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Automated laser excited atomic fluorescence spectrometer for determination of trace concentrations of elements
05~8~7~ $3LXt+.C# @ 1989. Prqtamon Press ptc
Spccnochimka Acts. Vol. MB. No. 3, pp. 253~2621989 Printed ia Great Britain.
TOPICS IN LASER SPECTROSCOPY
Automated laser excited atomic fluorescence spectrometer for determination of trace concentratiaus of elements V. M. APATIN,B. V. ARKHANGEL’SKII, M. A. Bot’s~ov,* V. V. ERMOLOV, V. G. KOLOSHNIKOV, 0. N. KOMPANETZ, N. I. KUZNETSOV, E. L. MIKHAILOV and V. S. SHISHKOVSKII Institute of Spectroscopy, Academy of Sciences of U.S.S.R., 142092 Moscow Region, Troitzk, U.S.S.R.
and C. F. BOUTRON Laboratoire de Glaciologie et Ghophysique de ~Environnement, 2, rue Moliere, Domaine Universitaire, BP 96, 38402 St. Martin D’Htres Cedex, France (Received
17 May 1988; in revised form 9 August 1988)
Abetract-A new versionof a computer-controlledlaser excitedatomic fluorescencespectrometerLAFAS-1is described.The laserpart of the spectrometerconsistsof a dye laserpumpedby an excimerlaser.Electrothermal ato~~~on in an argonatmosphe~and undervacuumconditionsmaybe employed.The spectrometerwastested by acidifiedstandard solutionsprepared in the Grenoble clean laboratory. These standards were based on ultrapure water. A limit of detection for Pb of 0.18 pg/ml was achieved. For a 20 ~1 aliquot volume this value gives an absolute limit of detection of 4 fg. A Pb content of 0.28 pg/ml in the Grenoble ultrapure water was measured by LAFAS-1 with a confidence interval of 0.05 pg/ml. This value is in excellent agreement with that obtained previously by isotope dilution mass spectrometry: 0.27 pg/ml.
1. INTRODUCTION RECENTLY,excellent analytical advantages of laser excited atomic fluorescence spectrometry (LEAFS) for determination of trace concentrations of elements have been demonstrated. Not only have extremely low limits of detection (LODs) been reached in the experiments with pure aqueous standards [i-6], but also some concrete analytical problems with real samples have been solved [7-10-J. Now that the physical fundamentals of the method have been developed reasonably well the design of a commercial apparatus is of considerable interest. At the Institute of Spectroscopy of the U.S.S.R. Academy of Sciences the foundations of the LEAFS-ETA method have been the object of investigations for some years and a laser excited atomic fluorescence spectrometer has been developed and modified. As a result of this work a new version of the spectrometer has appeared which differs appreciably from that described in [l]. It should also be noted that for a majority of elements the LODs in [l] were obtained by linear extrapolation of the calibration curve to the 3a noise level of the background. The content of the analytes in the deionized water used was much in excess of the LOD. Thus, the lead content in the deionized water was more than two orders of magnitude higher than the extrapolated LOD. Collaboration with the Laboratoire de Glaciologie et Gtophysique de l’Environnement (CNRS, Grenoble) which has vast experience with pure reagents permitted us to test the new spectrometer at a qualitatively higher analytical level. The present paper describes in detail the construction of the laser excited atomic fluorescence automated spectrometer LAFAS-1 and also some details of reference standards preparation as well as an analytical procedure with LAFAS-1.
*Author to whom correspondence should be addressed.
V. M. APA~N et al.
254
2. EXPERIMENTAL The spectrometer consists of four basic units: an optical unit-the source of laser resonance radiation, an analytical chamber-atomizer, a recording unit and a control microcomputer. The block diagram of the spectrometer is shown in Fig. 1. The basic units of the spectrometer are described below. 2.1, Radiation
source
Tunable dye lasers (DL) and their optical harmonics are the most widely used sources for LEAFS. DLs are most often excited by a solid-state Nd: YAG laser, excimer or N,-lasers. In contrast to the apparatus described in Ref. [l] in the LAFAS-1 spectrometer the DL was pumped by a XeCl excimer laser. This scheme provides the simplest means of obtaining tunable radiation over the range 20@400 nm. 2.1.1. Excimer laser. The discharge chamber of the excimer laser is a tube made of an aluminium alloy (outer diameter 270 mm, wall thickness 20 mm and length 600 mm). The tube is filled with a mixture of helium, xenon and hydrogen chloride (600:40:3) at a total pressure 1.6 atm. The discharge gap was formed by plane and profiled electrodes of 500 mm length made of a nickel alloy; the spacing between the electrodes was 25 mm. The laser resonator was formed by a dielectric mirror with reflection coefficient 99.98% and a quartz plane-parallef plate sealed at opposite ends of the tube. A discharge excitation scheme with preionization was used. The maximum voltage at the electrodes was 24 kV. The wavelength of the excimer laser radiation was 308 nm, the pulse energy up to 25 mJ, the repetition frequency up to 25 Hz, the pulse duration 10 ns, and the dimensions of the laser output in the near field were 30 x 8 mm. 2.1.2. l)ye Iuser. In the transverse-pumped dye laser an oscillator-amplifier scheme was used. The radiation energy of the excimer laser was split between the oscillator and the DL amplifier in the ratio 1:3. The resonator of the DL oscillator was constructed of a grating (1200 lines/mm), a 6-prism telescope (magnification 100 x ), a dye cell (15 mm long) and an optical wedge (2”) as an output mirror. An original grating assembly and a control unit [ill enable us to tune the DL wavelength by a microcomputer. Angular rotation of the grating is provided by a device based on an electrodynamic drive; this device is a positional system with automatic negative feedback by the angular displacement of the grating. The angular position of the grating is measured by a differential inductance transducer, the actuator is a linear micromotor of magnetoelectric type. The cores of the transducer and the motor are rigidly coupled with the driving lever of the grating holder. The grating is controlled by a specially
Fig. 1. Block diagram of LAFAS-1. (1) Excimer laser; (2)dye laser with d-meter and SHG unit; (3)atomizer chamber; (4)monochromator; (5) PMT with preamplifier; (6)mechanical telescopic system with the lenses and filters; (7) temperature control system; (8) analytical table with vacuum pump system; (9) computer control system with CAMAC interface modules and TV set; (10) clean chamber; (11) terminal; (12) matrix printer.
Trace concentrations of elements
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designed electronic module and a 15bit digital-analog converter (DAC) (DAC-15) which are parts of the computer-controlled unit. The grating assembly, the transducer and the motor may be mechanically rotated as a whole to choose the initial position corresponding to the centre of the generation band of the chosen dye. The range of the grating angle rotation controlled by the electrodynamic drive is 2”, the accuracy of the angle setting is 0.2 in., and the time required for the setting of the angle is 40 ms. The oscillator output was amplified in the second cell (25 mm long) placed at 150 mm from the output mirror of the resonator. The coefficient of amplification varied between 10-50 depending on the energy of the excimer laser. No matching optics were used between the oscillator and the amplifier. The spectra1 width of the amplified DL radiation was 0.8 cm - ‘, the efficiency of the excimer to DL energy conversion (at 570 nm) was 9-10%. Second harmonic generation (SHG) of the DL was used to obtain tunable UV radiation. Three KDP crystals (40 mm length, 15 x 30 mm* cross-section) with phase matching angles of 54,59 and 64” provided SHG in the range 340-260 nm. To increase the SHG efficiency the DL radiation was focused into the crystal by a lens F= 110 mm. The construction of the,SHG unit permitted us to tune the crystal angle both manually and by means of the computer controlled stepping motor. The radiation power of the second harmonic reached 3 kW. 2.1.3. Systemfor the DL wavelength control. An optical scheme of the I-meter is shown in Fig. 2. For calibration the DL beam was directed into the I-meter unit by means of a 90” prism. To obtain an optogalvanic signal the main part of the radiation was sent into the Ar glow-discharge lamp (GDL) and the part of the radiation reflected by the beam-splitter was fed to the stable Fabry-Perot interferometer with the quartz linings 2 mm thick. The central maximum of the interference pattern was separated by a diaphragm of 0.5 mm diameter and recorded by a photodiode (FD-7K) placed right after the diaphragm. In the course of calibration the program provided a linear increment of the DAC-15 code in the chosen range of codes and thus uniform scanning of the DL wavelength was possible. The signals from the photodiode FD-7K and from the load resistor in the GDL circuit were recorded and stored. A table of the strongest spectra1 lines of the argon discharge was stored previously. In the course of processing, the program found the DAC-15 codes corresponding to the maxima of the interferometer marks and those of optogalvanic signals from the GDL. The Fabry-Perot marks were used as the scale for the wavelengths. Hereby the program identified the recorded optogalvanic signals in terms of standard spectra1 lines. After this procedure the DL can be tuned by the operator by just setting the required wavelength into the computer. The program will calculate the required code for DAC-15 and set the grating in the DL oscillator to the appropriate angle. The whole calibration procedure takes 10-15 min depending on the laser repetition rate and the scanned spectra1 range. Owing to the relatively high long-term stability of the DL with electrodynamic positioning of the grating, the DL line need be calibrated no more than 2-3 times per day and an
Fig. 2. Optical scheme of I-meter. (1,3,9,11) diaphragms; (2) 90” prism; (4, 13) photodiode FD-7K; ($12) optical filters; (6,7) beam splitters, (8) Ar glow-discharge lamp; (10) stable Fabry-Wrot interferometer.
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appreciably smaller spectral range is scanned in the vicinity of the nearest GDL line (usually 0.2-0.3 nm). This operation takes l-2 min. The absolute accuracy of the DL wavelength setting is 0.01 nm. 2.2. Atomizer In the LAFAS-1 spectrometer electrothermal atomization of the sample is used. The atomizer design allows one to make analyses both in an inert atmosphere and under vacuum. The chamber is made of a stainless steel tube with welded upper and bottom flanges. A lid and a base are hermetically fixed to the flanges by means of vacuum seals. The lid is fastened by a hinge mechanism and the base is fixed with four screws. The base may be lowered along two guides with their rods downward; in this way free access to the atomizer is provided. The guides retain the centring of the atomizer relative to the laser beam and the monochromator optical axis when the base is removed or being set. The atomizer is attached to the base. It consists of two massive water-cooled metal holders with graphite electrodes 6 mm in diameter fixed in them. A graphite cup is pressed between the graphite electrodes. Cups of different sizes were used: outer diameter 5-6 mm, wall thickness 0.5-1.5 mm, height 4-7 mm. The cup can be quickly replaced through the open lid. To replace the graphite electrodes it is necessary to lower the base. The chamber has three side arms welded to it; they are provided with vacuum sealed quartz windows for the input and output of laser radiation and output of fluorescence radiation. A special pipe connects the chamber with the pumping system consisting of fore vacuum and diffusion oil-vapour pumps. The laser beam profile in the analytical zone over the atomizer cup is shaped by a diaphragm and a system of lenses in front of the input window of the chamber. In the present experiments the beam was of rectangular cross-section (4 x 2.5 mm). The lower edge of the beam was 2-4 mm above the brim of the cup. The cup temperature was stabilized by monitoring the light flux over the whole temperature range (50-3OOOC) by a specially developed system. A pyrocell MG-30 with a spectral sensitivity range of 2-20 pm was used as a photoreceiver. The radiation from the cup was fed to the pyrocell by means of a CaF, lens vacuum sealed on the chamber base right under the cup. The appropriate level of light flux at the pyrocell over the whole cup temperature range was maintained by diaphragms of different diameters. Diaphragms were automatically changed by a micromotor during the sample atomization. An automatic servo system controlled the attainment of the cup temperature and its stabilization. The pyrocell diaphragms, micromotor and electronic components of the servosystem were mounted in a separate mechanical unit on the outside of the chamber base. A system of electropneumatic valves mounted on the lid and on the base of the chamber provided the inlet and outlet of the buffer gas, connection of the chamber to the vacuum system and input of atmospheric air into the chamber before opening the lid after vacuum atomization. The atomizer power supply was designed for 4 kW and maximum current up to 400 A. The power supply was computer controlled. An interlock system turned off the power when the water flow and the buffer gas flow fell below a certain level. The sample atomization was carried out in five steps. At the first-evaporation-step the temperature could vary between 30-200°C for 10-120 s. At the second ashing step the temperature varied between 150-700°C for 1-60s; at the third atomization step the temperature varied between 50&3OOO”Cfor l-60 s; at the fourth firing step the temperature varied between 2000-3OOOC for l-10 s. The fifth step-pause was used to cool down the cup to room temperature. 2.3. Recording system The fluorescence radiation was collected from the analytical zone at an angle of 90” and directed to the entrance slit of the monochromator by a telescopic system consisting of two identical quartz lenses of 150 mm focal length and 60 mm diameter. The focal plane of the first lens coincided with the centre of the analytical zone, whereas the focal plane of the second lens coincided with the plane of the monochromator entrance slit. The spacing between the lenses was 100 mm. In this space up to five neutral light filters could be placed to attenuate the fluorescence radiation when samples with a high concentration of analyte were analyzed. The lenses and the filters were fixed in a special mechanical telescopic device connecting the chamber with the monochromator. This provided complete isolation of the monochromator entrance slit from stray light and scattered radiation from the room. The transmission of these commercial filters (NSl-8) varied between l&75%. The maximum attenuation of five filters was a factor of 3 112. Further attenuation of the fluorescence signal could be achieved by decreasing the PMT supply voltage. To reduce laser radiation scattered in the atomizer (A= 283.3 nm) a commercial cut-off filter BS-4 was used. It transmits more than 90% radiation above 380 nm, and attenuates radiation below 300 nm by a factor of 30.
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Trace con~ntrations of elements
A compact diffraction monochromator (260 x 145 x 135 mm3) with a relative aperture of 1: 3.7 and a reciprocal linear dispersion of 6.3 nm/mm was used. Behind the exit slit of the monochromator a photomultiplier tube (PMT) FEU-130 (or FEU-100) was placed. The PMT region of spectral sensitivity was 200--650nm, the maximum amplification coefficient 10’ and the width of a singie-electron pulse 6 ns. The supply voltage of the PMT varied from 1.2 to 1.8 kV. The current pulse in the anode circuit of the PMT during the fluorescence pulse was amplified by a preamplifier (amplification factor 10) and fed to the input of a charge-sensitive (CS) ADC. The gate of the CS-ADC was turned on by the leading edge of the pulse from the avalanche photodiode to which a part of the excimer laser radiation was fed. The duration of the gate pulse was varied between 20-130 ns. The ADC sensitivity was 5 x lo- ’ 3 Coulomb/count, the maximum output current 20 mA, the characteristic linearity 0.2%. The output signal of the ADC as a parallel code was fed to the computer for further processing. The CS-ADC output code A (numerical value) depends on the input electric pulse charge Q (Coulombs) as A= F(Q). The linear part of this function can be written in a form A= A, + kQ, where A, is ADC base line and k is the slope. We denote the end of the linear part A,,, as follows: A,,, differs from the “true” linear value (A, + kQ,,,) by 2%. Thus we estimate the linear dynamic range as the ratio Amnx13~Ao,where aAu is the standard deviation of the base line. The intrinsic pulse-to-pulse stability of the ADC base line is about 0.5% and provides an intrinsic ADC dynamic range of 1500. But the electric noises from the excimer laser discharge increase the fluctuations of the ADC base line up to 5% and thus decrease the effective linear dynamic range of the recording system (PMT-preamplifier-CS ADC) to 150. Numerical filtration of the ADC output was used to decrease the influence of the base line fluctuations on the detection limit. 2.4. ~arn~ter control system The system was based on a micro-computer “Elektronika 80”. It includes: a floppy disk unit, a display, a matrix printer, two CAMAC powered crates and a crate controller, base software RT-60, specially designed interfaces (interface modules) and a colour TV set for data display. A scheme of interaction of the programming modules and the CAMAC modules is shown in Fig. 3. An ADC-10 measured all analog signals in the spectrometer (except the fluorescence signal) and converted them into a digital code. A multiplexer module provided sampling of analog signals by the gate signal and their storing and subsequent output into the ADC-IO via a commutator. The DAC-15 module generated control voltages for the electrodynamic drive of the DL diffraction grating. SPS and SHPS modules provided stabilized supply voltages for the GDL (up to 400 V) and PMT (up to 2 kV). 2.5. Preparation and storing of standard solutions The LAFAS-1 calibration has been performed with synthetic aqueous standard solutions, both unacidified and acidified (0.1% HNO,). These solutions were prepared inside the Grenoble ultraclean laboratory [12J from J. T. Baker lOOO~g/ml Pb certified atomic absorption standards (Pb oxide in diluted HNO,), by dilution in ultrapure water. In addition to Pb, these standards contained
1
I I I I I / I
I I
____--------
I I J
Fig. 3. The structure of software engineering. (1) DAC-15; (2)analog memory unit; (3)ADc; (4) SPS-stabilized power supply; (5) control desk (6) module interface for monochromator control; (7) DL wavelength positioning; (8) DL wavelength calibration; (9) atomizer control; (10) monochromator positioning; (11) supervisory program;(12) terminal;(13) analytical signal processing;(14) analyte concentration calculation; (15) DAC-10; (16) CS-AN, (17) SHPS-stabilized high voltage power supply. (l-6) and (15-17) are the electronic modules; (7-14) are software packages.
258
v. bf, &A'TiN t?t ai
simultaneously 19 other metals or metdioids (Na, Mg, K, Ca, Al, Fe, Hg, Mn, Ni, Ti, Vd, Cu, Ag, Zn, Co, MO, Cd, As, Se). The Grenoble ultrapure water was produced by passing tap water first through activated charcoal and then successively through five successive mixed-bed ion exchange resins columns (one model “Universelle”, three models “Recherche” and one model “Micron”). All these columns are from Maxy Cor~~~o~ Saint Remy l&sChevreuses, France. Tbe Pb content of this water has been ~~~rate~~ measured by isotope dilution mass s~etrometry (~~~S~; it is 0.27 p&ml [rg.f+ Difutions were made using Eppendorf micropipettes with poiyprupylene tips and preconditioned conventional (low density) polyethylene 1000 and 2000 ml bottles. The standards were stored frozen inside preconditioned conventional polyethylene or FEP Teflon 30 ml bottles wrapped in sealed acid cleaned polyethylene bags. For the acidified standards (0.1% HNO,) we used ultrapure twice distilled HNO, prepared at the National Bureau of Standards (NNBS),Washington, DC, U.S.A. [14, IS]. The Pb content ofthis acid had p~ousi~ been shuwn to be fess than 10 pgjg ft5,16’j. The ~l~e~y~ene and T&on bottles had been cleaned with a CHCf, rinse, 2 weeks at 45% in 25% HNOB (E. Merck Suprapur grade in ultrapure water), 2 weeks at 45°C in 0.1% HNO, {NBS twice distified in ultrapure water), and finally 2 weeks at 45°C in 0.1% HNO, (NBS twice distilled in ultrapure water). They were then kept filled with 1% HN03 (NBS twice distilled in ultrapure water) for at least-six months at room temperature before use. The polypropylene micropipette tips were cleaned by dipping them in ~n~atrat~ Merck Suprapur HNO, for a few minutes, then ~ppjng them in ~on~~~rat~ NBS HNU, for a few minutes, then rinsing finally with ultrapure water. Pb ~o~~ntra~~ons in the standards ranged from 500 pg/ml down to 1 pg/ml. The Grenobie ultrapure water itself was used as an additional standard to determine the detection limit for the spectrometer, together with ultrapure water prepared at California Institute of Technology [ 171. This last ultrapure water was prepared by passing building distilled water through mixed bed ion exchange resins, then distillation in a cyclone scrubber boiling stiII made of quartz fl7f. Its Pb content has been determined by isotope dilution mass s~~~orn~tr~ (~~MS~ to be 0.16 p&ml f13-j. Afi these standards were ihtrodueed into the graphite cup of the spectrometer using a 20 pl ~ppendo~~~eropi~tte with polypropylene tips. These tips were cleaned by dipping them in concentrated reagent grade HNO, for a few minutes, then in multiple successive rinsings with 1% NBS HN03.
The instrument was placed in a special room where air was supplied by conditioners through a system af filters. In order to reduce the probability of contamination of the Grenoble standard in the course of the analytical procedure a special pure chamber was muunted over the anatytieal Gh~~r and the re~ordj~g system. The part of the speetrometer inside the clean chamber is shown in Fig. 1. The chamber dimensions were: 2500 x 1700 x 2800 mm3. It consisted of a metal frame with removable side walls, acrylic plastic doors and an upper box for special filters and ventilation units. The ventilation units drew the air from the room through a cellular plastic layer 20-mm thick and forced the air (free from coarse dust particles) into. the chamber through a system of fine filters. This provided a downward laminar flow of cleaned air. The air escaped from the eham~ through the slits between the side walls and the floor. The chamber had room For two operators. One of them worked in gloves and manipulated the micropipette, bottles for washing the tip and the bottles with the standards. The other one worked without gloves and was responsible for the operation of the whole spectrometer and the opening and closing of the upper lid af the a~a~~~~~ chamber and did not touch the mi~ropi~tte or the bottfes. LAFAS-1 was tested by anteing Pb in standard solutions from Grenobie. Pb atoms were excited at the 283.3 nm line guorescence was recorded at the 405.8 nm line. The solution (20~1) was introduced into the atomizer cup through the upper lid. After this the lid was closed and an argon flow was turned on (at the rate of 2l/min). The following regime of sample atomization was used: evaporation at 95°C for 3Os, preheating up to 6OOC for 10s [heating rate N gt.Y/$ atom~~tio~ at f6QO”Cfor 4 s theat~ng rate 7~~~s~, firing at 2~~C for 5 s, and coo&g for 20 s. Cups with pyrolytic coatings were used (outer diameter 6 mm> wall thickness 1 mm, height 5 mm). However, the coating thickness turned out to be insu!&ient, which resulted in an appreciable cup surface modification after 60-70 atomization cycles. As a result, the analytical signal decreased considerably (up to a factor of 2) and cup replacement was required.
Trace con~ntrations of elements
259
The laser was turned on and the signals were measured only at the third step. Fach pulse from the PMT was recorded by CS-ADC, stored and also displayed on the TV screen. Upon completion of the sample atomization cycle all information on the dynamics of the analytical signal was visualized on the screen. After this, the operator could call up a special subprogram for processing the results and choose from the whole data file the part in which the analytical signal reliably exceeds the background level. This operation is made by moving about the TV screen two vertical cursors which restrict the beginning and the end of the chosen section, Within the limits of the section the computer calculates the sum of all recorded pulses, stores the calculated value and outputs data on the display and the matrix printer. Each pulse recorded by CS-ADC and stored in the computer memory is the result of the simultaneous action of the fluorescent radiation and the background determined by the scattered laser radiation, the atomizer thermal radiation, electronics noises, etc. Two possible methods to eliminate the background and extract a “pure” analytical signal, determined only by the fluorescence radiation, can be used. The first method consists in measuring the signal according to the above technique when ultrapure water is fed into the cup and the DL line is tuned off the resonance line of the analyte. In our experiments the DL line was detuned for 3-4 line widths which corresponded to - 80 codes of DAC-15. The background value was calculated within the limits of the same interval where the total signal was calculated. The average value of the background and its standard deviation were determined from 10-15 measurements. In the second method for background correction the possibility of rapidly tuning the DL wavelength is used. In this case a subprogr~ for the periodic tuning of the DL wavelength from the present value is fed into the main measuring program. During the whole third step of atomization the laser radiates a fixed number of pulses “on the line”, then a fixed number of pulses “off the line”, etc. The detuning of the DL wavelength is obtained within - 40 ms, i.e. between the laser pulses. The recording of such a measuring interval is shown s~hemati~lly in Fig. 4. In+he course of data processing the program reconstructs by linear interpolation the time profile of the total signal and the background and, subtracting the second from the first, determines the “pure” analytical signal. Such a technique for (dynamic) background correction improves the accuracy of the measurements as it allows one to measure the background within the same cycle of the sample atomization. However, it works well only in the case that the duration of the sample atomization stage exceeds 45 s. In this case, at a repetition rate of 20 Hz, the total number of
T
1
m e, s
Fig. 4. Schematic diagram of background correction method. O-+-evaporation; t,-t,-atomization; t3-#.--firing.
t,-t,--ashing;
260
V. M. APATIN et al.
the recorded pulses is 80-100 and the DL tuning does not lead to an appreciable distortion of the time profile of the signal. In cases when full atomization of the analyte takes no more than 1 s (for highly-volatile elements) and when the total number of the fluorescence pulses exceeding the background level is 10-20, the DL line tuning may cause an appreciable distortion of the time profile of the signal and lead to additional errors when a “pure” analytical signal is being extracted. In the experiments with acidified standards, cups with pyrolytic coatings were used. In this case the dry residue of the standard aliquot remains in the thin subsurface layer of the cup. Atomization of Pb from the dry residue took 0.5s, i.e. approximately 9-10 laser pulses. Under these conditions the first method of background correction was chosen. The calibration curve was constructed with the following standard solutions: 1,2.5,5,7.5, 10, 25, 50, 100 and 250 pg/ml and also ultrapure water from Grenoble. In this series of experiments each standard solution was measured twice, pure water was measured 4 times. The results of these measurements are shown in Fig. 5. The standard with Pb content 7.5 pg/ml systematically gave underestimated values of the analytical signal not only in this, but also in all other measurement cycles. This is an indication of possible errors in its preparation. Determination of the Pb content in this standard by the constructed calibration curve yielded a value of 5.4 pg/ml and not 7.5 pg/ml. The point in Fig. 5 which corresponds to the standard of 1 pg/ml lies above the calibration curve. The Pb content in this standard determined by this curve corresponds to 1.28 pg/ml (see the arrow in Fig. 5). This value is in excellent agreement with the true Pb content in the solution with proper account for its content in the Grenoble ultrapure water (0.27 pg/ml). When the calibration curve was constructed by the least squares method, the points corresponding to 1 and 7.5 pg/ml were not taken into account. The slope of the double logarithmic analytical curves is 0.92 and the value Se characterizing the scattering of the experimental points with respect to the regression line equals 0.04 [18]. The Pb content in ultrapure water determined by the calibration curve and marked on it with an open circle is 0.28 pg/ml, with a confidence interval of 0.05 pg/ml (four parallel determinations). This value coincides with that obtained by the IDMS method in Ref. [13] within the limits of
Log Fig. 5. Calibration
cont.
pb,ng/l
curve for lead. O-lead concentration in the Grenoble depicts the “true” value for 1 pg/ml standard.
ultrapure
water. Arrow
Trace concentrations of elements
261
experimental error. A concentration of 0.28 pg/ml at a sample volume of 20 ~1 corresponds to an absolute Pb content of 6 fg (or N 1.6 x 10’ atoms per sample). It should be noted that such levels of Pb content are determined by means of the LAFAS-1 directly with a sample volume of 20 ~1. One determination takes 3-5 min. In the IDMS method the volume of the sample from which the metal being determined is extracted is 200 ml (with a concentration in the samples of the order of 10 pg/ml) and it is possible to make l-2 sample determinations per working day. The detection limit in these experiments was 0.18 pg/ml. This value was limited mainly by the fluctuations of the CS-ADC base line and by the scattering of the laser radiation. The atomizer blackbody radiation was rather small as the atomization temperature of the lead was relatively small (1600°C). The signal from the ultrapure water prepared at the California Technological Institute [17] did not exceed the 30 noise level of the background. One should keep in mind that the Pb content in this water was determined in Ref. [13] to be 0.16 pg/ml. A detection limit for the Pb analysis, namely 0.03 pg/ml was reported before [l]. The main reasons for the differences in the LODs reached are as follows. Firstly, when the LOD was determined in Ref. Cl, 191, the background fluctuations were calculated by the formula I
CT=
$ A;/@-
1)n, where n is the number of parallel measurements of the background, i.e.
the LOD was not determined by a single measurement, but by the average of a series of measurements. In accordance with the present IUPAC nomenclature we determined the background
standard deviation by the formula r~=
c Ai /(n - 1). When the number of J? measurements equal 10, these values (and, consequently, also the LOD) will differ by a factor of 3. Secondly, the use of a Nd:YAG laser in Ref. Cl] as an excitation source for the DL provided higher radiation power for the DL second harmonic (up to 20 kW in Ref. [l] compared to 3 kW in LAFAS-1). This, in turn, restricted the volume of the analytical zone in which the optimum density of the excitation energy could be provided [l, 191. The volume of the analytical zone in these experiments was one third as large as in Ref. [l]. The aperture ratio of the compact monochromator in LAFAS-1 was 1: 3.7 and in Ref. Cl] 1: 2.5. Finally, the laser repetition rate in Ref. [l] was 50 Hz, whereas in the present paper it was 18 Hz.
4. CONCLUSIONS The paper describes a new automated spectrometer LAFAS-1 for measuring trace concentrations of elements. For the first time a spectrometer was calibrated by acidified standard solutions over the range of concentrations l-250 pg/ml prepared under ultrapure laboratory conditions in Grenoble. Good linearity of the calibration curve and acceptable scattering of the experimental points with respect to the regression line show the correctness of the technique used for preparation, storing and transportation of the standards with such low elemental concentration. With the technique developed for the operation of LAFAS-1 in a clean chamber no contamination of the samples was noticed in the course of analyses. For the first time a Pb content as low as 0.28 pg/ml was measured by the LEAFS-ETA technique with a confidence interval of 0.05 pg/ml in a real sample-ultrapure water. For a sample of 20 ~1the Pb content measured in ultrapure water corresponds to an absolute lead content in the sample of 6 x lo-” g or N 1.6 x 10’ atoms. It is worthwhile to emphasize that the LODs of the LEAFS method published before [l, 3-53 where determined by linear extrapolation of the calibration curve to the 3a noise level of the background. For abundant elements such an extrapolation may span 2-3 orders of magnitude of concentration. In the present paper the detection limit reached is only about 50% lower than the actually measured concentration of Pb in ultrapure water. In this initial stage, the LAFAS-1 was tested by aqueous solutions. The possibilities of solid sample analysis by the LEAFS technique were successfully demonstrated in Ref. [7]. As the construction of the atomizer of LAFAS-1 is very similar to that used in Ref. [7] one can state that not only liquid but also solid samples can be analysed by LAFAS-1.
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V. M. APATINet al. REFERENCES
[l] M. A. Bolshov, A. V. Zybin and I. I. Smirenkina, Spectrochim. Acta 36B, 1143 (1981). [Z] J. P. Hohimer and P. J. Hargis, Anal. Chim. Acta 97, 43 (1978). [3] F. R. Preli, Jr., J. P. Dougherty and R. G. Michel, Anal. Chem. 59, 1784 (1987). [4] J. Tilch, H.-J. Paetzold, H. Falk and K. P. Schmidt, Analytiktreffen 1982, Neubrandenburg, Kurzreferateband, DY N 55. [S] K. Dittrich and H.-J. Stark, J. Anal. At. Spectrom. 2, 63 (1987). [6j N. Omenetto, P. Cavalli, P. Broglia, P. Qi and G. Rossi, J. Anal. At. Spectrom. 3, 231 (1988). [7] M. A. Bolshov, A. V. Zybin, V. G. Koloshnikov, I. A. Mayorov and I. I. Smirenkina, Spectrochim. Acta 41B, 487 (1986). [S] M. S. Epstein, S. Bayer, J. Bradshaw, J. Bower, E. Voigtman and J. D. Winefordner, Spectrochim. Acra 35B, 233 (1980). Appl. Spectrosc. 34, 372 (1980). [9] M. A. Bolshov, I. B. Gornushkin, E. I. Zilberstein, A. V. Zybin, Yu. B. Kiselev and I. I. Smirenkina, Zh. Anal. Chim. (in Russian) 42, 312 (1987). [lo] M. A. Bolshov, A. V. Zybin, Yu. R. Kolomiiskii, V. G. Koloshnikov, Yu. N. Loginov and I. I. Smirenkina, Zn. Anal. Chim. (in Russian) 41, 402 (1986). [ll] 0. N. Kompanets, V. I. Mishin and I. N. Nesteruk, Soo. Quant. Electr. (in Russian) 15, 455 (1988). [12] C. F. Boutron and S. Martin, Anal. Chem. 51, 140 (1979). [13] C. F. Boutron and C. C. Patterson, unpublished (1985). [14] E. C. Kuchner, R. Alvarez, D. J. Panlsen and T. Y. Murphy, Anal. Chem. 44,205O (1972). [15] P. J. Paulsen, E. S. Beary, D. S. Bushee and J. R. Moody, Anal. Chem. 60,971 (1988). [16] C. F. Boutron and C. C. Patterson, Geochim. Cosmochim. Acta 47, 1355 (1983). [17] C. C. Petterson and D. M. Settle, In Accuracy in Trace Analysis, Ed. P. La Fleur, National Bureau of Standards Special Publication, No. 422, p. 321 (1976). Cl83 V. V. Nalimov, The Application of Mathematical Statistics to Chemical Analysis (in Russian). Fismatgiz, Moscow (1960); English Transl.: Pergamon Press, Oxford (1963). [19] M. A. Bolshov Laser atomic fluorescence analysis. In Laser Analytical Spectrochemistry, Ed. V. S. Letokhov. Adam Hilger, Bristol (1986).
Spccnochimka Acts. Vol. MB. No. 3, pp. 253~2621989 Printed ia Great Britain.
TOPICS IN LASER SPECTROSCOPY
Automated laser excited atomic fluorescence spectrometer for determination of trace concentratiaus of elements V. M. APATIN,B. V. ARKHANGEL’SKII, M. A. Bot’s~ov,* V. V. ERMOLOV, V. G. KOLOSHNIKOV, 0. N. KOMPANETZ, N. I. KUZNETSOV, E. L. MIKHAILOV and V. S. SHISHKOVSKII Institute of Spectroscopy, Academy of Sciences of U.S.S.R., 142092 Moscow Region, Troitzk, U.S.S.R.
and C. F. BOUTRON Laboratoire de Glaciologie et Ghophysique de ~Environnement, 2, rue Moliere, Domaine Universitaire, BP 96, 38402 St. Martin D’Htres Cedex, France (Received
17 May 1988; in revised form 9 August 1988)
Abetract-A new versionof a computer-controlledlaser excitedatomic fluorescencespectrometerLAFAS-1is described.The laserpart of the spectrometerconsistsof a dye laserpumpedby an excimerlaser.Electrothermal ato~~~on in an argonatmosphe~and undervacuumconditionsmaybe employed.The spectrometerwastested by acidifiedstandard solutionsprepared in the Grenoble clean laboratory. These standards were based on ultrapure water. A limit of detection for Pb of 0.18 pg/ml was achieved. For a 20 ~1 aliquot volume this value gives an absolute limit of detection of 4 fg. A Pb content of 0.28 pg/ml in the Grenoble ultrapure water was measured by LAFAS-1 with a confidence interval of 0.05 pg/ml. This value is in excellent agreement with that obtained previously by isotope dilution mass spectrometry: 0.27 pg/ml.
1. INTRODUCTION RECENTLY,excellent analytical advantages of laser excited atomic fluorescence spectrometry (LEAFS) for determination of trace concentrations of elements have been demonstrated. Not only have extremely low limits of detection (LODs) been reached in the experiments with pure aqueous standards [i-6], but also some concrete analytical problems with real samples have been solved [7-10-J. Now that the physical fundamentals of the method have been developed reasonably well the design of a commercial apparatus is of considerable interest. At the Institute of Spectroscopy of the U.S.S.R. Academy of Sciences the foundations of the LEAFS-ETA method have been the object of investigations for some years and a laser excited atomic fluorescence spectrometer has been developed and modified. As a result of this work a new version of the spectrometer has appeared which differs appreciably from that described in [l]. It should also be noted that for a majority of elements the LODs in [l] were obtained by linear extrapolation of the calibration curve to the 3a noise level of the background. The content of the analytes in the deionized water used was much in excess of the LOD. Thus, the lead content in the deionized water was more than two orders of magnitude higher than the extrapolated LOD. Collaboration with the Laboratoire de Glaciologie et Gtophysique de l’Environnement (CNRS, Grenoble) which has vast experience with pure reagents permitted us to test the new spectrometer at a qualitatively higher analytical level. The present paper describes in detail the construction of the laser excited atomic fluorescence automated spectrometer LAFAS-1 and also some details of reference standards preparation as well as an analytical procedure with LAFAS-1.
*Author to whom correspondence should be addressed.
V. M. APA~N et al.
254
2. EXPERIMENTAL The spectrometer consists of four basic units: an optical unit-the source of laser resonance radiation, an analytical chamber-atomizer, a recording unit and a control microcomputer. The block diagram of the spectrometer is shown in Fig. 1. The basic units of the spectrometer are described below. 2.1, Radiation
source
Tunable dye lasers (DL) and their optical harmonics are the most widely used sources for LEAFS. DLs are most often excited by a solid-state Nd: YAG laser, excimer or N,-lasers. In contrast to the apparatus described in Ref. [l] in the LAFAS-1 spectrometer the DL was pumped by a XeCl excimer laser. This scheme provides the simplest means of obtaining tunable radiation over the range 20@400 nm. 2.1.1. Excimer laser. The discharge chamber of the excimer laser is a tube made of an aluminium alloy (outer diameter 270 mm, wall thickness 20 mm and length 600 mm). The tube is filled with a mixture of helium, xenon and hydrogen chloride (600:40:3) at a total pressure 1.6 atm. The discharge gap was formed by plane and profiled electrodes of 500 mm length made of a nickel alloy; the spacing between the electrodes was 25 mm. The laser resonator was formed by a dielectric mirror with reflection coefficient 99.98% and a quartz plane-parallef plate sealed at opposite ends of the tube. A discharge excitation scheme with preionization was used. The maximum voltage at the electrodes was 24 kV. The wavelength of the excimer laser radiation was 308 nm, the pulse energy up to 25 mJ, the repetition frequency up to 25 Hz, the pulse duration 10 ns, and the dimensions of the laser output in the near field were 30 x 8 mm. 2.1.2. l)ye Iuser. In the transverse-pumped dye laser an oscillator-amplifier scheme was used. The radiation energy of the excimer laser was split between the oscillator and the DL amplifier in the ratio 1:3. The resonator of the DL oscillator was constructed of a grating (1200 lines/mm), a 6-prism telescope (magnification 100 x ), a dye cell (15 mm long) and an optical wedge (2”) as an output mirror. An original grating assembly and a control unit [ill enable us to tune the DL wavelength by a microcomputer. Angular rotation of the grating is provided by a device based on an electrodynamic drive; this device is a positional system with automatic negative feedback by the angular displacement of the grating. The angular position of the grating is measured by a differential inductance transducer, the actuator is a linear micromotor of magnetoelectric type. The cores of the transducer and the motor are rigidly coupled with the driving lever of the grating holder. The grating is controlled by a specially
Fig. 1. Block diagram of LAFAS-1. (1) Excimer laser; (2)dye laser with d-meter and SHG unit; (3)atomizer chamber; (4)monochromator; (5) PMT with preamplifier; (6)mechanical telescopic system with the lenses and filters; (7) temperature control system; (8) analytical table with vacuum pump system; (9) computer control system with CAMAC interface modules and TV set; (10) clean chamber; (11) terminal; (12) matrix printer.
Trace concentrations of elements
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designed electronic module and a 15bit digital-analog converter (DAC) (DAC-15) which are parts of the computer-controlled unit. The grating assembly, the transducer and the motor may be mechanically rotated as a whole to choose the initial position corresponding to the centre of the generation band of the chosen dye. The range of the grating angle rotation controlled by the electrodynamic drive is 2”, the accuracy of the angle setting is 0.2 in., and the time required for the setting of the angle is 40 ms. The oscillator output was amplified in the second cell (25 mm long) placed at 150 mm from the output mirror of the resonator. The coefficient of amplification varied between 10-50 depending on the energy of the excimer laser. No matching optics were used between the oscillator and the amplifier. The spectra1 width of the amplified DL radiation was 0.8 cm - ‘, the efficiency of the excimer to DL energy conversion (at 570 nm) was 9-10%. Second harmonic generation (SHG) of the DL was used to obtain tunable UV radiation. Three KDP crystals (40 mm length, 15 x 30 mm* cross-section) with phase matching angles of 54,59 and 64” provided SHG in the range 340-260 nm. To increase the SHG efficiency the DL radiation was focused into the crystal by a lens F= 110 mm. The construction of the,SHG unit permitted us to tune the crystal angle both manually and by means of the computer controlled stepping motor. The radiation power of the second harmonic reached 3 kW. 2.1.3. Systemfor the DL wavelength control. An optical scheme of the I-meter is shown in Fig. 2. For calibration the DL beam was directed into the I-meter unit by means of a 90” prism. To obtain an optogalvanic signal the main part of the radiation was sent into the Ar glow-discharge lamp (GDL) and the part of the radiation reflected by the beam-splitter was fed to the stable Fabry-Perot interferometer with the quartz linings 2 mm thick. The central maximum of the interference pattern was separated by a diaphragm of 0.5 mm diameter and recorded by a photodiode (FD-7K) placed right after the diaphragm. In the course of calibration the program provided a linear increment of the DAC-15 code in the chosen range of codes and thus uniform scanning of the DL wavelength was possible. The signals from the photodiode FD-7K and from the load resistor in the GDL circuit were recorded and stored. A table of the strongest spectra1 lines of the argon discharge was stored previously. In the course of processing, the program found the DAC-15 codes corresponding to the maxima of the interferometer marks and those of optogalvanic signals from the GDL. The Fabry-Perot marks were used as the scale for the wavelengths. Hereby the program identified the recorded optogalvanic signals in terms of standard spectra1 lines. After this procedure the DL can be tuned by the operator by just setting the required wavelength into the computer. The program will calculate the required code for DAC-15 and set the grating in the DL oscillator to the appropriate angle. The whole calibration procedure takes 10-15 min depending on the laser repetition rate and the scanned spectra1 range. Owing to the relatively high long-term stability of the DL with electrodynamic positioning of the grating, the DL line need be calibrated no more than 2-3 times per day and an
Fig. 2. Optical scheme of I-meter. (1,3,9,11) diaphragms; (2) 90” prism; (4, 13) photodiode FD-7K; ($12) optical filters; (6,7) beam splitters, (8) Ar glow-discharge lamp; (10) stable Fabry-Wrot interferometer.
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appreciably smaller spectral range is scanned in the vicinity of the nearest GDL line (usually 0.2-0.3 nm). This operation takes l-2 min. The absolute accuracy of the DL wavelength setting is 0.01 nm. 2.2. Atomizer In the LAFAS-1 spectrometer electrothermal atomization of the sample is used. The atomizer design allows one to make analyses both in an inert atmosphere and under vacuum. The chamber is made of a stainless steel tube with welded upper and bottom flanges. A lid and a base are hermetically fixed to the flanges by means of vacuum seals. The lid is fastened by a hinge mechanism and the base is fixed with four screws. The base may be lowered along two guides with their rods downward; in this way free access to the atomizer is provided. The guides retain the centring of the atomizer relative to the laser beam and the monochromator optical axis when the base is removed or being set. The atomizer is attached to the base. It consists of two massive water-cooled metal holders with graphite electrodes 6 mm in diameter fixed in them. A graphite cup is pressed between the graphite electrodes. Cups of different sizes were used: outer diameter 5-6 mm, wall thickness 0.5-1.5 mm, height 4-7 mm. The cup can be quickly replaced through the open lid. To replace the graphite electrodes it is necessary to lower the base. The chamber has three side arms welded to it; they are provided with vacuum sealed quartz windows for the input and output of laser radiation and output of fluorescence radiation. A special pipe connects the chamber with the pumping system consisting of fore vacuum and diffusion oil-vapour pumps. The laser beam profile in the analytical zone over the atomizer cup is shaped by a diaphragm and a system of lenses in front of the input window of the chamber. In the present experiments the beam was of rectangular cross-section (4 x 2.5 mm). The lower edge of the beam was 2-4 mm above the brim of the cup. The cup temperature was stabilized by monitoring the light flux over the whole temperature range (50-3OOOC) by a specially developed system. A pyrocell MG-30 with a spectral sensitivity range of 2-20 pm was used as a photoreceiver. The radiation from the cup was fed to the pyrocell by means of a CaF, lens vacuum sealed on the chamber base right under the cup. The appropriate level of light flux at the pyrocell over the whole cup temperature range was maintained by diaphragms of different diameters. Diaphragms were automatically changed by a micromotor during the sample atomization. An automatic servo system controlled the attainment of the cup temperature and its stabilization. The pyrocell diaphragms, micromotor and electronic components of the servosystem were mounted in a separate mechanical unit on the outside of the chamber base. A system of electropneumatic valves mounted on the lid and on the base of the chamber provided the inlet and outlet of the buffer gas, connection of the chamber to the vacuum system and input of atmospheric air into the chamber before opening the lid after vacuum atomization. The atomizer power supply was designed for 4 kW and maximum current up to 400 A. The power supply was computer controlled. An interlock system turned off the power when the water flow and the buffer gas flow fell below a certain level. The sample atomization was carried out in five steps. At the first-evaporation-step the temperature could vary between 30-200°C for 10-120 s. At the second ashing step the temperature varied between 150-700°C for 1-60s; at the third atomization step the temperature varied between 50&3OOO”Cfor l-60 s; at the fourth firing step the temperature varied between 2000-3OOOC for l-10 s. The fifth step-pause was used to cool down the cup to room temperature. 2.3. Recording system The fluorescence radiation was collected from the analytical zone at an angle of 90” and directed to the entrance slit of the monochromator by a telescopic system consisting of two identical quartz lenses of 150 mm focal length and 60 mm diameter. The focal plane of the first lens coincided with the centre of the analytical zone, whereas the focal plane of the second lens coincided with the plane of the monochromator entrance slit. The spacing between the lenses was 100 mm. In this space up to five neutral light filters could be placed to attenuate the fluorescence radiation when samples with a high concentration of analyte were analyzed. The lenses and the filters were fixed in a special mechanical telescopic device connecting the chamber with the monochromator. This provided complete isolation of the monochromator entrance slit from stray light and scattered radiation from the room. The transmission of these commercial filters (NSl-8) varied between l&75%. The maximum attenuation of five filters was a factor of 3 112. Further attenuation of the fluorescence signal could be achieved by decreasing the PMT supply voltage. To reduce laser radiation scattered in the atomizer (A= 283.3 nm) a commercial cut-off filter BS-4 was used. It transmits more than 90% radiation above 380 nm, and attenuates radiation below 300 nm by a factor of 30.
257
Trace con~ntrations of elements
A compact diffraction monochromator (260 x 145 x 135 mm3) with a relative aperture of 1: 3.7 and a reciprocal linear dispersion of 6.3 nm/mm was used. Behind the exit slit of the monochromator a photomultiplier tube (PMT) FEU-130 (or FEU-100) was placed. The PMT region of spectral sensitivity was 200--650nm, the maximum amplification coefficient 10’ and the width of a singie-electron pulse 6 ns. The supply voltage of the PMT varied from 1.2 to 1.8 kV. The current pulse in the anode circuit of the PMT during the fluorescence pulse was amplified by a preamplifier (amplification factor 10) and fed to the input of a charge-sensitive (CS) ADC. The gate of the CS-ADC was turned on by the leading edge of the pulse from the avalanche photodiode to which a part of the excimer laser radiation was fed. The duration of the gate pulse was varied between 20-130 ns. The ADC sensitivity was 5 x lo- ’ 3 Coulomb/count, the maximum output current 20 mA, the characteristic linearity 0.2%. The output signal of the ADC as a parallel code was fed to the computer for further processing. The CS-ADC output code A (numerical value) depends on the input electric pulse charge Q (Coulombs) as A= F(Q). The linear part of this function can be written in a form A= A, + kQ, where A, is ADC base line and k is the slope. We denote the end of the linear part A,,, as follows: A,,, differs from the “true” linear value (A, + kQ,,,) by 2%. Thus we estimate the linear dynamic range as the ratio Amnx13~Ao,where aAu is the standard deviation of the base line. The intrinsic pulse-to-pulse stability of the ADC base line is about 0.5% and provides an intrinsic ADC dynamic range of 1500. But the electric noises from the excimer laser discharge increase the fluctuations of the ADC base line up to 5% and thus decrease the effective linear dynamic range of the recording system (PMT-preamplifier-CS ADC) to 150. Numerical filtration of the ADC output was used to decrease the influence of the base line fluctuations on the detection limit. 2.4. ~arn~ter control system The system was based on a micro-computer “Elektronika 80”. It includes: a floppy disk unit, a display, a matrix printer, two CAMAC powered crates and a crate controller, base software RT-60, specially designed interfaces (interface modules) and a colour TV set for data display. A scheme of interaction of the programming modules and the CAMAC modules is shown in Fig. 3. An ADC-10 measured all analog signals in the spectrometer (except the fluorescence signal) and converted them into a digital code. A multiplexer module provided sampling of analog signals by the gate signal and their storing and subsequent output into the ADC-IO via a commutator. The DAC-15 module generated control voltages for the electrodynamic drive of the DL diffraction grating. SPS and SHPS modules provided stabilized supply voltages for the GDL (up to 400 V) and PMT (up to 2 kV). 2.5. Preparation and storing of standard solutions The LAFAS-1 calibration has been performed with synthetic aqueous standard solutions, both unacidified and acidified (0.1% HNO,). These solutions were prepared inside the Grenoble ultraclean laboratory [12J from J. T. Baker lOOO~g/ml Pb certified atomic absorption standards (Pb oxide in diluted HNO,), by dilution in ultrapure water. In addition to Pb, these standards contained
1
I I I I I / I
I I
____--------
I I J
Fig. 3. The structure of software engineering. (1) DAC-15; (2)analog memory unit; (3)ADc; (4) SPS-stabilized power supply; (5) control desk (6) module interface for monochromator control; (7) DL wavelength positioning; (8) DL wavelength calibration; (9) atomizer control; (10) monochromator positioning; (11) supervisory program;(12) terminal;(13) analytical signal processing;(14) analyte concentration calculation; (15) DAC-10; (16) CS-AN, (17) SHPS-stabilized high voltage power supply. (l-6) and (15-17) are the electronic modules; (7-14) are software packages.
258
v. bf, &A'TiN t?t ai
simultaneously 19 other metals or metdioids (Na, Mg, K, Ca, Al, Fe, Hg, Mn, Ni, Ti, Vd, Cu, Ag, Zn, Co, MO, Cd, As, Se). The Grenoble ultrapure water was produced by passing tap water first through activated charcoal and then successively through five successive mixed-bed ion exchange resins columns (one model “Universelle”, three models “Recherche” and one model “Micron”). All these columns are from Maxy Cor~~~o~ Saint Remy l&sChevreuses, France. Tbe Pb content of this water has been ~~~rate~~ measured by isotope dilution mass s~etrometry (~~~S~; it is 0.27 p&ml [rg.f+ Difutions were made using Eppendorf micropipettes with poiyprupylene tips and preconditioned conventional (low density) polyethylene 1000 and 2000 ml bottles. The standards were stored frozen inside preconditioned conventional polyethylene or FEP Teflon 30 ml bottles wrapped in sealed acid cleaned polyethylene bags. For the acidified standards (0.1% HNO,) we used ultrapure twice distilled HNO, prepared at the National Bureau of Standards (NNBS),Washington, DC, U.S.A. [14, IS]. The Pb content ofthis acid had p~ousi~ been shuwn to be fess than 10 pgjg ft5,16’j. The ~l~e~y~ene and T&on bottles had been cleaned with a CHCf, rinse, 2 weeks at 45% in 25% HNOB (E. Merck Suprapur grade in ultrapure water), 2 weeks at 45°C in 0.1% HNO, {NBS twice distified in ultrapure water), and finally 2 weeks at 45°C in 0.1% HNO, (NBS twice distilled in ultrapure water). They were then kept filled with 1% HN03 (NBS twice distilled in ultrapure water) for at least-six months at room temperature before use. The polypropylene micropipette tips were cleaned by dipping them in ~n~atrat~ Merck Suprapur HNO, for a few minutes, then ~ppjng them in ~on~~~rat~ NBS HNU, for a few minutes, then rinsing finally with ultrapure water. Pb ~o~~ntra~~ons in the standards ranged from 500 pg/ml down to 1 pg/ml. The Grenobie ultrapure water itself was used as an additional standard to determine the detection limit for the spectrometer, together with ultrapure water prepared at California Institute of Technology [ 171. This last ultrapure water was prepared by passing building distilled water through mixed bed ion exchange resins, then distillation in a cyclone scrubber boiling stiII made of quartz fl7f. Its Pb content has been determined by isotope dilution mass s~~~orn~tr~ (~~MS~ to be 0.16 p&ml f13-j. Afi these standards were ihtrodueed into the graphite cup of the spectrometer using a 20 pl ~ppendo~~~eropi~tte with polypropylene tips. These tips were cleaned by dipping them in concentrated reagent grade HNO, for a few minutes, then in multiple successive rinsings with 1% NBS HN03.
The instrument was placed in a special room where air was supplied by conditioners through a system af filters. In order to reduce the probability of contamination of the Grenoble standard in the course of the analytical procedure a special pure chamber was muunted over the anatytieal Gh~~r and the re~ordj~g system. The part of the speetrometer inside the clean chamber is shown in Fig. 1. The chamber dimensions were: 2500 x 1700 x 2800 mm3. It consisted of a metal frame with removable side walls, acrylic plastic doors and an upper box for special filters and ventilation units. The ventilation units drew the air from the room through a cellular plastic layer 20-mm thick and forced the air (free from coarse dust particles) into. the chamber through a system of fine filters. This provided a downward laminar flow of cleaned air. The air escaped from the eham~ through the slits between the side walls and the floor. The chamber had room For two operators. One of them worked in gloves and manipulated the micropipette, bottles for washing the tip and the bottles with the standards. The other one worked without gloves and was responsible for the operation of the whole spectrometer and the opening and closing of the upper lid af the a~a~~~~~ chamber and did not touch the mi~ropi~tte or the bottfes. LAFAS-1 was tested by anteing Pb in standard solutions from Grenobie. Pb atoms were excited at the 283.3 nm line guorescence was recorded at the 405.8 nm line. The solution (20~1) was introduced into the atomizer cup through the upper lid. After this the lid was closed and an argon flow was turned on (at the rate of 2l/min). The following regime of sample atomization was used: evaporation at 95°C for 3Os, preheating up to 6OOC for 10s [heating rate N gt.Y/$ atom~~tio~ at f6QO”Cfor 4 s theat~ng rate 7~~~s~, firing at 2~~C for 5 s, and coo&g for 20 s. Cups with pyrolytic coatings were used (outer diameter 6 mm> wall thickness 1 mm, height 5 mm). However, the coating thickness turned out to be insu!&ient, which resulted in an appreciable cup surface modification after 60-70 atomization cycles. As a result, the analytical signal decreased considerably (up to a factor of 2) and cup replacement was required.
Trace con~ntrations of elements
259
The laser was turned on and the signals were measured only at the third step. Fach pulse from the PMT was recorded by CS-ADC, stored and also displayed on the TV screen. Upon completion of the sample atomization cycle all information on the dynamics of the analytical signal was visualized on the screen. After this, the operator could call up a special subprogram for processing the results and choose from the whole data file the part in which the analytical signal reliably exceeds the background level. This operation is made by moving about the TV screen two vertical cursors which restrict the beginning and the end of the chosen section, Within the limits of the section the computer calculates the sum of all recorded pulses, stores the calculated value and outputs data on the display and the matrix printer. Each pulse recorded by CS-ADC and stored in the computer memory is the result of the simultaneous action of the fluorescent radiation and the background determined by the scattered laser radiation, the atomizer thermal radiation, electronics noises, etc. Two possible methods to eliminate the background and extract a “pure” analytical signal, determined only by the fluorescence radiation, can be used. The first method consists in measuring the signal according to the above technique when ultrapure water is fed into the cup and the DL line is tuned off the resonance line of the analyte. In our experiments the DL line was detuned for 3-4 line widths which corresponded to - 80 codes of DAC-15. The background value was calculated within the limits of the same interval where the total signal was calculated. The average value of the background and its standard deviation were determined from 10-15 measurements. In the second method for background correction the possibility of rapidly tuning the DL wavelength is used. In this case a subprogr~ for the periodic tuning of the DL wavelength from the present value is fed into the main measuring program. During the whole third step of atomization the laser radiates a fixed number of pulses “on the line”, then a fixed number of pulses “off the line”, etc. The detuning of the DL wavelength is obtained within - 40 ms, i.e. between the laser pulses. The recording of such a measuring interval is shown s~hemati~lly in Fig. 4. In+he course of data processing the program reconstructs by linear interpolation the time profile of the total signal and the background and, subtracting the second from the first, determines the “pure” analytical signal. Such a technique for (dynamic) background correction improves the accuracy of the measurements as it allows one to measure the background within the same cycle of the sample atomization. However, it works well only in the case that the duration of the sample atomization stage exceeds 45 s. In this case, at a repetition rate of 20 Hz, the total number of
T
1
m e, s
Fig. 4. Schematic diagram of background correction method. O-+-evaporation; t,-t,-atomization; t3-#.--firing.
t,-t,--ashing;
260
V. M. APATIN et al.
the recorded pulses is 80-100 and the DL tuning does not lead to an appreciable distortion of the time profile of the signal. In cases when full atomization of the analyte takes no more than 1 s (for highly-volatile elements) and when the total number of the fluorescence pulses exceeding the background level is 10-20, the DL line tuning may cause an appreciable distortion of the time profile of the signal and lead to additional errors when a “pure” analytical signal is being extracted. In the experiments with acidified standards, cups with pyrolytic coatings were used. In this case the dry residue of the standard aliquot remains in the thin subsurface layer of the cup. Atomization of Pb from the dry residue took 0.5s, i.e. approximately 9-10 laser pulses. Under these conditions the first method of background correction was chosen. The calibration curve was constructed with the following standard solutions: 1,2.5,5,7.5, 10, 25, 50, 100 and 250 pg/ml and also ultrapure water from Grenoble. In this series of experiments each standard solution was measured twice, pure water was measured 4 times. The results of these measurements are shown in Fig. 5. The standard with Pb content 7.5 pg/ml systematically gave underestimated values of the analytical signal not only in this, but also in all other measurement cycles. This is an indication of possible errors in its preparation. Determination of the Pb content in this standard by the constructed calibration curve yielded a value of 5.4 pg/ml and not 7.5 pg/ml. The point in Fig. 5 which corresponds to the standard of 1 pg/ml lies above the calibration curve. The Pb content in this standard determined by this curve corresponds to 1.28 pg/ml (see the arrow in Fig. 5). This value is in excellent agreement with the true Pb content in the solution with proper account for its content in the Grenoble ultrapure water (0.27 pg/ml). When the calibration curve was constructed by the least squares method, the points corresponding to 1 and 7.5 pg/ml were not taken into account. The slope of the double logarithmic analytical curves is 0.92 and the value Se characterizing the scattering of the experimental points with respect to the regression line equals 0.04 [18]. The Pb content in ultrapure water determined by the calibration curve and marked on it with an open circle is 0.28 pg/ml, with a confidence interval of 0.05 pg/ml (four parallel determinations). This value coincides with that obtained by the IDMS method in Ref. [13] within the limits of
Log Fig. 5. Calibration
cont.
pb,ng/l
curve for lead. O-lead concentration in the Grenoble depicts the “true” value for 1 pg/ml standard.
ultrapure
water. Arrow
Trace concentrations of elements
261
experimental error. A concentration of 0.28 pg/ml at a sample volume of 20 ~1 corresponds to an absolute Pb content of 6 fg (or N 1.6 x 10’ atoms per sample). It should be noted that such levels of Pb content are determined by means of the LAFAS-1 directly with a sample volume of 20 ~1. One determination takes 3-5 min. In the IDMS method the volume of the sample from which the metal being determined is extracted is 200 ml (with a concentration in the samples of the order of 10 pg/ml) and it is possible to make l-2 sample determinations per working day. The detection limit in these experiments was 0.18 pg/ml. This value was limited mainly by the fluctuations of the CS-ADC base line and by the scattering of the laser radiation. The atomizer blackbody radiation was rather small as the atomization temperature of the lead was relatively small (1600°C). The signal from the ultrapure water prepared at the California Technological Institute [17] did not exceed the 30 noise level of the background. One should keep in mind that the Pb content in this water was determined in Ref. [13] to be 0.16 pg/ml. A detection limit for the Pb analysis, namely 0.03 pg/ml was reported before [l]. The main reasons for the differences in the LODs reached are as follows. Firstly, when the LOD was determined in Ref. Cl, 191, the background fluctuations were calculated by the formula I
CT=
$ A;/@-
1)n, where n is the number of parallel measurements of the background, i.e.
the LOD was not determined by a single measurement, but by the average of a series of measurements. In accordance with the present IUPAC nomenclature we determined the background
standard deviation by the formula r~=
c Ai /(n - 1). When the number of J? measurements equal 10, these values (and, consequently, also the LOD) will differ by a factor of 3. Secondly, the use of a Nd:YAG laser in Ref. Cl] as an excitation source for the DL provided higher radiation power for the DL second harmonic (up to 20 kW in Ref. [l] compared to 3 kW in LAFAS-1). This, in turn, restricted the volume of the analytical zone in which the optimum density of the excitation energy could be provided [l, 191. The volume of the analytical zone in these experiments was one third as large as in Ref. [l]. The aperture ratio of the compact monochromator in LAFAS-1 was 1: 3.7 and in Ref. Cl] 1: 2.5. Finally, the laser repetition rate in Ref. [l] was 50 Hz, whereas in the present paper it was 18 Hz.
4. CONCLUSIONS The paper describes a new automated spectrometer LAFAS-1 for measuring trace concentrations of elements. For the first time a spectrometer was calibrated by acidified standard solutions over the range of concentrations l-250 pg/ml prepared under ultrapure laboratory conditions in Grenoble. Good linearity of the calibration curve and acceptable scattering of the experimental points with respect to the regression line show the correctness of the technique used for preparation, storing and transportation of the standards with such low elemental concentration. With the technique developed for the operation of LAFAS-1 in a clean chamber no contamination of the samples was noticed in the course of analyses. For the first time a Pb content as low as 0.28 pg/ml was measured by the LEAFS-ETA technique with a confidence interval of 0.05 pg/ml in a real sample-ultrapure water. For a sample of 20 ~1the Pb content measured in ultrapure water corresponds to an absolute lead content in the sample of 6 x lo-” g or N 1.6 x 10’ atoms. It is worthwhile to emphasize that the LODs of the LEAFS method published before [l, 3-53 where determined by linear extrapolation of the calibration curve to the 3a noise level of the background. For abundant elements such an extrapolation may span 2-3 orders of magnitude of concentration. In the present paper the detection limit reached is only about 50% lower than the actually measured concentration of Pb in ultrapure water. In this initial stage, the LAFAS-1 was tested by aqueous solutions. The possibilities of solid sample analysis by the LEAFS technique were successfully demonstrated in Ref. [7]. As the construction of the atomizer of LAFAS-1 is very similar to that used in Ref. [7] one can state that not only liquid but also solid samples can be analysed by LAFAS-1.
262
V. M. APATINet al. REFERENCES
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