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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
Application of Microporous Membranes to Chemiluminescence Analysis Vance N a u and Timothy A. Nieman" School
of
Chemical Sciences, University
of Illinois,
Urbana, Illinois 6 180 1
A novel approach to reagent introduction into flowing streams is accomplished by using a microporous membrane to separate a reagent reservoir from an analyte stream. Reagent addition rates as low as a few pL/min are possible and can be controlled by the membrane porosity, membrane surface area, concentration of reagents in the reagent reservoir, and applied pressure gradient. The reaction zone is probably near, but not confined to the membrane surface; reagent concentrations in the reaction zone appear to be one tenth of that in the reagent reservoir. With Co(I1) as the analyte, efficient rinsing of the cell is limited by adsorption at the membrane surface, but can be improved if EDTA is present in the rinse solution. Sensitivity is maximized for analyte solutions of pH 7-9. Determinations of Co( 11) using luminol have a detection limit of about IO-'' M (10 part per trillion); typical precision is f3%.
Solution chemiluminescence (CL) has received increased interest as a method for determination of certain species (notably, metal ions, hydrogen peroxide, and certain other inorganics a n d organics) at trace concentration levels. However, routine use of CL in analysis is rare, due in part t o a lack of straightforward methodology and suitable instrumentation. Recent applications of CL have in large part been directed t o measurements in flowing streams ( I ) , either in continuous flow systems (2-6) or in liquid chromatography detectors (7-10). Such flow systems, while generally popular, require elaborate or expensive instrumentation t o deliver the analyte and reagent streams, and, in general, are not very conservative of reagents. W e have investigated the application of recently available microporous plastic membranes t o CL analysis. In this approach, the membrane separates a reservoir of CL reagent from a n analyte flow stream. T h e CL reagent is delivered through the membrane at a very low flow rate (about 1-10 gL per minute). T h e initial attraction t o use of such membranes in CL instrumentation was to evaluate several anticipated advantages: (1) small reagent volumes. ( 2 ) simplification of reagent delivery system, ( 3 ) negligible contamination of the analbte stream with the CL reagent, (4) minimization of interference from self-absorbance due t o a very low concentration of CL reagent in the hulk of the analyte solution, and ( 5 ) physical localization of the emission near the surface of t h e membrane. A continuous flow system using a microporous membrane was developed and characterized using the well known luminol-H,O,-Co(I1) reaction. This initial report on microporous membranes in CL instrumentation discusses that system and the investigation and optimization of the composition of t h e reagent and the analyte streams (concentrations, auxiliary complexing agents, pH), the absolute and relative flow rates of reagent and analyte, and the cell geometry. Future papers will be concerned with both specific applications and fundamental investigation of the emission properties of t h e membrane system. 0003-2700/79/0351-0424$01.OO/O
EXPERIMENTAL Reagents and Solutions. Luminol (Pfaltz and Bauer) and all other reagents (Mallinckrodt, Analytical Reagent) were used without further purification. Stock solutions of luminol (10-4-10-3M) were prepared in 4 x M KOH, then adjusted to pH 9.5 and stored under refrigeration. No deterioration was noted over the lifetime of the stock solution (about one month). Excluding the final working curve and the luminol concentration study, a small amount of EDTA (lo4 M) was added to the luminol stock solution to reduce the background emission level. Working solutions of Co(I1) were prepared by serial dilution from a M stock solution of C O ( N O ~ ) ~ . that ~ H ~contained O 1% nitric acid. Concentrations below M were achieved by diluting known aliquots of the lo-' M solution and using the resulting solution within 30 min after preparation. In general, the procedure outlined by Subramanian et al. (11) was used to construct the working curve. Each Co(I1) solution was adjusted to near neutral pH immediately before use (reason discussed in Results and Discussion section). Glass distilled deionized water (DDW) was used to prepare all solutions. Flow Cell. Figure 1 gives a schematic representation of the operational aspects of the flow cell. The reagent reservoir holds an essentially stagnant solution containing luminol, H202,and KOH at their selected concentrations. Because this reservoir is pressurized, a one-way flow of reagent is established, through the membrane, into the analyte flow chamber. Reaction occurs where the reagent and analyte mix and yields emission at an intensity which is characteristic of the level of analyte present. The actual flow cell is shown in Figure 2. This design minimizes the surface area that requires sealing, permits the large surface area to volume ratio desired in CL flow cells, and allows easy and reproducible assembly. Nylon was used for the construction because it is relatively inert, structurally rigid, and easy to machine. Because of the pressure required for proper sealing, the front window was Plexiglas, instead of glass. The cell holder was very similar to one previously described ( I ) . All solution ports have I/,, - 28 threads for compatibility with commercially available tube fittings. Delivery System. The analyte stream delivery system used in these studies is shown in Figure 3. Separate vessels for the analyte and the rinse (or background) solutions are connected through a three-way Teflon solenoid valve, \'I, (Angar Model 250) to the analyte flow chamber. Valve V1 automatically switches between the two solutions according to the preset pulse width (analyte volume) and duty cycle (ratio of analyte volume to total analyte plus rinse volumes). The vessels are pressurized with nitrogen to about 3 psi (20 kPa) to provide smooth, stable, and continuous flow, subject to only minor long-term drift of either rinse or analyte solutions. When the analyte solution is changed, valve V2 is activated to release the pressure from the analyte vessel without any perturbation to the rinse line. Release of V 2 and activation of V3 permits bleeding of any air that may have become lodged within the associated tubing. If air becomes trapped within the analyte flow chamber. the flow characteristics are substantially altered and the precision is degraded. This analyte delivery system is quite useful for characterization studies because it allows automatic control of the number of repetitions of a given sample, the rate of repetition, the duty cycle, and the sample volume per injection. For routine work or with small volumes of sample, the system could easily be simplified C 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
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Figure 1. Schematic diagram of membrane flow cell. Dashed arrows indicate direction of reagent flow through membrane
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Figure 2. Construction of membrane flow cell
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Figure 3. Analyte flow system t o syringe injection of sample into the flowing rinse stream.
The reagent reservoir is filled by delivering and mixing equal volumes of stock solutions of luminol, H 2 0 2 ,KOH, and DDW using a four-syringe stopped-flow solution delivery system as previously described ( 1 2 ) . The reservoir is then pressurized to the desired level (about 10 psi or 70 kPa) and maintained at that pressure using a regulated tank of nitrogen. The total amount of reagent consumed can be accurately measured with a modified graduated pipet positioned between the gas regulator and the reagent reservoir. Since the pipet is within the pressurized reagent system, it therefore increases the effective volume of the reagent reservoir. Membrane. The microporous membranes used in this study were manufactured by Moleculon Research Corporation (Cambridge, Mass.) and sold under the trade name of Poroplastic. Detection. The light emission was observed directly by a 1P28 PMT, without wavelength discrimination.
MEMBRANE CHARACTERISTICS Poroplastic membranes are an ultramicroporous network of cellulose triacetate with molecular size pores. Initially, the pores contain water, but a wide variety of solvents, both polar and nonpolar, can be substituted. The transparent membrane sheets are strong, flexible, non-ionic, and stable over a wide pH range. T h e membrane is available in a variety of thicknesses (5@500 km) pore sizes (14-200 A). Pore size and structural strength are inversely related and are both dependent upon the water content (70-97%) included in the original manufacture of the membrane (13). Diffusion through the membrane depends mainly upon its pore diameter and thickness. A controlled net flow, however. can be easily enforced by adjusting an applied pressure gradient, the upper limit of which is set by the structural rigidity of the membrane. Using a Tyvek (spunbound polyethylene fibers) reinforced membrane, pressure differ-
425
entials of up to 20 psi (140 kPa) can be applied across the membrane without significant deformation. Reverse diffusion against the net flow is negligible a t pressure gradients greater than 4 psi (28 kPa). In generai, mid-range pressures (9-10 psi) were used. Characterization studies revealed that the flow rates of both the solvent and the solute are equivalent, with no observed discrimination against any reagent used, irrespective of concentration, and that all flow rates are proportional to the applied pressure gradient. At 6 psi (42 kPa), the flow rate is 2.2 FL/min/cm2; a t 14 psi (98 kPa), the flow rate is 5.0 pL/min/cm2. (The membrane surface area in our cell is 1 cm2.) Substitution of membranes available with other porosities and thicknesses allows the flow-rate range to be shifted higher or lower by 1-2 orders of magnitude. There are, then, four methods to control the reagent flow rate (in moles per unit time) through the membrane: the porosity of the membrane (limited by structural rigidity), the total membrane surface area (limited by detector geometry and the rinsing efficiency of the cell), the concentration of the reagent solution (limited by reagent solubility and membrane integrity), and the applied pressure (limited by membrane rigidity).
RESULTS A N D DISCUSSION Explanation of the optimization of variables for this system can be readily divided into two separate although not entirely independent parts: (1)the reagent flow system, including the relative concentrations of luminol, H2O2,and OH-, as well as the flow rate through the membrane, and (2) the analyte flow system, including the necessity for a complexing agent in the rinse solution, the sample volume, the duty cycle, the sample pH, and the sample flow rate. Optimization was complicated by trade-offs between the signal intensity and either the background emission level or precision. All experiments used the luminol-H,02-OH~-Co(II) system. The order of the following discussion does not reflect the order in which the variables were investigated. Reagent Flow System. Exact knowledge of reagent concentrations is limited to those concentrations in the reagent reservoir. On the other side of the membrane, in the analyte flow chamber a t the position where reaction occurs, knowledge of reagent concentrations is less certain. If reaction is confined to a narrow region near the surface of the membrane, the reagent concentration at that position will be the same as in the reagent reservoir. However, if reaction occurs in the bulk of the solution in the analyte chamber, the reagent concentration will be 3 to 4 orders of magnitude lower due to dilution. (The reagent flow rate is only a few pL per minute while the analyte/rinse flow rate is about 10 mL per minute.) It is most likely that the reaction will occur in a continuum of conditions between these two extremes. T o demonstrate that the effective reagent concentrations in the reaction zone more closely approximate those at the membrane surface than in the bulk, the emission intensity from this reaction was measured using conventional stopped flow reagent delivery and compared to the intensity observed with the membrane. When the stopped flow delivered reagents (H20Z,OH , and luminol) that were a t the same concentration as in the membrane reservoir (see below), the observed intensity was 8 times larger than that observed with the membrane. When the stopped flow delivered reagents diluted by 1000 (to reproduce the bulk solution conditions with the membrane). the observed intensity was less than 0.1% of that observed with the membrane. (The above measurements all used M Co(II).) Experiments are planned to directly measure the emission intensity profile as a function of the distance from the membrane surface. Comparison of observed emission intensity as a function of reagent concentrations for the membrane flow system with
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
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Figure 5. Observed peak shape for large analyte volumes. Flow chamber volume = 0 1 mL; analyte volume = 8 mL; volume ratio = 80
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Figure 4. Ideal and observed rinsing efficiences of the analyte flow chamber. Flow chamber volume = 0.4 mL; analyte volume = 6 mL; volume ratio = 15. (1) Computer simulation of ideal peak shape; (2) observed peak with EDTA in the rinse solution; (3)observed peak without EDTA in the rinse solution; curves 2 and 3 are identical from 0-6 mL. For curve 1, the ordinate is in units of concentration of Co(I1). For curves 2 and 3, the ordinate is in units of observed photocurrent
published reports on this same chemical system using a conventional flow stream can offer probable upper limits on the effective reagent concentration in the reaction zone. It has been reported for the Co(I1)-luminol system that plots of CL emission intensity vs. reagent concentration pass M for luminol and M through a maximum a t about for both OH- and H202 (15). Because such plots for the membrane system show CL intensity still increasing a t concentrations nominally ten times higher, it is probable that the effective reagent concentration in the reaction zone is no greater than one tenth of the concentration in the reagent reservoir. Incomplete dissolution of luminol or precipitation, once the solution had been adjusted to a relatively neutral pH, became a problem a t concentrations greater than M. Any trials M or using both OH- and H 2 0 2 using OH- a t or below simultaneously a t or below M yielded either comparatively no response or substantially reduced response. T o ensure the integrity of the membrane, concentrations of OH- and H 2 0 2 were limited to lo-’M or lower, although no specific tests have been conducted to determine the necessity of this limitation. Thus, the reagent concentrations chosen for the remainder of this study were lo-* M OH-, lo-’ M H202,and 2 X lo-‘ M luminol. Unless stated otherwise, all reagent concentrations are those within the reagent reservoir. Addition of a small amount of EDTA! at concentrations approximately two orders of magnitude less than that of luminol, t o the reagent solution was found to reduce the background emission level by a factor of 50, without significantly degrading the response t o the higher concentrations of cobalt. The loss of linearity a t the lower Co(I1) concentrations, due to a finite amount of EDTA emerging from the membrane on the analyte side and tying up a significant portion of the total analyte present, precluded use of EDTA in the reagent solution during construction of a working curve. Analyte Flow System. Experimental verification of the rinsing efficiency of the analyte chamber was accomplished by comparing the observed response to a 6-mL “slug” of M Co(I1) analyte rinsed through the chamber (curve 3 in Figure 4) with that theoretically derived by computer simulation (curve 1 in Figure 4). The simulation assumed that the only variable involved in the signal intensity was the instantaneous Co(I1) concentration in the chamber and that
the Co(I1) concentration was homogeneous through the analyte chamber. The mathematical model used was that described by Seitz (14) for a CL reaction in a continuously stirred tank, but excluded depletion of Co(I1) due to reaction. This model differs slightly from our experimental situation but is quite useful because it provides a “best case” upper limit for comparisons. The observed (curve 3) and predicted (curve 1) peak shapes show much better agreement as the analyte enters the chamber than as it leaves the chamber. (Since the predicted curve has not been corrected for nonlinearity in the Co(I1) working curve, nor for Co(I1) depletion due to reaction, the actual agreement is somewhat better than is apparent.) I t was speculated that adsorption of the Co(I1) analyte on the interior surfaces of the cell, most probably a t the relatively polar membrane surface, caused the Co(1I) to be rinsed from the cell more slowly than predicted and produced the highly unsymmetrical, tailing peaks. EDTA (about 10-5 M) was added to the rinse solution to remove adsorbed Co(I1) from the cell walls by complexation. The resulting peak (curve 2 in Figure 4) followed the predicted exponential decay very closely. This method of establishing the rinsing efficiency of a continuous flow cell (by comparison to ideal peak shapes) could serve as a means t o directly compare the rinsing efficiencies of many flow cells appearing throughout the literature. Figure 5 shows the peak shape obtained with an analyte volume to flow chamber volume ratio more than five times larger than that used in Figure 4. The peaks begin with the expected exponential approach t o a maximum value, then abruptly change to a slower, linear increase that continues beyond the anticipated maximum. This behavior has been attributed to slow diffusion of the Co(I1) analyte into the pores of the membrane (against the reagent flow). The EDTA rinse is not able to remove the Co(I1) from the cell until the Co(I1) has diffused out of the membrane pores into the flow chamber. The mirror-image behavior during the rinse cycle demonstrates the diffusion of Co(I1) out of the membrane. This effect can be minimized by use of smaller analyte volumes and of rinse volumes two to five times larger than the analyte volume. If neither restriction is desirable, addition of a small amount of EDTA to the reagents (as discussed earlier) will help keep the Co(I1) analyte from accumulating within the membrane pores. It is obvious that the analyte pulse width (analyte volume) affects not only the peak heights, but also the rinsing efficiency and therefore the total rinse volume required to adequately flush out the analyte chamber. Figure 6 verifies that a 1:3 duty cycle with a lo-’M EDTA rinse solution was sufficient to completely flush out all traces of analyte from a 6-mL pulse. Each series of peaks was produced by an initial Co(I1) analyte pulse, at the indicated concentration. followed by the EDTA rinse solution and then a DDW pulse. If any Co(I1) remained in the analyte chamber after one complete cycle, either because of poor rinsing or desorption from the cell surfaces, the substitution of DDW for the next sample pulse would have shown some response (as in the first two traces in Figure 6); the cycle would repeat itself until eventually DDW gave no
ANALYTICAL CHEMISTRY, VOL. 51,
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I1
NO. 3,
MARCH 1979
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Figure 6. Effects of EDTA, in the rinse solution, on the rinse efficiency. Analyte volume = 6 mL (30-s pulse at a flow rate of 12 mL/min), flow chamber volume to analyte volume ratio = 15,duty cycle = 1.3. The upward arrows on the first and third traces indicate that the peak is considerably off scale; the two peaks are of equal height. The downward arrows in the third and fourth traces indicate the point at which a DDW pulse was injected
-
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Figure 8. Effect of analyte pH on peak intensity Analyte flow rate at 12 mL/min; 30-s analyte pulse width, reagent flow rate at 3.6 wL/min. (@-@-a) M Co(I1); (0-0-0)lo-' M Co(I1)
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Figure 9. Co(I1) working curve for the membrane CL system. All conditions given as in Figure 8
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Figure 7. Effect of analyte stream flow rate on peak intensity Analyte Co(I1) pulse width = 30 s
response. T h e fact that increasing the rinse EDTA conM had no effect on the response centration from to to M Co(I1) (second and fourth traces) provided further evidence that complete rinsing of the flow chamber occurred within the 6-mL analyte volume and that the additional time required for rinsing was likely due to analyte adsorption/ desorption. T h e volume of analyte solution introduced per pulse was governed by the time that V1 remained activated and by the analyte flow rate. I t was expected that response would increase with flow rate until a sufficient volume of analyte had been delivered to the cell to quantitatively displace the EDTA rinse solution. Figure 7 shows that the response does increase over the region of 4-10 mL/min. The reduction in signal intensity a t flow rates greater than 1 2 mL/min is due to the reduced residence time, relative to the rate of reaction, within the flow chamber reaction zone. T h e effect of the analyte solution's p H on the signal intensity is seen in Figure 8. In general, a plateau or maximum is observed a t values just basic of neutral (pH 7-9). Accordingly, in the construction of the working curve, all analyte solutions were adjusted to between pH 7 and 8 for maximum sensitivity. The p H yielding maximum response provides additional data concerning the location of the reaction zone within the analyte chamber and the effective reagent concentrations within that zone. The p H a t the membrane surface will be governed by the OH- concentration in the reagent solution (pH 12). The p H in the bulk of the solution will be determined by the analyte solution (about p H 8 a t the maximum sensitivity), the reagent solution (pH 121, and their relative flow rates. T h e flow rates in Figure 8 differ by 31/2 orders of magnitude, yielding a bulk solution p H of 8.6. I t
has been reported for the Co(I1)-luminol system that the CL intensity decreases a t pH values lower than pH 11 (15). This fact supports the earlier observation that the reaction zone is near to, but not confined to, the membrane surface, and that the effective reagent concentrations in the reaction zone are about one tenth of the concentrations in the reagent reservoir. Analytical Results. The optimum operating conditions, as indicated by Figures 4 and 7, consisted of a 6-mL analyte pulse driven at a flow rate of 12 mL/min. (The sample volume requirements can be lowered by designing a smaller volume cell.) Accordingly, to establish the stability of the reagents within the reagent reservoir, as well as the precision of the technique, 6-mL pulses of a M solution of Co(I1) were repeatedly introduced into the flow stream for 2l/, hours. The overall precision was 4.1 % RSD, with the worst case for any 30-min period being 3% RSD. Operation a t a 1:3 duty cycle and an analyte stream flow rate of 1 2 mL/min yields an analytical throughput of 40 samples (6 mL each) per hour. In one hour, 500 mL of rinse solution would be consumed, but! a t a typical reagent reservoir pressure of 10 psi (70 kPa), only 0.25 mL of reagent solution would be consumed. Reduction of analysis time could be made by improving the rinsing efficiency of the cell and/or by reducing the volume of the analyte sample. A working curve for Co(I1) determinations is given in Figure 9. The change in slope a t about 2 x M is of interest, and is perhaps related to the question of the reagent concentration in the reaction zone. The slope of a plot of CL intensity vs. Co(I1) concentration has been reported to change dramatically a t a luminol-to-metal ratio of about 1:1, presumably because of complex formation (16). However, in the present study, the luminol concentration (in the reaction zone) was between 2 X M (in M (at the surface) and 6 X bulk solution) and the luminol-to-metal ratio (at the bend in the working curve) is at least 30:l and possibly as high as 105:l. At present, the practical limit of detection for Co(I1) is slightly less than lo-'' M (10 parts per trillion) using the membrane flow cell. It should be noted that this is the lowest measured detection limit for Co(I1) with luminol reported to
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ANALYTICAL CHEMISTRY, VOL. 51,
NO. 3, MARCH 1979
date; a previously reported value of lo-’‘ M was obtained by extrapolation from higher concentrations ( 2 ) . The linear range (on a log-log plot) extends from 2 x M to 2 X M. The onset of leveling off can be seen below 2X M; however, significant slope still remains to push the detection limit below lo-’’ M (17). T h e authors believe that the linear range can be extended to include a wider concentration range through knowledge of, and control of, reagent and analyk concentrations and residence times in the reaction zone.
reagents, which respond to different analytes (or categories of analytes). Such an approach would extend CL to simultaneous multicomponent analysis. Finally, pressurizing a microporous membrane system using the energy stored in an inflatable elastomeric reservoir (18) could provide a portable reagent delivery system.
LITERATURE CITED (1) S. Stieg and T. A. Nieman, Anal. Chem., 50, 401 (1978). (2) W. R. Seitz and D. M. Hercules in “Chemiluminescence and Bioluminescence”,M. J. Cormier, D. M. Hercules, and J. Lee, Ed., Plenum Press, New York, 1973, pp 427-49. (3) D. Siawinska and J. Siawinski, Anal. Chem., 47, 2101 (1975). (4) U. Isacsson and G. Wettermark, Anal. Chim. Acta, 83,227 (1976). (5) T. L. Sheehan and D. M. Hercules, Anal. Chem., 49, 446 (1977). (6) S. N. Lowery, P. W. Carr, and W. R. Seitz, Anal. Lett., 10, 931 (1977). (7) 6.M. Strom, M.S. Thesis, Iowa State University, Ames, Iowa, 1974. (8) M. P. Neary, R. Seitz. and D. M. Hercules, Anal. Lett., 7 , 583 (1974). (9) R. Delumyea and A. V. Hartkopf, Anal. Chem., 48, 1402 (1976). (10) R. L. Veazey and T. A. Nieman, 29th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1978, paper 99. (11) K. S. Subramanian, C. L. Chakrabarti, J. E. Sueiras, and I. S. Maines. Anal. Chem., 50, 444 (1978). (12) S.Stieg and T. A. Nieman, Anal. Chem., 4 9 , 1322 (1977). (13) Poroplastic Data Sheets, Moleculon Research Gorp., Cambridge, Mass. (14) W. R. Seitz. R o c . Conf. Anal. Appl. Biolum. Chemilurn.,Brussels, 1978, to be published. (15) T. G. Burdo and W. R. Seitz, Anal. Chem., 47, 1639 (1975). (16) R. G. Delumyea, Ph.D. Thesis, Wayne State University, Detroit, Mich.,
CONCLUSIONS T h e reported method of reagent addition has been shown to provide the anticipated advantages, with reagent economy and good precision important in general, and localization of emission and high sensitivity of particular utility in CL analysis. T h e membrane system offers certain present advantages and possibilities for future applications that are not readily feasible with conventional use of a peristaltic pump for reagent delivery (although a peristaltic pump certainly has the advantages of wider availability, applicability, and general acceptance). Flow rates through the membrane are below the normal flow rates available with peristaltic pumps. Also, the membrane system is able to allow a reaction to take place under conditions where the “effective” reagent concentration is orders of magnitude higher than in the bulk. Future studies will examine storage and/or immobilization of reagents within the membrane pores. In addition, because of the localization of emission, it should be possible to have a sample stream containing several analytes flow along a row of membrane strips, where each membrane contains different
1974
(17) J,-D. Ingle, Jr., and R. L. Wilson, Anal. Chem., 48, 1641 (1976). (18) Chem. Eng. News, 55, 30 (July 11, 1977).
RECEIVED for review July 28, 19’78. Accepted December 8, 1978. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to Research Corporation for support of this research.
Trace Element Laser Microanalyzer with Freedom from Chemical Matrix Effect H. S. Kwong and R. M. Measures* Institute for Aerospace Studies, University of Toronto, 4925 Dufferin Street, Downsview, Ontario
Evaluation of trace concentrations of heavy metals in biological, medical, industrial and environmental materials has become a major analytical task ( I ) . The impact of heavy metal contamination on the biosphere is slowly being recognized as a potential health hazard of growing proportions (2). Consequently, there is an increasing demand for a trace element microprobe that can be used to rapidly monitor the concentration of a select group of elements in a wide variety of material samples. The ideal characteristics of such a trace element microprobe are: (1) Selective microsampling capability. ( 2 ) No sample preparation-in situ measurement. (3) Real time analysis. (4) High relative and absolute concentration sensitivity. ( 5 ) Freedom from matrix effects (chemical or physical) so that calibration is independent of substrate containing element. (6) Highly selective for element of interest. (7) Linearity of response over wide dynamic range of concentrations. (8) Minimum variation in sensitivity between elements. (9) Depth profiling capability. (10)Simultaneous multielement measurement possible. (11) Capable of distinguishing between isotopes. (12) Insensitive to the nature of substrate material.
A study on the susceptibility of TABLASER-Trace (element) Analyzer Based on Laser Ablation and Selectivity Excited Radiation to various chemical interference effects has been carried out. Tests were made on a diverse range of chromium doped samples to study source interference; anion-anion interference; cation-anion interference, and Ionization lnterference. The observed laser induced fluorescence signal was found to be relatively independent of the source materials used, even under conditions of doping with high concentrations of compounds such as copper sulfate and potassium sulfate. The calibration curves for chromium in three different matrix materials are linear with a 45’ slope. These observations provide evidence that our TABLASER is relatively free from chemical interference effects. The absence of these undesirable effects is attributed to (a) the extremely high temperature of the laser generated plasma (enabling complete dissociation of the compounds) and (b) the subsequent rapid expansion of the laser ablated plasma into the low vacuum region (which minimizes molecular association through interspecies collisions). This new technique has the potential for development into a new form of laser trace element microanalyzer that could be used for in situ high sensitivity measurements. 0003-2700/79/0351-0428$01 O O / O
M3H 5T6, Canada
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1979 American Chemical Society
