Anal. Chem. lQ82, 5 4 , 1113-1118
volatilization and technique implementation, respectively. Therefore, FAB/MS has considerable potential at least as a qualitative analytical technique in the higher mass range provided that sample siispension/dissolution techniques are developed which afford adequately intense ion beams. In this regard, i t should be noted that this exploratory research achieved no real success in producing FAB mass spectra of nitrogenous materials from heavy petroleum and coal liquids boiling in excesii of 500° C. Finally, conclusions concerning the use of FABIMS in quantitative analysis require detailed studies of the ionization/fragmentation mechanisms and of the intensities of significant ions per mole of neutral precursor as a function of molecular structure. However, the potential significance of F‘AB/MS to the emerging, significant field of high-mass/mass spectrometry applied to the analysis of liquid fossil fuels provides cogent justification for conducting basic research to determine its qualitative and quantitative capabilities.
ACKNOWLEDGMENT We thank P. E. Pulley and D. N. Pope, Data Processing Center, Texas h&M TJniversity, College Station, TX, for providing the program used to process the bar-graph spectra. LITERATURE CITED (1) Surman. D. J.: Vlckertnan, J. C. J. Chem. SOC., Chem. Commun. 1981, 324-325. Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. J. Chem. SOC., Chem. Commun. 1981, 325-327. Barber, M.; Bordoll, R. S.; Sedgwlck, R. D.; Tetler, L. W. Org. Mass Spectrom. 1981, 16, 256-260. Barber, M.; Bnrdoli, R. S.;Sedgwick, R. D.: Tyler, A. N. Nature (London) 1981. 293. 270-275. (5) Taylbr, L. C. E:. ind. ResJDev. 1981, 2 3 , 124-128. (6) Wllllams, D. H.; Bradley, C.; Bolesen, G.; Santlkarn, S.;Taylor, L. C. E. J. Am. Chem. SOC. 1981, 103, 5700-5704. (7) Scheppele, S.E.; Greenwood, G. J.; Panclrov, R. J.; Ashe, T. R. ACS Symp. Ser. 1981, No. 156, 39-73.
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(8) Beeck, 0. Ann. Phys. (Le@@) 1934, 19, 121-128. (9) Berry, H. W. Phys. Rev. 1949, 7 5 , 913-916. (10) McDowell, R. A.; Dell, A.; Morris, H. R. Presented at the 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, May 24-29, 1981; Paper No. WAMOA3. II) McLafferty, F. W. “‘Interpretation of Mass Spectra”, 3rd ed.; University Science Books: Mlll Valley, CA, 1980; pp 48-50. 12) McLafferty, F. W. “‘Interpretation of Mass Spectra”, 3rd ed.; University Science Books: Mill Valley, CA, 1980; p 35. 13) Budziklewlcz, H.; Djerassi, C.; Williams, D. H. “Mass Spectrometry of Organic Compounds”; Holden-Day: Can Francisco, CA, 1967; p 23. 14) Budzikiewicz, H.; Djerassi, C.; Williams, D. H. “Mass Spectrometry of Organic Compounds”; Holden-Day: San Franclsco, CA, 1967; Chapter 20. (15) Grigsby, R. D.; Schronk, L. R.; Grindstaff, Q. G.; Scheppele, S. E.; Aczel, T. Presented at the 27th Annual Conference on Mass Spectrometry and Allied Topics, Seattle, WA, June 3-8, 1979; Paper No. MAMOA3. (16) Brown, R. A. Anal. Chem. 1951, 2 3 , 430-437. (17) Lumpkin, H. E.; Thomas, B. W.; Elllott. A. Anal. Chem. 1952, 24, 1389-1391. (18) Lumpkin, H. E.; Johnson, B. H. Anal. Chem. 1954, 2 6 , 1719-1722. (19) Clerc, R. J.; Hood, A.; O’Neal, M. J. Anal. Chem. 1955, 2 7 , 868-875. (20) Lumpkin, H. E. Anal. Chem. 1956, 28, 1946-1948. (21) Hood, A.; O’Neal, M. J. Adv. Mass Spectrom. 1959, 1 , 175-192. (22) Carlson, E. G.; Paullssen, G. T.; Hunt, R. H.; O’Neai, M. J. M a l . Chsm. 1960, 32, 1489-1494. (23) Galiegos, E. J.; Green, J. W.; Lindeman, L. P.; Le Tourneau, R. L.; Teeter, R. M. Anal. Chem. 1967, 3 9 , 1833-1838. (24) Aczel. T.; Allan, D. E.; Hardlng, J. H.; Knlpp, E. A. Anal. Chem. 1970, 42, 341-347. (25) Scheppele, S. E.; Chung, K. C.; Hwang, C. S. submitted to Int. J. Mass Spectrom. Ion Phys. (26) Scheppeie, S. E. ’ Mass Spectrometry and Fossil-Energy Conversion Technology--A Review;” US. Department of Energy, FE 2537-7, Distribution Category UC-Sod, June 1978. (27) Lumpkin, H. E.; Aczel, T. Anal. Chem. 1964, 36, 181-184. (28) Scheppele, S. E.; Grlzzle, P. L.; Greenwood, G. J.; Marriott, T. 0.;Perrelra, N. B. Anal. Chem. 1978, 48, 2105-2113.
RECEIVED for review December 18,1981. Accepted February 17,1982. Presented in part at the 8th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, September 20-25, 1981.
Electronic Bubble Gate for Colorimetric, Air-Segmented, Continuous Flow Analyzers Chas. J. Patton, Martln Rabb, and S. R. Crouch* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
The electronlc bubble gate described permlts the use of low volume bubble-through flow cells wlth the photometrlc detectors of air-segmented continuous flow (CF) analyzers. Experlmental data on the performance of CF analyzers with 2 mm 1.d. and 1 mm 1.d. manifold components and debubbllng or bubble-through flow cells are reported. The wash of CF analyzers wlth bubble-through flow cells Is improved relatlve to those with dehubbling flow cells but Is somewhat poorer than predicted by theory because of mlxlng between unsegmented sample and wash slugs as they pass through the pump Into the manifold. A mlnlature CF analyzer Is described and used to determine micromolar concentrations of nltrlte In aqueous solution at an analysis rate of 360 h-’ wlth a preclslon of 0.7% relatlve standard deviation and 1.0% Interaction. The analytlcal performance reported for a flow-lnJectlonanalyzer used to assay nltrlte In the same concentration range Is compared with that of the mlnlature CF analyzer.
Most commercially available colorimetric air-segmented continuous flow (CF) analyzers are equipped with debubbling 0003-2700/82/0354-1113$01.25/0
flow cells to eliminate the erratic detector signal that would result from the repetitive passage of highly reflective air segments across the colorimeter’s light path. Unfortunately, this expedient contributes significantly to loss of wash in CF analyzers and thus reduces the rate at which analyses can be performed (1). Loss of wash due to flow cell debubblers can be eliminated by several techniques. Analog (2) or digital (3) curve regeneration of the detector signal can provide mathematical compensation for loss of wash due to flow cell debubblers and other unsegmented zones within the CF analyzer without physical modification of the apparatus. Alternatively, loss of wash can be eliminated by a technique known as “bubble gating”. Here, the segmented stream is passed directly into the flow cell and the detector signal is sampled only when the flow cell is completely filled by a liquid segment. Flow cells with volumes less than that of a single liquid segment are required in this technique. One major advantage of bubble gating is that the analytical stream remains segmented and thus maintains its integrity at the detector so that multiple detectors can be used with minimal loss of wash. Habig and co-workers (4) appear to have developed the first bubble gate, which was activated by conductance changes within a specially designed flow cell. They concluded, however, 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
that their design was preliminary and was not suitable for routine use. Subsequently a more robust electronic bubble gate was described by Neeley et al. (5);the same group later reported a slightly modified version of this circuit (6)which they incorporated into a high-performance colorimeter for a miniaturized CF analyzer. This bubble gate repetitively sampled and stored the detector signal a t a frequency determined by an adjustable internal time base. Then, for a fixed time interval, the stored signal level was compared with the real-time signal level by means of a window tomparator circuit. When the stored and real-time signal levels were within the limits of the window comparator, the real-time signal level was stored in a second sample-and-hold circuit connected to the readout device. We now report a much simpler bubble gate that uses the periodic fluctuations of the detector signal, caused by the successive passage of air and liquid segments through the flow cell, to synchronize the time of data acquisition and temporary storage with the brief interval during which each liquid segment completely fills the flow cell. Data are presented to demonstrate that improved performance of both conknercially available and custom-built CF analyzers can be achieved by using the bubble gate described. The analytical performance of a miniature CF (mCF) system with 1 mm i.d. manifold components and a bubble-gated detector used for the colorimetric determination of nitrite in water is presented, and results are compared with those reported for flow-injection analysis.
EXPERIMEN.TAL SECTION CF Systems. The filter photometer used with the 1mm i.d. manifold mCF system was built in our laboratory; it will be described in detail elsewhere. The photometer consists of a Model 03000 miniature tungsten-halogen lamp (Welch-Allyn, Inc., Skaneateles Falls,NY), a glass fiber optic bundle (Dolan-Jenner, Inc., Woburn, MA), a G178-B724-02 10 mm X 0.5 mm i.d. flow cell (Gamma Enterprises, Inc., Mt. Vernon, NY),a 540-nm nmow band-pass interference filter (Ditric Optics, Inc., Hudson, MA), and a UV-040B silicon photodiode (EG&G Electro-Optics, Princeton, NJ). The Model IP-12 variable speed peristaltic pump (Brinkmann Instruments, Westbury, NY) used with the mCF system was modified locally by replacing the standard (eight rollers) roller assembly with one containing 16 rollers. Following this modification the flow rates of SMA Flow-Rated pump tubes (Technicon Instruments, Inc., Tarrytown, NY) with nominal flow rates in the range of 0.03-2.0 mL m i d were determined gravimetrically as a function of the pump’s speed control setting. The measured flow rates were linear functions of the speed control setting, and at a setting of 40, the nominal and measured flow rates for all tubes agreed to within about 10%. Air segments were injected into the analytical stream each time a pump roller left the platten using the dual pump tube method described by Habig et al. (4). With this method of air injection, pump speed control settings of 42, 56, 70, and 84 consistently produced segmentation frequencies of 1.5,2.0, 2.5, and 3.0 s-l, respectively. A speed control setting of 42 is considered as “standard pump speed” in the remainder of this paper. The 2 mm i.d. manifold CF (AAII CF) system consisted of a Pump I11 (with air-bar) and an Industrial S.C. colorimeter equipped with 15 mm X 1.5 mm i.d. debubbling flow cells and 540 nm interference filters (Technicon Instruments, Inc., Tarrytown, NY). In bubble gating experiments with the AAII CF system, the colorimeter was modified by removing capacitors C-201, C-203, and C-204 from the “control module” circuit board. This reduced the risg time of the detector signal from about 0.3 s to about 10 ms. Also, the sample side debubbling flow cell was removed and replaced with a 15 mm X 1.0 mm i.d. flow cell (Gamma Enterprises, In&, Mt. Vernon, NY). The cell was mounted directly onto the phototube housing with an adaptor to occlude the phototube entrance and exclude stray light. The colorimeter was then adjusted in the usual manner. The ”telemetry output” (0-5 V) of the colorimeter was connected to
I
:t 0’
TIME
Flgure 1. Fluctuations in detector signal caused by the successive passage of liquid and air segments through the flow celi: position 1,
air segment completely within cell; position 2,air segment exiting cell; position 3, cell completely filled with liquid; position 4, air segment entering cell. the bubble gate described below through a unity gain inverting amplifier with offset. This made the logarithmic output of the colorimeter compatible with the logic of the bubble gate which is described in detail below. Manifold components for the AAII CF system were obtained commercially from standard sources, while those for the mCF system were for the most part custom-made. Technicon SMA Flow-Rated pump tubes were used for both systems. Data were recorded with a Model SR-255 10 in. strip chart recorder (Heath Co., Benton Harbor, MI). In some experiments data %ere simultaneously logged with a microcomputer. Reagents. The sulfanilamide (SAN) reagent was prepared by dissolving 10 g of SAN in 500 mL of deionized, distilled water (DDW) and 100 mL of concentrated HCl contained in a 1-L volumetric flask. This solution was diluted to the mark with DDW, mixed, and transferred to an amber bottle. Then 0.5 mL of Brij-35 wetting agent was added. The N-(1-naphthy1)ethylenediaminedihydrochloride (NED) reagent was prepared by dissolving 1.0 g of NED in 1L of DDW. This solution was transferred to an amber bottle, and 0.5 mL of Brij-35 was added. The diluent (DIL) consisted of 0.5 mL of Brij-35 in 1.0 L of DDW. Nitrite Standards. The primary nitrite standard was prepared by dissolving 0.345 g (5 mmol) of dried, analytical reagent grade sodium nitrite in 1 L of DDW. Working standards in two ranges were prepared by dilution of the primary standard using digital microliter pipets and volumetric flasks. Bubble Gate. When bubble-through flow cells are used with colorimetric, air-segmented CF analyzers, the detector signal closely resembles a square wave. With an air-segmented blank solution the detector output varies between the 100% T level (5.0 V in our case) and the 0% T level (0.0 V), as shown in Figure 1. As an air segment traverses the flow cell, it reflects most of the light away from the photodetector, and the signal approaches the 0% T level (0.1-0.4 V in our case). The actual magnitude of the signal observed in the presence of an air segment is somewhat erratic, and it decreases as the transmittance of the two adjoining liquid segments decreases. When the air segment exits the flow cell, the detector signal rapidly rises to a level that corresponds to the transmittance of the liquid segment (see Figure 1). The bubble gate described here uses the information encoded in the periodic fluctuationsof the detector signal to synchronize the time of data acquisition and temporary storage with the brief interval during which each liquid segment completeIy fiIls the flow cell. The schematic diagrams presented in Figure 2 depict the functional units of the bubble gate circuit. In the basic bubble gate circuit (Figure 2A) comparator IC1 converts the detector signal to a TTL logic level signal that indicates the presence or absence of an air segment in the flow cell. This is accomplished by setting the threshold level of the comparator at about 0.5 V. A HI logic level at the output of the comparator indicates the absence of an air segment in the flow cell. Note, however, that the logic level transitions of the comparator lag the entrance and exit of air segments into and out of the flow cell by several milliseconds because of the time required for the detector signal level to cross the threshold level of the comparator. The rising
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 10
ICZ-A
-
LED I
T& Fgure 2C
1 L J $ q J
Log S/HOut
S/HOut
P
P
S/H
LOG
Detector Signal 001
To B
G5" F i g l r e 2C
100k
i
-
D) Flgure 2. Generalized schematic diagrams of bubble gate clrcuitry; all resistances in ohms, all capacitances in microfarads: (A) basic bubble gate, IC1 = LF311 FET Input comparator, IC2 = 74LS123 dual monostable mufiibriator, IC3 = LF398 sampleandhold amplifier, IC4 = AD755 logarithmic amplifier, IC5 = n o 8 4 quad FET input operational amplifier: (B)differential edge sensor, IC5 = TL084 quad FET input Operational amplifier, IC6 = LF311 FET input comparator; (C) sample-and-hold amplifler update logic: IC7 = 7410 triple 3-Input NAND gate; (D) automatic threshold adjustment for comparator I C I , IC8 = TLO84 quad FET input operational amplifier.
edge of the comparator output triggers monostable multivibrator IC2-A, which generates a time delay pulse. The duration of this pulse is adjusted with potentiometer P1 to terminate at the approximate midpoint of the interval during which a liquid segment completely fills the flow cell. Two light-emitting diodes (LED1 and LEDS) facilitate this adjustment by giving visual indication of the logic states of the comparator and monostable, respectively. The Falling edge of the time delay pulse triggers monostable IC2-B which generates a pulse with a fixed width of approximately 100 ps. A gated version of this 100-ps pulse is used to update sample-and-holdamplifer IC3 which samples on a TTL HI and holds on a TTL LO. Irregularities in the segmentation pattern of the analytical stream, generated when the sample probe is cycled between samples and the wash solution, can produce two conditions which cause the basic bubble gate to malfunction. First, irregular liquid segmenta that completely fill the flow cell for a time interval much shorter than the selected time delay interval cause the comparator and the monostable to get out of phase; several liquid segments of normal length must pass through the flow cell before these two circuit elements are again synchronous. Erroneous sample-and-
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hold amplifier updates could occur in the interim. This problem was corrected by clearing monostable IC2-A with the falling edge of the Comparator output so that the time delay pulse is aborted in the event that it is still in progress when an air segment enters the flow cell. Second, irregular liquid segments that completely fill the flow cell for a time interval approximately equal to the duration of the delay pulse can cause incorrect sample-and-hold amplifier updates because of the lag between the initial entry of an air segment into the flow cell and the logic level transition of the comparator output as discussed previously. This problem was eliminated with the differential edge sensor circuit shown in Figure 2B, which generates a TTL HI at its output whenever the detector signal hm a nonzero derivative. Because the output of this circuit responds to changes in the detector signal level almost instantaneously, it can be used to eliminate the possiblity of sample-and-hold amplifier updates at the time of initial entrance or exit of an air segment into or out of the flow cell. Three conditions must be satisfied in order to update the sample-andhold amplifier: (1) The output of the monostable IC2-B must be HI. (2) The output of the differential edge sensor must be LO. (3) The output of comparator IC1 must be HI. Figure 2C shows the logic that prevents sample-and-hold amplifier updates when the above conditions are not satisfied. Our bubble-gate circuit will not operate when the signal level of a liquid segment is less than the threshold level of comparator IC1. If the threshold level of the comparator were to be fixed at 0.5 V, liquid segments with transmittances less than about 0.1 would be indistinguishable from air segments, and the bubble gate would be restricted to an operational range of 100% T to 10% T. However, because an air segment's effective transmittance is not constant but decreases as the transmittance of the two liquid segments adjoining it decreases, it is possible to extend the lower operational limit of the bubble gate considerably by using the detector signal level stored in the sample-and-hold amplifier for automatic adjustment of the comparator threshold to an appropriate level. This function is performed by the circuit shown in Figure 2D. A bufferecl voltage divider applies half of the sample-and-hold-amplifier output to the threshold level input of comparator IC1 through a unity gain buffer (IC8-D). Two active diode clippers limit the maximum and minimum threshold levels to about 2 V (fixed) and 0.1-0.5 V (adustable),respectively. This circuit extends the bubble gate's lower limit of operation to about 3% T (1.5 A ) , and in addition improves performance in t1.e high transmittance range because it provides greater separation between the comparator threshold level and the somewhat erratic detector signal produced by the passage of air segments through the flow cell. A logarithmic amplifier (Model AD755, Analog Devices, Norwood, MA) is also included in the bubble gate circuitry so that peak heights are linear functions of concentration. A detailed schematic diagram of the bubble gate is available from the authors on request. It should also be noted that bubble gating can be accomplished with a microcomputer and appropriate software. Apparently this approach was used by the designers of the Technicon SMAC clinical CF system.
RESULTS AND DISCUSSION The bubble gate was tested with the Technicon AutoAnalyzer I1 CF system (AAII CF system) and the mCF system assembled in our laboratory. The colorimetric determination of nitrite via a modified Griess reaction procedure (7) was chosen as the reference assay with which the performance of the two CF systems could be evaluated and compared. A composite manifold diagram for both CF systems is shown in Figure 3. The introduction of several air segments at the beginning of each sample and wash slug by rapid repetitive insertion and withdrawal of the sample probe ("pecking") had no discernible effect on the performance of either the standard or the bubble-gated AAII CF system; it did significantly improve the performance of the mCF system (see below). Therefore, the pecked sampling technique (four air segments a t the leading edge of each sample and wash slug) was used routinely with the mCF system. For both systems the time
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ANALYTICAL CHEMISTRY, VOL. 54. NO. 7. JUNE 1982
Table I. Major Variables" for Percent of Steady State Experiments AAII CF system
exptl variable flow cell path length (cm) full scale absorbanceb pump speed control setting r& 1-1 d.. cm
std
bubble gated
1.5
1.5 ~0.6
~0.5
mCF system w/o pecking 1.0
0.5 42
0.5 0.2 0.022 200
0.5 0.2
0.022 200 2.13
1.5 0.1 0.0067 130 1.08
with pecking 1.0 0.5 42 1.5 0.1 0.0067 130 1.08
1.0 0.5 56 2.0 0.1 0.0083 100 0.94
1.0 0.5 84 3.0 0.1 0.013 65 0.82
1.0 0.5 42 1.5 0.1 0.0067 110 0.99
1.0 0.5 84 3.0 0.1 0.013 110 1.07
Minor variables: viscosity ( q ) = 8.9 X 10.' P,surface tension ( 7 ) = 3.2 X 10' dyn cm-', effective diffusion coefficient ( D w . z 5=) 5 X 10.1 cm' s - ' . For the standard and huhble-gated AAII CF systems, Standard Calibration Control settings Were 2.32 and 2.02. reswctivelv. a
NOMINAL FLOW RATE lrnLlrni"1 WASTE
AIR
I
DllUD
Flgure 3. Composite diagram of various CF manifolds used. Values outsae M within parentheses refer to AAII or mCF manifolds, re-
spectively. To obtain flow rates for pump tubes at speed control settings of 56, 70, or 84 (n = 2.0. 2.5, or 3.0 s-') nominal flow rates are muniplied by 1.33. 1.67. or 2.00. respectivev. Manifold compcnents in the shaded area were omffled from the "analytical" mCF manifold and the nominal flow rate of the sample pump tube was increased to 0.23 mllmin. required to reach steady state and the percent interaction were determined as described below. Steady-State Experiments. A set of five aqueous nitrite standards with nominal concentrations of 5,15,25,35,and 45 p M were prepared, and the detector gain was adjusted such that these standards produced a response of approximately lo%, 30%,50%,70%, and 90% full scale, respectively, on the strip-chart recorder. With the M I system each standard and 60 s with was sampled in triplicate for 5,10,20,30,40,50, a 60-8 wash between each sample to minimii carryover. With the mCF system each standard was sampled in triplicate for 5,10,15,20,25,30, and 60 s with a 30-9wash between each sample. The average peak height for each sampling interval was divided by the steady-state peak height (60 s sampling interval) and multiplied by 100 to obtain the percent of steady state (%SS) as a function of sample time. A grand average of %SS for all five standards a t each sampling interval was then calculated. The results of this calculation showed that %SSwas independent of concentration to within about 1% for the 5-8and 10-ssampling intervals and to within about 0.5% for sampling intervals greater than 10 8. The increased scatter in the peak heights a t shorter sampling intervals is probably a result of the manual sampling employed. Results of the %SSexperiments are presented graphically in Figure 4. Values for major experimental variables can he found in Table I as well as the estimated dispersion, ot,of the air segmented sample slug during its passage through the CF system. The latter quantity was calculated according to the model of Snyder and Adler ( 8 , 9 ) . Values of minor variables
0
5
10 Sampling
I5
20
lntervol
25
30
,I)
Results of percent steady state (SS) experiments: 1, standard AAII CF system; 2. bubble-gated A A l l CF system; 3. b u h ble-gated mCF system whhout pecked sampling: 4-6, bubblegated mCF system with pecked sampling and n equal to 1.5. 2.0, and 3.0 8.'. respectively. See Table 1 for other experimental variables and text for specific details. Flgure 4.
shown in Table I were in keeping with a similar set of calculations by Snyder ( I ) . As shown in Figure 4, bubble gating reduced the sampling time required for the AAII CF system to reach 98% steady state by about 8 s. A similar reduction in the sampling time required for the mCF system to reach 98% steady state was achieved by using the pecked sampling technique. An apparent trend for the mCF system also shown in this figure is a decrease in the sampling time required to attain a given %SS as the segmentation frequency, n, increases. Note, however, that with the mCF system, n was increased by increasing the pump speed (and therefore the liquid flow rate, Fl)in order to keep air segmentation in phase with the roller lift off of the peristaltic pump, a condition necessary for uniform proportioning of samples and reagents (IO,11). Thus the residence time, t, of the sample slug in the manifold decreased as n and Flincreased (see Table I). T o differentiate between the effects of n and Fl, we repeated the experiment for n = 1.5s-l, Fl = 0.0067 mL s d and n = 3.0 s-l, Fl = 0.014 mL 8.' with t held constant at 110 s hy altering the length and number of mixing coils in the manifold. ks shown in Figure 5,the same trend was observed. This result is contrary to predictions of Snyder's model ( I ) under these experimental conditions (see Table I). However, Snyder's model does not consider contributions to the total observed dispersion of the sample slug caused by mixing effects in unsegmented zones of the CF system; these impose an exponential (12, 13) rather than
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
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~~
Table 11. Percent Interaction AAII CF system mCF system bubble n = n= n= stcl gated 1.5 s-' 2.0s-' 3.0s.'
wash interval(s) 5 10 15 20 30
1.'7 1.4 0.8 0.6 0.2
1.0 0.8 0.5 0.3 0.1
1.0 0.5 0.2 0.1
0.7
0.4 0.2
0.4 0.2
