Peak Table Based Methods

Peak table based methods first extract biomarker peaks from a spectrum and then use those peaks for organism identification. These methods have the ability to ignore peaks that are due to background or extraneous factors. 

However, they do generally require an effective peak detection routine to ensure that both large and small key peaks are used, and that noise spikes are not detected as peaks. The reader is referred to Jarman et al.16 for a discussion and comparison of some different peak detection routines applied to MAID I data of bacterial samples. 

Statistical studies of MALDI MS applied to bacterial samples show that some biomarker peaks are highly reproducible and appear very consistently, while others appear much less reliably.1719 In Jarman et al.20 and Wahl et al.21 a probability model for MALDI signatures is proposed that takes into account the variability in appearance of biomarker peaks. This method constructs MALDI reference signatures from the set of peak locations for reproducible biomarker peaks, along with a measure of the reproducibility of each peak. 

when a given organism A is present in the sample, biomarker peak i appears with probability ph Let xt = 0 if biomarker peak i is not observed in the unknown sample, and xt = 1 if biomarker peak i is observed in the unknown sample. Then the likelihood of the observed peak table is given by 

If we let H0 represent the hypothesis that organism A IS NOT present in the sample, and HA represent the hypothesis that organism A IS in the sample, then the likelihood ratio for H0 versus HA is given by the probability of the observed peak table under HA divided by the probability of observing the outcome under H0. Specifically, 

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With a peak table based method, the biomarkers are extracted from an unknown spectrum, and those biomarkers are then used to compare the unknown to a reference signature. 

For the precision of the HS-SPME method we can refer to the work of Vas et al. (1998) with a peaks evaluation based on percentage, and to the data of Carlin (1998) obtained more properly referring to internal standard method. Ulrich et al. (1997) also achieved precise results by using SPME analysis on fruit juices. Table 5.1, referring to the working conditions of one of the chromatograms in Figure 5.2, reports a study of repeatability for some wine compounds adsorbed in a HS-SPME 1 cm PDMS fiber and analysed by GC-MS. 

Szepesi et al. reported an ion-pair separation of eburnane alkaloids on a chemically bonded cyanopropyl stationary phase. As counter-ion, di-(2-ethyl hexyl)phosphoric acid or (+)-10-camphorsulfonic acid were used in a mobile phase consisting of hexane - chloroform -acetonitrile mixtures (Table 8.8, 8.9). Because of the poor solubility of the latter pairing ion, diethylamine (Table 8.9) was added to the mobile phase. Addition of diethylamine considerably reduced the k1 of the alkaloids, due to suppression of the ionization of the alkaloids. However, due to the strong acidic character of the pairing ion, ion-pairs were still formed under these conditions. The camphorsulfonic acid containing mobile phases were found to be very useful for the separation of optical isomers (Table 8.10, 8.11, Fig.8.8) 6. It was also found that the selectivity of the system could be altered by choosing different medium-polarity solvents (moderator solvents) as dioxane, chloroform or tetrahydrofuran. The polar component of the solvent system affected peak shape. Based on these observations, a method was developed to analyze the optical purity of vincamine and vinpocetine. For the ana-... 

Different aggregations of objective criteria have been developed for particular analytical methods. Table 4.2 gives examples of objective functions for chromatography and spectroscopy. The objective function for chromatography, the chromatographic response function (CRF) accounts for all m peaks of the chromatogram, the time t for elution of the last peak, the noise, Af , at the measurement point of peak i, and the selectivity of peak separation based on Kaiser s measure for peak separation fig (see Figure 4.5). For optimal separations, the CRF is maximized. 

For microbial identification, the mass spectra can be analyzed in two principally different ways database- or library-based methods compare mass peak tables from unknown bacterial strains by matching them against hbraries with validated microbial reference spectra (Holland et al. 1996 Arnold and Reilly 1998). Today,... 

Comparing the data of Table 2 with those of Table 3, one can observe that log Mp calculated by the rheology-based method is nearly equal to log Mp determined by the GPC method. Therefore, it is feasible and reasonable that the calculated M with Eq. (10) is regarded as the peak MW Mp) on the MFVD curve. Consequently, the reciprocal of the frequency is converted to the MW scale in the rheology-based method. 

Included in the table are all compounds for which information was available through the C, compounds. The mass number for the five most important peaks for each compound are listed, followed in each case by the relative intensity in parentheses. The intensities in all cases are normalized to the w-butane 43 peak taken as 100. Another method for expressing relative intensities is to assign the base peak a value of 100 and express the relative intensities of the other peaks as a ratio to the base peak. Taking ethyl nitrate as an example, the tabulated values would be... 

Physical Methods. Vitamins D2 and D exhibit uv absorption curves that have a maximum at 264 nm and an (absorbance) of 450—490 at 1% concentration (Table 8). The various isomers of vitamin D exhibit characteristically different uv absorption curves. Mixtures of the isomers are difficult to distinguish. However, when chromatographicaHy separated by hplc, the peaks can be identified by stop-flow techniques based on uv absorption scanning or by photodiodearray spectroscopy. The combination of elution time and characteristic uv absorption curves can be used to identify the isomers present in a sample of vitamin D. 

To summarise, a fractionation step allows the isolation of the compounds of interest from the other molecular constituents, particularly from the fatty acids that are well-ionised. To compensate for the low ionisation yield of some compounds, such as TAGs, the solutions may be doped with a cation. Samples are then directly infused into the ion electrospray source of the mass spectrometer. A first spectrum provides an overview of the main molecular compounds present in the solution based on the peaks related to molecular cations. The MS/MS experiment is then performed to elucidate the structure of each high molecular compound. Table 4.2 shows the different methods of sample preparation and analysis of nonvolatile compounds as esters and TAGs from reference beeswax, animal fats and archaeological samples. 

Based on the analytical figures of merit of the methods in Table 1, the best precision and selectivity are accomplished by using the decay rate rather than the formation rate or conventional CL-measured parameters such as the peak height or area under the CL curve. Table 2 gives the selectivity factor, expressed as decay-rate and peak-height tolerated concentration ratio, for the CL determination of hydrogen peroxide using SF-CLS. As can be seen, the selectivity factor was quite favorable in most instances. 

Samples for determination of ionic alky Head species in marine fauna were homogenized in the presence of salts and the alkyllead component was extracted with toluene and oxidized with HN03. Determination was by DPASV115. A method based on oxidation on Hg electrode has been described116 for analysis of alkylleads in gasoline. Alkylation of Hg is involved, of course, but as an oxidation the method does not suffer from the background of atmospheric oxygen. The peak potentials Ev for oxidation of tetramethyllead and tetraethyllead on various cathodes are well resolved (Table 5). 

Table 9.21 gives the yields, times to maximum yield, retention times and least detectable amounts of the herbicide esters or ethers prepared using the above method. In no instance was the standard error of the mean yield >2%. The least detectable amount is based on a peak giving a response of twice the background signal. 

The simplicity and clarity of CONCISE has been retained in the automated rule generator which creates CONCISE interpretation rules for PAIRS based on a representative set of IR spectra. The rule generator uses peak position, intensity, and width tables produced by an automated peak picking routine. This method reduces the dependency on published frequency correlation data and enhances the usefulness of data already available. All work was done using the version of PAIRS running on a Nicolet 1180 minicomputer and programs generated have been optimized for this system. 

Based on the fact that aromatic sulfonic and carboxylic acids were successfully separated by reversed-phase chromatography in the presence of organic electrolytes, Chaytor and Heal (158) developed a method for the separation of 15 synthetic colors using a mobile phase containing o-phosphoric acid (Table 7). The presence of the electrolyte provided lower variation in response and retention over a period of time. Furthermore, eluted peaks were sharper than those seen in ion-pair chromatography. 

Based on the method of internal normalization of peak areas, the percentage composition (less the diethyl ether solvent shown as the initial large component in Figures 1 and 2) of odor concentrates was estimated. The average composition of samples of odor isolates from individual preparations as well as from several different preparations of the same type—i.e., concurrent or noncurrent—was determined on the basis of all preparations which could be compared on a fair analytical basis (same gas chromatographic detector, column, and conditions). These results were then combined (traps plus distillate) to provide the rough estimations shown in Table IV. These data represent the best estimation of the composition of the total volatile odor concentrates from each processing method studied. 

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