Anal. Chem. 1907, 59,382-384
382
routine analysis for a large number of air samples. A second advantage is a high degree of specificity due to the characteristics of surface ionization. The present study is a succesul example of GC/MS combined with the surface ionization method. We believe that the GC/SIOMS method can be extended to analysis of other organic compounds in many fields that are able to be ionized efficiently on the SI emitter. These compounds are other alkylamines, diamines, and amino alcohols, hydrazines, and nitrosamines (20).
ACKNOWLEDGMENT The authors are grateful to D. S. Linton at Yokota Air Base for the manuscript preparation. Registry No. TMA, 75-50-3. LITERATURE CITED Fuselli, S.;Cerquiglini, C.; Chiacchierini, E. Chim. Ind. (Milan) 1978, 6 0 , 711. Brubaker, R . E.; Muranko, H. J.; Smith, D. B.; Beck. G. J.; Scovel, G. JOM, J . Occup. M e d . 1979, 21,688. Fuselli, S.;Benedetti, G.; Mastrangei, R . Atmos. Environ. 1982, 16, 2943. Mosler, A. R.: Andre, C. E.; Viets. F. G. Environ. Scl. Technol. 1973, 7, 642.
Van Langenhove, H. R.; Van Wassenhove, F. A,; Coppin, J. K.; Van Acker, M. R.; Schamp. W. M. Environ. Sci. Technol. 1982, 16,883. Fine, D. H.; Rounbehler, D. P.; Sawicki, E.; Krost, K. Environ. Scl. Technol. 1977, 1 1 , 577. Audurrsson, G.;Mathiasson, L. J . Chrornatogr. 1984, 315,299. Kuwata, K.; Akiyama, E.; Yamagali, Y . ; Yamagali. H.;Kuge, Y. Anal. Chem. 1883, 55,2199. Bouyoucos. S. A.; Melcher, R. G. Am. Ind. Hyg. Assoc. J . 1983, 44, 119. Kashihira, N.; Makino, K.; Kirita, K.; Watanabe, Y. J . Chrornatogr, 1982, 239, 617. Casselman, A. A.; Bannard, R. A. R. J . Chromatogr. 1974, 88, 33. Fujimura, K.; Kiranaka, M.; Takayanagi, H.; Ando, T. Anal. Chem. 1982, 54,918. Pons, J. L.; Rimbault, A.; Darbord, J. C.; Leiuan, G. J . Chrornatogr. 1985, 337, 213. Zandberg, E. Ya.; Ionov, N. I. Surface Ionization; Israel Program for Scientific Translations: Jerusalem, 1971. Zandberg, E. Ya.; Rasulev, U. Kh. Russ. Chem. Rev. (Engl. Trans/.) 1982, 51, 819. Fujii, T. Int. J. Mass Spectrom. Ion Processes 1984, 57,63. Billings, C. E.; Jonas, L. C. Am. Ind. Hyg. Assoc. J . 1981, 42,479. Fujii, T. J. Phys. Chem. 1984, 88, 5228. Dunn, S. R.; Simenhoff, M. L.; Wesson, L. G. Anal. Chem. 1976, 48, 41. Fujii, T.; Kitai. T. I n t . J . Mass Spectrom. Ion Processes 1986, 71, 129.
RECEIVEDfor review July 8, 1986. Accepted September 17, 1986. This work was supported in part by the Education Ministry in Japan, Contract No. 60880026.
Controtted Back Pressure Valve for Constant Flow and Pressure Programming in Packed Column Supercritical Fluid Chromatography K. R. Jahn and B. W. Wenclawiak*' Anorganisch Chemisches Institut, Westfalische Wilhelmsuniversitat, Corrensstrasse 36, 0-4400 Munster, FRG In supercritical fluid chromatography (SFC) it is necessary to maintain the entire eluent system at high pressure under supercritical conditions. It is common in capillary SFC to use small capillary tubes as flow restrictors at the outlet. Adjusting these small bore tubes (5-10 pm) to a certain length (I) or providing nitrogen back pressure (2)results in the desired flow rates. In packed-column SFC with its higher flow rate the use of a regulation valve is advantageous (3-6). In order to maintain the same pumping rates at different eluent pressure, the valve must be adjusted very carefully. Motorized valves are mentioned in ref 3 and 6, but the devices are either technically complicated (3) or still need manual adjustment. Adjusting the valve by hand is time-consuming, especially when using binary eluent mixtures with long equilibration times in the column (7). Because of the density effects in SFC, any supercritical system is very sensitive to any alterations in pressure. In praxis, the mass transport into and out of the system should be the same, thus avoiding any change in the system. This can be achieved by using a self-controlled back pressure valve, which is installed after the detection cell. The major criterion for its precise working is the reproducibility of retention times. The valve system described here uses a device where any changes in pressure are used to control the valve.
EXPERIMENTAL SECTION A dual piston pump (Altex 100) has been used and was operated in the constant flow mode. It was equipped with a cooling device (-10 "C) to avoid evaporation of liquid COz. As the density of solvents is pressure and temperature dependent, the pump had
Present address: De artment of Chemistry, University of ToToledo, OH 43606.
ledo, 2801 w. Bancroft
&.,
Ma
II
I If-
Figure 1. Back pressure control unit with valve: Ma, manometer; P, pressure puge; V, capillary needle valve (Chrompack); M, motor; PID,
(can be connected to a microcomputer). been calibrated prior to its use. The pump is equipped with a compressibility compensation control, which was adjusted according to the procedure as described in the manual. The obtained flows for C 0 2 have been checked after recalculation to normal temperature and pressure (NTP) conditions. A Rheodyne 7126 injection valve, with a 20-pL sample loop, was used throughout all experiments. The column, 250 X 4.6 mm id., was packed with 7-pm silica (Lichrosorb,Si 60). It was kept in an oven at the desired temperature. A Kontron 8-pL flow-through cell, modified for use at pressures above 15.0 MPa, was installed in a Zeiss PMQ I1 spectrophotometer, operated at 254 nm. A data system, started by the Altrex programmer at the same time the sample was injected, recorded simultaneously the output control unit
0003-2700/87/0359-0382$01.50/0 @ 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
383
a
-ub
Figure 2. Parts of the control circuit of the PID: R,-R, = 4.7 kQ; R , = 1 MQ; P,-P, = 0.1 MQ; C, = 2.2 pF; C2-C3 = 10 k ~A+,; All other parts (right of the dashed line) are dependent on the selection of the driving motor.
=~~084.
Table I. Reproducibility of Retention Times ( t R )Measured with Toluene in n-Heptane"
c I
injected
injected quantity of toluene,
tR7
ng
min
9 9 9
87
5.05 5.03 5.06 5.04
a7
5.07
quantity of toluene, ng
min
87 87 87 87 8,
5.09 5.04 5.05 5.03 5.04
tR3
av t R 5.05, std dev 0.02
"Conditions: 1 mL/min COz;37 "C; 13.1 MPa inlet; 12.6 MPa outlet; 254 nm; 1 bL injection volume. 10 min Figure 3. Pressure stability due to optimized (a) and nonoptimized (b, c) settings on the PID.
of the detector and the pressure in the back pressure control unit. A general schematic of the control unit and description of its parts is shown in Figure 1. The unit is connected to the detector outlet. The signal output of the pressure control point P is linearized and electronically set to zero. It delivers the current signal, which is compared to the reference values of the PID (proportional, integral, differential) control unit. In case of deviations, the PID adjusts the motor (M) to open or close the valve. Freezing of the valve is avoided by resistively heating its inlet capillary tube. The control circuit of the PID unit is in general the same as published in standard electronics books (8) (Figure 2). In the proportional part of the analog circuit the signal is amplified (AJ. The integral part corrects both long-term and small deviations of the set values (A3). Finally the differential part avoids maxima values and allows fast and smooth correction of the valve position (AJ. A digital voltmeter displays either the difference between the desired and the current pressure value while setting the initial parameters or the pressure during the run. The set value of the PID is adjusted by a 10-turn potentiometer or by an analog input connector from other devices. Instead of a potentiometer, a microcomputer (Apple IIe) with Adalab card and with a DAC (digital-to-analog converter) has been used, enabling pressure programming. Any type of pressure gradient is thereby possible. Similarly as described by Lee (9)an algorithm could be used to match density dependencies on the pressure.
RESULTS The integral and differential settings of the PID will influence each other. Under optimized conditions the pressure is stable (Figure 3a). If the integral and/or differential part are not properly optimized, large pressure fluctuations occur (Figure 3b,c). The reproducibility of retention times has been tested with toluene (Table I). For 10 injections with two different sample quantities, the relative standard deviation is only 0.40%. Considering the strong dependence of pressure on the density and thus on the mobile phase velocity, this is excellent precision. To demonstrate the pressure programming capabilities of the device described here, SFC's of a sample of "Superbenzin" under isoconfertic (same density) conditions and with a linear pressure program are shown in Figure 4. The effect of density programming is similar to temperature programming in GC and solvent gradient programming in HPLC.
DISCUSSION The device described here has been successfully applied to packed column SFC. Most piston pumps are not equipped with a pressure limiter; thus once reaching the maximum pressure, they shut off. Especially when the back pressure is adjusted by hand, such pumps are difficult to use in packed column SFC. The use of the PID control unit could help to overcome these problems. Controlling the unit with a small
384
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987 A
L -
+
I
5
IO
15
5
IO
15
2 O min
t
I
mln
Flgure 4. (a) Isoconfertic (isodensity) chromatogram of a sample of "Superbenzin" under the following conditions: CO, at 44 OC and 8.5 MPa; 0.5 mL/min; UV 254 nm detection. (b) Pressure programmed (dashed line).
computer (like an Apple) makes SFC with pressure programming possible, even to low-budgeted laboratories. So far, the flow through the column is always either temperature (GC) or pressure dependent (non-controlled-restrictor capillary SFC). With increasing densities in SFC, the mobile phase velocity (u)decreases. In a constant mass flow situation, the mobile phase velocity is inversely proportional to ita density (IO). One may want to adjust not only a certain pressure (density) but also a desired optimum flow velocity in the column. Thus, the valve and the control unit described herein may be used after some modifications for maintaining constant flow conditions in any GC or SFC. (These experiments are under study.) The device developed by Van Lenten and Rothman (11) controls pressure via the pump in front of the column. Our control unit is connected to the detector outlet and maintains a constant mass flow at the outlet. It allows the use of any type of fluid delivering system such as syringe or piston pumps or high-pressure reservoir tanks for pressure programming. The pressure drop along the column as given in Table I is about 0.5 MPa at 1.0 mL/min flow rate. As in HPLC the pressure drop is dependent on the material used for the packing. In other experiments the pressure drop was mea-
sured at 0.25 f 0.05 MPa. Nevertheless this is much less than under HPLC conditions.
ACKNOWLEDGMENT We wish to thank K. C. Brooks for reading the manuscript.
LITERATURE CITED (1) Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1984, 56, 619A-628A. (2) Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L.; Springston, St. R.; Novotny. M. Anal. Chem. 1982, 5 4 , 1090-1093. (3) Bartmann, D. Ber. Bunsen-Ges. Phys. Chem. 1972, 76, 336-339. (4) a r e , D. R.; Board, R.; McManigill, D. Anal. Chem. 1982, 5 4 , 736-740. (5) Bickmann, F.; Wenclawiak B.; Fresenius' Z. Anal. Chem. 1985, 320, 261-264. (6) Niemann, J. A.; Rogers, L. 8. Sep. Sci. 1975, IO, 517-545. (7) Jahn, K. R.; Wenclawiak, B.. unpublished work. (8) Tietze, U.; Schenk, C. Halbleiferschaffungstechnlk:Springer: Berlin, FRG. 1976. (9) Fjeldsted. J. C.; Jackson, W. P.; Peaden, P. A,; Lee, M. L. J . Chromatogr. Sci. 1983, 27, 222-225. (10) Peaden, P. A.; Lee, M. L. J. Chromafogr. 1983, 259, 1-16. (11) Van Lenten, F. J.; Rothman, L. D. Anal. Chem. 1976, 4 8 , 1430-1 432.
RECEIVED for review September 26,1985. Resubmitted August 14,1986. Accepted September 10,1986. We wish to thank the Ministerium fur Wissenschaft und Forschung des Landes NRW (FRG) for its support to our studies.
CORRECTION Probability Distributions of the Number of Chromatographically Resolved Peaks and Resolvable Components in Mixtures Michel Martin, David P. Herman, and Georges Guiochon (Anal. Chem. 1986,58, 2200-2207). On p 2201 the denominator in eq 4 should be (n- 2) rather than (n - 1). The equation should read as follows: op2 =
[
(
( p - 1) 1 + ( m - 2) 1 - n : 2)m-2
-
1
( P - 1)
