Page:NIOSH Manual of Analytical Methods - 5600.pdf/20

 ORGANOPHOSPHORUS PESTICIDES: METHOD 5600, Issue 1, dated 15 August 1994 - Page 20 of 20 Table 9 (Continued) - Notes on ANALYTICAL CHARACTERISTICS A.

CHEMICAL AND PHYSICAL 1. Amide or phosphoramide, some slightly acidic, very polar chemically. 2. Alkyl thioether, easily oxidized to sulfone and sulfoxide. 3. Sulfoxides, easily oxidized to sulfone. Also very polar chemically. 4. Phosphite, easily air oxidized to phosphate (Merphos → DEF). 5. Vicinal dibromide, easily debrominated (Naled → Dichlorvos). 6. Two or more isomers commonly exist (e.g. Demeton-O and Demeton-S; cis- and trans- mevinphos). 7. Relatively volatile, can be lost if media or vials are left uncapped for even a short period of time. General: Organophosphorous compounds are easily destroyed at mildly alkaline conditions (pH ≥ 8). Loses can occur for trace levels of compounds on alkaline glass surfaces. Glassware should be neutralized after washing, if alkaline detergent is used.

B.

DESORPTION AND SOLUTION 1. Solubility of concentrated solutions in toluene enhanced by the addition of 1% methanol or 10% acetone. Solubility in hexane very unfavorable even for dilute solutions. 2. Changing from 100% toluene to 90/10 toluene/acetone, desorption from glass fiber filters improved from 62% to 98%, desorption from quartz fiber filters improved from 30% to 101%. 3. Changing from glass fiber filters to quartz fiber filters, desorption in toluene improved from 16% to 88% and desorption in 90/10 toluene/acetone improved from 70% to 99%. 4. These compounds are more chemically polar than the other listed organophosphorous compounds; desorption from XAD-2 or from glass or quartz fiber filters in hexane was incomplete or non-existent. Desorption in toluene was adequate except as noted in 2 and 3 above. The use of toluene containing 10% acetone improved recoveries for all analytes to satisfactory levels. 5. The desorption characteristics of these compounds were not evaluated. General: a. The presence of acidic hydrogen or double bonded oxygen anywhere in the molecular structure greatly decreases solubility in non-polar solvents and increases the difficulty of desorption from polar surfaces and sorbents. b. While glass fiber filters and toluene desorbant were adequate for most compounds in preliminary tests, the method was given wider application for the more polar compounds by the use of quartz fiber filters and 90/10 toluene/acetone desorbant. c. Solvents with lower boiling points (e.g., methylene chloride, chloroform, methyl-t-butyl ether, and ethyl acetate) had good desorption power nearing equivalent to 90/10 toluene acetone, but rendered less satisfactory gas chromatographic responses for the analytes. This effect may be due to better analyte mass-transfer from the injection port to the capillary column with higher boiling solvents using splitless or direct injection techniques.

C.

GAS CHROMATOGRAPHY 1. Poor chromatography may be encountered with dirty or undeactivated columns or injection ports. Clean quartz wool plugs stuffed in the injection port liner are better than silanized glass wool at reducing losses within the injection port. 2. Multiple, shifted, irregular, or severely tailing peaks may be observed in the chromatogram if degradation or oxidation of the analytes occur prior to injection, within the injection port, or during chromatographic separation on-column. 3. Multiple peaks may be observed due to presence of isomers. 4. Short elution time, compound may co-elute with solvent if oven temperature is too high. 5. Long elution time, compound may be lost if run time is too short, column or injection port is too cool, or split-vent-valve opens too soon when injected in splitless or direct injection mode. 6. Azinphos methyl did not elute from DB-1701 even though Azinphos ethyl did elute. 7. Potential internal standards: Triphenyl phosphate is more favorable if multiple analytes are expected because it is less volatile and elutes in an area of the chromatogram having fewer competing analytes.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94