Anal. Chem. 1988,60,1354-1357
1354
Rechnitz, G. A.; Hameka, H. F. Fresenius' z . Anal. Chem. 1965, 2 7 4 , 252-257. Johansson, G.;Norberg, K. J. Electroanal. Chem . Interfacial Electrochem. 1988, 78, 239-250. Mertens, J.; Van den Winkel, P.; Massart, D. L. Anal. Chem. 1976, 48. 272-277. Pardue, H. L. Clin. Chem. (Winston-Salem, N . C . ) 1977, 23.
(12) Wiiiis. B. G.; Woodruff. W. H.; Frysinger, J. R.; Margerum, D. W.; Pardue, H. L. Anal. Chem. 1970, 4 2 , 1350-1355. * Author to whom correspondence should be addressed.
Harry L. Pardue* Paul J. McNulty
2189-2201. Greinke, R. A,; Mark, H. 0. Anal. Chem. 1978, 5 0 , 70R-76R. Mieling, G. E.; Pardue, H. L. Anal. Chem. 1978, 5 0 , 1611-1618. Mieling, G. E.; Pardue, H. L.; Thompson, J. E.; Smith, R . A. Clin. Chem. (Winston-Salem, N . C . ) 1979, 25, 1581-1590. Hamilton, S. D.; Pardue, H. L. Clin. Chem. (Winston-Salem, N . C . ) 1982, 2 8 , 2359-2365. Arnold, M. A. Anal. Chim. Acta 1983, 754, 33-39. Skoug. J. W.; Weiser, W. E.;Cyliax, I.; Pardue, H. L. Trac, Trends Anal. Chem. Pers. Ed. 1986, 5(2),32-34.
Department of Chemistry Purdue University West Lafayette, Indiana 41901
RECEIVED for review November 24, 1987. Accepted March 1, 1988. Financial support from the National Science Foundation via Grant No. CHE 8319014 is gratefully acknowledged.
TECHNICAL NOTES Design and Evaluation of an Interface between a Continuous Flow System and a Graphite Furnace Atomic Absorption Spectrometer Kenneth Backstrom* and Lars-Goran Danielsson Department of Analytical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden The use of flame atomic absorption spectrometry (FAAS) in conjunction with continuous flow systems is a well-established technique. FAAS, being a technique working with a continuous supply of sample, is well suited for such connections, and it is often used together with flow injection systems (93 references were given in a recent flow injection analysis bibliography ( I ) ) . Many of these deal with sample dilution. For the determination of metals at trace and ultratrace levels, however, the graphite furnace is often needed due to the requirement of better detection limits. Connecting a batch technique such as graphite furnace atomic absorption spectrometry (GFAAS) to a continuous flow presents some special difficulties. There are some applications where GFAAS has been used in conjunction with high-performance liquid chromatography (HPLC) (2) and three principal ways of connecting the systems to each other can be discerned. The first, and probably simplest, is to use a fraction collector ( 3 )and then transfer the collected fractions to the graphite furnace sampler. A more direct connection method was designed by Brinckman et al. (4)that makes use of a Teflon flow-through cell with a volume of 50 pL positioned in the graphite furnace sampler tray, transferring the sample to the graphite tube by the automatic pipet sampler. The third way is to use an injection valve for sample collection and to transfer the sample to the tube by a stream of gas (5, 6). A detailed description of such an interface has been given by Vickrey et al. (5),but no evaluation of the sample transfer efficiency, precision, and sample carry over between successive injections was presented. A system for the work up of aqueous samples by continuous flow extraction of metals has been developed at our laboratory (7). The system performs a two-step extraction giving a final aqueous extract free from alkali and alkaline-earth salts. Recent modifications of the system make it possible to obtain enrichment factors up to 100 for several heavy metals, and the extract is well suited for analysis with GFAAS. Our purpose with this work was therefore to develop a simple interface connecting such an extraction system to the graphite furnace.
EXPERIMENTAL SECTION Chemicals. The water used was obtained from a Milli-Q system (Millipore). The metal solutions were all prepared from Titrisol solutions (Merck) and acidified with 1mL of concentrated nitric acid per liter. The rinsing solution consisted of similarly acidified Millipore water. Apparatus. The GFAAS used was a Varian AA-1275 equipped with a graphite tube atomizer GTA-95. It was run in single-sample mode with the number of replicates set to 1 and with the sample volume set to 70 pL. Temperature programs were chosen according to the manufacturers recommendations. Pyrolytic coated graphite tubes with a small internal barrier, 63-1oooO8-00 (Varian), were used in order to prevent excessive spreading of the sample during deposition. The interface is schematically shown in Figure 1. Two six-port valve injectors (Rheodyne) (e and f) mounted on FIA-05 analyzers (Tecator) and connected in series were used for the injection of sample and rinsing solution to the graphite furnace ( I ) . The sample injector (0was connected to a spare sample delivery tubing (0.5 mm id., 1.1mm o.d., 57 cm length) from the GTA-95 sample dispenser (h). The other end of this tubing was attached to the sampler arm (i). In this way the sample dispenser could be used to place the end of the tubing in the graphite furnace before injection. The contents of the loops were transported through the sample delivery tubing by a flow of air (b) from a peristaltic pump (Gilson, Minipuls 2) (d). Silicon pump tubing (Technicon) was used for this channel and Tygon pump tubing (Technicon) was used to pump the rinsing solution (c). Greasing of the silicon pump tubing with Dow Corning stopcock grease prolonged its lifetime considerably and improved the constancy of the air flow. The timing of the injectors and the starting of the GTA-95 were controlled by the microcomputer Intel MCS-85 including the chip 8155 containing a 2048-bit static MOS RAM with 1/0 ports and timer (Figure 2). The 8-bit 1/0B port of the 8155 (h) was connected to a monolithic driving circuit, PBD 352303, suitable for 5-V relays (g). Each one of three relays, ELECTROL RA 30461051 (e), was connected to the driving circuit and to one of the pins of the B port. Two of the relays were connected to the injectors (a and c) and one to the start button of the GTA-95 (b). In this way each of the injectors and the GTA-95 could be started from different pins of the B port of the 8155. The time spent in inject position for the two injectors was set to 10 s on the FIA-05 units. The program for the graphite furnace was entered from the keyboard of the GTA-95.
0003-2700/88/0360-1354$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
1355
Table I. Working Cycle of the Interface
MCS-85event
time/s
GTA-95 event
starts the GTA-95 in single sample mode injects sample and rinse solutions simultaneously
0 20
sample dispenser arm begins to move to the rinse vessel and the temperature program of the graphite furnace starts
27
injects the rinsing solution through the sample delivery tubing to the rinse vessel injects the rinsing solution through the sample delivery tubing to the rinse vessel
46
61
t t
sample dispenser arm begins to move to the graphite furnace sample dispenser arm reaches inject position
+ t,
end of the GTA-95 single-sample program restart of time cvcle 9 a
m
I
n Flgure 1. Interface connecting a continuous flow with GFAAS: a, continuous sample flow; b, air flow (4 mL/min); c, rinsing solution flow (1.2 mL/min); d, peristaltic pump; e, injector for the rinsing solutlon (13 pL); f, injector for the sample solution (23 pL); g, waste; h, GTA-95 sample dispenser; i, dispenser arm during sample collection (rinse position): j, dispenser arm during injection; k, rinse vessel for the tip of the sample delivery tubing; I, graphite furnace; m, graphite tube atomizer GTA-95; n, microcomputer MCS-85. .5v 0
Y
Figure 2. Time control of the interface: a, injection valve for sample;
b, start of GTA-95; c, injection valve for the rinsing solution; d, connection terminal board; e, 5-V relays, ELEC-TROL RA 30461051; f, indicator diodes for the pins of the B port of 8155; g, monolithic driving circuit for the relays, PBD 352303; h, B port of the chip 8155 (Intel).
Operation of t h e Interface. The working cycle of the interface is given in Table I. The delay from the initiation of the GTA-95 program to the injection of sample and rinsing solutions was determined by the time it took for the sampling arm to reach ita injection position (j in Figure 1). The delay before the second injection of rinsing solution was determined by the time it took for the sampler arm to move back to its initial position at the rinse vessel (i in Figure 1). The shortest overall cycle time was determined by the time of the GTA-95 single sample program. The time tl could be chosen arbitrarily. The reason for setting the sample volume to 70 WLin the GTA-95 program was to allow for as long a time as possible for the transfer of the sample and rinsing solutions from the injectors to the graphite tube. If the speed of introduction is too high, sample losses due to splattering in the graphite tube might occur. In order to improve the sample transfer efficiency of the interface, a plug of rinsing solution was allowed to pass through the sample loop and the sample delivery tubing to the graphite furnace where it mixed with the sample. This was effected by
simultaneous triggering of both injectors. The two solution plugs were transported through the system separated by a plug of air. Further rinsing of the sample delivery tubing was possible by injecting plugs of acidified water when the sampler arm had returned to its standby position at the rinse vessel (k). Optimization and Evaluation. The parameters that were considered in the optimization of the system were the positioning of the tip of the Teflon tubing in the graphite tube during sample deposition, the air flow rate, the diameter of the sample delivery tubing, the wall temperature of the graphite tube during sample deposition, and the volume of the sample and rinsing solutions. The sample transfer efficiency was determined by simultaneous injections of a sample with a comparatively high concentration of metal and rinsing solution, and then, after the atomization of that sample, by making new consecutive injections of rinsing solution and performing new atomizations, thereby determining the amount of metal left in the sample loop and delivery tubing. The sample carry over between successive sample injections was determined in an analogous way but allowing for two extra injections of rinsing solution, according the working cycle, before determining the residue. Finally the overall precision of the combined interface-GFAAS system was determined. R E S U L T S A N D DISCUSSION Positioning of the Teflon T u b i n g T i p d u r i n g Sample Deposition. Since there is a continuous flow of air through the tubing, it is important that the tip will be above the liquid surface at the end of the liquid deposition in the graphite tube. On the other hand losses may occur at the sample input hole of the graphite tube if the tip is positioned too far up. With a total dispensed volume of 36 pL, it was found that positioning the tip at a height of about three-fourth of the internal diameter of the graphite tube yielded optimal results. A i r Flow Rate. In the first experimental setup of the interface the concept of Vickrey et al. ( 5 ) using a flow of nitrogen from a pressurized flask was tested. The pressure was 0.5 bar above ambient. This gave a very rapid transfer of the sample (300 mL/min) but also splattering in the tube. This method was therefore abandoned and replaced by air pumping as shown in Figure 1. Having observed the splattering that occurred during the high rate of deposition that was obtained with pressurised nitrogen, we assumed that this problem would be minimized if the delivery was made as slowly as practically possible. A slow rate of delivery would also facilitate a high sample transfer efficiency and a low sample carry-over between successive sample injections. On the other hand it would be convenient if the transfer time could be kept below 7 s, this being the longest dispenser arm dwell time in the inject position that could be obtained by direct programming of the GTA-95 (corresponding to a sample volume of 70 pL). A solution of 10 pg Cu/L was injected a t four different air flow rates measured by a soap film meter, and the results are given in Table 11. The mean absorbance values show no significant differences. I t is possible to use air flow rates up to 4.3 mL/min without loss of precision. Using an air flow rate of 4 mL/min ensures a sample and rinsing solution delivery to the tube within 7 s. Thus it is
1356
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
A
Table 11. Effect of Deposition Rate” air flow rate, mL/min 0.9 2.0 4.3
6.2
absorbance mean value 0.444 0.448 0.443 0.448
4
90 re1 std dev ( n = 10)
0.7 1.2
0.7 1.8
“ S a m ~ l econcentration 10 UP of Cu/L.
possible to deposit the sample within the limits set by the normal programming of the GTA-95. Diameter of the Sample Delivery Tubing. A test was made with a wider Teflon tubing (0.8 mm i.d., 1.4 mm 0.d.) instead of the more narrow tubing delivered with the graphite furnace sampler. However, it appeared to give greater losses as small droplets adhered to the periphery of the tubing tip after injection. We therefore used the original tubing throughout the rest of this work.
Wall Temperature of the Graphite Tube during Sample Introduction. A 13-pL portion of a solution of 25 pg of Cd/L was injected together with 23 pL of rinsing solution into the graphite tube at three different wall temperatures. This was also done for 23 pL of a solution of 10 pg of Cu/L together with 13 pL of rinsing solution (Figure 3). Among the temperatures tested the absorbance signal was highest and the precision best when the sample was injected at an ambient wall temperature. In all further experiments this was chosen therefore as the injection temperature. Volume of Sample and Rinsing Solution. It was obvious that volumes larger than 40 pL would lead to greater risks of sample loss due to splattering in the graphite tube because of the continuous flow of air in the sample delivery tubing. The total aqueous volume was kept therefore at 36 pL. With the injectors used it is not possible to obtain injection volumes below 10 pL. The two injectors were equipped therefore with loops of 13 and 23 pL, respectively. The sample transfer recovery was determined for injections of 13 pL of sample and 23 pL of rinsing solution. Then the experiment was repeated after exchanging the two injectors. In both cases very high degrees of sample transfer were obtained. In the former case it was 99.6%, determined by injection of 50 pg of Cd/L, and in the latter case 99.0%, determined by injection of 1000 pg of Ni/L. Since the sensitivity increases with increasing sample volume, the 23-pL loop was taken for samples and the 13-pL loop for rinsing solution in further experiments. Apart from assuring high sample transfer efficiency, the use of a rinsing solution opens a possibility to perform matrix modification with the proposed system. Evaluation of the Interface. The sample carry-over, determined for injections of 1000 pg of Cu/L, was less than 0.1 70. The precision of the overall interface-GFAAS system was determined by injecting a solution of 10 pg of Cu/L in 20 replicates. The relative standard deviation was 0.7% at a mean absorbance of 0.432. The time needed for a complete working cycle equals the time needed for the single sample program of the GTA-95. The maximum sampling rate is determined therefore by the length of the temperature program of the graphite furnace.
Performance of a System Not Using Rinsing Plugs. A test was made by injecting a sample (10 pg of Cu/L) with a modified system where the injector for the rinsing solution was excluded. This gave an absorbance 6% lower than that obtained with the proposed system, thus indicating a sample transfer of 93%. The standard deviation was, however, the same as that obtained with use of rinsing. The sample carry-over between successive sample injections was in the range 2-3% due to residues in the sample delivery tubing.
20
90 120 Temp. (‘CI
B
W
a
P
80.
20
95
120
Temp (Ti
Flgure 3. Absorbance readings after injection at different tube wall temperatures (mean absorbance and total range are given): (A) 23 pL of 10 pg of Cu/L, 100% corresponds to 0.435 absorbance unit, three injections at each temperature, A = 324.7 nm, slit = 0.5 nm, gas stop during atomization; (8) 13 pL of 25 pg of Cd/L, 100% corresponds to 0.269 absorbance unit, four injections at each temperature, X = 228.8 nm, slit = 0.5 nm, argon flow 1 L/min during atomization.
Injection of Nonaqueous Samples. The interface presented was primarily intended for the handling of aqueous solutions. Thus all experiments presented were performed with such solutions. On the basis of our experience of flow systems, we can still give some tentative guidelines for the adaption of the interface to nonaqueous sample solutions. In such cases it could be advantageous to exchange the Teflon sample delivery tubing for a quartz capillary, thus minimizing the carry-over due to residual sample adhering to the walls. Furthermore, the low surface tension of organic solvents might necessitate a lower rate of deposition in order to avoid splattering. The wall temperature of the graphite furnace should also be reconsidered in this case, as proper adjustment might reduce the spreading of the sample in the furnace. Interface Setup and Close Down. One of the objectives in the design of the interface was that it should be easy to switch between conventional operation of the GFAAS and operation by using the interface. With the present design the only modifications necessary are the loading of an appropriate temperature program and the replacement of the normal sample delivery tubing. During operation of the interface the ordinary sample delivery tubing is simply set aside to a waste bottle, used for the collection of the rinsing solution from the sample dispenser. This means that it only takes a few minutes to change between sampler and interface operation mode. If the interface is to be used for a prolonged period of time, the connections to the stepper motor of the syringe pump of the sample dispenser can be disconnected. Present Use of the Interface. The interface is currently used in our laboratory to connect a two-step extraction system to a GFAAS for the determination of heavy metals. Results from this work will be published later.
Anal. Chem. 1900, 60, 1357-1360
ACKNOWLEDGMENT The authors thank Folke Ingman and Lage Nord for fruitful discussions throughout this work.
LITERATURE CITED (1) FIAstar Flow Injection Analysls Bibliography, Tecator AB, 1985, and FIAstar Flow Injection Analysis Blbllography Supplement, 1985, Tecator AB, 1986. (2) Ebdon, L.; Hlll, S.; Ward, R. W. Analyst (London) 1987, 112, 1-18. (3) Kolzuml, H.; Hadelshl, T.; McLaughlin, R. Anal. Chem. 1978, 50, 387-392.
1357
(4) Brlnckman, F. E.; Blair, W. R.; Jewett, K. L.; Iverson, W. P. J . Chromatogr. Scl. 1977, 15, 493-503. (5) Vlckrey, T. M.; Buren, M. S.; Howell, H. E. Anal. Lett. 1978, 1 1 , 1075-1 095. (6) Stockton, R. A.; Irgollc, K. J. Inf. J . Environ. A n d . Chem. 1879, 6 , 313-319. (7) Bickstrom, K.: Danlelsson, L.-G.: Nord, L. Analyst (London) 1984. 109, 323-325.
RECEIVED for review
November 20, 1987* Accepted February
13, 1988.
Instrumentation for 7-Day Continuous Cycle Monitoring of Metals with Automated On-Line Sample Preparation, High-Performance Liquid Chromatography, and Electrochemical Detection A. M. Bond,* W. N. C. Garrard, I. D. Heritage,’ T. P. Majewski? and G . G. Wallace3 Division of Chemical a n d Physical Sciences, Deakin University, Waurn Ponds, Victoria 321 7, Australia
M. J. P. M ~ B u r n e y E. , ~ T. Crosher, and L. S. McLachlan Department of Defence, Instrument Services Section, Ordnance Factory, Maribyrnong, Private Bag No. 1, Ascot Vale Post Office, Victoria 3032, Australia High-performance liquid chromatography with electrochemical detection (HPLCEC) has become well accepted as a sensitive method for determining a wide variety of organic and inorganic compounds (1-6). In these laboratories, we have been interested in developing analytical methods for trace metal determinations of metals such as Cu, Ni, Cr(III), Cr(VI), Mn, Fe, Co, Hg, and P b by the HPLCEC method (5-11) in industrial effluents. However, the instrumentation developed in these earlier studies has not been fully automated as required for long-term “on-line” monitoring. This paper describes the design of a new completely automated HPLCEC system for on-line monitoring which offers the following: (i) The system can operate continuously for 7 days without maintenance. (ii) All procedures are completely automated including sampling from the industrial effluent. This has required the development of a newly designed, low-pressure mixing system, a string bead reactor coil to ensure complete mixing, a bubble chamber to remove bubbles formed during the mixing procedure, and automatic injection of metal complexes. (iii) The ex situ method of complex formation has been implemented in an automated version. Previously, in situ complex formation was used (6))which is inherently simpler from an instrumental point of view, but does not necessarily provide optimal sensitivity. (iv) A heating unit is provided as an option to increase the rate of formation of electroactive metal complexes when reaction rates are slow a t ambient temperatures. (v) The potentiostat and electrochemical cell have been integrated into a single unit in a Faraday cage to reduce noise and provide lower detection limits. Conventionally, HPLCEC systems are constructed in a modular fashion and the potentiostat and cell are physically separated by considerable distances. Noise associated with the measurement of picoampere currents can be a problem under ‘Present address: Shell Co. of Australia Ltd., Refinery Rd., Corio, Victoria 3214, Australia. On leave from: Institute of General Chemistry, Technical University, 90-924 Lodz, Poland. Present address: Department of Chemistry, University of Wollon ong, Wollongong, New South Wales 2500, Australia. %resent address: Department of Defence, Office of Defence Production, Canberra, A.C.T. 2600, Australia.
HPLCEC conditions. Recent work on minimization of noise via careful design of thermostated cells (12) and the use of battery-operated devices (13-15) provides convincing evidence that detection limits in HPLCEC have not generally been limited by background (charging) current but rather by electronic noise or drift. (vi) The flow rate has been decreased to well below 1 mL min-’ to provide substantial savings of expensive and toxic organic solvent. (vii) Microbore chromatography (16,17) has been introduced so that high resolution is retained a t the low flow rates.
EXPERIMENTAL SECTION All chemicals used were of analytical reagent grade. Acetonitrile was of HPLC grade. Conductivity grade water-buffer having an extremely low metal ion concentration was mixed with acetonitrile to form the running solvent. Solvent and dithiocarbamate ligand purification procedures have been described previously (9).
INSTRUMENTATION The fundamental chemical principle of the HPLCEC metal detection system described in this work is the formation of neutral metal-dithiocarbamate or similar complexes which can be separated on an HPLC column and electrochemically oxidized or reduced (5-11, 18, 19). The use of an oxidation process for metal determinations, rather than the usual reduction procedures, obviates the need for elimination of oxygen, which is a difficult and expensive procedure to automate. The neutral metal complexes may be prepared for an HPLCEC determination by two procedures which can be described as in situ or ex situ. With the in situ method, the ligand is included in the chromatographic solvent. At first glance this is the simplest approach to automate and forms the basis of a system described by Bond and Wallace (6). However, such a system has several distinct drawbacks. (a) The dithiocarbamate ligand, dtc-, is itself oxidized at potentials over the range of approximately 0.2-0.8 V vs Ag/AgCl so that the electrode is being continuously contaminated by ligand oxidation products. The background current in this potential region is sufficiently raised to accentuate minor fluctuations in flow conditions, thus precluding the use of cheaper single-piston pumps. At potentials more positive than 0.8 V vs Ag/AgCl a second irreversible ligand oxidation
0003-2700/88/0360-1357$01.50/0 0 1988 American Chemical Society
