VOLATILE ORGANIC COMPOUNDS BY VACUUM DIS

2.1 An aliquot of a liquid, solid, or tissue sample is transferred to a sample flask, which is then attached to the vacuum distillation apparatus (see Figure 1). Nominal sample sizes are 5 mL for water, 5 g for soil, 5 g for tissue, and 0.2 to 1 g for oil. The sample sizes may be varied, depending on analytical requirements, while using the same calibration curve. The surrogate corrections will compensate for variations in sample size. A total of 5 mL of reagent water is added to the aliquot of soil, tissue, or oil. 2.2 The sample chamber pressure is reduced using a vacuum pump and remains at approximately 10 torr (the vapor pressure of water) as water is removed from the sample. The vapor is passed over a condenser coil chilled to 5EC, which results in the condensation of water vapor. The uncondensed distillate is cryogenically trapped in a section of stainless steel tubing chilled to the temperature of liquid nitrogen (-196EC).After an appropriate distillation period, which may vary due to matrix or analyte group, the condensate contained in the cryotrap is thermally desorbed and transferred to the gas chromatograph using helium as a carrier gas. 2.4 Analytes eluted from the gas chromatographic column are introduced into the mass spectrometer via a jet separator or a direct connection. (Wide-bore capillary columns normally require a jet separator, whereas narrow-bore capillary columns may be directly interfaced to the ion source). 2.5 Quantitation is accomplished in three specific steps. 2.5.1 The first step is the determination of the response of each analyte at the mass spectrometer using external standard means. The amount (mass) of analyte introduced into the mass spectrometer is determined by comparing the response (area) of the quantitation ion for the analyte from a sample analyses to the quantitation ion response generated during the initial calibration. 2.5.2 The second step is the determination of surrogate recovery. The recommended surrogates are listed in Table 3. The recovery of each analyte can be predicted using the recovery-properties relationship solutions (see Sec. 12.8). 2.5.3 Finally, using the predicted recovery, sample size, and quantity of analyte detected at the mass spectrometer, the concentration of analyte is calculated. 2.6 The method includes specific calibration and quality control steps that supersede the general requirements provided in Method 8000. 2.7 It must be emphasized that the vacuum distillation conditions are optimized to remove analyte from the sample matrix and to isolate water from the distillate. The conditions may be varied to optimize the method for a given analyte or group of analytes. The length of time required for distillation may vary due to matrix effects or the analyte group of interest. Operating parameters may be varied to achieve optimum analyte recovery.