Лако-красочные материалы — производство

Table 1 lists a wide range of spectroscopic techniques with details on the type of informa­tion that these techniques can provide, their sampling depth, their sensitivity, and their major limitations. Some key references are provided for each technique. The techniques are classified in five major categories: ion spectroscopies, electron spectroscopies, x-ray spectroscopies, vibrational spectroscopies, and miscellaneous techniques. This distinction is, of course, arbitrary and is based on the type of signal that is recorded in each technique. It would be equally justifiable to classify the techniques on the basis of the primary beam or excitation source or even on sampling depth [3]. However, in each group a distinction has been made between those techniques that are well known and currently widely used and techniques that are either variations of the major techniques or still in a develop­mental stage. The techniques that are closely related or that are variations of the same main technique are grouped together.

Another way of comparing techniques is to group them according to the combina­tion of excitation (i. e., signal in) and response (i. e., signal out). This is frequently done in the literature [4]. This has been done here for a number of the techniques listed in Table 1. The results are shown in Table 2, which illustrates that a technique has been developed or proposed for almost all combinations of ions, electrons, and photons.

Table 1 indicates that the sampling depth for the various techniques varies from 1 monolayer to several millimeters. In general, the ion-based techniques, for instance SIMS and ISS, have the lowest sampling depths because the mean free path of ions in solids is not more than one or two monolayers. The penetration and escape depths for photons are much higher and, therefore, the techniques that are based on the detection of electromag­netic radiation, such as FTIR and XRF, give information on microns in metals and even millimeters in organics. This does not mean, however, that these techniques cannot detect monolayers. In suitable samples, both FTIR techniques and XRF can detect monolayers with high sensitivity, but it is difficult to restrict the signal acquisition to the monolayer only because of the larger sampling depths.

Methods based on electron detection have intermediate sampling depths. The sampled thickness in techniques such as AES and XPS is of the order of 50 A. Since the escape depth of an electron is dependent on its energy, the sampling depth in XPS and

(continued )

Sampling

Acronym depth Information Sensitivity Principle Limiatation Ref.

Attenuated total reflection ATR

spectroscopy

Reflection-absorption infrared RAIR

spectroscopy

Multiple reflection absorption MRAIR

infrared spectroscopy

Grazing incidence reflection GIR(S)

(spectroscopy)

Multiple reflection infrared MRS

spectroscopy

Multiple internal reflection MIR

(spectroscopy)

External reflection spectroscopy ERS

Surface reflectance infrared SRIRS

spectroscopy

Photoacoustic spectroscopy PAS

Emission spectroscopy EMS

Photothermal beam deflection PBDS

spectroscopy

Internal reflection spectroscopy IRS

Copyright © 2003 by Taylor & Francis Group, LLC

AES is not the same for all elements detected in the sample. Further, by varying the angle between the sample surface and analyzer, the sampling depth can be varied, resulting in a nondestructive quantitative concentration depth profile in the range 5-50 A. This feature is especially useful in XPS.

The type of information provided by the techniques listed in the tables also varies greatly. Many spectroscopic techniques give qualitative and/or quantitative elemental composition. The vibrational techniques, however, generally provide information on the molecular structure. SIMS, especially in the static mode (SSIMS or TOFSIMS), can yield information on molecular structures and even orientation of monolayers [5-10]. This is particularly useful for the study of the absorption of coupling agents on metals or to determine the effects of plasma treatments on polymer surfaces [11]. TOFSIMS instru­ments also have capabilities for determining the two-dimensional distributions of elements or molecular species at the surface, similar to the capabilities (for elements only) offered by AES and EDXA or WDXA.

The major technique for determining a depth profile of elemental compositions is AES. A newer technique for this purpose is SNMS, which has a better interface resolution (due to a lower sampling depth) than AES and a better sensitivity for many elements than AES [10,12,13]. In SNMS the neutrals emitted in the SIMS process are ionized and then mass-analyzed. The emission of neutrals is much less matrix-dependent than the emission of positive or negative ions detected in regular SIMS. Depth profiling can also be done in regular SIMS (the so-called dynamic SIMS version), but this technique then requires exten­sive calibration of sputtering rates and elemental sensitivities. Depth profiling in both AES and SIMS techniques is done by sputtering, usually by means of a beam of Ar+ ions.

The limitations of some of the more popular techniques are also given in Table 1. For many techniques, especially the more surface-sensitive ones (ion — and electron-based methods), an ultrahigh vacuum (UHV) environment is required. This requirement, of course, increases the cost of the equipment, but it also reduces the flexibility and applic­ability of the technique. Other limitations of certain techniques as indicated in the table are the difficult or complex sample preparation procedures, low sensitivity (long acquisition times), and poor resolution or element selectivity (e. g., ISS). Another limitation of some of the more sophisticated techniques is that they are not commercially available. To carry out certain techniques, it may be necessary to modify commercial instruments.