gas

Anal. Chem. lQ88, 58,2213-2217

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Carbon Hollow Tubes as Collectors in Thermal Desorption/Gas Chromatographic Analysis of Atmospheric Organic Compounds George P. Cobb and Robert 5.Braman* Department of Chemistry, University of South Florida, Tampa, Florida 33610

Ke Min Hua Southwest Institute for National Minorities, Chengdu, The People's Republic of China

Carbon hollow tubes are Introduced as preconcentrators for atmospherlc organlc compounds. The technlque has a demonstrated range of applkaMllty for stable compounds havlng bolllng points between 90 and 235 O C . Studles lndlcate a Itnear response from 3 to 770 nglsample. The collector's capacity was found to range from 0.4 to 2.6 pmol dependlng on the compound belng adsorbed. The method has been used to determine dmuskn coeffldents of organlc alr anaiytes and to record organlc component patterns.

Atmospheric organic compounds are analyzed by a multitude of methods. Probably the oldest method of collection involves impingers to remove components of interest from air (1-4). Many methods accepted today utilize filters. Filters are generally made of fiberglass or polyeurethane (4,5) and may be impregnated with various adsorbants (4). A series of filters arranged in order of decreasing pore size has been used to achieve size discrimination (4). Charcoal (6-12), diatomaceous earth (13),and commercial adsorbants such as XAD's (14-17) and Tenax (4, 17-22) are popular collection tube packings for preconcentration of atmospheric organics. The newest additions to the list of packings are metal chelates that selectively preconcentrate polar organics (23). Complications accompany most of the above preconcentration methods used before finishing analyses-gas chromatography (GC) (4, 24), gas chromatography/mass spectrometry (12, 20, 21), or high-performance liquid chromatography (4,251. Many recoveries from filters and packed collectors require lengthy extractions before chromatographic analysis (9,11,17,21). Thermal desorption has been presented as an effective means of sample introduction for GC analysis (17,18 21,22,26). Analyte decomposition has been observed during adsorption/thermal desorption on Tenax. Although no mechanism for this decomposition has been proved, it likely occurs during the desorption process (10, 21). At elevated temperatures, the natural atmospheric oxidants, O3and NOz, react with sorbed analytes. Under normal experimental conditions in the presence of oxidants, the Tenax adsorbant decomposes to produce various byproducts (10,Zl). Interior-coated hollow tubes (HTs) have been used as thermally reversible chemisorbers ( 2 7 , B ) . Many atmospheric nitrogen containing compounds, including amines, have been collected on HTs coated with thin metal oxide layers. A logical extention of this research is a preconcentration and analysis method for atmospheric organic compounds. The thermal desorption characteristics of the carbon hollow tube (CHT) system eliminates many of the problems encountered with various other systems for sampling atmospheric organics. First, the surface is sufficiently shallow for easy thermal desorption of the organic molecules, a fact that may aid in minimizing decomposition of trapped analytes. The analytes tested did not decompose during the desorption process. In

addition, the rapid speed of desorption makes direct interfacing into a GC feasible without requiring an intermediate trapping step. Finally, the tubular design of the CHT provides a high degree of separation of gases and particles. The basis of the size discrimination is the large difference in diffusion coefficients. Only gases (D, = 0.2-0.01 cm2/s) are removed by CHTs. Particles, 0.1 km in diameter and larger, have D,values on the order of cmz/s. Particle D,s are not large enough to allow these analytes to collide with the CHT walls with the frequency necessary to allow collection. Diffusion coefficients are of importance in speciation of atmospheric components (28). Knowledge of the D, of a compound analyte provides valuable information concerning possible dimerization, hydration, or large agglomerates formed.

EXPERIMENTAL SECTION

Tube Preparation and Coating Depth. The carbon surface was deposited on the interior of a 6 mm. 0.d. Vicor glass tube. The tube was first soaked in 10% aqueous NaOH for 1h and then thoroughly washed with deionized water. After the glass ends were fire polished and the entire tube was flame dried, the coating procedure began. Soot from the smoke of a benzene or toluene fire was drawn into the HT by means of a Neptune Dyna pump (UniversalElectric; Owosso, MI) or a pipet bulb. The procedure was carried out on both ends of the tube to create a more uniform coating. The layer deposited was estimated by weighing to be on the order of 100 nm in depth. With helium passing through the CHT, it was then heated to 270 "C for 30 min. A moderate amount of smoke and yellow oil was produced in this step. Finally the coating on the CHT ends was removed with a hot flame while passing oxygen through the tube. One inch was left uncoated on each tube's ends. Active coating lengths can vary. Tubes with 2040 cm coatings were used to allow good adsorption efficiency of analytes at sampling rates of 0.5-1.0 L/min. After 1month of intermittant use (200 or more cycles), deterioration in the CHT surface became apparent. Spaces barren of the carbon surface began to appear. Analyte capacities and measured collection efficiencies decreased. At this point, the coating procedure was repeated from the surface deposition step. Carbon coating depths were estimated by using CHTs prepared as described above. After a CHT's mass was recorded, the entire carbon surface was removed by use of a flame as in the final preparation step. The difference in coated and uncoated masses provided coating masses. By use of the density of amorphous carbon, 2 g/mL (29), the coating depth was easily calculated. Collectors packed with carbon-coated sand (60/80 mesh, precoated) were made in the following manner. A 6 mm 0.d. Vicor glass collection bed (27,28) was packed with the HC1-washed sea sand (28). The sane was carbon coated as were the CHTs. The preanalysis heat treating was done at 270 OC, and the duration was 1 h. The tube was then repacked to ensure clean glass wool in the collector. CHT-GC Interface. The inlet of an HP-3890 GC required modification to accomodate the CHT. The septum and retaining nut were replaced with a system capable of holding the CHT. The first step in the modification of the injection port (Figure 1)

0003-2700/86/0358-2213$01.50/00 1986 American Chemical Society

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 Gas Flow

101

I I-*

C B

Flgwe 1. Injection port modification: (A) port connector; (B) Swagelok nut; (C) Teflon O-ring: (D) injection port.

m I I

Figure 2. Experimentaldes@ for breakthrough capacities: (A) helium gas cylinder; (B) charcoal scrubber; (C) permeator in permeation chamber: (D) carbon hollow tube.

consisted of cutting 4.5 mm from the ferruled end of a port connector and filing the ends to smoothness. The reduction in ferrule length allowed the 0.25 in. Swagelok nut to reach the threads on the top of the injection port. The modification left a flat metal/metal interface that will not seal. A Teflon O-ring solved this problem although it required periodical replacement. A 0.25-in. Swagelok tee connector was used to attach the CHT to the new inlet in a verticle position (Figure 3). The first of two carrier gas lines was connected to the CHT with a 0.25 in.4.125 in. reducing union. The other was connected to the remaining fitting of the tee connector. Sample Preparation. All standards were 99% mol purity or redistilled over a range of not more than 1 OC. The permeation devices were one of two types: a screw top vial with a 0.125-in. hole drilled in the top and a Teflon diaphragm inside the cap serving as a permeation membrane or a Pasteur pipet with the large end sealed and the small end formed into a capillary. After addition of the standards to the permeators, they were placed in glass chambers (Figure 2c) and 100 mL/min of air was passed through the apparatus. Permeator masses were recorded periodically, usually daily, with a Mettler H20T balance until a constant permeation rate, &lo%/day, was achieved. Thereafter masses were taken periodically when in use. Samples from the permeators generally consisted of the entire effluent from the permeation chamber although occasional use of a glass tee and a Dyna-Pump allowed sampling of only a fraction of the total effluent (i.e., 100 mL/min of 1.0 L/min. All permeation flow rates were monitored with a calibrated Matheson Model 8160 flowmeter. Experimental Procedure. Hollow Tube Capacity. Hollow tube capacity was determined by using either a direct current discharge (DCD) (30) or a Bendix NOx analyzer. Nitrogen-containing compounds were analyzed by the NO, analyzer, and the remaining compounds were analyzed on the DCD. The total effluent from a permeation chamber was passed over a CHT and into the analyzer (Figure 2) a t a rate of less than 100 mL/min. Molecular Stability Experiments. The stability of analytes undergoing adsorption/ thermal desorption was studied by using two CHTs. Analytes from an air stream were first adsorbed onto one of the tubes. Analytes on this tube were then thermally desorbed directly into the analyzer. In a second experiment, the same amount of analyte was first adsorbed onto one CHT and

U

Figure 5. GC carrier gas Rows: (A) helium gas cylinder; (B) three-way valve; (C) carbon hollow tube; (D) gas chromatograph.

then transferred by thermal desorption onto the a second CHT. This second tube was then analyzed by GC, NO, analyzer, or DCD. Calibration curve comparisons utilized GC responses of a component injected as a solution standard and those of the same component adsorbed on CHT. The solutions for injection were 65.0 ppm toluene in H20along with 7 ppth and 0.7 ppth 1-decanol in benzene. A range of sample sizes were analyzed. A gas sampling rate of less than 100 mL/min was used in all CHT analyte sampling to ensure quantitative adsorption. Chromatography Effect. In the chromatography effect experiments, a breathing air flow of 40 mL/min was passed through the permeation chamber and through a CHT. For each analyte, direct analysis was performed as in the molecular stability experiments. Next the compound was collected on a CHT and then 600 mL/min of clean breathing air was passed through the CHT. The duration of exposure of the air flow was varied during compound testing. All samples were subsequently analyzed by GC. Sampling rates of less than 100 mL/min were used to ensure quantitative adsorption of the analytes on the CHTs. Diffusion Coefficients. Diffusion coefficient determinations required a known flow rate of breathing air,a permeation chamber, and two CHTs of known length. Optimal air flow rates ranged from 1 to 2 L/min for many atmospheric organic organic compounds, whose D,s generally range from 0.05 to 0.1 cm2/s. Tube lengths may vary but the best results were obtained when the first tube was at least 17 cm long and the second was 35-40 cm in length. Air was passed through the permeation chamber. Analytes were collected by passing the permeation chamber effluent through a pair of CHT in series. The quantity of analyte on each CHT was then determined by GC. Data analysis using a computer program allowed calculation of the best D, according to the Gormley-Kennedy equation (28). Organic Component GC Patterns. With a single CHT, organic components evolved from solid natural substances were preconcentrated from the effluent of a permeation chamber containing them. Indoor and outdoor air samples were taken by using a CHT(s) followed by a collection tube packed with carbon-coated sand (CPT). All sampling rates were 0.5 L/min. Each sample was analyzed by GC. GC Analysis. The GC used was an HP-3890 with a flame ionization detector (FID). An HP-3390A integrator and an HP19400A sampler/event controller module, which controlled a three-way Delta solenoid value (Fluorocarbon, Inc.; Aneheim, CA), were also used in combination with the GC. Separations were achieved by use of a 1m X 0.125 in. stainless steel 5% OV-101 column and a carrier gas flow rate of 22-24 mL/min. During GC analyses, typical temperature programs began at 30 "C and ramped to a maximum of 250 'C. The injection port was held at 250 ' C . The FID was held at 270 "C. Samples were desorbed into the GC at 270 "C for 2.5 min, with the carrier gas flowing along the analysis path (Figure 3). After heating, the carrier gas was switched to the bypass path where it remained until the next desorption began.

ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986

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Table I. Hollow Tube Capacity capacities compound acetonitrile benzene

hexylamine chlorobenzene nitrobenzene pyridine pyrrole

m-nitrotoluene nicotine

vapor pressure,

rg

rmol

bp, OC

mmHg at 23 "C

N

16.38 31.1 (12.2%) 59.6 (&4.4%) 140 (&2.8%) 181 (k2.4%) 181 ( i 1 7 % ) 197 (f1.7%) 257 (&lo%) 436 ( i 9 . 9 % )

0.397 0.398 (f2.2%) 0.589 (f4.4%) 1.25 (f2.8%) 1.47 (k2.4%) 2.30 (f17%) 2.93 ( i 1 . 7 % ) 1.87 (*lo%) 2.62 (f9.9%)

81.6 80.1 130 132 210.8 115.5 130 232.6 242.3

68.2 61.6 6.0 9.0 0.46 14.5 6.8 0.34 0.20

1 2 3 2 4 4 2 2 3

Table 11. Analyte Transfer compound 1-butanol 2,4-pentanedione chlorobenzene pyridine benzene

nitrobenzene

-

desorptions prior to detection single CHT CHTl CHT2 96.0 (i2.9%) 55.3 (k7.0%) 25.5 (k9.5%) 78.5 2.74 18.7

99.48 (f5.5%) 53.75 (+4.6%) 24.7 (i13.8%) 79.0 (116.4%) 2.77 18.2

Blanks. Helium gas was passed through CHT overnight at 40 mL/min to determine blanks. Repeated GC analyzes showed no measurable organics. When left opened overnight, CHTs did show substantial blanks as would be expected from inboard diffusion of organics from laboratory air.

RESULTS AND DISCUSSION CHT analyte capacities were measured via DCD or NO, analyzer. Capacities ranged from 16 to 440 pg depending on the analyte (Table I). CHT analyte capacity vs. analyte vapor pressure was fit to an exponential. The resulting correlation coefficient was 0.90. This dependence illustrates the importance of analyte vapor pressure as a collection parameter. The fact that the carbon surface of the CHTs slowly degraded upon repeated use points to the possibility that species adsorbed on the surface may also decompose. Reported evidence of analyte decomposition on Tenax increased this concern. Tests of a few compounds adsorbed onto a CHT and desorbed for analysis showed no decomposition losses as indicated by the data in Table 11. Analytes produce the same response whether desorbed from one CHT into the detector or from one CHT to a second CHT then into the detector. The quantitative readsorption of components transferred from one tube to the next illustrates the retention of components' integrity throughout the adsorption/thermal desorption process. If the analytes had fragmented, the products would likely have been too volatile to be readsorbed by the second CHT. In a similar experiment (Table 11) benzene and nitrotoluene were collected on CHTs and analyzed by GC. Virtually identical GC retention times (*0.05 min) and area responses were observed whether desorbed through one or two CHTs. The combination of reproducible retention times and peak areas reinforces the assertion that compounds are unchanged in the adsorption/ thermal desorption process. The final molecular stability study involved comparing chromatograms of injected samples to those of samples desorbed from CHTs. Table I11 shows the linear regression parameters for integration vs. analyte mass. This experiment showed that polar and nonpolar analytes, injected or adsorbed onto and then desorbed from CHTs, produce the same response. Calibration curves derived from injected and desorbed standards of toluene and 1-decanol, respectively, were linear at least over the range 10-770 ng. Slopes for the injected and desorbed toluene samples do not differ statistically. The

% recovery

N

detector

103 97.2 97.2 101 101 97.4

3 2 5 3 1 1

DCD DCD DCD NO, GC GC

Table 111. Calibration Curves ordinate intercept

GC

compound toluene

introduction injection TD"

1-decanol

injection TD"

slope ( x i 0 4 )

(xi041

1.42 i 0.057 1.26 i 0.13 1.12 f 0.086 1.61 f 0.066

-4.05 i 13 44.7 f 8.2 3.93 f 3.7 -18.3 f 13

Thermal desorption. difference in the 1-decanol slopes is 30%. Poor integration of broad decanol peaks and incomplete volatilization of decanol from the injection needle are possible reasons for the difference in decanol slopes. Most of the error is likely manifested in the 10% day to day variation in permeation rate. Retention times for the desorbed samples are found to be up to 0.2 min longer than those for the injected standards. This time lag is due to the delay in maximum heating during the desorption process. Thus all retention times for desorbed samples should be longer than those for the respective injection samples. The smallest sample from the toluene calibration curve was 23.7 ng. Decanol samples of 2.84 ng were analyzed and found to be near the detection limit. The calibration curve indicates a lower limit of detection at 10.5 f 7.5 ng. A more precise determination of the limit of detection will require a standard with a slower permeation rate, near 100 ng/min. This rate is necessary to ensure precision both in permeator weight loss and in sampling time. A study of the extent to which various species chromatograph from the carbon surface at room temperature provided interesting results. Figure 4 depicts the movement of organics sorbed on the surface of a CHT as a function of air volume passed through a loaded CHT. The chromatography effect seems to disappear above a boiling point of approximately 85-90 "C. Cyclohexane had little quantitative retention as seen by the loss of analyte after only 150 mL of air flow. Benzene retention was a bit better. Approximately 12 L of air was required to remove 10% of the benzene from the surface. Toluene and nitrotoluene (not shown) were not moved even after 65 min of air flow. This chromatography effect supports a view that strong physisorption is the mechanism of collection at the carbon surface. If there are

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 70 i

1

Table IV. Diffusion Coefficients

. . . . . . . . . . . . . . . . . . . . . . . . " ? " " " " '

. . . . . . . . . . . . . . . . .9. . . . . . . . . . . . . . . . . . . . . . . . . c\

;

.

,

.

.

.

.

. . . .

I

i i

compound

theoretical value, cm2/s

exptl value, cm2/s

pyridine chlorobenzene toluene 1,2-dichloroethane 1.2-dibromoethane

0.095 24 0.08146 0.085 07 0.093 12 0.083 81

0.109 (f3.85%) 0.0789 (f9.64%) 0.0802 (*1.50%) 0.0931 (f32.9%) 0.0744 (f12.8%)

N 2

3 12

5 4

p;;;xi*;i;;l

10 -

- A ^ .

J

0

.

. . . . .

5 10 Breathing Air Exposure (min.)

Flgure 4. Chromatography effect: circle, toluene; diamond, benzene; square, cyclohexane.

no bonds formed in the adsorption process, as in physisorption, the more volatile analytes should slowly chromatograph through a H T during sampling. One useful application demonstrated for this method is the determination of D,s. Gromley and Kennedy have described the adsorption of species, individual molecules or agglomerations, from a gas stream onto the surface of a H T through which the stream is passing. Their mathematical expression (28) is C,/C, = 0.819 exp(-3.6568rDLn/F) 0.0976 exp(-22.3rDLn/F) + ... (1)

+

where Co is the analyte concentration entering the hollow tube, C, is the concentration exiting hollow tube n, D is the diffusion coefficient of the compound in question, L, is the length of tube n, and F is the flow rate. Utilizing the definition

c1 = c, - s,

(2)

eq 1 may be written in the form

S1/Co = 1 - 0.819 exp(-3.6568rDLn/F)

+ ...

where SI is the amount of analyte adsorbed on tube 1. Utilizing the facts that Co is the total amount of a component arriving at the head of tube 1 and that SI may be treated as a fraction of that total, eq 1 reduces to

S , = 1 - Ea, exp{b,DL,/Fj

(3)

= 1 - R(D,Li,F)

(4)

where R(D,L,,F)-a function of D , L,, and F-is the summation portion from the left site of eq 3. For the second tube in the set

S2/C1 = 1 - R(D,L,,F)

(5)

Using eq 2, 4,and 5

s, = [1- R(D,L,,F)l

R(D,Li,F) (6) The iterative computer calculation of a D, began by assuming a D,of 0.1 cmz/s. The calculation of S1/Ssvalues was done using eq 4 and 6. This calculated value was then compared to the SI/&ratio from the experimental data. This comparison provided an index of precision of the two values. The iteration was complete when the difference in the ratios is below the selected precision threshold. Experimental diffusion coefficients were in good agreement with those calculated from physical properties using the Arnold method (Table IV) (31). It should be noted that this method of calculating theoretical D, values has a standard deviation of f8.4% when compared to experimental data (31). This fact shows the experimentally determined D,values to X

r

Imln.

5

I

I

I

I

10

15

1

I

20

25

I

Flgure 5. Spice odor patterns: (A) nutmeg; (B) cinnamon. Both samples separated with He flow of 32 mL/min, temperature program hold 40 OC, 1 min ramp at 10 OC/min to 210 OC for cinnamon and 250 OC for nutmeg, both held at their maximum temperature for 5 min.

be within the error range of the theoretical values. The average standard deviation of all D, experimental values was f12.1%, and the average absolute deviation between the experimental and theoretical was f6.91%. These data indicate CHT performance is in agreement with theoretical predictions. Chemical patterns of volatile organics evolved from fresh citrus and spices were obtained by use of a single CHT and subsequent GC analysis. Representative chromatograms are shown in Figure 5. These chemical patterns are readily distinguished from one another. With computer aid, sources of such patterns should be easily accessible. Figures 6 and 7 contain indoor and outdoor patterns, respectively. Inspection of indoor organic component patterns desorbed from the three collection surfaces points out the normal performance of this collection system. The components with medium volatility are collected on the CHTs. The higher volatility components chromatograph through the CHTs and thus show up on the CPT. The low volatility components are usually associated with particulates. The low D, of a particle is insufficient to allow efficient collision with the CHT collection surface. Of the components collected on the CHTs, those with higher D, values are collected more efficiently. Tubes with the same lengths have equal efficiencies, Thus, for a pair of CHTs, the amount of analyte(s) collected by the first CHT will be greater than that collected by the second. The only exception to this rule is the case where the CHT has been overloaded with analyte. The CPT collects the analytes not collected by CHT(s). The majority of these are of low and high volatility. The patterns from Figure 6 illustrate these properties. The patterns of Figures 6A and 6B are very similar; the main

ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986

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Gulf of Mexico. The high humidity in the atmosphere above these waters may have removed particulates, generally associated with anthropogenic sources of organic pollutants.

CONCLUSIONS The chromatography effect data shows that components with boiling points below 85-90 "C are not quantitatively retained by the CHTs. Molecular stability and calibration curve data indicate an upper limit of boiling point usefulness at 235 "C or more. The calibration curves also indicate a linear response over at least 2 orders of magnitude. Experimental D,values are within the error range for calculated theoretical values. And fiiaUy organic component patterns from different sources are readily discernible. These facts illustrate the potential of the CHT-GC method for analysis of diversified atmospheric organics. Registry No. Acetonitrile, 75-05-8;benzene, 71-43-2;hexylamine, 111-26-2;chlorobenzene, 108-90-7;nitrobenzene, 98-95-3; pyridine, 110-86-1; pyrrole, 109-97-7; rn-nitrotoluene, 99-08-1; nicotine, 54-11-5; 1-butanol, 71-36-3; 2,4-pentadione,123-54-6; toluene, 108-88-3;1-decanol, 112-30-1.

LITERATURE CITED Figure 6. Chromatograms of an indoor air sample: (A) first CHT; (B) second CHT; (C) CPT. Ail retention times are in minutes.

I /I

m e 7. Chromatograms of an outdoor air sample: (A) CHT; (B) CPT. All retention times are in minutes.

difference is in peak area. A comparison of components collected by a CPT to those collected by a CHT shows the low and high volatility analytes to be concentrated most heavily on the CPT. Analytes with retention times less than 4 min and greater than 11min are of little importance in CHT analyzes. However, in the CPT analysis, the peak a t 13.81 min is the largest and the peak at 2.4 min is the second largest. The same type patterns are observed by the outdoor chromatograms in Figure 7. The large amount of low volatility components is not present in the CPT analysis. One explanation is that the samples were collected on board ship in the

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RECEIVED for review November 18, 1985. Resubmitted May 8, 1986. Accepted May 8, 1986.