ritique of Some Conventional Evaluation Methods and Continuous Flow, Steady-State Blender for Evaluation of Gas Chromatography Detector Linearity William H. King, Jr., and Gerald D. Duprd Analytical and Information Division, Esso Research and Engineering Company, P.O. Box 121, LZnden, N . J. 07036
Conventional detector evaluation methods suffer various drawbacks which are thoroughly discussed. A new method using a steady-state flowing gas of known solute concentration is presented. The steadystate method eliminates adsorption problems and unequivocally determines detector linearity on an absolute basis. The technique presented employs a constant temperatwre saturator and a series of calibrated flowmeters to make blends of clean helium and solute-saturated helium. The lower range of concentration which was normally used was approximately 10-9 gram/see; the upper limit is set by the solute saturation pressure at approximately gramlsec. The accuracy is &5%. Several late model flame ionization detectors were evaluated. Some showed reasonable linear response up to gram/ sec., while others were grossly nonlinear over the entire range.
IN THE QUANTITATIVE analyses of organic compounds by a gas chromatographic method, one of the most important criteria in the performance of a gas chromatographic detector is its linearity of response over a wide dynamic range of concentration. A commonly used detector system has been the hydrogen flame ionization detector because of its high sensitivity, wide dynamic range, and its broad application to most organic compounds. In the evaluation of the response of the flame ionization detector, several techniques have been used. Direct sample injection of multiple component blends (1-31, the permeation tube ( 4 ) technique, and the exponential dilution flask method (5-7) have been employed in calibration systems. Because of some of the disadvantages found with these approaches, a new system has been developed to evaluate the flame ionization detector. This first-principle method for calibration is rapid, flexible, and unequivocal. It can be used over a wide range of concentrations with liquid or solid organic components to develop many data points quickly under many detector operating parameters. Our purpose in writing this paper is to point out some of the basic limitations in some of the current techniques used for determining linear performance of flame ionization detector systems (a flame ionization detector of commercial design and its associated electrometer amplifier and associated components). This should not discourage any individual from using his preferred method of calibrating his system. It should, however, alert him to some of the limiting factors which must be considered in his evaluation of the data. (1) W. A. Dietz, J. Gas Chromatog., 5 , 68-71 (1967). (2) 0. Hainova, P. Bovek, J. Novak, and J. Janak, ibid., p p 401-5. (3) D. R. Deans, Chromatographia, 1, 187-94 (1968). (4) A. E. O’Keefe and G. C . Ortman, ANAL.CHEM.,38, 770-3 (1966). (j) J. E. Lovelock, ibid., 33, 162-78 (1961). (6) J. M. Gill and C. H. Hartmann, J. Gas Chromatog., 5, 605 (1967). (7) I. A. Fowlis and R. P. W. Scott, J. Chromatography, 11, 1-10 (1963). 1936
EXPERIMENTAL
A schematic diagram of the system is given in Figure 1. The system is designed so that a stream of helium, saturated with the vapor of the sample, may be diluted with clean helium before being passed through the detector at a rate which simulates normal column flow. The rotameters are Brooks “sho-rate” 150 rotameters equipped with ELF needle valves which have been calibrated under conditions of the experiment. A four-way Teflon (Du Pont) valve is used for switching between dry helium and blended gas streams. The saturator, S, which is immersed in a constant temperature bath, B, is a 15-inch X l/&ch 0.d. stainless steel tubing containing two pipe cleaners on which the compound under study is applied. The overall repeatability of this method is approximately = t 5 % of the calculated response. In order to ascertain that this system was operating properly, experiments using a three-liter glass flask connected to an allmetal valve were conducted. The flask was evacuated and weighed using a five-place balance. Dry air was introduced, and the flask was reweighed. After again evacuating the flask, dry air was allowed through the saturator containing normal hexane before entering the flask. The difference in weight between dry air and the hexane-saturated air gave results which agreed quantitatively with values obtained from equilibrium vapor pressure calculations. RESULTS AND DISCUSSION
Of the many methods of obtaining linearity data, three of the better methods are injecting blends of varying sample sizes, exponential dilution experiments, and continuous flow blending. Injection is a fairly expensive method since time is required to wait for peaks to appear; however, by using many components in the blends, time can be saved if the objective is to test many substances. Also, employment of automatic injection systems can minimize this time factor. One point which should be appreciated when using the injection method is that the entire chromatograph is being tested and not just the detector. Adsorption problems in the system with some solutes can cause nonlinear performance in the low ranges. Frequently, adsorption is blamed for the nonlinear response of detectors at low concentrations. When this occurs it is difficult to determine whether the system or the detector is at fault. One way out of the dilemma is to use a different type of detector; another way is to change the system and solutes until adsorption is eliminated. There are added problems with the injection method when blends are injected. The volume of liquid measured by the plunger in addition to the volume held up on the needle is not necessarily constant for different components in the blend. This effect, which is probably due to fractionation, is demonstrated in Figure 2, where various size samples of 1.58 % water in methanol were injected. Although peak areas are shown, the same performance for peak heights was demonstrated.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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Figure 1. Schematic diagram of gas saturating and blending system for evaluating performance of flame ionization detector Absorption phenomena for water was not operative for a variety of reasons, the main one being the straightness of the water line. Our experience has been that the experimental data will deviate from the straight line, particularly at the lower concentrations, when a compound is subject to selective adsorption. The extrapolated needle holdup volume for methanol is 0.3 plywhile that for water is 0.15 p l . Data were taken with higher water concentrations, but the results were unavoidably the same. The problem does not disappear with other syringes as demonstrated in Table 1.
Figure 2. Thermal conductivity integrator counts as function of sample size for water and methanol injection by syringe Sample 1.6% water in methanol, Porapak Q column having 95 plates; eluted water 0.82 min, methanol 2.38 min
Table I. Extrapolated Needle Volumes for Various Syringe Models syringe Extrapolated needle volume, p1 volume, Water Methanol Syringe model PI 5 0.63 O.& Hamilton No. 75N 10 0.70 0.80 Hamilton No. 701N 10 0.70 0.78 Precision Sampling Series C 10 0.15 0.30 Precision Sampling Series A
In all cases the syringe and needle were flushed and filled with the indicated amount of sample; entrapped air bubbles were absent, and the plunger was retracted 2 pl before injection. The needle was inserted up to the hilt in a Hamilton Model 86800 inlet for 5 seconds with a septum temperature of 200 "C. When the syringe was removed, the plunger was retracted, and about 0.1 p1 of unevaporated sample could be seen in the bore. Repeatability was &5.2% with 95% confidence. An improved sample injection technique has recently been described by Kruppa (8). With this technique, many of the problems described above should be minimized. Exponential dilution has been used by many workers (5-7) and the problems due to adsorption are again operative, not in the chromatograph, but now in the dilution flask. Data from exponential dilution are often plotted on a log concentration OS. log response basis and a straight line is often misinterpreted as meaning linear response. Log concentration is easily calculated from the standard equation for exponential decay : - TF CiC, = e V where T i s time V is the volume of the flask F is the flow rate of the purge gas in units consistent with T and V C is the concentration relative to the starting concentration, C,. (8) R. Kruppa, Gas-Chrom Newsletter, 10, 1 (January 1969).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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10
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DETECTOR C h
10
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12 DETECTOR C IMPROPERLY ALIGNED
10 TIME
Figure 3. Log response os. time for theoretical system under evaluation by exponential dilution flask technique PEAK CONCENTRATION OF C O U W N E N T IN Tlbla II
Linear response can only be indicated if the line on log-log paper is 45 degrees (Slope or S = 1.00). For example, suppose a straight line at 46 degrees or slope = 1.04 is observed; then using the equation for a straight line, y = mx 6, where y is response (R),m is 1.04, x is concentration (C) and b is the intercept
+
Log R = 1.04 Log C
+ Log K
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Figure 4. Response in millicoulombs per gram us. concentration for different flame ionization detector situations 3 late model commercial detectors were used and adjusted for maximum response
Rearranging and taking anti-logs on both sides :
R =~
~ 1 . 0 4
It is therefore apparent that a linear response occurs only when the slope is 1.00, Although 0.04 seems like only a small deviation from unity, a slope of 1.04 means that the
Table 11. Advantages and Disadvantages of Some Detector Evaluation Techniques Technique Injection
response of the detector increases 10% for every 10-fold increase in concentration. This is a serious error and points out an important limitation of the exponential dilution technique and the log-log interpretation. There is an additional problem in exponential dilution which may not be appreciated by many people. The flow rate, temperature, and the flask volume must be very accurately known. Assume for illustrative purposes that two detectors are investigated by exponential dilution, the responses are plotted, and the slopes (S)determined to be Sl= 1.00 and S2 = 1.04, respectively. After listening to the foregoing argument, you might say that the first detector, S = 1.00 was linear having a response equal to R = KC and the second was not since S = 1.04 and its response formula is R = KC1.04. An exponent of 1.04 means 10% enhancement of the signal per decade of concentration. It turns out that both detectors could be identical in linearity since the difference could be explained by only a 3 % error in the volume or the flow measurement. To understand this, consider the data gathered €or the two cases plotted as log response US. time in Figure 3. The curves have been adjusted by a factor to make them coincide at zero time for illustrative purposes. In addition, the exponential formula for each line is shown. The ordinate can be considered either response or concentration. In the case of Detector No. l , the lower curve, the dilution flask had a volume of 67.3 and a flow rate of 6.91 ; thus, the response curve was parallel to the concentration curve which is the condition for linearity. For the "nonlinear detector," the dilution flask was thought to be a 67.3 volume, but in reality, it was 70.0, and this causes the response curve to diverge from the expected concentration curve, which in turn results in the wrong conclusion about linearity. A 3% error in flow rate could also reflect nonlinearity.
Advantage Convenient for most people Doesn't require special hardware Many components evaluated per injection Results are related to absolute known quantities of each compound Exponential Convenient for some dilution people Wide ranges quickly produced
Disadvantages Evaluates system as well as detector Adsorption clouds result Costly, time consuming Syringe fractionation is a problem
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VOL. 41, NO. 14, DECEMBER 1969
Unless great care is taken in knowing volumes and flows, results can be misleading Requires special system Not an absolute method (only relative concentration is known) Method forces the use of log-log plot which hides errors Continuous Absolute method Requires special system flow No absorption problems Accuracy limited to flowblending Detectors quickly optimeter accuracy mized Temperature on saturator Data produced quickly must be precisely conand conveniently trolled
B
ANALYTICAL CHEMISTRY,
Table 111. Errors in GC Analysis Due to Nonlinear Performance of Flame Ionization Detector C Found4
Optimum conditions
Improperly adjusted Knownwt % Wt A % Wt A % 5 84.7 89.2 0 89.2 39 13.6 9.74 1 nC0 9.85 63 1.58 1.02 5 nCs 0.97 78 0.016 0.011 20 nC7 0.009 SE a Normalized average data from 2 runs of a four-component blend on a 6-foot X l/&nch 0.d. stainless steel column packed with 3 30 on 80-100 mesh Chromosorb G at a helium flow of 30 ml/minute. The sample size was 0.1 pl, and the hydrogen and air flow rates were optimized for sensitivity.
z
z
Component nClo
~~
Continuous flow blending was chosen for the work reported here because the problems of adsorption disappear since the concentration is held at steady state until all adsorbers are satisfied. Of great importance is the point that the concentration is known on an absolute basis, which makes interpreting the data straight-forward. A summary of the advantages and disadvantages is presented in Table 11. In this table, only the more important items are stressed. For the critical evaluation of the response characteristics of any detector system, the treatment of the data on log-log plot gives an illusion of a minimal variation of response with concentration (5). Many of the subtle changes encountered by Dean (3), for example, would be neglected in this treatment. This effect will be described further in connection with Figure 4. In order to show up the subtle changes in linearity of response, it is advisable to use Halasz' (9) expression for response or factor of proportionality, R. This is essentially the detector response signal divided by the sample concentration. A plot of this factor as a function of concentration will give a horizontal straight line in the linear range of concentration. Any deviation from this horizontal straight line signals nonlinear performance of the detector. A positive slope means the detector is over-responding, while a negative deviation, which is normally encountered, means that the detector response is inadequate. This can be simply illustrated from the expression for a straight line.
where K is the slope and R,,Co is the intercept. Solving for response us. concentration yields the expression below :
R
R co
=KC2+2C
The only condition where true linear performance is demonstrated is where K , the slope, is zero. The magnitude of K is indicative of the magnitude of nonlinear response. TYPICAL APPLICATION
An evaluation of several detector systems is under way in our laboratory. Considering fairness to manufacturers and the incomplete nature of our investigations, only typical data showing the reliability of our evaluation technique will be given. Three commercially available detectors of recent (9) H. Bruderreck, W. Schneider, and I. Halasz, ANAL.CHEM., 34,
461-73. (1964).
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THEORETICAL LINEARITY CURVEACTUAL DATA
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Figure 5. Log-log response curve for flame ionization detector improperly adjusted showing that gross errors are hidden by this plotting technique
manufacture were tested. Acetone, hexane, and other solutes were used, and their response patterns suggest little effect of compound type. Acetone was chosen for illustrative purposes only. The response of acetone as a function of concentration when the detectors were operated at optimum conditions for sensitivity is shown in Figure 4. Detector A behaves nonlinearily over the entire range of concentration evaluated. The response of acetone increases approximately 20 % per order of magnitude of concentration in the range of loy8to gram/sec of acetone. This rate of change of response factor with concentration would indicate a response factor for a 1 % component peak would be approximately 40z less than that for a 99% component peak. Any attempts to perform quantitative analysis with this detector system, assuming a constant response factor as a function of concentration, would produce erroneous analytical results. With Detector B, however, the response of acetone appears to be linear from approximately gram/sec to approximately gramfsec with steady loss in response as the concentration was increased above the latter level. Although this may appear satisfactory, one must understand the implication of sample size and peak width on linear response. For example, if we assume that 1 pl of a sample containing acetone is injected into the chromatographic system and the acetone peak width-at-half-height is 100 seconds wide, the
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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response will be linear for concentrations of acetone up to approximately 12 %. Above this concentration nonlinear response can be expected. Higher concentrations can be assayed only if the sample size is reduced o r the peak width is increased. With Detector C, the response appeared linear through approximately 10-6 gram/sec acetone, a 40-fold increase in concentration. Thus, one would expect 1 pE*1 of acetone to respond linearily at peak width-at-half-height of only 20 seconds. When the mechanical configuration of the components of the detector C was not properly adjusted, however, the response varied with concentration as shown in Figure 4. for an improperly aligned system. There is no point in this curve where any degree of linearity is exhibited under these conditions,
These data are also presented on a six-cycle log-log plot in Figure 5 to emphasize how such a grossly nonlinear response curve can approximate an expected linear response curve. A theoretical linear response curve is shown as a dashed line in this figure. The impact of the nonlinear performance of flame detectors can best be shown by the data of Table 111. Were, analyses of normal paraffins were attempted on a known blend assuming equal response factors at all concentrations. With a nonlinear detector, errors of up to 80% were encountered, while goQd analyses of only a few per cent deviation resulted with a properly operated detector.
RECEIVED for review April 28,1969. Accepted September 17, 1969.
tion Isotherms an Giovanni Alberini, Fabrizio Bruner, and Giuseppe Devitofrancesco Isstituto di Chimica Analitica, Uniuersith di Roma, 00185 Roma, Italy The continuous flow system is used to determine the nitrogen adsorption isotherms of glass powders etched with hot alkaline solutions. The effect of alkali concentration on surface area, isotherm shape, and pore volume distribution is compared with the gaschromatographic properties of the glass powder. Etching conditions are found to be critical. Water vapepur is used as surface deactivant and its effect on surface Characteristics is related to the change on retention volumes and column efficiency.
EXPERIR#E"BL
(1) F, Bruner and G. P.Cartoni, ANAL.CHEM., 36, 1522 (1964).
A soda glass of the following composition was used: S O z : 69.4%; Na@: 13.9%; CaO: 9.5%; MgO: 1.6%; B203: 5.7% KzO: 0.02%. Sample Preparation. The powder used in the experiment was obtained by crushing and size grading the original tubing. The size-graded powder was then divided into three individual batches, each of which was etched with a solution containing either 5%, lo%, or 30% wt/vol sodium hydroxide. The etching was carried out by placing the powder in a large flat borosilicate container, adding the NaOH solution and then heating at 100 "C for eight hours. The sample was then cooled and washed with alkaline solutions of decreasing concentration Final washings were made with distilled water until they registered neutrality. The powder was then rinsed with ethanol and diethyl ether, dried at 120 "e,and then resieved. The reproducibility of the sample was not very good and was attributed to the etching conditions being a critical variable. Continuous Flow Sorptometric Measurements. The continuous flow method described by Nelsen and Eggertsen (5) and by Daeschner and Stross (6),was used for determining the surface area of the powders. This method is based on nitrogen absorption in a mixture of nitrogen and helium which continuously flows through the sample at liquid nitrogen temperature. When the adsorption equilibrium is reached the sample probe is rapidly heated to approximately 40 "C and the nitrogen desorbed into the moving nitrogen-helium stream. This increase of nitrogen in the gas mixture is detected by a thermal conductivity cell and i s recorded as a peak on the potentiometric recorder. The recorder response is also monitored during the adsorption process SO that the adsorption equilibrium is easily seen. The equilibrium is assumed to be complete when no baseline drift is observed after the adsorption. A calibration is made for each mea-
(3) J: F. K. Huber and A. I. M. Keulemans, in "Gas Chromatography 1962" M. van Swaay, Ed., Butterworths, London, 1962, p. 26. (4) A. V. Kiselev, ibid., p. XXXIV.
(5) F. M. Nelsen and F. T. Eggertsen, ANAL.CHEM.,30, 1387 (1958). (6) H.W. Daeschner and F. €3. Stross, ibid., 34,1150 (1962).
ETCHEDGLASS has been successfully used both as a liquid phase support for open tubular columns ( I ) or in high resolution gas-solid chromatography and gives very useful analytical results (2). Etched glass powders exhibit the same chromatographic properties, which allow this material to be used as a packing for adsorption columns. Particular characteristics of etched glass in gas-solid chromatography are its small surface area compared to other commonly used adsorbents, and the ease of modifying its chromatographic properties to the user's needs by proper deactivating agents. Adsorbents for gas-solid chromatography have been studied in terms of adsorption isotherms by Keulemans, who first indicated a way to directly correlate the peak tailing with the adsorption isotherm (3) and by Kiselev, who studied the gas chromatographic behavior of silica and its relationship with heats of adsorption and adsorption isotherms (4). In this paper we try to make a comparison between the physical properties of etched glass as they are obtained from the nitrogen adsorption isotherms and its gas chromatographic behavior-Le., retention volumes and column efficiency. 42) F. Bruner, 0.P. Cartoni, and A. Liberti, ibid., 38, 298 (1966).
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ANALYTICAL CHEMISTRY, VQL. 41, NO. 14, DECEMBER 1969
