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Electrically Heated Quartz Atomization Cell for Hydride Generation-Atomic Absorption Spectrophotometry D. Brooke Hatfield
US.Geological Survey, Box 25046, M S 928, Denver Federal Center, Denver, Colorado 80225 An ideal electrically heated cell design for routine use in hydride generation-atomic absorption spectrophotometry should have predictable operating characteristics between cells and a long usable life, require limited downtime due to replacement, and allow in-house replacement of the heating element. Deficiencies in these areas have motivated a reeexamination of the cell used in our laboratory and suggested by the U.S. Geological Survey (4, 8, 9). This design has a “T” configuration of clear fused quartz tubing with a central inlet tube. The inner surface of the atomization chamber is evenly sandblasted to increase surface area. It has an integral heating element, which is two 2-m lengths of Nichrome (Driver-Harris) wire connected in parallel, each coiled about opposite arms of the cell. The cell is covered by a cured, ceramic shell of ASTRO-CERAM (ChemoThermic Ind.). There has always a certain amount of black magic associated with the old ( 4 , 8 , 9 ) design, as no two cells performed alike in terms of sensitivity (concentration per 1% absorbance), drift (sensitivity change per unit time), and lifetime. In fact, some cells would never produce a signal a t all. The cells produced in this manner (4, 8, 9) had an alarming tendency toward electrical failure (loss of continuity in the heating element). This was compounded by the impervious nature of the ceramic shell, which prevented replacement of the heating element. The time required to build a new cell was about 4 days. Each new cell required about 1-2 days to bring the sensitivity to a maximum. The correction of these deficiencies was based on adressing two problems, namely understanding the factors that influence the sensitivity of a cell and the cause of stresses resulting in electrical failure of the heating element. Verlinden (1) has previously adressed the first problem, while a technical data sheet from Driver-Harris (IO) on heating-element design adresses the later. Verlinden ( I ) suggests that it is the vitreous phase of silica (SiOz) that, provides the active surface for the atomization of hydride species within the cell. He provides further evidence that the presence of a higher temperature phase, namely P-cristobalite, actually hinders the atomizaiton process. In the improvement of sensitivity, it is reasonable to focus on the removal of nonvitreous phases and the prevention of the formation of these phases. This is effected by exposing the atomization chamber to a dilute hydrofluoric acid solution for a short time. Higher temperature phases are thermodynamically less stable at room temperature and therefore react more readily. After the undesirable crystal structures have been removed, it is wise to remove any residual mechanical stresses, in order to reduce the probability of the formation of higher temperature phases by returning the surface to an even thermodynamic potential. This process is commonly called annealing. The annealing temperature for clear fused quartz is 1080 OC. This end is supported by Verlinden ( I ) who states that a cell’s sensitivity is improved by maintaining a temperature of about 1000 “C for several hours. Attempts to anneal the cell by using the integral heating element have proven futile as it generally burns out within a few minutes. The maximum operating temperature given
for Nichrome wire is 1010 OC (IO). The process may be successfully completed in a muffle furnace, prior to the incorporation of the Nichrome heating element. The resistive expansion of Nichrome wire a t normal cell operating temperatures (600-900 “C) is 150% of its cold length (10).
Clearly, the ceramic shell would induce large stresses on the confined wire. This problem is easily overcome by replacing the ceramic shell with a concentric outer jacket, producing an expansion void. The size of this void may be reduced by using less wire. If the use of a parallel resistance configuration is voided, the heating element may be reduced to a single 1-m strand. The expansion of the heating coil is accommodated by two mechanisms. Radial expansion against the outer jacket accounts for 50% of the total change in length. The addition of three extra coils by compression accounts for the remainder. This has the added advantage of providing a more uniform distribution of heat. The cell design incorporating these improvements has been tested over the last year. Electrical failures have been less common, and the heating element may be replaced in about 20 min. The cells achieve maximum sensitivity in about 15 min. Between cells, the sensitivity has been found to vary by less than 10%. The average life of a cell has increased 2-fold. In short, the improved design is more suited to routine applications than the previous design. More detailed information is available on request from the author.
EXPERIMENTAL SECTION The cell is fabricated from clear, fused-quartz tubing (CFQ), Nichrome wire, and woven quartz tape (available from Wale Apparatus). It is assembled from six parts; the atomization chamber, inlet tube, heating element, two furnace halves, and insulation (Figure 1). The equipment needed to fabricate the cell includes an oxygen-hydrogen torch, a diamond saw, and a sandblaster with 600-grain silicon carbide. The atomization chamber is constructed by sandblasting, to an even and milky translucence, the inner surface of a 100 mm length of 11- X 13-mm CFQ tubing. A 3-mm-diameter hole is then sandblasted through one wall, 50 mm from either end. A 30 mm length of 3- X 5-mm CFQ tubing is then attached in a “T” configuration over this hole, forming an inlet tube. Two loops are fused at the ends of the atomization chamber as anchors for the heating element. This entire assembly is then soaked in an acid bath (25% HNO, + 25% (v/v) HF) for 30 min, rinsed with deionized water, and heated overnight in a muffle furnace at 1100 “C. The heating element is made by tightly winding a 1.12-m length of 26 gauge (B&S) Nichrome wire around a 10-mm mandril, forming an even helix. This is then screwed onto the atomization chamber, with an even number of coils left on either side of the inlet tube. The coils are evenly spaced and secured by passing 30 mm of each end through the anchor loops. To prevent shorting and cold spots, caPe must be taken to keep the coils evenly spaced and snug against the atomization chamber. The outer jackets are made from two 49-mm lengths of 15- X 17-mm CFQ tubing. Clearance for the inlet tube is made by grinding a 6 mm radius semicircle in one wall of each furnace half. Three dimples are put in the opposite ends to concentrically support the atomization chamber. The two halves are secured by a piece of adhesive tape while wrapping four layers of insulation about the furnace, using 1.2 m of 25-mm-wide woven quartz tape.
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
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essary for the analytical decomposition of the various hydrides. It is this step that imparts predictable initial sensitivity between cells. The cell is operated by applying ac voltage across the Nichrome heating element with a variac. As expected, the interior cell temperature is a linear function with respect to applied voltage. The slope is 23 "C per root mean square V ac. The resistance of the wire is 11 R. At 850 "C the current is 3.6 A, yielding 145 W. Expansion of the heating element (150%) is accommodated by the double jacket design, minimizing shearing stresses on the thin and fragile wire during operation.
d
Figure 1. Electrically heated quartz atomization cell: (a) atomization chamber: (b) inlet tube: (c) heating element: (d) outer jacket (requires 2); (e) quartz tape.
The insulation is secured by a twist of Nichrome wire. DISCUSSION As this design has an integral heating element, rinsing with HF after assembly is not practical. This requires the presensitization of the atomization chamber, which involves sandblasting, removal of surface impurities by acids, and annealing the quartz. Sandblasting provides a large, fresh surface, which increases the reproducibility between cells and yields a longer cell life by increasing the active surface area. Soaking in a nitric and hydrofluoric acid bath removes both trace impurities and higher temperature phases of quartz, which act as devitrification "seeds" ( I ) and significantly reduce the longevity of the cell. Annealing the chamber at 1100 "C provides the uniform vitreous quartz surface, which is nec-
ACKNOWLEDGMENT I thank Steve Sweat and Pat Smythe of Precision Glass of Colorado, Denver, CO, for their invaluable aid in developing this cell. Registry No. Quartz, 14808-60-7. LITERATURE C I T E D Verlinden, M. Anal. Chim. Acta 1982, 740, 229-235. Welz, B.; Melcher, M. Analyst (London) 1983, 708, 213-224. Lee, D.S. Anal. Chem. 1982, 1682-1686. Open-file Rep. U .S . Geol. Surv. 1985, No. 85-495, 541. Thompson, A. J.; Thoresby, P. A. Analyst. (London) 1977, 702,9-16. Agemain, C.; Bedrek, E. Anal. Chim. Acta 1980, 779, 323-330. Wood, G. R.; Vijan. P. N. Taianta 1976, 23, 89-94. Briggs, P. H.; Crock, J. G. Open-File Rep. U . S . Geoi. Surv. 1986, No. 86-40. (9) Crock, J. G.: Lichte, F. E. Anal. Chim. Acta, 1982, 223-233. (10) Designing Heating Elements, Driver-Harris: Harrison, NJ, 1985. (1) (2) (3) (4) (5) (6) (7) (8)
RECEIVED for review October 2, 1986. Resubmitted March 16,1987. Accepted March 26,1987. Use of brand names does not constitute endorsement by the U.S.G.S.
CORRECTION S q u a r e Wave Voltammetry at t h e Mercury Film Electrode: Theoretical T r e a t m e n t S q u a r e Wave Anodic S t r i p p i n g Voltammetry at t h e M e r c u r y Film Electrode: Theoretical T r e a t m e n t
S. P. Kounaves, J. J. O'Dea, P. Chandrasekhar, and Janet Osteryoung (Anal. Chem. 1986,58,3199-3202; Anal. Chem. 1987, 59, 386-389). On the title page of each paper, the last name of the third author, Chandrasekhar, is spelled incorrectly as Chandresekhar.
