The Quadrupole Mass Spectrometer

Molecular ion mass interferences are not as prevalent for the simpler matrices, as is clear from the mass spectrum obtained for the Pechiney 11630 A1 standard sample by electron-gas SNMSd (Figure 4). For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfiictory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. 

The quadrupole mass spectrometer has been found to be particularly suitable for EGA in thermal analysis. Published reports include descriptions of the various systems used [153—155] and applications in studies of the pyrolysis of polymers [155], minerals [156] and many inorganic solids [157—159]. 

In the early 1970s, the introduction of the quadrupole mass spectrometer changed the landscape of residue analysis in the coming decades dramatically. The combination of GLC with the mass spectrometer as a detector proved to become the major tool for residue analysis for the next 20 years. 

Figure 1. Fast-flow reaction apparatus. Ions or ion clusters are introduced into the flow tube from various sources and reactions proceed after they encounter the reactants added through a ring injector located at a selected position in the flow tube. The disappearance of the reactant ions and formation of products is monitored with the quadrupole mass spectrometer/electron multiplier shown. Taken with permission from ref. 19.

Figure 9. The quadrupole mass spectrometer signal for volatile species released from 0.90 nm palladium acetate film as a function of 2 MeV He+ ion dose. Mass 15 is shown for both CH3 and CH4 because of overlap at m/e 16 with oxygen. Mass 31 is shown for C2H6 (13C isotope) because of overlap at m/e 30 with major fragments of other parent ions.

As with all spectroscopic methods discussed previously, this method is best suited to measurement and elucidation of the characteristics of pure compounds. For this reason, MS is often used as a detector for gas chromatographs. The GC separates the mixture into pure compounds and the MS then analyzes each pure chemical as it exits the column. The most common MS for this application is the quadrupole mass spectrometer. For this reason, it is discussed in Chapters 14 and 15. 

We discussed the fundamentals of mass spectrometry in Chapter 10 and infrared spectrometry in Chapter 8. The quadrupole mass spectrometer and the Fourier transform infrared spectrometer have been adapted to and used with GC equipment as detectors with great success. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometry (GC-IR) are very powerful tools for qualitative analysis in GC because not only do they give retention time information, but, due to their inherent speed, they are also able to measure and record the mass spectrum or infrared (IR) spectrum of the individual sample components as they elute from the GC column. It is like taking a photograph of each component as it elutes. See Figure 12.14. Coupled with the computer banks of mass and IR spectra, a component s identity is an easy chore for such a detector. It seems the only real... 

Schultz and Branson (S2), Schultz and Wiech (S3), Hogan (H12, H13), Cho (C2, Method No. 2), and Cohen (C5) all used the quadrupole mass spectrometer. This involves passage of accelerated aerosol particles between four longitudinal electrodes in high vacuum. These electrodes have imposed... 

The main requirements to the quadrupole mass spectrometer in the combined experiment are the following ... 

Mass calibration of the quadrupole mass spectrometers, Q1 and Q3, is based on poly(propylene)glycol in methanol as supplied by the instrument manufacturer and following instructions on calibration and tuning provided in the instrument operating manual. 

Many applications dI the quadrupole mass spectrometer use u gas or liquid chromalograph to inlroduce the sample into Ihc ionizer. When the speelromeict is used in this manner, it is nuisl common In scan a w ide inttss range (50-1 OCX) amul al rates on the order of 1000 amu/sceoiid for compound identification For process analyses, it is mosi ennunon lo introduce the sample directly inio the ionizer and scan a shorter mass range. For both applications, computer systems are needed to collect die enormous amounts of data produced. 

Tests on copper SRMs show that a stable signal is obtained when each isotope is measured using the peak jumping mode and one point per peak, with a dwell time of 25,000 ps. The quadrupole mass spectrometer scans three times the mass range per replicate and accumulates 10 replicates for a total acquisition time of about 1 minute. For this application, 22 isotopes were selected (Table I). 

The quadrupole mass spectrometer which was used for these initial studies could provide only limited insight into the formation or structures of these unusual ions from fluorene and amino PAH. Experiments using substituted fluorenes, such as 1-, 2-, and 9-methylfluorene, 9-phenylfluorene, and carbazole, revealed that the (M + 14) ion did not form if the C-9 position was blocked. Knowing that the (M + 14) ion was formed by a reaction at the C-9 carbon, two possible structures could be drawn for this anion. One possibility would be 9-methylfluorene (structure I), which could arise from the addition of CH2 to fluorene. Similar formation of adducts from methane buffer gas under NICI conditions has been reported (7, 8). Alternatively, fluorene could lose the two hydrogens at the 9-position and add oxygen to form 9-fluorenone (structure II)... 

A 15-fold glass tube parallel packed-bed reactor was introduced [67, 68], which is similar to conventional catalyst testing equipment (Figure 3.39). The premixed reactant gas is supplied by a 16-port valve either to the bypass or to one of the 15 reactors. By a second 16-port valve, the product gas stream of a selected reactor can be channeled to the quadrupole mass spectrometer. The full automation of the screening set-up allows the investigation of 15 catalysts per day [67]. 

The experimental setup used by our group is shown in Figure 7-1. This apparatus consists of a molecular beam source coupled to a chamber housing the quadrupole mass spectrometer. The continuous beam source consists of a Campargue-type nozzle, an expansion chamber, and a collimation chamber. The nozzle assembly itself is mounted on a micrometer and is fitted with a gas handling line which... 

The skimmed and collimated cluster beam passes into the quadrupole mass spectrometer (Extrel, C-50), which has a mass range of 0-1200 amu with unit mass resolution. The mass spectrometer chamber is pumped by a turbomolecular pump (360Is-1). The pressure in the mass spectrometer chamber (P3), when the beam is in operation, is always less than 1 x 10 6 torr. This is necessary to ensure that the contributions from reactions of the cluster ions with the background gas are not significant. The distance of the nozzle from the ion source varies in the range 20.5-22.5 cm, depending on the nozzle to skimmer distance. 

The laser is used in two ways. In the first strike, it is defocused and its beam is used simply to clear adsorbed impurities, films, etc., from the area, a small portion of which (say, a micron in dimension) is to be the object of a pothole excavation. In the second strike, the laser is intensely focused after passing through a lens system, and the intensity of its strike vaporizes metal to form a hole. The size of the potholes is as little as 0.1 pm in diameter and about 1 pm in depth. A hard vacuum (10 10 to 10 y mm Hg) is necessary to give the quadrupole mass spectrometer the required sensitivity. A diagram of the technique is shown in Fig. 12.95. 

The operation of the quadrupole mass spectrometer is only briefly described here. Excellent descriptions are available in the literature including an introduction to quadrupole mass spectrometers [121,122], analysis of the mathematics associated with quadrupole [123], and extensive treatises [124,125]. 

Some researchers have reported instrumental modifications to reduce chemical matrix effects, including a three-aperture interface [103,181] and removal of the ion optics [182]. These modifications appear to reduce the total ion current, and therefore, space-charge effects, before ions enter the quadrupole mass spectrometer. Modification of ion optic lens voltages and configurations may also reduce space-charge-induced chemical matrix effects [183-186]. 

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