2774
Anal. Chem. 1905, 57,2774-2779
Electrically Heated Cold Trap Inlet System for High-speed Gas Chromatography B. A. Ewels' and R. D. Sacks* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
An Inlet system for hlgh-speed gas chromatographythat uses a heated evaporatlon port wlth a splltter followed by an electrically heated and gas-cooled trap is described. A sample injected wlth a mlcrosyrlnge Is Immediately trapped and then relnjected by reslstlvely heating the metal trap. The trap is heated from a hlgh-current transformer using one or more half cycles of high current for rapld lnltlal heating followed by an Interval of lower current until the entire sample Is swept from the trap. The trap heatlng clrcult Is described. Trapping efficiency data are presented as well as solute reinjection proflles showlng the effects of sample she, compound bolllng polnt, and heating parameters. Trapplng efficlencles of 98 % and relnjectlon bandwidths of a few mllllseconds can be obtalned at gas velocltles of 150 cmh.
The separation of relatively simple mixtures by gas chromatography potentially can be accomplished in much less time than current practice would indicate. In the early 1960s, Desty and co-workers (1,Z) used short lengths of wall-coated open tubular (WCOT) columns operated a t high flow velocities to obtain good separations of mixtures containing 10-20 components in less than 10 s/sample. Operation a t very high flow velocities is practical with short WCOT columns because their much greater permeability relative to packed columns reduces the inlet pressure requirements to reasonable values, and their much less rapid loss in column efficiency with increasing gas velocity results in adequate resolution for many simple mixtures. Most of the GC separations currently done with packed columns should be amenable t o high-speed techniques, and the potential savings in time is very significant. In addition, certain classes of thermally labile compounds that are very difficult to determine by GC may be more easily determined by using high-speed techniques. Other potential applications include the rapid, repetitive analysis of industrial process streams and the rapid analysis of air samples in the industrial environment. Rapid and precisely controlled sample introduction is the key t o successful operation of a high-speed GC system. Sample injection bandwidths of only a few milliseconds must be obtained if extra-column band broadening is not to contribute significantly t o overall elution bandwidths. In order to obtain retention index values with a precision of a t least f0.1% for retention times of a few seconds or less, the time of sample introduction must be controlled to within a few milliseconds or less. Clearly, these requirements cannot be met with conventional microsyringe sampling techniques. Mechanical switching valves together with sample loops generally are too slow and cumbersome for the applications considered here (3, 4). Gas-flow switching systems using a solenoid valve located far from the actual switching point have Present address: E. I. du Pont de Nemours & Co., Agricultural Chemicals Department, La Porte Plant, P.O. Box 347, La Porte, TX
77571.
0003-2700/85/0357-2774$0 1.50/0
been used to inject narrow sample plugs (5, 6). However, reproducible sample size and the precise measurement of sampling time are difficult to achieve (7). Fluidic logic gates (8,9) cannot be used where quantitative sample transfer from a syringe or a purge-and-trap system is required. An electrically heated trap of capillary dimension should provide a versatile and efficient sample inlet system for high-speed GC. A gas or liquid microsample can be introduced directly into a conventional heated evaporator inlet system and the vapor immediately collected in the metal cold trap. A high-current pulse through the trap can resistively heat it in a few milliseconds (10). Several investigators have used traps of this type for pyrolysis GC (11) and heart-cutting techniques (12-14). However, very high flow velocities were not used in any of these studies. The prototype system described here uses a 0.5-mm-i.d. stainless steel tube as the cold trap. A relatively wide-bore trap was chosen because of its greater sample capacity without overloading and its greater volumetric flow rate for a given linear gas velocity. These features are useful for the rapid, direct collection of organic vapors from ambient air samples. This report describes the electrically heated cold trap inlet system and presents preliminary data on its operation at high gas velocities.
EXPERIMENTAL SECTION A diagram of the test facility is shown in Figure 1. It consists of a conventional heated injection port with a septum inlet and splitter, the electrically heated and gas-cooled stainless steel trap, a 60-Hz high-current power supply for trap heating, and a high-speed detector and readout system. A back pressure regulator (BPR) is used to obtain independent control of the split ratio and the gas flow velocity. A filter or trap between the BPR and the split point prevents organic compounds from deteriorating the rubber diaphragm. With no analytical column in the system, the inlet pressure is too low for proper operation of the BPR. A restrictor, typically a 5 m length of 0.35-mm-i.d. glass capillary, placed between the split point and the trap tube provides an adequate pressure drop. Cold Trap Design. The trap test facility consists of a 36-cm segment of 0.5-mm-i.d., 0.1-mm wall stainless steel tubing. A Teflon cooling sheath sets the length of the actual trap. The cooling sheath is formed by a 1.2 cm diameter hole in a 7.5 cm long Teflon block. The trap tube is sealed in the sheath by silicone septa at both ends. This is shown in the partial section drawing in Figure 1. Brass tubing in.) forms the inlet and outlet for the N2cooling gas. The flow of N2was always from the side nearer the injector to the side nearer the detector. The N2 was chilled by forcing it through l/s-in. copper refrigerator tubing in a Dewar of liquid N2. The flow of cooling gas was not interrupted during trap heating, and thus the trap rapidly cooled after completion of every heating cycle. The metal tube is clamped into 2.5 X 4.4 X 1.9 cm copper electrodes, which are connected by heavy-gauge cables to the electrical heating source. The electrodes can be heated by 2.5 cm long heating cartridges inserted near the trap. The temperature of the trap tube is measured at several points by microthermocouples (Omega, ChCo001). A 20-cm segment of the trap tube extending beyond the downstream electrode is used for connection t o the FID. This tube passes through the FID body and terminates just below the 0 1985 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOC. 57, NO. 14, DECEMBER 1985
2775
t5v T
I
I
u
FID
f N2
N2
Flgure 1. Test facility for the gas-cooled, electrically heated cold trap inlet system for high-speed GC: T, 0.5-mm4.d. stainless steel trap and connecting line; C, Teflon cooling sheath; P, power transformer used for trap heating; E, copper electrodes: A, silicone septa seals: H, heating cartridge: FID, zerodead-volume flame ionization detector: I, conventional heated inlet port: S, splitter: R, restrictor; BPR, back pressure regulator; G, pressure gauge; F, filter.
Table I. Component Values for the Trap Heater Circuit component R1 R2 R3 R4
c1 c2
c3 c4
T1,T2 SR1, SR2 IT AT MT
Figure 2. Trap heater control and power circuit. Component descriptions and values are given in Table I.
value/description 10 k3, 'I4 W 50 k 3 potentiometer 22 k3, J 4 w 83 3, 200 W
0.005 p F 0.001 p F 1.0WF 0.1 LF
LM555 timer zero-crossing solid-state relay (Midtex, 640-24LlOO) isolation transformer autotransformer muffle-furnace transformer
tip of the burner jet. Heat-shrink plastic tubing on the trap tube is used to provide electrical insulation between the trap tube and the FID. For most studies, the trap assembly was housed in an oven, which was maintained a t a temperature greater than the boiling points of the test compounds. T r a p Heater Circuit. The heater circuit shown in Figure 2 was adapted from a design by Hopkins and Pretorius (10). Component descriptions and values are given in Table I. Current to heat the trap is supplied from the secondary of a muffle-furnace transformer (MT). To obtain rapid trap heating without destructive overheating, two separate 60-Hz ac power sources are used. A high-current source, which is used for initial rapid heating, can deliver any number of current half cycles. After this initial heating phase, lower current is used to maintain the trap a t an elevated temperature for sufficient time to ensure that the entire sample has been swept out of the trap. The initial heating voltage is supplied by an autotransformer (AT) connected to a 208-V ac line. The sustainer voltage is obtained from an autotransformer connected to a 110-V ac line. A pair of zero-crossing solid-state relays (SR,and SR2)turn these voltages on and off under the control of a pair of 555 timer chips (T, and TJ. An R-S flip-flop initiates and terminates the entire heating cycle. Figure 3 illustrates the sequence of events during a heating cycle. Wave forms a and b are the 208 V and 110 V, 60-Hz line sources, respectively, from the two autotransformers. When the set NAND gate (S) is grounded, timer TI delivers a pulse which can be adjusted from 0 to 50 MS. This is indicated as wave form c. This pulse turns on SR1 at the next zero crossing of the 208-V
n (e)
V Flgure 3. Trap heater circuit wave forms: a, 208-V line voltage signal; b, 110-V line voltage signal; c, timer 1 output used to control the number of high-current half cycles for initial trap heating; d, timer 2 output used for a one half cycle delay between the high-current and the low-current heating phases; e, gating signal for the low-current heating phase; f, trap current wave form.
signal. In this example, a pulse width of 16.6 ms is used to obtain two half cycles of high current for initial trap heating. Relay SR1 turns off at the first zero crossing after the falling edge of the TI pulse. The pulse from Tatwhich is always 8.3 ms, begins a t the falling edge of the TI pulse. This is indicated as wave form d. This pulse delays the initiation of the sustainer signal for one half cycle. This allows the collapse of the magnetic field in the muffle-furnace transformer before superposition of a second signal. The outputs of the two timers and the set NAND gate generate the timing signal shown as wave form e. This signal turns on SR2 and starts the sustainer current to the trap. This current continues until manually interrupted by grounding the reset NAND gate. The resulting current wave form delivered to the trap is shown as wave form f.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14,DECEMBER 1985
Detection and Readout System. Since the solute bands reinjected from the cold trap should be only a few milliseconds wide for optimum operation of a high-speed GC system, the detector and readout system used to study the inlet system should have rise and fall times of 0.1 ms or less. A simple electrometer operational amplifier circuit was constructed from an Analog Devices AD515JH operational amplifier. The electrometer has rise and fall times of about 0.04 ms. The electrometer output and the voltage drop across the trap were monitored simultaneously with a Nicolet 1090A digital storage oscilloscope. Hard copy of the reinjection profiles was obtained on a Hewlett-Packard 7000A X-Y recorder. Experimental Procedures. Two methods were used t o estimate carrier gas velocity in the trap and the connecting line to the FID, but reported values may be in error by as much as 20%. The first method involved injecting methane with a syringe, measuring the total gas holdup time, and subtracting the estimated holdup time of the restrictor. Since the pressure drop through the entire system is quite small, no gas compression correction was applied, and the average gas velocity (a) through the trap and connecting line was computed from eq 1. Here rt and rb are
Table 11. Calculated Retention Times and Elution Bandwidths for Short Columns at High Gas Velocities
L, Lt + (rb2/rt2)Lb a=--= (1) tmt t, the trap and restrictor radii, respectively, L, and Lb the trap and restrictor lengths, and t , and t,, the total gas holdup time and the holdup time for the trap and connecting line only, respectively. The second method involved the measurement of arrival time a t the FID of a reinjected compound relative to the start of the heating cycle. While the time required to vaporize the sample following the start of the heating cycle is not accurately known, it is only a few percent of the total time from the start of the cycle to the time of band detection. This is particularly true for lowboiling-point compounds. Typically, this method will underestimate gas velocity by about 5-10%. To test the cold trap inlet system under chromatographic conditions, a 5.6-m column was prepared from 0.5-mm4.d. stainless steel tubing. The column was coated with SE-30 using the method of Tranchant (15). The coating efficiencywas measured at about 45%. The column was connected to the cold trap with heat-shrink plastic tubing.
from the Golay equation with D = 0.3 cm2/s. *Extra-columnband broadening for a 10% contribution to elution bandwidth.
R E S U L T S AND DISCUSSION T r a p Collection and Reinjection Requirements. To be useful for a broad range of analytical applications, the cold trap inlet system must have high trapping efficiency for compounds with a wide range of boiling points and must reinject these compounds as very narrow plugs. Both of these requirements become more difficult to meet a t high carrier gas flow velocities. The trapping process is not completely understood and probably involves some combination of condensation ( I 6 ) , diffusion of sample vapor to cold trap walls with subsequent sticking (IO), and in some cases aerosol formation (17). Considering only diffusion, the root mean square time (td) required for a sample molecule to diffuse from the center of the trap t o the trap wall is given by the Einstein equation
td = r 2 / 8 D
L, m
a, cm/s
k
r, mm
t,, s
utn, s
u I j l o b ,s
2 2 2 2 2 2 2 2
100 100 100
1 1
0.125 0.250 0.125 0.250 0.125 0.250 0.125 0.250 0.125 0.250 0.125 0.250 0.125 0.250 0.125 0.250
4.0 4.0 10 10 1.33 1.33 3.33 3.33
0.053 0.090 0.108 0.195 0.025 0.049 0.055 0.108 0.080 0.141 0.171 0.308 0.040 0.078 0.087 0.171
0.017 0.028 0.034 0.062 0.008 0.015 0.017 0.034 0.025 0.044 0.054 0.097 0.012 0.024 0.028 0.054
5
5 5 5 5 5 5 5
100
300 300 300 300 100 100 100 100 300 300 300 300
4 4 1 1
4 4 1 1
4 4 1 1
4 4
10 10
25 25 3.33 3.33 8.33 8.33
On-column band broadening (standard deviation) calculated
sample vapor from a 300 cm/s gas stream. The sample reinjection requirements are illustrated in Table 11. This table presents data on retention times ( t J ,on-column band broadening (at),and the maximum allowable extra~ ) latter is to contribute column band broadening ( u ~ , if~ the no more than 10% to overall elution bandwidths. Data are presented for 0.25-mm- and 0.5-mm-i.d. columns of 2 m and 5 m length for low-capacity-ratio ( k ) solutes a t carrier gas flow velocities of 100 and 300 cm/s. This represents the range of values that should be most useful for high-speed GC. The on-column band broadening was estimated from the Golay equation ( I @ , assuming a diffusion coefficient of 0.3 cmz/s and a 100% column coating efficiency. Note that in some cases extra-column band broadening, which includes the sample injection profile, should be under 10 ms (expressed as the standard deviation of a Gaussian band). The reinjection sample bandwidth (w,)when the trap is heated is given by (IO) w, = Wt at, (4)
+
where t, is the sample vaporization time. The reinjection bandwidth can be expressed in temporal units by
For a 1-cm sample bandwidth in the trap and a gas velocity of 200 cm/s, the w , / ~term has a value of about 5 ms. Thus t, should be no more than a few milliseconds if the reinjection profile is not to make an excessive contribution to the elution bandwidth. If heat loss is neglected, the trap temperature as a function of time T ( t )can be obtained from eq 6. In eq 6 I ( t ) is the
(2)
where D is the binary diffusion coefficient of the sample molecule in the carrier gas and r is the trap radius. The width (cut) of the sample band in the trap is given by
(3) where ii is the linear velocity of the carrier gas and 11 is the sticking probability. Assuming a value of 0.15 cmz/s for D for a typical organic molecule like benzene in He or Hz a t a trap temperature of about -100 OC, td for a 0.5 mm diameter trap is about 0.5 ms. Assuming that 1Otd is required for quantitative trapping and that the sticking probabilitv is 1.0, a trap length of only 1.5 cm would be required to remove the
heating current, R ( T ) and C ( T ) are the temperature-dependent resistance and heat capacity of the trap, respectively, rn is its mass, and To is its initial temperature. Core saturation of the muffle-furnace transformer often was observed with primary voltages greater than about 180 V. For most of the studies reported here, 160-180 V on the transformer primary was used for the initial trap heating phase. This results in a temperature increase in the range from 100 to 125 OC/half cycle. The initial trap temperature varies with the volumetric flow rate of the cooling gas but usually is in the range from -100 to -150 OC. Thus, a single-current half
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
n
Table 111. Trapping Efficiency of the 0.5-mm4.d. Trap % trapped
flow rate, cm/s
split ratio
pentane
hexane
octane
100 150
170:l 150:l 5Q1
99.5 f 0.0 98.5 f 0.6 95.9 f 0.7
99.8 f 0.1 98.4 f 0.3 95.1 f 0.1
99.4 k 0.1 97.5 f 0.2 94.7 f 0.4
200
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Table IV. Trapping Efficiency with Heated Electrodes % trappedu
solute
neither heated
pentane heptane nonane
99.9 f 0.1 99.9 f 0.2 lOOd
both heatedb 89.8 f 4.5 87.6 f 2.9 89.5 f 4.2
downstream only heatedC lOOd lOOd lOOd
uValues obtained for 0.l-wL injections using He at 120 cm/s. Downstream electrode heated to 133 "C and upstream electrode heated to 170 "C. CDownstream electrode heated to 147 O C . dTrapping efficiency of 100% implies that no untrapped peak was observed, and thus an average value could not be obtained.
I
1
cycle cannot heat the trap to reinjection temperatures except for very low-boiling compounds. However, if no solute vaporization occurs during the first half cycle, no contribution will be made to the reinjection bandwidth. Trapping Efficiency. Trapping efficiency was determined by Simpson integration of both reinjected and untrapped peaks, dividing the integral of the reinjected peak by the total. Table I11 lists trapping efficiencies for n-pentane, n-hexane, and n-octane at three different He velocities. Initial injections all were 0.5 pL. The split ratios also are listed in the table. Since order-of-magnitude differences in sample size are required to affect trapping efficiency significantly (16), the different split ratios should not be a significant factor here. At 100 cm/s, trapping is very nearly quantitative. Even at 200 cm/s, sample breakthrough is relatively modest. During these studies, the temperature of the trap wall was measured at about -150 OC. The somewhat reduced trapping efficiency with increasing molecular weight may be caused by increased aerosol formation ( 1 1 , 16, 1 7 ) . Table IV shows the effects of electrode heating on trapping efficiency. Injections of 0.1 pL were used with He as the carrier gas a t a flow velocity of 120 cm/s. Virtually quantitative trapping was achieved for all three compounds when neither electrode was heated or when only the downstream electrode was heated. However, heating both electrodes caused about a 10% decrease in trapping efficiency. This may be the result of higher trap temperature and an increase in the temperature gradient at the upstream end of the trap, which may increase the formation of aerosols (16, 17). Reinjection Profiles. Figure 4 shows an example of the trap-heating wave form and the reinjection profile from a 50-pL injection of acetone headspace vapors with a 50:l split ratio. The gas velocity was about 170 cm/s. Initial trap heating was accomplished by three half cycles of 68 A (root mean square). The reinjection peak occurs 193 ms after the start of the heating cycle. Most of this is the time required for the vapor plug to travel the 30-cm distance from the trap to the detector burner tip. For this very small sample, a narrow, well-formed, and nearly symmetric reinjection profile is obtained. The full width a t half-height is 13.8 ms. For a Gaussian peak, the corresponding standard deviation is only 5.9 ms. This is reduced to less than 5.6 ms when corrected for longitudinal diffusion during transit to the detector. The corresponding physical length of the vapor plug leaving the trap is about 0.95 cm (standard deviation).
I
50
0
100
I
I
I50
200
1
Tima,ms Flgure 4. Current wave form (a) and reinjection profile (b) for an acetone headspacg vapor sample using He at 170 crn/s. Initial heating was accomplished with three half cycles of 68 A rms.
(a)
n
H 5 0 ms Figure 5. Reinjection profiles for acetone using various size samples. The He flow rate was 100 crn/s, and initial heating used one 88 A rms half cycle. The split ratio was 400:l. Injection volumes were as follows: a, 0.5 pL; b, 2.0 pL; c, 5.0 pL.
Figure 5 shows the effect of sample size on the reinjection profiles. Again, acetone was used with He as carrier gas a t a flow velocity of 100 cm/s. Initial trap heating was achieved with a single 88-A half cycle. This is the highest current that can be obtained with this system. A 0.1-pL injection (not shown in Figure 5) produced a nearly symmetric peak with a 20-ms full width at half-height. The peak is wider than in Figure 4 because of the lower flow rate. A 0.5-pL injection (Figure 5a) still results in a nearly symmetric profile, but the width has increased to about 25 ms. Distortion of the profile begins with injections larger than about 1.0pL. For a 2.0-pL injection (Figure 5b), the reinjection bandwidth has increased to about 29 ms, and a shoulder has developed on the tailing edge of the band. A 5.0-pL injection (Figure 5c) results in a very broad and distorted reinjection profile. The leading edge is very sharp, and the rise time is about the same as for the smaller samples. What is not apparent in Figure 5 is that the arrival time of the band maximum is shifted progressively to
2778
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985 a
'
3
I
0
d H 25 ms
(C)
A H 50 ms
Flgure 7. Reinjection profiles for an equal-volume mixture of n-pentane, n-heptane, n-octane, n-nonane, and ndecane using (a) two initial half cycles, (b) four initial half cycles, and (c) six initial half cycles of 68 A rms. Injections of 0.1 pL were used with a split ratio of 150:1, and the He flow rate was 110 cm/s.
earlier values with increasing sample size, and about a 10-ms shift is observed in the range 0.1-2.0 pL. With a split ratio of 400:1, a 5.0-pL injection results in about mol being collected in the trap. Assuming that the 1.3 X gas in the trap is heated to 300 "C during the reinjection process, a 3 cm long plug of pure vapor at 1atm would occur if vaporization was instantaneous. However, the vaporization rate, no doubt, decreases significantly as the solute vapor pressure approaches saturation. These data indicate that overloading will occur with more than about 2.5 X mol of sample in the trap (a 1.0-pL injection with a 400:l split ratio for a solute with a molar volume of about 100 mL). Figure 6 shows the effect of solute boiling point on the shape of the reinjection profiles. Six half cycles of 68 A were used for initial trap heating. The He flow rate was 110 cm/s. Injections of 0.1 pL were used with a split ratio of 150:l. The four normal alkanes used here have a boiling point range of 36-196 "C. In Figure 6, pentane (a), heptane (b), and nonane (c) produce symmetric, nearly identical reinjection profiles. Undecane (d) shows a very distorted profile with a double peak. The bandwidths for pentane, heptane, and nonane are 20.8, 20.1, and 19.5 ms, respectively. The small decrease in bandwidth with increasing molecular size is the result of reduced longitudinal diffusion during transport to the detector. Figure 7 shows reinjection profiles for mixtures of five hydrocarbons using two (a),four (b), and six (c) half cycles of 68 A (root mean square) for initial trap heating. The He
I
IO
Flgure 8. High-speed chromatogram of an aromatic mixture using a 5.6-m SE-30 column at 100 OC. The carrier gas velocity was 215 cm/s. Components and retention data are given in Table V.
Table V. Components and Retention Data for Figure 8
Figure 6. Reinjection profiles for a, n-pentane; b, n-heptane; c, n-nonane; and d, n-undecane using He at 110 cm/s. Injections of 0.1 pL were used with a split ratio of 150:l. Initial heating used six half cycles of 68 A rms.
n
I
I
5 Ti m e , s
band
compd
1
n-pentane (solvent) toluene ethylbenzene impurity (?) isopropylbenzene n-propylbenzene n-butylbenzene
2 3 4 5 6 7 (I
retention time, s -
3.78 4.80 5.25 5.85 6.54 10.18
bandwidth," s -
0.17 0.30 -
0.34 0.42 0.69
Full width at half-height.
flow rate was 110 cm/s. With two initial half cycles, a very broad and distorted reinjection band is observed, which has a more gradual leading edge than the bands previously observed for single components, followed by a very gradual decrease that lasts about 200 ms. Clearly, this is not a satisfactory situation for high-speed operation. With four initial half cycles, the main peak is better defined and shifted to somewhat later time. The leading edge of the band is still distorted relative to that observed from a single component. Since the four high-current half cycles are complete in only 33.2 ms, it appears that for both profiles (a and b) the higher boiling components are not vaporized during the initial heating phase. This suggests that the trap continues to heat, but rather slowly, during the sustained low-current phase. The situation improves significantly with six high-current half cycles. Here the full width at half-height is about 35 ms. The falling edge of the band is quite satisfactory, but some distortion of the leading edge is still observed. This distortion suggests that all the components do not vaporize during the same half cycle and that a composite of two or more bands from different components is being observed. If vaporization of different components occurs during different half cycles of the heater wave form, a biasing of retention time values will be observed, but chromatographic resolution may not be adversely affected. The downstream electrode is cooled significantly by the flow of cold carrier gas exiting the trap. At lower oven temperatures, higher boiling solutes may recondense in this region of the trap tube, and severe tailing may be observed on the reinjection profiles. Heating this electrode to 150 "C nearly eliminates this problem with no significant loss in trapping efficiency (see Table IV). High-speed GC. As a preliminary test of the cold trap inlet system under chromatographic conditions, a 5.6 m long, 0.5mm-i.d. stainless steel column was connected to the trap with heat-shrink plastic tubing, and the chromatogram of a relatively simple aromatic mixture, shown in Figure 8, was obtained. The oven temperature was 100 "C and the downstream electrode was heated. The H2 carrier gas flow rate was
2779
Anal. Chem. 1985, 57, 2779-2787
215 cm/s. The components, retention times, and elution bandwidths are listed in Table V. Note that the chromatogram is complete in just over 10 s and that the first five sample components are well-resolved in about 6 s. Also note that the full width a t half-height of the toluene band is only 170 ms, corresponding to a standard deviation of 72 ms. Since the coating efficiency of the column used here was only 4570, considerably higher resolution and increased zone capacity could be achieved with a higher quality column. While the results reported here are preliminary, they do suggest that the electrically heated cold trap can be very useful as an inlet system for high-speed gas chromatography using short lengths of WCOT columns. Since the system operates a t very modest inlet pressures and uses conventional microsyringe sample introduction, the system should be applicable to a wide range of analytical and control problems, and it should be possible to convert almost any commercial gas chromatograph into a high-speed unit simply by installing a cold trap inlet system and a high-speed electrometer. While a stainless steel trap was used for the prototype system described here, a less active material such as Pt/Ir will be used in future studies. The principal limitations of the system are possible retention time biasing if sample components do not vaporize during the same high-current half cycle and significantly broadened and distorted reinjection bands for compounds with boiling points greater than about 190 "C. A trap with higher resistance or lower heat capacity should reduce these limitations. Alternatively, a more robust transformer, which would
allow the trap to heat to full operating temperature in a single half cycle, or a capacitive discharge heating technique could greatly extend the range of compound boiling points for which the inlet system would be useful.
LITERATURE CITED (1) Desty, D. H.; Goldup, A,; Swanton, T. W. I n "Gas Chromatography"; Brenner, N., Caiien, J. E., Weiss, M. D., Eds.; Academic Press: New York, 1960. (2) Desty, D. H. I n "Advances In Chromatography"; Giddings, J. C., Kelier, R. A., Eds.; Marcel Dekker: New York, 1965; Voi. 1, p 199. (3) Myers, M. N.; Giddings, J. C. Anal. Chem. 1965, 37, 1453-1457. (4) Doue, F.; Guiochon, G. Sep. Sci. 1970, 5 , 197-218. (5) Schomburg, G.; Husmann, H.; Weeke, F. J . Chromatogr. 1975, 9 1 , 603. (6) Deans, D, R. Chromatographia 1968, 7 , 18. (7) Gasper, G.; Arpino, P.; Guiochon, G. J . Chromatogr. 1977, 15, 256-26 1. (8) Wade, R. L.; Cram, S. P. Anal. Chem. 1972, 4 4 , 131. (9) Annino, R.; Leone, J. J . Chromatogr. Sci. 1982, 2 0 , 19-25. (10) Hopkins, B. J.; Pretorius, V. J . Chromatogr. 1978, 158, 465-469. (11) Jacques, C. A.; Morgan, J. J . Chromatogr. Sci. 1980, 18, 679. (12) Anderson, E. L. Ph.D. Dlssertation, University of Alabama, 1978. (13) Ducass, A.; Gonnord, M. J.; Arpino, P.; Guiochon, G. J . Chromatogr. 1978, 148, 321. (14) Biass, W.; Riegner, K.; Hulpke, H. J . Chromatogr. 1979, 172, 67-75. (15) Tranchant, J. "Practical Manual of Chromatography"; Elsevier: New York, 1969. (16) Leathard, D. A.; Shurlock, B. C. "Identification Techniques in Gas Chromatography"; Why-Interscience: New York, 1970; Chapter 9. (17) Kirsten, W. J.; Mattson, P. E.; Alfons, H. Anal. Chem. 1975, 4 7 , 1974-1979. (18) Giddings, J. C. "Dynamics of Chromatography": Marcel Dekker: New York, 1965.
RECEIVED for review March 26,1985. Accepted July 25, 1985.
Multiplex Gas Chromatography by Thermal Modulation of a Fused Silica Capillary Column John B. Phillips,* Derhsing Luu, and Janusz B. Pawliszyn' Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901
Glenn C. Carle Solar S y s t e m Exploration Office, N A S A - A m e s Research Center, Moffett Field, California 94035
Modulating the temperature of the first few centimeters of a fused slllca capillary gas chromatography column effectively modulates the concentratlons of retained substances flowlng contlnuously through the column. As the stationary phase Is heated and cooled, It alternately releases and adsorbs substances to and from the flowing carrler gas. The resulting chemical concentratlon signals follow the applled electrical signal. The slgnal form resembles a derivative of a chromatographic injectlon and results In a derlvatlve form chromatogram. Modulation efflclency and band length are dlrectiy dependent upon the capacity factor of the modulated substance and are predlctable. Large volume, contlnuously flowing, or headspace samples can be accepted directly without preconcentration. The minlmum detectable sample lntroductlon rate using a 9-m SE-52 column, a flame lonlzation g/s of detector, and a 1-h modulatlon signal Is 7.0 X p-dllsopropylbenrene. The linear dynamic range Is 6 orders of magnitude. The relative standard devlatlon Is less than 3%.
Present address: Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322.
It is common to think of the chromatogram as a signal and to use signal processing techniques in extracting information from it. But, the chemical processes occurring within a column are traditionally thought of primarily in terms of the physical transport of substances rather than in terms of the signals carried by these substances. A few exceptions appear in the literature. For example, Reilley et al. (1) demonstrated the response of a chromatographic column to a variety of imposed concentration signals. Considering a familiar subject from a different perspective often leads to new appreciation and understanding ( 2 ) . In this paper we consider gas chromatography from a signal processing point of view. The fundamental features remain unchanged, but different aspects are emphasized. Some new terminology is introduced to clarify which features of chromatography are fundamental and which are only conventional. Eliminating some common but unnecessary conventions leads to alternative techniques with significant advantages in some applications (3). Chromatographic Information and Signals. Information is carried by signals. A detector's output signal carries information which may be recorded and presented in the form of a chromatogram. The detector itself, however, is not the
0003-2700/85/0357-2779$01.50/00 1985 American Chemical Society
