(4) S. C. Jain and K. S. Krishnan, froc. R. SOC.(London),Ser. A,, 227, 141 (1955). ( 5 ) S. Dipierro and G. Tessari, Talanta, 18, 707-16 (1971). (6) M. P. Bratzel, Jr., and C. L. Chakrabarti, Anal. Chirn. Acta, 63, 1-10 (1973). (7) J. T. Vanderslice, S. Weissman, E. A. Mason, and R. J. Fallon, Phys. Fluids, 5, 155 (1962). (8) J. 0.Hirschfelder and M. D. Eliason, Ann. N.Y. Acad. Sci., 87, 451 (1957). (9) J. 0.Hirschfelder, “Molecular Theory of Gases and Liquids,” John Wiley & Sons, New York, NY, 1964, pp 952-3.
CORRECTION lime-Resolved Distrlbution of Atoms In Flameless Spectrometry: A Theoretical Calculation
In the paper by S. L. Paveri-Fontana, G. Tessari, and G. Torsi [Anal. Chem., 46, 1032 (1974)], there are the following misprints. In Formula 29, the factor appearing on page 1035 should read [l (4Dt/2dl) ( - . In the numerical computation which employed Equation 29, the correct version was actually used. In Formula A-5, dx dy should be replaced by dx dz.
+
RECEIVED for review August 5, 1974. Accepted December 19, 1974. Work done under contract No. 73/1123 of Consiglio Nazionale delle Ricerche (C.N.R.)-Roma.
a)].
Quick Laboratory Method for the Determination of Water Contents of Highly Viscous Liquors by Near Infrared Spectrometry Ernest Spinner Erling Riis Research Laboratory, lnternational Paper Company, Mobile, AL 3660 1
While evaluating different common methods of determining solids contents of varlous black liquor samples with respect to precision and accuracy, we developed an additional method to quickly monitor water contents of kraft black IIquor, NSSC brown liquor, wood molasses, and pulp. The near infrared spectrophotometric technique is slightly less precise than more conventional techniques. However, advantages include short analysis time and small sample size requirements. An analysis of black liquor, for example, can be performed in 15 minutes with a 0.5-gram sample. With areotropic distillation, the analysis would require about two hours with a 10-gram minimum sample; with oven drying (e.g., TAPPl standard), a minimum of six hours is requlred with about a 1-gram sample.
Laboratory methods of determining water contents of materials in the pulp and paper industry include heat drying (1-5), various distillation procedures (6-8), and Karl Fischer techniques (9, IO). Heat drying procedures are very good for determining the solids content of a given sample under certain well defined conditions of temperature, pressure, humidity, and atmosphere. Unfortunately they are often used to determine the water contents of various samples, by subtracting the solids content from 100, not amenable to this technique. For example, any of the following errors could be introduced into the analysis using heat drying: non-aqueous volatiles will be included in the analysis as “water.” water of hydration, which is usually not volatilized, will be determined as solids-not water. non-aqueous, non-volatiles that are oxidized during heating in an air atmosphere will gain weight. This will have the effect of reducing the determined water content of the sample.
If none of the above disadvantages apply to the particular sample under investigation, the procedure is quite useful because of its simplicity and precision. Distillation methods are also simple. One advantage distillation procedures have over the heat drying methods is t h a t water of hydration will not be disregarded, but will register as water. The main disadvantage of distillation techniques is that any water-soluble substance which forms an azeotrope with water will be determined as water. Also, organic acids are sometimes formed during distillation which were not prese n t in the mother liquid. These acids might also be counted as water here. Because the water-receiving vessels usually used with the various distillation procedures are relatively wide, small differences in water contents between samples cannot be easily verified. Precision, therefore, is inferior to that obtained with heat drying. Karl Fischer titration methods using the “two-solution reagent” have the advantages of being rapid and precise. T h e main disadvantage is ‘that strongly alkaline reagents, active aldehydes, and solutions containing unsaturated compounds react with the Karl Fischer reagents. This drawback can be eliminated with the “two-solution reagent” by a judicious choice of blanks from which correction factors can be generated. However, this reduces its attractiveness as a general purpose procedure for water determinations since each type of sample would require its own set of blanks. Methods used to determine the water contents of substances in other fields are numerous. Measurement of the amount of acetylene generated from the reaction of calcium carbide and water has been used for a long time ( 1 1 ) . Similar studies have been done monitoring the acetic acid generated from acetyl chloride (12), benzoic acid from benzoic anhydride ( 1 3 ) . However, most techniques used in other fields, which are not presently being used in the pulp and ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
849
STOPCOCK
0
l 115
I51
I10
I82
1
e6
2 (17
w i t h additional
2 20
”rnlll c r o n s j
Figure 1. Calibration curves for H20 in 100-ml DMSO
’ Absorbance paper industry, are not used because they are usually subject to interferences from labile hydrogen, such as found on organic acids, alcohols, etc. There are labile hydrogens found in most products in the pulp and paper industry. Contrary to all of the above-mentioned techniques, our near infrared method measures only water, free or bound, with no interferences yet found. The 1.93-pm absorption of water has been used to determine the amount of water in glycerol ( 1 4 ) , methanol ( 1 5 ) , several hydrazines ( 1 6 ) , dried vegetables ( I 7), starch hydrolyzates ( 1 8 ) ,and other food products (19). Other applications of the near infrared absorptions of water have been the study of the structure of water in various organic solvents in an attempt t o explain the often anomalous behavior of water (20). The effects of temperature, water concentration (20), and dissolved salts (21) on the near infrared band shape have also been investigated. The use of the near infrared region of the spectrum has not been used as an analytical tool in the pulp and paper industry. Although we have found the 1.77-pm absorption of water of as much utility as the 1.93-pm absorption, there is no evidence of prior use of the 1.77-pm band. Use of this absorption complements the use of the 1.93-pm band in t h a t a much wider range of water concentrations can be analyzed without added dilution of the unknown sample. Use of the 1.77-pm band extends the linear range of the calibration curve from 1.5 ml H20/100 ml DMSO to a t least 4.0 ml H20/100 ml DMSO. Use of this band also adds to the general utility of the technique since it could be used, under most circumstances, instead of the 1.93-pm band if there were any interference a t 1.93 pm. For t h a t matter, the same could probably be said for the 1.45-pm and 1.55-pm absorption bands of water (see Figure l ) ,although we have not worked up the data for these absorptions.
EXPERIMENTAL Near Infrared Studies. A Beckman DK-2 spectrophotometer was used for this study. Calibration Curves. Obtain calibration curves by adding from 0.15 to 4.00 ml of distilled water from a 0- to 5-ml buret to 100 ml of dry DMSO. The spectrum of some of these water-in-DMSO solutions in l-cm cells is shown in Figure l. Black Liquor. For samples of less than 65% solids, transfer about 0.5 gram of sample to a tared 200-ml Erlenmeyer flask. Pump DMSO (Figure 2) into a 100-ml volumetric flask. Quickly pour the 100-ml solution into the Erlenmeyer flask, cap, and shake, for about 5 minutes. Quickly pour about 5 ml of this solution into a 10-ml centrifuge tube. Cap the tube and centrifuge until clear (about 10 minutes). Pour the supernatant into a 1-cm cell. 850
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
Figure 2. DMSO transfer apparatus
Zero the spectrophotometer at 1.70 pm and read the absorption a t 1.93 pm. For black liquor samples greater than 65% solids, first mix the liquor with an equal weight of distilled water, and shake until thoroughly dispersed. Proceed as outlined above for samples containing less than 65% solids. The amount of water added is subtracted from the total amount detected to give the original amount of water present. Brown Liquor. It is not necessary to dilute any of the brown liquor samples before extracting with DMSO. Except for there being much less DMSO-insoluble solids in brown liquor which results in less centrifuging time needed to clarify the DMSO, brown liquor is handled the same as black liquor. Zero the DK-2 at 1.70 pm and use the absorption band at either 1.77 or 1.93 pm. We used 1.77 pm for these samples. Molasses. There are no DMSO insoluble solids in molasses, so it is not necessary to centrifuge. We used the 1.93-pm band for these samples. Pulp. Tear the pulp into approximately half-inch square pieces and extract with DMSO. No centrifuging is necessary.
RESULTS Table I compares black liquor results using various gravimetric techniques with azeotropic distillation and near infrared. The standard TAPPI gravimetric method is the most precise. Drying the samples on filter paper is less precise and always gives low percent solids values (Le., too high a moisture content), assuming the TAPPI standard is giving the “correct” answer. Azeotropic distillation and the near infrared method are about equally precise with black liquor samples less than 65% solids. Precision of the near infrared method is further lowered with liquors greater than 65% solids. Table I1 (brown liquor results) shows trends similar to those of Table I. Table I11 shows results with molasses and pulp. Here comparison was made with the recognized standard procedure (Association of Official Analytical Chemists, Method 31007) which is essentially a gravimetric technique. Although we have only single determinations for the AOAC method, the near infrared technique is probably less precise. Accuracy, as has been true of all other types of samples, is about the same when comparing the near infrared with the accepted standard method.
DISCUSSION For our needs in this study, the solvent had to meet two requirements: it must not absorb in the area of water absorption (1.93 and 1.77 pm), and water should be readily
Table I. Percent Solids, Black Liquola
Day sampled
To Oxidizer Filter paper 4-hr drying 24-hr drying TAPPI Azeotropic distillation Near infrared
36
2b
l b
43.0 42.8 43.8 44.7 44.7
k1.0 10.2 10.2 11.0 11.0
44.5 44.1 44.7 45.1 45.3
47.0 47.0 47.8 50.3 47.6
k0.4 f0.4 10.2 13.6 i2.2
48.2 48.0 48.8 48.7 49.6
Grand average
'
Day -to-day
Methods
Least
variation
variation
significant
significant
significant
difference
a t 95 ?4
a t 95 %
confidence
confidence
at 95 9; confidence e
0.2 0.8 f 0.2 1 1.0 1 1.4
43.7 f0.2 43.2 10.6 44.1 * O . O 44.2 1 1.6 43.6 11.6
43.7 f 1.4 43.4 1 1.2 44.2 f 0.8 44.6 1 1.4 44.5 i 2.0
Yes
Yes
0.6
0.4 0.0 1 0.2 1.2 f 1.6
47.7 47.9 49.1 49.1 51.2
47.6 47.6 48.6 49.4 49.5
Yes
Yes
0.8
f
i
From Oxidizer Filter paper 4-hr drying 24-hr drying TAPPI Azeotropic distillation Near infrared
i
f
10.2 10.2 fO.2 12.0 10.6
1.0 1.O i 1.2 12.6 1 3.4 1
To Burner Filter paper 65.3 10.2 4-hr drying 64.1 1 0.8 65.9 f 0 . 6 65.1 1 1.6 64.1 1 0 . 8 64.3 1 1.2 63.9 1 0.6 65.0 i 0 . 6 24-hr drying 65.5 k0.2 66.8 10.0 64.9 10.2 TAPPI Yes Yes 0.9 65.7 1 1.8 66.5 1 2 . 0 66.9 *1.4 Azeotropic distillation 67.3 10.8 65.4 i 0.6 64.3 1 4.2 64.9 12.0 62.6 f 4.2 65.5 i 4 . 6 Near infrared Percent solids values were determined by subtracting percent water from 100. All daily values are averages of three determinations f 2 standard deviations. Average of nine determinations f 2 ftandard deviations. Determined by a nested factorial analysis of variance. e Determined from a one-way analysis of variance: LSD = tdf e r r o r d / ( M S error X 2 ) / nwhere n is the number of observations in the average (in this case 9). Q
T a b l e 11.Percent Solids, B r o w n L i q u o F
Day sampled
l b
26
3b
Grand average
Day-to-day
Methods
Least
variation
variation
Significant
significant
significant
difference
a t 95 ?4
a t 95 ?4
a t 95 %
confidence
confidenced confidence e
Filter paper 4-hr drying 51.6 k0.4 54.1 1 1.2 51.0 i 0.8 52.2 -t 2.8 24-hr drying 51.8 10.4 54.6 i 0.2 51.0 10.0 52.5 1 3.2 51.7 1 0.2 53.1 f 3.2 Yes Yesf 1.6 TAPPI 52.4 10.2 55.1 1 0.2 Azeotropic distillation 52.7 11.6 56.9 1 2.8 55.5 1 6.0 55.0 f 5.0 Near infrared 53.1 1.4.2 58.5 f 2.4 57.4 4.2 56.3 1 5.8 a Percent solids values were determined by subtracting percent water from 100. Averages of three determinations f 2 standard deviations. Average of nine determinations f 2 standard deviations. d Determined by a two-factor nested design analysis of variance. e Determined from a one-way analysis of variance: LSD = tdferror \/(MS error X 2 ) / n where n is the number of observances in the average (in this case 9).f Significant at 99% confidence level.
T a b l e 111. Percent Solids, Molasses a n d Pulpa Sample
Molasses
Pulp
Day sampled 1 2 3 Oven drying, 4-hr minimum 92 .4b Near infrared' 59.3 1 4.0 61.9 1 2.2 49.6 1 4.0 92.4b AOAC~ 57.8 60.8 49.6 Percent solids were determined by subtracting percent water from 100. Single determination. ' Averages of three determinations f 2 standard deviations. Association of Official Analytical Chemists, Method 31.007. Done by Law & Co., Atlanta, Ga. This is the recognized standard method of determining percent solids in molasses. soluble in it. Dimethylsulfoxide (DMSO) satisfies both of these requirements; it does not absorb energy in t h e region of interest and is extremely hygroscopic.
As shown in Figure 1, four distinct bands are due t o water in the region studied. Three of the bands are of about equal intensity (i.e., extinction coefficients are similar) whereas the fourth band has a n extinction coefficient much greater than those of t h e others. Since the 1.77, 1.55, and 1.45-pm bands have similar extinction coefficients, there would be little, if any, advantage in studying all three. We arbitrarily chose t o study the 1.77-pm and 1.93-pm bands. Fortunately, t h e extinction coefficients of these two bands are very dissimilar; a wide range of water concentrations can be studied with a single determination. Multiple adjustments of the unknown t o DMSO ratio are not needed. If the unknown has enough water t o send t h e 1.95-pm band off the chart, the 1.77-pm band could still be used. DMSO is extremely hygroscopic. This does not affect the 1.77-wm calibration curve because this band is not hypersensitive t o small amounts of water. However, this is not ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
851
the case with the 1.93-wm band. Here, because of its relatively large extinction coefficient, trace amounts of water can Le noticed. Regardless of how quickly we manipulated our extractions, etc., the sample always picked up some water from the atmosphere. We could have forced the curve through zero by keeping the reference beam DMSO exposed to the atmosphere for an equal length of time, but did not think it worthwhile. Our main precaution in keeping the DMSO dry was the absorption tube arrangement shown in Figure 2. This arrangement prevented the DMSO source (Le., the gallon container) from picking up additional water with each sampling as would have happened without the absorption tubes. Although we did not do any azeotropic distillation of the wood molasses samples shown in Table 111, other molasses samples showed the futility of determining water in molasses by this technique. Wood molasses sometimes contains acetic acid which will distill with water (e.g., it forms an azeotrope with water) and is soluble in water. I t is therefore not possible to determine the water content of molasses by azeotropic distillation. However, this does point to an easy method for determining acetic acid content of wood molasses: water content can be determined by the near infrared method; the water plus acetic acid can be determined by azeotropic distillation; hence, the acetic acid content is known. This combination of techniques can be generalized to determine any water soluble azeotrope in any type sample if only one water soluble azeotrope is present. We first tried to use the 1.77-pm band to monitor the water content of black liquor, but results were consistently low. We hypothesized t h a t the large amount of solids which precipitated in the DMSO occluded some water. When we used the 1.93-pm band, much less black liquor was needed, very little solids precipitated in the DMSO, and no occlusion problems were noted. If only a trace of water is present and/or there is a large amount of precipitation in DMSO, the 1.93-wrn band should be used. Under all other circumstances, either band can be used. Although we did not use a temperature thermostated cell compartment, it is doubtful that this could cause our rela-
tively poor precision since it has been reported that small changes in temperature (5 O C or less) have a negligible effect on peak height ( 1 8 ) . All glassware was oven dried and blown with dry nitrogen before use. A likely cause of our precision difficulties was the small sample size itself. Black liquor especially is very inhomogeneous after it cools. Since the work described here was done in a laboratory, the black liquor samples were a t room temperature. We did investigate the possibility of dissolved salts in the black liquor interfering with our analysis. Although black liquor dissolved salts did extract into the DMSO, they had no noticeable effect on our precision.
ACKNOWLEDGMENT The author expresses his thanks and appreciation to the management of International Paper Co. for permission to publish this paper.
LITERATURE CITED TAPPI Standard T650 su-7 1, B. B. Edmonds. Jr., Anal. Chem., 19, 820 (1947). P. E. Borlew and S. J. Lancaster, South. Pulp Pap. Manuf., 16 (6). 44 (1953). P. B. Borlew and S.J. Lancaster, Tappl, 36(11), 504 (1953). J. L. Parker, R. P. Hensel, and C. L. Wagoner, Tappl, 53 (5),874 (1970). W. R. Fegzer, Anal. Chem., 23, 1062 (1951). TAPPI Standard T484 m-58. TAPPI Standard T208 m-60. J. H. Phillips, Tappi, 34 (10). l O l A (1951). J. H. Phillips and M. M. Rubright, Tappi, 36 (9), 392 (1953). R. M. West, Ind. Eng. Chem., 6, 31 (1916). D. M. Smith and W. M. 0.Bryant, J. Am. Chem. SOC.,57,841 (1935). John Ross, J. SOC.Chem. lnd., 51, 121-122T (1932). D. Chapman and J. F. Nacey, Analyst (London),83,377 (1958). J. D. S.Goulden and D. J. Manning, Analyst(London), 95, 308 (1970). H. F. Cordes and C.W. Tait, Anal. Chem., 29 (4), 485 (1957). E. R. Rader, J. Assoc. Offic.Anal. Chem., 50 (3), 701 (1967). D. W. Vomhof and J. H. Thomas, Anal. Chem., 42 ( l l ) , 1230 (1970). P. F. Vornheder and W. J. Brabbs, Anal. Chem., 42 (12), 1454 (1970). M. Violante, Ph.D. Thesis, The Florida State University, Tallahassee FL, 1970. H. Yamatera, E. Fitzpatrick, and G. Gordon, J. Mol. Spectrosc., 14, 268 (1964).
RECEIVEDfor review October 14, 1974. Accepted January 9, 1975.
Total-Reflection X-Ray Fluorescence Spectrometric Determination of Elements in Nanogram Amounts Peter Wobrauschek and Hannes Aiginger Atominstitut der Oestereichischen Hochschulen, A 1020 Wien, Schuettelstrasse 1 15. Austria
Energy dispersive fluorescence analysis enables a quick and simultaneous determination of different elements in a sample. The method presented here utilizes X-ray total reflection on the polished surface of the sample substrate and a special detector-sample geometry. That improves the signal to background ratio significantly and consequently the sensitivity too. The glass substrate combines its good mechanical strength, chemical resistance, and physical definiteness with the low background properties of thin foils. The effective thickness of some hundred Angstroms of the glass substrate is defined by the penetration depth of the X-rays in the case of total reflection. For one run, taking a 120-sec 852
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
counting period, 5 pl of sample volume are required. The application of the method seems to be advantageous where only small quantities of sample volume are available and concentrations ranging down to the parts per million level have to be measured.
Photon induced X-ray fluorescence spectra from a sample are used for nondestructive analysis. Compared with wavelength dispersive techniques, the energy dispersive spectrometer system provides simultaneous detection of different elements in a short time. Using mainly K-series
