Anal. Chem. l907, 59, 2922-2924
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CORRESPONDENCE Implementation of Ion-Exchanger Absorptiometric Detection in Flow Analysis Systems Sir: Ion-exchanger absorptiometry is based on the direct measurement of the degree of light absorption by an ion-exchange-resin phase that has sorbed a sample component. By direct application of the method, trace elements in water samples can be determined without preconcentration (1-8). Analytical methods for up to 20 elements have been developed and are summarized in a recent review (9). As reported in a previous paper, the sensitivity of this method was enhanced by using ion-exchanger layers much thicker than those previously employed (IO). Sensitivity is also enhanced by adopting a system in which the ratio of sample volume to the amount of ion exchanger is high, provided that the distribution ratio for sample species is high enough. This technique affords high sensitivity dispensing with large volumes of sample solution, if the sample species, concentrated in a very small volume of ion exchanger packed in a flow-through cell, is determined directly in the solid phase by absorption spectrophotometry. The implementation of on-line detection with ion-exchange retention should further simplify our method and broaden its flexibility for the determination of trace elements in water samples. The purpose of this study is to show how this method can be extended to flow analysis. EXPERIMENTAL SECTION Chemicals. All chemicals used were analytical grade. Ionexchanged water filtered through a 0.45-pm Millipore filter was used for the dilution of samples and reagents. All solutions were thoroughly filtered beforehand. A standard copper solution (loo0 ppm for copper) was prepared by dissolving cupric chloride dihydrate in a dilute nitric acid solution. The concentration was standardized by ethylenediaminetetraacetic acid (EDTA) titration with 1-(2-pyridylazo)-2naphthol (PAN) as indicator. Bio-Rad AG 5OW-X12 (100-200 mesh) in the hydrogen form was used. Apparatus and Procedure. A flow-throughcell was supplied by Nippon Sekiei Glass Co.; it was black-sided, 10 mm in length, and 1.5 mm in diameter. By means of a glass capillary tube with the same inner diameter as the flow-through cell, an amount of ion exchanger 3-5 mm in length was measured and then the resin beads were poured into the cell. The cell was blocked with nylon-fiber wool packing; therefore, only the light-path portion of the cell was filled with the ion exchanger (Figure 1). Light measurements were made with a Nippon Bunko double-beam spectrophotometer (Model UVIDEC-320), that was placed vertically in order to make the top part of the ion-exchanger column flat, as shown in Figure 1. An inside-mirrortube (12 mm i.d., 40 mm length) was placed between the cell holder and the light-detector window to recover part of the light scattered from the cell (10). A perforated metal plate of attenuance 2 was placed in a reference beam to balance the light intensities. A carrier solution (0.014 mol/dm3 nitric acid) stream was pumped by a middle pressure pump (Gasukuro Kogyo, Model MPD-3MG) with a diaphragm dampener. A sample (0.01-0.5 pmol of copper) was introduced into the stream by means of a Teflon six-way rotary valve. A sample loop of 4.41 cm3was made by using a Teflon tube (0.5-mm i.d.). The flow rate was maintained constant in the range from 1.2 to 1.5 cm3/min. All tubing was made of Teflon. The increase in attenuance was continuously monitored at 800 nm and recorded on a strip-chart recorder set
at 0.5 absorbance full scale. A solution for desorbing the copper in the ion-exchanger phase (2 mol/dm3 nitric acid) was introduced into the stream by means of another Teflon six-way rotary valve with a sample loop of about 5 cm3. Distribution Ratio Measurements. Various volumes of nitric acid and 0.50 g of Bio-Rad AG 5OW-X12 (100-200 mesh) in the hydrogen form were added to 100-cm3solutions containing 0.02 mmol of copper. After equilibration had been attained, the copper concentration in the solution was measured by atomic absorption spectrophotometry. The distribution ratio for copper is defined by the equation D = [(mol of copper sorbed)/(kg of resin)]/[(mol of copper) / (dm3of solution)] (1)
RESULTS AND DISCUSSION The present method can be applied to any system in which colored sample species can easily be sorbed on an ion exchanger and desorbed from it by an appropriate change in eluting conditions. For the ease of experiments, the applicability was first examined for copper and the d-d electron transition band (egmm = 12.1 dm3/(mol cm)) was used for measurements. Although the term “attenuance” can be used instead of absorbance when light loss is due to not only absorbance but also scattering, the attenuance value is the same as the absorbance read from the meter (9, IO). Distribution Ratio and Sample Volume. The distribution ratio of copper can be a measure of the copper retention in the flow-through cell. The present system makes it possible to change the D value only in relation to the concentration of nitric acid. Color-developing profiles of copper for different distribution ratios are shown in Figure 2, where the measured D value in each nitric acid solution is also shown. After the sample injection, the attenuance increased because of copper sorption in the cation-exchanger phase. In high distribution ratio systems, almost all the copper in the 0.17-cm3 sample solution was retained after passing at least 20 cm3 of 0.014 mol/dm3 nitric acid carrier solution (curve C). In systems with D values lower than 10000, however, the copper injected was eluted from the cell in a fairly short time. Therefore, a 0.014 mol/dm3 nitric acid carrier solution and a 2 mol/dm3 nitric acid copper-desorbing solution were used. Variations in sample volumes ranging from 0.2-8.7 cm3 resulted in proportional increases in absorbance. Much higher sensitivities can be obtained by employing larger amounts of sample solution under high distribution-ratio conditions. The sample volume employed was 4.41 cm3. Flow Rate. A change in flow rate from 0.38 to 2.51 cm3/min decreased the sensitivity by 30%. At a constant flow rate, however, the relationship between measured absorbance and copper concentration was strictly linear. High flow rates increased pressure and therefore the flow rate was kept constant in the range of 1.2-1.5 cm3/min. Within this range, the pressure was 1-2 kg/cm2. Calibration. Figure 3 shows typical examples for successive color development. It is not necessary to desorb the colored species after each measurement. The pressure change resulting from switching on or off the sample-injector valve causes a small change in background attenuance. The at-
0003-2700/87/0359-2922$01.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
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ion-excharger beads
Figure 1. Flow-through cell for ion-exchanger absorptiometry.
1
30
60
I
Time (rnin)
Figure 3. Calibration of copper by the flow system of ion-exchanger absorptiometry: sample, 4.41 cm3 (0.014 mol/dm3 “OB); (A) 0.048, (B) 0.096, (C) 0.144, and (D) 0.192 pmol of Cu; flow rate, 1.30 cm3/min.
Table I. Comparison of Sensitivity
diameter of flow-through cell S, cm2
V , cm3 SRdcda
Time (min)
SRobndb
Figure 2. Colordeveloping profiles of copper for different distribution ratks: sample, 0.17 cm3, 0.231 pmd of Cu; carrier solution, (A) 0.28, (B) 0.070, and (C) 0.014 mol/dm3 HNO,; flow rate, 1.17 cm3/min.
tenuance just after the sample injection can be measured from the background on the chart. Copper ions in the resin phase are easily desorbed with a 2 mol/dm3 nitric acid solution. The cell can be repeatedly used for measurements. The calibration curve obtained was reasonably straight for 0.01-0.5pmol of copper. Procedural blanks, if present, were determined and subtracted from the sample concentration. Sensitivity. In order to compare the present method’s sensitivity with that of conventional absorptiometry, it is sufficient to measure and compare each method’s absorbance ratio for sample solutions with the same target element concentration. If all the colored species injected are retained in the resin phase in the flow-through cell, the sensitivity ratio (SR)is closely related to
SR = ( F V ) / ( & ~ )
(2)
where e is the molar absorptivity, V the sample volume injected in cm3, S the cross-sectional area (normal to the light beam) of the flow-through cell in cm2, and 1 the light path length in cm for solution absorptiometry. The bar over c refers to the ion-exchanger phase. Equation 2 implies that the diameter of the flow-through cell and the sample volume are very important factors for obtaining high sensitivity. For copper ions in a 0.014 mol/dm3 nitric acid solution, the distribution ratio was sufficiently high (D= 6.2 x lo4),and z is approximately equal to e. With fixed sample volumes, the effect of S was examined by using two flow-through cells with different S. The SR values expected from eq 2 were in reasonably good agreement with the results obtained (Table I). Provided that the light intensity to be detected is not below the limits of the light detector used, a higher sensitivity can be obtained by using a cell with a smaller cross area. Particles
3.0 mm diameter
1.5 mm diameter
0.0707 4.41 62.2 49.3
0.0177 4.41 249 220
1 = 1 cm. Carrier solution, 0.014 mol/dm3 nitric acid solution; flow rate, 22 cm/min.
in the solution should be removed prior to measurement because their accumulation on the top of the ion-exchanger column gives rise to a background attenuance drift and sometimes to a change in sensitivity. Precision. Precision was measured with 4.41 cm3 sample solutions containing 0.048 pmol of copper. For six determinations, the absorbances obtained were 0.0236 f 0.0008. The relative standard deviation was 3.4%. Effects of Coexisting Ions. The applicability of the present system to copper determination is restricted to dilute salt or acid solutions. Hydrochloric acid and sulfuric acid did not interfere if the concentrations were lower than 0.01 mol/dm3. A calibration curve prepared at the same concentrations of salts and/or acids as those in the sample solution is recommended. Small amounts of cations that absorb light at 800 nm cause positive errors. Nickel interfers when its molar ratio to copper is 1:l or greater. Cobalt does not interfere when its concentration is up to 10 times that of copper. Comparison with O t h e r Methods. This method offers several advantages for determination of trace elements in water samples. In conventional flow analysis methods such as flow injection analysis, sensitivity is strongly affected by the sample-zone color dispersion. The problem caused by this dilution effect does not occur with the present method, because the concentration of colored species of a target element and absorptiometric measurements are carried out simultaneously. The signal from the spectrophotometer can be amplified because the background attenuance fluctuation is very small. This results in high sensitivity with smaller volumes of sample solution than those necessary for ion-exchanger absorptiometry by batch technique. The flow technique affords a repeatable measuring system involving simple operations and
Anal. Chem. 1987, 59, 2924-2927
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assures a precision that is often adequate for routine analyses. The present method can be applied to many systems. In a high molar absorptivity and highly selective system such as the one using chromium(V1) and 1,5-diphenylcarbazide, considerable trace levels of the target element can be determined in natural water (11). These systems will be discussed in detail in later papers (11, 12). Registry No. Cu, 7440-50-8. -
LITERATURE CITED Yoshimura, K.; Waki, H.; Ohashi, S. Talanta 1976, 23, 449-454. Yoshimura, K.;Toshlmitsu, Y.; Ohashi, Ta&n& 1980, 27, 693-697. Yoshimura. K.; Nlgo, S.; Tarutani, T. Talanta 1082, 29, 173-176. Waki, H.; Korklsch, J. Talanta 1983, 30. 95-100. Ishii, H. Fresenlus' Z . Anal. Chem. 1984, 319, 23-28. Capkan, F.; Valencia, M. C.; Capitan-Vailvey, L. F. Mkrochim. Acta 1984, III, 303-311.
s.
(7) Shrkdah, M. M. A.; Ohzeki, K. Analyst (London) 1985, 7 70, 677-879. (8) Yoshlmura, K.: Ishii, M.; Tarutani, T. Anal. Chem. 1986, 58, 591-594. (9) Yoshimura, K.; Waki. H. Telenta 1985. 32, 345-352. (10) Yoshimura, K.;Waki, H. Taknta 1987, 34, 239-242. (11) Yoshimura, K. Proceedings of International Symposium on New Sensors and Methods for Environmental Characterization, S1-02, Kyoto, 1986; submitted for publication in Analyst (London). (12) Yoshlmura, K. Bunseki Kagaku, in press.
Kazuhisa Yoshimura Chemistry Laboratory College of General Education Kyushu University Ropponmatsu, Chuo-ku, Fukuoka, 810 Japan RECEIVED for review
November
39
1986. Accepted
139
1987.
Characterization of Multilayer Thin Films by Laser- Induced Thermal Desorption Mass Spectrometry Sir: Identifying the molecular species adsorbed on a surface is a difficult analytical problem that is important in studies of heterogeneous catalysis, corrosion, and surface contamination. Most surface analytical methods reveal very little about the molecular identity of an adsorbate. For example, low-energy electron diffraction (LEED) is most sensitive to the structure and ordering of a surface, and Auger electron spectroscopy (AES) only reveals the elemental composition. X-ray photoelectron spectroscopy (XPS) can yield both the elemental composition and the oxidation state of species present on the surface. Surface spectroscopic techniques such as electron energy loss spectroscopy (EELS), Raman spectroscopy, and infrared spectroscopy have been used to identify surface molecular species. However, these techniques lack the sensitivity and specificity that are needed to identify complex molecular adsorbates, particularly if there is a mixture of several species present. A method that shows great promise for identifying molecular adsorbates is laser desorption coupled with mass spectrometry (1,2). Laser desorption experiments fall into two major categories: high laser power experiments in which ions are formed directly by the laser pulse (&5), and low laser power experiments in which only neutrals are desorbed (6-11). The first method is used by commercial laser mass spectrometers such as the LAMMA, which is manufactured by Leybold-Hereaus. The second method is often referred to as laser-induced thermal desorption (LITD) because the surface is subjected to a rapid temperature jump but is not ablated. In this paper we report on the use of laser-induced thermal desorption and postionization by an electron beam to characterize molecular species absorbed in the amorphous carbon layer which covers the surface of a computer magnetic hard disk platter. Adsorption of molecular species in and on hard overcoat layers has been found to be of great importance to the performance characteristics of the magnetic disk (12). Since the samples we have examined have a carbon overcoat layer, identifying carbon-containing molecules on the surface presents a particularly difficult analysis problem. Figure 1 is a schematic drawing of a typical laser-induced thermal desorption experiment. A pulsed laser beam is focused onto the surface to rapidly heat a small part of the sample. At low laser power, only neutral species are vaporized from the surface. A few centimeters away from the surface, an electron beam ionizes the desorbed molecules, and the ions
thus formed are detected by a Fourier transform (FT) mass spectrometer. Previous work in our laboratory has shown that this method can be used to identify a wide variety of organic molecules adsorbed on a single-crystal platinum substrate (9-11). In most cases, the electron ionization fragmentation patterns were indicative of the original surface adsorbates. Submonolayer sensitivity is possible with the FT mass spectrometer, and a complete mass spectrum can be obtained for each laser shot.
EXPERIMENTAL SECTION
Our aim in these studies was to identify the molecular species adsorbed on the surface of magnetic disk structures with carbon overcoats. The samples used are multilayer structures typical of commercial metal film hard disks (12), consisting of an aluminum base, a nickel and phosphorus layer, and a magnetic layer which is a metal film of Co/Cr/Ni. An overcoat layer of carbon is sputter deposited on top of the magnetic layer to act as a protective film and lubricant. Auger electron and X-ray photoelectron spectroscopy experiments were carried out by using a VG ESCALAB MK II surface analysis instnunent. Auger electron spectroscopydepth p r o f h g experiments showed that the carbon films were approximately 30 nm thick. The Fourier transform mass spectrometer utilized for these experiments has been described previously (9, 10). Basically, it consists of a large electromagnet, an ultrahigh vacuum chamber, and a data system manufactured by the IonSpec Corp., Irvine, CA. The magnetic field was normally set at 0.6 T, but it was lowered to 0.05 T to detect Hzat m/z 2 and was raised t o 1.2 T to obtain high-resolution mass spectra. The vacuum chamber is pumped to a base pressure of 6 X lo-" Torr by both an ion pump and a turbomolecular pump. The sample to be studied (typically 1 cm2) is positioned in front of a hole in one of the electrodes of the FTMS analyzer cell by a Varian sample manipulator. A focused laser beam (Lambda Physik Model EMG 103 MSC excimer laser, wavelength 248 nm, pulse width 20 ns) enters the cell through a hole opposite the sample and strikes the sample at near normal incidence causing heating of a 0.2 mm2 area. The reflected laser beam exits the system by retracing the incoming path. Molecules desorbed from the sample expand out to fi the volume of the chamber and some of them pass through a pulsed 10-fiAelectron beam, which traverses the cell perpendicular to the path of the laser beam and parallel to the magnetic field. Ions formed in the electron beam are trapped by the magnetic and electric fields and are mass analyzed by measuring their cyclotron resonance frequencies. Since all the desorbed species are detected simultaneously, the relative abundance5 of desorbed species can be compared directly, without concern for
0003-2700/87/0359-2924$01.50/00 1987 Amerlcan Chemical Society
