Electron ionization mass spectrometry: Quo vadis?

Mass spectrometry (MS) is a fundamental technique to identify compounds by their mass-to-charge ratio. It is known that MS can only detect target compounds when they are converted to ions in the gas phase. The ionization procedure is considered one of the most critical steps, and there are distinct techniques for it. One of them is electron ionization (EI), a widely used hard-ionization technique capable of generating several ions due to the excess energy employed. The existence of distinct ionization mechanisms turns EI capable of producing a fingerprint-like spectrum for each molecule. So, it is an essential technique for obtaining structural information. EI is often combined with chromatography to obtain a practical introduction of pretreated samples despite its excellent performance. EI–MS has been applied coupled with gas chromatography (GC) since the 1960s as both are very compatible. Currently, analytes of interest are more suitable for liquid chromatography (LC) analysis, so there are researchers dedicated to developing suitable interfaces for coupling LC and EI–MS. EI excels, as a reliable technique to fill the gap between GC and LC, possibly allowing them to coexist in a single instrument. In this work, the authors will present the fundamentals of EI–MS, emphasizing the development over the years, coupling with gas and LC, and future trends.     

Prospects for petroleum mass spectrometry and chromatography

Several recently developed analytical techniques, based on high-end mass spectrometry and chromatography, for dealing with challenges in petroleum characterization are reported. Folded flight path time-of-flight mass spectrometry provides resolving power up to 100000, enabling accurate mass measurement for molecular formula determination with high confidence. Atmospheric pressure chemical ionization (APCI) can be used in both gas chromatography (GC, as APGC) and liquid chromatography (LC) for analyzing non-polar hydrocarbons as well as polar compounds. The improvement in chromatography facilitates the mass spectrometric analysis through online coupling. Comprehensive two-dimensional gas chromatography (GC×GC) resolves overlapping components, rendering accurate identification and quantitation. Supercritical fluid extraction has been developed as an alternative method to replace traditional solvent extraction methods and eliminate the use of large volumes of solvents that can be harmful to health and environment. Supercritical fluid chromatography (SFC) has been developed as a convergence of GC and LC chromatographic techniques. The use of SFC for heavy oils and residua has been demonstrated. Prospective developments in the use of mass spectrometric and chromatographic methods for petroleum characterization are also described.     

Reversed-phase liquid chromatography coupled on-line with capillary gas chromatography use of an anion-exchange membrane to remove an ion-pair reagent from the eluent

Summary In order to enable the coupling of reversed-phase liquid chromatography (RPLC) with capillary gas chromatography (GC), the performance of an anion-exchange micromembrane device has been studied to remove the ion-pair reagent methanesulphonic acid from an acetonitrile/water LC eluent. The regenerant in the membrane was tetrabutylammonium hydroxide dissolved in acetonitrile/water, which effects an anion-exchange of methanesulphonate ions for regenerant hydroxide ions. The efficiency of the exchange process was found to be 99.9%. This enabled the direct introduction of the LC eluent, free of ions and with the proper acetonitrile/water ratio, into the GC. The applicability of the on-line LC-micromembrane-GC system has been illustrated for the potential drug eltoprazine, which is quantitatively recovered with a coefficient of variation for standard solutions of 3% at the 150 g/ml analyte level.     

Drying agents for water-free introduction of desorption solvent into a GC after on-line SPE of aqueous samples

Summary A small cartridge containing a drying agent is inserted between a solid phase extraction (SPE) column and a gas chromatograph (GC) to enable the introduction of water-free desorption solvent into the GC in on-line liquid chromatography (LC)-type enrichment of trace-level analytes from water samples. Some characteristics of the drying agents, such as their capacity to retain water and their re-usability after heating, have been tested. Possible interactions of the drying agent with the analytes, e.g., irreversible adsorption or catalyzed degradation, have been checked for a wide range of alkanes, alkylbenzenes chlorobenzenes, chlorophenols and phthalate esters. Using the on-line SPE-GC system with flame ionization detection (FID) and spiked samples containing different levels of the test compounds, the repeatability was shown to be satisfactory (6–17%). For 10 mL samples, the detection limits were lower than 0.1 g/L. The on-line SPE-GC-FID system here presented can be used for the repeated analysis of water samples without exchange of the dyring cartridge. The technique is applied to the analysis of tap water.     

Comprehensive two-dimensional chromatography in food analysis

Comprehensive two-dimensional (2D) chromatographic techniques can be considered innovative methods, only quite recently developed. Since their introduction to the chromatographic community, these techniques have been used in several fields and have gained an excellent reputation as valuable and powerful analytical tools. The revolutionary aspect of comprehensive multidimensional (MD) techniques, in respect to classical MD chromatography, is that the entire sample is subjected to the 2D advantage. The resulting unprecedented separating capacity makes these approaches prime choices when analysts are challenged with highly complex mixtures. Furthermore, in the case of automated systems, instrumental analysis times are roughly the same as in monodimensional applications. The present review reports various comprehensive chromatographic applications on different food matrices. The GC x GC section highlights two fundamental aspects for component separation/identification: the exceptional peak capacity and the formation of group types on the 2D space plane. The LC x LC section reports the employment in food analysis of a recently developed multidimensional normal-phase (NP)-reversed-phase (RP) high performance liquid chromatography (HPLC) system. Also reported are comprehensive LC x GC and packed column supercritical fluid chromatography (pSFC x pSFC) applications in this field.     

Data analysis programs for comprehensive two-dimensional chromatography

User-friendly and easy-to-use laboratory-written programs for visualisation and interpretation of comprehensive two-dimensional chromatographic data were developed. The programs that are not tied to any particular commercial instrument, and data obtained either by comprehensive two-dimensional liquid (LC × LC) or gas (GC × GC) chromatography can be analysed. Operations of the programs allow visualisation of 2D and 3D plots, comparison of two 2D plots at a time, as well as determination of retention times and peak heights and volumes.     

Derivatization of Drugs and Bioorganics for Improved Detection by Gas Chromatography and Photoionization Detection (GC-PID)

Abstract

Pentafluorophenyldimethylsilylchloride (flophemesyl chloride, FICI) has recently been used for derivatizations of alcohols, leading to significant improvements in detectability by gas chromatography (GC)-photoionization detection (PID) and electron capture detection (ECD). This off-line reaction has now been applied to a number of related alcohol derivatives, naturally occurring bioorganics such as cholesterol, androsterone, and similar sterols and steroids. Standard GC-ECD-PID analytical parameters have been determined, such as detection limits, calibration plots, ECD and PID relative response factors (RRF), ECD/PID relative response factor ratios, etc. for underivatized and derivatized substrates. Packed and capillary column GC conditions have been used with a 10.2 eV PID lamp. Application of the optimized derivatization-GC-ECD-PID method is reported for bioorganics in biological media. The overall results demonstrate the immediate applicability of such an approach for a number of alcohol derivatives, especially those having biological properties.     

Analysis of turkish lignite tar by coupled LC/GC,GC/MS,and capillary SFC

This work describes the analysis of a pyrolysis product of a lignite sample obtained from the Turkish Goynuk reserve. The aliphatic, aromatic and polar compounds present in the tar are separated and identified by various chromatographic techniques: Capillary gas chromatography/mass spectrometry (GC/MS), on-line high performance microbore liquid chromatography/capillary gas chromatography (LC/GC) and capillary supercritical fluid chromatography (SFC). The suitability of each technique for this particular application is discussed, and semi-quantitative results are presented for the major components detected.     

Development of novel fiber-packed needle interface for off-line reversed-phase liquid chromatography–capillary gas chromatography

For two-dimensional reversed-phase liquid chromatography–gas chromatography (2D RPLC–GC), a specially-designed needle packed with a polymer-coated fibrous stationary phase was introduced as a novel interface. The bundle of synthetic fibers coated with polydimethylsiloxane (PDMS) was packed into the head section of the needle, and served as the extraction medium. Using the post-column dilution of the LC eluent by water and subsequent extraction with the needle interface, the analyte was successfully concentrated to the PDMS phase on the fibrous support in the needle. The concentrated analytes were directly injected to GC system by inserting the needle to a heated GC injector. 2D separations of aliphatic and aromatic hydrocarbons, and also kerosene-extract were performed with the off-line RPLC–GC system interfaced by the needle extractor. The results suggested that the fiber-packed needle interface could be one of the simple and effective approaches to develop an on-line coupled LC–GC system.     

Preparation of Creatinine Standard Stock Solution

Abstract

The liquid chromatography isotope dilution mass spectrometry (LC/ID‐MS) has recently been used for the certification of organic reference materials. We are developing a new definitive method of LC‐ID/MS as to determination creatinine in serum. We prepared a stock standard solution of creatinine in 10 mmol/l of acetic acid at a concentration of 8.84 mmol/l. With this acetic acid concentration, creatinine dissolved completely in a few minutes. This stock standard solution was stable at least for 1 year and has widely applications (such as GC‐MS, LC‐MS, LC, colorimetric method, etc).     

Electrospray micro liquid chromatography – Fourier transform infrared micro spectrometry

Microcolumn liquid chromatography has been interfaced with infrared microspectrometry by means of a novel technique involving electrospray nebulization. The eluent from the LC column is subjected to a high positive potential relative to the surroundings and the electric field generated charges the surface of the emerging liquid causing it to disintegrate into charged droplets which are attracted to an earthed, infrared-transparent, zinc selenide deposition plate. Nitrogen sheath gas is used to evaporate the solvent in the droplets, and the separated components are deposited on the zinc selenide plate for subsequent analysis by means of a Fourier transform infrared microscope and spectrometer. Provided low flow rates (～4 μl/min) are used, total elimination of the solvent is achieved and easily identifiable infrared spectra can be measured from less than 500 pg of sample (caffeine).     

Determination of multiclass pesticides in food commodities by pressurized liquid extraction using GC–MS/MS and LC–MS/MS

Pressurized liquid extraction (PLE) was applied to the simultaneous extraction of a wide range of pesticides from food commodities. Extractions were performed by mixing 4 g of sample with 4 g of Hydromatrix and (after optimization) a mixture of ethyl acetate:acetone (3:1, v/v) as extraction solvent, a temperature of 100°C, a pressure of 1000 psi and a static extraction time of 5 min. After extraction, the more polar compounds were analyzed by liquid chromatography (LC), and the apolar and semipolar pesticides by gas chromatography (GC); in both cases LC and GC were coupled with mass spectrometry in tandem (MS/MS) mode. The overall method (including the PLE step) was validated in GC and LC according to the criteria of the SANCO Document of the European Commission. The average extraction recoveries (at two concentration levels) for most of the analytes were in the range 70–80%, with precision values usually lower than 15%. Limits of quantification (LOQ) were low enough to determine the pesticide residues at concentrations below or equal to the maximum residue levels (MRL) specified by legislation. In order to assess its applicability to the analysis of real samples, aliquots of 15 vegetable samples were processed using a conventional extraction method with dichloromethane, and the results obtained were compared with the proposed PLE method; differences lower than 0.01 mg kg⁻¹ were found.     

Persistent Organic Pollutants – Are Our Methods Sensitive and Selective Enough?

Due to the exponential growth of the world population and a strong worldwide economic development, humans are increasingly exposed to chemicals. This is particularly true for persistent, toxic chemicals that accumulate in organisms including humans. The presence of these persistent organic pollutants (POPs) has been reported since the 1960s. Analytical methods developed for the determination of POPs in wildlife, food, feed, sediments, dust, air, and humans have seen a tremendous improvement in sensitivity and selectivity. Detection limits of 0.1 mg/kg for pesticides in the 1960s have decreased down to < 0.1 ng/kg for dioxins presently. On the other hand, due to new toxicological insights, authorities nowadays often demand much lower detection limits than in the early days of discovery. This paper compares the lowest effectual levels of a number of POPs with the analytical capabilities of modern laboratories in terms of sensitivity and selectivity. Methods discussed are gas chromatography (GC), including multi-dimensional GC and liquid chromatography (LC), combined with electron capture detection (for GC) or with different mass spectrometric detectors. The conclusion is that the still improving sensitivity of current methods is sufficient to allow detection of POPs at a level clearly below the lowest observable adverse effect levels. Selectivity has also improved, but given the high complexity of some of the POPs and the multitude of POPs and other chemicals to which we are exposed, further developments in selectivity are still badly needed. To limit and focus the information for regulators, an effect-directed analysis is proposed as an alternative approach.     

Recent advances in microextraction by packed sorbent for bioanalysis

Microextraction by packed sorbent (MEPS) is a new format for solid-phase extraction (SPE) that has been miniaturized to work with sample volumes as small as 10 μL. The commercially available presentation of MEPS uses the same sorbents as conventional SPE columns and so is suitable for use with most existing methods by scaling the reagent and sample volumes. Unlike conventional SPE columns, the MEPS sorbent bed is integrated into a liquid handling syringe that allows for low void volume sample manipulations either manually or in combination with laboratory robotics. The key aspect of MEPS is that the solvent volume used for the elution of the analytes is of a suitable order of magnitude to be injected directly into GC or LC systems. This new technique is very promising because it is fast, simple and it requires very small volume of samples to produce comparable results to conventional SPE technique. Furthermore, this technique can be easily interfaced to LC/MS and GC/MS to provide a completely automated MEPS/LC/MS or MEPS/GC/MS system. This extraction technique (MEPS) could be of interest in clinical, forensic toxicology and environmental analysis areas. This review provides a short overview of recent applications of MEPS in clinical and pre-clinical studies for quantification of drugs and metabolites in blood, plasma and urine. The extraction of anti-cancer drugs, β-blockers drugs, local anaesthetics, neurotransmitters and antibiotics from biological samples using MEPS technique will be illustrated.     

Silicone rod and silicone tube sorptive extraction

This review focuses on the applications of silicone in the form of tubes or rods for sorptive extraction of organic compounds as sample preparation method in combination with various chromatographic techniques. Silicone rods (SRs) and silicone tubes (STs) have the advantage of being inexpensive, flexible and robust. SRs and STs with different sizes and volumes of silicone (8–635 μL) have so far been applied for the extraction/preconcentration of a large variety of organic micropollutants from different matrices. The theoretical principle of SR and ST extraction in comparison with similar microextraction techniques is presented as well as a summary of the published applications of SR and ST extraction in combination with gas chromatography (GC) or liquid chromatography (LC). Furthermore, the use of SRs and STs for time-integrated (passive) sampling is reported.     

Evaluation of the Plasma Chromatograph as a Qualitative Detector for Liquid Chromatography

Abstract

The applicability of the plasma chromatograph as a sensitive, qualitative detector for liquid chromatography (LC) is demonstrated. Only a fraction of an LC column effluent directly introduced is needed to produce a qualitative mobility spectrum. Most LC carrier fluids either exhibit no response in plasma chromatographic spectra or do not interfere with LC peak components. Using an indirect technique, components separated by TLC can be identified by plasma chromatography.     

High‐resolution GC/MS studies of a light crude oil fraction

The continuous development in analytical instrumentation has brought the newly developed Orbitrap‐based gas chromatography / mass spectrometry (GC/MS) instrument into the forefront for the analysis of complex mixtures such as crude oil. Traditional instrumentation usually requires a choice to be made between mass resolving power or an efficient chromatographic separation, which ideally enables the distinction of structural isomers that is not possible by mass spectrometry alone. Now, these features can be combined, thus enabling a deeper understanding of the constituents of volatile samples on a molecular level. Although electron ionization is the most popular ionization method employed in GC/MS analysis, the need for softer ionization methods has led to the utilization of atmospheric pressure ionization sources. The last arrival to this family is the atmospheric pressure photoionization (APPI), which was originally developed for liquid chromatography / mass spectrometry (LC/MS). With a newly developed commercial GC‐APPI interface, it is possible to extend the characterization of unknown compounds. Here, first results about the capabilities of the GC/MS instrument under high or low energy EI or APPI are reported on a volatile gas condensate. The use of different ionization energies helps matching the low abundant molecular ions to the structurally important fragment ions. A broad range of compounds from polar to medium polar were successfully detected and complementary information regarding the analyte was obtained.     

Analysis of polycyclic aromatic hydrocarbons (PAHs) in environmental samples: a critical review of gas chromatographic (GC) methods

Polycyclic aromatic hydrocarbons (PAHs) are frequently measured in the atmosphere for air quality assessment, in biological tissues for health-effects monitoring, in sediments and mollusks for environmental monitoring, and in foodstuffs for safety reasons. In contemporary analysis of these complex matrices, gas chromatography (GC), rather than liquid chromatography (LC), is often the preferred approach for separation, identification, and quantification of PAHs, largely because GC generally affords greater selectivity, resolution, and sensitivity than LC. This article reviews modern-day GC and state-of-the-art GC techniques used for the determination of PAHs in environmental samples. Standard test methods are discussed. GC separations of PAHs on a variety of capillary columns are examined, and the properties and uses of selected mass spectrometric (MS) techniques are presented. PAH literature on GC with MS techniques, including chemical ionization, ion-trap MS, time-of-flight MS (TOF-MS), and isotope-ratio mass spectrometry (IRMS), is reviewed. Enhancements to GC, for example large-volume injection, thermal desorption, fast GC, and coupling of GC to LC, are also discussed with regard to the determination of PAHs in an effort to demonstrate the vigor and robustness GC continues to achieve in the analytical sciences.     

Liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry: a highly sensitive method for the analysis of organic explosives

Gas chromatography/mass spectrometry (GC/MS) is applied to the analysis of volatile and thermally stable compounds, while liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (LC/APCI‐MS) and liquid chromatography/electrospray ionization mass spectrometry (LC/ESI‐MS) are preferred for the analysis of compounds with solution acid‐base chemistry. Because organic explosives are compounds with low polarity and some of them are thermally labile, they have not been very well analyzed by GC/MS, LC/APCI‐MS and LC/ESI‐MS. Herein, we demonstrate liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry (LC/NI‐APPI‐MS) as a novel and highly sensitive method for their analysis. Using LC/NI‐APPI‐MS, limits of quantification (LOQs) of nitroaromatics and nitramines down to the middle pg range have been achieved in full MS scan mode, which are approximately one order to two orders magnitude lower than those previously reported using GC/MS or LC/APCI‐MS. The calibration dynamic ranges achieved by LC/NI‐APPI‐MS are also wider than those using GC/MS and LC/APCI‐MS. The reproducibility of LC/NI‐APPI‐MS is also very reliable, with the intraday and interday variabilities by coefficient of variation (CV) of 0.2–3.4% and 0.6–1.9% for 2,4,6‐trinitrotoluene (2,4,6‐TNT). Copyright © 2008 John Wiley & Sons, Ltd.