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XRF analysis boosts QC in plastics processing.

XRF Analysis Boosts QC in Plastics Processing

How can one identify the amount of antimony and bromine in a polymer, or quantify the calcium stearate in polypropylene? What is the amount of manganese and titanium in nylon, and how much zinc is in polystyrene? How can one measure the amount of silicon in polyethylene? A new class of inexpensive X-ray fluorescence (XRF) analyzers is giving quality control engineers precise answers to these and many other questions concerning real-time process control.

This article provides an update and overview of current XRF technology, outlines a few typical applications, and presents some guidelines for making purchase decisions.

In the past, XRF spectrometers capable of performing the type of analysis required for quality control and quality assurance (QC/QA) were large and sophisticated. Because these systems required special installation sites and highly skilled technicians, on-line sampling or analysis was often impossible; the attendant cost and complexity put them beyond the practical reach of all but a few high capacity manufacturing operations.

Now there are smaller, more economical instruments designed specifically for QC/QA applications. This new generation of XRF analyzers requires minimal preparation of samples, and in many cases, none at all. Sample analysis may be performed in less than a minute right in the production area. The systems feature rugged designs and are easy to operate. Thus, they can be incorporated into routine process operations and do not require technicians highly trained in spectroscopy.

How Does XRF Work?

XRF is a technique used to identify and quantify atomic elements in a sample. It occurs at the atomic level and resembles the effect of illuminating fluorescent material.

In XRF, samples are irradiated by X-rays, which cause electrons in the atoms of the sample to jump to a higher energy level. Upon dropping back to the original energy level, the electrons emit X-rays of precise energy and wavelength that are characteristics of a particular element. This is the process of fluorescence.

The quantity of X-rays emitted from a sample is directly proportional to the concentration of the element that is present in the sample. By collecting the emitted X-rays and measuring their energies or wavelengths against their intensities, one can identify and quantify the elements in a sample (see Fig. 1). This, in a nutshell, is how XRF works.

There are many methods of elemental analysis, each of which has advantages and limitations. XRF, as are most other spectroscopy disciplines, is a comparative technique: The value of the analysis depends upon the accuracy of the standards or references with which the instrument is calibrated. XRF is renowned for its ability to make precise comparisons; its degree of precision is normally expressed as a standard deviation of a series of repeat measurements.

Applications in Plastics

In plastics, uses of XRF analysis include quantifying various pigment, flame-retardant, and UV-stabilizer additives that are based on elements such as titanium, copper, zinc, bromine, antimony, and phosphorous. XRF analyzers can simultaneously measure all of these elements directly on the granules in a typical span of 50 seconds or less. The operator simply places the samples into a cup, which is then placed in the analyzer. The results are available in less than a minute. Liquid samples are simply poured into cups that are fitted with windows transparent to X-rays. Each measurement is performed automatically, and again, usually in less than a minute. (See Figs. 2-4.)

What To Look for in an

XRF

Range of sample formats. Regarding preparation of solid, powder, and liquid samples, XRF can be the simplest of elemental analysis techniques. As in other spectroscopy techniques, the homogeneity of the measurement specimen must be representative of the entire lot of material.

Solid samples require flat surfaces for measurement. As explained above, holders fitted with X-ray-transmitting windows are used for liquids. If quick measurement is more important than ultimate accuracy, the holders may also be used for loose powders. Powders are better analyzed as pressed disks or, for even greater accuracy, as fused borax disks.

Excitation sources supply the energy that causes fluorescence. Essentially, there are two choices: a radioactive (isotope) element or an X-ray tube.

A sealed radioactive isotope source, which emits monochromatic X-radiation of a particular energy (wavelength), is the most compact X-ray excitation source. Unlike an X-ray tube, it requires no high voltage power supply or ancillary considerations. But to cover the full range of the periodic table, several such sources are needed. The result is an expensive though comprehensive system of analysis.

Because QC applications typically require a limited element range, only one excitation source is generally needed for a specific application. If it fulfills the objective, one source can result in significant cost savings.

X-ray tubes that have power levels ranging from a few watts to several thousand watts are now available. Research-grade spectrometers typically use the high wattage systems; QC/QA analyzers use very low wattage systems (less than 500 watts). Although the distribution of X-ray energy emitted by a tube depends upon the target material and the tube exit window, it is generally broad enough to cover many elements. Obtaining the best sensitivity for selected elements may involve choosing a specific anode, a number of which are available. Tubes fitted with a rhodium target, however, will cover essentially the entire periodic table. This configuration is used for more than 90% of all applications.

Initially and thereafter, the cost difference between a very low wattage X-ray tube source and a radio-isotope source is very little. However, a very significant cost difference exists between a system that uses a high wattage X-ray tube source and an analyzer that uses a radio-isotope source. And because the complexity of the analyzer also increases with the wattage, it is worthwhile to investigate carefully the need for a high wattage system before committing to one.

Detection Methods

The general term "dispersion" refers to the various methods of separation used for measuring emitted wavelengths (energies) of elements.

Non-dispersive systems are the simplest instruments: A gas-filled detector measures the intensity of radiation that it receives directly from an excited sample; its signal is directly proportional to the concentration of the element that is being measured. By using a multichannel analyzer as an integral part of the system, the operator can achieve discrimination (resolution) between elements.

The filler gas influences the level of response to light or heavy elements. Neon is used for the lighter elements and xenon for the heavier elements.

Energy-dispersive systems (EDX) are a special class of non-dispersive systems in which the design of the detector yields a higher (energy) resolution. EDX analyzers can record the entire spectrum simultaneously, an advantage that suits them well for survey applications.

Many detectors of this design require operating temperatures well below ambient; they are cooled electrically or by liquid nitrogen. However minor, this requirement increases somewhat the complexity of the analyzer. Because of their relatively low count-rate capability before experiencing saturation, EDX analyzers are at a disadvantage in performing trace analysis of a matrix that has major constituents near the element of interest.

Wavelength-dispersive systems (WDX) offer the best resolution of spectral lines of elements that require the highest count-rate capability. Because the precision of measurement is directly related to the square root of the counts collected for each element, WDX yields either the fastest analysis or the highest precision, depending upon the objective.

Nearly all of the large research-grade XRF analysis systems use the WDX principle. X-ray tubes of 3-kilowatt output are usually the excitation source for these spectrometers.

The design of wavelength-dispersive analyzers may be either sequential or simultaneous. That is, the instrument either sequentially records emission counts on a single detector as it scans from one wavelength to another, or it measures counts simultaneously with an array of fixed detectors. As the number of detectors increases, so do the cost and complexity of the system.

The excellent resolution capabilities of the simultaneous WDX analyzer enable the instrument to handle most situations, including those in which a sample contains a large amount of one element but only a trace quantity of an element of particular interest. The design of the analyzer allows all of the channels to collect data during the entire analysis period, thus permitting the use of a lower power (200 watts) X-ray tube. The lower power requirement reduces the cost and complexity of the system.

Sensitivity

Another general concern is the limit of detection (LOD). Although not competitive with atomic absorption spectrometry (AAS), inductively coupled plasma (ICP), or mass spectrometry (MS), XRF is capable of superb performance - its LODs range from a few parts per million to several hundred ppm, depending upon the element of interest and the matrix (see Fig. 5). It can also analyze at high percent levels without need of dilution or other quenching techniques. Figure 5 shows the LOD for a simultaneous WDX analyzer.

An energy-dispersive analyzer performs nearly as well for the heavy elements, but not as well for the elements that have low atomic numbers. Research-grade WDX spectrometers are capable of analyzing elements with atomic numbers as low as that of boron.

Conclusion

X-ray spectroscopy in the form of X-ray fluorescence is becoming less expensive and more accepted in QC/QA applications. Recognition of needs, and subsequent advances in technology, have led to analyzers that are particularly well suited to these applications.

XRF is not the method of choice for sub-ppm applications, nor for those in which calibration standards cannot be made or acquired. For many applications that depend upon elemental composition (sodium to uranium), XRF is an excellent choice.

The low power simultaneous WDX requires an initial investment higher than that of the non-dispersive analyzers, about the same as that of the EDX, and considerably less than that of the research-grade WDX spectrometers. Personnel considerations, such as training requirements and the operator's level of expertise, could be as important as the price and performance of the analyzer.

Because of their simplicity, reliability, and resolution of the fixed simultaneous channels, the simultaneous WDX analyzers warrant serious consideration, particularly for applications in which several elements are to be measured in every sample.

PHOTO : FIGURE 1. Fluorescence of a sulfur atom: Electrons emit X-rays characteristics of the element.

PHOTO : FIGURE 2. Various sample-holding cups and typical formats of samples used with XRF analyzers.

PHOTO : FIGURE 3. XRF analyzer includes samples holders (left), an automated sample-handling option (center), and dedicated microcomputer (right).

PHOTO : FIGURE 4. Preparing samples for XRF can be as easy as pouring specimens into a cup and then placing the cup in the analyzer.

PHOTO : FIGURE 5. The LOD for a simultaneous WDX analyzer, depending upon the element of interest and matrix, ranges from a few ppm to several hundred ppm.
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Title Annotation:X-ray fluorescence; quality control
Author:Price, Brian J.; Major, H.W.
Publication:Plastics Engineering
Date:Aug 1, 1990
Words:1781
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