TRACE ANALYSIS BY LASER OPTOGALVANIC SPECTROSCOPY

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TRACE ANALYSIS BY LASER OPTOGALVANIC SPECTROSCOPY

Yu. Kuzyakov, N. Zorov, V. Chaplygin, O. Novodvorsky

To cite this version:

Yu. Kuzyakov, N. Zorov, V. Chaplygin, O. Novodvorsky. TRACE ANALYSIS BY LASER OPTO- GALVANIC SPECTROSCOPY. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-335-C7-343.

�10.1051/jphyscol:1983731�. �jpa-00223288�

JOURNAL DE PHYSIQUE

Colloque C7, supplement au n O 1 l , Tome 44, novembre 1983 page C7-335

TRACE A N A L Y S I S BY L A S E R O P T O G A L V A N I C SPECTROSCOPY

Yu.Ya. Kuzyakov,

Zorov,

Chaplygin and

Novodvorsky

Department o f Chemistry, Lomonosov S t a t e U n i v e r s i t y , Moscow 117234, U. S.S. R.

Resume - On a u t i l i s e l a t e c h n i q u e s e l e c t i v e e t s e n s i b l e de l a s p e c t r o s c o p i e m o p t o g a l v a n i q u e pour l a d e t e c t i o n d'element ii l ' e t a t de t r a c e . P l u -

s i e u r s v a r i a n t e s d ' a t o m i s a t i o n de l ' e c h a n t i l l o n dans l a flamme o n t e t 6 u t i - l i s e e s y compris des m i c r o - t e c h n i q u e s . Les 616men s a1 a l i n s o n t e t @ c h o i s i s dans c e t t e etude. Des l i m i t e s de d e t e c t i o n de 10-4-10-6 p i n o n t e t 6 obtenus en s o l u t i o n acqueuse. La dependance du s i g n a l optogalvanique a 6 t 6 @ t u d i @ e en f o n c t i o n des d i f f e r e n t s schemas d ' e x c i t a t i o n l a s e r des atomes, de l a d i s - t r i b u t i o n du p o t e n t i e l dans l a zone d ' e x c i t a t i o n e t des i n t e r f e r e n c e s avec l e s el@ments f a c i l e m e n t i o n i s e s . C e t t e t e c h n i q u e a e t @ u t i l i s e e pour l ' a - n a l y s e de quel ques e c h a n t i l l ons n a t u r e 1

Abstract - The selective and sensitive technique of laser optogalvanic spectroscopy has been used in order to detect element traces. Different variants of sample atomization in a flame, including microtechniques, were applied. The alkali elements were chosen as objects of investigation. The limits of detection 10-4-1 0-6 ppm in water solutions have been achieved.

The effect of different excitation schemas of atoms, voltage gradient in the excitation zone, interferences of easily ionized elements on a optogalvanic signal has been studied;

Application of this technique to the analysis of some real samples has been demonstrated.

The possibility of determination of single atoms of individual substances in gaseous state has been shown using the technique of selective laser ionization. Generally used in experiments on detection of single atoms is evaporation of pure individual substances to vacuum from a special chamber, like the Knudsen cell, followed by the

treatment of the atomic beam by laser radiation. By selecting an appropriate frequency and power of the laser beam, atoms resonantly absorbing this radiation can undergo photoionization. Charged species thus formed are detected by the conventional methods.

In this case there will be no ions (electrons) which might result from collisions since the ions are not to occur in the atomic beam.'Limits of detection of elements are determined only by essentially

unremovable noises caused by multiphoton ionization as well as by direct photoionization of the atoms having the energy of electron detachment less than the energy of a quantum used for excitation and ionization of the atom to be detected. Assuming the matrix containing various elements from which the sodium is the most easily ionizable one (Vi = 5.14 eV), it may be shown that a direct photoionization will not occur with 82 elements since the excitation energy of the lowest levels for the atoms under analysis is less that the ionization

potential of sodium.

Because of complicated operation of vacuum equipment its time consumability in analysis and possible memory effects, - ^{all these}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983731

JOURNAIL D€ PHYSIQUE

preventes wide application of such a method of vaporization in analytical practice. Only in 1982 the first work of Letokhov et al.

/l/ appeared on the determination of aluminium in high purity germanium. The limit of detection obtained was about I O - ~ % . At present in analytical spectroscopy atomization with the use of

flame is the most widely used technique. The first experiments demonstrating analytical possibilities of optogalvanic spectroscopy were carried out in USA in 1977-78 /2,3/, in which one-step excitation was used. Since that time, the analytical applications of optogalvanic spectroscopy are attracting the interest of analysts.

The selectivity of detection of a certain type of atoms in flame as well as in the case of photoionization in vacuum is obtained by resonance excitation of atoms of a given type to one of the excited levels. The most probable process of ionization of the excited atoms is the collision ionization. The number of collisions of atoms in analytical flames is quite great (log collisions per second). The process of photoionization appears to be competitive though its importance decreases as long as the difference between the ionization potential and the energy of selectively excited levels decreases. We applied the two-step excitation in experiments with sodium /4/. Sodium atoms were excited to the 4 D state. The difference between the

ionization potential and the energy of 4 2 ~ level is equal to 0.85 eV.

The attempts to increase the degree of ionization of sodium irradiating the vapours by the first harmonic of neodymium laser (energy density of

1 ~ / c m ~ ) did not succeed although the energy of

the first harmonic quanta is quite sufficient to ionize sodium atoms being at their 4 2 ~ state. Van Dijk et al. carried out similar

experiments in 1981 /5/ studying the effect of nitrogen laser irradiation on the degre2 of ionization of sodium atoms, which were being at their 3 2 ~ and 4 D states. The ionization signal was found to increase hundred-fold provided the sodium atoms had been excited to their 3 2 ~ state while the irradiation by nitrogen laser had no effect on the ionization signal if the atoms were in the 4 2 ~ state.

Collisions of excited atoms of elements under analysis result in not only their ionization but deactivation so it is desirable to convert the atoms to such excited states where the difference between the excitation energy and ionization potential would be minimum. The stepwise excitation is frequently used for that purpose. This technique for the determination of series of elements was first published in 1979 from the Moscow State University /4/.

Limits of detection (LODs) of atoms in the flames are determined by noises related to the presence of ions from the flame itself.

Theoretical considerations of LODs were carried out in a number of studies. In 1980, Falk /6/ obtained the LOD value at ng/ml level; the value of 104-106 ions per 1 cm3 of flame was obtained at the Moscow State University /7/. Travis et al. /8/ believe that it is possible to determine

lo5 ions in 1 cm3 of flame which corresponds to

10-3 ng/ml. The best LOD obtained experimentally was the one for lithium

(1 -10-3 ng/ml) .

If,the analysis of real samples(where easily ionized elements are present in considerable amounts) will be performed the worse limits of detection due to the increasing background signal will be obtained.

The set-up used in our experiments consisted of dye lasers, an

electrode (cathode) placed directly in the flame, an electronic device

for registration of the ionization signal including a preamplifier, a gated integrator and a read-out unit.

The value of ionization signal depends on the spectral energy density of a laser beam. The most widely applied Hansch's desi n /9/ allows one to obtain the laser bandwidth ( A A ) of about 1 for a dye laser. We used /10/ the d e laser design with grazing incidence which made possible to obtain b.&

0.1 cme1 and to improve the efficiency of transformation of pumping energy. That enables the spectral energy density to be increased and so the limits of detection for a number of elements may be improved by one--two orders of magnitude. The results obtained with the Hansch design and with the grazing incidence design are presented in Table 1.

Table 1. - Limits of detection of K , Rb and CS (ng/ml) depending on the laser bandwidth

Precise tuning in to the absorption line of an element to be detected becomes an important problem for a narrow laser bandwidth. To solve it, we proposed /11/ a simple procedure based on the variation of impedance of laser irradiated electrodeless discharge lamps employed as high intensity light sources in atomic absorption spectroscopy. The change of impedance depending on the laser frequency scanning in the region of potassium doublet is shown in Fig. 1.

Element K Rb CS

Fig. 1 - Variation of a signal amplitude depending on the laser radiation wavelength for the resonant change in the impedance of potassium vapours in a sealed cell.

The position of the electrode deserves a special attention. A number of works are concerned with this problem now /8,12,13/. Various designs of electrodes and their arrangement with respect to the flame were studied. Analytical data were obtained presumably with the

electrodes besides the flame. Only in 1981, Turk /14/ applied a water- cooled cathode immersed directly in the flame.

d- = 1 cmd1 2 5

1 0.1

Since 1979 /4/, we are using in our studies the electrode inside the flame and the electrodes are not cooled. Electrode material had been selected so that to provide a prolonged stable operation. The electrode

AA

^{0. I cm-'}

0.1

0.1

0.004

C7-338 JOURNAL DE PHYSIQUE

was designed by such a manner that the laser beam passed in close proximity to the electrode surface. That provided the possibility for operation within the region of the cathode potential drop which greatly increased the efficiency of the process for collecting of charged species. The change in the potential depending on the distance from the electrode is shown in Fig. 2, which illustrates that on introducting of easily ionized atoms into the flame, the distance decreases between the electrode and the point in the flame, whereas the electric potential is still remain effective. In the analysis of complex real samples, the immersed electrode into the flame is of particular effectiveness.

Fig. 2 - Potential drop near the cathode for various concentration of charged species. 1 - pure flame; 2 - flame with sodium addition

(CNa

30 p g / m l ) .

The analytical signal appeared to be seen in this case in the presence of easily ionized matrix with its concentration 102 times exceeding the matrix concentration as compared with the results of experiments with the externally arranged electrode. The results of our

investigation of the effect of sodium on the ionization signal of caesium are presented in Fig. 3, where, for the sake of comparison the results of work /l21 are also given in which the electrodes were outside the flame.

Fig. 3 - The effect of sodium concentration on the signal for: 1 -

determination of indium (electrode outside the flame /12/); 2 -

deternination of caesium (electrode inside the flame).

From the same figure, it may be seen that the ionization signal first

increases on addition of easily ionized atoms, but until1 now the

phenomenon has not been satisfactorily explained. In this regard we

suggest an explanation for this non-monotonic dependence of the amplitude of ionization signal on the concentration of easily ionized element proceeding from a change in the parameters of closed

electrical curcuit of the optogalvanic detector. The corresponding equations are written and the dependence of evolved pulse power (6 p) resulted from the optogalvanic signal in the electrode was shown to be non-monotonic and depends on the ballast resistance and flame

resistance. This is shown in Fig. 4.

Fig. 4 - The plot of v ^{( d )} function in the (0.1) interval.

The pulse power is maximum for a certain ratio of these resistances.

Introduction of easily ionized atoms into the flame leads to a change in its resistance. Therefore, depending on the abovementioned

resistance ratio, there is the possibility of increasing of the signal as well as that of the transition of the signal value through its maximum with the concentration of easily ionized atoms in the flame changed.

We applied the technique of optogalvanic spectroscopy to determination of alkali metals in aqueous solutions. The LODs thus obtained as well as the LODs obtained in other laboratories are presented in Table 2.

For comparison here are given the results of determination of alkali elements by the technique of flame atomic emission spectroscopy /15/.

The reproducibility of the results of determinations was better that 3-4%.

Table 2. - Comparison of LODs for alkali elements obtained with the optogalvanic and atomic emission flame spectroscopy techniques

* The data are taken from /16/.

Element

Li Na K Rb C

For real samples to be analysed, the interaction of alkali elements was studied. For an example, the effect of sodium on the ionization

Limits of detection, ng/ml Present work

0.03 0.01 0.1 0.1 0.004

Other works / B /

0.001 0.05 1 1 -

/15/

0.02 0.1 0.5*

3

8

C7-340 JOIJRNAI- DE PHYSIQUE

signal of caesium is represented in Fig. 5.

Fig. 5 - The effect of CCs/CNa ratio on the caesium signal for various caesium concentrations: 1 - 100; 2 - 10; 3 - 1 ng/ml (the value of signal with no sodium addition is taken as 100%).

It can be observed the curves behaviour is the same for different levels of the analyte. From the above it is obvious that the less the concentration of analyte, the more the permissible ratio "analyte/

easily ionized matrix element". Thus, the ionization signal was shown to be affected only by the content of easily ionized element in the sample matrix. This important analytical conclusion made possible to solve the problem of analysis of industrial solutions with high salt content (up to 10%). In order to reduce the effect of easily i nized elements of matrix the analyzed solutions were just diluted 103 times and then the amount of, for instance, caesium was determined in them.

Another procedure of elimination of matrix effects consisted in working out a special atomizer. Its design /17/ is shown in Fig. 6.

Fig. 6 - Representation of the atomizer design with separate

vaporization and atomization of sample: 1 - introduction of fuel and air; 2 - a burner body; 3 - flame zone; 4 - an electrically heated loop; 5 - argon introduction.

Utilization of a heated wire loop allowed us to solve several problems.

First of all, introduction of small quantities of a substance produces

a small absolute amount of easily ionized matrix. Secondly, the use of

a programmed temperature control unit CRA-90 by "Varian-Techtron" made

possible to separate in temperature (and in time of appearence) the

signals resulted from different substances, namely from the analyte

and from the sample matrix, due to the difference in their

volatilization temperatures. The results of application of this atomizer are shown in Fig. 7. It should be noted that abso te LOD in the case of determination of caesium was equal to 5.10-tg g and the reproducibility was better than 5-6%.

Fig. 7 - Dependence of the caesium signal (A) and background currents (I) on temperature. 1 - caesium signal; 2 - background current of rubidium; 3 - background current of sodium. The noise level is 20 arbitrary units; flame conductance - ^{0.4 A} for vaporization of

5 of high purity water. p

The insertion of the cathode directly into the flame permitted us to use the procedure of putting the sample onto the electrode. The

advantage of this procedure is that excitation and ionization of atoms occurs in the sample atomization zone so the efficiency of sample utilization is high. Owing to this circumstance we managed to obtain fair absolute limit of detection. In the case of sodium and caesium, 1 - 10-I g and 2.10-' g, respectively, were detected. However, the reproducibility of determinations with this procedure is somewhat worse than upon sample aspiration into the flame and it composes 20-30%.

In practice one frequently encounters with the samples containing large amounts of silicon compounds which form thermally stable silicon dioxide Si02 on their combustion. The silicon dioxide precipitates on the electrodes and deteriorates the results of analysis even to such a degree that after some time the optogalvanic signal disappears at all.

We have suggested the procedure of analysis for the samples which contain elements forming thermally stable oxides. The idea of the procedure consists in additional heating of the cathode by electrical current. In this case, there is no precipitation of Si02 observed on the electrode surface. Using the above procedure, we determined the content of potassium in siliconcontaining samples at the level of

10-4-1 0-5% without timeconsuming procedure of separation of the matrix.

Using optogalvanic spectroscopy with sample atomization in the flames, we could analyze some real samples. Thus, the content of rubidium in caesium salts and vice versa, the content of caesium in rubidium salts, were determined at the level of 10-~-10- %; so was the content of

caesium, rubidium and potassium in various waters (of natural origin and the ones purified with the use of industrial installations like

"Milli-Q Water Purification System"). The limits of detection obtained made possible to perform direct determination of such amounts of

substances which could not be detected by other methods.

In conclusion, good prospects should be noted for application of the

methods of optogalvanic spectroscopy for photon detection. In one of

C7-342 JOURNAL Dli PHYSlQlJFl

our papers /18/, we reported such a photon detector to discover weak atomic absorption lines in the method of intracavity laser

spectroscopy. The propane-butane flame was used as a detector where the lithium solution with the concentration of 10,4g/ml was nebulized.

The flame was irradiated by two lasers with the wavelengths 670.7 nm and 610.3 nm so that to excite the lithium atoms to the 3 2 ~ state. The optogalvanic signal was distinctly registered with the use of cathode into the flame. A 10-cm acetylene-air slot burner was placed inside the resonator of the first laser. Standard lithium solutions were introduced into the acetylene flame. The change of lithium

concentration in solutions produced a change in the intensity of laser radiation (670.7 nm) and, correspondingly, a change of the

optogalvanic signal detected in the flame of the propane-butane burner.

With the use of a analytical curve, the characteristic concentration was determined, which was equal to 3 . I O - ~ /yg/ml. Thus, the techniques of direct analysis of real samples in the flame by the method of optogalvanic spectroscopy was demonstrated in this work.

The future application of optogalvanic spectroscopy to analytical purposes, apparently, has to be developed on the way of separation of the regions of atomization and excitation. That will make possible to use the higher temperatures for atomization and the lower temperatures for determinations.

References

/l/ Akilov R.O., Bekov G.I., Letokhov V.S., Maksimov G.A., Mishin V.I., Radaev V.N. and Shishov V.N., Kvant. Elektron. (sov.) 2

(1982) 1859

/2/ Green R.B., Keller R.A., Luther G.B. and Schenck P.K.,Abstrs.

Pittsburg Conf.Ana1. Chem. and Appl. Spectrosc. Cleveland, Ohio, 1977, p.126

/3/ Turk G.C., Travis J.C., De Voe J.R. and O'Haver T.C., Anal.

Chem. 50 (1978) 817

/4/ Gonchakov A.S., Zorov N.B., Kuzyakov Yu.Ya. and Matveev O.I., Anal. Lett. 12 (1979) 1037

/ 5 / Van Dijk C.A., Curran F.M., Lin K.C. and Crouch S.R., Anal.

Chem 13 (1981) 1275

/6/ Falk H., Progr. Anal. Atom. Spectrosc. 3 (1980) 181

/7/ Zorov N.B., Kuzyakov Yu.Ya., Matveev 0.1. and Chaplygin V.I., Zh. Anal. Khim. (sov.) 2 (1980) 1701

/8/ Travis J.C., Turk G.C. and Green R.B., Anal. Chem. 54 ⁽¹⁹⁸²⁾

1006A

/9/ Hansch T.W., Appl. Opt. 11 (1982) 895

/10/ Novodvorsky O.A., Zorov N.B., Kuzyakov Yu.Ya., Abstrs. XI All-Union Conf. on coger. and nonlin. optics, Erevan USSR, November 22-25, 1982, part 11, p.845.

/11/ Salsedo Torres L.E., Zorov N.B., Kuzyakov Yu.Ya., Matveev 0.1. and Novodvorsky O.A., Zh. Prikl. Spektrosk. (sov.) 37

(1982) 488

/12/ Green R.B., Havrilla G.J. and Trask T.O., Appl. Spectrosc.2 (1980) 561

/13/ Havrilla G.J. and Green R.B., Anal. Chem. 52 (1980) 2376

/14/ Turk G.C., Anal. Chem. 53 (1981) 1187

/15/ Weeks S.J., Haraguchi H. and Winefordner J.D., Anal. Chem. 50

(1978) 360

/16/ Plyushchev V.E. and Stepin B.D., Analytical Chemistry of Rubidium and Caesium, "Nauka" Moscow, 1975, p.138

/17/ Chaplygin V.I., Zorov N.B., Kuzyakov Yu.Ya. and Matveev O.I., Zh. Anal. Khim. (sov.) 38 (1983) 802

/18/ Matveev O.I., Zorov N.B. and Kuzyakov Yu.Ya., Talanta 27

(1980) 907