Anal. Chem. 1995,67,2901-2905
Direct Quantitation of Alkylketene Dimers Using Time-of=FlightSecondary Ion Mass Spectrometry Paul A. Zimmerman and David M. Hercules**+ Department of Chemistly, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
William 0. L w s * Hercules Research Center, Hercules Incorporated, Wilmington, Delaware 19808
The use of time-of-flightsecondaryion mass spectrometry OF-SIMS) to directly quanti@ organic compounds on paper is reported. Alkylketene dimers (AKDs)are widely used as sizing agents in the paper industry, and measuring relative surface concentrations is important. TOFSIMS spectra of AKDs on paper are compared with those of standard compounds. A calibration curve was established using papers with known loadings; relative standard deviations of "blinds"were -8%. Accuracies were comparable. Over 65% of the North American fine paper market has converted from acid to alkaline papermaking conditions over the past 6 years. Cost savings associated with the use of precipitated calcium carbonate, improvements in paper permanence, and increased closure of the wet end contributed to this transition.' Alkaline fine paper performs very well in most downstream applications. However, problems have been experienced in precision converting applications, such as forms bond and copy paper. Converting problems sometimes encountered include reduced operating speed, double feeds or jams in high-speed copiers, paper welding, and registration errors on envelope folding and high-speed printing equipment. The causes of alkaline paper converting and end-use problems have been grouped into three general classes, one of which is related to the sizing agent2 Sizing agents are used in the paper industry to slow penetration of water into the sheet. There are two types of sizing agents used in alkaline paper making systems: alkenylsuccinic anhydride (ASA)and alkylketene dimer (AKD). Converting problems have been encountered with both of these sizing agents. It is unclear how the sizing agent contributes to converting problems. One contributingfactor may be the distribution of the sizing agent and the hydrolysate through the sheet. Qualitative size test methods exist to measure the overall amount of sizing agent in paper and distribution on the ~ u r f a c e . ~However, -~ there is no method that gives direct quantitation of AKDs on paper Current address: Department of Chemistry, Vanderbilt University, Nashville, TN 37235. * Current address: AMP Inc., Materials Engineering & Research, WinstonSalem, NC 27102. (1) Walkden, S. A. Proceedings from the Tappi Neutral/Alkaline Papermaking Short Course; Tappi Press: Atlanta, 1990; pp 67-72. (2) Brungardt, C. L.; Gast, J. C. Proceedings from the Tappi Papermakers Conference;Tappi Press: Atlanta, 1994; pp 155-163. (3) Evans, B. In The Sizing ojPaper, 2nd ed.; Reynolds, W. F., Ed.; Tappi Press: Atlanta, 1989 Chapter 2. (4) Takeyama, S.; Gray, Cellul. Chem. Technol. 1982,16, 133. +
0003-2700/95/0367-2901$9.00/0 0 1995 American Chemical Society
H
Figure 1. Structure of alkylketene dimers (AKDs).
surfaces. Most spectroscopic techniques are of little use, primarily because of their inability to detect the low concentrations of AKDs used on paper (ca. tenths of a percent). One timeof-flight secondary ion mass spectrometry ("OF-SIMS) spectrum of AKD deposited on paper from toluene has been reported: along with the use of imaging to study the distribution of AKD on paper. The present study demonstrates that static TOF-SIMS can be used to directly quantify AKDs on paper after establishing a calibration curve. Furthermore, the stability of the samples and instrumentation allow the same samples to be analyzed on different days, weeks, or months with essentially identical results. EXPERIMENTAL SECTION
A. Sample Preparation. Standard and unknown paper samples containing AKD were supplied by Hercules Inc. (Wilmington, DE). Paper preparations were made on a 12 in. Fourdrinier pilot paper machine. Sheets were formulated using 70/30 hardwood/softwood pulp furnish, 0.5%Stalok-400 (cationic starch), 0.025% Reten 1523H retention aid (30 mol % sodium acrylate/polyacrylamide), and 12%precipitated calcium carbonate. Hercon 70 paper-sizing agent (AKD)was applied inline to the stock system (internal size) at levels of 0% (control) and over a range of 0.025% (0.23 kg/ton)-0.20% (1.82 kg/ton). One press at 45 PLI was used. The paper was dried over seven machine driers at varying temperatures in the range 57-76 "C. B. Instrumentation. The instrument used to obtain TOFSIMS spectra was an Ion-TOF time-of-flight selected ion mass spectrometer (Miinster, Germany), which has been described in great detail el~ewhere.~ During analysis, the paper samples were bombarded with 10 keV argon ions with an average target current of 0.4 PA for 200 s. The primary ion dose was -4.0 x lo8 argon ions, which corresponds to a static SIMS measurement. Several areas of 100 pm2 were analyzed from each sample. The spectra (5) Brinen, J. S.; Calbick, J. C.; Cody, R D. Surface Interface Anal. 1989,14, 245. (6) Brinen, J. S. Nord. Pulp Pap. Res. J. 1993,I , 123. (7)Niehuis, E.; Heller, T.; Feld, H.; Benninghoven, A. J. Vac. Sci. Technol. 1987, 5, 1243-1246.
Analytical Chemistry, Vol. 67,No. 17,September 1, 1995 2901
2
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Figure 2. Spectra of different concentrations of AKD on paper.
were accumulated in the ioncounting mode with a time resolution of 1.25 ns and a total time range of 160ps. Charge compensation was accomplished by a 10 eV electron beam with a current of 0.1 p A and a beam diameter of 1.5 mm. The charge compensation was pulsed out-of-phase with the 2 kV extraction field and the primary ion source at a repetition rate of 5 kHz. A -100 V grid was used to repel the excess electrons away from the extraction lens.
cellulose to form a covalent bond*or with water to form a ,&keto acid, which decomposes into a ketone 0 and carbon dioxidegas shown in eq 1.
RESULTS AND DISCUSSION
Figure 2 shows positive TOF-SIMSspectra for the region from 430 to 520 Da for three concentrations of AKD on paper. There are three main clusters in each spectrum, which corresponds to three species of AKD. The first AKD cluster (-450 Da) is from the condensation of two palmitic acid molecules, the second (-480 Da) from condensation of one palmitic and one stearic acid, and the third (-508 Da) from condensation of two stearic acid molecules. It should be noted that two species are represented in each AKD cluster. The f i s t peak in each cluster (Le., 449.5,
Figure 1 shows the structure of AKD. In this study, three AKDs are present on paper, where nl and n2 have values corresponding to 13 (palmitic acid) or 15 (stearic acid). Three combinations are possible: two palmitic acid molecules combining to form an AKD with a molecular mass of 476 Da; one palmitic and one stearic acid, for a molecular mass of 504 Da; or two stearic acid molecules, for a molecular mass of 532 Da. The distribution arises because commercial grade stearic acid was used as the starting material for the synthesis of AKD, and it contains a significant quantity of palmitic acid. Alkylketene dimers contain a four-membered lactone ring that has been shown to react with 2902 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
Hz0
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(8)Bottorff, K. J.; Sullivan, M. J. Nord. Pulp Pap. Res. J. 1993,I , 86 (9) Kamutzki, W.; Krause, T.Papier (Darmstadt) 1982,36, 311.
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490
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Figure 3. Spectrum of C36 AKD deposited on a paper surface.
+
477.5, and 505.5 Da) corresponds to [M H - COP (where M is the AKD molecule), which arises from protonation of the B-lactone, causing ring opening and subsequent loss of CO as shown in eq 2. Species I1 is probably that observed in the RCH=C-CHR'
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spectrum, as suggested by McGuire and Lynch.'O The third peak in the clusters (i.e., 451.5, 479.5, and 507.5 Da) corresponds to protonated ketones 0 . To interpret the spectra of Figure 2, TOF-SIMS spectra were obtained for an AKD derived from a single acid and its corresponding ketone.'O The spectrum of the C36 AKD (nl = n2 = 15 in Figure 1) is shown in Figure 3. It consists of two major clusters of peaks at 533.5 and 505.5 Da. The peak at 533.5 Da corresponds to the (M + H)+ peak of AKD and the 505.5 Da peak to (M H - 28)+. The latter was shown to correspond to eq 2 (loss of CO) by exact mass measurements (505.533 Da calcd vs 505.532 Da obsd). Figure 4 shows the TOF-SIMS spectrum of the ketone (11)derived from the C36 AKD. The main cluster is at 507.5 Da, corresponding to the (M H)+ ion of the ketone. Interpretation of the spectra in Figure 2 is straightforward based on the above spectra. Using the cluster at 480 Da as an
+
+
(10)McGuire, S . M.; Lynch, C. C. Proceedings of the 40th ASMS Conference on Mass Spectrometry and Allied Topics, Washington, DC, May 1592.
example, the main peak at 479.5 Da corresponds to the ketone derived from the AKD having nl = 13 and n2 = 15. The peaks at 480.5 and 481.5 Da are primarily the I3C isotope peaks from the ketone. The peak at 477.5 Da is from the (M H - CO)+ peak of the corresponding AKD. This could be either a fragment ion of the AKD (which is interfered with by the fragment ion in the next-higher cluster) or a fragment from the bound form of AKD. This latter interpretation seems to be reasonable, because we did not observe any cluster in the vicinity of 533.5 Da on any of the paper samples. The TOF-SIMS spectrum of AKD in Figure 3 is very similar to the methane CI spectrum of the C32 AKD acquired using a particle beam interface.'O For example, in the C36 spectrum, the peaks at 533.5 and 505.5 Da are exactly equal in intensity, as are the corresponding peaks at 449.5 and 477.5 Da in the C32 spectrum. Both show significant peaks due to loss of hydrogen. TOF-SIMS spectra of AKD reported earlier differ somewhat from those of the present study.6 The (M H)+ peak for C36 AKD was observed at 533.5 Da, along with peaks at 505.5 and 477.5 Da. The latter were attributed to subsequent losses of CH3CH2 and CH2CH2, respectively. This interpretation is at odds with the spectrum of Figure 3. The purity of the AKD sample in the earlier work is not known, and the peaks reported at 477.5 Da are probably due to the presence of palmitic acid in the sample. Also, Figure 3 shows that the loss of 28 Da is due to CO (exact mass, 505.533 Da), not CHzCHz (exact mass, 505.497 Da). A possible complication is that the earlier work used a different primary ion beam ("Ga) and primary beam energy (25 kv); the effect of these factors on the AKD spectrum is not known.
+
+
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
2903
10
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5000
Table 1. Time Dependence of Calibration Standards
intensity (counts/channel)
h
4000 C
a
concn (%)
June
September
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4360 f 170 2180 f 130 1150 f 40
4420 f 200 2380 f 180
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3
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Concentration of AKD on paper(percen1weight) Figure 5. Calibration curve for paper containing 0.05%-0.20% AKD
(m, counts). It is clear from the spectra in Figure 2 that the absolute intensities are proportional to the bulk concentration of AKD in the paper. The negative TOE-SIMS spectra of AKD on paper can also be obtained; however, the ion yield is lower than that for the positive TOF-SIMS. Figure 5 shows the calibration curve for AKD in paper in the range of 0.025%-0.20?6AKD. The calibration curve was generated by integrating the cluster from 477 to 481 Da. Both species in the cluster are unambiguously assigned to one palmitic and one stearic acid forming an AKD. The error bars show a standard deviation of less than &8%and a slope of 21 760 f 0.4%. Even over long periods of time, the variation in absolute intensities was no more than &15%,with the relative intensities varying less than f 7 % . As one example, Table 1 shows the absolute count levels obtained for AKD analysis on two different dates, 3 months apart. 2904 Analytical Chemistry, Vol. 67, No. 77, September 7 , 7995
This calibration curve is important because it represents the first time that direct quantitation has been accomplished on an insulating surface using TOF-SIMS. Furthermore, the quantitation required no internal standard; only an external standard (paper containing 0.20%AKD) was used to fine-tune the instrument for AKD analysis. Not only are these the first results for TOF-SIMS quantitation of insulating surfaces, but also, an additional advantage of this analysis is that no sample pretreatment (workup) is required. The actual time for sample introduction, analysis, and data interpretation was -20-30 min. It seems possible that industrial samples can be quickly analyzed during the manufacturing process. To test the AKD method, four blind unknowns within the same range as the standards were analyzed. The unknowns were prepared by the same method as was used for the standards. One external standard (0.20%)was analyzed with the unknowns. The results are listed in Table 2. The results agree quite well for papers containing 0.20%,Oslo%,and 0.075% AKD, while giving slightly lower than expected values for paper containing 0.05% AKD. Standard deviations were obtained from six measurements on three different spots for three samples of each concentration (i.e., w = 54).
Table 2. Expected and ExperimentalConcentration of Unknowns Values (YO)
expected
observed
0.20
0.20 f 8% 0.10 f 4% 0.073 8% 0.040 f 7%
0.10 0.075 0.050
*
CONCLUSIONS
Previous analysis of AKD depended on identifying a change in the C-H portion of the XPS Cls line; however, this provided no direct evidence for the AKD molecule. TOF-SIMS provides direct evidence of the AKD molecules and their related fragments and is able to resolve AKD peaks from the corresponding ketone. TOF-SIMS has been shown to be a valuable analytical tool for
the analysis and direct quantification of AKDs on paper. This study further shows that TOF-SIMS can be of value for analysis with no pretreatment of insulating surfaces containing low concentrations of organics such as those discussed in this work. ACKNOWLEDGMENT
This work was supported by NSF Grants CHE 9022135 and INT-9244276. The authors would like to thank Hercules Inc. for the standard and unknown samples. P.A.Z. would like to thank DuPont for the fellowship which helped him complete this work. Received for review June 1, 1995. Accepted June 13, 1995.a AC950575S @Abstractpublished in Advance ACS Abstracts, July 15, 1995
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