Lock-mass technique

Target compound analysis using high mass resolution, for example, for PCDD/ PCDFs, pesticides, POPs or pharmaceutical residues are typically performed by monitoring the compound-specific accurate mass ions at the expected retention time for each analyte. High resolution GC-MS target compound applications benefit from a unique technical feature referred to as the lock-mass technique for performing MID analyses. The lock mass technique provides ease of use, combined with a maximum quantitative precision and certainty in analyte confirmation. 

The exact ion masses of the reference compound are used in the MID acquisition windows for internal calibration. For the lock-and-cali-mass technique, two ions of the reference substance are individually selected for each MID window one mass which is close, but below the analyte target mass, and the second, which is slightly above the analyte or internal standard target masses. Although both reference masses are used for the internal calibration, it became common practice to name the lower reference mass the lock mass and the upper reference mass the calibration mass . 

Advantages of the Lock-Plus-Cali Mass Technique... 

Other acquisition techniques have been formerly used employing just one lock mass position. This technique requires a separate pre-run electrical mass calibration and does not allow a scan internal correction of the mass position which may arise due to long-term drifts of the analyser during data acquisition. 

For GC-HRMS (high mass resolution) systems, an internal mass calibration (scan-to-scan) for accurate mass determinations by control of the data system is employed. At a given resolution (e.g., 10 000), a known reference is used which is continuously leaked into the ion source during analysis. The analyser is positioned on the exact mass of the substance ion to be analysed relative to the measured centroid of the known reference. At the beginning of the next scan, the exact position of the centroid of the reference mass is determined again and is used as a new basis for the next scan (see Chapter 2.3.4.3 Lock-Plus-Cali Mass Technique). 

The sample introduction system used will depend on the type of sample and whether a chromatographic separation is required. The usual technique for separated and relatively pure samples is the direct insertion probe which carries a small amount of solid sample through an air lock into the high vacuum of the mass spectrometer. The sample would then be vapourised by applying gentle heat, and the vapours ionised in an ionisation source. Samples already in the gas phase, such as vapours or the eluent from gas chromatography, can be introduced directly into the ionisation source at low flows ( 1 mLmin ). Usually two ionisation techniques are used for these samples, electron ionisation (El) or chemical ionisation (Cl). 

The techniques used to obtain the untwisted SmC phase structure in low molar mass LCs and SCLCPs are limited to thin layers. In contrast to this, LC elastomers can be macroscopically uniformly oriented by mechanical deformations [230], and this orientation process is not limited to thin samples or suitable dielectric anisotropy of the material. Furthermore, for LC elastomers the oriented structure can be chemically locked in by crosslinking, resulting in the so-called liquid single crystal elastomers [231],... 

Since electron bombardment occurs in the gaseous phase, the volatility of the sample becomes a critical factor in mass spectrometry. This feature was mainly responsible for the slow development of mass spectrometry in organic chemistry, and more specifically in natural products chemistry. In 1955 the technique of direct sample introduction through a vacuum lock (13) into the ionizing chamber was applied (14-17). This modification allowed the study of samples of relatively low volatility and those which are thermally unstable. Polyfunctional compounds of low volatility can be rendered more volatile by a suitable selection of substituents or by chemical modification. 

The lock-plus-cali mass calibration technique provides extremely stable conditions for data acquisitions of long sequences even over days, for example, over the weekend and usually includes the performance documentation for quahty control documented in the data file. 

Both, the lock and call masses are monitored in parallel duringthe run providing an excellent confirmation of system stability for data certainty. Together with the constant resolution monitoring, this technique provides the required traceability in MID data analysis. 

As a consequence, with the one mass lock techniques, the mass jumps are less precise with increasing run times. Deviations from the peak top position when acquiring data at the peak slope result in less sensitivity, less reproducibility and poor isotope ratio confirmation. 

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