900
/
I
/'
600 r
1 0
io
20
30
-
TIME ( m i n l
'k"
Ib
2-
20
30
40
50
60
70
6Cl
CONCENTRATION ( pg/ml)
Figure 5. Graph illustrating response of UV detector to 4-oxide (solid
Figure 6. Chromatogram of lake water extracts Water spiked at concentration levels of 47 ng/ml 4-oxide ( l ) , 41 ng/ml 4.4dioxide (2), and 34 ng/ml Vitavax (3). Peak 4 is of unknown identity and was also present in blank
line) and 4,4-dioxide(dashed line) Figure 4 shows calibration curves for Vitavax as determined for the UV and fluorescence detection systems. The plots are quite linear over the range 1-57 pg/ml (correlation coefficients both greater than 0.99). Similarly Figure 5 shows that the UV detection exhibited good linearity (correlation coefficients greater than 0.999) for the photoproducts of Vitavax over the range of 1 to about 80 wg/ml. Thus, Vitavax and its photoproducts could be detected at concentrations lower than 1 pglml without prior concentration of the sample. Figure 6 shows a sample of lake water spiked with the three compounds under study. With the procedure employed, recoveries of about 70% were obtained (compare Figures 2 and 6). As can be seen, with suitable pre-concentration techniques, as little as about 20 ng/ml of Vitavax and about 5 ng/ml of the sulfone and sulfoxide can be detected fairly easily.
Work is presently proceeding on the separation and detection of compounds similar in structure to Vitavax.
LITERATURE CITED (1) M. C. Bowman and M. Beroza, J. Assoc. Off. Agric. Chem., 52, 1054 (1969). (2) H. G. Henkel, Chimia (Aarau), 18, 128 (1965). (3) P. 9. Baker and R. A. Hoodless, J. Chromatogr., 87, 585 (1973). (4) J. P. Frawley, J. W. Cook, R. Blake, and 0. G. Fitzhugh, J. Agric. Food Chem., 6, 28 (1958). (5) J. R. Lane, J. Agric. FoodChem., 18, 409 (1970). (6) R. K. Tripathi and G. Bhaktavasalam, J. Chromatogr., 87, 283 (1973). (7) W. T. Chin, G.M. Stone, and A. E. Smith, J. Agric. FoodChem., 18, 709 (1970). (8) W. T. Chin, 9. M. Stone, and A. E. Smith, J. Agric. FoodChem., 18, 731 (1970). (9) F. I. Onuska and M. E. Comba, J. Chromatogr., 100, 247 (1974). (10) A. W. Wolkoff and R. H. Larose, J. Chromatogr., 99, 731 (1974). ( 1 1 ) F. I. Onuska and M. E. Comba, Burlington. Ontario, un'published work 1974.
RECEIVEDfor review August 1, 1974. Accepted December 16, 1974.
Basicity of Aromatic Amines from Liquid Chromatographic Behavior Philip I?.Young NASA-Langley Research Center, Hampton, Va. 23665
H. M. McNair Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Va. 2406 1
Isomeric aromatic diamines are becoming increasingly important in studying the effects of chemical structure on the properties of polyimides ( I , 2), polyamides ( 3 ) ,epoxies ( 4 ) , and urethanes ( 5 ) .For example, glass transition temperatures can be significantly reduced without sacrificing thermal stability by polymerizing aromatic dianhydrides and diacid chlorides with meta- and perhaps ortho-oriented diamines instead of more common para-oriented monomers ( I , 2). 756
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
The importance of the reactivity or basicity of the amino group to the molecular weight of the resultant polymer has also been noted ( I , 3, 6). As a general rule, the more basic the amine group, the more reactive it is toward other monomers. Seven isomeric diamines have recently been synthesized in our laboratory where Z = C=O and five isomers
Table I. Properties of Column Packing M a t e r i a l s P a r t i c l e sire,
Surface a r e a ,
u
mZ'g
Type of s i l i c a
Name
Corasil I Corasil I1 Zorbax Si1 Porasil A Porasil B Porasil C
Porous Porous Porous Porous Porous Porous
layer beads layer beads microspheres beads beads beads
37-50 37-50 6-8 3 7-7 5 3 7-7 5 3 7-7 5
7 14 200+ 350-500 1 2 5-2 50 50-100
Table 11. S u m m a r y of Capacity F a c t o r Data and pKb Values Coras,l I
Isomer
Corasil I1
Zorbax Si1
0.489 0.616 0.840
0.756 0.886 1.179
"b
o-Phenylenediamine nz-Phenylenediamine p- Phenylenediamine o - Bromoaniline nz-Bromoaniline p-Bromoaniline o -Chloroaniline n?-Chloroaniline p-C hloroaniline
9.523 -0.085 9 .ooo 0.025 7.854 0.301 11.523 0.383 0.876 10.398 1.067 10.155 11.301 0.442 0.886 10.523 1.092 10.000 Porasil A
o-Toluidine nz-Toluidine p-Toluidine
9.585 9.301 8.921
-0.051 0.058 0.182
... ... ... ...
... ...
t
t
5.0
lU.0 TIME
Log of c a p a c i t y factor
I
I
0
15.0
20.0
lllLUTES
Figure 1. Separation of phenylenediamine isomers Column, Zorbax Sil, 25-cm X 2.1-mm i.d. stainless steel; mobile phase, 0.7% (v/v) methanol in chloroform; flow, 0.5 ml/min; pressure, 1600 psi; temperature, ambient; detector, UV; sample, 2 1 of 1.0 mg/ml of each isomer in chloroform
...
... ... ... ... ...
Porasil B
Porasil C
-0.126 0.081 0.334
-0.425 -0.305 -0.143
where Z = CHa. Additional series are being studied having a wide range of interconnecting Z groups. T h e direct measurement of Kh values for these isomers is not always satisfactory because it is often time consuming and lacks specificity. Thus, other techniques are needed which will help establish the relative basicity for series of diamines differing only in the point of attachment of the amino group or the interconnecting Z group. It is generally accepted that the adsorption of many basic organic compounds on silica gel may be treated as bases interacting with a n acidic surface. As part of a research effort on new monomers for polyimide synthesis, a liquid chromatographic investigation was conducted to determine if the adsorption of weakly basic aromatic amines on slightly acidic silica gel adsorbents could be used t o study their relative basicity. Liquid chromatography was employed because it provided a stable environment for these reactive monomers. The effect of sample structure on chromatographic behavior has been extensively investigated (7). However, with the possible exception of one thin layer chromatographic study ( 8 ) ,the correlation of sample basicity with retention time has received limited attention. Before complex monomers could be understood, it was necessary t o study the adsorption of simple aromatic amines on silica. Four series of amines, o-, m-, and p-bromoanilines, chloroanilines, toluidines, and phenylenediamines, were chromatographed under various conditions on several silica gel supports. Retention data were then correlated with respective pKh values. Under the proper conditions, a h e a r correlation between pKb and log of capacity factor was observed. This paper reports on the preliminary results of this study and discusses some of its implications. EXPERIMENTAL Apparatus. A Waters Associates Model ALC 202/R401 Liquid Chromatograph (Waters Associates, Milford, Mass.) with a Model 6000 Solvent Delivery System was used throughout this study.
Only the 254-nm fixed wavelength ultraviolet detector was employed. The chromatograph was operated under ambient temperature conditions. A 25-11 Series "B-110" Pressure-Lok Liquid Syringe (Precision Sampling Corp., Baton Rouge, La.) was used t o inject the sample. The chromatograms were recorded on a HewlettPackard 7100B Strip Chart Recorder at a 0.2 in./min chart speed. Columns. The Corasil I, Corasil 11, Porasil A, Porasil B, and Porasil C adsorbents (Waters Associates) were packed in 61-cm X 2.4-mm i.d. stainless steel columns fitted on the injection port end .with a silanized glass wool plug and on the detector end with a 5micron stainless steel frit. The columns were packed by the modified tap-fill technique (9). Low dead volume end-fittings were fabricated which made the pre-packed 25-cm X 2.1-mm i.d. stainless steel Zorbax Si1 column (E. I. du Pont de Nemours & Co., Instrument Product Division, Wilmington, Del.) compatible with the liquid chromatograph. The holdup time of each column was determined by injecting carbon tetrachloride into the mobile phase. Reagents. The o-, m-, and p - bromoaniline, chloroaniline, toluidine, and the m-phenylenediamine isomers were used as received from the supplier. The 0- and p-phenylenediamine isomers were recrystallized with charcoal treatment from ethanol and water, respectively. The reagent grade mobile phases were filtered prior to use. Procedure. Approximately 1 mg of each isomer within a series was dissolved in 1 ml of the mobile phase. The concentrations were adjusted to maintain approximately equal peak heights. Injections of 2-3 pl of each mixture were made and the identity of each peak was determined from retention time data. The capacity factor h' for each isomer was then calculated from the equation k' = It, t o ) / t o , where t , is the sample retention time and t o is the holdup time.
RESULTS A N D DISCUSSION T h e chromatographic conditions which provided a satisfactory separation in a reasonable time for each mixture were determined by changing the polarity of the mobile phase for one of the adsorbents and then, if necessary, repeating the process with a n adsorbent of higher or lower surface area as appropriate. A description of each adsorbent is given in Table I. The chromatogram obtained for the phenylenediamine isomers on Zorbax Si1 with a 0.7 per cent methanol in chloroform mobile phase is given in Figure 1. This figure is typical of all other chromatograms obtained with chloroform and cyclohexane as the mobile phases. In each case, the order of elution was ortho, meta, para. Table I1 lists pKb values ( 1 0 ) and summarizes the retention data. Figure 2 gives a plot of pKb us. log of capacity factor for the phenylenediamine isomers on Corasils I and 11, and Zorbax Sil. I t also lists the chromatographic conditions for the separation. Figure 3 gives a similar plot for the toluiANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
757
w.
Zorhax SI1
L
12 w
--
11 m
rn,
10 00 PKb
i 30
Br
CI 800
X
,050
0 130
0 53
:X
1.50
7 00
-1W
050
ow
050
100
150
1% k
Log r:
Figure 2. Plot of pKb vs. log of capacity factor for phenylenediamine isomers on three different adsorbents
Figure 4. Plot of pKb vs. log of capacity factor for chloroaniline and bromoaniline isomers
Conditions: Corasil I mobile phase, chloroform; flow, 2.0 ml/min; pressure, 1000 psi; Zorbax Si1 mobile phase, 0.7% (v/v) methanol in chloroform; flow, 0.5 ml/min; pressure, 1600 psi; Corasil I1 mobile phase, chloroform: flow, 2.0 ml/min; pressure 1000 psi
Conditions: Corasil I mobile phase, cyclohexane: flow, 2.0 ml/rnin; pressure, 1400 psi
11
w Poraril A
"2
10.03 PKb
9.w
8W
iW
1 IW
A -050
OW
050
1W
150
tog k'
Figure 3. Plot of pKb vs. log of capacity factor for toluidine isomers on three different adsorbents Conditions: Porasils A, B, and C mobile phase, 75/25 (v/v) chloroform/cyclohexane; flow, 0.5 ml/mtn: pressure, (A) 400 psi, (B) and (C) 350 psi
dine isomers on Porasils A, B, and C. Data for the bromoand chloroaniline isomers on Corasil I are given in Figure 4. Very few differences are noted in pKb values or capacity factors for the corresponding aniline isomers and a straight line fits all six points. In each of these cases, a direct correlation exists between amine basicity and chromatographic retention. This is probably explained on the basis of weakly basic amino groups interacting with slightly acidic hydroxyl groups on the surface of the column adsorbent. The more basic the amine, the greater this acid-base interaction and the longer the retention time. Therefore, the least basic ortho isomer elutes first, followed by the intermediate basic meta isomer, and finally the most basic para isomer elutes last. While this order of elution was not too surprising, the linear and empirical correlation between pKb and log of capacity factor was not anticipated. All of this suggests that the relative basicities between similar diamine isomers might be established simply from chromatographic retention data. This assumes, of course, that the amino groups are the only parts of the molecule to enter into the partitioning process. Recently, the separation of the toluidine and chloroaniline isomers was reported by Kunzru and Frei ( 1 1 ) . The purpose of their work was to study the retention characteristics and selectivities of various groups such as amines on cadmium-impregnated silica gel columns. Although a simi758
A N A L Y T I C A L C H E M I S T R Y , VOL. 47, NO. 4, APRIL 1975
lar order of elution was observed in their work, they reported that basicity was not the only factor determining the partitioning process for those particular columns. However, the present study suggests that basicity is the predominant factor when unaltered silica gel columns are used. This work did not clearly establish whether a correlation exists between different series of amines. The bromo- and chloroaniline isomers were separated under the same conditions, while an entirely different set was required to separate the more basic toluidine isomers. This indicates that monoamines with similar basicities should be separated under similar conditions. The phenylenediamine isomers, which have two active sites for interaction with the adsorbent, require a third set of chromatographic conditions. However, other isomeric diamines with similar basicities should separate under the same conditions. Except for the toluidine isomers on Porasil B, the retention volume was also related to adsorbent surface area. For example, in Figure 2, it is noted that the phenylenediamine isomers take longer to elute on Corasil I1 than on Corasil I. This can be understood since there are more active -OH sites available on the higher surface area Corasil 11. The Zorbax Si1 curve would undoubtedly lie far beyond the Corasil curves if all columns had been the same size. Both Corasil columns were 61-cm X 2.4-mm i.d. while the Zorbax Si1 column was only 25-cm X 2.1-mm i.d. The toluidine isomers, Figure 3, were not retained on Corasils I or I1 or Zorbax Si1 with chloroform as the mobile phase. However, the high surcace area Porasil absorbents and a less polar mobile phase (75/25 chloroform/cyclohexane) allowed these isomers to be separated. Finally, the bromo- and chloroaniline isomers, Figure 4, would not elute in a reasonable period of time on Corasil 11, Zorbax Sil, or the Porasils. Corasil I, a lower surface area adsorbent, was required. Separations can therefore be made by selecting the proper adsorbent area or by changing the mobile phase polarity. These two variables are most important in determining the slopes of the lines in Figures 2-4 and can be manipulated to improve resolution. An exception to this linear correlation of basicity and chromatographic retention was observed when ethyl acetate was used as the mobile phase. When the phenylenediamine isomers were chromatographed on Corasil 11, the order of elution was meta, ortho, para. Apparently, the more polar solvent competed for sites on the adsorbent and altered the partitioning process. Under the proper liquidliquid chromatographic conditions, the order of elution can
be made to be ortho, para, meta (12). This type of behavior is not uncommon ( 7 ) and emphasizes that there are conditions where a linear correlation between basicity and retention will not hold. However, it does hold for the amines studied here when chloroform and cyclohexane are the mobile phases.
CONCLUSION While this study was limited in scope, it does indicate that chromatographic conditions can be found which allow a direct correlation between pKb and log of capacity factor for aromatic amines. It established that the relative basicit y of some aromatic diamines can be determined from liquid chromatographic data. This may prove useful in helping to predict the relative basicity of closely related aromatic diamines, especially new amines being synthesized for polymer synthesis. This study also suggests that similar correlations may exist for aliphatic amines and even aromatic and aliphatic acids on slightly basic adsorbents. A
further insight into the separation mechanism of aromatic amines on silica adsorbents has also been obtained.
LITERATURE CITED (1)V. L. Bell, Org. Coatings Plastics Cbern., Prepr., 33 (I),153 (1973). (2)R . A. Dine-Hart and W. W. Wright, Makromol. Chem., 153, 237 (1972). (3)V. Guidofti and N. J. Johnston, Preprints, Amer. Chern. SOC.,Div. Po/ym. Chem., 15 (I),570 (1974). (4)H. Lee and K. Neville, "Handbook of Epoxy Resins,'' McGraw-Hill. New York, N.Y., 1967,Chap. 21,p 25. (5)C. V. Cagle, "Handbook of Adhesive Bonding," McGraw-Hill, New York. N.Y.. 1973,Chap. 9,p 4. (6) R. A. Dine-Hart and W. W. Wright, J. Appl. Polym. Sci.. 11, 609 (1967). (7)L. R. Snyder, "Principles of Adsorption Chromatography," Marcel Dekker, New York, N.Y., 1968,pp 257-334. (8) M. Przyborowska and E. Soczewinski, J. Chromafogr.. 42,516 (1969). (9)J. J. Kirkland. J. Chromatogr. Sci., 10, 129 (1972). (IO)R . T. Morrison and R. N. Boyd, "Organic Chemistry," 3rd ed , Allyn and Bacon, Boston, Mass.. 1973,p 730. (11)D. Kunzru and R. W. Frei. J. Chromatogr. Sci.. 12, 191 (1974). (12)Chromatographic Methods No. 820M6,E. I. du Pont de Nemours & Co., Inc., Instrument Product Division, Wilmington, Del.. March 30,1974,p 4.
RECEIVEDfor review August 15,1974. Accepted December 9, 1974.
A PDP 11 Computer System for the Multiterminal Processing of Several Analytical Instruments in an Interpretative Language Torbjorn Anfalt and Daniel Jagner Department of Analytical Chemistry, University of Goteborg, F a c k
S-40220 Goteborg 5,Sweden
Until recently, automation of analytical instruments by means of computers has mainly been concerned with instruments delivering data a t a very high speed, e g . , mass spectrometers and NMR instruments. Because of the constantly decreasing prices of minicomputers, it is, however, now justifiable to automate almost any analytical instrument. Most analytical instruments, e.g., titrators, spectrophotometers and gas chromatographs, deliver real time data a t a moderate speed, often less than 50 Hz; moreover, after the first reduction, the number of data is not very large (usually of the order of magnitude of less than lo?)). The limited number of data produced in each analysis means that it is not necessary to add an external memory to the system when automating these instruments, which considerably reduces the price. The slow flow of data makes it possible to use an interpretative language in the processing of the instruments, thus drastically decreasing programming difficulties. This is of particular advantage in teaching and in research laboratories where there is a need for frequent reprogramming. If, however, each of the above-mentioned instruments is assigned its own minicomputer, the full capacity of the computer is not exploited, since any minicomputer is capable of processing several such instruments. Simultaneous processing of entirely different analytical instruments, independently of one another has, however, not hitherto been considered in an interpretative language such as BASIC. This paper describes a soft-ware development on a P D P 11 which facilitates the simultaneous use of a maximum of eight processing terminals, all terminals being programmed in BASIC. Each processing terminal consists of an input/ output teletype unit, an A/D converter, a random mode analog multiplexer, and eight random mode programmable relays. Such a terminal is not only sufficient for the interfacing of the analog signals coming from most analytical instruments, but is also capable of processing most analytical
procedures, the programmable relays being used, for example, to process wavelength adjustments of a spectrophotometer by means of a step-motor.
HARDWARE DESCRIPTION The Computer. A diagram of the computer set-up is shown in Figure 1. The computer is a Digital Equipment P D P 11/10 with a 16-K core memory, the word size being 16 bits. All peripherals are connected to the computer unibus, i.e., they all have unique core memory addresses ( I ) . Real-time is obtained by means of a line-frequency clock, the time duration for one increment being 20 msec a t 50 Hz. The paper tape reader and punch (Digital Equipment PC-11) have maximum speeds of 300 and 50 characters/sec, respectively. Hitherto, only three processing terminals have been connected to the system ( c f . Fig. 1).A fourth terminal has been used for computation only. The Processing Terminals. The configuration of the terminals is shown in Figure 1. Input and output operation a t each processing terminal is facilitated by means of teletypes ASR 33 or Olivetti TE-308, both having a maximum speed of 10 charactershec. The terminals have access to the tape reader and punch of the computer to enable more rapid reading and punching. Each terminal has an integrating voltmeter as an A/D converter, Fluke 8300 or Fluke 8200, with maximum conversion speeds of 0.3 and 400 readingdsec, respectively, in the millivolt range. All voltmeters are equipped with automatic ranging, remote control, and isolated data output. The resolution is 1 p V and the input impedances are close to 10000 M a . Each terminal has a random mode scanner, facilitating measurements on eight different analog inputs, and a short-circuiting relay register, facilit,ating the random mode processing of eight different electromechanical devices. Scanners and relays a t all terminals are processed by ANALYTICAL CHEMISTRY, VOL. 47. NO. 4, APRIL 1975
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