The Laboratory Notebook

Answering important questions about quality.

By Robert Green

Q. I received heavy metal results, which are in terms of “parts per million.” What exactly does this mean, and how do I use it to determine how much heavy metal is in my product?

A. That’s an excellent and frequently asked question. First some background. If you send us a representative sample of capsules or tablets, we can tell you how much of a given ingredient is in a capsule. That’s because a capsule is a finite amount. We can calculate the average fill weight of your capsules which, so long as we have a representative sample, will hold true for the entire lot. Therefore, we can accurately state that you have a specified amount of a given material in each capsule.

Next, instead of sending us capsules or tablets, let’s say you send us a baggie with a raw material ingredient. We can, of course, determine how much of a given material is in the baggie, but what good does that do you? Since the baggie is not representative of anything, knowing how many milligrams of an ingredient are in it is useless information. Instead of reporting how many milligrams of the material are in the baggie, we report a level of concentration. That is, the amount of one material in a larger amount of another material. These results are expressed as concentrations rather than total amounts, so you can easily apply our findings to different (and relevant) situations. So, for example, if your baggie is filled with an orange drink powder, we can tell you that 20% of the powder is vitamin C. It doesn’t matter how much powder you have (so long as the sample we have is representative of the whole); 20% of it will consist of vitamin C. The nomenclature we use is “% wt / wt”. Whether you have 1 gram or 100 kilograms of powder, 20% of it will be vitamin C. This is a lot more useful than reporting that your baggie as 3.5 grams of vitamin C. Again, the key is that the sample we receive is representative of the whole.

Now let’s move on to your heavy metal example. Hopefully, the amount of heavy metals in your material is very small. Expressing it in terms of a percentage would be too cumbersome. Instead, we use parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt).

An illustration will help in understanding this “parts per” idea. If we divide a pie into 10 equal slices, then each slice would be a “part per 10”. Now divide that pie into one million slices and each slice becomes a part per million of the original pie. Take each one of these slices and cut them into a thousand slices, and each resulting slice becomes a part per billion of the pie. Do that one more time and you are at parts per trillion.

In our pie example each slice consists of the same material as the whole, so each slice is identical to every other slice. Instead, let’s say one slice out of our million-slice pie was contaminated. That’s one slice of “bad pie” and 999,999 good slices. So your concentration of bad pie is 1 ppm.

A ppm is so small it is hard to conceptualize. A ppm is:
•One in 1,000,000
•One inch in 16 miles
•One minute in two years
•One cent in $10,000

With 1 ppm consisting of 1,000 ppb, the ppb equivalents are:
•One in 1,000,000,000
•One inch in 16,000 miles
•One second in 32 years
•One cent in $10 million

If you multiply these amounts by 1,000 you are in ppt territory.

Okay, so you now understand the ppm/ppb/ppt concept. How do you determine how much heavy metal is in your capsule? We need to convert the parts per amount into a weight unit. This is easiest done using the metric system, since it goes by steps of 10, 100 and 1000.

For example, a milligram is a thousandth of a gram and a gram is a thousandth of a kilogram. Therefore, a milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram. So one ppm is the same as one milligram per kilogram, which is abbreviated as mg/kg.

Similarly, a microgram (which is abbreviated as ug) is a thousandth of a milligram. Since a gram is a thousandth of a kilogram, for smaller quantities we can express the mg/kg formula as ug/g, which is more useful to us when we are dealing with capsules and tablets.

Now let’s bring this baby home. You have a representative sample of material with 12 ppm of lead. How much lead is in each of your capsules, which contain 1 gram of material?

The calculation follows:
• 1ppm = 1 ug/g
• 12ppm = 12 ug/g
• You have 12 ug per capsule.
Let’s take it one step further; if your results are in ppb, since 1 ppb equals 1000 ppm, 1 ppb equals .001ug/g. The above formula is adjusted as follows:
• 1 ppb = .001 ug/g
• 12 ppb = .012 ug/g
You have .012 ug per capsule.
And that’s how you do it.NW