NATIONAL WATER QUALITY LABORATORY
TECHNICAL MEMORANDUM 2000.06
October 1, 2000
Subject: Elimination of Orthophosphate and Nitrate Analysis by Ion Chromatography
Author: Gary Cottrell, Inorganic Program
(303) 236-3490 (COTTRELL)
Purpose
Orthophosphate by ion
chromatography (IC), labcode 1262, and nitrate by IC (labcode 1261) will no
longer be offered after September 30, 2000.
Reasons for this change are the following: 1) better methods are
available; 2) there are concerns about sample stability; and 3) these methods
are low in demand (fewer than 250 tests per year).
The suggested replacements for
orthophosphate are labcode (LC) 1974, which has the same laboratory reporting
level (LRL) as the discontinued method (0.01 mg/L), or LC 1978, with an LRL of
0.001 mg/L. Labcodes 1974 and 1978 are
also less expensive than LC 1262. An
FCC bottle (125 mL, brown, polyethylene bottle) is required for either
replacement method.
Customers who require low-level
results for nitrate may still obtain them by requesting LC 1979
(nitrate-plus-nitrite) with an LRL of 0.005
mg/L and LC 1977 (nitrite) with an LRL of 0.001
mg/L, and then calculating the nitrate value.
If low-level data are not required, customers are encouraged to use LC
1975 and LC 1973 where LC 1975 has an LRL of 0.05 mg/L. In the case of low-level methods, costs for
the replacement labcodes are somewhat higher; however, customers who elect to
use labcodes 1975 and 1973 will realize small savings. By grouping the labcodes
as suggested the nitrate-plus-nitrite and nitrite will be analyzed
simultaneously under identical analytical conditions, thus providing the best
nitrate value. An FCC bottle is
required for the replacement methods.
Labcodes 1261 and 1262 will be
removed from existing schedules.
Customers will need to decide which replacement option will best fit the
data quality objectives of the project and add the replacement labcodes to each
Analytical Services Request form.
Customers may contact Steve Glodt (303-236-3721, srglodt) to update
existing schedules with desired replacement labcodes.
Background
The IC method that is being
eliminated uses a filtered, unchilled (FU bottle type) sample, but this bottle
type has never had a test of stability for orthophosphate or nitrate. The replacement (colorimetric) method is
performed on filtered, chilled (FCC bottle type) samples, and this sample type has
a proven stability (see Patton and Gilroy, 1999).
In addition to providing improved
results, the colorimetric methods for these tests will be less expensive for
customers who require both orthophosphate and nitrate.
During FY 1999 only 146 requests
for LC 1262 (orthophosphate) and 186 requests for LC 1261 (nitrate) were
received. The number of requests for both of these tests has been declining
each year.
Summary
After September 30, 2000, the NWQL
will no longer analyze samples for orthophosphate or nitrate by ion
chromatography. Improved methods are available, often at lower cost, and each
year there have been fewer requests for the IC methods.
All of the suggested replacement
methods require an FCC bottle, in which nutrients are stabilized by filtering
and chilling.
Customers with concerns should
contact Gary Cottrell (cottrell, 303-236-3490). Every effort will be made to minimize disruption during this
transition.
Reference
Patton, C.J., and Gilroy, E.J.,
1999, U.S. Geological Survey nutrient preservation experiment—Experimental
design, statistical analysis, and interpretation of analytical results: U.S.
Geological Survey Water-Resources Investigations Report 98-4118, 73 p.
For further information, please contact Gary Cottrell (COTTRELL) at (303) 236-3490.
Key Words
Orthophosphate, nitrate, ion chromatography, replacement, elimination, LC 1261, LC 1262
/Signed/
Gregory Mohrman, Chief
National Water Quality Laboratory
Branch of Analytical Services