtained for a variety of elements, it was difficult to quantify multiple elements simultaneously because the image dissector is not an integrating detector. In a more recent study (8),a higher (fifth) order of a conventional grating was used with a cross-dispersion grating and a one-dimensional photodiode array. Use of the higher order of the grating provided improved resolution (0.01 nm) and use of the integrating feature of the photodiode array permitted multiple elements to be quantified simultaneously. The principal problem with this system was the rather limited spectral range of 2.5 nm for each atomization step. The present study was undertaken to determine to what extent these problems could be resolved by adapting an echelle spectrometer to a two-dimensional integrating detector. Special features of the system described here involve use of an echelle grating with a blaze angle of 76" used in an offLittrow configuration (9) with a charge injection device (CID) and an electrothermal atomizer. This system offers advantages relative to all those reported previously. Relative to the echelle spectrometer/image dissector system (6, 7), it permits simultaneous multielement quantification with similar sensitivies and detection limits. Relative to the conventionalgrating/one-dimensional photodiode array system (8), it combines a 2.5- to 3-fold improvement in spectral resolution with a 16-fold improvement (2.5-40 nm) in spectral range.
Because this is the first application of the CID for CS-AAS, some of its salient features are discussed below. The CID is a solid-state imaging device that uses charge injection and intracell charge transfer to transduce image information (IO). Each element in the two-dimensional Sensor consists of a pair of metal-oxide semiconductor (MOS) capacitors which are reverse biased to collect and store photon-generated charge signals. The major advantage of the CID over the image dissector in detection of transient signals, such as those from an electrothermal atomizer, is the integrating capability. Consequently, spectral information at many different resolution elements can be measured and stored simultaneously in the CID. The image dissector has no integration capability and accordingly it is not possible to store signals at different locations simultaneously. Also, the electron beam from a selected pixel of the image dissector is focused onto the amplifying dynodes by electrical fields; accordingly, stray or drifting electric fields can cause inaccuracies in the addressing process in the image dissector. Because addressing in CID is achieved with solid-state electronics, the CID is less sensitive to electrical fields. Considerations that influence the choice of a detector and optical configuration are very different for absorption studies relative to emission studies. Whereas wide dynamic range is required for emission studies, the nature of the absorption process imposes inherent limitations on the dynamic range for which acceptable errors are obtained ( I I , I 2 ) . Accordingly, in the choice of a detector for atomic absorption spectroscopy, it is a reasonable compromise to sacrifice dynamic range in favor of improved spectral resolution. In addition to choosing a CID that emphasized spatial resolution, we also used the off-littrow configuration of the echelle spectrometer (9) to further enhance spectral resolution. Results are reported for simultaneous multielement quantification in the spectral range from 300 to 430 nm. EXPERIMENTAL S E C T I O N Reagents. All solutions were prepared by using distilled, deionized water and certified atomic absorption standards (Alfa Products, Danvers, MA). Multielement solutions were prepared by mixing aliquots from the stock solutions of the elements of interest and diluting to appropriate volumes. Concentrations for calibration studies were in the range from 0.1 ng mL-' through 10 pg mL-' with four standards in each 10-fold range. Instrumentation. A diagram of the instrumentational setup is depicted in Figure 1. The system consists of a xenon-arc lamp, an electrothermal atomizer, an echelle spectrometer, a charge injection device, an interface unit, and a microcomputer. The xenon-arc lamp (Model L2479, Hamamatsu Corp., San Jose, CA) was located in an air-cooled housing and powered by a current-regulated power supply (Model 533, Optical Radiation Corp., Azusa, CA) operated at 15 A and 20 V. The intensity of the continuum radiation was higher than that available from hollow cathode lamps at wavelengths above 350 nm. The electrothermal atomizer (Model HGA2100, Perkin-Elmer, Norwalk, CT) was equipped with pyrolytically coated graphite tubes. The temperature cycle was set to evaporate the solution at 100 "C for 20 s, char the residue at lo00 "C for 10 s, and atomize the sample at 2700 "C for 10 s. The atomization temperature was optimized for the least volatile element (vanadium) among those
0003-2700/90/0362-1963$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15. 1990
A=?=
Figure 1. Diagram for instrumentation setup: ARC, xenonarc lamp: L1, L2, imaging lens: ETA, electrothermalatomizer: M1, M2, cdlimatlng and focusing mirrors; EO, echelle grating; OG, order-sorting grating; CID, charge injection device; AID, interface unit; M, monitor; PC,
microcomputer. being studied. The flow rate of argon gas was 60 mL min-', which was interrupted during atomization. To reduce memory effects, the atomizer was heated at 2800 "C for 10 s and cooled with argon and running water for 2 min between samples. The echelle spectrometer was modified to achieve %fold image reduction because the size of the CID was about one-third that of the image dissector used in the previous study (7). This was accomplished by replacing the original 75-cm focal length (fl) mirror (M2) with a 25cm fl mirror. Orders in the echelle spectrum were separated by using an order-sorting grating (OS, first-order diffraction grating, 600 grooves mm-9. The positions of the echelle grating (EG)and order-sorting gratings were rearranged to eliminate aberrations. The width and height of the entrance slit for the spectrometer were set to 100 and 200 pm, respectively. The charge-injection device (Model TN2250, General Electric, Liverpool, NY)has a resolution of 512 rows by 512 columns. Each square pixel in the CID has the dimension of 15 pm X 15 pm. The surface of the CID sensor is covered with a fused-silica plate instead of regular glass to improve ultraviolet transmittance. The signal, photon-generated charge, was integrated by setting the CID to the injection-inhibit mode to enhance the signal-to-noise ratio. In this study, signal integration periods between 0.3 and 0.8 s were used. The selection of these operational conditions is discussed in a later section. The interface unit (Model PC-512, Poynting Products, Oak Park, IL) consists of a camera-support module and a memory buffer. The CID-support module provides power and timing pulses and converts analog signals from the CID to digital form. The analog-to-digital converter (ADC) has 8-bit resolution and a 14-MHzconversion rate. A display monitor was used to inspect the echelle spectrum visually and a microcomputer was used for communication, control, data collection, and data processing. Data Processing. The conversion of signals from the stored charges to the absorbances of spectral lines involved the following procedures. First, the stored charges in the entire CID were injected sequentially, converted to digital form, and stored in a 256K memory buffer located in the camera support module. Second, signals were then transferred from the 256K buffer to a 1-Mbyte buffer located in the microcomputer 1 / 0 slot. In this process, the image data representing dark current and light intensities before and during atomization were saved in separate locations of the 1-Mbyte memory buffer for computation of absorbances in a latter step. The image data transferred could be saved with reduced spatial resolution when more than four frames of images were needed to meet the required resolution in time. The resolution format for the transferred data is loaded into the control registers before the data acquisition. The timing of those processes is controlled by a clock in the camera-support module and the signal conversion has a frame rate of 30 Hz. The last step of data processing is to transform data that represents the integral of spectral signals into data that represents the absorbances of spectral lines. This can be done either by a random-access algorithm to obtain absorbances of selected absorption lines or by a sequential-access algorithm to obtain the
entire absorption spectra. In the sequential method, an image resolution format of 512 by 240 pixels is used and eight frames of spectral images are stored in the 1-Mbyte image buffer. The procedure for measuring signals in the sequential access algorithm is as follows. The dark current and signal at 100% T were measured in advance and stored in the first and second frames of the memory buffer. Then, the signals obtained for atomized samples were stored in the other frames of the buffer. After the data were stored in the memory buffer, the data were processed according to the following method. First, the 100% T signals were "scanned" vertically to locate the center of each order and to determine the total number of orders covered. The entire echelle spectrum was then scanned from the lowest order toward the highest order. Each order was scanned from the left to the right edge of the CID. Because the echelle spectrum was somewhat slanted, the center of each order was determined at every pixel scanned and assigned to the appropriate position. The absorbance ( A ) was computed as A = log (I,,- &)/(I - Id)], where Io is the 100% T signal, I is the absorption signal, and Id is the dark current. In the random-access method, the data retrieved to compute absorbance were only those from the pixels for preselected absorption lines. The time required to compute absorbances for the selected absorption lines was in the range of milliseconds and absorbances could be computed in real time. Also, it was possible to resolve the full 512 by 480 pixel array because it was necessary to store only one frame of image in the memory buffer when absorbances were computed in real time.
RESULTS AND DISCUSSION Unless stated otherwise, results were obtained with an electrothermal atomizer and uncertainties are reported at the level of one standard deviation. Echelle Spectrometer. Use of an echelle grating differs from conventional gratings in that high resolution is achieved by using high orders (lO-lOOO) rather than high-density ruling. Because the different orders are superimposed on each other, they are separated into a two-dimensional pattern by using a second grating or prism to disperse the radiant energy a t 90" relative to the dispersion axis of the echelle grating. In order to reduce the dimensions of the dispersed spectrum so that a complete free spectral range could be monitored along one dimension of the CID, the 63" 21' echelle grating used in earlier studies was replaced by a 76" grating. The 76" grating provides a free spectral range that is 4-fold smaller than the 63" 21' grating. The resulting improvement of the spectral coverage in the horizontal direction results in a trade-off of the spectral coverage in the vertical direction. In the same spectral range, the number of orders generated from the 76" echelle is 4-fold that from the 63" 26' echelle, thus requiring greater dispersion in the vertical direction to reduce overlap among diffraction orders. The greater dispersion was achieved by using a 600)-groove/mm first-order grating and by replacing the 75-cm fl focusing mirror (M2, Figure 1) used earlier (6, 7) with a 25 cm fl mirror. This change resulted in a reduction of the spectral range that can be monitored simultaneously and an increase in the aberration due to the increase in the solid angle (8) between the incident and the reflecting rays to M2. We were able to reduce the aberration by placing the smaller order-sorting grating next to the CID. This resulted in absorption profiles with very small asymmetric deviations. With this configuration, including a 25-cm focusing mirror and a 100-pm slit, the resolution in vertical dispersion was 1.1nm at 300 nm. At this wavelength, the overlap among orders was about 15% of the peak height. At longer wavelengths, the free spectral range and the spacing between orders increased, so that overlap among orders was reduced. Charge Injection Device. The dark current in a CID mainly results from thermally generated charges in the depleted storage regions of the semiconductor. This process competes with the photon-generation charges and reduces the
1985
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990 250 0 r'
300
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Flgure 2. Integrated signals (300 ms) across a row in the middle of sensor: (a) dark current; (b) at 300 nm; (c) at 350 nm; (d) at 400 nm.
dynamic range and signal-to-noise ratio (SIN). In applications to atomic emission spectroscopy the intensity is usually quite low and it is frequently necessary to cool the detector in order to reduce thermally generated dark current. However, because the xenon-arc lamp used in this study produces much higher intensities, the dark current represents a smaller fraction of the signal and we chose to evaluate the CID at room temperature. Dark Current/ Wavelength Dependency. Figure 2 shows some results for the CID operated at room temperature with an integration time of 300 ms. The spectral range covered by each plot is about 1.3 nm and the spectral resolution varies from about 0.003 nm at 300 nm to about 0.004 nm at 400 nm. The jagged nature of the plots represents variations among the various pixels in each row. The dark current (curve a) represents about 27, 7, and 4% respectively, of the signal amplitudes at 300, 350, and 400 nm, respectively. It is probable that improved results could be obtained, especially at shorter wavelengths, with a cooled detector. However, as noted above, our objective in this study was to evaluate the performance of the detector at room temperature. The results in Figure 2 also show how the combined effects of the source and detector influence the 100% T signal level at different wavelengths; the 100% T signal at 400 nm is about 3-fold that at 300 nm. To compensate for these variations, we used different integration times in different wavelength regions. For example, integration times varied from 800 ms at 300 nm to 300 ms at 400 nm. For the 40 nm monitored by the CID at any one time, the variation of intensity with wavelength was relatively small and fixed integration times could be used within each of the 40-nm ranges. Linearity. The linearity of the CID response was evaluated by using neutral density filters to vary the light intensity reaching the sensor. Transmittances of the filters were calibrated with a diode-array based spectrophotometer (Hewlett-Packard 4850A). For five transmittances between 20 and 100%, least-squares fits of CID values (y) vs results with the spectrophotometer ( x ) yielded y = (1.03 f 0 . 0 2 ) ~- (3.9 f 0.8) with S,, = 0.47 and r = 0.9997 a t 340 nm and
y = (1.05 f 0.06)~- (4.9 f 0.5) with S,, = 0.68 and r = 0.9999 a t 400 nm Plots of log y vs log x were linear with slopes of 1.00 f 0.01 at both 340 and 400 nm, respectively. The scatter about the least-squares lines as reflected by the standard errors of the estimates (S,,) and the values of unity for the log/log plots illustrate good agreement between the two instruments and
0 158
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HORIZONTAL PI XEL Flgure 3. Spectral response near 422.7 nm without and with calcium in a flame atomizer: (a) 100% T ; (b) intensity with calcium; (c) absorbance. b
WAVELENGTH (0 1 nm, ci
;
Flgure 4. Absorption spectra between 315 and 355 nm with the signals nm with the signals integrated b€%veen 3.2 and 4.0 s from the start of atomization (element and absorption wavelength, nm): (a) V, 318.3; (b) V, 318.4; (c) V, 318.5; (d) Li, 323.3; (e) Ni, 323.3; (f) Cu, 324.8; (9) Cu, 327.4; (h) Cu, 327.4; (i)Na, 330.2; (j) Na, 330.3; (k) NI, 337.0; (I) Ni, 341.5; (m) AI, 343.9; (n) Fe, 344.0; ( 0 ) AI, 344.4; (p) Yb, 346.4; (4)Ni, 349.3; (r) Ni, 351.5; (s) Ni, 352.5.
confirm a linear relationship between integrated current and light intensity. We were somewhat surprised by the negative intercepts for the linear least-squares fits; we are uncertain about the origin of these nonideal in intercepts, but a possible reason is that some of the photon-generated charge was not injected into the substrate during the readout period. Fixed Pattern Noise. As noted earlier, the jagged shapes of plots in Figure 2 reflect different sensitivities of the different pixels in each row. Figure 3 shows analogous data for the 100% T signal and absorption by calcium near 422.7 nm. The fixed-pattern noise in the transmittance data (curves a and b) is reduced substantially in the absorbance data (curve c). There is less fixed pattern noise in the absorbance plot (curve c) than the transmittance plots (curves a and b), suggesting that the pixel-to-pixel variations are partially canceled by the ratios of intensities used to compute absorbance. Atomic Absorption Spectra. Typical Spectrum. Figure 4 shows typical absorption spectra between 315 and 355 nm for several elements. Positions of spectral lines were calculated by using the equation X = Xo
+ (dX/dX) (X- Xo) + (dX/dM
( Y - Yo)
where X and Xo are computed and reference wavelengths, respectively, X and X o are the horizontal coordinates of the computed and reference wavelengths, respectively, Y and Yo are the vertical coordinates of the computed and reference wavelengths, respectively, and dX/dX and dX/d Y are the horizontal and vertical reciprocal dispersion factors, respectively. The vertical reciprocal dispersion factor was found to
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
Table I. Comparisons of Computed and Expected Wavelengths for Selected Absorption Lines
element V V
v Li Ni
cu cu cu Na Na Ni Ni AI Fe A1
Yb Ni Ni Ni
wavelengths, nm computed expected 318.173 318.340 318.414 323.162 323.176 324.580 327.246 327.219 330.124 330.155 336.825 341.412 343.921 343.061 344.239 346.318 349.269 351.379 352.447
difference
order
318.341 318.398 318.540 323.261 323.296 324.754 327.396 327.396 330.232 330.299 336.957 341.477 343.935 344.939 344.364 346.436 349.296 351.505 352.454
0.168 0.058 0.126 0.099 0.120 0.174 0.150 0.177 0.108 0.144 0.132 0.065 0.014 0.122 0.125 0.118 0.027 0.126 0.007
304 304 304 299 299 298 296 295 293 293 287 283 28 1 281 281 279 277 275 274
av
0.108 0.052
std dev
be virtually constant a t a value of 0.1735 nm per pixel. The horizontal reciprocal dispersion factor varied from order to order, as expected, as was computed as dA f d X = 0.605/m, where m is the diffraction order. Results for computed and expected wavelengths are presented in Table I. The average of differences among computed and expected values is 0.108 nm with a standard deviation of 0.05 nm. The observed bias probably results from the facts that the reference absorption line was not located at the center of the reference pixel and that coordinates of pixels were in integer values. A program is being developed to address coordinates of absorption lines as floating point numbers in the effort to improve the accuracy with which wavelength locations are predicted. The appearance of the Cu line at 327.4 nm in both the 295th and 296th orders (see Table I) shows that the free spectral range is fully covered in the horizontal direction. Time Dependencies. In addition to the multielement capabilities of systems that include imaging detectors, these systems also permit more efficient studies of effects of variables. For example, Mones et al. (8) used their system to evaluate effects of temperature on the atomization process. In this study we chose to examine effects of time on the atomization process. Figure 5 illustrates some time dependencies of the absorption spectra in the range from 395 to 435 nm. The signals for volatile elements (potassium, manganese, and gallium) appeared at about 0.6 s after the start of atomization (Figure 5A). Signals for less-volatile elements (aluminum, calcium, and chromium) appeared at about 1.2 s after the start of atomization (Figure 5B) a t about the same time that the signals for the more volatile elements reached their maxima. After 1.8 s (Figure 5C) the signals for the less-volatile elements were still increasing, while signals for the more volatile elements were decreasing. After 2.4 s (Figure 5D), signals for all elements except calcium were decreasing. More complete time dependencies are illustrated in Figure 6. The latter data were collected and processed with the random-access algorithm. Calibration Results. The quantification of multielement samples was done by evaluating both the heights and areas of the absorption peaks from the electrothermal atomizer. Some typical calibration plots for peak height and peak area
PVAVELENGT~ iC 1 nm,
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Figure 5. Absorption spectra at different times for several elements in an electrothermal atomizer. Time (s) alter start of atomization: A, 0.6; B, 1.2; C, 1.8; D. 2.4. Sample concentration (ng mL-I), absorption wavelength (nm): (a) AI, 200, 396.2; (b) Yb, 20, 398.8; (c) Mn, 100, 403.1; (d) Mn, 100, 403.3; (e) Mn, 100, 403.4; (f) K, 600, 404.4; (9) K, 600, 404.7; (h) Ga,400, 417.2; (i) Ca, 20, 422.7; (i) Cr, 800, 425.4; (k) Cr,800, 427.5; (I) Cr,800, 429.0.
vs concentration are shown in Figure 7 and numerical data are summarized in Table 11. These results were obtained by the random-access method by combining signals from three consecutive pixels along the vertical axis of the order for each absorption line. Characteristic concentrations were computed as O.O044/S, where S is the slope of each calibration curve
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
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Table 11. Performance Characteristics for Selected Elements Absorption Lines characteristic concentra-
detection
tion! ng
absorption integrated wavelength, signal at element 0
1
2
3
4
v v
5
TIME (SI Figure 8. Absorbance vs time traces of representative elements obtained by the random-access method with 0.3-s integration periods (sample concentration, ng mL-', absorption wavelength, nm): (a) Cr, 800, 427.5; (b) Cr, 800, 425.4 (c) Mn, 100, 403.4; (d) Mn, 100, 403.1; (e) Mn, 100, 403.3; (1) Ca, 20, 422.7.
0.2.
0
A
a
0
Ga
0
a 0.05.
n
0
0.1 .
a,
Cr Cr Cr
0 0
0
100
50
200
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C o n c e n t r a t i o n (ng/mL)
.
0.B
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.
.
.
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318.341 318.398 318.540 324.754 327.396 346.436 352.454 357.869 359.349 360.533 371.994 385.991 394.409 396.153 398.799 403.076 403.307 403.449 404.414 404.720 417.205 425.435 427.480 428.972
300 f 3 282 f 4 261 f 2 293 f 3 374 f 2 514 f 3 506 f 3 329 f 3 341 f 2 334 f 3 316f 3 548 f 3 600 f 3 616 f 3 419 f 3 445 f 3 427 f 3 416 f 3 384 3 412 f 7 418 f 2 478 f 3 510 f 3 471 f 3
*
22 25 24 1.6 4.6 5.8 26 1.6 2.6 3.5 19 28 16 5.2 1.5 2.9 1.8 4.3 26 83 9.0 3.8 5.4 6.3
7.0 7.7 8.2 1.0 2.8 3.5 14. 0.85 0.95 1.4 7.6 19. 10. 4.8 0.49 2.7 1.6 3.7 18. 49. 7.2 1.7 2.2 3.0
38 73 46 3.4 5.1 6.0 27 2.9 3.0 5.3 31 26 43 4.6 2.0 3.7 2.3 5.7 40 290 10 4.5 6.9 6.8
32 59 40 6.1 8.2 9.6 38 2.5 3.0 5.7 31 45 18 11 1.7 9.2 5.5 13 71 450 21 5.2 7.4 8.5
"In arbitrary units. Mean and standard deviation from 50 r e p hates. Integration times: 315-355 nm, 0.8 s; 355-395 nm, 0.5 s; 395-435 nm, 0.3 s. *The concentration corresponding to 1% absorption. 'The lowest concentration that can be determined at the 99.7% confidence level. Table 111. Comparison of Characteristic Concentrations for Results Obtained with Image Dissector and CID Systems characteristic
0
element
wavelength, nm
Cu
324.754 357.869 403.076 425.4 35
Cr
Mn
A
a,
a
Cr Cr
Mn Mn Mn K K
I
0
A O .
0.15
a,
m
Cr
Yb 0
0
I y
Ni
A1 A1
0 0
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Fe Fe
A
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V cu cu
nm
limit: ng mL-' peak peak peak peak height area height area
mL-'
0
1
0
Cr
concentration: na mL-' image dissectorb CID' 1.3 2.2 1.2 3.6
1.6 1.6 2.9 3.8
Concentration which gives 1% absorption. From ref 5. This study. 0
50
I00
150
200
C o n c e n t r a t i o n (ng/mL) Figure 7. Calbratkm curves for representative absorptbn llnes In both peak-height (A) and peak-area (B) methods (elementand absorption line, nm: (A)Cu, 324.8; (0)Cr, 357.9; (0) Mn, 403.1; (0)Cr, 425.4.
within the linear range (see Figure 7). Detection limits (DL) were computed as DL = Ei0/2.3i$, where Eio is three standard deviations in io, io is the signal a t 100% T, and S is the slope of the calibration curve a t low concentrations. Standard deviations (n = 50) for most elements/lines were in the range of 3% T, characteristic concentrations (corresponding to 1% absorption) were in the range of 1-25 ng mL-', and detection limits (except potassium a t 404.7 nm) were in the range of 3 ng mL-'. The high values of the detection limits for potassium probably result from the very high charring temperature (1000 "C) used. It is probable that some of the potassium was lost before the absorption measurements were made. Comparisons among these data show that peak areas give better linearity and detection limits than peak heights
for the less volatile elements, whereas peak heights give as good or better results for more volatile elements. Peak areas have higher nominal sensitivities (slopes) for all elements, though it is not very useful to compare sensitivities for different types of responses. A serious limitation of CS-AAS is the stray light which causes reduced sensitivities at higher concentrations. This was manifested in this work as well as that described earlier (8)by calibration plots that curved toward the concentration axes at absorbances as low as 0.10. A solution to this problem is discussed later. Table I11 compares results of characteristic concentrations obtained for selected elements by using the CID and an image dissector (7). Results with the image dissector were obtained in a single-element mode, whereas results with the CID were obtained in a multielement mode as described above. These and other results indicate that the CID/echelle combination can provide simultaneous multielement determinations with quantitative characteristics similar to results obtained by single-element methods with an image dissector. This is
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other elements. The larger range available with the echelle/CID combination relative to a one-dimensional detector is very useful in this regard.
DISCUSSION This study has elucidated advantages and limitations of the use of a two-dimensional charge injection device with an echelle grating spectrometer for elemental determinations by a, continuous-source atomic absorption spectroscopy. Some advantages include the ability to quantify multiple elements simultaneously by virtue of the integrating character of the CID, the ability to address different pixels randomly, the ability to use different integration times to compensate for different responses in different regions of the spectrum, the u 1'' . . i ability to monitor the time course of several responses si0 5 10 !5 20 25 30 multaneously, the ability to extend the useful concentration C z r c e c t r a t i o n (pq/rnL) range by using lines with different sensitivities for different concentration ranges, and simplified background correction. Flgwe 8. Callbratiin plots for a wide range of iron concentrations by Although the spectral range covered for a given optical using wavelengths with different sensitivities: 0, 372.0; 0,390.0;A, 382.1;X . 385.9 nm. setting is only 40 nm, this is 16-fold better than that described in the most recent application of an imaging detector for particularly gratifying in view of the fact that it was necessary CS-AAS (8). Also, the spectral resolution of 0.003 to 0.004 to reduce the echelle image %fold to adapt it to the CID. The nm at 300 and 400 nm, respectively, is a 2.5- to %fold immost probable reason why results were not degraded is that provement over the 0.01-nm resolution reported with a onethe physical size (15 pm) of each pixel in the CID is about dimensional array detector. A mechanized system analogous 2.5-fold smaller than the aperture (38 pm) in the image disto that described earlier (13) could be used to address different sector. Consequently, the reduced image in the CID system segments of the spectrum sequentially. did not seriously reduce the spectral resolution. Two of the LITERATURE CITED detection limits in Table I11 (Cr, 357.9 nm and Cu, 324.7 nm) Fassel. V. A.; Mossotti, V. G.; Grossman, W. E. L.; Kniseley, R. N. can be compared directly with results reported by Jones et Spectrochim. Acta 1966, 22, 347-357. al. (12). By taking account of the 20-pL sample sizes used in O'Haver, T. C. Analyst 1984, 709, 211-217. Marshall, J.; Ottaway, B. J.; Ottaway, J. M.; Littlejohn, D. Anal. Chim. this study, the concentration detection limits for Cr and Cu Acta 1986, 780, 357-371. reduce to mass values of 60 and 68 pg, respectively, compared Harnly, J. M. Anal. Chem. 1966, 58, 933A-943A. to values of 30 and 3 pg reported earlier (8). Our value for O'Haver, T. C.; Messman, J. D. P r q . Anal. Spectrosc. 1986, 9 , 483-503. Cr is about 2-fold higher than the earlier while our value for Masters, R.; Hsiech. C.; Pardue, H. L. Anal. Chim. Acta 1987, 799, Cu is about 23-fold higher than that reported earlier. 253-257. Extended Concentration Range. A s noted earlier, a serious Masters, R.; Hsiech, C.; Pardue, H. L. Talenta 1989, 36, 133-139. Jones, B. T.; Smith, B. W.; Winefordner, J. D. Anal. Chem. 1989, 67, limitation of continuous source atomic absorption is the lim1670- 1674. ited linear range which is caused primarily by stray light. One Masters, R.; Hsiech, C.; Pardue, H. L. ADD/. . . oDt. . 1988. 2 7 , 3895-3897. possible solution to this problem is to use different lines with Burke, H. K.; Michon, G. J. IEEE J. Solid State Circuits 1976, SC7 7 , different sensitivities for different concentration ranges. 121-127. Figure 8 shows calibration data at four different wavelengths Willard, H. H.; Merritt, L. L., Jr.; Dean, J. A. Instrumental Methods of Analysis; D. Van Nostrand Co., Inc.: New York, 1965; p 90. for a wide range of iron concentrations. By using these difPardue, H. L.; Hewltt, T. E.; Milano, M. J. Clin. Chem. 1974, ferent wavelengths, it is possible to quantify iron concentra1026-1042. Karanassios, V.; Horlick, G. Appl. Spectrosc. 1986, 4 0 , 813-821. tions over a range of a t least 3 orders of magnitude. Admittedly, iron is a special case in that it has a very rich spectrum from which one can choose lines with widely varying RECEIVED for review February 2, 1990. Revised manuscript sensitivities. However, to the extent that other elements have received June 6,1990. Accepted June 8,1990. This work was analogous distributions of absorption sensitivities, the same supported by Grant No. GM13326-20,21 from the National procedure can be used to extend the concentration range for Institutes of Health.
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