Quantitative Aspects of the Flame Ionization Detector in Flow Programmed Gas Chromatography R. L. Levy, J. Q. Walker, and C. J. Wolf McDonnell Research Labs., McDonnell Douglas Corp., S t . Louis, Mo. 63166
FLOW(pressure) programming is gaining popularity as a technique which adds versatility to gas chromatography. I n general, it is analogous to temperature programming although it is applicable to mixtures of narrower range of boiling points. Flow programming is particularly attractive when it is applied to problems requiring the use of high-selectivity liquid phases which exhibit poor thermal stability. The response of the flame ionization detector (FID) to a given amount of organic material, however, depends upon several parameters which change during a flow programmed analysis (1). One of the most important of these parameters is the composition of the fuel gas mixture as it leaves the flame jet (carrier gas plus hydrogen). I n conventional constant flow gas chromatography (isorheic), with a F I D , using nitrogen o r helium as carrier gas, a constant flow of hydrogen is added to the carrier gas stream at the outlet of the column. Thus, the ratio of carrier gas t o hydrogen remains constant during the analysis. Under the conditions of flow programmed gas chromatography, however, the ratio of carrier gas t o hydrogen is continuously changing during the analysis effecting a change in the FID response. As a result the peak areas of the individual components become dependent on their retention time. This causes a large discrepancy between results from isorheic and flow programmed analyses of the same sample. Many chromatographic analyses are conducted under the conditions of temperature programming; here the flow through the column varies with temperature because the viscosity of the carrier gas and porosity of the column change with temperature. Thus, unless one is careful in the system design and chooses a good flow controller, conventional temperature programmed chromatography also leads to a variation in the fuel gas mixture.
Scott (2) recognized the problem and schematically suggested a method requiring the use of two flow controllers for performing quantitative analysis under flow programmed conditions. A much simpler method was developed in our laboratory as a n alternative solution to the problem. A mixture of 60% N2 and 40% H Pwas used as a carrier gas which ensures a constant N2 to Hz ratio throughout the flow cycle and eliminated the major cause of variation of the F I D response. The air flow to the flame was constant and was high enough so that even at the highest carrier gas flow rates used (330 cm3/min) oxygen was in sufficient excess. In our FID system with a n air flow of 330 cm3/min, the maximum sensitivity to organic compounds occurs with a H2 flow of 23 cm3/min and a N Pflow of 25 cm3/min. The results obtained from the analysis of a mixture of 6 different normal alkane hydrocarbons under isorheic and flow programmed conditions using alternatively N2 and the N, + Hzmixture as a carrier gas are summarized in Table I. In all experiments, 0.1 p1 of the mixture was injected with a microsyringe. The average relative area of each peak based o n 5 determinations together with the standard deviation ( 1 ~ ) are given for the 4 different flow conditions investigated. The results from the analysis with Nz carrier gas at a constant flow of 60 cm3/min are shown in Column 2. The reproducibility of the relative areas is good, although the long elution times (approximately 30 minutes) of decane and undecane results in flatter broader peaks which are subject to more error. The actual integrator areas of hexane and heptane are essentially the same in the constant and flow programmed analyses with Nz as a carrier gas; however, the areas of the
(2) R. P. W. Scott, in “Progress in Gas Chromatography,’’ J. H. Purnell, Ed., Interscience, New York, 1968, p 271.
(1) I. G. McWilliam, J . Chromatogr. 6, 110 (1961).
Table I. Comparison of Results Obtained under Constant and Flow Program Conditions with N2 and Nz Carrier Gases
12-Hexane n-Heptane n-Octane n-Nonane n-Decane n-Undecane Carrier gas flow rate
+
+ H Pas Carrier Gas
+
Nz
Constant flowa
Nz Flow program”
Nz Hz Constant flow5
Nz Hz Flow program
16.7 f 0 . 2 10.1 i 0 . 2 14.0 f 0 . 2 16.5 =k 0 . 2 18.2 & 0 . 3 24.5 i 0 . 5
7.0 i 0 . 1 4 . 0 =k 0.1 8.8 f 0 . 1 17.7 f 0 . 2 29.5 f 0.1 33.0 i 0 . 1
17.3 & 0.1 8 . 9 i 0.1 12.6 i 0.1 16.9 i 0 . 2 20.8 i 0.2 23.5 f 0 . 2
15.5 i 0.1 11.1 i 0.1 14.5 f 0 . 1 19.1 =k 0 . 1 21.0 f 0.1 19.8 f 0.1
65 cm3/min
65-333 cm3/minc
60 cm3/minb
60-262 ~ m ~ / m i n * . ~
The numbers represent the average per cent relative to the total area of all 6 peaks together with the standard deviation from 5 separate determinations. * Hzfuel at a flow rate of 40 cm3/min was added at the end of the column. c The flow rate increased linearly at a rate of 34 psi/min.
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decane and undecane peak in the flow programmed studies are 3 times larger than in the constant flow case. Thus, the relative areas of the peaks eluting last are large. With the gas flows used in this study the sensitivity of the F I D t o organics with the Nz Hzmixture is more than twice that of Nz alone. We observed that the relative areas (shown in Table I) of the six peaks as well as the absolute areas are in reasonable agreement when the Nz Hz mixed carrier gas is used with either constant flow or flow programming. This observation indicates that the sensitivity does not appreciably change during a flow programmed analysis. Therefore, useful quantitative information can be gained from a flow programmed study when a Nz Hz mixture is used. It is important to note that the sensitivity varies slightly during the analysis but the variation can be neglected if a n accuracy of 10-15z is sufficient. In addition, the analysis
+
+
+
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time was reduced by more than a factor of two when flow programming was used, thereby yielding sharper chromatographed peaks. An additional problem in flow programmed gas chromatography with F I D is that for a flame jet of a given size there is a limited range of flow rates for which the flame remains lit. Therefore, the flow rate can be programmed only within these limits. In our system with a jet of 0.018-in. i.d., the flame extinguished at a flow rate of 260 cm3/min when using the Nz carrier gas. With the Nz HZmixture the flame did not extinguish at flow rates as high as 330 cm3/min although at this flow rate the flame became turbulent and the signal noisy.
+
RECEIVEDfor review July 14, 1969. Accepted August 25, 1969. This research was conducted under the McDonnell Douglas Independent Research and Development Program.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
