Rapid Secondary Analysis OffersGreat Potential for Improving Product Quality

AbstractThe rapid secondary analysis of wood products, such us

measuring the lignin content in pulp, the resin level in a woodcomposite or the amount of preservative in treated poles, is amuch more rapid process than if each sample were analyzed bythe traditional (primary) method. Consequently, a more uni-form product is manufactured.

The process (or processes, because there are several some-what similar ones) is hard for the lay person to understand, so ananalogy may be drawn: If you saw evidence of several gun shotsin one area, you would be justified in assuming that the targetwas somewhere in that area Scientists using some type of sec-ondary analysis can chemically test a number of samples andfind the range of for example, the adhesive level of a compositeboard, and calibrate the machine doing the testing accordinglyThey can then test samples on a production line to ensure thatthey fall within the set range. That is, they can employ rapid sec-ondary analysis using the electromagnetic spectrum to findwithin seconds whether each sample shows the same pattern thathas been established. Any sample showing a different pattern issuspect and can be analyzed by the slower, more traditionalmethods to find out why it is far from the norm. Thus a moreuniform product is manufactured, and processing costs aresometimes lowered.

The process is often called secondary analysis becausesamples are first analyzed by the primary (traditional chemical;method. Various methods such us near infrared reflectanceanalysis (NIRA), Fourier transform infrared spectroscopy(FTIR), and near infrared Raman spectroscopy (NIR-Raman)

Mississippi State University Forest Products Laboratory, Box 9820, Mississippi State, MS 39762

are being used. Non-scientists may not readily understand theseterms, but they are simply ways of using the electromagnetic spec-trum to swiftly check samples to ensure that they fall within the setlimits calibrated for that specific sample.

little “black box” and a greatdeal of “know-how” is helpingmake rapid secondary analysis ofmany wood products feasible. Ithas rnade the use of near-infraredreflectance analysis, referred to asNIRA, a complement to the moretraditional and definitive, butslower, analysis. The black boxtakes readings from the near-infrared portion of the electro-magnetic spectrum and helpsresearchers rapidly analyze sam-ples of pulp or wood in otherforms. Some sophisticated mathe-matics are necessary to programthe black box so that numbersemerging from it are significant.If numbers are off, it’s an indica-tion that something is wrongwith either the sample or themethod.

The operative word in theseanalyses is “rapid.” In fact, says Dr.

Tor Schultz, of the MississippiForest Products Laboratory(MFPL), the system can be used forat-line or on-line analysis. At-lineanalysis represents taking a sam-ple from the processing line to bechecked. On-line analysis is donecontinuously while the product isbeing made.

Rapid secondary analyses arealready widely used in agriculture.For example, every overseas grainshipment is now analyzed -- inseconds -- by NIRA for proteinand moisture content. Rapid sec-ondary methods are sometimesused in wood industries, but notas often. One reason, Dr. Schultzexplains, is that agricultural prod-ucts usually have a low level ofmoisture. In forest products, how-ever, the moisture content canvary widely in, for example, pulpsamples, and this causes errors inmeasuring other components.

This makes the analysis morecomplicated.

Dr. Philip Oldham, a chemistryprofessor at Mississippi StateUniversity, works closely with Dr.Schultz. He says, “NIRA does notgive as much chemical informationas infrared on the molecular levelbut can still provide acceptablequantitative results in most cases.

“The computer will give onlythe information asked for: it doesnot inform you about other com-ponents in the sample. Differentcomponents of the sample inter-act with different light frequen-cies in the electromagnetic spec-trum. By setting the parametersby which the sample is to be test-ed wide enough, you can often getreproducible analyses of the mate-rial being manufactured.”

Some possibilitiesDr. Schultz sees several possi-

ble areas for indirect analysis:(1) Analysis of pulp, both

before and after bleaching, forlignin content (researchers call itthe kappa number) and possiblyother factors such as physical

‘Professor Tor Schultz of the Forest Products Laboratory (FPL) prepares to examine spectral data (left photo). Associate'Professor Terrance E. Conners, FPL, prepares a presentation on his work (right photo).

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strength properties. (Lignin is thenatural “glue” that holds woodproducts together.)

(2) Determination of level ofadditives, such as resin and sizing,during the manufacture of vari-ous wood composites like orient-ed strand board (OSB), and flake-board

(3) Continuous quality con-trol of wood adhesive resins dur-ing manufacture for such thingsas molecular weight, viscosity,moisture content and freeformaldehyde.

(4) Quality control of woodpreservatives for such things ascontaminants, active ingredientsand moisture content.

(5) Monitoring chips beforepulping for such factors as storagelife and extractive content,

(6) Analysis of wood treatedwith wood preservatives or fireretardants for the retention level.

Indirect or rapid secondaryanalysis has the advantage ofbeing much faster than tradition-al methods but it does have somelimitations. Dr. Schultz reports,“The technician needs to recali-brate the instrument each timedifferent samples are analyzed,and this takes time. Thus, indirectanalysis is useful only if a largenumber of very similar samplesare run every week. Also, it is hardto transfer the procedure toanother laboratory.” Some forestproducts companies are alreadyusing rapid secondary analysis in aproduction setting.

A manufacturer’s practiceOne manufacturer of a wood

composite product is using NIR tomeasure at-line the level of anadditive. NIR is well-suited in thiscase since the manufactured prod-uct is hot-pressed and thus has auniform and very low moisturecontent, and the additive can easi-ly be measured to determine thelevel used.

In another example, a com-mercial NIR system has beendeveloped for the pulp and paperindustry, but no results are avail-able. In cases where the pulp

ter to use a moisture-insensitivemoisture can vary it may be bet-

instrument such as Raman. Priorto purchasing any system a man-ufacturing facility will need tocarefully consider its particularproduct, needs and limitations topick which instrument(s) may bebest, and then to run some stud-ies using that same instrumentwith some actual samples toensure that the system will work.

The second subset verifies or con-secondary analysis instrument.

firms results from the first,“This is very important because

of the ease of making a slighterror in calibration,” says Oldham."A calibration error often domi-nates the overall secondary analy-sis error and thus accuracy is ulti-mately dependent on the stan-dard analysis technique. Carefulanalysis of the standards by theprimary method is essential.”Schultz and Oldham say the

initial procedure is to obtain arepresentative set of samples froma diverse group. The samplesshould be analyzed using a tradi-tional chemical method, thendivided into two subsets. Thefirst subset is used to calibrate the

Consider other factorsHe goes on to say, “No manipu-

lation of the independent vari-ables such as wavelength, sensitiv-ity or sample preparation canimprove the secondary analysis

beyond the error in the originalcalibration.

“Another point is that thesamples analyzed by the sec-ondary technique must belong tothe same ‘universe’ as the calibra-tion set -- that is, must be similarin form and composition. Itoften occurs that the operatordoes not identify or recognizesamples whose composition orprocessing history lies outside thetraining set ‘universe’.”

Samples are critical!The reason calibration samples

are critical for indirect analysis isthat spectral differences in sec-ondary analysis are often veryminor, These minor spectralchanges are subjected to variousdata-analysis methods to detectsubtle differences which, in turn,predict the component levels. Ifreal samples are different from thecalibration samples in any appre-ciable way, such as in moisturecontent, the relatively small dif-ferences reflected in the spectrumare overwhelmed by large andextraneous changes. In otherwords, the analytical difference is“covered” by the differencebetween calibration samples and“real” samples. “Never trust,” saySchultz and Oldham, “always veri-fy.”

Surprisingly, no special inter-pretation is necessary with indi-rect analysis. When the calibra-tion has been set, the computerinterprets the data using special-ized software such as multiple lin-ear regression (MLR) or principalcomponent analysis (PCA). Instepwise MLR, the regression pro-cedure selects peaks difficult tojustify based on conventionalinterpretations but which givethe best statistical fit.

Plot of Kappa number vs. aver-age area for softwood and hard-

wood samples using NIR-Ramanspectroscopy.

Researchers point out that thetotal error of rapid secondaryanalysis or NIRA is the total errorof the primary error plus the addi-tional error of the secondaryinstrument and the laboratoryprocedures. “Thus,” they say, “thetotal error of indirect analysis willalways be greater than that of theprimary or traditional procedureused to calibrate the secondaryinstrument.”

Despite limitations, secondaryanalysis has potential for certain uses.

One advantage of NIRA overFTIR is that the near-infrared spec-trum has combination and over-tone bands of much lower intensi-ty than the fundamental infraredbands. Thus, no sample dilution isnecessary (one of the reasons sam-ples can be analyzed on-line).

Another problem is that thenear-infrared, like the infrared,spectrum of natural products isessentially a composite of the indi-vidual components. So, the

lower level for which a con-stituent part of the sample can beaccurately analyzed is limited fornatural and other complex prod-ucts, unless the constituent beinganalyzed has a unique spectralband -- one easily recognizable.

NIRA is the most-oftenusedmethod of secondary analysis. Butother methods are available, such asthe FTIR mentioned, ultravioletreflectance (UV) spectroscopy, mul-tidimensional fluorescence (MDF).and Raman spectroscopy. These lastthree are relatively insensitive tomoisture content, and thus helpfulin certain applications.

Dr. Terry Conners of the MFPLhas been using the NIR-Ramanmethod for determining lignincontent of pulp samples. He findsit very advantageous for its rapidi-ty, but prepares samples and cali-brates the instrument very care-fully. That, more than anything,makes it possible for secondaryanalysis to work.

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