1914
Anal. Chem. 1982, 5 4 , 1914-1917
Table I. Extractor Efficiency Column 1 % recovery of entire procedure Column 2 standard deviation Column 3 % recovery corrected for concentration losses LLE FUE FOE compound 1 2 3 1 2 3 1 2 3 chlorobenzene 72 8.9 96 78 10.5 104 43 4.3 107 phenol 72 5.4 90 81 6.8 101 no recovery 2-chlorophenol 73 5.9 91 85 8.0 106 50 4.9 79 2-nitrophenol 83 7.5 99 91 6.1 108 82 3.9 102 naphthalene 82 5.9 91 91 4.1 101 82 4.1 91 4-chlorophenol 69 2.9 83 76 4.7 91 32 3.3 42 1,2,3,4-tetrachlorobenzene 82 5.3 89 90 3.7 98 87 3.3 96 dimethyl phthalate 96 5.5 98 91 1.1 93 101 1.8 105 4-nitrophenol 98 7.6 102 92 2.7 97 norecovery pentachlorophenol 86 3.8 93 88 5.3 96 71 6.4 78
SDE
SCDE 1 2 1 2 3 59 1.8 82 7.9 90 no recovery 79 9.8 82 77 2.5 80 3.4 86 97 1.5 83 1.9 88 95 4.3 87 2.6 92 no recovery 66 1.7 72 80 6.5 87 4.7 93 64 45 7.2 62 4.5 no recovery no recovery 85 2.9 86 3.9 90
graphic column. A prediction of whether or not a particular compound will steam strip from water can be made on the basis of its “relative volatility to water” ( I ) . The small volumes of solvent normally used in steam distillation techniques (20 cm3 in SCDE vs. 700 cm3 in FUE) facilitates solvent concentration and minimizes interfences due to solvent impurities and/or preservatives. Temperature stability of each solute to be steam distilled must be demonstrated in the matrix. Extreme care must be exercised to be certain the species is neither formed nor destroyed during the extraction. Any samples that form emulsions during conventional liquid-liquid extractions may foam when used in a steam distillation apparatus. The extent of foaming can range from inconsequential to severe-filling the entire apparatus with foam. Experimentation is the only way to determine whether or not the sample will foam. Flgure 1. Flow over extractor: 1, solvent distillation pot; 2, 6 in. diameter X 4 In. helght; 3, 45/50 joint for sample addition and stirrer; 4, 24/40
joint to condenser.
Since no concentration of the SDE extract was made, losses of chlorobenzene can probably be attributed to volatilization through the condenser. Phenol and 4-chlorophenol, although they do distill, are insufficiently soluble in the hexane under these conditions to be concentrated. The low relative volatility of dimethyl phthalate with respect to water results in low recoveries with both distillation techniques. Normally, species that can be steam distilled can also be gas chromatographed. This results in a “cleaner” extract and reduces the buildup of residue at the front of the chromato-
CONCLUSION In general, the SCDE and FUE/FOE appear to be the most useful. For general purpose work where the aim is to extract everything possible with the solvent being used, the FUE/FOE would be optimum. When more specificity and a cleaner extract are desired, the SCDE can be used provided adequate precautions are taken (i.e., thermal stability, foaming problems, etc.).
LITERATURE CITED (1) Robbins, L. A. U.S. Patent No. 4236973, Dec 2, 1980.
RECEIVED for review January 18,1982. Accepted May 27,1982.
Determination of Water by an Automated Stopped-Flow Analyzer with Pyridine-Free Two-Component Karl Fischer Reagent Michael A. Koupparis’ and Howard V. Malmstadt”2 Department of Chemistty, University of Illinois, 1209 W. California St., Urbana, Illinols 6 180 1
Water or moisture content in a wide variety of materials is a matter of universal importance, and the determination of water has become one of the commonest routine procedures in chemical laboratories. Among the many methods used (I, Present address: Laboratory of Analytical Chemistry,University of Athens, 104 Solonos St., Athens (144), Greece.
‘-Present address: 75-5786 Niau Place, Kailua, Kona, HI 96740. 0003-2700/82/0354-1914$01.25/0
2 ) , e.g., gravimetric, spectroscopic (NMR, UV, IR), refractometric, titrimetric (Karl Fischer), etc., the last one, introduced in 1935, is the most generally used. The Karl Fischer reagent that consists of iodine and s u l f u r dioxide in pyridinemethanol solution is used as titrant and the end point is nearly always obtained electrometrically by biamperometric or bipotentiometric technique. Various types of apparatus have been designed to automate the titration. 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
There are several drawbacks associated with the standard Karl Fischer method. It is time-consuming because of the rather slow rate of reaction near the end point of the titration. Typically more than 5 min are needed per titration. The Karl Fischer solution is unpleasi%ntto work with because it contains toxic substances with a disagreeable odor. Also, since free sulfur dioxide is present, a yellow SOJ- species is formed, thereby making visual recognition of the end point rather difficult. Finally, there are several chemical interferences. Active carbonyl compoundls, for example, cause the formation of water through reaction with methanol. Also mercaptans and certain amines react with the iodine. These interferences become more intense because of the prolonged time of the titration. Various attempts have been made to develop pyridine-free Karl Fischer reagents by replacing pyridine with other bases (3-5). One of these consists of a methanolic sodium acetate-sulfur dioxide solution as solvent and an iodine solution in methanol as titrant (3). The oxidizable species in Karl Fischer reaction if3monomethyl sulfite ion CH3S03(6). The sodium acetate converts virtually all the sulfur dioxide into methyl sulfite
SO2
+ Ac- + CH,3QH+ HAC + CHp'303-
+
CH3SO3I2- HzO
-
CH3S04-+ 21-
+ 2H+
(1)
(3)
and the solution, in effect, contains an acetate-acetic acid buffer, required for the water reaction, eq 3. The advantages of this system are the good buffer action and the large methyl sulfite concentration which give a high reaction rate. The avoidance of the yellow EiO2I- complex formation makes a visual end point possible. The absence of toxic and unpleasant pyridine and the stability of the iodine reagent are major advantages. This reagent is now commercially available as ReAquant reagent system1 (J. T. Baker Chemical Co., Phillipsburg, NJ) (7). To minimize interferences associated with ithe standard Karl Fischer batch titration procedure and for a rapid routine method for the determination of water, we use this modified reagent with a relatively (simple computer-controlled stopped-flow analyzer (8). The automated sampling/mixing unit of this analyzer provides an excellent closed system for this determination thus avoiding contamination from atomospheric moisture. An adapatation of the conventional Karl Fischer reagent system has been proposed to flow injection analysis with potentiometric or ph1otometric detector (9). Organic solvents and reagents containing 1-10 mg/mL water are premixed with the Karl Fischer solvent and the stopped-flow analyzer is used to mix equal volumes of the sample mixture and iodine reagent. The absorbance is measured a t 620 nm (far sway from iodine absorption maximum because of its high concentration), 10 s after mixing. Results obtained by the stopped-flow method are compared with a titrimetric method using the same reagents and a visual end point. The absence of ,my colored product of this reagent system permits a precise and accurate visual end point.
1915
Table I. Absorbance Values for Different Delay Times delay time, s 5.0 10.0 15.0 20.0
absorbance 0.2789 0.2749 0.2738 0.27 34
RSD, % 0.97 0.18
0.12 0.06
a Measurement of 10 mg/mL water standard mixed Average of 1 0 integra1 + 4 with ReAquant solvent. tions during a 5-5 measurement time.
septum without any contamination by the atmospheric moisture. An interference filter with a 10-nm band-pass at 620 nm was used
in the photometer unit. Investigative and routine programs stored in a cassette recorder were used for optimization of the procedure for routine analysis. All measurements were carried out in an air-conditioned laboratory at a nominal temperature of 25 "C. Manual visual end point titrations were carried out with an automatic constant rate buret (Sargent, Model C) and a closed titration cell. The cell wag continuously purged with dried nitrogen during titrations. A mixture of Karl Fischer solvent with an excess of iodine reagent was found an effective drying agent for nitrogen. This solution is initially brown and turns colorless when spent. Reagents. A stabilized iodine methanolic solution 0.2 M, equivalent to -3.5 mg of HzO/mL (ReAquant,Titrant No 9029, J. T. Baker Chemical Co.) was used as iodine reagent. The ReAquant solvent (Baker, No. 9028), a 0.25 M methanolic solution of methyl sulfite ion in 0.5 M acetate-acetic acid buffer, was used as solvent. Both solutions were stored in tightly closed bottles in a refrigerator. Anhydrous methanol, distilled in glass and containing 0.009% HzO (Burdick & Jackson Laboratories Inc., Muskegon, MI, was used to prepare water standard solutions and dilute the samples if necessary. A stock water standard solution 25 mg/mL was prepared by weighing 2.500 g of deionized water and diluting with anhydrous methanol to 100 mL. Working water standard solutions 1-10 mg/mL were prepared everyday by appropriate dilutions of the stock solution with anhydrous methanol. Procedure. The 0.5-mL portions of water standard solutions or organic liquid samples are mixed with 2 mL of Karl Fischer solvent by injection with Hamilton accurate gastight syringes in 6-mL glass vials closed with a plastic (Tygon) septum. The vials are dried and filled with dry nitrogen before use. One-hundred-fifty microliters each of iodine reagent and the appropriate standard or sample mixture is aliquoted by the automaticsyringes of the stopped-flowanalyzer. The syringes then drive the solutions through the mixer and transfer the mixed solution to the observation cell. The absorbance is automaticallymeasured at 620 nm during a 5-s measurement time (ten 0.5-5 integrations) after a 10-sdelay time. The data are used to construct a working curve or provide quantitative concentrationinformation for the organic samples. Samples out of the concentration range of the working curve are diluted with anhydrous methanol before the mixing with ReAquant solvent. Basic substances (amines) required neutralization with glacial acetic acid before the analysis (7). Visual End Point Titration. The ReAquant titrant is standardized with 20 mg of HzO in 10 mL of pretitrated solvent. For the titration of the organic samples an aliquot containing up to 25 mg of HzO is added to a pretitrated 10-mL solvent volume and the titration is continued to a colorless to yellow visual end point.
EXPERIMENTAL SECTION
RESULTS AND DISCUSSION
Apparatus. The apparatus used was the automated microcomputer-based stopped-flowanalyzer described by Malmstadt et al. (8). This compact system utilizes an AIM 65 (Rockwell International, Anaheim, CA) microcomputer and provides for automatic aliquoting and mixing of sample and reagent and delivery of the mixed solution into the measurement cuvette (1-cm pathlength). About 150 pL of sample or reagent is delivered by each syringe, and four flushes are used for sample-to-sample change. Hypodermic needles were fitted to the ends of the sample and reagent valve tubes to accommodate the immersion of the Teflon tubes in the reagent and sample closed vials through a
By use of an investigative equilibrium program the optimum time parameters (delay and measurement time) were evaluated. The reaction was monitored for 25 s after the mixing of the solutions and 50 pointa (0.5-s integration per point) of the reaction curve were printed out. In Table I absorbance values using various delay times and a 5-9 measurement time are shown for a 10 mg/mL water standard solution. From these data it is shown that the reaction is fast, completed practically in less than 5 s. A delay time of 10 s was chosen as a compromise of good precision and short total measurement time. A slight turbidity occurs very slowly in the reaction mixture, especially with the high water
1916
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982 20
Table IV. Standard Addition of Water to Various Organic Solvents and Reagents amt of H,O, mg/mL
15
solvent methanol (0.1% H,O) Me,SO (0.08% H,O) glycerin
10
found
3.00 8.00 3.00 8.00 3.00
4.10 9.10 3.85 8.85 5.43 9.43
4.15 9.01 3.82 8.96 5.52 9.48
1.10
0.85 2.43
mean
0
1
I
I
0.05
0 IO
0 15
WATER
0 20
+
Table 11. Results Used for Water Working Curvea
H20, 1.00
2.50 5.00 7.50 10.00
absorbance
RSD, %
1.600 1.422 1.024 0.657 0.320
0.5 0.6 0.3 0.4 0.2
Working curve: slope = -0.1448, intercept = 1.758, Initial concentration, before the mixing r = 0.9994. with the solvent. Final concentration in the observation cell the 1/10 of the referred. Average of five determinations on a single solution. Delay time = 10 s, measurement time = 5 s. a
~
Table 111. Day-to-Day Stability of Water Working Curve day 1
absorbance day 6 day 1 0
day 15
1.823 1.564 1.034 0.598 0.1 39
1.681 1.394 1.034 0.639 0.268
1.600 1.422 1.024 0.657 0.320
1.522 1.373 0.976 0.607 0.258
-0.155 1.810 0.9993
-0.145 1.758 0.9994
-0.144 1.698 0.9990
slope -0.188 intercept 2.011 corr 0.9995 coeff ( r )
%
101.7 98.9 99.0 101.4 103.0 100.7 100.8
(w/w) titrationC diffd
% H,O
+ 3,(D) 1 + 1.
mg/mL
rec,
Table V. Comparison of Stopped-Flow with Visual End Point Titrimetric Method for the Determination of Water in Organic Solvents and Reagents a
rng/rnl
Figure 1. Effect of methyl sulfite and acetate buffer concentration on the working curve: (A) 0.225 M, (B) 0.20 M, (C) 0.15 M, (D) = 0.125 M. Water standardlsoivent mixing ratlo: (A) 1 4- 9, (B) 1 4, (C)2
10.00
total
7.00
05
H,O std, mg/mL 1.00 2.50 5.00 7.50
initially present added
concentration solutions but this has no effect on the measurement since the total measurement time is kept short. The methyl sulfite and acetate buffer concentration effect on the working curve was studied by using various mixing ratios of the solvent with the water standards. The results are shown in Figure 1. A linear working curve in the range of 0.02-0.2 mg/mL (water concentration in the solvent) can be obtained when methyl sulfite and acetate buffer are a t least 0.20 M. This is achieved by a 1+ 4 mixing of the standards and samples with the solvent. The solvent has a buffer capacity and methyl sulfite concentration for up to 2.5 mg of H20/mL. A more concentrated buffer/sulfite solvent is now commercially available with 0.5 M methyl sulfite (ReAquant 11) for up to 5 mg of H,O/mL (7). Typical results obtained for the working curve are shown in Table11and give a correlation coefficient of0.9994 and a relative standard deviation of 0.2-0.5%. The stability of the reagents was studied during a 15-day period of continuous use (Table 111). A slow decrease of the intercept of the working curve appears be-
sample
stoppedflowb
acetonitrile acetone Me, SO hexyl alcohol octyl alcohol ethyl acetoacetate isoamyl alcohol glycerin diethanolamine methanol
1.338 0.760 0.488 0.816 0.845 0.965 0.606 0.163 0.828 0.307
1.360 0.770 0.480 0.829 0.829 0.959 0.598 0.165 0.836 0.310
-0.022 -0.010
+0.008 -0.01 3 +0.016 +0.006 t 0.008 -0.002 -0.008 -0.003
a Solvents in use for a long time. Average of five measurements on a single sample with a mean RSD of 0.4%. Average of three titrations with a mean RSD of 1.5%. Stopped flow minus titration value.
cause of the increase of the solvent and anhydrous methanol water concentration by contamination from atmospheric moisture. The slope of the working curve also decreases, and the change is greatest during the first week the reagents are used. The accuracy of the stopped-flow method was examined by a recovery study using three organic liquid samples. Table IV shows that added water was quantitatively recovered with mean recovery 100.8% (range 98.9-103.0%). The method was also compared to a visual end point titration procedure using the same reagents system (7). Several organic solvents that were in use for a long period in the laboratory were analyzed for water using the stopped-flow and titrimetric procedures. The comparison results are shown in Table V with a good agreement. With four flushes to change from one solution to another (flush cycle time about 1.5 s),one measurement per sample, 5 s for the turntable position increment with manual immersion of the hypodermic needle in the sample vial through the septum, and 0.5 s for the computer calculation and printing time, the determination rate for the proposed method is about 130 samples/h. The total reagent volume consumed per determination is only 0.75 mL. The automated determination of water with microcomputercontrolled stopped-flow analyzer is rapid and can be used for routine analysis. There is less influence from interfering side reactions than in the conventional titration procedure. The closed system prevents contamination from atmospheric moisture. The cost per determination is very low because of the low consumption of reagents and high sample throughput. The sensitivity, about 0.5 mg/mL or 0.05% (v/v), is not as good at present as that obtained with the conventional titrimetric method, but the available range is applicable for many organic solvents and reagents that can be dissolved completely in the methanolic modified Karl Fischer solvent.
LITERATURE CITED (1) Harris, C. Talanta 1972, 79, 1523-1547. (2) ~, Mitchell. J.. Jr.: Smith. D. M. "Aauametrv Part 1: A Treatise of Methods for theDetermlnation of Water", 2nded.; Wiley: New York, 1977.
1917
Anal. Chem. 1982, 5 4 , 1917-1919
(9) Kagevall, I.; Astrom, 0.; Cedergren, A. Ana/. Chim, Acta 1980, 114, 199-208.
(3) Verhoef, J. C.; Barendrecht, E. Anal. Chim. Acta 1977, 94, 395-403. (4) Scholz, E. Fresenlus’ Z Anal. Chem. 1981, 306, 394-396. (5) Scholz, E. Am. Lab. (Fairfield,Conn.) 1981, 13 (8), 89-91. (6) Verhoef, J. C.; Barendrecht, E. J . Electroanal. Chem. 1978, 7 1 , 305-315. (7) J. T. Baker Chemical Co., product information bulletlns 15 and 17. (8) Koupparis, M. A.; Walczrrk, K. M.; Malmstadt, H. V. J . Autom. Chem. 1980, 2 (2), 66-75.
RECEIVED for review November 25.1981. AcceDted June
14.
lgg2*Research partially supported by Research Grant PHS-5RO1 GM 21984.
Miniature Ion-Source-Mounted Fast Atom and Ion Gun for Fast Atom Bombardment and Secondary Ion Mlass Spectrometry Martln A. Rudat Central Research & Development Department, Experimental Station, E.
The recent development of fast atom boimbardment (FAB) mass spectrometry ( I ) has led to an explosive growth of research in the application of the new technique to previously intractable molecules. At the same time, commercial versions of fast atom sources have become available to the mass spectrometry community. All of the commercial sources thus far offered have required appropriately placed flanges on the vacuum housing, with a direct line-of-sight to the end of a probe tip where the sample resides in the ion source. The distance over which the beam must traverse causes broadening and therefore lower incident flux than if the atom gun were in the vicinity of the sample and also induces alignment problems. The flange requirement greatly increases the cost and conversion time for instruments not equipped with an appropriately located blank flange. The most direct way to deviate these problems is to attach the atom gun directly to the mass spectrometer ion source, close to the sample and within the vacuum chamber. This paper describes such a miniature fast atom source which has been attached to a commercial high-voltage field desorption ion source. The present design could be used as an ion beam source for magnetic or quadrupole “organic” or “inorganic” secondary ion mass spectrometry (SIMS) and as a quadrupole FAB source if a deflector plate is added.
EXPERIMENTAL SECTION The fast atom source essentially consists of a miniaturized Capillaritron ion source, the operation of which has been described (2), and is shown in Figure 1. Capillaritron ion source tips were obtained from Phrasor Scientific, Inc. (Duarte, CA); these consist of a l/g in. diameter stainless steel tubing base with a pin-sharp came attached to one end. In the tip of this cone is a 25 pm diameter hole, through which gas flows and is the point at which a microdischarge is maintained by a high voltage applied to the tip. In the standard Capillaritron ion source, a counterelectrode consisting of a 0.75-in. tube with an end plate having a l/g in. diameter exit hole provides a shielded volume and contributes to the beam focusing. The counterelectrode in the miniature device is a stainless steel washer, 10 mm o.d., to provide a sufficient planar electrode surface, with a 2-mm hole in the center. The washer is attached to a support and conduction wire which is Connected to ground through a 2.2-MQresistor. The Capillaritron tip is positioned just inside the plane of the inside surface of the washer and centered with respect to the hole. The counterelectrode is positioned 2-5 mm from the entrance hole of tlhe ion source to allow good pumping speed in this region. Support for the entire source is provided by an insulating bracket made of Vespel resin, through which the tubing end of the Capillaritron tip passes. This bracket is screwed to the side of the VG Analytical (Altrincham, England) ZAB-2F field desorption (FD) ion source. Gas is supplied through silicone tubing and a glass capillary discharge suppressor and connected to the CL gas inlet connection. 0003-2700/82/0354-1917$01.25/0
I. du Pont de Nemours & Company, Wilmington, Delaware 19898 Argon gas flow rates, as measured by a floating-element type flowmeter (Show Rate), can be in the range 2-6 cm3/min. The applied discharge high voltage can be in the range 3-10 kV, and the discharge can be operated at 1-100 pA or more current. Current draw from the high-voltage supply is measured by a meter protected by a 2.2-Mil resistor. The gas flow rate and applied high voltage determine the operating current for the discharge; for the results presented here, the conditions were approximately 4 cm3/min argon, source housing pressure 4 X torr, and 4-5 kV applied and 5-10 pA discharge current. The discharge current is apparently a good measure of the ion beam current but does not directly measure the neutral beam flux. Nonetheless, the observed FAB ion intensities are linearly related to the discharge current. Figure 2 shows the arrangement of the new atom source with respect to the sample probe and the slightly modified FD source. The sample probe tip is a solid 3-mm steel rod, cut and roughly polished at an angle of 20° on one end, with support pins spot welded to the other end. With the atom gun mounted on the side of the source and the probe tip entering from the back (as is normal for FD work), the atom beam strikes the tip surface at an angle of 70’ from the surface normal. The atom beam enters the source via an existing electron entrance hole, and the probe tip passes through an FD extractor plate with an enlarged opening. The extractor plate is connected to the ion source potential. Glycerol was placed on the probe tip and approximately a microgram of sample was mixed into it. A small bubble of the solution was evident on the probe tip.
DISCUSSION The Capillaritron source, although originally designed as an ion beam source for sputtering work, also produces a significant flux of energetic neutrals. This is substantiated by these experiments with the miniature source, in which the ion beam must be deflected by the net 1-2 kV potential of the ion source with respect to the beam. Even with this deflection, a substantial secondary ion signal is observed, which therefore must arise from energetic neutral particle bombardment. The intense photon yield from the Capillaritron ion source is a further indication that excited species are present and that charge neutralization is probably taking place. In the region just past the end of the tip, relatively high argon gas pressures exist (ca. 1 torr). The gas flow is in the same direction as the ion extraction, possibly increasing the probability of neutralization. Although the neutralized beam flux and energy are difficult to determine, it seems likely that a substantial energy spread exists, as is also likely with other atom beam sources. The operation of the miniature source in the voltage range below the mass spectrometer ion source potential prevents ions from striking the sample. Simply increasing the voltage above this value and adjusting the gas flow to lower flow rates results in similar discharge operation but ion bombardment 0 1982 American Chemical Society
