134
AMI. Chem. 1981, 53, 734-135
in concentration (due to either anthropogenic perturbations of atmospheric chemistry (13) or direct emissions).
ACKNOWLEDGMENT The authors wish to thank J. E. Lovelock and R. A. Rasm w e n for helpful discussion on this subject and the National Science Foundation Atmospheric Science Section for support.
LITERATURE CITED Grlmsrud, E. P.; Miller, D. A. Anal. Chem. 1978, 50, 1141-1145. Miller, D. A.; Grimsrud, E. P. Anal. Chem. 1979, 51, 851. Rasmussen, R. A.; Rasmussen, L. E.; Khalll, M. A. K.; Dalluge, R. W. J. Geoohvs. Res. In Dress. Goldan; P: D.; Fehsenfeld, F. C.; Kuster, W. C.; Phlilips, M. P.; Sievers, R. E. Anal. Chem. 1980, 52, 1751-1754. Miller, D. A.; Grlmsrud, E. P. J . Cbromatogr. 1980, 133-135.
(6) Singh, H. B.; Salas, L. J.; Cavanagh, L. A. J. Air Polltit. Control Assoc. 1977, 27. 332-336. (7) Watson, A. J.; Lovelock, J. E.; Stedman, D. H. Proceedings of the NATO Advanced Study Institute on Atmospheric Ozone, FAA-EE-8020; U S . Department of Transportetbn, Federal Aviatbn Admlnistretlon, High Altitude Pollution Rogram: Washington, DC. 1980. (8) Singh, H. B.; Salas, L.; Shigelshl, H.; Smith, A. L. EPA-600/3-7&017; Envkonmental Protectbn Agency: Research Triangle Park, NC, 1978. (9) Singh, H. B.; Salas, L.; Shigeishl, H.; Scribner, E. EPA-800/3-75-100; Environmental Protection Agency: Research Triangle Park, NC, 1978. (10) Grlmsrud, E. P.; Rasmussen, R. A. Atmos. Envlron. 1975. 9 , 1014-1017. Lovelock, J. E. Nature (London)1975, 256. 193-194. Lovelock, J. E.; Maggs, R. J.; Wade, R. J. Natwe(London)1973, 241, 194- 196. Logan, J. A.; Rather, M. J.; Wofsy. S. C.; McElroy, M. B. Phlbs. Trans. R . Soc. London 1978, 290, 187-233.
RECEIVED for review August 29,1980. Accepted October 22, 1980.
Electrode Positioner for Laser-Enhanced Ionization Spectrometry
George J. Havrilia’ and Robert B. Green’ Deparfment of Chemistry, Chemistry BuiMing, University of Arkansas, Fayettevilie, Arkansas 7270 1
Recent studies (1,2) have demonstrated that an electrode positioner is necessary for optimization of the laser-enhanced ionization (LEI) signal. In LEI spectrometry, a dye laser tuned to a discrete absorption transition of an analyte atomized in a flame enhances the thermal (collisional) ionization of the analyte atoms. The laser-related increase in ionization is detected with electrodes held at a negative high voltage and measured with conventional electronics. The laser beam is directed between the electrodes and parallel to the slot of a premix burner. An electrode positioner is necessary to determine the position of the electrodes with respect to the burner head and laser beam once that relationship is fixed. The rest of this paper will describe the important features of an electrode positioner designed for LEI spectrometry and illustrate them with experimental data. The electrode positioner (see Figure 1) incorporates the necessary versatility for optimizing signals and the mechanical stability for precise positioning in a simple design. The primary functions of the positioner are the vertical translation and horizontal separation of the electrodes. Vertical translation of the electrode holder assembly (B) is accomplished by revolving a knurled wheel (A-4) which turns a 1/4 in. X 20 in. threaded steel rod (A-1) in the translation base (A-8). Twenty threads per inch provides about 1 mm travel of the vertical translation column (A-3) per revolution of A-1. Horizontal stability for the electrodes is maintained by the two 3/a in. diameter brass rods (A-2) which track parallel to the threaded rod (A-1). This prevents yawing of the electrodes and maintains them in a level, parallel position during vertical translation. The unique feature of the positioner is the electrode holder assembly (B). The basic element is the dual-threaded horiPresent address: Center for Analytical Chemistry, Rm. 212, Bldg. 222; U S . N a t i o n a l B u r e a u o f Standards; Washington, D C
20234.
0003-2700/81/0353-0134$01.00/0
Table I. Effect of Plate Separation on the LEI Signal LEI signal
separation
(mm)
(arbitrary u n i t s )
11.5 12.0 12.5
38 10 1
Table 11. Effect of Rod Separation on Matrix Concentration Range % LEI s i g n a l r e c o v e r y 2
5
20
mL
Pgl
Pgl
separation
rd
10 rgl
15
Pgl
&I
a/
(mm)
Na
Na
Na
Na
Na
Na
Na
9 10 11 12
158 159 144 133
161 152 127
158 134 8
147 63
139
47
1
mL
mL
mL
mL
30
mL
40
mL
1
25
zontal translation rod (B-1). B-1 is a 3/8 in. diameter brass rod with both right- and left-hand threads (24 threads/in.) originating at the center. This causes the threaded electrode holder mounts (B-2) to move in opposite directions along B1 when the adjustment knob (B-5) is turned, i.e., both electrodes move toward the center or toward the ends when B-5 is turned counterclockwise or clockwise. The electrode holder mounts maintain a fixed orientation as they move along B-1 because they slide along a track (B-6). The synchronous movement of the electrode holders maintains the flame in a centered position between the electrodes without additional adjustments. One revolution of El results in 1mm horizontal travel for each holder, Le., 2 mm separation or closure of the electrodes per revolution. A finer thread would, of course, yield greater resolution in electrode separation. A more advanced 0 1980 Amerlcan Chemical Society
Anal. Chem. 1981, 53. 135-138
-
I1 illustrates the dependence of the LEI signal for 100 ng/mL indium in sodium matrices on electrode separation. One millimeter diameter rods are used as electrodes with -800 V applied. At each electrode separation, the recoverable signal decreases to zero as the sodium matrix concentration is increased. ks the electrode separation is increased, 100% signal suppression due to the matrix occurs a t lower sodium concentrations. The sodium concentration range diminishes from approximately 40 pg/mL sodium a t a 9-mm electrode separation to approximately 20 pg/mL sodium at 10 mm. Thus, the sodium matrix concentration range was decreased by a factor of 2 by a 1-mm increase in cathode separation. Documentation of the need for precise sampling height adjustment is given in ref 2. The severity of these position effects are mitigated somewhat by operating a t higher applied voltages, but they still remain important for achieving maximum signal and precision. The position of the electrodes plays an important role in the LEI signal collection process. The use of this electrode positioner permits recovery of the maximum LEI signal with sufficient spatial resolution. The electrode position has been shown to be particularly important for low ionization potential matrices. The flexibility afforded by the electrode positioner is also necessary when different flames are used because of their different chemical and physical characteristics (2). Detailed plans for the electrode pwitioner may he obtained from the authors upon request.
I
i
135
Figure 1. Schematic diagram of helectrode positionw: (A) vertical translation assembly. (A-I) 'I, in. X 20 in. threaded steel rod, (A-2) ' I sin. brass support rods, (A-3) vertical kandabbn wlumn. (A-4) k n M wheel, (A-5) nylon washer, (A-6) ball bearing pivot, (A-7) spacer, (A-8) vertical translation column base, (A-9) positioner base: (6) electrode holder assembly. (El) right- and lefthand thread 3/8 in. X 2 4 in. brass rod, (6-2) right- and left-hand thread electrode holder mounts, (6-3) nonconducting spacers. (8.4) brass electrode holders, ( 8 5 ) knurled wheel. ( 8 6 ) track for electrode holder mounts, ( 8 7 ) end blocks. ( 8 8 ) base plate for electrode holder assembly. (6.9)tension and alignment boltlnut for 8 1 : stipple, brass: crosshatch, nonwnductw: slanted lines, threads: the rest is aluminum stock
ACKNOWLEDGMENT The authors acknowledge Carl Wise (West Virginia University) for machining the prototype model, Terry Trask (UA) for suggestions for improvement of the prototype, and George Kirsch (UA) for helpful suggestions and machining of the fnal product.
indexing system for both horizontal and vertical translations would he useful for reproducing settings. If the burner head is used as the anode, the high-voltage leads which are terminated with banana plugs are plugged into holes in the brass electrode holders (B-4). Figure 1depicts flat electrode (plate) holders but other types of electrodes may be used with minimal modification. The voltage applied to the electrodes is isolated from the rest of the assembly with nonconducting spacers (B-3).The electrode holders are also isolated from each other so that one electrode mav be used as the cathode and the other the anode. The importance of the electrode separation is illustrated by the data in the tables. In these experiments, the electrodes in the holder are held at a negative voltage and the burner head is used as the anode. Table I gives the LEI signals for 100 ng/mL indium (303.9 nm) for 30-mm plates a t various plate separations. An acetylene/air flame is used with -500 V applied. The LEI signal is significantly diminished when the separation is increased 1mm (0.5 revolution of B-5). Table
LITERATURE CITED (1) Green. R. 8.: HavriL, 0.J.: Trask. T. 0. Awl. Spechosc. 1980, 34,
561-569. (2) Havrilla. G. J.; Green.
R. B. Awl. Chem. 1980. 52. 2376.
RECEIVED for review J d y 28,1980. Accepted October 14,1980. This research was supported by the National Science Foundation under Grant No. CHE79-18626. This paper was taken in part from the d h r t a t i o n written by G. J. Havrilla in partial fulfillment of degree requirements for a Doctor of Philosophy in Chemistry from West Virginia University, Morgantown,
wv.
Differential Thermometric Titration Apparatus
Basil H. Vassos' and Re& A. Roddguez Chemistry Deparfment. Universily of puerto Rico, Rio Riedras, puerlo Reo 0093 1
Thermometric titration started with the pioneering work of Bell and Cowell (I) in 1914 and through the years has been 0003-2700/81/0353-0135$0i.00/0
developed into a competent analytical technique (2,3). In general, calorimetry is done with instruments of simple design 0 1980 American Chemical Society
