I.N.Izosimov

APPLICATION OF LASER SPECTROSCOPY FOR

DETECTION OF LANTHANIDES AND

ACTINIDES IN SOLUTIONS

Laser spectroscopy 1. Luminescence (TRLIF), Chemiluminescence (TRCH) in solutions. 238U LOD - 10-13 M, 1ml necessary for analysis or 10-16 mole in sample (or 6*107 atoms, or 3*10-10 Bq). Determination of type of molecules (TRLIF – U VI, TRICH) and valence state (TRICH). Can’t determine the isotope composition. TRICH - The Limits of Detection (LOD) for spectrometers using the registration of chemiluminescence are in the range from 10-6 mol/l till 10-13 mol/l depending on the type of solutions and type of detectable molecule. 2. Absorption optical spectroscopy. LOD for U-Pu 10-4М – 10-5М. Determination of type of molecules and valence state. 3. Laser-Induced Photoacoustic Spectroscopy (LIPAS). LOD - 10-6 – 10-7 M. 4. Thermal Lens Spectroscopy (TLS). LOD – 10-6 – 10-7 M. 5. RIMS (atomic beam). LOD 105 atoms, isotope composition determination.

Pu, Np, U. M = mole/litre. 10-13M ≈ 2.4x10-14g/ml ≈ 6x107atoms/ml 10-13M 239Pu = 5.4x10-5Bq/ml; 238U=3x10-10Bq/ml; 235U=1.9x10-9Bq/ml; 237Np=6.2x10-7Bq/ml

ICP-AES, LOD 10ng/ml Pu ICP-MS LOD 3х106 Pu atoms

DF-ICP-MS 40 pg/l 235U, 1.2 pg/l 239Pu, 241Am (3x106atoms/ml)

ICP-QMS 60 ng/l Np.

Global Level Pu – global level i.e. nuclear weapon tests in the 1950-1960s. Up to 10 9 atoms/g of Pu in soil or up to 10-3Bq/g or up to 4x10-13g/g. LOD DF-ICP-MS for Soil samples (Kosovo) 1.3 x 10-13 g/g (240Pu). One of the most commonly used methods for determining 240,239Pu, 238Pu, 234,235U, 238U, 241Am in environmental samples relative to the global level is alpha spectroscopy in combination with radiochemical separation. Analysis of a sample containing 108 nuclei of 239Pu requires no less than a day of measurements.

Pu Global Level in different Objects.

Pulmonary tissue – (1–50)x10-6Bq/g; Lymph nodes – (2-360)x10-6Bq/g;

Liver – (0.5-80)x10-6Bq/g; Bony tissue – (1-40)x10-6 Bq/g;

Soils – up to 10-3Bq/g; Seawater – up to 10-5 – 10-6 Bq/l.

238U (99.283%) + 235U(0.711%) + 234U(0.0054) – 2.5∙104 Bq/g natural uranium. 238U - 1.25∙104 Bq/g. 234U - member of 238U decay chain natural radioactivity N : 238U ≈ 234U ≈ 21x235U or N ≈ 2∙238U Bq. U in: Soils - 3·10-6– 5.1·10-4 %; Seawater – 3 ng/ml; Blood - 4∙10-10 g/g; Urine - 0.04-5 ng/ml; Kidneys – 1.3∙10-8 g/g – 5.3∙10-9 g/g; Liver - 6∙10-9 g/g; Heart - 10-10 g/g; Brain - 10-10 g/g; Lung - 6∙10-9 — 9∙10-9 g/g; Bony tissue – 10-9 g/g; Hair – 1.3∙10-7 g/g.

Typical concentration of uranium in blood plasma is about 0.05ng/ml – 0.5ng/ml,

in urine is about 0.2ng⋅ml-1 – 5ng⋅ml-1.

Compare in different time concentrations of the uranyl in blood plasma and in urine one can

estimate the time when the uranyl was got into organism.

• Sensitive and direct detection of the trace amounts of actinide detection and simultaneously

determination of the actinides oxidation states and type of molecules containing actinides present today major importance for ecology,

radwaste handling and control, rehabilitation of contaminated areas and risk assessment.

• Luminescence spectroscopy is very attractive, because its theoretical sensitivity is limited in principle only by intensity of excitation beam.

• Today the laser spectroscopy methods applied for trace analysis may be classified as methods for isotope

composition determination and methods for type of elements and valence states determination. In this report we discuss methods which allow to determine the

actinides valence states but not allow to determine the isotope composition.

Practical use: it is necessary to get the investigated element

(isotope) from the sample to the zone of interaction with laser radiation and have such element (isotope) in this zone for a long time.

Attractive from the practical point of view is to use the solutions of investigated samples.

Eu+3 , Tb+3 , Gd+3 , Dy+3 , Sm+3 , Ce+3 , Tm+3 lanthanides and UO2+, Cm3+,Am3+, Cf 3+, Es3+ , Bk3+ actinides ions give direct luminescence in solutions andmay be detected by TRLIF method

TRLIF – time resolved laser induced fluorescencepulse laser (~10-8s) for excitation and luminescence registration after delay (~10-6s).

No direct luminescence from Np, Pu in solutions. We have observed thechemiluminescence in solutions induced by the actinides comlexes (Pu, Np, U)excited by the pulse laser radiation with delay time after laser pulse.

The TR chemiluminescence technique may be used for non-luminescent actinides(Pu and Np) and non-luminescent molecules containing U, Pu, Np detection.

TRLIF allows to decrease considerably the background from laserpulse.and has a sensitivity up to 10-13 mol/l

Separation of peaks in the luminescence spectrum of uranyl

Element UO2+ Cm3+ Am3+

LOD in mol/l (M)

10-13M (27pg/l)

10-13M (24pg/l)

10-9M (240ng/l)

Table 1. Limit of actinides detection (LOD) by TRLIF.

Element Eu3+ Tb3+ Gd3+ Dy3+ Sm3+ Ce3+ Tm3+ LOD in mol/l 10-12 10-9 10-8 10-10 10-10 10-9 10-6

Table 2. Limit of lanthanides detection (LOD) by TRLIF.

Biological samples containing a large amount of organic substances should be preliminary mineralized. Typical concentration of uranium in blood plasma is about 0.05ng⋅ml-1 – 0.5ng⋅ml-1, in urine is about 0.2ng⋅ml-1-5ng⋅ml-1.

Without mineralization the limit of uranyl detection in blood plasma was 0.1 ng⋅ml-1. After mineralization was up to 0.008 ng⋅ml-1- 0.01 ng⋅ml-1.

The limit of uranyl detection in urine in our TRLIF experiments was up to 0.005 ng⋅ml-1. limit of detection of europium was 0.015 ng⋅ml-1 and samarium, 0.2 ng⋅ml-1.

Pu, Np, and a number of lanthanide and actinide compounds do not produce direct luminescence in solutions and traditional

procedure with time-resolution (TR) of analytical luminescence signal (TRLIF) cannot

be used for their detection. However, for detection of Pu, Np, and a lot of actinide and lanthanide compounds TR procedure can be used with registration of chemiluminescence of luminol (5-amino-1,2,3,4-tetrahydro-1,4-

phthalazindione) arising in light-induced excitation of actinide ions.

Chemiluminescence is one of the most sensitive methods of actinides and lanthanides detection in

solutions. In our experiments chemiluminescence arises as a

result of oxidation of luminol molecule with OH• radical generated in optical excitation of the element

to be detected and entering into composition of hydroxo complex. An excitation of hydroxo

complexes in a solution up to an energy allowing electron transfer from the ligand (OH−) to the central ion of actinide or lanthanide with subsequent splitting off of OH• radical is realized in successive resonance

absorption two photons with actinide or lanthanide ion.

Example of chemiluminescence observation

Only two uranyl complexes are in solution:

UO2F53- and UO2F4OH3-.

1. UO2F53- give the direct luminescence.

2. UO2F4OH3- or PuO2F4OH3-complex after excitation by

laser radiation give chemiluminescence .

When pH increase UO2F53- concentration and direct uranyl

luminescence decrease but UO2F4OH3- concentration and chemiluminescence increase.

450 500 550 600 650 7000

1000

2000

3000

4000

5000

6000

7000

2

1

Inte

nsity

Wavelength, nm

1 – pH 8.37; 2 – pH 10.8.

luminescence spectra of UO2F53-

Absorption spectra of 1-UO22+ (0.006M), 2- PuO2

2+ (0.006M), and 3-luminol in 42%CsF + H2O solution at pH=10.

0

0.1

0.2

0.3

0.4

0.5

300 500 700 900 1100

W a v e l e n g t h, n m

O p

t i c

a l

d e

n s

i t y

1

2

3

For this two-step excitation of hydroxo complexes we used tunable lasers. Two-step excitation allows us to avoid direct excitation of chemiluminogen (in our experiments, luminol) and bring energy sufficient for subsequent initiation of chemiluminescence reaction. It is significantly that in the first step of excitation f-f transitions can be used, which abruptly increases the procedure selectivity.

An increase of the power of laser radiation in multi-step scheme of excitation should in principle result in enhancement of the sensitivity of chemiluminescence procedure. However, beginning from some value of laser radiation power, higher-order effects play a significant role.

The behavior of actinides in environment is determined by actinides valence states and type of molecules. Information about actinide valence states is essential to fix the actinides emission source and the propagation history. The combinations of the chemiluminescence effects with high sensitivity (up to 10-9 – 10-13 mol/l) and high selectivity laser spectroscopy TR and multy-step excitation methods make it possible to carry out an effective detection both of luminescent and non-luminescent actinides and molecules containing actinides (especially U, Pu, Np) in different solutions.

Kinetic curve of luminol chemiluminescence induced by excited plutonyl complexes

From kinetic curve of luminol chemiluminescence induced by excited plutonyl complexes one may determine the optimal delay time after the laser pulse for chemiluminescence detection. After few microseconds the background from impurities luminescence and scattered laser radiation is practically absent and chemiluminescence may be detected by TR spectroscopy methods with high sensitivity.

Chemiluminescence in CsF2(42%) +H2O (pH =8,5)• – plytonyl + luminol [10-3M]; – luminol [10-3M].

Delay time 2 µs. Gate time 50 µs.

By proper selection of lasers radiation wavelengths, one or multi-step actinides complexes excitation scheme and chemiluminogenic label it is possible to induced chemiluminescence only by selective excitation of detectable actinide complexes. The sensitivity of the chemiluminescence methods is higher than sensitivity of other methods applied for Pu valence states determination (LIPAS for example).

Of actinides properties analysis interest was to use advantages of luminescence procedure for detection of actinides having no self-luminescence, as an example, by initiation of luminescence of some agents through excitation of actinide element to be detected. An effort was made to realize this approach by initiation of chemiluminescence of luminol through excitation of actinide ion with laser radiation. A key problem of chemiluminescence application for actinides detection in solutions is increase of the selectivity of detection.

Absorption spectra of plutonyl complexes in 42 % CsF + H2O solutions at different pH values. Two complexes are in solution:

PuO2F53- (band 839nm) and PuO2F4OH3- (band 846.5nm)

Absorption spectra of aqueous solutions (1) 7.3·10−3 M SmCl3,pH 5.6 (2) 6.8·10−3 M SmCl3 + 2.9·10-5 M luminol, pH 5.6 (3) 6.8·10−3 M SmCl3 + 8.79·10-5 M luminol, pH 5.6 (4) 10−3 M luminol, pH 5.6.

Absorption spectrum of aqueous solution 10−3 M SmCl3 + 2·10−5 M luminol: (1) 401.0, (2) 415.0, (3) 464.0, (4) 477.5 nm.

Chemiluminescence initiated by absorption of UV radiation is not selective. Appropriate selectivity in detection of actinides in solutions can be reached when absorption bands of ions in visible range caused by transitions of the inner 5f electrons are used for initiation of chemiluminescence. However, absorption of one quantum of visible light cannot impart the energy sufficient for chmiluminescence initiation. This energy can be reached when actinide complex absorbs two or more quanta of laser radiation in visible range.

(1) nitrogen laser OBB-1010, (2) beam splitter, (3) dye laser OBB-1011, (4) dye laser OBB-1012, (5) optical delay line OPD-1, (6) cuvette with solution, (7)

optical fiber, (8) monochromator DMR-4, (9) photomultiplier, (10) mirror

Experimental set-up for chemiluminescence spectroscopy of actinides in aqueous solutions

The experiments were performed on an installation consisting of pulse nitrogen laser OBB 1010 with a pulse length of 1 ns and generation power of approximately 1.4 MWt and two dye lasers OBB 1012 and OBB 1011. When using two dye lasers the radiation generated by nitrogen laser was simultaneously derived to both dye lasers through a beam splitter. This scheme allows synchronization of laser pulses in a cuvette within an accuracy of 10 ps at generation pulse length of 800 ps for laser OBB 1012 and 1 ns for laser OBB 1011. A laser beam splitter was oriented at an angle of 45º to the direction of laser beam generated by nitrogen laser and divided this beam to two beams with equal intensities. Laser beams generated by two dye lasers were aligned in the opposite directions and directed to a cuvette 1 cm in thickness.

Chemiluminescence of luminol was collected with a lense whose optical axis was oriented at an angle of 39° to the direction of laser beams and was transferred to the entrance slit of double prismatic monochromator with flexible optical fiber. Chemiluminescence was recorded in the quantum counting mode with the use of gating technique at a wavelength of 460 nm corresponding to the maximum of luminol chemiluminescence. The length of gating impulse (strobe) was 10 µs, delay time, 2 µs.

In our experiments we used dye lasers (models OBB-1011 and OBB-1012) pumping by nitrogen laser (model OBB-1010).

Model OBB-1010 Nitrogen Laser OBB’s Model 1010 Nitrogen Laser delivers a crisp pulse at 337 nanometers with a hefty 2.4 megawatts of peak power. With a pulse width of 1 nanosecond, that results in a pulse energy of 1.45 megawatts. We got the specified power with

the specified pulse characteristics every time. Model OBB-1011 High Intensity Dye Laser

OBB’s `1011 Dye Laser is built around a single stage Littrow configuration cavity, providing continuously tunable output from 360 to 900 nm. The pulse energy at 500 nm is 250 microjoules. With a pulse width of 1 nanosecond and a bandwidth of 1 to 3 nm, OBB’s 1011 model is perfect for

general spectroscopy.

Model OBB-1012 High Resolution Dye Laser OBB’s 1012 Dye Laser incorporates a grazing incident

design laser cavity for high resolution followed by a secondary amplifier cell to boost the power. The result is

a very narrow 0.04 nanometer output from 360 to 900 nm, a pulse width of 1 nanosecond, and an energy of

220 microjoules per pulse at 500 nm. With the addition of OBB’s OL-403 Frequency Doubler, tunable wavelengths

from 235 to 345 nm can be attained.

1. Luminol+U(IV)+HCl. Chemiluminescence intensity dependence on the wavelength of laser radiation at the first excitation step. When the wavelength of laser radiation corresponds to the wavelength of U(IV) absorption band than the intensity of luminal chemiluminescence is increased. The wavelength of laser radiation at the second step was fixed at 500 nm (two steps-two colors scheme).

2.Absorption spectrum of U(IV)+HCl solution.

The experiments were performed with hydroxo complexes of Sm(III). The wavelength of dye generation for the first step of excitation was chosen based on absorption spectra of solutions containing Sm(III) and luminol. Since the absorption end of luminol (sodium form) is close to the strongest line of samarium spectrum at 401nm, the experiments on reduction of samarium with laser radiation were performed at the line of 477.5 nm. In this case, the probability of excitation of luminol molecules with laser radiation is minimum.

Spectrum of excitation of luminol chemiluminescence (a) with dye laser in the range of absorption bands of Sm3+(two steps-one color scheme ).

Chemiluminescence is recorded at the wavelength of 460 nm. Absorption spectrum of Sm3+ is shown below.

At a power of laser radiation not exceeding 108 W/cm2 per pulse (the length of laser pulse in the order of 800 ps, spectral width δλ = 0.04 nm) chemiluminescence arises only under addition of samarium to the solution and when the wavelength of laser generation does not fall into the range of Sm3+ absorption chemiluminescence disappears. This means that absorption of two quanta with wavelength 477.5 nm is sufficient for initiation of luminol chemiluminescence.

1-Absorption spectrum of the solution to be irradiated. 2-Specific intensity of chemiluminescence of Pu(IV) solution as a function of wavelength of radiation generated by the first laser, wavelength of radiation

generated by the second laser 500 nm (two steps-two colors scheme). Solution composition: CsF 3.6 M, luminol 10–5 M, Pu(IV), solution pH was adjusted by

addition of CsOH.

Specific intensity of chemiluminescence excited by the mechanism two steps–one color in irradiation of Pu(IV) solution by laser OBB 1012 in the range of 630-660 nm.

1.We observed chemiluminescence of chemilumonogen (luminol) initiated by multi-quantum excitation of lanthanides and actinides in aqueous solutions by laser radiation.

The use of laser radiation with tunable wavelength

and multi-step schemes allows selective excitation of actinide or lanthanide species with subsequent registration of chemiluminescence.

2. A multi-step schemes of chemiluminescence

excitation open the way to makes this procedure not only highly sensitive but also highly selective procedure of detection of substances.

Conclusion

• Luminescence and Chemiluminescence: LOD up to 10-9 - 10-13mol/l, 1ml need for analysis.

• TRLIF: Without mineralization the limit of uranyl detection in blood plasma was 0.1 ng⋅ml-1 and after

mineralization was up to 0.008 ng⋅ml-1- 0.01 ng⋅ml-1.The limit of uranyl detection in urine in our TRLIF experiments was up to 0.005 ng⋅ml-1.

In pure solution the limit of detection of europium was 0.005ng⋅ml-1 and samarium,

0.07ng⋅ml-1. After addition of 0.2 ml of urine the limit of detection of europium was 0.015 ng⋅ml-1 and

samarium, 0.2 ng⋅ml-1. • We observed chemiluminescence in solutions induced by actinides selectively excited

by laser radiation. The Time Resolved method may be applied for Pu, Np and U induced chemiluminescence detection.

• Chemiluminescence technique can be used for non-luminescent actinides (Pu and Np) and non-luminescent molecules containing actinides (U, Pu and Np) detection in solutions.

In particular, non-resonance two-step excitation of hydroxo complexes through virtual intermediate level of the ion to be detected begins to manifest itself. In this case, the selectivity of procedure caused by the use of f-f transitions abruptly decreases.

However, with further increase of the power of laser radiation above 108 W/cm2 per pulse

chemiluminescence begins to burn even the wavelength of laser radiation is outside the range

of Sm3+ absorption. This means that two-quantum mechanism of excitation via virtual

intermediate level of Sm+3 ion begins to operate. Thus, when using multi-step scheme of

chemiluminescence excitation there is a need to choice the power of laser radiation to

provide required sensitivity and selectivity.

Oxidation states and type of molecule determination in solutionsapplied for actinides (M=mol/l):1. A.S. LOD = (10-4 – 10-5 )M.2. TLS. LOD = (10-7 – 10-8 )M.3. LPAS. LOD = (10-7 – 10-8 )M.

CLS (time resolved chemiluminescence). 1 ml need for analysis,LOD =(10-10–10-13 ) M. Applied in biology and medicine but wasnot studied and not applied for actinides.

U, Np, Pu: 10-13M – 2.4*10-14g/ml 237Np: 10-13M – 6.2*10-7Bq/ml 239Pu: 10-13M – 5.4*10-5Bq/ml 238U: 10-13M – 3*10-10Bq/ml 235U: 10-13M – 1.9*10-9Bq/ml

We observed and study the chemiluminescence in solutions

induced by U, Pu and Np excited by pulse laser radiation. It

open possibility to develop chemiluminescence spectroscopy for

actinides detection in solutions. Charge transfer (CT) transitions

excitation in molecules containing actinides led to the

chemiluminescence of chemiluminogenic molecules (we used

luminol) in solutions, multi-step chemiluminescence excitation

using for selective actinide detection

Example of chemiluminescence observation

Only two uranyl complexes are in solution:

UO2F53- and UO2F4OH3-.

1. UO2F53- give the direct luminescence.

2. UO2F4OH3- complex after excitation by laser radiation

give chemiluminescence .

When pH increase UO2F53- concentration and direct uranyl

luminescence decrease but UO2F4OH3- concentration andchemiluminescence increase.

0

1000

2000

3000

0 50 100 150 200

Delay time, microsecond

Inte

nsity

Kinetics of luminol luminescence in dry methanol.

0

3000

6000

9000

400 500 600 700 800

Wavelength, nm

Inte

nsity

3

2

1

Luminol luminescence: (1) chemiluminescence in aqueous

solution, (2) chemiluminescence and luminescence in aqueous ethanol,

and (3) luminescence in dry ethanol.

• However, for detection of plutonium, neptunium and uranium the TR procedure can

be used with registration of chemiluminescence of luminol (5-amino-1,2,3,4-tetrahydro-1,4-phthalazinedione), which arises under the action of OH radicals generated in solutions by light-excited actinide ions. We used this procedure for detection of actinides in solutions containing AnO2

2+ ions (An = U, Np, Pu). In this case, the chemiluminescence kinetics is characterized by burning up for three microseconds and decay of luminescence with a characteristic time of approximately two microseconds

0

50

100

150

0 5 10 15Delay time, microsecond

Inte

nsity

Depending on the lifetime of excited actinides complex the nanosecond or picosecond pulse

lasers may be used for multi-step excitation. After resonance excitation of actinide at the first step the hot molecules are formed. At the second step the hot molecules are excited by laser radiation. The excitation wavelength are selected in such way that only after resonance excitation of the hot

molecules at the second step the chemiluminascence will take place.

• For detection of small amounts of actinides it is necessary to exclude a possibility of registration of luminol luminescence having nature different from chemiluminescence.

It was found that with decreasing water content the intensity of luminescence having chemiluminescence nature decreases; in addition we observed a luminescence with red-shifted spectrum and kinetics having no burning-up stage typical for chemiluminescence. It should be noted that this luminescence different from chemiluminescence arises in single-quantum UV excitation of luminol molecule and can be significantly depressed in two-quantum excitation induced by radiation with longer wavelength since luminol has no absorption bands in visible region.