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Anal. Chem. 1986, 58,1566-1571
Table 11. T o l e r a n c e o f F o r e i g n Ions on t h e D e t e r m i n a t i o n af 5.20 mg/L C y a n i d e by the S p e c t r o p h o t o m e t r i c M e t h o d
tolerance, mg/L 5000
1000 500 400 200 100
ion
alkali and alkaline-earth metals, FC1-, NO,, Ala+,As3+,Tl(III), Mn2+,OCN-, SO-,: NOT, COS2-,SOa2-,Be2+ Br-, I-, BrO,-, IO; acetate, tartrate, citrate, oxalate AsOas, AsO,~-,MOO,”, WOd2-,B40$-, Fe(CN)6a-, Fe(CNh’, P,O$S2-, S202-i S20i2-; ClO,, CrOd2-; Cr20$-,” MnO;,” ’ SCN-, PO4”
OIn the presence of ascorbic acid. T a b l e 111. Quantity o f C y a n i d e P r e s e n t in t h e E f f l u e n t of C o k e O v e n and B y p r o d u c t Plant o f R o u r k e l a S t e e l Mill on Different Dates
dates 5-1-85 7-1-85 8-1-85 11-1-85 12-1-85
simple cyanide, mg/L spectrophotometric conductometric method method 9.26 8.71 7.13 8.91 6.73
9.12 8.53 6.83 8.65 6.40
std method 9.26 8.70 7.13
8.92 6.72
silver takes some more time and vigorous shaking for dissolution. Interferences. The effect of 32 anions and 13 cations on the determination of cyanide was tested (see Table 11) by the spectrophotometric method. The tolerance limits for interfering ions are summarized in Table 11. These ions do not affect the determination of cyanide when present in at least 20-fold excess for S2-,S2032-, Cloy, S202-, Cr2072-,and SCN-. S2- and SCN-, which interfere in most tests of cyanide, do not interfere in these determinations. Strong oxidizing agents like Cr042-and Mn04- do not interfere in the determination in the presence of ascorbic acid. In the absence of ascorbic acid the oxidizing agents easily oxidize silver sol, hence definitely interfere in the determination, while Cr20,2-reacts very slowly with the sol. Hence, the method is highly specific.
The determination of microamounts of cyanide in industrial effluents is of interest owing to its high toxicity. The pyridine-barbituric acid colorimetric method recommended (14) has a detection limit of 4 X mg/L for cyanide. To determine cyanide in water samples by this method, it is necessary first to separate it from interfering substances by distillation of hydrocyanic acid from the acidified samples. It is well-known ( 4 ) , and has been verified by us, that when the equipment and conditions described (14) for the distillation are used, the cyanide is not quantitatively recovered at levels below 1 mg/L in the original sample. The reason for this has been investigated (15),and it was found that the hydrocyanic acid is completely released by long distillation process but not totally absorbed in the absorbing medium. Applications. Several samples of water from a coke oven plant were analyzed by the spectrophotometric and conductometric methods (Table 111). Registry NO.AgNOB, 7761-88-8; H20,7732-18-5; CN-, 57-12-5; Ag, 7440-22-4; 02, 7782-44-7; ascorbic acid, 50-81-7.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9)
(10) (11) (12) (13) (14)
(15)
Kakovskll, L. D.; Vzerodov, SA, I. A. Tsvetn. Metall. 1979, 7 , 100. Tomozo, K.; Masahido, K. Anal. Chlm. Acta 1979, 709, 107. Guilbault, G. G.; Kramer, D. N. Anal. Chem. 1988, 38, 834. Brebee, M.; Delarue, G.; Santoni, B.; Sonat, P. Analusis 1975, 4 , 127. Kolthoff, I. M.; Sandeli, E. B. ”Textbook of Quantitative Inorganic Analysis”, 2nd ed.; Macmilian: New York, 1947; p 574. Epstein, J. Anal. Chem. 1947, 79. 272. Wli, F.; Han, B.; Shen, N. Analyst (London) 1984, 709, 167. Vernon, F.; Whltham. P. Anal. Chim. Acta 1972, 5 9 , 155. Ryan, D. E.; Hoizbecher, J. Int. J . Environ. Anal. Chem. 1971, 7 , 159. Rubio, S.; Gomez-Hens, A.; Valcarcel. M. Talenta 1984, 3 7 , 783. Pal. T. Inorg. Chim. Acta 1983, 7 9 8 , 283. Ciuhandu, G. Acad. Repub. Pop. Rom. Baza Cercet. Stllnt. Timlsoara Stud. Cercet. Stllnt. SOC.,Ser. 7 1955, 2, 133. Pal, T.; Ganguly, A.; Malty, D. S. Polymer Laboratory, Indian Institute of Technology, Kharagpur, unpublished work, 1985. “Standard Methods for the Examination of Water and Waste Water”, 13th ed.; American Public Health Association, American Water Works Association and Water Pollution Control Federation, 1971; pp 397, 404. Kirk-Othmer “Encyciopendia of Chemical Technology”; Uthea, Mexico, 1949.
RECEIVED for review September 16,1985. Accepted January 6, 1986. This work was presented at the 1985 Colloquim Spectroscopicum Internationale XXIV in Garmisch-Partenkirchen, FRG.
CORRESPONDENCE I sotope- Selective Trace- Element Detection with the Thermionic Diode Sir: During the last 15 years the development of laser spectroscopy affected strongly the field of atomic and molecular spectroscopy. New techniques allowed deeper and more precise examination of the structure of matter (1). Although analysts tried to adopt the laser as a tool for their work early (Z),laser spectroscopic methods did not really enter the laboratories where everyday problems in analytical chemistry have to be solved. There are several reasons for this. First, novel effective detection schemes had to be developed that permit trace-element determination in real samples. Second, in the present status, the application of tunable lasers allows the measurement of only one analyte species at the same time and is, because of narrow tuning
ranges, limited to only a few elements without changing the laser system or at least the active medium of the laser applied. Finally, tunable lasers are costly and complicated instruments, which are still not suitable for application outside research labs. They have to compete with established, powerful, and commercially available methods like optical emission spectroscopy (OES)using, e.g., ICP sources (3-5), or atomic absorption spectroscopy (AAS) (6). In the near future, as long as complicated high-cost laser systems have to be used, the opportunity to establish laser spectroscopic methods in analytical chemistry requires methods with very high detection sensitivities, methods that permit extreme trace-element detection in the femto- or even 0 1988 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
the attogram range, or which, e.g., enable the analyst to determine the abundances of isotopes (isotope dilution technique). Basically the spectroscopic techniques in analytical chemistry using tunable lasers are (i) laser-induced fluorescence (LIF),(ii) laser-enhanced ionization (LEI) or optogalvanic spectroscopy (OGS), and (iii) resonance ionization spectroscopy (RIS). While in LIF the fluorescence photons of the laser-excited analyte atoms are detected photoelectrically, in LEI and RIS the increase of electron current is measured when trace atoms are excited to higher lying electronic states or photoionized resonantly through states. There is also the possibility to couple RIS with mass spectroscopy (RIMS) for isotope-selective analytical spectroscopy. Extensive work in LIF has been performed, in particular, by the groups of Winefordner (7-9) and Omenetto ( 1 0 , I I ) . Pioneers in LEI are the scientists of the National Bureau of Standards (NBS) in Washington (12-14). We would like to mention also the work on LEI by the group of Lindgren in Goteborg (15-17) and Omenetto at Ispra (18). For RIS and RIMS the authors refer to ref 19 and 20, respectively. In this paper we report on first results applying a detection technique that is already well-established in atomic and molecular laser spectroscopy: the thermionic diode (see review paper ref 21 and references therein). Taking into account the extraordinary high detection sensitivity of the thermionic diode, it is likely that it becomes a tool for extreme traceelement detection in the near future.
THEORY Thermionic Diode Detector. Thermionic diodes are known to be extremely sensitive detectors for ions having a large dynamic range (see, e.g., ref 21 for detailed discussion). As few as about 10 ions/s produce signals that are above the noise limit, and the device was found to be linear at least over 4 orders of magnitude. In its simplest form the setup is a cylindrical anode with an axially mounted cathode filament that is heated directly by a dc current. Normally no bias is applied to the diode running in the space-charge-limited mode. If atomic (or molecular) ions are created, e.g., by photoionization they are trapped within the negative space charge. The trapping times can be very long (t 3 25 ms). The presence of ions reduces the space charge. As a result the diode current increases. The gain, G = Aje/Aji (Ajeand Aji are the increase of the diode and ion current, respectively), can be lo6 or even larger. The high gain factor is the major advantage of the thermionic diode spectroscopy (TDS)compared with OGS or LEI. Analytical Spectroscopy with the Thermionic Diode. Basically the physical processes leading to ionization of the analyte atom are the same in TDS and in OGS or LEI. By one- or two-step laser excitation the atom is excited to a high-lying level, from where it can be ionized more easily by collision with other atoms than from the initial level. The problem of a two- or many-level atom in a radiation field and a collisional atmosphere has been treated by different authors (two-level atom, see, e.g., ref 14;many-level atom, see ref 15). Here, only the exemplar results for a two-level atom will be given. Taking into account the rate equations for the population of the lower (NJ and higher level (N2)and neglecting collisional excitation of level 2 from level 1and, for the present, collisional ionization, in the steady-state case, the ratio of the number density in the upper level, N2, to the total density, Nt = Nl + N2is given by
NZ/Nt = B12P12/(B12(1 + gl/g2)P12
+ A21/D21)
(1)
where BE and A21 are the Einstein coefficients for absorption and spontaneous emission, respectively. DZl= Azl/(Az1+ CZ1), where Czl is the rate coefficient for collisional depopulation
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of level 2 to level 1. plz is the spectral energy density of the radiation field at the transition frequency between level 1and 2, and g1 and g2are the statistical weights of level 1 and 2, respectively. In the case of optical saturation by a laser radiation field,
& z ( l + gdg2)PlZ >> A21/D21
(2)
eq 1 simplifies to
N d N t = &?,/kl+ gz)
(3)
If we now take into account collisional ionization from the laser-populated level 2, but neglect electronic recombination, the rate equation for the ion number density, dNi/dt = CglN2) holds. With the total number density, NT = Nt + Ni, and F = N,/N, = Nz/(NT - Ni) the rate equation dNi/dt = C2jF(NT - Ni)
(4)
can now be integrated from t = 0 to t = Atl, where Atl is the irradiation time of the analyte atom by the laser light: Ni(Atl) = N T ( -~ exp(-CziFAtl))
(5)
It can be seen that all atoms may be ionized if the irradiation time At1 >> (C2iJ')-'
(6)
Of course, the dwell time of the analyte atoms in the laser beam has to be of the same order as At1. If the optical transition 1 2 is saturated (F = g2/(gl + g2)) and level 2 is near to the ionization limit (large Czicoefficient) relation 6 can easily be fulfilled. Note that Cziis depending on the colliding species and is a function of gas temperature. Because the atomic dipole moments for, e.g., transitions from the ground state to high-lyingRydberg states are small (for hydrogenic-like series, r ~ - ~it) ,is experimentally advantageous to use at least two lasers to excite the atoms via an intermediate level by two strong dipole transitions. Then saturation can be achieved with moderate laser powers. In addition, two-step excitation improves the selectivity of analyte atoms against wrong atoms of the matrix. Application of Tunable, Single-Mode CW Lasers. If the excitation of analyte atoms takes place in a low-pressure noble gas atmosphere there are crucial advantages to the use of single-mode CW lasers instead of pulsed laser systems. Because the collisidnal depopulation of the upper level by noble gas at low pressure is only weak it should be possible, in most cases, to saturate strong dipole transitions (e.g., resonance lines) with the moderate power of an Ar+ laser pumped CW ring dye laser or even with a few milliwatts of power obtained from frequency doubling of the laser light in a nonlinear crystal inside a cavity, With CW lasers we have optimum conditions to fulfill relation 6. If we investigate a medium-heavy element in a noble gas atmosphere with a gas temperature of about 1000 K the average velocity of the atoms is about 6 X lo4cm/s. Assuming a laser beam diameter of 3 mm that is transversed by the atoms, the dwell time in the beam is about 5 X lo4 s, whereas the pulse length of, e.g., an excimer laser pumped dye laser is typically 1.5 X s. Furthermore, the frequency of the pulsed laser is low (typically 50 Hz). To irradiate each transversing atom at least once, the frequency of the pulsed system should be of the order of 200 kHz. This is 4000 times higher than the assumed 50-Hz rate. The longer irradiation time as well as the better duty cycle yields a considerable advantage of a the CW over a pulsed laser system with low repetition rates. Another important advantage is the small laser line width that can be obtained with a CW system (AX 6 0.1 pm in the optical region). It permits utilization of the techniques of Doppler-free spectroscopy (1)to resolve the isotopic compo-
-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1988
Figure I. Experimental setup for isotope-selective anaiytical spectroscopy with the thermionic diode: (a)element under investigation, (b) sample, (c) activating material, (d) cathode filament.
nents of analyte lines. But one has to keep in mind that the pressure of the noble gas atmosphere has to be low. Otherwise the collisionally broadened Doppler-free isotopic componenb would merge. Neon is recommended as the buffer gas. It is known to have the smallest collisional cross sections of the noble gases (22).
EXPERIMENTAL SECTION The principle of a setup for isotopicallyselectivetrace-element detection with the thermionic diode is shown in Figure 1. With the beams of two frequency-stabilized,single-modeCW ring dye lasers (Spectra Physics 380 D) shining through the thermionic diode arrangement in a counterpropagating direction, the Yb analyte atoms were excited resonantly from the ground state 4P4 6s2lS0 via the 4f14 6s6p 3P1intermediate state to the 4f14 6s6d 3Dzfinal state. For the first step the vacuum wavelength of the laser radiation is 555.802 nm (dye, Rhodamin 110; typical power, 140 mW). The final step (A, = 457.749 nm) was induced by a Stilbene 3 dye laser (typical power, 30 mW). While the first dye laser was pumped by the visible lines of an Ar' laser, the second dye laser had to be pumped by the UV lines of a second Ar+ laser. The line widths of the lasers employed are about 1 MHz, which corresponds to roughly 0.1 pm in the wavelength scale. The diameters of the laser beams were typically 2 mm. While for the intercombination line at 555.802 nm the available laser power was just starting to saturate the transition, the degree of saturation of the strong dipole line at 457.749 nm was much higher. Using counter- (or co-)propagating beams and lasers of very narrow line widths, we are able to perform sub-Doppler spectroscopy. The technique is known as resonant Doppler-free two-photon spectroscopy (23). If the first laser is tuned into the Doppler profile of the first transition, one velocity group of atoms in respect to the direction of the laser field is excited to the intermediate level. The second narrow band laser is now interacting only with this group of atoms. If the direction of the second laser beam is opposite to the first one (or has the same direction) and if we detect the transition of the second step only, the lines are Doppler-free when scanning the second laser. Of course the second laser can also be tuned into the Doppler profile of the second transition and fixed in frequency while the first laser is being scanned. It should be noted that in contrast to saturation spectroscopy the technique of resonant Doppler-freetwo-photon spectroscopy does not need saturating laser fields. But as in saturation spectroscopy only one velocity group of thermal atoms is interacting with the resonant laser field. If the buffer gas pressure in the thermionic diode is high, collisional effects may
reduce the signal-to-background ratio. First there is collisional broadening leading to a larger line width of the analyte atom. Second, velocity-changingcollisions will produce a pedestral for the Doppler-free line, an effect which is well-known from saturation spectroscopy ( I ) . In our experiment pedestrals could only be observed at large amplification factors of the lock-in amplifierers. It should be noted here that the Yb atoms have been excited to a final state with an energy of 39 838.04 cm-l above the ground state. The energy gap to the ionization limit (Ei= 50 441 cm-I) is still over lo4 cm-I. Therefore the coefficient for collisional ionization is expected to be small at an experimental temperature of about 1100 K (24). Two stainless-steelthermionic heat pipe diodes were employed. The inner diameter was 25 mm and the length of the heated region 10 cm. They were filled with 300 mtorr of Ne buffer gas each. For activation of the cathode surface (low work function) we used Ba metal that was filled into small boats (c) (see Figure 1). The Ba vapor density could be optimized by shifting the boats more or less into the heated region of the pipes. This procedure has been discussed elsewhere (25). Mo filaments (diameter, 0.2 mm) were used as cathodes. The filaments were directly heated by dc current. Additionallywe introduced a second boat (a) in diode A containing pure Yb metal and a boat (b) in diode B with the sample under investigation. The diode signals were directly taken from the heat pipe walls and fed into lock-in amplifiers. The modulation frequency of the second laser beam was only about 7 Hz. Thermionic diodes are known to have a slow response time (21). The amplified signals of both diodes were recorded by a two-pen strip-chart recorder. The lasers were accuratelytuned into the centers of the Doppler profiles of the relevant transitions by using a high-precision vacuum wavemeter (A, meter), which is described elsewhere (26). With the help of diode A the laser frequencieshave to be tuned to the center of a particular Doppler-freeisotopic resonance and locked to this transition. Subsequently sample b in diode B is atomized by heating up the thermionic diode. The signal due to the selected isotope increases, and as the sample disappears from the heated part of the cell and condenses in the cooled regions the signal decreases. The shape of the analyte signal is similar to that known from atomic absorption spectroscopy ( U s technique). When the laser intensities change during the measurement or when there is a frequency drift of the lasers with respect to each other, the height of the signal of diode A changes. It can therefore serve for correction of the analyte signal of diode B. Using the experimental arrangement shown in Figure 1, we
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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17LY b
0)
600 MHz
time
-
Figure 3. Analytical diode signal from 173Ybin 7 mg of pure Eu evaporated from a platform and the signal from the calibrating reference diode. For further details, see text.
/
176Yb
I ......... u bi 600 M H Z
H
173Yb
-
Figure 2. Sections of the resonant Doppler-free two-photon spectrum 4f14 6s2 'So -,4f1' 6sBp 3P, 4fi4 6s6d 3D, in Yb. The asterisks mark the isotopic components that have been used for analysis.
started the investigation by placing small pieces of tantalum sheets into boat b. Areas of 1.2 X 0.7 mm2had been coated before with Yb layers down to 2.5 nm thickness. The thickness was measured simultaneously in the coating vessel by applying the oscillating crystal method. In the main experiment we investigated the content of different Yb isotopes in pure Eu and Sm and in very pure Ba metal. In these experiments boat b acted as a platform. The thermionic diode reached almost stable working conditions before the sample material started to evaporate. Unfortunatelythe measuring time for one sample was very long (typically about 45-60 min). There were two reasons why the time could not be shortened significantly. The heat pipe was made of steel (limitation of final temperature), and because of indirect heating the inertia of the heating procedure was large. On the other hand the pipe had to be opened to introduce the next sample. This procedure was also time-consuming. Therefore we modified diode B by introducingthe samples from outside the diode with a stainless-steel rod through a sealing system of Teflon disks into a prechamber. The sample was placed on a thin Re band with a shallow depression. After pumping down and filling the prechamber with the same noble gas pressure, a valve to the main cell was opened and the sample was shifted into the diode region running under stable conditions. By a dc current the Re band was heated up to high temperatures, and the sample material was evaporated into the region where the laser beams passed the diode. Now depending on the temperature of the Re band, typical measuring times were 1-2 min. To keep memory effects small, the operation temperature of the heat pipe was about 100 O C lower than in the earlier experiments.
RESULTS AND DISCUSSION In Figure 2 we show two different scans of the 4f14 6s2 lS0 4f14 6s6p 3P1-,4f14 6s6d 3Dz Doppler-free two-photon transition in ytterbium. Yb has seven natural isotopes (174Yb, 31.83%; 17'Yb, 21.82%; 173Yb,16.13; 171Yb, 14.31%; 176Yb, 12.73%; 170Yb,3.03%; and laYb, 0.135%). The isotope shift in the investigated transition is large, i.e., the Doppler profiles of the even isotopes are hardly overlapping. This can be seen in Figure 2a where the first laser was tuned to the center of the 174Ybcomponent. At that position the first laser excites
-
only a few 17Tybor 176Ybisotopes in the wings of their Doppler profiles. Both components can be seen as weak structures in Figure 2a. The situation is different in spectrum b where the first laser was tuned to a group of close-lying hyperfine component of the odd isotopes 171J73Yb.In both cases the first laser was fixed in frequency and the second was tuned across the resonances. The line widths of the Doppler-free twophoton transitions were found, depending on the laser powers applied, to be about 85 MHz in our experiment. As discussed above, in a first set of measurements we used the setup shown in Figure 1. When loading boat b with 7 mg of Eu metal and heating up thermionic diode B to about 1100 K, we obtained signals as displayed in Figure 3. The lasers were tuned to the hyperfine component of the 173Ybisotope marked by an asterisk in Figure 2b. It should be mentioned here that the line intensity of the 173Ybcomponent is a factor of 5.7 smaller than that of the 174Ybtransition. Before the evaporation process starts, the signal of the diode is noisy, but it becomes smooth later. Note that different amplication factors of the lock-in have been used. The analyte signal looks similar to signals known from AAS. Because thermionic diode A was run a t a very low Yb number density its signal was noisier. Furthermore, the time constant of the lock-in amplifier connected to diode A is about a factor of 3 smaller than that of the second amplifier. During the time of measurement (about 20 min) the second laser was not always locked properly. Therefore the signals of calibration diode A as well as of diode B decreased from time to time in the same manner, and the laser had to be relocked again. At the end of the measurement we tuned the second laser off resonance and increased the amplificationfactor of the lock-in to 1024 to get a measure of the background signal and the noise level. The signal-to-noise ratio was found to be of the order of 6 X lo3. We found no significant collisional shift of the Doppler-free two-photon lines measured with analytical diode B in respect to the lines detected with diode A at a lower gas temperature. Taking into account the values of 7 mg of Eu and the content of 460 ppm Yb in Eu which has been determined by flame AAS in a separate experiment, the analyte signal in Figure 3 correspondsto about 510 ng of 173Yb,With the given signal-to-noise ratio the detection limit would be 85 pg for 173Ybatoms. Because the even isotopes of Yb have no hyperfine splitting of the lines the detection limit for them would be about a factor of 2.9 lower (about 30 pg). With a less sluggish heating system, pipes of smaller diameter (to enlarge the interaction region between vapor and laser beams), higher laser powers for the first step of excitation (to get a higher degree of saturation for the weak intercombination line a t 555.802 nm), and a final state closer to the ionization limit it will be possible to lower the detection limit even much further. In a similar experiment we loaded boat b with 2.64 g of high-purity Ba metal (99.999%). By calibration of the integrated signal by the signals measured with the Eu samples,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
-
17'Yb i n .Irng Eu
1
A
I
1
i
ft e"
L -
...._ ..__" ..
time
Flgure 4. Analytical diode signal from 17'Yb In 0.1 mg of Eu evaporated from a Re strip and the signal from the calibrating reference diode.
the 1'3Yb content was found to be in the parts per trillion range. From shot to shot the analytical thermionic diode had to be cooled down, opened, and refilled with a sample. Slightly different detection sensitivities were found, probably due to oxidation of the activating Ba layer on the cathode surface or not exactly reproduced temperature of the heat pipe. As described above in our second approach to the problem we modified the thermionic diode by introducing the sample by a probe. Now the time from shot to shot was shorter and the thermionic diode did not have to be cooled down. Figure 4 displays the diode signals of the 174Ybisotope in 0.1 mg of Eu. Now after about 90 s the analyte signal is gone. We recognize in Figure 4 that the analytical signal is not as smooth as in Figure 3. It is due to a nonuniform evaporation of the solid sample from the Re strip. We see further in Figure 4 that there is a small memory effect. Because we evaporate from the Rh band into the colder region of the thermionic diode the Yb atoms stay here for a longer time. In practice however, this memory effect should be no problem. From time to time the hot part of the thermionic diode should be cleaned by heating up to a higher temperature. For elements with lower vapor pressure than Yb, memory effects should be correspondingly smaller. The dependence of the analytical signal on the distance from the laser beams to the Re strip was also studied. With laser beam diameters of typically 2 mm we found only slight changes in the integrated signal up to a distance of 6 mm. For larger distances there was a decrease of the signal (a factor of about 2 for 12 mm). This means that the major part of the material is evaporated into a small angle. Of course the size strongly depends on the shape of the depression in the Re strip that contains the solid sample. In spite of the shorter evaporation time, with the modified analytical diode the detection limit for 17*Ybwas found to be of the same order of magnitude as estimated for the setup used before. To smooth the analytical signal and to improve the detection limit, the analyte atoms have to be kept longer in the interaction region of the laser beams with the vapor. Modifications of the current setups are under way in our laboratories. The reproducibility of the integrated signals was found to be good. Figure 5 shows a plot of the integrated analytical signals for 174Ybin Sm vs. the sample mass. Unfortunately the balance used for mass determination was not very precise (estimated uncertainty about 0.1 mg). The uncertainty influences in particular the measurements of very small samples. The Yb content in the investigated Sm metal was 25 ppm, determined by flame AAS. Within the experimental uncertainties the ratio of the Yb content in Eu to Sm could be verified.
17iYb in Srn
/
Flgure 5. Integrated analytical diode signal of 174Ybin Sm plotted vs. the mass of the Sm samples.
Taking into account the high sensitivity of the thermionic diode, detection limits in the femto- or even attogram range for isotopes should be achievable, if the final state of the trace atom is closer to the ionization limit than in the present experiment (higher collisional ionization rate) and the first excitation step is a resonance transition instead of an intercombination line. Furthermore the atoms have to be kept longer in the laser beams. Atomization in tubes with inner diameters of the order of the laser beams should improve the detection sensitivity considerably. Because of the limited wavelength range of CW dye lasers (400-950 nm) the new method is restricted to only a few elements. Most of the resonance lines are located in the UV region of the spectrum. For future work it is therefore necessary to extend the range of application by using UV generation techniques like intracavity frequency doubling of laser radiation. The state of the art allows to gain about 5 mW of UV light in the wavelength region 270-400 nm. With laser beams of small diameter this power is sufficient for saturation of resonance transitions in low-pressure noble gas atmospheres. For the long run we are convinced that the high-cost CW dye laser systems can be replaced by easy-to-operate, longlived, and low-cost semiconductor lasers. First experiments inducing the second transition in a two-step excitation processes by a diode laser are under way in our laboratories.
ACKNOWLEDGMENT We thank J. Richter of the Institut fiir Experimentalphysik of Kiel University for the hospitality during the realization of this work and J. Messerschmidt for performing the flame AAS analysis. 14041-51-1; Eu, Registry No. IT4Yb, 14683-29-5; 173Yb, 7440-53-1; Sm, 7440-19-9; Ba, 7440-39-3.
LITERATURE CITED Demtroder, W. Laser Spectroscopy; Springer-Verlag: New York, 1981. Omenetto, N. Analytlcal Laser Spectroscopy; Wiley: New York, 1979. Greenfield, S.; McGreachin, H. McD.; Smith, P. B. Talanta 1975, 22, 1-15, 553-562. Greenfield, S.; McGreachin, H. McD.; Smith, P. B. Talanta 1976, 2 3 , 1-14. Scott, R. H.; Strasheim, A. I n Applled Atomic Spectroscopy; Grove, E. L., Ed.; Plenum: New York, 1978; p 73. Price, W. J. Spectrochemlcal Analysis by Atomic Absorptlon ; Heyden & Sons: Philadelphia, PA, 1979. Omenetto, N.; Nlkdel, S.; Bradshaw, J. D.; Epsteln, M. S.; Reeves, R. D.; Winefordner, J. D. Anal. Chem. 1979, 51, 1521-1525. Epstein, M. S.;Nikdel, S.; Omenetto, N.; Reeves, R. D.; Bradshaw, J. D.; Winefordner, J. D. Anal. Chem. 1979, 51, 2071-2077. Uchida, H.; Kosinski, M. A,; Winefordner, J. D. Spectrochlm. Acta, Part B 1983, 3 8 8 , 5-13. Omenetto, N.; Human, H. G. C. Spectrochlm. Acta, Part B 1984, 3 9 8 , 1333-1343. Human, H. 0. C.; Omenetto, N.; Cavalli, P.; Rossi, G. Spectrochim. Acta, Part W 1984, 3 9 8 , 1345-1363. Green, R . B.; Keller, R. A,; Schenck, P. K.; Luther, G. G.; Travis, J. C. J . Am. Chem. SOC. 1978, 9 8 , 1517-1518. Schenck, P. K.; Hastie, J. W. Opt. f n g . 1981, 2 0 , 522-528. Travis. G. C.: DeVoe. J. R.: Schenck, P. K.; van Dijk, C. A. -. -, .I.- C.: - , Turk. Prog. Anal. At. Specfrosc. 1984, 7 , 199-241.
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Anal. Chem. 1988, 58, 1571-1572 (15) Axner, 0.;Bergllnd. T.: Heully, J. L.; Lindgren, I.; Rubinsztein-Dunlop, Ha J . APPl. PhyS. 1984, 55, 3215-3225. (16) Axner, 0.;Llndgren, 1.; Magnusson, I.; Rublnzszteln-Dunlop, H. Anal. Chem. M85, 57,776-778. (17) Axner, 0.;Magnusson, I . Phys. Scr. 1985, 37, 587-591. (18) Omenetto, 0.; Berthoud, T.; Cavalll, P.; Rossl, 0. Anal. Chem. 1985, 57. 1256-1281. (19) Hurst. G. S. Anal. Chem. 1981, 53, 1448A-1456A. (20) Fassea, J. D.: Moore, L. J.; Travis, J. C.; De Voe, J. R. Science (Washington. D.C.)1985, 230, 262-267. (21) Nlemax, K. Appl. Phys. 6 1985, 38, 147-157. (22) Brlllet, W. L.; Gallagher, A. Phys. Rev. A 1980, 22, 1012-1017. (23) Llao, P. F.; Bjorkholm, J. E. Phys. Rev. Lett. 1978, 36, 1543-1545. (24) Nlemax, K. Appl. Phys. 6 1983, 32,59-62. (25) Niemax, K.; Weber, K.-H. Appl. Phys. 6 1985, 36, 177-180. (26) Lorenzen. C.J.; Nlemax, K.; Pendrlll, L. R. Opt. Commun. 1981, 39, 370-374.
' Present address:
ERNO, Bremen, FRG.
Kay Niemax* J o r g Lawrenz Andreas Obrebski Karl-Heinz Weber' Institut fur Spektrochemie und angewandte Spektroskopie Bunsen-Kirchhoff-Strasse 11 D-4600 Dortmund, Federal Republic of Germany RECEIVEDfor review November 22,1985. Accepted February 13,1986. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Determination of Total Nitrogen in Oil Shale Sir: Cooper and Evans (I)report that fixed NH4+-Noccurs along with organic N in samples of Green River Formation oil shale from Colorado Core Hole No. 1. Total N was determined by a modified Kjeldahl method employing concentrated H2S04in a sealed tube at 400 "C for 3 h (2, 3). A digestion temperature of 400 "C was chosen rather than the 420 "C used in earlier work (2) so that the results would be more comparable to those for sediments from the Argentine Basin (3). In addition, the oil shales have a high organic content and there is less loss of tubes in the muffle furnace at the lower temperature (2). Fixed NH4+-Nwas determined by a KOBr-HF method (2,4). Because exchangeable NH4+-N was found to be negligible in these samples, the difference between total N and fixed NH4+-Nis reported as organic N. In a recent attempt to establish the mineralogical residence of the fixed NH4+-N in oil shale samples, ",+-containing minerals were concentrated by treating with dilute HCl to remove carbonates and with alkaline NaOCl (household bleach) to remove organic matter. When fixed NH4+-Nand total-N were determined on a sample of spent shale from Fischer assay that had been treated with HC1 and NaOCl (sample 781.5st), fixed NH4+-N was found to be 0.59% whereas total N was found to be only 0.15% (Table I). Clearly, not all of the fixed NH4+-Nwas extracted by the method for total N; therefore, a new method of analysis for total N content of oil shale was developed. EXPERIMENTAL SECTION Total N was determined by combining techniques for determining total N by sealed-tube digestion and for extracting fixed NH4+-Nwith HF (3). A sample (ground to pass a 170-meshsieve) of a size estimated to give a titer between 3 and 5 mL in the final step was weighed into a 10 X 75 mm2 Pyrex test tube. A 0.1-mL portion of concentrated H2S04was added to the sample, which was allowed to stand for 1h at room temperature before it was placed in an oven at 50 "C for 1h. The sample then was placed in a vacuum oven at 50 "C for 16 h. After the sample was removed from the vacuum oven, 0.4 mL of concentrated HzS04and 1 drop of HzS04diluted 1:l with water were added. The tube was sealed with a propane-oxygen torch, placed in a protective casing made from pipe, and the casing was placed in a muffle furnace maintained between 400 and 425 'C for 3 h. The casing (which contained several tubes) was removed from the oven, and, after it had cooled, the tube was removed and opened with a propane-oxygen torch to release pressure. The tube was scored with a file and the top was broken off. The contents of the tube were transferred completely to a 50-mL polypropylene tube with sufficient demineralized water to make the volume about 5 mL. To this was added 5 mL of 10
Table I. Total N, Fixed NH4+-N,and Organic N in Samples of Oil Shale from Colorado Core Hole No. 1 digestion fixed sample NH4+-N Only deDth," total N,* m % %' % o f N %d %ofN 772.3 781.5 781.5s 781.5st 789.3 813.2 819.4 819.4s
0.37 0.91 0.70 0.64 0.60 0.64 0.68 0.48
0.26 0.62 0.46 0.15 0.46 0.39 0.44 0.29
70 68 66
23 77 61 65 60
0.12 0.41 0.32 0.59 0.23 0.33 0.18 0.13
32 45 46 92 38 52 26 27
organic N %ofN
%'
0.25 0.50 0.38 0.05 0.37 0.31 0.50 0.35
68 55 54 8 62 48 74 73
"Depth to top of sample: s denotes spent shale from Fischer assay; t indicates that sample was treated first with HC1 and then with NaOC1. *By sealed-tube digestion + HF treatment. Standard deviation is estimated to be 0.020 by the method of Youden (6). cBy sealed-tube digestion only. Data from ref 1. Standard deviation is estimated to be 0.067 by the method of Youden ( 6 ) . dData from ref 1. Standard deviation is estimated to be 0.019 by the method of Youden (6). eBy difference between total N and fixed NHat-N because exchangeable NHdt-N is negligible. N HF:0.2 N HC1. The tube was stoppered and shaken on a mechanical shaker for 16 h. At the end of the shaking period no solid material remained. Three drops of thymol blue indicator (1%alcoholic solution) were added, and the pH was adjusted to the first color change of the indicator by cautious addition of 5 N NaOH. The solution became cloudy upon addition of the NaOH solution. The mixture was transferred to a 100-mL Kjeldahl distillation flask, and the pH was adjusted by adding 5 N NaOH sufficiently past the second color change of the indicator that the color remained blue throughout the distillation which followed. The distillation and determination of NH, were carried out by using a method described elsewhere (5). The flask containing the sample was attached to a steam distillation unit, and 25 mL of distillate was collected with 5 mL of boric acid-indicator solution. The NH3content was determined by titrating with 0.005 N HZSOb
RESULTS AND DISCUSSION When sealed-tube digestion was followed by HF treatment, the total N content of the HC1- and NaOC1-treated sample of spent shale from Fischer assay was found to be 0.64% rather than the 0.15% found by using sealed-tube digestion alone (Table I); thus, 4 times more N was extracted from this sample when H F treatment was added. Because extraction of total
0003-2700/86/0358-1571$01.50/00 1986 American Chemical Society
