740
Anal. Chem. 1986, 58, 740-743
0 = qLJ2DA/(kT)2
(48)
The sign of 6 is generally negative because A is negative, making convenient the use of the absolute value notation, lei, as found in the final term of eq 47. When eq 47 is solved for S, we obtain
S=
S"
(1
+ (O(So2t)1/2
(49)
which shows the time-dependent form required for S in order to maintain a constant A. An equation of this form was suggested previously on intuitive grounds as yielding a constant relative departure of R from its steady-state value (5). This departure, expressed by A, can be assigned any desired value; the appropriate program is then specified by eq 49. However, if the chosen A is too small, then the program will evolve too slowly to conform to the needs of rapid analysis. The above treatment is a general formulation valid for any subtechnique of FFF. The factors involved in selecting the initial value, So, of field strength have been detailed elsewhere (5). Because S values programmed according to eq 49 decrease only gradually for large t , it might be profitable to phase in flow programming during the latter parts of the field program.
NOMENCLATURE
A, amount of particulate material above unit area of accumulation wall B,, integration constant B2, integration constant e, concentration e*, steady-state concentration steady-state concentration at accumulation wall D , particle diffusion coefficient d, effective (Stokes) diameter G, sedimentation field strength k, Boltzmann's constant 1, cloud thickness m , particle mass R, retention ratio R*, retention ratio under steady-state conditions S, generalized field strength T , temperature t , time t,, time necessary to generate 1 theoretical d a t e ti, retention time
U , displacement velocity of particles due to field v, particle migration velocity down channel u, flow velocity ( u ) , mean flow velocity x , coordinate position above accumulation wall
w ,channel thickness A, fractional departure of R from steady-state value am, error incurred in steady-state calculation of mass m at,, gain in retention time due to secondary relaxation e, fractional departure from steady-state concentration q, viscosity 6, constant defined by eq 48 A, retention parameter p , carrier density T , relaxation time constant T ~ primary , relaxation time T ~ secondary , relaxation time TS, time scale of program 4, field-particle interaction parameter LITERATURE CITED (1) Giddings, J. C.; Myers, M. N.; Caldwell, K. D.; Fisher, S. R. I n "Methods of Biochemical Analysis"; Gllck, D., Ed.; Wiley: New York, 1980; Vol. 26, p 79. (2) Giddings, J. C. Anal. Chem. 1981, 53, 1170A. (3) Giddings, J. C. Sep. Sci. Techno/. 1984, 19,831. (4) Yang, F. J. F.; Myers, M. N.; Glddings, J. C. Anal. Chem. 1974, 4 6 , 1924. (5) Giddings, J. C.; Caldwell, K. D. Anal. Chem. 1984, 56,2093. (6) Castellan, G. W. "Physical Chemistry", 3rd ed.; Addison Wesley: Reading, MA, 1983, p 828. (7) Yang, F. J.; Myers, M. N.; Giddings, J. C. Anal. Chem. 1977, 49, 659. (8) Giddlngs, J. C. Anal. Chem. 1985, 57,945. (9) Giddings, J. C. J. Chem. Phys. 1968, 4 9 , 81. (10) Giddings, J. C.; Karaiskakis, G.; Caldwell, K. D.; Myers, M. N. J. Colloid Interface Sci. 1983, 92,66. (11) Giddings, J. C. J. Chem. Ed. 1967, 44, 704. (12) Giddings, J. C.; Martin, M.; Myers, M. N. J. Chromatogr. 1978, 758, 419. (13) Yau, W. W.; Kirkland, J. J. Anal. Chem. 1984, 56,1461. (14) Giddings, J. C. J. Chem. Phys. 1959, 31, 1462. (15) Giddings, J. C. "Dynamics of Chromatography"; Dekker: New York, 1965. (16) Giddings, J. C. J. Chem. Phys. 1957, 26, 1210. (17) Giddings, J. C.; Shin, H. K. Trans. Faraday SOC. 1981, 57,468. (18) Giddings, J. C.; Yoon, Y. H.; Caldwell, K. D.; Myers, M. N.; Hovlngh, M. E. Sep. Sci. 1975, 10, 447. (19) Kirkland, J. J.; Yau, W. W. Science 1982, 218, 121. (20) Giddings, J. C.; Caldwell, K. D.; Moellmer, J. F.; Dickinson, T. H.; Myers, M. N.; Martln. M. Anal. Chem. 1979, 51,30.
RECEIVED for review September 10,1985. Accepted November 11, 1985. This work was supported by Grant GM10851-28 from the National Institutes of Health.
Capillary Supercritical Fluid Chromatography with Dual-Flame Photometric Detection Karin E. Markides,* Edgar D. Lee, Randy Bolick, and Milton L. Lee*
Department of Chemistry, Brigham Young University, Provo, Utah 84602
A dual-flame photornetrlc detector was modified to be used In a capillary supercrltlcal fluid chromatographic system. The detection limn was found to be 25 ng for the sulfur mode and 0.5 ng for the phosphorus mode when supercrltlcal carbon dloxlde was used as the mobile phase. The advantages and llmltatlons are dlscussed and demonstrated with several appllcatlons.
The flame photometric detector (FPD) has found extensive use in high-performance gas chromatography for the analysis 0003-2700/86/0358-0740$01.50/0
of pollutants in air and water, pesticides, and coal hydrogenation products (I). Selective detection of compounds containing either sulfur or phosphorus has been reported for which the FPD response was in the subnanogram range and at least loo0 times that elicited from hydrocarbons. The flame photometric detector was fwst described by Brody and Chaney (2). Excess H,created a low-temperatureplasma that supplied sufficient energy to produce simple molecular species and excite them to higher electronic states at a low signal-to-noise ratio. Band emissions from the excited molecular species HPO and S2were detected as they returned to their ground state. 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 741
-
A dual-flame FPD, introduced by Patterson et al. (3),allowed significant improvements in the performance of the FPD. The dual-flame FPD consists of a primary air-H2 flame that partly combusts and decomposes the solute molecules. A secondary air-H2 flame then excites produced S2 and HPO species to light-emitting species for photometric detection. The design was shown to be unaffected by large sample volumes, and it could handle large volumes of solvent without extinguishing the flames. Typical additional problems in detector response (4),such as quenching of the signal by other coeluting organic compounds, were also absent in the dual-flame FPD. It was therefore reasoned that this detector design could be compatible with chromatography utilizing a supercritical mobile phase. Capillary supercritical fluid chromatography (SFC) extends the possibilities of high-efficiency separations beyond the limit of gas chromatography. The combination of mobile phases with low viscosities, high solute diffusivities, and high solvating strengths and open tubular columns with low pressure drops has made capillary supercritical fluid chromatography one of the most attractive alternatives today for the analysis of thermally labile and/or high molecular weight samples. The technique, because of the low operational flow rate ( ~ 1 - 1 0 p L min-'), has been successfully coupled directly to several valuable detectors used in either gas or liquid chromatography, e.g., mass spectrometric (5,6),UV absorbance and fluorescence (7), FTIR (8), and flame-based detectors (9) including the flame ionization and alkali flame detectors. Carbon dioxide and nitrous oxide were found to be compatible mobile phases with the flame detectors. Decompression of the mobile phase is necessary before it is introduced into the flame, and restrictors of different configurations have been reported for this purpose. Prevention of condensation in the restrictor can be accomplished by using a tapered fused silica restrictor (10) and by carefully eliminating cold spots in the detedor through additional heating (11). Sulfur, being a common element in complex high molecular weight fossil-fuel-derived extracts, can selectively be detected by flame photometric detection (12). Use of capillary SFC would increase the range of high molecular mass compounds that could be analyzed. Phosphorus, being an element in many thermally labile pesticides, could preferably be detected by using the mild capillary SFC technique. In this paper, we present results from the coupling of capillary SFC to a dual-flame photometric detector. The detector response linearity and detection limits were determined for both sulfur and phosphorus. The advantages and limitations are discussed and demonstrated with several applications.
EXPERIMENTAL SECTION The chromatographic system consisted of a modified Varian liquid chromatographic syringe pump (Model 8500) controlled through a pressure feedback by an Apple 11+ microcomputer and a Varian gas chromatographic oven (Model 3700) equipped with a modified dual-flame photometric detector (Varian, Walnut Creek, CA). The sulfur filter used was a broad band-pass filter with peak transmission at 365 nm. For phosphorus detection, the filter used was a narrow band-pass filter at 530 nm. Modification of the detector involved shortening of the detector base and insert by 4 cm and adding a makeup gas line slightly above the primary air inlet; see Figure 1. The helium makeup gas was set at a flow rate between 30 and 40 cm3min-' during operation. The hydrogen and air gases were set at flow rates 25% higher than the reported optimum fllows ( 3 ) . The supercriticalfluid, COzor NzO,was delivered by the syringe pump to a 75 pm i.d. fused silica column, which was housed in the chromatographic oven. Fused silica capillary tubing, 15 m X 75 pm i.d. (Hewlett-Packard, Avondale, PA), was statically coated with poly(methy1-n-octylsiloxane) (13) to give a film thickness of 1.0 pm and cross-linked3 times using azo-tert-butane (14).
9
wT Figure 1. Schematic diagram of the dual-flame photometric detector before and after modification for supercritical fluid chromatography: (1) secondary flame jet, (2) primary flame jet, (3) secondary air inlet, (4) H2 gas inlet, (5) modified detector base, (6) makeup gas inlet, (7) primary air inlet, (8) column (restrictor),and (9) detector base.
t
1
L
/
I
h Y
a,
0
0 0
Amount lnlected (ngl
700
Plots of chromatographicpeak area for benzo[b]thiophene as a function of sample amount injected at 83, 104, and 167 atm, respectively. Conditions are as follows: 15 m X 75 pm i.d. fused silica column, n -0ctyl polysiloxane stationary phase ( 1.O-pm film thickness) cross-linked with azo-t-butane, C 0 2 mobile phase at 40 O C . Figure 2.
Samples to be analyzed were introduced into the capillary column with a 0.2-pL internal sample volume valve (Valco, Instruments, Houston, TX). An inlet splitter (15) was restricted to give a split ratio of 4:l between split and column outlet. A 10.5 cm X 7 pm i.d. fused silica capillary was used as a restrictor between the column and detector. Connectionto the column was achieved by means of a butt connector (SGE,Austin, TX). The restrictor end was inserted into the detector base and positioned approximately2 mm below the tip of the primary flame jet. The detector temperature was set at 240 "C. A base-line correction was used for density-programmed analysis in the sulfur mode. Several points on the drifted base line were manually entered into a chromatographic software program that immediately performed the correction. RESULTS AND DISCUSSION The commercially available dual-flame FPD was modified to make it compatible with capillary SFC demands. For optimum sensitivity, the temperature of the diffusion flame must be as cool as possible (3). This is achieved by decreasing
742
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
2500
met homyl
B
CH3C=NOC N HCH3 500
I
1
I
I
SCH3
I
Density Ig nL-11
0.3
0.7
Flgure 3. Plots of chromatographlc peak area as a function of density for injections of 150 ng of benzo[b ]thiophene. Chromatographic conditions are as in Figure 2.
0
Time lrninl
36
Figure 5. Supercritical fluid chromatogram of methomyi. Conditions are as follows: column as In Figure 2, C02 mobile phase at 60 OC, isoconfertic at 0.55 g mL-'.
Density (Q/mL) 0.55 Time (min)
t
Density (g/mLl 0.55 t Time (minl 0
0.55
0.60
0.65
16
0.55
0.60
0.65 26
Figure 4. Uncorrected (A) and computer base-line corrected (B) chromatograms of a PASH standard mixture. Conditions are as follows: column as in Flgure 2, COP mobile phase at 120 "C,density programmed from 0.55 g mL-l to 0.65 g mL-l at 0.005 g mL-' min-' after an initial 8-mln isoconfertic period. the flow of fuel gases and by adding high thermal conductivity gases such as helium and hydrogen. A gas line for helium makeup was added to the modified detector base to dilute the fuel gas and to improve the transfer of sample to the flame. A shortening of the detector was also necessary in order to
minimize the length of the restrictor between the column and the lower flame. This burner design could be used at pressures up to 160 atm at a temperature of 60 "C without any solvent flame-out. The linearity of the FPD signal in the sulfur mode was determined by injecting benzo[b]thiophene at 40 "C and by using supercritical carbon dioxide as the mobile phase. Plots of peak area vs. concentration at three different pressures are shown in Figure 2. The peak area increased nonlinearly with an increase in concentration, and the peak area increased faster when using a higher pressure mobile phase. An exponential increase in the peak area with respect to the sulfur concentration is what would be expected from compounds containing a single atom of sulfur (1). This nonlinear, nonexponential relationship can be explained if formation of S2molecules in the lower flame is dependent on how close the sulfur-containing molecules are to each other during decomposition. A plot of peak area vs. density at a constant concentration of 150 ng of benzo[b]thiophene and with supercritical COz as the mobile phase at 40 " C is shown in Figure 3. This graph verifies an increase in peak area with respect to increased density. An increase in density increases the concentrationin the detector at any point in time by narrowing the solute peak width, causing the sulfur atoms to associate more efficiently, which results in a larger signal. A relatively poor detection limit of 25 ng for benzo[blthiophene (SIN = 2) was found for the dual-flame FPD when operated in the sulfur mode. A combination of several factors can be responsible for this high detection limit. Several articles have been published that describe the interferences from coeluting hydrocarbons (4,16,17). CO, has almost the same emission intensity as hydrocarbons at 394 nm (18). The mobile phases, carbon dioxide and nitrous oxide, were experimentally found in this study to give some emission at 365 nm that interfered with the sulfur response and contributed to a high background noise level. Another Sz band could possibly give better performance. Quenching of the signal from a high concentration of CO can also explain the high
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
1. parathion
B
2 2. chloropyrifos
743
corrected chromatogram of several sulfur heterocyclic compounds. Use of the FPD in the sulfur mode at constant density is demonstrated in Figure 5 by the elution of the thermally labile pesticide methomyl. No interferences from the mobile phase even during density programming were found when the detector was operated in the phosphorus mode at 530 nm. The phosphorus mode possessed high sensitivity with a detection limit of 0.5 ng (SIN = 2) for parathion. Only a slight base-line rise was detected during a program from 50 to 200 atm, and no base-line correction was required. Figure 6 shows an example of a separation of three pesticides in the sulfur and phosphorus modes, respectively. In conclusion, the dual-flame FPD can be a useful detector in capillary SFC. Density programmed analysis with C02as mobile phase and sulfur selective detection at 365 nm demands a base-line correction. The sulfur mode is less sensitive than the phosphorus mode because of light emission of C 0 2 at 365 nm. Sulfur- and/or phosphorus-containing thermally labile pesticides can be chromatographed and selectively detected by using capillary SFC with a sulfur- or phosphorus-selective FPD.
LITERATURE CITED
Figure 6. Supercritical fluid chromatogram of a standard mixture of sulfur- and phosphorus-containing pesticides: flame photometric detection in (A) sulfur mode (365 nm) and (B) phosphorus mode (530 nm). Conditions are as follows: column as in Figure 2, C 0 2 mobile phase at 120 OC, isoconfertic at 0.5 g mL-’.
detection limit. With the flame temperature as low as possible, the CO and CH formation should be minimized. The burner design is, therefore, a factor that greatly influences the performance of the FPD. An increase in base-line noise and a rising base line were also observed with an increase in density when the detector was operated in the sulfur mode. Because of the COzemission at 365 nm, it would be expected that higher detection limits should result at higher densities. However, this effect is not large enough to offset the trends shown in Figures 2 and 3. A drifting base line may cause errors in peak detection, seriously affecting measurements of peak areas and retention times. The base-line rise can be compensated for by a computer base-line correction routine. Figure 4 shows a comparison of the uncorrected chromatogram and the base-line-
Lee, M. L.; Yang, F. J.; Bartle, K. D. “Open Tubular Column Gas Chromatography”;Wiley: New York, 1984. Brcdy, S. S.; Chaney, J. E. J . Gas Chromatogr. 1966, 4 , 42-46. Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1978, 50, 339-348. Patterson, P. L. Anal. Chem. 1978, 50, 345-348. Smith, R. D.; Fleklsted, J. C.; Lee, M. L. J . Chromafogr. 1982, 247, 231-243. Smith, R. D.; Udseth, H. R.; Kalinoski, H. T. Anal. Chem. 1984, 56, 2971-2973. Fjeldsted, J. C.; Richter, E. E.; Jackson, W. P.; Lee, M. L. J . ChromafOgr. 1983, 279,423-430. Novotny, M.; French, S. E.; Olesik, S. V. Fifteenth International Symposium on Chromatography,Nurnberg, FRG, 1984. Fjeldsted, J. C.; Kong, R. C.; Lee, M. L. J . Chromatogr. 1983, 279, 449-455. Chester, T. L. J . Chromatogr. 1984, 299,424-431. Richter, E. E. HRC CC, J. High Resolut. Chromatogr. Chromafogr. Commun., in press. Nlshloka, M.; Campbell, R. M.; Lee, M. L.; Castle, R. N. Fuel 1986, 65, 270-273. Kuei, J. C.; Tarbet, E. J.; Jackson, W. P.; Bradshaw, J. S.; Markides, K. E.; Lee, M. L. Chromatographia 1985, 20,25-30. Kong, R. C.; Fields, S. M.; Jackson, W. P., Lee, M. L. J . Chromafogr. 1984, 289, 105-116. Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L.; Springston S. R.; Novotny, M. Anal. Chem. 1982, 5 4 , 1090-1093. Dagnall, R. M.; Thompson K. C.; West, T.S. Analysf (London) 1982, 92,506-512. Farwell, S. 0.; Rasmussen, R. A. J . Chromafogr. Sci. 1976, 74, 224-234. Hunt, R. J., personal communication.
RECEIVED for review August 26,1985. Accepted November 12, 1985. This work was supported by the Gas Research Institute (GRI), Contract 5081-260-0586.
