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“Q-&vq?yfl+l+Development of Laser Excited Atomic Fluorescence and IonizatiProgress Report for DOE-DE-FG05-88-ER13881

Aefl

J. D. Winefordner, PI 4s C9We have .rzo olijec~fon from a patent

Department of Chemistry standpoint to the publication orUniversi~ of Florida dissemination of this material.

Gainesville, FL 32611(352) 392-0556 w? @-VQZ&~Jec~UaJ

Propkr~ CounselDOE.Field office, Ch a

Research ObjectivesdLaserexcitedatornicfluorescenceand ionizationspectrometryarefimdament y%emost

sensitive ofatomic spectroscopic methods. Inprinciple, s~gleatomdetection can bedernonstratedfor bothtechniques. ~egoaloftis resewch pro~mis todevelop tiesemetiods forpmcticdultratrace elemental analysis in a wide variety of matrices. This is being done through furidamentalstudies of atomization processes with the aim of developing optimum atom reservoirs for practicalanalysis, studies of laser excitation processes for fluorescence and ionization with the aim ofenhancing our understanding of the dynamics of atom and ion populations in these reservoirs andfinding eftlcient single and multi-step excitation schemes for many elements, and studies oftechniques for photon and ion detection with the aim of improved analytical performance.

Overview of past four years workSince the initiation of DOE support for this research program in 1988, a large number of

topics related to the goal of the work have been explored. A total of 49 publications relating to thisresearch have been published or are in press. In preface to this renewaI proposal, we will onlyreview our work of the past three years.

Promess Report (Jam arv 1.1995- December 31. 1998)The progress report is divided into 8 sections including (1) laser ionization, (2) laser

fluorescence in a glow discharge, (3) glow discharge ernissiotifluorescence, (4) magneticallyenhanced glow discharges for optical err&sion spectrometry, (5) electrothermal atomization-laserexcited atomic fluorescence, (6) 3-dimensional number density profiles of species in a glowdischarge, (7) laser induced breakdown spectroscopy, LIBS, (8) fimdamental studies, and (9) otherstudies.

Laser Ionization. Petrucci, et al’ studied and characterized a resonance ionization detector(RID) based on the two-step enhanced ionization of indiurn atoms in an atmospheric pressureairheetylene flame. Practical utilization of the RID is shown by recording a partial excitationspectrum of the OH radical in the same air/acetylene flame used as the detector cell andmeasurement of the flame temperature (2516+175) by the Boltzmann plot method. OH transitionswere excited in the ~IIi + A2Z+(1,0) vibronic band in the wavelength range 281-288 nm.

A schematic of the general principles of the 2 step-LEI approach for photon detection isshown in Fig. 1. The transition 1+ 2 of the atom M, henceforth termed the detector elemen~ servesas the signal transition. An atom, M, promoted directly (or ind~ectly through another level NQ tolevel 2, M“,by any means, is simultaneously illuminatedby an intense laser beam, henceforth termedthe detection laser, tuned to the excited state transition 2 + 3. These highIy excited atoms, if

1

—.. —

DISCLAIMER

This repo~ was prepared as an account of work sponsoredbyanagency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

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promoted to energy levels of the detector element within approximately 3kT (- 5000 cm-] for anair/acetylene flame) of the ionization continuum (IC), are efficiently ionized by collisions with flamespecies. The attractiveness of this approach to photon detection lies mainly in its high lightthroughput, narrow frequency response, determined by the absorption bandwidth of the detectorelement signal transition inmost cases, and subsequently high stray light rejection. Also, since theionization signal is produced only in the flame volume illuminated by both the excit@ion anddetection lasers, exceptional spatial resolution, of the order of micrometers, can be obtained bymaking the two laser beams spatially orthogonal in the flame.

‘We also reported the &st observationof the excitation spectrum of a native flamespecies by the resonance ionization detectionapproach. The principal aim of this workwas to show the practical utility of such anRID, based on In as the detector elementj inrecording OH excitation spectra (see Fig. 2)and measuring flame temperatures. Also, thehigh spatial resolution afforded by the IUDapproach is demonstrated by recording atemperature profile of the miniature flameused with a spatial resolution on the order of100 pm. Finally, two potential mechanismsof energy transfer from the excited OHmolecules to the indiurn atoms werediscussed in addition to experimentsperformed towards the elucidation of thepredominant energy transfer pathway.

Petrucci, et aL2used the same iridiumresonance detection (RID) for themeasurement of flame temperatures with aspatial resolution of less than 100 ym. Thedetector, based on the two-step excitation ofiridium atoms, with subsequent collisionalionization, was used to record rotationalexcitation scans of OH in an atmospheric-pressure acetylene/air flame. The OHexcitation spectra were recorded by scanningan “excitation” laser in the A*Z++ X211i(1,0)vibronic band in the wavelength range, 281-288 nm, while simultaneously illuminating

2

1

n

I

M’

M“

Ma

M*

M

the same fkune rf@OII With the “detection” Fi~~~ 10 Gene~~d Wo+tepLJZ1excitationlaser, tuned to the 6p2Pqn + 10d2~D5nscheme.

excited-state transition of In at 786.4 nm.The excitation and detection laser beams were made orthogonal in the flame, defining the resolutionto be limited by the waist of the excitation beam (100pm), whose diameter was always sm+ler thanthe detection laser beam. A temperature profile of the flame was recorded with the use of both theRID approach and a more conventional laser-induced fluorescence (LIF) approach for comparison

2

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53

h.%m 2:z

1Cl-1

0

-1281.:12 281.74a 282.384 283.020 283.6S6

Excitation wavelength (rim)

Figure 2. Partial excitation spectrum of OH in an atmospheric pressure acetylene/air flamerecorded using the Iridium RID.

(see Fig. 3). A more structured temperature profile was recorded with the IUD owing to its highspatial resolution, whereas the LIF method produced a rather featureless temperature distributionacross the flame. Anomalowly high flame temperatures were recorded at the flame edge withresonance ionization detection (see Fig. 4).

Avalanche detection ofalaserenhanced ionization (LEI) signal wasstudied3’4in a resonance ionization ~detector (RID) ceil containing zmercury vapor at room temperature. ~Figure 5 shows the experimental set ~up. h avalanche multiplication ~.factor of more than 8000 was ~achieved. The limit of detection of –Hg resonance radiation (A = 253.7run) was at the level of 0.5 quantumduring the lifetime of the excited63P1*state. The excitation process isshown in Fig. 6. The aval~che

-6 [

I:#

-10 L2000 3000 4CO0 5000

Ground stote energy (cm-’)

process is shown in Fig. 7 ~d tie Figure 3. Typical Boltzmann plot from RID data useddetection of a singIe photoelec~on to calculate rotational OH temperatures.pulse is shown in Fig. 8. Detectionof radiation from a conventional CW Hg discharge lamp source with a signal-to-noise ratio of morethan 104was achieved.

3

I i

1000’-4 -3 -2 -1 0 1 2 3 4

Distance from flame center (mm)

Figure 4. Temperature profde of a small air/acetyleneflame measured using the IUD and LIF.

With the goal of optimizingthe efficiency ofresonance ionizationof Hg, we made several studies of thetemporal behavior of the laserenhanced ionization signal ofmercurys’bin a quartz cell under lowbuffer gas pressure. Using fastelectronics and a short (34 ns) laserpulse, it was possible to distinguishbetween the non-selectivephotoionization component of thesignal and that which was due tocollisional ionization from selectedlevels in onetime-resolved ionizationwaveform. Experimental resultswere shown to agree with thoseobtained by computer simulation, and

optimal conditions for deconvolution of the two components were Studied. This work hasd~monstrated the possibility of obtaining simultaneo& and yet separate Morrnation aboutphotoionization and collisional ionization in a single time-resolved waveform, obtained when atoms.are excited in a low pressure gas cell. It has been shown that the signal due to photoionizationresembled the laser pulse shape that created it and therefore occurred during the time duration of thispulse, while the collisional ionization signal has alongertemporal duration (200-800 m), dependingon the buffer gas pressure and the Rydberg level fromwhich collisionaI ionization occurs. This experimentalevidence agrees with than obtained by simulating theactual conditions with the DensMat density matrixmodel. Observations ranged from n = 10, from whichlittle detectable collisional ionization could be observed,to n = 26, where the collisional ionization occurred sorapidly that this part of the signal could not bedistinguished from the photoionization component. Itwas I%rtherobserved that this type of measurement mustbe made at s 10 Torr; above this, the lower electronmobility causes difficulty in extracting the specificprocesses from the signal. From these results, we canconclude that any analytical measurements made usingthis technique, for a buffer gas at room temperature,must be made at a gas pressure ofs 10 Torr and withionization born levels n= 10-20 which results in greaterselectivity.’ Future work will include studying Rydbergline broadening, the temperature dependence of the rate Signal

of ionizatio~ and fluorescence dip spectroscopy toestimate the efficiency of collisional ionization from Figure 5. Experimental setup for the

resonance ionization detection-of Hg.

4

excited Rydberg levels. In the projects which are proposed below, we plan to develop and evaluatea practical sample introduction approach which can take advantage of the extremely high detectionpower which is available in this ionization technique.

In other work involving the ionization of mercury, a method for LEI detection is describe8’9which is based on the optical emission from buffer gas atoms which are collisionally excited byinteractions with electrons and ions in a strong electric field (see setup in Fig.9). The firstobservations of this phenomenon are reported here, along with comparisons between optical andelectrical detection. Advantages of a pulsed high voltage field over a continuous field are described.A wide range of possible applications for this type of gas phase ionization detector are suggested.

F— 133P?

-(A3 = 489.Onm

73s~ .12=435.8nm

~ 63q0/...

.....”””””””2.1= 253.7nm/

/..””’”””61~o

In the optical emission LEI detector7’8,a proportional,position-sensitive LEI signal was detected by observing theoptical emission of a btier gas (neon) after the acceleration ofelectrons in a constant and pulsed field. Further experimentsshould be carried out in order to study the linearity of theemission signal. In principle, this emission method “couldbeapplied for the detection of single atoms or molecules in anytype of buffer gas. This technique of LEI gas phase detectionwas amenable to a wide range of applications. These includedetecting the energy and the tracks of high energy particlesmoving through a gas target in a nuclear experiment, detectingmacro and micro objects from Raman scattering andfluorescence radiation, and detecting u-, ~-, y- and x-rayradiation. In addition, the method could find use in globalmercury monitoring and in backgroundless multichannel spacecommunication systems, as well as in such techniques as deep

Fi~re 6. Excitation scheme for Vacuu microscopy

the three step ionization of Hg..

In a related study, a method was described10involving electrospray nebulization of liquidsamples on the surface of an atomizer or vaporizer, which can enable quantitative and uniformdeposition and also matrix modification of the solid residue of the sample. Ultralow limits ofdetection of atoms could be achieved in combination with laser excited fluorescence, RIS and LEImethods of atomic and molecular analysis with ES deposition of the sample on a wire atomizer orvaporizer.

Laser Excited Fluorescence in a Glow Discharge. Davis, et al” designed and evaluated aminiature glow discharge atom reservoir for laser excited atomic fluorescence spectrometricmeasurements of nanoliter-sized solution residues. Figure 10 shows a schematic of the experimentalsetup. The copper vapor laser-pumped dye laser was used to measure the fluorescence of Pb atomssputtered from the Ni cathode of the discharge. Excitation of Pb occurred at 283.3 nm, and

fluorescence was monitored at 405.8 nm. The optimum discharge operating pressure and currentwere 5.5 torr and 20 mA with continuous fill gas introduction. No improvement was found in MNwith stop-flow versus flowing operation, however, considerable improvement in the S/N wasachieved when gated peak integration, in contrast to peak detectiom was employed. The temporalprofiles indicated that the Pb atoms were rapidly sputtered from the surface of the cathode and that

5

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.— L-

1000

800

20C

(

.I 1 i I r

P-10 - gas 450 Tc

10

{i

54”Torr

i)0 30 Torf

i !4

Argoni

102 Torr

~,/ 4001/:

2;: .~f” -

o 1000 2000 3000

50 Torr 200 Torr* .*~1

~ooo 1000

Voltage, V

Figure 7. Measured signal vs voltage between the electrodes for the Hg ionization cell whenfilled with argon or P-10 gas. . . . . . . . . . . . . . +.. . . . . . . . . . . . . . .b. . . .

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Time, 25 Ps/div

Oscilloscope traces showing the detection of a single photo-election event. a) typical

noise waveform, b) single photoelectron pulse, c) average of 72 pulses.

a high percentage of these atoms difised back toward the cathode. The redeposition of the Pbatoms led to peak tailing with signak lasting more than 60s (see temporal profile in Figure 11). Ineffect, atoms were sputtered, atomized and excited several times during a measurement period. Thelimit of detection for Pb was 0.6 pg based on peak detection and 0.03 pg based on peak areameasurements. These detection limits were several orders of magnitude higher than the theoretical,intrinsic detection limit due to the interfering background emission of molecuhr impurities, such asNz and HZO,present in the discharge.

Davis, et al[z also used the same miniature glow discharge as an atom reservoir for laser-excited atomic fluorescence spectrometric measurements of EU Y, and Tm. Nanoliter aqueoussamples were deposited and dried on the Ni cathode and atomized upon ignition of the GD. The

d!!7”&:L_5!!!,................

Oscilloscope

Figure 9. LEI detection via optical detection of collisionally excited buffer gas.

atom population was probed by a copper vapor laser pumped dye laser, and direct-line fluorescencewas detected. The optimum chamber pressures and operating currents were 1 Torr and 10 rnA forEu, 2 Torr and 10 rnA for Y, and 1 Torr and 40 mA for Tm. Similar to the results obtained for Pb,the fluorescence temporal profiles were found to consist of short (-20 ms duration) transient spikes,followed by long tails which lasted more than 60s as a result of sample redeposition on the cathodestiace. The limits of detection (3u) were 2 fg, 1.2 pg, and 0.08 fg for Eu, Y and Tm, respectively,with the use of signal area integration over a 6s duration. Theoretically simulated signals for aparticular sample mass, considering the geomehy of the GD atomizer, were found to fdl within oneorder of magnitude of those obtained by experiment.

7

.

l\Trigga

-c

pwfq

FigurelO. G1owdischarge atomreservoir for LEAFS.

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40

0

J&UG5Htac1XDP Cl@

l+,

I

4DyeI

Vapxbscr

[ I I I I I 1-200 0 200 40U 600 800 loco

Time (ins)

Figure 11. Temporal profile of a 0.03 ng Pb sample with continuous gas introduction.

Glow Dischame Emission/Fluorescence. Walden, et al*3designed and evaluated a glowdischarge, as an atom source for the direct sputtering of solid samples for atomic fluorescencespectrometry. Conventional, broadband light sources have advantageous features for an inexpensive,GD atornizerfluorescence spectrometry technique with capability formulti-element analysis. Figure12 shows a schematic of the experiment. In this work the xenon arc lamp and xenon flashlamp wereused as spectral continuum sources. With these lamps, improvements in signal-to-background ratio(S/B) compared with GD atomic emission spectrometry were observed. Figure 13 compares spectra

8

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obtained from the same copper cathode in the emission and fluorescence modes. Simultaneouspulsing of the GD and delaying of the flashlamp gave larger S/B ratios resulting from a reductionin emission background.

c1X13ArcLampf

XeFMllamp

Figure 12. Experimental setup for multielement GD-LEAFS.

Walden, et al.14also designed and Cu324.7Nllevaluated a microsecond pulsed glowdischarge with high pulse magnitude and

\

small duty cycle for optical emissionspectrometry. Time resolved emissionand absorption spectroscopy was appliedto study the processes of atomization,excitation and ionization in this glowdischarge. Figure 14 shows the temporal hi 4

I

Cu 327.4nm

y’

L.evolution of the atomic argon and copper I & I 1 I (m 250emission and the copper ion emission, for

W 3s0 m 4s0Wavelength(rim)

a 10ps discharge pulse duration.Experimental restits showed that, withoutoverheating the sample, the emission peakintensity was several orders greater than .aa4.7“~that obtained in the conventional dcmode.

\

[

327.4nmThe signal-to-noise improvement of themicropulsed GD over the DC-GD is

/

shown in Figure 15.

Figure 13. Comparison of GD emission and-fluorescencespectra for a copper sample.

9

..,...”. ,.-.T,7T. !,,,,..,. ,.,.‘., -,- . . . . ., 9 . . . . . . . . a,..,... . . . . . .m-mrz

20

0u~

..-

(Cu II224.7-20 L I ! I 1 I 1 1

-20 0 20 4C x. 80 100time (Ms)

Figure 14. Temporal evolution of atomic and

Magnetically Enhanced Glow Discharges forODtical Emission S~ectrome~. Raghani, et al. 15

ionic emission from the microsecond-pulsed glowdischarge.

I I 1 I I I I 1 I I

140w-. . -A-.. WithoutMagnetic Field

- A~ WithMagneticField

0

i ii 1 I I I I I0123456;; I I I9 10 11

Pressure (tOrij

Figure 16. Pressure dependence of themagnetic field enhancement in the glowdischarge.

0.6 “

0.4 “

02 -E

DC

c Iu a

Fu

A3IICuI

.; 12 II I I 1 I

I1

II I

i I1

I I i 1 1

Wavelength (rim)

Figure 15. Glow discharge emission spectra ofcopper in the pulsed and d.c. mode, both with 2 Waverage power.

designed a miniature magnetically-boostedmicrocavity hollow cathode discharge forAES whichresulted in an enhancement of the atomic emissionintensity of magnesium in an aluminum alloy up to afactor of about three. The enhancement of theemission intensity output was dependent on thepressure, as shown in Figure 16. Reduced pressure ofthe fill gas lowered the ambipolar diffisioncoefficient in the magnetic field which helped toefficiently trap the electrons inside the rnicrocavity.Collisions of the sputtered analyte with the trappedelectrons contributed to the enhancement in emissionintensity of the analyte.

..-, -.

Ragha.ni, et al16 also described and evaluated a compact magnetron glow discharge. Thecompact design, shown in Figure 17,was used as a source for simultaneous multi-elemental analysisof nanoliter samples by atomic emission spectroscopy. The sensitivity of the magnetically-enhancedglow discharge was greater than the conventional source. The limits of detection for five elements,europium, copper, silver, boron, and magnesium from the magnetically-coupled glow discharge were3 to 40 times lower than for the glow discharge source without the magnetic field when thecompromised conditions for each discharge were used. These ranged from 0.5 ng for silver to 13ng for europium. The better detection power of the boosted discharge was attributed to the formationof a localized discharge in the form of a ring which resided on the locus of the maximum magneticfield strength on the cathode surface. The plasma ring formed in the presence of the magnetic field,has a higher current density than the system without the magnetic field. As a result, there wasgreater sputtering of the cathodic material. The discharge was operable form 0.25 torr to 1 torrwhere the plasma ring was formed. A pressure above 1 torr did not result in enhancement of theemission signal when the magnetic field was applied. Discharges, with and without magnetic field,could be operated up to 150 mA of discharge current without overheating the cathode; however, thedischarge, without the magnetic field, was unstable above 150 rnA. From 150 to 250 mA, thedischarge with the magnetic field was quite stable but the cathode overheated. The magnetically-boosted discharge could not be operated above 250 mA due to the limited output of the powersupply. A higher discharge current would be advantageous because greater sputtering rates wouldbe possible which should result in greater sensitivity of the system. A more efficient cooling systemshould be designed such that the magnetically-boosted glow discharge could be operated with ahigher discharge current, at the same timekeeping the volume of the discharge chamber as small aspractically feasible.

O-Ring

Disk Magnets\ Pole piece A

Cathode

I3.0 cm\ 4

I N’

I3.6 cm

Electric FeedthroughI J,/

I F 1

I4 --- .-—>

Chilled Water

m wRing Magnets

\I i

Ceramic Disk l.2 cm

Teflon Sleeve

Figure 17. The cathode assembly for the compact planar magnetron glow discharge.

11

If optimized discharge conditions for the discharges with and without the magnetic field areused for each element with higher currents (>200 mA), then it maybe possible to attain still lowerdetection limits. The high background emission from the cathode was a limiting factor in thesestudies. Instead of viewing the discharge end-o~ as was done in the present case, it could beviewed side-on so that the background emission could be greatly reduced; it should then be possibleto further enhance the sensitivity and detection power of the boosted glow discharge.

We have also evaluated a magnetically coupled hollow cathode discharge device for itsanalytical use as a boosted atomic emission source. 17 A magnetic field from an electromagnet wasapplied perpendicular to the axis of the microcavily hollow cathode, as shown in Figure 18. Thesample was dried as a residue in the bottom of the microcavity cathode and the emission wasobserved end-on through a quartz window mounted on the left end, as viewed in Fig. 18. Theintensity of atomic emission of magnesium increased with increasing magnetic field until it reacheda maximum. Figure 19 shows the signal enhancement in a temporally resolved spectrum of copperresidue where the magnetic fieId was applied after 8s. Further increase in the field strength did notenhance the emission intensity. The attainment of the maximum was attributed to the increase inthe electron temperature and radial difision of the electrons horn the center of the rnicrocavity axis.Electron temperatures in the presence of magnetic field calculated based on the semicorona modelwere shown to be proportional to the square of the reduced field strength. Furthermore, thesemaxima were correlated to the energies of the upper levels of the transitions studied.

To Vacuum

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Magnet Pole

A ldicrocavi~ Hollow Cathode

-—..—

J (“)

——

\

!4

(+)Argon Gas

n’

MagnetPde Ceramic Sleevein

FiWre 18. Microcavity hollow cathode discharge mounted in an external magnetic field.

12

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323 324 325 326 327

Wavcknglb(nrn)

Figure 19. Temporally resolved copper spectrum (100 ng) in the microcavity hollow cathodedischarge. The magnetic field is applied after 8s.

Electrothermal Atomization-LaserExcited Atomic Fluorescence. OurETA-LEAFS projectshave continuedin several directions, both with com.mercid and specialized atom reservoi.rs. LEAFSwas used to study atomization and diffusion mechanisms in a novel diffusive graphite tubeatomizer. 18The atomizer design included a hollow graphite cylinder mounted be~een two graphiterods which served as electrodes. One of the rods had a small graphite insert with a sampling hollowand could move backwards and forwards. After the sample was introduced into the hollow, theelectrodes tightly sealed the graphite cylinder ensuring that the insert was directly in the center ofthe furnace. The furnace assembly was then heated and the vaporized sample diffhsed through thehot graphite wall. The atomic species of the sample vapor were excited by a laser beam which wasdirected along the graphite tube surface so that no gap remained between the beam and the tubesurface.

Fluorescence vs time profiles for three elements - CU,Ag and Ni were obtied witi atemperature range of 1400 K -2600 K. The rate constants of the released atoms were measured fromthe decay portions of the fluorescence signal undert.he assumption of first-order kinetics. Arrheniusplots were constructed and the activation energies, E-were evaluated from their slopes. The plotsobtained for Cu and Ag consisted of two linem parts, the corresponding values of E~were: 195kJ/mol and 77 kJ/mol for Cu (1550K~Q600 K) and 238 kJ/mol and 97 kJ/mol for Ag (1430K~Q280 K). The Arrhenius plot for Ni was linear over a temperature range of 1770 K-2530 Kresulting in an E, equal to 161 kJ/mol.

Diffbsion coefficients were evaluated on the basis of a steady-state difl?usionmodel out ofa hollow cylinder. The values for the diffusion coefficients were: 3.7s 10-3cm2/s (1750 K-2600 K)for Cu, 6.50103 cm2/s - 1.4*103 cm2/s (1750 K-2280 K) for Ag, and 5.6*10-5cm2/s - 1.5*10-3cm2/s(1770 K- 2530 K) forNi.

Laser excited atomic fluorescence spectrometry was used with the difiive graphite tubeelectrothermal atomizer for the dete~ation of silver in seawater and three NIST soil referencematerials (SRM 2709,2710,2711).19 An excimer laser-p~ped dye laser was used with excitationat 328.07 nm and fluorescence detection at 338.29 mn. The samples were contained in a smallgraphite boat which was attached to one of @vographite electrodes. The boat was inserted into thecenter of a graphite tube which was then sealed by the electrodes and heated. The vaporized sample

13

diffused through the heated graphite walls and was excited by a laser beam which passed a few mmabove the tube. The seawater matrix caused a two-fold suppression in the silver fluorescence signalcompared to a pure aqueous standard of the same concentration. The depression was constant overa concentration range of 6 orders of magnitude. When aqueous standards were used for thedetermination ofAg in the solid samples, no significant difference between the measured values andthe certified values were found. A limit of detection of 40 fg (4 pptr) was obtained for pure aqueoussolutions and 90 fg (9 pptr) in a 1:1 diluted seawater matrix. A concentration of silver of 14 rig/Lwas determined in a sample of coastal Atlantic water.

Our LEAFS measurements with a conventional electrothermal atomizer (Perkin Ehner)continue with the copper vapor laser-pumped dye laser as the excitation source. With carefi.doptimization and a good spectroscopic scheme (strong transitions with a large separation betweenthe excitation wavelength and the direct line fluorescence wavelength), limits of detection below 1fg could generally be attained. We recently used this system to detect Pb in blood with a very simplesample treatment consisting only of a 21-fold dilution.20No matrix modifier was required, whichis important if one wishes to take advantage of the exceptional detection power of the ETV-LEAFStechnique. Atomization was carried out directly, without an ashing step. The large amount of smokewhich was generated along with the atomization of the sample caused a modest loss in signal dueto scattering of the laser and the returning fluorescence; however, the background could beaccurately corrected for by simple use of a diode after the ETA to monitor the laser intensity. Thehigh repetition rate of the copper vapor laser system (9 kHz in this work) and the excellentspectroscopic scheme (excitation at 283.3 nm and fluorescence at 405.8 nrn) resulted in a limit ofdetection of 10 f~rnL (100 ag absolute). Excellent correlation was obtained for Pb in certifiedreference blood at the low ppb level. Figure 20 shows a schematic of our present ETA-LEAFSexperimental setup.

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Figure 20. Experimental design of the ETA-LEAFS system.

14

7

A very sensitive ETA-LEAFS method for determination of germanium has also beendeveloped.2* The same copper vapor laser pumped dye laser was used and the analyticalcharacteristics were carefully optimized. Two analytical non-resonant fluorescence schemes werestudied. The iniluence of some common matrix modifiers as well as the effect of possibleinterferences were evaluated. The standard addition method was recommended when the presenceof chlorine in the sample was suspected. Differences between the slopes of calibration curvesobtained in water and blood samples indicated that the components of the matrix tiected the atomicfluorescence signal. Absolute limits of detection at femtogram levels were obtained for watersamples and at picogram levels in blood samples without the need of a preconcentration step.

Three Dimensional Number Densitv Profiles of Species in a Glow Dischame. Completethree dimensional density profiles of sputtered tantalum atoms and corresponding ions have beenmeasured in a direct current glow discharge by laser induced fluorescence spectroscopy.= Atomicabsorption measurements were also performed to check the fluorescence results for the atomicspecies. The discharge was studied for a range of voltages, pressures and currents. Theexperimental data have been compared with results of mathematical simulations for the samegeomehy; in general, satisfactory agreement is reached. Experimental observations and modelingcalculations allowed insight into the complex interactions occuning in a glow discharge.

Three-dimensional density profiles of the argonmetastable atoms (A.*) were also measuredby laser induced fluorescence in a direct current glow discharge for a range of voltages, pressuresand currents.23 The profile is characterized by two distinct peaks, at 2-4 mm and at about 12 mmfrom the cathode, respectively. These peaks were explained as being caused by local Ar.”production and loss processes, giving rise to local maxima which were not completely spread outby diflhsion. The experimental data were compared with results of a mathematical model. Thetheoretical profile also showed two peaks, but at somewhat different positions, and the first peak wasmuch more intense. This suggests that the model is not yet able to describe the behati-or of themetastable atoms exactly, and that the glow discharge is hence more complex than ofien assumed.Nevertheless, comparison of the overall Ar.” number density in the rest of the discharged volumeindicated to us that a general reasonable agreement was reached between experiment and theory.

Laser Induced Breakdown Suectroscorw--Laser Excited Atomic Fluorescence. Thecombination of LIBS-LEAFS (Laser Induced Breakdown-Laser Excited Atomic FluorescenceSpectrometry) with the use of simple open air W ablation has been investigated in terms of itsanalytical possibilities for the determination of cobalt in three solid sample matrices: graphite, soiland steel.24 Figure 21 shows the general experimental setup. The fluorescence of cobalt was excitedfrom a level which was already populated in the ablation plasma and was monitored at the Stokesshifled wavelength (see Fig. 22). The optimal time delay between the ablating and exciting pulseswas 16 ps. Detection limits in the ppb to ppm range and linearity over about four orders ofmagnitude were obtained. Figure 23 shows calibrations plots for cobalt in graphite, soil and steel.Excellent correlation of the LIBWLEAFS with LA-ICP-MS and the certified value for Co (13.4ppm) in an SRM-soil showed that both the precision and the accuracy of the LIBWLEAFS methodwere satisfactory. The method has the advantage, shared by all laser ablation techniques, of needingno sample preparation.

...uzk.i.-d i. “. .- ... ..+.

Computer

MonochrornatorExcimer laser2 Excimerlaser 1

Dye laser M

Figure 21. Sirnpliiled schematic diagram of the LIBS-LEAFS experimental setup.

—

cm-l

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Figure 22. Wavelength schemes for the LEAFS determination of cobalt.

16

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-1 0 1 2 3 4 5Log [Concentration, ppm]

Figure 23. Calibration plots for the determination of cobalt in graphite, soil and steel by LIB-LEAFS.

In fi.u-therstudies, Gornushkin, et a/.25measured concentrations of lead in the range of 0:15ppm -750 ppm in metallic matrices (copper, brass, steel, and zinc) by laser excited atomicfluorescence combined withUV laser ablation in a Iowpressure argon atmosphere. No matrix effectwas observed providing a universal calibration curve for all samples with a 20’%relative standarddeviation. Figure 24 shows calibration data for the determination of Pb for all four matrices. Therelative and absolute limits of detection were 22 ppb and 0.5 fg, respectively. Also, the lifetime ofthe metastable 6pz*Dlevel of lead was measured and found to be in good agreement with literaturedata.

Fundamental Studies. Laser enhanced ionization of mercury atoms in an inert atmospherewith avalanche amplification of the signal was carried out by Clevenger, et al.2GThe method, whichwas based on the avalanche amplification of the signal resulting from the ionization from a selectedRydberg level reached, by a three-step laser excitation (see setup in Figure 25) of mercury vapor ina simpIe quartz cell, could be applied to the determination of this element in various matrices by theuse of conventional cold atomization techniques. The overall (collisional + photo) ionizationefficiency was investigated at different temperatures, and the avalanche amplification effect wasreported for Ar and P-10 gases at atmospheric pressure. It was shown that the amplified signal wasrelated to the number of charges produced in the laser-irradiated volume. Under amplifier noise-limited conditions, a detection ltilt of-15 Hg atorndlaser pulse in the interaction region wasestimated. The avalanche effect increased the ionization signal by neady lOOOX.

17

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Log [Concentration, ppm]

Figure 24. Calibration data forPb in copper, brass, steel and zinc samples using LIB-LEAFS.

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18

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Matveev, et aL27have experimentally studied the characteristics of a novel uhranarrowbandimage detector. The principle of this new imaging approach was to detect a resonance-ionizationand (or) fluorescence-imaging signal formed on a thin planar cell filled with atomic vapor. A planarvapor cell with a thickness of 1.6 mm was used for imaging 253.7 nm radiation by Hg atoms. One-and two-dimensional images were produced and detected with ionization and fluorescence-signalacquisition. The feasibility of atomic-vapor image detectors with a spectral resolution of severalmegahertz was discussed. The excititiodioti=tiotifluorescence scheme for mercury is shown inFig. 26 and the experiment setup for the observation of fluorescence is shown in Fig. 27.

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Ionization of Hg Atoms

Matveev, et aL28described an unique method involving electrospray nebulization of liquidsamples on the surface of an atomizer or vaporizer, which enable quantitative and even depositionand also matrix modification of the solid residue of the sample. Ultralow limits of detection ofatoms could be achieved in combination with laser excited fluorescence, RIS and LEI methods ofatomic and molecular analysis with ES deposition of the sample on a wire atomizer or vaporizer.

The 308 nm ablation system was also used for a study of time-resolved resonance shadowimaging of plasmas formed on Pb and Sn samples.29The laser breakdown plasma was illuminatedwith an expanded beam from a pulsed dye laser tuned to a resonance transition of the matrix element(283.3 nm for Pb and 286.3 run for Sri). The image of the strongly absorbing plasma and post-plasma plume was then formed on a fluorescent screen andrecordedby using a CCD-TV camera andvideo recorder. By varying the delay between the ignition of the plasma and the firing of the probedye laser, images of the temporal development of the plasma could be obtained with ca. 10 nsresolution. Timing jitter between the two excimer lasers limited the data acquisition to times greaterthan about 1 ps after the initiation of breakdown. Figure 28 shows atypical image of the LIB plumeformed on the surfaces of Pb and Sn pellets at 100 mbar argon pressure, at several delay times.Image b) in the figure was taken with the probe laser detuned by 0.1 nm from the resonance

19

,,

Pb

m

Pb

Sn

a)Sn

b)

Figure 28. Shadow images oflead andtin laser plasmas at different delay timesat 100 mbar argon pressure. (a) In thesix top images, the laser is tuned inresonance with the lead and tin atomictransitions. (b) In the two bottomimages, the laser is detuned from theresonance by 0.1 nm. 1, resonanceabsorption (a) or laser beam deflection(b) within the plasma; 2, resonanceabsorption due to cluster decomposition;3, the shockwave.

transition. During these studies, we alsoobserved the W photodecomposition of lead andtin dimers or large clusters, present in theablation chamber born preceding laser shots.The evolution of the plasmas was studied over arange of argon pressures (50 mbar to 1000 mbar).The shockwave produced by the laser ablationwas also observed and its speed was measured asa fimction of the argon pressure and ~e delaytime between the ablating and imaging lasers. Inthe proposed work described below, we describesimilar experiments with the temporally gatedTi:Sapphire laser which will provide improvedtemporal resolution and range as well asquantitative absorption images.

Smith, et al.30 have combined laserablation sampling with laser excited atomicfluorescence for isotonically selectivemeasurements of lithium in solid lithium oxalate.The dye laser line (10 pm FWHM) was scannedacross the 670 nm transition (see Figure 29)which had a 15 pm separation between isotopicfine components. The laser plasma was createdby an excirnerlaser operating neart.he breakdownthreshold in a low pressure (10 mTorr) argonatmosphere. The plasma was probed by a dyelaser (see Figure 30), at an optimal distance fromthe target surface (1 cm) and at optinyd delaytime (1.6 ps). The 7LVLi ratio (12.1) wasdetermined with a reasonable precision (4%RSD) and was close to the typical naturalabundance ratio of the two lithium isotopes(12.477). The results of this study show theusefulness of high spectral resolutionmeasurements inlaserbreakdown plasmas for therapid, isotonically selective determination oflithium in solid samples. Such a system could beportable and will provide analytical results forlithium concentrations above 1 ppm.

King, et aL3’used laser atomic absorption to measure rubidium isotopes in a laser inducedplasma. A Nd:YAG laser was used to produce the plasma on the surface of solid samples placedinside a low pressure chamber (see setup in Figure 31). A narrowband Ti:Sapphire laser wasscanned across the 780.02 nmtransition of the rubidium isotopes (see ener~ level diagram in Figure32). The plasma conditiom were op-d in order to provide the best sensitivity and resolution

20

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(see Figure 33~b). The resolved isotope spectrum was obtained, as well as the isotope selectivecalibration plots (see Figure 34 and 35). A limit of detection of 25 ppm for the individual isotopeswas obtained. The optimization studies and the likely mechanisms of line broadening are discussed.

36623 —

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0 cm-!

J Li2.5

Figure 29. Partial energy level diagram forlithium.

---

Figure 30. Isotope-resolvedspectrum of lithium obtainedby scanning a 10 pm-Iinewidth dye laser acrossthe 670.8 nm transition.Circles representexperimental points; solidlines represent deconvolutedLorentzian profiles; and thedashed line represents aLorentzian muki-peak fit tothe experimental points.

670.75 670.76 670.77 670.78 670.79 670.80 67;.81

Wavelegth, nm

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Figure 34. Resolved rubidiumisotope spectrum. This spectrumwas obtained at a pressure of 150mTorr and a delay time of 120 p.s.

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1.2Q1.1{

Figure 35. Isotope selectivecalibration plot of rubidium. Theslope of the ~Rb curve is 9.9 x 104

a 0.3~ Abs. units/ppm, and the slope of the‘Rb curve is 3.6 x 104 Abs.units/ppm. The ratio of the slopes is

omm~~looo 2.75

Concentrationof Rb (pPm)

It was shown that isotopes such as 85Rband 87Rbwhich are characterized by small isotopicshifts can be resolved in a laser induced plasma under optimized conditions. With an argonatmosphere of less than 10 Torr, and working at a delay time of 100 ps, the D2 transition lines of85Rband 87Rbhave been successfully resolved and measured. The isotope concentrations weredetermined in solid calcium carbonate samples to be 2.7 +0.2 with a RSD of 5% and with a limit ofdetection of 25 ppm or 20 pg per laser shot for each isotope. The analysis of a basalt rock samplewas also done. The isotope ratio was found to be 2.8+0.3, and the total rubidium concentration wasdetermined to be 77 ppm with a precision of 5 percent RSD.

In general, this technique provided a rapid, non-invasive method for isotope ratiodeterminations in solid samples. The precision of the isotope ratio measurements could be greatlyimproved by probing a single plasma simultaneously with two diode lasers tuned to the 85Rband87Rbtransitions, respectively, instead of measuring each isotope individually in sequential plasmasas was done in this case.

24

.,-,-..>:,- ,-r —-~ ... .—-. -— --

Gomushkin, etal.31have applied the curve of growth (COG) method for the first time to alaser-induced plasma. The plasma was produced by a Nd:YAG laser on a surface of steel samples(MST) containing 0.007-1.3% of Cr. The emission was collected horn the top of the plasma bymeans of a 45 ‘-angle pierced mirror and aligned onto an intensified charge-coupled device (ICCD)with the gate width set at 1 VSand with a variable delay time. The resonance 425.4 nm Cr line wasused for construction of the COG. The temperature of the plasma (-8000 K at 5 ASdelay) wasdetermined from a Boltzmann plot. The damping constant a, proportional to the ratio of theLorentzian to the Doppler line widths, was found from the best fit of a series of calculated COG tothe experimental data points and was equal to 0;20+0.05. The number density of neutral Cr atoms,corresponding to the transition between low and high optical densities, was estimated as 6.5’10’2cm-3.The cross section for broadening collisions of Cr atoms with atmospheric species (presumably,Nz) was calculated to be (66+16) ~2. The shape of the 425.4 nm Cr line was additionally checkedby scanning the ultra narrow Ti:Sph laser across the atomic transition and found to be in agreementwith preliminary estimates. The potential of the COG method for laser breakdown spectroscopy wasdiscussed.

Aucelio, et al.32have studied the interaction between a laser beam from a high repetition ratecopper vapor laser (CVL) and a metal target. The influence of several buffer gases and their pressureon the generation of the plasma was evaluated and the importance of the presence of air on theprocess of the formation of the plasma was observed. Diagnostics of the plasma were made. Severalparameters were optimized in order to maximize the emission intensity of the 327.75 nm line of thecopper target. Temporal studies were performed in order to observe the evolution of the signal tobackground ratio.

The formation of the plasma induced by a focused copper vapor laser beam on a target isstrongly dependent upon physical (absorption coefllcient, thermal conductivity, etc.) and chemicalreactivity (to oxygen) characteristics of the target, the atmosphere surrounding the target and thelaser fluence. In general, the maximum intensity of atomic lines can be observed for samples whichare reactive to oxygen, forming an absorbing oxide layer which increases the absorption of energyfrom the laser. The melted materkd evaporates, forming a cavity in the sample where atoms and ionsare formed due to the heat from the laser and from the exotherrnic reaction between metal andoxygen. Linearity of copper emission calibration curves is strongly dependent on the matrix. Theprecision obtained for 18 runs, with 2 x 105laser shot accumulations each, was 2.9%. This resultwas for signals acquired during the entire evolution of the plasma. Better precision would beexpected if the acquisitions had a delay time to coincide with the maximum signal to background(around 3.3 ps) with a 300 ns gate time.

Matveev, et al. have compared the luminosity-resolving power product of several ~es ofmodem spectrometric imaging and non-imaging systems. It WaSshown that n~owb~d signaldetected in the presence of strong, spectrally continuous background luminosity-resolving powerproduct which was the critical figure of merit controlling the potential signal-to-noise ratio. Withina spectral resolving power range of 105-109,the signal-to-noise ratio which can be attained by atomicvapor detectors and filters, and by resonance ionization detectors in particular, can be 2-3 orders ofmagnitude higher than for the best traditional spectroscopic approaches. A comparison ofluminosilyresolving power products for a number for spectrometric systems is shown in Figure 36.

25 I

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!,

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Hg RIID cooled atoms

10°

c~6n

E10“2

o

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Planar Fabry-Perot or Michelson F@re 36. Luminosity-resolvinginterferometer power plot for several

spectrometers● Diffraction grating (ideal)Conventional 4 m

grating spectrometerInfrared 1pm

‘u ~ Heterodyne detection “v ~z~um

,.-10 ~J I I I v

105 106 107 108 10’ 10’0

Resolving power

Other Studies. Glow discharge atomic emission and atomic fluorescence and microwaveplasma atomic emission spectrometric methods were reviewed and compared to the conventionalatomic approaches of electrothermal atomization-atomic absorption spectrometry, inductivelycoupled plasma-atomic emission spectrometry, and inductively coupled plasma-mass spectrometry.~Diagnostic characteristics and analytical figures of merit are given for a number of plasma types andspectrometric methods, respectively. Theoretical efficiencies of detection and measurement weregiven for the glow discharge and microwave plasma methods. Quantitative methods were discussedand future predictions of a number of atomic spectrometic methods are given in tabular form.

Hollow cathode glow discharge atomic emission spectrometry was applied to thedetermination of silicon coupled with a novel gaseous hydride generation technique, involvingdrying of an aqueous solution of silicate (sample) and mixing with powdered LiAlHq?5 Sampleintroduction into the glow discharge chamber was performed via a pinhole at the center of thecathode which was connected to the hydride generator. The detection limit for silicon was 6 yg at288.1 nm and 30 Vgat 251.6 nm.

A method to determine ukratrace amounts of platinum in biological and environmentalsamples based on electrothermal atomization laser-excited fluorescence spectrometry (ETA-LEAFS)was described by Aucelio, et al.36A high repetition rate copper vapor laser was employed as a dyelaser pump in order to probe more efficiently the platinum atoms generated in a graphite furnace.The L’vov platform, the Katskov type graphite filter and wall atomization were evaluated to obtainthe best atomization technique for complex samples. Atomization and ashing temperature studieswere performed to obtain the highest signal-to-noise ratio andor efficient separation of the analytefrom complex matrix components. An absolute limit of detection of 50 fg was achieved based onfluorescence values of aqueous standard solutions. The linear dynamic range was 1.0 to 250 ng g-l;the 250 ng g-l was limited by detector sa-tion. A precision of 4.5% at the 10 ng g-l level wasobtained for water solutiou increasing 8.O’XOfor complex sa.mpIes. Recoveries between 100 and108’%owere obtained for urine, blood, soil and used auto catalyst pellet samples.

26

~..., . _. ,.,.,.e: :’..’-,.:f:.%.,-,’.-”.,”’ -:: : ,.2:.. . . . . . —..

A dye laser pumped by a high repetition rate copper vapor laser was used by Aucelio et aL37as the excitation source to determine iridium at part-per-trillion level by electrothermal atomizationlaser-excited atomic fluorescence. Wall atomization in pyrolytic and non-pyrolytic graphite tubesas well as platform atomization were compared as the atomization reservoirs. The influence ofseveral chemical modifiers either in solution or pre-coated in the graphite tube were evaluated. Theinfluence of several acids and NaOH in the analyte solution were also studied. Optimization of theanalytical conditions was carried out to achieve the best signal-to-noise ratio and consequently anabsolute limit of detection of 1 fg. Some possible intefierents of the method were evaluated. Themethod was evaluated by determining iridium in blood, urine, soil, and urban dust samples.Recoveries between 99.17 and 109.17% are reported. A precision of 4.1% at 10 ng g-l level in waterstandards was achieved.

A microsecond-pulsed glowdischargetime-of-flight mass spectrometer was constructed andevaluated for elemental analysis by Hang, et aL38Mass spectra from the instrument show significantadvantages, including higher signal-to-noise ratios than those of a dc glow discharge source.Important temporal advantages result from the pulsed discharge and pulsed mass analyzer. Massdiscrimination among different elements is very small. The instrument currently has a resolvingpower of 360 in linear mode and 1600 in reflection mode (fill width at half maximum). Presentdetection limits are at the low ppm level, limited primarily by the detection and data acquisitionsystem. Because the detector is easily saturated, the present data acquisition system has limiteddynamic range and sensitivity. Possibilities exist to overcome this constraint.

We have also recently published a comprehensive review of methods for the tracedetermination of mercury.39

27

.- . .=-..,>.. .. ,,,-........ zma,‘......Vi,...-.,.

.- .-’. --

Bibliography (Publications supported by DOE Grant)

1. G.A. Petrucci, D. Imbroisi, B.W. Smith, and J.D. Winefordner, “Detection of OH in anAtmospheric Pressure Flame via Laser Enhanced Ionization of Iridium,” Spectrochim. Acts,49B, 1569-1578(1994).

2. G.A. Petrucci, D. Imbroisi, RD. Guenard,B.W. Smith, andJ.D. Winefordner, “High-Spatial-Resolution OH Rotational Temperature Measurements in an Atmospheric-Pressure FlameUsing an Iridium-Based Resonance Ionization Detector,” Appl. Spectrosc., 49, 655-659(1995).

3. 0.1. Matveev, B.W. Smith, N. Omenetto, and J.D. Winefordner, “Single Photo-Electron andPhoton Detection in a Mercury Resonance Ionization Detector,” Spectrochim. Acts, 51B,563-567 (1996).

4, 0.1. Matveev, W.L. Clevenger, B.W. Smith, N. Omenetto, and J.D. Winefordner,“Resonance Ionization Detection of 253.7 nm Photons from Mercury Atoms,” ProceedingsoflUS 1996,353-356 (1997).

5. W.L. Clevenger, L.S. Mordoh, 0.1. Matveev, N. Omenetto, B.W. Smith, and J.D.Winefordner, “Analytical Time-Resolved Laser Enhanced Ionization Spectroscopy:Collisional Ionization and Photoionization of the Hg Rydberg States in aLowPressure Gas,”Spectrochim. Acts, 52,295-304 (1997).

6. W.L. Clevenger, L.S. Mordoh, 0.1. Matveev, N. Omenetto, B.W. Smith, &d J.D.Winefordner, “Temporal Behavior of Mercury LEI Signal in a Buffer Gas,” Proceedings ofRZS 1996,315-318 (1997).

7. W.L. Clevenger, 0.1. Ma~eev, N. Omenetto, B.W. Smith, and J.D. Winefordner, “LaserEnhanced Ionization Spectroscopy of Mercury Rydberg States,” Spectrochim. Acts, 52B,1139-1149 (1997).

8. 0.1. Matveev, L.S. Mordob W.L. Clevenger, B.W. SmitlZ and J.D. Winefordner, “OpticalEmission Detection of Charged Particles After Selective Laser Ionization of Mercury in aBuffer Gas,” Appl. Spectrosc., 57,798-803 (1997).

9. 0.1. Matveev, L.S. Mordob W.L. Clevenger, B.W. SmitlL and J.D. Winefordner, “PlasmaEmission in a Pulsed Electric Field After Resonance Ionization of Atoms,” AmericanInstitute of Physics, Proceedings of RIS 1996,171-174 (1997).

10. 0.1. Matveev, I.B. Gomushkin, W.L. Clevenger, B.W. Smith, and J.D. Winefordner,Electrospray Ionization Source for Highly Sensitive Resonance and Laser-EnhancedIonization Analysis,” American Institute of Physics, Proceedings of RI.. 1996,.435-438(1997).

28

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11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

210

C.L. Davis, B.W. Smith, and J.D. Winefordner, “A Miniature Glow Discharge for LaserExcited Atomic Fluorescence Detection of Lead,” A4icrochem.J, 52,383-395 (1995).

C.L. Davis, B.W. Smith, M.A. Bolshov, and J.D. Winefordner, “Laser-Excited AtomicFluorescence of Eu, Y, and Tm in a Miniature Glow Discharge Atom Reservoir,” Appl.Spectrosc., 49,907-916 (1995).

W.O. Walden, W.W. Harrison, B.W. Smith, and J.D. Winefordner, “Multi-Element GlowDischarge for Atomic Fluorescence Using Continuum Sources,” J Anal Atom. Spectrom.,9,1039-1043 (1994).

W.O. Walden, W. Hang, B.W. Smit.QJ.D. Winefordner, and W.W. Harrison, “Microsecond-Pulse Glow Discharge Atomic Emission,” Fres. J Anal. Chem., 355,442-446 (1996).

A.R. Raghani, B.W. Smith, and J.D. Winefordner, “Spectroscopic Evaluation of a MiniatureMicrocavity Cylindrical Magnetron Source for Atomic Emission Spectroscopy,” AppLSpectrosc., 50,417-420 (1996).

A.R. Raghani, B.W. Smi@ and J.D. Winefordner, “A Miniature Planar Magnetron GlowDischarge Source for Analysis of Submicroliter Volume Aqueous Samples Using AtomicEmission Spectroscopy,” Spectrochim. Acts, 51B, 399-409 (1996).

A.R. Raghani, M.A. Bolshov, B.W. Smith, and J.D. Winefordner, “Evaluation of aMagnetically Coupled Microcavi~ Hollow Cathode Discharge for Atomic EmissionSpectroscopy,” Talanta, 42,1817-1825 (1995).

LB. Gornushkin, B.W. Smith, and J.D. Winefordner, “A Kinetic Study of Difision in theElectrothermal Atomizer with a Graphite Filter by Laser Excited Atomic Fluorescence,”Spectrochim. Acts, 51B, 1679-1693 (1996).

I.B. GornushkirL B.W. Smith, and J.D. Winefordner, “Use of Laser Excited AtomicFluorescence with a Novel Diffbsive Graphite Tube Electrothermal Atomizer for the DirectDetermination of Silver in Sea Water and in Solid Reference Materials,” Spectrochim. Acts,51B, 1355-1370 (1996).

E. Wagner, B.W. Smith, and J.D. Winefordner, “Ultratrace Determination of Lead in WholeBlood Using ElectrothennalAtomintionLaserExcitedAtom.icFluorescence Spectrometry,”Anal. Chem., 68,3199-3203 (1996).

R.Q. Aucelio, V.N. Rubin, E. Bece~ B.W. Smith, and J.D. Winefordner, “ElectrothermalAtomization Laser-Excited Atomic Fluorescence Spectrometry for Direct h“alysis ofGermanium in Water and Blood Samples,” Anal. Chim. Acts, 350,231-239 (1997).

29

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A. Bogaerts, E. Wagner, B.W. Smith, J.D. Winefordner, D. Pollrnan, W.W. Harrison, andR. Gijbels, “Three Dimensional Density Profiles of Sputtered Tantalum Atoms and Ions ina Direct Current Glow Discharge: Experimental Study and Comparison with Calculations,”Spectrochim. Acts, 52B, 205-218 (1997).

A. Bogaerts, R.D. Guenard, B.W. Smith, J.D. Winefordner, W.W. Harrison, and R. Gijbels,“Three-Dimensional Density Profiles of the Argon Metastable Atoms in a Direct CurrentGlow Discharge: Experimental Study and Comparison with Calculations,” Spectrochim.Acts, 52b, 219-230 (1997).

I.B. Gornushkin, J.E. Kim, B.W. Smith, S.A. Baker, and J.D. Winefordner, “Determinationof Cobalt in Soil, Steel, and Graphite Using Excited State Laser Fluorescence Induced in aLaser Spark,” Appl. Spectrosc., 51,1055-1059 (1997).

I.B. Gornushkin, B.W. Smith, and J.D. Winefordner, “Determination of Lead in MetallicReference Materials by Laser Ablation with Laser Excited Atomic Fluorescence,”Spectrochim. Acts, 52B, 1653-1662 (1997).

W.L. Clevenger, 0.1. Matveev, S. Cabredo, N. Omenetto, B.W. Smith, and J.D.Winefordner, “Laser-Enhanced Ionization of Mercury Atoms in an Inert Atmosphere withAvalanche Amplification of the Signal,” Anal. Chem., 69,2232-2237 (1997).

0.1. Matveev, B.W. Smiti and J.D. Winefordner, “Narrowband Resonance-Ionization andFluorescence Imaging in a Mercury Vapor Cell,” Optics Lett., 23,304-306 (1997).

0.1. Matveev, I.B. Gomushkin, W.L. Clevenger, B.W. Smith, and J.D. Winefordner,“Electrospray Ionization Source for Highly Sensitive Resonance and Laser-EnhancedIonization Analysis,” Proceedings ofRZS, 1996,435-438 (1997).

I.B. Gomushkin, M. Cl- B.W. Smit4 J.D. Winefordner, U. Panne, and R. Neissner, “TimeResolved Resonance Shadow Imaging of Laser Produced Lead in Tin Plasmas,”Spectrochim. Acts, 52B, 1617-1625 (1997).

B.W. Smith, I.B. Gomushkin, L.A. King, and J.D. Winefordner, “A Laser Ablation-AtomicFluorescence Technique for Isotonically Selective Determination of Lithium in Solids,”Spectrochim. Acts, 53,1131-1138 (1998).

I.B. Gomusbkin, J.M. Anzano, L.A. King, B.W. Smith, N. Omenetto, and J.D. Winefordner,“The Curve of Growth Applied to Laser-Induced Plasma Emission Spectroscopy,”Spectrochim. Acts, in press.

R.C).Aucelio, B.C. Castle, B.W. Smith, and J.D. Winefordner, “Study of the Characteristicsan~Factors ~uencing the Emission from a Copper Vapor Laser Induced Plasm~” Appl.Spectrosc., submitted.

30

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0.1. Matveev, B.W. Smith, N. Omenetto, and J.D. Winefordner, “A Comparison of theLuminosity-Resolving Power Produced for Several Imaging andNon-Imaging SpectrometricSystems,” Appl. Spectrosc., submitted.

J.D. Winefordner, E.P. Wagner, and B.W. Smith, “The Status and Perspectives onMicrowave and Glow Discharges for Spectrochemical Analysis,” J Anal. Atom. Spectrosc.,11,689-702 (1996).

K. Fujiwar~ E.P. Wagner, B.W. Smith, and J.D. Winefordner, “Determination of Silicate byHollow Cathode Glow Discharge-Atomic Emission Spectrometry with Hydride GenerationTechnique,” Anal. Lett., 29,1985-1992 (1996).

R.Q. Aucelio, V.N. Rubin, B.W. Smith, and J.D. Winefordner, “Ultratrace Determination ofPlatinum in Environmental and Biological Samples by Electrothermal Atomization Laser-Excited Atomic Fluorescence Using a Copper Vapor Laser Pumped Dye,” J Anal. Atom.Spectrosc., 13,49-54 (1998).

R.Q. Aucelio, B.W. Smith, and J.D. Winefordner, “Electrothermal Atomization Laser-Excited Atomic Fluorescence Spectroscopy for the Determination of Iridium,” Appl.Spectrosc., submitted.

W. Hang, C. Baker, B.W. Smith, J.D. Winefordner, and W.W. Harrison, “Microsecond-Puked GlowDkchargeTirne-of-Fl@t Mass Spectrometry: Analytical Advantages,” J AnalAtom. Spectrosc., 12,143-149 (1997).

W.L. Clevenger, B.W. Smith, and J.D. Winefordner, “Trace Determination of Mercury: AReview,” Cr~ical Reviews in Analytical Chemistry, 27,1-26 (1997).

31

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Personnel Involved in the Above Research:Dr. J.D. Winefordner, PIDr. B.W. Smith, Research ScientistDr. Kobus Visser, Postdoctoral AssociateDr. Mikhail Bolshov, Postdoctoral AssociateDr. Annemie Bogaefis, Postdoctoral AssociateDr. Oleg Matveev, Visiting Research ScientistDr. Kitao Fujiw~ Postdoctoral AssociateDr. Alexie Podshivalov, Postdoctoral AssociateDr. Susanna Cabredo, Postdoctoral AssociateDr. Giuseppe Petrucci, Postdoctoral AssociateDr. E&on Becerra, Postdoctoral AssociateDr. Nicolo Omenetto, Visiting Distinguished ProfessorDr. Igor Gornushkin, Graduate Research Assistant/Postdoctoral AssociateMr. Eugene Wagner, Graduate Research AssistantMr. Rob Guenard, Graduate Research AssistantMs. Denise Imbroisi, Graduate Research AssistantMr. Anil Ragani, Graduate Research AssistantMs. Cheryl Davis, Graduate Research AssistantMr. Bryan Castle, Graduate Research AssistantMr. Ricardo Aucelio, Graduate Research AssistantMr. Scott Baker, Graduate Research AssistantMs. Wendy Clevenger, Graduate Research AssistantMs. Leslie King, Graduate Research AssistantMr. Jason Kim, Graduate Research AssistantMs. Leah Mordoh, Undergraduate Research AssistantMs. Valeria Rubin, Undergraduate Research AssistantMr. Dimitri Pappas, Undergraduate Research Assistant/Graduate Research AssistantMs. Celeste Johnson, Undergraduate Research Assistant

32

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