Tim J. Jenkins is with Waters Corporation, Manchester, UK.

Tim J. Jenkins

Food safety is a key concern of governments, food producers, and manufacturers. Recent events involving compounds such as melamine (1) and dioxins (2) highlight the risks posed by poor agricultural practices, accidental contamination, and deliberate adulteration. Heightened vigilance goes hand-in-hand with more effective and efficient solutions for the comprehensive screening of food products for chemicals that might pose a risk and to improve the detection and identification of new or unexpected compounds of concern.  

Mass spectrometry (MS) is playing a significant role in residue testing. Tandem quadrupole MS-MS has become the accepted, and in some regulatory frameworks, the required technique, for quantitative analysis and confirmation of identity. The main limitation of tandem quadrupole MS-MS and of the targeted nature of so many of the experimental methods of analysis is the number of residues that can be included in a single experiment due to the nature of quadrupole-based instrument duty cycles. Targeting specific residues in an MS-MS experiment on a tandem quadrupole instrument does provide high selectivity and enables accurate quantification of compounds at trace levels even in complex food matrices. However, the nature of the experiment means that data is collected only for those compounds that have been targeted. There is no capability to look at and detect untargeted compounds, meaning potential contaminants can escape detection. Also, data cannot be reinterrogated historically postacquisition for the presence of additional compounds that might be relevant. This is one reason that analysts seek to target the maximum number of compounds with multiresidue methods that can target hundreds of compounds (3). 

There is now a clear requirement for very large multiresidue screens and the desire to capture "all of the data, all of the time" thereby generating as comprehensive a dataset as possible. This has become a major driver in the application of high-resolution MS, notably time of flight (TOF)-MS, to food safety residue and contaminant screening and discovery of new or emerging compounds of concern. Here we present a workflow for a generic TOF-MS screening scenario and illustrate it with an example of the targeted screening of animal products for veterinary drug residues. 

The workflow shown in Figure 1 describes a generic TOF-MS screening scenario, in which a nonselective sample preparation is used to generate an extract suitable for screening analysis. Such a nonselective extraction can maximize the number of residues that can be determined but results in a complex sample extract with many coextracted matrix components. This places extreme demands upon the ability to separate and detect each component. The quality of the final result will be directly related to the quality of the initial chromatographic separation. The better that matrix and residue components are separated from one another, the more effective detection is and the better the data obtained. The introduction of columns packed with sub-2-μm particles in 2004, and the development of a new generation of instruments capable of maximizing the potential of these sub-2-μm particle columns and design to perform under back pressures up to 15,000 psi means that separations are now routinely performed with much greater speed, resolution, and sensitivity (4). Resolution is a key factor in developing a screening method in complex samples. Food samples typically contain hundreds of components and sufficient separation of residues of interest from the matrix requires sufficient peak capacity during the sample analysis. While speed is important, it is meaningful only when sufficient separation resolution is obtained. For these reasons, ultrahigh-pressure liquid chromatography (UHPLC) is the preferred chromatographic technique for screening work.  

UHPLC coupled with quadrupole TOF-MS provides both high chromatographic peak capacity and comprehensive mass spectral datasets containing full spectrum, exact mass information. The work described here employs a quadrupole TOF-MS system that acquires spectra using a unique acquisition mode. 

The second step of our workflow (Figure 1) is to generate a comprehensive dataset with independent precursor (molecular ion) and product (fragment) ion information. This is achieved using the quadrupole TOF-MS system in MS

  acquisition mode. This data acquisition strategy allows the mass spectrometer to obtain two distinct but parallel data sets, one at low collision energy (CE) and the other at high CE. The resulting raw data file contains two distinct chromatograms; one contains data with the molecular ions while the other contains fragment ion information. This means that from a single UHPLC injection, the MS full scan, containing molecular ion data, and fragment ion information can be obtained simultaneously.  

Simultaneous acquisition of molecular ion and product ions improves the efficiency of the screening experiment because it enables fragment ions to be used during the targeting and identification of known compounds. MS

  also provides critical structural information that assists in the identification of unknown compounds. 

After the data have been acquired, data processing is then performed using POSI±IVE software (Waters Corporation, Milford, Massachsuetts). There are two basic steps in targeted residue screening (see Figure 1). The first is simply "presence or absence screening," that is, are the compounds of interest present? If compounds are present then the second step is "quantitation": at what level are the detected compounds present in the sample? To perform the presence–absence step, the acquired dataset is interrogated using a database containing target compound information. The database, which is edited and updated easily by the user, contains target compound names and associated parameters such as the exact mass of the targeted molecular ion, exact masses of product ions, and retention time. For each compound in the database, the software will construct a narrow mass range extracted ion chromatogram (see Figure 2) within target retention time window. It also will look for the presence of specified fragment ions within the same time window. For any molecular ion peak detected in the extracted ion chromatogram, the isotopic profile will be compared with the theoretical profile based upon the compound elemental composition and the result reported as an isotope fit or "i-Fit" value. To develop effective screens  (no false negatives, limited false positives) exact mass, isotopic patterns, product ions, and retention time filters are used. These are setup in the processing software and should be used to optimize method performance.  

The result of this presence–absence screen is a list of prospective positive compounds based upon the parameters described above. It also is possible to have the software assign compounds as "tentative" detections based upon how well the retention time, mass accuracy, fragment ions, and i-Fit conditions are met. The positive (and tentative, if desired) compounds are then automatically processed using the quantification part of the software workflow.  

A broad scope TOF-MS screen performed in this way requires minimal information in advance of data collection and allows the use of limited standards. This reduces labor costs by reducing handling of chemicals and time spent preparing and running extensive calibration standards. For some situations, calibration is a single point standard containing the most commonly detected target compounds at a concentration of regulatory relevance, in which case, the quantification can be regarded as semi quantitative. In this case, positive detections can then be quantified fully using a validated confirmatory method (such as MS-MS with a tandem quadrupole mass spectrometer using complete calibration curves). Alternatively, it also is feasible to run full calibration curves on the TOF-MS instrument and quantify directly. 

Veterinary Drugs in Products of Animal Origin

Residual levels of veterinary drugs, widely used to treat or prevent disease in animals, are sometimes present at trace levels in meat, fish, milk, eggs, or honey. The presence of drug residues in the food chain is of concern due to their potential allergenic effects or of the potential to cause bacterial resistance to similar antibiotics in humans. To protect consumer health and to ensure the high quality of animal products, regulatory authorities have set maximum residue limits (MRLs) for allowable drug residues in animal products (5–7). 

The application presented here describes the use of an ACQUITY UPLC system (Waters Corp.) coupled to a Xevo QTof quadrupole TOF-MS system (Waters Corp.) for the screening of more than 150 veterinary drug residues and metabolites including avermectins, benzimidazoles, β-agonists, β-lactams, corticosteroids, macrolides, nitroimidazoles, quinolones, sulfonamides, tetracyclines and some other veterinary medicinal products. The data were processed using the software described earlier. 

The sample preparation method for milk was reported previously (8). For liver, blood, fish, and meat samples, a 5-g sample was mixed with 20 mL acetonitrile and 5 g of anhydrous Na

. After centrifugation, 0.4 mL of DMSO was added to 4 mL of supernatant. Acetonitrile was then evaporated and samples were reconstituted to 0.8 g with water and ultracentrifuged before injection.  

The LC separation was performed on the system mentioned previously with an ACQUITY UPLC BEH C18 column (1.7 μm, 100 mm × 2.1 mm, Waters Corp.). The column temperature was set at 40 °C. Mobile phase A was 0.1% formic acid (aqueous), and mobile phase B was acetonitrile with 0.1% formic acid. The initial gradient was held at 95% A for 0.25 min, then stepped to 5% A at 6.00 min and held for 1 min before being stepped back to 95% A at 7.20 min then held for a 1.80 min equilibration. Total run time was 9.00 min, and the injection volume was 20 μL full loop. The flow rate was kept at 0.4 mL/min. 

The MS detection was performed on the MS system described earlier. The data acquisition mode was MS

. The ionization mode was positive electrospray (ESI

). The source temperature was set at 120 °C, and the desolvation temperature was set at 400 °C with desolvation gas flow set at 800 L/h. The capillary voltage was set at 2.4 kV. The cone voltage was set at 30 V. The MS collision energy was set at 6 V and the MS

  collision energy ramp 25–35V. Waters IntelliStart Software (Waters Corp.) for the quadrupole TOF-MS system automated the set-up of mass calibration using sodium formate and lock mass checks. 

The data were acquired and processed using Waters software. In MS

  acquisition mode, data is collected in two channels all of the time; low CE for molecular ion information; and high CE for product ions.  

The selectivity in complex sample matrices comes from the ability to reconstruct exact mass chromatograms with narrow mass windows and is the result of the high chromatographic resolution enabled by the LC system together with the high mass spectral resolution of the quadrupole TOF-MS system. An example of the total ion chromatogram (TIC) and corresponding exact mass chromatogram for danofloxacin in a cow liver extract is illustrated in Figure 2. 

High resolution is very useful for achieving selectivity and hence, confidence in results. However, in some cases, extra information, such as product ions, is of significant benefit to unambiguously identify compounds of interest. The added confidence in results is illustrated in Figure 3 by the case of xylazine and morantel, which share the same exact mass (

S) and have similar retention times, 2.44 and 2.52 min, respectively. Xylazine and morantel share one product ion (

 164.0536) but can have their identity assigned with the unique product ions (

 90.0366 and 150.0381, respectively). Even very high mass spectral resolution would be unable to distinguish between these compounds so additional structural information must be used to unambiguously assign an identity to the residue of interest.  

Product ion information can be obtained by performing in-source fragmentation, MS-MS, or MS

. In-source fragmentation suffers from reduced sensitivity compared to other approaches and MS-MS is a data-dependent technique that suffers from reduced duty cycle and its use requires prior knowledge of the sample contents. MS

, as described previously, is a data-independent acquisition technique that provides a simple, unbiased, parallel route to delivering both exact mass molecular ion and product ion information on every detectable component without the need for multiple injections. 

Figure 4 illustrates the differences in the spectra obtained by low CE and high CE in MS

  mode. The low-CE spectrum is dominated by the molecular ion while the high-CE spectrum contains product ions that add confidence to the assignment of identity.  

The samples were analyzed with a total cycle time of 9 min, including equilibration time, for a sample throughput of more than 100 samples per day. However, this level of sample throughput, together with even just 150 residues per sample, will yield significant bottlenecks in data processing. 

The software used has been developed specifically to reduce the data processing burden for review of TOF-MS screening data (9) by ensuring that only positive (exact mass and retention time within definable tolerances) and tentative (retention time OK with flagging indicating exact mass out of tolerance) detections are quantified automatically. A target compound list containing compound name, formula and retention time is all that is needed. This list can be very large, enabling very many compounds to be screened. 

During automated processing, the software performs a qualitative search to generate presence–absence results for the compounds in the target list, using mass accuracy and retention time to determine if compounds are positively, tentatively or negatively detected. All positive and tentative detections are then automatically quantified and listed within an application manager browser report with a measure of their isotopic fit (iFit). Figure 5 illustrates the screening results for a cow blood extract where >150 veterinary drugs have been reduced qualitatively and quantitatively to just three: hydrocortisone, sulfadoxine, and trimethoprim. The displayed sample shows the positive identification and quantification of trimethoprim in a blood extract.  

Figure 6 illustrates part of the report generated from a cow blood extract where 150 residues have been reduced to just three that were positively identified by retention time, exact mass, and iFIT.  

A generic workflow for residue and contaminant screening has been described using the combination of UHPLC and exact mass TOF-MS with MS

  acquisition to generate a comprehensive record of precursor and fragment ion data. Dedicated software is then used to provide automated presence–absence screening and quantification of detected compounds.  

The workflow was demonstrated using an LC system and a quadrupole TOF-MS system for the rapid screening of more than 150 veterinary drug residues at the relevant reporting limits in products of animal origin. MS

  acquisition provides additional confidence when assigning identity to the residue of interest and overcomes the limitations of conventional data-dependent approaches. 

The information-rich nature of TOF-MS data places increasing demands upon data processing software, with the reduction in manual processing and automation of repetitive tasks key to improving the quality of results and accessibility to TOF-MS. 

The software significantly reduces the bottleneck of data processing for reviewing TOF-MS screening data, by ensuring that only the positive and tentative detections are quantified automatically. The automated nature of processing also reduces the possibility of errors by removing manual transcription steps from the workflow. 

Tim J. Jenkins, Keith Worrall, Peter Hancock,

 are with Waters Corporation, Manchester, UK. 

 are with the Food Authority Control of Geneva, Geneva, Switzerland.

Articles

Mass spectrometry plays an increasingly significant role in the analysis of residues and contaminants in food. Here we will illustrate how the combination of ultrahigh-pressure liquid chromatography (UHPLC) and high-resolution time-of-flight-mass spectrometry (TOF-MS) is used to generate a screen of veterinary drug residues in products of animal origin. The use of UHPLC–TOF-MS and dedicated, workflow directed software allows rapid screening for large numbers of residues and automated quantification of positive samples. In addition, we illustrate how the data generated using MSE acquisition mode enable critical structural information to be collected, which offers additional selectivity and confirmatory data for compound identification and facilitates elucidation of the structure of newly discovered compounds.

Advances in TOF-MS-Based Screening for Food Safety Residue Analysis with a Positive Approach

Gas chromatography–mass spectrometry (GC–MS) and liquid chromatography (LC)–MS are widespread successful approaches, based on single-quadrupole MS, for the routine detection, identification, and quantitation of compounds. There has, however, been increasing interest in the use of tandem MS in more challenging, complex matrices such as those commonly found in food, environmental, and biological analyses. The combination of GC with tandem-quadrupole MS (MS-MS) is discussed, where the inherent increase in selectivity and sensitivity of the approach has enabled rapid, confident compound detection and quantitation for such demanding applications.

Over the last decade, tandem mass spectrometry (MS-MS) techniques have become firmly established in laboratories supporting a wide range of activities, including the life sciences, pharmaceutical, clinical, food and beverage, environmental, petrochemical, and chemical materials industries. Both liquid chromatography (LC) and gas chromatography (GC) have been used successfully as inlets for MS-MS systems, and the technique has become the dominant workhorse for quantitative LC–MS-MS in many laboratories. When used in combination, LC- and GC-based MS-MS are complementary and enable analysts to detect the widest possible range of compounds in a sample. This article presents GC coupled to tandem-quadrupole MS (GC–MS-MS), describing both the core technology and experimental modes of operation. Several applications will then be discussed, demonstrating the ability of GC–MS-MS to provide effective solutions for the analytical challenges faced by many laboratories.  

MS-MS techniques require the use of two mass analyzers in series or a single analyzer that can be used sequentially, such as an ion trap, to analyze precursor and product ions (1). The mechanism of MS-MS involves mass selection of a precursor ion in the first analyzer, fragmentation of the precursor (typically using collisionally induced dissociation [CID]), and then mass analysis of the resulting product ions in the second analyzer. Operated in this way, MS-MS can provide useful structural information by establishing relationships between precursor ions and their fragmentation products, and it can achieve better selectivity and sensitivity for quantitative analysis.  

Various combinations of analyzers have been investigated and many commercialized, including the use of magnetic sector, quadrupole, time-of-flight, and trap technologies (2,3). Originally, such instruments often were found only in specialist MS groups with high levels of expertise and substantial funding. However, developments in technology and demand from the market have resulted in vendors being able to offer lower cost, high-performance, benchtop MS-MS instruments. This newer generation of instruments has made MS-MS techniques accessible to many more laboratories, including those undertaking routine work, and has ensured the widespread adoption of the technology.  

One of the most popular types of MS-MS instrument has been the triple-quadrupole mass spectrometer, first developed by Yost and Enke (4). The introduction of the first commercial triple-stage quadrupoles in the early 1980s greatly helped to popularize MS-MS, and today tandem-quadrupole MS-MS is one of the most widely used tandem techniques as a result of its lower relative cost, ease of use, and ability to carry out a wide range of MS-MS experiments. 

A schematic diagram of a GC–MS-MS tandem-quadrupole instrument is shown in Figure 1. The diagram shows a heated GC interface bringing eluent from the GC column into the MS source. Ionization is achieved under electron ionization (EI) or chemical ionization (CI) conditions and the EI and CI inner sources are interchanged readily without the need for venting the full system. The mass analyzer consists of two quadrupoles (labeled here as MS1 and MS2) separated by a hexapole collision cell. Tandem quadrupoles use two stages of mass analysis, the first to preselect an ion (the precursor ion) and the second to analyze fragments (product ions) induced by collision with an inert gas, usually argon, in the collision cell. The collision cell shown here is a linear hexapole, but other designs exist such as 90° and 180° curved cells. An off-axis photomultiplier detector is utilized in this schematic, but electron multipliers also are employed.  

Tandem-quadrupole MS-MS offers great flexibility in the type of experiments that can be carried out on ions generated in the source. Figure 2 shows the schematic of the instrument ion optics and can be used to explain the different operating modes.  

The first experiments we consider are the MS operating modes. In MS1 mode, the first quadrupole (MS1) is used as the mass filter with the collision cell and second quadrupole operated in transmission-only mode. (This is achieved by turning off the collision gas in the cell and using MS2 in what is known as Rf-only mode. Under these conditions, ions exiting MS1 will be transmitted directly through the cell and MS2 to the detector.) This mode provides the highest transmission and is directly analogous to using a single quadrupole mass spectrometer. It is possible to use the MS1 mode for acquiring both full scan spectra (by using MS1 in scanning mode), and selected ion monitoring (SIM) data (by using MS1 in static mode).  

The ability to generate full scan spectra and SIM data is a useful feature of tandem-quadrupole MS, but the real benefits of the technique are realized fully only when the MS-MS operating modes are employed.  

Table I: Summary of MS-MS operating modes			

There are four common MS-MS operating modes, which are summarized in Table I and Figure 3. Product ion mode (Figure 3a) is the most commonly used MS-MS scanning mode and typically is used for structural elucidation and in method development for multiple reaction monitoring (MRM)-based studies, which are used for demanding quantitative applications. When developing MRM methods, product ion scanning experiments are used to identify the product ions for use in MRM transitions and to optimize CID tuning conditions to maximize the yield of a specific product ion to be used in MRM analysis, thereby improving the quantitative performance.  

Figure 3b shows the precursor ion-scanning mode. This technique will enable all precursor ions that give rise to a specific product ion to be recorded. Typical applications, therefore, often involve the detection of compounds having closely related structures, which give rise to a common product ion. The benefit of the MS-MS experiment is that it filters out chemical noise and potentially interfering ions, greatly improving the sensitivity of detection in complex sample matrices and simplifying structural confirmation and elucidation. 

The multiple reaction monitoring (MRM) mode (Figure 3c) is a highly selective MS-MS experiment and is the most commonly used mode in quantitative applications. Because both MS1 and MS2 are static, greater dwell time on the ions of interest is allowed; therefore, better sensitivity is achieved compared to scanning MS-MS. MRM mode provides very high selectivity if appropriate transitions are selected, and in combination with high sensitivity, this is an ideal technique for trace-level quantitation of target compounds in highly complex or dirty matrices.  

The final MS-MS mode is constant neutral loss (or gain) (CNL), which is shown in Figure 3d. CNL detects the loss (or gain) of a specific neutral fragment or functional group from an unspecified precursor or precursors. Applications would include screening mixtures for a specific class of compound that is characterized by a common fragmentation pathway. The scans of MS1 and MS2 are synchronized. When MS1 transmits a specific parent ion, MS2 "looks" to see if that parent loses a fragment of a certain mass. If it does, it registers at the detector. The result is that spectrum shows the masses of all parents that actually lost a fragment of a certain mass. 

Tandem quadrupole–based GC–MS-MS provides the widest possible range of MS-MS experimental modes, making it a powerful tool for both structural elucidation and targeted quantitative analysis. In practice, the most popular use of the technique, and that which has contributed most to its growth, has been the use of MRM mode for low-level quantitation of compounds in complex matrices. The following sections will focus on some of the specific application areas of significance and will demonstrate the benefits of the technology over conventional approaches.  

International trade in food products has increased rapidly in recent years (5), with many nations worldwide placing strict legislative demands on the quality of imported foods. Exporters are thus required to check the quality of products before they are shipped or risk rejection and even destruction of the shipment at considerable cost to the producer. The net result has been an increase in monitoring, much of which is for chemical residues at trace levels, requiring MS techniques. Two very extensively legislated, and therefore, commonly monitored, groups of residues are veterinary drugs and pesticides. 

There are over 800 pesticides registered for use worldwide and there is an ever-increasing requirement to monitor levels of these compounds in foodstuffs. Given the wide range of pesticides and product types to be analyzed, the analytical challenge is to maximize the number of residues that can be determined, minimize the variety of methods required, keep run times short, and achieve limits of detection at or below the legislated reporting levels. For these reasons, multiresidue methods in which generic sample preparation procedures are employed have become very popular. Inherent to this approach is that cleanup of extracts is only possible to a very limited extent. When complex matrices such as baby food, spices, or tobacco are analyzed, enhanced selectivity in detection is required to make up for the low selectivity in sample preparation.  

Traditionally, much pesticide analysis has been carried out by GC–MS using single quadrupole detection and SIM experiments. This can, however, prove limiting when more complex matrices are encountered. Figure 4 illustrates this by comparing the SIM and MRM data of the pesticide malathion in several matrices of varying complexity. Selecting appropriate conditions for an MRM involves choosing an appropriate characteristic precursor ion and optimizing the CID conditions to give suitable fragmentation and selection of product ions. This process is illustrated in Figure 5. Here we see the EI spectrum of methyl-pirimiphos and a product ion spectrum resulting from fragmentation of the 

 305 molecular ion. Many applications require more than one transition to be used in order to confirm the presence of a target analyte, and so two MRM transitions are shown in Figure 5, enabling both quantitative and confirmatory analysis to be carried out in a single injection.  

As discussed previously, the limited sample cleanup required by multiresidue methods can result in very complex sample matrices such as those shown in Figure 6a. The selectivity of GC–MS-MS in overcoming matrix interferences can enable over 100 residues to be determined in such matrices (Figure 6b). The ability to quantify and confirm pesticides, sometimes at levels below 0.01 mg/kg, is extremely challenging in such applications, but the sensitivity and selectivity of MRM mode can meet this challenge effectively.  

A wide range of drugs, including hormones and antibiotics, are used in farm animals for therapeutic purposes but also by some farmers to alter the characteristics of the meat by improving growth performances. European countries have implemented screening programs to reduce the abuse of such drugs and to ensure that meat sold for human consumption is free from residues. However, concentration levels are ever-decreasing, and so traditional methods of detection will need to be improved to maintain efficient control of such substances within the food chain. Similar compounds are also used illegally to enhance performance in sport, and the analytical strategies adopted by the World Anti-Doping Agency (WADA) laboratories closely parallel those described here for food safety.  

Steroid growth promoters often are analyzed by GC–MS with single quadrupoles, using selected ion monitoring (SIM). Results must satisfy the current EU legislation on confirmation criteria, Commission Decision 2002/657/EC, which stipulates that four structure-related ions for each analyte with the correct ion ratio must be monitored when using SIM mode. When the analysis requires the unambiguous identification of these compounds at trace concentrations from highly complex matrices, difficulties can be encountered. These are often interferences from co-eluting compounds, which appear on the SIR trace, making accurate quantification prone to error (Figure 7).  

Selectivity can be gained through the use of the MRM technique. When MRM is used, legislation stipulates that monitoring of two structure-related transitions for each analyte suffices for confirmation of identity. Figure 8 clearly illustrates the advantage of MRM because the analyte at the same concentration and in the same extract is resolved from all matrix interferences and can be quantified easily and confirmed with improved signal-to-noise ratio (S/N).  

Environmental Application: Synthetic Pyrethroids in Wastewater

Synthetic pyrethroids were introduced as a less hazardous and less persistent alternative to organochlorine and organophosphate insecticides on the basis of their low acute toxicity to mammals. Although pyrethroids have a low acute toxicity to mammals, they are extremely toxic to aquatic organisms, including fish such as the bluegill and lake trout, with LC50 values less than 1 ppb (6). Two of the synthetic pyrethroids, cyfluthrin and permethrin, are listed in the Dangerous Substances Directive (76/464/EEC), which requires the reduction or elimination of discharges of certain substances to the environment. Under the directive, the environmental quality standard (EQS) for cyfluthrin is 0.001 μg/L and for permethrin is 0.01 μg/L.  

Analytical methodologies for the low-level determination of pyrethroids in water generally require significant sample preconcentration. This also can result in concentration of matrix components, which can adversely affect quantitation of the target compounds. The use of sensitive MRM-based methods has proved highly successful, and good sensitivity can be achieved in EI mode. However, pyrethroids ionize well in negative CI mode (NCI), and the selectivity of NCI is greater than EI for the pyrethroids because fewer compounds are ionized in this manner. Therefore, the matrix interference/background noise is reduced greatly, which allows very low LODs to be achieved (Figure 9). A further example of another pyrethroid, deltamethrin, determined in a real wastewater matrix at 0.007 mg/L, is shown in Figure 10.  

Two MRM transitions were chosen so that quantification and confirmation could be performed in a single run. The confirmation criteria were based upon the measured ion ratio of the transitions with a tolerance of ±10% set. The expected ion ratio was calculated from the mean of the 10 calibration standards. For deltamethrin, the expected ion ratio was 0.987. The presence of deltamethrin was considered confirmed if the observed ion ratio from any extract did not deviate by more than 10% from this expected value. The ion ratio between the quantification transition (4546) and the confirmation transition (4412) for deltamethrin in Figure 10 is 0.971. The confirmation criteria have been passed with a percentage difference of 1.6% between the expected and measured ratios. This is significantly less than the acceptable limits of ±10%.  

In summary, GC–MS-MS in NCI mode using MRMs provides high selectivity to reduce any matrix interferences, high sensitivity to reach the low reporting levels required, and quantitative and confirmatory data in a single injection even for potentially dirty samples such as wastewater. 

Geochemical Application: Biomarkers in Oils 

Biomarkers are natural products that can be traced to a particular biological origin and have a wide variety of applications, for example in petroleum exploration. Specifically, biomarkers in oil can reveal parameters such as the age of the source rock and the environment of deposition (such as marine or terrigenous). One of the main classes of biomarkers is the tetracyclic steranes, which originate from naturally occurring sterols in animals and plants that undergo diagenesis in sediments to form sterenes and, ultimately, steranes.  

GC–MS is the principal method used to detect and identify sterane isomers and other biological markers. Where the concentration of biomarkers of interest is relatively high, analysis of characteristic fragment ions in single ion monitoring (SIM) mode (for example, using a single-quadrupole instrument) provides useful results. 

Tim J. Jenkins

Articles

Advances in TOF-MS-Based Screening for Food Safety Residue Analysis with a Positive Approach

Improving on GC–MS Performance for Demanding Applications: The Use of GC Coupled to Tandem-Quadrupole MS