Anal. Chem. 1984, 56, 1043-1046
m/z 290
7MIN
RETENTION TIME
18 MIN
Figure 3. Reconstructed ion chromatograms for Aroclor 1242 and surrogate at 250 ppb in NBS transformer oil. Dibromobiphenyl was added at 1 ppm as an internal standard. Selected ion chromatograms for tri-, tetra-, and pentachlorobiphenyl isomers, as well as the 2,2'-dibromobiphenyl recovery surrogate standard, clearly reveal characteristic peak profiles free of interferences. These chromatograms have very similar isomer distribution patterns and relative intensities to that of 2 ppm Aroclor 1016 in hexane. The only interferences observed arise in the latter portion of the trichlorobiphenyl chromatograms, mlz 256 and mlz 258 (Figure 21, beyond the retention region of the respective PCBs. This interference can be easily differentiated from PCBs not only by its longer retention time but also by its lack of consistently corresponding isotope peaks at mlz 256 and m/z 258. These data resulted from spiking 2 ppm Aroclor 1016 and 1 ppm 2,2'-dibromobiphenyl into a transformer oil. The various PCBs which comprise Aroclor 1016 are of course present in lesser amounts. For example, the total tetrachlorobiphenyl isomers represent only 12% of total Aroclor and therefore are present at 240 ppb. The individual peaks detected are within
1043
the 100 ppb range (Figure 2, m/z 290, 292). In a second case a mixture of Aroclor 1242 from SRM 1581 was diluted to 2 ppm with the corresponding NBS transformer oil and was then spiked at 250 ppb with [l3CI2]-3,3',4,4'tetrachlorobiphenyl as a recovery surrogate. Dibromobiphenyl was added to the cleaned HPLC fraction at the 1 ppm level as an internal standard. A comparison of the [13C]tetrachlorobiphenyl recovery surrogate chromatogram, m / z 312, with the chromatogram of the tetrachlorobiphenyl in Aroclor 1242, m / z 292 (Figure 3), indicates clearly individual isomers at approximately 80-150 ppb. Exact quantitation would require the determination of response factors between individual PCB chromatogram peaks and the surrogate peaks as described by Erickson et al. (4). Interfering background noise is more pronounced but is beyond the expected GC retention time for tetrachlorobiphenyl. The major advantage of this method is first that it permits at least a 100 ng detection in the trichloro- to hexachlorobiphenyl range using [13C]tetrachlorobiphenylas a surrogate. To preclude the effect of any possible PCB fractionation during the recovery process, the lower and higher PCB ranges would best be quantitated against [13C]chlorobiphenyl, [13C]octachlorobiphenyl,and [13CJdecachlorobiphenyl,which are also included in the EPA surrogate package. The second advantage is that the HPLC fraction collection procedure is fully automated. Although the GC/MS runs were performed on an individual basis, this step also could be made more efficient by using an automated GC/MS inlet system. Furthermore, even though the mass spectrometric analysis was carried out on a high-resolution instrument, it was operated under low-resolution conditions and ions were monitored over approximately half of an atomic mass unit. Therefore a mass spectrometer having unit resolution with good sensitivity could have been substituted.
LITERATURE CITED Cairns, T.; Slegmund, E. G. Anal. Chem. 1981, 53. 1183A. Cairns, T.; Siegmund, E. G. Anal. Chem. 1981, 53, 1599, Voyksner, R. D.; Hass, J. R . ; Sovocool, G. W.; Bursey, M. M. Anal. Chem. 1983, 55, 744. Erlckson, M. D.; Stanley, J. S.; Turman, K.; Radolovich, G.; Bauer, K.; Onstof, J.; Rose, D.; Wlckham, M. "Analytical Methods for By-products PCB's"; Literature Review and Preliminary Recommendations; Interim Report No. 1 ; Office of TOXICSubstances, U S . EnvironmentalProtection Agency: Washington, D.C.; EPA-560/5-82-005, 1982. "ASTM Method D-2786,ASTM Standards, Part 24"; American Society for Testing and Materials: Philadelphia, PA, 1981. ASTM Method D-3239,ASTM Standards, Part 25"; American Society for Testing and Materlals: Philadelphia, PA, 1981. Chmlelowlec, J.; George, A. E. Anal. Chem. 1980, 52, 1154. Bluemer, 0. P.; Zander, M. Fresenius' 2. Anal. Chem. f977, 288, 277.
RECEIVED for review October 28,1983.
Accepted January 25,
1984.
Gas Transfer Device Utilizing a Mechanlcai Piston Compressor Eric R. Sirkin' Department of Chemistry, University of California-Berkeley, Berkeley, California 94720 Quantitative analyses of gases produced by chemical reactions generally involve collections and analysis of only small portions of the total sample. For some applications it is desirable to abstract all, or nearly all, the sample for analysis. 'presentaddress: CA 94086.
vale,
zorancarp., 710 Lakeway, Suite 170, sunny-
When the gases cannot be readily condensed by liquid N2,this can be a considerable problem. Previously a Toepler pump has been employed in such applications, however, this device suffers from the necessity of using large quantities of Hg in a fragile glass collection vessel, thereby providing an unwieldy system with a considerable safety risk and little portability. In addition some applications such as organometallic photo-
0003-2700/84/0356-1043$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
1044
Table I. Gas Compressor Components desig.
mater i a1
dimensionQ
description
140 o.d x 12.7 thick 57 long X 38 wide x 9.5 high 57 long x 38 wide x 9.5 high in. gyrolock/pipethread
welded to W, in. pipe thread on bottom screwed onto L, supports F shaped like tuning fork, locks onto R monted on L, allows evacuation of volume between P and L to 130 torr 8-10132 clearance holes matches W fits into S, provides a smooth surface for motion of R supports Quad-X O-ring, 1/2-20 NC thread in center forms movable vacuum seal 1 2 slots machined at 10 mm intervals, 12.5 mm o.d., 1/2-20 NC thread (male) on end welded onto L, accommodates F Kanigen electroplated, threaded for 8 of 10/32 screws on top
B C F K
cylinder bottom braces2 fork pump port
L N
cylinder lid bushing
P
piston
Q R
Quad-X O-ring piston rod
Viton
5 in. 0.d. X .265 in. thick 1 9 0.d. x 372 long
S
piston rod support cylinder wall
aluminum
23 i.d. X 48 long X 4.5 thick 130 i.d. X 243 long x 5 thick
W a
part
brass nylon
140 0.d. X 12.7 thick
23 0.d. x 54 long x 2 thick
129.9 0.d. x 19 thick
-
All dimensions are given in millimeters except where indicated otherwise, To GC
Compressor Volume CV=284iO,OlJ Vent
0 053 f
0 0031
0 Whitey SS Valves = 1/4" SS Tubing To Vacuum Line
118" SS Tubing 1/4" Poly-Flo Tubing
Figure 2. Gas compressor, vaiving network, and flashlamp assembly (see text).
C End View
Figure 1. Gas compressor schematic (see Table tions).
=
I for part designa-
chemistry may exclude the use of Hg due to potential reactions. Alternatively the entire sample may be condensed by using either liquid Hz or He. However, their use excludes the possibility of using reaction mixtures which might contain Hz or He as either reactants, products, or inert collision partners. If the collected samples are analyzed with gas chromatography, an inert gas other than He would be present considerable problems for detecting some species due to an unstable base line. Moreover, there are applications where the facilities for obtaining, storing, or transferring the cryogenic fluids may be impractical. In this paper we describe a totally mechanical device for transferring a gaseous sample from one vessel, a t a lower pressure, to one smaller in volume, at a higher pressure, for subsequent quantitative analysis. Its operation is simple, safe and, with the exception of a modest mechanical vacuum pump, completely portable. Since the surfaces that come in contact with the gas are either stainless steel, nickel, or Viton, the system is inert to most reactive gas mixtures. The main purpose for constructing such an apparatus was to analyze the products resulting from the vacuum ultraviolet photolysis of CHzCHF:He and CHzCFz:He(CHzCFz:Hedenotes a mixture of CHzCFzand He), where only 7% or less of the parent molecules react.
EXPERIMENTAL SECTION A schematic of the gas compressor showing all the individual parts is given in Figure 1. The letter designations refer to parts listed in Table I together with their dimensions. Gases are expanded under vacuum into the compressor volume CV, defined
by the cylinder bottom B, the cylinder wall W, and piston P. W and B were welded for a vacuumtight seal. The mobile vacuum seal betvieen W and P is formed by a Quad-X 0-Ring (Viton, 5 in. 0.d. X 0.265 in. wide, Minnesota Rubber Co., Minneapolis, MN). The inner surface W was carefully machined and then honed to ensure a smooth sealing surface. It was then Kanigen electroplated in order to prevent scratching of the surface and coated with a thin film of Kel-F grease to ease the motion of P. A 0.250 in. stainless steel gyrolock fitting was attached to the cylinder via the 0.50 in. pipe thread in B. In this way gases could be introduced into evacuated CV and compressed by moving piston P toward B. The motion of the piston rod R, screwed onto the back of P, is restricted by a support S welded onto the cylinder lid L. To ease the movement of the piston rod R a nylon bushing N was inserted into the support. Eight 10/32 hexagonal screws distributed around the perimeter of the lid L secure it to the end of the cylinder wall. The piston position, and as a consequence the compressible volume CV, is fixed by locking R to L. This is accomplished by inserting fork F through two braces C attached to the cylinder lid and locking it around 1to 12 notches cut into R. In this way any 1of 12 expandable volumes ranging from 0.25 to 2.84 L can be used in the compressor. The force holding F aganist L is reduced by applying a vacuum to the region on the other side of CV through pump port K. The vacuum also provides a means for controlling the rate at which piston P moves. The cylinder is clamped horizontally onto a mount and positioned as close as possible to the vessel from which the gases are transferred. The entire gas collection system including the initial sample chamber (flashlamp),gas compressor, final sample chamber, and valve networking are shown in Figure 2. Details of the flashlamp and discharge circuitry are given elsewhere ( 1 , Z ) . Initially valves 7, 8, and 9 are closed and the entire gas collection system is evacuated through valve 1 by a standard high vacuum glass manifold to 10-6-104 torr. A gas sample is introduced into the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
photolysis portion of the flashlamp by closing valve 3 and expanding the gas to a set pressure from the vacuum line. After pressure equilibration valve 2 is closed, the volume bounded by valves 2 and 3 is evacuated and the sample exposed to the UV light from the flashlamp discharge. After exposure valve 1is closed and valves 2 and 3 are opened. After 1 min was allowed for pressure equilibration and good sample mixing (only ca. 50% of the sample was exposed to the flash), most of the original sample is now contained in the 2.84-L CV. Next, valve 5 is closed, valve 1opened, and the network again evacuated. The sample contained in CV is compressed by removing the fork F and slowly allowing the piston to move to the cylinder bottom B. A vacuum is applied to the hack of the piston in order to provide control of its motion during compression. Once the piston P stops moving, the vacuum behind the piston is turned off and the piston moves to its final position. After valve 6 is closed, 85% of the original sample is contained in the 0.0302-L sample chamber (defined by valves 5, 6, and 7 ) at atmospheric pressure. In principle it could be expanded into an IR or UV absorption cell for spectroscopic analysis or collected for mass spectral analysis, etc. In the present work the collected sample is injected directly into a GC by closing valve 4, setting the sampling valve on the GC to "inject", and opening valve 8. Then valve 5 and (after a short delay) valve 7 are opened, thereby injecting the sample into the He gas stream. The sample chamber consisted of a single 1-m coil of 0.25 in. stainless steel tubing coiled in a single loop in order to provide for a smooth clean flow of the carrier gas. After the last species emerges from the GC column, a pressurized He flow is applied to the sample chamber through the GC gas sampling valve and valve 8. The piston P returns to its original position by closing valve 7 and opening valve 6 to a 40 psi He source. After the piston reaches the full open position, valve 8 is closed, valve 4 is opened, and the entire system is evacuated. The tubes interconnecting the flashlamp, vacuum line, GC, and compressor consisted of either 0.25 or 0.125 in. 0.d. 316 stainless steel tubing. Stainless steel vacuum fittings and right angle valves were used as indicated. In order to remove eddy currents in the He carrier gas, valve 7 was straight rather than at right angles. All volumes relevant to calculating the percentage of initial photolyzed gases collected by the system are indicated in Figure 2. Except for CV all volumes were determined by weighing the amount of liquid methanol displacement. The gas chromatograph used in these experiments was an Aerograph Model 202 equipped with a thermal conductivity detector (TC). The separation column was made from a 5 f t X 0.125 in. stainless steel column packed with Porapak N and operated at room temperature. The detector response was amplified by a lOOOX dc amplifier and sent through a low pass RC filter. The signal coming from the dc amplifier was converted into digital form by an analog to digital converter and then routed by an Intel 8080 microprocessor and stored on RAM memory. At least 1000 points were summed every 60 ms and then sent as a packet to a Microdata 321s minicomputer. CHzCHF (99.9%), CH2CFZ (99.0%), C2H2 (99.6%), and CHzCHz(99.5%) were all purchased from Matheson Gas Products and subjected to several freeze-thaw cycles prior to use. The He (99.99+%) used both for preparing samples and as a carrier gas was purchased from the University of California College of Chemistry. All samples were prepared at least 6 h prior to use and stored in blackened gas bulbs. Pressures were measured with either a Hg Mcleod gage or a Hg manometer. Gases were handled with a conventional glass vacuum line.
RESULTS AND DISCUSSION Figure 3A shows a gas chromatograph taken of a 2.55 torr C2Hz sample placed in the 0.0302-L final sample chamber. Peaks which appeared immediately after opening valves 8, 5, and 7 (such as peaks 1, 2 and 3 in Figure 3A) can be identified with changes in the He carrier gas flow. They were observed whenever the pressure in the final sample chamber was insignificantly different from the 80 psi He delivery pressure. For example, in Figure 3A their heights were minimized by introducing He at the delivery pressure into the final sample chamber immediately after isolating the chamber from the vacuum line and CzHz source. Thus there was
1045
4
250 mV
11
T
A
1
0
3
100
200
300
400
Time After lniection (sec)
/ " ' I " ' I " ' I " ' I " ' / '
0
100 200 300 400 Time A f t e r Injection ( s e c )
500
Figure 3. Gas chromatograms of (A) C,H, 2.55 torr in initial sample chamber and (B) products from a single 400-J flash of 5.50 torr
CH,CHF:He 1 : l O in initial sample chamber.
minimal pressure drop in the carrier gas delivery. The actual height and position of the peaks varied slightly from one experiment to the next depending on the peaks varied slightly from one experiment to the next depending on the precise timing of the valve sequence. When He at reduced pressure was introduced into the final sample chamber, these peaks were also observed. Attributing these peaks to air present in the chamber cannot be completely ruled out. However, when air was intentionally introduced into the sample chamber and injected into the GC column in the manner described above, a single sharp peak appeared within 20 s of injection. In fact air peaks were observed on occasion in the GC spectrum of actual photolyzed samples. Air leakage during the compression cycle was eliminated by actively purging the atmospheric side of the piston with He during compression and by lubricating the inner walls of the cylinder with Kel-F grease every 20 cycles. Peak 4 in Figure 3A is assigned as C2H2 Integration of peak 4 for a range of C,H, pressures established a calibration curve. Similar calibration were established for CH,CHF, CH2CF2, and CHzCH2. The 85% gas collection efficiency calculated from the measured sample and transfer volumes, as designated in Figure 2, was experimentally verified by filling the initial sample chamber with C2Hzto a set pressure (as measured by a Hg Mcleod gage), collecting the gas as described previously, and injecting it into the GC. By use of a calibration curve the GC integrated peak area corresponding to CzHz can be related to the C2H2pressure in the final sample chamber. This pressure could then be related to the derived initial sample pressure by using the relationship Two measurements were made a t 0.153 and 0.205 torr CzH2 initial sample chamber pressures which agreed with the values derived from the GC to within 2 and 3% of the derived pressures, respectively. A further test of the gas collection apparatus was made in conjunction with the flash photolysis work. A 5.50 torr sample of CH,CHF:He at a mixed ratio of l:lO, was introduced into the initial sample chamber, exposed to a single 400-5 photolytic flash, collected in the gas compressor, and injected into the GC. The resulting chromatogram is shown in Figure 3B. Peaks 1through 4 result from interruptions in the He carrier flow. The identities of peaks 5, 6, 7 , 8, and 9 as CH,CH2, C2HF, CH,CF,, C2H2,and CHzCHF were determined either
Anal. Chem. 1984, 56, 1046-1050
1046
by comparing their retention times with intentionally injected samples or by mass spectral analysis ( 2 ) . A reliable determination of the CH2CH2and CzHF peak areas was not possible. However, peaks 7, 8, and 9 indicated the following sample chamber pressures: 0.034,0.29, and 3.28 torr. By use of the initial and final sample chamber volumes and the 85% transfer efficiency, the sum of these pressures corresponds to a CHzCHF initial sample chamber pressure of 0.54 torr or 8% higher than the actual measured value. This is within the combined accuracies of all the known volumes and pressures.
CONCLUSIONS A conceptually simple device not requiring the use of cryogenic fluids or Hg has been shown capable of abstracting large gaseous sample for analytical purposes. With minor improvements the operation of the device can be made completely automated and fully portable. Although the technique has been demonstrated in conjunction with only GC and mass spectral techniques, it can be readily extended to other applications such as IR or UV absorption spectrometry,
fluorescence spectrometry, Raman, radioactive trace, etc.
ACKNOWLEDGMENT We appreciate the assistance of Andy Anderson and the University of California-Berkeley College of Chemistry machine shop staff in construction of the gas compressor. We also appreciate the assistance of Donald Stone and Yuan-Pern Lee in the use of a microprocessor-based data collection system. Registry No. CHzCHF, 75-02-5.
LITERATURE CITED (1) Sirkin, E. R. PhD. Dissertation, University of California-Berkeley, 1980. (2) Sirkin, E. R.; Pimentel, G. C. J . Chem. Phys. 1981, 75, 604.
Received for review August 15,1983. Resubmitted September 30, 1983. Accepted January 16, 1984. Support for this research was provided by the U S . Air Force Office of Scientific Research under Grant No. 78-3535.
Two-Phase Flow Cell for Chemiluminescence and Bioluminescence Measurements J e r r y L. Mullin and W. Rudolf Seitz*
Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824 There are a number of CL analyses reported in the literature which involve contact between two distinct phases. Examples include the use of immobilized enzymes as solid phase catalysts for solution reactions ( I , @ and the analysis of gases by contact with a liquid or solid reactant phase (3, 4 ) . Two-phase CL measurements offer several advantages. The reagent phase can be recovered and reused offering significant cost advantages for measurements involving expensive catalysts. The reagent phase can be tailored to exclude interferences. For example, oxygen in aqueous solution can be determined by CL from its reaction with tetrakis(alky1amino)ethylenes by using a hydrophobic 02-permeable membrane to exclude nonvolatile species ( 4 ) . Two-phase systems can also be potentially used for continuous sensing. For example, a system based on the luminol reaction has been developed to continuously measure the amount of glucose in a flowing stream (5). The major drawback of two-phase measurements is that they are relatively insensitive because CL is only generated at the boundary between the two phases where analyte and reagent are in contact. We report here a new approach to two-phase CL measurements. A magnetically stirred reagent phase is separated from the analyte phase by a dialysis membrane so that only smaller molecules can go from one phase to the other. The system is designed so that the analyte phase flows through a spiral groove on an aluminum block that is flush against the dialysis membrane. As solution flows through the spiral groove, analyte diffuses into the reagent phase where it reacts to produce light. A simple model is developed to predict how this system will behave. Experimentally, the system is evaluated by using the luminol reaction catalyzed by peroxidase, the firefly reaction, and the bacterial bioluminescence reaction. Recent reviews describe the basic chemistry of these reactions along with analytical applications (6-8).
mass transfer in the dialysis membrane so that concentration gradients only develop in the membrane but not in the analyte or reagent phases. It is also assumed that the fraction of analyte entering the reagent phase is so small that analyte depletion is not significant. Since mass transfer in the membrane is due only to diffusion while mass transfer in analyte and reagent phases occurs by convection, these are reasonable assumptions although in practice they are not realized. With these assumptions, one can write the following expression for R, the flux of analyte into the reagent phase
where D is the diffusion coefficient for analyte in the membrane, 6 is the membrane thickness, Co is the analyte concentration in the sample, and Ci is the analyte concentration in the reagent phase. Initially, there is no analyte present in the system, and Ci = 0. At time t = 0, analyte solution at concentration Co is introduced into the flow system and begins flowing past the membrane. Analyte diffuses across the membrzne into the reagent phase and reacts. Analyte concentration in the reagent phase increases until a steady state is reached. The rate of change of analyte concentration inside the reagent phase equals the rate at which analyte enters the cell minus the rate at which it reads. If the reaction is first order in analyte, then
where K , is the rate constant, A is the area of phase contact, and V is the volume of reagent phase. Solving eq 2 yields the following expression for Ci as a function of time.
THEORY Figure 1shows the model for the two-phase CL measurement with a stirred reagent phase. It is assumed that mass transfer in the analyte and reagent phases is fast relative to
The exponential term establishes the response time of the
0003-2700/84/0356-1046$01.50/00 1984 American Chemical Society
