A simple, inexpensive computer-controlled slew-scan atomic fluorescence flame spectrometer for multi-element determinations

Analytica Chimica Acta, 173 (1985) 51-62 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A SIMPLE, INEXPENSIVE COMPUTERCONTROLLED ATOMIC FLUORESCENCE FLAME SPECTROMETER MULTI-ELEMENT DETERMINATIONS

SLEW-SCAN FOR

LORI A. DAVIS, R. J. KRUPA and J. D. WINEFORDNER* Department

of Chemistry,

University

of Florida,

Gainesville,

FL 32611

(U.S.A.)

(Received 29th January 1985)

SUMMARY The system consists of a continuum xenon arc lamp source, a chopper, an argonseparated air/acetylene flame atomizer, a high-throughput, medium-resolution grating monochromator under Apple II+ wavelength control, a photomultiplier detector, and a photon counter with an Apple II+ for data collection and statistical treatment. The computer-controlled system is shown to give semiquantitative results (within *50%) for 19 elements at two wavelengths each (one wavelength for 7 elements) in less than 15 min; quantitative results (within + 5%) for each element at a selected wavelength were obtained in about 5 min. The system was characterized by determining 10 and 14 elements in synthetic mixtures and by determining a number of elements in NBS standard reference materials (orchard leaves and two steels).

Presently, graphite-furnace and flame atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry are the most commonly used atomic spectrometric methods for the determination of the concentration of elements in a variety of samples; atomic fluorescence spectrometry (a.f.s.) is less widely used. Presently there is only one commercially available atomic fluorescence spectrometer, the Baird ICP AFS-2000 [I]. While atomic absorption spectrometry (a.a.s.) provides low limits of detection, it has a relatively small linear dynamic range, approximately one to three decades [2]. Multi-element a.a.s. with individual hollow-cathode line sources is also time-consuming, especially with the conventional single-channel instruments. Modern atomic absorption flame spectrometers are capable of rapidly measuring 2, 6 or 12 elements sequentially and rapidly with no changes in instrumentation, such as hollow-cathode lamps. The xenon source, wavelength modulated, echelle, flame/furnace atomic absorption spectrometric system of Harnly et al. [3] allows up to 20 elements (wavelengths) to be measured simultaneously. While a.f.s. has been noted as a valuable tool in multi-element determinations, it has not been commonly used [2, 41. However, it has several potential advantages over a.a.s. Detection limits by a.f.s. are generally equal to or better than a.a.s., and a.f.s. has a linear dynamic range of three to six decades. 0003-2670/85/$03.30

o 1985 Elsevier Science Publishers B.V.

52

Just as in a.a.s., a.f.s. has high selectivity. The presence of chemical (matrix) interferences depend on the atomizer used. Because of the high spectral selectivity, a multi-element atomic fluorescence spectrometer is easy to operate and can be constructed at a relatively low cost. Several years ago, Ullman et al. [5] proposed the use of a wavelengthmodulated computer-controlled atomic emission/fluorescence spectrometer for multi-element determinations [5] . Based on this design, our system, a computer-controlled, slew-scan, multi-element atomic fluorescence spectrometer (m.e.a.f.s.), was constructed at a low cost and for ease of operation. An Eimac (Cermax) xenon arc lamp, which is a continuum source of radiation, was used as the excitation source. This eliminated the need for a different source for each element which is measured, and thus eliminated the normally long warm-up time for individual lamps. The m.e.a.f.s. system in the present case sequentially slew-scans to thirty-one different fluorescence wavelengths, and determines the presence of and concentrations of nineteen elements in a water matrix. The scanning of the monochromator and the data acquisition are all under computer-control. The signal data are converted to concentrations by using calibration curves stored within the computer programming. The system was designed for ease of operation, time efficiency, wide applicability, and low cost while yielding semi-quantitative results (results within a factor of 2X the known results). EXPERIMENTAL

Instrumental system A block diagram of the system is shown in Fig. 1. Much of this system was based upon systems previously described [ 5, 61; a list of instrumental components is given in Table 1. A 300-W Eimac (Cermax) xenon arc lamp (the excitation source) was focused onto an argon-separated air/acetylene flame. The optical train consisted of an f/4,4-in. focal length Suprasil lens focusing FLAME

CHOPPER

ARC LAMP POWER

SUPPLY

I

AMPLIFIER DISCRIMINATOR

PHOTON COUNTER

Fig. 1. Block diagram of the m.e.a.f.s. system.

APPLE II

+

53 TABLE

1

Components of the m.e.a.f.s.

instrument

Component

Model No.

Company

Monochromator Photomultiplier Photomultiplier power supply Photomultiplier housing Photon counter Amplifier/discriminator Digital synchronous computer Nebulizer Capillary burner Xenon arc lamp Lamp power supply Computer Interfaces Video terminal Printer Dual disk drives

218 EM1 9789QB 244 1151

GCA McPherson Thome Keithley SSR Instruments

1120 1110 303-0110

SSR Instruments SSR Instruments Perkin Elmer Laboratory-made EIMAC, Div. of Varian EIMAC, Div. of Varian Apple Laboratory-made Zenith Vista Computer Co. Apple Rana Superior Electric Co. Superior Electric Co. Ithaca

Stepping motors Translator module Chopper

300 uv PS300-1 II’

V300-25 Disk II Elite I M063-FC06 STM 101 382 B

C2

the light from the xenon arc lamp onto the flame and an f/5.3,2-m. focal length Suprasil lens in a 2f configuration focusing a 1:l image onto the entrance slit of the monochromator. The fluorescence signals were amplitude-modulated by means of a chopper placed between the xenon arc lamp and the first lens. Several precautions were also taken to reduce scattered light reaching the monochromator. Two sets of cylindrical snouts, painted with flat black paint, enclosed the light path. The first set of snouts was placed between the xenon arc lamp and the first lens, enclosing the light path. These snouts were also purged with nitrogen gas to reduce the formation of ozone. A l-in. aperture (diameter of the collimated beam emitted by the xenon arc lamp) was placed on the end of one snout prior to the first lens. The second set of snouts enclosed the light path from the flame to the monochromator. A second aperture was placed prior to the second lens and was 0.75 in. to match the angle of acceptance of the monochromator. The pre-mixed argon-separated air/acetylene flame was fuel-rich to reduce the background in the OH region [7]. A laboratory-constructed stainlesssteel capillary burner was used. Both the air and acetylene gas flows were doubly regulated to reduce fluctuations in the flame. Nupro fine-metering valves were also placed after the rotameters to adjust and set the gas flows to the burner. The gas can, thus, be shut off from the burner at the rotameters

54

without changing the flow settings. The argon sheath was also used around the flame to minimize entrainment of air into the flame. Several precautions were taken to reduce radiofrequency (r.f.) noise. In addition to a photomultiplier housing which was magnetically and r.f. shielded, a doubly shielded coaxial cable was constructed for the high voltage cable to the photomultiplier tube. All other cables were wrapped in aluminum foil and electrical tape and grounded to reduce rf. pickup. The Eimac xenon arc lamp was also housed in a ventilated, grounded, aluminum box. The photon counter was mounted in a bench which was surrounded on three sides with grounded copper screen acting like a Faraday cage. These precautions helped reduce r.f. noise and thus lowered the background count rate. An Apple II’ microcomputer controlled the sequential slew scanning of the monochromator by means of a stepping motor attached to the grating drive lead screw [6]. The slew scan (ca. 16 nm s-l) was controlled by an assembly language program. Acquisition of the fluorescence signal from the photon counter was also under computer control. The experimental parameters (used with SEMI-QUANT) chosen to reduce noise and fluctuations in the system are given in Table 2. Computer system and progmmming A listing of the computer programs is available from the authors on request. Two programs, SEMI-QUANT and QUANT are written in BASIC for use with the m.e.a.f.s. system. The SEMI-QUANT program also used an assembly language program, which is callable from BASIC, to control the sequential slew scan. The QUANT program is a modification of an LOD program written by Long, based on a propagation-of-error approach to limits of detection [8]. TABLE 2 Experimental

conditions used with the SEMI-QUANT program

Component

Condition

Monochromator

0.3-m Czerny-Turner, f/5.3; 600 grooves/mm grating blazed at 300 nm; 90-Mm slit width; l-cm slit height; 0.4~nm spectral bandpass

Source power arc voltage arc current Chopper frequency Photon counter Flame gases (1 min-‘) Nebulizer aspiration rate Observation height

300 w 15 v 19.6 A 141 Hz Chop mode; difference display select; preset N = 1410 (10 s); chopper sampling time = 3.2 ms Air atomization = 5.6; air auxiliary = 0.8; acetylene = 1.2 ; argon = 8.5 5 ml min-’ 1.8 cm above burner

55

In this program, calibration graphs for an element at a single wavelength are obtained. Upon completion of the calibration graph, the limit of detection (LOD) fork values of 2 and 3, a propagation-of-error LOD, and the equation for the calibration graph are printed out. Finally, samples are introduced, the fluorescence signal at the specific fluorescence line used for the calibration curve is measured and converted by means of the program into an analyte concentration, and the results (concentrations) are printed. To use the QUANT program at a single wavelength requires about 5 min for determination of concentration (*5%). The SEMI-QUANT program is written to allow rapid sequential slew scanning to 31 fluorescence wavelengths and to facilitate collection of data for both qualitative and semi-quantitative analysis of 19 elements using stored (up to 6 months) calibration data. The experimental conditions for use with this system (see Table 2) are chosen to reduce noise and fluctuations in the system over time. The SEMI-QUANT program resulted in slew scanning of the monochromator to each wavelength and in the collection of two measured signals; each measured signal is the number of counts in a 10-s integration period. The first signal is compared to the sum of three times the standard deviation of the blank signal plus the average blank signal, i.e., 3s,, + %J, based on the IUPAC definition of limit of detection [9]. If the first measured signal is less than the value of 3sb + 3c,,, then the second value is not collected. Otherwise, the two measured signals are averaged, and the average is stored. These averages are then compared to the stored log-log calibration graphs and concentrations are calculated. For the concentrations to be accepted, they must fall within the previously established linear dynamic ranges at each wavelength. A table of the elements detected, their wavelengths, and the concentration calculated at that wavelength is then printed out. The program was also adapted to the collection of blank signals and/or standard signals prior to running a sample. The m.e.a.f.s. system was found to be background shot-noise limited. Thus, the standard deviation of the fluorescence signal is equivalent to the square root of the sum of the fluorescence and emission of the flame [lo]. The values for the square root of the sum at each wavelength were also stored within the program. The sum of the blank signal plus three times the square root of the background is then used for each line of each element as the minimum signal with which to compare the measured signals for qualitative and semi-quantitative measurement. The fluorescence signal of one standard solution (located in the middle of the calibration data) collected at each fluorescence line is used to correct the intercepts of the log-log calibrations. Blank signals and standard signals may be collected as often as necessary (in this case, once every two weeks). The slopes of these log-log calibrations are very close to unity (1.00 + 0.05). By periodically measuring blank signals and signals of standard solutions, the m.e.a.f .s. system can thus be corrected for long-term drift.

56

R eagen ts The 1000 pg ml-’ stock solutions of each element were prepared by dissolving the pure metal or reagent-grade salt in dilute acid, and diluting with deionized water which was further purified using a Barnstead Nanopure filtration system (14 Ma cm-’ resistivity). Volumetric dilutions of the stock solutions were made to obtain standard solutions of the desired concentrations. Two multi-element solutions were also prepared to evaluate the instrument. One such solution, a ten-element mixture called lOM, was prepared by dissolving the metals in nitric acid. An aliquot of this solution was then diluted to be within the linear dynamic range of the system. The second solution, a fourteen-element mixture called 14M, was prepared by taking aliquots of standard solutions and diluting them in a volumetric flask. This solution was also diluted further to be within the linear dynamic range of the instrument. Solutions of National Bureau of Standards (NBS) Standard Reference Materials (SRM) were prepared by established procedures [ 111. Aliquots of these solutions were then diluted so that the concentration of all major constituents was 100 pg ml-’ or less, to approximate a water matrix. Water was used as the blank in all cases. RESULTS

AND DISCUSSION

Detection limits In Table 3 are listed limits of detection obtained under optimized conditions (applicable to the QUANT program) and constant conditions (Table 2, applicable to the SEMI-QUANT program) for all lines of all elements studied. Two lines were used for each element when possible to increase the reliability of the results. The linear dynamic ranges are also indicated in Table 3; this range corresponds to the range of concentrations for which the slope of the log-log plot is 1.00 + 0.05. It should be noted that the linear dynamic range for sodium at 589.6 nm is only one decade; this is caused by a high background and the maximum count rate of 85 MHz for the photon counter used. In Tables 4 and 5, the recoveries of 10 and 14 elements, respectively, in two synthetic solutions are given. Except for iron (at 252.285 nm), all elements are recovered within &16% in the case of the 10M synthetic mix. Except for Au, Pt, Tl, and Rh (at 350.252 nm), all elements were measured within +23% in the case of the 14M synthetic mixture. In the 10M mixture, the high iron result at 252.285 nm may be due to a spectral interference by cobalt at 252.136 nm. In the 10M mixture, the somewhat low result for copper occurs at both copper lines, indicating a non-spectral interference. In the 14M mixture, the low results for thallium may have been the result of thallium(I) chloride precipitating out of solution, because of a high concentration of hydrochloric acid before dilution. The very high results for gold at 267.595 nm and platinum at 265.945 nm are certainly due to spectral interferences. The system also does not sufficiently resolve the cobalt and gold lines at 242.493 nm and 242.795 nm, respectively.

57 TABLE 3 Limits of detection (LOD) and linear dynamic ranges obtained by the m.e.a.f.s. system Element

Wavelength (nm)

LODa (ti g ml-’ )

Upper conc.b (clg ml” )

LOD a.a.sc (pg ml-’ )

Cd Ca Cr

228.800 422.673 357.869 359.349 240.7 25 242.493 324.754 327.396 242.795 267.595 303.936 451.131 248.327 252.285 283.306 405.783 285.2 13 279.482 232.003 352.454 265.945 306.471 343.489 350.252 328.068 338.289 589.592 460.733 276.787 377.572 213.756

0.01 0.002 0.007 0.03 0.005 0.3 0.05 0.02 0.2 0.0003 0.003 0.04 1.0 0.08 0.004 0.06 0.003 0.04 0.02

1 1 5 10 10 10 5 5 40 100 50 10 100 100 50 50 1 10 5 10 100 1000 100 100 5 5 1 5 50 50 1

0.03 NA (0.003) 0.02

co cu Au In Fe Pb

Mg

Mn Nl Pt Rh

& Na Sr Tl Zn

(0.01) (0.004) (0.009) (0.01) (0.06) (0.09) (0.008) (0.02) (0.3) (0.3) (0.2) (0.08) (0.05) (0.07) (0.2) (0.2) (0.0006) (0.005) (0.06) (0.1) (1) (5) (0.2) (1) (0.004) (0.007) (0.1) (0.01) (0.2) (0.04) (0.02)

0.07 0.01 0.17

0.07 0.1 0.001 0.01 0.07 1 0.07 0.007 0.003 NA (0.03) 0.1 0.07

‘Values obtained for optimized flame conditions; values in parentheses were obtained for m.e.a.f.s. SEMI-QUANT experimental parameters. bConcentration at which linearity of calibration curve has changed by 5% (see text). CValues taken from O’Haver [12] for simultaneous multi-element atomic absorption flame spectrometry. NA means nitrous oxide/acetylene flame; ah other values are for air/acetylene flames. The upper concentration for linear response is obtained by combining several calibration lines obtained at different points on the absorption profile during wavelength modulation. Those upper concentrations are usually lo’-10’ the limits of detection.

58 TABLE 4 Determination gram Element

Cd Ca co CU Fe Pb MII Nl &

zn aError = ((cont.

of ten elements in a synthetic mixture (10M) with the SEMI-QUANT pro-

Wavelength (nm) 228.802 422.673 240,725 242.493 324.754 327.396 248.327 252.285 283.306 405.783 279.482 232.003 352.454 328.068 338.289 213.856

found -actual

Concentratlon(pg ml-') Actual

Found

0.433 0.290 2.38

0.378 0.292 2.35 2.50 0.574 0.573 5.61 6.41 2.77 2.79 0.722 1.86 2.17 0.121 0.129 0.163

0.669 5.03 2.75 0.858 2.01 0.135 0.157

conc.)/actual

ErrI+ (46) -12.7 0.7 -1.0 5.5 14.2 -14.3 11.5 27.5 0.8 1.6 -15.8 -7.6 8.0 -10.4 -4.4 3.8

cont.) x 100.

TABLE 5 Determination program

of fourteen elements in a synthetic mixture (14M) with the SEMI-QUANT

Element

Wavelength (nm)

Cd Ca Cr

228.802 422.673 357.869 359.349 240.725 242.493 242.795 267.595 303.936 451.131 285.213 279.482 265.945 306.471 343.489 350.252 589.592 460.733 276.787 377.572 213.756

co AU In Mg Mll Pt Rh Na Sr Tl ZIl

Concentration(Fgml-I') Actual

Found

0.100 0.100 1.00

0.097

1.00 1.00 5.00 0.0500 0.100 10.0 5.00 0.500 1.00 5.00 0.100

0.103 0.969 1.00 0.956 (0.988jb (3.13)b 0.701 4.91 5.00 0.0439 0.100 22.0
Errora m) -2.9

3.0 -3.1 0.3 -4.4 (-1.2)b (21.3)b -29.9 -1.8 0 -12.2 0 120 -2.6 41.4 -22.8 3.5 -54.0 -61.2 -4.2

%ee Table 4. bThe monochromator does not have sufficient resolution fully to separate these lines. C
59

It must be stressed that the results in Tables 4 and 5 were obtained by using the SEMI-QUANT approach, involving stored calibration curves. A 19component mixture can be processed in about 15 min. By means of a faster stepping motor and by decreasing the integration time to 5 s, a 19-component mixture (at 3 1 wavelengths) could be processed in 3.5 min but the imprecision is doubled. If greater precision is needed, then the QUANT program must be used involving the preparation of fresh calibration data for each line of each element prior to sample processing. Table 6 shows a comparison of the recoveries for the SEMI-QUANT and QUANT approaches; the superior accuracy of the QUANT approach is apparent. However, the QUANT approach is considerably more time-consuming as discussed above. Analysis of reference materials In Table 7, the concentrations of several elements in three NBS-SRMs (Orchard leaves and two carbon steel samples) measured by the m.e.a.f.s./ SEMI-QUANT approach are given and compared with certified values. For most elements, the results are reasonable, especially when viewed from the semi-quantitative aspect. In all cases, the concentrations are within a factor of two of the certified values. Reproducibility The 10M and 14M synthetic mixtures were used to obtain estimates of the relative standard deviation (RSD) for measurements taken over a period of time (ranging from a week to a month). In Table 8, the RSD values are given for all elements, except for platinum and gold (at 267.595 nm) which were not reproducibly detected. These measurements were all obtained by using the SEMI-QUANT approach (with constant input blank and standard values and therefore no recalibration). The average RSD for all lines is 10.7%. It is obvious that the reproducibility of the m.e.a.f.s. system in the SEMIQUANT program mode is considerably better than the accuracy (especially if Table 7 is considered). TABLE 6 Comparison of recoveries for m.e.a.f.s. with the SEMI-QUANT and QUANT approaches Element

Cd Ca Cr co Au Pt Zn

Wavelength (nm)

Actual cont. (clg ml-’ 1

SEMI-QUANT (pg ml” 1

228.802 422.673 357.869 240.725 267.595 265,945 213.756

0.100 0.100 0.100 1.00 1.00 10.0 0.100

0.100 0.098 0.054 1.14 1.18 2.91 0.094

aSee footnote to Table 4.

Errora (a) 0 -2 -46 14 18 -71 -6

QUANT (kg ml-‘)

Errora (%)

0.107 0.103 0.106 1.15 1.05 9.41 0.112

0 3 6 15 5 -6 12

60 TABLE 7 Analysis of NBS standards by m.e.a.f.s. and the SEMI-QUANT approach Element

Wavelength (nm)

SRM 1571 (Orchard Leaves)a Ca 422.673 Mg 285.213 SRM 361 (High Carbon Steel,Jb cr 359.349 cu 324,754 327.396 Fe 248.327 252.285 Mn 279.482 Ni 232.003 352.454 SRM 364 (High Carbon Steel)d cr 359.349 CU 324.754 327.396 Fe 248.327 252.285 Mn 279.482 & 328.068 338.289

Concentration Actual

(pg ml-‘) Found

1.20 0.267

0.677 0.267

1.40 0.086

0.94 0.113 0.124 > LDRC 69.0 1.31 5.10 5.34

1.35 4.08

0.07 0.293 118 0.305 0.008

0.12 0.266 0.284 85 69 0.353 0.010
%oncentration (rg g”) of analyte in orchard leaves is calculated as the concentration (pg ml-‘) of analyte measured X 250 ml of solution X loo-fold dilution/O.8609 g of sample. bConcentration (rg g-i) of analyte in SRM 361 is calculated as the concentration (rg ml-‘) of analyte measured x 250 ml of solution x 50-fold dilution/2.5596g of sample. VLDR means concentration is above the range of linear response. dConcentration (pg g-‘) of analyte in SRM 364 is calculated as concentration (pg ml-*) of analyte measured X 250 ml of solution X loo-fold dilution/3.0515 g of sample.

Conclusions The m.e.af.s. system with the SEMI-QUANT program based on stored calibration curves gives reasonable precision and accuracy for low concentrations. The QUANT program improves the accuracy and precision but at the expense of speed. For samples such as steel, the accuracy is degraded because the matrix causes spectral interferences in some cases. The m.e.a.f.s. system is simple to operate and easy to adapt to specific problems, because of the use of a flame atomizer. The system could be improved by better r.f. shielding to reduce the background even more, by a photon counter with a higher maximum counting rate, by a higher-resolution grating in the monochromator and by a more powerful light source. For higher light levels, as are found in nitrous oxide/acetylene flames, a lock-in amplifier could be used rather than the photon counter.

61 TABLE 8 Reproducibility time

(RSD for concentration)

of the SEMI-QUANT

mode over a period of

Element

Wavelength (nm)

RSD (%)

Wavelength (nm)

RSD (%)

Cd Ca Cr co cu Au

228.802 422.673 357.869 240.725 324.754 242.795

8.3 17.8 22.8a 10.6 5.5 2.ga

359.349 242.493 327.396 267.595

8.1a 7.7 5.5

In Fe Pb Mg Mn Ni Pt

303.936 248.327 283.306 285.213 279.482 232.003 265.945

4.9s 4.3 9.4 8.4 10.6 7.5 -a

451.131 252.285 205.783

9.7a 4.8 14.3

352.454 306.471

4.6

Rh Ag Na Sr Ti Zn

343.489 328.068 589.592 460.733 276.787 213.856

11.9a 5.7 33.2a 20.7a 12.2a 26.7

350.252 338.289

1.0s 11.8

377.572

9.5a

4

4

aThese values are based on three readings taken over a week for solution 14M. The other values are based on six readings taken over a month for solution 10M. bConcentrations not reproducibly detected.

The authors acknowledge the help of Dr. Benjamin W. Smith and Dr. Edward G. Voigtman in the design of this system. This work was supported by AF-AFOSR-F49620-84-C-0002. REFERENCES 1 D. R. Demers and C. D. Allemand, Anal. Chem., 53 (1981) 1915. 2 J. D. Winefordner, J. J. Fitzgerald and N. Omenetto, Appl. Spectrosc., 29 (1975) 369. 3 J. M. Harnly, N. J. Miller-Ihli and T. C. O’Haver, Spectrochim. Acta, 39B (1984) 305. 4 J. D. Winefordner, in K. Fuwa (Ed.), Recent Advances in Analytical Spectroscopy, Pergamon Press, New York, 1982, pp. 151-164. 5 A. H. Uliman, B. D. Pollard, G. D. Boutilier, R. P. Bateh and P. Hanley, Anal. Chem., 51(1979) 2382. 6 P. Wittman, J. Bower, J. J. Horvath, A. Uliman and J. D. Winefordner, Can. J. Spectrosc., 26(5) (1981) 212. 7 K. Fujiwara, A. H. Ulhnan, J. D. Bradshaw, B. D. Pollard and J. D. Winefordner, Spectrochim. Acta, 34B (1979) 137. 8 G. L. Long and J. D. Winefordner, Anal. Chem., 55 (1983) 712A. 9 Nomenclature, Symbols, Units and Their Usage in Spectrochemical Analysis - II, Spectrochim. Acta, 33B (1978) 242.

62 10 B. D. Pollard, A. H. Ullman and J. D. Winefordner, AnaL Chem., 53 (1981) 330. 11 R. Mavrodineanu (Ed.), Procedures Used at the NBS to Determine Selected Trace Elements in Biological and Botanical Materials, NBS Spec. Pub. 492, Department of Commerce, Washington, DC, 1977. 12 T. C. O’Haver, Analyst (London), 109 (1984) 211.