Disposable sample cups for x-ray fluorescence analysis

Reprinted from American Laboratory, November 1984

By Dr. Monte J. Solazzi

TECHNOLOGICAL advancements in both wavelength-dispersive (WDXRF) and energy-dispersive (EDXRF) x-ray fluorescence instrumentation have given the spectroscopist the means to accommodate virtually all types of sample materials. Higher degrees of analytical accuracy and precision and lower limits of detection and concentration levels can be achieved. As a result, refinements in sample presentation methods have been made and, in many instances, new systems and adjunct equipment have been introduced. Sample analysis times decreased dramatically, and x-ray fluorescence is now recognized as a versatile laboratory tool useful for both infrequent and routine sample analysis.

Sample preparation is frequently more time-consuming than the actual analysis. This is particularly true with powdered solid sample materials as a result of the need to reduce sample particle size differences and inhomogeneities to insignificant levels. Solution samples, in most cases, simply require transfer to an appropriate device for containment and presentation to the instrument. Unlike powdered solid or solid samples, the constituent elements in solution samples are assumed to be in complete states of dissolution: as long as the laws governing critical depths of penetration are observed, no further processing is usually required.

This paper describes a line of disposable plastic XRF sample cups (Chemplex® Industries, Inc.) for use with powdered solid, liquid, and solid sample retention. The cups are used to present samples for x-ray fluorescence analysis, and contribute to efficiency in sample handling methods and diversification of applications.

Product Development

The first Chemplex disposable x-ray fluorescence sample cups were called Dispoza-CupsTM. The body, or cell, of the cup shown in Figure 1a, was fabricated of tubular cellulose acetate butyrate cut into appropriate lengths to fit into metallic sample holders on the instrumentation. The ends of the cell were thermo mechanically turned down and under to form smooth, rounded beads to which thin-film sample supports with nitrile "O"-rings were affixed (Figure 1b). A conically shaped aluminum device was designed to stretch the nitrile "O"-ring as it was guided to roll over the beaded edge of the cell to effect a seal.

Dispoza-Cups were useful in eliminating time consuming and costly clean-up operations, and for reducing the possibility of cross contamination. Their use and applications were mainly in the oil and petrochemical industries. Significant drawbacks prohibited their use in effectively retaining the types of sample materials normally conducive to x-ray fluorescence analysis. The cellulose acetate butyrate material did not offer adequate resistance to chemical attack and degradation or softening from intense energy source excitation. Also, since the cell end obverse to the thin-film sample support was open, analyses of many types of sample materials were limited to applications in air or inert gas environments. To resolve the difficulties associated with pressure inequalities, a gas-permeable, chemically unreactive polypropylene membrane, called microporous film, was used as a cover for the open end of the Dispoza-Cup, and was attached similar to a thin-film sample support with a nitrile "O"-ring (Figure 1c).

Microporous film is characterized by 0.1µm channels (35% porosity) that permit the permeation of gaseous-size molecules, while at the same time prohibiting the penetration and withdrawal of material from the cell. Microporous film maintains continuous equalization of pressure within the sample cup and sample chamber, thereby averting potential distention or retraction of the thin-film sample support and subsequent alteration of the sample-to-excitation source distance. The use of other plastic materials was then investigated, and a complete cell design change was made that incorporated a type of clamping ring to firmly and easily secure a thin-film sample support. Injection molding was found to be the best method for manufacturing sample cups in volume, a procedure that was adaptable to a large variety of thermoplastic materials.

Fifteen different disposable XRF sample cups are currently available for use with a wide range of commercial x-ray spectrochemical analyzers. The cups are made of polyethylene, which is resistant to chemical degradation, deterioration by excitation of source exposure, and thermal softening by x-ray bombardment. Polyethylene also resists thin-film distortion during and after assembly, is elastic or ease in firmly securing thin-film sample supports without formation of pinholes, and is pure, particularly with regard to sulfur content.

Thin-Film Sample Support Attachment

The design of the cell neck on Chemplex XRF sample cups, in conjunction with the snap-on ring, retains the thin-film sample support in position and maintains it continuously taut up to the completion of assembly. Figure 2 shows the mechanism responsible for effective thin-film sample support preparations. One end of the snap-on ring (Figure 2a) has a semi-spherically shaped "bead" around the interior circumference. Below the bead, the diameter decreases toward the opposite end to form a taper on the inside of the snap-on ring. The cell neck (Figure 2b) has a similar taper on the outer diameter that increases beginning from the edge and extending toward a semispherical indentation around the circumference. The inside diameter of the bead is slightly larger than the outside diameter of the cell neck at the edge. Extending a small distance from the cell neck edge, the bead of the ring meets resistance to further assembly from the increasing cell neck taper. Thus the thin-film sample support material is initially grasped and held taut at all points of contact by the bead and is temporarily stretched (illustrating the need for thermoplastic elasticity) until the bead finally locks into the cell neck (Figure 2c). The thin film sample support thus formed is leak-resistant, wrinkle-free, and taut, reducing the chance of contamination.

The distances from the cell neck edge to the indentation, and from the bead to the opposite end, are slightly dissimilar; the ring extends a slight distance beyond the cell neck. The difference in length prevents the sample support from contacting the surface preparation area and introducing contamination or accidental puncturing. In addition, the snap-on ring will not fit the cell neck unless positioned properly.

Open cell 1500 XRF Sample Cups

The 1500 Series XRF sample cups, the successors to the Dispoza-Cup, consist of a cell open at both ends and two identical snap-on rings (Figure 3). The ends of the cell incorporate tapered and beaded snap-on rings for attachment of thin-film sample supports, microporous film, and the 1600 Series sample cup caps. A ridge at the outer circumference adjacent to the indentation at one end of the, cell shows which is the appropriate side for installing microporous film. As a general rule, microporous film and sample cup caps are attached to the ridged end of the cell, and thin-film sample supports are secured to the opposite end.

A thin-film sample support is positioned over the cell, and a snap-on ring is placed on it and pushed downward to complete assembly, as indicated by a clicking sound. The cell is inverted and a sample is introduced through the top open end and presented for analysis. To avoid spillage during handling or analysis, the open top end of the cell is covered using either microporous film with a second snap-on ring or a 1600 Series sample cup cap. Some analysts reported use of a thin-film sample support on the open top end, but this is not recommended because of significant pressure differentials that may affect the excitation source-to-sample distance or cause a thin-film rupture.

The 1500 Series cups are available in both 32 mm (12 ml sample capacity) and 40 mm (18 ml capacity) diameters, 23 mm in height. The aperture (inside diameter of the cell measured at the opening) is 25 mm and 32 mm respectively for the two sizes. Translated into sample surface areas available for analysis, the 32 mm cups provide 493 mm2 and the 40 mm cups 807 mm2.

Sample Cup Caps

The 1600 Series XRF sample cup caps are used with the open cell sample cups. The caps maintain continuous equalization of pressure between the sample cup and sample chamber concurrent with sample containment.

Sample cup caps, when assembled to a 1500 Series sample cup (Figure 4), are designed to keep a specimen from escaping. The underside of the cap contains a trough between a circular baffle adjacent to an outer circumferential ring. The inside surface of the outermost ring has a semi spherically shaped bead that mates with the indentation in the cell neck. The flat plane of the sample cup cap contains four narrow slits that coincide with the circular narrow trough. When the cap and cup are assembled, the cell neck positions itself in the trough, leaving a narrow passageway to the slits for pressure equalization. The winding passageway prevents material from escaping.

Closed Cell Sample Cups

The 1400 Series closed cell XRF sample cups* (Figure 5) are thermo plastically sealed at one end during the injection molding process and incorporate special features that enhance their versatility.

As with all Chemplex XRF sample cups, the design for the closed-cell cups includes the thin-film sample support "clamping" method for attachment. The closed outside end contains two small circular depressions that are for vent-holes for pressure equalization purposes. One vent-hole is centrally located and the other is off-center. Both are sealed during the injection molding process and can be easily ruptured by a blunt instrument or an automatic vent-hole punch, supplied as an accessory, for establishing pressure equalization. For routine applications, the cell is filled with a sample material and a thin-film sample support is attached with a snap-on ring. The assembled cup containing the sample is inverted and the central vent-hole seal is punctured to establish pressure equalization in air, helium, or vacuum atmospheres.

For special applications, the underside of the closed end contains a receptacle in the center directly opposite the exterior center vent-hole provision (Figure 6) for insertion of a capillary tube or rod to which a micro sample or internally positioned reference material may be attached. The inside diameter of the receptacle is 4 mm, and the receptacle is slightly tapered to accept and firmly retain a rod or capillary tube of similar outside diameter. Different lengths of rod or capillary tube can be used. The off-center vent-hole serves to equalize pressure in applications using the built-in central receptacle. The secondary function of the receptacle is to reduce sample escape during evacuation and to impede the flow of oil-type specimens, which tend to adhere and creep along a surface,

The 1400 Series cups have a collar that entraps or "sandwiches" powdered solid or evaporated liquid specimens between two pieces of thin film (Figure 7), The collar is first introduced to attach a thin-film substrate to the cell; the specimen is deposited on the substrate, and a second thin-film sample support is secured directly over this with a snap-on ring, thereby entrapping the specimen. A third piece of thin film is frequently used to uniformly spread or distribute sandwiched powdered solid sample, to encompass as much surface area as the quantity of specimen allows, The collar is also used to position a sample-impregnated filter paper prior to final assembly with a snap-on ring, Many users initially position a thin-film sample support with a collar prior to final assembly with a snap-on ring, as an added convenience. Closed cell cups are available in 32 and 40 mm diameters with volumetric capacities of 12 and 18 ml respectively, The sample exposure or excitation impingement area is 493 mm2 for the 32-mm size and 807 mm2 for the 40-mm version, Both sizes are 23 mm in height.

Sample Cups for Heat-Sensitive Liquid Samples

The 1800 Series XRF sample cups (Figure 8) accommodate heat-sensitive liquid samples that tend to expand when subjected to intense irradiation or reduction in atmospheric environment. The top end of the closed cell has an overflow reservoir to collect and retain thermally expanded sample materials or oil specimens, which are characterized by their tendency to "creep," and which may escape through the punctured vent-hole, The 1800 Series sample cups have all of the features of the 1400 Series cups, with the exception of the interior receptacle for micros ample mounting, The cups have snap-on rings and collars for thin-layered sandwiched evaporated liquid or powdered solid sample preparations.

Two sizes are available: 32 mm and 40 mm diameters. The 32 mm size accommodates 7 ml of sample material, and the overflow reservoir accepts up to 3 ml. The 40 mm cup will contain 12 ml of sample, with an overflow reservoir of 5 ml. The 32 mm diameter cup exposes 493 mm2 of sample surface area for analysis, and the 40 mm size exposes 807 mm2. Both are 23 mm in height.

Specially Designed Sample Cups

Requests from spectroscopist and x-ray fluorescence instrumentation manufacturers have led to the development of specially configured XRF sample cups, all incorporating the "clamping" method of thin-film sample support attachment.

The 1850 Series disposable, polyethylene XRF sample cup (Figure 9) is designed for use with Horiba SLFA Series instrumentation. The cup features a vent-hole for pressure equalization, an overflow reservoir of 6 ml capacity, and a cell that contains 15 ml of sample. The outside diameter of the 19-mm-high assembled unit is 47 mm. The aperture of the 1850 sample cup is considerably larger in diameter-36 mm-than other cups, and permits a sample exposure area of 1022 mm2. This sample cup is not limited to use with Horiba instrumentation. Many analysts have found the large sample exposure area beneficial in decreasing the influence of powdered solid sample surface irregularity on x-ray data and in reducing integration time. The instrument, however, must be able to accept a sample at least 50 mm in diameter.

Instruments manufactured by Oxford Analytical require disposable sample cups 37 mm in length to accommodate the long depth of excitation source penetration and to avoid impingement with the sample cup. Two disposable sample cups were developed for Oxford Lab-X spectrometers. The 1440-L cup has a closed cell and the 1540-L is open on both sides, with one end containing a slight ridge for convenience of handling. The closed 1440-L cell has a vent-hole provision for pressure equalization. Both cups are 37 mm long and 40 mm in diameter, with volumetric capacities of 25 ml and a sample surface exposure area of 493 mm2.

Convertible Closed/Open cell Sample Cups**

Another unique sample cup design is a convertible closed/open cell (Figure 10). The cup consists of a snap-on ring for attaching a thin-film sample support, an open cell, and a leak-resistant snap-on cap for insertion into the upper open end, thereby converting the open cell to a closed version. The snap-on cap is secured to the cell by the bead and indentation design and can be rotated by force.

A small cutaway in the snap-on cap at the circumference corresponds to a vertical groove in the inside cell wall that extends a short distance to the upper surface. When these are fitted together, an unobstructed passageway from the interior of the cell is established for pressure equalization. Misalignment of the cutaway with the vertical groove converts the cell to a closed unit (Figure 10b).

In practice, the snap-on cap is first inserted in place in a sealed position. The cell is inverted and a liquid or powdered solid sample is introduced, followed by attachment of a thin-film sample support. Prior to analysis, the cap is rotated to the appropriate position for pressure equalization. An overflow reservoir built into the snap-on cap collects heat sensitive liquid samples that may expand as a result of heat generated by intense excitation, This XRF sample cup style is available in 32 mm and 40 mm diameter sizes, both 23 mm in height. The 32 mm cup exposes 493 mm2 of sample area, and the 40 mm version exposes 807 mm2.

Push-Plunger ***, Self-Venting XRF Sample Cups

A disposable XRF sample cup similar to the 1400 Series features a grooved plunger attached in the center to the closed end of the cell (Figure 11). The plunger punctures the thermoplastic seal to equalize pressure within the sample cup and its external environment. This cup features an overflow reservoir; other specifications are similar to those for the 1400 Series cups, including availability of both 32 mm and 40 mm diameter sizes.

Thin-Film Sample Supports

As the range of elemental detect ability to lower atomic numbers and concentrations has been extended, the need for thin-film sample supports characterized by greater analyte-line transmittance properties and greater sample retention strength has also increased. Polyethyleneterephthalate (Mylar®), available in 6.3, 3.6, and 2.5 µm gauges, together with 6.3 µm XRF polypropylene film and 7.5 µm polyimide (Kapton) thin-films, give analysts a range of sample supports for a variety of needs. Figure 12 shows the relative percent transmittance for each thin-film sample support material and gauge to analyte-line wavelength up to 14 Angstroms.

Summary

A broad range of disposable XRF sample cups have been described that were developed in response to advances in WDXRF and EDXRF instrumentation. The cups aid in sample handling and increase efficiency. The cups feature a unique tapered snap on ring with a bead and indentation assembly that clamps the thin-film sample support. Supports come in a variety of materials and gauges.

XRF sample cups are manufactured in the United States in a range of styles, sizes, and specialized features, and can be shipped on the day of order. Engineering expertise, tooling, and supporting equipment are available at the manufacturer to accommodate the special needs of instrumentation manufacturers and users for quality XRF sampling accessories.

Dr. Monte J. Solazzi is President, Chemplex® Industries, Inc., 2820 SW 42nd Avenue, Palm City, Fl. 34990, USA. Tel: (772) 283-2700.

Dispoza-Cup is a trademark of Chemplex Industries, Inc.
* Registered US Patent Number 238,693
** Registered US Patent Number 4,698,210
*** Registered US Patent Number 4,409,854