Anal. Chem. 1987, 5 9 , 1066-1069
1U66
found from wt of watern
re1 std dev, ppt
each tip used is calibrated by the operator. Manufacturer's claims for precision can generally be met by individual users, but volumes delivered tend to be below nominal values. We conclude that care should be taken in precise work to ensure that the operator employ consistent, careful technique when using mechanical action micropipets.
0.95 (tip 1) 0.97 (tip 2) 4.99 (tip 1) 5.01 (tip 2)
21 10 4 2 1
The assistance of students and staff at the University of Alberta in the collection of the data reported here is gratefully acknowledged.
Table IV. Summary of Calibration Data for a Positive-Displacement Adjustable 1-10-mL Pipet (Single Operator) volume, mL setting
1 5 10 a
10.02 (tip 1) 10.03 (tip 2)
1
ACKNOWLEDGMENT
LITERATURE CITED
Each value is the average of six measurements.
technique of the operator. We recommend that tips be calibrated by weighing the water delivered in all situations where an accuracy of a part per hundred or better is necessary. In summary, analytical data obtained by using mechanical-action micropipets may possess considerable bias unless
(1) Emanuel, C. F. Anal. Chem. 1973, 45, 1568. (2) Bo-Rad Cafalog 1886; Bio-Rad Laboratories: Richmond, CA, 1984; pp 26-27.
RECEIVED for review May 21, 1986. Accepted November 24, 1986.
Enhancement of Osmium Detection in Inductively Coupled Plasma Atomic Emission Spectrometry J. M. Bazan Lawrence Livermore National Laboratory, Livermore, California 94550 Use of 18'Re as a cosmochronometer has been investigated primarily by Luck et al. (1-5). As part of a project to develop 187Re-1870s chronometry (6,7), it became necessary to develop a method for the determination of nanogram quantities of osmium. The instability of osmium in various chemical environments limited the type of separation and purification chemistry that could be used. Previously reported osmium detection limits obtained in inductively coupled plasma atomic emission spectrometry (ICP-AES) vary from 38 ng/mL to 1 pg/mL (8-10). This variation was generally attributed to the unexplained chemical behavior of osmium. Summerhays et al. (11)observed, when using nebulization, that the signal intensity of osmium inThey attributed this rise creased with time in 9 M "OB. in intensity to the formation in solution of the volatile species Os04. The signal intensity did not reach a stable value over the period of investigation. We decided to take advantage of the volatility of Os04 and to introduce it directly into the plasma torch. A stable, enhanced signal for Os04was achieved after identifying (1)a stable matrix solution, (2) an efficient oxidant, (3) the ideal temperature for the reaction, and (4) the optimum instrumental operating parameters. Two distinct apparatus were designed to produce Os04 for on-line chemistry. Each setup was used for a specific purpose. A continuous-flow generator (12,13)(Figure 1)was used for samples contained in relatively unlimited volumes, i.e., 1 1 0 mL, for osmium concentrations 11 pg/mL, and for determination of the optimum instrumental operating parameters. A discrete batch sparging apparatus (14-16) (Figure 2) was used for samples of < l o mL volume and for osmium concentrations 51 pg/mL. Another desirable feature of the batch system was the total recovery of the sample solution, which could then be used for further elemental analysis. Since both apparatus generated a dry-torch environment, and required identical argon gas flows, the optimum operating conditions determined by continuous flow were directly applied to the discrete batch sparging. This on-line chemistry
technique had several advantages: (1)increased sensitivity for the detection of osmium, (2) removal of potential interferences caused by nonvolatile elements, and (3) simplified sample preparation. The increase in signal intensity obtained when using the continuous-flow apparatus was a factor of 10 over conventional nebulization. In the discrete batch sparging technique the increase in intensity ranges from 10 to lo3 over nebulization and all the osmium appeared to be removed from the sample.
EXPERIMENTAL SECTION Reagents. A standard solution of OsC1, in 10% HCl(2.7 N) at 1000 pg of Os/mL was obtained from Spex Industries, Inc., Metuchen, NJ. Water for solution preparation was double deionized, first, in large batch mode and, second, with a Millipore deionization system. Periodic acid (H5106)was purchased from Eastman Kodak Co., Rochester, NY. All other reagents were analytical grade and were used as received. Instrumentation. A Jarrell-Ash Model 975 Atomcomp direct reading spectrometer equipped with a 2400-grooves/mm concave grating was used. The rf generator has a frequency of 27.12 MHz and incident power set at 1.15 kW with the reflected power less than 25 W. The osmium wavelength used was 225.585 nm. The two OSO,generators (Figures 1and 2) were used as the sample sources for the ICP torch. The demountable torch had a thickwalled capillary-tip sample channel with a tip inside diameter of 1.2 mm. The argon flow rates for the dry plasma torch maintenance were coolant gas at 16.2 L/min, plasma or auxiliary gas at 0.5 L/min, and sample gas at 0.72 L/min. Procedure. The continuous-flow apparatus (figure 1)required that the reaction chamber (6) be maintained at an elevated temperature during the flow of the reactants. This was monitored by a thermocouple (7). The multichannel peristaltic pump (4) delivered 1 mL/min of the oxidant solution (3) and 1 mL/min of the sample solution (2). These were pumped continously through the mixing valve (5),through the heated reaction chamber at 135-140 "C, and into the separation chamber. The flexible connecting tubing from the peristaltic pump to the mixing valve and through the heated reaction chamber was made of Teflon, since it is nonreactive with the OsO, produced in this technique.
0003-2700/87/0359-1066$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
1067
c E
4l 2t HZ "4
0
60
30
I
2 90
A Time id)
Flgure 3. Continuous flow time study of the signal intensity generated by OsO, from 10 pg/mL Os in acidic solutions and 10% H,IOB. Figure 1. Continuous flow apparatus: 1, ice water; 2, sample: 3, oxidant; 4, multichannel peristaltic pump; 5, mixing valve: 6, heated reaction chamber; 7, thermocouple; 8, argon sample gas; 9, to the torch; 10, water circulating ports of the condenser: 11, separation chamber; 12, to waste tank.
Since the plasma torch of the ICP required a constant flow of sample gas to maintain its integrity, the discrete batch sparging apparatus (figure 2) had an argon flow rate of 0.6 L/min through the sample carrier argon inlet (2), while the auxiliary or plasma gas flow was kept at 0.5 L/min when the system was not being sparged. With stopcocks 1(A), l(C), and (4) closed, and 1(B) opened, 1mL of the oxidant solution was loaded into the reaction vessel; then 1 mL of the osmium solution was added. Stopcock 1(A) was opened for -5 s to back flush the reaction vessel with argon to eliminate any oxygen. Stopcock 1(B)was closed, followed by closing l(A). The plasma or auxiliary gas flow t o the torch was turned off via an external valve. Simultaneously, the 0.12 L/min sparging gas flow line (3) of the three-way stopcock (4) and stopcock 1(A) were opened. Immediately, the heating coil was turned on and data were collected. In -5 min the external reaction vessel temperature rose to -125 "C, measured by a thermocouple (9) placed between the heating coil and the wall of the reaction vessel. A final temperature of -135 "C was maintained for the remainder of the run. The reaction vessel was fitted with an ice-water-cooledcondenser for the previously explained reason. When the osmium tetroxide (0~0,) signal was depleted, stopcock 1(A) was closed and the three-way stopcock (4) was opened to the drain position with the heat turned off. The plasma or auxiliary gas flow was initiated. The spent solution can either be collected for further analysis or discarded. Stopcock l(C) was opened when a total wash of the apparatus was desired. This was done between samples without disturbing the parameters of the ICP plasma torch. The apparatus cooled in -4 min, and a new sample could then be loaded and the process repeated.
RESULTS AND DISCUSSION The stability of the signal generated by osmium with Figure 2. Discrete batch sparging apparatus: 1, stopcocks made of Teflon; 2, sample carrier argon inlet; 3, sparging argon inlet: 4, three-way Teflon stopcock; 5, sample injection port; 6, drain; 7, frit; 8, heating coil; 9, thermocouple: 10, water circulating ports of the condenser; 11, sample gas line made of Teflon; 12, to the torch.
In the ice-water-jacketed separation chamber, the surface of the liquid was swept by a continuous flow of argon sample gas, which carried the osmium tetroxide vapor to the ICP ionizing torch. The ice-water condenser removed most of the water vapor that would have been otherwise transported along with the OsC4,thus the dry-to-semidry torch environment. The drain tubing of the separation chamber had an inside diameter small enough to keep a residual amount of the reactants in the chamber base during the sample exposure. The response of the analyte was recorded for 10 s followed by a 5-s data transfer; therefore, four sample exposures per minute were collected. Between samples, the chamber was easily flushed with reagent blanks and reached background levels in 3 min, while maintaining the integrity of the plasma torch.
nebulization, and generated by osmium with the Os04 continuous-flow generator from various matrices, was investigated. Figure 3 illustrates that acidic solutions did not yield a constant signal intensity for Os04 but increased with time. A signal intensity enhancement for osmium was noted when HCl solutions were nebulized (Table I). This behavior was similar to that noted by Summerhays (11). Four separate new dilutions a t 10 pg/mL for each HCl (0.6, 3, and 6 N) solution studied were prepared. A 22-day time study of signal intensities resulting from nebulization and continuous flow of each of the four samples in each acid concentration yielded exactly the same signal intensities from time = 0. X-ray fluorescence measurements were done on the osmium standard solution to ensure its stability in relation to the osmium concentration. They were 1000 pg/mL f 5% in 1979,960 pg/mL A 5% in 1983, and 990 pg/mL f 5% in 1984; therefore, the total amount of osmium present in the 10% HC1 standard solution was constant within the analytical uncertainties of 5 % . The
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
1068
-r-
Table I. A Time Study of the Signal Intensity of 10 ppm Os Solution in Various Matrices Using Nebulization and Continuous Flow matrix 10 pg/mL Os
days 22
1.3 x 104 2.0 x 104
8.5 X 10' 2.2 x 104
N HC1
1 30
1.2 x 10' 2.1 x 104
8.0 x 103 1.6 x 104
6 N HCI
1 22
1.2 x 104 2.2 x 104
5.7 x 103 1.3 x 104
3
1
1 22
2.1 x 104 2.0 x 104
2.2 x 105 2.1 x 105
0.1 N ",OH
1 2 22
2.3 x 104 2.3 x 104 2.3 x 104
7.0 x 103 2.0 x 105 2.0 x 105
1 17
1.9 x 104 1.4 x 104
2.1 x 105 2.2 x 104
i
I
0
20
40
,
10
60
Minutes
20
30
,
s
lo'-1
s
1
2
4
6
B
10
12
11
I6
18
20
22
1 24
,
, 26
1 28
1
,
30
1 32
,,A 34 36160min
Tima ,m ""In
1 N ",OH
0.1 N NaOH
----
intensity NEB tetroxide
time,
0.6 N HC1
--------
I
40
Days Time
Figure 4. Continuous flow time study of the signal intensity generated by OsO, from 10 pglmL ammoniacal solutions and 10% H,IO, increasing osmium intensities observed by ICP-AES suggest a kinetic reaction mechanism, which facilitated the production of osmium tetroxide in solution with time in the acidic matrices investigated. The magnitude of the Os signal from the nebulized sample was 1-100 times greater than the signal produced by the tetroxide in continuous-flow mode from the same acidic solution (Table I). The reverse was true for basic solutions: Table I shows that the signal intensity from the osmium tetroxide was greater than from the nebulized sample by a factor of 10. It should be noted that the osmium signal intensity from the OsO, continuous-flowgenerator was greater in the basic solutions than in the HC1 solutions by factors of 10- 103. It was found that the basic solutions had a definite time of "growth" before the maximum signal intensity was attained (Figure 4). A 1N ",OH and 0.1 N NaOH solution reached maximum intensity in -1.5 h, while a 0.1 N ",OH solution required 24 h for the same response a t identical osmium concentrations. Once the maximum signal intensity was attained, it remained constant for -4 months for the 0.1 N ",OH solution, -1.5 months for the 1N NH40H solution, and -1 week for the 0.1 N NaOH solution. I t appeared a strong base such as NaOH required a lower concentration to reach a maximum signal intensity for the same time of "growth of a ",OH solution. The maximum signal intensity is maintained for a much shorter time than with the ammoniacal solutions. The longer stability times of the ammoniacal
Figure 5. Os signal intensity by discrete batch sparging of 1 mL of 0.1 N ",OH at 10 pg/mL Os and 1 mL of 10% H,IO,. solutions would allow the preparation of standard solutions for the determination of the osmium concentration of an unknown sample. When the ammoniacal solutions at 10 pg/mL Os were used, it was extremely helpful to have the naturally occurring solution color change. The first color change, light yellow-green to clear, indicated when the solution would yield a maximum signal intensity. The second color change, clear to light yellow-orange, indicated when the solution was becoming unstable and beginning to lose osmium tetroxide signal. This observation reinforced our hypothesis that an unexplained kinetic reaction had been occurring with the acid solutions and now within the basic solutions. The time required for this mechanism to take place and to yield a stable maximum signal intensity was much shorter in basic solutions. There was a slight color change noted in the acid solutions after approximately 10 months. This change was not as prominent as the ammoniacal solutions. The continuous-flowapparatus was used to investigate eight oxidizing agents: 2.5% H,I06, 5% H5106, 10% H5106,10% KMnO,, 10% K2Cr207,30% HzOz,concentrated "OB, and The generated OsO,, signal intensity and the fuming "OB. ease of handling were used as criteria in determining the most suitable oxidizing agent. Varying the oxidants, but maintaining the same osmium solution matrix, resulted in signal intensities that were comparable. Varying the osmium solution matrix, Le., molarity, acid, or base, but maintaining the same oxidant resulted in distinctly different maximum signal intensities for the OsO, produced. Figures 3 and 4 illustrate these differences. Periodic acid was chosen for the following reasons: (1)ease of handling, (2) long shelf life, (3) high and equal signal intensities for the three concentrations of oxidant used, and (4) least number of foreign elements introduced into the effluent if further analysis was required. Data from the discrete batch sparging apparatus can be plotted as an evolution curve of OsO, as the temperature is raised and then maintained at a maximum of 135-145 "C. Figure 5 illustrates the OsO, evolution from 1 mL of 0.1 N ",OH containing 10 pg/mL osmium and 1 mL of 10% H,I06. Various concentrations of osmium ranging over 4 orders of magnitude were studied. The OsO, evolution curves of these solutions yielded a linear relationship between the integrated peak areas and sample concentration, but the peaks' heights were not linear with concentration. Evolution times were shorter for less concentrated solutions. Figure 6 shows a typical plot for OsO, generated from 1 ng/mL of 0.1 N ",OH and 1 mL of 10% H5106. The 1-ng sample's signal intensity reached background in -24 min, while that of the 10-pg sample reached background in -1.5 h. It can be seen from figures 5 and 6 that the concentration of an osmium unknown solution can be determined by integrating the area under the curve and comparing it to a standard. No attempt was made to measure the detection limit directly, but extrapolation of Figure 6 strongly suggested the picogram level.
Anal. Chem. 1987, 5 9 , 1069-1071 I
1
I
I
l
l
I
l
I
l
I
l
1069
many helpful suggestions, and (3) R. Brown, Analyteck Instrumentation and Service, Inc., for discussions regarding the technique of sparging. Registry No. Os, 7440-04-2; OsO,, 20816-12-0.
LITERATURE CITED
1 .
(1) Luck, J. M.; Birck, J. L.; Allegre, C. J. Nature (London) 1980, 283, 258-259. (2) Luck, J. M.; Allegre, C. J. Nature (London) 1983, 302, 130-132. (3) Allegre, C. J.; Luck, J. M. Earth Planet. Sci. Lett. 1980, 48, 148-154. (4) Luck, J. M.: Allegre. C. J. Earth Planet. Sci. Lett. 1982, 6 1 , 291-296. (5) Luck, J. M.; Turekian, K. K. Science 1983, 222,613-615. (6) Lindner, M.; Leich, D. A.; Borg, R. J.; Russ, G. P.; Bazan, J. M; Simons, D. S.; Date, A. R. Nature (London)1988, 320,246-248. (7) Russ, G. P.; Bazan, J. M.; Leich, D. A,; Date, A. R. Presented at the
Heated.
I
27th Rocky Mountain Conference, of the Society for Applied Spectrometry, Denver, CO, 1985. (8) Barnes, R. M. ICP I n f . Newsl. 1980, 5 (E),416. (9) Wenge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979,
L
Background blank
3
x
102
l
1
1
1
1
1
1
1
1
1
1
t
33 (3),206. (10) Boumans, P. W. J. M.;Barnes, R. M. ICP I n f . Newsl. 1978, 5 (ll), 445. (11) Summerhays, K. D.; Lamothe, P. J.; Fries, T. L. Appl. Spectrosc. 1983, 37,25. (12) Oda, C. E.;Ingle, J. D. Anal. Chem. 1981, 53,2030-2033. (13) Hawley, J. E.; Ingle, J. D. Appl. Spectrosc. 1975, 47,719-723. (14) Lehrer, L. H.; Ind. Eng. Chem. Process Des. Dev. 1968, 7 (2), 226-239. (15) Knobloch, J. 0.US. Patent 3 956 373, 1976. (18) McWhirter, J. R.; Albertson, J. G. U.S. Patent 3775307, 1973.
RECEIVED for review May 12,1986. Resubmitted November 3, 1986. Accepted December 2, 1986. This work was performed under the auspices of the US. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. This invention is owned by Lawrence Livermore National Laboratory and a patient application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to Patent Counsel, Department of Energy, Livermore, CA 94550. Refer to IL-7727.
Optical Fiber Interface between a Laser and a Tandem Mass Spectrometer W. Bart Emary, Karl V. Wood, and R. Graham Cooks*
Chemistry Department, Purdue University, West Lafayette, Indiana 47907 Laser desorption (LD) is increasingly used in mass spectrometry (MS) to transform condensed-phase molecules into gas-phase ions. Samples of particular interest include biological materials (1-5) and polymers (6, 7). Modification of the ion source configuration and surrounding vacuum manifold is usually necessary to accommodate a laser. We report on a fiber optical interface which utilizes a direct insertion probe and provides an inexpensive and simple means of achieving laser desorption. The desorbed ions can be examined directly or the neutrals can be independently ionized by chemical ionization (CI) or electron impact (EI) prior to mass analysis. Results obtained by using this device with a Nd:YAG laser and a triple quadrupole mass spectrometer are presented. A brief description of an alternative approach using fiber optics for laser desorption has appeared recently (8).
EXPERIMENTAL SECTION The fiber optic probe assembly (Figure 1)consists of two parts, a 1/2-in.hollow stainless-steel probe and the optical fiber. O-ring seals are used for vacuum isolation. A standard Finnigan chemical ionization (CI) volume fits snugly to the end of the probe so that samples can be changed simply by withdrawing the probe and interchanging ion volumes. The optical fiber (100-cm length) was a PCS 1000 (Quartz Products, Watchung, NJ) plastic-clad fused silica core (1mm) type. The 8-mm beam of the Nd:YAG laser
(Quanta Ray) was focused onto the end of the fiber, from which approximately 0.5 cm of the plastic cladding had been removed, by using a germanium lens (Oriel Corp. of America, 20-cm focal length). The laser was operated at 1.06 pm, 10 Hz, with 200-ps pulse widths of 15 mJ/pulse. The power used was about IO4 W/cm2 (up to IO' W/cm2 was available). The fiber terminated 1-2 mm from the sample and created a 0.01-cm2spot size. The Finnigan triple quadrupole (TSQ) mass spectrometer was scanned at an average rate of 1 dalton per laser pulse. Laser irradiation was initiated a few seconds after the data acquisition program was started, and no hardware has been added to synchronize the laser and mass analyzer. Ion emission generally persisted for 0.5-1 ms as determined on an oscilloscope. Isobutane (0.5 torr) was found to increase ion currents and was always present in the ion source. Collision induced dissociation (CID) spectra were obtained by mass selecting the ions of interest with the first analyzer and subjecting them to 20-eV collisions with argon (1.5 mtorr) in the second quadrupole. The third quadrupole was used as a mass analyzer to characterize the products from collisional activation. Five to ten scans were summed together to generate the spectra shown. Compounds were purchased from Sigma Chemical (St. Louis, MO) and either were deposited as solutions on the inside of the ion volume with a 10-pL syringe and subsequently dried or were electrosprayed (9)into the volume. Good quality, full mass spectra could be obtained with 1-10 pg of material. These quantities of sample are on the same order as in other laser desorption ex-
0003-2700/87/0359-1089$01.50/0 0 1987 American Chemical Society
