Enriching Volatile Compounds by a Temperature Gradient Tube R. E. Kaiser lnsfitute of Chromatography, 0-6702 Bad Duerkheim, West Germany
Environmental analysis needs quantitative determinations and identification at concentration levels which are out of reach of most direct working analytical systems. Ethylene in air, a very active hormone for plants, kills some flowers within 24 hours when its concentration level reaches 10-7% (v/v\. Many other reactions of ethylene with plants have been reported by D. Zimmerman ( I ) , H. 0. Kunkel (2), and F. B. Abeles (3), showing that.ethy1ene in air has to be controlled to well under its lethal dose of l O - 7 % (v/v) for plants. Even a t a distance of 15 miles from a chemical factory, ethylene emission can reach levels higher than 10-5T0 (w/w). This is only one example where specific analysis in environmental control is necessary by specific enrichment, separation, and detection of a single compound. Identification of parts-per-billion contaminants by gas chromatography-mass spectrometry is troublesome. Enrichment to 10-8 gram of substance or a mass flow rate of 10-9 gram per second helps in many cases. In .u-ater, air, food, plants, and animals, nanotraces ( i e . , traces in the concentration range 10-9% w/w) are almost always accompanied by more than lo4 contaminants; therefore, specific separations are often the only way to solve the analytical problem. Chromatographic separations usually dilute the sample even more so; therefore enrichment prior to separation and again before detection is a must for many of the determinations in environmental analysis. Equation 1 expresses the limitations of chromatographic detectors, giving the theoretical basis for the above conclusions:
fili, x d t = limit of 3 sigma value of the smallest signal, coulomb q x = sensitivity of detector, coulomb per gram of compound E = total amount of sample, grams compound x q = yield of enrichment, YC %,x)lim = lowest measurable concentration of a compound x in the original sample, ’70w/w For example: Integral i l i m x t = 10-’3 coulomb q x = 10-2 coulomb/gram gram E = 20 ml air = 2.4 X f = 95% than %cx,lim, the analytical limit, = 1.05 X 1 0 - V ~w/w. A powerful procedure for enriching volatile compounds from gases is given by using a temperature gradient along ( I ) Don Zimmerrnan, Goteborg. private communication. 1972. ( 2 ) H. 0. Kunkel. Acting director. “Regulation of Ethylene as an Agricultural Practice, ’ Texas A&M University, The Texas Agricultural Exper-
iment Station, College Station, Texas. (3) F. B. Abeles, Plant Air Pollution Laboratories. U.S. Department of
a sorbing bed. This leads to a concentration-focusing effect, as the migration speed of substances through the sorbing material depends exponentially upon the temperature. The focusing effect is superposed by a collection a t different places within the sorbing bed for each compound-a so-called locally separated collection, thus preventing chemical reaction of the enriched traces with each other. Elution of the enriched compounds is accomplished by using a flowing clean gas under temperature gradient conditions. The focusing effect continues to operate, and the local separation in the temperature gradient tube is maintained. This keeps compounds both concentrated and separated, although the separation power of a gradient tube, about 20 to 30 cm long, is poor and allows separation of only homologs. Both enrichment and elution are made with continuous flowing gases, which is another “must” for trace analysis ( a t about 10-9% w/w) in the environment.
EXPERIMENTAL Apparatus. This consists of two concentric tubes. The inner one is packed with sorbing material of 30 to 40 mesh size in lengths from a few centimeters to 30 cm depending on the use of the system. T h e tube itself is made of stainless steel, glass, quartz, or other materials. I t is thin walled and has a n inner diameter of 2 to 4 mm. The outer tube, stainless steel or brass, is thin walled and acts as a cooling or heating device. Its inner diameter is 2 to 3 mm greater than the outer diameter of the inner tube. It has an inlet and outlet connections for the cooling or heating gas. Reagents. The sorbing material in the inner tube is chosen depending on the analytical problem and the compounds to be enriched as well as on the temperatures necessary. Carbon molecular sieves type B, better C (4), Tenax GC (51, Dexsil GC 300 on Chromosorb W (AW, HMDS), Silicone oil DC 200 on Chromosorb or Camag “Kieselgur” and many other packings have been used. Column bleeding limits the enrichment factor which can be achieved and, therefore, becomes a factor in choosing the proper sorbing material. Procedures. For enriching ethylene or hydrocarbons including methane to Cq from air, carbon molecular sieves is used in a bed length of 10 cm, 40 mesh, in stainless steel, 3-mm inner diameter. For enriching non- and medium polar impurities in air from C* u p to CIS, Dexsil 300 GC, 5% w/w on Chromosorb AW HMDS, 25-cm length, is used. At the enrichment step, the temperature gradient is produced by counter current flow of cold nitrogen, bubbled through liquid nitrogen a t about 1200 l./hr flow rate. Within 20 min, about 4 1. of air is passed through the gradient tube (pump a t the outlet). About 200 grams of liquid nitrogen is necessary to keep the lowest temperature of the gradient a t about -160°C. The amount of air sample pumped through the tube is measured carefully by volume, temperature, and pressure, as these data are the basis for calibration-free quantitative data processing. The pump, installed behind the gradient tube must be gastight too. T h e tube is now sealed. If absolutely gas-tight, the enriched compounds can be stored one week without noticeable change of the analytical results, as long as storage is done at -20 “C and no chemical reaction occurs. Sampling and enrichment, therefore, can be made in the field, in an airplane. or a t sea.
Agriculture, Agriculture Research Division, Plant Science Research Division, Beltsville. Maryland. Ann. Rev. Pian? Physioi.. 23, 259-92
(4) R E
(1 9 7 2 ) .
(5) A Zlatkis, Chromatographla, 6, in press (1973)
Kaiser, Chrornatographm 3, 38-40 (1970)
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'Gmdientenrohr '- technique
-
---
diluted sample
clean g a s
6
2000 / / h N2gas
N2 liquid
very clean carrier gas
enriched
--
to detector or old column
4
t
9 0 watts
2000 / / h heated air Figure 1. Schematic of temperature gradient tube, enrichment, and elution steps
2 I air
s H, 0 u,
E=64!
Figure 2. Ethylene in traffic air, detector, FID; gradient tube 10 crn of carbon molecular sieve, 30/40 mesh. Cryogenic TGC from -20 to $200 "C on 30-cm column, Carbon molecular sieve. Blind: 0.5 ppb ethylene in nitrogen as carrier gas
Elution is made with highly clean carrier gas, 4 l./hr, after recooling the tube and connecting it to a specific detector or to the inlet of a chromatdgraphic system, temperature programmed from cryogenic temperatures. The tube is heated under gradient conditions with flowing air, about 500 watts of energy at 1000 to 2000 l./hr flow rate. Overheating must be prevented for precise ethylene trace analysis. In this case, even at temperatures around 200 "C, higher organic compounds start to decompose on carbon black or carbon molecular sieves, producing secondary ethylene. By using a very clean carrier gas, volatiles from any source of materials can be extracted and transferred to a temperature gradient tube where enrichment takes place. So impure water or volatiles from dispersions, mud, plants, or foods can be sampled quantitatively, as the partition coefficient is independent of the concentration of compound x in the sample, but surface controlled. Rates of migration of volatiles through walls or from inner parts of solids can be measured in grams per second per gram sample. In no case is calibration necessary at the level of primary concentration, but the value for q of the detector (coulomb/gram or volt X ml/gram) has to be known. Figure 1 shows the gradient tube in schematic form.
RESULTS AND DISCUSSION Figure 2 shows a typical result using carbon molecular sieves a t the enrichment step and a temperature gradient 966
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6 , M A Y 1973
starting from -20 "C. The enriched sample is than eluted into a cooled chromatographic column, 30-cm length, packed with carbon molecular sieves also. The detector is a FID. The elution into the GC column is done under 200 "C at the gradient tube and the separation of the C1 to Cs compounds is done under temperature programmed conditions. Figure 3 is a typical result when analyzing air, enriched on a Dexsil GC 300 gradient tube, 25-cm length, gradient starting at -140 "C. GC separation on 4-m Dexsil 300, 5% w/w on Kieselgur Camag, FID, temperature programmed from -80 "C to + E O to 200 "C. Figure 4 shows a quantitative result, obtained after GC separation and detection with a quanttitative specific element detector (Cl) by eluting 100 grams of river water with a flow rate of 4 l./hr at 20 "C for 10 minutes. The standard deviation of repeated enrichment (GC) analysis is at the 0.01-ppb level, better than *lo% relative and about *3% relative when analyzing hydrocarbons. The systematic error is smaller than -10% relative, when analyzing volatile chlorine-hydrocarbon compounds by enrichment at the 0.01 ppb = 10-9% level (w/w). In this type of analytical problem, the yield of enrichment from the 0.01-ppb level was better than 94% absolute. This was tested by coupling more than two gradient tubes together, filled with a gradient of sorbing materials (this is an additional gradient!) and coupled on a reversion chromatograph (S), which automatically measures traces in the 10-10% (w/w) range. We enriched in differing analytical runs 1 or 2 or 4 or 8 or 16 1. of compressed cylinder air. The absolute amount of organics eluted thereby, measured as coulombs with an FID after elution and separation by GC, gave the relation 1 to 2 to 4 to 8 to 16 with a standard deviation of 1 3 % and an error of smaller than -5% a t the higher sampling rates. This was the total of all peaks, so it included each possible error of the GC procedure itself. For the quantitative calculations, a computer was used (7) because of the complex chromatograms obtained. The diagrams had been stored on magnetic tape. (6) R E (7) R E
Kaiser, Chromatographla Kaiser Chromatographa
1, 199-207 (1968) 5, 177-181 (1972)
nn total Chlorine: 8.5-105
2 I air
I
I
tS Iminl
t,
min
20
10
Figure 3. Sample, 2-1. Compressed cylinder air; detector, FID, gradient tube 25-cm Dexsil GC 300 on Chromosorb AW H M D S . Cryogenic TGC from -80 to 4-150 "C on 4-m column Silicone oil DC 200 on Kieselgur Camag
By eluting a t very limited temperature ranges, the compounds analyzed represent only a fraction of all that have been enriched. Backflushing a t higher temperatures is the clean u p procedure for the gradient tube. This again makes clear that environmental analysis done this way is limited in the type of contaminants measurable.
CONCLUSIONS Micro fog is often produced when cooling flowing gases too suddenly or to too low a temperature even in the ppb-concentration range, where the micro fog cannot be seen. This leads to erratic results. Even when using freshly activated molecular sieves 3 A, 4 A, 5 A, and 13 X in a 4-m column, 4-mm diameter, stainless steel, kept a t the temperature of liquid nitrogen, a gaseous sample contaminated with hydrocarbons flowing a t 2 l./hr, can loose micro fog representing a constant loss of about 10 ppb hydrocarbons in this gas. In other words, this long molecular sieve bed cannot clean up the flowing gas to better than 10 ppb contaminants of hydrocarbon type or: when using such a trap, the analytical result will have a noticeable systematic error. We used a tempera-
30
n 1
20
10 I 4-
Figure 4. River water, 100-gram sample, detector Dohrman coulometer CI, enriched and separated as in Figure 3, contarninants transfer by clean nitrogen 4 lL/hr flow rate for 10 minutes at 20 "C. Blind value as good as in Figure 2
ture gradient system, which helped analyze such a gas behind the 4-m long cold trap. This shows the power of gradient enrichment for preventing micro fog production. By enrichment of many liters of air, one obtains a lo-, or 15-, or 20-minute sample as a medium result. This does not fulfill all requirements in environmental analysis; therefore, high speed sample enrichment methods need to be developed, but often a 15-minute value is useful. The temperature gradient can be combined with a gradient in sorbing materials, thus making the enrichment effective for a wider scale of compounds, but often the few-compound enrichment, the very specific enrichment, coupled with multidimensional separation, again enriched after separation by mini gradient tubes-to achieve high rates of grams per second flow-into specific detectors is a powerful tool for correct environmental analysis a t the concentration level where effects really start to influence life quality.
ACKNOWLEDGMENT The author thanks his former employer, Badische Anilin-& Soda Fabrik AG, LudwigshafenjRhein, Germany, for the agreement to publish parts of these results and W. Stoll for his valuable experimental help. Received for review November 29, 1972. Accepted January 29, 1973.
Gas Chromatographic and Mass Spectrometric Studies of S-Alkyl Derivatives of N,N-Dialkyl Dithlocarbamates Francis I . Onuska and Walter R. Boos UniRoyai (Canada) L t d . , Research Laboratories, Guelph, Ontario
In the past decade, gas chromatography has been used as a very powerful analytical technique in the separation of trace amounts of organic compounds. However, it has the disadvantage of being vague in qualitative interpretation of a chromatogram. Mass spectrometry, on the other hand, has the advantage of being unambiguous in the structural identification of organic compounds in extremely minute quantities. Yet, it loses its potential if mixtures have to be identified. The combination of these two techniques, i.e., the identification of components separated by tandem GC/MS, has become close to the ideal solution for residual analysis.
In the course of our investigations into the identification of S-alkylated N,N-dialkyl dithiocarbamates (DTC) in wastewater, we found that a mixture of these compounds can easily be separated by gas chromatography. Gas chromatography has been recently applied to the separation of S-n-propyl N,N-dialkyl dithiocarbamates ( I , 2) and S-methyl and S-ethyl derivatives as a part of a more extensive investigation. These compounds are technically important as herbicides and rubber chemicals. (1) J Hrivhak and V KoneEny, Collect Czech Chem Cornrnun 4136 (1967) ( 2 ) F I Onuska, lntern J Environ A n a / Chem In press
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