Fluorescence emission from mechanical pulp sheets II. Estimation of quantum yields

J. Phorochem. Photobiol. A: Chem., 73 (1993) 67-14

Fluorescence emission from mechanical II. Estimation of quantum yields Jian H. Zhu and Derek

pulp sheets

G. Gray

Paprican and Department of Chemistry Pulp and Paper Research Centre, McGill University, 3420 University Street, Monfrial, Q&,., H3A 2.47 (Canada) (Received November 30, 1992; accepted

February

25, 1993)

Abstract The luminescence of dry paper sheets made from lignin-containing peroxide-bleached wood pulps was investigated by steady state spectrofluorometry. The emission was predominantly fluorescence, although a very weak phosphorescence was also detected. Fluorescence spectra, corrected for incident light intensity and detector response and recorded in emission and excitation modes, revealed the existence of more than one emitting excited state for excitation wavelengths between 300 and 470 nm. A relative method for determining the fluorescence quantum yield using rhodamine-6G (R6G) as standard was developed, and the data thus obtained indicate that fluorescence is a very inefficient deactivation pathway for the excited states.

1. Introduction Light-induced yellowing of mechanical pulps has long been attributed to reactions taking place in lignin [l-3]. Demands for the extended use of lignin-containing high-yield pulps have motivated research on the structural details of the lightsensitive chromophores present in lignin, and on the reactions leading to yellowing on exposure to light [4]. The photochemistry of lignin model cornpounds and extracted soluble lignin has also been studied [5-S]. However, the mechanisms governing the yellowing process are not completely understood, and efficient, economic inhibition of yellowing remains an elusive goal. The first step in any photoyellowing process is presumed to be the absorption of light of specific wavelengths by the chromophores in lignin. The excited chromophores may lose their energy by reacting chemically or thermally, or they may emit light at longer wavelengths. This luminescence process may be either fluorescence or phosphorescence, from the singlet or triplet states respectively. Luminescence has been reported from solid paper [!9-111, from soluble lignin adsorbed on a solid cellulosic substrate [ 121 and from lignin model compounds in solution 1131. As solid paper is a strongly scattering medium, both the incident light and the luminescence are scattered, making quan-

lOlO-6030/93/$6.00

titative work difficult. In this paper, the steady state luminescence spectra for handsheets of several bleached and unbleached thermomechanical pulps (TMP) and chemithermomechanical pulps (CTMP) are reported, and a simple method of estimating the emission quantum yield of these highly scattering solid samples is proposed.

2. Experimental

details

2.1. Spectral measurements Emission spectra were recorded on a Spex modelF112 spectrofluorometer with a 450 W xenon iight source. Luminescence was detected using the front face mode at an angle of 22.5” from the excitation light beam. Light of a designated wavelength was obtained by focusing the incident light through a single-grating monochromator. Spectra were corrected for both the fluctuation of the incident light intensity and the response of the detector as a function of wavelength using a solution of rhodamine,B (8 g in 200 ml of ethylene glycol) as an internal reference and the correction factors provided by the manufacturer respectively. Stray light from the light source was eliminated by inserting a bandpass filter of the designated wavelength between the source and the sample. For fluorescence measurements, samples consisting of &lo handsheets (depending on the thickness and

D 1993 - Elsevier Sequoia. All rights

reserved

68

I. ItI. Zhu, D. G. Gray I Fluorescence from mechotaical pulp

the absorbance of each individual sheet) were used to give an optically thick sample. A Spex accessory with a programmable pulsed light source (duration, 3 ~LS), whose intensity decays to zero after 20 ps, was used for phosphorescence measurements. The total diffuse reflectance R was measured on a Philips PU8800 UV-visible spectrometer equipped with an integrating sphere. The reflectance of the samples, measured for a solid angle defined by the instrument geometry, was also obtained with the Spex spectrofluorometer by simultaneously scanning the excitation and emission wavelengths (a synchronous scan) with a wavelength offset a zero. Both types of reflectance measurements were made on optically thick samples. 2.2. Sample sheets Bleached CI’MP from aspen and softwood and unbleached TMP from black spruce were obtained from eastern Canadian sources. Hydrogen peroxide bleaching of the TMP was carried out following the method described in ref. 14. The bleached TMP (BTMP) was reduced by treating the pulp (at 1% consistency) with 1% charge (based on oven dry weight) of a 10% (w/v) aqueous solution of NaBH, for 72 h, with oxygen being removed by bubbling N2 continuously through the suspension. All pulps were extracted with diethylenetriamine-pentaacetic acid (DTPA) solution (2% on oven dry weight basis), before making handsheets or bleaching, in order to remove heavy metals contained in the pulps. Handsheets were made from these pulps with an average basis weight of about 20 g m-‘. The fluorescent dye, rhodamine-6G (Schmid and Co., Stuttgart, Germany), was dissolved to give a range of concentrations from 10e4 to 5X low3 g 1-l in water-methanol (30:70). Sheets were immersed for about 1 min in the solution, and dried in the dark at room temperature while being held flat.

sheets

the angular distribution of the intensity, on the surface optical properties of each sample. Both factors must be considered in order to make a meaningful comparison of the emission intensities from different paper samples. Fluorescence is normally quantified as the emission quantum yield, @, i.e. the ratio of emitted light intensity to incident light intensity. Unlike solution samples, forwhich the calculation methods are well established [15], and for which standards with rigorously determined values.of @ are available, methods for solids are rare and they apply only to specified cases under limited conditions. They can be grouped under two categories: direct determination of emission and absorption signals and applications of Kubelka-Munk theory. In a method applied to the determination of quantum yields for analytes loaded on filter paper, the absorption of the analytes is obtained by simply subtracting the reflectance of the analyte sample from that of a filter paper blank [16]. This method implicitly assumes that the fluorescence and reflectance of a given sample have the same angular distribution factor. Methods developed by Bonham [17] and Fukshansky and Kazarinova 1181 are examples of the application of Kubelka-Munk theory. These methods apply only to the emission of a single fluorescent dye loaded onto a lightscattering substrate and illuminated by polychromatic incident light. We require a method to measure quantitatively the emission from a solid sample sheet using a regular spectrofluorometer. The key element in emission quantification for paper is the determination of absorption. For a strongly scattering substance, such as paper, direct determination of absorption 1, is difficult. However, for optically thick paper sheets, for which transmission is ciose to zero, we have 1, =lo-I, =&I(1 -LlZ0)

3. Method of quantifying solid paper

the fluorescence

of

In studies of solid state fluorescence, a direct comparison is often made between emission intensities of different samples [lo]. This can be misleading, since a variation in absorption can also result in a difference in emission intensity. In addition, fluorescence measured with a typical spectrofluorometer is only a portion of the total emission, and the exact percentage of that portion depends on the instrument geometry and, through

=I,,(1 -Rh)

(1)

where I0 is the incident light intensity, 1, is the total intensity of scattered light and Rk is the total diffise reflectance at the excitation wavelength A. When measuring fluorescence, the intensity recorded if is only part of the total emission intensity 1, aif = z,

(2)

where (Yis the emission angular distribution factor for the experimental detector angle. The emission quantum yield is then

J. H. Zhu, D. G. Gray

i Fluorescence fiwm mechanical pulp sheers residual derived

=

=

v.lR,;i

1

_

absorption of the substrate paper, from the Kubelka-Munk equation

(1-R~‘)‘IR~’

As LYcannot be measured with conventional spectrometers, we introduce a;, the angular distribution factor for scattered light reflectance, and i,, the partial intensity of scattered light at the same experimental detection angle 1, = c&,

(4)

K0 and S, in the above expression are the absorption and scattering coefficients respectively for the undyed paper, and Kd and Sd are the corresponding coefficients for the dyed paper. RA’ is the reflectance of the undyed paper, measured at the same wavelength as R,. Combining eqns. (8) and (9) leads to

Consequently @=

and is [19]

(10)

(1- RJ2/R,

-R,)

69

UW

c&R,

(5)

GL(l - Rh)

If we can assume CY~ = CY, i.e. the angular distribution factors for reflectance and emission are the same for a given sample, then

This approach is similar to that of Ramasamy et al. [16]. By measuring i, R, and i, for a given sample, its emission quantum yield @ can be calculated by eqn. (6), if its validity can be confirmed. An alternative approach is based on the use of a fluorescent dye as an internal standard. For an optically thick sample, the overall emission intensity 1, can be written as If = @I= =@I&-R,)

(7)

and the detected

emission

intensity

i, is

01

’ al-Rdf @+jd,___ (A

cyd 1 -R,

P

WI

where 0 is the ratio of the excitation light intensities (to take into account the fact that i, and (i& are recorded with different excitation wavelengths). If @., is known and IY can be considered as equal to %, then if

l-R, --

f

@=@d(i3dl-Rhf?

(12)

Rhodamine& (R6G) has been chosen as the fluorescent dye. It has an emission quantum yield @,=l when adsorbed on cellulose [19]. If we assume that there is no complexation or other reactions between lignin and R6G, either in the ground state or excited state when A,,= 500 nm iS used for recording (i&, @d Will be unity on the mechanical pulp substrates.

i, = &la = @IO(1 - R,)la

4. Results

Analogously, when the same paper is dyed with a fluorescent compound and excited at an appropriate wavelength so that the detected emission (i& is solely from the dye, we have (& = @&I

-&)$‘Q

where @d is the emission quantum fluorescent compound on the paper Rd are the emission angular distribution the total diffuse reflectance of the respectively. The factor f is a factor for the light absorbed by the dye

and discussion

(8)

(9) yield of the and Q and factor and dyed paper accounting and for the

4.1. Quantitative methods 4. I. 1. Method based on eqn. (6) Equation (6) depends on the assumption that the angular distribution of intensity is the same for luminescence and scattering from the paper sheets. With the current apparatus, it was not possible to vary the angle of observation, but changing the slit widths indirectly samples the light intensity over different angles. As an experimental test of eqn. (6), the ratios of fluorescence and scattering intensities ii/i, were measured at different emission slit widths, keeping a constant excitation

70

J. H. Zhu,

D.

G. Gray

I Fluorescence

slit width. (The scattering intensity i, was measured in the spectrofluorometer by scanning the excitation and emission monochromators with zero wavelength offset.) The overall intensities If and 1, are independent of the emission slit width, and so any variation in the if/i, ratio must stem from the different angular distributions for fluorescence and scattering intensities, i.e. cr and s (from eqns. (2) and (4), for a given slit width, (iJ&> = (1&J (a!/ q)). Our experimental results show that the ratio if/i, varies by as much as 200% when the emission slit width is changed. This is presumably due to variations in ff/c~~.If 4% is not constant, it must in general differ from unity, and so the assumption that (Y= ag in eqn. (6) cannot be used for the determination of the fluorescence quantum yield for solid paper. 4.1.2. Method based on eqn. (12) The fluorescence from several dyed paper sheets containing a range of very low concentrations of R6G gave a series of values for (i&, Rd and f (see eqn. (9)), for an excitation wavelength of 500 nm. If a plot of (i& against (1 -&)f is linear up to the origin of the (1 -&)f axis, then od is independent of R6G loading and characteristic of the sheet. Thus for the blank substrate, i.e. the same sheet before R6G loading, a = ad at 500 nm. If we assume that the angular distribution of fluorescence is independent of wavelength, then eqn. (12) can be used to determine the quantum yield of fluorescence of paper, provided that a standard sample, dyed with RBG, is made from the same sheets whose fluorescence is to be studied.

0

0.02

0.04

0.06

0.08

from

mechanical

pulp

sheets

Figure 1 shows the curves of (i&r vs. (1 -RJf for a bleached thermomechanical pulp (BTMP) and two bleached chemithermomechanical pulps, made from aspen (BCTMP-1) and softwood (BCTMP-2). A sample of Whatman No. 1 cotton filter paper was run for comparison. The results give straight lines, passing near the origin, as required by eqn. (12). The different slopes indicate a different spatial distribution of emission from each type of sheet. Hence eqn. (12) appears to be valid. By measuring if and R, for a paper sample at a chosen excitation wavelength, and (i&, R, and f for the same sample loaded with R6G at 500 nm, the emission quantum yield at the chosen wavelength of the paper sample can be calculated from eqn. (12). 4.2. Phosphorescence Phosphorescence was detected from BCI’MP and BTMP in air at room temperature (an example for BTMP is shown in Fig. 2). The broad phosphorescence peak is in the region of 500 nm. It lasts about 10-20 ms, the same order of magnitude as that observed by Shul’ga et al. [20]. The intensity is less than 10m4 of that of the fluorescence, and so the observed static emission in our study is considered to be exclusively fluorescence. 4.3. Fluorescence quantum yields Fluorescence quantum yields @ for the five mechanical pulps, calculated according to eqn. (12), are listed in Table 1. For comparison, when excited at a wavelength of 320 nm, the Whatman No. 1 filter paper gave: a fluorescence peak with a broad maximum at 415-435 nm, and an intensity corresponding to a quantum yield of 0.11 f 0.01.

0.1

U-Rd) f Fig. 1. Plot of (i& ~3. (1 -Rd)f for Whatman No. 1 filter paper ( l ), BCTMP-1. (B). BCTMP-2 (e) and BTMP (A) loaded with rhodamine-6G.

400

450

500

550

wavelength

600

650

700

nm

Fig. 2. Phosphorescence spectra of BTMP. A, (nm): curve 1, 320; cmve 2, 430 (100 ps after excitation pulse).

71

J. H. Zhu, D. G. Gray I Fluorescence from mechanical pulp sheets TABLE 1. Values of the apparent fluorescence quantum yield (a) for BCI’MP-1, BCTMP-2, BTMP, NaBHcreduced BTMP (R-BTMP) and TMP BCTMP-1

BCTMP-2

BTMP

R-BTMP

TMP

0.024 0.041 0.061 0.098 0.147 0.181 0.259 0.183 0.148

0.028 0.056 0.076 0.10 0.11 0.14 0.25 0.27 0.21

0.021 0.023 0.026 0.031 0.038 0.055 0.108 0.110 0.082

0.039 0.051 0.053 0.050 0.049 0.047 0.040 0.029 0.019

0.014 0.015 0.017 0.017 0.020 0.017 0.038 0.045 0.039

2;;m, 300 320 340 360 380 400 420 450 470

For all these pulps, emission is very inefficient for excitation with near-UV light between 300 and 360 nm, where incident irradiation causes the most efficient yellowing of mechanical pulps [21]. It should be noted that although 0 is still small at A>400 nm, no yellowing occurs when pulps are irradiated with visible light. The photochemical changes taking place under visible light result only in uncoloured products, or even photobleaching [22]. Also, the emission observed on excitation between 420 and 470 nm implies that lignin contains long-wavelength absorbers which are fluorescent. For near-UV excitation, bleaching causes a minor increase in the @ value of TMP; reduction of BTMP by NaBH, gives a further slight increase in @ (Table 1). This slight increase in photochemical activity is reminiscent of the observation that bleached pulps, even after reducing treatment, are still vulnerable to light-induced yellowing [14, 23, 241. Quantum yields of fluorescence are clearly low. The initial singlet excited states must decay by other mechanisms. These include unimolecular and bimolecular chemical reactions, intersystem crossing to the triplet state, internal conversion and bimolecular deactivation processes, notably energy transfer. Energy transfer has been invoked in lignin photochemistry [25]. Energy acceptors in this process can be either fluorescent or non-fluorescent groups, but the emission spectra for the different excitation wavelengths presented below indicate that energy transfer resulting in emission from the acceptors is not significant. The rate constants for intersystem crossing depend strongly on the nature of the singlet states. For (n, #?r*)singlets, intersystem crossing is relatively efficient. The only structures in lignin that have excited states with (n, r*) nature are cY-carbonyl units and coniferyl aldehydes. However, (n, I?) singlets generally fluoresce

very weakly. This point is supported by calculation and experimental results on vanillin and coniferyl aldehyde [26]. The absence of evidence for triplet emission indicates that intersystem crossing is a minor contribution to the decay process. All other potential fluorophores in lignin, such as biphenyl, phenylcoumarone and stilbene structures, form (rr, 7~*) singlets. These fluoresce more efficiently, and have rate constants for intersystem crossing which are much lower than those for (n, ti) singlets. Since bimolecular decay and intersystem crossing appear to be unimportant for fluorescent structures in solid lignin, the low @ data may signify that chemical reactions from excited singlet states are relatively efficient, assuming that internal conversion is insignificant. The singlet state reactivity of lignin chromophores has been demonstrated by Schmidt et al. [27]. 4.4. Fluorescence spectra of mechanical pulps The peak shapes for the corrected emission and excitation fluorescence spectra of BCTMP-1, BTMP and BCTMP-2 are shown in Figs. 3-5 (in which spectra are scaled for visibility rather than for their original relative intensities). Not unexpectedly, the spectra are complex. The emission spectra indicate that there is more than one emitting species in paper under light of h> 300 nm. This is confirmed by the corresponding excitation spectra, which shift with observation wavelength. Examples are given in Fig. 6. For the three types of bleached pulp, the emission spectra show two distinct regions: one for 3OCL360 nm excitation and a second for excitation above 360 nm. There are two striking observations in the first region. Firstly, for each type of pulp, the emission spectra

300

400

500

wavelength

GO0

nm

Fig. 3. Emission spectra of BLIMP-1. A.. (nm): (1) 300; (2) 320; (3) 340; (4) 360; (5) 380; (6) 400; (7) 420; (8) 450.

J. H. Zhy

300

400

560

wavelength

D. G. Gray I Fluorescence f?om mechanical pulp sheels

600

nm

Fig. 4. Emission spectra of BTMP. A, (ran): (1) 300; (2) 32O; (3) 340; (4) 360; (5) 380; (6) 400; (7) 420; (8) 450.

45

,

have very similar band profiles and band maxima regardless of the excitation wavelength used. Similar fluorescent characteristics also occur for unbleached TMP (Fig. 71, but at excitation wavelengths of 340-380 nm. Secondly, these emission bands are very similar for BTMP, BCTMP-1 and BCTMP-2. The observed emission may thus originate from one common absorbing chromophore. This is not the case for the same pulps on excitation at wavelengths longer than 360 nm, as the emission spectra shift to longer wavelengths as the excitation wavelength is increased. Bleaching, in the case of TMP for example, results in a blue shift of the emission band on excitation between 300 and 360 mn. Reduction of BTMP by NaBH_, modifies the emission spectral distribution. Most notably, on excitation at 300-340

I

... i 400

500

wavelength

300

600

spectra of BCTMP-2. A,, (nm): (1) 3M1; (2) 32% (3) 340; (4) 360; (5) 380; (6) 400; (7) 420; (8) 450.

wavelength

480

500

wavelength

nm

Fig.5. Emission

380

400

Fig. 6. Excitation spectra of BTMP. A, (nm): (1) 3% (3) 430; (4) 450; (5) 480; (6) 500; (7) 520; (8) 550.

560

wavelength

nm (2) 410;

700

nm

Fig. 7. Emission spectra of unbleached TMP. A, (nm):(1) 300; (2) 320; (3) 340; (4) 360; (5) 380; (6) 400; (7) 420; (8) 450.

460

580

600

600

7

nm

Fig. 8. Emission spectra of NaBW-reduced BLIMP. A.. (nm): (1) 300; (2) 320; (3) 340, (4) 360; (5) 380; (6) 400; (7) 420; (8) 450.

J. H. Zhu, D. G. Gray I Fluorescence from mechanical pulp sheets

nm, a new emission profile, with maxima at 388 and 408 nm, results from the reduction treatment (Fig. 8). The new emission band could originate either from a reduced chromophore, e.g. an (Yhydroxyl aromatic, or from an emitting group present in BTMP, whose excited state is efficiently quenched by an NaBH,-reducible group such as aromatic carbonyl. However, considering the overall blue shift and shape change in the emission spectrum of reduced BTMP compared with that of unreduced BTMP, it seems that emission from a new species is observed in reduced BTMP. It seems that the excited states probed by emission detection are groups structurally sensitive to NaBI%, treatment. At the present stage, we cannot assign the observed emission from these mechanical pulps to definite chromophores. Of the lignin structural groups already identified, only a-aromatic ketone, biphenyl, coniferyl, stilbene and phenylcoumarone structures absorb at A > 300 nm in the solid state. a-Aromatic ketones generally fluoresce very inefficiently because of rapid intersystem crossing. Quinone-like structures absorb in the visible region, but are normally non-fluorescent because of their low-lying (n, r*) singlet state nature, However, quinone-like structures may become fluorescent when they are linked to large conjugated aromatic structures, so that the low-lying singlet changes to (r, 7i”) in character. The “energy sink” phenomenon reported for soluble lignin fluorescence [13], in which all the excitation energy is transferred to a single emitting site of lower excitation energy, does not seem to exist in solid paper; there is little evidence for energy transfer to emitting acceptors, because changing the excitation wavelength does not give fluorescence in a single longer wavelength band. (Energy transfer to acceptors which are non-fluorescent cannot be ruled out.)

Acknowledgments J.Z. thanks Paprican for a Postdoctoral Fellowship. We thank Dr. A. Koukoulas for helpful discussions, Dr. J. Schmidt for providing pulp samples and for assistance with the PU8800 UV-visible spectrometer and Dr. P. Whiting for providing TMP samples. The work was supported by the Government of Canada through the NCE Mechanical Pulps Network.

References

4

5 6 7 8 9 10 11 12

13

14 15 16

5. Conclusions 17 18

A new method, based on the incorporation of an efficient fluorescent dye, has been developed for measuring the fluorescence quantum yield @ of paper sheets. The @ data for lignin-rich mechanical pulps indicate that emission is not an efficient depletion pathway for the singlet .excited states.

73

19 20 21 22

P. Nolan, J. A. Van den Akker and W. A. Wink, Paper Trade J., 121 (1945) 101. G. J. Leary, Tappi, 50 (1967) 17. C.Heitner, Light-inducedyellowingofwood-containing papers _ a review of fifty years of research, in Proc. 6th Inf. Symp. on Wood and Pulping Chemiwy, Melbourne, Australia, 1991, 1 (1991) 131. (a) S. Y. Lin and K. P. Krinstad, Tappi 53 (4) (1970) 658. (b) T. P. Schutz and W. G. Glasser, Ho~fotxhung, 40 (1986) Suppl. 37-44. (c) K. Lundquist, Zntematianal Symposium on Wood and Pulping Chemistry, Stockholm, 1981, Preprint V.l 81-85. (d) T. M. Gatver, Jr. and S. Sarkanen, Ho&forschu~~g 40 (1986) Suppl. 93-100. C. Vanucci, P. Fomier de Violet, H. Bows-Laurent and A. Castehan, I. Photochem. Photobiol. A: Chem., 41 (1988) 251. J. Gierer and S. Lin, Sven. Pappersddn., 75 (1972) 233. G. Gellerstedt and E. Pettersson, Sven. Pappersridn., 80 (1977) 35. L. Hemra and K. Lundquist, Acta. C&m. Stand., 27 (1973) 365. R. S. Davidson, L. A. Dunn, A. Castellan and A. Nourmamode, I. Photochem. Photobiol. A: Chem., 58 (199X) 349. H. Tylli, I. Forsskehl and C. Olkkonen,J. Photo&em. Photobiol A: Chem., 67 (1992) 117. J. A. Olmstead and D. G. Gray, J. Photochem. Photobio!. A: Chem., 73 (1993) 59-65. A. Castehan, A. Nourmamode, C. Noutary, C. Belin and Ph. Former de Violet, J. Wood Chem. Technol., 12 (1) (1992) 19. (a) K_ Lundquist, B. Josefsson and G. Nyquist, Hubzforschung, 32 (1978) 27. (b) K. Lundquist, Cel[ul. Chem. Techno!., 15 (1981) 669. G. Gellerstedt, I. Pattersson and S. Sundin, Sven. Papperstidn., 86 (1983) R3.57. J. N. Demas and G. A. Grosby, .Z Z’hys. C/tern., 75 (1971) 991. S. M. Ramasamy, V. P. Senthilnathan and R. J. Hurtubise, Anal. Chem., 58 (1986) 612. J. S. Bonham, Color Res. Appl., 11 (1986) 223. L. Fukshansky and N. Krzarinova, J. Opt. Sot. Am., 70 (1980) 1101. L. F. Vieira Ferreira, M. Rosari Freixo, A. R. Garcia and F. Wilkinson, I. Chem. Sec., Famdoy Tram-., 88 (1992) 15. V. I. Shul’ga, T. M. Rykova, P. I. Zelikman and E. I. Chupka, KhZm. Rrev, Z (1991) 26. 1. Forsskahl and .I. Janson, Pmt. 6th Int. Symp. on Wood and Pulping Chemzkhy, Melbourne, Australia, 1991, 2 (1591) 2.55. A. L. Andrady, Y. Song, V. R. Parthasarathy, K. Fueki and A. Torikai, Tappi J., 8 (1991) 162.

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J. H. Z/m, Q. G. Gray / Fluorescence from mechanical pulp sheets

23 I. A. Schmidt and C. Heitner, J. Wood Chew. Technol., II (1991) 397. 24 M. Ek, H. Lennholm and T. Iversen, Nerd. Pulp Pup. J., 5 (1990) 159. 25 E. I. Cbupka and V. M. Burlakov, Mu’m. Drev.. 2 (1984) 31.

26 V. M. Burlakw, E. I. Chupka, D. Ratovskii, Cellul. Gem. TechnoL, 27 J. A. Schmidt, A. B. Berinstain, L. J. Johnston and J. C. Scaiano, 104.

D. Chuvashev and G. V. 6 (1986) 651. F. de Rege, C. Heitner, Can. J. Chem., 69 (1991)