Spectrometer, mass calibration

Appropriate maintenance of a mass spectrometer makes sense from a scientific viewpoint and also for financial reasons, since the mass spectrometer is by far the most expensive single apparams in the laboratory. The mass spectrometer and peripheral systems snch as GC, LC pump, autoinjectors etc. need to be weh maintained with the objective of keeping them at a performance level at or near to what it was on the first day of instaUa-tion. In addition, procedures need to be in place to ensure that the instrument performance does not deteriorate or drift over time. Any routine maintenance procedures that are suggested by the instrument manufacturer should be foUowed in addition to any routine maintenance procedures that are established at the laboratory based upon how the instruments are used, what types of samples are typically run and the number of users that have access to the instrument, along with a consideration for their level of training and expertise. 

Maintenance procedures should be distinguished as routine or nonroutine. In the event of a nonroutine operation, the record should indicate the nature of the failure, how and when it was discovered and any remedial action taken in response to the failure. All records must contain the date of maintenance and the initials of the person performing the procedure. A record of all routine and nonroutine maintenance should be maintained for each piece of equipment or system, e.g. LC-MS/MS with autoinjector, in the laboratory. An instrument maintenance logbook should be maintained for each mass spectrometer system and be readily available. In addition to any requirements in the laboratory s SOPs, the following information shonld be inclnded in the equipment logbook the equipment identification, model number, serial number, the date the equipment was put into service, current location, reference(s) to any relevant SOP(s) and contact information for the equipment manufacturer and/or service organization. 

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Insert the sample probe in the MALDI-TOF mass spectrometer. Calibration is performed in external mode with peptides covering the mass range of 500 Da to 5 kDa (see Note 15). 

One should note that some of the kinetic rate constants in all of these models are derived from Peeters and Mahnen mass spectrometric results therefore, it is not surprising that the theoretical fits to this data are rather good. It is reassuring that the model of Ref. 1 also exhibits overall good agreement with the following laser probe results that are free of mass spectrometer calibration estimates and flame perturbation. 

Calibration can be done every time you start a system up but for general leak detection, calibration is less important. However, whenever any general repair or replacement maintenance is performed, such as replacing the filament on a mass spectrometer, calibration becomes mandatory. See your owner s manual for specific instructions. 

The apparatus used for these experiments has been described previously (10). In a typical TPD experiment, 25 mg of sample were placed in a quartz microreactor which was mounted inside a furnace. Following evacuation for 1 h at room temperature, helium was flowed over the sample at a rate of 100 cc/min (STP) and the temperature was raised at 0.5 K/s. During heating, the desorption products were swept from the reactor by the helium stream and monitored downstream with a UTI Model 100 C quadrupole mass spectrometer. Upon completion of each TPD experiment, the mass spectrometer was calibrated for oxygen as described below, and then a TPR experiment was performed using a hydrogen flow rate of 200 cc/min (STP). After each TPR experiment, the mass spectrometer calibration was repeated. 

The stable carbon isotope ratios of dissolved inorganic carbon (DIC) and benthic foraminiferal calcite generally are determined with isotope ratio gas mass spectrometers calibrated via NBS 19 international standard to the VPDB (Vienna Pee Dee Belemnite) scale. All values are given in 8-notation versus VPDB with an overall precision of measurements including sample preparation usually better than +0.06 and +0.1%o for calcite and DIC carbon isotopes, respectively. Except one single-specimen based dataset (Hill et al. 2004), all stable isotope data from papers referred to in this overview are from species-specific multi-specimens analyses. The number of specimens used for a single analysis depended on size and weight of species but usually varied between 2 and 25. 

Both gas/solid adsorption and gas/liquid partition chromatography can be used for GC-MS, but GC is by far the most common. Because, in GC, the stationary phase is a liquid, usually a polymer, its vapor pressure will cause a continual low flow, or bleed into the ion source of the mass spectrometer. This bleed, which usually consists of decomposed stationary phase, will produce a spectrum whose intensity increases with column temperature. Stationary phases should therefore be of the high-boiling, low-bleed type. Most currently used stationary phases for routine GC-MS are based on alkyl-polysiloxanes or alkyl-phenyl-polysiloxanes that are chemically bonded to the column wall to increase stability. Columns containing such phases can, in some cases, be used at temperatures of up to 400°C. One advantage, however, to the presence of bleed peaks in the spectrum is that they enable a continual check to be made on the mass spectrometer calibration. For the alkyl siloxanes, ion peaks are present, in decreasing relative abundance, at miz 73, 207, 281, 355, 429,... 

Methods of Abundance Measurement. Mass spectrometry (Table 2) is the common form of analysis for lead isotope abundances. Samples suitable for mass spectrometer calibration are in Table 3. Neutron activation ( ° Pb/ ° Pb) and alpha particle activation ( ° Pb/ Pb and ° Pb/ ° Pb) show promise of getting some data on materials having very low lead contents. 

Ethylene-vinyl acetate copolymers were investigated by simultaneous thermal analysis and pyrolysis gas chromatography. Both devices were coupled to a mass spectrometer. Calibration with polyvinyl acetate led to sufficient accuracy in relation to the vinyl acetate content, because of the linear calibration plot. Quantitative determination of vinyl acetate was carried out at both 500 and 700 C. The calibration plot was linear provided that the mass of vinyl acetate did not exceed a critical value. 18 refs. 

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

Other techniques for mass measurement are available, but they are not as popular as those outlined above. These other methods include mass measurements on a standard substance to calibrate the instrument. The standard is then withdrawn, and the unknown is let into the instrument to obtain a new spectrum that is compared with that of the standard. It is assumed that there are no instrumental variations during this changeover. Generally, this technique is less reliable than when the standard and unknown are in the instrument together. Fourier-transform techniques are used with ion cyclotron mass spectrometers and give excellent mass accuracy at lower mass but not at higher. 

Before sample preparation, the laboratory must demonstrate that the mass spectrometer is operating satisfactorily. First, the instrument must be tuned by calibration using one of two compounds. 

This calibration procedure, which must be demonstrated at the start of each working period (or 12-hour shift) ensures that all samples are analyzed with respect to a known reference point of the mass spectrometer. 

A considerable amount of time is necessary to reach the point at which sample analyses can commence, and it is essential that the stability and reliability of the mass spectrometer be high to ensure maximum sample throughput during the limited time available between calibration checta. 

The U.S. Environmental Protection Agency publishes sets of Series Methods that describe procedures for detecting and estimating the quantity of environmentally hazardous substances. There are strict requirements for accuracy, reproducibility, and for calibration of mass spectrometers. 

Calibration and tuning of the mass spectrometer are achieved using either bromofluorobenzene (BFB) or decafluorotriphenylphosphine (DITPP). 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Under many experimental conditions, the mass spectrometer functions as a mass-sensitive detector, while in others, with LC-MS using electrospray ionization being a good example, it can behave as a concentration-sensitive detector. The reasons for this behaviour are beyond the scope of this present book (interested readers should consult the text by Cole [8]) but reinforce the need to ensure that adequate calibration and standardization procedures are incorporated into any quantitative methodology to ensure the validity of any results obtained. 

In many cases when methods involve internal or external standards, the solutions used to construct the calibration graph are made up in pure solvents and the signal intensities obtained will not reflect any interaction of the analyte and internal standard with the matrix found in unknown samples or the effect that the matrix may have on the performance of the mass spectrometer. One way of overcoming this is to make up the calibration standards in solutions thought to reflect the matrix in which the samples are found. The major limitation of this is that the composition of the matrix may well vary widely and there can be no guarantee that the matrix effects found in the sample to be determined are identical to those in the calibration standards. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

Accessibility to Cu sites was determined by temperature programmed desorption of NO (NO TPD), using an experimental setup similar to that used for TPR, except the detector was a quadrupole mass spectrometer (Balzers QMS421) calibrated on standard mixtures. The samples were first activated in air at 673 K, cooled to room temperature in air, and saturated with NO (NO/He 1/99, vol/vol). They were then flushed with He until no NO could be detected in the effluent, and TPD was started up to 873 K at a heating rate of 10 K/min with an helium flow of 50 cm min. The amount of NO held on the surface was determined from the peak area of the TPD curves. 

In a separate set of experiments designed to follow the gas phase reactions of CHj-radicals with NO, CHj- radicals were generated by the thermal decomposition of azomethane, CHjN NCHj, at 980 °C. The CH3- radicals were subsequently allowed to react with themselves and with NO in a Knudsen cell that has been described previously [12]. Analysis of intermediates and products was again done by mass spectrometry, using the VIEMS. Calibration of the mass spectrometer with respect to CH,- radicals was carried out by introducing the products of azomethane decomposition directly into the high vacuum region of the instrument. 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the mass spectrometer detector. The purity of all standards must be verified before preparation of the stock solutions. All standards should be refrigerated (2-10 °C) in clean amber-glass bottles with foil/Tefion-lined screw-caps. The absolute volume of the standard solutions may be varied at the discretion of the analyst, as long as the correct proportions of the solute and solvent are maintained. Calibrate the analytical balance before weighing any analytical standard material for this method. 

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