Flow Injection Dispersion Characteristics with Inductively Coupled

Deviations in the dispersion heights and widths, especially at low flow rates observed for normal and reverse. FI with the continuous-flow mode, were ...
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Anal. Chem. 1994,66, 3937-3943

Flow Injection Dispersion Characteristics with Inductively Coupled Plasma Atomic Emission Spectrometry. 2. Influence of Flow Modes and Manifold Configuration Yecheskel Israel and Ramon M. Barnes' Department of Chemistryl Lederle Graduate Research Center, University of Massachusetts, Box 345 IOl Amherst, Massachusetts 0 1003-45 10

A flow injection (FI) dispersion study was conducted with inductively coupled plasma atomic emission spectrometric (ICPAES) detectionusing continuous- and stopped-flowmodes. The influence of molecular diffusion on the overall dispersion behavior was examined in diffusion models devised for normal and reverse FI. Deviations in the dispersionheights and widths, especially at low flow rates observed for normal and reverse FI with the continuous-flow mode, were enhancedin the stoppedflow mode. At conditions promoting diffusion, opposing deviations in the dispersion height and width behaviors were observed between the two FI modes. A practical sample with matrix composition unmatched to the countersolutionexhibits pronounced width contraction for normal FI and broadening accompanied by a sharp splitting of the minimum signal for reverse FI. A flow configuration was applied at high FI rates that enables the choice of optimal flow rates for either continuous- or stopped-flow injection and detector devices. However, continuous sample division before the ICPAES detector leads to some dispersion height and width deviations as a result of secondary dispersion. The flow injection (FI) dispersion height and width characteristic behavior with an inductively coupled plasma atomic emission spectrometry (ICPAES) detector and the methodology involved were examined previously.' A continuous-flow (CF) configuration was employed to supply the sample directly to the ICP nebulizer, which was used to investigate the influence of various parameters (Le., injection volume, flow rate, and reactor dimensions) on both the normal (n) and reverse (r) FI dispersion height and width characteristics. Various indicators were measured to define the transient dispersion width characteristics. Deviations from conventional dispersion behavior were observed, and differences between n- and r-FI modes occurred at low flow rates and/or large reactor volumes. The influence of molecular diffusion on the overall flow injection dispersion behavior was not examined in our earlier study.' The combined dispersion action of convection and molecular diffusion was quantitatively described by In another previous study with a solution spectrophotometric d e t e ~ t o r the , ~ stopped-flow (SF) method was employed to investigate the influence of molecular diffusion on the overall (1) Israel, Y.; Barnes, R. M. Analyst 1994, 119, 1011. (2) Taylor, G.Proc. R . SOC.London Ser. A 1953 219, 186. (3) Taylor, G. Proc. R. SOC.London Ser. A 1954, 225, 413. (4) Israel, Y.; Barnes, R . M. Mikrochim. Acta [Wien] 1990, 1 , 17.

0003-2700/94/0366-3937$04.50/0 0 1994 American Chemical Society

dispersion behavior. The injection of a defined sample volume into the counterstream undergoes dispersion in the flow manifold depending on the residence time upstream, flow parameters, and dimensions. However, if the counterstream flow is stopped suddenly, dispersion by convection is halted as well, and only the influence of diffusion remains. As a result, the stopped-flow mode can separate the influence of diffusion on dispersion. In that in~estigation,~ a mathematical derivation indicated an identical dispersion behavior for nand r-FI (expressed in absoluteunits) at thesame flow injection conditions, provided the dispersion is controlled by only convection. In this instance, the resulting sample concentration profiles are identical, though opposite in sign, and display exponential signal-time relationships. In other words, the transient front and rear sections of both modes are mirror images that do not exhibit peak or minimum splitting or shoulders. However, deviations between them will result if dispersion along with convection is controlled by other processes (Le., diffusion and/or other secondary mixing phenomena) that are liable to differ for n- and r-FI. In this situation, the sample dispersion profiles are not predictable and may undergo peak and/or minimum splitting. Considering these conclusions, the influence of molecular diffusion on the overall dispersion of normal and reverse flow injection was found to cause small discrepancies in the dispersion height^.^ These were enhanced by the stopped-flow period, especially when unmatched inorganic matrix compositions of the reagent solutions and the countersolutions were examined. For this mismatch, the influence of increased concentration gradients confined the dispersion height for n-FI and expanded it for r-FI. In the present investigation, two flow configurations with an ICPAES detector, with and without a separate nebulizer pump and stream splitter, are employed. Various conditions of continuous- and stopped-flow modes are applied to examine n- and r-FI dispersion behavior. The same principles as described4 for the stopped-flow mode to study the influence of diffusion on the overall dispersion behavior were applied in the present investigation. However, in the previous work4 the stopped-flow period was initiated only on part of the sample dispersion zone. Only dispersion height measurements were considered. In the present study, the stopped-flow period was initiated when the whole sample zone profile was located in the reactor. Adopting this approach provides data on the dispersion width as well as dispersion height indicators. Analytical Chemistry, Vol. 66, No. 22, November 15, 1994 3937

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Reactor

C

Pump 1

waste

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Waste

Pump2

Flgure 1. Schematic diagrams of two flow manifolds for flow nIdispersion studies. (a, top) Conventional with one pump. (b, bottom) Dual pump with flaw Injection and nebulizer pumps. Pump 1 is the peristaltic pump of the flow injection device for sample introduction. Pump 2 is the ICPAES peristaltic pump for the nebulizer. S is the sample in the normal mode or the countersolutionin the reverse mode. C is the countersolution in the normal mode or the sample solution in the reverse mode. Reactor is usually a straight-channel tube with a 1-junction for splitting the sample solution before it reaches Pz. Waste is either for the split sample or for excess analyte from the spray chamber. ICP is the nebulizer, spray chamber, torch, and emission spectrometer.

EXPERIMENTAL SECTION Instrumentation. The flow manifold configuration in the previous dispersion studies' (Figure 1a) employs a peristaltic pump to drive in the n-FI mode the carrier (C) and the sample (S) solutions. For the valve position indicated in Figure la, the valve loop is filled with the sample, dispensing the excess to waste. During the injection period, the valve position is changed to inject the sample from the valve loop in a plug into the carrier solution. The sample undergoes dispersion in the straight reactor tube. In the r-FI mode, the functions of C and S are reversed. The net result is to inject the carrier solution during the injection period into the sample. The dispersed sampleof either mode is then supplied to the detector (ICP) consisting of a pneumatic nebulizer, spray chamber, and ICP torch. In Figure la, the flow injection peristaltic pump is used to drive the whole sample stream to the ICPAES nebulizer.' A peristaltic pump for the nebulizer is unnecessary; hence, the whole sample dispersion reaches the nebulizer. Conventionally, a small fraction of the aspirated sample reaches the plasma from the spray chamber. The majority is dispensed to waste. This flow configuration (Figure la) is inadequate when high flow rates are required, because flow exceeds the optimum nebulizer flow rate. An alternative flow configuration was employed when excessive flow rates were involved (Figure lb).5 Both the flow injection pump (PI) and the nebulizer peristaltic pump (P2) are operated simultaneously with a single channel, straight tube and two different reactor mixing volumes Vr (0.37and 0.83 mL, described previously1). The manifold flow rate Q1 of PI is always higher than the flow rate Q2 of P2. To reduce the differencebetween Q1 and Q2, a T-junction (5) Israel, Y.; Lgsztity, A.; Bames, R. M. Anolyst 1989, 114, 1259.

3938 Ana!ytical Ct?emisiry,Vol. 66, No. 23,November 15, 1994

is inserted before P2 to remove excess flowing solution to waste. With a T-junction inserted in the stream, the flowing solution is continuously split before it is pumped into the nebulizer. Only a fraction of the sample flow reaches the nebulizer. Therefore, the optimum flow rates for the flow manifold and the nebulizer can be selected independently. Two commercial flow injection instruments (Fiatron Systems, Model SHS-200and Tecator FIAstar 5020 analyzer) described previously' were used for dispersion studies with these two flow manifold configurations(Figure 1). Both flow arrangementswere examined, with each instrument operated in the continuous- or stopped-flow mode. A sequential ICPAES system (Perkin-Elmer Plasma 11) was used with conditions described earlier.' Reagents. Two test solutions were used. An Sc stock solution (1000 mg L-I) in 5% v/v HNO3 and diluted stock Sc solutions (40-200 mg L-I) were prepared as described previous1y.l A 1g L-l compositesoil reference samplesolution containing 50 mg L-I Sc was prepared by LiB02 (10 g L-I) fusion, also described earlier.6 The countersolution was 5% V/V HNO3. Procedure. n- and r-FI modes were investigated with flow configurations in Figure 1 using the continuous-flow, as we used previously,' and the stopped-flow modes. Measurements of the dispersion-time relationship for either continuous- or stopped-flow modes by ICPAES were used to calculate the dispersion heights and widths for various FI conditions. The dispersion heights (DP and Dm for n- and r-FI, respectively) were calculated for n-FI from the ratio of IO,the steady-state emission intensity, and IP, the transient peak emission intensity (DP = IO/IP), and similarly for r-FI by substitutingF for IP (DP= IO/F);P" is the negative emission intensity signal between IO and the transient minimum, Fin.' The dispersion width characteristic time periods were calculated from the recorded dispersion-time relationships. They include c P 1 , the time measured between the front and rear transient sections at 61% of Jp, or P";tf, the time of the front transient section between 10 and 100%of IP or F ;tr, the time of the rear transient section between 10 and 100% of IP or P";and A P ,the time measured between the front and rear transient sections at 10%of IP or P" and equal to the sum of tf and tr. In the stopped-flow mode, the flow was halted after sample injection into the stream for 100 s for the Fiatron system and 99 s for the Tecator analyzer before the resumption of the flow. Thestopped-flow period is initiated after the termination of the injection period, ti. However, special attention was devoted to thechoiceof experimental parameters(Le., of higher Vr and/or lower injection volume K) to prevent the sample dispersion front from reaching the detector during ti. The dispersed sample undergoes further dispersion by diffusion during the stopped-flow period. This procedure allows the determinationof the dispersion width characteristicsincluding tf and AZO-'. The latter cannot be determined when part of the sample reaches the spray chamber before initiation of the stopped-flow period. Full description of the nomenclatureis given in our previous paper.' (6) Israel, Y.; Bames, R. M. Anulyst 1990,115, 1411.

Table 1. Dlrperdon Characterlstlcs for Normal and Reverse Flow Injection ICPAES wlth Slngle- and Two-Pump Flow Conflguratlons' no. mode Ql (mL min-I) Q2 (mL min-1) vi (rL) D 8 . 6 ' (s) If (SI 1' (s) A10.l (s) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

n-CF n-CF n-CF n-CF n-CF n-CF n-CF r-CF r-CF r-CF r-CF r-CF r-CF r-CF r-CF r-CF r-CF

1.14 1.14 1.14 0.65 0.65 1.10 1.10 1.14 1.14 1.14 1.14 1.14 1.14 0.65 0.65 1.10 1.10

b 0.8 0.5 b 0.5 1.o 0.5 b 1.o 0.8 0.7 0.6 0.5

b 0.5 10 0.5

200 200 200 100 100 310 310 200 200 200 200 200 200 100 100 310 310

2.87 3.07 2.15 5.35 5.50 1.60 1.62 2.56 2.81 3.01 2.77 2.83 2.96 4.16 4.75 1.59 1.59

21.7 22.8 20.1 24.0 25.4 31.3 27.6 19.7 17.2 22.4 20.4 16.9 17.3 26.1 31.8 34.4 31.1

15.4 15.3 17.6 12.9 19.0 28.5 27.0 14.5 13.8 19.1 18.0 17.8 15.2 30.4 30.7 32.0 36.3

61.8 50.3 48.8 78.0 82.4 62.3 44.1 52.7 31.8 45.8 41.3 51.3 32.3 92.8 89.6 68.3 74.7

77.2 65.6 66.4 90.9 101.4 90.8 71.7 61.2 45.6 64.9 65.3 69.1 47.3 123.2 120.3 100.3 111.0

0 A sample solution of 50 mg L-1 of Sc in 5% v/v HN03 and a 5% v/v HN03 countersolution were used. Q c and Q 2 are the flow rates of pumps 1 and 2, respectively. Only continuous-flow experiments are reported for reactor volume V,of 0.83 mL (see Experimental Section). In flow manifold configuration, Figure la, pump 2 is not used.

*

For the two-pump arrangement, the values of Q1 and Q2 were independently set for Q1 > Q2 in each application. The Sc I1 361.384 nm intensity for the sample solution was measured every 4.6 s, with a 25 ms integration period for each reading.'

RESULTS AND DISCUSSION Influence of Flow Manifold Configurations on Dispersion. Only one flow manifold configuration (Figure l a ) was used in the first part of this study.' An anomalous phenomenon for r-FI was observed during the injection period, ti, displaying a dip at the steady-state emission signal, I o . However, with the two-pump flow configuration (Figure lb), the dip in Io was avoided, producing normal behavior during the ti. The T-junction probably also eliminates pressure fluctuations imposed on the nebulizer during sample injection. The dispersion behavior of the two-pump configuration was not studied before, and sample splitting may adversely influence it. The influence of the two-pump configuration on dispersion was investigated, and the results are listed in Table 1. The dispersion heights and widths obtained at various values of the nebulizer pump flow rate Q2 are compared with the corresponding results of the single-pump arrangement (Figure la) for identical flow parameters. Deviations were observed in the dispersion heights and widths. Very small influence on DP but more significant deviations for Dm(i.e., not exceeding 18% for Q1 of 1.14 mL min-I) were observed. However, more pronounced deviations were obtained for some of the dispersion width indicators in the nand r-FI modes. Nevertheless, the two-pump configuration can be used for ICPAES applications influenced by the choice of parameters Ql and vi. However, in practice the value of Q2 may have to be optimized. These deviations between flow configurations are caused by the continuous splitting of the manifold stream in Figure lb. The split was between 12 and 74% of the total stream flow. Dividing the main stream into two leads to a secondary mixing phenomenon dependent on the ratios of the flow rates Q2/(Q2 - QI)at the T junction. This influences the sample

dispersion before its introduction to the ICP nebulizer. Splitting the sample in the nebulizer and spray chamber before its introduction to the plasma also may involve a secondary mixing phenomenon that modifies the overall dispersion behavior. However, this phenomenon is specific to the use of a nebulizer, and it affects the dispersion behavior of both manifold flow configurations. Influence of Stopped-Flow Mode on Dispersion Behavior. The stopped-flow mode with the single-pump flow configuration (Figure la) was used to investigate the influence of diffusion on the overall dispersion characteristics for various flow conditions (Le., Q, vi, and Vr). The results obtained with continuous- and stopped-flow modes for either n- or r-FI are summarized in Table 2. The influence of stopped flow on a relatively large, 200 pL injection volume (entries 1-8) was to decrease DP slightly and significantly increase D m at a relatively high flow rate (Le,, 1.14 mL min-l, entries 2 and 4). These deviations were considerably enhanced for DP and only slightly enhanced for Dm at a low flow rate (Le., 0.45 mL min-l, entries 6 and 8). Most marked for the n-FI dispersion width indicators at stopped flow was the increase in tf along with the decrease in tr at 1.14 mL min-l that was enhanced at 0.45 mL min-I. This greatly decreased the Ato.' value. These data for the dispersion width deviations indicate a considerable symmetry improvement of the n-FI transient dispersion profiles resulting from the use of the stopped-flow mode, especially at low flow rates. The dispersion behaviors of the n-continuous-flow and the n-stopped-flow modes are illustrated in Figure 2. The stopped-flow mode gives a sharp peak bordered by shoulders on both sides (Figure 2, curve B). Curve B shows improved symmetry compared to the continuous-flow mode transient (Figure 2, curve A). The transients in Figure 2 appear to provide additional details concerning the dispersion-time relationship complementing the characteristic dispersion width indicator data. Deviations in the dispersion width indicator data were observed for r-FI in the stopped-flow mode, with vi of 200 pL at 1.14 mL min-I (Table 2, entry 4). The trend of these deviations opposes the n-FIbehavior under the same conditions, Analytical Chemistry, Vol. 66, No. 22, November 15, 1994

3939

Tabb 2. Dbpmkn Charactw&tk$ tor Normal and Revor80 Fbw InJecthICPAES wlth con#rmour F b w (CF) and Stopp.bFbw (SF) Mode8 In the ShgbPump F b w Connguratkn of Flgure 18. no. mode Q (mL m i d ) vi (PL) Vr (mL) D 00.61 (s) tp (SI t' (SI A P (s)

1.14 1.14 1.14 1.14 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.66 0.66 0.66 0.66

n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF n-CFb n-SFb n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF

1

2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

200 200 200 200 200 200 200 200 200 200 100 100 100 100 100 100 100 100 30 30 30 30 260 260 260 260

0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.37 0.37 0.37 0.37

2.88 2.75 2.56 2.80 3.48 3.06 2.53 2.87 3.47 3.48 6.44 6.08 5.14 4.83 5.35 5.84 5.06 5.72 12.9 14.9 12.6 10.8 1.57 1.55 1.64 1.69

21.9 22.8 19.6 22.1 67.7 64.0 50.6 65.3 52.0 57.5 58.7 53.3 59.0 49.6 24.1 33.1 34.1 33.5 22.0 30.1 20.4 23.8 23.0 22.5 25.9 23.4

15.1 20.9 14.7 16.8 48.9 51.3 49.3 54.2 38.9 46.2 45.2 45.6 44.7 36.0 12.9 20.4 24.1 19.9 15.4 20.1 23.9 29.9 20.2 23.4 21.1 20.9

60.7 52.4 52.6 56.8 128.8 108.4 84.5 116.8 93.3 88.0 110.3 96.6 91.5 87.5 78.0 92.8 85.3 80.4 86.4 83.1 54.6 73.6 32.7 44.4 32.6 38.2

75.8 73.3 67.3 73.6 177.7 159.7 133.8 171.0 132.2 134.2 155.5 142.2 135.5 123.5 90.9 113.2 109.4 99.3 101.8 103.2 78.5 103.5 52.9 67.8 53.7 59.1

Various flow rates ( ) and Vi and V,values were used. Sample solutioncontained 50 mg L-l of Sc in 5%v/v HNOs. Composite soil sample solution contained 50 mg L-1 of g c in 5% v/v HNOo (cf. Rea ents). Countersolution was 5% v/v HNOo. Vr was 0.83 and 0.37 mL. b Composite soil sample solution was used. The computed DP values are douttful and have only a relative value (cf. Results).

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60 80 100 120 140 160 ReadingNumber Flgurr 2. Dependence of the dispersion behavlor of normal flow Injection on time. with flow configuration In Flgure la, and matched matrix composition of 50 mg L-I of Sc in 5% HN03 with 5 % HNOs countersolution. (A) Continuous-flow mode. (6) Stopped-flow mode. 4,200 p l : 0, 0.45 m t min-I; 4,0.83 mL. O

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40

indicating an increase in both tr and Ato.'. This trend was considerably enhanced at 0.45 mL min-I (entry 8). The effect of decreasing the manifold flow rate in the above experiments with the continuous-flow mode at the same vi and V,is to enhance the influence of diffusion on the overall dispersion, since it increases the sample dispersion zone residence time in the manifold. The stopped flow magnifies the influence of diffusion depending on the length of the stopped-flowperiod. These experimentswith the ICPdetector and continuous- and stopped-flow modes (entries 1-8) clearly indicateopposing diffusion trends for n- and r-FI. These trends are essentially similar to the behavior encountered previously with a solution spectrophotometricdetect~r:~ diffusionopposes dispersion by convection, decreasing the overall dispersion heights for n-FI and increasing them for r-FI. Furthermore, because the dispersion width behaviors were also determined, 9940 Anal)rHcal Chemisby, Vol. 66, No. 22, November 15. 1994

we concludethat diffusion serves to improve the peaksymmetry for n-FI transients and to degrade it for r-FI. With a vi of 100 pL and Q of 0.45 mL min-l (entries 11-1 4), the stopped-flow mode exhibits dispersion deviations for n-FI &e., a decrease of DP,AtO.', and tr values; entry 12) similar to those observed at the same flow conditions for a 200 pL injection (entry 6). However, the deviations obtained for a 100 pL volume with the r-FI stopped-flow mode (entry 14) oppose those obtained for a 200 pL injection (entry 8) at otherwise the same conditions. Essentially no change was observed for both DP and Dm when the stopped-flowmode was applied for a vi of 30 pL and Q of 0.65 mL m i d (entries 19 and 22). These latter results clearly demonstratethe influence of changing vi on the diffusion behavior. For any set flow conditions increasing the injection volume increases the dispersion zone concentrations. Hence, the concentration gradient throughout the dispersion zone is increased. This increases the influence of diffusion in continuous- or stoppedflow modes. Negligible deviations for both DP and Dm result from the stopped-flow mode with a small (0.37 mL) reactor volume (entries 23-26). However, a minor increase is observed in Dm over DP for either the continuous- or the stopped-flowmodes. Slightly more significant deviations were found for some of the dispersion width indicators. At set flow conditions, the use of a small reactor volume decreases the sample residence time in the flow manifold. Negligible influence of diffusion results from combininglow reactor volume with high manifold flow rates and/or low injection volumes. However, some influence of diffusion can be observed for the 0.37mL reactor volume resulting from the use of a fairly low manifold flow rate and a high injection volume (entries 23-26).

These experimentswere conducted with a diluteScstandard solution with a matched matrix composition of the countersolution (5% v/v HNO3). Therefore, the concentration gradients are low. A high concentration gradient condition occurs with a digested soil compositesample and a 5% HNOs countersolution. Results for the n-FI dispersion height and width characteristics of the soil sample are given in Table 2 (entries 9 and 10). A considerable decrease of between 20 and 28% (entries 9 and 5 ) is observed for the n-continuousflow dispersion width indicators. A lower decreaseof between 10 and 19% (entries 10 and 6)is found for all the n-stoppedflow dispersion width indicators. The results with the soil sample for stopped- and continuous-flow modes (entries 10 and 9) show an increase in the and tf periods and some decrease in tr. The net effect of using the stopped-flow mode is to improve the symmetry of the front and rear slopes of the n-FI transient. Reproducible transient peak emission intensity values, P, were obtained for the soil sample (entries 9and 10). However, owing to a continuous drift of the steady-state emission intensity signal 10,the absolute values of D p for both methods are doubtful and may have only relative significance that indicates no deviation. The n-FI dispersion characteristics for soil solution with continuous and stopped-flow modes are illustrated in Figure 3. Despite the similarity of the transient peak heights, a substantial broadening w a r s for the stopped-flow mode near the transient peak (Figure 3, curve B). However, a sharp peak is obtained for the n-continuous-flow mode (Figure 3, curve A). Thus, large dispersion behavior deviations result with the soil sample (Figure 3) instead of the Sc solution (Figure 2). With the soil solution in the r-continuous-flow mode, a strikingly different dispersion behavior is encountered. The dispersion width increases considerably, and severe splitting results. Thus, Dm and dispersion width indicators could not be calculated. With the soil sample, the disparity of the diffusion characteristics is significantly enhanced. The dispersion of the composite soil sample with high salt composition and the dilute countersolutions produces high gradient concentrations in the dispersion zone profile of nand r-FI in both continuous- and stopped-flow modes. Therefore, a high diffusion is exerted on the overall dispersion. However, if at the same flow conditions the dispersion of the Sc standard sample and countersolution is involved, very low concentration gradients prevail in the dispersion zone that arise from the Sc in the sample. Thus, the influenceof diffusion is very low. High inorganic matrix composition samples, especially those with high salt content, can adversely affect the smooth operation of the ICP nebulizer and spray chamber. One of the advantages of combining FI with the ICP detector is the ability to dilute high salt matrix composition samples with dilute countersolution before their introduction to the ICP nebulizer. However, for these conditions,undesirablediffusion effects were observed, such as the unusual increase in the transient dispersion widths as well as the minimum splitting that took place for r-FI (at low Q and high E). Hence, a knowledge of the effect of various flow parameters on the diffusion behavior becomes valuable for the design of proper

”AL 0 ‘ 0

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60 80 100 120 140 160 Reading Number

Flgum a. Dependence of the dispersion behavbr of normal flow on time with flow configuration In Flgwe l a using unmatched lnj” matrix compositkn of a soil sample with 5% HNOs countersdution. (A) Continuow-flowmode. (B) Stopped-flow mode. V;, 200 pL; Q, 0.45 mL min-’; V , 0.83 mL. (Plotted on the same scale as Flgure 2.)

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flow

I

flow

Flgure 4. Diffusion models for (a, top) normal flow i n j ” and (b, bottom) reverse flow Injection. Key: a, axial flux; r, radialflux; 1, man&# flow direction. The arrows above the flow manlfold tube refer to the sample zone dispersion flow dhection. The inner arrows point to the direction of the gradual decease in the sample dispersion zone concentrationand the gradual decrease In the diffusion flux rate. Axial and radial diffusion are represented by horirontaland vertical arrows, respectively.

flow and reactor conditions for specific applications. Therefore, for high salt matrix composition samples, a small reactor volume, small injection volume, and/or high manifold flow rates are necessary to avert undesirable diffusion effects. Diffusion Flux Models. In an attempt to explain these findings, two diffusion flux models are proposed.. Axial and radial dispersion tendencies for the n- (Figure 4a) and r(Figure 4b) FI modes are illustrated in Figure 4. The inner arrows point to the direction of the gradual decease in the sampledispersionzone concentration and the gradual decrease in the diffusion flux rate. Axial and radial diffusion are represented by horizontal and vertical arrows, respectively. For the dispersion zone front and rear sections of each mode, opposite axial and radial diffusion flux conditions prevail, and the same diffusion flux conditions exist for n- and r-FI modes. These deviations in the diffusion tendencies can significantly influence the overall dispersion behavior when A n a ~ l W ~ m & &Vd. y ~66 Na. 22: ”ber 15, 1994

5941

Table 3. Dispersion Characteristics for Normal and Reverse Flow InJectlon ICPAES with Continuous-Flow (CF) and Stopped-Flow (SF) Modes In the Dual Pump Manlfold Flow Conflgurallon of Flgure lb* no. mode Ql (mL min-l) Q2 (mL min-I) K (A) D dJ.61 (s) if (s) 2' (s) Are,' (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF n-CF n-SF n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF n-CF n-SF r-CF r-SF

2.41 2.41 2.41 2.41 2.41 2.41 2.41 2.41 1.14 1.14 1.14 1.14 1.14 1.14 1.10 1.10 1.10 1.10 1.10 1.10 1.10 1.10

1.o 1.o 1.o 1.o 0.8 0.8 0.8 0.8 0.8 0.8 0.5 0.5 0.3 0.3 1.o 1.o 1.o 1.o 0.5 0.5 0.5 0.5

200 200 200 200 200 200 200 200 200 200 200 200 200 200 310 310 310 310 120 120 120 120

3.05 3.82 2.8 1 3.01 3.35 3.30 2.81 2.46 3.07 3.15 2.75 3.67 2.76 2.81 1.60 1.52 1.59 1.70 4.43 6.53 3.42 6.06

16.2 17.2 11.2 16.3 16.1 14.7 16.5 14.5 22.8 27.4 21.3 22.8 23.5 29.5 31.3 32.2 34.4 29.3 23.1 20.2 19.5 21.1

13.0 13.7 13.8 14.2 15.2 13.1 13.8 11.3 15.3 16.1 16.8 15.3 19.3 35.8 28.5 28.6 32.0 31.8 15.2 15.2 16.5 14.7

21.5 23.7 31.8 24.7 23.0 22.8 24.5 20.9 50.3 56.0 48.8 60.1 47.4 41.3 62.3 64.2 68.3 57.4 61.3 58.8 65.2 45.9

34.5 37.4 45.6 38.9 38.2 35.9 38.3 32.2 65.6 72.1 65.6 76.5 66.7 77.1 90.8 92.8 100.3 89.2 76.5 74.0 81.7 60.6

Various flow rates Q1 and Q2 and 6 values were used. Sample solution was 50 mg L-' of Sc in 5% HNO3. Countersolution was 5% HN03. VI was 0.83 mL.

a high diffusion rate is involved or at relatively low diffusion rates for a long manifold residence time. The diffusion rate is dependent on sample composition and the extent of sample solution and countersolution composition matching. The axial diffusion flux enhances the overall dispersion if the axial flux coincides with the sample zone flow direction but retards it when they diverge. The diffusion flux model illustrated by Figure 4a, therefore, indicates that axial diffusion for the n-FI mode enhances the overall dispersion front section and retards it for the rear section, and conversely for the r-FI mode (Figure 4b). However, the influence of the radial diffusion flux is more intricate. The front radial diffusion flux for the n-F1 mode seems to enhance the overall front dispersion (Figure 4a). The rear radial diffusion flux is known to contain it. Therefore, the influences of axial and radial diffusion coincide for the n-FI mode, increasing the overall front dispersion (i.e., increasing tq and decreasing the rear dispersion (i.e., decreasing tr). The FI transients for both modes exhibit a significant lack of symmetry (Le., tr > tf; cf. Tables 1-3). Therefore, the influence of diffusion on n-FI is to increase the overall front dispersion and to decrease the rear dispersion, thus improving the transient front and rear section symmetry and decreasing the overall dispersion widths. An increase in Z P and a decrease in DP will consequently follow for high diffusion flux rates. The diffusion flux model in Figure 4b indicates opposite tendencies for the r-FI mode. Hence, the influence of high diffusion rates would impair r-transient symmetry and enhance the overall dispersion widths. These diffusion models help us to visualize the influence of axial and radial diffusion on the overall front and rear dispersions for n- and r-FI modes. High Flow Rates with Two-Pump Manifold. Dispersion studies at high manifold flow rates are more effectively carried out for both continuous- and stopped-flow modes with the two-pump flow configuration (Figure lb). The influence of 3942

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applying the stopped-flow mode with this flow configuration on the dispersion characteristic behavior of both n- and r-FI was investigated. The results are listed in Table 3. The experiment in which a Q1 of 2.41 mL min-1 is used with continuous- and stopped-flow modes (entries 1-8) is of special interest. These conditions were not studied earlier with the single-pump flow configuration (Figure l a ) for V, of 0.83 mL.' With the continuous-flow mode and the two-pump flow configuration, no significant decrease is found in DP or Dm as a result of the increase of Q1 from 1.14 to 2.41 mL min-I at otherwise the same conditions (Le., compare entries 9 and 11 with entries 1 and 5 for the n-FI mode and entry 13 with entries 3 and 7 for the r-FI mode). This contradicts the disputed dependence, which suggests that DP decreases as Q is increased, but agrees with the results obtained with the single-pump configuration on the influence of increasing Q on DP.l While the increase in Q from relatively low values results in a linear decrease in DP values, a break in the dependence occurs before DP reaches 1.14 mL min-'.' The stopped-flow mode resulted mostly in an increase in DP for Q1 of 1.14 and 2.41 mL min-' (entries 2, 10, and 12). These results contradict those obtained for the stopped-flow mode at 1.14 mL min-l with the single-pump configuration at otherwise the same conditions. In all cases except for entry 8, applying the stopped-flow mode increased Dm for the twopump configuration. This agrees with the results obtained for the single-pump arrangement at 1.14 mL min-'. Applying a vi of 200 pL and Q1 of 2.41 mL min-l (Table 3, entries 1-8) with the two-pump configuration produces the lowest values for all dispersion width indicators compared to those obtained at otherwise the same conditions, expect for 1.14 mL min-l. These dispersion width results are expected but could not be determined before with the single-pump configuration. They may serve to increase the sample dispersion throughput of n- and r-FI with the ICP detector.

Perspective. Considerable effort has been devoted to the study of parameters influencing FI dispersion mainly with solution spectrophotometry. Convection is the main process promoting dispersion at short residence times; otherwise, convection and diffusion have a superimposed effect. Therefore, separating their influence on dispersion is difficult with the continuous-flow method. In earlier studies with solution spectrophotometric d e t e ~ t i o nthe , ~ stopped-flow method was used to study the influence of diffusion on the overall dispersion heights of both n- and r-FI. However, the FI dispersion behavior in general and the influenceof diffusion on the overall dispersion with ICPAES detection have not been investigated previously. This study is justified because of the peculiar construction and operation of the plasma detectors that are assumed to influence FI dispersion more than conventional FI detectors. Therefore, methodology to combine FI with ICPAES and various parameters influencing n- and r-FI dispersion for continuous flow were examined previously.' The present investigation applies the stopped-flow technique on the whole sample zone dispersion to detect the influence

of diffusion on the overall dispersion of n- and r-FI modes. The dispersion heights and width indicators at various flow parameters and injection volumes were measured, and results were consistent with qualitative diffusion flux models. An alternative manifold flow configuration with two pumps was particularly suitable to apply high FI flow rates with the ICPAES. Because of the continuous sample zone division in the manifold before the ICP nebulizer, some deviations were observed for both FI techniques in the continuous- and stoppedflow modes. However, these deviations do not hinder its application for chemical analysis, and an increase in sample throughput results.

ACKNOWLEDGMENT Research was sponsored by the ICP Information Newsletter. Received for review April 8, 1994. Accepted August 15, 1994." *Abstract published in Advance ACS Absfroefs. October 1, 1994.

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