Interview with Dr. Ursula E.A. Fittschen
An interview with Dr. Ursula E.A. Fittschen of the Institute of Inorganic and Applied Chemistry at the University of Hamburg in Hamburg, Germany. Fittschen’s research involves the use of total reflection X-ray fluorescence (TXRF) spectrometry for environmental, aerosol, and elemental analysis.
Your presentation at the 2013 Denver X-ray Conference discussed your research using TXRF to analyze aerosols containing nanoparticles, both PbSe nanoparticles of various shapes and airborne Ag nanoparticles from fabrics. What are the advantages of using TXRF over techniques such as inductively coupled plasma–mass spectrometry (ICP-MS) and laser-induced breakdown spectroscopy (LIBS) for the elemental analysis of nanoparticle samples?
First a few words on the overall procedure: The original nanoparticles were <50 nm in diameter, although it was found that those agglomerate to give larger particles in the micrometer range. The size was determined by collecting the aerosols off-line using a low-pressure impaction device (10 size fractions from 16 µm to 15 nm, which is the aerodynamic particle size). The elemental content was determined using TXRF and the presence of nanoparticles was confirmed using scanning electron microscopy (SEM) backscattering.
One unique feature of X-ray–based techniques is that they are nondestructive. We were able to determine and screen Ag in samples from all size fractions using TXRF, and then confirm the presence of heavy-element nanoparticles using SEM backscattered electron images (Ag could not be detected using SEM–energy dispersive X-ray [EDX] spectroscopy). This would not have been possible using ICP-MS or LIBS.
Alternatively, one may think of on-line size-segregated mass spectrometry. But usually with such an instrument either the particle size or the elemental amounts in terms of mass per air volume are reliably determined.
How does the geometry of TXRF differ from that of conventional XRF spectrometry?
In TXRF the primary beam impinges on the sample carrier (reflector) in a very shallow angle below the critical angle of total reflection. As consequence the beam penetrates only a few nanometers into the carrier, which leads to a very low background compared to conventional setups. The specimen, which is located on the surface of the reflector, is excited by both the incoming beam and the outgoing beam, which leads to the well known increase of the signal (2X) in the regime of total reflection.
How and why are nanoparticles used in commercial fabrics, and what are the potential problems of these nanoparticles if they become airborne?
Ag nanoparticles in consumer products are usually applied because of their antimicrobial properties. Accordingly they are used in fabrics for sportswear but also in cloth used for cleaning. Hazards to human health from airborne Ag nanoparticles may occur when the nanoparticles are inhaled. Because of their small size, nanoparticles are comparable in size to macromolecules and viruses; they have the potential to penetrate deep into the lungs — for example, into the alveoli. Because of the large surface of the nanoparticles they may become active in catalyzing chemical reactions that have the potential to damage cells and tissue.
The good news: We found that the Ag nanoparticles are released mainly in form of larger particles, probably attached to fiber material. The same situation applies for the PbSe nanoparticles, which accumulate rapidly to give larger particles in the size range of 250–500 nm.
What type of sampling procedure do you use to collect and prepare aerosol particle samples from fabrics for analysis by TXRF?
To enhance aerosol mobilization, fabric treated with Ag nanoparticles was abraded over 2 h using the standardized Martindale method (1). During this procedure the aerosols were collected using a low-pressure Berner impactor (10 size fractions from 16 µm to 15 nm). The impaction plates were modified to hold Si reflectors. The Si reflector with the impacted aerosols was inserted directly into a commercially available TXRF instrument and analyzed.
What challenges or limitations have you encountered in using TXRF for aerosol analysis?
To be able to analyze Ag in aerosols with acceptable limits of detection, it was necessary to eliminate the Ar K-alpha line signal. Ar is present in the ambient air and therefore is detected. We built a glove box for the TXRF instrument and analyzed the samples in nitrogen atmosphere. We were able to improve the detection limits from 50 ng (200 s lifetime) in air to 200 pg (200 s lifetime) in nitrogen atmosphere. The latter result corresponds to a limit of detection (mass per air volume) of 4 ng/m3h for the smallest particle size fraction and is even lower for the larger particles.
What are the next steps in your research?
We have already started to microdigest the Ag nanoparticle samples and submit them to ICP-MS analysis. We expect to obtain an additional improvement of the detection limit. Unfortunately, because of the limited amount of sample material it is difficult to obtain the reliability usually associated with this method. We will improve that in the future.