Types of Samples
The following are various types of sample that can be collected.
(i) Grab Samples: These are single samples collected at specific spot at a siteover a short period of time. These types samples represent a “snap shot” in both space and time of the sampling area.Grab Sampling can be
(a) discrete grab samples (ie) samples taken at a selected location depth and time,
(b) depth- integrated sample which are collected over a predetermined part or to entire depth of an area with respect to location and time. Grab Sample is suitable either when the source is known to vary with time or when source composition varies in space.
(2) Composite Samples: These types of sample provide more representative sampling of heterogeneous matrices in which the composition of the analyte of interest may vary over a period of time and or space. Composite samples can be
(i) sequential (time) composites are collected using continuous and constant equipment (like pumping machine or by mixing equal volume of water collected at a regular time interval,
(ii) Flow-Proportional Composite Samples are collected at a rate proportional to flow rate.
(3) Integrated Samples: At times, information or result desired may be best provided by analysing mixture of grab samples collected from different points simultaneously or as nearly so as possible using discharge- weighted methods such as equal width increment (EWI) or equal discloses increment (EDI). Generally sampling can be done manually or with the aid of instrument depending on various factors such as cost of the analysis, size of the sampling site and the numbers of samples to be procured.