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Laboratory Application Data LAD 001-015 DETERMINING ENVIRONMENTAL CONTAMINATION With the CENTURY Programmed Thermal Desorber and Organic Vapor Analyzer Time weighted averages of environmental contamination can be determined using the CENTURY Programmed Thermal Desorber PTD-132A and Organic Vapor Analyer OVA-128 more conveniently and accurately and in less time than with chemical desorption methods. SORBENT COLLECTOR TUBES--BACKGROUND The use of sorbent collector tubes is a well accepted technique for evaluating the amount of environmental contamination as a time weighted average (TWA). These tubes are generally packed with a compatible sorbent me dium such as activated charcoal, Tenax, or carbon spheres. In use, the tube is opened on both ends either by breaking off the tips, as is the case with glass tubes, or by removing polyethylene caps as in the case of metal tubes. See Figure 1. The tube is then connected with plastic tub ing to a portable air pump that draws a known volume of environmental air through the tube during the monitoring period. 1983 by The Foxboro Company tOXBORb Reg|jtered Trademark UCC 094397 LAD 001-015 Page 2 o `--.v; ." " :f' V, . lii%:--uli Figure 1 After such exposure, the collector tube is capped to pre vent the loss of sample by diffusion. The sample is then analyzed in one of several ways to determine the TWA. The most often used method for tube desorption is the use of chemicals (such as carbon disulphide) to physi cally extract adsorbed contaminent chemicals for later analysis by gas chromatography. THE PROBLEM Though widely utilized, chemical desorption has several disadvantages that hinder rapid and effective timeweighted-average determination. These are: 1. Long, sample analysis time. 2. Expensive, individual sample handling. 3. Careful laboratory procedures required. 4. The use of toxic chemicals. 5. Elaborate calculations required. THE SOLUTION--THE CENTURY PROGRAMMED THERMAL DESORBER (PTD) The PTD represents an attractive method for sample han dling in preparation for analysis by an analytical instru ment. See Figure 2. This device thermally desorbs the adsorbed chemicals from the adsorbent at a temperature preselected by the user, stores the desorbed sample in a small chamber of known volume, and automatically in jects aliquots of the sample into a gas chromatograph or other analytical instrument. At each stage in this process, the PTD method proceeds under user preselected conditions that have been deter mined by experiment to yield optimum results. PROCEDURE The contaminated collector tube is placed in the PTD de sorption oven which is then sealed. Upon pressing the "start" button, a small pump begins to withdraw air from a sealed chamber over a piston. As the piston begins to rise, a partial vacuum is formed that draws air from the environment through a charcoal scrubber, through the heated tube, and into the expanding volume under the pis ton in the storage chamber As the piston reaches the top of the chamber, the pump stops automatically, and the chamber is sealed. The volume of the storage chamber is 300 mL in the fully desorbed position. See Figure 3 After desorption is complete, the user may choose to re move aliquots of sample from the chamber for analysis either manually or automatically. If the automatic method is chosen, a pump integral to the PTD may be used to draw sample from the storage chamber through a sample loop of 2.5 mL volume, which is then made part of the carrier gas line of a laboratory gas chromatograph. This is ac complished by means of a solenoid-operated sample valve programmed to inject the sample after a given period of time. * For the manual mode, a pump external to the PTD may be used to extract sample from the storage chamber. The sample is then injected into a portable gas chromatograph, such as the OVA-128, or other analytical instrument. In the first case, the PTD contains an integral sample loop that is automatically flushed utilizing a part of the 300 mL sample in the chamber. Since the volume between the sample storage chamber and the sample loop is only a few mL, multiple sample injections are possible before the storage chamber is completely emptied. For manual injection, as in the case where the PTD is uti lized with the Century Portable Organic Vapor Analyzer (OVA), a pump in the OVA draws sample out of the cham ber for analysis. > UCC 094398 LAD 001-015 Page 3 Figure 2 THE BENEFITS OF USING THE PTD SYSTEM The PTD has several advantages, over currently used chemical desorption techniques, that benefit the user with higher accuracy and savings in cost, time, and man power. It also relieves laboratory personnel from repeti tious and exacting determinations: 1. Desorption and storage of samples is automatic. 2. Sample tubes are reusable. 3. Time-weighted-average calculations are simple. 4. Exacting laboratory technique is unnecessary. 5. No chemicals are necessary. 6. Sample injection is automatic. 7. Replicate sample injections are possible for in creasing accuracy. AIR SCRUBBER OVEN SAMPLE PUMP COMPRESSED GAS Figure 3 UCC 094399 LAD 001-015 Page 4 THE CENTURY ORGANIC VAPOR ANALYZER (OVA) The OVA is a portable gas chromatograph that uses a flame ionization detector that responds to organic va pors. It may be used either as a survey tool or to quantify specific compounds by means of a chromatographic col umn and sample-injection system. See Figure 4. As a gas chromatograph, the OVA incorporates a voltage output sufficient for use with a variety of chart recorders. In this way, chromatograms are produced that serve to both identify a compound (retention time) and quantify it (peak height measurement). As shown in Figure 5, peak height is proportional to con centration of sample. This relationship can be expressed as a calibration curve as shown in Figure 6. When expressed as a calibration curve, the operator can rapidly determine concentration of sample thereby speeding up analysis. When used with the PTD, the OVA-128 GC is attached di rectly to the desorber by a teflon tube. (See Figure 7.) When the "read" button on the PTD is depressed, a sole noid valve opens the storage chamber allowing the OVA sample pump to withdraw sample to be used in flushing its sample loop. A steady reading on the OVA meter indi cates that the sample loop has been thoroughly flushed and that the manual sample valve may be actuated to be gin chromatography. 1 Figure 4 I UCC 094400 BENZENE SAMPLES 140 ppm LAO 001-015 Page 5 BENZENE CALIBRATION G'8 COLUMN oc RETENTION TIME - 80s Figure 5 CHROMATOGRAPHIC PEAK HEIGHT VS SAMPLE CONCENTRATION 0C PIPG`8 COLUMN Figure 6 Figure 7 UCC 094401 LAD 001-015 Page 6 EXAMPLES The chromatograms shown in Figures 8 through 13 illus trate the use of the PTD with the OVA-128 GC to produce chromatograms that serve both to identify and quantify environmental contaminants. In each example, a sample of vapor of known concentra tion was prepared in a sampling jar and a chromatogram was generated using the OVA. In Figure 8, the chromato gram was run on 223 ppm of isopropyl alcohol on an 8-inch tris column at a temperature of 0C, Under these conditions, the retention time was 214 seconds. After the initial chromatogram was run, 300 mL of sample was drawn through a collector tube. The sample was then desorbed in the PTD under a known set of conditions, Since the storage chamber of the PTD is 300 mL, the con centration in the chamber should be identical to that in the original sample, assuming 100% desorption effi ciency. In Figure 8, the collector tube was packed with CS-5 (car bon spheres) and was desorbed at 200C at the rate of 1 mL/s. A comparison of peak heights (peak height is proportional to concentration) for the chromatograms of sample and desorbed sample gives a direct measure of desorption ef ficiency. In the example given in Figure 8, desorption effi ciency is 100% since the peak heights of the two chromatograms are identical. TIME WEIGHTED AVERAGE CALCULATION Calculation of the time weighted average (TWA) for a compound of interest is greatly simplified by use of the PTD. The formula can be expressed as: jyjfr _ PTD Chamber Volume Volume of Air Sampled Concentration of Desorbed Sample As an example, an air sampling pump was utilized over an eight-hour period to draw environmental air through a metal sample tube at the rate of 20 mL per minute. The tube was later desorbed on the PTD, and the desorbed sample was found to contain 120 ppm of benzene. The time weighted average (8 hours) is found using the formula: TMA = 0^760 x 8 X 120 Ppm = 375 ppm The analytical instrument utilized to determine the con centration of desorbed sample can be a gas chromato graph, an infrared analyzer, or another instrument capable of the desired degree of accuracy for the mea surement at hand. sample SAMPLE AFTER DESORPTION COMPOUNO COLUMN TEMPERATURE RETENTION TIME SAMPLE CONCENTRATION DESORPTION TEMPERATURE DESORPTION RATE DESORPTION TUBE DESORPTION EFFICIENCY ISOPROPANOL 10% TRIS PROPANE ON SUPELCOPORT 0"C 214i 223 ppm 200C 1 mL/t CS-5 100% Figure 8 SAMPLE SAMPLE AFTER DESORPTION COMPOUND COLUMN TEMPERATURE RETENTION TIME SAMPLE CONCENTRATION DESORPTION TEMPERATURE DESORPTION RATE DESORPTION TUBE DESORPTION EFFICIENCY ACETONE 10% TRIS PROPANE ON SUPELCOPORT 0C 117 f 223 ppm 200C 1 mL/t CS-5 100% Figure 9 i UCC 094402 LAO 001-015 Page 7 SAMPLE SAMPLE AFTER DESORPTION COMPOUND METHANOL COLUMN TEMPERATURE 10% TRIS PROPANE ON SUPELCOPORT 0C RETENTION TIME 150 s SAMPLE CONCENTRATION DESORPTION TEMPERATURE 668 ppm 200*0 DESORPTION RATE 1 mL/s DESORPTION TUBE DESORPTION EFFICIENCY CS-5 100% Figure 10 SAMPLE SAMPLE AFTER DESORPTION COMPOUND COLUMN TEMPERATURE RETENTION TIME SAMPLE CONCENTRATION DESORPTION TEMPERATURE DESORPTION RATE DESORPTION TUBE DESORPTION EFFICIENCY CHLOROFORM 10% TRIS PROPANE ON SUPELCOPORT 04C 115s 223 ppm 200C 1 mLVf CS-5 98% Figure 12 SAMPLE SAMPLE AFTER DESORPTION COMPOUND COLUMN TEMPERATURE RETENTION TIME SAMPLE CONCENTRATION DESORPTION TEMPERATURE DESORPTION RATE DESORPTION TUBE DESORPTION EFFICIENCY ETHYL ACETATE 10% TRIS PROPANE ON SUPELCOPORT 0C 140 s 223 ppm 200C 1 mL/s CS-5 95% Figure 11 SAMPLE SAMPLE AFTER DESORPTION COMPOUND COLUMN temperature RETENTION TIME SAMPLE CONCENTRATION DESORPTION TEMPERATURE DESORPTION RATE DESORPTION TUBE DESORPTION EFFICIENCY 8ENZENE 10% SP-21 ON SUPELCOPORT 0C 80s 111 ppm 250C 1 mL/s CS-5 95% Figure 13 UCC 094403 LAD 001-015 Page 8 CONCLUSION The Organic Vapor Analyzer, and the Programmed Ther mal Desorber are powerful analytical tools for collection and analysis of environmental samples, on-site, by rela tively inexperienced personnel. The combined use of the two instruments provides a sys tem that yields real-time answers to questions of worker exposure to hazardous gases and vapors. As applications are broadened, these instruments will as sume a dominant position for environmental monitoring where accurate data, rapidly obtained, is required for flexible response. REFERENCE LITERATURE PSS6-12Z1 B CENTURY Programmed Thermal De sorber (PTD-132 A) PSS6-12Z1 A CENTURY Dual-mode Organic Vapor Analyzer (OVA-108 and 128) PSS6-12Z1D CENTURY Organic Vapor Analyzer (OVA-88) CENTURY is a trademark of The Foxboro Company. Tenax is a registered Trademark of Enka N. V,, The Netherlands. MB 042 Printed in U,S A ,i-JTEK CC-.FCRATION o r-' 0,y< /.IT 321/606 ' y Tc-^ 77042 l0 ; T':/.2y Road Hous.c.i, 77099 713/ 438-6655 0183 UCC 094404