Laser Raman Molecular Microprobe (MOLE)

P. Dhamelincourt and F. Wallart Laboratoire de Spectrochemie Infrarouge et Raman, C.N.R.S., Université de Lille, B.P. 36, 59650 Villeneuve d'Ascq, France

Most microanalysis physical meth­ods are based on atomic properties and only indirectly provide informa­tion on how atoms link together to form polyatomic structures. This is the case for electron or ion microprobe methods, which are based on the anal­ysis of electron X-rays or secondary ions emitted by samples under elec­tron or ion beam impact. These meth­ods often fail to yield information on chemical bonds and conformation of molecules.

In the laser molecular microprobe (1, 2), photons generated by a laser are used to excite the sample and cause the emission of Raman lines of differ­ent components. By use of these Raman lines, each component can be detected, identified, and then located by forming a micrographie image that gives the " m a p " of its distribution in the sample. This is the principle of the nondestructive microanalytical meth­od described here.

Instrument Description The basic instrument (3-6) com­

bines a conventional optical micro-

M. Leclercq and A. T. N'Guyen I.S.A. Nord 35 rue Inkermann 59000 Lille, France

scope (with bright and dark field illu­mination), an optical filter possessing a very low stray light level, and a mul­tichannel and/or monochannel detec­tion system. This configuration gives a very versatile instrument because it permits different modes of opera­tion without disturbing the sample. A block diagram of the instrument is shown in Figure 1.

A laser (argon, krypton, dye laser, etc.) is the monochromatic source of light used for irradiating the sample.

A conventional optical microscope provides an illuminatory stage and forms part of the imaging system. The sample under study is placed on the microscope stage. All modes of obser­vation available in a standard optical microscope are available in the instru­ment; therefore, the samples can be studied in air, liquid, or t ransparent media.

A double monochromator spectrom­eter with two concave aberration cor­rected gratings is the central part of the instrument. This device permits spectral separation of the chosen spec­tral line, which corresponds to a par­ticular vibrational mode, and it trans-

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mits good images in a wide spectral range without changing components.

The detection system is composed of two separate assemblies. One, which has a photomultiplier, records the spectrum on a chart recorder. The other has a three-stage image intensi­fier, a TV-type detector, and a visual­ization system that displays micro-graphic images. A computer and disc or tape memory can be attached to the system for information processing and storage. Figure 2 gives the optical scheme of the instrument.

Modes of Operation The two modes of operation cur­

rently used depend on the type of irra­diation of the sample.

Punctual Illumination with Mo­nochannel or Multichannel Detec­tion. By use of the bright field illumi­nation system of the microscope, the same objective (50X, ΝΑ, 0.85-100Χ, N/A, 0.90) focuses the laser beam into a spot on the component of the sample to be identified and collects the scat­tered light. In this case, the top aper­ture of the objective, the grating sur­faces, the sample, and entrance slit are simultaneously optically aligned by the transfer optic. To obtain the Raman spectrum, the optical filter can function as a Raman spectrometer when the detector is a photomultiplier followed by an amplifier and a chart recorder, or as a Raman spectrograph when the detector is an intensifier phototube, followed by a low-level TV camera (SIT or SEC tube).

Global Illumination and Imaging System. Selecting, in the Raman spec­trum, radiation characterizing one particular component forms a micro-

This new microanalysis technique for detecting, identi­fying, and obtaining the distribution of the different com­ponents of an heterogeneous sample is similar to electron or ion microprobe methods. However, an important advan­tage is the ability to study samples in air, under a con­trolled atmosphere, or even inside transparent media rath­er than just in a vacuum

414 A · ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 0003-2700/79/0351-414A$01.00/0 © 1979 American Chemical Society

Instrumentation

graphic image that gives the distribu­tion of this component. The aperture of the microscope objective (O) (Fig­ure 2) is optically aligned with the three slits (Oi, 02 , 03) of the optical filter. The image (Si) of the sample (S) given by the objective is formed on the gratings (S2, S3) and goes through the aperture of the exit slit (O3) to the photocathode of the inten­sifier tube (S4). A rotating laser beam feeding the objective annular illumi­nator (dark field illuminator) homoge­neously illuminates the sample and suppresses the "speckle noise" of the images. The spatial resolution of the images is about 1 μηι.

The Spectral Mode. In the mono-channel spectral mode classical Raman spectra can be recorded of the entire sample or of a specific portion selected to identify positively the na­ture of that portion.

In the multichannel spectral mode the Raman spectra can be observed in spectrographic form on an oscillo­scope in real time in up to 100-cm-1

segments. This mode is particularly useful for monitoring the evolution of a sample.

The Imaging Mode. To locate one of the components of the sample, an image of the surface obtained from a characteristic line of this component can be directly observed on the TV monitor screen. This allows "map­ping" the distribution of a given sub­stance in a heterogeneous sample.

Some Applications of the Laser Raman Microprobe

To show the many possibilities opened by the laser Raman micro-probe, we now present some examples of applications of the instrument.

Geochemistry and Geophysics. The first example is a fragment of a natural stone of celestine (collection of the Faculty of Science of Lille, France). The stone has been cut and roughly polished to make it approxi­mately plane. By examination under an optical microscope (dark field illu­mination by reflected white light), an area comprising crystal inclusion is se­lected.

In the monochannel spectral mode, using punctual illumination (spot di­ameter: 2 μπι), we obtain a character­istic Raman spectrum for each of the. two areas of interest (Figures 3 and 4).

In the multichannel spectral mode, using global illumination, we isolate the 473-cm-1 wavelength characteris­tic of sulfur, and the image is observe/1 at this wavelength in the imaging mode (Figure 5). Similarly, the 1000-cm_1 wavelength characteristic of SrSC>4 is isolated, and the image is ob­served at this wavelength (Figure 6).

Multichannel /

^ O

\ Micrographie

Image

ΛΑΛ Image

Intensifier and

TV Camera

Image Intensifier

and TV Camera

\

.•s"

System

Monochannel Detection (Photomultiplier)

Input Optical System

Microscope \

Double Spectrometer with Concave Holographic Gratings

Mode Commutation Control

Monochannel Detection Mode Control Module

Display Module

Spectrometer Control

Multichannel Detection Mode Control Module

To Computer

Laser

Figure 1. Block diagram of laser Raman microprobe

Double Concave Holographic Grating Filter

o2

,* Semitransparent Laser l\ . p ! a t e Beam * 7 r i r * / ^ c -

t l i \ y Laser jTJ' 1 1 I 11· Roam 7N ' '

S4 - .

Image Intensifier Phototube

SIT or SEC Camera

Photomultiplier

Amplitude

- Sample -Punctual Global

Illumination Illumination

TV Monitor

θ Oscilloscope

_ / U _ ^ _

Illumination

Figure 2. Optical scheme of instrument

Multichannel v Detector

Chart Recorder Monochannel

Detector

Detection

-4a, >ΙΓ]51 159 219 473

28

215

1 «

84

63

I

1 VI 187

249 440

100 200 300 400 Afcrrv 1

Figure 3. Raman spectrum of Celestine crystal inclusion: sulfur

ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 · 415 A

450 1000

100 -f f 7k / lh r4i 450 600 700 1000 1100 AScrrr1

' Figure 4. Raman spectrum of area around same inclusion: Strontium sulfate

When the image at the characteris­tic line of SrSC>4 is observed, the sul­fur is no longer obvious but its trace is still visible. The sulfur crystal in the plane of the surface is thin and there­fore transparent to the radiation emit­ted by SrS0 4 . Further impurities at the surface appear as dark spots on the micrographie image. Fluid inclu­sions can also be directly analyzed (7) provided that the matrix is sufficient­ly transparent to the laser beam and scattered light.

Archeology and Gemnology. Knowing the exact nature of a stone object without taking a sample (even a microscopic one) would be greatly beneficial in any museum of art and archeology. With the laser Raman mi­croprobe, it is possible to identify di­rectly the nature of the stone.

Examining the precise location of a small area of the stone can prevent contact with fluorescent impurities, which are often present in natural ma­terials and which, in most cases, pre­vent the recording of Raman spectra with a classical Raman spectrometer.

Figure 7 shows the comparison be­tween Raman spectra obtained from

50 μΠΊ Figure 5. Raman image of sulfur inclusion obtained with 4 7 3 - c m _ 1 line

50 μϊΥ\ Figure 6. Raman image of same area obtained with 1000 -cm _ 1 characteristic line of sulfate

416 A · ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979

a precious Chinese vase and from a sample of serpentine (8).

Another important application is the determination of authenticity and geographical origins of gems by identi­fying in situ the solid inclusions present in precious stones.

Investigation of Defects in In­dustrial Materials. The formation of bubble-like inclusions is a problem currently encountered in the commer­cial production of glass. The in situ analysis of bubble content aids in properly selecting materials and pro­cess parameters which will inhibit their formation.

Figure 8 represents one example of the investigation of solid deposits on the wall of a bubble inside a sample of industrial glass (in collaboration with the Inorganic Chemistry Labora­tory E.N.S.C. Strasbourg, France). Spectra a, b, and c represent sulfur, polysulfide, and sulfate of sodium, re­spectively. Each deposit can be im­aged by use of a characteristic Raman line.

Pollution Analysis. This new non­destructive analytical tool is also per­fectly suited to the study and identifi­cation of urban and industrial dusts. Figure 9 shows the results obtained from dust collected on a highway. The two broad maxima near 1400 and 1600 cm - 1 (Figure 9c) are often observed

Figure 7. Raman spectra obtained from a) precious Chinese vase, b) specimen of serpentine

on the spectra of air particulate dust. They can be attributed to degraded organic compounds or soot coating (9).

Industrial Quality Control. In the industrial field, it is very important to control the quality of manufactured products. Local defects in synthetic films or fibers impair their clarity and can cause rending. The compounds re­sponsible for these defects can be identified with MOLE (molecular op­tics laser examiner) by simply record­ing in situ the Raman spectra of the

defects. Generally, the defects are caused by atmospheric dusts, local dif­ferences of crystallinity, degraded areas, or local concentration of copoly­mers.

These studies are useful for quality control of semiconductors and inte­grated circuits. Figure 10 shows the spectra of different parts of an inte­grated circuit and the pollutants dis­covered on its surface. The major pol­lutant found was lead acetate mixed with organic compounds.

Figure 8. Identification of solid deposits contained in a bubble inside a commercial glass Different areas appear on wall of bubble when it is observed through a mi­croscope. By use of punctual illumination a characteristic Raman spectrum can be recorded from each of these areas. Raman spectrum characteristic of a) sulfur, b) sodium polysulfide, c) sodium sulfate

ANALYTICAL CHEMISTRY, VOL. 5 1 , NO. 3, MARCH 1979 · 417 A

Figure 9. Raman spectra of some particles (2 -10 μ in size) removed from surface of a collection filter placed near a highway Raman spectrum of a) quartz particle ((V-Si02), b) calcite particle (CaC03), c) Dolomite particle [CaMg(C03)2], d) Feldspar particle (Orthoclase)

Figure 10. Analysis of pollutants found at surface of a com­mercial integrated circuit

420 A · ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979

Figure 11. Micrographie image (X300) of part of an histologi­cal section of Blatella germanica L

Figure 12. Raman spectra obtained from a) commercial sample of uric acid (parti­cle ~ 5 μ in size), b) fat body spherocrystal (~5 μ in diameter), c) commercial sam­ple of potassium urate (particle ~ 5 μ in size)

Biology. Important information unattainable by electron or ion micro-probe can be obtained on biological samples. For example, Figure 11 shows spherocrystals (10) (1-12 μ) of fat body of Blatella germanica L. (In-secta Dictyoptera). Fat body can be roughly considered as the liver of in­vertebrates, and puric catabolism is

important. The samples are histologi­cal sections of fixed and paraffin-em­bedded material. Traditional methods of study have given the following re­sults (10, 11): biochemistry, uric acid; X-ray, amorphous structure; and Castaing microprobe, abundant Κ and small quantities of Ca and P. Figure 12 represents the spectra obtained

from reference materials (uric acid, potassium urate) and fat body sphero­crystals. A comparison between these spectra shows that fat body is mainly composed of uric acid with a small amount of potassium urate.

Conclusions The purpose of the results present­

ed here is simply to show the wide­spread interest in this new microana-lytical technique.

Several other fields will be investi­gated, and studies are already in prog­ress (i.e., corrosion, and photochemi­cal and electrochemical reactions). We expect the laser Raman microprobe to find application in laboratories using such microprobe techniques as Auger (AES), low-energy electron dif­fraction (LEED), X-ray photoelectron spectroscopy (ESCA), secondary ion mass spectrometry (SIMS), and elec­tron microscopy. The high-energy in­tensity required in some of these tech­niques and sample treatment prevent their use on delicate samples. The Raman microprobe probably provides the only approach to surface analysis on living materials. In addition, the instrument performs nondestructive analysis in various media (air, con­trolled atmosphere or liquid) for small quantities of sample down to a few pi-cograms.

The versatility of this new instru­ment should greatly facilitate micro­analysis and structural determination, and complement other techniques.

References ( 1 ) M. Delhaye and P. Dhamelincourt, J.

Raman Spectrosc, 3, 33 (1975). (2) G. J. Rosasco, E. S. Etz, and W. A. Cas

satt, Appl. Spectrosc, 29, 396 (1975); in Proc. of the Fifth Int. Conf. on Raman Spectroscopy, E. 0 . Schmid et al., Freiburg, Germany, Sept. 2-8, 1976.

(3) M. Delhaye, P. Dhamelincourt, and Y. Moschetto, U.S. Patent 4,030,827 (1977).

(4) M. Delhaye, P. Dhamelincourt, and E. Da Silva, French Patent ANVAR 762 1539(1976).

(5) P. Dhamelincourt and P. Bisson, Mi-crosc. Acta, 79, 267 (1977).

(6) P. Dhamelincourt, "Lasers in Chemis­try," Elsevier, Amsterdam, The Nether­lands, 1977.

(7) G. J. Rosasco, E. Roedder, and J. H. Simmons, Science, 190, 557 (1975).

(8) P. Dhamelincourt and H. J. Schubnel, Rev. Gemnol., 52, 11 (1977).

(9) E. S. Etz and G. J. Rosasco, "Environ­mental Analysis," Academic Press, New York, N.Y., 1977.

(10) C. Ballan-Du Français, doctoral the­sis, Paris, France, 1975.

(11) C. Ballan-Du Français, Cellule, 70, 317(1974).

This study financially supported by C.N.R.S., University of Lille, the Direction Générale de la Recherche Scientifique, and the Agence Nation­ale pour la valorisation de la Recherche of France.

From left to right: P. Dhamelincourt has played a major role in the conception and realization of the molecular microprobe. He specializes in micro Raman spectroscopy. M. Leclercq was in charge of the Mole Laboratory at Lirinord Di­vision Instrument S.A. He is now technical manager at D.I.L.O.R. (rue des Bois Blancs à Lille, France). A. T. N'Guyen worked in the Mole Laboratory at Liri­nord Division Instrument S.A. He is now in charge of the Application Labora­tory at D.I.L.O.R. F. Wallart is assistant professor at the University of Lille. He specializes in Raman spectroscopy and instrumentation. D. O. London (not shown) is manager of research and development at Instruments S.A., Metuch-en, N.J. His interests include optical spectroscopy and instrument design.

ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 · 421 A