E.P. Grimsrud

Effect of oxygen on the response of a constant-current 63Ni electron-capture detector

Abstract A constant-current electron-capture detector (ECD) with a 63 Ni ionization cell has been used in a study of the effect of oxygen in nitrogen carrier gas on its response to several compounds. Because of the greatly increased linear dynamic range and the high temperature capabilities of this instrument, oxygen contamination of the carrier gas was found to be much less harmful to the chromatogram baseline than had been previously reported for earlier ECD models. Measurements of the changes in the molar responses of several compounds caused by the addition of up to 2000 ppm of oxygen to the nitrogen carrier gas have been made. For the chlorinated hydrocarbons studied, an effect of oxygen on their molar responses was observed only with the highest oxygen dopings. Anthracene behaved differently, showing an increased molar response at very low levels of oxygen doping. These results suggest that constant-current ECD analyses of polynuclear aromatics may be very sensitive to uncontrolled oxygen contamination of carrier gas, while those of halocarbons will not. For 1-chlorobutane a greatly enhnaced response factor is caused by doping of the carrier gas with large amounts of oxygen. As this effect is observed at high detector temperatures where the baseline frequency is only moderately impared by oxygen, oxygen doping is suggested as a means of improving the ECD sensitivity to molecules that have only one or a few halogen atoms. Two reaction mechanisms are proposed to account for the oxygen effects observed.

Spacial distribution of ions and electrons within 63Ni ionization cells

Abstract Measurements of ion densities within a specialized 63 Ni ionization cell are described where the distance between a radioactive foil and the point of ion density measurement is continuously variable. Air and dichlorodifluoromethane are used as the ionizing medium. Measurements are made with an atmospheric pressure ionization mass spectrometer. The results are shown to be in excellent agreement with a simple quantitative model of the experiment. The model is then used to predict the distribution of ions throughout the active volumes of several cylindrical cells of typical dimensions.

Alteration of the electron-capture detector response to polycyclic aromatic hydrocarbons by oxygen doping of the carrier gas

Abstract A constant-current electron-capture detector (ECD) has been used to study the effects of oxygen added intentionally to the carrier gas on the response of the instrument to five polycyclic aromatic hydrocarbons (PAHs). At a detector temperature of 250°C the responses of anthracene, pyrene and 1,2-benzanthracene are increased very substantially, while those of phenanthrene and tetracene are increased only slightly. The basis of these observations is discussed, and mass spectrometric measurements of the negative ions formed in an ECD as a result of oxygen doping are reported. It is suggested that the structure-dependent oxygen-caused response enhancements observed for PAH molecules might assist in the identification of these in their analysis by gas chromatography.

Improvements in the detection and analysis of CF3-containing compounds in the background atmosphere by gas chromatography-high-resolution mass spectrometry

An improved method for the gas chromatography/mass spectrometry analysis of CF3-containing compounds in air is described. This method replaces a GS-Q porous layer open tubular (PLOT) column previously used with a 30 m x 0.32 mm GS-GasPro PLOT column. For this exceedingly volatile set of compounds the GS-GasPro column provides improved peak shapes, better signal-to-noise responses and no coelution of compounds. These improvements have allowed eleven CF3-containing compounds to be detected in background air, including CF4 (FC 14), C2F6 (FC 116), CF3Cl (CFC 13), CF3H (HFC 23), CF3Br (Halon 1301), C3F8 (FC 218), CF3CF2Cl (CFC 115), CF3CHF2 (HFC 125), CF3CH3 (HFC 143a), CF3CH2F (HFC 134a), and CF3CFCl2 (CFC 114a). Three of these compounds have not been previously detected in background air, to our knowledge. Quantitative determinations for each of these compounds in the background atmosphere of Montana are also reported.

Non-linear responses of the electron-capture detector to alkyl monochlorides

Abstract Calibration curves for the response of a constant-current electron-capture detector with a 63 Ni ionization cell to methyl and ethyl chloride are reported to be non-linear. The molar response to low-concentration samples is shown to be very much greater than to high-concentration samples. Oxygen contamination of the carrier gas is investigated as a likely cause of the low-concentration, increased responses, but this possibility is not supported by the experiments reported here. Other possible causes are discussed. Superior calibration curves for alkyl monochlorides are obtained by the use of intentionally oxygen-doped carrier gas.

Physical parameters affecting the quantitative response of the constant current electron-capture detector

Abstract While the response of the constant current pulsed (CCP) electron-capture detector reflects the extent of reaction between electrons and analyte molecules, it can also be dependent upon other non-analyte electron loss processes. These might include electron losses by reaction with carrier gas impurities, by recombination with positive ions, by diffusion to walls, by ventilation with carrier gas flow, and by migration through small fields generated by contact potentials. These physical factors are examined in detail here with regard to their potential affect on the quantitative response of the CCP electron-capture detector. Experiments indicate that as long as the base frequency of pulsing is moderate to fast, non-analyte electron loss processes within the CCP electron-capture detector will be well-behaved and in harmony with the expectations of simple and idealized theory, and a constant molar response to well-behaved analytes can be expected over the entire dymamic range of the instrument. With the use of slower base frequencies of pulsing, however, non-analyte electron losses are more complicated and non-ideal behavior in the form of non.linear calibration curves is observed. An attempt is made to identify the specific causes of the non-ideal behavior which accompanies the use of the slower pulse frequencies.

The effect of trialkylamines on the repsonse of the electron-capture detector to acridine

Abstract The normal electron-capture detector response to acridine is shown to be complicated by a positive ion-molecule reaction sequence. Because of its very high proton affinity, acridine is readily protonated inthe electron-capture detector when clean carrier gas is used. This positive ion-molecule sequence has an effect on electron density which is dominant over the electron capture reaction with small concentrations of acridine so that an inverted response is then observed. At higher concentrations of acridine, complex peak shapes are observed which reflect the competition between the electron capture reaction and the positive ion sequence. The positive ion sequence is partially stabilized and the electron-capture detector reesponse to acridine is simplified by the addition of trimethylamine to the carrier gas. The positive ion sequence is completely stabilized by triethylamine. The positive ions formed by the presence of the alkylamines are not simply the (M + 1) + ions as were expected. Rather, for trimethylamine the (M − 1) + ion and for triethylamine the (M − 15) + ion are dominat. It is shown that these ions are produced in abundance only in a relatively pure nitrogen medium. These ions are shwn to be more stable than the corresponding (M + 1) + ions which are also observed in lower abundance.

Effect of applied fields on positive ions in the pulsed electron-capture detector

Abstract Experiments are reported which indicate the effect of positive ions on the measured standing current and response of the pulsed electron-capture detector (ECD). These experiments include measurements taken with a specialized ion source on an atmospheric pressure ionization mass spectrometer, and also measurements of standing current obtained from two ECDs of coaxial cylindrical geometry but having different length-to-diameter ratios. Two opposing views of the means by which positive ions might be removed from a pulsed ECD are considered. The results of this study are shown to strongly support one of these.

Analysis of brominated compounds in background air by gas chromatography-high-resolution mass spectrometry

Abstract The application of a gas chromatography–high-resolution mass spectrometry (GC–HRMS) system to the detection and quantitative analysis of bromine-containing compounds in the Earth’s atmosphere is described. By this technique, air samples are introduced to the HRMS system by GC while the HRMS system is tuned to the exact masses of the ions expected in the electron impact spectrum of each compound. By the method described here, the mass spectrometer is assured of focusing precisely on the centroid of the exact mass selected because the mass scale is continuously (two times per second) recalibrated during the chromatographic analysis using n -dodecane as a mass reference. By this technique, the positive identification and quantitative analysis of several brominated compounds consistently found in the background air of rural Montana is demonstrated. In addition, a large volume standard addition technique is described and shown to be well-suited to the quantitative analysis of atmospheric components that have extremely low concentrations. Most of the brominated compounds identified here are present in background air at the sub parts-per-trillion (ppt) level. One of them, CF 2 Br 2 , is shown to have an atmospheric presence of only about 40 parts per quadrillion (ppq). Detection limits for all of the brominated compounds identified here were very low (in the low ppq range) and even lower detection limits could be achieved, if desired, by use of sample volumes that are larger than those routinely used here (340 cm 3 ).

Mass spectrometric measurements of the negative ions formed in the thermionic ionization detector with nitrogen carrier gas

Abstract A specialized ion source for an atmospheric pressure ionization mass spectrometer was constructed so that ionizing conditions within it were identical to those existing within the nitrogen carrier gas mode of the thermionic ionization detector for gas chromatography. During the passage of several substituted nitrobenzenes through this ion source, an effort was made to identify the negative ions which are thought to be responsible for this detectors response. For all compounds studied no negative ions having masses within the 1–500 a.m.u. mass range of the mass spectrometer were detected. However, negative ions of unknown masses greater than 500 a.m.u. were detected in each case. This unexpected result provides additional basis for speculation concerning the mechanism of response of the thermionic ionization with nitrogen mode.