Instrument for the Generation of Reactive Gases Wing Tsang* and James A. Walker Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234
An instrument has been constructed which is capable of generating reactive gases over wide concentration ranges [from ppm on up]. its operational characteristics and performance have been evaluated with specific application to the generation of formaldehyde, acetaldehyde, and acroleln vapors at micrograms/minute rates.
This report is concerned with the continuous generation of accurately known quantities of reactive gases in dilute concentrations. It is a problem that is intimately connected with the assessment of environmental or occupational health hazards and the development of abatement strategies. At the most basic level, such a capability is needed for the calibration of measuring instruments, the evaluation of analytical methodologies or for carrying out toxicity investigations. From another point of view, it permits the investigation of the properties and reactivities of such species in detail, so that the most efficient abatement method may be developed. The basic problem in generating such mixtures lies in the requirement for dilute concentration and the natural reactivity of the compound of interest. Thus, standard static methods ( 1 ) of sample preparation are unsatisfactory. The need is for a dynamic method, where the required samples are prepared immediately before use. In the present paper, we report on the construction and operation of an apparatus that performs this function. It involves, first, the generation of a dilute mixture of a large organic molecule in an inert diluent and then flowing this mixture through a hot tube where complete stochiometric decomposition occurs and the reactive gas of interest is formed. The former is achieved using the well established diffusion cell technique ( 1 ) . The latter may be considered an extension of earlier work (2) on the formation of reactive gases by pyrolysis. In the previous study, dilute concentrations of a suitable parent molecule in toluene were injected into a chromatograph, pyrolyzed in a gold tube reactor (inserted into the chromatograph between injection port and column) and the products, containing the reactive gas of interest, separated and detected. The “parent mo1ecuie”of choice is one that undergoes pyrolytic decomposition exclusively via the reaction, parent molecule
-
hydrocarbon
+ reactive compound
Thus, the quantity of the reactive gas delivered to the detector was deduced from the amount of the hydrocarbon that was present. Calibration of the latter was straightforward. The presence of this internal standard is an added feature since it minimized the possibility of operator errors. Furthermore, by varying the amount of “parent molecule” injected or the extent of reaction, such a method had, for practical purposes, infinite dynamic range. A complete discussion can be found in the earlier paper (2). However, in this mode of operation, the sample was introduced as a pulse and was limited in its range of potential applications. For toxicity and physicochemical investigations, continuous generation is an absolute necessity. Furthermore, even for calibration purposes, the ideal system is that which most closely approximates the situation being studied, and here again the continuous generator is the most useful. This is the prime motivation for this in-
vestigation: to translate an interesting concept into a useful device. With regard to the molecules of specific interest, we focus once again on the three pollutant aldehydes: formaldehyde, acetaldehyde, and acrolein. The choice of these substances for study is dictated by the absence of any standard method for their generation in trace quantities. The present study serves not only to demonstrate the method but may well be of immediate use. For example, in the context of pollution problems, the three aldehydes have been found in automobile exhausts ( 3 ) . Furthermore, recent studies have identified formaldehyde ( 4 ) as a key intermediate in the generation of photochemical smog. Finally, we note that the OSHA (5) limits for these compounds are 5 ppm, 50 ppm and 0.1 ppm for HCHO, CHsCHO, and acrolein, respectively.
EXPERIMENTAL A schematic diagram of the reactive gas generator is presented in Figure 1. The key design decisions have been made with the intent of using the same instrument to generate a wide variety of reactive gases. At the same time, we have sought to construct a device that is reasonably compact. Fundamentally, the generator consists of two constant temperature compartments. The first, containing the metal diffusion and buffer cells can be maintained a t temperatures from ambient to 250 “C. Heat is supplied by a 4-in. X 10-in. silicone rubber heater fixed to the bottom of the aluminum box holding the cells. Temperature control of hO.1 “C is achieved using a RFL-70 controller. The second compartment holds the lh-in. 0.d. gold tubing in which pyrolysis is carried out. A cartridge heater provides the thermal input. The temperature range is from 0 to 700 “C and can be held to f 1 2 “C using a Burling A-1S thermostat. The upper portion or head of the metal diffusion cell contains the diffusion column. A variety of heads, with i.d. up to 0.63 cm may be used. The length of the diffusion column is of the order of 6.3 cm. The substance to be pyrolyzed is placed in a glass cup which sits on the lower portion or base of the diffusion cell. The total volume of sample that can be accommodated is of the order of 15 cm3. This is sufficient for practically all conceivable applications. A 0.31-cm hole at the bottom of the base of the diffusion cell provides a means of purging the cell of air so that oxidation will not make any contribution to the pyrolytic process. T h e role of the buffer cell is to take out pressure surges which arise from the switching of valves or the connection of fittings. The entire system is operated a t about 1/2 atmospheres above ambient pressure. Control of system pressure and flow is effected by means of a needle valve a t the exit of the pyrolyzer. This has the advantage of providing some isolation of generator from downstream events. The elevated pressure also makes it easy to check for leaks in the system. Experience from these studies suggests that leaks are the most likely source of error using this device. The conversion of formaldehyde to CO has been noted when the generator is operated a t atmospheric pressure. This is especially serious when low concentrations of formaldehyde are needed. Increasing pyrolyzer pressure appears to minimize this effect. At first, the conversion to CO had been interpreted as indicative of oxygen passing through the thin walled gold tube a t high temperature (as is the case for silver). However, sheathing of the gold tube with a stainless steel tube did not appear to have an appreciable effect. The effect is still not understood. ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
13
DIFFUS
- T O DETECTOR
I DILUENT GAS INLET DIFFUSION C E L L
BUFFER C E L L
BLOCK SUPPORTS
DIFFUSION H O L E
PURGEVALVE
+FURNACE
E XIT
TUBE
CONTROLLER
L-HOUSING OUTER BOX
Flgure 1. Schematic of reactive gas generator
The relationship between the diffusion rate and the physical parameters of the material and the diffusion cell is ( I ) Qd=-h(-) DA P
L
P-P,
where Qd = diffusion rate, vol/time; D = diffusion coefficient, A = diffusion tube cross-sectional area; L = diffusion tube length: P = pressure in diffusion cell and P, = partial vapor pressure of diffusing vapor. It is important to note that the diffusion rate can be varied by changing the diffusion tube cross-sectional area A and/or by changing the cell temperature. The latter arises from the three-halves power dependence of the diffusion coefficient on temperature (approximate) and the exponential relationship between vapor pressure and temperature (Clapeyron Equation). For vapor pressures sufficiently lower than the cell pressure, this is equivalent to an exponential dependence between diffusion rate and temperature. Thus, by suitable adjustment of the physical parameters, an enormous range of diffusion rates can be generated. Inversely, this means that for a given range of diffusion rates, the present apparatus can accommodate a wide variety of substances through the proper selection of diffusion cell cross-section and temperature. The sensitivity of the diffusion rate to the latter requires controls of the diffusion cell temperature to f O . l OC. On the other hand, it also means that diffusion rate can be set easily and continuously over a wide range. This is in contrast to the situation of generation using static methods. Finally, it will be noted that the concentration of reactive gas is related to the diffusion rate by the equation
c = Qd/Q where Q is the flow rate across the diffusion cell. Thus the concentration can also be controlled by the flow rate. The compounds and decomposition reactions that have been used to generate the three aldehydes are
+
CH~=C(CH~)CHZCHZOHHCHO i-CdHs CHz=CHCHz CHOH CH:, CH:,CHO C 3Hfi CH*=CHCHO z-C~HX CHzeCHCHOH CH*C(CH+CH* +
+
+
+
+
Since all three processes proceed as written, the extent of conversion is dependent only on the reaction time ( t ) ,temperature T and the thermal stability of the appropriate compound. The relation is 14
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
In (C,/Cf) = t A exp(-E/RT) where A and E are Arrhenius parameters for unimolecular decomposition (6) and characteristic of the alcohol of interest and C, and Cf are the initial and final concentrations. In the present context, interest is focused in the region of high conversions or where C,/Cf is greater than 50. Under these conditions, the concentration of products is relatively insensitive to changes in reaction variables. Thus these variables need not be strictly controlled and the operational procedure to locate the minimum temperature necessary for complete conversion is to increase the temperature until the product yield reaches a maximum and/or when the parent molecule disappears. It should be noted that the success of these experiments is critically dependent upon the choice of the parent compounds. This has been discussed in great detail in an earlier paper ( 2 ) .For the present purposes, it is sufficient to state that possible gas phase side reactions are a factor of 10 000 to 100 000 times slower. Possible contributions from surface reactions are minimized by the use of the gold reactor. Finally, we note that all of the product compounds are thermally stable under the reaction conditions. Gas chromatography with flame ionization detection is used for analysis and products are identified by retention times. Formaldehyde is virtually insensitive to this detector. Thus it was necessary to hydrogenate formaldehyde to methane before passage into the flame ionization detector. For this purpose the method of Stevens, O'Keeffe, and Ortman (7) was used. This procedure quantitatively converts carbon monoxide (at all relevant levels) to methane and it was assumed that this will also be the case for the carbonyl groups. This assumption is consistent with the results of this study. Separation was effected with a 6-ft Poropak Q-S column operating in the temperature range 80-140 OC. Even with this column, formaldehyde peaks in the parts per hundred thousand range showed pronounced tailing. Other Poropak columns (without silanation) showed tailing a t concentrations as high as 1%.Peak areas were determined using electronic integration. The three alcohols were purchased from Chemical Samples Company. The stated purities were 97%, 3-methyl-3-butene-1-01; 98%, 4-pentene-2-01; and 97%, 5-rnethyl-1,5-hexadiene-3-01. All substances were used without further purification. Calibration was carried out with a purchased bottle of 190 ppm C3H6 in He. The uncertainty limits was stated to be &lo%.Throughout the present study, the linearity of the flame detector, the linear relation between the number
Table I(a). Experimental Results on the Production of j-CdH8 and HCHO from CHZ=C(CH~)CH~CH~OH Area ratio
T°C Generator A
Generator B
iC4Hs/HCHOa
29 40.6 40.6 50 59.3 69.0 79.5
4.25 4.50 4.15 4.5 4.25 4.10 4.12
88
4.16
41.5
1.43 2.92 2.91 5.18 8.85 14.7 26.5 38.8 1.55 2.88 3.03 13.70 30.8
39.7 40.4 67.5 80.5 a
T h r u -put, dmin
HCHO converted t o CHI (see text).
(b) Experimental Results on the Production of C3H6 and CHsCHO from CHz=CHCHzCH(OH)CH3 Area ratio
Thru-put, CSH~/CH~CHO wg/min
T "C Generator B
Generator C
30.2 44.3 48.6 60.3 62.4 72.5 89
2.91 2.89 2.88 2.92 2.89 2.94 2.93
125.3
34 39.8
2.94 2.78 2.84 4.92
4.9 7.6 23.3, 125.0
58.5 88.8 Generator B With hypodermic needle for diffusion col. (0.025-in. i.d.)
4.1
10.0 11.9 26.3 29.4
53.2
0.33
39.4 57.8 76.6 94.5
1.16 3.35 11.3
( c ) Experimental Results on the Production of i-C4Hs and CHzCHCHO from CH~=CHCHOHCHZC(CH~)=CH~ Area ratio
T "C
i-C4H4/CH2CHCHO
30.4 40.6 50.2 60.4 71.0 82.5 90.0
1.86 1.87 1.89
1.88 1.92 1.93 1.93
Thru-put, pg/min 0.86 1.75 3.28 6.15 12.21 22.2
31.0
of CH, groups and the detector response, and the approximate detector insensitivity to carbonyl groups were assumed. The experimental program had as its goals the determination of the optimum conditions for the operation of the generator and the validation of the design concepts. More specifically, the aim was to demonstrate the capability of delivering the aldehyde a t the levels of fig/min and higher. Convenient flow rates for the cell geometry and chosen compounds were in the range of 20-200 cm3/min. This implies concentrations in the range of 10-20 ppm and above. Helium was the carrier gas. Obviously other nonreactive gases such as Nz or Ar would have been equally satisfactory. Oxygen had to be rigorously excluded from the system. With the present detection system, 10-20 ppm was the minimum concentration for which it is possible to make accurate quantitative measurements in a routine manner. These constraints together with the vapor pressure of the parent substances dictate the use of diffusion columns of a t most 0.125-in. i.d. Although most test resulis were obtained using this diameter column, a few runs were also carried out with a hypodermic needle as the diffusion column (i.d. = 0.025 in.).
Table 11. (a) Variation of Concentration as a Function of Flow Rate. HCHO Generation at 88 "C;7.5 psig Cell Pressure; Pyrolyzer Temperature, 675 "C Flow rate cm3/min
(flow rate)-l (min/cm3)
HCHO, PPm
33 81 210
0.030 0.0124 0.0048
881 397 148
(b) Variation of Concentration as a Function of Pressure, HCHO Generator at 88 "C,210 cm3/min Flow Rate, Pyrolyzer Temperature 655 "C Pressure, psia
HCHO, ppm
26.4 22.1 18.3
122 148 172
Table 111. Long Term Behavior of Reactive Gas Generators (a) HCHO Generation at 40 "C. Flow Rate = 30 cm3/min. Pyrolyzer at 650 "C. System Pressure = 7.2 psig Hours
0 2 6 72
Concentration, ppm 75 75 74 76.5
(b) CH3CHO Generation at 30.2 "C. Flow Rate = 40 cma/ min. Pyrolyzer at 680 "C. System Pressure = 7.0 psig Hours Concentration, ppm 0 96 120 168
56 58 58.5 56.5
( c )Acrolein Generation at 31.0 "C.Flow Rate = 40 cm3/ min. Pyrolyzer at 620 "C. System Pressure = 7.0 psig Hours Concentration, p p m 0 24 48 72 96
9.7 9.6 9.7 9.8
10.0
For the three compounds of interest HCHO, CHJCHO, and acrolein, substantially complete reaction (>97% conversion from disappearance of parent) a t the lower flow rates (-30 cmi/min) was achieved a t temperatures of -600, 660, and 600 "C, respectively. For the higher flow rates (up to 200 cm3/min), a minimum temperature of -50-60 "C higher was necessary. Inasmuch as the softening temperature of gold is about 700 "C, this means that for aceteldehyde the maximum flow rate was about 100 cm3/min. We have, however, carried out experiments a t temperatures of 720-730. "C without untoward effects. These limits were established by carrying out sets of runs a t various pyrolyzer temperatures and noting the values a t which the yields of aldehydes became optimum and invariant. Further confirmation of complete decomposition was offered by the disappearance of the parent compound under this condition. We also noted the existence of a satisfactory material balance as evidenced by the equality of the area of the parent peak under conditions where no reaction occurred (very low pyrolyzer temperature) and the total area of the two product peaks when 100%conversion was achieved.
RESULTS The experimental r e s u l t s a r e summarized on Tables I, 11, and 111. I n T a b l e s Ia, b, and c, we d e m o n s t r a t e t h e e x p e c t e d stochiometry as well as the d y n a m i c range of t h e instrument. It should b e emphasized that t h e s e r e p r e s e n t essentially all ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
15
Table IV. (a) Formaldehyde Output from the Reactive Gas Generator Using a Formalin Solution at 60.5 “C Mass Flow Rate Concentration thru-put, Time, h cm3/min PPm pglmin 0 25 678 20.9 66 25 671 20.7 68 100 167 20.5 70 200 83.5 20.6 (b) Formaldehyde Generation from Formalin Solution as a Function of Temperature Mass thru-put, T”C clg/min 28.8 2.08 36.5 3.78 47.0 8.55 60.6 20.8 69.0 37.6
the reaction products detectable by gas chromatography and are indicative of the absence of side reactions. It can be seen that over the entire range of the experiments, we were able to establish the essentially 1to 1correspondence of the appropriate aldehydes and olefins. The actual area ratios are for isobutene and formaldehyde, 4.22 f 0.12 to 1;propylene and acetaldehyde, 2.89 f 0.05 to 1; and isobutene and acrolein 1.898 f 0.002 to 1. With respect to the number involving formaldehyde, this is derived from experiments where the formaldehyde is converted to methane (see above). The failure to obtain an exact 4 to 1ratio is probably due to problems in obtaining 100% conversion, decomposition to CO, and the pronounced tailing of the formaldehyde peak, making exact integration uncertain. However, for the present purposes, the results are satisfactory. One interesting observation that is peripheral to the present study is that formaldehyde is not totally inert to a flame detector. The actual sensitivity in comparison with methane being -0.02. The ratios for the other aldehydes were carried out without hydrogenation of the carbonyl groups and were as expected. The small difference from integral values does suggest that CHO groups are not totally inert to flame ionization detection as noted for formaldehyde. In several runs, the pyrolyzer products were hydrogenated and for propylene and acetaldehyde, the expected 3 to 2 area ratio was obtained (the actual ratio is 1.485). On the other hand, the isobutene and acrolein ratio was only 1.45 to 1instead of the expected 1.33 to 1.We have no explanation for the observation, The present results are of importance not only as confirmation of the postulated stochiometry but also as demonstration that in this type of reactive gas generation an accurate internal standard is available. This may well be the key advantage of this method of generation. With respect to the dynamic range of the instrument, it can be seen from the data in Tables Ia, b, and c that for diffusion cell temperatures in the range of 30-100 “C, the mass output per unit time varies by a factor of 30. Taking into account the range of flow rates that may be used, this means that the range of concentrations is over two orders of magnitude. Note that these concentrations can in effect be “dialed”. This is in striking contrast to samples derived from static systems where only one fixed concentration is available. In comparison to evaporative methods of generation, there is the additional option offered by the use of diffusion columns of different internal diameter. We have also included in Table Ib data for the acetaldehyde system as obtained using a hypodermic needle. It can be seen that the difference in mass output is close to the expected ratio of areas of diffusion columns. The 16
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
relationship between cell temperature and aldehyde output (with l/g-in. i.d. diffusion column) is
+
loglo HCHO(pg/min) = -2704 f 30 (l/T) 9.09 f 0.09 loglo CHsCHO (pg/min) = -2814 f 30 (1/T) 9.86 f 0.09 loglo C2HsCHO (pg/min) = -2901 f 27 (l/T) 9.49 f 0.08
+ +
The standard deviation over the entire range is 4%. This can be considered to be the uncertainty if one seeks to dial a particular concentration. It should be noted that the exponential dependence derived here is an empirical one for presenting the data. Theoretically the actual temperature dependence should be some power of the temperature multiplied by the exponential dependence. The data in Tables I1 and I11 provide further confirmation of the proper behavior of the apparatus. More specifically, the numbers in Table I1 demonstrate fairly well the expected dependence of concentration vs. flow rate and cell pressure. The long term behavior of this system is summarized in Tables IIIa, b, and c. It can be seen that over periods of several hundreds of hours, material output can be held invariant. This is hardly surprising. The literature on gas generation by diffusion cells is replete with testimony to their long term stability and pyrolysis experiments are known to behave with increasing reproducibility during the course of an experiment (6). During the course of these experiments, three reactive gas generators were assembled. The results for the formaldehyde and acetaldehyde have each been derived from a pair of these generators. From Table I, it can be seen that there were no systematic deviations. Furthermore, the acceptably small uncertainty limits in the derived relationship between the output of aldehydes and the diffusion cell temperature demonstrate that, in accordance with theory, the former is uniquely determined by the cell temperature. In a sense, this reproducibility renders unnecessary the provision for an internal standard. This presents other possibilities with regard to reactive gas generation. For formaldehyde generation, standard formaldehyde solutions may be a possible candidate. The results of such studies are summarized in Table IV. From the latter, one notes the long term stability. Note that quite fortuitously the yields of formaldehyde from this and from the alcohol pyrolysis are in the same range. The above is based on a fresh formaldehyde solution. Surprisingly, we find that although the shelf life of formaldehyde solution is only 3 months, a two-year old bottle of formalin gave very similar results. Returning now to the role of the internal standard, we emphasize that its presence is rendered somewhat unnecessary with the kind of reproducibility displayed here. More properly, these observations demonstrate the possibility of making cross checks on the operation of the generator. This is not a trivial luxury.
CONCLUSION We have constructed and demonstrated the operation of a reactive gas generator. Our studies show that the accuracy with which the instruments can be set is f4%; the long term stability is of the order of f 2 % and the accuracy that is attainable with the internal standard is 1-3% (the last number is due to the formaldehyde results and as noted earlier is probably due to analytical problems). All of these numbers compare very favorably with the best that can be attained with commercially prepared mixtures of unreactive gases. In a sense, this instrument can also be considered a very satisfactory olefin generator. Furthermore, unlike the static mixture, a whole range of concentrations is available. Although we have used this instrument for the generation of the three pollutant aldehydes, we emphasize that these are
used here purely for demonstration purposes. Indeed, there is good reason to suppose that most thermally stable reactive species can be generated in this manner. In a sense, the present results remove the “hardware” aspects from consideration in reactive gas generation and focus attention on the thermal properties of organic molecules. We also note that the majority of substances that are presently considered hazardous are, in fact, liquid or solids a t ambient conditions and are not particularly unstable in the bulk. For such substances, generation can be effected without the pyrolyzer and obviously our observations with respect to reproducibility, precision, accuracy, and range will hold with equal, if not greater, force. Thus, this instrument may well provide a simple general solution to a difficult analytical problem. LITERATURE CITED (1) G. 0. Nelson, “Controlled Test Atmospheric, Principles and Techniques”, Ann Arbor Science Publishers, Ann Arbor, Mich., 1971. (2) W. Tsang, J. Res. Natl. Bur. Stand., Sect. A, 78, 157 (1974).
B. Dimitriades, C. F. Ellis, and 0. E. Seizinger. in “Advances in Chrornatography”, Vol. 7, J. C. Giddings and R. A. Keller, Ed., Dekker. New York. 1968. J. Calvert and McQuigg, Int. J. Chern. Kinet., Symposium No. 1 Proc. of the Symposium on Chem. Kin. Data for the Upper and Lower Atmosphere, 1974, p 113. Fed. Regist., 36,No. 157, Aug. 13, 1971. S. W. Benson. and H. E. O’Neal, “Kinetic Data in Gas Phase Unimolecular Reactions”, Natl. Stand. Ref. Data Ser., Natl. Sur. Stand. ( U .S .), 21, 645 pages (Feb. 1970). R. K. Stevens, A. E. O’Keeffe, and G. C. Ortman, unpublished results.
RECEIVEDfor review April 26,1976. Accepted July 22,1976. Supported in part by the Office of Air and Water Measurement and the Center for Fire Research of the National Bureau of Standards. Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
Pumping Pressure and Reagent Consumption in Continuous Flow Analysis with Unsegmented Reaction Streams Marvin Margoshes Technicon Instruments Corporation, Tarrytown, N. Y. 1059 1
Three research groups have independentlydeveloped equipment for rapld chemical analysis by continuous flow without segmentatlon by air bubbles. All three systems rely on rapld flow of a reagent stream in narrow tubing to llmlt the spreading of a sample that is injected as a bolus of liquid. Elementary considerations show that an unsegmented continuous flow analysis system will either consume considerably more reagent than a segmented system or else will require very hlgh pumping pressures.
At least three independent groups have recently described systems for chemical analysis based on continuous flow in an unsegmented fluid stream. The system described by White and Fitzgerald ( I , 2 ) employs a photochemical reaction in a flowing stream, with the sample introduced by gravity ( I ) or by injection via a motor-driven syringe ( 2 ) . The reagent stream is pumped by applying a small air pressure (5 psi) to the reagent reservoir. The system of Ruzicka and Hansen ( 3 ) employs a peristaltic pump and rapid, manual injection of the sample into the flowing stream. Stewart, Beecher, and Hare ( 4 ) also used gas pressure pumping, but at a much higher pressure of 400 to 500 psi; they introduced the sample into the reagent stream by means of a liquid chromatography valve. The three systems have similarities. All employ rapid flow in narrow tubing to control peak spreading. All are used for automation of analyses with simple chemical steps and short reaction times. The purpose of this publication is to discuss some limitations of unsegmented continuous flow analytical systems, particularly the relation between pumping pressure and reagent consumption. BACKGROUND Unsegmented reaction streams have been employed for many years in chromatography. The amino acid analyzer of Spackman, Stein, and Moore (5)incorporated such a method
for the reaction between the column effluent and the ninhydrin reagent. These authors recognized that band spreading in the flowing stream could be controlled by keeping the tube diameter small at all points along its length. They observed that a 0.5-ml aliquot of the effluent (equivalent to 1 min of flow from the column) occupied a length of 150 cm in the narrow tubing they used. It was soon recognized that turbulence is the key to control of band spreading in a fluid stream. This can be seen in publications by Sternberg and Poulson (6),by Giddings (7),and by Pretorius and Smuts (8).In laminar flow, the fluid in the center of the tube moves at twice the mean fluid speed, leading to rapid longitudinal mixing which is seen as peak spreading. In turbulent flow, the fluid velocity is more nearly constant across the diameter of the tube and peak spreading is greatly reduced. Segmented continuous flow analysis, devised by Skeggs (9), employs air bubbles to divide the sample stream into many individual portions, effectively preventing longitudinal mixing. The recent publications on analytical systems that employ unsegmented streams show that continuous flow analysis can be accomplished without air bubbles. The considerations described in the next section indicate that the penalty that is paid in unsegmented flow is either increased reagent consumption or greatly increased pumping pressure. PUMPING PRESSURE AND FLOW VOLUMES
An exact description of longitudinal mixing in unsegmented continuous flow analysis systems is beyond the scope of this publication. Lane and Sirs (IO) have shown how a bolus of an indicator will spread under flow conditions not unlike those in the unsegmented analytical systems. The analytical systems are not exactly described by the equations of Lane and Sirs, which apply only to the spreading of a bolus of dye. There is an extra complexity in the analytical systems, where the color does not form until the bolus of sample mixes with the reagent ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
17
