Anal. Chem. 1688, 58, 2057-2064 2057

Performance of Capillary Restrictors in Supercritical Fluid Chromatography Richatd D. Smith,* J o h n L. Fulton, Robert C. Petersen, Andrew J. K o ~ r i v a , and Bob W. Wright Chemical Methods and Separations Group, Chemical Technology Department, Battelle, Pacific Northwest Laboratories, Richland, Washington 99352

The operating charact~rldlcs of capillary flow redrktors to control hear v.kdty and for trMIIpart andytee to gas phase analysis devices have been Investigated for wo In ruporcrl- tkal W chromaography (SFC). Expsnrkn of the fluid from too low a temperature can result @ formation of a condensed solvent phase and problems In flow regulation. Fluid flow rates for expandons that avoid two-phase regions can be estimated to Wlthln -30%. The expansion proteas has been directly observed and solvent droplet size was found to be strongly on fiulsI temperature, with average dropkt size decreasing to under 0.2 pm above the crltkal tempera- ture and being negQlble for reduced temperatwe8 X . 3 . The source d detector nolse with flame knlzatkn detecton (FID) and the loss of less volatile compounds In SFC results from precipitation and collection of the anatyte on the redrlctor during expansion of the fluid. The transport of low volatlllty amilytes bfacNtated by heating the fluid prbr to the rerMctor or, w erfecthrdy, by heating in the regtrlctor. The succesm tranrpolt and Wectlon d nonvotatlb comoounds by FID with SFC are demonstrated.

Supercritical fluid chromatography (SFC) allows fast or high-resolution separations of inany less volatile or labile compounds not amenable to gas chromatography (1-10). The importance of preasure in controlling fluid density (and, hence, fluid solvating power) is well-established and fine control of this parameter is vi td for obtaining high-remlution separations for wide ranges of compounds (11). Another advantage of S W compared to HPLC is much greater compatibility with gas- phase detectors. Capillary SFC with flame ionization detection (FID) has been shown to be applicable to even relatively high molecular weight compounds (5,7) when using fluids giving negligible detector response. The mass spectrometer has also been demonstrated to be a nearly universal detectot for SFC that is both sensitive and selectiue (1-4). It provides com- patibility with virtually any fluid or fluid mixture and a variety of mass spectrometric methods to enhance selectivity (e.g., alternate ionization modes, high resolution, and MS/MS) (3, 4, 9, 10).

Flow control for small diameter (<lo0 pm) capillary SFC is typically obtained by use of a capillary restrictor with di- mensions empirically selected to give the desired linear ve- locity. The capillary restrictor can be short with a relatively small inner diameter or longer with a larger inner diameter; the latter is more desirable for situations in which the re- strictor follows the detector, as is the case with on-column W and fluorescence detectors. The behavior of drawn (tapered) or frit restrictors (12) can also be approximated by a capillary tube, since the degree of taper is thically quite small near the end of the restrictor (5), while frit restrictors function by providing numerous convoluted paths, and likely behave similar to an array of much smaller diameter capillaries.

The ideal restrictor for capillary SFC would have the fol- lowing characteristics: (a) uniform pulse-free flow, (b) im- munity from plugging, (c) easy replaceablity or capability of flow rate variation, and (d) provision for complete transfer

of labile or nonvolatile solutes to gas phase detectors without pyrolysis or formation of analyte particles not amenable to the detector. Larger diameter restrictors have more repro- ducible fluid flow characteristics and are less likely to be plugged by particulate matter (from samples, the injector valve rotor, column unions, the stationary phase, a poorly filtered fluid supply, or precipitation of analytes) than would small diameter capillaries. However, long restrictors are generally unsuited for gas-phase detectors since less volatile analytes wil l precipitate before reaching the analysis region. For fluids cooled to a subcritical liquid prior to the restrictor (where the analyte may still precipitate due to lower solubility in the subcritical liquid than in the supercritical fluid), it is necessary to incorporate a method of solvent vaporization, obviating the advantage of a supercritical mobile phase.

Although restrictor performance is clearly vital to capillary SFC, little has been reported regarding selection of restrictor dimensions and operating parameters. Earlier we qualitatively described the requirements for capillary restrictors (9, 10) relevant to SFC-MS. In the present work we examine various aspecta of restrictor performance. Many of the previous (apparently) contradictory observations regarding fluid linear velocity are shown to be readily explainable. The fate of analytes during the decompression process is considered and the source of "spiking noise" in previous studies with FID detectors determined. Finally, the criteria for capillary re- strictors that approach the ideal situation And implications for other restrictor designs (e.g., tapered and porous frit) are described, and the difficulties of meeting all requirements for all possible analytes at low flow rates are discussed.

EXPERIMENTAL SECTION The instrumentation for capillary SFC with COz and FID is

the same as that described previously (1-3, 11). The system consisted of a computer-controlled syringe pump to generate the pressure regulated flow of purified and distilled carbon dioxide. Temperature regulation and flame ionization detection were provided by a Carlo Erba 4160 gas chromatograph. A Valco C14W HPLC injection valve with a 60-nL rotor volume was used for sample introduction. Flow restrictors were connected to the end of the chromatographic column using a zero dead volume union. Linear velocities were measured by use of methane to assure negligible retention. Various restrictor designs were explored and moat frequently consisted of either a short length of -5-9 pm i.d. fused silica capiuary or a short length of 25 pm or 50 pm tubing drawn using a reproducible procedure similar to that described by Chester (5). The F D base was modified to provide improved restrictor temperature regulation independent of the oven. Ca- pillary columns were prepared as described previously (1-3).

The high flow rate studies (more relevant to packed columns of >1 mm diameter) with propane and pentane utilized a similar system but employed a conventional HPLC pump. Analyte particles from expansion of supercritical fluid solutions were prepared by pumping solutions (10-50 ppm) at the desired pressure through -20 m of 250 pm i.d. stainless steel tubing to allow thermal equilibration prior to expansion through the re- strictor. Fused silica capillaries of 5-75 pm i.d. were used. An optical microscope was used for direct observation of the restrictor exit. Particles were collected on glass slides for optical and scanning electron microscopy (SEM). The supercritical water expansions used a Haskel high-pressure piston pump with a

0003-2700/86/035&2057$01.50/0 Q 1986 American Chemical Society

2058 ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986

Figwe 1. Schematic illustration of the supercritical fiuid expansion for an adiabatic process where the initial and final states of the fluid are known.

Tescom back pressure regulator. The water was heated to su- percritical conditions by passing through 1 m of 3.2 mm o.d., 1 mm i.d. stainless steel tubing, heated using a regulated electric current, and connected to a 10 mm long, 100 Mm i.d. stainless steel nozzle (23).

RESULTS AND DISCUSSION Expansion from Supercritical Conditions: Thermo-

dynamic Considerations. The flow properties of a restrictor, and to a significant degree the transfer of leas volatile solutes to the gas phase during an expansion, depend upon the state of the fluid prior to expansion, the dimensions of the restrictor, restrictor heating (and heat transfer properties of the re- strictor), and the state of the fluid during and after the ex- pansion. In this discussion we assume linear capillary re- strictors; later we will briefly consider the implications of more complex tapered or porous frit restrictor designs. Figure 1 illustrates the process schematically for an adiabatic process in which the enthalpy of the fluid is the same before and after the expansion (HJ . Expansions relevant to SFC occur under conditions where the Joule-Thomson coefficient, ( ~ T / ~ P ) H , is generally positive and net cooling results (14). If the re- strictor is very short, the process may be considered adiabatic. Conversely, for a very long restrictor at constant temperature, the expansion may be considered isothermal. In either case, the equilibrium state of the fluid in the restrictor and after expansion (if also isolated from surroundings) can be predicted from thermodynamic data. Both limiting cases may be rea- sonable approximations for restrictor designs used in SFC. However, the state of the fluid during and shortly after ex- pansion depends upon physical processes related to the ex- pansion which are kinetically controlled and inherently non- equilibrium.

For a flow process a t steady state, the energy balance can be written as

where AH is the enthalpy change between the inlet and outlet streams of the system, u b is the turbulent flow bulk velocity, g, is the gravitational constant, g is the gravitational accel- eration, and z is the vertical position. Q is the heat added and W is the work extracted from the system (which is zero). For adiabatic flow through a sufficiently short restrictor, Q = 0, and the process is only reversible during the brief isen- tropic expansion phase, which begins a t the end of the re- strictor. For horizontal tubes the potential energy term (Azg/g,) is zero and eq 1 can be written as

When the fluid is accelerated to the speed of sound, the kinetic energy term is typically <lo% of the initial enthalpy or -20 cal/g. This is usually a small correction that is discarded after

Entropy (kcalikg, O K )

Figure 2. Temperature-entropy diagram for C02. Dashed lines show constant enthalpy ( H ) paths for a reversible adiabatic expansion.

viscous dissipation of the jet (AH = 0). Enthalpy and pressure data for carbon dioxide as a function

of entropy and temperature are given in Figure 2 (15). The figure shows two-phase regions where the vapor and condensed phase exist in equilibrium. Flow through a short restrictor will be nearly isenthalpic until the end of the restrictor, where isentropic expansion at supersonic velocities occurs (10). Shock phenomena and interaction with gas in the expansion region largely convert the kinetic energy of the gas jet back to internal energy (9,10). For COz an initial enthalpy of <175 kcal/g (e.g., a fluid temperature of 70 "C and a pressure >lo0 bar) results in the expanding fluid traversing the two-phase region during an adiabatic expansion. In order to obtain good flow characteristics and avoid a two-phase region during COz expansion, a fluid temperature of >lo0 "C is usually adequate.

When expansion results in two phases, the assumption of adiabatic conditions allows the fraction of fluid in each phase to be calculated. Figure 3 gives the fraction of COz in a condensed phase after expansion to low pressure as a function of the initial reduced pressure and temperature (Xi), and provides general guidance for other single-component fluid systems. The minimum pressure and temperature required to avoid passage through a two-phase region for any isen- thalpic process corresponds to a reduced enthalpy of 1.15 for COz (solid line). The thermodynamic prediction of a two- phase product is entirely consistent with our previous ob- servations of extensive cluster formation in the electron impact ionization mass spectra of COz a t lower expansion tempera- tures (16).

The expansion of supercritical water from 600 bar and various temperatures through a stainless steel restrictor (10 mm long, 0.1 mm id.) into air a t 100 "C is shown in Figure 4. Supercritical water has potential analytical applications including controlled pyrolysis mass spectrometry, high-tem- perature inorganic SFC, and introduction for plasma emission analysis and other spectroscopic methods. The identity of the fluid is relatively unimportant since the degree of con- densation during expansion is governed primarily by the re- duced temperature and pressure of the fluid. The gas or steam

1 7 , 1.0,

I I I I I 2 1 5

I d U C e d ?mule

ngm 3. ~aducsd temperawe-reduced presswe diaprn for a w P e r m fkld (cod showfrg hes Of -m VaPa-RqUld composltbn after expansion to low pressures.

jet formed has an average droplet size that decreases as fluid temperature increases, due to a decreasing fraction of water in the liquid phase, and gradually becomes less visible. At a temperature just above where a two-phase product is pre- dicted (500 "Cat 600 bar) the jet is nearly invisible (Figure 4, bottom) even though the flow rate is about 20 mL/min (liquid). The contribution of isentropic cooling to the droplet size during expansion is difficult to estimate due to kinetic limitations. The observed behavior is consistent with ex- pectations based upon Figure 3.

Flow-Through SFC Capillary Restrictors: Kinetic Considerations. The state of the supercritical fluid solvent before and after the restrictor can be approximated by the equilibrium situation. However, the expansion is a none- quilibrium process where the rate of droplet formation or evaporation, when a two-phase region is entered, will be ki- n e t i d y limited. A more complex situation prevails for dilute analytes (and less volatile fluid modifiers), which tend to rapidly nucleate a t lower temperatures and precipitate. De- viations from adiabatic conditions due to heat transfer a t the restrictor walls can also be critical to SFC applications. The details of this p r e s s are beyond the scope of this work, but it will be shown that these nonequilibrium processes can be vital to the successful application of gas-phase detectors in SFC.

Estimation of Flow Rates through Restrictors. In addition to dowing efficient analyte transfer to the detector, a primary requirement for capillary SFC restrictom is to provide acceptable flow rates or linear velocities over the range of pressures relevant to a given separation. For short re- strictors the flow rate during an adiabatic expansion cannot exceed the speed of sound until within a few nozzle diameters of the end of the capillary (17. 18). A convenient method of estimating flow rates of compressible fluids through straight-walled tubes is based upon friction losses from the Fanning friction factor

(3)

where F is the friction loea, f is the Fanning friction factor, G is the flow rate, p is the fluid density, L is the capillary length, and d in the inner diameter (19). For a compressible

ANALYTICAL CHEMISTRY, VOL. 58. NO. 9. AUGUST 1986 M5B

I

A- I

I a I

Flgure 4. Photographs of water expansions from 600 bar at various preexpansion temperatures (T,) Showing the liquid droplet size de- crease rapidly.

fluid the Fanning friction factor has been used successfully in differential form (18). The differential form of the energy balance (eq 1) then becomes

vdp+ u - d u = [ E - "1 &

where Vis volume, P the pressure, and u the instantaneous velocity. Integrating this equation allows estimation of flow rates in the adiabatic and isothermal limiting cases (18). If ideal gas behavior is assumed, the V dP term is only a function of V and does not introduce large errors (>30%) in most practical applications where the two-phase region is avoided. A more exact solution could be obtained by a similar deriv- ation using an appropriate equation of state. The following relationship was developed by Lapple (18) using the Fanning friction factor to determine maximum mass flux

(4 ) g c gc gcd

Gma=P1(&)(&) (5 )

where P, is the inlet pressure, TI is the inlet temperature, M is the molecular weight, and G, is the maximum maas flux (g/(scmz)) attainable through an infinitely short restrictor.

Equation 4 and 5 can be evaluated to yield a simple rela- tionship for estimating mass or volumetric flow (Q) of a fluid through a capillary restrictor

Q (mL/min) = 31.3Fr- 'Idz( !!)"' (6) P

where PI is in bar, d is capillary diameter in mm, TI in K, and

2060 ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986

' - I I

0.8 b 4 0.4 dc" ; I - \

f5 1 5 0.2

Capillary Aspect Ratio, Lld

Flgure 5. Flow reduction parameter from eq 6 as a function of ca- pillary aspect ratio (L I d ) for an adiabatic expansion to sonic velocities at the capMary exit. Also ghren Is the maximum pressure drop ratio (f *If ,) resuitlng in sonic conditions at the exit.

F, is the flow reduction parameter based upon fluid viscosity and capillary dimensions (18). Figure 5 gives typical values for F, in terms of the capillary length to diameter ratio (L /d ) applicable to adiabatic expansions of supercritical fluids such as COz and H20 when sonic conditions exist at the end of the restrictor. The calcualtions assume an abrupt edge entrance to the capillary and f = 0.0047. Also shown in Figure 5 is the maximum capillary outlet/inlet pressure ratio (P2/Pl) for sonic velocities at the capillary exit. When P2/Pl is below the minimum for sonic flow (as is usually the case), then the pressure at the capillary exit (P& can be estimated since the ratio becomes Pexit/Pl. For such an adiabatic expansion the fluid temperature at the capillary exit (Texit) is given by (18)

(7)

where y = C,/C,, the ratio of heat capacities at constant pressure and constant volume. Note that when y = 1.4, the approximate value for C 0 2 and H20, then Texit/T1 = 0.83. Thus, the fluid density at the capillary exit can be estimated as

P1

Figure 6 gives flow rate measurements obtained for super- critical pentane (T, = 196.6 "C) at 250 "C through 6.4 mm long restrictors, with 9 pm, 25 pm, 50 pm, and 75 Fm i.d., for decompression to atmospheric pressure. Estimated flow rates were calculated from eq 6 with F, = 0.66 and show reasonable agreement with experiment. Greater deviations at high pressures probably reflect the limitations of the ideal gas assumption. Good agreement was also observed for super- critical water expansions at 600 bar and 500-600 "C. It is interesting to note that calculated fluid density at the end of the capillary restrictor can still be quite large. For example, at 250 "C and 240 bar ( p = 0.47 g/cm3) and a flow rate of 150 pL/min (liquid) for the 9 pm i.d. capillary, the pentane density at capillary exit was approximately 0.12 g/cm3. Under these conditions the average fluid density (0.29 g/cm3) can be used to estimate the average velocity in the 9 pm restrictor at 8400 cm/s, corresponding to a residence time of 7.5 x s in the restrictor. Such estimates of the fluid flow rate and expansion conditions give guidance in selection of restrictor dimensions and conditions for maximizing fluid phase solubility at the capillary exit.

Restrictors for Capillary SFC. The flow rates reported in Figure 6 are approximately 2 to 3 orders of magnitude larger

1 C

E

E

7

- 6

E E Y 5

3 E 4

-

cc

3

2

1

0

u nn

0 AA

0 0 / 0 M

Q "25 pm" I D. A "50 pm" I.D. 0 "75 prn" I.D. 0 "9 pn" I.D. - Theoretical in all Cases

100 200 300 Pressure lbarl

Flgure 6. Flow rate data as a function of pressure for supercritical pentane at 250 "C for various restrictor diameters. all 6.4 mm long, and predictions calculated from eq 6.

300

5 cm, 8 pm ID at 400T / /'

Theory

200 1 - m m e 2 E n

100

/ /'

A I I I I " 0 1 2 3 4 5

Mass Flux ( p x VI, g/cm2 * s

Figwe 7. Mass flux of COP in a short, smalldiameter heated (400 "C) restrictor and a longer, larger diameter restrictor thermostated at 20 "C. The dashed line shows the predicted mass flux for the smaller 5 cm X 8 pm 1.d. restrictor based upon the adiabatic assumption.

than desirable for capillary SFC and are more appropriate for conventional packed HPLC columns. Lower flow rates require longer or smaller inner diameter restrictors. For very long restrictors the assumption of an adiabatic process becomes invalid due to greater restrictor residence times, although the speed of sound limitation to maximum velocity still applies. The analyte will precipitate and ultimately plug a long re- strictor if it is not heated to vaporize the analyte or cooled to maintain sufficient density for solubility.

Figure 7 shows typical data for the variation in flow rates (mass flux) with carbon dioxide for two restrictors where significant deviations exist from the adiabatic assumption.

ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986 2061

C 0 2 Mass Flux

100 150 200 250

Pressure (Bar)

Figure 8. SFC linear velocity ( v ) as a function of pressure at various temperatures for a 5 cm X 8 pm i.d. restrlctor at 275 "C.

The lower curve was obtained with a 36 cm X 50 r m i.d. fused silica capillary restrictor maintained at -20 "C. The second curve was generated by use of a 5 cm X -8 pm i.d. capillary in a temperature regulated FID block at 400 "C. The first m e is a complex situation in which the fluid was cooled during expansion to the liquid (at high pressures) and underwent a liquid/gas transition in the restrictor at a location dependent upon pressure. Although a relatively linear relationship was obtained between mass flux and pressure, significant devia- tions are expected a t both higher and lower pressures (note the nonzero intercept). Flow rates in the short 8 pm i.d. restrictor were sufficiently fast that more nearly adiabatic conditions existed. Here fluid temperature in the restrictor was a complex function of the entering fluid at the SFC column temperature, the heat transfer properties of the re- strictor, and physical configuration (e.g., heating method, restrictor outer diameter, etc). Figure 7 gives the calculated mass flux (dashed line) from eq 6 using F, = 0.17 and the restridor heater temperature (400 "C) as the fluid temperature prior to decompression. The predicted mass flux is larger than experimentally observed, with the greatest difference at lower pressures, but still provides qualitative guidance in restrictor selection. The deviation probably reflects both the temper- ature variations and the limitations of the assumptions.

Increased restrictor temperature reduces the mass flux (eq 5) if the fluid is a gas. If the fluid is cooled to the liquid state in the restrictor, as is often the case for spectroscopic detection, mass flux will increase with temperature. This behavior re- flects the temperature dependence of fluid viscosity. Figure 8 shows typical SFC column linear velocity data for various SFC temperatures using FID with the restrictor heater at 275 "C, well above the fluid critical temperature of 31 "C. These data show that linear velocity had a nearly direct dependence upon pressure at higher temperatures, as predicted from eq 6.

If the SFC column and restrictor are independently ther- mostated, SFC mass flux (linear velocity times density, up) should be independent of column temperature. Figure 9 shows the SFC mass flux at six pressures as a function of column temperature for a restrictor heater temperature of 275 "C. At lower pressures the expected behavior was observed with mass flux nearly independent of SFC temperature. At higher pressures an increasingly significant decrease in mass flux was observed at higher SFC temperatures. This result implies that the effective restrictor temperature was increased by the greater mass flux. Such behavior is consistent with data obtained at a higher restrictor heater temperature (400 "C), shown normalized to the lower temperature data in Figure 9 (dashed lines). The lower mass flux and the higher restridor

250 Bar @ - 7 - \ L - 275% Restrictor

3 I --- 400% Restrictor \ 275% Restrictor 400% Restrictor

e I 150Bar -e-----==

0 1 I I I I 0 50 100 150 200

SFC Column Temperature I°C)

Flgure 0. SFC mass flux at various temperatures as a function of column temperature for a restrictor heater temperature of 275 OC and for similar measurements at 400 OC (dashed lines).

heater temperature reduced the heating effect of the fluid. These results support the assumption that heating can be inefficient for short restrictors due to both the high mass flux and the poor thermal conductivity of fused silica.

The relationship between restrictor temperature and mass flux also explains previous observations of SFC linear velocity either increasing or decreasing during pressure programming. A constant temperature fluid entering the restrictor results in a linear dependence of mass flux on density. However, if the SFC temperature is close to the critical point, the density increase in the SFC column with pressure can be much greater than the increase in mass flux through the restrictor. Under such conditions linear velocity decreases as pressure is in- creased. Therefore it is possible to select restrictor temper- ature (as far as compatible with detector requirements) to control the variation in linear velocity during pressure pro- grammed operation.

Transport of Nonvolatile Compounds to Gas-Phase Detectors. A vital property of SFC restrictors is their ef- fectiveness for transporting "nonvolatile" compounds to various gas-phase detectors. Success depends on the restrictor design, fluid pressure and temperature at the entrance of the restriction, the particular demands of the detector, and (under some conditions) the volatility or melting point of the "nonvolatile" compounds.

It has been observed by some capillary SFC workers (5,20, 21) that detection of less volatile or higher molecular weight compounds can become erratic, with the FID signal producing large spikes with a frequency on the order of 0.1-10 s-l. Lee and co-workers (20) have suggested an origin due to "clustering" and have shown that incorporation of a suitably long time constant (a few seconds) can produce a reasonable chromatogram. Chester (7) has also observed that increased fluid temperature delays the onset of spiking to later eluting (and typically less volatile) analytes and suggested a "spiking

2002 ANALYTICAL CHEMISTRY, VOL. 58. NO. 9, AUGUST 1986

delivery rate threshold", which varies with solute volatility. Heating of the fluid after the SFC column and prior to en- tering the restrictor is also effective. Heating of the fluid through the relatively long ( L / d > 1ooO) restrictors is some- what less effective due to the limitations upon heat transfer and the brief residence time in the restrictor. Restrictom drawn from larger diameter capillaries have performance fa- cilitated by heating both prior to the drawn restriction and perhaps some smaller contribution of heating in the restrictor due to the thinner capillary walls.

For truly nonvolatile compounds two distinct modes of operation appear feasible, the firat leading to spiking and the second to operating in a 'nucleation" regime where spiking is unlikely. In the spiking mode, the nonvolatile analyte tends to collect on restrictor walls and (if the local temperature is above the analyte melting point) flows toward the end of the restrictor. The liquid will collect a t the end of the restrictor and be periodically entrained in the high shear gas flow and transported to the detector. If the analyte has a high viscosity or can adhere to the capillary walls, it will tend to collect and often rapidly plug the restrictor (although sporadic detection may be briefly possible). This process is likely to depend on the phase dynamics, liquid viscosity. and restrictor geometry. Below the analyte melting point, operation becomes more sporadic, with larger irregularly shaped particles being ob- served and resulting in frequent plugging of the restrictor.

The fluid expansion process haa been directly ob~erved and various materials expanded from supercritical fluid solutions through 5-75 pm i.d. fused silica capillaries have been col- lected. Figure 10a shows a photograph of a solute droplet, which m be seen as a distinct stream emitted from the flat base of a 25 pm i.d. fused silica capillary restrictor. The polycarbosilane (Dow Corning), from a 30 ppm solution in pentane (T, = 196.6 OC), was observed to collect a t the ca- pillary exit and be periodically entrained in the gas flow, collecting on the head of a pin in Figure loa. The poly- carhosilane had a mean molecular weight of approximately 1430 and a melting point of -240 "C and is not volatile hut decomposes yielding silicon carbide a t >900 "C (22). Ex- pansion of polycarhilane in pentane a t 350 "C and 100 bar produced short fibers (Figure lob), due to the high shear forces that can elongate liquid polymer droplets. The particle for- mation rate is directly proportional to solute concentration. Particle size (either fibers or spheres depending on the solute and restrictor conditions) was found to increase with capillary diameter. Thew observations are consistent with the ~bserved FID spiking phenomena and demonstrate an origin due to complex heterogeneous processes involving the restrictor walls and solute melting point. The average particle diameter was in the 1-10 pm size range for 5-10 pm i.d. restrictors. As- suming an average of 1 ng of a nonvolatile solute in capillary SFC, on the order of l-lO* particles might be formed during elution of this compound. The particle production rates based upon elution of such an analyte over 10 s are entirely con- sistent with observed spiking rates for FID.

A second mode of operation producing much smaller par- ticles exits when analyte nucleation is delayed to near the end of the restrictor. Figure 1Oc shows polycarhilane collected under such conditions (30 ppm pentane solution a t 240 bar, 250 "C with a 6 mm X 25 pm i.d. restrictor). In this ease large quantities of nearly monodisperse particles with an average diameter of -0.02 pm were collected; 1 ng of analyte will produce on the order of 108-109 particles of this size. Clearly these particles are too small and produced a t too high a fre- quency ta result in FID spiking.

.' 1 1 1

Flgure 10. Photographs showing (a) the expansion of polycarbc- silane-pentane supercritical solution at -300 "C showing a fiber entrained in the high shear gas flow. (B) Fibers collected from a polycarbosiiane solution expanded from 350 "C and 100 bar. (C) Ultrafine poiycarbosiiane powder formed by expansion from 250 OC and 240 bar.

and fluid conditions enhancing solubility. Increased pressure always serves to enhance the transport of nonvolatile analytes, but in practice maximum pressure is determined by desirable SFC conditions. It is important to recognize that optimum conditions do not necessarily correspond to elevated restrictor temperatures. For example, the polycarbosilane particles shown in Fieure lob were formed at 350 "C and 100 bar. while

The major requirement for operation in the 'nucleation" mode is tn maintain solvating conditions to near the end of the restrictor. which is facilitated by the we of short restrictom

the very fine, nearly monodisperse, particles were produced at 250 "C and 240 bar. The solvating power of pentane for polycarhosilane is much greater under the latter conditions.

ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986 2063

L , , I I

Pressure (bar) ~ I I , I

I I

Polycarbosilane CO,, 1 5OoC

4 1 - 822

888 \ OET - 4 o o o c

1030

I 1 I I I , I 72 89 1- !E-_-- 219 318 360

Pressure (bar)

Flgure 1 1. Capillary SFC chromatograms of polybutadiene ( 1000 av mol wt) wfth restrictor heater temperatures of 325 O C and 400 O C .

Peak labels indicate molecular weights as determlned by SFC-MS analysis.

In addition, the particles were formed at gas temperatures below their melting point due to cooling during expansion. When pressure was decreased to less than 150 bar, the nu- cleation mode disappeared and large (10-100 pm) particles were observed to form sporadically at the tip of the restrictor, presumably by buildup of material that had precipitated earlier in the restrictor. Under these conditions extreme detector spiking might be expected and a very long restrictor (L/d >> l O o 0 0 ) would plug rapidly; although at temperatures >300 "C particles would be produced (as in Figure lob) and more regular FID spiking would occur.

The effect of temperature on fluid phase solubility is more difficult to predict (IO, 23), but some guidance is provided by analyte vapor pressures. For more volatile analytes a greater increase in solubility is typically observed with temperature. The improved performance obtained from drawn restrictors is explained in part by the improved heating (both prior to and in the restriction region) and increased fluid phase sol- ubilities for such analytes. In addition, few of the compounds that can be solvated by fluids such as COz are truly nonvolatile at temperatures of 100-400 OC. Compounds with quite low volatilities can be transported in the fast gas flow, and one would expect to find few analytes separated with C02 which could not also be volatilized from a direct-inlet probe at 400 "C for mass spectrometric analysis. Even very labile com- pounds will typically show little decomposition due to the brief period a t elevated temperatures. Figure 11 illustrates the effect of increased restrictor temperature on the detection of polybutadiene (average molecular weight N 1000). A signif- icant extension in the oligomer size that can be detected was observed when the restrictor heater temperature (RHT) was increased from 325 to 400 O C . Polybutadiene is representative of "less volatile" compounds for which this approach can be used to improve transport to gas-phase detectors.

The effect of restrictor temperature upon FID detection of polycarbosilane is shown in Figure 12. The most successful detection using drawn restrictors was accomplished a t the higher linear velocities and lowest detector temperatures,

1 I I I I I 50 204 272 340 390

/ I I

I I I I I 1 50 190 290 390

Pressure (bad

I I I I

I 0 10 20 30 40 50

50 200 300 3SO Pressure (bar)

Flgure 12. Capillary SFC chromatograms of poiycarbosilane under various restrictor heater temperatures (RHT). At higher RHT no sig- nificant FID signal was obtained.

although significant spiking was still evident. (No electronic filtering was used to smooth the FID signal.) At higher tem- peratures significant detector spiking was observed, which is consistent with the expected particle formation rate. At even higher restrictor heater temperatures, which correspond to lower solubility conditions for this compound, a complete loss of signal was observed, which is opposite the behavior observed for somewhat volatile compounds.

Very short restrictors (<1 mm) suitable for capillary SFC are difficult to fabricate and couple to capillary columns. The smaller orifice size also makes them somewhat more subject to plugging. This tendency is particularly a problem at fluid conditions leading to saturation during expansion (Le., lower pressures and temperatures), where frozen or condensed solvent can disrupt flow and cause plugging. At higher flow

2064 ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986

rates, such as used to produce the fine polycarbosilane powder in Figure lob, the requirements are relatively easy to meet, and the major difficulty is the large gas flow rate to the de- tector. Extended operation (>weeks) in the nucleation mode with flow rates >>lo0 hL/min have been demonstrated for nonvolatile compounds for systems as diverse as supercritical water with silica and germanium dioxide solutes, and su- percritical pentane with solutes such as polycarbosilane.

CONCLUSIONS In general, the best flow conditions for capillary SFC re-

strictors are obtained when relatively short restrictors are used and where the expansion is nearly adiabatic and the enthalpy of the fluid is sufficiently high to avoid transition through a two-phase region. Reduced temperatures of >1.3 generally avoid two-phase solvent systems for expansions from typical SFC pressures. For fluids such as C02 these conditions have the additional advantage of yielding improved FID perform- ance by reducing the cooling effect of the fluid and incidence of solvent clusters in mass spectrometry (16). Flow rates and fluid densities in capillary restrictors may be estimated by using relatively simple relationships based upon the adiabatic assumption and the speed of sound limitation.

Presently the most widely used fluid for capillary SFC is carbon dioxide. Due to the limitations in the solvating properties of this fluid, few compounds can be separated which cannot be readily detected by FID using a simple fused silica restrictor (5-10 pm i.d., 1-5 cm length) with effective heating. Improved performance can be ascribed to the often greater analyte solubility or vapor pressure a t the elevated restrictor temperature. The fluid can be most effectively warmed prior to the restrictor or much less efficiently warmed during de- compression where thinner fused silica walls appear to improve heat transfer.

For truly nonvolatile compounds, optimum conditions are obtained for very short restrictors and higher flow rates. In this case a “nucleation” regime exists compatible with FID detection. Operation in this mode is feasible for longer re- strictors, but optimum conditions are expected to depend on the solubility behavior of the analyte. If the temperature and pressure dependence of analyte solubility are known, the methods that have been described provide a basis for esti- mating optimum restrictor parameters.

A mode of operation, due to early analyte precipitation that produces particles from collection on restrictor surfaces, has been found to cause the troublesome detector spiking reported previously. In this case, higher pressures and either higher or lower restrictor temperatures can lead to improved oper- ation, depending upon analyte melting point and volatility.

The approximate behavior of more complex drawn (or ta- pered) and porous frit restrictors can also be predicted based upon these results. In the case of drawn restrictors with relatively gentle tapers, as typically used (5), the restrictor L / d ratio (Figure 5) can be reasonably approximated by using the dimensions of the drawn portion nearest the exit (where the diameter is smallest) and a diameter slightly larger than the actual exit diameter. The primary advantage of a drawn restrictor, compared to a similar length of small diameter capillary, results from the fact that the undrawn portion of the capillary allows effective heating of the fluid prior to expansion. This eliminates the need for an additional zero dead volume union in a heated region just prior to expansion. A similar benefit is obtained by using the porous frit restrictor. This design provides numerous convoluted flow paths, which may be pictured as an array of very small capillaries. Since

the porous frit is formed in a region at the end (- 1 cm long) of a length of capillary tubing, the larger diameter region before the frit can used to be effectively heat the fluid, fa- cilitating transfer of less volatile compounds. The porous frit design may have the additional advantage of being unlikely to plug due to the presence of a single particle of sufficient size to plug normal capillary restrictors. However, the very high effective L / d ratio (probably >IO6 due to the tortuosity of the fluid path) and the resulting subsonic velocity at the capillary exit suggest that t rdy nonvolatile compounds would rapidly be lost or plug porous frit restrictors.

The development of mass spectrometric detectors for ca- pillary SFC, however, presents a more demanding situation due to the wider range of fluid solvents and nonvolatile an- alytes. At present, the limited understanding of the particle growth process and complex phase behavior during expansion often makes compromises necessary for successful operation (e.g., very high flow rates and more frequent restrictor plug- ging) with nonvolatile compounds. Currently new SFC-MS interface designs are under development with the dual aims of exploiting and extending understanding of these phenom- ena.

ACKNOWLEDGMENT We thank H. R. Udseth, H. T. Kalinoski, E. K. Chess, and

E. G. Chapman of this laboratory for helpful comments and discussions.

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RECEIVED for review February 13, 1986. Accepted April 9, 1986. The authors gratefully acknowledge the support of the U.S. Army Medical Research Institute of Infectious Diseases and the U.S. Army Research Office under Contract DAAG29-83-K-0172.