X-ray photoelectron spectroscopy (XPS) can be used to determine the contaminants present on laboratory gloves and to evaluate the type and amount of contamination transfer from gloves to other surfaces.
Surface contamination on analytical samples or other handled materials can come from sources such as human hands, inappropriate packaging materials, airborne dust fallout, contaminated sample handling tools, or contaminated sample holders. An often overlooked source of contamination is disposable laboratory gloves, which are frequently worn for the purpose of protecting handled materials from fingerprints and other contaminants present on bare fingers. However, the gloves themselves may be a source of potential contamination transfer equal to or even greater than bare fingers. This study shows how X-ray photoelectron spectroscopy (XPS) can be used to determine the contaminants present on laboratory gloves and also evaluate the type and amount of contamination transfer from gloves to other surfaces.
Disposable elastic gloves are ubiquitous in scientific laboratories and are also widely used in many industries while handling critical surfaces. Disposable gloves are typically made from nitrile or latex rubber and offer their users protection from various aqueous acids and bases, biological and medical fluids, organic solvents, and other potentially harmful substances. A second major use of disposable gloves is to protect manufactured products and analytical samples from contamination caused by the transfer of skin cells, oils, salts, cosmetics, hand lotions, or other residues resulting from contact with bare hands. However, disposable gloves can also be an overlooked potential source of contamination on handled surfaces. In addition to the primary polymer structure, many types of common laboratory gloves also contain a variety of inorganic salt additives in the glove formulation. For example, zinc oxide is often added as an accelerator and calcium nitrate is used as a coagulant (1). Post-forming processes such as chlorination are often used to oxidize the outer glove surface to reduce surface tackiness (2). Silicone-containing mold-release agents that allow powder-free gloves to be easily stripped from the glove formers during fabrication may be present on glove surfaces. Silicones are leachable and can be easily transferred to any object they contact (2). Furthermore, the inner surfaces of disposable gloves may have various polymeric surface coatings for improved donning properties or other specialized uses (2). These inner coatings or bulk glove components may permeate the glove material and segregate to the outer glove surface after exposure to certain solvents. Contact with solvents during rinsing of items being held may also transfer glove components to the surfaces of those items. Contamination resulting from surface residues on gloves can adversely affect materials used in industries in which surface cleanliness is essential for optimum product performance and can also interfere with sample analyses depending on the specificity and sensitivity of the analytical technique being used. Therefore, it is important to know if the various components within a particular glove material are leached out by certain solvents or if manufacturing residues present on the surfaces of gloves are easily transferred to other materials.
Studies regarding contamination on laboratory gloves have been previously reported. Roberts and colleagues (3) studied the organic and inorganic components extractable by ethyl alcohol from a variety of latex and polyvinyl chloride (PVC) gloves using infrared (IR) spectroscopy and optical emission spectroscopy (OES), respectively. This study also reported on the elemental composition of fingerprints left by gloves on aluminum foil using Auger electron spectroscopy (AES) (3). All of the gloves examined by Roberts and colleagues were found to contain mobile organic species (for example, plasticizers) and trace metal components (for example, Na, Mg, Si, K, Ca, Fe, Cu, and Zn) (3). AES and dynamic, quadrupole-based, secondary ion mass spectrometry (SIMS) were used in another study to investigate the transfer of elemental components (such as Li, C, Na, Mg, Al, K, and Ca) present on latex, nitrile, and vinyl cleanroom gloves to semiconductor wafer surfaces (4). It was found that the contamination levels transferred from gloves to the wafers could be substantial and comparable to those observed from bare hands (4). Friel and colleagues (5) studied inorganic contamination encountered in trace elemental laboratories resulting from the use of latex and vinyl gloves by flame atomic adsorption spectroscopy (AAS) and inductively coupled plasma–mass spectrometry (ICP-MS). Results indicated that many latex gloves contained such high levels of biologically important elements (for example, Mn, Fe, Co, Cu, Zn, Se, and Mo) that they were unsuitable for trace-element work (5). Vinyl gloves were also found to contain high levels of many elements and it was recommended that vinyl gloves should be routinely acid-washed to remove the contamination before being used in a trace-element laboratory (5). High performance liquid chromatography (HPLC) was used to determine extractable nitrite and nitrate species present on latex, neoprene, polyethylene, and other gloves as potential sources of contamination interfering with nitrite and nitrate medical assays (6). Sovinski (7) studied contact transfer of nonvolatile residues (NVR) from latex, nitrile, and polyethylene gloves to steel surfaces using gravimetric methods and Fourier transform infrared (FT-IR) spectroscopy. FT-IR was also used by Castino and colleagues for analyzing organic residues on latex and nitrile gloves (8).
Figure 1: XPS survey spectra of the outer surfaces of example nitrile and latex laboratory gloves and the qualitative and quantitative (atomic %) results. (N.D. denotes "not detected".)
The above studies have demonstrated that techniques like FT-IR are useful for determining specific types of organic contamination on gloves and techniques such as AES, SIMS, AAS, OES, and ICP-MS are useful for determining the presence of metals and some anions. Each of these techniques, however, has some drawbacks for analyzing glove materials. FT-IR, for example, generally cannot provide much useful information on inorganic species or trace metals. AES and dynamic SIMS analyses are usually difficult to perform on insulating materials like polymeric gloves and do not provide much chemical state information. In addition, SIMS spectra are often quite complex and difficult to quantify. Trace-elemental techniques such as AAS, OES, and ICP-MS require digestion of the glove sample and provide total contaminant information within the bulk of the gloves, but they do not adequately represent the extent to which those elements are on the surface of the gloves or the extent to which those elements might transfer to another surface. These bulk elemental techniques also do not provide chemical state information or information on the organic species that may be present on glove surfaces. Hence, a better approach for evaluating potential surface contamination on gloves would be to use a technique that readily provides information about both the organic and the inorganic species present on the glove surface as well as surfaces that come into contact with the gloves being tested. X-ray photoelectron spectroscopy (XPS) is a qualitative and quantitative surface-sensitive (nominal sampling depth ≤ 10 nm) technique that can detect all elements, except for hydrogen and helium, and can also provide information about metal and anion oxidation states, chemical bonding, and general types of organic functional groups (9). The technique is rapid (for example, typical survey analysis times of <5–10 min/sample) with a detection limit of ~0.1 atomic % for most elements, nondestructive, can be used for insulators as well as conductive samples, and typically requires minimal sample preparation (9). XPS is much more surface sensitive than FT-IR and has a sampling depth and detection limits similar to AES (9). In this study, XPS was used to characterize the elemental and chemical surface compositions of the outer and inner surfaces of a variety of common disposable laboratory gloves. Changes in the surface composition of the gloves following exposure to several common laboratory solvents were also investigated. The transfer of surface components from the gloves to clean aluminum foil surfaces by light touching was also evaluated.
Table I: Surface compositions (atomic %), as determined from the XPS survey spectra, for the outer (O) and inner (I) surfaces of various gloves examined in this study
We examined 15 different types of powder-free laboratory gloves (seven nitrile, seven latex, and one neoprene) in this study. The various gloves will be generically referred to in this report only by type and number (for example, nitrile 1) keeping the manufacturer's identity anonymous. Fresh glove samples were obtained from unopened boxes of each type to avoid potential external sources of contamination on the glove surfaces. Sample sections (approximately 1 cm × 1 cm) were cut with scissors from the index finger region of each glove. No other sample preparation was necessary. The outer and inner surfaces of each glove were analyzed. In addition, the outer surfaces of each glove were reanalyzed following a 5-min rinse (and subsequent air dry) in four different commonly used laboratory solvents: acetone, chloroform, hexane, and methanol. Potential transfer of glove components on the outer surface of the gloves to other surfaces was tested by analyzing the surface of the bright side (smoother side) of clean household aluminum foil (Reynolds Wrap) after lightly touching the foil surface with a gloved finger.
Figure 2: High-resolution C 1s spectrum fitted with three peaks for the outer surface of nitrile glove 7. Note that trace amounts (that is, 0.2 atomic %) of potassium were also detected on this sample.
XPS was performed with a Thermo Scientific Model K-Alpha XPS instrument. The instrument uses a monochromated, microfocusing, Al K? X-ray source (1486.6 eV) with a variable spot size (that is, 30–400 µm). Analyses of the glove and aluminum foil samples were all conducted with a 400-µm X-ray spot size for maximum signal and to obtain an average surface composition over the largest possible area. The instrument has a hemispherical electron energy analyzer equipped with a 128-channel detector system. The base pressure in the analysis chamber is typically 2 × 10-9 mbar or lower. Samples were mounted to the sample platten using double-sided tape. Areas were chosen for analysis by viewing the samples with a digital optical camera with a magnification of approximately 60–200?. Survey spectra (0–1350 eV) were acquired for qualitative and quantitative analysis and high-resolution spectra were acquired for appropriate elements for chemical state characterization. The survey spectra were acquired with a pass energy of 200 eV and the high-resolution spectra were acquired with a pass energy of 50 eV. While not providing the maximum spectral resolution for the XPS instrument, the 50-eV pass energy used for the high-resolution spectra provided high sensitivity for low level components and sufficient spectral resolution to allow general chemical state identification. All spectra were acquired with the charge neutralization flood gun turned on to maintain stable analysis conditions on the insulating glove samples. The flood gun uses a combination of low-energy electrons and argon ions for optimum charge compensation. The typical pressure in the analysis chamber with the flood gun operating is 2 × 10-7 mbar. Data were collected and processed using Thermo Scientific Avantage XPS software. Spectra were charge corrected using the main C 1s peak because of adventitious hydrocarbon set to 285.0 eV. Peak fitting was performed using mixed Gaussian and Lorentzian peak shapes and a Smart Shirley type background. Depth profiling analyses were conducted with a Thermo Scientific EX06 argon ion gun operated at 1000 eV and rastered over a 2 mm × 4 mm area. Sputtered depths were calibrated with a 100-nm SiO2/Si standard.
Figure 3: High-resolution S 2p spectrum fitted with two peaks for the outer surface of nitrile glove 2.
XPS survey spectra allow straightforward and rapid qualitative and quantitative elemental characterization of glove surfaces. Examples of XPS survey spectra are shown in Figure 1 for the outer surfaces of a nitrile (nitrile 4) and latex (latex 2) glove. These results demonstrate that the surfaces of different laboratory gloves can vary to a large degree. In this case, the nitrile glove had only a small amount of oxygen present plus a few minor surface components (for example, Si, S, and Ca). In comparison, the latex glove had a much lower surface carbon concentration and approximately 10 times the oxygen present along with substantial amounts of N, Mg, Si, S, Cl, and Ca, plus minor amounts of P and Zn. Table I shows the qualitative and quantitative results obtained for the outer and inner surfaces of all the glove types used in this study. Substantial differences in the qualitative and quantitative surface compositions were observed among the different brands and types of nitrile, latex, and neoprene gloves. The wide variations in surface composition among the nitrile and latex gloves most likely result from a combination of different nitrile or latex rubber formulas used by the manufacturers, differences in processing conditions, and the use of different surface coatings. Substantial differences in surface composition were also observed between the outer and inner surfaces for the majority of the gloves investigated in this study. However, in two cases, (that is, latex 4 and latex 7), the surface compositions of the outer and inner glove surfaces were similar for a given glove (see Table I) and indicate that the inner surfaces on these two types of gloves were most likely uncoated. Elements detected on the surfaces of one or more of the gloves included C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, and Zn.
Table II: Chemical degradation resistance of various types of laboratory glove to various solvents (from reference 12)
As mentioned previously, FT-IR can be used to identify various classes of organic residues on glove surfaces and in some cases can identify specific organic compounds (8). In comparison, high-resolution XPS cannot unequivocally identify exact organic compounds, but it can be used to determine the general chemical states of carbon on glove surfaces as shown in Figure 2 for a nitrile glove (nitrile 7). In addition to the main hydrocarbon peak (285.0 eV), which corresponds to only C-C or C-H bonding, peaks characteristic of nitrile (C≡N) or ether/alcohol (C-O) functional groups (286.6 eV) and carbonate species (290.2 eV) were observed at higher binding energies (10,11). Note that trace amounts (0.2 atomic %) of potassium (as K+) were also detected on this particular sample, as indicated by the presence of the K 2p3/2 and 2p1/2 peaks at binding energies above the C 1s region. A peak corresponding to carbonate species was observed in the high-resolution C 1s spectra for several other gloves examined in this study as well. Castino and colleagues (8) also reported the presence of carbonate species on latex gloves. Knowledge of the organic functional groups present on glove surfaces can be useful for determining compatibility with other materials and when tracing sources of contamination on handled materials.
Figure 4: High-resolution N 1s spectrum for the outer surface of nitrile glove 6 showing two peaks characteristic of organically bound nitrogen species and nitrate species (total N surface concentration = 0.4 atomic %).
High-resolution XPS also can be used to investigate the chemical states of other elements present on glove surfaces. For example, Figure 3 shows the S 2p spectrum obtained for a nitrile glove (nitrile 2). Sulfur was detected on this glove at a surface concentration of 3.6 atomic % and was detected in two chemical states characteristic of sulfate (SO32-) or sulfone (R2SO2) species (168.7 eV) plus sulfide (S2-) or mercaptan (R-SH) species (163.2 eV) (10,11). Two peaks corresponding to these types of sulfur species were observed in the S 2p spectra obtained for all of the gloves in this study; however, the relative intensities of the peaks (corresponding to the relative concentrations) varied widely. In addition to sulfur chemical state identification, high-resolution XPS results obtained for the various types of gloves in this study indicated that in general, when the following elements were detected: nitrogen was present as organically bound nitrogen species such as nitrile or amine groups (~399-401 eV) plus, in some cases, metal nitrates (~407–408 eV); silicon was present as silicone or silicate species (~102–103 eV); phosphorus was present as phosphates (~132–133 eV); chlorine was present as organically bound chlorine (C-Cl) or metal chloride species (~199–200 eV); and all detected metals were present in their highest stable oxidation state (10,11). Figure 4 shows a high-resolution N 1s spectrum obtained for nitrile glove 6 where organically bound nitrogen and nitrate species were both readily detectable at a total nitrogen surface concentration of only 0.4 atomic %. Makela and colleagues (6) reported that low-level nitrate contamination on latex gloves can affect the accuracy of medical nitrite and nitrate assays if samples are mishandled. The examples described above demonstrate how XPS can readily provide chemical state information even for elements that are present in low-to-trace surface concentrations.
Table III: Surface compositions (atomic %), as determined from XPS survey spectra, for as-received and solvent rinsed (5 min) nitrile glove 7 and latex glove 3
Use of laboratory gloves is the most common approach to protecting individuals from contact with hazardous chemicals. However, some chemicals may remove components from the glove surface or they may cause bulk components to segregate to the glove surface. Therefore, the effects of various chemicals on the surface composition of laboratory gloves are of interest and XPS is an excellent technique for conducting such studies. Table II shows general recommendations for the chemical compatibility of latex, neoprene, and nitrile gloves with acetone, chloroform, hexane, and methanol (12). The recommendations shown in Table II indicate that the chemical resistance of latex, neoprene, and nitrile vary greatly for a given solvent. Figure 5 shows wide range (0–250 eV) high-resolution XPS spectra obtained for an as-received latex glove (latex 3) and the same type of glove following a 5-min rinse with acetone, chloroform, hexane, or methanol. The spectra indicate that the solvent rinses affected the surface composition of the glove in all four cases. Table III shows examples of the qualitative and quantitative XPS results obtained for the solvent rinse studies on a nitrile (nitrile 7) and latex glove (latex 3). (All solvent-rinse analyses were conducted on the outer glove surfaces.) The results shown in Table III indicate that the surface compositions of the rinsed gloves varied widely with the type of glove and the solvent, regardless of the reported chemical compatibility. For example, all four solvents removed Cl from the surface of latex glove 3 at least below the detection limit of XPS. In contrast, only methanol removed Cl from the surface of nitrile glove 7. Rinsing nitrile glove 7 with either chloroform or hexane increased the Cl concentration several fold compared to the as-received glove, whereas rinsing with acetone had only a minor effect (see Table III). All four solvents also partially removed Si from the surface of nitrile glove 7. In contrast, all four solvents substantially increased the surface concentration of Si on latex glove 3. Similar variable composition results were obtained for the other gloves studied in this investigation. In any case, it is clear that common laboratory solvents can alter the surface composition and the contaminants present on laboratory glove surfaces even in cases where the particular solvent and glove type are considered to be compatible with respect to chemical degradation of the bulk glove material. It is therefore important to consider secondary transfer of glove contaminants removed by processing solutions as a potential route of glove components onto the surfaces of other materials. These results also suggest that washing gloves before use with appropriate solvents (followed by drying) could be a way to remove the native contamination present on the gloves and minimize or eliminate subsequent contamination transfer to handled materials. Washing laboratory gloves to remove contamination prior to use has been recommended by others (5,8).
Figure 5: High-resolution XPS spectra (~0â250 eV) for the as-received outer surface of latex glove 3 and following a 5-min rinse in various solvents.
Surface contamination on analytical samples or other handled materials resulting from the direct transfer of residues present on gloves is probably overlooked in most laboratories and industries, but it can be important and should be considered. For example, Figure 6 shows XPS survey spectra obtained for a clean, untouched sample of aluminum foil and the same foil after being lightly touched with a gloved finger (latex 3). The main surface components detected on the clean aluminum foil were C, O, Mg, and Al, plus a trace amount of Ca. After the touch test, the surface concentration of C increased substantially compared to the clean surface (55.4 atomic % vs. 6.1 atomic %, respectively), and Si, which was undetected on the clean foil, was now a major surface component (20.9 atomic %). After the touch test, the main surface components detected on the foil were C, O, and Si, plus only a small amount of Al and a trace amount of Ca. The observed binding energy of the Si 2p peak (102.5 eV) on the glove-touched sample would be consistent for silicone species (10,11), which suggests that a silicone oil surface contamination was present on this type of glove.
Table IV: Surface compositions (atomic %), as determined from XPS survey spectra, for as-received, untouched aluminum foil and aluminum foil after light touching by bare fingers and fingers covered by various laboratory gloves
Table IV summarizes the qualitative and quantitative XPS results obtained for the aluminum foil touch tests for all of the gloves studied in this investigation. After touching a foil sample with a bare finger (bare finger 1), XPS indicated that C, N, Na, Si, S, and Cl had been transferred to the foil surface. A second touch test with a bare finger after first lightly wiping the finger on the individual's forehead (bare finger 2) gave similar results (plus a trace amount of Ca); however, in the second test the surface concentration of C was much higher than that found for the bare finger test without the preceding forehead wipe. These results are consistent with the transfer of skin oils from the forehead to the bare finger in the second bare finger test. The results shown in Table IV indicate that C, N, Na, Si, S, Cl, K, Ca, and Zn may all potentially be transferred to other touched surfaces depending on the type of glove used. The amounts of these transferred elements detected on the foil surfaces varied greatly among the different glove types. Results indicated that carbon was the most frequently and presumably the most easily transferred element detected, regardless if the glove tested was nitrile, latex, or neoprene. In one case (nitrile 4), the transfer of carbon species to the foil surface was such (that is, 91.2 atomic % C) that the underlying aluminum foil was barely detectable (that is, 0.2 atomic % Al). Only one of the 15 gloves tested (nitrile 2) showed no detectable surface contamination transfer to aluminum foil, other than a small increase in the amount of surface carbon species (that is, 11.0 atomic % C vs. 6.1 atomic % C). These results indicate that in cases where surface cleanliness is essential, avoiding contact between laboratory gloves and the handled material will eliminate direct transfer of potential contamination.
Figure 6: XPS survey spectra for clean, untouched aluminum foil and foil lightly touched by latex glove 3.
Figure 7 shows an XPS depth profile for an aluminum foil sample that was touched by latex glove 1. The depth profile indicates that the silicon-containing species was concentrated in the topmost ~2 nm of the sample surface and a carbon or oxygen-containing material was detected to a sputtered depth of ~40 nm. In contrast, Figure 8 shows the XPS depth profile obtained for an aluminum foil sample that was lightly touched by latex glove 3. In this case, Si was concentrated in the topmost ~10 nm of the foil surface and a carbon- plus oxygen-containing material persisted to a sputtered depth of >100 nm on the foil surface. Note that the relative C and O profile shapes were different for the foil samples touched by latex glove 1 and latex glove 3 (see Figures 7 and 8, respectively), indicating different compositions for the transferred carbon- plus oxygen-containing materials. These results also indicate that the thickness of surface contamination layers resulting from laboratory glove contact varies substantially with the type of glove.
Figure 7: XPS depth profile for an aluminum foil sample that was lightly touched by latex glove 1.
XPS was used to investigate the surface compositions of a wide variety of laboratory gloves. XPS results indicated that in addition to the expected rubber polymer components, silicones and numerous other compounds containing O, Na, Mg, Al, P, S, Cl, K, Ca, and Zn may also be present on the outer glove surfaces. Many of these additional surface components can be readily removed from the glove surfaces or may diffuse from the bulk glove material to the glove surface by contact with common laboratory solvents. In addition, many of the glove surface components may also easily transfer to other surfaces when only lightly touched. In this study, only one out of the 15 gloves studied did not transfer substantial amounts of contaminants to aluminum foil during a touch test. Therefore, when handling samples for XPS or other surface sensitive analyses or when handling materials where surface cleanliness is a priority, it is always best to make use of clean handling tools rather than gloved hands. If you use gloves in your laboratory or manufacturing process, XPS is the ideal analytical technique for investigating the surface composition of the gloves, for identifying potential contaminants, and for evaluating the potential for contamination transfer to handled surfaces.
Figure 8: XPS depth profile for an aluminum foil sample that was lightly touched by latex glove 3.
Brian R. Strohmeier and Alex Plasencia are with Thermo Fisher Scientific in Madison, Wisconsin. John D. Piasecki was formerly with RJ Lee Group, Inc., in Monroeville, Pennsylvania, and is currently with the Pennsylvania State Police. Please direct correspondence to: firstname.lastname@example.org.
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