3.11  Sampling of heavy metals in particles

3.11.1  Introduction

The recommended sampling frequency for heavy metals in particles is weekly and is recommended to collect the PM10 fraction. A cost efficient strategy may be to combine PM10 measurements that also are part of EMEP’s measurement program with particulate heavy metal sampling. However, since the PM10 measurements are to be done on a daily frequency extracts should be combined into weekly samples to be analysed for heavy metals. Analysis on daily samples should be carried out during campaign studies. It is though important to realize that when collecting particles for heavy metal analysis it is necessary to take extra precautions to prevent contamination.

Either high- or low volume sampler may be used. The choice depends on the sampling period. For short sampling periods (daily) a high volume sampler is usually needed. With a high volume sampler it is possible to collect samples of 1600 m3 per 24 hours. Low volume samplers are in comparison in the range of 1-3 m3 per hour. The filter pack method, used for sampling of the main air components is a low volume sampler and can be used for heavy metal sampling if the sampling period is longer, i.e. weekly sampling. For clarification, many particle samplers that today are called low volume sampler have also been classified as medium volume sampler since they have been compared with the flow rate of absorption solution which has even slower rate.

Another important issue is which particle size should be sampled, fine (PM2.5), coarse + fine (PM10) or total particulate matter (TSP). Most of the high or low volume samplers provide the possibility to collect either total suspended particle matter or a fraction with a defined cut-off; which to prefer depends on the aim of the measurements. In order to obtain the best estimate of the deposition of heavy metals, a size distribution is preferred. However, several studies indicate that the mass distribution is different for the elements, and to obtain an informative size distribution there should be sampled 7-8 fractions. This is very expensive and for monitoring purposes it is generally sufficient to sample one fraction. Sampling of particles less than 10 mm will to a large extent contain the main fraction of long range transported heavy metals and it is recommended that this fraction is collected. A size distribution should be done at a few stations in Europe.

When collecting particles for heavy metal analysis it is necessary to take precautions to prevent contamination. All equipment used has to be cleaned carefully and gloves must be used whenever the filters are treated.

3.11.2  Sampling equipment

3.11.2.1  The air sampler

The same sampler as for PM10 measurements described in 3.15 can be used. 

In addition low volume sampler for the filterpack system can be used. A general description of this sampling equipment is written in the Chapter 3.2 under sampling of the main components.

3.11.2.2  Filters

See general description under 3.15.3. It is strongly recommended to use either teflon or quartz filters for heavy metal sampling; however the teflon filters can not be used for high volume sampling with a prescribed flow, but may be used for low volume sampler. Glass filters do often have high blank values for certain elements. Teflon holders are recommended since they can be washed with acid before use and generally have low blank values. Quartz glass filters do have the advantage that they can be baked at 500oC prior use. Cellulose filters may be used as well; however, it must be checked whether the flow rate is maintained at given level.

All handling of filters should be made in a clean air facility (clean room or at least clean bench). Plastic gloves and acid washed utensils are necessary. All equipment should be stored in double plastic bags in a dust free environment. All parts that may be in contact with the filters should, if possible, be cleaned in nitric acid before the filters are inserted, and after exposure the filter should be stored in acid cleaned equipment.

3.11.3  Sampling procedure

In addition to the general siting criteria given in Chapter 2, it is important that the inlet is located far from any obstruction that might influence on the airflow, like building walls and trees etc. Some pumps have shown to give some dust containing copper. Air from the pump should therefore be transferred in a separate tube at least 10 metre from the filter intake.

The sampling procedures may be somewhat different from one air sampling system to another. Standard operating sampling procedures (SOP) should therefore be based on the sampler’s operator manual. The procedure is also dependent on whether the filters need to be weighted for PM10 measurements or not, see Chapter 3.15.

It is very important that the filters are stored within a plastic bag with zippers when it is transported between the laboratory and the field. One should also use tweezers made of not metallic material or covered with teflon when touching the filter. Never touch the filters with the fingers. After exposure the high volume filters are folded in two with the exposed side against each other, put in the plastic bag and sent to the laboratory for analysis. For low volume sampler the filter holder is sent to laboratory where it is dismounted.

Field reporting forms should always be put in a separate plastic bag in case of accidental leaks from precipitation samples, which may be contained in the same transportation box.

3.11.4  Field blanks

Field blanks should be taken regularly, at least every quarter a year. They should be handled similar as the other filters from the batch and given the same chemical treatment and analysis as the exposed filters. If blank values exceed 20% of the concentration normally measured at the site measures should be taken to reduce the blanks (i.e. exchange or cleaning of sampling devices). The yearly average blank values are used to determine the detection limit and should be reported to CCC.

3.11.5  Extraction from filters

Filters used in high volume samplers should be cut into smaller pieces before extraction. The partition may be done by folding and cutting 1/4 or 1/8 of the filter using non-metallic tools. Another option is to use a well-defined form to cut a piece of e.g. 10% of the filter. The filter part is cut into smaller pieces and 6-7 ml of concentrated nitric acid is added to the filter in a teflon bomb. To extract particles from filters in a low volume sampler, the whole filter is included and 1.5 ml concentrated nitric acid is added to the bomb.

The bomb is kept at 150-170oC for 6-8 hours. The solution is cooled to room temperature and transferred to a 50 ml volumetric flask. If the analyses are to be performed using ICP-MS the internal standard is added to this solution and then diluted to the mark with distilled water. The solution should now contain 10% HNO3.

3.11.6  Filter blanks

It is recommended that 5% randomly selected samples from each new batch of filters are analysed as laboratory filter blanks. The purpose of the filter blanks is to control the quality of the filters rather than to estimate the laboratory detection limit. Normally, the blank values should be sufficiently low that their values can be ignored. If high blank values are found a problem has occurred which has to be identified and solved, e.g. by using filters or chemicals from another batch, and by inspection of the routines in the laboratory.

3.11.7  Calculation of results

The flow volume is given in cubic feet per minutes (cfm) for some samplers, it should be given in cubic meter per hour.

          1 foot3/min = 0.02832m3 * 60 min/hour = 1.698 m3/h.

When calculating airflow the average between start and stop flow is used. The total air volume in cubic meter is then given as:

          Total air volume = Average flow * total sampling time (h)

The concentrations of the element in the air sample should be expressed in ng /m3; this is given by:

         

C         is metal concentration in ng/m3
a          is concentration of the element in ng/ml
vextr       is the extraction volume in ml
vair           is the air volume from the sampler, in cubic meter at approximately 20°C, and corrected for height from elevated sites.
Fb                    is average amount of metal in field blank (ng)

3.11.8  Quality assurance

See chapter 3.10.7


Last revision: November 2001