Mass spectrometer contamination

A quantitative yield of a pure gas is usually necessary for the mass spectrometric measurement in order to prevent not only isotope fractionation during sample preparation, but also interference in the mass spectrometer. Contamination with gases having the same molecular masses and similar physical properties may be a serious problem. This is especially critical with CO2 and N2O, (Craig and Keeling 1963), and N2 and CO. When CO2 is used, interference by hydrocarbons and a CS + ion may also pose a problem. 

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

A gun is used to direct a beam of fast-moving atoms or ions onto the liquid target (matrix). Figure 4.1 shows details of the operation of an atom gun. An inert gas is normally used for bombardment because it does not produce unwanted secondary species in the primary beam and avoids contaminating the gun and mass spectrometer. Helium, argon, and xenon have been used commonly, but the higher mass atoms are preferred for maximum yield of secondary ions. 

Instead of the fast-atom beam, a primary ion-beam gun can be used in just the same way. Generally, such an ion gun emits a stream of cesium ions (Cs ), which are cheaper to use than xenon but still have large mass (atomic masses Cs, 139 Xe, 131). Although ion guns produce no fragment ions in the primary beam, they can contaminate the mass spectrometer by deposition with continued use. 

Eventually, multipliers become less sensitive and even fail because of surface contamination caused by the imperfect vacuum in the mass spectrometer and the impact of ions on the surfaces of the dynodes. 

For the naturally occurring elements, many new artificial isotopes have been made, and these are radioactive. Although these new isotopes can be measured in a mass spectrometer, this process could lead to unacceptable radioactive contamination of the instrument. This practical consideration needs to be considered carefully before using mass spectrometers for radioactive isotope analysis. 

In earlier procedures, the ReO anion was precipitated from water as the relatively insoluble potassium salt. Reduction of KReO with hydrogen gas gives rhenium metal, but the metal is contaminated with ca 0.4 wt % potassium that cannot be separated easily. Although suitable for some purposes, rhenium formed from KReO is found to be unsatisfactory in appHcations such as those for use in filaments in mass spectrometer systems. The route involving NH ReO avoids this problem. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

Because the vacuum in the mass spectrometer and the cleanliness of the ion source, transfer line, GC column, and so forth are not perfect, a mass spectrum will typically have several peaks that are due to background. All GC/MS spectra, if scanned to low enough mass values, will have peaks associated with air, water, and the carrier gas. Other ions that are observed in GC/MS are associated with column bleed and column contamination. 

It is crucial in quantitative GC to obtain a good separation of the components of interest. Although this is not critical when a mass spectrometer is used as the detector (because ions for identification can be mass selected), it is nevertheless good practice. If the GC effluent is split between the mass spectrometer and FID detector, either detector can be used for quantitation. Because the response for any individual compound will differ, it is necessary to obtain relative response factors for those compounds for which quantitation is needed. Care should be taken to prevent contamination of the sample with the reference standards. This is a major source of error in trace quantitative analysis. To prevent such contamination, a method blank should be run, following all steps in the method of preparation of a sample except the addition of the sample. To ensure that there is no contamination or carryover in the GC column or the ion source, the method blank should be run prior to each sample. 

Specifications for modem detectors in HPLC are given by Hanai [538] and comprise spectroscopic detectors (UV, F, FUR, Raman, RID, ICP, AAS, AES), electrochemical detectors (polarography, coulometry, (pulsed) amperometry, conductivity), mass spectromet-ric and other devices (FID, ECD, ELSD, ESR, NMR). None of these detectors meets all the requirement criteria of Table 4.40. The four most commonly used HPLC detectors are UV (80%), electrochemical, fluorescence and refractive index detectors. As these detectors are several orders of magnitude less sensitive than their GC counterparts, sensor contamination is not so severe, and... 

Magnetic sector mass spectrometers accelerate ions to more than 100 times the kinetic energy of ions analysed in quadrupole and ion trap mass spectrometers. The higher accelerating voltage contributes to the fact that ion source contamination is less likely to result in degraded sensitivity. This is particularly important for analysis that requires stable quantitative accuracy. 

Multiply charged proteins can also be partially sequenced, and microsequences of proteins isolated from several microorganisms have been reported, accomplished with electrospray ionization and FTMS.23,90 Nonadjacent fragment ions may be used to identify bacterial proteins in these top-down strategies.91 In all cases these sequences could be related by bioinformatics to the parent species. An obvious extension would be to characterize proteins from intact microorganisms in this way. In at least one instance a microsequence has been obtained from a protein released from a contaminated intact bacteriophage sample (MS2) to provide a chemotaxonomic identification.77 This work was carried out in an ion trap mass spectrometer. 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

Two-dimensional GC can be used to separate complex mixtures of polyaromatic compounds, and MS used to subsequently identify the compounds. In this method, the original sample is injected into a gas chromatograph with one type of column. As the components exit the first GC, they are fed into a second GC, with a different column, for further separation and finally into a mass spectrometer. In this way, compounds that coeluted from the first column are separated on the second. Focant et al. [19] were able to separate polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated dibenzofuran (PCDF), and coplanar polychlorinated biphenyl (cPCB) using this type of analytical procedure, including isotope dilution TOF-MS. These compounds are frequently found as contaminants in soils surrounding industrial settings thus, the ability to separate and identify them is extremely important [6,12,19],... 

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