micro methods in infrared and raman spectroscopy

Fresenius J Anal Chem (1990) 337:824- 829 Fresenius' Journal of

© Springer-Verlag 1990

Micro methods in infrared and Raman spectroscopy Bernhard Schrader

Institut fiir Physikalische und Theoretische Chemie, Universit/it Essen, D-4300 Essen 1, Federal Republic of Germany

Summary. Employing the concept of the optical conductance [1 - 3 ] and taking into account the spectroscopic properties of the sample, firstly the features of sample arrangements are given, which are using minimal amounts of sample determined by the condition o f complete illumination of the different types of spectrometers and using different techniques for infrared, near-infrared and Raman micro spectroscopy. Secondly, arrangements of a micro sample are discussed, the minimal size of which is determined by the laws of diffraction. In this case the spectrometers are usually not fully illuminated. The minimal amount of sample necessary is for infrared microscopy in the order of 1000 gm 3 (1 pg), for Raman microscopy of the order 100 gm 3 (0.1 pg). Examples demonstrate the state of the art and finally developments, which can be anticipated, are outlined.

1 Introduction

Optical micro methods have an enormous potential of supplying analytical information: For every volume element complete spectra may be recorded. Since these techniques are non-destructive repeated analyses may be made even from precious samples.

Micro methods using radiation in the visible and UV- range have been known for some time. Micro fluorescence spectrometry is an invaluable tool for trace analyses.

Micro techniques applying the methods of vibrational spectroscopy are far less well accepted, although they may supply considerably more detailed information concerning the composition of a volume element of a sample and further the identity and the structure of its components. This is due to the fact that these techniques are quite new and usually need expensive instruments. The success of spectroscopic investigations of micro samples is still often dependent on personal experience. Only sporadic information about the optimization of micro methods is given by the literature and the manuals of the instrument producers. In order to fully exploit the capabilities o f the modern spectrometers the features determining the limits of detection and the best method of sample preparation must be taken into account. The minimal amount of sample which is needed to fully illuminate the spectrometer can be derived by means of the optical conductance of the spectrometers. The absolute mini- mum amount of sample necessary is given by the laws of diffraction and the properties of the detection system. This is

outlined in the following paragraphs, some examples are given and developments to be anticipated are discussed.

2 The optical conductance

The radiant flux ~b (power) transported by any optical system from the source to the detector may be described by the following equation [1 - 3]:

~b = LGz . (1)

Here L is the radiance (power/solid angle - area) of the light source, G the optical conductance (solid angle • area) and

the transmission factor of the system. For the flux through a spectrometer the equation (2) may be used with the spectral radiance L~ (radiance per wavenumber) and the spectral optical conductance G~ (optical conductance per wavenumber) and with A ~, the bandwidth of the instrument (in wavenumber units):

q5 = L~ G~< (A ~)2 ~. (2)

For a correctly designed optical instrument the optical conductance between any two apertures in sequence is invariant (Fig. 1). Therefore also the radiance observed for any cross-section of the beam of an optical instrument is invariant, except for losses by reflection, absorption or scattering (taken into account by the transmission factor). The optical conductance of some part of the arrangement which cannot be enlarged for theoretical or technical reasons determines the effective optical conductance for the whole instrument.

lI" Gij = c o n s t

Fig. 1. For a correctly designed optical instrument the optical conductance between any two apertures in sequence is constant, presupposed that all apertures are fully illuminated

Let dF1 be a surface element of the source with an area F1, dF2 a surface element of the sink with an area F2, cq and ~2 the angles between the normals of the surface elements and the direction between them with the intervening distance a12 (Fig. 2). The optical conductance is defined by the integral:

G = n 2 ~ ~ (cos cq cos c@a22) dVldF2. (3) F1 Fz

An area F radiating into a cone with the half angle O has an optical conductance of:

G = n 2 F 2~(1 - c o s O) = n z F 47r sin 2 (0/2). (4)

The approximation G -~ F 7z N A z with N A the numerical aperture (NA = n sin c~) is only valid for small angles c~. Another approximation is used for a system with the areas F1 and F 2 at a distance a12 when F1, F2 << a22:

G ~ n 2 F1 Fa/a~2. (5)

3 The optical conductance of different types of spectrometers

For the following discussion equation (5) is regarded to be a sufficient approximation. We take F2 to be the area of the collimator mirror (approximately equal to that of the grating of a spectrometer) or the mirrors o f an interferometer. F2 is the area of the slit of the grating instrument or, respectively, the 'Jacquinot stop' for an interferometer (Fig. 3). For simplicity we assume n = I in the following equations, the optical conductance is then equal to the geometrical conductance.

Any aperture stop in an optical system produces a diffraction pattern. Light of the wavelength 2 in a collimator of width B and a focal length f produces a pattern, the distance between the central maximum and the first mini- mum of which is given by [6]:

So = 2 f/B. (6)

This is defined as 'optimal slit width z of a grating spectrometer [6]. Its theoretical resolving power Ro = 2/A 2o is determined by:

Ro = m Nr (7)

with m the order of the spectrum and Nr the number of grooves on the grating. When a grating spectrometer is operating in first order B is in rough approximation given by Ro " 2, the number of grooves times the wavelength (which is usually of the order of the grating constant) [3]. Therefore:

so ~ f/Ro (S)

with f the focal length of the collimator mirror and Ro the resolving power. The practical resolving power is usually smaller, determined by a larger slit width:

s ~ f /R (9)

Thus the area of the slit, F2 ~, given by the slit width s times the slit height h is:

F2 c _-__ fh /R. (10)

The slit height h is usually 1/50... 1/10 of the focal length f. The optical conductance of a grating spectrometer operating with a practical resolving power R (determined b y the slit width s > So) is, using equation (5) with al2 = f:

G G = Flh/Rf . (11)

"12 N /

Fig. 2. Parameters for the calculation of the optical conductance

825

[3

A

Fig. 3A, B. The elements determining the optical conductance of an interferometer A and a grating spectrometer B

For the interferometer the radius of the Jacquinot stop is dependent on the necessary practical resolving power R by [3- 5]:

r = f / l /R (•2)

R cannot be larger than the maximal resolving power Ro, determined by the path difference (given by twice the me- chanical amplitude of the moving mirror A 1) divided by the wavelength (or multiplied with the wavenumber):

Ro = 2 All2 = 2 AI~. (13)

The area F2 is given, using (12) by:

F~ = r 2 ~ = f2 zc/R. (14)

The optical conductance of the interferometer is then:

G I = F1 zt/R. (15)

Supposed both spectrometers are operating with the same resolving power, the same area F1 and the same focal length, then the ratio of the optical conductance for both instruments is:

GO/G I ~ i = Fz /Fa = h/f~. (16)

Since the ratio h/f is, as mentioned above, about 1/10... 1/50, the optical conductance of the interferometer is larger by

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Table 1. Volume of samples for infrared absorption spectroscopy using normal sample conditions, 5 : 1 beam condensors under different resolution and a microscope for the middle and near infrared region, radius of the necessary sample, relative flux and approximate mass (relative density = 1)

Thickness R = 1000 R = 16000 Microscope b/gm

1:1 5:1 1:1 5:1 500 cm -1 4000 cm -1 10000 cm -~

5 0.35 i.tl 0.01 gl 0.02 gl 0.001 gl 4.5 " ] 0 - 9 ~tl 7 6 • 1 0 - 1 2 g l 11 • 10 -12 gl 5000 353 gl 14 gl 22 ~tl 0.9 gl 4.5 " 1 0 - 6 g l 7 6 " 1 0 - 9 ~tl 11 " 1 0 - 9 gl 0 4.74 mm 0.95 mm 1.18 mm 0.24 mm 17 gm 2.2 gm 0.86 ~tm ~rel 1 t 0.06 0.06 1.3 " 10 9 21 " 10 12 3.3 " l0 12 m (5 gm) 0.35 mg 10 ~g 20 gg 1 I.tg 4.5 pg 76 fg 11 fg

about a factor of 30... 150 compared to the grating instrument. This is called the Jacquinot advantage. For a more convincing comparison the different transmission factors have to be taken into account [9]. However, they may be taken as approximately equal.

Interference filters are used in photometers and spectrometers as fixed wavelength or tunable wavelength filter. Their optical conductance is given by an expression equal to that for interferometers [15]. Since the path difference is small also the resolving power is smatk Therefore the Jacquinot stop F2 is large. This means a large optical conductance.

Tunable lasers are sometimes used instead of spectrometers. They have a low optical conductance, see equation (5), but a very large radiance, compared to 'classical', thermal light sources.

4 Micro arrangements for infrared spectroscopy

A sample for absorption spectroscopy produces band peaks with a largest signal/noise ratio when the transmission is about 33% or the absorbance is about 0.5 [9, 10]. Since the molar decadic absorption coefficients e have a value of about 10 to 103 1 - cm -1 - mo1-1 for the infrared region [11], the thickness for optimal absorption cells fo r pure substances is given by:

b = 0.5/~" c. (17)

For pure substances c is of the order 10 mol/1, therefore b is in the range 0 .0001-0.005 cm or 1 - 5 0 gm; for solutions with about 1 mol/1 b is about 1 0 - 500 gm. For near-infrared spectroscopy the appropriate thickness is, for substances, of the order 0.1 - 1 cm [12].

The optical conductance for a typical Michelson interferometer for the infrared region is given as follows: The diameter of the mirrors o f the interferometer is taken to be 3.5 cm, the focal length of the collimator mirrors 15 cm. According to (12) for a resolving power of 1000 (this means 4 cm-1 at 4000 cm-1) the radius of the Jacquinot stop is 0.474 cm. According to (5) the optical conductance is then 0.030 cm 2 • sr. For a resolving power of 16000 (0.25 cm -1 at 4000 cm-1) the Jacquinot stop has a radius of 0.118 cm and the optical conductance is 0.0018 cm z. sr. At the position of the detector a 5 : 1 reduced image of the Jacquinot stop is produced with a radius of 0.095 or 0.024 cm. This is also the radius of the beam at the sample position when a 5 : 1 beam condensor is employed.

In Table I the volumes of the samples of different thickness are given, which are matched to the optical conductance of an interferometer with a resolving power o f 1000 and 16000. Also the relative light flux q~rel is given which passes the sample and represents the signal. With a beam condensor the necessary volume is reduced considerably but the relative light flux is constant, when losses due to additional reflections and aberrations are neglected. The last row gives the necessary amount of sample which is needed to fit the optical conductance of the spectrometer when the sample has a thickness of 5 gm. it is interesting to compare these values to the figures of the limit of detection given elsewhere [3, 8].

Infrared microscopes are really micro sample arrangements which allow visual inspection and adjustment of a sample. A standard objective, which is often used, has a magnification of 15 and a numerical aperture N A = n • sin e of 0.58. It can be related to the spatial resolution with the help of Abbe's equation:

~ = 2 / 2 N A (18)

These values are given in Table 1 for different wavelengths used in infrared and near-infrared microscopes. With these values the optical conductance and the relative light flux can be calculated for samples the diameter of which is given by the diffraction limit. It is evident that when ordinary spectrometers would be employed, the signals would be buried in the noise (usual S/N-factors are o f the order of 103 [13]). Therefore larger samples are usually employed. A very interesting approach to the given figures would be possible, if tunable infrared lasers would be employed. Their whole light flux can be concentrated in foci of the given dimensions. In addition, a reduction of the size of the detector is recommended, since the whole surface contributes noise and only a small part is necessary for the detection of radiation.

When grating spectrometers would be employed instead of interferometers the necessary amount of sample would be smaller, because of the smaller optical conductance. How- ever, grating spectrometers need a higher quality of illumination than interferometers, which are more tolerant against aberrations [14].

There are several other infrared techniques which may be applied to micro samples. They cannot be discussed ex- haustively in this paper. However, a great part of the discussion in this paper can be applied also to the optimization of micro sample arrangements for other infrared techniques capable for investigations of micro samples. Their merits are

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2

a b ~-'~Y

Rel. radiant flux 1 0,01 Resolution/]um 10 1

0,1 100

Fig. 4a-e Scanning micro arrangements for Raman

spectroscopiy: a normal sample arrangement; b arrangement using microscope with reflecting objective and fiber optics; e scanning of surfaces with fiber optics and half-spheric mirror

discussed elsewhere: infrared emission [15], diffuse reflection [16], attenuated total reflection (ATR) [17], reflection-absorption spectrometry (IR-RAS) [18], photoacoustic spectroscopy [17, 19], limits of detection of micro methods [20].

A large number of publications deals with applications of micro methods in different fields. Only a few typical papers of special interest are recorded here: general applications [21-23], micro-phase-transitions [24], functional group maps [25], identification of fibers by infrared and Raman spectroscopy [26], investigation of optical fibers [27, 28], micro-ATR by employing optical fibers [29], miniature high-pressure cells for infrared and Raman spectroscopy [30, 31], near-infrared microscopy of semiconductors [32], comparison of microscopy of ATR for the analysis of multilayer polymer films [33].

5 Micro methods for Raman spectroscopy

Raman spectrometers combine two beams: Firstly, the exciting radiation, usually from a laser source is transported to the sample. Secondly, the Raman radiation, excited in the sample is transported through the spectrometer to the detector. This beam is essentially analogous to that already discussed for infrared absorption spectroscopy. A significant difference is, that the Raman signal is emitted from the sample [3]. The 'classical' Raman spectroscopy uses exciting radiation in the visible (or UV) range of the spectrum. The spectra are usually analyzed with grating instruments: double or triple monochromators or polychromators combined with monochromator-arrangements (with re- ciprocal dispersion) for the elimination of false light. This is due to the fact that Raman spectra are of very low intensity and are buried in the scattered radiation, which mainly con- sists of 'unshifted' exciting radiation. Its flux is by 6 or more orders of magnitude stronger than that of the Raman radiation. Presently Raman spectra, excited in the near-infrared region find special interest. In this field interferometers (developed for the infrared region) with their Jacquinot advantage are used sucessfully instead of the grating instruments.

One of the most delicate problems is the proper design of a sample arrangement for a Raman spectrometer. It has

to produce Raman radiation of maximal intensity [3] from a minimal amount of sample. Nevertheless, the sample is to be protected against overheating by the laser radiation. For a given light flux of a laser source the flux of the Raman radiation is inversely proportional to the diameter of the focus of the laser beam at the sample. This means that an optimized Raman sample is a micro sample [3, 8]. The minimal focal diameter of the laser beam can be in the order of the wavelength of the laser radiation.

There are several possibilities of matching a micro sample to a spectrometer. The most straightforward arrangement is using the ordinary sample arrangement (Fig. 4a). The spatial resolution is given by the illuminated volume of the sample and the resolution of the observing system. In practice laser beams can be concentrated to foci with a diameter of about 8 times the wavelengths [34], e. g. 4 gm for excitation by laser radiation at 488 or 515 nm and 8 gm at 1064 nm. Even when the focus of the laser beam is of this magnitude, the effective volume is determined by the nature of the sample. In Fig. 5 the typical cases are shown. A sample which may internally reflect the laser beam (Fig. 5 a) will show a halo around the focal region. This can be prevented by an absorbing layer on the back of the sample. A sample which is transparent will send Raman radiation to the spectrometer, which comes from the whole lobe which is illuminated (Fig. 5 b). A sample like a crystal powder which scatters the radiation by reflection and refraction at the surface of its grains will show a blurred halo, the diameter of which will be larger, when the size of the grains is large (Fig. 5 c). The size of this diffusion halo may be calculated by a procedure similar to that of Kubelka-Munk [35]. When the spatial resolving power has to be large, then the Raman radiation must be observed by using microscope objectives (Fig. 4b). Unfortunately, their optical conductance is somewhat smaller than that of the normal sample arrangement. Therefore the observed Raman light flux is much smaller than that of the spectrometers. The situation is analogous to that for infrared beam condensors versus microscopes. Microscopes can be connected to the spectrometer by a mirror system or with optical fibers as shown in Fig. 4b. Optical fibers are applied for remote sampling in Raman spectroscopy [36, 37], they are especially useful for NIR FT Raman spectroscopy since the transmission of the fibers may be largest just in the range of the

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I1

\ \ \ \ '

~b

Fig. 5 a - e. Volume of sample observed with normal sample arrangement: a layer with internal reflection; b transparent absorbing material; e diffusely scattering material

I A B C

WgVENUMBERS CM-I

Fig. 6 A - C . Micro Raman spectra of different sections A - C of coronar artery. Standard scanning optics; exciting radiation 400 mW at 1064 nm; resolution 4 cm-1; recording time for each spectrum 10 min; Bruker Raman module FRA 106 with FTIR spectrometer IFS 66

Raman spectrum, excited by a Nd: YAG laser [36]. Scanning of the sample can be achieved by an x - y-drive transporting the sample about the focal region of the spectrometer. In Fig. 4 a sample arrangement is shown which makes use of a fiber-optical connection from the laser to the sample and back to the spectrometer. The arrangement shown in Fig. 4 c is specially designed for scanning of surface layers, e.g. of precious prints or paintings. The half-spheric mirror (Fig. 4c) reflects the part of exciting radiation back to the sample which has been scattered by the sample. It also reflects the Raman, radiation which is not collected by the opticM fiber. Thus the mirror enhances - as multiple reflection system - the observed Raman intensity by a factor of 2 to 8, depending on the properties of the sample. The spatial resolution of this arrangement is small, the resolved spot has a diameter

3500 3090 2Sg0 208B 1 5 ~ 10g0 $00 WAgENUHBERS CM-I Fig. 7 A-F. Micro Raman spectra of human cataractous lens A, B,

C, D, silicone rubber E, bovine lens F. Recording conditions as for Fig. 6

of about two times the diameter of the fiber, which transports the laser radiation.

The recently developed NIR FT Raman spectroscopy [38] has the advantage of being essentially immune against fluorescence of impurities or products of decomposition. It is making use of interferometers which are commercially available [39]. In addition to the Jacquinot advantage some of them have another very useful feature. The temperature of the observed spot of the sample can be monitored (by evaluation of the 'Stokes/anti-Stokes' intensity ratio) without the expense of extra recording time [40]. This procedure is highly recommended when temperature-sensitive samples are to be investigated or when the sample may show interfering thermal emission according to Kirchhoff's and Planck's law. The temperature can be reduced by reducing the laser power, by putting a half-sphere of sapphire upon the sample in thermal and optical contact by a drop on an immersion fluid or by blowing cold nitrogen gas upon the sample.

Figures 6 and 7 show examples of Micro Raman spectra recorded from different spots of biological samples by using a Raman modul and a FTIR spectrometer which are commercially available [40, 41, 60].

The combination of a Raman spectrometer with a microscope has first been demonstrated by Delhaye and coworkers [42]. This instrument was applied successfully to the study of inclusion in minerals [43, 44]. Problems of the quantitative analysis of the inclusions have been discussed [45] and their automatic identification [46]. Kiefer has described the Raman spectroscopy of single particles of

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aerosols with the help of the optical levitation technique [47]. The Raman spectra of some materials in monomolecular layers can be observed by the method of surface-enhanced R a m a n spectroscopy (SERS) [ 4 8 - 51]. Investigations of the curing of dental composites have been described [52, 53]. R a m a n spectra of p lant cells have been recorded by R a m a n microscopy and by scanning coherent anti-Stokes R a m a n microscopy [54, 55]. Other biological samples have been investigated by fiber-optic assemblies [56]. The spectra of intact chromosomes have been observed by a microscope with water immersion [57]. Prel iminary results of applications of N I R FT R a m a n microscopes have proved its usefulness [58, 59]. The instrument companies give many further examples. Details of the present-day instruments may be found in a recent collection of da ta [39].

6 Conclusion

Micro methods in infrared and R a m a n spectroscopy supply useful complementary informat ion concerning the nature and structure of mater ia l in small particles or on surface layers. The min imum volume o f sample necessary is, for infrared spectroscopy, of the order 1000 ~tm 3 (]0 -9 ~tl), for R a m a n spectroscopy of about 100 ~tm 3. Scanning techniques allow mapping of samples. F o r the future the development of miniature spectrometers may be ant icipated and the em- p loyment of optical fibers. Thus the flexibility and mobi l i ty of these methods will be improved considerably, so that they can be employed where this may be necessary - in many fields of science and technology.

Acknowledgements. Financial help by the Deutsche Forschungsge- meinschaft, the Fonds der Chemischen Industrie and the Bundes- ministerium ffir Forschung und Technologie is gratefully acknowledged. I further thank the representatives of the instrument producers who supplied interesting new material.

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micro methods in infrared and raman spectroscopy

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