800
Anal. Chem. 1983, 55, 800-802
(5) Drushel, H. V.; Sommers, A. L.; Cox, R. C. Anal. Chem. 1063, 35,
2166. (6) Berlman, I. B. “Handbook of Molecules”, 2nd ed.; Academic: (7) Berlman, I . B. “Handbook of Molecules”, 2nd ed.; Academlc: (8) Berlman, I. B. “Handbook of
Fluorescence Spectra of Aromatic New York, 1971; p 220. Fluorescence Spectra of Aromatic New York, 1971; p 306. Fluorescence Spectra of Aromatlc
Molecules”, 2nd ed.; Academic: New York, 1971; p 238.
RECEIVED for review October 1,1982. Accepted December 23, 1982. This work was supported by the U.S. Department of Energy.
Hydrogen-Tritium Exchange between Glass-Fiber Filters and Tritlated Water William P. Bryan Department of Blochemistry, Indiana University School of Medicine, Indianapolis, Indiana 46223
Hydrogen exchange between glass and water is an occasional problem in analytical procedures. For example, work on the infrared spectra of solutions of D 2 0 in CCll was complicated by the presence of HOD bands due to hydrogen exchange between D20 and borosilicate glass present in the infrared cell (I). Our own measurements of hydrogen-tritium exchange between biological membranes and tritiated water involved filtration of exchanged samples onto Whatman GF/C glass fiber filters, thorough washing with very dry tetrahydrofuran to remove all water, and liquid scintillation counting of residual tritium exchanged into the membranes. Because the amount of membrane preparation which could be filtered was limited, unacceptably high blank counts due to tritium exchange into the glass fiber filters were observed. This exchange occurred during the brief period between filtration and washing. The problem was largely eliminated by preheating the GF/C filters at 500 OC for a t least 1 day. It was thought that this hydrogen exchange between water and glass might be of sufficient interest to warrant further study.
EXPERIMENTAL SECTION The general method involved incubation of GF/C filters with tritiated water for varying periods followed by freezing and freeze-dryingto remove the water. Nonradioactive water was then added. This was followed by a period of back-exchange long enough for complete uptake of tritium by the water. The water was then liquid scintillation counted. Contact of the tritiated filters with air was avoided since exchange between tritium in the glass and atmospheric water vapor can occur. The procedure is based on a freeze-drying method for studying hydrogen exchange between proteins and water (2). Whatman glass microfiber filters are made from borosilicate glass microfibers of circular cross section. They have a mean diameter of 0.05 fim upward. No binder is present. The filters can be heated up to 500 OC without adverse effects. The GF/C grade has a medium retention efficiency (3). Pieces of GF/C fiiters (3-6 mg) were placed in borosilicate glass “minivials” used for liquid scintillation counting. These vials had been preheated at 500 OC for at least 1 day to minimize blank counts. To start exchange, 0.2 or 0.4 mL of deionized tritiated water (lo4 Ci/mL) was added to each vial, which was then tightly capped. Constant temperature for exchange at 1 “C was maintained with a constant temperature bath. A Scientific Products Temp-Blok module heater was used for constant temperatures above ambient. Several blank vials, not containing GF/C filters, were included in each run so that a blank correction curve could be established. After various periods of exchange, samples were frozen in dry ice, caps were removed, and the “minivials”placed in holders (11 cm closed tubes with 24/25 standard taper female joints) for freeze-drying. The linear vacuum manifold had a number of 24/25 male joints directly attached through stopcocks, so it was only necessary to add a holder and open a stopcock to start freezedrying a sample. Vacuum was obtained with a Cenco 7 mechanical pump. Two dry ice-ethanol traps, in series between the manifold and pump, were used to collect the tritiated water. Pressures below torr were obtained with this system. Samples were
freeze-dried and then dried overnight so that no physically adsorbed tritiated water was present. After drying, the samples and manifold were closed off and very dry COBwas passed into the manifold until a pressure somewhat above that of the atmosphere was attained. The gas was dried by passage through Aquasorb (Mallinckrodt Chemical Works). Special precautions were used to ensure that the water content of the gas was very low (4). The heavier than air COz acts as a protective blanket and eliminates any hydrogen exchange between the glass and atmospheric water vapor. Water was added to each sample by admitting C02to the sample, removing the holder from the manifold, adding 0.50 mL of water directly to the “minivial”, and capping it tightly. After water had been added to all the samples, they were set aside for back-exchange. All samples were allowed to back-exchange for a period at least three times the duration of the exchange run at a temperature greater than or equal to that of the run. A sample for determining the radioactivity of the Ci/mL tritiated water used in an exchange run was prepared by diluting 0.100 mL of this water to 100.0 mL with ordinary water and adding 0.100 mL of this dilution along with 0.40 mL of water to a “minivial”. Five milliliters of Bray’s solution was added to each sample. In order to ensure that each sample was counted with the same efficiency, pieces of GF/C fiiters were added to the blank vials and the vial used for water activity determination. After the samples were counted, a blank curve was prepared so that blank values due to tritium exchange with the glass of the vials could be subtracted from the total counts given by GF/C filter sample vials. Results were calculated from the expression 0.0111c hcalcd
=
where C is the corrected counts per minute in a GF/C filter sample, Co is the counts per minute in the sample for water radioactivity determination, w is the weight of the GF/C filter in grams, 0.0111 represents the grams of hydrogen present in 0.100 mL of water, and hc&d is the apparent number of grams of hydrogen per gram of glass which have exchanged with water. Equation 1takes no account of isotope effects. After complete exchange each chemically equivalent group of hydrogens in the glass will show an equilibrium isotope effect given by (GT)i(H20) Ii = (2) (GWi(HTO) where (GT)iand (GH)i correspond to concentrations of tritium and hydrogen in the group and (HTO) and (H,O) correspond to concentrations of tritium and hydrogen in the water. Upon attainment of equilibrium for each equivalent group of hydrogens (3)
where hi represents the true number of grams of hydrogen per gram of glass in the group (2).
RESULTS AND DISCUSSION Hydrogen exchange curves at several temperatures are shown in Figure 1. At 1 OC there is an immediate exchange of surface hydrogens. The 1OC curve also shows a slight slope which is presumably due to exchange of interior hydrogens.
0003-2700/83/0355-0800$01.50/00 1983 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983 120
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Flgure 2. Exchange curves between GF/C filters and tritiated water at 1 OC: no prior treatment, open circles; 1 h prior treatment with water at 100 "C, filled circles; 7 h prior treatment with water at 100 "C, open triangles.
Additional exchange of interior hydrogens is shown at 50 "C, 80 "C, and 100 "C. The scatter in data points a t higher temperatures is at least partly due to uncertainties in the relatively large blank corrections. The curves show no indication of leveling off due to complete exchange of all preexisting hydrogens in the glass. The appearance of the samples run at higher temperatures is interesting. A surprisingly large amount of fluffy lyophilized material was seen in both filter-containing and blank vials. This material is freeze-dried solubilized glass. It immediately dissolved upon readdition of water. Since solubilization is accompanied by hydrolysis (5))some part of h&d at 80 and 100 "C and perhaps also at 50 "C is due to reaction of the glass with water rather than to exchange of preexisting hydrogens in the glass with water. The production of new hydrogens in the glass due to reaction with water is clearly demonstrated by the data shown in Figure 2 where exchange curves at 1 "C are shown for untreated GF/C filters and filters which were heated with water at 100 OC for 1h or 7 h and then freeze-dried prior to the start of the exchange run. The rapid exchange values obtained by extrapolating the curves to zero time are 2.4 X 10" g of H/g of glass for untreated filters, 3.0 X lo4 g of H/g of glass for fiiters pretreated with water for 1h, and 4.0 X lo4 g of H/g of glass for 7 h of pretreatment. The higher values for the pretreated filters are due to rapidly exchangeable hydrogens in the solubilized glass and the production of additional rapidly exchangeable hydrogens at the glass surface. Pretreatment of the GF/C filters at 500 OC for 20 h results in a dramatic decrease in hcdcdas shown in Figure 3. The extrapolated value for 1 O C exchange drops from 2.4 X g of H/g of glass to 0.27 X lo4 g of H/g of glass. Far less exchange is also observed at 100 "C. The average value for
the exchange points at 100 "C is 2.55 X g of H/g of glass and there is no clear indication of any additional exchange in the period from 20 min to 7 h after the start of exchange (compare with Figure 1). This lorn of hydrogens from the glass is due to removal of water by heating (6, 7). Experiments at 50 and 100 "C in which exchange was carried out in the presence of M HC1 instead of deionized water gave points which fall on the corresponding curves in Figure 1. This shows that the exchange is not acid catalyzed. Hydrogens in glass are mainly present as silanol ( d i - O H ) groups (8). In the glass interior such groups can be either free or hydrogen bonded. It is also possible that molecular water might be present to some extent in the glass interior. Free and hydrogen bonded silanol groups are also present on the glass surface, but water adsorbed on the surface is removed by vacuum drying in the present experiments. It would be expected that surface hydrogens would exchange more rapidly than interior hydrogens and that hydrogen bonding would slow the rate of exchange. Water can react with glass by hydrolysis of silicon-oxygen bonds or by ion exchange of hydrogen ions for alkali ions in the glass (5). Water could also diffuse into the glass interior. All these processes would give rise to the production of additional glass hydrogens during exchange experiments. Not much work has been done on the use of tracers in the study of glass-water hydrogen exchange. A study of hydrogen exchange between D20 vapor and silica gels at room temperature and 1200 "C has been reported (9). Surface and interior silanol groups could be distinguished. Tritiated water vapor at temperatures from 700 to 1300 "C has been used to determine tritium penetration profiles into silica glass under conditions where rapidly exchanging tritiums are washed out prior to measurement (10). The present study of hydrogen exchange between liquid water and borosilicate glass fibers allows a determination of hcdcd for preexisting surface hydrogens, but exchange of preexisting interior hydrogens cannot be distinguished from the production of additional hydrogens due to reaction with water. Some preexisting interior hydrogens may only exchange if they are sufficiently exposed to water by hydrolytic processes (5, 11). Whatman GF/C glass filters show a relatively large number of preexisting hydrogens. Normally borosilicate glass shows a water content of 0.07% or less (12). The surface hydrogens of untreated GF/C filters (2.4 X lom4g of H/g of glass) correspond to an apparent water content of 0.22% alone. The large apparent water content of these glass fibers may be due to the manufacturing process. However, some of this apparent water content may be due to equilibrium isotope effect values greater than 1.0. It has been shown that deuterium is preferentially taken up over hydrogen at a glass surface (1).
802
Anal. Chem. 1983, 55, 802-805
The dramatic reduction of the hydrogen content of GF/C filters by heating a t 500 "C shows that heat treatment is a useful way of reducing problems due to the presence of hydrogen in glass. Most of the water lost by heating probably results from two silanol groups which are sufficiently close for hydrogen bonding rather than from isolated silanol groups. Isolated silanol groups cannot be removed by heating silica at 500 "C (13). The results in Figure 3 also show that heat treatment may be useful in reducing the reactivity of glass with water. The freeze-drying method described here would be useful in conjunction with surface area measurements, since the apparent surface density of hydrogens exchangeable with water in various insoluble solid materials could then be obtained. Registry No. Water, 7732-18-5; hydrogen, 1333-74-0.
LITERATURE CITED (1) Glasoe, P. K.; Bush, C. N. Anal. Chem. 1972, 4 4 , 833-834. (2) Byrne, R. H.; Bryan, W. P. Anal. Biochem. 1970, 33, 414-428. (3) "Glass Microflber Fllters"; Bull. No. 400, Whatman, Inc.: Clifton, NJ. 1976. (4) Bryan, W. P.; Rao, P. B. Anal. Chim. Acta 1976, 8 4 , 149-155. (5) Doremus, R. H. "Glass Sclence"; W h y : New York, 1973; pp 242-248. (6)Todd, 8. J. J . Appi. Phys. 1955, 2 6 , 1238-1243. (7) Todd, B. J. J . Appl. Phys. 1956, 2 7 , 1209-1210. (8) Boulos, E. N.; Kreldl, N. J. J . Can. Ceram. SOC. 1972, 4 7 , 83-90. (9) Davydov, V. Y.; Klselev, A. V.; Zhuravlev, L. T. Trans. Faraday SOC. 1964, 6 0 , 2254-2264. (10) Burn, I.; Drury, T.; Roberts, J. P. Silic. Ind. 1965, 30, 403-407. (11) Lanford, W. A.; et al. J. Non-Cryst. Solids 1979, 33, 249-266. (12) Wllllams, J. P.; et al. Am. Ceram. SOC. Bull. 1976, 55, 524-527. (13) Armistead, C. G.; et 81. J . Phys. Chem. 1969, 73, 3947-3953.
RECEIVEDfor review September 27,1982. Accepted January 17, 1983.
Thermal Vaporization for One-Drop Sample Introductlon into the Inductively Coupled Plasma Ellchl Kltazume Central Research Laboratoty, Hitachi Ltd., Kokubunj;, Tokyo 185, Japan
Inductively coupled plasma (ICP) atomic emission spectrometry has become one of the most useful techniques for determining multielement traces. However, the nebulizer commonly used for sample introduction is inefficient and requires a solution of at least 0.1 mL. More efficient sample introduction systems are required that can provide an analysis using less solution. If only one drop of sample could be effectively introduced into the ICP, absolute detection limits and analytical capability would be greatly improved. Recently, electrothermal vaporization techniques that use a tantalum filament (1)and graphite rod (2, 3) were investigated. However, their evaporation chambers were above 100 mL in volume because of the structure of the large electric current devices used for the thermal vaporization systems. Large chamber dead volumes will lead to temporal peak broadening due to vapor diffusion mechanisms. For improved detection limits, a smaller evaporation chamber volume is better for preventing sample aerosol dilution by the carrier gas. In addition, it is necessary to provide a sufficiently high heating rate to the filament to ensure a rapid rate of removal of analyte from the surface (2). This paper describes the application of a wire filament electrothermal vaporization technique for introducing samples into the ICP. In this technique, a microliter sample solution is vaporized on a filament heated by momentary condenser discharge (4)in a small evaporation chamber. Detection limits for six elements were measured and the effects of sodium, potassium, and lithium on analyte emissions were studied.
EXPERIMENTAL SECTION Apparatus. A block diagram of the overall apparatus is shown in Figure 1. The ICP atomic emission spectrometer system is a Jarrell-Ash Model 975 plasma spectrometer which has 40 fixed-wavelength channels, and an additional variable-wavelength channel. For observation of the signal profile, a 1-m grating monochromator (Nippon Jarrell-Ash, M-l,1200 grooves/mm) and a storage oscilloscope (Hitachi, Model V-038) were mainly used. The wire filament vaporization apparatus employed is shown in Figure 2. The filament was platinum or tungsten wire, 0.25-0.30 mm in diameter, and was placed in a small quartz evaporation chamber (about 4.5 mL in volume). The regular operating con-
Table I. Operating Conditions coolant gas flow rate plasma gas flow rate carrier gas flow rate generator forward power generator reflected power observation height desolvation current condenser charging voltage sample volume integration time
18 L of Ar/min 1.0 L of Ar/min 1.0 L of Ar/min 1.1kW 5W 16 mm above load coil 3.0-4.0 A 9.75 V for Pt filament 8.00 V for W filament 10 ML 3s
ditions are described in Table I. The filament temperature was measured by an optical pyrometer. Procedure. After the plasma was generated and stabilized, the argon flowing through the filament chamber was adjusted to the value established as optimal. The monochromator was set at the desired wavelength for the emission profile that was observed. A 10-pLsample was deposited at the top of the wire filament with an Eppendorf microliter pipet. The sample was dried slowly by passing a 3-4 A current through the filament and vaporized by pulse heating from a high capacity condenser (0.22 F, 16 V). The condenser had been charged to an appropriate voltage that was determined experimentally, and the filament temperature quickly increased to approximately 1400 "C for the platinum filament and to more than 1500 "C for the tungsten filament. The vaporized specimen was introduced into the ICP torch through polypropylene tubing and a three-way stopcock. The emission intensities of elements of interest were then integrated in the polychromator system and printed. (The condenser was discharged immediately after integration was started.) At the same time, the emission signal of the element of interest could be observed by the monochromator and storage oscilloscope. After the measurement, the condenser was discharged twice with no sample on the filament to clean the filament. Standard Solutions. A boron standard solution was prepared by dissolving reagent grade potassium tetrafluoroborate in water, because boron in samples prepared from boric acid was apt to evaporate when the solution was dried on the filament. A phosphorus standard solution was prepared by dissolving reagent grade potassium dihydrogenphosphatein water. Other standard
0003-2700/83/0355-0802$01.50/00 1983 Arnerlcan Chemlcal Soclety
