Anal. Chem. 1983. 55, 959-963
959
dominate in this pentacyclic chrysene series. The quantification of such compounds can be done in an absolute way by the method of constant additives, with an internal standard, allowing the determination of absolute quantities of the order of a nanogram. This HRF is now being developed by us (M.E.) on other series of biogeochemical markers in complement with GC/MS investigations, especially in the case of the elucidation of complex mixtures of isomers.
AUTHENTIC M O L F C U L E OF CHRYS-a
ACKNOWLEDGMENT We thank d. Joussot-Dubien for comments and careful examination of the manuscript. ' 7 Registry No. Chrys-a, 59279-20-8.
W A V E L E N G T H (nm)
r-
> -
B
in
LITERATURE CITED
I-
I C H R Y S - a FROM 5OUTH A R A B I A 5 E A W
0
2
p-365
370
375
380
385
W A V E L E N G T H (nm)
Flgure 1. (A) Filrorescence spectrum in n-heptane at 4 K of an authentic molecule of chrysene-a (Chrys-a). C = M. Excltation was at 276.5 nm. (B) Fluorescence spectrum in n-heptane at 4 K of the tetraaromatic fraction, obtained by HPLC,from a marine sediment (Sea of Oman, South Arabia). Excitation was at 276.5 nm.
n-alkane crystals, even though sterical hindrance is not very favorable to insertion. In fact, what is occurring could most likely be related to the findings of Rima et al. (5) in the case of PAH such as phenanthrene whose molecules do not fit in the crystal lattice of a n-alkane. In that case, the authors assume that a t low concentration and under the condition of fast freezing a new type of inclusion of guest molecules is formed in the alkane matrix (5). The technical aspects of the methodology, which includes a home made spectrofluorimeter working with a liquid helium cryostat, have been reported elsewhere (4-6).The HPLC is straightforward and has been described ( 4 ) . The marine sediments were cored from surface sediments during the Orgon IV cruise at several water depths varying between 200 and 4000 m in an area where the organic matter is mainly of authochthonous origin (7). This origin would explain the predominance of chrys-a, a biogeochemical marker formed by the aromatization of hopanoid triterpenes occurring in procaryotes (bacteria and blue-green algae), which appears to
(1) Pltts, W. M.; Merle, A. M.; El-Sayed, M. A. Chem. Phys. 1979, 3 6 , 437-446. (2) Greiner, A.; Spyckerelle, C.; Albrecht, P. Tetrahedron 1976, 32, 257-260. (3) Palewska, K.; Ruziewlcz, 2 . Chem. Phys. Lett. 1979, 64, 378-382. (4) Ewald, M.; Moinet, A.; Bellocq, J.; Wehrung, P.; Albrecht, P. In "Gochimie Organique des Skiiments, Marins Profonds"; Orgon IV, Golfe d'Aden, Mer d'oman, Ed. CNRS: Paris, 1981; pp 405-414. (5) Rima, J.; Lamotte, M.; Joussot-Dubien, J. Anal. Chem. 1982, 52, 2093-2095. (6) Ewald, M.; Lamotte, M.; Redero, F.; Tissier, M. J.; Albrecht, P. Adv. Org. Geochem. 1980, 12, 275-279. (7) Caratini, C ; Beliet, J.; Tlssot, C. In "GOochimie Organique des Skilments Marlns Profonds"; Orgon I V , Goife d'Aden, Mer d'Oman, Ed. CNRS, Parls, 1981; pp 265-307.
Marc Ewald* Groupe d'Oc6anographie Physico-Chimique de 1'ERA No. 167 Laboratoire de Chimie Physique A Universit6 de Bordeaux I 33405 Talence Cedex, France
ERA CNRS No. 278-Laboratoire et Chimie Marines Universit6 Pierre et Marie Curie 75230 Paris Cedex 05, France
Alain Moinet Alain Saliot de Physique
Pierre Albrecht LA 31-Laboratoire d'Etudes des Substances Naturelles Institut de Chimie, Universit6 de Strasbourg 67008 Strasbourg Cedex, France RECEIVED for review November 1, 1982. Accepted January 26, 1983. We are indebted for partial financial support to Centre de GBochimie Marine who sponsors ORGON cruises on N/O J. Charcot, allowing new experiments on sediment cores.
Electron Pulse Shape in Laser-Enhanced Ionization Spectrometry Sir: Recently, the optogalvanic effect first observed by Penning (I)has provided, in conjunction with the development of dye lasers, a new detection tool which avoids most of the optical difficulties usually encountered in fluorescence and emission spectrometry. Flame optogalvanic spectrometry or laser-enhanced ionization (LEI) spectrometry has already been demonstrated (2-5) to be a very powerful technique in the trace analysis field. Different theoretical aspects of optogalvanic signals in flames (photoexcitation, charge production,
and current detection) have been investigated in recent papers (3, 5-8). In a conventional air-acetylene flame with low concentration of Na (100 ppb) and a two-step laser excitation, we have observed the temporal shapes of the LEI signal, using a fast current detector. These studies have been made by varying two parameters: the high dc voltage and the position in the flame of the two laser beams. Analysis and interprlatation of the profiles observed for the first time without significant electronic distortion for short (6 ns) laser pulses are
0003-2700/83/0355-0959$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL.
55, NO. 6, MAY 1983
Rebtive ntensity ?Okv
-
TIME
Figure 1. Temporal shape of the LEI electron current for different high voltage (-2 kV, -1.6 kV, -1.2 kV, -0.8 kV, 4-03 kV, +1.2 kV, + l . 6 kV, +2 kV) at different heights for the laser beam. x o = 8 mm.
Figure 2. Same as Figure 1. x,,
= 20
mm.
TIME i n s
given.
EXPERIMENTAL SECTION The two-step excitation is provided by two tunable dye lasers pumped by a nitrogen laser with a 3 0 - H ~repetition rate. The time duration of the laser pulse is about 6 ns and the dye laser line width is 0.1 A. In our experiment with Na, one laser is tuned to the first step transition 3Sl/p3p3/2 at 5889.9 A with an energy of 50 fiJ/pulse. The second laser is tuned to the second step 3Pa/2-5d3/2,6/2 at 4982.8 A and the energy per pulse is approximately 400 pJ. The fine structure of the 5d level is not resolved due to the laser line width. The measurement cell is composed of a Perkin-Elmer premix burner with three slits operated under recommended conditions for an air-acetylene flame. The 100 ppb Na solution is prepared with ultrapure grade NaCl and doubly distilled water. The nebulization consumes 10 mL/min of solution. The electric field is created between the burner and a water-cooled molybdenum electrode following the design proposed by Turk (9). The high voltage is provided by a stabilized dc power supply. The time delay between the two laser pulses in the interaction zone is adjusted to obtain the best efficiency of the LEI signal. The two beams are counterpropagating in the flame for convenience. A multipass optical device (horizontal and vertical) allows us to increase the amplitude of the LEI signal in proportion to the number of passes when it is needed for analytical use of the technique. With the 100 ppb Na solution, a single pass is sufficient. The burner head is used as the second electrode, from which signal currents are successively processed by current and voltage amplifiers built in the laboratory. For a typical 15-V output signal the rise time is below 25 ns. The temporal shape of the LEI signal is recorded on a Hewlett-Packard X-Y plotter through a PAR 162/165 boxcar averager with a 5-11s gate width. The signal is normalized to the second step laser intensity by a ratiometer card incorporated in the boxcar. The distance between the burner and the molybdenum electrode has been fixed to 25 mm for all the following experiments. All the equipment, including,the lasers and the flame LEI detector used in the experiment, is manufactured by SOPRA Co. (SociBt6 de Production et de Recherches AppliquBes-68, rue Pierre Joigneaux F., 92270 Bois Colombes, France).
RESULTS AND DISCUSSION The amplitude of the LEI signal has been measured as a function of the time delay between the first and second laser pulses. It follows closely the expected convolution function in time for two laser pulses in the flame. This confirms that the collisional life time of the intermediate 3p3p level is very short in an air-acetylene flame (0.3 ns) (IO)compared to the 16-ns radiative decay time. So the experimental arrangement has been fixed for the time coincidence of the laser pulses where the best signal is recovered. Smyth and Mallard (11, 12) have shown the existence of two temporal components of the LEI signal with short laser pulses, They observed a short electron signal followed some microseconds later by a longer and weaker ion signal. We observed the same features. Our study only concerns the temporal shape of the electron current pulse, which is a t least
\
+ 1.2 k v 400 -
300 -
200
+ 1.6 k v +20 kv
i-
1001
I
b
Ib
i5
do
25
mm DHE’GH’
Figure 3. Second maxima of the LEI current vs. height above the burner for posltlve applied voltage.
10 times greater in amplitude and shorter in duration than that due to the ions. The temporal shapes of the electron current have been studied for dc voltages varying from -2000 V to +2000 V and for different heights in the flame given by the distance xo between the burner and the laser beams. Figures 1and 2 show the set of curves obtained respectively for xo = 8 mm and xo = 23 mm at different voltages. Curves for intermediate values of xo show a continuous evolution between the extreme cases presented. The sign of currents on Figures 1 and 2 are the real signs of observed currents. Strong dependence of the pulse shapes with voltage and height above the burner clearly appears: Figure 1 shows different profiles for opposite applied voltages and a similar feature is observed on Figure 2. When the pulse duration is over 150 ns, the profiles always show two maxima, the first one at about 100-130 ns and the second one varying in the range of 150-600 ns. Actually, there is a continuous evolution from the double positive peak of Figure 1to the single positive peak of Figure 2 with increasing values of xo. A similar phenomenon occurs for negative voltage, an evolution from the double pulse of Figure 2 to single pulse in Figure 1. That means that the time position of the second peak continuously varies with xo and the cases of a single peak correspond to a superposition of two pulses. The behavior of the two peaks can be summarized as follows: the position of the first peak is independent of voltage and xo, with an average position of 100 ns. Time delay of the maximum of the second peak with respect to the laser excitation is a function of xo and voltage and is represented in Figures 3 and 4,respectively, for negative and positive polarization. It should be noticed that, although we collect the signal on the burner, we can observe the arrival time of the electrons
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
961
npplled potentkl
TIME
t
+ns
400
*
1 -08kv -1 2 k v -1 6kv -2 0 kV
200
1
Figure 5.
Experimental result of capacitive effect (temporal shape).
t
100
h
L 5
h
15
25
mm
Second maxima of the LEI current vs. height above the burner for negative applied voltage. on the electrode when positive voltage is applied. This indicates that, at a given time, the density of current at any height in the flame is controlled by the potential of the electrode. This is clearly understandable taking into account the time T of establishment of a steady-state current
+ kv' = p g
with m = mass density of moving electrons, u = speed of the electrons, p = clharge de_nsityof moving electrons, k = viscous force coefficient, and .E = electric field. In the steady state we have dv/dt = 0 Assuming u' = /LE p = mobility of the electrons so
k = P/P Integrating eq 1, we get
- e-b/")t)
L, =:
(2)
For the electron we have p / m =
e/M
with e = charge of the electron and M = mass of the electron. So the time constant for establishment of a steady-state current is 7e =
P m
-- = --"p = (5.68X 1O-l2)p in ISU C'
e
Substituting the value of p calculated below from experimental results p = 5 X lo3 cm2.V-ld = 0.5 m2-V-1d, we get 7, = 2.84 X s. A similar calculation for Na+ ions with p = 25 cm2-V-l.s-' leads to 7Na+
1 Relative
1 Intensity
HEIGHT
Figure 4.
dv' mdt
A
Relative 1ntenr;ity
= 5.87 x lou1' 8
All these time constants are below the nanosecond range and therefore we can assume that all the phenomena we are dealing with occur in a steady-state scale of time. This means that, at any time, the density of current at any height in the flame and particularly at the burner depends on the events occurring a t the electrode. This explains why we observed the arrival of the electrons at the electrode. We propose the following mechanism to explain the observed phenomena: (1) The atoms are excited by a two-step excitation during the 6 ns of the laser pulse duration. ( 2 ) Instantanetously (picosecond scale), the excited atoms are ionized with an efficiency given by the Arrhenius law.
0
4000~ time
0
Observed pulse
Figure 6.
400"s Real pulse
time
Interpretation of LEI current observed pulses.
(3) Electrons and ions are separated by the electric field cregted between the electrode and the burner by the applied dc voltage. (4) The transient electron and ion currents are observed through two main processes: direct charge collection and capacitive reaction of the burner-flame-electrode system to the instantaneous creation and separation of charges. This capacitive effect has been investigated with an experiment where a potential step was applied to a tungsten rod located at the position of the laser beam between the electrode and the burner. A similar capacitive effect has been obtained with and without the flame and is given in Figure 5. When we change the sign of the potential step, we get the same signal but with the opposite sign. This experiment indicates that the first-and only the first-maximum of the observed double pulses of Figures 1 and 2 may be expected to arise from capacitive coupling at the moment of the charge creation and separation. Furthermore, the circuit response to the step function (Figure 5 ) may be deconvoluted from experimental pulse shape to yield the underlying pulse shape, as illustrated in Figure 6. This interpretation indicates that the pulse of current is observed with little or no delay after the laser pulse. Actually this is easily explained by the movement of the electrons toward the electrode or the burner. As they get closer, they increase the capacitive charge already created by the phenomena just described. This charge increases until the arrival of the electrons, at which time capacitive charge and electron cancel each other. So we interpret the second maximum (usually followed by a quick return to zero) of the current as the arrival time of the electrons. It should be noticed that the first capacitive effect may create some parasitic peak that has to be distinguished from the real second peak. The dependence of the time of the second maximum of double pulse on voltage and position strongly suggests that it really corresponds to the arrival of the electrons at the positive electrode, in close agreement with optogalvanic measurements of ion arrival times (11, 12). Figures 3 and 4 indicate a significant difference in the behavior of the electron arrival time as a function of position for positive and negative high voltage. Assuming that the electrical field follows this equation E(x) = a bx (3)
+
for negative applied voltage E(c - x) = a
+ b(c - x)
(4)
for positive applied voltage, with x the distance from a point in the flame to the burner ( x = 0), a the value of the electric field at the burner, b the slope of the electric field, and c the distance from the burner to the electrode.
962
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
This type of linear electric field in a flame has been described by Lawton and Weinberg (13) and already used in different models of LEI phenomena (8, 9). The arrival time of the electrons created by the laser beam at x o is given by
dx/dt = p(a
+ bx),
6mm -
RELATIVE INTENSITY
I L
-HV
dx/dt = p(a + b(c - x ) ) , +HV so
to =
2C7Lb log (1 +
?)
for negative applied voltage RELATIVE INTENSITY
1 to = Pb log (1
I
+ %$)
+2.0kV
for positive applied voltage. The curves given in Figures 3 and 4 indicate that for positive applied voltage we must have b(c - xo)
c - xo
TIME
> 0 so t o = - log 1 + -xo U
Pb
With the following relation:
and
where tl and t zare the times corresponding to x1 and x 2 which are any value of x o a t a definite voltage using V(c) = '/2bc2
+ ac.
When the above relationships are used to extract values for a, b, and p from the data of Figure 4, the indicated values of p vary with applied voltage and fall in the range of X lo4 c m 2 . V 1 d some 2 to 3 time higher than the
lo4 to 1.5
expected value. This is most likely due to the fact that the field may not exactly follow the E = a bx equation, particularly near the electrode. That the electric field configuration differs radically for positive and negative high voltages is further confirmed by the fact that the dc current is much greater for a given positive high voltage than for an equivalent negative high voltage.
+
Flgure 7. LEI current temporal shape in the presence of KCI (50 ppm).
The long overshoot of the electron pulse is probably due to reequilibrium phenomena of the electron current through the flame. A confirmation of some aspects of this interpretation has been obtained by measurements made by introducing 50 ppm of KC1 in the 100 ppb Na solution to modify the electrical properties of the flame. The pulse shapes obtained under such conditions are presented in Figure 7 . For negative applied voltage, the pulse duration is reduced in contrast to increase observed for the positive applied voltage. Assuming that the introduction of KC1 increases b, the slope of the field, the experimental results agree with the equation proposed in both cases. An important reduction of the intensity of signal appears when the laser beam is directed in a zone of very low electric field. This confirms that the separation speed of electrons and ions after the laser pulse is greatly influenced by both the ions and electric field distribution. The two calibration curves obtained linear a t least on 2 orders of magnitude show an enhancement of a factor of 4.5 for the sensitivity, when KC1 is added, all other conditions being identical. The measurements record the peak values of the pulses. The total charge collected appears to be the same with and without KC1 and the enhancement seems to be due to short collection time. It should be noticed that the enhanced sensitivity, in this case, does not necessarily mean decreased limit of detection, due to increase in limiting noise. This last point should be clarified by a complete study of the limiting noise. The following characteristics of the electron pulse have been demonstrated: The leading edge of the electron pulse occurs instantaneously, corresponding to capacitive coupling to the external circuit. The second maximum in the electron pulse for positive voltage applied to the immersed electrode is consistent with predicted electron arrival time at the electrode for an appropriate value of electron mobility and nearly a constant field. The simple model successfully used for the positive applied voltage fails to provide self-consistent results for negative high voltage. This may well reflect the role of the flame reaction zone in distorting the electric field in the flame. A possible approach for this case could be the classical model of virtual
Anal. Chem. 1983, 55,963-965
electrode used for thermoionic diodes. CONCLUSION Detailed understanding of the response of the LEI pulse to electric field and laser beam position is an important step in ensuring the sensitivity and the accuracy of the method. This is especially true since easily ionized substances have been demonstrated ,to modify the electric field in the flame. ACKNOWLEDGMENT The authors ;are greatly indebted to J. C. Travis for helpful discussions, access to his team's theoretical work prior to publication, anid comments on our manuscript. Acknowledgment is made to G. Baudin and G. Delarue for help and suggestions received throughout the course of this study. We are grateful to B. Fleurot for advice in the high-speed electronic devices realization. LITERATURE CITED (1) Penning, F. hd. Physlca (The Hague) 1028, 8, 137-140. (2) Green, R. B.; Keller, R. A.; Luther, G. G.; Schenck, P. K.; Travis, J. C. Appl. Phys. Lett. 1 9 W 29, 727-729. (3) Turk, G. C.; Travis, J. C.; De Voe, J. I?.; Oblaver, T. C. Anal. Chem. 1078, 50, 817-820. (4) Turk, G. C.; 'Travis, J. C.; De Voe, J. R.; O'Haver, T. C. Anal. Chem. 1979, 51, 1890-1896. (5) Turk, G. C.; Mallard, VI/. G.; Schenck, P. K.; Smyth, K. C. Anal. Chem. W7g, 51. 2408-1410. (6) Green, R. B.; Havrilla, G. J.; Trask, T. 0. Appl. Specttrosc. 1980, 34, 56 1-569.
963
(7) Gonchakov, A. S.;Zorov, N. B.; Kuzyakov, Yu. Ya.; Matveev, 0.I . Anal. Lett. 1079, 12, 1037-1048. (8) Schenck, P. K.;Travls, J. C.; Turk, G. C.; O'Haver, T. C. J . Flhys. Chem. 1981, 85. 2547-2557. (9) Turk, G. C. Anal. Chem. 1981, 53, 1187-1190. ( 1 0 ) Mavrodineanu, R.; Boiteux, H. "Flame Spectroscopy"; Wiley: New York, 1965. (11) Smyth, K. C.; Mallard, W. G. Combust. Sci. Techno/. 1981, 26, 35-41. (12) Mallard, W. G.; Smyth, K. C. Combust. Flame 1982, 44, 61-70. (13) Lawton, J.; Welnberg, F. J. "Electrlcal Aspects of Combustion"; Clarendon Press: Oxford, 1969.
Thierry Berthoud* Jacek Lipinsky Centre d'Etudes NuclBaires Fontenay aux Roses, France Pierre Camus Laboratoire Aim6 Cotton
CNRS Orsay, France Jean-Louis Stehle SOPRA Bois Colombes, France
RECEIVED for review May 17, 1982. Resubmitted November 15, 1982. Accepted December 30, 1982. This work is supported by the DGRST-MinistBre de la Recherche et de 1'Industrie under No. 80.7.0569 and 81.Y.0805.
Cationic Indicator Bases for the H+ Scale in Concentrated Sulfuric Acid Sir: The Hammett acidity function, HO( I ) , was introduced to describe the prototropic behavior of neutral bases in concentrated mineral acid. The Ho function measured the tendency of the imediunn to transfer a proton to an uncharged base. It was felt (2), however, that this function would not adequately describe the prototropic equilibria of charged bases such as singly or doubly charged cations. Thus attempts were made to establish the H+function based on the protonation of singly charged cationic bases. Theoretically, the H+function was expected to be more negative than the Ho function because of the electrostatic contribution of the charges on the indicator ions to their activity coefficients (3). One study ( 4 ) showed that plots of log ([base]/[acid]) vs. percent sulfuric acid for several anilines as well as 4-nitro1,2-phenylenediamine (inflection region in 30-55% sulfuric acid) and 4-aminoacetophenone (inflection region in 75-95% sulfuric acid) were parallel. From these results, these investigators concluded that the H+and Hoscales were either identical or differed by a small constant amount. Other workers (5) found that the second protonation of the aminopyridines followed the Ho function. In investigation of the prototropic behawior of singly charged cations from 0.02 to 15 M sulfuric acid (6),it was found that the H+ acidity function was slightly more negative than the Ho function up to about 7 M acid. Thereafter, the former became less negative than the latter and did not increase as rapidly with increasing molar acid concentration. Below 7 M acid, primary anilines were used to establish the H+ function. However, above 7 M acid quinoxalines, except for 3-nitr0-1,2phenylenediamine, were used as indicators. It seemed likely to us that the leveling off of the H+ function was due to the use of quinoxalirie cations.
In this work the second protonation of several cations in sulfuric acid was studied. The aim was to use these indicators to check the accuracy of and to extend the published H+ function to 18 M sulfuric acid using the protonation of primary arylamines as the exclusive class of indicator reaction. A reliable H+function is necessary in order to extend the hydration parameter treatment which was applied previously to carboxamides (7,8) and lactams (9) to charged bases other than primary amineEi. To date there has been no study of the protonation behavior of doubly positively charged cations. Hence a limited study of two such indicators was also conducted. EXPERIMENTAL SECTION Chemicals. Proflavine (Allied Chemical and Dye Corp., New York, NY), Thionine (K&K Laboratories, Plainview, NY), 1,2diaminoanthraquinone, 2,6-diaminopyridine, 3-nitro-1,2phenylenediamine, 4-aminopyridine, 2-amino-6-methylpyridirte, and 2-aminopyridine were used without further purification. 4-Bromo-6-nitro-1,2-phenylenediamine was synthesized from 2,6-dinitroaniline (Pfaltz and Bauer, Stamford, CT) by the following procedure. 2,6-Dinitroaniline was dissolved in acetic acid and excess bromine added to the solution. This mixture was then refluxed on a water bath for 2 h (10). The precipitate was filtered and shown to be 4-bromo-2,6-dinitroaniline by melting point (150-162 OC) (11) and NMR. This compound was dissolved in ethanol and treated with ammonium sulfide (12). This solution was heated on a water bath for an hour during which time it became deep red. The solution was concentrated under vacuu.m and the precipitate filtered. This was checked for purity by TLC using methano1:chloroform (10:90) as developing solvent. This precipitate was found to be 4-bromo-6-nitro-1,2-phenylenediamine by melting point (360 "C) and NMR. Method. A stock solution of each indicator was prepared in
0003-2700/83/0355-0963$01.50/00 1983 American Chemical Society
