Use of Sample Loops with Micro-Packed Columns in Gas Chromatography D. E. Durbin and Jackie Fruge Honeywell, Inc., 6625 McGrew, Houston, Texas 77017
IN RECENT YEARS, there has been an increase in the use of micro-packed columns (1-3) and packed capillary columns (3, 4-7) in gas chromatography (GC). While these columns have low loadability, their primary advantage is that the analysis time is on the order of a few seconds. Such analysis time requires a G C system having a very fast response time, approximately 10 to 20 times less than the time base width of the narrowest peak (5). For ionization detectors, this requirement necessitates the use of expensive low time-constant amplifiers. Low dead volume catharometers, however, have low time-constants and are rather inexpensive to construct or purchase. Preau and Guiochon (6) described a simple catharometer having a dead volume of 40 pl and a time constant of 70 milliseconds. Aptech's MTC-01 Micro Thermal Conductivity Detector has a cell volume of 0.03 pl and a time constant of less than 16 milliseconds (3). Servomex's Micro Katharometer MK 158 has a cell chamber of 2.6 p1 and has been used for rapid quantitative analyses (7). These detectors should be adequate for measuring chromatographic peaks having base width as narrow as 0.5 second. However, the primary disadvantage of high speed G C systems employing micro catharometers is that the limit of detection is only about 0.1 with a gas sample size of 1-10 p1 (3, 6, 7). In order to decrease dead volume and to increase the speed of injection, the sample is usually injected onto the column by means of a liquid sampling valve. This paper reports on the use of sample loops to increase the sensitivity of G C systems employing low dead volume catharometers and high speed columns.
I
50
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0
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6
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SAMPLE LOOP LENGTH (IN 1
Figure 1. Effect of sample loop length on effective sensitivity at constant sample loop i.d.
0
1
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SAMPLE LOOP I 0 IIN
I
5
6
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Figure 2. Effect of sample loop internal diameter on effective sensitivity at constant sample loop volume
EXPERIMENTAL
The detector system was an Aptech MTC-01 Micro Thermal Conductivity Detector and associated constant current bridge/ amplifier. Samples were introduced with a Valco AMFSV10-HP liquid sampling valve having internal connecting slots of 3 pl each. The valve was air actuated with a Rotary Operator obtained from Arcus, Inc. (Houston, Texas). Sample loops were constructed from 1/16-o.d.stainless tubing obtained from Tube Sales, Inc. The volume injected was calculated from the sample loop dimensions to which was added the volume (6 pl) of the internal connecting slots in the sample valve. The carrier gas was zero Helium (Big Three Industrial Gas and Welding, Houston, Texas) and filtered through indicating silica gel and Molecular Sieves Type 5A. Chromatograms were recorded on a Techni-rite Model TMD-25 Recorder having a frequency response of dc to 125 Hz and a risetime of 4 to 5 milliseconds. The micropacked columns used in this work are described in Table I. (1) W. F. Wilhite, J . Gas Chromatogr., 4, 47 (1966). (2) A. Zlatkis, H. Kaufman, and D. E. Durbin, J. Chromatogr. Sci., 8, 416 (1970). (3) D. E. Durbin, Honeywell, Inc., Houston, Texas, unpublished results, 1969. (4) I. Halasz and E. Heine, ANAL.CHEM., 37,495 (1965). ( 5 ) L. J. Schmauch, ibid., 31, 255 (1959). (6) G. Preau and G. Guiochon, J . Gas Chromatogr., 4, 343 (1966). (7) E. Davidson, Chromatographin, 3,43 (1970). 1502
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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PLATE NUMBER
Figure 3. Comparison of plate number us. optimum sample loop length RESULTS AND DISCUSSION
An increase in injected sample volume increases the detector signal, but only at the expense of column efficiency. It is, therefore, convenient to define an effective sensitivity (S,) of a chromatographic system as: S, = S / H
(1)
where S is the detector signal measured as a peak height and H is the column plate height calculated in the usual manner.
Table I. Column Parameters Column 1
Dimensions, in. Packing Porapak S 18 X 0.035 2 13.25 X 0.035 Porapak S Porapak S 3 6.5 X 0.035 Porapak P 4 10 X 0.051 Chromasorb 102 5 12 X 0.023 6 18 X 0.035 Porapak S 7 30 X 0.035 a 1 5 % Dimethylsulfolane on Chromosorb P, AW, DMCS. Measured for 1,3-butadiene. (1
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40-56
125-149
Optimum sample loop volume, p1 173 110 71 151 27 173
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Particle size, 40-56 40-56 4@56 44-74 53-63
Optimum sample loop Theoretical plates (ethylene) length, in. 1350 11 loo0 7 490 4.5 4.5 650 4 610 11 1187 8.5 1089b
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Figure 4. Effect of sample loop length on peak height at constant sample loop i.d.
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SECONDS
Figure 6. Separation of light gases G . C. conditions: 7.75 X 0.035 in. packed with 40-56 p Porapak S; carrier flow = 9.6 ml/min helium; carrier pressure = 30 psia; temp. = ambient; sample volume = 3 p l ; theoretical plates: 710 (ethylene); attenuation = X 1
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Figure 5. Effect of sample loop length on column plate height at constant sample loop i.d. Both S and H are measured in identical units. A typical plot of S, us. sample loop volume is shown in Figure 1. The sample loop i.d. was held constant (0.035 in.) while the length was varied. An optimum volume was obtained at which S, is maximum. As will be discussed later, little is gained by increasing the sample volume above this optimum value. Sample Loop Internal Diameter (i.d.). The effect of sample loop i.d. at constant volume on S , was studied using Col-
umn 7 (0.035-in. id.). The sample loop volume was held constant at the optimum sample volume, which for this column was found to be 158 111. The results are plotted in Figure 2 and show that, for this column, sample loops smaller than 0.047-in. i.d. have little eRect on S,. A similar plot using Column 5 (0.023-in. i d . ) showed that S , decreased at sample loop i.d.'s greater than 0.027 in. These results indicate that a good rule of thumb for choosing a sample loop i.d. would be to use one having the same i.d. as that of the column. Column Plate Number. Plots of S , us. sample loop length (SLL) were constructed for the several cclumns listed in Table I. The sample loop i.d. used for a particular column was kept identical with the i.d. of that column. The sample loop length at optimum S , was then plotted against the column plate number (n). The sample volume used for the determination of column plate number was 3 pl of gas. This sample size was obtained by operating the Valco valve in the liquid sampling mode. The results are shown in Figure 3. These data indicate that a linear relationship of at least a semi-quantitative nature exists between the optimum sample loop length (OSLL) and n. The slope of the plot is 8 X inch/plate and Equation 2 can be used to calculate the OSLL for a given column: ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1503
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Figure 7. Separation of trace light gases Column and G. C.conditions same as Figure 6. Sample size: 5.7 X 0.035 in.; sample loop (90 pl)
OSLL
=
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(2)
A plot of OSLL us. effective plate number did not show a better data fit. From Table I, it can be seen that the columns used for the determination of Equation 2 consisted of various lengths, i.d.’s, and phases, both solid and liquid. Columns 4 and 6 were operated at twice their optimum flow rates as determined from their Van Deemter plots, while the others were operated at their optimum flow rates. The significance of OSLL can be readily observed by examining the effect of SLL on detector response (peak height)
and on column plate height. This is illustrated using Column 3. The OSLL for this column is approximately five inches (Figure 1). In Figure 4, it is shown that the peak height increases to a maximum with increasing SLL and that the maximum is slightly less than twice the peak height value at OSLL. The plate height, on the other hand, has more than doubled for the same increase in SLL (see Figure 5). At OSLL, the loss in column efficiency is only 15 to 20%. This behavior was typical of all the columns listed in Table I. Consequently, the OSLL represents a good compromise between the desirable increase in sensitivity and the undesirable degradation in column efficiency. The use of OSLL is illustrated for the rapid analysis of ethylene in a light gas mixture. The n required for the separation of ethylene and ethane on Porapak S was first determined. To this value, an additional 20% was added in order to offset the loss of column efficiency due to OSLL. The total plate number required was then found to be 710. By using Equation 2, OSSL is calculated to be 5.7 inches. In Figures 6 and 7 are compared an analysis using a 3-11 sample size and one using the OSLL, respectively. Comparison of the concentrations of the components shows that the sensitivity has increased by a factor of fifty while base-line separation between ethylene and ethane was still maintained. ACKNOWLEDGMENT The authors thank Ken Kee for the art work.
RECEIVED for review June 23, 1971. Accepted March 30, 1972. This paper was presented at the Seventh International Symposium on Advances in Chromatography, held in Las Vegas, Nevada, November 29-December 3,1971.
Neutron Activation Analysis of Copper by Substoichiometric Extraction with Neocuproine R. A. Nadkarni and B. C. Haldar Inorganic and Nuclear Chemistry Laboratory, Institute of Science. Bombay-32, India COPPERIS ONE of the seven essential trace elements necessary for normal animal and plant metabolism. Copper deficiency in humans causes Wilson’s disease. Copper acts as an activator of lipid enzyme systems, particularly those concerned with phospholipid synthesis. At the same time copper is an important metal in various industries and technologies, and mineral prospecting for copper is important geochemically, Nondestructive activation analysis of copper is not feasible since 64Cubeing a positron emitt’er, any other activity in the irradiated material which decays by positron emission will interfere seriously in this determination. Hence, radiochemical separation of copper is necessary for its determination. By incorporating the substoichiometric principle in the radiochemical separation scheme, several purification steps can be eliminated, and the time of separation shortened, provided a suitable reagent is available. Dithizone ( I ) and diethyldithiocarbamate (2) are two reagents which have been (1) J. Ruzicka and J. Stary, “Substoichiometry in Radiochemical Analysis,” Pergamon Press, New York, N.Y., 1968, p 87. (2) M. Krivanek, F. Kukula, and J. Sluencko, Talanta, 12, 721 (1965). 1504
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
used for substoichiometric activation analysis of copper. As is well known, the former is a very unstable and sometimes unpredictable reagent, while in the case of the second reagent, there is interference from several elements which can be avoided only by employing a multichannel analyzer. Neocuproine has been known for long time as an almost specific spectrophotometric reagent for copper (3, 4 ) . We have developed a method for the determination of trace amounts of copper based on neutron activation and substoichiometric extraction of copper with neocuproine in chloroform. EXPERIMENTAL Copper standards were prepared by dissolving 99.999 pure copper metal in H N 0 3 and diluting by weight to the desired concentration. Aliquots of this solution were taken in quartz vials and dried. The samples were dried (except for blood serum which was freeze-dried) at 80 “C for several hours. About 50-200 mg of the sample and about 10 pg (3) C. L. Luke and M. E. Campbell, ANAL.CHEM., 25,1588 (1953). (4) A. R. Gahler, ibid., 26, 577 (1954).
