1713
Anal. Chem. 1981, 53, 1713-1715
Chemical Kinetic Contributions to Practical Dispersion in Flow Injection Analysis Sir: Applications of flow injection determinations have mushroomed since the mid-70s when RiiiiEka and Hansen (1) coined the name that has now been almost universally adopted to signify unsegmented continuous-flow determinations under controlled dispersion. The number of papers devoted to fundamental aspects of these approaches, however, is limited. Dispersion (of the injected or intercalated plug) has received some attention recently, but such studies focused on the mechanical transport effects related to reactor geometry, on injector and detector cointributions to peak broadening, and on axial dispersion effects of coiled and straight tube reactors (2-5). Recently, Ru5iEk.a and Hansen recognized that (sic) “It is obvious that the comprehensive theory of the Flow Injection method will eventually combine the theory of mixing of liquids in continuous moving streams with the theory of chemical kinetics” (6). The effect of chemical kinetics, however, has been practically ignored in efforts to describe the shape of the transient peak profiles typical of flow injection determinations. The same is true of attempts to associate such profiles to an analytical parameter of relatively easy experi. mental extraction. One such parameter is the practical dispersion proposed by RuiiEka and Hansen (6). They have associated the numerical value of this parameter with different characteristic shapes of peak profiles (6). The numerical value of practical dispersion is calculated by dividing the signal value of the expected equilibrium level for the injected sample in the absence of dispersion by the maximum signal value of the recorded peak. Such practical dispersion values will be always larger than 1. The suggested procedure for extracting this numerical value consists in calculating the ratio of the absorbance of a dye solution-provided Beer’s law is obeyed in the concentration range in use-before a plug of it is injected and its absorbance at the maximum of the recorded transient peak. The value of peak rnaximum is decreased by the amount of dispersion taking place from the point of injection to the pont of detection, resulting from differences in velocities and concentration gradients between the injected plug of dye solution and the transporting carrier stream. The usefulness of this practical dispersion number is decreased by chemical reactions caused by concentration gradients; mixing in coiled reactor tubes (effect of secondary flow phenomena) can obscure, even invalidate, its significance. From an analytical viewpoint the two relevant parameters are the peak height or peak area and the time needed for return to base line after peak development; these are intimately related to the method sensitivity and number of injections possible per hour (7).The practical dispersion value, when complicated by chemical knetics, conveys no direct information on the return to base line, and depending on the case of monitoring a product or reactant (both cases commlon to flow injection determinations), without qualifications, it may confuse the issue. Selecting as model a chemical reaction of moderate speed, the oxidation of L-ascorbic acid by dichromate ion, we have measured the contributicln that chemical kinetics may have on practical dispersion and made certain observations of analytical interest in flow injection determinations. EXPERIMENTAL SECTION Apparatus. The peristdic pump used was a Minipuls I1 (Gilson Medical Electronicn, Inc., Middleton, WI). Photometric detection was provided by a Beckman Model 25 spectrophotometer with the Beckman recarder/controller unit as readout. A Model E-178-Q-10 (Markson Science, Inc., Del Mar, CA) flow cell 0003-2700/81/0353-1713$01.25/0
of 10 mm path length and 80 pL chamber volume was used as the detection reservoir. All tubing was Tygon microbore, formulation S-54-HL for surgical and hospital use (Norton Plastics and Synthetics Division, Akron, OH). Samples were introduced with a custom-made rotary valve constructed according to information available in the literature (8). A block diagram is shown in Figure 1. Reagents and Solutions. All chemicals used were of AR grade. The water used for solution preparation was deionized water that was further purified by distillation in an all-borosilicate still with a quartz immersion heater. Both reagent solutions, L-ascorbic acid and K2CrzO7,were prepared daily and kept in the dark or protected from direct exposure to light. The pH of the injected sample and that of the carrier stream were adjusted to the desired value just before use. Procedure. The carrier stream was pumped from a closed reservoir at constant flow rate. The sample was intercalated by means of the sliding (rotary) valve located at a very small distance downstream from the reservoir and defining the start of the so-called “reactor”. The dispersed or dispersed-reacted sample plug was monitored at the exit of the “reador” as it passed through the flow cell. The peristaltic pump was located after the point of detection, and the stream was sent to waste. The carrier stream was a 0.00500 M L-ascorbic acid (in experiments to evaluate the chemical reaction effect) or aqueous solutions (HC1or HC104)of the same pH as the intercalated plug; the intercalated sample consisted of 0.000 500 M potassium dichromate. The flow rate was varied in the range 0.050-6.0 mL/min. The reactor tube diameters were 0.50, 0.80,1.00, and 1.30 mm, corresponding to tube volumes of 1.96, 5.03, 7.85, and 13.2, pL/cm. Straight reactors had lengths between 30 and 400 cm and coiled reactors were all of lengths 30 cm (straight)-l20 cm (coiled)-30 cm (straight) with coil diameters of 3.4,6.7,12.2, 15.6, and 22.3 mm. The length of sample loops was varied within the range 7-600 cm, corresponding to sample volumes of 33.7-1198 pL. All stated volumes were determined by calibration with mercury.
RESULTS AND DISCUSSION Intercalation of a dichromate solution into an aqueous stream of equal pH is taken to lead to dispersion resulting solely from physical mass transport as a result of concentration gradients and velocity profiles. On the other hand, intercalation of similar samples into aqueous streams containing L-ascorbic acid adds the chemical effect of the overall chemical reaction: Crz0T2- + 3 C & 3 0 6 -k 8H’
2Cr3+
+ 7H20 + 3CeH606 (1)
Increases of values in the practical dispersion, D, have different analytical implications in each case. As shown in Figure 2, in the absence of chemical reactions an increase in the value of D results in a decrease in recorded peak height but in a substantial increase in the time needed for return to base line. An increase in value of D when a chemical reaction adds to concentration gradients also implies lower signal heights, but the time for return to base line becomes significantly smaller as a result of chemical consumption of the monitored species (Cr2072-in this case, monitored a t 350-360 nm depending on the pH) in the front leading part of the plug and in its tailing portion. The effect is more pronounced in the tailing part of the plug, so that the larger concentration of monitored species resides in the leading front (Figure 3). Since the reactant in excess is the one in the carrier stream (L-ascorbic acid in this case) the probability for effective collisions seems 0 1981 Amerlcan Chemlcal Society
1714
ANALYTICAL CHEMISTRY, VOL. 53,NO. 11, SEPTEMBER 1981
1
WASTE /FLOW
CELL
Figure 1. Instrumental setup used in the studies described in this paper. For details see text.
B 3.0
Figure 2. Comparative trend in peak profiles as the value of practical dlspersion increases. Curves A, B, and C, no chemical reaction. pH 3.83;sample volume = 61.2pL, flow rate = 0.65 mL/min. Reactor: 0.8 mm i.d. straight tube of lengths (A) 30 cm, (B) 90 cm, (C) 250 cm. Absorbance monitored at 351 nm. Curves D, E, F, chemical effect. Flow rate = 0.65 mL/min. Reactor: straight tubing 0.5 mm 1.d. and 90 cm length. Sample volume = 61.2pL. pH (D) 4.76,(E) 4.57,and (F) 3.83. Absorbance monitored at (D) 352 nm, (E) 352 nm, and (F) 351 nm. Values for D : (A) 4.22; (B) 5.58; (C) 8.87; (D) 4.82; (E) 6.47; (F) 24.6.
Flgure 3. Comparative, superimposed peaks showing the effect of chemical reaction on peak leading front and tall: (A) no chemical reaction; (B) chemical reaction. Both A and B are under the following conditions: pH 4.76;sample volume = 61.2 pL; flow rate = 0.65 mL/mln; reactor, straight tube, 0.5 mm i.d., 400 cm length. Absorbance monitored at 352 nm. Values for Dare (A) 6.45and (B) 24.63. to be greater in the tail portion of the plug. It may be noted that if the monitored species is one of the products of the reaction, the expected correlation between D and the time for return to base line is the inverse of that observed when a dye (or any other colored species) is intercalated without chemical reaction contribution to dispersion. That is, a small extent
4.0
5.0
6.0
FLOW R A T E , mL/min
Figure 4. Variation of Dhemwith flow rate: carrler stream, L-ascorbic acid (0.0050M), pH 5.55;Injected sample, Cr,O$-, 0.00050 M and, pH (A) 5.56,(B) 5.01,(C) 4.58,(D) 4.25,(E) 3.33,(F) 2.54. Sample volume = 33.7 pL. Reactor was a straight tube 0.5 mm 1.d. and 150 cm length. of reaction would result in small time for return, small peak, large D, a large extent of reaction, large peak, small D. It is clear that the value of D must be qualified and is unable by itself to convey information regarding the return to base line, the analytical parameter that dictates the determination rate (7). The value of D, however, does give direct information with respect to signal sensitivity. As an example of the kind of effect introduced by a chemical reaction in the practical dispersion value, Figure 4 shows a plot of Dchem vs. flow rate. The value of Dchemis derived as
D being calculated as described earlier for the intercalation of the same chemical species at stated experimental conditions. Note that the minimum effect (chemical) can be observed as the flow rate increases but at relatively low flow rates the chemical contribution to dispersion increases dramatically. Figure 4 also shows quite clearly the effect of pH gradient, which is proportional to the distance along the line g (note that curve A represents the same pH in carrier stream and intercalated plug, but from curve B to curve F the pH of the intercalated plug is progressively smaller). The use of coiled reators, frequently found in flow injection systems, introduces an additional dimension: extensive secondary mixing over the cross section of reactor tubing (9). Additional flow is generated perpendicularly to the centrifugal forces active in a curved flow channel and stimulates mass transfer in a radial direction. This should reduce peak dispersion because mixing across the stream tends to oppose dilution of the solute by longitudinal dispersion. This is true
Anal. Chem. 1981. 53. 1715-1717
T l Y l I ..conds)
Figure 5. Comparative, superimpoJed paaks BlusWating the effect of straight and coiled reactors on practical dispersion wimout chemical effect: Row rate, 0.42 mLlmin; reactw. 30 cm362 cm (2.5 cm o.d., coiledt30 cm [422 cm total length]; skaight reactw length. 422 cm. Curve A, coiled reactor [ D = 9.371. Curve E. straight reactor [ D = 12.941.
Table I. Comparative Values for Practical Dispersion and Time for Return to Base Line for Straight and Coiled Reactors (Flow Rate, 0.16 mC/min; Reactor, Tygon tubing, 180 cm length, 0.5 mm i.d.; pH 5.05) coiled reactor straight 3.4 mm 22.3 mm reactor iden idc' Dispersion without Chemical Effect tbasr 26 7 219 230 D
4.36
4.08
4.17
Dispersion with Chemical Effect S
166
Dchem
7.44 3.08
tbas,
D
153 6.24 2.16
154
1715
for return to base line, th. as a result of increased radial diffusion. When a chemical reaction is operative its effect can be characterized by change in D- as described above. Table I shows that Deb., (chemical contribution to practical dispersion) is larger for straight reactors as a result of an increase in mixing with the flowing stream (background reactant) since axial diffusion is greater. The same is true for the increase of D,,, when the diameter of the coil reactor is increased. We hope this paper adds additional insight to earlier helpful descriptions of dispersion in flow injection analysis and points to the need for further work to lead to a more thorough understanding of parameters and concepts describing the transient signal observed in unsegmented, continuous-flow systems. LITERATURE C I T E D (1) ROfi6ka. J.: Mnsen. E. H. A w l . CMm. Acta 1975, 78, 145-157. (2) Tljssen. R. AMI. Chlm. Acts 1080. 114. 71-89. (3) Van Der Berg. J. H. M.; DeslJa. R. S.: E g M k . H. G. M. Aml. CMI. Acta 1980. 114. 91-104. (4) Reijn. J. M.; Van Der unden.W. E.: P m . H. AMI. C M . Acta 1910. 114. 105-118. (5) Vandersllce. J. T.: StewaBn. K. K.; QeWy Rossnfeld, A,: Higgs. D. J. rabnra mi. 28. 11-18, (6) Ruii&a. J.: Hansen. E. H. AMI. C h h . Acts 1978, 99. 37-76. (7) Motlola. H. A.: brim. A. AMI. Chlm. Acta 1978. 100, 167-180. (8) Fansen. E. H.; RuiiEka. J. J. C h m . Educ. 1979. 56. 677-680. (9) Cam, C. 0. J. mysbl. i988, 185. 501-519.
C. C. Painton Horacio A. Mottola* Department of Chemistry Oklahoma State University Stillwater, Oklahoma 74078
6.69 2.52
idc = internal diameter of coil. for the injection of sample transported without imposed chemical reaction adding to changes to concentration levels of the monitored species. Figure 5 confirms this effect coiled reactors produce sharper peaks (smaller D ) and shorter time
RFCEIVEDfor review March 6,1981. Accepted June 2,1981. This work is being supported by a grant from the National Science Foundation (CHE-7923956). This paper was presented a t the 1980 Southeast/Southwest Regional Meeing of the American Chemical Society, New Orleans, LA, Dec 12. 1980.
Carbon- 13 Enriched Nuclear Magnetic Resonance Method for the Determination of Hydroxyl Functionality in Humic Substances Sic We wish to report a method for the characterization of hydroxyl functional groups in humic substances. Treatment of humic and fulvic acids with W e n r i c h e d methylating reagents followed by 13CNMR spectroscopy permits one to identify and estimate the relative abundances of the various OH groups. Humic substances, especially fulvic and humic acids, are present in soil,water, and coal ( I , 2). They are active chelating agents (3-5),adsorb hydrophobic compounds, and promote plant growth. In natural waten, they affectaquatic organisms, form toxic haloforms upon chlorination, transport insecticides, and remobilize metal ions from sediments. They appear to interfere with the flotation process in copper miniig and c a w engineering problems in coal gasification. T h e activity of these polymeric humic substances resides in their hydroxyl functionality, which is composed of carboxyl, phenolic, hydroxyl, and saccharide groups. Amino nitrogen may also be present in some humates. Hydrogen bonding may 0003-270018110353-1715t0 1.2510
affect their state of aggregation and may also influence their chemical behavior (6. i?.Despite many years of m a r c h , little is known about the chemical structure of these ubiquitous compounds or the exact nature of oxygen functionality. Direct spectral measurements reveal very little information. Recent IT NMR studia @,9)show four broad bands, which indicate aliphatic, hydroxyl, aromatic, and carboxyl regions. One can make rough estimates of relative abundances of these four regions from the CPIMAS (cross polarization/magic angle spinning) NMR spectra (9). In order to enhance and separate the NMR signals of the hydroxyl functions from the broad featureless spectra of humic substances, we have methylated humic and fulvic acids with "C-enriched reagents. This derivatiration labels hydroxyl groups. eliminates hydrogen bonding and facilitates signal assignment in the 13C NMR spectrum. Estimation of the relative ahundance of functional group types can also be inferred from integral absorptions. A variety of model com0 1981 Amwkan UWkA Soclay
