Response of flame ionization detectors to different homologous series

Summary Hydrogen flame-ionization detectors (FIDs) are the most widely used type of detector in gas chromatography. The FID signals is proportional to the number of carbon atoms in a hydrocarbon molecule; the presence of heteroatoms usually reduces the signal. If the extent of the signal-reducing effect of heteroatoms were known, it would be possible to measure compounds which are not available as pure standards, or cannot be prepared, or their preparation is very expensive. The sensitivity of a detector to an organic molecule containing heteroatoms is referred to normal hydrocarbons by means of the effective carbon-atom number (ECN) value. By use of the values of increments inECN for heteroatoms and functional groups, theECN can be calculated for any organic molecule. For this, exact values of theECN increments are needed, and the effects of different factors on the increments must be known. In this study a wide range of homologues of normal paraffins, alcohols, amines, and esters was investigated, with emphasis on differences between the behaviour of lower and higher homologues. Studies were extended to theECN values of ketones, and aromatic and halogenated compounds. For all types of compound investigated the difference between the actual carbon number and the calculated effective carbon number (dECN) was compared with literature data, and an attempt was made to interpret the differences.     

Selectivity enhancement of a metal-sensitive flame ionization detector for capillary gas chromatography

A novel modification of a hydrogen-atmosphere flamo ionization detector (HAFID) is presented which attenuates response to hydrocarbon compounds, significantly enhancing selectivity towards organometallic compounds by more than an order of magnitude. Chromatograms of an organometallic compound test mixture and regular leaded gasoline are presented to depict the specificity of the response.     

Characterization of the flame photometric detector in the sulphur mode

Summary The effect of experimental conditions such as gas flow rates, detector and column temperatures on sulphur response are discussed. It is concluded that the response of the flame photometric detector is affected by the O/H ratio and the total gas flow. Column temperature has an important influence upon S₂ emission. It is shown that the sulphur response of the FPD is dependent on the compound containing the sulphur.     

Comparison of methods to enhance flame ionization detector (FID) signals in gas chromatography

The performances of some numerical methods to improve the signal to noise ratio are compared and applied to enhance noisy signals obtained in gas chromatography with capillary columns and a flame Ionization detector. Several methods have been considered: cutoffs In the Fourier transform of the recorded signal; real time numerical filtering; theoretical model curve fitting; and the correlation of a chromatogram recorded from a pseudorandomly injected sample with the pseudorandom injection function. Numerical real time filtering is shown to be the most convenient method when the main periodic component of the noise has been determined by Fourier analysis.     

Characterization of a microelectromechanical systems-based counter-current flame ionization detector

This work is concerned with the influence of different operating parameters on the response of a counter-current micro flame ionization detector (cc-μFID) with low gas consumption for mobile applications. At cc-μFID flow rates (<10ml/min hydrogen), the response depends mainly on the oxygen flow. At 7.5ml/min hydrogen flow, highest sensitivity (13.7mC/gC) is obtained with the smallest flame chamber and nozzle size, moderate sample gas flow (2.0ml/min), and an oxygen flow above stoichiometry (9.4ml/min, λ=2.5). The largest absolute signal is obtained at increased sample gas flow (8.0ml/min). However, to prevent parting of the micro-flame by the sample gas stream, largest nozzles (smallest outflow velocity) give the best result (4.37nA). Whereas cc-μFID sensitivity is comparable with conventional FID sensitivity, peak-to-peak noise of 1pA is relatively large. Therefore, the minimum detectable carbon mass flow of 1.46×10(-10)gC/s and the minimum detectable methane concentration of 3.43ppm are larger than typical FID detection limits. μGC-μFID experiments show the difference between premixing the sample with the hydrogen or with the oxygen with respect to sensitivity and response factors. Sensitivity is decreased considerably when the column effluent is added to the oxygen instead of to the hydrogen. For hydrogen premixed samples the response factor to butane can be increased up to 0.81 (methane=1), whereas for oxygen premixed samples it is maximally 0.31. This smaller sensitivity to oxygen premixed samples and the larger variation of response factors shows the importance of the hydrogen atom during breakdown of organic molecules to single-carbon fragments before ionization.     

Modified flame ionization detector for the analysis of large molecular weight polar compounds by capillary supercritical fluid chromatography

The modifications made to a flame ionization detector (FID) to facilitate the detection of large molecular weight polar compounds analyzed by capillary supercritical fluid chromatography are described. Some specific examples are given to demonstrate that polar compounds can be effectively eluted using deactivated fused silica capillary columns and supercritical carbon dioxide at 40°C.     

Effect of ammonia carrier gas on the response of the flame ionization detector

The effect of using ammonia as a carrier gas on the response of the flame ionization detector (FID) has been investigated. It was found that the FID response, calculated as the effective carbon number (ECN), increased for all the compounds studied when ammonia, rather than helium, was used. The change was 0–0. 9 carbon atom for hydrocarbons, one carbon atom for alcohols and diphenyl ether, and 0.4–1 carbon atom for phenols and ketones. The increase in ECN was larger for amines (0. 8–5 carbon atoms), but these numbers also reflected an improvement in chromatographic performance as a result of reduced adsorption on the column. The largest change in signal-to-noise ratio, a six-fold increase, was obtained for octyl-amine; ratios for hexyl methyl ketone, diisobutyl ketone, dihexyl-amine, dibutylamine, and N-methyloctylamine increased by a factor of 2–3 when ammonia was used as carrier gas. To determine the extent to which the effect on detector response was solely attributable to ammonia, a mixture of 5 % ammonia in nitrogen was used as detector make-up gas with helium as carrier gas. Under these conditions the noise in the FID increased but for most of the compounds studied the signal-to-noise ratio also increased.     

Quantitative gas chromatographic analysis of perhalogenated compounds using a flame lonization detector

Summary In the determination of several perhalogenated compounds after gas chromatographic separation on an Apiezon L column, using a flame ionization detector, all the compounds tested gave linear area-mass plots although the coefficients observed do not lead to extrapolations which would permit quantitation of perhalogenated compounds for which no standard is available. Of the compounds tested, dibromodichloromethane, tetrachloroethene and tetrabromoethene gave good linear correlations of area ratio-mass ratio (using n-decane as the internal standard). Bromotrichloromethane and tetrabromomethane gave continuously curving area ratio-mass ratio plots which were very reproducible and could be used for graphical interpolation.     

Effects of hydrocarbon quenching in gas chromatography when using the flame photometric detector

Summary The long time retardation of the main hydrocarbon peak in the chimney of the flame photometric detector greatly reduces the responses of later-eluting sulfur compounds. In the absence of hydrocarbons in the flame, the slope (s) of the log I vs. log [S] plot (where I is the sulfur response and [S] is the sulfur concentration in the sample) is of the highest value and is constant for all experimental conditions tested. Flame hydrocarbons cause the s value to decrease, and this is dependent on the oxygen to hydrogen ratio in the flame (O/H) and, under certain conditions, also on the sulfur to carbon ratio (S/C) of the sample. The abnormalities observed in the determination of methyl thiol in natural gas are explained on the basis of the present study.     

Determination of cyanogen and cyanogen chloride using gas chromatography with a flame ionisation detector

Summary The response of a flame ionisation detector (FID) to cyanogen ((CN)₂) and cyanogen chloride (CNCI) has been studied. The lower limits of detection for (CN)₂ and CNCl were 6×10⁻¹² moles and 2×10⁻¹¹ moles respectively. In a comparative study a thermal conductivity detector gave as its lower limit of detection for both (CN)₂ and CNCl 4×10⁻¹¹ moles. Both detectors showed excellent linearity over the range 0 to 10 micromoles. The ratio of the gradients of the FID calibrations (CN)₂/CNCI were found to be approximately two suggesting the response depended on the cyanide group. The response and stability of the FID was investigated as a function of air and hydrogen flow-rates. The response increased with increasing air flow-rate for both (CN)₂ and CNCI up to 300cm³min⁻¹, thereafter no significant increase was observed. On raising the hydrogen flow from 10 to 25cm³min⁻¹ only a slight increase in the sensitivity of the FID towards both (CN)₂ and CNCI was observed. At higher flow rates the response for (CN)₂ increased markedly whereas at the same flow rates the response for CNCI increased only slightly.     

Investigating the performance characteristics of the barrier discharge ionization detector and comparison to the flame ionization detector for the gas chromatographic analysis of volatile and semivolatile organic compounds

Abstract

The response behavior and performance characteristics of the recently introduced barrier discharge ionization detector (BID) for gas chromatography (GC-BID) were investigated by analyzing different classes of organic compounds such as alcohols, alkanes, cycloaliphatic compounds, polycyclic aromatic hydrocarbons (PAHs), and others. The results obtained by GC-BID were compared with those of gas chromatography with flame ionization detection (GC-FID), aiming to demonstrate the particular merits of the new BID detector over the well-established FID. The response of the BID not only was found to be strongly dependent on the detector settings, but also shows a high dependence on the analyte class and the individual analyte. The sensitivity of the BID detector compared to the FID was higher by a factor of ca. 4 on average when considering all compounds analyzed. The relative standard deviation (RSD) was better than 5% for the majority of the cases. The BID detector showed better precision (lower RSD) in comparison with the FID for the investigated compounds. Linear calibrations were obtained for the analytes over more than four orders of magnitude with coefficients of determination typically higher than 0.999 and the limits of detection varied from 0.04 to 1.48?ng/s for the GC-BID.     

Prediction of response factors for gas chromatography with flame ionization detection: Algorithm improvement,extension to silylated compounds,and application to the quantification of metabolites

We previously showed that the relative response factors of volatile compounds were predictable from either combustion enthalpies or their molecular formulae only 1 . We now extend this prediction to silylated derivatives by adding an increment in the ab initio calculation of combustion enthalpies. The accuracy of the experimental relative response factors database was also improved and its population increased to 490 values. In particular, more brominated compounds were measured, and their prediction accuracy was improved by adding a correction factor in the algorithm. The correlation coefficient between predicted and measured values increased from 0.936 to 0.972, leading to a mean prediction accuracy of ± 6%. Thus, 93% of the relative response factors values were predicted with an accuracy of better than ± 10%. The capabilities of the extended algorithm are exemplified by (i) the quick and accurate quantification of hydroxylated metabolites resulting from a biodegradation test after silylation and prediction of their relative response factors, without having the reference substances available; and (ii) the rapid purity determinations of volatile compounds. This study confirms that Gas chromatography with a flame ionization detector and using predicted relative response factors is one of the few techniques that enables quantification of volatile compounds without calibrating the instrument with the pure reference substance.     

Linearity of a flame ionisation detector for capillary gas chromatography

Summary The influence of the nitrogen and hydrogen flow rates on the linearity of the flame ionisation detector is investigated and flow-rates which give a linear response over five orders of magnitude are determined.     

High resolution gas chromatographic determination of permanent gases with a helium ionization detector

The capability of the helium ionization detector (HID) to operate in connection with capillary columns for trace gas analyses has been evaluated. Two different capillary columns were considered: a PLOT fused silica column with molecular sleve and a thick film WCOT glass column with PS-255. The determination of trace impurities in gases can be achieved with evident advantages over classical adsorption columns, even using a split injection system. Direct on-column injections also have been investigated with promising results.     

Determination of selected fatty acids in dried sweat spot using gas chromatography with flame ionization detection

A method is described for the determination of fatty acids in dried sweat spot and plasma samples using gas chromatography with flame ionization detection. Plasma and dried sweat spot samples were obtained from a group of blood donors. The sweat was collected from each volunteer during exercise. Sweat was spotted onto collection paper containing butylated hydroxytoluene. Fatty acids were derivatized with acetyl chloride in methanol to form methyl esters of fatty acids. The fatty acids in dried sweat spot samples treated with butylated hydroxytoluene and stored at –20°C were stable for 3 months. Our results indicate that sweat contains, among fatty acids with short chain, also fatty acids with long chain and unsaturated fatty acids. Linear relationships between percentage content of selected fatty acids in dried sweat spot and plasma were observed.     

Quantitative and fingerprinting analysis of Atractylodes rhizome based on gas chromatography with flame ionization detection combined with chemometrics

This study assessed the feasibility of gas chromatography with flame ionization detection fingerprinting combined with chemometrics for quality analysis of Atractylodes rhizome. We extracted essential oils from 20 Atractylodes lancea and Atractylodes koreana samples by hydrodistillation. The variation in extraction yields (1.33–4.06%) suggested that contents of the essential oils differed between species. The volatile components (atractylon, atractydin, and atractylenolide I, II, and III) were quantified by gas chromatography with flame ionization detection and confirmed by gas chromatography with mass spectrometry, and the results demonstrated that the number and content of volatile components differed between A. lancea and A. koreana. We then calculated the relative peak areas of common components and similarities of samples by comparing the chromatograms of A. lancea and A. koreana extracts. Also, we employed several chemometric techniques, including similarity analysis, hierarchical clustering analysis, principal component analysis, and partial least‐squares discriminate analysis, to analyze the samples. Results were consistent across analytical methods and showed that samples could be separated according to species. Five volatile components in the essential oils were quantified to further validate the results of the multivariate statistical analysis. The method is simple, stable, accurate, and reproducible. Our results provide a foundation for quality control analysis of A. lancea and A. koreana.