Quadrupole, mass selective detector

Chromatographic systems were finally coupled with relatively inexpensive, yet powerful, detection systems with the advent of the quadrupole mass selective detector (MSD). The operational complexity of this type of instrumentation has significantly declined over the last 15 years, thus allowing routine laboratory use. These instruments... 

Gas chromatograph fitted with a thermionic nitrogen-specific detector Gas chromatograph fitted with a quadrupole mass-selective detector... 

The analytes separated on GC column are determined by a halogen-specific detector, such as an electrolytic conductivity detector (ELCD) or a microcoulo-metric detector. An ECD, FID, quadrupole mass selective detector, or ion trap detector (ITD) may also be used. A photoionization detector (PID) may also be used to determine unsaturated halogenated hydrocarbons such as chlorobenzene or trichloroethylene. Among the detectors, ELCD, PID, and ECD give a lower level of detection than FID or MS. The detector operating conditions for ELCD are listed below ... 

Ion Trap MS The evolution of ion trap mass spectrometry started also in 1953 with the same patent of Paul and Steinwedel that described the quadrupole mass selective detector. The applicability of a three-dimensional quadrupole to trap ions was recognized in the late 1950s, followed by several comparable observations using a circular two-dimensional (linear) ion trap. ... 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

In the past decade, as systems have become simpler to operate, mass spectrometry (MS) has become increasingly popular as a detector for GC. Of all detectors for GC, mass spectrometry, often termed mass selective detector (MSD) in bench-top systems, offers the most versatile combination of sensitivity and selectivity. The fundamentals of MS are discussed elsewhere in this text. Quadrupole (and ion trap, which is a variant of quadrupole) mass analyzers, with electron impact ionization are by far (over 95%) the most commonly used with GC. They offer the benefits of simplicity, small size, rapid scanning of the entire mass range and sensitivity that make an ideal detector for GC. 

Solutes were tentatively identified by atmospheric pressure ionization (API)-electrospray-mass selective detector (gas temperature 350°C, flow rate 101/min, nebulizer pressure 30 psi, quadrupole temperature 30°C, capillary voltage 3 500 V). 

The GC detector is the last major instrument component to discuss. The GC detector appears in Fig. 4.7 as the box to which the column outlet is connected. Evolution in GC detector technology has been as great as any other component of the gas chromatograph during the past 40 years. Among all GC detectors, the photoionization (PID), electrolytic conductivity (EICD), electron-capture (ECD), and mass selective detector (MSD) (or quadrupole mass filter) have been the most important to TEQA. The fact that an environmental contaminant can be measured in some cases down to concentration levels of parts per trillion (ppt) is a direct tribute to the success of these very sensitive GC detectors and to advances in electronic amplifier design. GC detectors manufactured during the packed column era were found to be compatible with WCOTs. In some cases, makeup gas must be introduced, such as for the ECD. Before we discuss these GC detectors and their importance to TEQA, let us list the most common commercially available GC detectors and then classify these detectors from several points of view. 

Because volumes have been written concerning mass spectrometry over the past 40 years, our approach here is to focus on the type of mass spec instrumentation required to perform EPA methods and those systems can be described as being of a low-resolution nature and the most affordable. The quadrupole mass filter or mass selective detector (MSD) and quadru-pole ion trap mass spectrometer (ITD) fit this criteria and will be the only mass specs discussed. Onuska and Karasek have given a good definition and description of the importance of gas chromatography-mass spectrometry (GC-MS) to TEQA (76) ... 

Fig. 39 d. Quadrupole Mass spectrometer or mass-selective detector 1) =... 

In current work the TA Instruments thermal analyst 220 was interfaced [46] to the Hewlett-Packard 5972 series mass selective detector (Figure 15) equipped with a hyperbolic quadrupole mass filter and vapor diffusion high-vacuum pump used in conjunction with a LaserJet 4 Plus printer. The TG analyzer s effluent tube was modified to terminate in a straight 1/4 in. OD glass tube. A 1/4 to 1/6 in. tube reducing union... 

Quadrupole GC/MS Instrument Agilent 7890 GC coupled to Agilent 5975C TAD mass selective detector (Agilent Technologies, Palo Alto, CA). GC is equipped with 30 m x 0.25 mm x 0.25 pm DB-5 column (J W, Folsom, CA), split/splitless injector, and Agilent 7860 autosampler. 

The main analytical instrumentation in the vehicle is a benchtop GC/MS by Hewlett Packard, HP 5890 GC/5970 MSB (mass selective detector). A basic configuration for analyzing liquid samples includes a heated injection port and a capillary fused-silica column interfaced directly to the MS via a heated transfer line. The MS is a quadrupole design operated under vacuum provided by a diffusion pump and backed by a mechanical rotary pump. System operation and data analysis are performed by a Pentium-level personal computer loaded with proprietary software and a NBS spectral library to aid with identification of unknowns. Depending on the mission, a simpler installation may consist of a GC equipped with an appropriate detector such as the electron capture detector (ECD). 

Downstream of the thermal processor is an in-line analytical system capable of cryogenic trapping, separation, and detection of thermal decomposition products. For the replacement fluids, the thermal decomposition products were trapped using liquid nitrogen coolant at the head of a capillary GC column housed within an HP 5890 GC, The GC was then used to separate the products, and detection was accomplished using an HP 5970B mass selective detector (MSD). The MSD is a compact quadrupole mass spectrometer which permits analytes to be identified via their fragmentation patterns and quantified via peak areas. 

This instrument is essentially a coupling of an HPLC with a mass selective detector and as such offers a significant enhancement in performance to the polymer chemist. Although such combinations have been around for many years, they were complex and expensive (i.e., effectively research tools only), and it is only within the last five years or so that single quadrupole benchtop LC-MS systems have become reasonably common place due to their relative ease of use and cost. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

Fig. 11.16. Representation of three tandem mass spectrometry (MS/MS) scan modes illustrated for a triple quadrupole instrument configuration. The top panel shows the attributes of the popular and prevalent product ion CID experiment. The first mass filter is held at a constant m/z value transmitting only ions of a single mlz value into the collision region. Conversion of a portion of translational energy into internal energy in the collision event results in excitation of the mass-selected ions, followed by unimolecular dissociation. The spectrum of product ions is recorded by scanning the second mass filter (commonly referred to as Q3 ). The center panel illustrates the precursor ion CID experiment. Ions of all mlz values are transmitted sequentially into the collision region as the first analyzer (Ql) is scanned. Only dissociation processes that generate product ions of a specific mlz ratio are transmitted by Q3 to the detector. The lower panel shows the constant neutral loss CID experiment. Both mass analyzers are scanned simultaneously, at the same rate, and at a constant mlz offset. The mlz offset is selected on the basis of known neutral elimination products (e.g., H20, NH3, CH3COOH, etc.) that may be particularly diagnostic of one or more compound classes that may be present in a sample mixture. The utility of the two compound class-specific scans (precursor ion and neutral loss) is illustrated in Fig. 11.17.

In a linear ion trap one of the most efficient ways to perform mass analysis is to eject ions radially. Hager [60] demonstrated that, by using fringe field effects, ions can also be mass-selectively ejected in the axial direction. There are several benefits for axial ejection (i) it does not require open slits in the quadrupole, (ii) the device can be operated either as a regular quadrupole or a LIT using one detector. A commercial hybrid mass spectrometer was developed based on a triple quadrupole platform where Q3 can be operated either in normal RF/DC mode or in the LIT ion trap mode (Fig. 1.24). 

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