Mass Spectra Quality

Mass spectral quality is an important consideration in performing a successful GC-MS analysis. The quality of the mass spectra depends on (1) the concentration of the constituents in the sample solution, (2) the GC operating conditions used to resolve the peaks, and (3) excessive pressure fluctuation in the MS unit of the GC-MS system leading to distortion of the mass spectrum, especially an El mass spectrum, as reflected in the relative abundance of the ion peaks. The implication of (3) is that distortions of this type could lead to misinterpretation of the spectrum even though a library search is performed. 

Furthermore, good mass spectra of the minor constituents in a sample are difficult to obtain by GC-MS analysis if the relative intensity of these constituents is less than 2% of the area above the base line in the corresponding total ion chromatogram (TIC) because of the presence of background spectra. However, this problem can be circumvented by applying specific mass number plotting as described in Section 9.1.3.4. 

---

MCLAFFERTY, F.W., STAUFFER, D.A., LOH, S.Y., WESDEMIOTIS, C., Unknown identification using reference mass spectra. Quality evaluation of databases, J. Am. Soc. Mass Spectrom., 1999,10, 1229-1240. 

Preliminary tests of these polySi-based nano-nibs have shown outstanding results in terms of analysis conditions and mass spectra quality (pattern and signal-to-noise ratio). We have demonstrated that HV can be decreased down to 0.5-0.7 kV, still keeping acceptable ionization conditions for peptide samples at 1 pM. These nibs are also compatible with analysis of solutions with a high amount of water without any degradation in analysis quality. 

The solvent condensation/MALDl matrix recrystallization procedure has shown the ability to improve mass spectra quality by reducing the number and intensity of inorganic salt ion adducts typical for traditional MS imaging sample preparations. This reduction in potassium and sodium salt adducts creates less complex mass spectra (20). 

Sehumaker S, Borror CM, Sandrin TR. Automating data acquisition aifects mass spectrum quality and reproducibility during bacterial profiling using an intact cell sample preparation method with matrix-assisted laser desorption/ionization time-of- light mass speetrometry. Rapid Com-mun Mass Sp. 2012 26(3) 243-53. 

Space charge effects and ion-molecule reactions, leading to a poor-quality mass spectrum... 

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. 

Analytical potency method development should be performed to the extent that it is sufficient for its intended purpose. It is important to understand and know the molecular structure of the analyte during the method development process, as this will facilitate the identification of potential degradation impurities. For example, an impurity of M + 16 in the mass spectrum of a sample may indicate the probability of a nitrogen oxide formation. Upon successful completion of method development, the potency method will then be validated to show proof that it is suitable for its intended purpose. Finally, the method validated will be transferred to the quality control laboratory in preparation for the launch of the drug substance or drug product. 

According to OPCW procedures, all GC/MS runs of a sample or a blank involve the coinjection of an internal standard (hexachloroben-zene contained in the OPCW HCB mixture). This is a quality control measure, which is applied to assess the validity of the run, especially when operating in blinded mode. When AMDIS processes data from the internal standard run, it uses a small auxiliary Internal Standard Library (ASCII file onsite.isl). This library contains data on HCB (mass spectrum, RI, etc.). HCB is defined as detected if the net match factor for HCB is >80 (this is the threshold for identification, which is fixed in this version of AMDIS). Everything so far is done by AMDIS in the background. 

Generally, MALDI is more tolerant than ESI to many contaminants. This can in part be due in part to some separation occurring during the crystallization of the sample with the matrix [30], Whatever the ionization method, the quality of the mass spectrum is higher if the contamination is reduced. 

However, is not the analysis of minor wine volatiles that still presents difficulties. With the level of sensitivity and automation of the analytical techniques, the determination of many odorants at /rg/L level is a simple analysis. The difficulties come when the analytes of interest cannot be easily determined using a single non-selective-preconcentration step. This will happen when the analytes are difficult to extract because they are very polar and/or not very volatile or when they are present at very low levels. The concentration level at which the analysis of an aroma compound becomes difficult is related to its polarity and to the quality of its mass spectrum. Eor instance, the analysis of 2,4,6-trichloroanisol (TCA) at, let s say, 20 ng/L is not a very difficult analysis, because this molecule is quite nonpolar (easily extractable, relative volatile) and has a mass spectrum with abundant high mass ions.In contrast, the analysis of methional or of sotolon at 1 /rg/L is quite difficult because these compounds are very polar (difficult to extract, not very volatile) and their mass spectra lack powerful ions. For these difficult analytes, some of which are very important wine impact aromas, specific strategies must be developed ... 

Figure 11-4. Purity assessment is a critical component in the decision process by the chemist as to whether their isolated compound is of sufficient quality to be submitted for compound registration and biological testing. To facilitate automated and rapid purity assessment of compound libraries, applescripts and visual basic scripts are used. (A) Total ion current chromatogram shows two components. (B) Extracted ion chromatogram for the expected product identifies its retention time. (C) Mass spectrum observed for the expected product. (D) UV 220-nm chromatogram indicates the expected product is approximately 75% pure. (E) UV 254-nm chromatogram indicates the expected product is approximately 66% pure.

More often, as described earlier, compound purity is reported taking into account the purities determined from the UV, ELSD, and CLND detectors. In some instances, purity assessment has been made based on the intensity of the expected ion in the mass spectrum relative to the sum of the intensities of all ions in the spectrum. This method, however, is only a very crude estimate of purity, because ionization efficiencies for compounds can vary widely within and between classes of compounds. Though LC/MS (with UV and/or ELSD detection) has been adopted as the method of choice for assessing the quality and quantity of material prepared by parallel synthesis techniques, a decision still needs to be made by each respective organization as to what constitutes acceptable quality before submitting a sample for biological testing. 

It is seen that an excellent separation is obtained and apparently little resolution is lost in the capillary interface. The mass spectrum of the peak marked T2 in the chromatogram is shown in figure 30. It is clear that good quality spectra can be obtained up to ion masses of at least 900. Such a combination of techniques can be invaluable for the structure elucidation of compounds generated in biochemical research. 

Following ionization, the charged molecular, cluster, or fragment ions are accelerated and focused into a mass analyzer. The type of mass analyzer influences the region and quality of the mass spectrum. Some... 

For the last two methods, there is the assumption that extraneous material from the chromatogram does not unduly and adversely affect the quality of the mass spectrum which is recorded. The chromatogram is placed, intact, within a source housing and a spatially... 

Figure 5 Correlation of FT-ICR-MS scan number and mass signal quality. The more MS scans are acquired (left), the higher the signal-to-noise ratio in the corresponding mass spectrum (right).

---