Bill is correct about differences likely to occur among different species, even varieties, and there are also differences by soil type, the weather for any given year, etc.


However, there's another important issue.  NIST (aka the National Bureau of Standards) has a specimen bank program, part of a multi-national effort, in which biological samples are in long term storage (decades or more).  The purpose is to enable retrospective analysis of samples for chemicals.  A major reason for this is to have benchmark samples so if something new is discovered, such as pesticide build-up in bird eggs, they can go back in time, look at bird eggs from earlier times.  For example, in the Great Lakes region, Fish and Wildlife folks in the early 80s thought they had discovered another DDT-like pesticide.  Fortunately, someone had bird eggs from 20 years earlier.


The rest of the story was that the chemical was found at about the same level in the 20 year old samples - so it's presence wasn't increasing in the environment.  Older analytical methods and instrumentation hadn't detected its presence, the modern techniques did, but found that nothing had really changed in terms of environmental exposure levels.


When I started 40 years ago, most instruments had limits of detection (LODs) in thelow ppm, sometimes ppb levels.  One had to concentrate samples or use other laborious techniques to improve detection levels.  Now labs like NSL at Gastonia routinely report LODs of 1-5 ppb for hundreds of chemicals - an unattainable detection limits of only a decade or so back.


Thus, when making comparisons to chemical analysis data from many years past, one has to ask - is the change real or is it an artifact of improved analytical technologies?





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