fluorescence

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979

Computer-Controlled Multielement Atomic Emission/Fluorescence Spectrometer System A. H. Ullman,’ B. D. Pollard,* G. D. B ~ u t i l i e r R. , ~ P. Bateh, P. Hanley, and J. D. Winefordner“ Department of Chemistry, University of Florida, Gainesville, Florida 326 7 7

A versatile system for multieiement analysis and analytical studies of noise sources, flames, modulation techniques, and statistics of error propagation has been constructed. The instrument conslsted of an EIMAC xenon arc lamp for fluorescence excitation, a flame (most commonly airlacetylene or nitrous oxlde/acetylene), and a wavelength modulated, slewed-scan monochromator with data acquisition by a photon counter. The entire Instrument was under computer (PDP 11) control. Analytical figures of merit (detection limits and growth curves) for 25 elements are presented along wlth a discussion of instrumental design, flexibility, mode of modulation, and operation. Applicability to real samples Is demonstrated with respect to trace metal analysis in jet engine oils (SOAP samples) and NBS Standard Reference Materials.

T h e importance of “simultaneous” multielement analysis to the fields of medicine, agriculture, energy, environment, etc. has become more and more important in recent years. Various spectroscopic techniques, such as direct readers, multiplexing, vidicon tubes, and diode arrays, have been applied to multielement analysis. These approaches have been summarized in several papers (1-3). A slewed-wavelength scan (programmed scan) has been shown to be a viable method for multielement analysis ( 4 , 5 ) particularly because of its efficient use of analysis time as compared to conventional linear scan spectrometry. In the vast majority of spectroscopic instrumental systems, amplitude modulation (“chopping”, pulsing, or sinewave modulation) of a light source was used to reduce the noise and correct for some of the background. Recently, wavelength modulation has been shown to have some theoretical advantages in increasing the signal/noise (S/N) ratio (6), and in compensating for spectral interferences (7-9) and Rayleigh and Mie scatter of exciting radiation (9). In addition, wavelength modulation gives a signal consisting of all the onwavelength information; that is, the signal consists of emission plus fluorescence. Amplitude and wavelength modulation have been experimentally compared and the advantages and disadvantages of each were noted for atomic fluorescence and/or emission signals for different flames. We have found the added flexibility provided by the emission plus fluorescence signal quite useful in determining trace elements in real samples. Programming for our P D P 11 controlled system was designed for ease of use and ease of modification. Control of the monochromator stepping motor and of the photon counter were accomplished by use of machine language subroutines callable from BASIC. All other programming was in BASIC which permitted extensive file storage and manipulation on disks Present address: Industrial Chemical Division, Procter & Gamble 11530 Reed Hartman Highway, Cincinnati, Ohio 45241. $Present address: Department of Chemistry, Marquette Universit Milwaukee, Wis. 53233. xPresent address: Department of Chemistry, University of Georgia, Athens, Ga 30602. 0003-2700/79/0351-2382$01 .OO/O

Table I. Components of t h e MEAFS Instrument Company address component model no. GCAiMcPherson, monochromator 218 Chicago, Ill. Gencom Division, photomultiplier EM1 62569 Plainview, N.Y. 1 1 8 0 3 SSR Instruments, photomultiplier 1151 Division of Princeton housing Applied Research, Princeton, N.J. photomultiplier 1 1 0 5 power supply amplifier/ 1120 discriminator 1110 digital synchronous computer Perkin-Elmer, Norwalk, nebulizer 303-0110 Conn. 06852 ___ Laboratory constructed capillary burner EIMAC, Div. of Varian, xenon arc 300 U V San Carlos, lamp Calif. 94070 lamp power PS300-1 supply Digital Equipment computer PD-11 (24k) Corp. Maynard, Mass. 0 1 7 5 4 interfaces DEC Kit 2-H parallel binary I/O DR-11 A general purpose Consul 580 Analog Digital Data video Systems, Inc., terminal Hauppague, N.Y. 11787 LA-36 Digital Equipment printer Corp., Maynard, Mass. 01754 Advanced Elect. Design, dual disk drive 2500 Sunnyvale, Calif.- 94086 stepping motor HDM-12-480-4 USM Corp., Wakefield, Mass. 01880 _._ Laboratory constructed modulator electronics R4-155 M F E Corp., Salem, modulator N.H. 03079 motor

as well as rapid and simple program modification (ascompared to machine language, or even to FORTRAN which must be compiled and relocated in core). Our system is based upon a computer-controlled, slewedscan monochromator, continuum source (EIMAC xenon arc lamp) excited atomic fluorescence with wavelength modulation yielding a net signal consisting of emission plus fluorescence, synchronous photon counting, and a choice of analytical flames as the atomization cell and exciter (in the case of emission). The analytical utility of the system for rapid multielement analysis, fundamental noise studies, flame characterization, 1979 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979

PC

ClxrRx

2383

Lrl-m PDP I1 U4KI

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Figure 2. Block diagram of computer control system

LKm W

Figure 1. Block diagram of the multielement atomic emission/atomic fluorescence spectrometer (MEAFS)system

Table 11. General ExDerimental Conditions Used with MEAFS Instrument condition component 0.3-m Czerny-Turner, monochromator fl5.3, 600 grooveslnm, grating blazed a t 300 nm slit width 75 pm 1 cm slit height 0.7 nm spectral bandpass source 300 W power arc voltage 15 V arc current 20 A amplitude modulation 265 Hz frequency 0.5 duty factor wavelength modulator 1 2 . 5 Hz frequency 0.4 duty factor i 0 . 4 nm wavelength range flame air /acetylene neb. air 10.5 L/min aux./air 1.0 L/min (fuel-rich flame) C,H, 3.5 L/min N* 1 2 . L/min N,O/acetylene neb. N,O 5 . 5 L/min (fuel-rich flame) aux. N,O 2.7 L/min 4.6 L/min C,H, N, 12. Limin nebulizer aspiration rate 5 mL/min a n d general system optimization is described.

EXPERIMENTAL Instrumental System. A block diagram of the multielement atomic emission/fluorescence spectrometer system (MEAFS) is shown in Figure 1. Much of the system was described previously ( 4 , I O ) and is therefore merely outlined in Table I. A 300-W EIMAC xenon arc lamp was used as the excitation source with an optical system consisting of a 3.5-inch focal length lens focusing the light at the flame and a 3-inch focal length lens a t 2 f transferring a 1:l image of the flame to the entrance slit of the monochromator. For wavelength modulation, a 0.3-cm thick quartz plate (1.2 cm X 2.0 cm) was positioned within the monochromator just behind the entrance slit. A torque motor was mounted on top of the monochromator with its shaft extending down inside the monochromator and attached to the quartz refractor plate. The motor was driven by a “stepped-square” wave with a reference signal triggering the photon counter (11). Thus, we were able to wavelength modulate both “above” and “below” the analytical wavelength (further details on the system are available in ref. 11). Amplitude modulation was performed by means of a mechanical chopper operating at 265 Hz. A three-lens optical train was used for both modulators. Collimated light from the EIMAC lamp was focused onto a stop where the mechanical chopper was placed, recollimated by a second lens, and then focused onto the flame. When wavelength modulation was being performed, the optical chopper was fixed in an open position. In Table 11, modulation

Table 111. Modes Available on the MEAFS Instrument 0. Instructions 1. Calibrate Monochromator

2. Parameter File Set-Up 3. Multielement Calibration Curve Runs 4. Multielement Analysis Runs 5 . Spectral Scan A . Ordinary Scan i. Store Scan On Disk 1 1 . Remove From Disk B. Noise Scan a. Plot b . Manipulate Scans by adding together, subtracting, or merging

} {,,

6. 7. 8. 9.

Quick Qualitative Survey Limits of Detection Single Element Calibration/Analysis Performance Optimization and Statistics (Noise) Studv

parameters for the amplitude and wavelength modulation devices are given. With the exception of the gas flows and sample interchange (both of which could readily be automated), the entire system was controlled by the computer using simple commands, input at the video (CRT) terminal. Computer System a n d Programming. (See ref, 11 for all machine language and BASIC source listings). Monochromator control was via a stepping motor attached directly to the grating drive lead screw, disengaging the normal wavelength drive (Figure 2). A general purpose interface to the stepping motor driver was controlled by an assembly language program callable from BASIC. Typically, the slew rate was =13 nm 5-l (e624 steps s-’). A “ramping” function to start the stepping motor as in ref. 4 was not used and no problems with missed steps were encountered at this speed. The photon counter was also under computer control (Figure 2). Program Mode (see Table 111). Monochromator calibration served two purposes: to accurately define the number of steps (nm-’) and more importantly, to accurately position the monochromator a t a known wavelength (calibrate monochromator mode). In a previous work ( 4 ) , the atomic lines of Hg (from a Hg pen lamp) were used for calibration; however, in our case, a flame was more convenient as the calibration source; the flame was placed on the same position as the calibration source or a wavelength offset occurred. A solution of Mg and Ca was aspirated into the flame, and the monochromator slewed from its normal starting position (at 200.0 nm) to just below the 285.213 nm line of Mg; the monochromator wavelength was then stepped slowly upward in wavelength taking data at each step (=#2 s counting time/step). The counts at each step were compared to the previous step until the maximum was located. To be certain the peak had been located, the stepping continued a short distance beyond the peak, and then rapidly slewed to =422 nm where, in an analogous manner, the Ca 422.673-nm line was located. Using the location of the two peaks as “benchmarks”, the monochromator was now calibrated and was returned to 200.0 nm to await further instructions. The experimental procedure for a multielement analysis utilized a file prepared in the parameter set-up mode. Such a file contained the count times to be used, the elements and their wavelengths, and other pertinent information. Of particular interest

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ANAL rICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979

WAVELENGTH

Inml

284 284 284 284 284 284 284 284 285 285 285 285 285 285 285 285 285 285 286 286 286

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INTENSITY OF

3

4

5

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6

7

8

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189 293 397 501 605 709 814 918

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126 23 334 439 543 647 751 355 959 064 168 272

Figure 3. Histogram (second derivative spectrum) of magnesium (0.1

ppm) in N,-separated premixed airlacetylene flame with wavelength modulated MEAFS system was the “S/N cutoff’ which allowed a time savings during an analysis. When the S / N of a metal being measured reached the predetermined S / N (for example S / N = loo), the instrument continued onto the next element. The file was stored on disk but was readily accessible to modification. The monochromator under computer control slewed from 200.0 nm to the wavelength of the first element, and the signal was counted for the prescribed time period. The monochromator was then slewed to the next element, the signal again counted for the prescribed time period, and so on until all elements, in ascending order of wavelength, were measured. When the first blank had been completed, the same procedure was used for the standard. Before and after each run of a standard, the operator might choose to run another blank. The average of the two most recent blanks was subtracted from each standard before running the next one and, after all had been completed, the least square fit was calculated. If an internal standard had been measured, all signals would have been ratioed to it, and a “corrected” least square fit generated. The calibration data could then have been printed out and/or stored on the disk. The determination of metals in unknown samples was then performed by measuring the unknowns in a manner analogous to the standards. The same options concerning S/Ncutoff, number of blanks, and use of an internal standard were available. After each unknown was completed, the data, consisting of counts (corrected for the blanks), concentrations (multielement calibration curve mode), and precision (standard deviation, relative standard deviation, etc.) were then printed out. The flexibility of the computer programming is shown by the number of modes of operation listed in Table I11 (the interested reader can obtain more information from Reference 11). Two scanning modes (see Table 111) and their associated programming were used to study such things as flame emission or fluorescence backgrounds, lamp output, and noises on these, and to check for spectral interferences. The scan mode allowed the operator to input the lower and upper wavelength limits of the scan, the resolution (minimum = 1 step or ~ 0 . 0 2nm) and the count time per resolution increment. In operation, the monochromator slewed to the low wavelength limit; data were then taken while the monochromator stepped upwards from the lower wavelength limit. After completing the scan, the operator either chose to print out the raw counts (wavelength and counts from all four photon counter channels), to print out a histogram of any of the channels (Figure 3), or to store the entire scan on the disk. If the latter option was selected, the scan could be printed out later, added to or subtracted from other scans, or merged with other scans to expand the wavelength range, and then printed out. The noise scan mode operated in the same manner but took more than one measurement at each wavelength increment. Statistical analysis of the replicates at each wavelength interval allowed estimation of total noise, shot noise, and flicker noise which could be determined over any selected spectral region and printed out as a spectral noise distribution. The use of this mode

for determination of spectral noise distrihutions was reported previously ( 2 2 ) . The MEAFS system was also used for a quick qualitative analysis (quick qualitative survey mode) which was similar to the multielement analysis mode except that the system was programmed to monitor up to 35 elements. The operator selected the elements of interest and input the count time. Beginning at 200 nm, the monochromator slewed (in ascending wavelength order) to the analytical wavelengths of each element selected. This procedure was followed first for a blank and then for the sample. The printout listed each element, the signal (corrected for the blank), and the S / N (for simplicity of calculation, noise was assumed to be only shot noise). From the signal and S / N , a semiquantitative estimate of the concentration of each element in the sample could be made. This mode has been found to be extremely useful in surveying a wide variety of samples and for “previewing” an unknown to be determined quantitatively (allowing for appropriate dilution of the sample and choice of the proper concentration range for standards). The limits of detection were determined, one element at a time, with the operator designating the element, wavelength, counting time, and concentrations of standards to be used. Before measuring the blanks or standards, the monochromator was slewed to the analytical wavelength and automatically “peaked” by scanning across the atomic line while the sample was being aspirated. The detection limit was determined by measuring several standards with a total of 16 blanks interspersed between the standards; typically, the order was: four blanks; 1st standard; four blanks; 2nd standard; etc. The detection limit was calculated for a S / N of 3 where the noise was the total noise on t,he blank, Le., the standard deviation of the 16 replicates. This estimation procedure was more conservative than those used by many others, but it was felt to be more realistic for analytical situations. The single element calibration mode was used where only one element was to be determined. After the wavelength was “peaked”, as in the limit of detection mode, the measurement began. Standards and samples were measured in any sequence with blanks measured before and/or after each. When the measurements were completed, two different linear regressions (log-log and normal) were performed and the concentrations of the metal were calculated by using both regressions and by interpolating between the two surrounding standards. The statistics and optimization mode involved, at any given wavelength, any number of measurements (of any duration) and estimation of averages, standard deviations, etc. For optimization of parameters, such as aligning optical components or adjusting the flame, short counting times were used. Statistics of the measurements (average, standard deviation) were used to study different noise sources (13). Experimental Conditions and Solutions. Instrumental and flame conditions used are given in Table 11. Gas flows for the air/acetylene flame were adjusted for a slightly fuel-rich flame, which was optimal for most elements; a more fuel-rich flame generally has a lower background and noise than a leaner flame (12). The nitrous oxide/acetylene flame was also slightly fuel-rich; however, for elements other than those determined here, a leaner flame might have been better because the high flame background regions occurred a t lower wavelengths (12). Multielement standard solutions were made from 1000 ppm stock solutions (prepared with nitric acid from the appropriate compounds or preferably, metals, according to the recommendations in ref. 14 and 15) and diluted/mixed serially. In all analytical work, blank measurements were made prior to each standard or unknown; the reagent blank consisting of a dilute acid solution was used in all measurements. All solutions were prepared with double-deionized water. National Bureau of Standards Reference Materials were digested by established procedures (16). Jet engine oil samples from the Spectrographic Oil Analysis Program (SOAP) of the Air Force were diluted 1:20 with 2,2,4trimethylpentane (isooctane), and if necessary, additionally 1:lO with 5% base oil (no added metals)-95% isooctane. The blank was 5% base oil--95% isooctane and was, in addition to a small amount of acetylene, fuel to the flame. The wavelength modulation system was optimized with respect to peak-to-peak voltage applied to the torque motor and to the frequency with which the torque motor could be driven; the Ca

ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979

+-

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Flgure 4. Analytical calibration curves for several elements (Na, Sr, Ca, Mg, Mn, Fe, Cd, and Z n ) in the air/acetylene flame

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2385

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Figure 6. Analytical calibration curves for several elements (TI, Ni, Pb,

Cu, I n , and Co) in the air/acetylene flame

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