Tube type samplers

Other types of diffusive sampler have been less widely applied for indoor air studies. These include tube type samplers that are solvent desorbed and radial type samplers consisting of a cylindrical adsorbent surface that has a short diffusive path resulting in an effective uptake rate that is typically 100 times that of the tube type sampler (Cocheo, Boaretto and Sacco, 1996). One type of radial sampler developed for measuring carbonyl compounds in indoor air comprises silica gel coated with 2,4-DNPH as the adsorbent within a sintered polyethylene tube that acts as a diffusive membrane (Uchiyamaa, Aoyagi and Ando, 2004). 

The tube type samplers have lower diffusive uptake rates and are therefore less prone to starvation effects. In consequence they require longer exposure periods to collect the same mass of analyte. The occurrence of reverse diffusion depends on the analyte-sorbent interaction, but for weaker adsorbents such as Tenax TA this does occur with more volatile compounds for example, compounds more volatile than toluene. 

An example of an evaluation undertaken according to this protocol is for the measurement of benzene in ambient air by tube type samplers subject to thermal desorption (EN, 2005b) and samplers subject to solvent desorption (EN, 2005a). These standards were developed specifically to provide measurement methods meeting requirements for checking compliance with the air quality standard for benzene set by the European Ambient Air Quality Directive (Directive 96/62/EC, 1996). Evaluation of published data concerning the samplers under the test conditions enabled an evaluation of the expanded relative uncertainty of measurement at a limit value (5 pgnT3) for benzene in air and for solvent and thermally desorbed samplers a value of 13.4% was obtained. 

Jia et al. (2005) undertook measurements in 30-50 homes in each of 3 cities in Michigan, USA using a tube type sampler that was thermally desorbed and an exposure period of 3-4 days. 49 compounds were identified indoors and indoor/ outdoor ratios ranged from 1 to 82. Significantly higher benzene, toluene and naphthalene concentrations occurred in homes with attached garages and D-limonene was higher in homes with smokers. Concentrations in air reported in one city are summarized in Table 3.2. 

Tube-type samplers, which are also used for active pumping, have a long axial diffusive path and a small cross-sectional area. These are also called axial diffusive samplers because diffusion of the analytes takes place in the axial direction. 

Badge samplers show greater uptake rates than tube-type samplers due to their greater cross-sectional area. This agrees with the given definition of uptake rate, which is directly proportional to the cross-sectional area and inversely proportional to the diffusive path length. 

As for workplace monitoring, there have been two main types of diffusive sampler used for monitoring of indoor air (i) badge type samplers containing a strong adsorbent such as charcoal that requires solvent desorption for GC analysis and (ii) tube type samplers with weaker adsorbents such as the porous polymer Tenax that can be thermally desorbed. The samplers most widely used have been developed for monitoring of workplace atmospheres and applied to indoor air through modification of the exposure period and the analytical method. 

Tube type samplers with thermal desorption... 

Figure 2 Types of diffusive samplers. From the left a colorimetric length-of-stain sampler a tube-type sampler for thermal desorption a badge-type sampler with multiple channels and no windscreen, containing a removable sorbent pad for solvent desorption a badge-type sampler with multiple channels and an internal windscreen, containing granular sorbent for in situ solvent desorption a badge-type sampler with an external vtrind-screen, behind which is an open diffusion path, and containing a sorbent pad for in situ solvent desorption.

In general, the sorbents used in the tube-type samplers for thermal desorption tend to be porous... 

Figures 3 and 4 display the paired results obtained with Ab-cor and 3M diffusion-type samplers, respectively. These results were analyzed through use of the "t" test for paired samples and the calculation of correlation coefficients and regression equations, with the results of these analyses shown in Table I. A statistically significant correlation is seen between the data set for each type of diffusion sampler and the corresponding tube/ pump sample data set, and the "t" test fails to refute the null hypothesis that there is no significant systematic difference between each of the diffusion sampler data set and the corresponding tube/pump data set.

Samples can be extracted from the product stream by the projeetion of a sample tube, containing a nozzle or orifice, into the flow. The particles impact on the tube and fill the open eavity. The sampling head is out of the stream when not sampling. The snorkel type sampler (Figure 1.9) is available for vertical or inclined applieations and can be pre-programmed on sampling frequency. It is not possible to sample non-homogeneous streams representatively with this type of device. 

The JL900 type samplers are equipped with an alarm system with two GM-tubes above the filter. The readings from the GM-tubes are updated in 1 min intervals and an alarm level of 0.5 pSv/h for a 15 min average reading is in use. For preparedness purposes there are in addition three samplers of type JL150 in reserve. Two of them are located in Helsinki and one is located in Rovaniemi. If needed, they can readily be brought into use and operated anywhere in Finland with the standard 230V (1 kW) power supply. One of the reserve samplers is equipped with a timer. 

ORSA 5 sampler (tube-type diffusive sampler) 400 mg of activated charcoal contained in a 0.5 cm diffusive path and 0.88 cm cross sectional area sampler OVM 500 sampler (batch-type diffusive sampler) 180 mg of activated charcoal contained in a 1 cm diffusive path and 7.07 cm cross sectional area sampler 3 M organic vapor monitor 3500 [OVM]... 

Van de Hoed N. and Asselen O.L. (1991) A computer model for calculating effective uptake rates of tube-type diffusive samplers. Ann. Occup. Hyg., 35, 3, 213-285. 

One important consideration is the possibility of reuse of thermal desorption tubes, since they are cleaned in the desorption step. Charcoal badge-type samplers are of single-use nature but cheaper. Solvent desorption also allows multiple analyses from a single sample to give greater accuracy. 

If a cork-borer type sampler is available, push the tube through the material all the way, so that all layers of material are sampled. Some samplers are intended to be disposable. These should be capped and sent to the laboratory. If a non-disposable cork borer is used, empty the contents into a scintillation vial and send to the laboratory. 

GO-FLO PVC tube. Teflon coating optional, external rubber spring, for single use and in rosette-type samplers, COC, reduced flushing 1.7-100 3.9 7 ul Std., TM GO... 

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

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