Sensitivity polymer analysis

Applications of ISS to polymer analysis can provide some extremely useful and unique information that cannot be obtained by other means. This makes it extremely complementary to use ISS with other techniques, such as XPS and static SIMS. Some particularly important applications include the analysis of oxidation or degradation of polymers, adhesive failures, delaminations, silicone contamination, discolorations, and contamination by both organic or inorganic materials within the very outer layers of a sample. XPS and static SIMS are extremely comple-mentar when used in these studies, although these contaminants often are undetected by XPS and too complex because of interferences in SIMS. The concentration, and especially the thickness, of these thin surfiice layers has been found to have profound affects on adhesion. Besides problems in adhesion, ISS has proven very useful in studies related to printing operations, which are extremely sensitive to surface chemistry in the very outer layers. 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

Recently a decreased level of CE activity has been noticed with a shift of attention towards other separation techniques such as electrochromatography. CE is apparently not more frequently used partly because of early instrumental problems associated with lower sensitivity, sample injection, and lack of precision and reliability compared with HPLC. CE has slumped in many application areas with relatively few accepted routine methods and few manufacturers in the market place. While the slow acceptance of electrokinetic separations in polymer analysis has been attributed to conservatism [905], it is more likely that as yet no unique information has been generated in this area or eventually only the same information has been gathered in a more efficient manner than by conventional means. The applications of CE have recently been reviewed [949,950] metal ion determination by CE was specifically addressed by Pacakova et al. [951]. 

For the analysis of nonvolatile compounds, on-line coupled microcolumn SEC-PyGC has been described [979]. Alternatively, on-line p,SEC coupled to a conventional-size LC system can be used for separation and quantitative determination of compounds, in which volatility may not allow analysis via capillary GC [976]. An automated SEC-gradient HPLC flow system for polymer analysis has been developed [980]. The high sample loading capacity available in SEC makes it an attractive technique for intermediate sample cleanup [981] prior to a more sensitive RPLC technique. Hence, this intermediate step is especially interesting for experimental purposes whenever polymer matrix interference cannot be separated from the peak of interest. Coupling of SEC to RPLC is expected to benefit from the miniaturised approach in the first dimension (no broadening). Development of the first separation step in SEC-HPLC is usually quite short, unless problems are encountered with sample/column compatibility. 

Isolation may occur by liquid-solid interaction (extraction, dissolution) or heat (thermal, pyrolytic, laser). Extraction methods easily handle qualitative screening for low- to medium-MW compounds fail for high-MW components or polymer-bound functionalities and are less reliable quantitatively (analyte dependent). When applicable, dissolution methods suffer from sensitivity, because of the dilution effect on account of the polymer. In-polymer analysis performs well for qualitative screening, but is as yet not strongly performing for quantitative analysis, except for some specific questions. 

Mass spectrometers are used as detectors in gas chromatography offering the capability of compound quantitation and identification with exceptionally good sensitivity. For this reason, pyrolysis-gas chromatography/mass spectrometry (PY-GC/MS) is an excellent tool for polymer analysis. When a pyrolyser is used at the front end of the chromatograph, no special problems related to the GC/MS analysis are really added. 

HS-SPME is a very useful tool in polymer analysis and can be applied for absolute and semi-quantitative determination of the volatile content in polymers, for degradation studies, in the assessment of polymer durabihty, for screening tests and for quality control of recycled materials. For quantitative determination of volatiles in polymers, SPME can be combined with multiple headspace extraction to remove the matrix effects. If the hnearity of the MHS-SPME plot has been verified, the number of extractions can be reduced to two, which considerably reduces the total analysis time. Advantages of MHS-SPME compared to MAE are its higher sensitivity, the small sample amount required, solvent free nature and if an autosampler is used a low demand of labor time. In addition, if the matrix effects are absent, the recovery will always be 100%. This is valuable compared to other techniques for extracting volatiles in polymers in which the recovery should be calculated from the extraction of spiked samples, which are very difficult to produce in the case of polymeric materials. 

A similar situation does not presently exist for multilayer retortable food packages. In our initial study of LEP and PEP food packages we found that there are very few experimental studies on water permeation in polymers. The studies which we did find were usually limited to low temperature, far removed from retort conditions. The effect of water activity on transport and sorption behavior has not received much attention especially for water-sensitive polymers such as EvOH. In EvOH-type systems the problem is especially acute because EvOH exhibits case II sorption at moderate humidities and its effective glass transition is depressed below storage temperatures at high humidities. Thus, modelling the transport in this type of material is very complex and does not lend itself to analysis with existing models. 

At one time, the progress in the field of Raman spectroscopy of polymers was heavily dependent upon laser technology. The advent of accessible laser sources made it possible to replace the mercury-discharge lamp as an excitation source. Other developments, such as photomultipliers, computerization, or, most recently, sensitive array detectors made a tremendous impact on applications of this method in polymer analysis. 

Varlot, K., J.M. Martin, D. Gonbeau and C. Quet. 1999. Chemical bonding analysis of electron-sensitive polymers by EELS. Polymer AO 5691-5697. 

The method plays an important role in polymer analysis also in view of the recurrent difficulties encoxmtered by the conventional methods of characterization, such as size exclusion chromatography (SEC) or nuclear magnetic resonance (NMR), e.g., lack of calibration standards, low sensitivity, and the like. 

Mass spectrometry allows performing compound characterization in a sensitive, specific, and rapid manner (high-throughput) that is particularly important in CombiChem applications.The development of ionization techniques such as EST and MALDI allows the analysis of high molecular weight compounds broadening the field of polymer analysis by mass spectrometry. ... 

ATR-FTIR spectroscopy was used to monitor the uptake of urea into a silicone polymer. Analysis of the time-dependent changes in the IR absorbances of urea and silicone leads to an estimate of the diffusion coefficient for urea that is in close agreement with a value obtained using a bulk transport method (involving radiolabelled permeant). The silicone polymer was medical grade silicone pressure-sensitive adhesive (X7 201). ATR-FTIR is proposed as a rapid and accurate method of rapidly and accurately determining solute diffusion within a polymer matrix. 12 refs. 

Because of the poor sensitivity of FTIR, the flow-cell interface has been mainly used with packed-column SFC for analysing sample mixtures [22]. Unfortunately, the identification capacity of this type of interface is limited by interfering mobile-phase absorptions, which reduce the sensitivity of measurements. Solvent elimination has therefore been applied to real-life applications such as polymer analysis [24]. 

The example shown in Figure 8.2 illustrates that the selection of a proper matrix is important to generate a MALDI spectrum reflective of the polymer sample composihon. In this case, if only DHB were used, the MALDI spectrum produced would not reveal the low mass oligomers (m/z<10000) actually present in this sample. As only a handful of matrices are found to be practically useful for polymer analysis, it is often worthy spending the time to test these matrices on a given sample to identify the best matrix that provides good sensitivity, mass resolution, and reproducibiUty over a broad mass range. 

Because the amount of polymer samples available is usually not limited, it is possible to underestimate the sensitivity issue in MALDI polymer characterization. In reality, the use of a MS instrument that provides high sensitivity and a wide dynamic range of ion detection is pivotal to the success of polymer analysis. This is true not only for the measurement of polymer average mass, but also for the determination of polymer composition [110, 113-121]. With limited detection... 

Because it is necessary to reduce the molar amount of polymer loading as the molecular mass increases, this would suggest a practical hmit to the mass range for polymer analysis by MALDI. As molecular mass increases, the sensitivity of the instrument is challenged on two fronts (i) decreased sensitivity due to a loss in detector efficiency and (ii) decreased sensitivity from the requirement of lower (molar) sample loading. In the analysis of PS, this hmit appears to occur at -1.5 milHon Da. In order to obtain the MALDI mass spectrum of PS with a nominal mass of 1.5 miUion Da [29], 5 fmol of total polymer was loaded onto the probe. 

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