High-Speed GC Analysis of VOCs - American Chemical Society

ES&T

High-Speed GC Analysis of VOCs: Tunable Selectivity and Column Selection Part 2 of a Two-Part Article

V

olatile organic compounds (VOCs) are widely used in the manufac turing and service industries as well as at government facilities. Even at very low concentrations, many of these materials are toxic or carcinogenic and thus represent a direct health risk to workers. Because virtually all VOCs adversely affect overall air quality, the Clean Air Act Amendments of 1990 require increased monitoring of major emission sources. State and local regulations regarding air quality and emission abatement as well as standards for clean-up operations at chemical waste and spill remediation sites are changing and will be in a state of flux for the foreseeable future. For all of these reasons, there needs to be increased monitoring of VOCs. Many environmental VOC monitoring problems focus on a small number of target compounds. This is particularly true for field and screening applications and for industrial hygiene monitoring. For these applications, high-speed gas chromatography (GC) may be extremely useful in reducing analysis time and increasing sample throughput. Instrumentation requirements and inlet systems for high-speed GC were discussed last month in this journal. Although these techniques promise analysis times of a few seconds to a few tens of seconds, the probability of coincidental peak overlap is quite large (i). Therefore, careful attention 428 A

must be paid to column selectivity and special selectivity adjustment techniques to take full advantage of the capabilities of these high-speed GC instruments. Using tandem or serially linked columns with different stationary phases, it is possible to tune the selectivity of a separation by changing either the relative lengths of the columns (2, 3) or the pressure at the junction points between the colu m n s (4, 5). This provides a straightforward way to minimize overlapping peaks and completely separate more complex mixtures. Recently, high-speed column switching has been used to further improve the selectivity of high-speed GC analysis for VOCs. These tech-

Environ. Sci. Technol., Vol. 28, No. 9, 1994

RICHARD MICHAEL

SACKS AKARD

University of Michigan Ann Arbor, MI 48104

niques begin with the rapid, partial separation of the analytes on a nonpolar capillary separation column. The partially separated compounds are automatically transferred in groups to either a polar or a nonpolar capillary column to complete the separation (6). A separate detector monitors each of the two columns used for the final separation. The combination of high-speed column switching and pressure-adjustable selectivity yields a complete s e p a r a t i o n of m i x t u r e s containing as many as 20—30 compounds in less than one minute. In this report, these techniques will be discussed and applied to mixtures of VOCs of environmental interest. Stationary phases for VOC analysis High-speed GC separations require high-speed inlet systems with relatively short capillary columns. The price paid for reduced analysis time and increased sample throughput is a significant loss in chromatographic resolution, reducing the number of components that can be completely separated and increasing the probability of peak overlap. This makes the choice of stationary phase more important in highspeed GC than with conventional GC. Most VOC analyses are performed using the stationary phases described in Table 1. Many capillary GC separations of VOCs use 0.25 or 0.32 mm i.d. bonded-phase capillary columns. Microbore capil-

0013-936X/94/0927-428A$04.50/0 © 1994 American Chemical Society

lary columns (< 0.1 mm i.d.) offer greater resolution and zone capacity (i.e., the number of peaks, if per fectly spaced, that will fit in the chromatogram at a specified resolu tion) for comparable analysis times, but they are more difficult to use and require smaller samples than do capillary columns. Microbore columns are only beginning to be widely applied to VOC analysis of environmental samples. Mixed phases and tandem columns For relatively simple mixtures, a single stationary phase may be ade quate for high-speed GC analysis. However, in many cases, significant time can be saved by using mixed stationary phases or tandem col umns of different selectivity. In this scheme, the phase mixture or col umn length ratios can be tuned for the optimal separation of the target compounds. Several mixed-phase capillary columns for analysis of specific sets of VOCs, as required by U.S. regulatory agencies, are avail able commercially. Continuously adjustable selectivity can be achieved with a pressure-tunable tandem column ensemble. Figure 1 shows a diagram of a pressure-tunable system for high speed GC. A high-speed cryofocusing inlet system (described in more detail in Reference 7) introduces a narrow sample vapor plug into the column ensemble. A nonpolar cap illary column Cj (blue) is followed by a highly polar column Cz (red). Varying the pressure of carrier gas supply Ga adjusts the selectivity of the tandem column ensemble. If this pressure increases, the pressure difference along the nonpolar col umn decreases and along the polar column increases. This reduces car-

rier gas velocity in the nonpolar col umn and, thus, increases retention times in that section. At the same time, gas flow velocity in the polar column increases, leading to shorter retention times on that column. The result is a decrease in the influence of the polar column on the overall

separation. A pressure reduction at the column midpoint has the oppo site effect. Column switching The instrument shown in Figure 1 also allows high-speed column switching after the initial separa-

FIGURE 1

Diagram of a high-speed GC system using tandem-column tunable selectivity and selective peak switching

Effluents from a preliminary separation on nonpolar column C, can be switched to either a polar column C or a second nonpolar column C

TABLE 1

Properties of stationary phases frequently used for VOC analysis Stationary phases Polarity

Dimethyl polysiloxane (DMP)

Phenyl methyl polysiloxane

Trifluoropropyl methyl polysiloxane

Nonpolar

Low

High

Relative efficiency Basic structure

CH3 —

Si—

Biscyanopropyl polysiloxane (BCP)

Polyethylene glycol (PEG)

Intermediate

High

High

Intermediate

Low

High

σιΦ)

CF3

Ô

CH3 —

Si—Ο

C2H4 —

I Si—Ο

N

I (CH2)3 —Si —Ο

| CH3

CH3

C=

/

(CH 2 ) 3

Ο Η— C — Η Η— C — H

0ΞΞΞΝ


429 A

tion on the nonpolar column. Column switching increases the zone capacity and enhances the tuning characteristics of the nonpolar/ polar tandem pair. For the instrument described here, components or groups of components that elute from column Ca can be switched to another nonpolar column C3 (blue) using carrier gas source G2 to divert these components to the second nonpolar column. Because columns Ca and C3 have the same selectivity, temperature, and stationary phase film thickness, no tuning is possible for this tandem pair. For the sample chromatogram shown from column C a , the wellseparated peaks 1, 2, 5, 6, and 9 were switched to column C3. These compounds were then detected on FID2 (for flame ionization detection) as shown in the figure. Component pairs 3 and 4, and 7 and 8 cannot be completely separated on C3 and were instead selectively switched to column C2. The component pairs are passed through the tandem combination of C t and C2, which was previously tuned to completely separate these components, and the components were detected by FID a . The pressure at G1 was earlier optimized for separating these components.

k = (t R -t m )/t m = k1

+

( t m / U ( ^ 2 - ^ (2) Plots of the overall capacity factor k versus t m /t m are straight lines with slopes equal to k2—k1 and intercepts equal to Jca. Thus, the slopes of such plots provide a useful comparison of the polarity of the polar column C2 relative to the nonpolar column C1. Optimization strategy A wide range of selectivities can be obtained by coupling columns with different stationary phases in series. In many cases it is possible to tune the selectivity for a particular mixture and drastically reduce analysis time. The DMP and PEG columns show significantly greater efficiency (smaller plate height) than the BCP

Selectivity tuning with tandem column ensembles The instrument shown in Figure 1 allows for the direct measurement of the carrier gas hold-up times, t m and t m , from the individual columns Ca and C2, respectively. [Hold-up time is the carrier gas transport time through the column. It is determined by measuring the retention time for a compound, which is completely unretained on the column and thus travels at the same velocity as the carrier gas. Methane is usually chosen when detection is by FID.] These key parameters aid in the optimization of the tandem columns. The overall hold-up time t m , the sum of t m and t m , is related to the overall retention time t R and an overall capacity factor k as described in Equation 1. [Capacity factor is the mass of a sample component in the stationary phase ratioed to the mass in the moving phase (carrier gas).] (1)

The overall capacity factor can be described in terms of the capacity factors k^ and k2 for columns Ca and C2, respectively, and the time fraction t m /t m as shown in Equation 2. 430 A


Rr = ( V W ^ + i )

O)

Here, ka and kh are the overall capacity factors for components a and b on the tandem column ensemble, and A:av is their average value. This function is a reliable measure of chromatographic resolution for all capacity factor values. Figure 2(a)

FIGURE 2

Plots of overall capacity factor k versus time fraction tm2/tm (a) and relative retention window diagram (b) for seven chlorinated hydrocarbon compounds using a DMP-PEG tandem column ensemble (a)

s-

î

î

I

c .g j5 ο 03

ω

Ι

ω

tR = W * + 1)

and TFP columns. This feature, coupled with the very large selectivity range available for many environmentally important compounds, makes the DMP-PEG tandem ensemble particularly useful. Optimization refers to the selection of the column hold-up time fraction t m /t m that will provide the best separation of the worst-case component pair (critical pair). Optimization is based on Equation 2 and a relative resolution function Rr as given in Equation 3.

IX.

See Table 2 for mixture components

shows plots of overall capacity fac tor versus t m /t m for a mixture of seven chlorinated hydrocarbon compounds using a DMP-PEG tan dem combination. Table 2 lists the c o m p o u n d s and t h e i r boiling points. Figure 2(b) shows plots of the relative resolution (Equation 3) of the critical pair using the same t m /t m axis. Different component pairs constitute the critical pair for different tm /t m values. For example, for t m / t m values near 0.1, c o m p o n e n t s Ε and F (1,1,2-trichloroethane and tetra chloroethylene) are the critical pair; for t m ./t m values near 0.45, compo nents ÎD and Β (1-chloropentane and 1,2-dichloroethane) form the criti cal pair. In Figure 2, the critical pairs in (a) and the corresponding regions in the relative resolution plot in (b) are identified by various colors. Note that there are seven points, corresponding to t m /t m values, where pairs of plots cross in Figure 2(a). For each of these points, the corresponding compounds co-elute, and the relative retention is zero. Also note that there are several "windows" of large relative resolu tion of the critical pair. Plots of the type shown in Figure 2(b) often are called window diagrams (8). The t m /t m value of 0.58 gives the great est relative resolution and would al low for the best separation of the worst case pair of components with a specified total column length. A high degree of selectivity con trol for these chlorinated hydrocar bon compounds is obtained using the DMP-PEG combination. Monochlorinated compounds have nega tive slopes for the plots in Figure 2(a), as does the symmetric tetra chloroethylene molecule. Positive slopes are obtained for all other polychlorinated c o m p o u n d s . A large positive slope is obtained for 1,1,2-trichloroethane. Figure 3 shows chromatograms obtained with t m ./t m values of 0.26 (a), 0.37 (b), 0.402 (c), and 0.48 (d). The critical pair in each chromato gram is indicated by contrasting colors. For chromatogram (a), com ponents A and C are not adequately separated; note that plots A and C in Figure 2(a) cross at a t m2 /t m value of about 0.22. For chromatogram (b) in Figure 3 all components are well separated; note the peak at t m /t m = 0.37 in Figure 2(b). Using this lmJtm value, the overall column length could be significantly reduced with a further reduction in analysis time. For chromatograms (c) and (d),

TABLE 2

Component identification for Figures 3 and 4 Label

Component

A Β C D Ε F G

c-1,2- Dichloroethylene 1,2- Dichloroethane 1 -Chlorobutane 1-Chloropentane 1,1,2-Trichloroethane Tetrachloroethylene 1 -Chlorohexane

Boiling point (• C)

Nominal concentrations, ppm (vol/vol)

60 84 78 108 113 121 133

100 66 49 130 110 100 57

FIGURE 3

Chromatograms of the seven-component chlorinated hydrocarbon mixture using the DMP-PEG tandem column ensemble with tm2/tm values of 0.26 (a), 0.37 (b), 0.40 (c), and 0.48 (d)

î g

Time (s) See Table 2 for mixture components.

components Β and D are not ade quately separated. Tunable selectivity with column switching For more complex mixtures, even tunable selectivity may not allow a complete separation on a time scale of 30—60 s. Figure 4 shows chro matograms of a 21-component mix ture using an 8-m-long nonpolar DMP column (a) and the tandem combination of a 4.0-m-long DMP column, and a 4.0-m-long PEG col umn (b). The components, their

boiling points, and their concentra tions are listed in Table 3. For both chromatograms, the column tem perature was 40 °C. On the 8.0-m DMP column sev eral unresolved or poorly resolved peaks are observed. In particular, note unresolved pairs C-D, J-K, and L-M. However, many of the compo nents are separated in about 40 s. On the tandem column ensemble, the separation is poorer, and more overlapping components are ob served. Figure 5(a) shows plots of overall


431 A

FIGURE 4

Chromatograms of a 21-component mixture using only a DMP column (a) and using the DMP-PEG tandem column ensemble (b)

ι

8

Time (s) See Table 3 for mixture components.

TABLE 3

Component identification for Figures 5, 6, and 7 Label

Component

A B Β C D Ε F G H I 1 J Κ L M Ν Ο Ρ Q R Ft S Τ U υ

Methylene chloride M ,2 - Dichloroethylene c-1,2- Dichloroethylene Chloroform 1,2-Dichloroethane Carbon tetrachloride Bromodichloromethane Methylcyclohexane c-1,3 - Dichloropropene M ,3-Dichloropropene 1,1,2-Trichloroethane 2-Fluorotoluene 1,3- Dichloropropane Cycloheptane Tetrachloroethylene Methyl pentanoate Chlorobenzene 1 -Chlorohexane m-Xylene mXylene 4-Heptanone o-Xylene

c a p a c i t y factor k v e r s u s h o l d - u p time fraction t m / t m for ail 21 com ponents. The components repre s e n t e d b y b l u e l i n e s are all a d e quately separated on the 8.0-m DMP column. If all 21 c o m p o n e n t s (red a n d blue lines) are separated on the 432 A

Boiling point (°C)

Nominal concentrations, ppm (vol/vol)

40 48 60 61 84 77 90 101 105 105 113 114 120 118 121 127 132 133 139 144 144

81 67 68 65 52 54 62 41 47 36 56 38 44 43 51 39 110 75 84 81 85

t a n d e m c o m b i n a t i o n of t h e DMP a n d the PEG c o l u m n s , no h o l d - u p time fraction will adequately sepa rate the mixture in the short time frame considered here. This is illus trated by the large n u m b e r of lowa m p l i t u d e w i n d o w s s h o w n in the


blue-line w i n d o w diagram plot in Figure 5(b). If only the eight c o m p o n e n t s rep resented by red lines in Figure 5(a) are switched from the nonpolar C1 c o l u m n to the polar C 2 c o l u m n a n d t h e o t h e r 13 c o m p o n e n t s are al lowed to continue on the n o n p o l a r C3 c o l u m n , t h e w i n d o w d i a g r a m s h o w n by the red plot in Figure 5(b) is obtained. Here, there are a variety of hold-up time fractions that result in an adequate separation. Figure 6 shows chromatograms ob tained simultaneously from the two FIDs w h e n the 8 c o m p o n e n t s indi cated by the red plots in Figure 5(a) are switched from column C1 to col u m n C 2 and the pressure set to give a t m /t m value of about 0.50. Note that all 21 components are adequately sep arated in about 40 s. The use of col u m n switching and two parallel mea surement channels increases available zone capacity and more efficiently uses available zone capacity. Conclusions Tunable selectivity is necessary for the high-speed separation of complex mixtures. Even for relatively simple mixtures the probability of peak over lap is large, and tunable selectivity of ten can significantly reduce required column length and analysis time. The t a n d e m combination of a nonpolar column and a highly polar column provides a relatively w i d e polarity range. The use of a nonpolar dimethyl polysiloxane column in tandem with a p o l y e t h y l e n e g l y c o l c o l u m n pro vides high selectivity and a wide tun ing range for chlorinated hydrocar bons. The work described here e m p l o y e d t w o series-coupled col u m n s . Extension of these principles to three or more series-coupled col u m n s is in progress and should pro vide additional tunability with fur ther reductions in analysis times. High-speed column switching can be e x t r e m e l y useful for t h e high speed separation of mixtures involv ing more than 10 or 15 components. T h e i n c r e a s e d z o n e capacity pro vided by a d u a l - c h a n n e l detection system, together w i t h selective peak s w i t c h i n g , r e d u c e s the n u m b e r of components separated in the tuned portion of the system. The combination of selective peak switching and t a n d e m column tun able selectivity m a y be extremely useful in increasing sample through p u t for m a n y e n v i r o n m e n t a l sam ples. The techniques are relatively straightforward to i m p l e m e n t , a n d the optimization strategies described in this report are readily employed

FIGURE 5

Plots of overall capacity factor k versus time fraction t m2 /t m (a) and relative retention window diagrams (b) for a 21-component mixture using a DMP-PEG tandem column ensemble

u s i n g e i t h e r e m p i r i c a l d a t a from r e a l s a m p l e s or library c a p a c i t y factor v a l u e s for t a r g e t c o m p o u n d s o n t h e individual columns. Because each s e p a r a t i o n is v e r y r a p i d , m e t h o d s d e v e l o p m e n t is a l s o r a p i d , a n d t h e t i m e a n d cost savings c a n be e n o r m o u s . These techniques could have a major i m p a c t o n t h e a n a l y s i s of V O C s i n e n vironmental samples.

s

I I

Richard Sacks, co-founder and president of Chromatofast and professor of chemistry at the University of Michigan, received his Ph.D. in analytical chemistry from the University of Wisconsin in 1969. He has published more than 100 papers on atomic spectroscopy, high-speed GC, and chemical instrumentation. He has been working in the area of high-speed GC since 1982 and served as a consultant to several chemical companies.

I The blue-line window diagram is for the case where all 21 components are separated on the tandem column ensemble. The red-line window diagram is for the case where only the eight components indicated by red lines in part (a) are separated on the tandem column ensemble. See Table 3 for mixture components.

FIGURE 6

Chromatograms of the 21-component mixture using column switching and tunable selectivity: (a), chromatogram obtained from the nonpolar DMP column; (b), chromatogram obtained from the PEG column

Michael Akard, a research chemist at Chromatofast, received his B.S. degree in chemistry and mathematics from the University of New Mexico in 1990. In 1994, he received his Ph.D. in analytical chemistry from the University of Michigan. His dissertation work focused on selectivity tuning strategies for highspeed gas chromatography. References (1)

I Time (s) See Table 3 for mixture components.

Davies, M.; Giddings, J. C. Anal. Chem. 1983, 55, 418-24. (2) Vililobos, R.; Pearson, R. 1ST Transaction 1986, 25, 55-60. (3) Sandra, P. et al. HRC&CC 1985, 8, 782-97. (4) Deans, D.; Scott, I. Anal. Chem. 1973, 45, 1137-41. (5) Maurer, T.; Engewald, W.; Steinborn, A. /. Chromatogr. 1990, 517, 77-86. (6) Akard, M.; Sacks, R. /. Chromatogr. Sci. 1993, 31, 297-304. (7) Klemp, M.; Akard, M.; Sacks, R. Anal. Chem. 1993, 65, 2516-21. (8) Laub, R.; P u r n e l l , J. Anal. Chem. 1976, 48, 1720.


433 A

High-Speed GC Analysis of VOCs - American Chemical Society

Recommend Documents