Automatic Gas Chromatographic Monitor for the Determination of Parts-per-Billion Levels of Bis(chloromethy1) Ether L. S. Frankel and R. F. Black" Rohm and Haas Company, 5000 Richmond Street, Philadelphia, Pa. 19 137
An automatic continuous gas chromatographic monitor capable of selectively measuring 0.1 ppb of bis(chloromethy1) ether in air samples is descrlbed. The monitor utilizes gas phase enrichment on a solid adsorption material and subsequent thermal elution. Two gas chromatographic columns in sequence were needed to obtain the required selectivity. The effluent from the first analytical column during a selected time period is vented into a second analytical column and analyzed. The automatic monitor can analyze eight different sample locations and generate 48 data points a day. The monitor has been in continuous automatic Operation for over 700 days. This system could be used to selectively study most organic air pollutants at the ppb levels.
Bis(chloromethy1) ether (BCME) and chloromethyl methyl ether (CMME) have recently been termed human carcinogens by the United States Department of Labor ( I ) . This followed laboratory tests with these chemicals on small animals by Gargus et al. ( 2 ) , Van Duuren et al. ( 3 ) , and Laskin et al. ( 4 ) . Laskin et al. reported a high incidence of cancer in rats subjected to inhalation of 100 ppb of BCME in air under controlled laboratory conditions ( 4 ) . Although CMME is not stable in humid air, BCME is significantly more stable and consequently of major concern (5,6). L. Collier (7) of our laboratory has reported a highly selective method for the determination of BCME in air a t 0.1 ppb. This method utilizes gas phase enrichment in conjunction with high resolution mass spectrometry. The intensity of the m / e 78.9950 ion (ClCH20CH2) is measured to determine the BCME content. More recently, two gas chromatographic/mass spectrometric methods ( 9 , I O ) and a derivatization/electron capture technique (8) have been reported. In this paper, we describe a totally automatic gas chromatographic monitoring system which is capable of selectively measuring BCME in air a t 0.1 ppb. This system can be adapted to selectively measure most organic compounds a t ppb levels. Gas Phase Enrichment. Initial gas chromatographic studies compared the relative sensitivity of a radioactive foil type 63Ni electron capture detector and a flame ionization detector. This type of electron capture detector was somewhat more sensitive than a flame ionization detector a t comparable background levels. The basic problem with the foil type electron capture detector is that it is not very durable, does not have the long term stability needed for continuous automatic operation, and is not amenable to temperature programming. I t is also typically very sensitive to trace quantities of water and can easily be fouled by large quantities of chlorinated material. The sensitivity of the flame ionization detector was found to be of the order of 25 pprn with a 200 111 injection of a gaseous sample of BCME. I t is apparent that gas phase enrichment is needed to reach ppb sensitivity. The organic components of a gaseous sample may be enriched by two methods: 1)enrichment in a cold organic sol732
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
vent via a gas impinger and gas sampling train; 2) adsorption on a solid or liquid-coated solid support. The enriched organic components are subsequently thermally eluted or solvent extracted and analyzed by any suitable means. In practical terms, gas enrichment in a cold organic solvent cannot be conveniently adapted to determine most air pollutants a t levels significantly less than 1 ppm. By selectively adsorbing the organic molecules on a solid and subsequently thermally eluting them into a gas chromatograph, the entire sample is used. This results in a gain of sensitivity of up to 5 magnitudes over direct injection. Adsorption from 7.5 1. of air will give a sensitivity of better than 1 ppb for compounds similar to BCME. When the adsorbed compound is solvent extracted from a solid adsorbent, sensitivity is lost, because only an aliquot of the solvent, therefore of the sample, is used. The basic requirements of an adsorption system for BCME are: 1) Large quantities of water must not be adsorbed and introduced into the GC system. 2) The adsorber must have sufficient dynamic capacity to retain BCME from large volumes of air sampled a t a fairly rapid flow rate. 3) Thermal elution of BCME must be rapid and quantitative. 4) The adsorber must be capable of being recycled many times. 5) Under required thermal conditions, it must not contribute interferences. Preliminary experiments indicated that macroreticular adsorbent copolymers might satisfy the above basic requirements. These adsorbents elute water very quickly with no tailing. Several adsorbents have been evaluated under a standard set of conditions to facilitate a comparison of their relative dynamic capacity. The experimental conditions were designed to cause some breakthrough of BCME. Significantly higher levels of BCME were utilized than would be actually encountered. BCME was introduced into a 1-1. round bottom flask a t the start of the collection period. The flask was subsequently continuously purged with ambient air. The entire air sample is used to cause breakthrough. The adsorbents investigated were: Chromosorb 101 (Johns-Manville Products Corp., Celite Div.), 80/100 mesh; Chromosorb 104, 60/80 mesh; and Porapak Q (Waters Associates, Inc.), 50/80 and SO/lOO mesh. Five-mm. 0.d. by 75-mm glass tubes containing 175 mg of adsorbent were analyzed by high resolution mass spectrometry. A BCME gas aliquot equivalent to 87 ppb (10 cm3 of 130 ppm) in a 15-1. sample was utilized in all experiments. The results for Porapak Q are summarized in Table I. Similar experiments were performed for each adsorbent in the presence of the equivalent of 100 ppm (5-111liquid aliquots) 1,2-dichloroethane in a 15-1. sample. Two enricher columns were utilized in tandem. Porapak Q, SO/lOO mesh, was the most efficient adsorber. The results in Table I show that massive concentrations of 1,2-dichloroethane do reduce the efficiency of the adsorber. Porapak Q is a macroreticular styrene-divinylbenzene copolymer with a very high surface area, 840 m2/g, and very small average pore diameter, 75 8, ( 1 1 ) . The Porapak Q was further evaluated under actual operating conditions in the
Table I. Evaluation of Porapak Q, 80/100 Mesh Adsorber Flow rate, l/min
Sampling time, min
1.0 1..5 1.5 1.0
5 10 20 5
1.5
10
1.5
20
Adsorber
1,2-Dichloroethane present
...
... ... ...
... ...
1st 2nd 1st 2nd 1st 2nd
X X X
X X
X
Table 11. Evaluation of Tenax-GC 60/80 Mesh Adsorbera % BCME retained
100 100 80 60 30 45 15 25 35
monitor. After 500 days, replacement frequency (15 days average) was high and background from the adsorber troublesome, but not detrimental, to detecting BCME. Some lots of Porapak Q were rejected because of poor recovery of BCME. Laboratory experiments indicated the primary problem with Porapak Q is oxidative degradation of the adsorbent. An alternate porous polymer adsorber based on 2,6-diphenyl-p-phenylene oxide, Tenax-GC (Enka N.F., The Netherlands), was subsequently evaluated in the monitor. Results indicated Tenax-GC fulfilled all the previously established requirements for an adsorbent. Recovery of BCME (in the presence of large quantities of n-hexane) over a range of 10-80 ppb (Table 11) from an air stream was quantitative (requirements 2 and 3). This adsorber has been in operation for 180 days without requiring repacking (requirement 4). Since its upper temperature limit is 375 "C, the adsorbent can be decontaminated to a greater extent than Porapak Q (requirement 5). Tenax-GC will not enrich water (requirement 1). Sample Transfer from Adsorber to the Gas Chromatograph. The components enriched on the adsorber will spread through the length of the adsorber as the air sample is collected. I t is necessary to concentrate the sample into a single plug prior to gas chromatographic analysis. If this is not achieved, the width of the chromatographic peaks will be large and poor resolution will result. This is accomplished by diverting the carrier gas flow through the adsorber which is subsequently heated a t a specific temperature for a specific time. The thermal elution conditions need be sufficient only to remove BCME. The temperature is maintained after the analytical process begins in order to avoid contamination in subsequent runs. The column is subsequently temperature programmed to an elevated temperature and the components are sepa-
Recovered Introduced,* ng
ng
%
349 698 1396 2792
387 729 1431 2956
111
104 103 106
A n-hexane solua Flow rate, 0.5 l./min. Sample time 15 min. tion containing 0.349 r(Lg BCME/pl was utilized as a standard. This standard is equivalent to 10 ppb BCME in a 7.5 1. air sample.
rated. After BCME elutes, the column is programmed to a higher temperature and purged of other higher boiling air pollutants. The column is then cooled and prepared for the next sample. Inadequacy of Single Column GLC. The selectivity of a single gas chromatographic column for BCME at ppb levels was evaluated in and around our production facilities at many different sampling sites. When a positive response for BCME was obtained, a sample would be taken for mass spectrometry, to confirm that the peak was BCME. A column was required to perform with no significant interference for 5 days to be considered useful. No single column was found that could satisfy this requirement. The columns studied were Porapak Q (6 ft, 50/80 mesh), Chromosorb 101 (6 ft, 80/100 mesh), 15% Polyterg B-300 on Chromosorb P (12 ft, 60/80 mesh), 7% OV-225 on Gas Chrom Q (12 ft, 80/100 mesh, silylated), and 20% Carbowax 20M TPA on Chrom W (15 ft, 60/80 mesh). It became very evident that selectivity and not sensitivity would be the limiting factor of a gas chromatographic system. Dual Column Gas Chromatography. In an effort to enhance selectivity, a dual column gas chromatographic system was investigated. Basically, the adsorber is thermally eluted into a fore column a t 25 "C and the column is then temperature programmed. A gate period is defined which starts immediately before, and ends immediately after, the BCME elution time. All components eluting during this critical gate time are diverted into a second column maintained at 25 "C in a second oven. The effluent from the fore column is plugged on the second column which is subsequently temperature programmed. The effluents from the fore column eluting before, and after, the critical gate period never enter the second analytical column. Selection of Column Systems. Information was accumulated on possible major organic compounds present in
Table 111. Gas Chromatographic Columns-Retention Time Data Column
10%DC-200 7%0V-225 QF-1 (1.95%)/ OV-17 ( 1.5%) 1%SP-1000
Lenth, ft Program
Isopropanol
Ethylene dichloride
Toluene
Butyl acetate
10 15 20
1 2 3
108 170 160
210 260 244
243 290 257
285 390 286
348 290 312
390 340 373
20
3
213
289
345
382
281
305
Programs
Initial, " C
Rate, O/min
Final, "C
1
35 35 35
10 15 15
150 90 95
2 3
Compound R.T. (Sec.) Butanol BCME
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4 , APRIL 1976
733
Table IV. Permeation Rate of BCME vs. Temperature Permeation rate, ng/minn
Concentration, ppb
Response raw counts
55 40 35
772.4 65.4 29.7
1.3 6.7 10.1 20.2 40.4 50.5
1371 10 615 13 318 27 580 50 565 62 885
Calculated vs. gas chromatographicresponse to an injection of a known concentration of BCME. (I
~
~
the environmental samples collected for BCME analysis. Four column systems were selected and retention times of the major contaminants and BCME determined (Table 111). All columns appeared to yield adequate resolution under laboratory test. However, only the combination of DC-200 (fore column) and OV-225 (analytical column) was sufficiently selective under actual operating conditions in the monitor. Column lengths and programs were optimized under operating conditions. Both columns could adequately plug BCME over a thermal desorption time span of 2 min. The BCME peak base width eluting from the fore column was determined to be 30 s permitting a narrow gate time of 40 s for quantitative transfer from the fore column to the analytical column. The columns are thermally decontaminated after each analysis over a temperature range of 110-240 OC for a 10-min period. These columns have been in use for over 10 000 analyses with only a 3% negative drift in retention time. Calibration of the Automated System. The system is automatically calibrated after each sampling cycle of seven analyses. This is accomplished by the use of an Analytical Instruments Development Inc., Route 41 and Newark Road, Avondale, Pa. 19311, permeator, the temperature of which was adjusted to nominally deliver 23.5 ng/min into a 500 ml/min clean air stream resulting in a BCME concentration of 10 ppb in air. The permeation line was connected to position 1 of the 8-port sampling valve which enabled automatic calibration during each 4-h cycle. The permeator consists primarily of a thermostated glass chamber containing BCME sealed in a Teflon tube. The rate of permeation through the Teflon as a function of temperature is summarized in Table IV. A plot of log permeation rate ( p ) vs. l / K o is linear and indicated a temperature of 34 OC was required to introduce 352 ng into a 7.5-1. air stream over a 15-min period to achieve a 10-ppb standard. Once equilibrium was established, a 52-h test of reproducibility indicated the permeation rate averaged 23.8 f 1.2 ng/min ( N = 12) which is equivalent to 10.1 ppb with a standard deviation of s = 0.3 ppb absolute. Linearity of BCME Concentration of FID Response. Linearity of BCME concentration to FID response was tested over a range of 1.3 to 51 ppb. A plot of concentration vs. integrator response indicated the response was linear over this range (Table V). Sensitivity. Under normal operating conditions, a 10ppb standard analyzed at 8 X A/mV sensitivity yields A/mV, a 0.7-ppb a signal of 1.0 mV (Figure 1).At 4 X standard gave a response of 80 pV (Figure 2). The noise level a t this attenuation is 3 pV. A 0.1-ppb standard should yield a signal of 12 pV which is still four times the noise level. BCME Recovery Experiments. At 10-ppb quantitative recovery of BCME through the entire system, including the 300-ft sampling lines, was demonstrated by connecting a second permeator to the inlet of the sampling line a t the source. All lines had been in use for a t least 6 months. The air from the source entering the permeator was decontaminated with a zero gas cartridge consisting of layers of charcoal and silica gel. Samples were analyzed a t 4-h intervals 734
Table V. Linearity of BCME to FID
Temperature, “C
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
w
I
u
m
Figure 1. FID response to 387 ng BCME in 715 I. of air (equivalent to 10.7 ppb in 15 I.)
- 100
- 80 -
60
- 40 W
I 0
m
1 - 20
Figure 2. FiD response to 25.8 ng BCME in 7.5 I. of air (equivalent to 0.7 ppb in 15 I.)
under automated conditions. Recoveries on the three lines tested indicated a range of 89 to 104%. Mean recoveries for the three lines tested were 97 f 5% (19 data points). Sample Stream Drying. As sampling of the various areas moved into the summer months, a significant drop in the permeation standard occurred. Tests with injected BCME standards eliminated the analytical system as the cause. Mass spectral analysis of the air stream indicated the permeation of BCME was still nominally 10.1 ppb. Suspecting that moisture was collecting on the adsorber because of high humidity (70-90% RH), several means of eliminating moisture were evaluated. A quantitative sample stream dryer developed by Perma Pure Products, Inc., Oceanport, N.J., 07757, was installed in the sampling sys-
__ MODE I
DISCARDING U N D E S I R A B L E C O M P O N E N T S
L
' DUMMY
COLUMN
GLC*Z
FROM COL *I
-
I
Figure 3. Sampling system MODE
I
COLLECTING
FILTER
SAMPLE V3
VCDE
2
EXAYIN,hG
GCMPONENTS
OF
IN'ERESI
DRYER
1 VENT "DUMMY"
ADSORBER
COLUMN
COL'I GLC
*
FROM C O L *I
I
TO VALVE * Z
MODE 2
ANALYZING
SAMPLE
Table VI. Matrix Temperature Program of Analytical Columns GC Sequence
*1 1
Figure 5. Valve V2
SAMPLE
FLOWMETER
He
I
4
Temp. Rate Time Isothermal Temp. Time Second Program Rate Time Isothermal Purge Temp. Time Isothermal First Program
REG
COL'l
TO
VALVE '2
DC-200
25OC 16 OC/min 5 min 105°C 5 min 26 OC/min 5 min 235 "C 3 min
OV-225
25 O C 16 OC/min 5 min 105 OC 5 min 24 "C/min 5 min 225 O C 3 min
Figure 4. Valve VI
tem in front of the adsorber. The dryer is a bundle of permeation tubes enclosed in a stainless steel shell which permits water to permeate through the tubular membrane walls. Continuous drying is accomplished by passing the moist sample stream a t 500 ml/min through the tubes and dry air a t 1500 ml/min through the shell in the opposite direction of the sample air stream. Under routine operating conditions, quantitative recovery of the BCME permeation standard was achieved when humidity in the sampling areas was as high as 95%).The dryer also eliminated a minor component which occasionally interfered with BCME. Several spiking experiments indicated that this compound was acetic acid. Sampling System. A diagram of the sampling system is shown in Figure 3. Environmental air samples originate from 7 different sampling sources. The air sample is pulled into the trailer monitor through %-in. o.d. Teflon lines that vary in length from 150-300 ft. Saran, polyethylene, and polypropylene were not satisfactory because of cracking and crazing under adverse weather conditions and contribution to background interference. Teflon was durable under adverse conditions and did not exhibit any background interference. The lines enter the trailer and are connected to a
stainless steel porous trap which prevents particulates > 2 0 - p diameter from entering the analytical system. A t the outlet of the trap, a Teflon diaphragm vacuum pump draws the sample through the line and traps a t approximately 2-3 l./min. The pump pushes the sample into a Yg-in. stainless steel line, heated to 50 "C to prevent condensation. Each sample is then fed into individually regulated back pressure systems which permit all the sample to dump to a decontamination system in the nonsampling mode and a portion of the sample (500 ml/min) to feed to the adsorber in the sampling mode. From the 8 back-pressure regulators, a l/Is-in. stainless steel line feeds the sample to an 8-port valve (V3). At any one time, 7 lines are blocked which forces all the sample to dump to the decontamination system. The eighth port feeds to a valve (VI) a t a rate of 500 ml/min. Valve three (V3) is moved forward 1 step every 30 rnin by a Tenor programmer. The single line feeding through V3 passes through the Perma Pure dryer to valve V1. For a period of 2 min, the stream passes through V 1 (mode 2) to dump, permitting the sample system to equilibrate to the new sample. Valve V1 switches and the stream passes through V1 for 15 min to the adsorber. The calibration standard is sampled in essentially the ANALYTICAL CHEMISTRY, VOL. 48,
NO. 4, APRIL 1976
735
same manner. An air flow of 500 ml/min is supplied to the BCME permeation chamber where 352 ng is introduced into the sample stream over a 15-min period, equivalent to about 10.1 ppb in a 7.5-1. sample. Adsorber System. The adsorber is a stainless steel tube which is 51/2 inches long by %-in 0.d. packed with 300 mg of 60/80 mesh, Tenax GC. The adsorbent is preconditioned by heating a t 240 "C under a helium flow for 15 min. The adsorbent occupies about 1.5-inch length in the mid-section of the adsorber. The adsorbent is kept in place with glass wool plugs. The adsorber is connected to valve VI, ports 3 and 4. Heavy gauge cables are attached to the ends of the adsorber with brass clamps. An iron constantan thermocouple is silver-soldered a t the midpoint of the adsorber. Direct resistance heating is utilized to thermally elute the enriched organic components. The adsorber is heated by a voltage step-down transformer which feeds high current a t low voltage. The adsorber elution temperature and time is 180 "C for 2 min. These elution conditions give quantitative transfer of the enriched components into the DC-200 column. Carle Valve V1. A detailed diagram of the valve (VI) located between the adsorber and the first chromatograph is given in Figure 4. The valve has 2 positions and 8 ports. In sampling mode 1, the air is passed from V3 and the dryer through positions 1-3 of V1 to the adsorber where the organics are enriched. The exiting organic-free air is passed through ports 4-2 to a rotameter which measures the sample flow and exits to a decontamination system. In the same mode, helium is fed to the first gas chromatographic column via V1 ports 6-8-7-5. After 15 min a t a flow rate of 500 ml/min, V1 rotates to mode 2. Helium back-flushes the adsorber for 3 min a t 25 "C to remove residual oxygen from the adsorber via ports 6-4-3-5. The temperature of the adsorber is now raised. The enriched organics pass from the adsorber through ports 3-5 and are plugged on the first gas chromatographic column. The Analytical System and Valve V2. The analytical system consists of a fore column DC-200, a gate, V2 (Figure 5), and an analytical column OV-225. A description of the columns and glc parameters is presented in Table VI. The organic enriched sample is thermally eluted from the adsorber and plugged on the DC-200 column a t 25 "C. The column is programmed for initial separation of BCME from other organics. The organic effluent preceding BCME is vented via ports 4-1 of valve V2 (mode 1)through a pressure regulator (dummy column) to the decontaminator. The back pressure on the DC-200 column supplied by the regulator is set 2 lb greater than the head pressure on the analytical OV-225 column. This permits a slight forward thrust of the effluent from the fore column when the V2 valve gates to transfer the BCME to the analytical column. At a precise time, which is governed by a timer initiated when programmer No. 1 is started, V2 rotates (mode 2) permitting the BCME to pass through ports 4-3 to the OV225 column where it is plugged a t 25 "C. At the same time, a timer is initiated which maintains the valve in mode 2 position for 40 s (gate time). With the termination of the 40 s, v 2 rotates back to mode 1, GLC 1 is temperature-programmed rapidly to clean the column, GLC 2 door closes, and the programmer is initiated for analysis of the gated sample. The effluent from the OV-225 column is monitored by a flame ionization detector and its output is fed to an Autolab (Spectra-Physics, 2905 Stender Way, Santa Clara, Calif. 95051) integrator. Tenor Programmer. The entire sequence of events is controlled by the Tenor Programmer. The program is outlined in Figure 6. The numbers running across the top of 736
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
the table correspond to a 30-min cycle period. Each minute will be referred to as "step." Various operations are entered in the left hand column and will be considered in detail. Index V3
The rotating disc selector valve, V3, will move to a new sampling station a t step 24 stopping 2 min before sampling to permit equilibration of the system. V1 Mode Indicates time sequence of position of valve V1; mode 1 (collect, step 26) or mode 2 (dump, step 11). Adsorber Heat Thermal elution of the adsorber is initiated a t the beginning of step 14 and terminated a t the beginning of step 16. Note the heat is supplied an additional 5 min to.decontaminate the adsorber. The temperature program sequence in GLC No. 1 Start the first chromatograph is indicated. The temperature program starts at the beginning of step 16 and terminates a t the end of step 2. GLC No. 1 Oven Indicates opened (0) and closed (C) Door position of chromatographic oven door in first gas chromatograph. The gate initiation timer is started siGate Start Time multaneously with the first gas chromatograph a t the beginning of step 16. This timer is set a t 6 min, 55 s. This means that 6 min and 55 s after step 15, valve 2 switches into mode 2 permitting GLC effluent to vent to GLC No. 2. V2 Mode Indicates time sequence of position of valve V2; mode 1 (discard undesirable components) mode 2 (examining components of interest). The temperature program sequence in GLC No. 2 the second chromatograph is indicated. Indicates open ( 0 )and closed (C) posiGLC No. 2 Oven tion of oven door in second gas chroDoor matograph. The gate start timer is reset for the Timers Reset next cycle during step 29. The Autolab Integrator starts simultaAutolab neously with the temperature programIntegrator mer of the second gas chromatograph Start during step 2. The recorder chart drive is initiated in Recorder Start step 23. The recorder is active for 10 min. I t is apparent that many operations occur simultaneously. The easiest way to find out exactly what is going on a t any time is to read down the program step. For example, reading down step 14 indicates that V1 is in mode 2, the
adsorber is being heated, GC No. 1 door is open and the chromatograph a t 25 OC for the next run, V2 is in mode 1 (closed), GC No. 2 is cooling to 25 O C . I t is apparent that samples are being simultaneously collected and analyzed in a consecutive repetitive automatic pattern. I t is important to note that the adsorber is purged with helium for 3 min prior t o being thermally eluted t o drive off any oxygen before heat is applied. Performance Experience. The monitor has been in continuous operation since July 1972. Between installation and the end of 1974, 35 000 data points have been generated. This monitor has played a major role in ensuring safe operating conditions in our plant. The automatic monitor described in this report could be readily adapted to selectively monitor almost any gaseous pollutant a t the ppb level.
LITERATURE CITED (1) "Occupational Safety and Health Standards: Part Ill", Department of Labor, Occupational Safety and Health Administration, Fed. Regist., 39, 3756-3797. Tuesday, January 29, 1974.
(2) J. L. Gargus, W. H. Reese, Jr., and H. A. Rutter, Toxic. Appi. Pharmacol.. 15, 92 (1969). (3) B. L. Van Duuren, A. Sivak, B. M. Goldschmidt, C. Katz, and S. Melchionne, J. Natl. Cancer inst., 43, 481 (1969). (4) S. Laskin, M. Kuschner, R. T. Drew, V. P. Cappiello, and N. Nelson, Arch. Environ. Health, 23, 135 (1971). (5) R . W. Nichols and R. F. Merritt, J. Natl. Cancer lnst., 50, 1373-1374 (1973). (6) J. C . Tou and G. J. Kallos, Anal. Chem., 46, 1866-1869 (1974). (7) L. Collier, Environ. Sci. Techno/., 6, 930 (1972). (8) R . A. Soloman and G. J. Kallos, Anal. Chem., 47, 955 (1975). (9) K. P. Evans, A. Mathias. N. Mellor, R. Silvester, and A. E. Williams, Anal. Chem., 47, 821 (1975). (10) L. A. Shadoff, G. J. Kallos, and J. S. Woods, Anal. Chem., 45, 2341 (1973). (11) S. B. Dave, J. Chromatogr. Sci., 7, 389 (1969).
ACKNOWLEDGMENT We wish to acknowledge the substantial contributions of L. J. Gibson, E. M. Sioma, L. Walecki, and K. Wallisch to the success of this project.
RECEIVEDfor review July 9, 1975. Accepted December 24, 1975. This analytical monitoring system is covered by United States Patent No. 3,807,217, April 30, 1974.
Rate Dependence of Statistical Moments of Chromatographic Profile on Solute-Solvent Interactions Kuang-Pang Li" and Yue-Yue H w a Li Department of Chemistry, University of Florida, Gainesville, Fla. 326 7 7
Statistical moments and related parameters, such as the skew, S, and the excess, E, of a chromatographic profile carry all macroscopic information about the kinetics of oncolumn interactions. To establish the rate dependence of these parameters, the steepest descent approximation method is employed. Simplified equations for these parameters are derived on the basis of a hypothetical model In which complexation of solute molecules with the stationary phase is included. Computation with these equations indicates that the statistical moments are always less involved than S and E. Accordingly, they may be more useful in the real-time determination of on-column reaction rates even though S and E may be more conveniently measured experimentally.
Chromatography has been recognized as a powerful tool for chemical separations and determinations. I t has been regarded as a device which most nearly approaches the chemist's dream of a magic tube into which a complex sample is placed and out of which emerges a complete analysis ( 1). This impressive achievement of chromatography has tended to overshadow its other potentialities to study reaction kinetics directly on the column. The use of a chromatographic column both as a reactor and as an analyzer has a number of advantages. First, since separation of reaction products is initiated instantaneously and effected continuously during the entire course of chromatography of the reactant, reverse reaction and side reactions are minimized. The forward reaction can be studied with little interferences. Second, retention data and relative concentrations (or rates of formation) of all eluting re-
action products are available. This information may throw important light on the mechanism of the reaction. Third, stop-flow technique can easily be employed (2, 3 ) . With this method, it is possible to follow in one experiment the rate of reaction from the first 0.1% of reaction to the stage where the reaction is 99.9% complete. In addition, the chromatographic reactor benefits from other features inherent in chromatography, i.e., small sample size and simple control of operating variables. Since chromatography is based upon rate processes, each physical-chemical reaction which the eluting molecules undergo, will have profound influences on the distribution of the molecules. In other words, the resulting chromatogram should carry all of the macroscopic information about these reactions. Earlier theories of chromatography have clearly demonstrated that pure partitioning, or simple sorptiondesorption processes, gives rise to a gaussian distribution in the limit (4-6). Thus, substantial deviations from gaussian distribution may be attributed to processes which contribute to the non-linearity or non-ideality of the column. These processes could be dissociation (7, 8 ) , rearrangement (91, etc., of the eluting molecules, or mutual interactions, such as solvation or complexation of the eluting molecules with the stationary phase (IO). If the nature of the on-column process is known, then an appropriate profile analysis would reveal the kinetics of the reaction. There are several ways to characterize a peak profile. The use of statistical moments and related parameters has been the most widely adapted in chromatography. Several authors (11-15) have discussed the importance and theoretical evaluation of these parameters. Some of them have demonstrated their usefulness in the discernment of overlapping chromatographic peaks ( 1 6 ) and in the diagnosis of ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
737
