Mass reference compound

Any mass spectrometer requires mass calibration before use. However, the procedures to perform it properly and the number of calibration points needed may largely differ between different types of mass analyzers. Typically, several peaks of well-known m/z values evenly distributed over the mass range of interest are necessary. These are supplied from a well-known mass calibration compound or mass reference compound. Calibration is then performed by recording a mass spectrum of the calibration compound and subsequent correlation of experimental m/z values to the mass reference list. Usually, this conversion of the mass reference list to a calibration is accomplished by the mass spectrometer s data system. Thereby, the mass spectrum is recalibrated by interpolation of the m/z scale between the assigned calibration peaks to obtain the best match. The mass calibration obtained may then be stored in a calibration file and used for future measurements without the presence of a calibration compound. This procedure is termed external mass calibration. 

Many manufacturers now offer other sample injection systems compatible with the vacuum lock used for the solids probe. These include small (e.g., 75-ml) heatable batch inlet systems, usually accessible via syringe (gas syringe or GC microliter syringe for liquids), which can be particularly useful as inlets for mass reference compounds. Other probes are designed as flexible, easily removed connections to a gas chromatograph via some form of interface. 

Derrick and coworkers have investigated polyglycols (PEG, polybutadiene,polystyrene, and poly(methylmethacrylate) by FD-MS. Prokai and Scrivens et al. have looked at various phenol-formaldehyde (novalak and resole) resins. Wile and Cook obtained FD spectra of lactone, lactam, and carbonate polymers." Matsuo et al. investigated polystyrene and poly(propylene glycol) as high mass reference compounds." Faffimer and Schulten studied several hydrocarbon polymers (polybutadiene, polyiso-prene, polyethylene, polystyrene)." Evans et al. used FD-MS to investigate mechanistic aspects of fhe mefal-cafalyzed polymerization of ethylene and other olefins. 

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

In addition, synthesis of reference compounds and a comparison of HPLC retention characteristics and mass spectral data is often reqnired. 

FTICR-MS is capable of powerful mixture analysis, due to its high mass range and ultrahigh mass resolving power. However, in many cases it is still desirable to couple a chromatographic interface to the mass spectrometer for sample purification, preconcentration, and mixture separation. In the example given above, DTMS under HRMS conditions provides the elementary composition. Apart from DTMS, PyGC-MS can be performed to preseparate the mixture of molecules and to obtain the MS spectrum of a purified unknown. Direct comparison with the pure reference compound remains the best approach to obtain final proof. 

A further common problem the analyst faces when integrating SPC into analytical procedures for the determination of LAS is the scarcity of available reference compounds, thereby complicating their determination. Therefore, the identification of the analytes has to be performed by comparison of retention time and absolute peak area ratio between the deprotonated molecular ion and the fragment ion, relative to the ratio obtained from the authentic standard ( 20%). Retention times of SPC, for which no standards were available, can be determined once by mass spectrometric identification in full-scan mode. 

The elution volume of a solute is determined mainly by its relative molecular mass and it has been shown that the elution volume is approximately a linear function of the logarithm of the relative molecular mass. It is possible to determine the relative molecular mass of a test molecule using a calibration curve prepared from the elution volumes of several reference substances of known relative molecular mass. This should be done using the same column and conditions (Figure 3.37) and in practice it may be possible to calibrate the column and separate the test substance at the same time by incorporating the reference compounds in the sample. Such a method is rapid and inexpensive and does not demand a highly purified sample, provided that there is a specific method for detecting the molecule in the eluate. 

In a subsequent study, Schnitzer and Spiteller [15] hydrolyzed each fraction with 2 M H2S04. After neutralization of the soluble materials, the latter were reduced with NaBH4 and then acetylated. The resulting acetates were analyzed by capillary gas chromatography/mass spectrometry, and identified by comparing their mass spectra with those of reference compounds of known structures and with literature data. Eighteen N-heterocyclics were identified. These compounds induded hydroxy-and oxy-indoles, quinolines, isoquinolines, aminobenzofurans, piperidines, pyrro-lines, and pyrrolidines. In addition, a number of benzylamines and nitriles were also identified. It is noteworthy that the N heterocyclics were isolated and identified without the use of pyrolysis. 

Maeder, H. Gunzelmann, K.H. Straight-Chain Alkanes As Reference Compounds for Accurate Mass Determination in Isobutane CI-MS. Rapid Commun. Mass Spectrom. 1988,2, 199-200. 

Brinded, K.A. Tiller, P.R. Lane, S.J. Triton X-100 As a Reference Compound for Ammonia High-Resolution CI-MS and as a Tuning and Calibration Compound for Thermospray. Rapid Commun. Mass Spectrom. 1993, 7, 1059-1061. 

As the monitored m/z values are selected to best represent the target compound, SIM exhibits high selectivity that can be further increased by high resolution SIM (HR-SIM) because this reduces isobaric interferences. [41-44] As HR-SIM requires precise and drift-free positioning on narrow peaks, one or several lock masses are generally employed although rarely explicitly mentioned. [44,45] The role of the lock mass is to serve as internal mass reference for accurate mass measurement. (Examples are given below.)... 

An example of the spectrum/substructure determination process is illustrated for the reference compound di-n-octylphthalate. Daughter spectra were acquired for every major ion (above 1 relative intensity) that appeared in the conventional mass spectrum (Figure 2) of the reference compound. All the daughter spectra were then matched against the reference daughter spectra of the same parent mass (but from different compounds) in the data base. The results of some of the matches are described below. 

Fig. 2.16 GPC spin column binding assay of RGS4 and G alpha proteins with WY817 (MW 450 Da). Positive ion ESI mass spectra for compound WY817, a weak binder to RGS4 protein and non-binder to G alpha protein. A miniature P6 GPC spin column was used. (A) ESI-MS response for 250 pg of reference compound WY817 (no GPC spin column used), (B) ESI-MS response for GPC spin column (P6 gel, 1 cm long, 100 pL volume) eluate when 100 pg of WY817 were passed...

More informative, perhaps, are the marked differences in relative FI signal intensities between the mass spectra shown in Figures 2c and 2d. In agreement with the previously mentioned results reported by Chakravarty et al (15) and Yun et al (16). the mass spectra of the low temperature component (Figures 2b and c) appear to be dominated by homologous series of aromatic and hydroaromatic compounds. Chemical identification of many of the compounds up to MW 350 has been accomplished by high resolution GC/MS (19, 20). although precise identification of the many possible isomeric structures involved will have to await the availability of suitable reference compounds. 

Selectivity is important, if interferences are present. For example a reference compound and an analyte compound generate a peak in a chromatogram at the same retention time. Either columns of different polarity or a mass spectrometer as detector can help to identify a single compound peak. 

Temperature programmed reaction (TPR) experiments were carried out by adsorbing allyl alcohol and allyl iodide on a 9.0 wt% Mo03/Si02 sample and monitoring the evolved products by mass spectrometry. The Raman spectra of the pure liquid reference compounds are shown in Fig. 2. They agree well with those reported earlier (18-20). 

The contribution of individual compounds to ozone depletion is characterized by the ozone depletion potential (ODP). The ODP of a compound as normally defined is the ratio of the global loss of ozone (i.e., integrated over latitude, altitude, and time) from that compound at steady state per unit mass emitted relative to the loss of ozone due to emission of unit mass of a reference compound, usually taken as CFC-11 (CFC13) (Wuebbles, 1983 Fisher et al., 1990 Solomon et al., 1992). The ODP thus provides a relative measure of the overall impact of a compound on ozone destruction over the long term. Values for ODPs have been derived using a variety of models (e.g., see World Meteorological Organization, 1995, 1999). 

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