(305
Anal. Chem. 1982, 5 4 , 605-607
/
d
I
I
I
c
n
/ / I
,
b
Flgure 1. Drawing of OTTLE: (a)quartz seallng tape, (b) gold minigvld, (c) Interior edge that approximately defines electrode width, (d) hem-
itube.
05
OTTLE is finally assembled, is ground into the edge that was originally the bottom portion of the Beckman cuvette. The other flat is cut to size from a quartz microscope slide. Two applications of quartz sealer are fixed to the ground surfaces of the Beckman flat. (The quartz sealer comes as a ribbon, 25 km thick, and is held in place by its own adhesive which burns off during the sealing process.) With the flat face down, an appropriately sized gold minigrid is laid across the flat so that a portion of the grid material overlaps both raised lateral edges of the flat. The overlapping grid material provides electrical contact to the interior portion of the grid (vide infra). After the other quartz flat is positioned on top, a larger quartz flat and a 60-g weight are placed on the entire assembly, which is then placed in an oven at room temperature. The weight, in addition to securing the assembly from misalignment, provides adequate pressiure to force the tape through the grid material when the tape melts. Melting the tape to form a seal normally requires an oven temperature above 1000 "C. This temperature is impracticable because of the softening point of gold is below 1OOO OC. However, the pressure of the weight is sufficient to achieve a vacuum seal without deformation of the gold minigrid, when the temperature of the oven is allowed to rise to only 850 "C and then cool down to room temperature. At this point the OTTLE is a fused assembly and therefore impervious to separation by normal use. Electrical connection is made to the grid material extending beyond the lateral edges of the OTTLE. This portion of the grid is isolated from the solvent with quartz hemitubes (d in Figure 1) sealed to the OTTLE edges. The hemitubing is made from 2 mm diameter quartz tubing closed at one end and then cut in half lengwise. The cut edges of the hemitube are ground flat and quartz sealing tape is applied (vide supra).
00
-1 0
05
E, V vs
-1 5
-2 0
-2 5
AgIAgCIO,
Figure 2. Thin-layer cyclic voltammogram of 1.04 mmol/L mei hyl viologen in propylene carbonate 0.5 mol/L TEAP supporting electrolyte vs. AgIAgCIO,. Scan rate = 2 mV/s.
The grid material is trimmed leavirAgapproximately 1-2 mm exposed. With the quartz hemitubing positioned appropaiately, a seal is formed with gentle heating from a torch. Sirice the edge of the OTTLE is not ground smooth, additional applications of the quartz sealer are made to the exterior seam. The procedure is repeated on the opposite edge. Electrical connection is made by filling the quartz tube with conducting cement (ACME Chemicals-Insulation Co., New Haven, CT) and inserting a length of wire. An OTTLE constructed in this manner has been used c'xclusively in DMF solutions for over 50 h in our laboratory. No discernible change in the OTTLE has been detected. A typical thin-layer cyclic voltammogram with this OTTLE is shown in Figure 2. We have also used this procedure to seal quartz tubing to a quartz flat with a gold minigrid between and subjected it to several freeze-thaw cycles from 77 to 373 K. Since the seal maintained a vacuum (0.01 torr), it is evidence that this method of sealing gold mesh to quartz introduces no sevme stress.
LITERATURE CITED (1) DeAngeiis, T. 594-597.
P.; Heineman, W. R. J . Chem. Educ. 1976, !53,
RECEIVED for review July 31,1981. Accepted September 2!3, 1981. The identification of commercial products does not imply endorsement by the National Bureau of Standards.
Detachable Hydride Introduction Device for Inductively Coupled Plasma Torch Scott Stieg
and Allan Dennis2
Laboratory of the Government Chemist, Cornwall House, Stamford St., London SEI 9NQ, United Kingdom
The preconcentration method of hydride evolution is used extensively for extending detection limits and reducing matrix effeds in atomic absorption and plasma emission spectroscopy. 'Present address: Department of Chemistry, Harvey Mudd College, Claremont, CA 91711. 2Present address: Warrell Spring Laboratory, Gunneb Wood Rd., Stevenage, Herts, U.K.
The method, which converts an analyte to a volatile hydride, introduces a much larger fraction of analyte to the atom reservoir than the small fraction provided by droplets from a nebulizer spray. Two approaches are used to form arid introduce analyte hydrides (1). In the batch approach, the hydrides are formed above the reacting liquid and then suddenly swept into the detector along with other headspace
0003-2700/82/0354-0605$01.25/00 1982 American Chemical Society
606
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 Solution
Ar
2
iiln l l
1
l l
u Figure 1. Detachable interface.
gases. In the continuous approach, the analyte forms the hydride in a continuous flowing stream of reagents. Large amounts of excess Hzand water vapor are formed by both approaches. In the second approach, these gases are formed continuously, enabling a continuous determination of the analyte blank, before and after analyte introduction. The continuous flow method has been used to determine several elements in a variety of matrices (2-5). Where continuous flow methods have been used with an inductively coupled plasma (ICP), the gaseous hydrides are injected directly into the sample argon flow of the plasma torch by means of an external injection flow of argon. This direct introduction requires torch shutdown and reassembly when samples are to be introduced through the nebulizer and spray chamber. In a laboratory receiving a variety of samples in which 10-20 species must be determined, the hydride method would be, at best, only a small complement to the overall work of the instrument. Generally, then, the hydride method in a nonresearch laboratory is applied to the atomic absorption spectrometer, using commercial, detachable batch hydride generators. Although the continuous generation of analyte hydrides would increase the sensitivity of a single-element atomic absorption method, this instrument must likewise be dedicated to detection of evolved hydrides. The generated gases must be carefully dried, as the absorption bands of water occur in the region of absorption for most hydride-forming elements. The ICP experiment, on the other hand, is insensitive to water vapor and, as will be seen, lends itself more readily to occasional use of the hydride generator. In this laboratory, we have investigated the utility of continuous flow hydride generation with an ICP instrument routinely dedicated to nebulizer-introduced samples and have developed an easily detachable device for continuously introducing hydrides through the nebulizer/spray chamber. EXPERIMENTAL SECTION Source and Detector. A Plasma-Therm 2500 autotuning ICP source was operated at 1.6 kW forward power with the supplied torch. No “plasma gas” was supplied; the cooling argon flow rate was 15 L/min. The Meinhard concentric glass nebulizer operates at a critical pressure of 40 lb/in.,, which gives a 1 L/min cross flow of Ar into the torch. The nebulizer has a solution uptake flow rate of 3.4 ml/min at this pressure. The detector was a Spex monochromator, Model 1704, with an EM1 9781B photomultiplier tube at 640 V and associated data
acquisition electronics. The monochromator was manually tuned to the arsenic line at 228.81 nm and to the selenium line at 196.03 nm during spray introduction. Hydride Generator. The continuous flow method of Goulden and Brooksbank (3) was used with the following differences. Discrete pumps were used instead of a manifold pump. One ampere was supplied to an 8 4 heating wire wrapped around the stripping column. The A.R. grade reagent concentrations and rates of supply were: 2% KI, 0.45 mL/min; 0.15% w/v SnCl,-H,O/HCl, 3 mL/min; 0.5% w/v A1 slurry (Goodfellow Metals
