Units Of Measure Commonly Used In A Microbiology Laboratory
Molarity (M):
Molarity is the number of moles of solute per liter of solution. A 1M solution contains 6.023 x1023
molecules per liter. To calculate M, you need to know the molecular weight of the material.
For Example: Glucose (dextrose), a carbohydrate, has a formula (molecular) weight of 180.16 g. This
means that 6.023 x1023 molecules would weight 180.16 g. The molecular weight number comes from the
formula for D-glucose: CH2OH(CHOH)4CHO
6 carbons @ 12 g each
72
12 hydrogens @ 1 g each 12
6 oxygens @ 16 g each
96
TOTAL
180
If you were to weight out 180.16 g of D-glucose and dissolve it in enough water to give a final volume
of 1 liter, you would have a 1 M solution. [Note that solid material will contribute to the final volume of a
solution. Therefore, it is important initially to dissolve the glucose in less than 1 liter and then add water to
adjust the final volume to 1 liter (1000 ml).] More dilute solutions are written as follows:
1 mmole (millimole)
1 µmole (micromole)
1 nmole (nanomole)
1 pmole (picomole)
1 mM
1 µM
1 nM
= 10-3 moles
(1/1000 of a mole)
= 10-6 moles
(1/1000000 of a mole)
= 10-9 moles
(1/1000000000 of a mole)
-12
= 10 moles
(1/1000000000000 of a mole)
= 10-3 M = 1 mmole/liter = 1 µmole/ml
= 10-6 M = 1 µmole/liter = 1 nmole/ml
= 10-9 M = 1 nmole/liter = 1 pmole/ml
To make a 1 mM solution of D-glucose, you could weigh out 0.180 g
(or 180 mg) of D-glucose and dissolve it in enough water to give a
final volume of 1 liter, you would have a 1 mM solution.
% Weight or Volume:
The conventions above are the same for
other measurements (i.e. grams):
1 gram
1 milligram = 10-3 g
1 µgram = 10-6 g
Some recipes call for a certain percentage of material. For most
purposes in microbiology, this will be a weight/volume (w/v) or
volume/volume (v/v) recipe.
A X% (w/v) means X grams per 100 ml
A X% (v/v) means X ml per 100 ml
For Example: A 10% (w/v) D-glucose solution contains 10 g in 100 ml of liquid (usually water). You
can covert this to molarity (M) as follows: 10 g/100 ml is the same as saying 100g in 1000 ml (100 g per
liter). Because a 1M solution of glucose is 180 g/liter, a 100 g/liter solution = 100/180 or 0.55 M solution of
D-glucose. Therefore 10% (w/v) glucose is approximately equal to 0.55M glucose
Summary Of Most Common Abbreviations
weight (based on gram,
abbreviated g)
length (based on meter,
abbreviated m)
volume (based on liter,
abbreviated l or L)
DNA (based on base
pairs, abbreviated bp)
time (based on second,
abbreviated sec)
smaller----------------------------------to-------------------------------------larger
10-12
10-9
10-6 µ
10-3
103 K
microg
picog (pg)
nanog (ng)
millig (mg)
g
Kilog (Kg)
(µg)
microm
millim (mm) m Kilom (Kg)
(µm)
microl
l
nanol (nl)
milli (ml)
(µl)
(L)
Kilobp
bp
(Kbp)
nanosec
microse
millisec
picosec (psec)
sec
(nsec)
c (µsec)
(msec)