Anal. Chem. 1989, 67.1414-1419
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composition for mixtures of cations. For higher ionic strength solutions, standard addition procedures are recommended (2). The calculated LOD for Pb2+with the use of FIDD-FAA was about 1ng/mL. This LOD was of particular interest since the EPA limit on P b in drinking water was recently lowered to 5 ng/mL. The LOD for direct aspiration FAA was typically 90 ng/mL. To test the applicability of the FIDD-FAA experiment to drinking water analysis, tap water samples were spiked at the 5 ng/mL level and acidified to pH -3.5. The 16%, indicating the average recovery of Pb2+ was 103 usefulness of this approach to drinking water analysis.
*
CONCLUSIONS Flow injection Donnan dialysis has been shown to be a simple, rapid means for obtaining large (>loo) improvements in LOD for flame atomic absorption analysis. The degree of LOD enhancement can be controlled for a particular application through adjustment of the membrane tubing length, dialysis time, sample volume, carrier flow rate, and membrane thickness. Only about 8 min is required (dialysis time plus flush) to obtain near-optimum enrichment factors (-100) with the thin-wall membrane. By the use of multiple dialysis cells, the effective analysis time could be reduced to the time required for signal generation and cell flushing, thus providing high sample throughput (20-30 per hour). Based on the simple hardware requirements, the FIDD approach readily lends itself to automation, unlike more traditional enrichment techniques, such as precipitation or solvent extraction ( I O ) .
The Donnan dialysis technique also has advantages over ion-exchange preconcentration since enrichment factors have been shown to be comparable for a wide range of cations (2, 4 , 7),while those for flow injection ion-exchange procedures have been shown to vary substantially due to differences in cation selectivity coefficients ( I I ) . Like Donnan dialysis, ion-exchange preconcentration methods are effectively subject to sample ionic strength limitations (11). Registry No. Cu, 7440-50-8;Pb, 7439-92-1;water, 7732-18-5. LITERATURE C I T E D Wallace, R. M. Ind. Eng. Chem. Process Des. Dev. 1987, 6 , 423. Cox, J. A.; Gray, T.; Yoon, K. S.; Kim, Y.; Twardowski, 2. Analyst (London) 1984, 709, 1603. Cox. J. A.: Twardowski, 2 . Anal. Chim. Acta 1W0, 779, 39. Cox, J. A.; DiNunzio, J. E. Anal. Chem. 1977, 4 9 , 1272. Cox, J. A.; Twardowski, 2. Anal. Chem. 1980, 52, 1503. Cox, J. A.; Carnahan, J. W. Appi. Spectrosc. 1881, 35, 447. Koropchak, J. A.: Dabek-Zlotorzynska, E. Appl. Spectrosc. 1987, 4 7 , 1231. Koropchak. J. A.; Dabek-Zlotorzynska. E. Anal. Chem. 1988, 60. 328. Biaedel, W. J.; Haupert, T. J. Anal. Chem. I98S, 38, 1305. Mizuike, A. Enrichment Techniques for Inorganic Trace Analysis : Springer-Veriag: New York, 1983. Hartenstein, S. D.; Christian, G. D.: Ruzicka, J. Can. J . Spectrosc. 1985, 3 0 , 144.
RECEIVED for review January 17, 1989. Accepted March 24, 1989. This research was supported in part by a grant from the Office of Research Development and Administration, SIU-c.
Automated Slurry Sample Introduction for Analysis of a River Sediment by Graphite Furnace Atomic Absorption Spectrometry M. S. Epstein,' G.R. C a r n r i c k , and Walter Slavin* T h e Perkin-Elmer Corporation, Norwalk, Connecticut 06859
N. J. Miller-Ihli U S . Department of Agriculture, Nutrient Composition Laboratory, Beltsuille, Maryland 20705
A prototype automated slurry sample introduction (SSI)system Is used with a graphlte furnace atomk absorptlon spectrometer (GFAAS) and Zeeman-effect background correction to determJne lead, manganese, arsenk, and Iron In a standard reference materlal (SRM) river sedlment (SRM 2704). Ditferent methods of slurry preparation are tested, optimum analysls parameters are determlned, and sources of varlaMlity in the GFAAS measurements are characterized. Measurement varlabllity is found to Increase in proportion to the percent of analyte not extracted Into the aqueous phase of the slurry solution and Is highly dependent on the homogeneity of anaiyte distrlbutlon In the sample. Analytlcai results for the four elements determined In SRM 2704 are in good agreement with certlfled values and confirm the utility of SSI comblned with GFAAS for analysls of a complex matrix. On leave from the Center for Analytical Chemistry, National Institute of Standards and Technology (formerly the National Bureau of Standards), Gaithersburg, MD.
INTRODUCTION The slurry sampling method of material introduction into a graphite furnace atomizer has been proposed as a rapid and effective technique to reduce the preparation requirements for samples to be analyzed by graphite furnace atomic absorption spectrometry (GFAAS) (I). This paper evaluates the application of an automated slurry sample introduction (SSI) device for the determination of arsenic, iron, manganese, and lead in a National Institute of Standards and Technology (NIST) standard reference material (SRM) river sediment (SRM 2704) (2, 3) by GFAAS, using stabilized temperature platform furnace (STPF) technology (4). The automated slurry sampling system (5)uses a retractable ultrasonic probe for mixing of slurry solutions prior to sampling and deposition into a graphite furnace. The accuracy and precision of the slurry sample introduction system are evaluated, and different methods to prepare the slurry are compared. Sources of experimental variability in slurry sampling are identified and quantitated.
0003-2700/89/0361-1414$01.50/0@ 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
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Table I. Instrument Conditions Used for Slurry Sampling element parameters
As
Fe
Mn
Pb
sample size, (rL) wavelength, nm spectral bandpass, nm radiation source integration time, s furnace program steps drying, OC, s pyrolysis cooldown atomize c1eanup matrix modifiera sonification time, s vortexing time, sb concn of high std, mg/Lc M o(obsd)d Mo(theoret)d measurement precision, %f std/sample supernate slurry
10 193.6 0.7 EDL (8W) 10
5 346.6 0.2 HCL (12mA) 10
5 403.1 0.2 HCL (20mA) 10
10 283.3 0.7 HCL (12mA) 5
120,50 800,30 20,15 2100,lO 2600,5 Ni 15
120,50 1400,30 20,15 2400, 10 2600,8 Pd, MgNO3 15
120,50 1000,30 20,15 2200,lO 2600,7 pod, MgNOB 10 30
120,50 650,30 20,15 1800,5 2600,5
0.1 14 15
150 5700
1.5 4.5
4.0 8.8
1
24 21e 0.9 4.3
pod, MgNO3 10 30 0.3 17 12
0.6 1.2
"Composition of matrix modifier in sample solution: Ni = 0.3% nickel; Pd, MgN03 = 0.06% Pd + 0.04% Mg(N03)z; PO4, MgNO3 = 0.8% (NH4)HZPO4 + 0.04% Mg(N03)z.bVortexingwas only used for slurry sampling of Pb and Mn. cConcentration of the highest standard used to establish the calibration curve, dCharacteristic mass, picograms, for 0.0044A s. eEstimated from relative sensitivities given for flame analysis. /Typical relative standard deviation of three replicate measurements at the concentration of analyte in the sample.
EXPERIMENTAL SECTION Instrumentation. All determinations were made on a Perkin-Elmer Model Zeeman 5100 PC atomic absorption spectrometer equipped with an HGA-BOO graphite furnace atomizer, AS-60 autosampler, and IBM PC-AT computer. A prototype ultrasonic probe mixer synchronized to the operation of the autosampler, after the design of Miller-Ihli (5), was used to suspend sediment particles in the sample cups. The graphite furnace used maximum power heating for atomization with pyrolytic graphite tubes and L'vov platforms. Peak area measurements were used for all analyses. Instrument parameters are summarized in Table I. Slurry Preparation. Samples were either weighed directly (1-2 mg) into polyethylene autosampler cups or sampled from a vortexed suspension (10-50 mg), as described by Miller-Ihli (I). Both methods were employed for studies involving lead and manganese. Since no significant difference in res& was observed between the two methods, the simpler method involving direct weighing into the autosampler cups was used for studies of arsenic and iron. Weighing of samples was done on an electronic balance to an accuracy of hO.01 mg. Each sample was weighed into the appropriate container, and a diluent solution consisting of 5% HNOBand 0.04% Triton X-100 was added to the sample in a weight ratio of approximately 1part sample to 500 parts diluent. Matrix modifiers were also added directly to the samples in the autosampler cups. Standards were prepared from NIST SRM spectrometric solutions or other commercial standards that were certified in weight/volume units (Le., mg/L). All dilutions were made by using weight aliquots delivered with a micropipet and weighed on an electronic balance. When required for conversion to volume units, densities were determined by replicate measurements of the weight of solution in a calibrated (to contain) 1-mL vessel. The river sediment reference material (SRM 2704) was not dried, but analyzed as received, and corrected for moisture (0.8%) determined on separate samples. When vortexing and aliquoting procedures were used to prepare slurries for sampling, two aliquots were removed from each slurry preparation and weighed into autosampler cups. Three replicate samples were taken from each autosampler cup for an analysis set, and one or two analysis sets were performed. The fraction of each element extracted into the solvent phase during preparation of the slurry was estimated by allowing the slurry to settle and adjusting the autopipetor tip so that only the solvent phase was sampled. Design for Evaluation of Sources of Experimental Variability. An analysis of variance (ANOVA) nested design, which analyzes the effect of one or more factors on one response variable
Direct
Weighing
V o r t e x - Allquot Method
IO
( a )
-
1
'I1
121
ib)
Figure 1. Experimental design for the analysis of variance. (A) Direct weighing: (1) two weighings from each bottle into autosampler cups: (2) three to six instrument nneasurements of each weighing. (B) Vortex-aliquot method: (1) SIX weighings into test tubes: (2) two
aliquots from each vortexed sluny into autosampler cups; (3) three to six instrument measurements of each aliquot. (the measured analyte concentration in this case), was used to examine all sources of variability in the GFAAS-SSI system. Figure 1 illustrates the experimental design for (a) the direct weighing procedure and (b) the vortexing and aliquoting procedure. The design was meant to separate sampling variability caused by analyte inhomogeneity from variability due to weighing errors, aliquoting errors, and instrument noise. In the direct weighing procedure, eight 2-mg samples of SRM 2704 were weighed from four different bottles (two samples from each bottle) directly into the autosampler cups. In the vortexing and aliquoting procedure, six 10-mg samples were weighed into plastic test tubes, and two aliquots of each slurry were removed and weighed into autosampler cups. Lead and manganese were chosen as the test elements, since lead was reported by isotope dilution mass spectrometry to be inhomogeneous in SRM 2704,while manganese had been shown to be homogeneous to better than 0.4% RSD for a 1-g sample size by X-ray fluorescence (XRF) (2, 3).
RESULTS A N D DISCUSSION Accuracy and Precision of the GFAAS-SSI System. The effect on measurement accuracy of SSI for the GFAAS determination of arsenic, iron, manganese, and lead in SRM 2704 can be assessed from the data in Table 11. The analysis results for iron, lead, and manganese are in good agreement
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989 8.105
0
Time ( s e d
18.88
lire (ret)
6.88
A\
E
B
line (sec)
18.88
Flgure 2. Absorbance versus time profiles for slurry and standard measurements, illustrating differences observed for release of analyte from the solid slurry matrix: upper left panel, Fe; upper right panel, Pb; lower left panel, Mn; lower right panel, As.
Table 11. Analytical Results for Slurry Sampling of Sediment (SRM 2704) certfd Val, elem.
As
Fe Mn
Pb
conditions” N b 2,DW 2, DW 2,DW 10, VTX 50, VTX 2,DW 10, VTX 50, VTX
8 8 8
%E‘ anal. Val, pg/gd
68 22 64
6 3
8 6 4
88
pg/g
21.7 i 0.6 23.4 i 0.8 40600 f 1700 41100 1000 558 i 18 555 f 19 558 f 19 522 i 24 171 f 24 161 i 17 171 f 35 165 f 11
Sample size is given in milligrams, and methods of preparation are as follows: DW, direct weighing into autosampler cup; VTX, vortex mixing and transfer of slurry into autosampler cup. Samples were diluted in a volume of solution (5% HNO, and 0.04% Triton X-100) corresponding to a weight ratio with the sample of 500:l. bNumber of discrete samples of sediment weighed. Percent of analyte extracted into diluent solution. Uncertainty exmessed as confidence limits at a 95% confidence level. with the certified values for those elements. The analysis result for arsenic is slightly lower than and slightly outside the confidence limits for the certified value. Absorbance versus time profiles of samples and standards for all four elements are shown in Figure 2. Sample and standard peak profiles and appearance times are identical for iron. Profile shapes are identical for lead, but the slurry signal appears about 0.6 s after the standard signal. Manganese profiles vary somewhat in both shape and appearance time. For these elements the STPF conditions have been properly optimized to eliminate bias caused by sample matrix effects. The arsenic absorbance versus time profile for the sediment has not quite returned to the base line, which may cause the small bias observed in the analytical results. Alternatively, the bias may be caused by the loss of arsenic during the charring step, since the nickel matrix modifier cannot interact with the solid slurry component as well as with the solution component. In general, the precision of results for the analytes that are known to be homogeneously dispersed in the sample (As, Fe,
Mn) is equivalent to what might be expected from GFAAS using an acid digestion method of sample preparation. Relative precision, expressed as 95% confidence limits, is 3-4% of the analyte concentration for these elements. The inhomogeneous distribution of lead in SRM 2704 causes the measurement precision to vary from 7 to 15% of the analyte concentration. The inhomogeneity of lead distribution was reported by analysts participating in the certification of SRM 2704 ( 4 ) and has been suggested to be caused by segregation of lead in specific mineral fractions, such as zircon (6). The precision of the lead measurements reported in Table I1 is a good indication of the degree of lead homogeneity at the weighed sample size (i.e., 2-50 mg), since almost 90% of lead in the sample is extracted into the solvent phase during the ultrasonic mixing step. When the analyte extraction efficiency is close to loo%, SSI measurement precision is largely determined by the degree of material homogeneity at an “effective” sample size (for homogeneity considerations) corresponding to the total amount of sample weighed to prepare the slurry. For a homogeneously distributed analyte in a finely ground sample, the SSI measurement precision would be expected to approach the precision defined by instrument noise. However, for most ”real-world” samples, degree of analyte homogeneity is likely to limit measurement precision. At low extraction efficiencies, the SSI measurement precision is determined by the degree of homogeneity at a sample size corresponding to only the amount of slurry introduced into the furnace. Since this “effective sample size” will be a t least 2 orders of magnitude smaller than the amount of sample weighed into the autosampler cup, the measurement precision would be expected to be significantly poorer for analytes with low extraction efficiency and inhomogeneous distribution in the sample. The precision of replicate measurements for standards of slurry supernate solutions is contrasted with measurements of the slurries at the bottom of Table I. In all cases, precision is poorer when the analyte is measured in the slurry samples. The difference in the measurement precision of standards compared to that of the slurry samples is a function of the
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989 /
/"
Table 111. Evaluation of Sources of Measurement Variability i n GFAAS-SSI
/
conditions"
As.
.Mn
lead, VTX lead, VTX' manganese, VTX lead, DW manganese, DW iron, DW arsenic, DW
/
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percentage of the total variability resulting from slurry bulk aliquot- instrum sampling total samplingb ing' measmt measmtd measmt 99.7 96.5 23.5
0.2 2.3 69.1
0.1
1.2 7.4
92.8 12.6
1.8 3.8
5.4 83.6
7.2 87.4
1.7 17.5
20.3 9.2
78.0 73.3
98.3 82.5
"Element and slurry preparation method (VTX is vortexing and aliquoting; DW is direct weighing into autosampler cups). Replicate weighing of SRM 2704 into test tubes or autosampler cups. Replicate volumes of RSM 2704 taken from vortexed test tube. dReplicate autosampler aliquoh of slurry taken from autosampler cup. e One very high lead value was rejected in this sample set to note the effect on the ANOVA.
/ 0
/
,
,
1 20
/
,
,
I 40
,
,
,
l 60
,
,
/
1 80
Percent A n o l y l t No1 Eatrocled
Flgure 3. Correlation plot of the measurement precision of slurry
sampling versus the percent analyte remaining unextracted after ultrasonic probe treatment of the sample. amount of analyte extracted into solution, the degree of analyte homogeneity in the sample, and the level of instrument noise. The measurement precision for the standards or slurry supernate solutions shown in Table I can be deconvoluted from the slurry measurement precision by subtracting in quadrature (i.e., the square root of the difference of the squares), assuming the variability of the standard measurements is not related to the variability of the slurry sampling (i.e., independent sources of variability are involved). The resulting precision is due only to sampling of the slurry and is highly correlated with the fraction of unextracted analyte, as shown in Figure 3. This would be the case if the degree of homogeneity for all four elements in the sample was similar. This is not true at large "effective" sample sizes, as illustrated by the analysis results for lead in Table 11. However, assuming the lead is distributed as a small number of discrete particles of high lead concentration, the probability of sampling a lead-containing mineral fraction is small a t the effective sample size ( 10 pg) aliquoted by the autosampler. Alternately, the lead-containing mineral fraction may be soluble in the "OB diluent. Thus, the measurement variability observed for replicate aliquots from a single slurry preparation will be determined by the homogeneity of the analyte in the insoluble fraction of the sample (e.g. a silicate). In either case, the composition of the insoluble fraction of the slurry appears to be similar for the measured elements, and the degree of homogeneity of that fraction dominates the measurement process. Direct Weighing versus Vortex Mixing/Sampling. Direct weighing of sample into autosampler cups is preferable to vortex mixing and aliquoting of the slurry into the cups, unless weighing to fO.O1 mg or better is not possible. Vortex mixing and aliquoting will result in a negative bias of results if the extraction efficiency of analyte into the diluent solution is low and the analyte is associated with large particles that settle out of suspension before a representative sample can be taken (I). An example of this phenomenon is the low value for manganese in Table I1 a t a 50-mg sample size. The test tube used for the 50-mg samples could not be sampled during vortexing without loss of sample. The slurry had to be aliN
quoted after vortexing, which resulted in a nonrepresentative sample of manganese. This was not a problem for lead because of the high extraction efficiency. It was also not a problem for the measurements of manganese when the smaller test tube and 10-mg sample weights were used, since that tube could be aliquoted during vortexing. The vortex mixing method of slurry sample would be advantageous when material homogeneity is a critical concern. That method can provide a larger "effective" sample size if the extraction efficiency of analyte into the diluent solution is high. However, if the extraction efficiency is low, the "effective" sample size is the amount of sample aliquoted from the slurry by the autosampler. That "effective" sample size would be similar to one taken by direct weighing, so there would be no advantage to use of the vortex mixing method. Furthermore, the chance of error is also increased due to contamination or aliquoting errors resulting from the increased sample handling required by the vortex mixing method. Evaluation of Sources of Experimental Variability Using a n Analysis of Variance (ANOVA). All sources of variability in the SSI process were examined by using the ANOVA as described in the Experimental Section. Table I11 shows the percentage breakdown of the sources of variability from the ANOVA obtained for lead and manganese when both vortexing and direct weighing procedures were used and for arsenic and iron with only the direct weighing procedure. Sample-to-sample variability in the bulk sample weighing step is the dominant source of random error for all lead measurement methods, reflecting the material heterogeneity for lead. The principal source of variance in all other cases was the instrumental measurement process. However, the instrumental measurement process (i.e., replicate within-sample measurements) involves components of variance due to both the instrument "noise" and the variability in autosampler aliquoting of the slurry. The latter operation is a sampling procedure that depends on the degree of analyte homogeneity in the slurry and the percentage of analyte extracted into the diluent. If the andyte is completely extracted into the diluent, there is no inhomogeneity contribution to the variance of the replicate within-sample measurements. Only the precision of the delivered volume is significant. Alternately, if the analyte is not extracted into the diluent, analyte inhomogeneity will determine the variability of the replicate withinsample measurements, as was demonstrated in Figure 3. By deconvolution of the slurry sampling function from the instrument noise, as described previously, using the measurement precisions shown in Table I, the actual contribution of
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989 2-
'I
1.6 -
1.2
* u
-
v 2
L
-
. 0.8 -
0.4
-
0-
5
4
3
z w
?! 2
1
0
[ I t I I I I ~ , I, l , , , , l , , , , l , , , , l , , , , J 2.9
3 4
39
4 4
4 9
54
5 9
II
Iron (ut % )
Figure 4. Frequency histograms of data from slurry sampling, illustrating the nonnormaldistribution of the data, for (a) Mn (top) and (b) Fe.
material inhomogeneity to the total variance can be seen to be greater than 90% in all cases.
Frequency Distributions of Analysis Results from SSI. Figure 4 shows frequency histograms of results from the direct weighing experiments in slurry sampling. The manganese distribution in Figure 4a is severely skewed, and the iron distribution in Figure 4b appears slightly skewed. The distribution for lead was severely skewed, similar to that of manganese, while the arsenic distribution appeared normal. These distributions are similar to histograms of data from geochemical exploration of ore bodies (7).The skewed distributions are characteristic of inhomogeneous analyte distributions, and many of the same principles used in geostatistics may therefore be applicable to data treatment and evaluation in slurry sampling. In contrast to the skewed
Flgwe 5. Frequency histograms of data taken by sampling from the supernate above the slurry after treatment with ultrasonic probe and settling of the particles, illustrating the normal distribution of the data, for (a) Mn (top) and (b) Fe.
histograms from the slurry sampling, Figure 5 shows histograms of data from sampling of the supernate from the slurry after the particles have been allowed to settle. Both iron and manganese show normal distributions, which indicate the dominance of instrument noise and the random nature of the extraction process.
CONCLUSIONS GFAAS-SSI has been shown to be a useful method for very rapid and quantitative evaluation of element concentrations in a complex sediment matrix. The ultrasonic probe mixer allows direct weighing of samples into autosampler cups and adequately suspends the solids for sampling into the graphite furnace. It is also clear that the precision and accuracy of
Anal. Chem. 1989, 61, 1419-1424
analytical results obtained by using SSI for GFAAS will be highly dependent on the analyte homogeneity in the sample material. Since this has been shown to be critical for a reference material that has been extensively sieved and blended, it may be a far more serious concern for real samples of more questionable homogeneity. Therefore, to obtain accurate analyses when SSI is used on a material of unknown homogeneity, particularly for elements of low extraction efficiency, a relatively large number of carefully chosen, discrete samples must be taken. A sampling protocol such as described by Ingamells (7) for geological samples will ensure that material homogeneity is properly evaluated and the measured analyte concentration and uncertainty reflect the true values. Such a protocol emphasizes that a relatively large number of discrete samples must be taken to obtain accurate analyses, and that “abnormally” high data points cannot be arbitrarily discarded, as might be done for conventional analytical work.
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Registry No. As, 7440-38-2;Fe, 7439-89-6;Mn, 7439-96-5;Pb, 7439-92-1. LITERATURE C I T E D (1) Miller-Ihli, N. J. J . Anal. At. Spectrom. 1988, 3 , 73. (2) Standard Reference Material 2704 (River Sediment), NIST Certificate of Analysis, National Institute of Standards and Technology, Gaithersburg, MD, 1986. (3) Epstein, M. S.; Diamondstone, B. I.; Gills, T. E. Tahnta 1980, 3 6 , 141. (4) Slavin, W.; Carnrick, G. R.; Manning, D. C. At. Spectrosc. 1981, 2 , 137. (5) Miller-Ihli, N. J. Federation of Analytical Chemistry and Spectroscopy Societies Meeting, Paper 355. Oct. 1987. (6) Barnes, I.L., National Institute of Standards and Technology, private communication. (7) Ingamells, C. 0.; Pitard, F. F. Applied Geochemical Analysis; John, Wlley and Sons: New York, 1988; Chapter 1.
RECEIVEDfor review December 27,1988. Accepted March 23, 1989.
Measurement of Caustic and Caustic Brine Solutions by Spectroscopic Detection of the Hydroxide Ion in the Near-Infrared Region, 700-1 150 nm M. Kathleen Phelan, Clyde H.Barlow,’ J e f f r e y J. Kelly,’ Thomas M. Jinguji, a n d J a m e s B. Callis* Center for Process Analytical Chemistry, Department of Chemistry, BG-IO, University of Washington, Seattle, Washington 98195
We have explored the feaslblilty of caustlc measurement by direct detection of hydroxlde Ion using optlcal absorption spectroscopy In the near-lnfrared wavelength range 700-1150 nm. unfortunately, the spectral features of the hydroxide Ion are obscured by strong bulk water absorptlons whose Intensltles and peak shapes are dependent upon temperature and the presence of electrolytes. Nevertheless, with the aid of difference spectra, second-derivative technlques, and m u l variate spectral reconstruction, we have obtained a clear lndlcatlon of the spectrum of the hydroxlde Ion. Its features Include (a) a sharp absorptlon band at 965 nm that arises from the second overtone of the OH stretching motlon localized on the hydroxlde Ion, (b) a broad absorption band centered at 1100 nm that a r k s from the Mndlng of two water molecules to the hydroxide Ion, and (c) a second sharp absorption band that is attributed to a comblnation stretch-bend transltlon arislng from the concerted motion of the hydrated ion. Using thls knowledge as a guide, we have developed multlvarlate analysis methods for determlnlng hydroxlde concentration of caustic brlnes In the range 0.01-5.0 M that are successful even In the presence of a large variable excess of NaCI. Such methods are sultable for lmplementatlon as process monitoring tools.
INTRODUCTION Measurement of hydroxide concentration in caustic and caustic brine solutions is an important industrial problem. In Permanent address: Department of Chemistry, The Evergreen State College, Olympia, WA 98505. 0003-2700/69/0361-1419$01 SO10
the past, a number of techniques have been employed for caustic analysis (1). Among these methods, pH measurement by glass electrode is most common. However, a t high pH, conventional glass electrodes become unstable and begin to suffer from interferences caused by the presence of other cations, most notably sodium (2). As a result, other techniques have been developed for caustic determination, such as index of refraction, conductivity, on-line titration, and flow injection analysis. All of the above methods have the drawback that they require physical invasion of the process with some sort of probe or sampling device. In a review on the current status of process analysis, the potential of remote, noninvasive methodologies was described (3). One particularly promising approach to noninvasive analysis involves the use of shortwavelength near-infrared (SW-NIR) spectroscopy in the wavelength region 700-1150 nm. This technique has the following advantages: (a) Measurements can be made remotely through quartz windows, using fiber optics to guide the light to and from the window; (b) path lengths can be long (many centimeters); (c) scanning fiber-optic spectrophotometers are available that are rugged, small, and relatively inexpensive (4); (d) signal-to-noise ratios are very high; and (e) good quantitative results are obtained on highly scattering samples. The major disadvantage of SW-NIR spectroscopy is low spectral resolution, which results in severe overlap of absorption spectra. This disadvantage is overcome with the aid of multivariate calibration methods (5),which usually work well in this spectral region due to the excellent signal-to-noise ratio. Unfortunately, it is not straightforward to develop a spectroscopic method for determining hydroxide ion concentration because the major spectral features of this ion are expected to overlap the broad bands of water. To make 0 1989 American Chemical Society
