Wipe sampling techniques

Equipment and area should be verified as clean using wipe sampling techniques (when compound specific analytical methods are available)... 

In situations where health risk concerns are of greatest importance, surface and air contamination standards will often predominate. This is particularly true in commercial office buildings where PCB contamination is present in paint or concrete on walls, furniture and equipment. These are considered high contact areas for employees and, along with air, represent the most direct route of human exposure. Because of the porous nature of concrete, standard solvent-based wipe sampling techniques are not as applicable to concrete surfaces as they would be to bare metal surfaces and other nonporous materials. 

ASTM. 1998f. ASTME 1728. Standard practice for field collection of settled dust samples using wipe sampling methods for lead determination by atomic spectrometry techniques. American Society for Testing and Materials. 

The wipe sample from the organic solvent extraction is then extracted with polar solvent (e.g. water, methanol, or acetonitrile) in the manner described above for nonpolar solvent (Figure 3, fraction 2A). For analysis with GC and GC-hyphenated techniques, the polar extract must be derivatized. A part of the acetonitrile extract can be silylated with BSTFA in the same way as the organic extract of the wipe sample however, methanol or water extracts... 

Black, K.G. and R.A. Fenske (1996). Dislodgeability of chlorpyrifos and fluorescent tracer residues on turf Comparison of wipe and foliar wash sampling techniques. Arch. Environ. Contam. Toxicol, 31, 563-570. 

C) Hmited wipe sampling or other environmental sampling techniques ... 

The subj ect oiwipe sampling is not well defined in standard industrial hygiene reference books. The use of wipe sample datais open to interpretation.However, wipe sampling is of interest to the semiconductor industry because of certain metals used in the manufacturing process such as arsenic, antimony, chromium, and lead. A variation on this technique is also used in the evaluation of surface contamination with sealed radioactive sources (see Ch. 6, Radiation Safety). 

The terms "wipe sampling", "swipe sampling , and "smear sampling" are all used synonymously to describe the techniques used for assessing surface contamination. However, the term "wipe sampling" is one which will be used in this chapter. 

More recently, solid phase microextraction (SPME) [22] has been applied to the analysis of bug pheromones, using two techniques. In the first, headspace volatiles are trapped on the SPME fiber, analogous to trapping on SuperQ [e.g., 23]. Alternatively, if the source of the pheromone is known, the SPME fiber can be wiped on the cuticle to directly adsorb the compounds [24]. In either case, the fiber is then thermally desorbed directly into a GC or GC-MS. Whereas this method is excellent for analysis, with good recoveries, it does not provide a sample that can be used for bioassays or for isolation of an active compound. 

Finally, proper handling technique is very important, especially when it comes to wiping the outside of the needle and the droplet at the tip of the needle prior to injection. Any residual liquid on the outside of the needle will be caught in the septum puncture and will slowly enter the column. This produces broad tailing, especially of the solvent, making separations difficult as well as introducing an unknown amount of sample. On the other hand, liquid in the needle cannot be removed via capillary action of the wiping towel. 

Some autosampler models use air pressure or a piston to push sample out of the vial into the injection loop others pull the sample into the loop with vacuum or a syringe barrel. Various techniques are employed to wipe or wash the outside of the injection needle before the next injection to prevent sample-to-sample contamination. Other systems just wash the inside of the needle by pulling in some of the next sample and spitting it out to a waste vial or back into the last vial used. 

The choice of dust sampling method depends on the purpose of the study. A number of publications compare various wipe and vacuum methods for Pb sampling (e.g., U.S. EPA, 1995 Sutton etal., 1995 Laxen et ah, 1988). Wipe methods have the advantage of being simple and inexpensive compared to vacuum techniques. However, vacuum methods have some distinct advantages over common wipe methods vacuum... 

A method for the detection of nerve agent metabolites ba,sed on GC coupled with an atomic emission detector (AED) has been reported (Creasy etal., 1995). The nerve agent degradation products were extracted from spiked water, wipes, and soil samples. The extracted samples were deriva-tized with 1% trimethylchlorosilane in bis-(trimethylsilyl) trifluoroacelamidc. The GC/AED technique was used for separation, detection, and determination of OP nerve agent metabolites. 

Two further techniques that can be included under the title of direct analysis ate DART and DESI. The focus of these two techniques is slightly different from those already presented in this chapter. What distinguishes DART and DESI is that they do not foctts on gas phase VOCs but instead extract or wipe off analyte mol-ecttles from samples by exposing the sttrface to the ionizing gas or aerosol. The ionization still occms at atmospheric pressure. This enables the direct examination of completely improcessed samples or even entire objects. Both DESI and DART... 

The purpose of this injection technique is to introduce the entire injected sample into the column and use it for trace determination. Different techniques can be used, but the most common is the solvent effect technique, which uses the same instrumentation as used for spht injection (Figure 2.4). In splitless injection, the sample is introduced into the heated liner as in split injection and brought into the gas phase. Contrary to the spht injection, the splitter outlet valve is now dosed. Hence, the total sample volume (1-2 ml of gas) is transferred to the column. When splitiess injection is carried out, the column inlet temperature is kept at a temperature that is 20-50 °C lower than the solvent Bp. Hence, when the sample arrives at the column inlet, the solvent condenses as a thick film on the column wall. This film will act as a plug of stationary phase into which the sample components will be dissolved. Following the sample transfer to the column, which will take 2 min when 2 pi is injected and the carrier gas flow rate is 1 ml min , the column oven temperature is increased. The solvent evaporates first from the column entrance and thereafter the analytes, which will subsequently be separated in the column. The sphtter valve is opened when the whole sample has been transferred to the column in order to wipe out remains of the sample before the next injection. This injection technique is used for trace determinations and can only be carried out in combination with temperature programming. 

If the spectrogram shows that there is too little mull between the windows, the window-mull sandwich should be removed from the cell, the windows separated, and more mull added. (For those mulls where there is a shortage of sample, the available mull can be scraped into a small rectangle in the center of the window). If there is too much mull between the windows, the window-mull sandwich should be removed from the cell and the windows rotated in opposite directions under firm pressure from the fingers. This will squeeze out some of the mull and leave a thinner layer between the windows. If the sample thickness cannot be sufficiently reduced by this technique, the windows can be separated and one of them cleaned by wiping it very gently with a soft, lint-free tissue. The cell is then reassembled and the sample rerun. 

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