Hadamard Spectrometer for Ultraviolet-VisibleSpectrometry F. W. Plankey, T. H. Glenn,’ L. P. Hart, and J. D. Winefordner2 Department of Chemistry, University of Florida, Gainesville, Fla. 326 1 7
A Hadamard Transform Spectrometer (HTS) for use in the UV-Visible spectral region is described. The spectrometer is connected on-line to a digital computer to accumulate spectral information and to perform a Fast Hadamard Transform (FHT). The HTS system has been evaluated with several line sources as well as with a continuum source and for the atomic fluorescence measurement of cadmium and zinc in an air/CnH2 flame. The present HTS system covers a spectral range of 25 nm in about 25 sec. The HTS system consists of a conventional Czerny-Turner grating monochromator which has been modified only by removal of the folding mirror at the exit slit and installation of a 255-cyclic slot Hadamard (509 total slots) mask at the exit focal plane. An appendix is also given to outline the Fast Hadamard Transform used in this work.
Hadamard Transform Spectrometry (HTS) gives useful spectral information in the infrared region, where the major source of noise is in the detector ( I , 2 ) . However, in the UV-Visible region, the limiting noise is experimentally photon noise. In this case, the measuring of all the spectral intervals a t the same time becomes a disadvantage as the photon noise from the flux a t each spectral interval contributes to the noise of each signal. Thus, a small signal may be buried in the noise from a large signal(s). Because of the numerous references (3-5) to the possible use of Hadamard spectrometry for multielement analysis by atomic spectrometry and in order to evaluate HTS for use in multielement atomic spectrometry, a UV-Visible HTS system has been constructed and evaluated. The present system consists of a cyclic-255-slot Hadamard mask a t the exit plane of a grating monochromator with a photomultiplier and a mini-computer for the Fast Hadamard Transform (FHT). This system has been initially evaluated for several excitation sources and for atomic fluorescence flame spectrometry. In Hadamard spectrometry, a specially-constructed mask is used to block some spectral intervals and allow others to pass to the detector; the total signal yL (also see Appendix) a t the detector for a given mask position i will be given by ;= s y, = a,,x, ;=l
where the a,,’s 0’ = 1 to N ) are either 0 (if the light is blocked a t the j t h spectral interval) or 1 (if the light is allowed to pass a t the j t h spectral interval and x J is the intensity at the j t h spectral interval). If a set of A4 orthogonal equations can be formulated with various combinations of a,,’s, then the individual xJ’s can be determined by solvDeceased.
* Author to whom reprint requests should be sent. ( 1 ) R N lbbett D Aspinall and J F Grainger Appi O p t 7, 1089 (1968) ( 2 ) J A Decker and M 0 Harwlt A p p l O p t 7 , 2205 (1968) ( 3 ) K W Busch and G H Morrison Ana/ Chem 45, 712A (1973) ( 4 ) G Horlick and E G Codding A n a l Chem 45, 1490 ( 1 9 7 3 ) ( 5 ) R E Santinl M J Milano and H L Pardue A n a l Chem 4 5 , 915A ( 1 9 7 3 )
1000
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 8, J U L Y 1974
ing the set of simultaneous equations given by N intensity readings with N different mask configurations. Hadamard matrices give the relationships necessary to form independent equations with a set of N different masks. Or alternatively, one mask (constructed so that an orthogonal set of coefficients (a,,’s) can be created by moving the mask one slot width) can be used at N positions to form the set of equations.
EXPERIMENTAL The Cyclic Exit Mask. A cyclic mask can be designed for any 1; n any integer) according to the method of primitive polynomials given by Nelson and Fredman (6). For this work (N = 28 - 1 = 255), 255 spectral intervals are covered in a range of about 25 n m resulting in a resolution of 1 interval per 0.1 nm. A cyclic mask then would have a total of 2N 1 = 509 transparent and opaque slots. The order of 1’s (open slots) and 0’s (closed slots) is determined once the first n (8 in our case) coefficients are given. In our case, the first 8 coefficients are lMH)1110, and the ninth coefficient is (in the case of n = 8) the sum mod 2 (i.e.,0 + 0 = 0, 1 + 0 = 1, 0 + 1 = 1, and 1 + 1 = 0) of the first, third, fourth, and fifth coefficients or 1 + 0 + 0 + 1 = 0. Next the first coefficient is dropped, and the eight remaining ones are used to compute the 10th coefficient. This process is repeated until all 509 coefficients are determined. For our case, the coefficient matrix is as shown in Table I. The coefficients of the first mask position are determined from a portion of the mask consisting of the first 255 coefficients 10001 , . . to 00000 (see array in Table I). The coefficients of the second mask position are formed by shifting one to the right and so are 00011 . to 00001 and so forth until the last (255th) mask position is made up of 0 1 0 0 0 , . . to 00000. The Hadamard Transform Spectrometer. The Hadamard Spectrometer (see Table I1 for description of components) was constructed from a Czerny-Turner Scanning Monochromator, a single-pass, 350-cm focal length, 5 / 6 3 mount with a 48 mm X 48 mm 1180 lines per mm grating blazed for 250 nm. The reciprocal linear dispersion with this configuration was approximately 2.0 n m j m m . The slits are straight-edged and bilaterally-adjustable in width from 5 to 2000 p m and in height with values of 12, 5, 3, 1, or 0.5 mm. The folding mirror a t the exit slit was removed, and a laboratory-fabricated translation stage was mounted at the front panel with the front edge of the slide assembly (as shown in Figure 1) mounted a t the exit focal plane. A field stop 0.510 in. X 0.5 in. was aligned to allow approximately 12.5 nm on either side of the exit focal point to fall on the mask which was connected to the slide assembly. The mask consisted of a copper-nickel bimetallic strip 1 x 3 X 0.010 inches. The 255 slot cyclic mask (509 total slots) described earlier was inscribed on the strip with each slot encompassing a width of 0.002 in. (50 pm) and a height of 0.387 in.; the total mask code length was 1.018 in. The slide assembly was spring-loaded in the translation stage and was driven by a 40-turns-per-in. micrometer which in turn could be stepped in increments of 1/19200 in. by a 480-steps-per-revolution stepping motor. The detector was a 30-mm diameter end-on photomultiplier with S-13 spectral response and was operated a t room temperature and at 800-1000 VDC. The photoanode current was measured with a constant bandwidth nanoammeter (7) modified to give an RC time constant of about 1 msec. The output of the nanoammeter was inverted and further filtered by an amplifier and processed with a laboratory-fabricated computer interface consisting of a sample and hold (S/H) amplifier and a 10-bit, 0- to 10-V analog-to-digital converter (ADC). The transformation described in the Appendix is used with a PDP-11/20 computer with ( 6 ) E. D. Nelson and M L. Fredman, J . O p t . Soc. Amer., 60, 1665
N (N = F -
(1970). (7) T . C. O’Haver and J (1969).
D
Winefordner,
J.
Chem. Educ.. 46, 241
8K of core memory to compute the Fast Hadamard Transform (FHT).The spectrum can be displayed on an oscilloscope or plotted on an x-y recorder by means of two 10-bit, 0- to 10-V digitalto-analog converters (DAC). Or, the digital spectrum for the 255 locations may be printed out by an ASR-33 Teletype.
RESULTS Initial Adjustments. The H T S system described above has been initially tested with several light sources to determine its speed, accuracy, wavelength range, and other optical characteristics. It should be emphasized here that all alignment procedures were performed visually with the aid of a microscope and all tolerances of machined parts were of the order of 0.001 in. (25 pm) except for the mask itself where tolerances were of the order of 0.0001 in. (2.5 pm). This indicates a definite advantage of HTS systems over interferometric spectrometers in that construction tolerances are not highly critical. The green 546.1-nm line of a Hg pen light was an excellent source for alignment and adjustment of the mask and translation stage. It was necessary to set the mask slots parallel to t h e entrance slit image and also to establish the initial position for t h e translation stage so that the field stop would allow passage of radiation through the first 255 slots. After alignment, the spectrometer was adjusted to give signal levels between 20 and 7070 of full scale (with the visible lines from the mercury pen light) as the mask was stepped over the entire range. The effect of variation of the monochromator slit width was investigated with the 546.1-nm line from the pen
*J
a-y--.--
mnoommetcr
i"*Wll,
Figure 1. Block
diagram of the UV-Visible Hadamard Transform
Spectrometer light. With a slit width of 300 pm, the image a t the exit focal plane (i.e., the image resulting from the transformation of the signals) occupied 8 channels which is probably a result of the slight distortion of the 12-mm high optical image. In fact, when the slit width was narrowed to 5 pm, the image still occupied 2 channels (100pm). The wavelength range of the HTS system was 27.5 nm near 250 nm and 24.5 nm near 550 nm. Also, for the present HTS system, the peak signals decreased by about 40% as the peaks were shifted from the long wavelength end of
Table I. 255-Slot Cyclic Mask Code. 10001 10110 01101 11011 10101 00000 00010 01000 01001 01110 10100
11000 11010 10001 10110 01111
10001 10110 01101 11011 10101 0000
10010 11001 11100 01111 10010 11000 11010 10001
11100 01100 11100 01111 10000 10010 11001 11100
00001 00111 11000 11010 10011 11100 01100 11100
10010 11011 10110 01100 11111 00001
10110 01111
01111
01111
10010
10000
11010 10011
00111 11000
01001 01111 10010 11010 10000
10010 11011 10110 01100 11111
10111 01011 00101 10001 10111 01001
00100 10100 00101 10000 10001
01111 10010 11010
01011
10000
10111
00101 10001 10111
00010 01000 01001 01110 10100 00100 10100 00101 10000 10001
a ''1" denotes transparent slot and "0" denotes opaque slot. Note that the 255-slot cyclic mask implies 609 total mask slots. The italic portion represents the slots illuminated at the first mask position.
Table 11. Components of UV-Visible Hadamard Transform Spectrometer Item
Mask Monochromator Photomultiplier Stepping Motor Computer and Peripherals
x-y Recorder Oscilloscope Mercury Pen Lamp and Supply Electrodeless Discharge Lamp (EDL) E D L Power Supply and Antenna (A) Hollow Cathode Discharge Lamp (HCDL) HCDL Power Supply Xenon Lamp Xenon Lamp Power Supply
Description (Model Number)
Source
509 Total Slot 255 Cyclic Code EU-700 EM1 9526B HDM-12-480-4 PDP-11/20 A 618 (DAC) A 404 (S,") A 811 (ADC) 715 M ITT 1735D 11 s c - 1 c
Dynamic Research Corp., Wilmington, Mass. 01587; under license from Spectral Imaging, Concord, Mass. Heath Co., Benton Harbor, Mich. 49022 Gencom Div., Plainview, N.Y. 11803 USM Corp., Wakefield, Mass. 01880 Digital Equip. Corp., Maynard, Mass. 01754
MFE Corp., Salem, N.H. 03709 ITT Corp., Van Nuys, Calif. 91409 Ultraviolet Products, Inc., San Gabriel, Calif. 91778 Laboratory constructed Raytheon Co., Norwalk, Conn. 06856 Varian Tektron, Palo Alto, Calif. 94303
82-135 150X8R VL-150-2
Jarrell-Ash, Waltham, Mass. 02154 Varian Eimac, San Carlos, Calif. 94070 Varian Eimac, San Carlos, Calif. 94070 ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
1001
t
FL 2 4 8 3
I
60 70
-
100-
\
I
2395 WAVELENGTH (nml-
Figure 2. Wavelength region covered by the UV-Vis HTS shown by use of the wavelength centered at 239.5 n m , 253.7 n m , and Figure 4. Spectrum of a multielement (Fe, Co, Ni) EDL obtained in 25 sec with the base line averaged to eliminate noise in the spectrum
266.8 nm
'Ot
9+ 40-
1. t
?
..
--*
u
ii'
ZD-
IY
Y
30-
r c
z
z
,
/--, ' S "
P
h'
-z 10-
~
#,"\
I-
-1
-
-,, '
i"
-_
--b\
Y
" "
0
Y
324'7nm
-
3274nm
WAVELENGTH
Figure 3. Effect of deliberate misalignment of the starting position of the mask ( A ) Mask aligned by eye to correct position, (8)0.001-inch ('/2-slot width) misalignment, (C)0.005-inch (2% slot widths) misalignment
the range to the short wavelength end (see Figure 2). This was probably a result of misalignment in the optical system. The effect of mask misalignment was studied with the 324.7 nm-327.4 nm doublet from a copper hollow cathode discharge lamp (see Figure 3). An error in the starting position by 0.005 in. (250 pm) (i.e., two and one half slot widths) resulted in peak shift by 3 channels as well as in a smaller S/N-ratio. It is remarkable that usable spectra result even when the mask is misaligned by up to 4 or 5 slot widths. Nevertheless, our apparatus for displacing the mask was automatically-aligned after each scan to at least 0.0001 in. (2.5pm). Base-Line Averaging. Averaging was useful for smoothing the base-line noise. The spectrum of a multielement electrodeless discharge lamp is shown after base-line smoothing in Figure 4. Recording of Continuum and Broad Band Spectra. Although primarily intended for line spectra, the system was also used to record the spectrum from a con1002
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
C d 228 Bnm
J
I
!
W
> -
t
Zn 213 Bnm
W
WAVELENGTH (nm)--r
Figure 6. Atomic fluorescence spectrum of Zn and Cd in CzHz/ air flame excited by electrodeless discharge lamps; note two
separate spectra-with base-line averaging and the noisier, without base-line averaging tinuum with a bandpass filter, with center wavelength of 400 nm. The results are shown in Figure 5 . In the case of a broad band spectrum with no fine structural features, the
I
la
'
I
b
i
vg cas 2
* I
Figure 7. Demonstration of multiplex disadvantage. Atomic fluorescence of Mg and M n in C2Hz/air flame excited via Eimac 150 W xenon continuum: a. 50 ppm Mg and 50 ppm M n ; b. 60 ppm Mg and 20 ppm M n
large constant signal is bucked out so that small changes in signal with slot position can be scale-expanded before processing. Atomic Fluorescence Spectra. Simultaneous excitation of the atomic fluorescence of zinc and cadmium was effected by Cd and Zn electrodeless discharge lamps; the results for 10 ppm Zn plus 10 ppm Cd introduced into a CgHg/Air flame are given in Figure 6. No attempt was made to optimize source or flame conditions. With the HTS system described here, the spectrum over a range of approximately 25 nm is determined in approximately 25 sec; the time is primarily dependent on the mechanical speed involved in stepping the mask. With a faster driving mechanism and faster electronics, the stepping time could be reduced by at least 10-fold.
DISCUSSION The spectra show typical output from the UV-Visible HTS. Although the speed of spectral analysis (about 1 nm/sec) is relatively fast, the quality of the spectra from the intense sources used is such that HTS is expected to be of limited application in the UV-Visible region. A s a result of the large noise component from a strong signal, a t each spectral element, it is difficult to distinguish weak lines (see Figure 7a, b ) ; a signal with intensity -3% of the most intense signal is lost in noise, giving a dynamic range of about 30-fold. Similar difficulties arise with the broadband continuum as evidenced in Figure 5. For these reasons, the HTS system will probably be of limited use in multielement atomic spectrometry in the UV-Visible range, where it is necessary to have the capability of distinguishing weak lines in the presence of many strong lines. Of course, the HTS has applications in the IR region (2, B ) , not considered here, where detector noise is predominant.
APPENDIX The Hadamard spectrometry concept can be summarized as follows (6). Let XI, x2, . . . , X N denote the signal (intensity) of the desired spectral elements. Instead of measuring each separately as in conventional dispersive spectrometry, signals, yl, . . . , Y N , which are combinations of the intensities XI, x 2 , . . . , xAv,of each spectral element, are measured, Le., (8)J . A . Decker,Appb Opf., 10, 510 (1971)
yi =
a,lxl
+ ... +
a,.\X~y =
Ca,,x,
The N-equations are linearly independent, a result of the design at the mask which gives the appropriate a,, coefficients. The N-simultaneous equations can be solved by one of several matrix inversion methods (9) to determine , iterative methods are the values of X I , x g , . . . , x , ~ Such quite simple to achieve with a digital computer, but are still relatively-slow for large values of N. Alternatively, the Fast Hadamard Transform (FHT) may be applied to calculate the N-spectral intensities from the N-measurements, i. e.,
The matrices la,, 1 and {b,, 1 used are closely-related to the Hadamard matrices, and appropriate permutations (6) of the signal values allow use of the FHT for calculation of the intensities X I , . . . , The details of the program used here for the HTS follow. Measurement of yL. The sequence of programmed instrumental functions consists of the following. The Hadamard mask is stepped to the first position. The combination of stepping motor and micrometer drive results in a linear motion of 1/19200 in. per motor step, and so. to move the mask the equivalent of one slot width ( i e . , 0.002 in.), 38.4 steps would be needed. Because only integer steps are available by the stepping mechanism, a sequence of 38-39-38-39-38 (repeated as a cycle of 5) steps/ slot was designed to maintain a maximum error of 0.4 step (0.4 step = 0.000021 in. or about 1%of the slot size). Because the stepping rate is 1200 steps/sec, 38 steps require about 32 msec, and thus the total time used in stepping the mask 255 slot widths consumes approximately 8.2 sec. Each time after the mask has been stepped one mask position, the signal level at that position is determined; 64 analog-to-digital conversions are performed (with provision for delay between each conversion), and the average of the 64 conversions is computed. The average obtained a t each mask position is compared with individual values (each of the 64)used to compute the average; if individual values exceeded some set deviation, 64 new individual values are acquired, and the above process repeated. x\r.
(9) T. Sheid, "Theory and Problems of Numercal Analysls." Schaum's Outline Series, McGraw-Hill Book Co , New York, N Y , 1968, pp 343-347.
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, JULY 1974
1003
Accumulation of ye's. When a sufficiently precise average measurement for the mask position is obtained, the average is either added to or subtracted from the 255 (the number of slot locations of the mask) double-precision accumulator (storage) locations according to a scheme determined by the 8-bit binary code word describing the first 8 slots of the Hadamard mask as it is then positioned. For example, for the first position of the mask, the first 8 bits (bo to b7) are lOOO1110. The accumulator number, initially 1, is determined as an 8-bit binary word with bits a0 to a7 (00000001 for the first accumulator). Corresponding individual bits are added, mod 2, according to (a0 bo) + (a1 + b l ) + . . . (a7 + b 7 ) . The sum determined in this way will be 1 if the number of matching set (1) bits in these two words is odd, and the average signal a t this slot position is then added to the respective accumulator. If, on the other hand, the number of matching set bits is even (0), then the average signal is subtracted from the particular accumulator. (It can be seen that for the first code word (10001110) there are no matching 1’s for the first storage location (00000001), and so the signal is subtracted from the first accumulator.) The accumulator number is then incremented by one, and the new set of bits representing the new accumulator number is matched with the code word, and the average signal is again added to or subtracted from the next accumulator (10001110 has 1 matching set with OOOOOO10 so the signal is added to the second accumulator). The code word remains the same (until the mask is stepped to the next location), and the accumulator number is increased by 1; then the add/subtract process is repeated until the average signal a t this mask position has either been added to or subtracted from each of the 255 accumulator locations. Measurement of Other ye's. At this point, the code word describing the first 8 slots of the next mask position is determined; this is achieved by adding, mod 2, the bits bo bz b3 bq of the first code word. By using the results of this process as bs, a new word bo’ to b7‘ is determined from bl to bs of the first word. Thus, if 10001110 describes the first code word, b8 = bo bz b3 bq = 1 0 0 1 = 0 and so bo’ to b7’ = 00011100. The new word is stored, and the mask is stepped to the next mask position, and the addition/subtraction process is repeated. After 255 mask steps, the spectral signal values are stored as double precision octal (base 8) integers with any noise portion of the signal readings resulting in a base-line fluctuation around 0. To eliminate problems caused by negative numbers being routed to a unipolar DAC, each signal value is digitally offset by a predetermined value. Procedure to Determine x,. A final permutation is needed to obtain the actual signal us. wavelength spectrum. The real channel (I) number of each signal is now related to the stored channel (1) by an 8-bit binary number which can be found according to the following process. The intensities of the first 8 spectral elements (c,;j = 1 to 8), increasing monotonically with wavelength, are contained in the storage channels (accumulators) SL as follows (calculated signal C, is an estimate of the spectral intensity x,) :
+
+
+
+
+
+
+ + +
+
+
c, = SI where 1
=
2j-I Cj= 1 t o 8 ) .
c, = s,
s, c, = s,
CL =
Ch
1004
= SIB
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 8, J U L Y 1974
Subsequent spectral intensities (C,; 9 5 j 5 255) may then be found in storage locations calculated from the binary representations of the previous eight storage locations. Spectral intensity C, (9 5 j 5 255), with binary representation ( c j , l c , , 2 c ~ , 3 c j , * c ~ , g c 1 , 6 c ~ , 7 cwill ~ , 8 ) be found in accumulator Sl where the binary representation of 1 is (s1s2s3s&s6s7sg) and where sk
=
Ck+i-9.1
+ ch+i-9.3 + c h + j - 9 . 4 + ck t,-9,j
(mod 2 ) h = 1 to 8
where s k is the value of the j t h binary bit of the S I accumulator and ciz + - 9,1 is the value of the first binary bit of the ( k j - 9)’th spectral interval and the other cterms are correspondingly defined. For example for c9 (the ninth spectral interval), we need the eight previous accumulators in binary notation
+
,
C, = 00000001, Le., C, c, = 00000010 c, = 00000100
= ~~.lc~,2~~,~~1,~~l,j~l,6~l,ic
c, = 00001000 ci = 00010000 cg = 00100000 c; = 01000000
Ca = 1OOOOc)OO, i.e.. CB= ca,ic8,2c~.~c~,4c~,jcg,~cd,~c,,o Thus for i = 9 si =
CB+l-Y,1
+
C9+1-9,2
+ C!lil-9.4 +
c9+1-9.i.= c1.1
C1.J
+
c1.4
+0+0+ 0= 0 + c2,j + e?,, + c2.j = 0 + 0 + 0 + 0 = 0 + cj,j + + c,j,j = 0 + 0 + 0 + 0 = 0 + + + c,,j = 0 + 0 + 0 + 1 = 1 + + cj,* + cj,j = 0 + 0 + 1 + 0 = 1 + + + =0+ 1 + 0 + 0= 1 + ci,3 + + c;,j = 0 + 0 + 0 + 0 = 0 + + + = 1 + 0+ 0+ 0= 1
s, =
0
s.L =
c2.1
Sj
+
= CjJ
Cl,5
CJ,*
s, = sj =
c4.1
C4,J
cj.1
cj,g
Sg
=
C6.1
cg.3
s;
= cy,1
s g = cg.1
+
c4.4
cg,,
C6.j
cy.4
C&J
ca,4
cg,5
Therefore, 1 = (00011101) = 358 = 2g10 so Cg = s 2 9 or the ninth spectral element is found in the 29th sequential storage location. Each subsequent location can therefore be determined from the 8 binary words describing the 8 previous storage locations. The storage locations are determined and the final values are stored in a sequential (increasing with wavelength) buffer area. At this time, the transform is complete, and the digital data are available for processing. It can be seen that the FHT can be accomplished in this way with N X N additions (or subtractions). For this reason, the FHT can be programmed on any computer without the need for a hardware multiply/divide or a slow, software multiply/divide package. This one-step arithmetic process is the basis for the order of magnitude increase in speed (10) compared with the Fast Fourier Transform which requires N log2 N multiplications (or divisions) (11). Since the spectral intensity values are stored, they are available for digital manipulation. The data can be read out on an oscilloscope or an x-y plotter. Alternatively, the entire vector of the signal can be printed on paper tape for storage. Also, the data can be smoothed, and the base line averaged. Peaks can be located and integrated, and comparisons with calibration standards or previous spectra
(10) J. A D e c k e r , A n a / , Chern., 44 ( 2 ) ,127A (1972) (11) J. W . Cooley and J W. T u k e y , Math. Cornput.. 19. 297 (1965)
can be made. Multiple scans can be performed, and the results added or a background scan can be generated and subtracted.
ACKNOWLEDGMENT We would like to thank John Decker for allowing us to use his design for the 509 slot mask. We also acknowledge
the assistance of Art Grant and his men a t the Chemistry Department Machine Shop. Received for review September 19, 1973. Accepted March 4, 1974. Research sponsored by Air Force AFOSR-74-2574. One of us (FWP) appreciates support from an NDEA Title IV Fellowship.
Determination of Zinc and Cadmium in Environmentally Based Samples by the Radiofrequency Spectrometric Source Yair Talmi Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn
The applicability of the radiofrequency furnace (RFF) spectrometric source to the analysis of trace amounts of cadmium and zinc in environmental samples is described. Modifications in the original design of the system were employed to enhance sensitivity and reduce interferences. Both atomic absorption ( A A ) and atomic emission (AE) spectrometric modes have been successfully used with samples pretreated by various methods such as wet ashing, on-substrate wet ashing and Soluene solubility, and by direct analysis. Samples analyzed include coal, fly ash, bunker oil, gasoline, soil, bovine liver, orchard leaves, and fish gonad. The samples, once prepared, were analyzed at the rate of five per minute with an average overall accuracy of 6.8% and reproducibility of 5.5%. Detection limits for Cd and Zn were 5 pg with the A A mode and 6 and 8 pg, respectively, with the AE mode. Relative sensitivities are in the 0.001-0.5 ppm range. Interferences in the two operating modes are compared.
Among the various non-flame spectroscopy devices, the radiofrequency furnace (RFF) source has a unique position due to its ability to employ both atomic absorption (AA) and atomic emission (AE) modes. The fundamental processes involved in RFF analysis and the parameters by which they are governed were previously described ( I ) . A presentation of the practical applicability of the system to the direct analysis of trace elements in geological, metallurgical, and biological materials has also been given ( 1 ) . The present study represents the progress to date using this system for the analysis of trace amounts of Cd and Zn in various environmental samples. The natural association of zinc and cadmium in the geological and biological environment and the fact that cadmium toxicity may be largely associated with its ability to replace zinc in important metalloenzymes or other metalloproteins are the factors which promote environmental concern (2). The samples analyzed by the RFF system included both Cd donors-i. e., coal, oil, gasoline, and tobacco products-as well as Cd accumulators-i. e., soil, sediment, and biological and plant tissues. (1) (2)
Yair Talmi and G. H. Morrison, Ana/. Chem.. 44, 1455 (1972) William Fulkerson and H. E Goeller. "Cadmium, The Dissipated Element," ORNL-NSF Environmental Program Report, Oak Ridge, Tenn , 1973, p 29.
EXPERIMENTAL Reagents. Redistilled HzO and "03 were used in the standard preparation and wet ashing procedures. "Soluene-100," a quaternary ammonium hydroxide prepared as a xylene and toluene solution by Packard Scientific, or a 25% tetramethyl ammonium hydroxide-ethanol solution served as biological tissue solubilizers ( 3 ) . High purity, 5-9's, cadmium and zinc were dissolved in " 0 3 to prepare the corresponding 1000 pg/ml standard stock solutions. Tantalum Substrates. Tantalum substrates of Ys-in. to 5hs-in. diameter with volume capacities of 5-50 r l were punched from 0.005-in. thick tantalum sheets by simple dies. Such relatively shallow substrates stack very well and up to 400, Yg-in. diameter, can accumulate inside the furnace before their removal is required. T o reduce zinc blank levels, the freshly cut substrates were prebaked in the RFF a t 1500 "C for 15 minutes. Up to 1000 substrates can be treated simultaneously. The time required for the total procedure of cutting, cleaning, and prebaking is approximately 2 hours per lo00 substrates. Solutions a t the 0.5- to 25-pl volume range were transferred to the substrates by means of a microsyringe. When highly acidic solutions were treated, Teflon-plunger type syringes were preferred to improve reproducibility and reduce blank values. Solid samples, including fine powders, were directly weighed on the tantalum substrates and then immobilized by means of 1-2p1 of a dilute collodion solution. Apparatus. The primary instrumentation utilized in this newly built RFF system (Figure 1) is similar to that used previously ( 1 , 4 ) . It includes the following modifications: A more powerful RF generator; a substantial reduction in the number of coil turns when t h e system is utilized for AA; a newly designed crucible; and attachment of the helium exhaust outlet t o a vacuum pump. The major components of the RFF system are listed in Table I. The graphite crucible is surrounded by a layer of carbon black for insulation, continuously heated by induction with an R F field and flushed with helium. The hot graphite crucible serves as the principal source of atomization, while excitation occurs in the helium plasma discharge located above the mouth of the crucible. By selecting the proper working conditions, such as the induction coil and crucible design, location of the crucible mouth in relation to the optical path, helium flow rate, and pressure, either the AA or AE mode can be optimized. Samples in the form of solids or evaporated solutions are deposited on small tantalum substrates which are fed to the furnace via the introduction chamber. Procedure. Direct Analysis. Liquids such as blood, oil, and gasoline a t the 1- to 20-pl range and solids such as freeze-dried liver, coal, and soil between 0.1-3.0 mg were transferred t o the substrates as previously described. All organic samples were then ashed in a muffle oven a t 450-500 "C for 8-12 hours, after which they were ready for analysis. (3) A. J. Schumacher, Department of Environmental Health, University of Cincinnati, College of Medicine, Cincinnati, Ohio 45219, private communication, 1972. ( 4 ) G . H . Morrison and Yair Talmi, Anal. Chem.. 42, 809 (1970) ANALYTICAL C H E M I S T R Y , VOL. 46, N O . 8, JULY 1974
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