Anal. Chem. 1993, 65, 2403-2406
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Water-Based, Gravimetric Method for the Determination of Gas Sample Loop Volume R J. Wilke,' D.W.R. Wallace, and K. M.Johnson Oceanographic and Atmospheric Sciences Division, Brookhven Notional Laboratory, Upton, New York 11973
Gas sampling loops (GSLs) for gas chromatography valves (CCVs) are supplied with a nominal volume which suffices formanygaschromatographicdeterminations. Theuser need not calibrate them to further tolerances because the same loop is filled with standarda of known concentrations to generate calibration curves which are subsequently applied to quantify unknown sample concentrations. However, gas sample loops are frequently used in calibration applications for which their volumemust be knownaccurately. Examples include the following: (a) gas-phase analyses for which gasphase standards are not available a t multiple concentration levels. In thesecases, calibration curves can begenerated via injection of mulitple aliquots of a known volume or separate injections of a seriesof different volumes; (h) calibration using gas-phase standards of analyses of a separate phase, e.g., purge-and-trapanalysis ofaqueous samples. A very exacting use of the GSL is required by the on-going effort to access the ocean's role in climatic change by surveying the spatial and temporal oceanic distribution of total dissolved carbon dioxide (CT) during the multinational World Ocean Circulation Experiment-World Hydrographic Program (WOCEWHP). The method chosen for the CT survey is continuous gas extraction of the CO? liberated from acidified seawater followed by coulometric titration with a COz coulometer.l.2 Agreement has been reached on a goal of 1 part in 2000 for the accuracy and precision of the CT determination? T h e most convenient way of calibrating this method is to analyze knownmassesofCOzfromaGSLofknownvolume(C3filled with pure CO, a t some temperature ( r )and pressure (P) and coulometrically titrating its content for comparison with the known amount. For this purpose, we developed a reliable and straightforward means using water to calibrate the volume of GSLs to this required level of accuracy. Subsequently, the accuracy of the CSL volume determination was checked through the analysis of liquid-phase certified reference materials on the same apparatus. This paper describes the method used and our experimental results. The technique should prove relevant and useful for the many branches of analytical chemistry in which GSLs are employed.
4
CG
in
8 . p a r l C h n m I o ~mlm
Flgure 1. sdnrmatic fw an a n a w l system In whkh p v e CQ Rlllng a gas sample bop of known volume at a known temperature and pressure b cwiometricallytitrated (determined) and compared to the calculated mass of Co2contained in the loop (true). Abbrevlaths: carrier gas. CG;isolation valve, IV; personal computer, PC, pressure relief valve, Pt7C union cross fming. UC.
EXPERIMENTAL SECTION Figure 1 is a achematic of an automated arrangement of a GCV and GSLs, barometer, temperature sensors, process eomponents, and coulometer for gas calibration developed and modified3 from an earlier system' and suitable for the analysis of CT in seawater. Carbon dioxide originating from the GSLs or degassed from acidified seawater dispensed from the pipet into the fritted stripper follows the same pathway to the COn coulometer. In this way the combined error for the coulometric titration and losses from the process components (glassware, drying agents, etc.) can be determined. After the GSL in filled, the loop ispurgedwithcarriergasand itscontent coulometrically (1) Johnson, K. M.; King, A. E.; Sieburth, J. McN. Mor. Chem. 1986, 16,61. (2) Handbook of methodsfor the awlpis of the voriaus parameters of the carbon d i o d e system in sea water, version 1.0; Dickson, A. G., Goyet, C., Eds.; DOE,GPO Washington, DC, 1991. (3) Johnson, K. M.; Wallace, R. W. DOE Research Summary No. 1% Carbon Dioxide Information Analysis Center, Oak Ridge National Laborstory: OaL Ridge, TN 37831,1992; pp 1-4. (4) Johnson, K. M.; Williams, P. J. IeB.; Brandstrom, L.; Sieburth,J. McN. Mar. Chem. 1987.21,117.
DDI
-02. Calibratiaweady~~t~valves~wingthei~~ gas sample loops connected to a tharmostated calibrating flukl.
Abbreviations: countercl~kwlsa.cC:clockwi~, CW dehlzedwater.
DDI.
titrated. The ratio (mass of COI injeded (calculated))/(mans of COPdetermined (titrated)) in then defined as the gas calibration factor (GCF), which is used to correct subsequent titrations for small departures from theory. The coulometric titration has beenfound tobe highlylinearandafteryearsofexperimentation a consensus has been reachedPrequiring a two-point calibration using an eightport in-line GCV with two loops of '/s in. 0.d. by 0.08 in. i.d. (hereafter, called the valveloop pair). The loop volumes should differ by at least 0.5 mL and deliver masses of
0 0 0 3 - 2 7 ~ / 9 3 / 0 ~ 5 . 0 0 / 0(0 1993 Amaican Chrrrnlcd soclav
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 17, SEPTEMBER 1, 1993
C02which bracket the mass of C02 extracted from the seawater samples. In this case, the "to deliver" volume of the sample pipet shown in Figure 1is approximately 29 mL, which yields ca. 700 pg of carbon for seawater (S= 35960, CT = 2000 pmol/kg), while the GSLs shown have nominal volumes of 1.25 and 1.75 mL yielding ca. 500 and 800 pg of C02 carbon, respectively, at STP. The mass of CO2 injected is obtained by iteratively calculating the molar volume of C02 at the gas temperature (2') and pressure (P)using the first viral coefficient of C02 (BT), converting this result to the gas density (fw of carbon, 12.0115), and multiplying the density (g/mL) by the calibrated loop volume in milliliters.2 Obviously, the determination of V, P, and T is pivotal for gas calibration. The latter two depend on very accurate commercially available devices, but the absolute volume determination must be performed in-house because the manufacturer of the chromatography valve and loops does not offer this service at the required accuracy and precision. For this work, 316 stainless steel or nickel GSLs were obtained from VICI Corp. (Houston,TX). Theywere ordered withnominal volumes from 1.25 to 3.0 mL, and installed on eight-port chromatography valves (8UWP, VICI Corp.) after cleaning with a series of solvents followed by combustion. For comparison, we prepared our own GSLs from 316 stainless steel (Supelco Inc., Bellefonte, PA) or from electropolished 316 stainless steel (Tech 50, Cardinal Systems, Inc., St. Louis, MO) by square cutting the tubing with a metallographic saw into lengths consistent with the desired nominal volume and then polishing the ends. The untreated stainless steel loops were cleaned by soaking for 5 min in 6 N "03, rinsing with deionized water and methanol, and then drying overnight with N2. For the electropolished tubing, the acid rinse was omitted. The loops were carefully connected to the l/s-in. porta of the chromatography valves using either Hastalloy (ZFPHC, VICI Corp.) or gold-plated (ZFZGP, VICI Corp.) ferrules. Care was taken to see that the loops were fully and correctlyseated in the fitting port. By convention? the larger of the two loops was installed between porta 6 and 2, and the smaller between porta 8 and 4. After installation, the fittings were checked for leak-tightness by pressurizing the system to 40 psi with helium and checking the ports with a Model 333 helium leak detector (Gow-Mac Instrument Co., Bridgewater, NJ) at the high sensitivity setting. Next, the chromatography valves along with two loose port plugs (ZC2, VICI Corp.) were placed in an open Ziploc freezer bag and dried in a vacuum oven (Model 1410, VWR Scientific) at 228 in. of Hg and ambient temperature until constant weight. This weight was used as the net weight of the valve-loop combination. The term weight, as used here, is the mean of five replicate weighings. The valves were kept in the bags except during weighing, and surgical gloves were worn when the valve was manipulated. Weighings were made on a Model R300S analytical balance (Satorious AG, G6ttingen, Germany) with a 300-g capacity capable of weighing to 0.1 mg (repeatableto h0.2 mg) which was self-calibrating via an internal weight traceable to NIST weights. Once constant weight was achieved, the port plugs were installed on porta 3 and 7, while Teflon tubing was connected to ports 1and 5 of the valve to allow flushing and filling of the loop. The calibration-ready eight-port chromatography valve is shown in Figure 2. The water used for calibration is distilled and then deionized by a Milli-Qwater purification system (MilliporeCorp., Bedford, MA). It is degassed by sparging with He at 1200 mL/min for 30 min. The valve shown in Figure 2 is double-wrapped in plastic bags (the external valve surface must be kept scrupulously dry) and immersed in a constant-temperature bath at 25 h 0.1 "C for 1h until thermally equilibrated (testsshow equilibration to within 0.4 O C requires 30-45 min). Equilibration is also promoted by keeping the laboratory at or near 25 OC. Following equilibration, 30-50 loop volumes of thermally equilibrated (25 OC) water are drawn through the valve to fill the loop connected to the fluid path. Loop filling is accomplished by means of suction via a 100-mL syringe located on the vent side of the fluid path (port 1,Figure 2). The valve is then manually switched as smoothly and rapidly as possible to isolate the filled fluid path (loops and valve porta). The valve is removed from the bath, and the inlet and vent lines are disconnected. Any water droplets remaining on these port sealing surfaces are removed with cotton swabs, while water remaining in the nonisolated pathway is expelled
with dry compressed N2 and two 50-mL methanol rinses followed by a dry compressed N2 purge for 30-45 min at 200 mL/min. The The valve is then reweighed to obtain the gross weight ( Wg). apparent mass (M.)of the water contained in the isolated pathway is calculated by subracting the net weight from the gross weight. The volume (V)of water in milliliters is calculated by correcting Ma to the weight under vacuum (buoyancy correction) and dividing the result by the density (d) of water at 25 OC5
V = [Ma(0.0012/d- 0.0012/8.000) + Mal/d (1) where 0.0012 is the density of moist air at 1 atm and 25 OC and 8.000 is the density of stainless steel weights in air. The water in the isolated pathway is expelled as above, and both pathways are rinsed with methanol and dried with compressed gas. The valve and the port plugs are returned to the vacuum oven and weighed at l/2-h intervals until constant weight (W,) is once again obtained. If the difference W, - Wo is 10.0007 g, the calibration result is considered valid, and the procedure is repeated as required. The C02 coulometer shown in Figure 1is the Model 5011from UIC Corp. (Joliet, IL). The barometer is the Model 216B-101 from Paroscience Inc. (Redmond, WA) calibrated for pressures between 11.5 and 16 psia. It is protected from damage due to overpressures (25 psi above ambient) by a pressure relief valve (PRV) Model 559T1-1M-5 (Circle Seal Controls, Anaheim, CA). Barometric pressure is acquired by an IBM-type personal computer (PC)through an RS 232 port. The temperature sensors shown in Figure 1are semiconductor devices (Model LM34CZ, National Semiconductor, Santa Clara, CA) with a voltage output of 10 mV/OF calibrated with sensors factory-certified to hO.O1 "C (CSP60BT103M,Thermometrica,Edison,NJ). The glassware is from G. Finkenbeiner Inc., Waltham, MA. Coulometric titrations of the carbon dioxide extracted from certified reference materials (CRMs) are used to verify the valveloop pair V, P, and T determinations, gas calibration factors, and the system accuracy. The CRMs are sterile salt or seawater solutions prepared by A. Dickson at the Scripps Institution of Oceanography (SIO)which are spiked with carbonate.2 The CRM concentration of total dissolved carbon dioxide (C)is determined (certified) to hl.0 pmol/kg by C. D. Keeling of S I 0 by vacuum extraction-manometry.2 For our work,the pipet shown in Figure 1is filled with the CRM and then ita contents are pneumatically injected into the stripper and acidified. The resultant C02 is scrubbed, dried, and coulometrically titrated. At the end point, the result is corrected by multiplying it by the gas calibration factor (GCF)determined for the system. A complete description of the automated instrumentation used to make the analyses reported in this work is reported elsewhere.
RESULTS AND DISCUSSION Calibration data from 29 chromatography valves with 58 sample loops are complete. Calibrated volumes range from 1.0066 to 3.0532 mL, and the mean volume is 1.6448 mL. Assuming homogeneous variance, the pooled standard deviation (Sp2) of the method calculated from
E n i-K 8-1
is f0.0008 mL, where K is the number of loops calibrated (581, n is the number of replicates for each loop (2-6), and In1 = 189. The aggregate precision (relative standard deviation) is 0.051 5%. Outlying data points (7 of 196) were rejected if they failed the &-testat the 90% confidence level. Results for valve-loop pairs recalibrated from 1to 8 months after the original calibration are shown in Table I. For valves ( 5 ) Skoog, D. A.; West, D. M. Fundanentak ofdmlytical Chemistry; Saunders College: Philadelphia, PA, 1976; pp 686-689.
ANALYTICAL CHEMISTRY, VOL. 85, NO. 17, SEPTEMBER 1, 1993
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Table I. Reproducibility of Gas Sample Loop (GSL) Volumes after Installation on Chromatography Valves vol (mL) difference n SD SE nominal calib valve no. data RSD (%) (mL) (%)
JT1235 JT1235 overall JT1235 JT1235 JT1235 overall JXO138O JXO138O overall JXO138" JXOl38O overall JY1253 JY1253 JY1253 overall JY1253 JY1253 JY1253 overall a
Jun 91 Jul91
1.0
Dec 90 May 91 Jul91
1.5 1.5
1.0
1.5
Dec 91 Aug 92
1.25 1.25
Dec 91 Aug 92
1.75 1.75
May 91 Jun 91 Jul91
1.25 1.25 1.25
May 91 Jun 91 Jul91
1.75 1.75 1.75
1.0453 1.0441 1.0446 1.4222 1.4228 1.4226 1.4225 1.2059 1.2050 1.2054 1.7064 1.7066 1.7065 1.3074 1.3065 1.3070 1.3070 1.8246 1.8253 1.8242 1.8245
4
O.OOO6
5
0.0007 0.0009 0.0004 0.0002
9 4 3 4
0.0005
11
0.0004
3 3 6 3 3 6 3 3 3 9 3
0.0005
0.0008 0.0007 0.0004 0.0005
0.0004 0.0003 0.0004 0.0003 0.0005
0.0001
1 3
O.OOO6
7
0.0005
0.0003 0.0003 0.0003 0.0002 0.0001 0.0002 0.0001 0.0003 0.0004 0.0003 0.0002 0.0003 0.0002 0.0001 0.0003 0.0001 0.0003 O.oo00
0.061 0.063
0.0004 0.0003
0.034 0.030
-0.0012
-0.110
+O.OOO6 +0.0004
+0.042 +0.028
-0.0009
-0.074
+0.0002
+0.011
-0.0009 -0.0004
-0,068 -0,030
-0.0004
-0.021
0.084
0.027 0.016 0.032 0.030 0.042 0.063 0.061 0.021 0.027 0.023 0.023 0.033 0.019 0.037 0.003
Nickel GSL.
Table 11. Mean Gas Calibration Factors (GCF) for Nine Valve-Loop Pairs with Gas Sample Loop (GSL) Volumes Calibrated at the Brookhaven National Laboratory. Calibrated vol (mL) SP2 GCF mean nb mean ( % ) c period valve small (S) large (S)
JT0730 JT0731 JT1231jd JY1251 JY1252 JY1253 JT1255 JX0138d KN1300
1.5334(0.0008) 1.5211(0.0006) 1.0446(0.0008) 1.3000(0.0004) 1.3066(0.0015) 1.3070(0.0005) 1.3031(0.0002) 1.2054(0.0007) 1.4965(0.0002)
2.7851(0.0024) 3.0532(0.0013) 1.4225(0.0004) 1.8041(0.0005) 1.8239(0.0005) 1.8245(0.0005) 1.8573(0.0001) 1.7065(0.0003) 2.4978(0.0004)
0.0020 0.0010 O.OOO6 0.0005
0.0012 0.0014 0.0002 0.0005
0.0003
1.006 03 1.00367 1.004 41 1.004 91 1.005 84 1.005 66 1.006 50 1.004 10 1.004 20
5
2 26 50 8 8
6 2 6
0.068 0.017 0.028 0.093 0.057 0.042 0.032 0.100 0.008
1&13 Jan 1992 24 Jan 1992 Feb-Mar 1991g May 1992f 15-16 Oct 1991 16-18 Sep 1991 5-6 Sep 1991 23 Jan 1992 26-27 Feb 1992
a The precision of the volume determination (n 2 3) is shown next to the individual loop volumes. The pooled standard deviation (S,*) for each loop pair (n 1 6) is also given. See text for description of gas calibration. Number of gas calibrations during the study period, where n is the mean of the large and small loop calibration factors. Mean agreement (%) between large and small loop calibration factors. See Table I for GSL volume calibration data. e Calibrations made at sea aboard the F/S Meteor. f Calibrations made at sea aboard the R/VKnorr.
JT1235 (large loop only) and JX0138, recalibration took place after the valves had been used on research cruises and had been shipped to and from overseas. Due to damage in transit, the smaller loop on valve JT1235 could not be recalibrated. There is no evidence for change in the calibrated loop volumes. The analytical results obtained with the calibrated valveloop pairs are given in Tables I1 and 111. Inspection of Figure 1shows that gas calibration, which depends on the calibration of the temperature sensors, barometer, and the loop volumes, is not the sole determinate for analytical accuracy. Additionally, the "to-deliver" volume of the pipet must be accurately known in order to analyze the CRM and samples. Very small errors in the calibration of any one component or the aggregate error for all components calibrated could prevent the attainment of the 1 part in 2000 goal (0.05%) of this work. The criterion we employ for a successfulgas calibration is an agreement between the GCFs obtained from each loop to 50.1 % whereupon the mean (n = 2) GCF is considered a valid correction.2 Table I1 shows that all nine valve-loop pairs meet the 0.1% criterion for GCF agreement between the individual loops of the valve-loop pair. This is equally
true for the factory and in-house prepared sample loops. There is no relationship between the loop agreement (7% ) and the pooled standard deviation (Sp2)for the volume calibration even though the latter varies 10-fold (0.0002-0.0020), indicating there is no consistent bias in either the large or small loop volume determination. Table I11 compares the gas calibration-corrected CRM determinations with the certified concentrations on the nine systems using the calibrated valveloop pairs shown in Table 11. Remarkably, the overall mean difference is 0.14 pmoVkg with mean absolute deviation being 0.82 pmol/kg. These data encompass CRM concentrations ranging from 1960.67 to 2304.57 pmol/kg analyzed in our laboratory and a t sea. In the case of two valve-loop pairs, KN1300 and JY 1251,two different CRM concentrations were analyzed at the same time with equally good results. Table IV gives the correlation coefficients ( r ) and significance levels when the variables associated with the determination of GSL volume (Table 11) are correlated with the absolute mean difference of the determined and certified CRM concentrations in Table 111. There is no correlation between the pooled standard deviation (S?) of the GSL volume
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 17, SEPTEMBER 1, 1993
Table 111. Mean Difference between the Coulometrically Determined and Certified Total Carbon Dioxide of the Certified Reference Materials Concentration (6) (CRMs). CT (mol/kg) SD differencec valve CRM determined Nb (gmol/kg) (pmol/kg)
JT0730 JT0731 JT1235 JY1251 JY1251 JY1252b JY1253b JT1255b JX0138 KN1300 KN1300
1960.67 1960.67 1978.70 1960.67 2188.77 2304.40 2304.40 2304.40 1960.67 2304.40 1960.67
1958.43 1960.95 1978.07 1960.30 2189.77 2305.10 2304.97 2306.84 1960.70 2304.70 1960.17
6 2 11
20 24 9 12
9 2 2 2
0.38 0.07 0.79
-2.24 +0.28 -0.63 -0.37
1.21
0.67 0.97 0.83 1.29 0.70 0.33
+LOO +0.70
+0.57 +2.44 +0.03 +0.30 -0.50
0.59
mean difference mean absolute difference
+0.14 0.82
a These data are for nine independent instruments gas-calibrated (GCF) with the valve-loop pairs given in Table 11. b N is the number of CRM bottles analyzed with a single replicate from each bottle except for valvesJ Y 1252,JY1253, and JT1255,wherethree replicates were run from each CRM bottle. For these values, the number of individual CRM bottles analyzed is N/3. Determined - CRM.
Table IV. Correlation ( r )between Calibrated Valve-Loop Pair Parameters (Table 11) and Mean Difference (Table 111) for the Determined and Certified CRM Total Carbon Dioxide (6) Concentration calibr param (X) mean diff (Y) r df signif % agreement
%F
diff (pmollkg) diff (gmoVkg) diff (pmol/kg)
0.013 0.270 0.796
9 9 9
NS NS 0.01
goal of 1 part in 2000 (f1.0 pmol/kg) is not compromised (Table 111). The positive correlation between the GCF and the difference (Table IV) is probably due to a combination of small calibration errors (temperature, pressure, and volume). Our method offers at least five significant advantages over other calibration techniques: (1) The volume measured includes the volumes of the internal valve porta; (2) it eliminates potential changes in volume due to the distortion of the loops during installation; (3) mercury, poisonous and difficult to manipulate, is not used; (4) the precision of the technique (0.05%) exceeds by a t least 8-fold the 0.4% precision for a gas displacement technique6 and by nearly 20-fold the 0.9% given for a method of additions technique7 on loops of similar volumes; (5) accuracy. Even assuming that the mean error (0.82 pmollkg) between determined and certified CTconcentrations shown in Table I11was due entirely to errors in the calibration of the loop volumes used to determine the GCF, our technique is accurate to at least 1 part in 2000 (0.05%). Neither of the techniques above6~7 provides information on their absolute accuracy. From a chromatography standpoint, the specific loop installation scheme in Figure 2 (GSLs between porta 8 and 4 and 6 and 2) confers an additional advantage. With the exception of the steel GSL, the carrier gas (CG) never shares a pathway with the COZcalibration gas. The calibration gas is introduced into the GSLs via a path distinct from the CG route into the GSLs. This eliminates the possibility of desorption of COa from polar thermoplastic valve rotors during the purging of the GSLs. Avoiding this “memory effect” is important in other GC applications such as tracer-level chlorofluorocarbon analysis.
ACKNOWLEDGMENT determination or the valve-loop pair GCF agreement (% ) and the mean difference. The mean difference, however, is positively correlated with the magnitude of the GCF. Importantly, within the range of precision shown for the determination of GSL volume (Tables I and 11),the accuracy
We thank Robert Ramirez and David Chipman for their help and advice concerning the plumbing and manipulation of the gas sampling valves, and Andrew Dickson for providing the CRM. This work was supported by the Department of Energy through Contract DE-AC02-76CH000016.
(6) Smirnova, S. A,; Vitenberg, A. G.; Stolyarov,B. V. J. Chrornatogr.
1979, 170, 419.
(7) Cuddeback,J. E.; Birch, S. R.; Burg, W. R.A n d . Chern. 1975,47,
355.
RECEIVED for review December 29, 1992. Accepted April 14, 1993.
