Anal. Chem. 1994,66, 2180-2186
Novel Technique To Reduce Electrical Interference Inherent in Laser-Enhanced Ionization Detection by Using Flow Injection Analysis Shau-Chun Wang and King-Chuen Lln'
Department of Chemistry, National Taiwan University, Taipei, Taiwan 10764, and Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23- 166, Taipei, Taiwan 10764, Repubiic of China
The influence of electrical interference induced by the easily ionized sodiummatrix upon thelaser-enhancedionization (LEI) detection of indium analyte is studied. As the LEI signal is electrically suppressed but still detectable, the original sample concentration may be effectivelyrecovered by using the standard addition method. Nevertheless, this method fails to apply when the added Na matrix reaches a threshold level, at which the LEI signal is completely suppressed. Under such a condition of severe electrical interference, our design of an LEI device interfaced by a flow injection analysis (FIA)can be a successful substitute to recover a significant LEI signal, so that the standard addition method may be resumed. The FIA-LEI technique is capable of detecting the In sample in a solution containing a Na matrix of more than 40 ppm (pg/mL), about 20-fold that the conventional LEI apparatus can tolerate. It possessesanother remarkable advantage, having a much wider range of linear dependence of indium concentration than that achieved by the conventional LEI. For instance, the linearity of indium concentration dependence measured by the conventional LEI method is restricted within 5 ppm, which can extend to 30 ppm with a mixture of 8 ppm of Na by means of the FIA-LEI apparatus. A good reproducibility and a rapid sampling rate are also advantages possessed. In addition, the new technique is able to be integrated with those methods used previously for reduction of the electrical interference to make the best use of this apparatus. Laser-enhanced ionization (LEI) technique has proven to be capable of detecting trace analytes in the sub parts per billion (ng/mL) concentration of aqueous solution for At least two advantages possessed by this technique make it as comparableas any optical spectrometers in the aspect of trace analysis. First, LEI detection can avoid (1) Travis, J.C.;Turk,G.C.;Grctn,R. B.AMI. Chem. 1982,54,1006A-l018A. (2) Axner, 0.;Lindgren, I.; Magnuason, I.; Rubinsztein-Dunlop,H. AMI. Chem. 1985,57, 773-776. (3) Omenetto, N.; Berthoud, Th.; Cavalli, P.; Rossi, G. AMI. Chem. 1985, 57, 1256-1 260. (4) Axner, 0.;Magnusson, J.; Peterson, J.; Sjcstrom. S . Appl. Spectrosc. 1987, 41, 19-26. ( 5 ) van Dijk, C. A.; Curran, F. M.;Lin, K. C.; Crouch, S . R. A M / .Chem. 1981, 53, 1275-1279. ( 6 ) Su,K. D.;Lin, K. C. Appl. Spectrosc. 1994, 48, 241-247. (7) Turk, G. C.; Travis, J. C.; DeVoc, J. R.;OHaver, T. C. AMI. Chem. 1979, 51, 1890-1896.
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optical interference, which is a severe drawback to most optical spectroscopic methods.'** Second, the ion detector used in the LEI technique can be potentially made as sensitive as to have almost unity quantum efficiency, while optical detectors such as a photomultiplier tube are restricted to a small quantum efficiency.l.9 Since LEI detects the species in the ionized form, the electrical interference inherent in this technique may diminish its sensitivity to a trace detection. This interference results from such different sources as ions and electrons produced in the flame background and the easily ionized matrices mixed in the anlytes. The influence of the interference upon the LEI technique is to reduce the signal-to-noise ratio and to block the electric field applied from reaching the radiation-interactive regime. The effect of the reduction of the signal-to-noise ratio is similar to that occurring in a mass spectrometer, as interference by background charged particles, and therefore a weak ion signal may be buried in the background noise. The second effect caused by the interference may prevent the electric field, formed between two voltage-biased metal electrodes, from reaching the interactive regime, where the sample ions and electrons are generated upon irradiation of a laser. Since the sample ions and electrons cannot be set apart and then collected toward the opposite electrodes by a nonzero electric field, the LEI signal appears to be suppressed and even ~ndetectable.~J~J~ Study on electrical interference becomes crucial for improving the sensitivity of LEI to trace analysis. Thus far, several methods have been applied to reduce this effect: for instance, sample pretreatment,l2J3 improvement of electrode d e ~ i g n , ~increase J ~ v ~ ~of biased ~ o l t a g e , ~ and J & ~employment ~ of pulsed voltage-biased c0llectors.1~These current methods, although effective in diminishing the electrical interference, may somehow cause such side effects as disturbance of the (8) Fraser, L. M.;Winefordner, J.
D.AMI. Chem. 1972, 44, 1444-1454. (9) Hunt, G. S.;Payne, M.G.; Kramer, S.D.; Young, J. P. Rm.Mod.Phys. 1979, 51, 767-818. (10) Schenck, P. K.;Travis, J. C.; Turk, G. C.; O'Haver, T. C. J. Phys. Chem. 1981,85,2547-2557. (11) Turk, G. C. AMI. Chem. 1992, 64, 1836-1839. (12) Trask, T. 0.;Green, R. B. A w l . Chem. 1981,53,32&324. (13) Bykov, I. V.;Chekalin. N. V.;Tikhomirova, E. 1. 1.A M / . Chcm. USSR (Engl. Trawl.) 1986, 40, 1579-1584. (14) Turk, G. C. AMI. Chcm. 1981,53, 1187-1190. (15) Havrilla, G. J.; Green, R. B. A w l . Chem. 1980.52.2376-2983. (16) Green, R. B.;Havril1a.G. J. Trask,T. 0.Appl. Spectrosc. 1980,34,561-568. 0003-2700/94/03852 180$04.50/0 Q 1994 A m r k a n Chemical Society
flamestr~cture,'~ complication of the flame composition,'* or lack of reproducibility in LEI detecti~n.'~ In an effort to eliminate the electrical interference, in this paper we design a new apparatus by interfacing a flow injection analysis (FIA) technique to the LEI device. Using this new apparatus, the electrical interference can be effectively reduced, and thereby the easily ionized matrix added can be increased to 20-fold as high a concentration as that tolerated in the conventional LEI. The linear dynamic range of sample concentration can also extend much further. In addition, the new apparatus possesses the advantages of good reproducibility and rapid sampling rates.'* In this work, we have applied a standard addition method under the condition of electrical interference to recover the sample concentration. Although addition of matrix in the sample solution may suppress the resulting LEI intensity and lead to a change of some physical properties in the system, the original concentration of sample can be effectively determined by employment of the method of standard addition. Nevertheless, if the concentration of matrix is increased to a threshold level, at which the LEI signal is completely suppressed, then the standard addition method fails to apply. Under such a condition of severe electrical interference, our design of an FIA-LEI apparatus can be a successfulsubstitute to recover a significant LEI signal, so that the standard addition method may be resumed. Wedescribe the experimental setup of the FIA-LEI apparatus under Experimental Section, then state briefly the method of standard addition employed, and finally characterize the FIA-LEI apparatus under Results and Discussion. EXPERIMENTAL SECTION A. ConventionalLEITechnique. The basic LEI apparatus is composed of a flame burner assembly, a pair of metal rod electrodes as a collector of charged particles, and a laser radiation source. Flame System and Reagents. As illustrated previa commercial burner assembly with a 100 mm X 0.5 mm slot burner head was employed. The fuel C 2 H 2 and air were regulated to have flow rates of 0.5 and 12.5 L/min, respectively, and were premixed prior to reaching the burner head. The flame temperature was measured as about 2500 K.19 The indium salt solution was prepared at various In concentrations, in which the sodium salt was added as the matrix, composed of 0.1,0.5, 1,2,4,8,40, and 50 ppm (pg/ mL) of Na concentration. The mixed solution was then nebulized into the flame with a rate of 4.5 cm3/min. Laser. A tunable dye laser, pumped by a 10-Hz frequencydoubled Nd:YAG laser at 532 nm, was used as radiation. The output frequency was then doubled through a KDP crystal emitting at 325.62 nm for excitation of the In atom in the 52P3p-52D5/2 transition. The dye laser has a pulse duration 5-8 ns and a line width of 1 cm-l. The output energy was
-
(17) Vippoldt, M. A.; Green, R. B. Anal. Chcm. 1983,55,554-557. (18) Ruzich, J.; Hansen. E. H.Flow Injecrion Analysis, 2nd ai.;Wiley: New York, 1988; Chapters 1-4. (19) Su,K. D.; Chcn. C. Y.;Lin, K. C.; Luh, W. T. Appl. Specrrarc. 1991, 45, 1340-1343. (20) Su. K. D.; Lin, K. C.; Luh, W. T. Appl. Specrrosc. 1992, 46, 1370-1375.
maintained at 80-350 pJ, which was constantly monitored with a surface absorbing disk calorimeter. Data Collection. A pair of metal rod electrodes, 0.2-cm diameter by 8 cm long, was set 12 mm apart and suspended 10mm above the burner head in the vicinity of the flame. One electrode was biased at -1000 V, and the other was connected to a current-to-voltagecurrent amplifier, by which the collected ions were amplified before being fed into a boxcar integrator for improving signal-to-noise ratio. The LEI signal of In analyte was displayed on an oscilloscope or stored in a personal computer for data treatment. B. FIA-LEIApparatus. An FIA technique was employed to couple to the LEI device to eliminate the electrical interference effect, as the matrix concentration reached its threshold level at which no indium LEI signal can be detected. The design of the FIA-LEI apparatus is illustrated in Figure 1 . A single-channel FIA system was implemented to handle the solution prior to flowing toward the nebulizer.18 In this manner, a segment of distilled water flow was injected into the carrier stream of sample solution; the mixed solution was then nebulized into the flame with a flow rate of 4.5 cm3/min in the same process as that without the FIA system involved. The single-channel manifold was composed of a peristaltic pump, which was used to load the distilled water in a volumefixed loop, and a four-channel-based injection port, by means of which a well-defined volume of distilled water was injected into the sample carrier. During the water loading, the sample flow with the original concentration was passed by this segment and continuously nebulized into the burner head. While the valve was switched to release the water in the loop, a 250-pL water zone was instantanenously dispersed into the carrier stream of sample solution. Diffusion and possibly convection mixing at the interface between the sample and the injected water plug cause a concentration gradient to develop along the direction of flow. The water-injected mixture was transported toward the flame, in which the In atoms contained in the carrier stream were collisionally ionized following optical excitation to the 5 2 D s / 2 state. The gradients on each side of the injected water plug gave rise to the double peaks observed in the analytical signal. The injected sampling rate was kept at about 2-3 sample/min; the speed was mainly limited by a low repetition rate of the laser used. With increased laser repetition rate and flow rate of the nebulizer, the sampling rate may be enhanced. Theoperation procedure for the FIALEI detection is otherwise the same as that in the so-called conventional LEI apparatus without implementation of the FIA system. RESULTS AND DISCUSSION A. StandardAdditionMetbod. The influenceof Na matrix upon the indium LEI intensity is shown in Figure 2, of which the curves A, B, C, and D represent the indium concentration dependence of LEI detection in 0.0,O.1,0.5, and 2 ppm of Na solutions, respectively. The LEI response appears to be suppressed with increasing Na matrix. As Na concentration is increased to 2 ppm, the indium LEI signal is suppressed completely, despite its concentration reaching as high as 10 ppm. (Figure 2, curve D) Note that the LEI detection as a function of In concentration with Na added as the matrix Analytcat Chemistry, Vot. 66,No. 13, July 1, 1994
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a. Water Loading
532 nm
fi
651.23 nm
325.62nm
17 0Power meter
Water injecting
Figure 1. (a) Schematic dlagramof FIA-LEI apparatus. (b) Four-channel-based1n)ectionport: (1) sample fbw passes by the segment of dlstllled water IOOD:(21 as the valve Is switched to release the dlstltled water In the loop, the segment of water flow Is Injected Into the carrier stream of sampd s&;tlon. 120 r 110
T a b 1. Apptkatbn of Standard Addnbn Method to the Conventional LEI Detection of 2.0 ppm of Indium Analyte In a 1.0 ppm of Na MMure
*
i
’03
test no.
80-
?
a
5
’
BO 70
-
1
2 3
L
standard deviation ppm
weighing factoP
1.99 1.90 2.14
0.06 0.03 0.30
244 1075 11.1
(
4
9
eo1
Weighing factor = I/.;. 5040-
In Concentration (ppm)
Fbwe 2. Indlum concentratlon dependence of conventional LEI detection In a solution with a varlety of Na concentratbns added as the matrix: curve A denotes the In solution without Na added, (0);B, 0.1 ppm of Na added (0):C, 0.5 ppm of Na added (0); D, 2 ppm of Na added
m.
follows a linear relationship with the intercept equal to zero. Accordingly, the standard deviation associated with the standard addition method is estimated to be 0.19 ppm for curve Band 0.32 ppm for curve C.21As expected, the precision resulting from the method of standard addition becomes worse as a higher concentration of matrix is added. In the following we made up a mixed solution of 2 ppm of In and 1ppm of Na and invoked the standard addition method (21) Lamen, I. L.; Hartmann, N . A.; Wagner, J. J. AMI. Chem. 1973,45,15ll1531.
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AnalyticalChemistry, Vol. 66, No. 13, July 1, 1994
to find the original concentration. Three sets of tests were carried out, and the results are given in Table 1. According to the parameters evaluated in Table 1, one obtains a value 1.92 f 0.07 ppm under 95% confidence leve1.22123 The above treatment is valid under the condition that the standard deviation carried by the instrument response is independent ofits intensity. Otherwise, the weighed regression method must be used to improve the precision evaluated. To compare with the results obtained above, we have added a weighing factor to each measured LEI response, which is defined as l/u; (uj is the standard deviation for each individual sample point).24 The outcome is 1.93 f 0.07 ppm under 95%confidencelevel. Both treatments are in agreement with each other and apparently demonstrate that the standard addition method can be effectively applied to the LEI detection to determine the unknown, original sample concentration, if the system is not so severely interfered with that the LEI signal is completely buried. (22) Bader, M. J. Chem. Educ. 1980,57, 703-706. (23) Ratdaff, K. L. AMI. Chem. 1979, 51, 232-235. (24) Berington, P. R. Data Reduction andError Analysisfor the Physic41 Sciences; McGraw-Hill: New York, 1969; Chapter 6.
a. 18 18 c /
+
t
t
b. 3213 nm
InJectbn
32575 nm
Injection
T
I
1
3
4
5
6
7
8
In Concentration (ppm) Figuro 4. FIA-LEI detection of the lndlum analyte In a mlxtue with a variety of Na concentrations: A, 4 ppm of Na (0);B, 8 ppm of Na (0):C. 20 ppm of Na (0);D, 40 ppm of Na 0.
C.
Injection 1
Injectton 2
Injectton 3
Figure 9. (a) FIA-LEI detection of 8 ppm of indium In a 4 ppm of Na solutkn. A weKdeftnedvokmeof dlstiHedwater was injectedIn trlpWcate intoasinglechennelmanlfoidofFIAsystem. (b)LEIsigneldiseppeared as the laser was downed to the wing wavelength elther at 325.50 nm or at 325.75 nm. (c) No Wkrm LEI swl was observed In a blank SOMion compowJ of 4 ppm of Na alone, folbwlng optical excltatbn to the 5 Q I 2 state.
B. FIA in Combinationwith LEI. Resistance to Electrical Interference. Using the new FIA-LEI apparatus described under ExperimentalSection, and injecting a segment of water flow into thecarrier stream of the sample, one obtains a doublepeak LEI signal, as shown in Figure 3a, in a 4 ppm of In solution mixed with 4 ppm of Na as the matrix. To further examine whether the double peak is associated with the In sample, we have tuned the laser slightly off-resonance to the wavelengths at either 325.50 or 325.75 nm and found that the LEI peak disappears (Figure 3b). As expected, the LEI signal also disappears when a blank solution, which is composed of 4 ppm of Na concentration alone, is irradiated with a laser at the resonance wavelength of 325.62 nm (Figure 3c). As recalled in the preceding section, the conventional LEI device fails to detect any In trace as the Na matrix is increased to 2 ppm. To our surprise, however, a significant LEI signal is resumed by using our FIA-LEI apparatus, which can detect In sample in the solution containingNa concentrationof more than 40 ppm, about 20-fold that the conventional LEI apparatus can tolerate. The In concentration dependence of FIA-LEI detection in a 4, 8, 20, or 40 ppm of Na solution, respectively, is shown in Figure 4, in which the peak height of the late-appearing larger one of the double peaks is used for analysis. The dynamicconcentrationdependence appears to be linear within the In concentration studied; its slope tends to decrease with the Na concentration. Comparison of linear dynamic range of concentration between conventional LEI and FIA-LEI detection is shown in Figure 5 . Under otherwise identical experimental conditions, the linearity of In concentration dependence with the FIA-LEI detection in a 8 ppm of Na solution can extend to a concentrationof 30 ppm, about
In Concentration (ppm) Flgrrre 5. LEI measurement as a function of In concentretion: A, conventionel LEI detection of the In sample In a sokrtkn wlthout any Na matrlx added (Intensity is scaled down by a factor of 0.25) (0); B, FIA-LEI detectlon of the In sample in a 4 ppm of Na sdutkn 0; C, FIA-LEI detectlon of the In sample In a 8 ppm of Na sdutbn (0).
6-fold larger than that achieved by the conventional LEI apparatus. Concept of FIA-LEI. Using the design of pulsed voltagebiased collectors, Vippoldt and Green17 have reported that a transient LEI signal of the In atom with K added as the matrix appears in the early few hundreds of milliseconds before reaching the steady-state condition, where the LEI signal is totally suppressed. Although the reproducibility of such a transient LEI signal is doubted, the result communicates a message that in a matrix-interfered solution the LEI signal may penetrate through the resistance of electrical interference and become detectable, if some method is applied to disturb the homogeneous mixture from its equilibrium. The FIA technique is one of these methods, popularly employed to generate a concentration gradient in the mixture of reagent and sample.** As a well-defined volume of distilled water in the loop is injected into the sample carrier stream, the water dispersion through the solution makes the sample concentration AnaWcaI Chemistry. Vol. 66, No. 13. Ju& 1, 1994
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difference of mobility of ions and electrons such that the positive space charge is around the cathode. The extent of ion sheath is associated with the ionization rates generated by time scan the species in the flame; that is, the length of the ion sheath will decrease with increasing ionization rates of matrices. Under steady-state approximation, the ionization rate becomes proportional to the matrix concentration. Accordingly, the increase of matrix concentration may shorten the ion sheath by producing more ions and electrons. When the ion sheath is shorter than the distance between the cathode and the nearside edge of the radiation-interactive regime, the nonzero C. electric field cannot reach the produced ions and electrons 1.25 These and then separate these charged parti~les.~J~J A A , particles without motion in the field result in a zero current ‘ U to be detected. In light of this concept regarding the LEI t’ detection, the generation of concentration gradient by the FIA technique is believed to help to dilute the matrix concentration, causing the ion sheath to stretch out into the charged particle regime. On the other hand, such a diluting process is also at the expense of dilution of the sample concentration. The resulting FIA-LEI intensity recovered is t reduced to some extent, as compared to that detected by a Flguro 6. (a) Dlspersbn behavior for the Injected water zone along the passage of flow. (b) Concentration gradlent profile formed by conventional LEI technique under an interference-free condil n j ” of a segment of water flow Into the sample carrier In a slngb tion. channel FIA system. The solid cvve denotes the b e h a wof sample concentration gradlent, w h k the dashed cwve denotes the behavior FIA us Bench-Dilution Operation. The FIA technique of the water concentration gradient. (c) A double-peak LEI a n a l introduced herein has a great advantage over the manual appears wrth a small distance between, Indicating that the sample bench-dilution operation. By generating a dilution gradient, carrler is much dlMed to allow for penetrationof nonzero electrk flekl through resistance of electrical Interference. (d) A doublepsak LEI the FIA-LEI is capable of automatically searching for the signal appears with a large distance between, lndbtlng that the optimalcondition under which the electric field may penetrate electrical Interference is not so serious that a si@htdllutlon m y bad to reach the ionized sample target at ‘the least” cost of sample to recovery of the sample signal. dilution. The peaking position of the double peaks (see Figure 3) isindicativeof thisoptimalconditionasachieved. Ofcourse, this work can be replaced by a manual operation involving a gradients occur on each side of the injected water plug along multiple-step, tedious bench-dilution procedure. If the matrix the passage of flow into the flame. The dilution gradient profile is depicted in Figure 6. Such behavior of a sample concentration is changed, the above tedious trial-and-error procedure should be repeated. In contrast, with the FIA concentration gradient may explain the appearance of a technique, a segment of water plug injection may find the double-peak LEI signal as shown in Figure 3a. The early optimal condition no matter what concentration of the matrix appearance of the small peak is caused by the gradual is changed. decreasing process of In concentration, while the late, large peak comes from the gradual increasing process of In Linear Dynamic Range. FIA-LEI detection possesses concentration. The valley between two peaks indicates the another remarkable advantage, having a much wider range lack of In sample in the center of the water zone to be detected. of linear dependence of sample concentration than that The character of a double peak is usually found in a singleachieved by the conventional LEI technique. For instance, channel manifold of FIA design.l8 It can be changed to become Figure 5 reveals that the linearity of In concentration a single peak by using a device having a two-channel manifold, dependence measured by the conventional LEI method is allowing the process for the sample injection into the reagent restricted within 5 ppm, while it can extend to 30 ppm (or 10 stream to occur prior to merging with another reagent stream ppm) with a mixture of 8 ppm (or 4 ppm) of Na by means from a second channel. In this manner, the concentrationof of the FIA-LEI apparatus. It is worthwhile to understand reagent remains invarient throughout the passage of flow. qualitatively the concentration dynamic behavior found in It should be noted that the generated ions and electrons, the new apparatus with respect to the conventional LEI. In while moving toward the opposite electrodes as a result of the the latter measurement, the leveling off of the linearity for the electric field applied, may produce a current for detecti~n.~JOJ~ In concentrationdependence may be attributed mainly to the If the electric field is prevented from reaching the charged space charge effect, which causes the shielding of the electric particles, then the produced current may diminish or even field.7J0J1 The more the charged particles are formed in the disappear. According to the theory of Lawton and Wei~~berg?~ flame, the more significant the space charge effect may the shielding effect of electric field caused by the interference become. The influence of space charge is ascending with the of those easily ionized species existing in the flame may be In concentration; this consideration may explain why the indicated by the ion sheath, which is formed due to the detected LEI intensity levels off and then tends to decrease with In concentration. In the FIA-LEI apparatus, the (2s) Lawton, J.; Wcinkrg. F. J. Electrical Aspects of Combustion; Clarendon concentrations of In and Na are simultaneously diluted, such Press: Oxford. U.K., 1969; Chapter 8.
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a.
b.
Table 2. AppUcatbn d 81.nd.rd AWltkn Mothod to ttm FIA-LEI Dotoctbn ol 9.0 ppm ol Indkm An8Iyto h a 4.0 ppm ol Na M b ” determined value standard deviation teat no. (Xi), ppm (dP P ~ weighing factop 1 2 3
2.60 3.25 2.91
0.30 0.39 0.22
11.1 6.51 20.1
Weighing factor = l/t$.
I Figure7. (a) Observeddoublepeek FIA-LEI signal of 2 ppmof Indium in a 4 ppm of Ne mixture, corresponding to the case (Figure 6d) in whichthe sample solution is weakly lnterferedwith electricetly and only sllght dilution is needed. (b) Observed double-peak FIA-LEI slgnal of 2 ppm of IndiumIn a 50 ppm of Ne mixtwe, corre8pOndins to the case Figure 6c) In which the sample solution is severely Interfered with electrically and an extreme dHutlon Is needed.
that the nonzero electric field may interact with the radiationactive regime. The character of dilution gradient leads to a decrease of the total ion density in the flame, and thereby the space charge effect is reduced significantly. Accordingly, the linear range can stretch to a higher concentration of indium before leveling off from the linear relationship. In terms of the dilution effect, we may also interpret why a larger linear range of In concentration dependence is found in a higher Na concentration mixture (Figure 5). Along the concentration gradient profile shown in Figure 6, if the doublepeak LEI appears at a smaller concentration of In, the solution must be mixed with a larger concentration of Na matrix (the case of Figure 6c). In this manner, the concentrations of In and Na need to be diluted to some extent with which the nonzero electric field can finally penetrate to reach the charged particle regime. In contrast, the appearance of the doublepeak LEI at the higher concentration of In suggests that the solution should be interfered with at low Na concentration, because the ion sheath can reach the interactive regime without much dilution (the case of Figure 6d). The distance between two peaks becomes indicative of the extent of dilution. A double-peak LEI signal appearswith a smalldistancebetween, indicating that the sample carrier is much diluted to allow for penetration of nonzero electric field through resistance of electrical interference (Figure 6c). On the other hand, the double-peak LEI signal appearswith a large distance between, indicating that the electrical interference is not so serious that a slight dilution may lead to recovery of the sample signal (Figure 6d). This is evidenced by the FIA-LEI detection of 2 ppm of In as mixed with 4 and 50 ppm of Na under otherwise identical experimental conditions (Figure 7a,b). A small distance between two peaks is found in the 50 ppm of Na solution, while a large distance appears in the 4 ppm of Na solution. Accordingly, a large linear dynamic range found in a high matrix concentration is apparently caused by an extreme dilution process. The resulting reduction of ion density between electrodes lengthens the concentration dynamic range. Note that the detection limit of the In sample with the use of the FIA-LEI technique is presumed to be essentially the same as the conventional LEI, since they both have otherwise identical setups except for the implementation of the FIA
system. A larger linear dynamic range is meant on the basis of this point. Standard Addition Method. Since the measured indium FIA-LEI signal is linearly proportional to its concentration, we can apply the standard addition method to recover its real Concentration. For instance, by measuring the FIA-LEI intensity of 3 ppm of In in a 4 ppm of Na solution and applying the standard addition method for correction, we have obtained a recovered value of 2.88 f 0.38 ppm under 95% confidence level. The related parameters are listed in Table 2. As treated in the preceding section, for the case shown in Figure 4, the standard deviations associated with the standard addition method is estimated to be 0.25, 0.63, 0.75, and 1.65 ppm, respectively, for the In analyte with Na concentrations of 4, 8,20, and 40 ppm. The precision of recovery for the original analyte concentration seems to be less reliable with increasing suppression effect of the Na matrix. Advantages and Rod Electrode Consideration, The current methods normally used to reduce the electrical interference may somehow disturb the flame structure,I4 complicate the flame composition,12or fail in the reprod~cibi1ity.l~However, in this paper the FIA-LEI technique has proven to be a very effective tool to eliminate the electrical interference without causing those side effects. In the meantime it can also be integrated with those methods used previously and incorporated with various types of electrode configurations to make the best useof this new technique. Note that the rod electrodes demonstrated herein are regarded as the worst configuration to resist against the electrical interference.IJ4 That is why we chose to demonstrate the potential development of this FIA-LEI apparatus, since only a small amount of matrix added can result in a complete suppression of LEI signal, the very condition for an effective application of the FIA-LEI technique. Despite the fact that more matrix may be needed to meet the criteria, the FIA-LEI can also be equipped with water-cooled immersed electrode or plate electrodes, much better behaved electrode configurations than the rods for resistance to electrical interference.l.14
CONCLUSION In conclusion,we have applied the standard addition method to the LEI detection of the In analyte as interfered with by the Na matrix. When the resulting electrical interference is not serous, and the LEI signal can still be detectable, the use of this method may result in a satisfactory recovery of the original analyte concentration. However, as the Na matrix is increased to completely suppress the LEI signal, the standard addition method is no longer valid. Then our design of an AmWai Chemlsby, Voi. 66, No. 13, &v!
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FIA-LEI apparatus can be a successful substitute, capable of detecting In sample in a solution with Na matrix as condensed as >20-fold that theconventional LEI can tolerate. The linear dependence of indium FIA-LEI detection can also stretch by 6-fold more condensed than that achieved by the conventional LEI technique. With the aid of the new apparatus, a significant LEI signal can be recovered from an environment with severeelectrical interference,thereby leading to resumption of the standard addition method.
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ACKNOWLEDGMENT This work is financially supported by the National Science Council, the Republic of China,under Contract No,NSC82o1 5-M00170, R e d ~ e for d t w k w JenMV 14, 1994. Accepted April 9, 1994.' Abstract published in Advance ACS Absrrucrs, May IS, 1994.
