Standard Sample Preparation

Example The choice of a matrix and optimized conditions of sample preparation have substantial influence on the appearance of MALDI spectra. Even when employing standard matrices such as CHCA or DHB, significant improvements can be achieved, e.g., by appropriate mixing of the two substances (Fig. 10.6). [71] 

Note The conventional co-crystallization is usually termed dried droplet preparation. The original thin layer technique involves preparation of a thin HCHA layer from solution in acetone on top of which the analyte is placed in a second step without re-dissolving the matrix. [95,97] 

Example Assuming a typical MALDI matrix with an Mr of about 200 g mol dissolution at 10 mg ml in a suitable solvent yields a matrix solution that is 0.05 M in concentration, which is equal to 5 x 10 mol pl An average peptide of Mr around 2000 g moL dissolved at 0.01 mg mT results in 5 x 10 mol pl (5 pmol pl ). Mixing the matrix with the analyte solution 1 1 (vA ) results in a molar matrix-to-analyte ratio of 10,000 1 for preparation on a target. Pipetting 1 pi of this mixture per spot onto a MALDI target corresponds to 2.5 pmol of sample per spot (Fig. 11.11). In fact, MALDI-TOF-MS of peptides can be extended to 1/lOOOth of this amount, and thus, may routinely deliver useful spectra down to a few fmol of peptide per spot, which is equal to a molar matrix-to-analyte ratio of 10,000,000 1. 

The crystallization process is a critical parameter in LDI and MALDI sanple preparation [50,105,106]. The conventional co-crystallization is usually termed dried droplet preparation. Dried droplet preparation yields comparatively large crystals, especially when slow evaporation, e.g., from aqueous solutions, is involved. Unfortunately, large crystals are detrimental for good shot-to-shot reproducibility and mass accuracy. 

Finally, (nano)electrospray deposition can be used to deposit the analytes onto different kinds of predeposited matrix layers. MALDI sample preparations where the analyte solution is deposited on top of a previously prepared matrix layer are generally termed sandwich methods. The base layer of matrix may be prepared either by the standard dried droplet technique or by thin layer preparation. For (nano)electrospray deposition of peptides, for example, a 10 M solution is sprayed from a (nano)electrospray capillary onto the solid matrix layer. The advantage of nanoelectrospray over conventional electrospray is that very small droplets are formed, which arrive at the target as dry particles, and thus, do not wet and redissolve the matrix surface [41]. 

Metal ions, in particular singly charged ions such as Na, Cs , and Ag are sometimes added to the matrix-analyte solution to effect cationization of the neutral analyte [111]. This is advantageous when the analyte has a high affinity to a certain metal ion, e.g., towards alkali ions in case of oligosaccharides [6]. Addition of individual cations can also lead to a concentration of ions of a particular 

Silver ions (from silver trifluoroacetate or trifluoromethanesulfonate), Cu , and other transition metal ions in their 1+ oxidation state [112,113] are frequently employed to obtain [M+metal] ions from nonfunctionalized or at least nonpolar hydrocarbons [114], polyethylene [115,116], or polystyrene (for an example see Chap. 11.6.1) [112,113,117-119]. 

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Internal methods of quality assessment should always be viewed with some level of skepticism because of the potential for bias in their execution and interpretation. For this reason, external methods of quality assessment also play an important role in quality assurance programs. One external method of quality assessment is the certification of a laboratory by a sponsoring agency. Certification is based on the successful analysis of a set of proficiency standards prepared by the sponsoring agency. For example, laboratories involved in environmental analyses may be required to analyze standard samples prepared by the Environmental Protection... 

Whilst sample preparation may not be the most interesting aspect of NMR spectroscopy, it is nonetheless extremely important as it will have a huge bearing on the quality of the data obtained and therefore on your ability to make logical deductions about your compounds. This is particularly true when acquiring the most straightforward 1-D proton spectra. The most typical manifestation of sub-standard sample preparation is poor line shape. It is worth remembering that in terms of 1-D proton NMR, the devil can be very much in the detail . Detail , in this context, means fine structure and fine structure is always the first casualty of poor sample preparation. 

The solution to the problem was discovered when a titrated sample (clear solution) was left on the bench and, after a period, it started changing back to a faint yellow color. We hypothesized that air oxidation may have caused that effect and, consequently, air may have interfered with analysis. Standard samples prepared and purposely delayed during the analysis showed that end-point volumes were larger, indicating that some of the iodide ions turned into free-iodine by air oxidation which, in turn, required more thiosulfate for titration and, therefore, larger end-point volume. The following chemical equations obtained from the literature 8) show what happens before, during, and after titration. The reaction of a chlorinated isocyanuric acid compound with potassium iodide in acidic pH is ... 

Sample solution composition does not match the standard solution, resulting in a low response bias for samples. This can be due to a standard/sample preparation scheme or to negative formulation matrix effects in the dissolution media (e.g., pH change). 

Initial and continuing calibration verifications and acceptance limits Raw data for each sample (including reanalysis and second column confirmation), blanks, LCS/LCSD, MS/MSD, and calibration standards Sample preparation bench sheets Gel permeation chromatography clean-up logs / (Summary only) ... 

The Human Proteome Project (HUPO, www.hupo.org) initiated the Plasma Proteome Project (PPP) in 2002, and numerous laboratories have contributed to this ambitious project of deciphering all proteins contained in the human plasma. Plasma, the soluble component of the human blood, is believed to harbor thousands of distinct proteins that originate from a variety of cells and tissues through either active secretion or leakage from blood cells or tissues. For the reasons described above, HUPO recommends use of plasma instead of serum, with EDTA (or citrate) for anticoagulation and standardized sample preparation. HUPO proposes combinations of serum depletion, fractionation procedures, and MS/MS technologies, with explicit... 

In the preferred procedure, samples of vinyl acetate on Chromosorb 107 were prepared. A glass U-tube was connected to the inlet end of a clean sampling tube, the outlet of which was connected to a small pump. As an aliquot of vinyl acetate in hexane was injected into the U-tube, the pump drew air through the system at 0.2 L/min such that the vapors were swept onto the sorbent bed. After 2-3 minutes, or approximately five volume changes, the pump was turned off and the apparatus was disassembled. Standard samples prepared in this manner were thermally desorbed and analyzed. A calibration curve thus... 

Spectra acquired from PE membranes are of equal or better quality as those acquired from metal sample stages under standard sample preparation conditions. The PE membrane provides access to higher molecular weights than the more common transfer membrane materials (PVDF, nylon, and nitrocellulose). This permits the mass analysis of the large proteins for which MALDI-TOF MS is ideally suited. Mass accuracy and reproducibility approaches that obtained with standard sample preparations. Furfeermore, the use of PE reduces the severe ion suppression effects typically observed in the MALDI analysis of high mass mixtures. This also permits more accurate mass measurements to be made via the use of internal calibration. 

Numerous experimental setups to perform GIXS were recently described in the open literature [31-34]. We just recall that in almost all cases, a high brilliance synchrotron radiation X-ray beam is needed, especially from 3 generation synchrotron radiation sources like the ESRF in Grenoble (France), and they always couple a large UHV chamber with standard sample preparation with a heavy duty high precision goniometer that allows the orientation of the sample and the detector. 

One of the main constraints on conventional procedures for sample preparation is their lack of uniformity. In fact, many users introduce specific modifications in the general procedures that eventually result in enormous differences in the way they are implemented. Such a lack of uniformity precludes comparison of results obtained using what was seemingly the same procedure and validation of the ensuing methods. Comparability of the results can thus only be achieved by using actually identical procedures this justifies the efforts currently in progress at standardizing sample preparation procedures. 

Figure 2. Standard sample preparation procedure for the study of papaya (C.papaya, L.) fruit volatiles (17).

In the air quality laboratories of EAE and REI chemieal analyses are performed in accordance with standardized analytical methods. A Quality Handbook, which is periodically updated, is also available. Control of the measurement accuracy is performed (calibration curve settings, verification with standard samples prepared in the laboratory). Comparative assessment of the results of the chemical analyses from different measuring devices is performed. The laboratories of EAE are accredited by the Bulgarian Accreditation Body. 

Each sample (100 xL) was injected into the HPLC apparatus, and four replications were performed. Residual monomer concentrations were determined from a regressed calibration curve that was obtained from standard samples prepared with recrystaUized and dried monomer over the range 0-100 ppm. All samples from a given experiment were analyzed within 10 h. 

After standard sample preparation, detection and quantitation of both solutions are done by LC/MS/MS. The expected problem of compounds sticking to the membrane or container can occur, but careful quantitation will at least tip one off that this is happening. Another problem that crops up is that the compounds need to be stable to plasma over an hour long time course or the results will be wrong and misleading. Equilibrium dialysis is a bit time- and labor-intensive, but it is possible to improve the throughput to some extent, for example by using pooled samples or 96-well dialysis plates. 

The absolute quantities of elements contained in both the soluble and insoluble fractions were determined by particle induced X-ray emission (PIXE) analytical method. PIXE analysis was performed with a proton beam of a 6 mm diameter and a 2.0 MeV energy from a Tandem Cockcroft accelerator installed at the Quantum Science and Engineering Center of Kyoto University. Beam intensities from 10 to 60 nA were employed, and the total dose was about 20 pC. An X-ray with an energy of up to 14.8 keV emitted from the target was detected by a Si (Li) detector with a resolution of 152 eV at 5.9 keV. The calibration method of PIXE was similar to that described elsewhere (Kasahara et al. 1993) and is described here briefly. The relationship of an X-ray yield and the mass thickness was measured first using the 18 single element standard samples prepared by a vacuum... 

There are many reasons for incorrect data, but many times the source of error is traceable to either standards, sample preparation, or contamination. 

To increase the precision of quantitative analysis, the plasticizer sample is diluted with carbon disulfide, its infrared absorption measured, and compared with absorptions standard of standard samples prepared also in CS2 to cover the range of concentrations from 0.5 to 3 mg/ml. For each suspected (identified) plasticizer, a series of standards has to be prepared and measured. It is also important to select a suitable wavelength for quantitative analysis. For dioctyl phthalate bands at 1725 and 1121 cm" are usually used. For tricresyl phosphate band at 1191 cm is used. Similar to gravimetric method, the results are subject to various interferences when mixtirres of plasticizers or mixture of plasticizers with other additives ate used. 

The weakly nitric-acid solution obtained as a result of concentration of the water sample is extracted with ethyl acetate following the addition of aluminium nitrate. The organic phase containing the uranium is separated off, evaporated in a platinum crucible, and the uranium is melted at 630 °C after adding sodium fluoride/potassium carbonate/sodium carbonate as a fluxing agent. The yellow-green fluorescence of the melt which appears when irradiated with an ultraviolet lamp is compared with that of standard samples prepared under equivalent conditions. 

Facdiiano, A., et al. (2010) Standardized sample preparation phases for a quantitative measurement of plasma peptidome profiling by MALDl-TOF. 

Standard sample preparation and measurement scheme for spent fuel samples of high burnup (IAEA 2004)... 

Fig. 7.4 Standard sample preparation protocol MALDI-TOF mass spectra of Acinetobacter baumannii and Acinetobacter nosocomiaiis strains with one highlighted species-specific signal. Dendrograms constructed on the basis of the MALDI-TOF mass spectra of the strains of the ACB complex...

Fig. 7.6 Dendrograms calculated on the basis of cluster analysis of MALDl-TOF mass spectra of sm Staphylococcus aureus strains analyzed in triplicate (marked as 1, 11, and 111 ) using a standard sample preparation protocol and b microwave-assisted digestion...

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