Evaluation of a pulsed glow discharge time-of-flight mass spectrometer as a detector for gas chromatography and the influence of the glow discharge source parameters on the information volume in chemical speciation analysis

The figures of merit of a pulsed glow discharge time-of-flight mass spectrometer (GD-TOFMS) as a detector for gas chromatography (GC) analysis were evaluated. The mass resolution for the GD-TOFMS was determined on FWHM in the high mass range (²⁰⁸Pb⁺) as high as 5,500. Precision of 400 subsequent analyses was calculated on ⁶³Cu⁺ to be better than 1% RSD with no significant drift over the time of the analysis. Isotope precision based on the ⁶³Cu⁺/⁶⁵Cu⁺ ratio over 400 analyses was 1.5% RSD. The limits of detection for gaseous analytes (toluene in methanol as solvent) were determined to be as low as several hundred ppb or several hundred pg absolute without using any pre-concentration technique. Furthermore, the different GD source parameters like capillary distance, cathode–anode spacing, and GD source pressure with regards to the accessible elemental, structural, and molecular information were evaluated. It was demonstrated that each of these parameters has severe influence on the ratio of elemental, structural, and parent molecular information in chemical speciation analysis.     

A glow discharge ion source with fourier transform ion cyclotron resonance mass spectrometric detection

A glow discharge (CD) ion source has been coupled to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer using a four-element electrostatic lens to accelerate and focus ions generated external to the instrument’s high magnetic field into its analyzer cell. Like other CD mass spectrometers, GD-FT-ICR can provide a quantitative measure of bulk analyte concentration with good precision and accuracy. Although detection limits currently attainable are several orders of magnitude higher than the commercially available magnetic sector-based instrument, CD-FT-ICR holds promise for ultrahigh resolving power elemental mass analysis. Several schemes are proposed to lower the detection limits of the technique while still providing high enough resolution to resolve isobaric interferences.     

Emission spectral characteristics of Cu,Ag, Zn,and Cd neutral atoms in a glow discharge

Detailed spectra highlighting the neutral atom emission characteristics (i.e. I lines) for Cu, Zn, Ag and Cd in a glow discharge device are presented in this study. A particular focus is the presentation of spectra that document the many high excitation energy neutral atom lines that are observed in these spectra. For Cu, several spectral lines originating from levels close to the ionization potential of copper are observed including lines from the so-called autoionizing levels which are actually just above the ionization potential for copper. Generally similar results are seen for Ag, Zn and Cd, including the observation of many high excitation energy neutral atom lines of Zn originating from the upper levels on the triplet side of the energy level diagram. The spectral data point to ion–electron recombination processes followed by stepwise de-excitation and radiative decay as a key mechanism in setting the spectral character of neutral atom emission in a glow discharge device. Unambiguous identification of spectral lines for specific transitions was facilitated by the acquisition of all spectral data utilizing a UV–visible Fourier transform spectrometer. This spectrometer provided complete and continuous coverage of the spectral region from 200 to 650 nm and allowed spectral lines to be identified with an accuracy of 1–2 pm.     

The spectral detectability of fluorine in a helium glow discharge

The theoretical basis of the excitation of the fluorine emission spectrum is discussed, and the negative effects of easily excited impurities are demonstrated. Fluorine can be detected by means of its emission spectrum if it is present as a minor impurity in helium. The amount of water vapor and/or organic compounds in the helium must be minimized because these substances produce easily excited fragments in the glow discharge which dissipate the energy needed to excite fluorine atoms. Direct excitation of a diphenyldifluorosilane residue permits detection of about 70 pg of fluorine; if fluorine is volatilized as hydrofluoric acid and injected directly into the glow discharge, a photocurrent peak of 1.8 Ag^-1 of fluorine is obtained.     

Characterization of a glow discharge ion source for the mass spectrometric analysis of organic compounds

A glow discharge ion source has been constructed for the mass spectrometric analysis of organic compounds. Characterization of the source has been made by studying the effect of pressure and discharge current on ionic distributions by anodic ion sampling along the discharge axis. Ion and electron densities and electronic temperatures have been calculated by using the single Langmuir probe technique to correlate the extraction efficiency with measured ion distributions and gain some insight into the ionization of organic molecules. The spectra obtained for several classes of organic compounds show that formation of parent-molecular ions by proton transfer, resulting partly from the background water molecules, is a major low energy process while charge transfer, Penning ionization, and electron ionization ace probably responsible for the fragmentation observed. The spectra result from the simultaneous occurrence of high and low energy reactions, and their structural information content is very high, yielding both molecular and extensive fragment ion information. The glow discharge ion source has proved to be essentially maintenance-free, easy to operate, stable, and can be used at reasonable mass resolution (up to 70001. The source also provides picogram range detection limits and has a linear response range of about six orders of magnitude, which makes it an interesting ion source for routine analysis. Preliminary work conducted with chromatographic interfaces indicates that its use can be easily extended to both gas and liquid chromatography.     

Laser-excited fluorescence of diatomic lead in a glow discharge source

Glow discharge sources provide simple and convenient means for laser-excited atomic spectroscopic studies of atoms sputtered from the cathode material. Presence of molecular species was observed in the course of investigation on this source as an atom cell for the laser-excited atomic fluorescence spectroscopic studies of lead. Spectral studies revealed characteristic band spectra, indicating the presence of molecular species in the discharge. The spectra were identified as belonging to the lead dimer species (diatomic lead molecule). It is, therefore, essential to examine the presence and possibility of interferences from molecular species while using glow discharge sources as atomization cells in atomic spectroscopic applications.     

Application of a glow discharge detector to the determination of trimethylsilyl-derivatized complex organic mixtures

A helium glow discharge is employed as a gas Chromatographie detector which is selective for silicon. Operated with spectral background compensation at 251.6 nm or 288.2 nm, the detector shows a lower limit of detection of 5 ng of silicon per injection. The detector is applied to the rapid screening of trimethylsilylated fractions of shale-derived liquids being prepared for biossay.     

Emission spectroscopic studies of Grimm-type glow discharge plasma with argon-helium gas mixtures

The effect of argon/helium pressure ratios on the emission intensity of various Ar II lines is investigated for a Grimm-type glow discharge radiation source, operated with Ar-He mixtures. The relative intensities of the Ar II lines are altered significantly by mixing helium with argon. It is found that the population of the Ar⁺ excited states can be redistributed through He-Ar collisional energy transfer. The energy level of the He singlet metastable state (¹S₀,20.62 eV) is very important for these processes. If the excitation energy of Ar II lines is higher than that of the He singlet metastable, strong quenching of the Ar II line intensity is observed. However, when the excitation energy is slightly lower, some of the Ar II lines are enhanced by adding helium to the argon plasma. Energy exchanges between the Ar⁺ doublet term states and the He singlet metastable are favoured because the total spin remains unchanged before and after the He-Ar collisions. Furthermore, the helium mixing also exerts a great influence on the emission intensities of the elements sputtered from the cathode of the discharge lamp. The enhancement of Al I and Al II emission intensities at suitable Ar-He mixture ratios is discussed for when aluminum is employed as a cathode material.     

Analytical performance of a radiofrequency-powered glow discharge excitation source associated with bias voltage modulation

A 13.56-MHz Grimm-type glow discharge plasma on which external d.c. voltages are superimposed is investigated for the application to optical emission spectrometry. With a voltage modulation technique associated with phase-sensitive detection, the emission intensities can be measured at very low noise levels, enabling the detection power to be improved. The experimental parameters: d.c. voltage, modulation frequency, and Ar pressure, are investigated for obtaining the optimum conditions. It is possible to obtain a detection limit (Cu I 327.40 nm) of 8 × 10^–4 wt.% for Cu in Fe-based alloys, while 6.6 × 10^–3 wt.% in the case of conventional detection. Received: 3 August 1998 / Revised: 21 September 1998 / Accepted: 24 September 1998     

Analytical performance of a radiofrequency-powered glow discharge excitation source associated with bias voltage modulation

A 13.56-MHz Grimm-type glow discharge plasma on which external d.c. voltages are superimposed is investigated for the application to optical emission spectrometry. With a voltage modulation technique associated with phase-sensitive detection, the emission intensities can be measured at very low noise levels, enabling the detection power to be improved. The experimental parameters: d.c. voltage, modulation frequency, and Ar pressure, are investigated for obtaining the optimum conditions. It is possible to obtain a detection limit (Cu I 327.40 nm) of 8 × 10^–4 wt.% for Cu in Fe-based alloys, while 6.6 × 10^–3 wt.% in the case of conventional detection. Received: 3 August 1998 / Revised: 21 September 1998 / Accepted: 24 September 1998     

The pulsed glow discharge as an elemental ion source

A pulsed glow discharge, rather than a conventional constant dc voltage discharge, is used as an ion source for a quadrupole mass spectrometer. Both sputter yield and ion signal are enhanced by using the pulsed system because of an increase in the voltage necessary to maintain a constant average current at the cathode over the pulse period. Irregularities are seen in the pulse spectrum that appear as rapid surges in the ion signal for both sputtered and contaminant gas species. These peaks appear at the beginning of the pulse for gaseous species but are limited to the postpulse period for sputtered species. Differences in the signal forms allow for the discrimination against selected types of ion signals by using narrow data collection gates placed over different portions of the pulse period.     

Study of a pulsed glow discharge ion source for time-of-flight mass spectrometry

This paper describes a new type of glow discharge (GD) ion source coupled to a time-of-flight mass spectrometer (TOFMS). The GD is operated in the microsecond pulse (μs-pulse) mode. The operational parameters of the μs-pulse GD were optimized against the ion signals, giving 180 Pa for the discharge pressure, 3 A for the transient discharge current, 1.75 kHz for the discharge frequency and 2 μs for the discharge pulse duration. Experimental results show that the discharge current in the μs-pulse mode can be one order of magnitude higher than that obtained in the d.c. mode. The structure of the interface between the μs-pulse GD and the mass spectrometer was found to be critical, and a Macor disc must be applied in front of the sampling orifice in order to shield the sampling plate from the anode of the GD to achieve both a good vacuum and the best sputtering. A transient sputtering rate of 24.4 μs s⁻¹ mm⁻² was reached in the μs-pulse mode and was significantly higher than that for the d.c.-GD. Typical mass spectra of brass and nickel samples were studied and are discussed. © 1997 Elsevier Science B.V.     

Chemical ionization mass spectrometry using a glow discharge ion source combined with a nebulizer sampling system

A glow discharge chemical ionization (CI) source equipped with a pneumatic nebulizer for sample introduction has been constructed. A comparative study of the discharge CI and conventional CI by electron impact from a hot filament is made for various polar compounds using oxygen-containing reagents such as water and methanol. The potential utility of the discharge ion source to liquid chromatography/mass spectrometry is also discussed.     

Development of a laser ablation-hollow cathode glow discharge emission source and the application to the analysis of steel samples.

A novel atomic emission spectrometry comprising laser ablation as a sampling source and hollow cathode plasma for the excitation of ablated sample atoms is proposed. In this arrangement, a conventional Grimm-type discharge lamp is employed, but the polarity of the power supply is reversed so that the cylindrical hollow tube acts as a cathode and the glow discharge plasma is produced within this tube. A laser is irradiated to introduce sample atoms into the discharge plasma. Ablated atoms are excited by collisions with electrons and gas species, and emit characteristic radiation upon de-excitation. The experiments were conducted only in an atmosphere of helium gas so as to avoid a rapid erosion of the cathode hollow tube. Phase-sensitive detection with a lock-in amplifier was utilized to reject the continuous background emission of the plasma gas and emissions of sputtered atoms from the tube material. The unique feature of this technique is that the sampling and excitation processes can be controlled independently. The proposed technique was employed for the determination of Cr, Mn, and Ni in low-alloyed steel samples. The obtained concentrations are in good agreement with the reported values. The relative standard deviation (RSD), a measure of the analytical precision, was estimated to be 2-9% for Cr, 3-4% for Mn, and 4-11% for Ni determination.     

The application of a double voltage modulation technique to a glow discharge source with a supplementary electrode

The Ar spectral lines are suppressed in glow discharge source atomic emission spectroscopy by a double voltage modulation technique with a supplementary electrode. The voltage is modulated between two levels, typically 350 and 700 V. At the lower voltage level mainly the Ar emission occurs where at the higher voltage level both the Ar and the metal atoms sputtered from the sample contribute to the emission. The power supply of the supplementary electrode is switched on when the glow discharge passes from operation at 700 V to 350 V. The intensity of argon gase lines is regulated by the current applied to the supplementary electrode at that period. Therefore, there is no need for electronical amplification and lower voltage adjustment of two operation modes. This modified DVM technique has been applied to the determination of Si and Cu in Al- samples. The suppression of Ar lines is possible, and the elements can be determined without Ar interferences.     

The application of a double voltage modulation technique to a glow discharge source with a supplementary electrode

The Ar spectral lines are suppressed in glow discharge source atomic emission spectroscopy by a double voltage modulation technique with a supplementary electrode. The voltage is modulated between two levels, typically 350 and 700 V. At the lower voltage level mainly the Ar emission occurs where at the higher voltage level both the Ar and the metal atoms sputtered from the sample contribute to the emission. The power supply of the supplementary electrode is switched on when the glow discharge passes from operation at 700 V to 350 V. The intensity of argon gase lines is regulated by the current applied to the supplementary electrode at that period. Therefore, there is no need for electronical amplification and lower voltage adjustment of two operation modes. This modified DVM technique has been applied to the determination of Si and Cu in Al- samples. The suppression of Ar lines is possible, and the elements can be determined without Ar interferences.     

Effect of driving frequency on the operation of a radiofrequency glow discharge emission source

Several characteristics of the r.f. glow discharge were examined to determine their dependence on driving frequency. The cathode potential exhibited a strong dependence on frequency, as would be expected, because of the capacitive nature of the discharge. As the frequency was dropped from 13 to 6 MHz there was a dramatic rise in the r.f. voltage of the discharge (with a conductive sample), attributed to a change in the mode of power coupling. The sputtering rates of a conductive sample were dramatically greater at the lower frequencies, in part due to higher energy of the sputtering ions. The emission characteristics of the source also changed as the frequency was varied from 6 to 13 MHz. At the higher operating frequencies, atomic emission peaked at a particular r.f. power level, whereas at lower frequencies the neutral-atom signal generally increased monotonically with power. The highest signal levels were found at 20 MHz, the highest frequency studied. Detection limits were determined for both conductors and insulators; in both cases they are detector-noise-limited because of the low throughput of the spectrometer. Detection limits for a conducting sample ranged from 0.1 μ g⁻¹ at 20 MHz to 20 μ g⁻¹ at 3 MHz. The emission from an insulating sample showed the same trends as those from a conducting sample but required higher r.f. power; the greatest signals were found at 6 and 13 MHz because not enough power was available from the r.f. amplifier to reach the optimum power for the 20 MHz discharge. Detection limits for elements in a Macor^® ceramic sample ranged from 30 to 110 μg g⁻¹.     

RF-pulsed glow discharge time-of-flight mass spectrometry for glass analysis: Investigation of the ion source design

Radiofrequency (RF) millisecond pulsed glow discharge (PGD) coupled to time-of-flight mass spectrometry (TOFMS) was investigated for direct elemental analysis of glass samples. Aiming at achieving highest elemental sensitivity, appropriate discrimination from polyatomics, and good crater shapes on glasses, a new Grimm-type GD chamber (termed from now “UNIOVI GD”, designed and constructed in our laboratory) was coupled to TOFMS, and the results compared with those obtained with the former GD design (here denominated as “GD.1”) of the initial RF-PGD-TOFMS prototype. The critical differences distinguishing the two GDs under scrutiny are the GD chamber thickness (15.5 mm for the GD.1 and 7 mm for the UNIOVI GD) and the “flow tube” which is inserted in the GD.1 and inexistent in UNIOVI GD.     

Radio-frequency glow discharge ion source for high resolution mass spectrometry

A radio-frequency powered glow discharge ion source has been developed for a double-focusing mass spectrometer. The sputtering and ionization of conducting, semiconducting and insulating materials have been realized using a 13.56 MHz generator to supply the discharge operating potential. The glow discharge ion source operates stably at argon pressures of 0.1–1 hPa and radio frequency powers of 10–50 W. The influence of discharge parameters and gas inlet system on sputtering rates and ion signal intensities for semi-insulating gallium arsenide wafers has been investigated.