Anal. Chem. 1995, 67, 87R-95R
Pulp and Paper B. Bruce Sithole Process Chemistry Division, Pulp and Paper Research Institute of Canada, 570 St. John’s Boulevardj Pointe Claire, Quebec, Canada, H9R 3 9 Review Contents
Standard Methods On-Line Analytical Techniques Surface Analytical Techniques Charge Analysis Wood Components Inorganic Components Additives Halogenated Compounds Analysis of Ancient Papers
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This is an inaugural review covering analytical topics in the pulp and paper industry from October 1992 through October 1994. The review covers areas that are of direct concern to the industry and not forestry topics per se. Hence, the extensive literature on lignin chemistry is it may be argued that the omission is somewhat arbitrary. In any case, there are excellent reviews covering analytical techniques in lignin chemistry. For example, one report reviews the role of analytical chemistry in the forest products industry (I), and another gives 594 references on methods available for the chemical analysis of wood (2). Despite the foregoing preamble, analysis of residual lignin in Pulp and paper matrices is covered since this does have a direct bearing on the pulp and papermaking process. The objective of the review is to give a brief overview of some significant, interesting, or useful applications that are of interest to researchers, chemists, environmental professionals, and engineers in the pulp and paper field. As this is a first review of its kind for Analytical Chemistry, suggestions for improvements in future issues are welcome. STANDARD METHODS The CPPA (3), TAPPI (4), and ASTM (5) have issued new or
revised test methods for the analysis of pulp and paper matrices. Their publications should be consulted for current methodologies and testing procedures. Notable among the new test methods is a CPPA one that now recommends using acetone instead of dichloromethane for determining the amount of extractives in wood and pulp (6). This is good news for laboratory personnel because they now do not have to worry about exposure to halogenated solvents. However, mill personnel still insist on using dichloromethane because it is what their customers use to judge the quality of their products. ON.LINE ANALYTICAL TECHNIQUES
The pulp and paper industry needs on-line methods to monitor various process parameters. Such techniques can help mills be more efficient and save them from expenses incurred due to production of off-grade or contaminated products. FT-IR spectroscopy, spectrophotometry, and charge analysis are prime candidates for on-line developments. 0003-270019510367-0087$15.50/0 0 1995 American Chemical Society
An on-line system for measuring organic compounds in the wet end was successfully tested in a mill (7). Sample preparation is by centrifugation and the floated organics are measured by FTIR Although there was good correlation with other analytical techniques (e.g., COD measurement), the authors believe that sensitivity can be improved by constructing a simpler, more robust spectrometer based on narrow-band filters at k e d wavelengths. An IR wet-end gauge for troubleshooting wet-end problems has been developed. The instrument gives the paper maker a highdefinition moisture profile and enables correction of streaking and dry-edge problems -(8). Better control of kraftcooking parameters can yield better pulp quality as well as significant decreases in production costs. A diffusion system capable of operating in the harsh cooking conditions has been developed (9). The system requires oxygenfree acceptorsolutions to negate changes in sulfide speciation, a simple oxygen removal unit, and a rapid-scanning photometer. Simultaneous determination of several sulfur species was demonstrated. A recently issued patent describes a direct monitoring and control method which eliminates frequent sampling and equip merit maintenance as well as providing for on-line measurement of effective &ali. carbonate, sulfate, and thiosulfate concentrations in kraft pulping liquors (10). The method includes the steps of withdrawing liquid samples from the pulping process and measuring the concentrations of the listed compounds by infrared spectrometry at predetermined wavenumbers. Experiences with an OPTI-KAPPAon-line lignin analyzer have been reported (11). The data show that the system measures lignin content independently of variations in raw materials and process conditions in mills. Since there are problems with standardization and longevity of such on-line analyzers, a new calibration solution has been suggested (12). The solution uses known amounts of refined lignin in ethylene glycol and offers good reliability as well as reasonable service lives for the instrumentations. Reproducible and accurate methods for measurement of sulfide and alkali consumptions during kraft cooks have been developed (13). The methods are based on conductivity analysis and on-line ion chromatography. There is vigorous competition in the development of on-line charge analyzers. An on-line fiber charge analyzer that measures charge in the entire fiber suspension furnish has been developed (14). It quickly responds to alterations of the electrokinetic input to paper machines such as a change form cationic to anionic dye. Another analyzer, a 5 potential instrument based on the streaming current potential principle, was shown to give better results than microelectrophoretic measurements of cellulosic fibers (15). Still another analyzer is the particle charge detector that not only measures anionic trash in paper machine whitewaters but also enables the paper maker to determine the sources of charges within the production process (16). Analytical Chemistry, Vol. 67, No. 72, June 15, 1995 87R
An apparatus that enables on-machine quantitation of ash content (inorganic fillers and pigments) on paper surfaces as the paper leaves the paper machine has been developed (17). The two-chambered apparatus, which is useful for instantaneous measurements of pigments on both sides of paper surfaces, deaerates paper in the first chamber and bombards it with X-rays on both sides, and the reflected rays are detected by sensors placed in the walls of the second chamber.
SURFACE ANALYTICAL TECHNIQUES Incorporation of low levels of organic reagents into paper generally leads to improved performance properties in formation, wet strength, sizing, and printability. The development of surface spectroscopic techniques provides a means of detecting the presence and distribution of these additives on the paper surface. Although paper surfaces are not ideal for classical surface analytical techniques, XPS and SIMS are two techniques that can be used successfully to characterize defects in paper that are caused by changes in surface chemistry. One report discusses some problems associated with these techniques and gives examples detailing successful characterization of surface defects in paper (18). SIMS was also used to study the adsorption of calcium and aluminum ions on unbleached and peroxide-bleached groundwood pulps (19). Calcium was evenly distributed on the fibers while silicate and aluminum were adsorbed mainly on the fines fraction. The information content of ESCA and SIMS for characterizing paper surfaces was compared in a recent report (20). Another report described the use of SIMS for.nondestructive analysis of colorants in paper (21). The use of static analytical conditions had no detectable effects on the paper and made SIMS suitable for nondestructive analyses. Rapid differentiation of the colorants was achieved, and some components were identified qualitatively. Combined thin-layer chromatography/SIMS was useful for compounds that were not properly resolved by SIMS alone (21). Another report on SIMS used the technique to image the spatial distribution of organochlorine in fully bleached kraft pulp fibers (22). The analysis showed that inextractable chloride was present over the entire fiber surfaces and enriched in the middle of the secondary wall. ESCA was used to study the distribution of extractives in pulp fibers of several mechanical and chemical pulps (23). The analysis showed that there were more extractives on the fiber surfaces than inside the fiber cells. The technique was also used to study the distribution of alkylketene dimer, an internal surface sizing agent, in handsheets (24). In this case, the analysis showed that the dimer only covered a part of the fiber surfaces and its film thickness was -3 nm. Sufficient sizing was achieved when -15% of the fiber surfaces were covered by the dimer. Acid groups on kraft fibers of varying lignin content were determined by ESCA, polyelectrolyte titration, and potentiometric titration (25). The titrations showed that the fiber surfaces contained different acid groups with pKa)s of > 7.25, 3.6, and 5.7 while the ESCA analysis showed that the carboxylic groups and alkyl carbon groups on the surfaces decreased with decrease in lignin content. Thus, a combination of ESCA analysis and highprecision titrations revealed a detailed picture of the effect of cooking conditions on the surface characterization of cellulose fibers. This information is of direct relevance to paper properties. 88R
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ESCA has also been used to study the chemical composition of paper surfaces coated with different pigments, namely, clay, calcium carbonate, and latex binders (26). The surfaces were rich in carbon, and -90% of the C signal was due to the latex binder. The analysis enabled calculation of the surface areas covered by the different binders. Energy-dispersive X-ray fluorescence (EDXRF) is suitable for determining amounts of pigments, such as talc, clay, and gypsum, coated on paper (27). The method is accurate and rapid and can replace traditional gravimetric methods that are long and tedious. However, the analysis is rather inconvenient as it requires both base and coated papers. EDXRF has also been used (in conjunction with ash content determinations) to characterize Japanese papers with the aim of identifying papers from differing manufacturing processes (28). A report from Israel used X-ray fluorescence W) to quantitatively estimate the health risk of lead in US. bank notes since they contain lead-based ink driers (29). The analysis showed that the lead concentration has progressively decreased &fold in the 1980s and bills printed in the 1990s showed very little amounts of lead. XRF has also been used to study potential occupational hazards to workers using abrasives from papers coated with corundum and silicon carbides (30). X-ray dfiaction has been used for quantitative analysis of talc, kaolin, TiOz, and BaS04 inorganic fillers in paper (31). A review on newly developed FT-IR instrumentation and techniques for studying lignocellulosic materials and paper products showed that diffuse reflectance, attenuated total reflectance, and photoacoustic techniques can give information on the chemistry of surfaces of wood and pulp samples (32). Although the techniques do not allow for quantitative determinations, their qualitative spectra are similar to normal transmission spectra. Near-infi-ared spectroscopy has been used to measure chemicals in paper (33). Use of the full near-IR spectrum from 1100 to 2500 nm permits the detection of several constituents simultaneously, especially when derivatives are employed to mathematically manipulate the stored spectra. Fourier transform Raman spectroscopy has been used to differentiate papers made from sulfate and sulfite hardwood and softwood pulps (34). Sulfate papers differed from sul6te ones by a band at 510 cm-l that was attributed to the presence of disulfide bonds. Surface grinder attenuated total reflection IR spectrometry (ATR-IR) and quantitative X-ray microanalysis were used to characterize single- and multiplecoated papers produced on a pilot coater (35). The former provide information on the chemistry of the binders and pigments in the coatings while the latter provide information on the composition of the pigments. Inverse gas chromatography is a tool that can be used to thoroughly characterize surfaces of materials. For example, the technique was used to characterize surfaces of untreated pulp fibers and fibers treated with an internal surface size, alkenylsuccinic anhydride (36). The properties monitored were surface energetics and acid/base characteristics; IGC did indeed detect the changes induced by the suing. IGC was also used to characterize the surface of wood birch meal (37). The values of the acid and basic parameters of the substrate surface suggested that an extractives-free wood meal surface was amphoteric, with predominantly acceptor donor sites.
IR thermography was used to detect temperature changes that occur in paper during tensile tests (38). The technique is better than calorimetry for this purpose and is useful for studying local deformations in paper manufacturing. One form of surface analysis measured the distribution of additives such as starch, SBR latex, and polyacrylamide found in paper coatings (39). The methodology entailed abrading a coated paper sample at inclined angles, slitting the paper into small pieces, and dipping the pieces into reagents that are specific to the additives, namely, iodine for starch, o-toluidine blue for SBR and eosine for polyacrylamide. CHARGE ANALYSIS
Analyses of charges in pulp suspensions can provide powerful tools for optimizing the wet end of a papermaking process. The techniques currently used for charge analyses to characterize and monitor the paper machine wet end as well as the fundamentals and practical applications of each technique have been discussed (40). A report on newly developed measuring techniques for characterization of polymers, surfactants, and polyelectrolytes discusses the theoretical background and methodological details of an apparatus for determining the wetting kinetics, contact angle, and free surface energy of fibers and fillers in papermaking (41). The apparatus, a potential meter, includes automatic measuring techniques and options for quantitative measurement of surfactants, polyelectrolytes, and functional groups present in polymers. The fiber charge and acidic groups of 17 different radiata pine pulps were determined by polyelectrolyte and conductometric titration, respectively (42). The fiber charge, assessed by the ability of fibers to absorb the cationic polyelectrolyte polybrene, varied between 20 and 300 mmol/kg, depending on the pulp type and the pH at which it was measured. For pulps with high lignin content, conductometric titrations with NaOH and NaHC03 gave different results. But there were good correlations between acidic content from the NaOH titration and the fiber charge at pH 8.5, and between acidic content from the NaHC03 titration and fiber charge at pH 6. A report from India described an electrokinetic potential meter that measures charges of pulp suspensions at consistencies (0.2-2.0%) that are difficult to measure by conventional methods (43). The instrumentation relies on the streaming potential technique, which enables determination of charge on both fines and long fibers. Another report from the same place compared data from the instrument and a widely used laboratory meter instrument that works by microelectrophoresis (44). Fiber charge can also be determined by the concept of mass ionic demand. This concept was used to show the reaction between the specific hydrodynamic surface area of pulp fibers and the specific consumption of polyelectrolyte (poly-DADMAC) required to neutralize the accessible charge on the fibers (45). The relation is stronger at lower pulp consistencies.
c
(c)
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WOOD COMPONENTS Only components that are released during pulping or are
byproducts of the pulping processes will be reviewed here. They are divided into oleophilic or lipophilic components (extractives) and nonlipophilic components such as carbohydrates and sugars. Halogenated compounds resulting from bleaching practices will be covered under a separate heading.
Extractives. Lipophilic constituents in benzene/ethanol or acetone extractives of two Chinese softwood species were extracted with ether and further separated into neutral, weakly acid, and strongly acid fractions by ion-exchange chromatography (46). The fractions were further characterized by GC/MS and GC/IR/ MS. It is alarming though that the report used a solvent mixture that includes benzene, a solvent long banned in the western world because of its carcinogenic properties. Methods for the analysis of rosin, a byproduct of the kraft pulping process, abound. Proton NMR has been used to determine abietic acid, dehydroabietic acid, and total resin acids in rosin (47). The technique dispenses with the need for pure standards that are not easily obtained in some countries. GC has been used for the analysis of dehydroabietic acid in disproportionated rosin (48). The method entails converting the acid into its methyl ester derivative before analysis by capillary GC and can also be used for quantitative determination of the acid in rosin and other rosin derivatives. The sterols content of tall oil was determined by near-IR spectroscopy and multivariate calibration (49). Application of the multivariate calibration methods led to development of mathematical models for estimating the sterols content and some other properties of tall oil from near-IR spectra. Mills that pulp eucalyptus are often plagued with pitch deposition problems caused by precipitation of ellagic acid soaps of magnesium and sodium ions. Rapid identification of the acid in deposits by infrared spectrometry is feasible (50). SEC with tetrahydrofuran eluent can be used for quantitative analysis of the acid in the matrix, and further identification of the compound can be achieved by GC analysis of its trimethylsilyl derivative. HPLC can also be used to analyze for ellagic acid (51). A gravimetric procedure for quantitative estimation of pitch in whitewaters has been advocated by Japanese researchers (52). The method measures dispersed pitch by adsorption onto activated carbon fibers. It is claimed that the procedure can be used for removal of pitch on a commercial scale. Extractives released on steam drying of bark residues at a Finnish pulp mill were studied to evaluate their role in the formation of deposits in the dryer (53). The lipid components of bark, condensate, and deposit samples in a back-pressure steam dryer were analyzed by GUMS. Both the bark and condensate contained fatty acids and resin acids dominated by CISacids in the bark and dehydroabietic acid in the condensate. The tackiness of the acids was presumed to be a major factor in the formation of deposits in the dryer (the chemistry of the deposits was similar to that of the incoming bark). A review article was published on methods used to detect defects in manufactured paper (54). The review paid particular emphasis on techniques, systems used, and processing of the information obtained. A GC procedure for monitoring volatile compounds in food packaging was published (55). Samples are heated to enable desorption of volatiles in a dynamic headspace system connected to a GC equipped with a high-resolution capillary column and a flame ionization detector. Determination of aldehydes in cardboard was enabled by a multiple injection method. Automation of the sampling and detection steps results in highly reproducible data. The thermostability of paperboard food packaging was determined by heating the paper for 30-60 min at 150 "C and analyzing Analytical Chemistry, Vol. 67,No. 72,June 15, 7995
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the resultant volatiles by solvent extraction. These were subsequently trapped onto adsorption tubes followed by G U M S analysis (56). The adsorption procedure gave better results than the extraction methods. Another method for monitoring volatiles involved FT-IR measurement of emission of terpenes from chip piles at a Swedish pulp mill (57). The method uses a novel polymer lilm wind tunnel and measures a-pinene, /%pinene,and y-3-carene on a semicontinuous basis with a detection limit of 1 mg/m3 for each of the compounds. Total hydrocarbons were monitored simultaneously with a conventional flame ionization instrument. Emission of terpenes from a freshly cut chip pile decreased 4-fold within 30 min, and the fresh chips emitted 30 times more terpenes than older (12 h-3 weeks) chips. A recent Japanese patent discloses a method for measuring the efficiency of pulp washing by monitoring light transmittance of colloidal pitch in the filtrate (58). The method is claimed to be more reliable than a conventional conductometric one. Resin acids are toxins to fish at very low concentrations (1-2 mg/L). Consequently, the pulp and paper industry needs methods that can be used to monitor residual concentration of these compounds in mill effluents destined for discharge into receiving waters. A colorimetric method for resin acid determination was automated using flow injection (59). Three solvent extraction flow injection manifold designs were constructed to implement the colorimetric method (reaction of the acid with HzS04 and acetic anhydride to yield a transient pink coloration), and their attributes compared. The best method utilized a flow-through cell design which combines reaction($, extraction, and detection. This provided a sample throughput of 30/h, a detection limit of 20 mg/ L, a usable dynamic range of 20-1000 mg/L, and a relative standard deviation of 6.3%at 500 mg/L. Unfortunately, since the detection limit of the technique is much higher than the fish toxic values, it cannot be used for accurate monitoring of residual resin acids in mill effluents. Validation work was carried out to develop analytical protocols for fatty and resin acids in water, sediment, and fish bile samples g/g, range respectively (61). in the g/L, lo-’ g/g, and Application of the methods to samples from mills in northem Canada yielded satisfactory results. Similar studies by the same authors on chlorophenols and related compounds in water, sediment, fish muscle, and mill samples gave good results also (62). A rapid spectrophotometric procedure for the determination of total resin and fatty acids in pulp matrices was developed (63). The procedure was demonstrated for wood chips, whitewaters, and effluents and can be used by mill personnel since it does not require sophisticated instrumentation other than a simple spectrophotometer. Resin acids in wastewater from a paper mill in Australia were determined by gas and high-performance liquid chromatographic analyses of their 7-methoxycoumarin-4-yland 7-acetoxycoumarin4yl methyl esters (64). The results confirmed the presence of resin acids derived from the paper mill wastewater, and the major ones were dehydroabietic, palustric, abietic, and pimaric acids. The study concluded that dehydroabietic acid could be used as a tracer for organic matter derived from the paper mill. Extractives can sometimes cause impurities in pulp and paper products. Such impurities can be determined by pyrolysis gas chromatography/mass spectrometry with oncolumn methylation 90R
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of the extractives components (65). The technique was able to distinguish wood resin from other nonwood resin extractives such as rosin, forti!ied rosin, and akylketene dimers. Infrared microscopy can also be used to identify contaminants associated with raw stock supplies and various stages of pulping and papermaking operations (66). Such contaminants include insects, plastics, and process additives. Pyrolysis gas chromatography has been used for quantitative determination of synthetic polymer impurities in deposits (67). A specially developed procedure that enables determination of the amounts of the polymers in mill deposit samples was described. A method for quantitative determination of “harmful rosin” in sulfite pulp was reported by Russian authors (68). The method measures pitch on the fibers by agitating a pulp suspension (at pH 6.5, 20 “C), filtering the suspension, and determining the absorbance of the filtrate at 400 nm. It is claimed that the method is rapid and is suitable for determiniig pitch in bleached and unbleached sulfite pulps. A review on some of the modem analytical methods that can be used in the analysis of extractives from wood and pulp was prepared (69). The methods were classified into two broad categories, that is, sample isolation procedures and analytical procedures. A scheme for the identification and quantitation of the components in the acetone extractives in wood and bark samples was presented (70). It includes solvent extraction, fractionation of the extracts by ion-exchange chromatography, analysis by GC using short capillary columns, and use of a spreadsheet program to identify and quantitate the extractives components. The use of the short capillary columns (2 m) enabled elution of high boiling point lipid fractions and fatty acid methyl esters in a reasonable time frame (35 min). Solid phase extraction can be a fast, inexpensive, versatile, and accurate method for separating and quantifyingacetone extracts from different wood species and pulps (71). Extractives in papermaking process waters were determined by size exclusion chromatographyhandem mass spectrometry (SEC/MS/MS). The analysis includes detection of a lignan and its fragmentation while operating in the chemical ionization MS/ MS mode (72). Nonlipophilic Components. The degradation of sugars during acid hydrolysis and the effects of other aspects of sample preparation on measurement of wood sugars by anion-exchange HPLC using pulsed amperometric detection were investigated (73). Loss of sugars in standards ranged from 6.4%for arabinose to 15.7%for mannose. Measurements of wood samples to which standards were added before hydrolysis were very close to the sum of wood samples and standards hydrolyzed and analyzed separately except in the case of galactose, which was in sign%cantly lower amounts than in the combined samples. A rapid and sensitive method was developed for the determination of monosaccharides present in acid hydrolyzates of wood, pulp, Mg-based spent sulfite liquor, kraft weak black liquor, and newsprint paper machine whitewaters (74). The hydrolyzates were purified by solid phase extraction before analysis by highperformance anion exchange chromatography with pulsed amperometric detection. The results obtained were in good agreement with GC methods. Carbohydrates were measured in rayons and a dissolving pulp by using acid hydrolysis, anionexchange chromatographicsepara-
tion of the resulting monosaccharides, and pulsed amperometric detection of the separated monosaccharides (75). Analysis of the hydrolyzates without acid neutralization or monosaccharide derivatization proved effective in separating and analyzing low concentrations of D-xylose and D-mannose in the presence of D-glucose. This allowed close approximations of hemicellulose 0 in cellulosic products. A method for microquantitative determination of sugars in pulping black liquors was reported (76). The method uses aniline and phthalate to detect 0-5 mg/L sucrose and 0-2.5 mg/mL glucose and fructose with relative ease, and no interferences from the deep color of the liquors and foam-forming substances in the liquors. The monosaccharide composition of exopolysaccharides extracted from mixed slimes from process waters of paper machines was determined by gas chromatography (77). The results showed that the microbial slimes were heteropolysaccharides, each composed of more than one monosaccharide. Monosaccharides idenmed in deposit samples were glucose, mannose, galactose, rhamnose, fucose, and glucuronic acid. The results have implications for slime control strategies in pulp and paper mills. To reduce the amounts of chlorinated compounds in pulp and paper mill effluents, alternative bleaching agents are used or being developed. However, such bleaching agents should not lead to cellulose degradation. Bleaching with ozone was studied to analyze for cellulose and kraft pulp ozonolysis products by anionexchange chromatography with pulsed amperometric detection (78). The results showed that increasing amounts of ozone led to release of oligosaccharides, monosaccharides, and possibly oxidized forms of the compounds. Pulps made from kraft pulping of Pinus radiata chips were studied by 13C NMR (79). A “delayed contact” pulse sequence was used to separate 13C CP/MAS NMR spectra into subspectra of pulp components. The results showed that there was preferential loss of disordered cellulose in the early stages of pulping and cellulose damage was observed for pulp yields of
