An Autosampler and Field Sample Carrier for Maximizing Throughput Using an Open-Air, Surface Sampling Ion Source for MS

A commercially available, open-air, surface sampling ion source for mass spectrometers provides individual analyses in several seconds. To realize its full throughput potential, an autosampler and field sample carrier were designed and built. The autosampler provides analyses of 76 swabs in 7.5 min. The field sample carrier simplifies sample collection and provides the cotton swabs nearly ready for analysis upon receipt. These devices were easily fabricated from less than $350 worth of materials using a 10-in. table saw, 10-in. drill press, and small combination lathe and mill. In addition, ion correlation software based on exact masses and relative isotopic abundances was written to deconvolute composite mass spectra that occur in the absence of prior component separation.

Three requirements for rapid analyses are: 1) rapid data acquisition, 2) short intrasample times, and 3) short sample preparation times.

Autosampler

Figure 1 - a) Autosampler; b) magnified view of the weight, a vertical adjustment, pulley, and 7-rpm dc motor; and c) ion source with cotton swab heads inserted through an aluminum bar that rides through the source on two N-scale model railroad flatcars. The two plexiglass and aluminum post, alignment devices fix the bar location along the ionizing beam axis and prevent rotation of the bar about its axis.

Figure 1 shows an autosampler for a Direct Analysis in Real Time (DART™) ion source (IonSense, Saugus , MA) inter faced to an AccuTOF™ time-of-flight mass spectrometer (TOFMS) (JEOL USA, Inc., Peabody, MA). The system provides a 300 °C beam of metastable helium atoms that graze the leading and trailing edges of each cotton swab as it is pulled through the DART. An analyte is desorbed and ionized, and the ions are pulled into the TOFMS through the hole in the cone to be mass-analyzed. The two chromatographic peak areas shown in Figure 2 for each swab in the base peak chromatogram for sulfamerazine are summed to provide a semiquantitative measure of the amount of analyte on the swab. DART/TOFMS analysis eliminates the complex and time-consuming sample extraction, extract cleanup, and chromatography associated with conventional mass spectrometric methods. The instrument is stable and has no capillary tubing prone to breakage or blockage.

Figure 2 - a) Partial m/z 265 ion chromatogram for the precursor ion from sulfamerazine. b) Approximate locations of the swabs when the chromatographic peaks were recorded.

The moving parts of the autosampler in Figure 1a are the 3-ft, ¼-in. square, aluminum bar (Small Parts, Inc., Miami Lakes, FL) mounted on two N-scale model railroad flatcars that ride on N-scale track (Bachmann Industries, Philadelphia, PA). Holes in the bar support the top portion of the cotton swab, wipe samples. The hole-spacing ensures rapid introduction into the ionizing beam of subsequent cotton swabs. In Figure 2, the intrasample time is similar to the time during which the ionizing beam is blocked by each swab.

As shown in Figure 1b, the bar is pulled by 15-lb test, monofilament fish line that is wrapped once around a drive wheel with a 0.273-in.-diam groove on the shaft of a 7-rpm motor (Edmund Scientific, Tonawanda, NY), which pulls the bar at 0.2 cm/sec. The 15-V dc motor is powered by a model railroad transformer (Model Rectifier Corp., Edison, NJ). Four calibration swabs and 72 analyte swabs require 7.5 min to traverse the DART (6 sec/ swab). The autosampler provides rapid analysis and short intrasample times.

To provide straight alignment of the track, plastic-backed, simulated-gravel track was used. The track was tacked to 4-ft and 5-ft lengths of a 1.87-in.-wide, 0.79-in.-deep PVC channel (Small Parts, Inc.), which was supported by pairs of 1-in., 6-32 threaded screws, each with a nut that provided vertical adjustment. The nuts rested in shallow holes in small pieces of wood that fit into slots in the main horizontal wood support, which can be moved forward and backward. Slots in the end vertical supports also provided vertical adjustment of the PVC channel. Wide slots in the horizontal support provided left–right and forward–backward adjustment for the horizontal wood support relative to the two vertical supports affixed to the TOFMS by 2-in., 6-32 screws. These adjustments allowed the track to be aligned and level along the entire distance of travel. In addition, the adjustments permit the autosampler to be backed off in order to clean the inlet orifice of the TOFMS without damage.

Because the height for the horizontal support was less than 3 ft above the floor, 6-ft vertical supports with pairs of pulleys (Palmer pulleys, Science Kit & Boreal Laboratories, Tonawanda, NY) were used to provide nearly 6 ft of travel for the 3-ft-long bar. The bolts on each end of the fish line supplied tension to keep the fish line taut, which ensured a constant speed. To rapidly return the bar to its loading position in front of the data system, the fish line was removed from around the drive wheel, the line was pulled in the reverse direction, and the fish line was replaced around the drive wheel.

This improved autosampler design was based on experience with the prototype described in Ref. 1. Now track derailments are rare and result only from clumsy manipulation of the bar during its placement and removal from the flatcars.

Field sample carrier

Figure 3 - The field sample carrier as it would appear after collection of four wipe samples. The vial has been removed for the fifth cotton swab, which was pushed upward just prior to collecting the wipe sample.

If hundreds of double-bagged, 6-in.-long cotton swab, wipe samples were received with some fraction of illegible labels, the error-prone tasks of unpackaging, trimming, sorting, and inserting the swabs into the correct holes in multiple bars would require many hours. Instead, the field sample carrier2 shown in Figure 3 was designed and built to provide rapid unpackaging and to eliminate the other three tasks. The bar loaded with 6-in. cotton swabs is the core of the carrier. Each swab is protected by a 1.8-mL glass vial that is held in place by a linear cell array made from manila folders and packaging tape. Affixed to both the bar and cell array are labels prepared by a laser printer to ensure legibility. In the field, the stick is pushed upward, the vial is removed and stored in a hole in the carrier, the wipe sample is acquired, the vial is placed over the swab, the swab and vial are reinserted into the bar and cell array, and the stick is trimmed at the base of the cell array. Trimming ensures that the same swab is not used twice, and readies the cotton swab for analysis. The swab is already the correct length and located correctly in the bar when it reaches the laboratory or van.

Figure 4 - a) Cutting of two precut and retaped slits removes the bottom of the linear cell assembly. b) The remainder of the cell assembly and the vials are then lifted upward away from the swabs to prepare the swabs for placement on the flatcars and analysis. (Figure reproduced with permission from Environmental Forensics, www.informaworld.com.)

Figure 4 illustrates the rapid sample preparation after all wipe samples have been collected and all swabs have been trimmed. Two precut slits covered with packaging tape are recut with a hobby knife (Figure 4a), the bottom of the cell assembly falls away, and the vials and top of the cell assembly are lifted upward to reveal the swabs ready for their analytical rail journey (Figure 4b). Calibration swabs are placed in the 1st, 26th, 51st, and 76th holes; the bar is placed onto the two flatcars; fish line hooks are placed on posts on the ends of the bar; the bar is moved so that the first calibrant swab is an inch to the right of the ionizing beam; and data acquisition begins as the motor is started. The motor and data acquisition are stopped about 8 min later. Although this sample preparation has only been done once, it is not likely to require more than 5 min for each bar. Notice that the operator does not label or resort swabs, thereby eliminating these sources of error. The requirement for rapid sample preparation is met by the field sample carrier.

Applications

The need for such high throughput is to rapidly identify major contaminants; map their distribution with high spatial resolution; guide remediation; and document cleanups after deliberate, accidental, or weather-related dispersive events. Other applications for which the cotton swab, wipe samples, autosampler, and field sample carrier could be useful include mapping and remediation of “hot spots” in Superfund sites; characterization and cleanup of clandestine drug laboratories; location of leaks from caps over contaminated sediments; detection of pesticides on fruit skins; and detection of adulterants such as melamine in pet food.3

Mass spectral deconvolution

The trade-off for using autosampler/DART/TOFMS analyses to provide high-speed analyses is that only a few prominent contaminants will dominate the mass spectra. Composite mass spectra are common. The TOFMS provides exact masses accurate to within 2 mDa and relative isotopic abundances accurate to within 20%,4 which are sufficient to determine the elemental compositions of the ions in mass spectra. Using in-house software to correlate precursor ions with product ion:neutral loss pairs, the compositions of the precursor ions for 21 standards and three- and seven-component mixtures have been found.4 The software starts with text files from the data system of m/z ratios and ion abundances and prints out the compositions of the ions in the spectra and lists of unique product ion:precursor ion correlations when multiple analytes are present. This ion correlation and deconvolution software greatly decreases the time required to identify tentatively compounds not present in exact mass libraries.5 Software to rapidly print out a multicolor, semiquantitation map based on nondetect, low, moderate, or high levels of an analyte has also been developed.

References

  1. Grange, A.H. An inexpensive autosampler to maximize throughput for an ion source that samples surfaces in open air. Environ. Forensics 2008, 9, 127–36.
  2. Grange, A.H. An integrated wipe sample transport/autosampler to maximize throughput for a DART™/oa-TOFMS. Environ. Forensics  2008, 9, 137–43.
  3. Vail, T.M.; Sparkman, O.D.; Jones, P.R. Rapid and unambiguous identification of melamine in contaminated pet food based on mass spectrometry with four degrees of confirmation. J. Anal. Toxicol. 2007, 31, 304–12.
  4. Grange, A.H.; Sovocool, G.W. Automated determination of precursor ion, product ion, and neutral loss compositions and deconvolution of composite mass spectra using ion correlation based on exact masses and relative isotopic abundances. Rapid Commun. Mass Spectrom.  2008, 22, 2375–90.
  5. Ojanperä, S.; Pelander, A.; Pelzing, M.; Krebs, L.; Vuori, E.; Ojanperä, L. Isotopic pattern and accurate mass determination in urine drug screening by liquid chromatography/time-of-flight mass spectrometry. Rapid Commun. Mass Spectrom.2006, 20, 1161–7.

Dr. Grange is a Research Scientist, U.S. EPA, National Exposure Research Laboratory, Environmental Sciences Division, P.O. Box 93478, Las Vegas, NV 89193, U.S.A.; tel.: 702-798-2137; fax: 708-798-2142; e-mail: grange.andrew@epa.gov. Note: The United States Environmental Protection Agency (U.S. EPA), through its Office of Research and Development (ORD), funded and performed the research described. This manuscript has been subjected to the U.S. EPA’s peer and administrative review and has been approved for publication.

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