How Does the 900 Series® Interface Convert Analog Signals to Digital Values?

This author is most familiar with TotalChrom® (upgraded from Turbochrom® PE-Nelson) from PerkinElmer Instruments. TotalChrom is a complete software package for non-mass spectro- metric chromatographic data acquisition, processing, and reporting, and for instrument control. Most leading analytical instrument manufacturers have their own complete software packages. Agilent has ChemStation®, etc. Some software packages such as EZ Chrom Elite® (ESA, Inc.) can be downloaded onto a PC, and if this PC can be properly interfaced, the software can be used with a variety of manufacturers’ chromatographs.

According to PerkinElmer Instruments:36

Each 900 Series Interface can be connected to one or two chromatographic detectors usually on a single instrument ... The 900 Series Interface converts an analog voltage signal to a frequency that varies in proportion to the signal voltage. The interface then counts the pulses and records a value every 0.01 second. The count accumulated during the interval is called a time slice. The value of each time slice, or the sum of two or more time slices, becomes a data point on the chromatogram ... Because the interface always records a count every 0.01 seconds, its fundamental sampling rate is 100 points per second. However, you can define a lower sampling rate in the method ... If you use a rate that is slower than the fundamental rate, the interface sums the appropriate number of slice values taken at the fundamental rate. This integrated value becomes a data point. The number of time slices that are summed to derive a data point depends on the desired sampling rate. For example, if the method calls for a rate of 10 points/sec, the interface sums 10 time slices taken at the fundamental rate.

Nine distinct tasks are identified below if a GC has been downloaded with TotalChrom. Each task is executed starting at the top and proceeding to the bottom.

  • • Baseline subtraction
  • • Peak detection
  • • Peak integration
  • • Component identification (optional)
  • • Calibration
  • • Quantitation
  • • Report generation (optional)
  • • Replot generation (optional)
  • • Post analysis programs (optional).

Let us focus on peak integration while leaving the other eight topics to the user who must operate a given instrument. This assumes that the software has identified a chromatographically resolved peak in a plot of instrument response against run time, i.e., the time after injection of the sample. A sketch of a chromatographically resolved peak is shown in Figure 2.29.

Referring to Figure 2.29, the peak area is found by first dividing the area beginning at a raw data point that corresponds to the peak stop and extending horizontally backward to the data point that corresponds to the peak start. In the above sketch, eight area slices of equal width are shown. The start point of the peak does not contribute to the peak’s area. To integrate the area under this curve, the software first sums the area slices from the peak start to the peak end. Initially, these slices extend vertically from the level of a data point to the zero-microvolt level. Next, the software corrects this sum of the height of the baseline by subtracting the baseline area. This baseline area is the area of a trapezoid between the baseline and the zero-microvolt level. A correct peak area results. This peak is proportional to the amount of analyte injected into the chromatograph,

FIGURE 2.29

and this forms the most fundamental basis of TEQA. In the above sketch, note that the peak is somewhat skewed; i.e., the peak is slightly unsymmetrical or is said to be not perfectly Gaussian.

What Are Integration Parameters?

One other aspect of using chromatographic software that the author wishes to address in this chapter is that of integration parameter settings. Every software package has some sort of integration parameter settings protocol. The settings should be established after a preliminary chromatogram has been obtained and reviewed. Much of a chromatogram will have unnecessary and even unwanted noise and peaks. The integration parameter settings assist in eliminating these undesirable portions of the baseline. These parameter settings also assist the analyst who wants to properly integrate a pair of partially chromatographically resolved peaks. Tabulated below are those parameters as found in Agilent’s ChemStation® software.

Integration Parameter Settings

Initial area reject

Initial peak width

Shoulder detection

Initial threshold

Area reject

Area sum off and/or on

Baseline all valleys off and/or on

Baseline back

Baseline hold off and/or on

Baseline next valley

Baseline now

Integrator off and/or on

Negative peak off and/or on

Peak width

Solvent peak off and/or on

Tangent skim

Threshold

Integration parameters whose entry includes the word initial are default settings and are initiated without user intervention whatsoever. Integrator enables integration to start and stop at preselected time events across the chromatogram. The area sum parameter enables the analyst to sum multiple peaks across a specific time interval in the chromatogram. The user of such software must understand how most of these parameters can be used to yield an analytical method

in the software that is efficient as well as to minimize computer memory and disk storage

FIGURE 2.30 in the software that is efficient as well as to minimize computer memory and disk storage. The manner by which some of these parameters are employed is seen in Figure 2.30 for a simulated chromatogram that reveals undesirable regions along the time axis. The arrows pointing upward represent those timed events that should be programmed in to eliminate unwanted peak integration due to the presence of baseline noise, interfering peaks, and certainly the initial solvent peak (sometimes called a solvent front) that is present when more universal GC detectors are used.

How Are Analytical Results Reported?

The organization of sample results that follows data reduction and interpretation into a tabular format for reporting purposes has evolved significantly over the past 30 years. Laboratory PC software that controls and acquires data all has some type of reporting protocols. The analyst today has numerous options with regard to reporting and presenting analytical results. Reports can be printed out on the accompanying printer right in the laboratory. Various summary versions of sample batches can be printed out in tabular formats. Raw data and results can be transferred to a thumbnail or flash drive and transported to a PC (earlier 3!/2-in. disks were used and even earlier PCs used floppy disks that were 514 in. in length and did indeed flop). Older flash drives can store up to 15 GB while newer ones can reach 30 GB! Software such as MassHunter® (Agilent Technologies) provide numerous unique opportunities for analysts to report results! Most chromatography data acquisition and control software commercially available has algorithms that enable a specific raw data or result file to be converted to an ASCII (American Standard Code for Information Interchange) file. This ASCII file can be imported into an Excel spreadsheet or an Access database. In recent years, the computer architecture has changed so as to make archiving on hard or portable disk obsolete. The client-server architecture in which the laboratory PC becomes the client will succeed in making even flash drives obsolete in the future. Interested readers may find an earlier resource by Dyson of interest.37 McDowell and colleagues have addressed this topic and continue to write a continuous column on this subject.38-39

 
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