Portable Spectrophotometer / Colorimeter (Field Use)

SOP-F208 Revision 0.2
Author

Marc Los Huertos

EA Program, Pomona College

Published

2026-06-26

1 Purpose

This SOP describes the field operation of the Hach DR900 portable colorimeter for on-site water quality analysis (Hach Company, 2024a, 2024b). The DR900 enables rapid, field-based measurement of nutrients, metals, and other parameters using Hach pre-packaged reagent kits. Field colorimetry provides same-day results when samples cannot be transported to the lab within required hold times and supports real-time decisions during stormwater events, spill response, or reconnaissance sampling (APHA et al., 2012).

This SOP focuses on field-specific procedures (transport, outdoor operation, field QC). For detailed method chemistry, reagent handling, the Method Registry, and laboratory use of the DR3900 and DR900, see SOP-L303 Hach Colorimetric Analysis.

2 Scope

  • Applies to: Field use of the Hach DR900 portable colorimeter at EA Program field sites for grab-sample analysis of nutrients (PO4, NO3, NH3), metals (Fe, Mn, Cu), and general chemistry (hardness, chloride, silica, chlorine)
  • Does not apply to: Laboratory bench-top analysis (see SOP-L303); field turbidity (see SOP-F205); in situ sonde measurements (see SOP-F200)
  • Location: All EA Program surface water, groundwater, and stormwater field sites

3 Responsibilities

Role Responsibility
PI / Field Lead Select analytes and approve field methods
Trained Personnel Operate the DR900 per this SOP; record results
New Students Complete training on both this SOP and SOP-L303 before
independent field use

4 Definitions

  • Beer-Lambert law: The linear relationship between the absorbance of a solution and the concentration of the absorbing analyte, expressed as \(A = \varepsilon c l\), where \(\varepsilon\) is the molar absorptivity, \(c\) is concentration, and \(l\) is path length; the physical basis of colorimetric analysis (APHA et al., 2012).
  • Blank (reagent blank / DI blank): A deionized water sample carried through the complete analytical procedure; used to zero the instrument and establish the baseline absorbance of the reagent system; must read at or near zero before sample measurements begin.
  • Detection limit (MDL): The minimum concentration reliably distinguishable from the blank; Hach method-specific MDLs are listed in the Method Registry in SOP-L303 (Hach Company, 2024b).
  • Field duplicate: A second independent measurement of the same sample, prepared through the full analytical procedure, to assess precision; RPD must be < 20% for concentrations above 5 times the detection limit.
  • Reaction time: The incubation period specified in the Hach method protocol after reagent addition during which the color-forming reaction proceeds to completion; reading before reaction time has elapsed gives systematically low results (Hach Company, 2024a).
  • RPD (relative percent difference): |A - B| / ((A + B) / 2) × 100; used to assess duplicate precision.
  • Stored program: A factory-programmed method on the DR900 that specifies wavelength, calibration curve, and units for a specific Hach reagent kit; accessed by program number on the instrument (Hach Company, 2024a).
  • Zeroing: The process of inserting a DI water blank into the DR900 sample chamber and pressing Zero to set the baseline absorbance to zero for the selected program; required before reading samples.

5 Safety and Hazards

WarningHazards
  • Chemical: Hach reagent powder pillows and liquid reagents may contain acids, oxidizers, or heavy metal indicators. Review the SDS for each kit before field use. Cadmium-containing reagents (older NitraVer 5 formulation) are hazardous waste.
  • Physical: Glass sample vials can break during transport; pack in padded cases.
  • Biological: Water samples may contain pathogens. Wear gloves.

5.1 Required PPE

5.2 Emergency Procedures

For reagent splashes to skin or eyes, flush with clean water for at least 15 minutes. Carry a squeeze bottle of DI water for emergency rinsing. Refer to SOP-F001 Field Safety and SOP-L003 Hazardous Materials Handling.

6 Equipment and Materials

Item Notes
Hach DR900 Portable Colorimeter Charge before field day (Hach Company, 2024a)
Sample vials (10 mL, 25 mL, 1-inch round) Matched to the methods being run
Hach reagent kits Check expiration dates; select analytes per project
DI water (squeeze bottle, ≥ 500 mL) For blanks, rinsing, and dilutions
Lint-free wipes (Kimwipes) For cleaning vial exteriors
Silicone oil and applicator cloth For 1-inch round vials
Timer Phone timer or stopwatch for reaction times
Waste container (sealable) For used reagent vials and waste solution
Padded carrying case For protecting vials during transport
Field notebook Per SOP-F003

7 Procedure

7.1 Pre-Field Preparation

  1. Identify the analytes to be measured; pack the corresponding reagent kits and verify none are expired.
  2. Pack at least 500 mL DI water and two extra clean sample vials per method.
  3. Charge the DR900 battery and confirm stored programs for your methods are accessible (Hach Company, 2024a).
  4. Print or photograph method procedure cards for each analyte (from the kit box or SOP-L303 Method Registry).

7.2 Field Setup

  1. Place the DR900 on a flat, stable surface out of direct sunlight. Direct sunlight on the sample chamber causes erroneous readings.
  2. If no shade is available, shade the instrument with a jacket or tarp during each reading.
  3. Turn on the DR900 and allow it to complete its self-check.
  4. Select the stored program for your analyte.

7.3 Blank and Calibration Check

  1. Fill a clean sample vial with DI water to the fill line.
  2. Wipe the vial exterior with a lint-free cloth.
  3. Insert the vial, close the lid, and press Zero.
  4. The display should read 0.00 (within the method detection limit); if not, clean the vial and re-zero.
  5. If a known mid-range standard is available, read it immediately after zeroing; the result must be within ±10% of the stated value. Record the result in the field notebook (Hach Company, 2024b).

7.4 Sample Analysis

Follow the method-specific procedure from the reagent kit or SOP-L303 Method Registry:

  1. Collect a grab sample per SOP-F300.
  2. Fill the sample vial to the fill line with sample.
  3. Add the reagent per method instructions.
  4. Cap, invert, and mix as directed. Start the timer for the required reaction time.
  5. When the reaction time is complete, wipe the vial exterior (apply silicone oil for round vials).
  6. Insert the vial into the DR900, close the lid (shield from sunlight), and press Read.
  7. Record the result, analyte, Hach method number, sample ID, units, date, and time in the field notebook.
  8. Rinse the vial three times with DI water before the next sample.

7.5 Waste Management

  1. Collect all used reagent packets, vials, and waste solution in a sealable container.
  2. Do not discard reagent waste in the field.
  3. Return waste to the lab and dispose of per SOP-L003.

8 Quality Control

  • Zero the instrument with DI water before each batch of samples.
  • Run one DI blank and one mid-range standard at the start of each field session; record results in the field notebook (Hach Company, 2024b).
  • Analyze one field duplicate per 10 samples (minimum one per event); RPD must be < 20% for concentrations above 5 × the detection limit.
  • If the field blank reads above the MDL, investigate contamination (dirty vials, contaminated DI water) and re-run.
  • Note ambient temperature; some Hach methods are temperature-sensitive. If the sample is outside the method-specified range (typically 20–25 °C), note the potential bias (Hach Company, 2024a).

9 References

APHA, AWWA, & WEF. (2012). Standard methods for the examination of water and wastewater (22nd ed., p. 1360). American Public Health Association.
Hach Company. (2024a). DR900 portable colorimeter: Procedures manual. https://www.hach.com/dr900-portable-colorimeter/product?id=56440352476
Hach Company. (2024b). Water analysis guide (7th ed.). https://www.hach.com/wag

11 Training Record

Before using the DR900 in the field independently, personnel must:

  1. Read this SOP and SOP-L303 in full
  2. Demonstrate a complete field analysis (zero, standard check, sample analysis, waste collection) under supervision
  3. Be signed off by the PI or field lead