Characteristics of Interrupted Elution Gas Chromatography John 0 . Walker and Clarence J. Wolf McDonnell Douglas Research Laboratories, McDonnell Douglas Corp., Si.Louis, Mo. 631 66
Interrupted elution gas chromatography is a relatively new technique whereby the separation process in a gas chromatographic column is stopped for a specified period of time while a detailed chemical analysis is performed upon a particular eluate. This method has proved of value for qualitative identification. Scott has successfully utilized the interrupted elution method to couple both a mass spectrometer and an infrared spectrometer to a GC ( I ) . Thus, he was able to use a less expensive mass spectrometer because it did not require a rapid scan mode. In addition, he was able to scan the infrared spectrum for 1G15 min realizing a useful spectra. Cacace and Perez (2) combined interrupted elution with radio GC to measure the radioactive peaks under static conditions, thereby minimizing statistical fluctuations with improved sensitivity and eliminating errors arising from variations in the flow rate. Walker and Wolf (3, 4 ) used interrupted elution combined with vapor phase pyrolysis gas chromatography to identify GC eluates in a manner analogous to GC/MS. Several different experimental procedures have been proposed for stopping and restarting the carrier gas flow during the interruption interval. Specifically, Scott et al. ( I ) and Walker and Wolf (3) reduced the pressure in the GC column to atmospheric pressure during the stop mode. However, Cacace and Perez ( 2 ) and Wolf and Walker ( 4 ) did not vent to the atmosphere during the stop interval, but maintained the gas pressure in the column during the interruption interval. In addition to the venting and nonventing procedures described previously, a third technique for conducting interruption elution is described. This last technique utilizes a second GC column identical to the original separation column to minimize backflushing and pressure surges, during the stop and start action. The most efficient technique as well as the differences in the proposed methods have not been reported. This paper describes operating characteristics for the separation of similar compourids with three different systems employing interrupted elution gas chromatography. The column efficiency and resolution for a pair of similar compounds was measured as a function of both "stop-time" and the number of interruption intervals.
EXPERIMENTAL The gas chromatograph used in all experiments was a HewlettPackard Model 5750 equipped with flame ionization (FID)and thermal conductivity (TC) detectors. The packed column was 8-m by 0.165-cm ( i d . ) stainless steel containing 2% (by weight) Triton X-305 on 80/100 mesh Chromosorb "G". Helium carrier gas at a flow rate 25 cm3/min was used with this column. The support coated open tubular (SCOT) column was 84-m by 0.051cm ( i d . ) stainless steel coated with squalene. Helium carrier gas a t a flow rate of 4.0 cm3/min was used with the SCOT column. A modified "on column" injection system ( 5 ) was used with both R . P. Scott, F. A. Fowles, D. Welti, and T. Wilkins, "Gas Chrornatography 1966," A. 6 .Littlewood, E d . , The Institute of Petroleum, London, 1967, p 318. F. Cacace and G. Perez, Ana/. Chem.. 39, 1863 ( 1 9 6 7 ) . J. Q. Walker andC. J . Wolf,Ana/. Chem. 40, 710 (1968). C. J. Wolf and J. Q. Walker, "Gas Chromatography 1968," C. L. A. Harbourn. E d . , The Institute of Petroleum, London, 1969, p 385. D. E. Willis and R . M. Englebrecht, J . Gas Chrornatogr.. 5, 435 (1967).
the packed and open tubular column experiments. A 2-m by 0.025-cm (i.d.) restrictor between the SCOT column and the FID was required to prevent a rapid gas surge during the stop-start action from extinguishing the FID flame. When a microthermal conductivity cell was used as the GC detector, a more stable base line was observed during the stop-start action with the restrictor. Both columns were held isothermally a t 80 "C; the injector was heated to 275 "C. The hydrogen and air flow rates to the FID were 20 cm3/min and 275 cm3/min. respectively. The block diagram for the one-column vented interrupted elution arrangement is shown in Figure 1 and is similar to that described previously ( 3 ) .This system will be referred to as 1C-V. In normal operation, helium carrier gas flows through the No. 1 regulator into the four-port valve along the path indicated by the solid line. The vent and valve to the No. 2 regulator are closed. The liquid sample, 0.02 pl, is introduced in a conventional manner with a microsyringe at the injector. The separation column resolves the mixture into individual components which are detected by the detector. When the compound from the column leaves the detector to pass into the spectrometer, the four-port valve is rotated so that the helium from the No. 1 regulator passes through the valve in the direction shown by the lower dashed line. While the compound is analyzed by the spectrometer, the GC column is isolated from the gas flow. The helium in the GC column is released to the atmosphere by venting through the pre-column; the vent is then closed. Scott ( I ) indicated t h a t compound band spreading in the column during the stop action is minimized when the pressure is relieved. Therefore, a stainless steel restrictor, 5 . 2 m long and of a 0.22-mm i.d., was placed between the four-port valve and the vent. This restrictor prevents any rapid gas surge from occurring and removing compounds from the end of the column during vent. When the qualitative analysis of the spectrometer is completed, the GC column is repressurized by opening the valve connecting the S o . 2 regulator to the GC column. After the pressure reaches its original value (about 20 sec), the valve near the No. 2 regulator is closed and the four-port valve is rotated to its initial position. This procedure is repeated for each peak emerging from the column. The purpose of the pre-column is to serve as a trap for any sample material which may be removed from the separation column when the helium is vented. This material returns to the GC column when the system is repressurized. The block diagram of the one-column nonvented system is shown in Figure 2 and will be referred to as 1C-NV. The system is quite similar to t h a t of the vented system (see Figure 1) except that only one regulator and flowmeter are required and the precolumn is not needed. When a compound elutes from the separation GC, the four-port valve is rotated, isolating the column from the carrier gas flow. As soon as the spectrometric analysis is complete, the valve is rotated back to its original position until another compound emerges. Although there are many different ways to arrange a column and the necessary valves to perform interrupted elution GC, the system illustrated in Figure 1 represents a convenient method to reduce the pressure during the stop mode while the system illustrated in Figure 2 eliminates the need for the venting system. Neither of these arrangements is satisfactory when a TC detector is used instead of the FID. With TC detectors the base line is unstable for approximately 40-60 sec after each position change in the four-port valve. The base-line instability, which is a direct result of the flow sensitive nature of the T C detector. prevents satisfactory GC operation. Therefore, another method which minimizes pressure differentials in .the TC system during operation was designed, This system, called the two-column nonvented system, is illustrated in Figure 3 and is referred to as 2C-NV. The following experimental procedure is used with the 2C-NV system, When a peak is observed in the detector, the valve is rotated 90" and the chromatographic process in the column ceases
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
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Table I . Effect of Stop Time on Efficiency and Resolution of 2,2,4- and 2,3,4-Trimethylpentane (TMP) with a (8-m X 0.165-cm i.d.) Packed Column Stoo time. min Modeb
0
0.5
2.5
5.0
25.0
150.0
50
1.5c
1. E f f i c i e n c p 2. E f f i c i e n c p 3. Resolutiond
2,2,4-TM P 2,3,4-TM P
1cv 1cv 1cv
9950 9545 3.1 5
9140 9250 2.70
71 40 6690 2.52
41 55 4630 2.41
1558 2040 1.34
0.63
7445 7045 2.84
4. E f f i c i e n c p 5. E f f i c i e n c y 6. Resolutiond
2,2,4-TM P 2,3,4-TM P
lCNV lCNV 1CNV
9850 8370 2.96
9145 8130 2.56
6900 7255 2.24
5925 5660 2.08
1995 21 75 1.28
111 101 0.41
71 30 8525 2.57
7. E f f i c i e n c p 8. E f f i c i e n c p 9. Resolutiond
2,2,4-TM P 2,3,4-TM P
2CNV 2CNV 2CNV
11,020 9650 2.90
6955 8335 3.40
3895 4250 2.63
2860 3325 2.38
930 1026 1.37
97 95 0.45
5530 471 0 2.75
70
Calculated plates, average of two analyses. 1CV = one column vented, 1 C N V = one column nonvented, 2CNV = two column nonvented. A total of 1.5-min stop time is the accumulated time from three separate 0.5-min intervals. Average of two resolution numbers.
Table I I . Effect of Stop Time on Efficiency and Resolution of Normal Pentane and Cyclopentane with a 84-m (0.5-mm i d . ) SCOT Column Stop time 2.5
5.0
25.0
150.0
1 .!iC
1. E f f i c i e n c p 2. E f f i c i e n c p 3. Resolutiond
n-C5* cyc5**
Modeb 1cv 1cv 1cv
84,700 78,600 8.8
66,500 60,900 8.5
51.700 49,600 7.9
41,900 39,400 6.8
35,800 33,900 6.0
24,700 23,000 5.1
56,500 51,000 8.0
4. E f f i c i e n c p 5. E f f i c i e n c p 6. Resolutiond
n-C5 CYCS
1 CNV 1 CNV 1CNV
85,600 79,000 9.0
67,500 61,800 8.6
52,100 48,300 8.1
41,600 38,900 7.1
36,800 34,200 6.2
24,100 22,700 5.0
56,500 44,800 8.3
7. E f f i c i e n c y 8. E f f i c i e n c p 9. Resolutiond
n-C5 CYC5
2CNV 2CNV 2CNV
110,400 102,000 9.2
106,600 95,800 9.1
90,100 78,000 8.3
68,400 67,800 8.0
56,100 44,600 7.9
35,700 32,200 5.4
78,300 73,400 9.0
0
0.5
Calculated plates, average of two analyses. 1CV = one column vented. 1 C N V = one column nonvented. PCNV = two column nonvented. A total of 1.5-min stop time is the accumulated time from three separate 0.5-min intervals. Average of two resolution numbers. * Normai pentane. ** Cyclooentane.
NO 2 regulator 2nd flowmeter
n
W
I
I/cx I v y I
I
Injector I
I I I I
-
Spectrometer
He carr,er
NO. 1 regulator and flowmeter
I
Figure 3. Block diagram of the two-column nonvented (2C-NV) interrupted elution system
r-
I I
I
Detector
L------------------l
Figure 1. Block diagram of the one-column vented (1C-V) interrupted elution system
I I
I
II
tector
RESULTS AND DISCUSSION
I
I H e carrier
i------------------J
I
Figure 2. Block diagram of the one-column nonvented (1C-NV) interrupted elution system 2264
since the flow is stopped by a tubing plug (illustrated in Figure 3 by “X”). The carrier gas flow to the detector and spectrometer continues through the dummy or reference column and the open side of the four-port valve. When the spectrometer analysis is completed, the four-port valve is rotated to the original position to restart the GC separation column. Figure 3 shows the arrangement used with a FID which normally burns approximately 10% of the effluent. The FID was used rather than the TC so that a valid comparison of the 1C-V, 1C-NV, and 2C-NV systems could be made. However, when the TC detector is used, all the effluent passes into the spectrometer and the base-line disturbance is minimal and lasts 5-7 sec.
T h e e f f e c t of i n t e r r u p t e d e l u t i o n o n t h e n u m b e r o f t h e o r e t i c a l p l a t e s ( N ) a n d r e s o l u t i o n ( R ) f o r t h e s e p a r a t i o n of a p a i r o f similar alkane hydrocarbons for packed a n d SCOT c o l u m n s was d e t e r m i n e d . T h e n u m b e r of t h e o r e t i c a l plates for each c o m p o u n d
ANALYTICAL CHEMISTRY, VOL. 45. NO. 13, NOVEMBER 1973
-- - -3 -
U O N E COLUMN, NOT V E N T E D Q - ONE COLUMN, VENTED TWO C O L U M N , NOT V E N T E D
104
THEORETICAL PLATES
\ 102
I
1
1
-
~
TIME MIN
Figure 4. The number of theoretical plates for the elution of 2,3,44rimethylpentane on a packed column as a function of stop time with
the vented and nonvented interrupted elution systems
C --
- - -& - -
I
-C-
O N E C O L U M N , NOT V E N T E D ONE COLUMN, VENTED TWO C O L U M N NOT V E N T E D
I
THEORETICAL PLATES
104 -
i070
1~ 10 0
10
100 0
1000 0
TIME M I N
Figure 5. The number of theoretical plates for the elution of n-pentane on a SCOT column as a function of stop time with the vented
and nonvented interrupted elution systems
and the resolution ( R ) for the separation of each similar pair
were calculated directly. Here, tr, and tr,+l correspond to the retention time of the ith and (i 1)th peaks while W L and correspond to their respective peak width at the base line. When calculating the retention time, the period of the stop interval is subtracted from the clock time, and the retention time corresponds to that time in which the carrier gas flows. The number of plates and resolution for the separation of 2,2,4-trimethylpentane and 2,3,4-trimethylpentane with the 8-m packed column using the 1C-V, 1C-NV, and 2C-NV systems are given in Table I. The number of plates for 2,3,4-trimethylpentane as a function of stop time with the three systems is shown in Figure 4. The optimum he-
+
lium flow rate for this column was 25 cm3/min and the number of plates for both compounds was approximately 10,000. After a stop interval of 150 min, N decreased by more than two orders of magnitude with all three systems. The resolution also decreased drastically from approximately 3 to about 0.5 during the 150-min stop period. It is interesting to note that the pressure change induced by the stop-start action itself has little effect on either N or R. This conclusion is justified by noting that three consecutive 0.5-min interruption periods produce the same loss in N and R as that found by a single 1.5-min interruption. The number of plates and resolution for the separation of n-pentane and cyclopentane as a function of stop time with the 84-m SCOT are given in Table 11. The number of theoretical plates for n-pentane as a function stop time with the three systems is shown in Figure 5. The two-column system exhibits a slightly higher N and R than the other two systems, possibly because of a higher pressure with this system. With all three systems, N decreases a factor of three while R decreases approximately one-half
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 13, N O V E M B E R 1973
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during the EO-min stop interval. As observed with the packed column, the number of stop intervals has little effect on either N or R. Five significant observations can be concluded about interrupted elution gas chromatography from these studies: 1) Regardless of the particular type of column used, i e . , either packed or SCOT? it is desirable to use the simpler nonvented system because venting serves no useful purpose. 2 ) SCOT, i.e., open tubular columns, retain a higher number of theoretical plates and resolution during extended stop periods while packed columns degrade rapidly. 3) Although pressure surges may produce deleterious effects with TC detectors, these effects are minimized
with a matched two-column vented system; and 4) the number of stop-start intervals has little effect on the separation ability of a particular GC column. 5) The deleterious effects of the pressure surges resulting from the stopstart action on the number of theoretical plates and resolution are much smaller with open tubular columns than with packed columns. Received for review March 26, 1973. Accepted July 2, 1973. This research was conducted under the McDonnell Douglas Independent Research and Development Program.
Differential Kinetic Analysis of Nitric Oxide-Nitrogen Dioxide Mixtures by Reaction with Iron(l1) in Sulfolane as Solvent J. F. Coetzee,' D. R. Balya, and P. K. Chattopadhyay Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa. 75273
The determination of nitrogen(I1) and nitrogen(1V) oxides in ambient air and a t emission sources is of major current concern. These gases are highly toxic and play a significant role in the formation of photochemical smog. Both are emitted in hazardous amounts from sources such as incineration, combustion of fossil fuels, and operation of internal combustion engines. In the immediate vicinity of such sources, NO, levels up to several parts per thousand may be produced. Several current methods for the determination of nitrogen(I1) and nitrogen(1V) oxides have been reviewed recently ( 1 ) . Methods fall into two broad categories (2): (a) indirect colorimetric procedures for total NO, in which nitrite or nitrate is actually determined and preoxidation of NO to NO2 (actually N204) is required. and ( b ) direct instrumental methods, such as gas chromatography ( I ) and nondispersive infrared (3) and chemiluminescence ( 4 ) spectrometry for the determination of nitric oxide. Gas chromatographic methods are not yet sensitive enough for direct measuremea a t atmospheric concentrations, although future developments in column design may minimize this limitation. It appears that chemiluminescence methods in particular hold much promise. However, a t this time no single method is the method of choice in all applications because of varying relative importance of features such as cost and complexity, accuracy, sensitivity, selectivity, speed, and the ability to determine N O and NO2 individually. We report here a differential kinetic method which has certain useful features. It is based on the formation of the iron(I1) nitrosyl ("brown-ring") complex by both NO and KO2. which react a t different rates. This reaction has 1Please address all correspondence to this author. (1) P. K . Mueller, E. L. Kothny, L. E. Pierce, T. Belsky, M. Imada. and H. Moore, Anal. Chem., 4 3 ( 5 ) . 1 R (1971). (2) D. R. Baiya, M. S.Thesis, University of Pittsburgh, 1972. (3) C. J. Halstead, G. H. Nation, and L. Turner, Analyst (London). 9 7 , 64 (1972). (4) A. Fontijn, A. J. Sabadeli, and R. J. Ronco, Anal. Chem., 42, 575
(1970).
2266
been used in aqueous solution for the colorimetric determination of total NO, in concentrations above 100 ppm (5, 6). The stoichiometry of the reactions was shown to be Fez+ 3Fe2'
+
NO,
+
+
2H'
NO Z Fe(N0)" f
Fe(N0)"
+
2Fe3-
+
H,O (2)
The method is relatively free of interferences, including sulfur dioxide at concentrations in air below 6000 ppm and hydrogen sulfide below 400 ppm (6); higher concentrations of the latter cause some reduction in color. We have studied reactions 1 and 2 in sulfolane containing (for practical reasons, discussed below) 1.6 vol YC of water. Sulfolane appears to offer certain advantages for air pollution studies, particularly for the analysis of "grab" samples collected a t emission sites. For example, NO and particularly NO2 have much higher solubilities in sulfolane than in water, and sulfolane is highly resistant toward oxidation and a wide variety of other forms of chemical attack. Since it also has a very low vapor pressure, the possibility exists that it also may be a useful medium for the collection of air samples by aspiration, but collection efficiencies may be unduly low.
EXPERIMENTAL Apparatus. Spectra of the iron(I1) nitrosyl complex in both water and sulfolane were taken with a Perkin-Elmer Model 124 double beam recording spectrophotometer equipped with temperature control to zt0.5 "C. Kinetic data were obtained with a Durrum Instrument Company (Palo Alto, Calif.) Model D-110 stopped-flow spectrophotometer equipped with a Kel-F flow system and temperature control to f O . l "C. Reagents. Sulfolane was purified as described before (7), even though such elaborate purification probably was not essential. For example, sulfolane suitable for acid-base work has been ob(5) E. L. Kothny and P. K. Mueller. 7th Conference on Methods in Air
Pollution Studies, Calif. State Department of Public Health, Jan. 1965. (6) G. Norwitz, Analyst (London), 91, 553 (1966). (7) J. F. Coetzee, J. M. Simon, and R. J. Bertozzi, Ana/. Chem., 41, 766 (1969).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
