1414 (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978 Y. Takata and G. Muto, Anal. Chem., 45, 1864 (1973). D. G. Swartzfager, Anal. Chem,, 48, 2189 (1976). D. L. Rabenstein and R . Saetre, Anal. Chem., 49, 1036 (1977). R. C. Buchta and L. J. Papa, J . Chromatogr. Sci., 14, 213 (1977). T. Wasa and S. Musha, Bull. Chem. Soc. Jpn., 48, 2176 (1975). R. N. Adams, "Electrochemistty at Soli Electrodes", Marcel Dekker, New York, N.Y., 1969. R. Keller, A. Oke, I. Mefford, and R . N. Adams, Life Sci., 19. 995 (1976). R. E. Panzer and P. J. Elving, J, Nectrochem. Soc., 119, 864. (1972). R. E. Panzer and P. J. Elving, Electrochim. Acta. 20, 635 (1975). 8.Fleet and C. J. Little, J. Chromatogr. Sci., 12, 747 (1974). J. Lankelma and H. Poppe, J. Chromatogr., 125, 375 (1976). J. Randin and E . Yeager, J. €/ectroana/. Chem., 38, 257 (1972). S. Sasa and C . L. Blank, Anal. Chem., 49, 354 (1977). J. F. Evans, T. Kuwana, M. T. Henne, and G. P. Royer, J . Electroanal. Chem., 80, 409 (1977). J. F. Evans and T. Kuwana, Anal. Chem., 49, 1632 (1977). B. D. Epstein, E. Dalle-Molle, and J. S. Mattson, Carbon, 9, 609 (1971). G. Mamantov. D. B. Freeman, F. J. Miller. and H. E. Zittel, J Nectroanal. Chem., 9, 305 (1965). W. J. Biaedel and R. A. Jenkins, Anal. Chem., 47, 1337 (1975).
(21) (22) (23) (24) (25) (26)
I. Morcos and E. Yeager, Electfochim. Acta, 15, 953 (1970). C. L. Blank, J . Chromatogr.. 117, 35 (1976). W J. Koehl, J . Am. Chem. Soc., 88, 4686 (1964). J Randin and E. Yeager, J. Electroanal. Chem., 58, 313 (1975). G. W. Jackson and J. S. Dereska, J. Electrochem. Sac., 112, 1218 (1965). J. P. Zanetta, 0.Vincendon, P. Mandel, and G. Gombos, J. Chromarcgr.. 51, 441 (1970).
RErElVED for review May 1, 1978. Accepted June 16, 1978. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Department of Chemistry, Indiana University, for support of this research. E.C.P. was a participant in the 22nd annual High School Summer Science Institute, Indiana University, Bloomington, Ind., 1977. M.A.D. was a combined Medical-Ph.D. candidate, Indiana University, Rloomington. Ind.
Evaporative Analyzer as a Mass Detector for Liquid Chromatography John M. Charlesworth' Department of Industrial Science, University of Meibourne, Victoria, Australia 3052
A study of the variables which influence the response of the evaporative analyzer has been undertaken. The instrument appears to function adequately as a mass detector In liquid chromatographic applications, provided the solute is considerably less volatile than the solvent at the operating temperature. Furthermore at the normal atomization air pressure of 1.4 X 10' kPa, the calibration curve is very nearly linear for concentrations of solute in the range 1 X to 1.5 X g ~ m - ~Evidence . has been found indicating that in the thlrd zone of the instrument, light is deflected predominantly by reflection and refraction although in the case of solute droplets less than approximately 0.9 pm in radius, Mie scattering is the most likely mechanism. For a model system, calculations have shown that the sum of the intensities of the light reflected and refracted at an angle of 135' to the incident beam is almost Independent of the refractive index of the solute, which in turn explains the experimentally observed approximate independence of response and chemical composition.
During the course of an investigation into the distribution of low molecular weight species produced in the early stages of the formation of diamine-diepoxide network polymers, the need to separate mixtures of structurally complex compounds became apparent. Gel permeation chromatography (GPC) has been used as a highly effective tool for separating moderately low molecular weight material ( 1 ) including compounds similar to those anticipated in the first stages of the reaction ( 2 ) ,but accurate quantitative detection of the separated components presents a problem. The majority of commercially available liquid chromatographic instruments are equipped with one or more of the following detectors ( 3 ) : differential refractometers, conductance bridges. transport 'Current address, Department of Chemical Engineering. University of Melbourne, Victoria, Australia, 3052, 0003-2700/78/0350-14 14$01. O O / O
detectors, fluorimeters, and UV, visible, and IR photometers. Apart from the need to use a noninterfering solvent in several of these, difficulties also arise through the necessity to calibrate the instruments with each of the compounds which are to be rmined. This requirement arises because the sensitivity vary w i t h changes in the chemical structure of each of the eluted components and the response may not always vary linearly with changes in concentration. Bearing these limitations in mind, none of the above detectors could be considered as ideally suited to the task a t hand, because of the considerable difficulties associated with isolating sufficient amounts of each polycondensation product for calibration purposes. However, preliminary observations by Ford and Keniiard (1)indicate that the evaporative analyzer (EA) (5) could provide an acceptable solutioii to this problem. These workers have shown that a variety of low molecular weight polymers produce almost equivalent responses when passed through the instrument, irrespective of their chemical composition. This suggests that, under certain operating conditions, the EA instrument may function as a pure mass detector and as such it may be very suitable for determining the concentrations of compounds which are difficult to isolate for calibration purposes. The aim of this paper is therefore to report the results of some systematic investigations into the factors which might influence the response, with particular reference to the polycondensation products formed by the reaction between diepoxide and diamine monomers. Furthermore, since the mechanism by which the EA detector functions has not been well defined ( 4 ) ,sufficient analysis of the data is presented to enable a working explanation to be postulated for most of the phenomena observed.
EXPERIMENTAL Instrument Description. A diagram illustrating the main features of the EA detector as used in this study is shown in Figure 1. The instrument consists of three zones, the first of which, 3, is a continuous sampler constructed from a stainless steel capillary tube, 1, (0.45-mm i.d, 0 81-mm o.d) carrying the effluent from the GPC columns. This is surrounded by a larger tube, 2, (1.49-mm i d) through which filtered, dried, and pressurized air 'C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
1415
P
:
-
IO
Pressure
CPd x j6'
Flgure 2. Plot of the volumetric air flow rate through the atomizer as a function of pressure __r ~-
-@
Cross section at
0
Figure 1. Illustration of the main features of the evaporative analyzer detection system. (1) capilhry tube, (2) atomizer air inlet, (3) atomization zone, (4) heated wire gauze tube, (5)evaporation zone, (6) photomultiplier, (7) light deflection zone, (8) light source, (9) thermistor, (10) secondary air inlet, (1 1) air outlet, (12) light trap
is forced. The droplets produced by the ensuing Venturi atomization process are carried by a low pressure secondary air supply, also dried and filtered, into the second zone, 5. Here the suspension is passed through a resistance heated wire gauze tube, 4, in order that the volatile material present in the particles may be vaporized. In the final zone, 7, the remaining suspension of the less volatile solute particles is illuminated by a light source, 8, consisting of a standard 6-V projector bulb (Philips) coupled to a voltage stabilized power supply. The light deflected by the particles at an angle of 135" to the incident beam is detected and converted to an electric current by an HTV IP-28 cadmium sulfide photomultiplier, 6. The light which is not deflected is trapped and dissipated by a light horn, 12. The signal from the photomultiplier is amplified by a circuit obtained from an atomic absorption instrument (Varian) and displayed on a chart recorder. In practice the air pressure to the atomizer could be varied up to 2.40 X lo2 kPa and the position of the tip of the capillary tube could also be varied relative to the nozzle of the air tube. Satisfactory atomization was achieved using a pressure of 1.38 X 10' kPa with the capillary tube protruding a distance of 0.5 mm from the air nozzle. A plot of the volumetric air flow rate through the atomizer as a function of pressure for this configuration is shown in Figure 2. From this, the flow rate at the normal operating pressure is determined to be 2.1 X 10 dm3m i d . The temperature of the secondary air flow could also be raised to approximately 150 "C by increasing the voltage to the resistance heated wire gauze tube. Since the eluant used for all the analyses was the relatively volatile solvent, chloroform, this temperature was normally set at 25 "C. The secondary air flow rate was held constant at 15 dm3 min-'. Gel Permeation Column System. The gel permeation column system coupled to the EA detector consisted of five stainless steel tubes (123-cm length, 1.0-cm o.d), each packed with soft, cross-linked polystyrene gel (Waters Associates' Styragel). Columns containing beads with nominal pore sizes of 20 and 10 nm (1:l mixture by volume); 10 nm; 10 and 6 nm (1:l mixture by volume); and 6 nm in duplicate were slurry packed according to a previously published method ( 6 ) . The order of columns as used in the instrument ran from lowest to highest pore size packing. Degassed chloroform was pumped by a Milton-Roy low pressure minipump through the columns at a flow rate of 1.67 X lo-' cm3 sec-l under a pressure of 5.5 X 10' kPa. Pressure fluctuations were removed using a bellows pressure damper, and samples were introduced into the columns by means of a six-port injection valve equipped with a 2 cm3 capacity sample loop. Reagents and Purity. Commercial grade chloroform was fractionally distilled and the material boiling in the range of 61.5 to 62.5 "C was collected. A small amount of AR grade ethanol (0.2% v/v) was added to inhibit the formation of phosgene. The
compound p,p'-diaminodiphenylmethane, obtained as a commercial grade material, was recrystallized using a water/decolorizing charcoal mixture. The purified substance had a melting range of 91-93 "C and microanalysis yielded the following values: C, 78.5%; H, 7.0%; N, 14.1%. Theoretical: C, 78.8%; H, 7.1%; N, 14.1%. The diglycidyl ether of bisphenol A was obtained as a commercial grade liquid (Shell Epon 826) and was purified by solvent/nonsolvent recrystallization from a mixture of methyl ethyl ketone and methanol. The white crystalline product bad a melting range of 40 43 O C and microanalysis yielded the following values: C, 73.7%; H, 7.0%; 0 , 18.9%. Theoretical: C, 74.1%; H, 7.1%; 0, 18.8%. The remainder of the reagents employed in this study were either AR or LR grade compounds which were used without further purification. RESULTS A N D DISCUSSION T h e possibility that several compounds, typical of those anticipated in the polycondensation reaction, produce equal detector response was first investigated. T h e compounds studied were p,p'-diaminodiphenylmethane(DDM), the diglycidyl ether of bisphenol A (DGEBA), the product of the reaction between two moles of N-methyl aniline (NMA) and one mole of DGEBA ((NMA),(DGEBA)), the product of the reaction between two moles of phenyl glycidyl ether (PGE) and one mole of aniline ((ANIL)(PGE),), and the product of the reaction between one mole of NMA and one mole of P G E ((NMA)(PGE)). These materials were injected into the column system as standard solutions in chloroform in the concentration range 7.5 x to 3 x g ~ m - ~T h. e response, defined as peak area, was measured from the trace produced by the chart recorder. All peaks were sharp and symmetrical and no impurity peaks were detected. T h e variation of response with concentration is shown in Figure 3. This reveals that, apart from (ANIL)(PGE),, the responses of the compounds when interpolated to any particular concentration in the indicated range are within approximately i 5 % of each other and the master curve appears to be linear u p to a concentration of approximately 1 x g~m-~. At first glance, the linear portion of the curve appears to extrapolate through the origin, but upon examination of the and 1.5 X response at concentrations between 1.5 x g by amplifying the detector output 25 times it can be seen that below a concentration of approximately 1 x g cm13 the response again varies in a nonlinear fashion (see Figure 4). Thus the calibration curve has a sigmoidal shape, with a very nearly linear section between 1 X lo-* and 1.5 X g A similar sigmoidal shaped calibration curve was also obtained when the solutions were injected directly into the detector. T h e origins of this behavior can best be understood if the light deflection mechanism by which the instrument functions is examined. Light Deflection Mechanism. There are four main processes by which the path of electromagnetic radiation can change direction when passing through a medium containing
1416
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
Table I. Calculated Particle Radii for Selected Atomization Pressures atomization pressure, kPa < 1 0 -
a' /
/
i.. 1
2
-3
3
3
g crnxio
Concentration
Figure 3. Plot of the detector response as a function of concentration, g c d . (0) for concentrations in the range 7.5 X to 3 X DDM, (0)DGEBA, (A)(NMAIADGEBA), (A)(NMAXPGE), (m) (ANILXPGE),
prn
r, r m ( c = f lo--g cm-')
1.33
63.7
5.06
2.76
15.3
6.90
15.3 10.3 5.8
9.66
25.6 20.9
3.59 2.04 1.66
4.7
13.80 17.91
16.6 14.1
1.32
20.70
12.8
3.8 3.0 2.9
D,12,
3
1.11 1.01
rlh
(11AX)
plitudes and phases and the induced oscillating dipoles produce waves which interfere with each other (7). Because of this, the scattered light can have a greater or lesser intensity than comparable Raleigh scattering, depending on the angles a t which observations are carried out. Once the particle size approaches the wavelength of the incident light, then reflection and refraction begin to prevail. These two always occur together and are due simply to the deviations of t h e light quanta as they encounter the boundaries between phases. The sum of the intensities of the light reflected and refracted is equal to the total intensity of the incident light. Clearly, in order to decide which mechanism is responsible for the "scattering" observed in the third zone of the instrument, an estimate of the size of the particles involved compared to the wavelength of the incident light must be made. This is provided by an examination of the theoretical drop size produced in the atomization process as predicted by the equation developed by Nukiyama and Tanasawa (8). This equation has been found to describe the mean drop diameter for a variety of Venturi type atomizers (9), and Atkinson ( I O ) has shown t h a t t h e equation gives good agreement with the experimentally determined drop sizes for an atomizer of similar design to that used in the present study. T h e equation is originally stated as follows
= xn,D3/xn,D2 4
/
L/' ~
I/
CD"CB"ll.tlO"
l:
cm'i,a5
"
Figure 4. Plot of the detector response for DDM solutions in the concentration range 1.5 X to 1.5 X g ~ r n - ~
a suspended particulate phase (7). These are Raleigh scattering, Mie scattering, reflection, and refraction. The importance of each process is governed by the radius of the particles ( r ) compared to the wavelength (A) of the incident light (7). Raleigh scattering is most predominant when the particles are very much smaller than the wavelength of the incident light, i.e. r/A < 5 X lo-'. In this case the incident light quanta induce oscillating dipoles in each particle they strike, and these in turn radiate comparatively low intensity light in all directions. Once the dimensions of the particles are approximately greater than X/20 they no longer behave as point sources and Mie scattering becomes the predominant mechanism. In this process, different points on the same particle are exposed to incident light with a variety of am-
where Do is the mean drop diameter, n, is the number of drops in the size range with diameter D,CJ is the liquid surface tension, p1 is the liquid density, fi is the liquid viscosity, u is the relative velocity between the air stream and the liquid stream, Ql is the volumetric flow rate of the liquid, and Q, is the volumetric flow rate of the air. For chloroform a t 25 "C, CJ is 2.71 x 10 dyne cm-', pl is 1.48 g cm-j, and fi is 1.48 x 10.' poise ( 2 2 ) . Using these data together with the dimensions of the annular gap constituting the air nozzle, the flow rate to the atomizer at each pressure setting (see Figure 2 ) and the solvent pumping rate enables the relative velocity ( u ) to be calculated for a variety of atomizer pressure settings. T h e values of the radii of the chloroform solution particles (D,/2) thereby calculated are listed in Table I. Also listed in this table are the final solute particle radii ( r ) calculated by multiplication of D o / 2 by the factor ( c / p , ) l I 3 where c is the concentration of solute (g ~ m - and ~ ) ps is the density of the solute (g ~ m - ~A) .value of 1 g cm for ps was assumed in these cm-3 calculations and c was fixed a t a typical value of 5 X (see later discussion). T h e tungsten filament lamp used as a light source in the instrument did not produce monochromatic light but rather a distribution of wavelengths. According to the manufacturers specifications, the filament temperature for the applied potential of 6 V should be approximately 2000 K. The maximum emissivity of tungsten at this temperature occurs
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
1417
Table 11. Calculated Particle Radii for Selected Solute Concentrations m - ~i-, p m (pressure = x 10‘ 1.38 x 10’ kPa)
c, g ~
0.5 2.5 7.5 20.0 100.0 200.0
0.29 0.49 0.70 0.97 1.66 2.10 2.40
300.0
r/h
(MAX) 0.81 1.39 2.01 2.78 4.75 5.99 6.85
Table 111. Experimental Response of DGEBAa and DDM‘ Solutions for Selected Atomization Pressures atomization DDM response, atomization DGEBA response, pressure, cm2 ( c = 5 x pressure, cm2 ( c = 5 x kPa x lo-’ g cm-’) kPa x lo-’ g cm-’) 2.07 3.45 5.18 6.90 8.63
1.8
3.1 4.5 5.8 6.5
1.38 2.76 4.14 5.52 6.90 8.28 9.66 11.04 13.80 15.18 16.56 17.94 20.70
1.0 2.5 3.7
4.8 5.6 10.35 6.8 6.1 13.80 6.8 6.3 17.25 6.8 6.6 20.70 6.2 6.8 24.15 6.1 6.7 6.6 6.4 6.1 DDM = p,p’-diaminodiphenylmethane. DGEBA = diglycidyl ether of bisphenol A. a t a wavelength (X(MAX)) of 0.35 pm (11) and the ratios of t h e solute particle sizes to this wavelength are also listed in Table I. In addition to altering the particle size by varying the gas velocity, it is also possible to change their size by varying the initial solute concentration. Table I1 lists values of r/h(MAX) calculated for solute concentrations ranging from 5 x lo4 to 3X g cm-3 a t the normal operating pressure of 1.38 X 10’ kPa. I t is evident from each of these tables that within the range of concentrations and pressures examined, r is approximately equal to or greater than h(MAX). This suggests t h a t t h e “scattering” is most probably due to reflection and refraction for all but the smallest particles. T h e experimentally measured dependence of response on air pressure is shown in Table I11 for DGEBA and DDM g ~ m - These ~. solutions, each a t a concentration of 5 X values divided by the concentration (defined as sensitivity) are plotted in Figure 5 against values of r/X(MAX) calculated a t each pressure setting. Also included in the Figure 5 are the responses of the DDM solutions shown in Figures 3 and 4 divided by their respective concentrations. I t is clear from Figure 5 that all the points lie close to a single curve, thereby indicating that changes in the solute concentration and variations in the atomizer air pressure influence the final solute particle size in the same manner. Furthermore it is evident that the instrumental sensitivity passes through a maximum around a value of r/X(MAX) equal to 4, and declines rapidly above and below approximate values of 5 and 2.5, respectively. I t would therefore appear that for r/h(MAX)
