LITERATURE CITED (1) J. G. O'Connor and M. S . Norris, Anal. Chem., 32, 701 (1960). (2) J. G. O'Connor, F. H. Burow, and M. S. Norris, Anal. Chem., 34,82 (1962).
(5) V. R. Sista et al., IIP Internal Report No. 20.008(unpublished). (6)"Modern Petroleum Technology",Institute of Petroleum,London, 1962,p 438.
Determination of Aromatic Amines by an Adsorption Technique with Flame Ionization Gas Chromatography Barry E. Bowen Polymer Intermediates Department, E. 1. du Pont de Nemours and Company, Experimental Station Laboratory, Wilmington, Del. 19898
Quantitative adsorption and desorption of aromatic amines using Tenax GC has been demonstrated at the nanogram level. Linear calibration data were obtained over two to four decades and detailed response curves are reported for the first time. The sensitivity of an adsorption-FlGC technique to detect sub-ppm levels of aromatic amines in organic or aqueous solutions or in air is shown to approach that achieved by electron capture and nitrogen specific detectors.
The toxicity of aromatic amines to man has been studied for many years ( I ) . Recently, the Occupational Safety and Health Organization (OSHA) established exposure limits for 14 human carcinogens (2, 3 ) . This list includes 4-aminodiphenyl and several other amines. Numerous other aromatic amines are cancer-suspect agents. Secondary and tertiary amines, when reacted with nitrous acid, yield nitrosamines, another cancer-suspect class of compounds. Numerous workers have recently published trapping and/or detection schemes for aromatic amines, using GC, LC, or TLC, sometimes coupled with ancillary techniques like IR and UV spectrometry (4-15). Tenax GC has been found to be a good general purpose adsorbent for efficiently adsorbing and desorbing organics. This study presents a general analytical technique for detection and quantitation of trace aromatic amines in dilute solutions, in air, and in solids. Well-defined limits of detection and linearity ranges, work not previously reported, for aromatic amines up to bp 300 "C have been established for our system which uses only commercially available components. Tenax GC, a 2,6-diphenyl-p-phenylene oxide solid GC support stable to 375 "C, was used to absorb submicrogram amounts of aromatic amines from dilute organic and aqueous solutions. The solvent was vented using a heated injection valve. The amines were then desorbed and backflushed onto a glass Silar 1OC GC column, then detected by flame ionization.
EXPERIMENTAL The amines listed in Table I were obtained from various chemical suppliers. They were purified by crystallization and sublimation to at least 99 mol % purity. A Model 1047 Concentrator (Chromalytics Corp., Unionville, Pa.) was used to concentrate the amines either by trapping with 15 cm X 0.5 cm 0.d. glass sampling tubes filled with 8 cm of adsorbent or by trapping with 30 cm X 3/s in. 0.d. stainless steel U-tube filled with 10 cm of adsorbent. Adsorbents tried were Tenax GC, Porapak QS, Chromosorb 103,and Silar 1OC on Chromosorb WHP obtained from Applied Science Laboratories, Inc., State College, Pa. 1584
The Model 1047 is a heated gas sampling valve that attaches to many commercial gas chromatographs. The device permits normal injection onto the GC column or concentration of a dilute gaseous or liquid sample by injection onto the U-trap. A sampling tube, used for collection of airborne amines in a remote location, can be attached to the unit. The amines are desorbed from the adsorbent by rapid heating and backflushing onto the GC column. The temperature of the concentrator oven is limited to 250 "C by the internal Teflon seals. The temperature of the adsorption tube or trap can be controlled separately and can be held at "raised ambient" levels for venting solvents. The rate of temperature rise of the sampling tube or U-trap, for rapid desorption, is obtained from a variable potentiometer and is nominally set at 1-5 O C / s . Several other sample concentrators are commerciallyavailable and adaptable to GC work besides the Model 1047 (Chromalytics Corp., Unionville, Pa. 19375)used in the work reported here. They include the Single Hollow Fiber Concentrator for liquids and the Model 105 T for gases (MDS Scientific, Inc., Park Ridge, I11 60068),the DC-50 series (Environtech. Dohrmann Division, Santa Clara, Calif. 95050), the Model LSC-1 (Tekmar Co., Cincinnati, Ohio 45222) and the Bendix Personnel Monitoring Collection Column with accessories (National Environmental, Inc., Warwick, R.I. 02888). Silar 1OC (Applied Science Laboratories, Inc., State College, Pa.), a polar liquid phase stable to 250-275 "C, was coated at 8%by weight onto Chromosorb WHP by slowlywithdrawingthe solvent chloroform using a rotary vacuum apparatus. Single 6 f t X 6 mm 0.d. X 2 mm i.d. glass columns were placed into a Perkin-Elmer 3920 gas chromatograph equipped with a flame ionization detector. The hydrogedair pressure was optimized to 16/45 psi using the methane bleed method. Chromatographicdata were collected by the Du Pont Experimental Station Real Time Computer System (16).
RESULTS Linearity of Response. Dilute xylene solutions of the aromatic amines in Table I (to be used as a glossary for abbreviations in the discussion below) were adsorbed onto the Tenax U-trap. The xylene was vented and the amines were desorbed. Our data show that most of these amines can be quantitatively adsorbed and desorbed from Tenax GC. Figures 1and 2 show that response of some amines for our analytical system is linear in the 10-104 ng range. Silar 1OC on Chromosorb WHP gives sharp well-defined peaks (Figure 3), so that peak height response varies linearly with the amount injected, similar to area response. Our original work with Tenax GC as the analytical column proved to be less satisfactory, since peaks were broad and higher mol wt amines could not be eluted within a reasonable time period. Diamines (OPD, PPD) are apparently "irreversibly" adsorbed to some degree on the Tenax trap below the 500-ng level, since the response curves take on a new slope near that point. Similar behavior was observed in this laboratory using
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
~~
~~~
~~~~
~
Table I. Aromatic Amines Desorbed by Tenax GC Abbreviation
Name
Molwt
Bp,'Ca
AN Aniline 93 184 2-ADP 2-Aminodiphenyl 169 299 254'35 3-ADP 3-Aminodiphenyl 169 19115 4-Aminodiphenyl 4-ADP 169 169 302 DPA Diphenylamine 3 0 4dec 138 rn-Nitroaniline MNA MPD 108 282 m-Phenylenediamine OAAB 197 o-Aminoazobenzene ... ONA 138 o-Nitroaniline 284 108 o-Phenylenediamine 256 OPD 197 PAAB p-Aminoazobenzene > 360 332 138 PNA p-Nitroaniline PPD p-Phenylenediamine 267 108 a At 760 mm Hg, except where noted by superscript.
0
4-ADP
0 OPD
10
' " " " ' ~
'
'
I
'
'
'
-
'
'
t
I
v PAAB 0
lo-'
'
IO
'
'
' ' 1 8 "
lo*
'
' '
ng
io3
PPD '
'
' ' "'
io4
Figure 2. Peak height response of aromatic amines desorbed from Tenax GC adsorbent using the Chromalytics Model 1047 Concentrator with a 90-s purge (150 OC) of the solvent xylene from a 2-pl injection The lowest useful peak height is approximately 1 chart division on FlGC range 100 X 1. The best straight line fits for the 4-ADP, PAAB. PPD, and OPD data are shown
i
4-ADP
/I
0
v PAAB 0
PPD
Figure 1. Peak area response of aromatic amines desorbed from Tenax GC adsorbent using the Chromalytics Model 1047 Concentrator with a 90-s purge (150 O C ) of the solvent xylene from a 2 4 injection The lowest useful computer area is approximately 0.01 mV-min on FlGC range 100. The best straight line fits for the 4-ADP, PAAB, PPD, and OPD data are shown
the conventional column for amines, Carbowax 20 M-KOH, and also on mixed SE30/Silar 5CP columns. The diamines chromatographed on Silar 1OC alone gave slightly more linear response curves than when incorporating the Tenax trap. Ideally, parallel lines of 45" slope would be obtained for all compound responses on a log-log plot. The distance between the lines would represent the difference in relative response factors, commonly employed in quantitative GC. This work showed a f 5 % reproducibility over a three-month period for the response factors of most aromatic amines studied using the Tenax trap-Silar 1OC combination. Essentially identical chromatograms and response curves were obtained when xylene solutions were desorbed from a Tenax sampling tube onto the Tenax U-trap and then onto the analytical column. This sequence of events simulates capture of airborne materials on remote sampling tubes, followed by quantitative FIGC analysis. Other sampling tube adsorbents tried (Porapak QS,Chromosorb 103, and Silar 1OC on Chromosorb 750) offered no particular advantage over Tenax GC. Silar 1OC in the sampling tube, as well as in the trap, eliminated the characteristic pattern of small peaks obtained in the scan after rapidly heating Tenax. The carrier gas was routinely cleaned using a pre-trap made of Tenax, so
4
8
I2 I6 20 24 28 RETENTION TIME j minutes1
3'2
36
4b
Figure 3. Aromatic amines chromatographed on a 6 4 glass Silar 1OC column with flame ionization detection The 2-pl sample volume, containing about 1 pg of each amine, was injected onto a 15-cm Tenax sampling tube, transferred to the Tenax U-trap, and then thermally desorbed. Program rate 4'/min from 120 to 260 'C. Range 100 X 4. Detector P-E 3920 FID
that hydrocarbon impurities were not concentrated a t the head of'the analytical trap. The peaks from Tenax posed no interference problem above the 10-20 ng level for the amines studied.
APPLICATIONS Aqueous Waste Streams. Figure 4 illustrates the desorption of 2-ADP using 100-500 pl injections of dilute aqueous solutions onto the Tenax trap. Water is vented quantitatively in 2 min a t 130 "C. This concentration technique allows up to 500-fold improvement in sensitivity over the normal 1-2 ~1 injection for a FID, such that the practical range is extended t o 10-100 ng (20 ppb-2 ppm). Aqueous waste streams with
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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-
Y
.
n w
-
IO
I
I
I02
103
ng
4
-1
Figure 4. Peak area and peak height response of 2-ADP from pI injections of aqueous solutions onto the Tenax U-trap
Water was purged at 130 ‘C for 2 min and then 2-ADP was thermally desorbed onto a Silar 1OC GC column and detected by a P-E 3920 FID. Range 100 for area and 100 X 1 for peak height
ppb levels of aromatic amines were analyzed directly, with no sample treatment in many cases. Ambient Air. Five Tenax air sampling tubes were prepared having a pressure drop of 42.4 cm HgO (f1.8%relative, 2u) for a 1.3 l./min sampling rate. They were used to monitor for aromatic amines in air near a distillation unit. PPD, 2-ADP, DPA, and 4-ADP were not detected a t the 0.5-ppb level after sampling for 1-2 h at 1.3 l./min. Figure 5 ( A )shows the presence of 1.3 ppb aniline in a sampled working atmosphere in contrast to the “blank” ( B )obtained by heating a clean Tenax sampling tube. This particular scan ( 5 B )is the second rapid heating cycle of the sampling tube used for the Figure 5A experiment, showing that aniline is completely desorbed, but numerous small peaks are produced by thermal degradation of Tenax. With the demonstrated analytical sensitivity of the adsorption-FIGC technique (Figures 1 and 2), 1ppb of any of these amines could be detected and quantitated in less than 1 h by sampling 20 1. of air. I t may be necessary to use less absorbent in order to conform with the controllable flow rates of existing personnel monitoring pumps. Impurities i n Solid Reagent. The limit of detection for 2-ADP, DPA, and 4-ADP as impurities in P P D was found to be 0.2 ppm by using an extraction-adsorption technique. P P D dissolved in water and extracted with xylene will quantitatively transfer these three aromatic amines into the xylene layer when present in the solid in the 0.1-100 ppm range. Large injections of the xylene solution onto the Tenax trap and FID detection gave limits of detection equal to those obtained by amine derivatization followed by 3-pl injections into a Silar 5CP column with electron capture detection (Hewlett-Packard, Model 18713A, Ni). Vapor P r e s s u r e Measurements. Work is continuing in this laboratory to determine vapor pressures of solids amines. The technique is to saturate a flowing gas stream with the amine. The vaporized amine is collected on a Tenax sampling tube and then desorbed giving a GC peak area which can be related to ng of amine collected and, thus, to the concentration in the flowing stream. It should be possible to determine vapor pressures for many solid aromatic amines, as well as for other relatively nonvolatile compounds, at ambient conditions, data which are extremely sparse in the literature. With the 10-ng FID sensitivity, a gas stream saturated with a compound of mol w t 200 and vapor pressure of 0.01 ppm (8 X mm Hgo) would only have to be sampled for 100 min at 1cm3/min. This 1586
4
100-500
Q 11
16
2b
24
21*
RETENTION TIME (minutes)
Figure 5. ( A ) Chromatogram of the volatiles, desorbed from 8 cm of Tenax adsorbent,from an actual working atmosphere. Sample size was 42 I. at 1.3 I./min. Program rate 8’/min from 120 to 260 O C . Range 100 X 1. Detector P-E 3920 FID. (B)Chromatogram of a “blank” Tenax sampling tube (no amines present), cycled with the same GC conditions as for scan A
would eliminate much of the tedium associated with conventional vapor pressure techniques. Comparison to Specific Detectors. The above adsorption-FIGC technique is not specific for aromatic amines; however, it does compare favorably with the sensitivity often obtained when using specific detectors. Convenience and reproducibility are often important considerations for scouting a wide range of analytical problems as well as for repetitive analyses. Our laboratory optimized the sensitivity of electron capture detection (ECD) of aromatic amine derivatives (heptafluorobutyrates) on a glass Silar 5CP GC column to about 5 ppb in hydrocarbon solvents. Our practical limits of detection for trace aromatic amines were 5 ppb in air using a toluene scrubber and 100 ppb in solid amine reagent using xylene extraction. We were unable to devise a suitable derivatization scheme for water analysis. Workers in this laboratory demonstrated a 5-ppb sensitivity for 2-ADP and 4-ADP in water using a Du Pont Model 836 fluorescence/absorbance LC detector (17). The improvement in sensitivity was mainly due to the increased selectivity afforded by selecting the optimum analytical excitation and absorption wavelengths. The response of the Perkin-Elmer Nitrogen/Phosphorous detector (NPD) is generally 10-104 times more for nitrogencontaining compounds than for hydrocarbons. This increased selectivity should enable one to achieve higher sensitivities for aromatic amines on the tail of hydrocarbon solvents and eliminates the need to vent the solvent as in the adsorptionFIGC method. However, the sensitivity gained by the N P D over the technique reported here is only about 10-fold for P P D and 4-ADP, for example (18). The NPD, and ECD as well, has been noted to lose sensitivity with continuous operation. They are subject to contamination when used by analysts without expertise. Practical injection volumes are 1-5 p1 for the NPD and ECD, and as much as 100 p1 may be injected into the LC. For the adsorption-FIGC technique described above, repetitive 500-pl samples can be injected if necessary, venting the solvent and concentrating aromatic amines at the head of the cool trap. This technique is convenient and reproducible, as described above, and allows adequate sensitivity for a variety of applications.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
LITERATURE CITED (1) T. S. Scott, "Carcinogenic and Chronic Toxic Hazards of Aromatic Amines", Elsevier Publishing Co., New York, 1962. (2) J. H. Stender, Fed. Regist., 38, 20074 (1973). (3) J. H. Stender, Fed. Regist., 39, 3756 (1974). (4) A. Ziatkis, H. A. Lichenstein. and A. Tishbee, Chromatographia, 6 , 67 (1973). (5)J. P. Mieure and M. W. Dietrich, J. Chromafogr. Sci., 11, 559 (1973). (6) A. Savitsky and S. Siggia, Anal. Chem., 46, 153 (1974). (7) T. A. Bellar and J. J. Lichtenberg, Am. Water Works Assoc. J., 739 (1974). (8) J. Janak, J. Ruzickova, and J. Novak, J. Chromatogr., 99, 689 (1974). (9) P. W. Jones, R . D. Giammar, P. E. Strup, and T. B. Stanford, 68th Meeting of the Air Pollution Control Association, Boston, Mass., June 15-20, 1975. (IO) J. S. Parsons and S. Mitzner, Environ. Sci. Techno/.,9, 1053 (1975). (11) S. L. Yasuda, J. Chromatogr. Sci., 104, 283 (1975).
(12) G. Ivan and R. Ciutacu, J. Chromatogr., 88, 391 (1974). (13) J. F. Dacher, J. P. Guenier, B. Herve-Bayin, and 0. Moulut, Chromatographia, 8, 228 (1975). (14) P. Haefelfinger, J. Chromatogr., 111, 323 (1975). (15) I. M. Jakovljevic, J. Zynger, and R . H. Bishara, Anal. Chem., 47, 2045 (1975). (16) J. S.Fok and E. A. Abrahamson, Chromatographia,7, 423 (1974). (17) C. C. Milionis, Polymer Intermediates Department, E. I. du Pont de Nemours and Co., Experimental Station, Wilmington, Del., unpublished work, 1975. (18) H. D. Deveraux, Plastics Products and Resins Department, E. I. du Pont de Nemours and Co., Experimental Station, Wilmington, Del., unpublished work, 1975.
RECEIVEDfor review March 30, 1976. Accepted June 17, 1976.
Programmed Thermal Field-Flow Fractionation J. Calvin Giddings,* LaRell K. Smith, and Marcus N. Myers Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
The role of programmed thermal field-flow fractionation in making tractable a greater range in sample molecular weights is established. The theory of retention is described, and retention plots are given. Experiments are described using nine polystyrene solutes ranging in mol wt from 4000 to 7 100 000. These nine components are resolved in a single run using either linear or parabolic programming. The effects of various experimental parameters are described.
Thermal field-flow fractionation (TFFF) (1-4), among all the techniques of field-flow fractionation (FFF) (5-8), has so far shown the greatest versatility for polymer separations. However, like all elution systems based on a positive retention (as opposed to exclusion, or negative retention), FFF must be modified to handle samples of a broad molecular weight range. Otherwise the early peaks are inadequately resolved and the late peaks are strung out over increasing increments of time. This general problem is usually handled through various kinds of programming systems in chromatographic work (9, I O ) . Earlier we described two programming systems for sedimentation FFF (SFFF) (11). These employed variations of the centrifugal field and variations of solvent density, respectively. These systems succeeded in speeding up the heaviest components in a mixture of polystyrene beads, improving peak spacing, and reducing analysis time. In that TFFF is applicable to a wider molecular weight range of macromolecules (down to mol wt = 500 ( 4 ) ) than SFFF, programming in this system should be especially useful. Here we report the development of a programmed TFFF system. Programming is achieved through the variation of the external field: in this case, the temperature increment, A T . Parameter A T controls retention; it can be varied externally and continuously according to any reasonable program, extending all the way to zero increment and thus zero retention. Here, as in SFFF, one can, in theory, program solvent parameters as well as external fields. However, with SFFF, the relevant solvent parameter (density) has a precisely understood role, whereas in TFFF, no single solvent parameter can be identified with predictable changes in retention. Hence rational solvent programs must await more fundamental developments in our understanding of thermal diffusion parameters.
THEORY OF RETENTION In the cited paper on programmed SFFF ( I I ) , general programmed FFF was divided into two categories: uniform programming and solvent programming. Both are theoretically applicable to TFFF. However, only the former concerns us here, for the reason just discussed. In uniform programming, variations occur as a function of time but not of position. This is achieved in TFFF by keeping the temperature increment equal throughout the column, but forcing it to vary in some predesigned manner with time. The time-program is therefore some function AT = AT(t)
(1)
Inasmuch as retention parameter, R , depends on A T , R also undergoes systematic change with time, R = R ( t ) ,and programming is achieved. The general equation for peak retention time, t,, is ( 1 1 )
where L is column length and ( u ) is mean flow velocity. The particular form of the variation of R ( t ) must now be established. In FFF, solute is compressed into a layer of exponential form, and of characteristic thickness 1. The ratio of 1 to column width w is the dimensionless thickness, X = l/w.Studies of T F F F have shown that X is approximately inversely proportional to A T ( 2 ) .Explicitly ( 4 ) , X = d / A T MI12
(3)
where 4 is a solvent constant, different for each polymersolvent series, and M is molecular weight. For polystyrenes in ethyl benzene, q5 = 1420 "C (g/mol)l/*. For integration purposes, Equation 3 is written in the form,
X = A/AT
(4)
We must now relate R to X to make Equation 2 useful. This is done by the standard retention equation (7).
(5) which, with Equation 4, becomes
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